576 research outputs found
The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Taurus
By combining deep optical imaging and infrared spectroscopy with data from
the Two-Micron All-Sky Survey (2MASS) and from previous studies (e.g., Briceno
et al.), I have measured the Initial Mass Function (IMF) for a
reddening-limited sample in four fields in the Taurus star forming region. This
IMF is representative of the young populations within these fields for masses
above 0.02 Msun. Relative to the similarly derived IMF for the Trapezium
Cluster (Luhman et al.), the IMF for Taurus exhibits a modest deficit of stars
above one solar mass (i.e., steeper slope), the same turnover mass (~0.8 Msun),
and a significant deficit of brown dwarfs. If the IMF in Taurus were the same
as that in the Trapezium, 12.8+/-1.8 brown dwarfs (>0.02 Msun) are expected in
these Taurus fields where only one brown dwarf candidate is found. These
results are used to test theories of the IMF.Comment: to be published in The Astrophysical Journal, 24 pages, 6 figures,
also found at http://cfa-www.harvard.edu/~kluhman/taurus
Radiation-Hydrodynamic Simulations of Collapse and Fragmentation in Massive Protostellar Cores
We simulate the early stages of the evolution of turbulent, virialized,
high-mass protostellar cores, with primary attention to how cores fragment, and
whether they form a small or large number of protostars. Our simulations use
the Orion adaptive mesh refinement code to follow the collapse from ~0.1 pc
scales to ~10 AU scales, for durations that cover the main fragmentation phase,
using three-dimensional gravito-radiation hydrodynamics. We find that for a
wide range of initial conditions radiation feedback from accreting protostars
inhibits the formation of fragments, so that the vast majority of the collapsed
mass accretes onto one or a few objects. Most of the fragmentation that does
occur takes place in massive, self-shielding disks. These are driven to
gravitational instability by rapid accretion, producing rapid mass and angular
momentum transport that allows most of the gas to accrete onto the central star
rather than forming fragments. In contrast, a control run using the same
initial conditions but an isothermal equation of state produces much more
fragmentation, both in and out of the disk. We conclude that massive cores with
observed properties are not likely to fragment into many stars, so that, at
least at high masses, the core mass function probably determines the stellar
initial mass function. Our results also demonstrate that simulations of massive
star forming regions that do not include radiative transfer, and instead rely
on a barotropic equation of state or optically thin heating and cooling curves,
are likely to produce misleading results.Comment: 23 pages, 18 figures, emulateapj format. Accepted to ApJ. This
version has minor typo fixes and small additions, no significant changes.
Resolution of images severely degraded to fit within size limit. Download the
full paper from http://www.astro.princeton.edu/~krumholz/recent.htm
The Kinematics of Molecular Cloud Cores in the Presence of Driven and Decaying Turbulence: Comparisons with Observations
In this study we investigate the formation and properties of prestellar and
protostellar cores using hydrodynamic, self-gravitating Adaptive Mesh
Refinement simulations, comparing the cases where turbulence is continually
driven and where it is allowed to decay. We model observations of these cores
in the CO, NH, and NH lines, and from
the simulated observations we measure the linewidths of individual cores, the
linewidths of the surrounding gas, and the motions of the cores relative to one
another. Some of these distributions are significantly different in the driven
and decaying runs, making them potential diagnostics for determining whether
the turbulence in observed star-forming clouds is driven or decaying. Comparing
our simulations with observed cores in the Perseus and Ophiuchus clouds
shows reasonably good agreement between the observed and simulated core-to-core
velocity dispersions for both the driven and decaying cases. However, we find
that the linewidths through protostellar cores in both simulations are too
large compared to the observations. The disagreement is noticably worse for the
decaying simulation, in which cores show highly supersonic infall signatures in
their centers that decrease toward their edges, a pattern not seen in the
observed regions. This result gives some support to the use of driven
turbulence for modeling regions of star formation, but reaching a firm
conclusion on the relative merits of driven or decaying turbulence will require
more complete data on a larger sample of clouds as well as simulations that
include magnetic fields, outflows, and thermal feedback from the protostars.Comment: 18 pages, 12 figures, accepted to A
Magnetically Regulated Star Formation in 3D: The Case of Taurus Molecular Cloud Complex
We carry out three-dimensional MHD simulations of star formation in
turbulent, magnetized clouds, including ambipolar diffusion and feedback from
protostellar outflows. The calculations focus on relatively diffuse clouds
threaded by a strong magnetic field capable of resisting severe tangling by
turbulent motions and retarding global gravitational contraction in the
cross-field direction. They are motivated by observations of the Taurus
molecular cloud complex (and, to a lesser extent, Pipe Nebula), which shows an
ordered large-scale magnetic field, as well as elongated condensations that are
generally perpendicular to the large-scale field. We find that stars form in
earnest in such clouds when enough material has settled gravitationally along
the field lines that the mass-to-flux ratios of the condensations approach the
critical value. Only a small fraction (of order 1% or less) of the nearly
magnetically-critical, condensed material is turned into stars per local
free-fall time, however. The slow star formation takes place in condensations
that are moderately supersonic; it is regulated primarily by magnetic fields,
rather than turbulence. The quiescent condensations are surrounded by diffuse
halos that are much more turbulent, as observed in the Taurus complex. Strong
support for magnetic regulation of star formation in this complex comes from
the extremely slow conversion of the already condensed, relatively quiescent
CO gas into stars, at a rate two orders of magnitude below the maximum,
free-fall value. We analyze the properties of dense cores, including their mass
spectrum, which resembles the stellar initial mass function.Comment: submitted to Ap
Star formation in clusters: early sub-clustering in the Serpens core
We present high resolution interferometric and single dish observations of
molecular gas in the Serpens cluster-forming core. Star formation does not
appear to be homogeneous throughout the core, but is localised in spatially-
and kinematically-separated sub-clusters. The stellar (or proto-stellar)
density in each of the sub-clusters is much higher than the mean for the entire
Serpens cluster. This is the first observational evidence for the hierarchical
fragmentation of proto-cluster cores suggested by cluster formation models.Comment: 11 pages, 3 Figures, ApJ Letters in pres
The "Mysterious" Origin of Brown Dwarfs
Hundreds of brown dwarfs (BDs) have been discovered in the last few years in
stellar clusters and among field stars. BDs are almost as numerous as hydrogen
burning stars and so a theory of star formation should also explain their
origin. The ``mystery'' of the origin of BDs is that their mass is two orders
of magnitude smaller than the average Jeans' mass in star--forming clouds, and
yet they are so common. In this work we investigate the possibility that
gravitationally unstable protostellar cores of BD mass are formed directly by
the process of turbulent fragmentation. Supersonic turbulence in molecular
clouds generates a complex density field with a very large density contrast. As
a result, a fraction of BD mass cores formed by the turbulent flow are dense
enough to be gravitationally unstable. We find that with density, temperature
and rms Mach number typical of cluster--forming regions, turbulent
fragmentation can account for the observed BD abundance.Comment: 11 pages, 3 figures, ApJ submitted Error in equation 1 has been
corrected. Improved figure
A graph theory-based multi-scale analysis of hierarchical cascade in molecular clouds : Application to the NGC 2264 region
The spatial properties of small star-clusters suggest that they may originate
from a fragmentation cascade of the cloud for which there might be traces up to
a few dozen of kAU. Our goal is to investigate the multi-scale spatial
structure of gas clumps, to probe the existence of a hierarchical cascade and
to evaluate its possible link with star production in terms of multiplicity.
From the Herschel emission maps of NGC 2264, clumps are extracted using getsf
software at each of their associated spatial resolution, respectively [8.4,
13.5, 18.2, 24.9, 36.3]". Using the spatial distribution of these clumps and
the class 0/I Young Stellar Object (YSO) from Spitzer data, we develop a
graph-theoretic analysis to represent the multi-scale structure of the cloud as
a connected network. From this network, we derive three classes of multi-scale
structure in NGC 2264 depending on the number of nodes produced at the deepest
level: hierarchical, linear and isolated. The structure class is strongly
correlated with the column density since the hierarchical ones
dominate the regions whose Ncm. Although
the latter are in minority, they contain half of the class 0/I YSOs proving
that they are highly efficient in producing stars. We define a novel
statistical metric, the fractality coefficient F that measure the fractal index
describing the scale-free process of the cascade. For NGC 2264, we estimate F =
1.450.12. However, a single fractal index fails to fully describe a
scale-free process since the hierarchical cascade starts at a 13 kAU
characteristic spatial scale. Our novel methodology allows us to correlate YSOs
with their multi-scale gaseous environment. This hierarchical cascade that
drives efficient star formation is suspected to be both hierarchical and rooted
by the larger-scale gas environment up to 13 kAU
The Mass Function of Super Giant Molecular Complexes and Implications for Forming Young Massive Star Clusters in the Antennae (NGC 4038/39)
We have used previously published observations of the CO emission from the
Antennae (NGC 4038/39) to study the detailed properties of the super giant
molecular complexes with the goal of understanding the formation of young
massive star clusters. Over a mass range from 5E6 to 9E8 solar masses, the
molecular complexes follow a power-law mass function with a slope of -1.4 +/-
0.1, which is very similar to the slope seen at lower masses in molecular
clouds and cloud cores in the Galaxy. Compared to the spiral galaxy M51, which
has a similar surface density and total mass of molecular gas, the Antennae
contain clouds that are an order of magnitude more massive. Many of the
youngest star clusters lie in the gas-rich overlap region, where extinctions as
high as Av~100 imply that the clusters must lie in front of the gas. Combining
data on the young clusters, thermal and nonthermal radio sources, and the
molecular gas suggests that young massive clusters could have formed at a
constant rate in the Antennae over the last 160 Myr and that sufficient gas
exists to sustain this cluster formation rate well into the future. However,
this conclusion requires that a very high fraction of the massive clusters that
form initially in the Antennae do not survive as long as 100 Myr. Finally, we
compare our data with two models for massive star cluster formation and
conclude that the model where young massive star clusters form from dense cores
within the observed super giant molecular complexes is most consistent with our
current understanding of this merging system. (abbreviated)Comment: 40 pages, four figures; accepted for publication in Ap
The Initial Mass Function of Low-Mass Stars and Brown Dwarfs in Young Clusters
We have obtained images of the Trapezium Cluster (140" x 140"; 0.3 pc x 0.3
pc) with the Hubble Space Telescope Near-Infrared Camera and Multi-Object
Spectrometer (NICMOS). Combining these data with new ground-based K-band
spectra (R=800) and existing spectral types and photometry and the models of
D'Antona & Mazzitelli, we find that the distributions of ages of comparable
samples of stars in the Trapezium, rho Oph, and IC 348 indicate median ages of
\~0.4 Myr for the first two regions and ~1-2 Myr for the latter. The low-mass
IMFs in these sites of clustered star formation are similar over a wide range
of stellar densities and other environmental conditions. With current data, we
cannot rule out modest variations in the substellar mass functions among these
clusters. We then make the best estimate of the true form of the IMF in the
Trapezium by using the evolutionary models of Baraffe et al. and an empirically
adjusted temperature scale and compare this mass function to recent results for
the Pleiades and the field. All of these data are consistent with an IMF that
is flat or rises slowly from the substellar regime to about 0.6 Msun, and then
rolls over into a power law that continues from about 1 Msun to higher masses
with a slope similar to or somewhat larger than the Salpeter value of 1.35. For
the Trapezium, this behavior holds from our completeness limit of ~0.02 Msun
and probably, after a modest completeness correction, even from 0.01-0.02 Msun.
These data include ~50 likely brown dwarfs. We test the predictions of theories
of the IMF against various properties of the observed IMF.Comment: 34 pages, 13 figures, for color image see
http://cfa-www.harvard.edu/~kluhman/trap/colorimage.jp
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