EDP Sciences
Free access
Volume 394, Number 2, November I 2002
Page(s) 459 - 478
Section Stellar clusters and associations
DOI http://dx.doi.org/10.1051/0004-6361:20021118

A&A 394, 459-478 (2002)
DOI: 10.1051/0004-6361:20021118

The mass function of the Arches cluster from Gemini adaptive optics data

A. Stolte1, 2, E. K. Grebel1, W. Brandner2 and D. F. Figer3

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
    e-mail: stolte@mpia-hd.mpg.de; grebel@mpia-hd.mpg.de
2  European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
    e-mail: wbrandne@eso.org
3  Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
    e-mail: figer@stsci.edu

(Received 8 March 2002 / Accepted 30 July 2002 )

We have analysed high resolution adaptive optics (AO) science demonstration data of the young, massive stellar cluster Arches near the Galactic Center, obtained with the Gemini North telescope in combination with the University of Hawai'i AO system Hokupa'a. The AO H and K' photometry is calibrated using HST/NICMOS observations in the equivalent filters F160W and F205W obtained by Figer et al. (1999). The calibration procedure allows a detailed comparison of the ground-based adaptive optics observations against diffraction limited space-based photometry. The spatial resolution as well as the overall signal-to-noise ratio of the Gemini/Hokupa'a data is comparable to the HST/NICMOS data. The low Strehl ratio of only a few percent is the dominant limiting factor in the Gemini AO science demonstration data as opposed to space-based observations. After a thorough technical comparison, the Gemini and HST data are used in combination to study the spatial distribution of stellar masses in the Arches cluster. Arches is one of the densest young clusters known in the Milky Way, with a central density of ~ $ 3 \times 10^5\,M_\odot\,{\rm pc^{-3}}$ and a total mass of about $10^4\,M_\odot$. A strong colour gradient is observed over the cluster field. The visual extinction increases by $\Delta A_{\rm V} \sim 10$ mag over a distance of 15´´ from the cluster core. Extinction maps reveal a low-extinction cavity in the densest parts of Arches ( $R \leq 5\arcsec$), indicating the depletion of dust due to stellar winds or photo-evaporation. We correct for the change in extinction over the field and show that the slope of the mass function is strongly influenced by the effects of differential extinction. We obtain present-day mass function slopes of $\Gamma \sim -0.8 \pm 0.2$ in the mass range $6\!<\!M\!<\!65\ M_\odot$ from both data sets. The spatial analysis reveals a steepening of the mass function slope from close to zero in the cluster center to $\Gamma \sim -1.7 \pm 0.7$ at $R > 10\arcsec$, in accordance with a Salpeter slope ( $\Gamma = -1.35$). The bias in the mass function towards high-mass stars in the Arches center is a strong indication for mass segregation. The dynamical and relaxation timescales for Arches are estimated, and possible mass segregation effects are discussed with respect to cluster formation models.

Key words: open clusters and associations: individual: Arches -- stars: luminosity function, mass function -- stars: early-type -- stars: formation -- ISM: dust, extinction -- instrumentation: adaptive optics

Offprint request: A. Stolte, stolte@mpia-hd.mpg.de

© ESO 2002