A&A 394, 459–478 (2002) Astronomy DOI: 10.1051/0004-6361:20021118 & c ESO 2002 Astrophysics 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 f¨ur Astronomie, K¨onigstuhl 17, 69117 Heidelberg, Germany e-mail: [email protected]; [email protected] 2 European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany e-mail: [email protected] 3 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA e-mail: [email protected] Received 8 March 2002 / Accepted 30 July 2002 Abstract. 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 K0 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, 5 3 4 with a central density of 3 10 M pc− and a total mass of about 10 M . A strong colour gradient is observed over the ∼ × cluster field. The visual extinction increases by ∆A 10 mag over a distance of 1500 from the cluster core. Extinction maps V ∼ reveal a low-extinction cavity in the densest parts of Arches (R 500), 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 Γ 0:8 0:2in ∼− ± the mass range 6 < M < 65 M from both data sets. The spatial analysis reveals a steepening of the mass function slope from close to zero in the cluster center to Γ 1:7 0:7atR > 1000, in accordance with a Salpeter slope (Γ= 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 1. Introduction environment to be most efficient. In particular, the formation of high mass stars and massive clusters is more successful than in The Galactic Center (GC) is the most extreme star forming en- any other region of the Milky Way. vironment within the Milky Way. High stellar and gas densi- ties, turbulent motion, tidal torques exerted by the steep gravi- A detailed study of star formation processes and the stel- tational potential, magnetic fields and an intense radiation field lar content of the GC region has until recently been limited to determine the physical environment of star formation in the GC the brightest and most massive stars due to the large amount of region. Although disruptive forces exerted by the gravitational extinction (A 30 mag) along the line of sight. Additional and radiation fields counteract the agglomeration of material, V constraints are imposed∼ due to the spatial resolution at the GC the high gas and dust densities cause star formation in the GC distance of 8 kpc (DM = 14:47 0:08 mag, e.g., McNamara ∼ ± Send offprint requests to: A. Stolte, et al. 2000), much farther than nearby star forming regions such e-mail: [email protected] as the Orion or ρ Ophiuchi star forming complexes, which have ? Based on observations obtained with the Gemini North Telescope been studied in greater detail to date. Only with the advent of and the NASA/ESA Hubble Space Telescope. deep, high resolution near-infrared instruments, the analysis of Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20021118 460 A. Stolte et al.: Mass function of the Arches cluster stellar populations in young star clusters near the GC has be- as well as growing accretion rates depending on the mass of the come feasible. accreting protostar (Behrend & Maeder 2001), allow stars of up During the past few years, it has become evident that three to 100 M to form in the densest regions of a rich star cluster. out of four young starburst clusters known in the Milky Way are In case of the GC environment, a higher gas density may lead located in the GC region – namely, the Arches and Quintuplet to a higher accretion rate and/or to a longer accretion process clusters, as well as the Galactic Center Cluster itself. With in the protostellar phase. As long as the gravitational potential a cluster age of only a few Myr for Arches and Quintuplet, is strongly influenced by the amount of gas associated with the the question arises how many clusters do actually form in the cluster, gas infall causes a decrease in cluster radius and sub- densest environment of the Milky Way. The 2MASS database sequent increase in the collision rate, reinforcing the formation yielded new insights into the estimated number of star clusters of high-mass stars. Physical processes such as gravitational col- hidden in the dense stellar background. Dutra & Bica (2000, lapse or cloud collisions scale with the square root of the local 2001) report the detection of new cluster candidates of vari- density, √ρ (Elmegreen 1999, 2001), causing an enhanced star ous ages located in the innermost 200 pc of the Galaxy found formation rate (SFR) in high density environments. Elmegreen in 2MASS. Numerical simulations by Portegies Zwart et al. (2001) shows that the total mass as well as the maximum stel- (2001) suggest that clusters with properties similar to the mas- lar mass in a cluster strongly depends on the SFR and local sive Arches and Quintuplet may have formed in the past in the density. This is confirmed by observations of high-mass stars innermost 200 pc, but were then dispersed and are now indis- found predominantly in the largest star forming clouds (Larson tinguishable from the dense stellar background. As dynamical 1982). evolution timescales are short due to the strong tidal field in the Both the growing accretion and the collision scenario pre- GC region (Kim et al. 1999), young star clusters are disrupted dict the high-mass stars to form in the densest central region of quickly after formation, contributing to the Galactic bulge pop- a cluster, leading to primordial mass segregation, which may ulation. Thus, only the youngest clusters remain intact for the be evidenced in a flat mass function in the dense cluster center. study of star formation in this extraordinary environment. As an additional physical constraint, both scenarios require the The Arches cluster, at a projected distance of only 25 pc lower-mass stars to form first, and the highest-mass stars last in from the GC (assuming a heliocentric distance of 8 kpc to the the cluster evolution process. As the strong UV-radiation field GC), is one of the most massive young clusters known in the originating from hydrogen ignition in high-mass stars expells 4 Milky Way. With an estimated mass of about 10 M and a cen- the remaining gas from the cluster center, the accretion process 5 3 tral density of 3 10 M pc− , Arches is the densest young star should be halted immediately after high-mass star formation. cluster (YC) known× (Figer et al. 1999). From physical proper- The short dynamical timescales of compact clusters are, ties of Wolf-Rayet stars, the age of the cluster is estimated to however, influencing the spatial distribution of stellar masses be between 2 and 4.5 Myr (Blum et al. 2001). The stellar con- as well. On the one hand, high-mass stars are dragged into the tent of Arches has been studied by Figer et al. (1999) using cluster center due to the gravitational potential of the young HST/NICMOS data. They derived a shallow initial mass func- cluster. On the other hand, low-mass stars may easily be flung tion in the range 6< M <120 M with a slope of Γ= 0:7 0:1, out of the cluster due to interaction processes, especially given but with significant flattening observed in the innermost− part± of star densities as high as in the Arches cluster. The result of these the cluster (Γ= 0:1 0:2). processes would also be a flat mass function in the cluster cen- Most young− star clusters± and associations in the Milky Way ter, steepening as one progresses outwards due to dynamical display a mass function close to a Salpeter (1955) power law mass segregation. Dynamical segregation is predicted to occur with a slope of Γ= 1:35. Several such star forming regions within one relaxation time (Bonnell & Davies 1998), which for have been studied by− Massey et al.