By Giovanni Sostero and Ernesto Guido Remanzacco Observatory () CARA ( Meeting on Asteroids and...

by Giovanni Sostero and Ernesto Guido

Remanzacco Observatory (www.afamweb.com)

CARA (http://cara.uai.it/)

Meeting on Asteroids and Comets in Europe - MACE 2006

Vienna, May 12-14, 2006

“Some thoughts about cometary CCD photometry”

Summary

• Some issues about comets & CCDs photometry

• Photometry and physical studies of comets

• The CARA standardization approach

• Some results

• Conclusions

• Which photometric aperture has to be selected for their measurement?

• Which is the influence of the observing conditions?

Comets are diffuse objects

• Stacking of multiple subframes

• Need to provide suitable reference stars sequences

Comets do have significative proper motions

• Need to use narrowband filters (objects with strong emission lines)

• Possibility to use broadband filters (objects with negligible emission lines)

• Possibility to use ulfiltered photometry (limited amount of objects and/or under particular conditions)

Comets are emission line objects

The Afrho connection

Photometry and physical studies of comets

•Afrho, is a quantity introduced by A'Hearn et al. in 1984 with the aim of comparing measurements concerning the dust continuum produced under different observing conditions, times and instruments.

•The Af[ρ] is the product of the albedo (A), filling factor (f) of grains within the field of view and the linear radius of the field of view [ρ] at the comet.

•We can consider it only as a “proxy” of the dust abundance within the coma. This means that we cannot establish a simple and secure link between Afrho and the dust production rate.

What is Afrho?

The Afrho quantity is calculated by the ratio between the luminous flux received from the comet, and the solar flux. Using the following equation:

Af[ρ] = ρ*(2DR/ρ)2 Fcom/Fsun

Where:

A is the albedo f is the filling factor, that is how much the powder grains fill the field of view ρ is the coma radium considered in the measurement, usually expressed in cm D is the geocentric distance Earth-Comet, expressed in cm R is the eliocentric distance, expressed in Astronomical Unit Fcom is the observed comet light flux (or flow) Fsun is the solar flux at 1 AU

CCD comet photometry Do we really need some kind of filtered photometry?

Narrowband Interference Filters

Central Wavelength (nm) Molecule

390 CN

405 C3

430 CO+

450 Blue Dust Continuum

515 C2

620 H2O+

647 or 650 Red Dust Continuum

CCD comet photometry C/2001 Q4 (NEAT): strong gas contamination

Aperture radius Lowell Obs. CARA Difference

5,000 to 20,000 ~4450 cm ~ 4320cm ~3%

Comparison with D. Schleicher results (Lowell Observatory)

Comet C/2001 Q4 (NEAT) on May 12, 2004

Strong gas contaminations narrowband filter @647nm

Example of Afrho determinations - 1

CCD comet photometry 9P/Tempel: negligible gas contamination

Comparison with D. Schleicher results (Lowell Observatory)

Aperture radius Lowell Obs. CARA Difference

~ 25,000 Km ~166 cm ~ 189 cm ~13%

~ 15,000 Km ~193 cm ~ 215 cm ~11%

~ 10,000 Km ~234 cm ~ 245 cm ~5%

Comet 9P/Tempel on May 6, 2005

Negligible gas contamination R & I broadband photometric filters

Example of Afrho determinations - 2

CCD comet filtered photometryA standardization tentative from the CARA collaboration

WINDOW APERTURE

• Main window size for aperture photometry: 100,000 km @ comet distance

• (arcsec)= 138/circular apertures)

• (arcsec)= 122.3/(square apertures)

• Multiples and submultiples apertures (e.g. 200,000 Km, 50,000 km, 25,000 km, etc.)

+

REFERENCE STARS MAGNITUDES SOURCES

• Johnson B & V: Hipparcos/Tycho catalogue magnitudes recommended

• Cousins R & I: polynomial extrapolation from catalogued B-V values (+/- 0.1 magn.)

=

TARGET

• Attempt to produce uniform photometry (+/- 10%) from various observers

Historic roadmap of the CARA collaboration

•TA Annual General Meeting, September 2001. Basingstoke, UK.

•BAA Comet Section Meeting, February 2002. London, UK.

First CCDs filtered lightcurves of comets

Presentation of the “fixed aperture window” technique for CCD photometry

•Meeting on Asteroids and Comets in Europe (MACE2002), May 2002. Visnjan, Croatia.

Improvements in photometric methods

The first experiments with the Afrho procedure are presented

•Meeting on Asteroids and Comets in Europe (MACE 2003), May 2003. Mallorca Observatory, Spain.

Presentation of the CARA collaboration

Suggestion about the opportunity of the Afrho procedure approach also for the amateurs

•IWCA III, June 2004. Paris, France.

Some observing campaigns results based on the Afrho method are presented

•BAA Comet Section Meeting, May 2005. Cambridge, UK

Discussion about the need to choose some kind of standardization for CCDs comets photometry

Developed by Roberto Trabatti (CARA)

Wafrho: the CARA workhorse

0009P 20050426.86 0.715 1.656 18.85 R 11.51 025580 0000182 00001 HIP Sos www.afamweb.com HIP 64792 G0Vs 0.45m reflector data -0069.45











0009P 20050501.91 0.712 1.637 21.38 R 11.23 049954 0000116 00005 HIP Sos www.afamweb.com HIP 63215 G5 -0064.40











Output: tabulated data

Some examples:

• C/2002 T7 (LINEAR)

• 97/Tempel

• 73P/Schwassmann-Wachmann

From theory to practice

• The dust environment of comet 67P/Churyumov-Gerasimenko, Fulle, M.; Barbieri, C.; Cremonese, G.; Rauer, H; Weiler, M.; Milani, G.; Ligustri, R., Astronomy and Astrophysics, v.422, p.357-368 (2004)

• A neck-line structure in the dust tail of Comet C/2004 F4 (Bradfield), Pansecchi, L.; Scardia, M.; Astronomy and Astrophysics, v.430, p.1129-1132 (2005)

• Photometry of comet 9P/Tempel 1 during the 2004/2005 approach and the Deep Impact module impact, Milani G.A., Sostero G., Trabatti R., Ligustri R., Nicolini M., Facchini M., Tirelli D., Carosati D., Vinante C. and Szabo’ Gy. M.; Icarus, submitted

• CBET nr.464, 473, 361; IAUC nr. 8668, 8660, 8659, 8658, 8557, 8543

Publications so far

• Some kind of standardization for cometary photometry is needed

• It’s unlikely to establish an “all purpose” method

• Different kind of approaches are possible, according to the targets

• Different approaches must (hopefully) converge to comparable results

• Cometary CCDs photometry is well suited for amateurs

Conclusions

Acknowledgements

Collaborating astronomers

Marco Fulle (Trieste Astronomical Observatory, Italy), Gian Paolo Tozzi (Arcetri Astronomical Observatory, Italy), Luigi Pansecchi (Merate Astronomical Observatory, Italy), Mauro Barbieri (Padova Astronomical Observatory, Italy), Gyula Szabo’ (University of Szeged, Hungary), Laurent Jorda (Lab. d‘Astrophysique de Marseille, France)

CARA dedicated staff

Giannantonio Milani (Coordinator), Carlo Vinante (Webmaster), Roberto Trabatti, Martino Nicolini & Mauro Facchini (Software)

CARA observing stations

Giannantonio Milani (Padova, Italy), Diego Tirelli (Vicenza, Italy), Toni Scarmato (S. Costantino Briatico, Italy), Filip Fratev (Bulgaria), Descartes Observatory (Pavia, Italy), Campo dei Fiori Observatory (Varese, Italy), CAsT Observatory (Talmassons, Italy), Remanzacco Observatory (Remanzacco, Italy), G. Montanari Observatory (Cavezzo, Italy), Cor Caroli Observatory (Vicenza, Italy), Armenzano Observatory (Perugia, Italy), Crn Vrh Observatory (Crni Vrh, Slovenia), Terry Lovejoy (Brisbane, Australia), Erik Bryssinck (Belgium), New Mexico Skies (USA)

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By Giovanni Sostero and Ernesto Guido Remanzacco Observatory () CARA ( Meeting on Asteroids and...

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