The Stellar Initial Mass Function in the Galactic Center

Massive stars define the upper limits of the star formation process, dominate the energetics of their local environs, and significantly affect the chemical evolution of galaxies. Their role in starburst galaxies and the early Universe is likely to be important, but we still do not know the maximum mass that a star can possess, i.e.``the upper mass cutoff.'' I will discuss results from a program to measure the upper mass cutoff and IMF slope in the Galactic Center. The results suggest that the IMF in the Galactic center may deviate significantly from the Salpeter value, and that there may be an upper mass cutoff to the initial mass function of $\sim$150 Msun.Comment: To be published in the IMF@50 conference proceeding

Discovery of new Milky Way star cluster candidates in the 2MASS point source catalog III. Follow-up observations of cluster candidates in the Galactic Center region

This paper is part of a project to search the inner Milky Way for hidden massive clusters and to address the question of whether our Galaxy still forms clusters similar to the progenitors of the present-day globular clusters. We report high angular resolution deep near-infrared imaging of 21 cluster candidates selected from the catalogues of Bica et al. (2003) and Dutra et al.(2003) in a region around the Galactic Center. These catalogues were created from visual inspection of the 2MASS images. Seven objects appear to be genuine clusters, and for these objects we present estimates of extinction, distance and in some cases age and mass. Our estimated masses range from 1200 to 5500 solar masses. These clusters are thus significantly smaller than any Galactic globular cluster, and indicate that the formation of massive young clusters such as Arches and Quintuplet is not common in the present-day Milky Way. The remaining 14 objects are either not clusters or cannot be classified based on our data.Comment: 8 pages, 19 figures, accepted for publication in A&

Mid-infrared imaging and spectroscopy of the enigmatic cocoon stars in the Quintuplet Cluster

In an attempt to determine the nature of the enigmatic cocoon stars in the Quintuplet Cluster, we have obtained mid-infrared imaging and spectrophotometry of the cluster, using the CAM and SWS instruments on ISO, using SpectroCam-10 on the Palomar 5m telescope, and NICMOS on HST. The spectra show smooth continua with various dust and ice absorption features. These features are all consistent with an interstellar origin, and there is no clear evidence for any circumstellar contribution to these features. We find no spectral line or feature that could elucidate the nature of these sources. Detailed modeling of the silicate absorption features shows that they are best reproduced by the mu Cep profile, which is typical of the interstellar medium, with tau(sil) \sim 2.9. The high spatial resolution mid-IR images show that three of the five cocoon stars have spatially extended and asymmetric envelopes, with diameters of \sim 20,000 AUs. A reddening law similar to that of Lutz (1999) but with silicate features based on the mu Cep profile and normalized to our value of tau(sil) is used to deredden the observed spectrophotometry. The dereddened energy distributions are characterised by temperatures of 750-925 K, somewhat cooler than determined from near IR data alone. Models of optically thin and geometrically thick dust shells, as used by Williams et al. (1987) for very dusty, late-type WC stars, reproduce the observed SEDs from 4 to 17 mic, and imply shell luminosities of log(L/L(sun)) \sim 4.5-4.9 for the brightest four components. An analysis of the various suggestions proposed to explain the nature of the cocoon stars reveals serious problems with all the hypotheses, and the nature of these sources remains an enigma.Comment: 16 pages, 11 figures, A&A style. Accepted by A&

A Hidden Population of Massive Stars with Circumstellar Shells Discovered with the Spitzer Space Telescope

We have discovered a large number of circular and elliptical shells at 24 microns around luminous central sources with the MIPS instrument on-board the Spitzer Space Telescope. Our archival follow-up effort has revealed 90% of these circumstellar shells to be previously unknown. The majority of the shells is only visible at 24 microns, but many of the central stars are detected at multiple wavelengths from the mid- to the near-IR regime. The general lack of optical counterparts, however, indicates that these sources represent a population of highly obscured objects. We obtained optical and near-IR spectroscopic observations of the central stars and find most of these objects to be massive stars. In particular, we identify a large population of sources that we argue represents a narrow evolutionary phase, closely related or identical to the LBV stage of massive stellar evolution.Comment: 23 pages, 9 figures, accepted for publication in A

H-Band Spectroscopic Classification of OB Stars

We present a new spectroscopic classification for OB stars based on H-band (1.5 micron to 1.8 micron) observations of a sample of stars with optical spectral types. Our initial sample of nine stars demonstrates that the combination of He I 1.7002 micron and H Brackett series absorption can be used to determine spectral types for stars between about O4 and B7 (to within about +/- 2 sub-types). We find that the Brackett series exhibits luminosity effects similar to the Balmer series for the B stars. This classification scheme will be useful in studies of optically obscured high mass star forming regions. In addition, we present spectra for the OB stars near 1.1 micron and 1.3 micron which may be of use in analyzing their atmospheres and winds.Comment: Accepted by AJ, 16 pages Latex (aastex4.0) including 4 figures and 2 tables. A complete PostScript copy is available at ftp://degobah.colorado.edu/pub/rblum/Hband

A Multiwavelength Study of Evolved Massive Stars in the Galactic Center

The central region of the Milky Way provides a unique laboratory for a systematic, spatially-resolved population study of evolved massive stars of various types in a relatively high metallicity environment. We have conducted a multi-wavelength data analysis of 180 such stars or candidates, most of which were drawn from a recent large-scale HST/NICMOS narrow-band Pa-a survey, plus additional 14 Wolf-Rayet stars identified in earlier ground-based spectroscopic observations of the same field. The multi-wavelength data include broad-band IR photometry measurements from HST/NICMOS, SIRIUS, 2MASS, Spitzer/IRAC, and Chandra X-ray observations. We correct for extinctions toward individual stars, improve the Pa-a line equivalent width measurements, quantify the substantial mid-IR dust emission associated with WC stars, and find X-ray counterparts. In the process, we identify 10 foreground sources, some of which may be nearby cataclysmic variables. The WN stars in the Arches and Central clusters show correlations between the Pa-a equivalent width and the adjacent continuum emission. However, the WN stars in the latter cluster are systematically dimmer than those in the Arches cluster, presumably due to the different ages of the two clusters. In the EW-magnitude plot, WNL stars, WC stars and OB supergiants roughly fall into three distinct regions. We estimate that the dust mass associated with individual WC stars in the Quintuplet cluster can reach 1e-5 M, or more than one order of magnitude larger than previous estimates. Thus WC stars could be a significant source of dust in the galaxies of the early universe. Nearly half of the evolved massive stars in the GC are located outside the three known massive stellar clusters. The ionization of several compact HII regions can be accounted for by their enclosed individual evolved massive stars, which thus likely formed in isolation or in small groups.Comment: Accepted for publication in MNRA