CV: Morten Andersen form number 76740 LAM - Laboratoire d’Astrophysique de Marseille Pole de l’Etoile Site de Chˆ ateau-Gombert 38, rue Frd´ eric Joliot-Curie 13388 Marseille cedex 13 FRANCE Employment History 2014 - Present Postdoctoral Researcher, LAM, Marseille 2012- 2014 Postdoctoral Researcher, IPAG, Grenoble 2009 - 2012 Internal Research Fellow, ESTEC 2007 - 2009 Postdoctoral Scholar, Spitzer Science Center, California Institute of Technology 2004 - 2007 Research Associate, University of Arizona, Steward Observatory 2000 - 2004 PhD student, Astrophysical Institute, Potsdam Academic Education 2005 PhD Astrophysics, Potsdam University, Germany Advisers: Dr. H. Zinnecker (SOFIA) & Prof. M. J. McCaughrean (ESA) 2000 MSc Astronomy, Copenhagen University, Denmark Advisers: Dr. Jens Knude (Copenhagen University) & Dr. Bo Reipurth (IfA) Research Interests The Initial Mass Function in Young Star Clusters Triggered Star Formation Formation and Evolution of Low-mass Stars and Brown Dwarfs Dust Production, Destruction and Evolution in Supernova Remnants Core shine, grain growth Teaching Experience PhD Course: Star Clusters as Astrophysical Laboratories, Grenoble University 2013 Teaching assistant in “Experimental astrophysics”, 1999, Copenhagen University Teaching assistant in “Computer science for physics students”, 1998, Copenhagen University Mentoring Co–supervision of Alan Aversa, undergraduate student at the University of Arizona, 2006/2007 Grants Obtained HST GO programme 11708.01 entitled ”Determining the Sub-stellar IMF in the Most Massive Young Milky Way Cluster, Westerlund 1” was awarded 20 orbits and an HST grant of $115.000. 1
Transcript
CV: Morten Andersen form number 76740
LAM - Laboratoire d’Astrophysique de Marseille Pole de l’Etoile Site de Chateau-Gombert
38, rue Frderic Joliot-Curie 13388 Marseille cedex 13 FRANCE
Sep. 2007 IPAC, Pasadena: Constraints on the sub–stellar IMF
Nov. 2006 Cool stars 14 meeting, Pasadena: Ratios of stars to sub-stellar objects:
Constraining the low–mass IMF from observations of young clusters
June 2006 ESO, Santiago: Is the initial mass function universal?
May 2005 IAU symposium 227: The IMF in extreme Star-forming environments:
Searching for variations vs. initial conditions
Feb. 2005 NOAO: The low mass stellar content in the young massive star clusters
NGC 3603 and 30 Dor
Nov. 2004 Astrophysical Institute Potsdam: Determining the low–mass stellar content in
massive star clusters
Feb. 2004 Sheffield University: The low mass stellar content in R136 and NGC 3603
Nov. 2003 ESO, Garching: The low mass stellar content in the young massive star clusters
NGC 3603 and 30 Dor
Nov. 2002 Danish Physical Society Annual Meeting: Young massive star clusters and their
low mass stellar content.
Nov. 2002 Copenhagen University: Star formation in M16, a typical example
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Publications:Morten Andersen Form number 76740
Refereed papers
Andersen, M., Gennaro, M., de Marchi, G., Meyer, M. R., Brandner, W., Stolte, A. & Zinnecker, H.
2015, The very low mass stellar content in the young super-massive Galactic star cluster Westerlund 1
Astron. Astrophys. Accepted
Lefevre, C., Pagani, L. et al., Juvela, M., Paladini, R., Lallement, R., Marshall, D. J., Andersen, M.
et al., 2014, Dust properties inside molecular clouds from coreshine modeling and observations Astron.
Astrophys. 572, 20L
Andersen, M., Thi, W.-F., Steinacker, J., Tothill, N., 2014, A common column density threshold for
scattering at 3.6 µm and water-ice in molecular clouds Astron. Astrophys. 568, L8
Holverda, B., Trenti, M., Clarkson, W., Sahu, K., Bradley, L., Stiavelli, M., Pizkal, N. de Marchi, G.,
Andersen, M., Bouwens, R. & Ryan, R. 2014, Milky Way Red Dwarfs in the BoRG Survey, Galactic
scale-height and the distribution of dwarfs stars in WFC3 imaging Astrophys. J. 788, 77
Steinacker, J., Ormel, C. W., Andersen, M., Bacmann, A. 2014, Coreshine in L1506C - Evidence for a
primitive big-grain component or indication for a turbulent core history? Astron. Astrophys. 564, A96
Parker, R. & Andersen, M. 2014, Spatial differences between stars and brown dwarfs: a dynamical
origin? Mon. Not. Roy. Astron. Soc. 444, 784
Steinacker, J., Andersen, M., Thi, W.F, & Bacmann, A. 2014, Detecting scattered light from low-mass
molecular cores at 3.6 µm Astron. Astrophys. 563 A106
Andersen, M., Steinacker, J, Thi, W-F, Pagani, L Bacmann, A. & Paladini, R. 2013, Constraining dust
grain parameters through near-infrared cloudshine and coreshine Astron. Astrophys. 559 60A
Robberto et al. 2013, The Hubble Space Telescope Treasury Program on the Orion Nebula Cluster
Astrophys. J. Suppl. 207, 10
Sala, G., Haberl, F., Jose, Parikh, A., Longland, R. & Andersen, M., 2012, How strange is the Rapid
Burster? Constraints on the mass and radius of the accreting object in MXB 1730-3345 Astrophys. J. 752,
158
Kudryavtseva, N., Brandner, W., Gennaro, M., Rochau, B., Stolte. A., Andersen, M., et al. 2012,
Instantaneous starburst of the massive clusters Westerlund 1 and NGC 3603 YC Astrophys. J. 750, L44
Cottaar, M., Meyer, M. R., Andersen, M. & Espinoza, P., 2012, Dynamical state and stability of
Westerlund I Astron. Astrophys. 539, A5
Andersen, M., Rho, J., Reach, W. T., Hewitt, J., Bernard, J–P. & Tappe, A. 2011, Dust Processing in
Supernova Remnants: Spitzer MIPS SED and IRS Observations Astrophys. J. 742, 7A
Andersen, M, Meyer, M. R., Roberto, M. R., Bergeron, L. E. & Reid, N., 2011, The low-mass initial
mass function in the Orion Nebula cluster based on HST/NICMOS III imaging Astron. Astrophys. 534, 10
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De Marchi, G., Paresce, F., Panagia, N., Beccari, G., Spezzi, L., Sirianni, M., Andersen, M. et al. Star
formation in 30 Doradus Astrophys. J. 739, 27
Spezzi, L., Beccari, G., De Marchi, G., Young, E. T., Paresce, F., Dopita, M. A., Andersen, M., et al.
2011, Detection of Brown Dwarf Like Objects in the Core of NGC 3603 Astrophys. J. 731, 1
Beccari, G., Spezzi L., De Marchi, G., Paresce, F., Young, E., Andersen, M., et al., 2010, Progressive
star formation in the young galactic super star cluster NGC 3603 Astrophys. J. 720, 1108
Andersen, M., Zinnecker, H., Moneti, A., Brandl, B., Brandner, W., Meylan, G. & Hunter, D. 2009,
The low–mass IMF in the 30 Doradus starburst cluster, Astrophys. J. 707, 1347
Hewitt, J., Rho, J, Andersen, M & Reach, W. T., 2009. A Spitzer view of interacting supernova
remnants Astrophys. J. 694, 1266
Rho, J., Jarrett, T., Reach, W., Tappe, A., Andersen, M. & Gomez, H. 2009, Carbon monoxide in the
Cassiopeia A Supernova Remnant, Astrophys. J. Lett. 693, 39
Andersen, M., Meyer, R. M., Greissl, J. & Aversa, A. 2008, Evidence for a Turnover in the Sub–stellar
IMF. Analysis from an Ensemble of Young Clusters, Astrophys. J. Lett. 683, 183
Fontaine, G., Green, E. M., Chayer, P., Charpinet, S., Andersen, M. & Portouw, J. 2008, Radiative
Levitation: A likely explanation for the presence of pulsations in the unique hot O subdwarf star SDSS
J160043.6+074802.9, Astron. Astrophys. 486, 39L
Linsky, J.L., Gagne, M., Mytyk, A., McCaughrean, M.J, & Andersen, M. 2007, Chandra Observations
of the Eagle Nebula. I. Embedded Young Stellar Objects near the Pillars of Creation, Astrophys. J. Lett.
654, 347
Andersen, M., Meyer, M.R, Oppenheimer, B.R, Dougados, C. & Carpenter, J.M. 2006, NICMOS/HST
Observations of the Embedded Cluster Associated with Mon R2: Constraining the sub–stellar Initial Mass
Function, Astronom. J. 132, 2296
Andersen, M., Knude, J., Reipurth, B., Castets, A., Nyman, L. A, McCaughrean, M. J. & Heathcote, S.
2004, Molecular cloud structure and star formation near HH 216 in M16, Astron. Astrophys. 414, 969
Brandner, W., Martin, E. L., Bouy, H., Kohler, R., Delfosse, X., Basri, G. & Andersen, M. 2004,
Astrometric monitoring of the binary brown dwarf DENIS-P J1228.2-1547, Astron. Astrophys. 428, 205
McCaughrean, M. J. & Andersen, M. 2002, The Eagle’s EGGs: fertile or sterile? Astron. Astrophys.
389, 513
Scholz, R.-D., Szokoly, G. P., Andersen, M., Ibata, R.& Irwin, M. J. 2002, A New Wide Pair of Cool
White Dwarfs in the Solar Neighborhood, Astrophys. J. 565, 539
Beuther, H., Schilke, P., Gueth, F., McCaughrean, M., Andersen, M., Sridharan, T. K. & Menten, K.
M. 2002, IRAS 05358+3543: Multiple outflows at the earliest stages of massive star formation, Astron.
Astrophys. 387, 931
Andersen, M. & Kimeswenger, S. 2001, NOVA Sco 2001 (V1178 SCO) , Astron. Astrophys. 377, 5L
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Contributed papers and abstracts
Andersen, M., Steinacker, J., Thi, W,-F,, Pagani, L., Bacmann, A.; Paladini, R., 2013, Scattering in
molecular clouds: Constraining the dust grain size distribution through near-infrared cloudshine and mid-
infrared coreshine, Protostars and Planets VI, Heidelberg, July 15-20, 2013. Poster #1S041
Steinacker, J., Andersen, M.; Thi, W.-F. & The SEED Team, 2013, SEED - Exploring the seed popula-
tion of dust grains for planet formation with coreshine, Protostars and Planets VI, Heidelberg, July 15-20,
2013. Poster #1B012
Paladini, R, Pagani, L., Steinacker, J., Lefevre, C., Andersen, M. et al., 2013, Hunting Coreshine with
(Warm) Spitzer: from grain growth to planet formation, Protostars and Planets VI, Heidelberg, July 15-20,
2013. Poster #1S060
Spezzi, L. Beccari, G., De Marchi, G., Paresce, F., Sirianni, M., Andersen, M. & Panagia, N., 2012,
Stellar Populations in the Super Star Clusters NGC 3603 and 30 Doradus, Star Clusters in the Era of Large
Surveys, Astrophysics and Space Science Proceedings. Springer-Verlag Berlin Heidelberg
Cottaar, M., Meyer, M. R., Andersen, M., & Espinoza, P. 2011, Dynamical state of Westerlund 1, Stellar
Clusters & Associations: A RIA Workshop on Gaia, 113
Andersen, M., Meyer, M. R., Robberto, M., Reid, I. N., & Bergeron, L. E. 2011, The Low-Mass
Initial Mass Function in the Orion Nebula Cluster Based on HST/NICMOS III Imaging, Stellar Clusters &
Associations: A RIA Workshop on Gaia, 28
Rho, J., Hewitt, J., Reach, W., Andersen, M & Bernard, J.-P., 2011, Shock-Induced Molecular Astro-
chemistry in Dense Clouds Int. Symposium On Molecular Spectroscopy, 66th Meeting, Ohio
De Marchi, G., Paresce, F., Panagia, N., Beccari, G., Spezzi, L., Sirianni, M., Andersen, M. et al. 2011,
Recent star formation in 30 Doradus Bulletin of the American Astronomical Society, 36, 1117
Rho, J., Andersen, M., Tappe, A., Reach, W. T., Bernard, J. P. & Hewitt, J. 2011, PAH and Dust
Processing in Supernova Remnants, EAS Publications Series, Volume 46, 2011, pp.169-175
Spezzi, Loredana, Beccari, G., Young, E., De Marchi, G., Paresce, F., Sirianni, M., Andersen, M. et al.
2010, Using HST-WFC3 Photometry To Classify Brown Dwarfs In The Field Of NGC3603 Bulletin of the
American Astronomical Society, 36, 1117
Beccari, G., Spezzi, L., Young, E., De Marchi, G., Paresce, F., Sirianni, M., Andersen, M. et al. 2010,
A panchromatic study of NGC3603 Bulletin of the American Astronomical Society, 36, 1115
De Marchi, G. Paresce, F., Sirianni, M., Spezzi, L., Andersen, M. et al. 2010, Star formation in 30
Doradus Bulletin of the American Astronomical Society, 36, 1114
Rho, J., Reach, W. T., Tappe, A., Rudnick, L., Kozasa, T., Hwang, U., Andersen, M., Gomez, H.,
Delaney, T., Dunne, L., & Slavin, J. 2009, Dust Formation Observed in Young Supernova Remnants with
Spitzer Conference proceedings: Cosmic Dust - Near and Far ASP Conference Series, Vol. 414
Fontaine, G., Brassard, P., Green, E. M., Chayer, P., Charpinet, S., Andersen, M. & Portouw, J., Pul-
sational instabilities in the hot sdO star SDSS J1600+0748: The key role of radiative levitation Journal of
Physics: Conference Series, Volume 172, Issue 1, pp. 012079
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Rho, J, Jarrett, T., Reach, W., Gomez, H. & Andersen, M. 2009, Carbon Monoxide in the Cassiopeia
A Supernova Remnant Bulletin of the American Astronomical Society, 41, 217
Linsky, J. L., Gagne, M., Mytyk, A., McCaughrean, M, & Andersen, M. The Eagle Nebula: Pillars
of Creation, EGGs, and PMS Stars in NGC 6611 Proceedings 14th Cambridge Workshop on Cool Stars,
Stellar Systems, p59
Andersen, M., Rho, J., Hewitt, J. & Reach, W. 2007, PAH Features in Supernova Remnants, Bulletin
of the American Astronomical Society, 213, 403.01
Hewitt, J., Rho, J., Andersen, M. & Reach, W. 2007, The Nature of Shocks in Supernova Remnants
3C 396, Kes 69 and G346.6-0.2 revealed by Spitzer, Bulletin of the American Astronomical Society, 211,
115.01
Andersen, M., Rho, J., Hewitt, J. & Reach, W. 2007, PAH Features in Supernova Remnants, Conference
Proceedings: The Evolving Interstellar Medium in the Milky Way and Nearby Galaxies
Andersen, M., Meyer, M.R., de Grijs, R., Portegies Zwart, S. & Greissl, J. 2006, Featured image of
Westerlund 1 from NACO/VLT, for “Science of the Solstice”, Nature, 441, 1040
Andersen, M., Meyer, M.R., Bergeron, L.E., Robberto, M., Smith, K. & Reid, I.N. 2006, HST/NICMOS
imaging of the Orion Nebula Cluster: Constraining the low mass IMF, Bulletin of the American Astronom-
ical Society, 37, 1376
Andersen, M., Meyer, M. R., Greissl, J., Oppenheimer, B. D., Kenworthy, M. A. & McCarthy, D. W.
2005, The IMF in Extreme Star–forming Environments: Searching for Variations vs. Initial Conditions,
IAU Symp. No. 227, 285
Andersen, M. & Zinnecker, H. 2004, The 30 Doradus Starburst Cluster: Infrared Luminosity Function
and Low-Mass IMF in a Spatially Resolved Dense Young Stellar System, ANS 325, 35
Brandl, B. R. & Andersen, M., 2004, The Starburst IMF – An Impossible Measurement? IMF@50:
The Initial Mass Function 50 years later, eds: Corbelli, E., Palla, F. & Zinnecker, H., Springer Verlag
Poteet, C, Marchenko, S., Corcoran, M & Andersen, M, 2004, Revealing the Nature of Faint X-Ray
Sources in the Giant Star–forming Region NGC 3603, Bulletin of the American Astronomical Society, 36,
1380
Andersen, M., Zinnecker, H., Brandl, B., Meylan, G. & Moneti, A. 2002, The H band luminosity
function of the centre of the 30 Dor cluster, Extragalactic Globular Cluster Systems, ed Kissler-Patig, M,
Springer-Verlag series “ESO Astrophysics Symposia”
McCaughrean, M., Zinnecker, H., Andersen, M., Meeus, G. & Lodieu, N., 2002, Standing on the
shoulder of a giant: ISAAC, Antu, and star formation, ESO Messenger 109, 28
Zinnecker, H., Andersen, M., Brandl, B., Brandner, W., Hunter, D., Larson, R. , McCaughrean, M. J.,
Meylan, G. & Moneti, A. 2002, The Infrared Luminosity Function in the 30 Dor Cluster, IAU Symp. No.
207, 531
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Previous ResearchMorten Andersen form number 76740
My main fields of research are observational studies of star formation in clustered environments and of dustgrowth and destruction in molecular clouds. I am in particular interested in the shape of the Initial Mass Functionin massive star clusters and whether the IMF varies as a function of environment. My studies of star clustersis predominantly clusters in the Galaxy or in the MagellanicClouds due to their proximity and the possibilityto resolve the individual stars instead of having to rely of the integral properties which is necessary for moredistant systems. This provides a direct measure of the cluster content much less model dependent than possiblethrough integrated properties. The observations are challenging, requiring high spatial resolution and sensitivityin the near-infrared and longer wavelengths to fully characterise the clusters and their environment.
Understanding the star forming regions and in particular massive star forming regions is important to bothunderstand the stellar and brown dwarf outcome of the star formation process but equally important understand-ing the feedback on the surrounding Interstellar Medium (ISM). Metals from the most massive stars are shortlyafter their birth distributed into the surrounding gas and the gas is being heated and compressed setting the stagefor subsequent star formation.
I have taken advantage of the optical and in particular near-infrared capabilities from the ground on largeaperture telescopes as well as from space usingSpitzer, Herschel and in particular theHST. In the short termI plan to use these facilities in my science together with therecent generation of near-infrared spectrographsand imagers from the ground. Of particular interest is the opportunities with the multiplexing capabilities in thenear-infrared. Below I provide an overview of my previous research and accomplishments.
1 Star formation in clustered environments
It has become clear that star formation rarely occurs in isolation but most often in clustered environmentsranging from sparse regions like e.g. the Taurus star forming region to massive star burst environments observedin interacting galaxies. Thus, our understanding of star formation is not complete unless we understand it inclusters and clustered environments. One particular important question is whether the Initial Mass Function(IMF), the distribution of stellar and brown dwarf masses at birth, is universal or if it depends on environment.Determining the shape of the IMF is one of the most important problems in astronomy and has implications farbeyond star formation. It is a key ingredient in our understanding of the assembly of galaxies, their chemicalevolution, and in the interpretation of the mass-to-light ratios of stellar populations.
Theoretically it is expected that the IMF should depend on environment, for example pressure and metallicity(Krumholz 2011, ApJ, 743, 110) or cluster density (e.g. Bate2009, MNRAS, 392, 590, Elmegreen, 2004,MNRAS, 354, 357) or as a function of turbulent Jeans Mass (Chabrier, Hennebelle & Charlot, 2014, ApJ, 796,75). However, studies of relatively low-mass (less than 1000 M⊙ and all solar metallicity) clusters within some2 kpc indicate that the derived stellar IMFs are similar (e.g.Bastian, Covey & Meyer, 2010, ARA&A, 48, 339).Thus, to search for variations (or lack thereof), in the stellar IMF, I observe young star clusters, both nearbylower mass regions and in particular massive star clusters,in order to compare their derived IMF with those ofnearby star forming regions and the field. Further, by extending the studies to the Magellanic Clouds one canstudy metal-poor environments in details. These are a proxy for star formation in the early Universe where themetallicity is lower than currently in the Milky Way. Since the peak of star formation occurred at high redshiftit is of importance to understand how the star formation process depends on metallicity.
The nearest massive clusters are relatively distant which, combined with their larger density than nearbyregions, make high spatial resolution together with high sensitivity essential to probe the low-mass IMF in thesesystems. Only with the spatial resolution of theHST or with adaptive optics observations from the groundcan we currently probe the low-mass content in the clusters.For example, the Large Magellanic Cloud is at adistance of 50 kpc, so 1′′ corresponds to 0.25 pc.
1.1 Young Massive Clusters
The most massive star forming region in the Magellanic Cloudsis 30 Doradus which is also the most luminousoptically visible HIIregion within the local group. It thus serves both as a directprobe of star formation in a low-metallicity environment and as a local resolvable templatefor the more distant starburst environments. I haveanalyzed HST/NICMOS H band observations of the central 1 arcminute square of the cluster, covering R136,to determine the low-mass IMF in a massive low-metallicity environment. The high spatial resolution possiblewith the HST combined with the capability of a stable point spread function across the cluster is fundamentalfor these studies and to be able to quantify the effects of crowding and limited sensitivity.
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P1311 MK34
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Figure 1: Left: NICMOS II F160W (H band) mosaic of the central 1′ square (corresponding to 15 pc) of R136 within 30 Doradus in theLarge Magellanic Cloud. The boxes indicate the regions previously studiedby Sirianni et al. where they claimed a flattening of the IMFwas detected at 2 M⊙. The two circles indicate a radius of 1 and 5 pc, respectively. Right: The result from a simulation of the effectsof differential reddening on the derived IMF from optical data if not taken into account. The straight line is the input Salpeter IMF, theplus symbols are the measured IMF by Sirianni et al. The arrow indicatesthere 50% completeness limit quoted as their lower limit inthe survey. After adding differential reddening of AV = 1− 3mag the input IMF is transformed into the dashed line which correspondswell with the data down to the completeness limit (Andersen et al. 2009, ApJ,707, 1347).
I derived star counts for the cluster and corrected for incompleteness due to crowding. The correction forincompleteness is crucial for studies of the low-mass end ofthe IMF. Due to limited resolution and sensitivityobjects will be missed due to low signal to noise and overlapping sources in projection. Both effects pointstowards a systematic bias in that the faintest, lowest mass,objects are undetected. This will again result in aderived IMF that is artificially deficient in low-mass objects compared to the true underlying IMF. Thus, it iscrucial to quantify this effect and correct for it before conclusions on the cluster IMF can be deduced.
The correction is done through a series of artificial star experiments. Stars with random but known mag-nitudes and spatial location are placed in the observed frames using the telescope point spread function, eithercreated from models or from stars within the field of view. In this particular case we opted for the syntheticpoint spread function available from the Space Telescope Science Institute due to the limited number of isolatedhigh signal to noise stars, whereas in other studies with a larger field of view I have used point spread functionscreates from the data. The point source identification and photometry is then re-done on the frames with artifi-cial stars using exactly the same detection criteria and photometry setting as for the original data. The numberof recovered artificial stars compared to the number put in as afunction of magnitude and surface density ofsources then provides the statistical fraction of stars detected to those lost due to crowding and bright stars. Inorder to obtain a statistical significant ratio, many artificial star experiments have to be done in order to samplethe observed field well across the whole magnitude range.
The resolution of the HST in the near-infrared is sufficient to resolve the stellar population down to 1 M⊙.Down to this limit, I found the IMF to be consistent with a Salpeter IMF wi th no indication of a flatteningat the low-mass end (Andersen et al. 2009, ApJ, 707, 1347).The result was in contradiction with a previouslyidentified flattening at 2 M⊙ from optical observations (Sirianni et al. 2000, ApJ, 533, 203). I showed that thediscrepancy in the derived IMF can be due to differential extinction which is stronger in the optical than in thenear-infrared and which was not fully taken into account in the optical study, see Fig. 1. Previous studies haveshown that the cluster is indeed affected by line of sight dust (e.g. Brandl et al. ApJ, 466, 254).
Due to the distance of the LMC (50 kpc), current studies are limited to the scale-free part of the IMF abovethe mass where the slope of the Galactic field IMF changes (∼0.5 M⊙ for a Kroupa IMF). To reach the low-mass end of the IMF in a massive cluster one currently has to resort to the Galactic candidates. However, thewell known clusters such as NGC 3603 and the Arches were foundto be too dense or too extincted for thecurrent imaging capabilities to reach much deeper than was possible in R136. The discovery that Westerlund 1,see Fig. 2, at a distance of only 4 kpc, is the most massive youngcluster known in the Galaxy has opened up
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the possibility to, for the first time, probe the IMF in a massive star cluster below the peak of the field IMF at0.25 M⊙. Due to its age of 3-5 Myr it is not as dense as for example NGC 3603 making it feasible to resolvethe individual objects to lower masses. To this end, I was rewarded 20 orbits of HST/WFC3 near-infraredobservations of a 4.5′×5′ field of Westerlund 1 covering the cluster out to a 3 pc radius together with a controlfield to account for the field star contamination.
Figure 2: Westerlund 1 as seen with theHST (Andersen et al.accepted). The field of view is 4.5′
×5’. The faintest clustermembers detected are below the brown dwarf limit.
Objects below the brown dwarf limit are detectedbut incompleteness due to crowding etc. means thatonly a fraction of the lowest mass objects are detected.After careful separation of the field star contamina-tion from the cluster content and characterization ofthe completeness of the data as a function of radius,I derived the IMF down to a mass limit of 0.15 M⊙(Fig. 3). The peak of the IMF is found to be verysimilar to that of the field and a log-normal fit tothe IMF provides a width comparable to that ofthe field IMF (Bochansky et al. 2010, AJ, 139,2679, Chabrier 2005, AASL, 327, 41), providingthe strongest evidence yet that the stellar IMF inGalactic massive star clusters is similar to the field(Andersen et al. A&A, accepted). Determining aphotometric mass of 50 000 M⊙, I further confirmedthat Westerlund 1 is the most massive young Galacticstar cluster known.
Will the currently massive star clusters beingformed survive as entities and thus be present analogsto globular clusters formed billions of years ago orwill they disperse into the field on relatively short timescales? A necessary condition for their survival is thatat least presently their dynamical mass is not largerthan their photometric mass or they will disperse. Tothis end, the total mass derived through star counts forWesterlund 1 was compared with the dynamical mass,determined through radial velocity measurements using Magellan MIKE optical spectra proposed for and ob-tained by me.After correcting for the effects on the velocity dispersion of tight binaries, we found that thecluster is bound which implicates that it can survive to old age and potentially become a globular clusterif not disrupted by external event in the Galactic disk (Cottaar, Meyer, Andersen & Espinoza 2011, A&A,539, A5). This may indeed be typical for massive clusters. Later studiesof R136 showed that this clusterindeed also appear bound presently (Henault-Brunet et al. 2012, A&A 546, 73).
Figure 3: The derived IMF for Westerlund 1. The solid-line his-togram is the completeness corrected IMF which is similar to thefield IMF (Andersen et al. A&A, accepted).
One outstanding question is whether massive starclusters are coeval of if there is a measurable agespread within the cluster. This has important im-plications for the formation mechanisms of massiveclusters since it will trace whether the clusters areformed in a monolithic collapse of if there has beenmerging of sub-systems. In addition, our under-standing of extra-galactic unresolved clusters dependsstrongly on the assumed age distribution since the rel-ative number of giants and supergiants depends on thestar formation history as well as the age of the clus-ter. In principle the determination of any age spreadcan be determined utilizing the near-infrared color-magnitude diagrams also used for the mass determi-nation for the individual stars. For a single age pop-ulation there will be a unique transition region wherestars evolve from the pre-main sequence to the mainsequence. Identification of this region will then pro-vide the age to the cluster. For example, for Wester-lund 1 this transition happens for stars in the mass range 2-3M⊙, providing an age estimate of 3-5 Myr.
However, for observed clusters this transition is never a single line which in principle is expected for a
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single star single age population. There are several reasonsfor this, including binarity, differential extinction,variability, and field star contamination, in addition to a possible age spread. Multi-epoch high spatial resolutionimaging has been shown to be a powerful tool to separate the field and cluster populations through propermotion for massive star clusters, e.g. for NGC 3603 and the Arches. We have combined previously obtainedNAOS/CONICA observations of a few selected regions in Westerlund 1 with the new HST data to obtain a sevenyear baseline to remove field star contamination and obtain a catalogue of likely cluster members.Based on thesample with field objects removed and different assumptions on the binary characteristics in the clusterwe determined the age spread in Westerlund 1 to be 0.5 Myr or less, where0.5 Myr is the accuracy ofthe method (Kudryavtseva et al. 2012, ApJ, 750, L44).It is currently not clear if this is typical for massiveclusters. Evidence for merging has been reported for R136 where two different ages (2 Myr difference) may bepresent (Sabbi et al. 2012, ApJL, 753, 34). Future studies of massive clusters are necessary to characterize howcommon cluster merging is compared to single star formationevents.
1.2 The Brown Dwarf Initial Mass Function
The shape of the IMF in the brown dwarf regime is still uncertain and a larger set of nearby clusters is necessaryto reveal any potential variations as a function of environment. I have led studies using HST observations todetermine the brown dwarf content in nearby embedded star clusters having different physical conditions interms of cluster mass and cluster density. One study included observations of a water band absorption featurein the near-infrared to obtain effective temperature estimates of each individual low-mass object, both to placethe objects in the HR diagram but also to effectively excludefield objects. The use of medium band filters isan efficient approach to obtain a list of candidate members forfollow-up spectroscopic observations. Futureobservations of star forming regions will benefit strongly from this approach to discriminate between field andlow-mass members. Multiobject near-infrared spectroscopy can then be obtained of the objects for more detailedstudies than possible with medium band filters.
I determined the IMF for two clusters, namely Mon R2, an intermediate mass cluster and the Orion NebulaCluster (ONC), the nearest star forming region containing Ostars. Although the ONC has been studied inthe past this study was aimed at determining the brown dwarf IMF out to much larger radii than previousstudies (out to 1.5 pc). The HST photometry was sufficiently deepin both cases to detect brown dwarfs lessmassive than 0.02 M⊙ (20 MJup). The mass of each object in the clusters was determined from the near-infraredphotometry through comparison with theoretical isochrones of the appropriate age. For Mon R2 we adopted themeasurements of water band absorption to place the low-massobjects in the HR diagram. A comparison withthe theoretical isochrones then provided an age of 1 Myr. The ratio of brown dwarfs to low-mass stars (lessthan 1 M⊙) was determined for each region and compared with the similar ratio for other nearby regions andthe field. I found for both regions, Mon R2 and the Orion Nebula Cluster, that the ratio of brown dwarfsto stars was consistent with each other (Andersen et al. 2006, AJ, 132, 2296, Andersen et al. 2011, A&A534, 10) and that the ratios were consistent with the field IMF.
The finding of a similar ratio of stars to brown dwarfs for Mon R2,the ONC and for other nearby embeddedclusters encouraged me to combine the results with other results from the literature to place constraints on thecombined low–mass IMF (Andersen et al. 2008, ApJ, 683, 183L).Such an average IMF is a proxy for the fieldIMF, assuming an underlying universal IMF. I compared the distribution of the derived ratios of low–mass starsto brown–dwarfs for a total sample of 7 clusters ratio distribution predicted by the functional forms of the IMF(Kroupa 2002, Science, 295, 82; Chabrier 2003, PASP, 115, 763). I found that the measured ratios of stars tobrown dwarfs were consistent with each other, they are fit by an underlyinglog–normal IMF (Chabrier2003), i.e. an IMF that is falling into the brown dwarf regime and that the ratio of stars to brown dwarfsis 5 to 1.
1.3 Triggered star formation.
What are the effects of massive stars on star formation in their vicinity? Will the strong radiation pressurecompress surrounding gas and thus trigger star formation? Is the final mass of objects near massive stars affectedby the strong UV flux such that disks evaporate faster (e.g. Robberto et al. 2004, ApJ, 606, 952)? Thesequestions can be addressed by studying areas where triggered star formation is expected. This would in turninfluence the final IMF for the star formation event. Detailed studies of the impact of the ionisation from on thesurrounding material is therefore necessary.
I examined a part of the Eagle Nebula (M16), where the NGC 6611 star cluster is creating an HII region andsculpting protrusions of molecular material, known as ’elephant trunks’ using mm line observations and opticalnarrow band imaging. The goal was to combine millimeter line observations of the molecular gas with optical[SII] and Hα narrow-band images to examine the physical conditions in the molecular material south-east of
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the elephant trunks. Using the optically thick CO line to measure the core temperatures and the optically thinCS and C18O lines to then determine the column density of the moleculargas, I measured the masses of themolecular cores and searched for signs of star formation in them. The core masses were determined to be in therange 50-100 M⊙ and I found that one of the cores harbors the driving source ofthe Herbig Haro object HH216and we detected the counter jet of HH 216 (emission originating from material ejected from the protostar in ajet which collides with the surrounding interstellar medium). Despite a core mass of∼50 M⊙, it had only twopoint sources associated with it (Andersen et al. 2004, A&A,414, 969).
We subsequently extended this project to include VLT ISAACJS , H, KS and L-band imaging of theelephant-trunk structures and a large part of the NGC 6611 cluster. The immediate aim was to test whetherstars form in the small globules of molecular material called Evaporating Gaseous Globules (EGGs), seenalong the trunks in the HST images.These were suggested to be stellar hatching sites and if true wouldbe common places to find young stellar objects not visible in the optical butexposed when observed inthe near-infrared. However, only roughly 15% of the EGGs contain a point source. It appears that themajority of these objects are brown dwarfs (McCaughrean & Andersen, 2002, A&A, 389, 513).
The IR-detected sources within the EGGs were not detected in the X-rays, indicating that the X-ray luminos-ity at early evolutionary stages of very young objects is perhaps smaller than that observed in the ONC (Linsky,Gagne, Mytyk, McCaughrean & Andersen 2006, ApJ, 654, 347).
H II regions in the Hi-GAL surveyHow many HIIregions are there in the Galactic plane, how are they distributed and, and what are their charac-teristics? These questions are of importance for our understanding of the global picture of star formation and todetermine the effects of massive stars on their environments. However, due to the large amounts of extinctionin the Galactic plane embedded nature of many HIIregions our knowledge has been limited to the most nearbyregions. The advent of theHerschel Hi-Gal survey has changed this. For the first time is a completeGalacticplane survey available sensitive to the presence of hot dustsuch that the HIIregions can be identified.
Figure 4: Example of the active contour fitting to a bubble detectedin the Hi-Gal survey. The initial guess for the contours is the best fitellipse to the regions identified having large gradients.
I am undertaking the identification and character-isation of bubbles in the 70µm HI-Gal data. The de-tection is automatic to avoid selection biases and tobe able to determine the completeness of the data. Byidentifying sharp gradients in the 70µm images thebubbles are detected. A first characterisation of themare then performed by elliptical fitting. Subsequentanalysis includes more detailed shape fitting using ac-tive contours.The catalogue is the first full Galac-tic plane bubble catalogue produced. I identifiedalmost 3000 bubbles in the Galactic plane. Com-parison with previous catalogs of only parts of theGalactic plane shows that I recover previously iden-tified regions and new previously unknown regions.The parameters determined by rough elliptical fittingare refined using Active Contours through collabora-tions in Leeds (see Fig. 4).
The catalogue is currently being complementedwith millimetre line data where available. Largescale Galactic surveys such as NANTEN 2 andSEDIGISM are utilized to correlated the bubblesidentified through dust emission with the velocity in-formation available from the millimetre observations.
2 Dust growth and destruction in molecular clouds and cores
Dust is an important component of the interstellar medium due to its extinction of light affecting observationsof other astrophysical objects, and as a tracer of the interstellar medium through its emission. Understandingthe dust properties, in particular its size distribution, is of fundamental importance in order to interpret thedust observations and to describe the extinction law. I haveinvestigated the destruction of dust in molecularclouds due to supernova shocks and I have used scattered light to probe the grain size distribution in molecularcores emphasizing on the large grains. The use of scattered light provides an independent measure of the dustproperties which are typically inferred from its thermal emission or through the extinction law.
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2.1 Dust destruction in supernova remnants interacting with molecular clouds
Supernovae of type II are in general expected to occur near thebirthplace of their progenitor, i.e. near a molec-ular cloud complex. It is therefore expected that a significant fraction of supernova remnants (SNRs) are as-sociated with a molecular cloud and that the shock wave is interacting with the cloud. SNRs interacting withmolecular clouds are ideal regions to study dust destruction mechanisms in the interstellar medium. However,until the advent of theSpitzer GLIMPSE survey of a large part of the Galactic plane only few wereknown dueto the problems of confusion and line of sight extinction. Then a sample of 15 interacting SNR candidates wereidentified based on theSpitzer IRAC colors suggesting emission from molecular hydrogen orPAH emission(Reach et al. 2006, AJ, 131, 1479).
To determine the origin of the dominant emission from the 15 SNRs, and to determine the physical parame-ters of the shocked gas and dust, we obtained low–resolution(R∼60-120)Spitzer IRS spectra in the wavelengthrange 5–40µm. Further,Spitzer MIPS SED observations were carried out, covering the wavelength range 50–90µm. The SED observations cover the OI line at 63µm, a dominant cooling line.We found a total of 12 SNRsthat showed molecular hydrogen in the spectra.The range of vibrational molecular hydrogen lines coveredby Spitzer enabled us to determine the temperature and density in the shocked molecular gas. The Boltzmanndiagrams show that at least two temperature components are necessary to model the shocked molecular gas.We found a warm component of∼ 300K and a hot component of1000 − 2000K are necessary to reproducethe H2 observations (Hewitt, Rho, Andersen, et al. 2009, ApJ, 694,1266). Further, the ortho–to–para ratio forthe warm component is found to be lower than the equilibrium value of 3, indicating that the shocked gas hasonly recently been heated. The hydrogen line ratios are best fitby a 2–component C shock model, one shockat relatively low velocity (20kms
−1) into a high density medium (106 cm−3) and a faster shock into less densematerial is necessary to fit the hot component,50− 70kms
−1, 103cm−3
Figure 5: An example of the dust model fitting for the supernovaremnant Kes 17. TheSpitzer IRS and MIPS SED data points areshown together with the best fit dust model. The red line showsthe contribution of the largest grains, green the stochastically heatedsmall grains and blue the PAH contribution. A large relative abun-dance of small grains were found for Kes 17, suggesting shatteringof the large grains.
The large wavelength coverage bySpitzer fur-ther allowed me to sample the dust properties in theshocked material. Several theoretical studies have in-dicated the slope of the dust mass spectrum wouldsteepen by dust processing by the shock (Guillet et al.2007, A&A, 476, 263). To characterize the shockeddust, I adopted a 3-component dust model consist-ing of PAHs, very small grains, and big grains (e.g.Bernard et al. 2008, AJ, 136, 919, see Fig. 5). Dueto the long slit nature of the IRS spectra and the nod-ding of the MIPS SEDs, we were able to successfullysubtract the local background which can be substan-tial in the Galactic plane in order to obtain only emis-sion from the shocked gas. Similarly, the on–off ob-serving technique for the MIPS SEDs ensure that theline–of–sight background is removed. The observa-tions cover the emission from 5–80µm, thus includ-ing most of the hot dust emission.I found evidencefor dust destruction in the interacting SNRs. Foryoung SNRs associated with a strong radiation field,the abundance of PAHs and very small grains rela-tive to the big grains appear to be strongly suppressedcompared to the interstellar values. This suggests thatthe dust grains are being sputtered which mainly affect the smallest grains. Conversely, for some of the slightlyolder SNRs interacting with dense molecular material we find anoverabundance of the very small grains com-pared to the big grains. This is consistent with predictions (Jones et al. 1996) that shattering is destroying thebig grains for shock velocities of∼50 kms−1 as indicated by the ionic line measurements (Andersen et al.2011,ApJ, 742, 7A).
2.2 Scattering at 3-5µm as a tracer of large grains in molecular cores
Although scattered light is well known from molecular clouds in the optical and the near-infrared it was asurprise that it was also identified in theSpitzer in the 3.6µm and 4.5µm bands since large grains (on the orderof at least 1µm) are necessary to efficiently scatter at these wavelengths.Further, the albedo for the grains haveto be high suggesting they are silicates and not carbon grains. Such large silicates are larger than expected fromthe diffuse interstellar medium, suggesting grain growth during the life time of the molecular core. Scattered
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light thus pose an independent diagnostic of the grain properties that can be used in conjunction with the moretraditional dust tracers such as (sub) mm emission and extinction measurements.
By combining the 3-5µm scattering with similar scattering at shorter wavelengths one can probe the grainsize distribution within the cores. I have usedSpitzer imaging of the starless molecular core L260 togetherwith ground-based archival near-infrared J and Ks band observations in a study to explore the possibilities ofcombining the short- and mid-infrared observations to place limits on the large grain size distribution. Scatteringat different wavelengths probe different grain sizes and itis therefore possible to use the large wavelengthcoverage to determine the ratio of large to small grains thatare present in a molecular core. Adopting a 3dimensional radiative transfer code I found that although grains up to at least 1µm are necessary to reproducethe scattering seen from L260, the surface brightness profilesfor the different wavelengths are inconsistentwith a larger upper grain size for the grain size distribution (Andersen et al. 2013, A&A, 559, A60). This isdue to a balance between scattering and extinction. As the grain size distribution is extended to larger grainsthe scattering efficiency increases to a certain point whereas the extinction increases further at which point theobserved emission will decrease. This effect is strongest for a core with a central column density such that theoptical depth is around 1.
Only a fraction of the cores surveyed so far showed evidence for emission due to scattering. In order forthe scattering to be visible in emission it has to be strongerthan the extinction of the background radiation. Wehave calculated the balance of the two effects across the skyin order to identify molecular cores where scatteredlight should be observed if large grains are present (Steinacker, Andersen et al., 2014, A&A, 563A, 106).
Figure 6: Correlations between water-ice optical depth (left) andscattered light (right). Red symbols are after correction from self-absorption in the scattering measurements. There is a commonthreshold for the onset of water-band absorption and scattering seenin emission over the absorption, suggesting a common origin (An-dersen, Thi, Steinacker & Tothill 2014, A&A, 568L, 3A).
The origin of the larger grains is still unclear.Most coagulation models suggest that the smallergrains cannot grow unless there is a material presentto make them stick. This is commonly assumed tobe water ice. Combining water band absorption mea-surements with coreshine measurements in the Lu-pus area I investigated a possible connection betweenwater-ice abundance and mid-infrared scattering. Theline of sight scattering and water-band absorptionwere compared with the silicate optical depth at 96µm. After correcting scattering for self-absorption Ifound that the onset for water-absorption and scatter-ing is the same at a threshold extinction of AV ∼ 2,strongly suggesting a common origin for water-ice and efficient scattering (Andersen et al. 2014,A&A, 568L, 3A), see Fig. 6. There were further indi-cations that the scattering efficiency per unit of extinc-tion increased as a function of optical depth suggest-ing larger grains. This opens for new complementarystudies of dust grains through the possibility to com-bine fully independent measurement techniques.
I further investigated the influence of simply icecoating the grains and the effects on the grain scatter-ing. By adopting an ice accretion model I showed that the presence of water-ice can increase the albedo of thegrains by a factor of three, comparable to that obtained by extending the grain size distribution to oneµm.
7
Future Research
Star cluster formation and evolution and the low-mass Initial Mass Function
Morten Andersen form number 76740
The topics of star formation and massive stellar cluster formation are fundamental to modern astrophysics.Most stars are formed in clusters and to understand their origin we have to understand their formation. Thisis challenging endevaour since it required simoultaneously high spatial resolution observations of the densestparts and large-scale mapping of the entire complexes spanning many parsec. Further, star formation is notsimultaneous and a complete overview requires observations from the near-infrared to millimetre observations.Current large-scale surveys of the Galactic plane from the mid-infrared to millimetre wavelengths provide in-formation on the large scale structures now. Combined with the near-infrared high spatial resolution capabilitiesfrom the ground provide a basis for characterising the stellar content in great detail. The multi-plexing spectralpossibilities now coming online provides means of obtaining large sets of spectra of the stellar content.
In the coming years, the advent of JWST, the E-ELT, and a fully developed ALMA will be transformativeand will allow groundbreaking advances in our knowledge of the formation mechanisms. Current Galacticstudies will be possible within the Local Group and will allowus to probe star formation across a wider rangeof parameters in terms of metallicity and specific galactic star formation rates. The studies of the Galactic andLocal Group star forming events will provide the templates for the distant unresolved star forming events. Theresults are therefore fundamental to understand the underlying stellar population from extragalactic resolvedstellar population studies and to understand galaxy evolution. The science proposed therefore overlaps withmany topics within LAM ranging from the Galactic interstellar medium and HII region studies over the physicsof galaxies. There is further overlap with the instrumentation projects at LAM, especially through the use ofAdaptive Optics (AO). Collaborations have already begun including Gemini GeMS observations of Westerlund1 and N159W with AO in the LMC in addition to the characterisation and utilization of the Hi-GAL detectedbubbles in the Galactic plane.
Below I outline some of the more immediate projects to pursuein the very near future and some longer termgoals as telescopes and instrumentation become available.There are many opportunities to involve students inthe projects, both at the master and PhD level and for further collaborations at LAM.
1 Short-term goals
With the current telescopes and instrumentations the first steps can be taken towards a much better understandingof the clustered star forming process. The large-scale field ofviews will allow an overview of the star formingregions. With these the star formation history of the largest complexes can be mapped and the importance ofcluster-cluster interactions investigated. The first steps towards an understanding of the initial conditions ofcluster formation can be probed which will lead to insight inthe the cluster formation mechanisms.
1.1 Star clusters at the earliest stages of star formation
Tracing clustering at the earliest stages of the star formation process has been difficult in the past due to thecombination of a lack of good candidate massive star formingcore and the sensitivity to probe its stellar con-tent. Systematic mm-line surveys of the Galactic plane in thelast few years have revealed large (10 000 M⊙)molecular cores either not forming stars yet (e.g. Longmore et al. 2012) or cores showing gas infall motion aswell as a forming star cluster in the center (Barnes et al. 2010). The core BYF73 stands out as a particularlypromising candidate (see Fig. 1). Millimeter line observations have shown this core to not only be massive(more than 10 000 M⊙) but also to be a core with strong molecular infall (Barnes etal. 2010). It is thus aunique opportunity to directly study cluster formation at its very early stages. Deep near-infrared observationare able to penetrate the extinction from the core and revealthe stellar content such that the stellar positions andmasses can be accurately determined. By observing the molecular core in its collapse I will be able to dissectthe star-forming region, establish the stellar distribution at birth and identify the sub-clustering of the stars. Thetotal stellar content across the region will be determined.Stellar sub-structuring will be identified and the agesof the sub-structures derived. This will in turn allow to trace the sub-clustering through the collapse of the core.The amount of sub-clustering is an indication of the time-scale for the collapse. A smooth distribution indicatesa long star formation time compared to the free fall time whereas a large degree of substructure indicates afast collapse (e.g, Tan, Krumholz & McKee 2006). Further information will be obtained from proper motionstudies of the cluster. Multi-epoch HST observations beforeits retirement will be able to identify sub-structurekinematically, further placing constraints on the formation mechanisms.
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Figure 1: VLT HAWK-I 2.2 micron near-infrared image of BYF 73, located at 2 kpc, and its surrounding (PI Andersen). The contoursshow the HCO+ column density which is a dense gas tracer. A deeply embedded cluster is located at the peak of the HCO+ emission.To the west of the core is located a slightly older H II region which itself might be sub-clustered.
A similar study is being under of a cluster in the N159W complex in the LMC. The cluster is at the edge of anH II region and is possibly a second generation of star formationwithin the complex. We have obtained GeminiSouth GeMSJHK laser guide star multi-conjugate adaptive optics observations of the cluster. The combinationof large field of view (one arcminute square, corresponding to15 pc square at the distance of the LMC), highspatial resolution, stable point spread function and sensitivity enables an unprecedented view of the area. Thestellar content can be probed below 1 M⊙ and will allow a detailed study of the region. The spatial distributionof stars can be established in a second generation star forming event and the IMF can be established.
In addition to the direct and unique scientific goals of the project it also serves as preparation for upcomingobservations at e.g. the E-ELT with laser guide star aided adaptive optics is fundamental in reaching the fullpotential of the facility. The current GeMS facility and the upcoming four laser guide star facility at the VLT (tobe used with MUSE and HAWK-I) will provide us the necessary know-how to optimally use the E-ELT systemin the future while at the same time obtain unprecedented data now.
Collaboration with: A. Zavagno & B. Neichel (LAM), J. Tan & P. Barnes (Florida), J. Kainulainen (MPIA).
1.2 The total cluster and stellar content in a starburst environment
What fraction of stars are forming in bound or unbound structures in a starburst environment? How is starformation spread in time throughout the region? Starbursts are common occurrences in e.g. interacting galaxies,especially at higher redshifts and have been a major mode of star formation in the early Universe. Despite theirimportance little is known of their total star and cluster content. Although the Young Massive Clusters can beobserved in interacting galaxies, for example the Antennae, they are too distant to resolve individual stars or thelower mass clusters (for the distance to the Antennae of 20 Mpc, 0.1 resolution corresponds to∼ 10 pc). Thuswe need to study local analogs to not only determine the totalcontent of the complexes but equally important toidentify potential future cluster mergers. A complete census of a resolved starburst environment would provide afundamental template for the extra-galactic systems. The 30Doradus system in the LMC is such an environment(see Fig. 2). At a distance of 50 kpc it is possible to resolve the individual stars and clusters in the complex withthe best spatial resolution available today.
There is ample evidence that there is a large population of clusters within the complex. Firstly, several can beseen directly even in shallow observations, including the clusters R136 (3-4 Myr old) and Hodge 301 (20-25Myr,see Fig. 2). Second, X-ray observations have shown a rich morphology of hot gas originating from supernovaeexplosions (Townsley et al. 2006). Massive stars must therefore have been present and for a standard InitialMass Function, many lower mass stars are expected. Third, massive young stellar objects have been identifiedpreviously in the infrared (e.g. Maercker & Burton, 2005, Walborn, 2013). The massive stars detected areexpected to be associated with clusters as well.
Recently, optical HST imaging of the complex has been complemented with WFC3 near-infrared observa-tions such that there now is a pancromatic high spatial resolution map of the whole region. I propose to utilizethis dataset in tandem with near-infrared ESO VLT data obtained by us to determine the stellar content and the
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Figure 2: 30 Doradus seen in the optical (B, V, R) with the TRAPPIST telescope (ESO press release eso1023a, 2010). The shown regionis 100pc×100pc. At the center is R136 and even with these shallow observations with aground based 60cm telescope are several otherclusters visible. The picture is from the TRAPPIST telescope, La Silla. R136is at the center of the image and Hodge 301 is a bit up andto the right.
cluster content across 30 Doradus. With the dataset I will detect all stars down to 1 M⊙ across the complexand all the clusters can be identified. Cluster ages can further be determined across the whole starburst regionand the star formation history and spatial distribution will be established. For the first time will it be possibleto measure the fraction of stars in clusters in a starburst environment from resolved photometry. This will thenbe an important template to understand unresolved starburst events. Star burst templates have to assume an agedistribution for the complex. This can be tested with a regionlike 30 Dor.
Full coverage from the optical (HST) over the near-infrared (HST+VLT) together with spectroscopic infor-mation for all O and B stars provide a unique opportunity to dissect a star-burst. The cluster mass function canbe traced from the super-massive R136 in the center to low-mass clusters in a complete sample. This is notpossible in the Galaxy. Double or multiple cluster candidates with separations down to typical cluster sizes of1 pc will be detected. Measuring the radial velocity of the apparent cluster pairs will eventually establish if theyare physically bound. The fate of the cluster pairs can then bedetermined through simulations of their orbits.Will they merge or be disrupted during their lifetime? On theshorter time scale, spectroscopy of a sample ofstars will help determine the ages more accurately and to determine the masses of the higher mass stars.
The obtained cluster catalog will be combined with the VLT FLAMESTarantula survey results where theproperties of the massive O and B stars have been measured andtheir radial velocities have been determined.This will allow a search for the host cluster of the apparent isolated O and B stars. Where a massive star is foundin the imaging survey to be directly associated with a cluster I will use the radial velocity determined by theVLT FLAMES survey. The VLT FLAMES Tarantula survey data are also publically available.
Many sub-projects are possible with a census of star formation in 30 Dor, including at LAM with for exampleBouret. Since the sample is homogeneous in terms of spatial resolution all clusters can be treated on an equalbasis. Further, a crucial feature of the sample is that all theclusters are at essentially the same distance (thedistance to the LMC is 50 kpc whereas the depth of 30 Dor region is on the order of 100 pc). Galactic samplesare plagued by uncertainties on the distance to each individual cluster. Thus, this sample will be ideal to evaluatedistributions of cluster parameters, e.g. cluster sizes asa function of age. One particular test is whether clustersfollow two sequences in cluster radius/central density diagrams as suggested from Galactic clusters. One groupof clusters was termed the starburst like clusters and the other leaky clusters since they were disrupting. If thetwo sequences are real they are of great importance for our understanding of the origin of field stars and thefraction of stars in clusters compared to the field. Distance is crucial in such a diagram as well as separation ofcluster and field population to derive the central cluster densities. With a homogeneous, complete, sample ofstar clusters this can be tested.
In collaboration with: W. Brandner (MPIA), H. Zinnecker (SOFIA), B. Brandl (Leiden), N Bastian (Liver-pool), M. Gennaro (STScI).
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1.3 Star cluster mergers
Studies of the distribution of binary clusters in the LMC have shown a surprising large number of them beingin apparent multiple systems (defined as their separation being less than 20 pc, Dieball et al. 2002). Someof these are expected to be change alignments on the sky, especially close to the bar in the LMC. However,statistical tests by Dieball et al. showed that most of the systems are not chance alignments. Around 10% ofthe cluster pairs in the outer LMC were estimated to be chance alignments whereas it is some 50% near the bar.Thus binary clusters are abundant and if they merge they will form an apparent single cluster but with the mixedstellar populations which could explain the complicated stellar populations in some globular clusters. Thesemeasurements are possible with the VLTs using the multi object high spectral resolution capabilities. Observingthe clusters at several epochs over 1-2 years will identify tight binaries that otherwise would complicate themeasured velocity dispersion due to the binary motion around the center of mass.
The Magellanic Clouds are ideal for studies of cluster systems. They are sufficiently close that the individualstars can be resolved and due to the orientation of the LMC the contamination is a much smaller issue. Acomparable study in the Milky Way would be severely limited.Clusters are today predominantly formed in theGalactic disk and thus the large amount of extinction would require infrared observations. In addition line ofsight chance alignments would be a large issue.
For an established physical cluster pair sample their fate can be estimated once their radial velocities areestablished. The physical pairs can merge over their lifetime into a single cluster and thus show the integratedcharacteristics of a combined cluster. Of particular interest would be a double cluster where the two clusters havedifferent ages. One such apparent double cluster is NGC 1850where the age difference between the clustersis 45 Myr (primary is 50 Myr, secondary 4-5 Myr). The clusters are at a projected separation of only 7.5 pc.Establishing that the clusters are a pair their fate can be determined and it can be shown if they will merge andform a single cluster. Such systems could explain the age spreads observed in some globular clusters.
The apparent cluster pair catalog contains a large number of systems where the clusters appear to havesimilar ages within the uncertainties of the age estimation, typically 10%. For cluster pairs around a few 100Myr that translates into similar ages within 10 Myr, that is,much longer than the time for the first cluster tohave experienced supernova explosions before the second isformed. They could therefore have been formedout of the same giant molecular cloud. Recent work have shownmetals are effectively mixed on short timescales into the surrounding medium, even for a giant molecular cloud (Vasileiadis et al. 2013) and thus the laterformed cluster could have a different abundance pattern in terms of alpha elements. Confirmed physical pairscan then be followed up to determine the metallicity of each cluster to search for differences in their metallicityand verify if a multiple stellar populations within a singlecluster can be produced through merging.
Collaboration with: M. Gieles (Surrey), N. Bastian (Liverpool).
1.4 The characterisation of triggered star formation sites
Triggered star formation has been suggested for many years but have been difficult to study in detail due topoor spatial resolution and sensitity in the infrared and sub-millimetre and the theoretical models are thereforeseverely lacking better observational input. This has changed dramatically in recent years with the adventof Spitzer, Herschel and sub-mm surveys of the Galactic plane. Combined with highspatial resolution near-infrared observations the characteristics of the newly formed stars and of the properties molecular material stillforming stars can be established.
The catalogs of HII regions currently being build by me within the interstellarmedium group at LAM will beinstrumental in future studies. Combined with millimetre line data, to which the group has access, the velocitystructure can be obtained and the material unrelated along the line of sight can be removed from the analysis.This is crucial for studies in the Galactic plane where dust emission is being identified from the intersection withseveral different spiral arms along the line of sight.
Isolating the emission from the HII region allows for a much more detailed study of the regions expansionspeed, the amount of swept up material in the shell and density estimated in general than is otherwise possible formost HII regions in the inner parts of the Galaxy, where most recides.This will in turn provide possibilities forboth detailed studies of individual HII regions where the pre-main sequence and proto-stars in the compressedmolecular material can be compared with the total star formation inside the HII region to provide estimates ofthe relative star formation activity in the compressed gas.
The formed stars associated with the HII can then be analyzed with high spatial resolution near-infraredobservations. To fully characterise the stellar population one has to reach the low-mass content. This is funda-mental for studies of the IMF in triggered star forming event. One has to reach the peak of the mass function toestablish if the IMF in triggered star forming events is the same as in the field and in first generation star formingevent. Deep, high spatial resolution near-infrared imaging is the only way to resolve the individual objects and
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to determine their ages and mass. This calls currently for theuse of Adaptive Optics and direct imaging fromspace, both areas where the application is experienced.
Typically a problem of determining the IMF for individual triggered regions is the relatively low number ofobjects per event which then provides limited statiststical significance to show similarity or differences in theIMF. This can now be addressed due to several improvements. First of all through the increased sensitivity inthe near-infrared over larger fields of view than before. This will both drastically increase the sample size andwill provide diagnostics of the relative number of low-massto higher-mass objects as probed in the past for theprimary star formation events by me. Along the same lines will ensemble IMFs of the triggered star formationsites be investigated and used to make an IMF based solely on triggered star formation to be compared with thefield and cluster IMF.
Collaboration with: A. Zavagno & D. Russeil (LAM), M. Hoare (Leeds), S. Molinari (UNAF).
2 Longer term outlook
My slightly longer term future plans will be focused around three main areas: a) What are the initial conditionsfor clustered star formation? b) How are the star formation characteristics in triggered star formation events? c)Is the shape of the Initial Mass Function universal or does it depend on environment? All three research areasdescribed above fits perfectly well into the research conducted at LAM and will be developped in a transversemanner, increasing the interaction with LAM researchers combining their plural expertises. Below I discuss thelonger term prospects in more detail.
It is becoming feasible to probe in detail the shape of the brown dwarf IMF in embedded young star clus-ters. Observing clusters at their earliest stages is fundamental to determine the initial spatial distribution withinthe cluster which provides strong limits on star formation theories. For example, are brown dwarfs and starstwo different populations or can they be described by a single continuous function? Is the velocity dispersionand spatial distribution the same for brown dwarfs and starsor are they already segregated at birth? Since theyoungest clusters are still embedded it is necessary to observe them in the near-infrared to overcome the ex-tinction. Further, since the frequency of brown dwarfs is expected to be relatively low, the contamination offield objects at the low-mass end is often the limiting problemin quantifying the low-mass IMF. The infraredcolors of cluster members and reddened background objects are very similar making it an insufficient criterionto exclude field objects. Spectroscopy is necessary to determine the objects spectral type and hence their mem-bership. The multiplexing near-infrared spectral capabilities now becoming available are crucial to pursue thisin a time efficient manner with for example K-MOS on the VLT or FLAMINGOS2 on Gemini south.
The brown dwarf content can currently be probed in nearby embedded clusters. 10 metre class telescopesdo not have sufficient spatial resolution or collecting powerto perform similar studies in more extreme starformation event. Therefore, the lowest stellar mass and browndwarf content in massive and metal poor clustersis currently unknown. Higher sensitivity is necessary to beable to detect the lowest mass members. Further,higher spatial resolution fundamental to resolve the individual stars in the cluster center such that the wholecluster IMF can be determined and not only the low-mass content far from the center. The advent of the JWSTwill provide the spatial resolution necessary to resolve individual brown dwarfs in Westerlund 1 and extendmy previous work with the HST deep into the brown dwarf regime.It will be possible for the first time todirectly compare nearby low-mass regions with massive starclusters across the whole stellar and brown dwarfIMF. This will in turn provide fundamental tests on star formation theories and the importance of feedback frommassive stars and cluster density. NIRCam and NIRSpec onboard JWST will be a powerful combination toidentify the lowest mass objects and to obtain unbiased samples of the lowest mass objects due to the relativelylarge field of view. Absorption features from e.g. water vaporwill be identified with medium band imagingwith NIRCam to discriminate between the field and cluster members. The approach will be very similar to thatsucessfully utilized in Andersen et al. (2006) where HST datawere used targeting theH band. Follow-up multi-object spectroscopy will then be possible with NIRSpec for detailed studies of the objects and for an accurateplacement in the HR diagram. Complete samples of well characterized objects are crucial to deduce the slope ofthe brown dwarf IMF, to determine if the objects are coeval, and to determine the bottom of the mass function.
Studies of metal-poor clusters will be possible with JWST in the Large and Small Magellanic clouds but dueto the larger distance the higher spatial resolution possible with 25 meter class telescopes with adaptive opticsis necessary. The E-ELT will be instrumental for these studies.The high Strehl ratios obtainable with modernadaptive optics systems combined with the large apertures will enable unprecedented spatial resolution in thenear-infrared. With adaptive optics it is expected that thebrown dwarf regime can be reached for young clustersin the Magellanic Clouds.
The work will directly build on my experience with similar analysis from ground-based and space data.In particular will the experience and algorithm I have established with HST and ground based data be directly
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applicable for both the JWST and ground-based adaptive optics observations. Structure in the Point SpreadFunction due to the segmented mirrors will produce structurein the diffraction rings that can be mistaken forfainter point sources unless properly taken into account. Ihave shown in previous work the effectiveness ofthe algorithms in discriminating point sources from instrumental artifacts (e.g. Andersen et al. 2009, ApJ, 707,1347 and Andersen et al. 2015, A&A, accepted).
Probing triggered star forming regions provide insight intoconditions not normally probed in first genera-tion star forming events. The compressed and heated gas changes the fundamental parameters that may changethe outcome of star formation. The regions are therefore of great importance to understand in detail. Combiningvery high spatial infrared and sub-mm observations of the regions are necessary for the studies. By probing boththe molecular gas to small scales and the stellar/brown dwarf content, a complete picture of the star formationevents can be determined. The combination of ALMA and the E-ELT will be transformative for our understand-ing. Only with the sensitivity and spatial resolution will it be possible to resolve the individual cores and probethe full IMF.
Our current work on characterisation of Galactic HII regions will provide the best regions of triggered starformation to study. Follow-up observations with ALMA will reveal the molecular core content down to smallscales and subsequent deep near-infrared observations will provide the full stellar and brown dwarf content.This will in turn, for a large sample of regions, enable a systematic study of the star formation characteristics insecondary star formation event.
Combining the studies of the dust and stellar populations with X-ray data will provide further insight on theregions. ATHENA+ will provide the sensitivity to rapidly probe deep down the IMF at the distance of massivestar forming regions. The IFU unit will further provide full spectral information of both the stars and the gas.This will in turn provide a census of the stellar content, a good understanding of the energetics of the HII region,and hence the effects of the massive stars, and the ionizing star properties. The field of view of the IFU of 5′
ensures the majority of the HII regions are mapped in only a few pointings enabling the possibility to probeseveral regions in a short amount of time.
The research is a natural extension of the current research topics at LAM and of the application. Thecombined knowledge across the electro-magnetic spectrum is necessary to unveil the physical processes at playand their relative importance. The team will be ideal to obtain time on the major facilities and to internationallylead the topic.
3 Proposed Institute
I propose Laboratoire d’Astrophysique de Marseille (LAM), asthe research institute. The research topics inthe institute have substantial overlap with my current and previous research, both in terms of studies of theInterstellar Medium and HII regions but also branching out to the extra-galactic community at the institutestudying e.g. star forming galaxies.
I already have several collaboration with staff and personel at the institute including A. Zavagno and D.Russeil studying the interstellar medium and HII regions and with B. Neichel using adaptive optics data ofmassive star forming regions in the Galaxy and the Magellanic Clouds. The combination of multi-wavelengthanalysis and competence in adaptive optics at the instituteis very powerful together with my expertise in stellarpopulation, analysis and interpretation of crowded fields and of the characterisation of star forming events.
The collaborations currently ongoing are expected to naturally continue in the years to come and will betaking advantage of the facilities available through ESO and non-european facilities through the strong inter-national colloborations both through me and the current staff at LAM. The short-term goals will be the use ofthe VLTs with Adaptive Optics and the multi-object spectroscopic near-infrared capabilities. The collaborationsare expected to naturally extend with the upcoming telescopes and instrumentation, for example, JWST and theE-ELT where the current proven expertise of the application and the groups at LAM will make ensure the groupis very competitive for the highly over subscribed facilities.
