44 research outputs found
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 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
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
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
A Keck LGS AO Search for Brown Dwarf and Planetary Mass Companions to Upper Scorpius Brown Dwarfs
We searched for binary companions to 20 young brown dwarfs in the Upper
Scorpius association (145 pc, 5 Myr, nearest OB association) with the the Laser
Guide Star adaptive optics system and the facility infrared camera NIRC2 on the
10 m Keck II telescope. We discovered a 0.14" companion (20.9+-0.4 AU) to the
<0.1 MSun object SCH J16091837-20073523. From spectral deconvolution of
integrated-light near-IR spectroscopy of SCH1609 using the SpeX spectrograph
(Rayner et al. 2003), we estimate primary and secondary spectral types of
M6+-0.5 and M7+-1.0, corresponding to masses of 79+-17 MJup and 55+-25 MJup at
an age of 5 Myr and masses of 84+-15 MJup and 60+-25 MJup at an age of 10 Myr.
For our survey objects with spectral types later than M8, we find an upper
limit on the binary fraction of <9% (1-sigma) at separations of 10 -- 500 AU.
We combine the results of our survey with previous surveys of Upper Sco and
similar young regions to set the strongest constraints to date on binary
fraction for young substellar objects and very low mass stars. The binary
fraction for low mass (<40 MJup) brown dwarfs in Upper Sco is similar to that
for T dwarfs in the field; for higher mass brown dwarfs and very low mass
stars, there is an excess of medium-separation (10-50 AU projected separation)
young binaries with respect to the field. These medium separation binaries will
likely survive to late ages.Comment: 37 pages, 6 figures, accepted to Ap
Substellar Objects in Nearby Young Clusters (SONYC) IV: A census of very low mass objects in NGC1333
SONYC -- Substellar Objects in Nearby Young Clusters -- is a program to
investigate the frequency and properties of young substellar objects with
masses down to a few times that of Jupiter. Here we present a census of very
low mass objects in the ~1 Myr old cluster NGC1333. We analyze near-infrared
spectra taken with FMOS/Subaru for 100 candidates from our deep, wide-field
survey and find 10 new likely brown dwarfs with spectral types of M6 or later.
Among them, there are three with >~M9 and one with early L spectral type,
corresponding to masses of 0.006 to <~0.02 Msol, so far the lowest mass objects
identified in this cluster. The combination of survey depth, spatial coverage,
and extensive spectroscopic follow-up makes NGC1333 one of the most
comprehensively surveyed clusters for substellar objects. In total, there are
now 51 objects with spectral type M5 or later and/or effective temperature of
3200 K or cooler identified in NGC1333; 30-40 of them are likely to be
substellar. NGC1333 harbours about half as many brown dwarfs as stars, which is
significantly more than in other well-studied star forming regions, thus
raising the possibility of environmental differences in the formation of
substellar objects. The brown dwarfs in NGC1333 are spatially strongly
clustered within a radius of ~1 pc, mirroring the distribution of the stars.
The disk fraction in the substellar regime is <66%, lower than for the total
population (83%) but comparable to the brown dwarf disk fraction in other 2-3
Myr old regions.Comment: 16 pages, 7 tables, 12 figures, accepted for publication in Ap
Mergers of multi-metallic globular clusters: The role of dynamics
Hubble Space Telescope observations of globular clusters (GCs) in the
Antennae galaxy show clusters of clusters, or regions in the galaxy that span
hundreds of parsecs, where many of the GCs are doomed to collide, and
eventually merge. Several such objects appear likely to present a significant
range in ages, hence possibly metallicities, and their merger could plausibly
lead to multi-metallic GCs. Here we explore this process with direct-summation
N-body simulations with GPU hardware. Our results reveal that colliding GCs
with different metallicities and ages can produce a GC with multiplicity and
occupation fractions not unlike those observed in multi-metallic clusters. In
our simulations, the merged clusters have a phase with a larger amount of
flattening than average, as a consequence of rapid rotation- thus suggesting
that relatively recent mergers may play a role in producing highly flattened,
multi-metallic clusters. We additionally explore the role of the King parameter
of the cluster in the occupation fractions with a set of 160 direct-summation
simulations and find that for equal size clusters the King parameter of the
progenitor clusters determines the occupation fractions in the merger product,
while in unequal size mergers the size of the clusters dominates the
distribution of stars in the new GC. In particular, we find that the observed
distribution of populations in Omega Cen can be described to some extent with
our dynamical models.Comment: Accepted for publication MNRA
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
The Massive Star Clusters in the Dwarf Merger ESO 185-IG13: is the Red Excess Ubiquitous in Starbursts?
We have investigated the starburst properties of the luminous blue compact
galaxy ESO 185-IG13. The galaxy has been imaged with the high resolution
cameras onboard to the Hubble Space Telescope. From the UV to the IR, the data
reveal a system shaped by hundreds of young star clusters, and fine structures,
like a tidal stream and a shell. The presence of numerous clusters and the
perturbed morphology indicate that the galaxy has been involved in a recent
merger event. Using previous simulations of shell formation in galaxy mergers
we constrain potential progenitors of ESO 185-IG13. The analysis of the star
cluster population is used to investigate the properties of the present
starburst and to date the final merger event, which has produced hundreds of
clusters younger than 100 Myr. We have found a peak of cluster formation only
3.5 Myr old. A large fraction of these clusters will not survive after 10-20
Myr, due to the "infant mortality" caused by gas expulsion. However, this
sample of clusters represents an unique chance to investigate the youngest
phases of cluster evolution. As already observed in the analog blue compact
galaxy Haro 11, a fraction of young clusters are affected by a flux excess at
wavelengths longer than 8000 \AA. Ages, masses, and extinctions of clusters
with this NIR excess are estimated from UV and optical data. We discuss
similarities and differences of the observed NIR excess in ESO 185-IG13
clusters with other cases in the literature. The cluster ages and masses are
used to distinguish among the potential causes of the excess. We observe, as in
Haro 11, that the use of the IR and the (commonly used) I band data results in
overestimates of age and mass in clusters affected by the NIR excess. This has
important implications for a number of related studies of star clusters.Comment: 23 pages, 16 figures, accepted for publication in MNRA