995 research outputs found
Ring Formation in Magnetically Subcritical Clouds and Multiple Star Formation
We study numerically the ambipolar diffusion-driven evolution of
non-rotating, magnetically subcritical, disk-like molecular clouds, assuming
axisymmetry. Previous similar studies have concentrated on the formation of
single magnetically supercritical cores at the cloud center, which collapse to
form isolated stars. We show that, for a cloud with many Jeans masses and a
relatively flat mass distribution near the center, a magnetically supercritical
ring is produced instead. The supercritical ring contains a mass well above the
Jeans limit. It is expected to break up, through both gravitational and
possibly magnetic interchange instabilities, into a number of supercritical
dense cores, whose dynamic collapse may give rise to a burst of star formation.
Non-axisymmetric calculations are needed to follow in detail the expected ring
fragmentation into multiple cores and the subsequent core evolution.
Implications of our results on multiple star formation in general and the
northwestern cluster of protostars in the Serpens molecular cloud core in
particular are discussed.Comment: 25 pages, 4 figures, to appear in Ap
High Mass Starless Cores
We report the identification of a sample of potential High-Mass Starless
Cores (HMSCs). The cores were discovered by comparing images of the fields
containing candidate High-Mass Protostellar Objects (HMPOs) at 1.2mm and
mid-infrared (8.3um; MIR) wavelengths. While the HMPOs are detected at both
wavelengths, several cores emitting at 1.2mm in the same fields show absorption
or no emission at the MIR wavelength. We argue that the absorption is caused by
cold dust. The estimated masses of a few 10^2Msun - 10^3 Msun and the lack of
IR emission suggests that they may be massive cold cores in a pre-stellar
phase, which could presumably form massive stars eventually. Ammonia (1,1) and
(2,2) observations of the cores indicate smaller velocity dispersions and lower
rotation temperatures compared to HMPOs and UCHII regions suggesting a
quiescent pre-stellar stage. We propose that these newly discovered cores are
good candidates for the HMSC stage in high-mass star-formation. This sample of
cores will allow us to study the high-mass star and cluster formation processes
at the earliest evolutionary stages.Comment: 7 pages, 3 figures, 1 table, to be published in ApJL, author names
replaced with comma separatio
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
Recommended from our members
Changing lives, changing systems: a report evaluating Opportunity Nottingham in its first two years of project delivery, 2014-16
Massive Quiescent Cores in Orion. -- II. Core Mass Function
We have surveyed submillimeter continuum emission from relatively quiescent
regions in the Orion molecular cloud to determine how the core mass function in
a high mass star forming region compares to the stellar initial mass function.
Such studies are important for understanding the evolution of cores to stars,
and for comparison to formation processes in high and low mass star forming
regions. We used the SHARC II camera on the Caltech Submillimeter Observatory
telescope to obtain 350 \micron data having angular resolution of about 9
arcsec, which corresponds to 0.02 pc at the distance of Orion. Our analysis
combining dust continuum and spectral line data defines a sample of 51 Orion
molecular cores with masses ranging from 0.1 \Ms to 46 \Ms and a mean mass of
9.8 \Ms, which is one order of magnitude higher than the value found in typical
low mass star forming regions, such as Taurus. The majority of these cores
cannot be supported by thermal pressure or turbulence, and are probably
supercritical.They are thus likely precursors of protostars. The core mass
function for the Orion quiescent cores can be fitted by a power law with an
index equal to -0.850.21. This is significantly flatter than the Salpeter
initial mass function and is also flatter than the core mass function found in
low and intermediate star forming regions. Thus, it is likely that
environmental processes play a role in shaping the stellar IMF later in the
evolution of dense cores and the formation of stars in such regions.Comment: 30 pages, 10 figures, accepted by Ap
Massive Protoplanetary Disks in the Trapezium Region
(abridged) We determine the disk mass distribution around 336 stars in the
young Orion Nebula cluster by imaging a 2.5' x 2.5' region in 3 mm continuum
emission with the Owens Valley Millimeter Array. For this sample of 336 stars,
we observe 3 mm emission above the 3-sigma noise level toward ten sources, six
of which have also been detected optically in silhouette against the bright
nebular background. In addition, we detect 20 objects that do not correspond to
known near-IR cluster members. Comparisons of our measured fluxes with longer
wavelength observations enable rough separation of dust emission from thermal
free-free emission, and we find substantial dust emission toward most objects.
For the ten objects detected at both 3 mm and near-IR wavelengths, eight
exhibit substantial dust emission. Excluding the high-mass stars and assuming a
gas-to-dust ratio of 100, we estimate circumstellar masses ranging from 0.13 to
0.39 Msun. For the cluster members not detected at 3 mm, images of individual
objects are stacked to constrain the mean 3 mm flux of the ensemble. The
average flux is detected at the 3-sigma confidence level, and implies an
average disk mass of 0.005 Msun, comparable to the minimum mass solar nebula.
The percentage of stars in Orion surrounded by disks more massive than ~0.1
Msun is consistent with the disk mass distribution in Taurus, and we argue that
massive disks in Orion do not appear to be truncated through close encounters
with high-mass stars. Comparison of the average disk mass and number of massive
dusty structures in Orion with similar surveys of the NGC 2024 and IC 348
clusters constrains the evolutionary timescales of massive circumstellar disks
in clustered environments.Comment: 27 pages, including 7 figures. Accepted by Ap
Search for starless clumps in the ATLASGAL survey
In this study, we present an unbiased sample of the earliest stages of
massive star formation across 20 square-degree of the sky. Within the region
10deg < l < 20deg and |b| < 1deg, we search the ATLASGAL survey at 870 micron
for dense gas condensations. These clumps are carefully examined for
indications of ongoing star formation using YSOs from the GLIMPSE source
catalog as well as sources in the 24 micron MIPSGAL images, to search for
starless clumps. We calculate the column densities as well as the kinematic
distances and masses for sources where the v_lsr is known from spectroscopic
observations. Within the given region, we identify 210 starless clumps with
peak column densities > 1 x 10e23 cm^(-2). In particular, we identify potential
starless clumps on the other side of the Galaxy. The sizes of the clumps range
between 0.1 pc and 3 pc with masses between a few tens of solar masses up to
several ten thousands of solar masses. Most of them may form massive stars, but
in the 20 square-degree we only find 14 regions massive enough to form stars
more massive than 20 solar masses and 3 regions with the potential to form
stars more massive than 40 40 solar masses. The slope of the high-mass tail of
the clump mass function for clumps on the near side of the Galaxy is 2.2 and,
therefore, Salpeter-like. We estimate the lifetime of the most massive starless
clumps to be 60000 yr. The sample offers a uniform selection of starless
clumps. In the large area surveyed, we only find a few potential precursors of
stars in the excess of 40 solar masses. It appears that the lifetime of these
clumps is somewhat shorter than their free-fall times, although both values
agree within the errors. In addition, these are ideal objects for detailed
studies and follow-up observations.Comment: 15 pages plus appendix, in total 44 pages, accepted for publication
in Astronomy & Astrophysics, full tables will be added soo
Evidence of triggered star formation in G327.3-0.6. Dust-continuum mapping of an infrared dark cloud with P-ArT\'eMiS
Aims. Expanding HII regions and propagating shocks are common in the
environment of young high-mass star-forming complexes. They can compress a
pre-existing molecular cloud and trigger the formation of dense cores. We
investigate whether these phenomena can explain the formation of high-mass
protostars within an infrared dark cloud located at the position of G327.3-0.6
in the Galactic plane, in between two large infrared bubbles and two HII
regions. Methods: The region of G327.3-0.6 was imaged at 450 ? m with the CEA
P-ArT\'eMiS bolometer array on the Atacama Pathfinder EXperiment telescope in
Chile. APEX/LABOCA and APEX-2A, and Spitzer/IRAC and MIPS archives data were
used in this study. Results: Ten massive cores were detected in the P-ArT\'eMiS
image, embedded within the infrared dark cloud seen in absorption at both 8 and
24 ?m. Their luminosities and masses indicate that they form high-mass stars.
The kinematical study of the region suggests that the infrared bubbles expand
toward the infrared dark cloud. Conclusions: Under the influence of expanding
bubbles, star formation occurs in the infrared dark areas at the border of HII
regions and infrared bubbles.Comment: 4 page
Fragmentation in the Massive Star-Forming Region IRAS 19410+2336
The Core Mass Functions (CMFs) of low-mass star-forming regions are found to
resemble the shape of the Initial Mass Function (IMF). A similar result is
observed for the dust clumps in high-mass star forming regions, although at
spatial scales of clusters that do not resolve the substructure found in them.
The region IRAS 19410+2336 is one exception, having been observed at spatial
scales on the order of ~2500AU, resolving the clump substructure into
individual cores.
We mapped that region with the PdBI in the 1.4mm and 3mm continuum and
several transitions of H2CO and CH3CN. The H2CO transitions were also observed
with the IRAM 30m Telescope. We detected 26 continuum sources at 1.4mm with a
spatial resolution down to ~2200 AU, distributed in two protoclusters. With the
lines emission we derived the temperature structure of the region, ranging from
35 to 90K. With them we calculated the core masses of the detected sources,
ranging from ~0.7 to ~8 M_sun. These masses were strongly (~90%) affected by
the interferometer spatial filtering. Considering only the detected dense cores
we derived a CMF with a power-law index b=-2.3+-0.2. We resolve the Jeans
length of the protoclusters by one order of magnitude, and only find little
velocity dispersion between the different subsources.
Since we cannot unambiguously differentiate protostellar and prestellar
cores, the derived CMF is not prestellar. Also, because of the large missing
flux, we cannot establish a firm link between the CMF and the IMF. This implies
that future high-mass CMF studies will need to complement the interferometer
continuum data with the short spacing data, a task suitable for ALMA. We note
that the method of extracting temperatures using H2CO lines becomes less
applicable when reaching the dense core scales of the interferometric
observations because most of the H2CO appears to originate in the envelope
structure.Comment: 17 pages, 12 figures, accepted by A&
Flaring Up All Over -- Radio Activity in Rapidly-Rotating Late-Type M and L Dwarfs
We present Very Large Array observations of twelve late M and L dwarfs in the
Solar neighborhood. The observed sources were chosen to cover a wide range of
physical characteristics - spectral type, rotation, age, binarity, and X-ray
and H\alpha activity - to determine the role of these properties in the
production of radio emission, and hence magnetic fields. Three of the twelve
sources, TVLM513-46546, 2MASS J0036159+182110, and BRI0021-0214, were observed
to flare and also exhibit persistent emission, indicating that magnetic
activity is not quenched at the bottom of the main sequence. The radio emission
extends to spectral type L3.5, and there is no apparent decrease in the ratio
of flaring luminosities to bolometric luminosities between M8-L3.5. Moreover,
contrary to the significant drop in persistent H\alpha activity beyond spectral
type M7, the persistent radio activity appears to steadily increase between
M3-L3.5. Similarly, the radio emission from BRI0021-0214 violates the
phenomenological relations between the radio and X-ray luminosities of
coronally active stars, hinting that radio and X-ray activity are also
uncorrelated at the bottom of the main sequence. The radio active sources that
have measured rotational velocities are rapid rotators, Vsin(i)>30 km/sec,
while the upper limits on radio activity in slowly-rotating late M dwarfs
(Vsin(i)<10 km/sec) are lower than these detections. These observations provide
tantalizing evidence that rapidly-rotating late M and L dwarfs are more likely
to be radio active. This possible correlation is puzzling given that the
observed radio emission requires sustained magnetic fields of 10-1000 G and
densities of 10^12 cm^-3, indicating that the active sources should have slowed
down considerably due to magnetic braking.Comment: Accepted to ApJ; Two new figures; Minor text revision
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