280 research outputs found
Dense Gas Formation via Collision-induced Magnetic Reconnection in a Disk Galaxy with a BiSymmetric Spiral Magnetic Field
Recently, a collision-induced magnetic reconnection (CMR) mechanism was
proposed to explain a dense filament formation in the Orion A giant molecular
cloud. A natural question is that whether CMR works elsewhere in the Galaxy. As
an initial attempt to answer the question, this paper investigates the
triggering of CMR and the production of dense gas in a flat-rotating disk with
a modified BiSymmetric Spiral (BSS) magnetic field. Cloud-cloud collisions at
field reversals in the disk are modeled with the Athena++ code. Under the
condition that is representative of the warm neutral medium, the cloud-cloud
collision successfully triggers CMR at different disk radii. However, dense gas
formation is hindered by the dominating thermal pressure, unless a moderately
stronger initial field G is present. The strong-field model,
having a larger Lundquist number and lower plasma , activates the
plasmoid instability in the collision midplane, which is otherwise suppressed
by the disk rotation. We speculate that CMR can be common if more clouds
collide along field reversals. However, to witness the CMR process in numerical
simulations, we need to significantly resolve the collision midplane with a
spatial dynamic range . If Milky Way spiral arms indeed coincide
with field reversals in BSS, it is possible that CMR creates or maintains dense
gas in the arms. High-resolution, high-sensitivity Zeeman/Faraday-Rotation
observations are crucial for finding CMR candidates that have helical fields.Comment: 21 pages, 11 figures, 1 table, accepted by Ap
The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration
The deuterium fraction [ND]/[NH], may provide information
about the ages of dense, cold gas structures, important to compare with
dynamical models of cloud core formation and evolution. Here we introduce a
complete chemical network with species containing up to three atoms, with the
exception of the Oxygen chemistry, where reactions involving HO and its
deuterated forms have been added, significantly improving the consistency with
comprehensive chemical networks. Deuterium chemistry and spin states of H
and H isotopologues are included in this primarily gas-phase chemical
model. We investigate dependence of deuterium chemistry on model parameters:
density (), temperature, cosmic ray ionization rate, and gas-phase
depletion factor of heavy elements (). We also explore the effects
of time-dependent freeze-out of gas-phase species and dynamical evolution of
density at various rates relative to free-fall collapse. For a broad range of
model parameters, the timescales to reach large values of , observed in some low- and high-mass starless cores, are
relatively long compared to the local free-fall timescale. These conclusions
are unaffected by introducing time-dependent freeze-out and considering models
with evolving density, unless the initial 10. For fiducial
model parameters, achieving requires
collapse to be proceeding at rates at least several times slower than that of
free-fall collapse, perhaps indicating a dynamically important role for
magnetic fields in the support of starless cores and thus the regulation of
star formation.Comment: 23 pages, 18 figures, accepted by Ap
Structure, Dynamics and Deuterium Fractionation of Massive Pre-Stellar Cores
High levels of deuterium fraction in NH are observed in some
pre-stellar cores. Single-zone chemical models find that the timescale required
to reach observed values () is longer than the free-fall
time, possibly ten times longer. Here, we explore the deuteration of turbulent,
magnetized cores with 3D magnetohydrodynamics simulations. We use an
approximate chemical model to follow the growth in abundances of NH and
ND. We then examine the dynamics of the core using each tracer for
comparison to observations. We find that the velocity dispersion of the core as
traced by ND appears slightly sub-virial compared to predictions of the
Turbulent Core Model of McKee & Tan, except at late times just before the onset
of protostar formation. By varying the initial mass surface density, the
magnetic energy, the chemical age, and the ortho-to-para ratio of H, we
also determine the physical and temporal properties required for high
deuteration. We find that low initial ortho-to-para ratios ()
and/or multiple free-fall times () of prior chemical evolution are
necessary to reach the observed values of deuterium fraction in pre-stellar
cores.Comment: 20 pages, 18 figures; accepted for publication in Ap
Zooming in to Massive Star Birth
We present high resolution (0.2", 1000 AU) 1.3 mm ALMA observations of
massive infrared dark cloud clump, G028.37+00.07-C1, thought to harbor the
early stages of massive star formation. Using (3-2) we resolve the
previously identified C1-S core, separating the bulk of its emission from two
nearby protostellar sources. C1-S is thus identified as a massive
(), compact (pc diameter) starless core, e.g., with
no signs of outflow activity. Being highly deuterated, this is a promising
candidate for a pre-stellar core on the verge of collapse. An analysis of its
dynamical state indicates a sub-virial velocity dispersion compared to a
trans-Alfv\'enic turbulent core model. However, virial equilibrium could be
achieved with sub-Alfv\'enic conditions involving mG magnetic field
strengths.Comment: 19 pages, 15 figures, 4 tables, accepted by Ap
X Marks the Spot: Nexus of Filaments, Cores, and Outflows in a Young Star-Forming Region
We present a multiwavelength investigation of a region of a nearby giant
molecular cloud that is distinguished by a minimal level of star formation
activity. With our new 12CO(J=2-1) and 13CO(J=2-1) observations of a remote
region within the middle of the California molecular cloud, we aim to
investigate the relationship between filaments, cores, and a molecular outflow
in a relatively pristine environment. An extinction map of the region from
Herschel Space Observatory observations reveals the presence of two 2-pc-long
filaments radiating from a high-extinction clump. Using the 13CO observations,
we show that the filaments have coherent velocity gradients and that their
mass-per-unit-lengths may exceed the critical value above which filaments are
gravitationally unstable. The region exhibits structure with eight cores, at
least one of which is a starless, prestellar core. We identify a low-velocity,
low-mass molecular outflow that may be driven by a flat spectrum protostar. The
outflow does not appear to be responsible for driving the turbulence in the
core with which it is associated, nor does it provide significant support
against gravitational collapse.Comment: Accepted for publication in the Astrophysical Journa
The CARMA-NRO Orion Survey
We present the first results from a new, high-resolution ^(12)CO(1–0), ^(13)CO(1–0), and C^(18)O(1–0) molecular-line survey of the Orion A cloud, hereafter referred to as the CARMA-NRO Orion Survey. CARMA observations have been combined with single-dish data from the Nobeyama 45 m telescope to provide extended images at about 0.01 pc resolution, with a dynamic range of approximately 1200 in spatial scale. Here we describe the practical details of the data combination in uv space, including flux scale matching, the conversion of single-dish data to visibilities, and joint deconvolution of single-dish and interferometric data. A Δ-variance analysis indicates that no artifacts are caused by combining data from the two instruments. Initial analysis of the data cubes, including moment maps, average spectra, channel maps, position–velocity diagrams, excitation temperature, column density, and line ratio maps, provides evidence of complex and interesting structures such as filaments, bipolar outflows, shells, bubbles, and photo-eroded pillars. The implications for star formation processes are profound, and follow-up scientific studies by the CARMA-NRO Orion team are now underway. We plan to make all the data products described here generally accessible; some are already available at https://dataverse.harvard.edu/dataverse/CARMA-NRO-Orion
The Core Mass Function in the Massive Protocluster G286.21+0.17 revealed by ALMA
We study the core mass function (CMF) of the massive protocluster
G286.21+0.17 with the Atacama Large Millimeter/submillimeter Array via 1.3~mm
continuum emission at a resolution of 1.0\arcsec\ (2500~au). We have mapped a
field of 5.3\arcmin5.3\arcmin\ centered on the protocluster clump. We
measure the CMF in the central region, exploring various core detection
algorithms, which give source numbers ranging from 60 to 125, depending on
parameter selection. We estimate completeness corrections due to imperfect flux
recovery and core identification via artificial core insertion experiments. For
masses , the fiducial dendrogram-identified CMF can be fit
with a power law of the form
with , slightly shallower than, but still consistent with, the
index of the Salpeter stellar initial mass function of 1.35.
Clumpfind-identified CMFs are significantly shallower with
. While raw CMFs show a peak near ,
completeness-corrected CMFs are consistent with a single power law extending
down to , with only a tentative indication of a shallowing
of the slope around . We discuss the implications of these
results for star and star cluster formation theories.Comment: 11 pages, accepted by Ap
The Core Mass Function Across Galactic Environments. II. Infrared Dark Cloud Clumps
We study the core mass function (CMF) within 32 dense clumps in seven
infrared dark clouds (IRDCs) with the Atacama Large Millimeter/submillimeter
Array (ALMA) via 1.3~mm continuum emission at a resolution of 1". We have
identified 107 cores with the dendrogram algorithm, with a median radius of
about 0.02 pc. Their masses range from 0.261 to 178 . After applying
completeness corrections, we fit the combined IRDC CMF with a power law of the
form and derive an index of
for and
for , which is a significantly
more top-heavy distribution than the Salpeter stellar initial mass function
(IMF) index of 1.35. We also make a direct comparison of these IRDC clump CMF
results to those measured in the more evolved protocluster G286 derived with
similar methods, which have and in
these mass ranges, respectively. These results provide a hint that, especially
for the range where completeness corrections are modest,
the CMF in high pressure, early-stage environments of IRDC clumps may be
top-heavy compared to that in the more evolved, global environment of the G286
protoclusters. However, larger samples of cores probing these different
environments are needed to better establish the robustness of this potential
CMF variation.Comment: Accepted to ApJ, 15 pages, 7 figure
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