Constraining the IMF in Extreme Environments: Detecting Young Low Mass Stars in Unresolved Starbursts

We demonstrate the feasibility of detecting directly low mass stars in unresolved super-star clusters with ages < 10 Myr using near-infrared spectroscopy at modest resolution (R ~ 1000). Such measurements could constrain the ratio of high to low mass stars in these extreme star-forming events, providing a direct test on the universal nature of the initial mass function (IMF) compared to the disk of the Milky Way (Chabrier, 2003). We compute the integrated light of super-star clusters with masses of 10^6 Msun drawn from the Salpeter (1955) and Chabrier (2003) IMFs for clusters aged 1, 3, and 10 Myr. We combine, for the first time, results from Starburst99 (Leitherer et al. 1999) for the main sequence and post-main sequence population (including nebular emission) with pre-main sequence (PMS) evolutionary models (Siess et al. 2000) for the low mass stars as a function of age. We show that ~ 4-12 % of the integrated light observed at 2.2 microns comes from low mass PMS stars with late-type stellar absorption features at ages < 3 Myr. This light is discernable using high signal-to-noise spectra (> 100) at R=1000 placing constraints on the ratio of high to low mass stars contributing to the integrated light of the cluster.Comment: Accepted for publication in the Astrophysical Journal Letter

Evidence for a Turnover in the IMF of Low Mass Stars and Sub-stellar Objects: Analysis from an Ensemble of Young Clusters

We present a combined analysis of the low-mass Initial Mass Function (IMF) for seven star forming regions. We first demonstrate that the ratios of stars to brown dwarfs are consistent with a single underlying IMF. Assuming the underlying IMF is the same for all seven clusters and by combining the ratio of stars to brown dwarfs from each cluster we constrain the shape of the brown dwarf IMF and find it to be consistent with a log--normal IMF. This provides the strongest constraint yet that the sub-stellar IMF turns over (dN/dM M^(-alpha), alpha < 0).Comment: 12 pages, 2 figures. Accepted in ApJ Letters Revised version have Column 7 modified from previous versions and gramatical errors have been correcte

The IMF in Extreme Star-Forming Environments: Searching for Variations vs. Initial Conditions

Any predictive theory of star formation must explain observed variations (or lack thereof) in the initial mass function. Recent work suggests that we might expect quantitative variations in the IMF as a function of metallicity (Larson 2005) or magnetic field strength (Shu et al. 2004). We summarize results from several on-going studies attempting to constrain the ratio of high to low mass stars, as well as stars to sub- stellar objects, in a variety of different environments, all containing high mass stars. First, we examine the ratio of stars to sub--stellar objects in the nearby Mon R2 region utilizing NICMOS/HST data. We compare our results to the IMF by Kroupa (2002)]} and to the observed ratios for IC 348 and Orion. Second, we present preliminary results for the ratio of high to low mass stars in W51, the most luminous HII region in the galaxy. Based on ground--based multi--colour images of the cluster obtained with the MMT adaptive optics system, we derive a lower limit to the ratio of high-mass to low-mass stars and compare it to the ratios for nearby clusters. Finally, we present the derived IMF for the R136 region in the LMC where the metallicity is 1/4 solar using HST/NICMOS data. We find that the IMF is consistent with that characterizing the field (Chabrier 2003), as well as nearby star--forming regions, down to 1.0 Msun outside 2 pc. Whereas the results for both Mon R2 and R136 are consistent with the nearby clusters, the ratio of high to low mass stars in W51 tentatively indicates a lack of low--mass objects.Comment: 6 pages, 3 figures, to appear in the proceedings of IAU Symposium 227: "Massive Star Birth: A Crossroads of Astrophysics

The Hubble Legacy Archive NICMOS Grism Data

The Hubble Legacy Archive (HLA) aims to create calibrated science data from the Hubble Space Telescope archive and make them accessible via user-friendly and Virtual Observatory (VO) compatible interfaces. It is a collaboration between the Space Telescope Science Institute (STScI), the Canadian Astronomy Data Centre (CADC) and the Space Telescope - European Coordinating Facility (ST-ECF). Data produced by the Hubble Space Telescope (HST) instruments with slitless spectroscopy modes are among the most difficult to extract and exploit. As part of the HLA project, the ST-ECF aims to provide calibrated spectra for objects observed with these HST slitless modes. In this paper, we present the HLA NICMOS G141 grism spectra. We describe in detail the calibration, data reduction and spectrum extraction methods used to produce the extracted spectra. The quality of the extracted spectra and associated direct images is demonstrated through comparison with near-IR imaging catalogues and existing near-IR spectroscopy. The output data products and their associated metadata are publicly available through a web form at http://hla.stecf.org and via VO interfaces. In total, 2470 spectra of 1923 unique targets are included in the current release.Comment: 18 pages, 21 figures, accepted for publication in Astronomy & Astrophysic

Star Formation History of a Young Super-Star Cluster in NGC 4038/39: Direct Detection of Low Mass Pre-Main Sequence Stars

We present an analysis of the near-infrared spectrum of a young massive star cluster in the overlap region of the interacting galaxies NGC 4038/39 using population synthesis models. Our goal is to model the cluster population as well as provide rough constraints on its initial mass function (IMF). The cluster shows signs of youth such as thermal radio emission and strong hydrogen emission lines in the near-infrared. Late-type absorption lines are also present which are indicative of late-type stars in the cluster. The strength and ratio of these absorption lines cannot be reproduced through either late-type pre-main sequence (PMS) stars or red supergiants alone. Thus we interpret the spectrum as a superposition of two star clusters of different ages, which is feasible since the 1" spectrum encompasses a physical region of ~90 pc and radii of super-star clusters are generally measured to be a few parsecs. One cluster is young (<= 3 Myr) and is responsible for part of the late-type absorption features, which are due to PMS stars in the cluster, and the hydrogen emission lines. The second cluster is older (6 Myr - 18 Myr) and is needed to reproduce the overall depth of the late-type absorption features in the spectrum. Both are required to accurately reproduce the near-infrared spectrum of the object. Thus we have directly detected PMS objects in an unresolved super-star cluster for the first time using a combination of population synthesis models and pre-main sequence tracks. This analysis serves as a testbed of our technique to constrain the low-mass IMF in young super-star clusters as well as an exploration of the star formation history of young UC HII regions.Comment: 26 pages, 5 figures, accepted for publication in the Astrophysical Journa

HST/NICMOS Observations of NGC 1333: The Ratio of Stars to Sub-Stellar Objects

We present an analysis of NICMOS photometry and low-resolution grism spectroscopy of low-mass stars and sub-stellar objects in the young star-forming region NGC 1333. Our goal is to constrain the ratio of low-mass stars to sub- stellar objects down to 20 Mjup in the cluster as well as constrain the cluster IMF down to 30 Mjup in combination with a previous survey of NGC 1333 by Wilking et al. Our survey covers 4 fields of 51.2" x 51.2", centered on brown dwarf candidates previously identified in Wilking et al. We extend previous work based on the use of a water vapor index for spectral typing to wavelengths accessible with NICMOS on the HST. Spectral types were derived for the 14 brightest objects in our fields, ranging from <M0 - M8, which at the age of the cluster (0.3 Myr) corresponds to a range in mass of >0.25 - 0.02 Msun. In addition to the spectra, we present an analysis of the color-magnitude diagram using pre-main sequence evolutionary models of D'Antona & Mazzitelli. Using an extinction-limited sample, we derive the ratio of low-mass stars to brown dwarfs. Comparisons of the observed ratio to that expected from the field IMF of Chabrier indicate that the two results are consistent. We combine our data with that of Wilking et al. to compute the ratio of intermediate-mass stars (0.1 - 1.0 Msun) to low-mass objects (0.03 - 0.1 Msun) in the cluster. We also report the discovery of a faint companion to the previously confirmed brown dwarf ASR 28, as well as a possible outflow surrounding ASR 16. If the faint companion is confirmed as a cluster member, it would have a mass of ~ 5 Mjup (mass ratio 0.15) at a projected distance of 350 AU, similar to 2MASS 1207-3923 B.Comment: 33 pages, 6 figures, accepted for publication by A

Star cluster formation and evolution in Mrk 930: properties of a metal-poor starburst

We present a HST multiband analysis of the large population of star clusters in the blue compact galaxy (BCG) Mrk 930. We have reconstructed the spectral energy distributions of the star clusters and estimated age, mass, and extinction for a representative sample. We observe a very young cluster population with 70% of the systems formed less than 10 Myr ago. The peak in the star cluster age distribution at 4 Myr is corroborated by the presence of Wolf-Rayet spectral features, and by the observed optical and IR lines ratios [OIII]/H_beta and [NeIII]/[NeII]. The recovered extinction in these very young clusters shows large variations, with a decrease at older ages. It is likely that our analysis is limited to the optically brightest objects (i.e. systems only partially embedded in their natal cocoons). We map the extinction across the galaxy using low-resolution spectra and the H_alpha/H_beta ratio, as obtained from ground-based narrow band imaging. We find that the mean optical extinction derived in the starburst regions is close to the averaged value observed in the clusters, but locally, do not trace the more extinguished clusters. Previous HST studies of BCGs have revealed a population of young and extremely red super star clusters. We detect a considerable fraction of clusters affected by a red excess also in Mrk 930. The nature of the red excess, which turns up at near-IR wavelengths remains unknown. We compare the cluster and the star formation history, the latter derived from the fit of spectral population synthesis models to the spectra. We find a general agreement between the two independently estimated quantities. Using the cluster properties we perform a study of the host environmental properties. We find that the cluster formation efficiency is significantly higher, suggesting a key role of the environment for the formation of these massive objects.[Abridged]Comment: 21 pages, 16 figures, accepted for publication in MNRAS. For high-resolution figures ask the author

Stellar, Brown Dwarf, and Multiple Star Properties from Hydrodynamical Simulations of Star Cluster Formation

We report the statistical properties of stars, brown dwarfs and multiple systems obtained from the largest hydrodynamical simulation of star cluster formation to date that resolves masses down to the opacity limit for fragmentation (a few Jupiter masses). The simulation is essentially identical to that of Bate, Bonnell & Bromm except that the initial molecular cloud is larger and more massive. It produces more than 1250 stars and brown dwarfs, providing unprecedented statistical information that can be compared with observational surveys. We find that hydrodynamical/sink particle simulations can reproduce many of the observed stellar properties very well. Binarity as a function of primary mass, the frequency of very-low-mass (VLM) binaries, general trends for the separation and mass ratio distributions of binaries, and the relative orbital orientations of triples systems are all in reasonable agreement with observations. We also examine the radial variations of binarity, velocity dispersion, and mass function in the resulting stellar cluster and the distributions of disc truncation radii due to dynamical interactions. For VLM binaries, we find that their frequency when using small accretion radii and gravitational softening is similar to that expected from observational surveys (approximately 20 percent). We also find that VLM binaries evolve from wide, unequal-mass systems towards close equal-mass systems as they form. The two main deficiencies of the calculations are that they over produce brown dwarfs relative to stars and that there are too few unequal mass binaries with K and G-dwarf primaries. [Abridged]Comment: Accepted by MNRAS, 28 pages, 26 figures. Animations available at http://www.astro.ex.ac.uk/people/mbate

Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters

Recent progress in studies of globular clusters has shown that they are not simple stellar populations, being rather made of multiple generations. Evidence stems both from photometry and spectroscopy. A new paradigm is then arising for the formation of massive star clusters, which includes several episodes of star formation. While this provides an explanation for several features of globular clusters, including the second parameter problem, it also opens new perspectives about the relation between globular clusters and the halo of our Galaxy, and by extension of all populations with a high specific frequency of globular clusters, such as, e.g., giant elliptical galaxies. We review progress in this area, focusing on the most recent studies. Several points remain to be properly understood, in particular those concerning the nature of the polluters producing the abundance pattern in the clusters and the typical timescale, the range of cluster masses where this phenomenon is active, and the relation between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review

How does a low-mass cut-off in the stellar IMF affect the evolution of young star clusters?

We investigate how different stellar initial mass functions (IMFs) can affect the mass-loss and survival of star clusters. We find that IMFs with radically different low-mass cut-offs (between 0.1 and 2 M⊙) do not change cluster destruction time-scales as much as might be expected. Unsurprisingly, we find that clusters with more high-mass stars lose relatively more mass through stellar evolution, but the response to this mass-loss is to expand and hence significantly slow their dynamical evolution. We also argue that it is very difficult, if not impossible, to have clusters with different IMFs that are initially ‘the same’, since the mass, radius and relaxation times depend on each other and on the IMF in a complex way. We conclude that changing the IMF to be biased towards more massive stars does speed up mass-loss and dissolution, but that it is not as dramatic as might be thought