Extrasolar planets : Today and TomorrowASP Conference Series, Vol. 321, 2004J.-P. Beaulieu, A. Lecavelier des Etangs, and C. Terquem

What Can We Learn from Protoplanetary Disk Frequencyin Young Clusters?

S.R. Fernandes, P. S. Teixeira, J. C. Correia

Faculdade de Ciencias da Universidade de Lisboa, Ed. C8, CampoGrande, 1749-016 Lisboa, Portugal

J. F. Alves

European Southern Observatory, Karl-Schwarzschild-Str.2, D-85748Garching bei Munchen, Germany

F.D. Santos

Faculdade de Ciencias da Universidade de Lisboa, Ed. C8, CampoGrande, 1749-016 Lisboa, Portugal

E.A. Lada

University of Florida, USA

C. J. Lada

Harvard-Smithsonian Center for Astrophysics, 60 Garden St., MA02138, USA

Abstract. The origin and evolution of circumstellar disks is one of the mainscientific quests intimately related with planetary formation, since disks areknown to be planetary nurseries. A study of statistically significant young stel-lar populations, in different evolutionary stages and astrophysical environments,can provide fundamental tests for theories of disk and planet formation. Weare presently conducting a systematic broadband infrared wavelength study often young clusters of different ages, in order to compare their circumstellar diskfrequency. We report our results of JHK photometry of three nearby clusters ofour sample, RCW38, NGC2316 and NGC2547.

1. Project Goals

This work is part of a project called The Nearest Planet Nurseries, which aimsto study near–infrared (NIR) excesses in young clusters of different ages andenvironments due to the presence of circumstellar material surrounding youngstellar objects (YSOs). The excess frequencies can be used to establish timescalesfor disk and circumstellar matter dissipation and be connected with the presenttheories of planetary formation. The project sample has 10 young clusters. Wepresent here results for three clusters: RCW38 (the youngest in our sample, withan age < 1Myr), NGC 2316 (2–3Myr) and NGC2547, the oldest cluster in thesample (≈ 25Myr).

237

238 Fernandes et al.

2. Results

2.1. RCW38

Figure 1. (a) JHKs RGB colour composite image of RCW 38. Data takenwith ISAAC, VLT. (b) Colour-colour diagram. The solid lines define the locusof the main sequence and giant stars. The dotted lines define the reddeningband.

RCW38 is located at coordinates (α, δ)(J2000) = (08h59m 47s, −47◦31′57′′)and at a distance of 1700 pc. This cluster is very young, with less than 1 Myr.It shows a great amount of nebulosity that can be seen in Figure 1(a) due tothe fact that RCW38 is an heavily embedded cluster in its parental molecularcloud with some strong ionizing sources.

Our preliminary results for this young cluster are depicted in Figure 1(b) inthe form of a JHK colour-colour diagram. Sources that fall to the right ofthe reddening band have detectable excess emission arising from protoplanetarydisks. We determine a 42 ± 6% fraction of sources with excess emission in theNIR.

2.2. NGC2316

The second cluster analized was NGC2316, located at 1100 pc (Felli et al. 1992)with coordinates (α; δ)(J2000) = (06h59m:40s; −07◦46′36′′).

Figure 2(a) shows NGC2316’s shape and extension. We calculate the radiusto be approximately 0.63 pc. In the same Figure, some nebulosity is visible ina spherical distribution around the central B3 zero age main sequence (ZAMS)star (Noguchi et al. 1993; Fukui et al. 1993). This nebulosity is less than theone seen in RCW38 due to NGC2316’s older age. The average extinction forthis cluster is < AV >=4.5 magnitudes (Teixeira et al. 2003).

The colour-colour diagram is plotted in Figure 2.(b), considering only sourceswithin the cluster radius determined above. There is a spread of sources alongthe reddening band, corroborating the previous statement that the cluster ispartially embedded. Its average extinction is < AV >=4.5 magnitudes (Teixeiraet al. 2003). Comparing its colour-colour diagram with that of RCW38 we can

Protoplanetary Disk Frequency in Young Clusters 239

Figure 2. (a) JHKs RGB colour composite in grey scale. Data was takenwith SOFI on ESOs NTT (La Silla). (b) Colour-colour diagram of NGC2316(Teixeira et al., 2003). The maximum error is plotted in the upper right.

see that the average extinction is lower implying that NGC2316 is older thanRCW38. We find 16%± 3% of cluster members with NIR excess characteristicof young stars with circumstellar disks.

To estimate the age for NGC2316, we compare it’s K–band luminosity func-tion (KLF) with those of two other young clusters: the Trapezium in Orionand IC 348. These KLFs are plotted in Figure 3. The comparison betweenthe three KLFs shows good agreement between their overall shapes, in partic-ular, NGC2316 and IC 348 have a striking similarity (apart the total numberof sources) as they roughly show the same broad peak at the same absolutemagnitudes. Assuming a universal Initial Mass Function (e.g. Lada & Lada,2003), we infer that NGC2316 has approximately the same age as IC 348, about2–3 Myr but is more evolved and older than the Trapezium.

2.3. NGC2547

Claria (1982) has placed this cluster at a distance of 450 pc with an estimated ageof 20–30 Myr (Naylor et al. 2003). It is located at(α; δ)(J2000) = (08h10m25.7s;−49◦10′03′′).

Negligent nebulosity is visible (see Figure 4(a)) and as such, all the sourcesare located in the main sequence and no spreading along the reddening band isnoted in the colour-colour diagram of Figure 4(b). Only sources identified aspossible members are plotted, in order to decrease field contamination. Thereare only a small number of sources with NIR excess. From this analysis, weconsider the fraction of sources with NIR excess to be < 1%, which means thatthe star forming process is almost (if not totally) completed in NGC2547.

In order to estimate age and membership of NGC2547, all sources from ourdata were plotted in a K vs. J−K colour-magnitude diagram. Using X-raysources detected for this cluster by Jeffries et al. (1998) with confirmed mem-bership, and matching these with our data, we made an isochrone fitting on thediagram of Figure 5(a) using the stellar atmosphere models from Baraffe et al.

240 Fernandes et al.

Figure 3. K–band luminosity function compared to that of IC 348 and theTrapezium clusters.

Figure 4. (a) Ks–band image (5′×5′). Data taken with SofI at theNew Technology Telescope, NTT, La Silla. (b) Colour-colour diagram ofNGC2547.

Protoplanetary Disk Frequency in Young Clusters 241

Figure 5. (a) K vs. (J − K) colour-magnitude diagram for X-ray sourcesof NGC2547 identified by Jeffries et al. (1998). (b) K vs. (J − K) colour-magnitude diagram of NGC2547.

(1998). From this, we estimate an age of 20–30Myr for this cluster, which isin agreement with previous results (Naylor et al. 2003). Presenting all sourcesobserved in the diagram on Figure 5(b), we selected the sources between theisochrones of 20 and 30 Myr, most likely to be cluster members, and use thissample to construct the colour-colour diagram of Figure 4(b).

3. Timescales for disk dissipation

Our results so far indicate that the NIR excess fraction decreases with cluster ageand that, by observing a large enough sample of clusters, a determination of thetypical lifetime of the protoplanetary disk phase of evolution can be measured.

The plot on Figure 6 shows how the NIR excess fraction decreases with clusterage for RCW38, NGC2316, NGC2547, where we also included data obtainedfor NGC2362 (Alves et al. 2003; Moitinho et al. 2001). We note that the NIRexcess fraction in the K–band decreases almost completely within 5 Myrs. Thisis in agreement with a similar plot from Hillenbrand et al. (2002).

A similar plot by Haisch et al. (2001) using L–band data also gives a lineartrend that translates into 50% disk dispersion by 3 Myr and 90% disk dispersionby 5 Myr. Since the conclusion is the same, using either K–band or L–bandphotometry to assess emission from circumstelar disks, we infer that using K–band photometry is representative of the total number of stars with disks, apartfrom a scale factor which is ∼ 2.

242 Fernandes et al.

Figure 6. Disk fraction vs. age for RCW 38, NGC2316 and NGC2547.Errors in age and NIR excess fraction are plotted for each cluster. Dataobtained for NGC2362 is also plotted (Alves et al. 2003).

4. Discussion

From Figure 1 it is shown that RCW38 is an extremely young embedded clusterwith a considerable disk frequency, 42± 6%. The presence of strong ionizingsources may cause photodissociation of nearby disks. We also need to considerthe possibility that there can be some contamination due to emission from thenebula itself.

The second cluster that has been analized from our sample also shows someremnant of its parental molecular cloud. We determine a disk frequency forNGC2316 of 16± 3%. NGC2316 may also have disk dissipation due to thecentral B3 ZAMS star that is creating an HII region due to photoionization,although at a smaller scale compared to RCW38.

NGC2547, on the other hand, has evolved sufficiently to have dissipated what-ever disks may have existed, clearing also all the residual gas and dust from theparental molecular cloud.

The values obtained are underestimated due to the fact that our data arefrom Ks band, even though we converted this to K–band values using calibra-tion equations determined specifically for the SOFI camera (Lidman et al. 2000).The shorter wavelength filter Ks (2.16µm) detects the warmer emission from theinnermost region of the circumstellar disk. Some of this emission may be misseddue to possible disk holes or because the inner disk emission is small comparedto the stronger stellar emission (near its peak) and hence difficult to detect. The

Protoplanetary Disk Frequency in Young Clusters 243

longer K–band filter (2.2µm) may detect emission from a larger inner region ofthe circumstellar disk. This efficiency of excess emission is increased for longerwavelengths so the L–band (3.5µ) is able to detect most of the emission of cir-cumstellar disks. Data collected in the K–band or, even best, in the L–bandcould have uncovered more excess sources (Lada, 2003, private communication).The advantage in using Ks–band lies in the fact that this band does not includewavelengths where atmosphere emission and absorption is stronger. Also, weneed to consider de possibility that there can be some contamination due toemission from the nebula itself, specially for RCW38.

The results obtained for RCW38 (preliminary), NGC2316 and NGC2547follow the general trend between disk frequency and age derived from K–bandobserving (e.g., Hillenbrand et al. 2002). These results show that disk dissipationmay occur between 5–10 Myr. Using K–band photometry, half the populationin young stellar clusters loose their disks in the first 3 Myrs, and most of themat 5 Myrs. If this disk dissipation is directly related to the disk clearing dueto coagulation of planetesimals, then we can constrain the timescale for planetformation to the first 5 Myrs.

Acknowledgments. This research is financially supported by the Funda-cao para a Ciencia e Tecnologia (FCT), Portugal, under the project PESO/P/-PRO/40154/2000. J. C. Correia gratefully acknowledges the financial supportfrom FCT through the grant SFRH/BPD/3614/2000.

References

Alves, A., Lada, C., Lada, E., & Muench, A. 2003, in prep.

Baraffe, I., Chabrier, G., Allard, F., & Hauschildt, P. 2002, A&A, 382, 563

Claria, J. 1982, A&ASS, 47, 323

Felli, M., Palagi, F., & Tofani, G. 1992, A&A, 255, 293

Fukui, Y., Iwata, T., Mizano, A., Bally, J., & Lane, A. 1993, PPIII, 603

Haisch, Lada, & Lada 2001, AJ, 122, 2065

Hillenbrand, L. 2002, Origins 2002, ASP Conf. Ser., in press

Jeffries, R., & Tolley, A. 1998, MNRAS, 300, 331

Lidman, C., Cuby, J., & Vanzi, L. 2000, SOFI User’s Manual, Issue 1.3

Naylor, T., Totten, E., Jeffries, R., Pozzo, M., Devey, C., & Thompson, S. 2002, MN-RAS, 332, 291

Noguchi, K., Qian, Z., Wang, G., & Wang, J. 1993, PASJ, 45, 65

Teixeira, P., Fernandes, S., Alves, J., Correia, J., Lada, C., Lada, E., & Santos, F. 2003,A&A, submitted