96 research outputs found
Rotating filament in Orion B: Do cores inherit their angular momentum from their parent filament?
Angular momentum is one of the most important physical quantities that govern
star formation. The initial angular momentum of a core may be responsible for
its fragmentation and can have an influence on the size of the protoplanetary
disk. To understand how cores obtain their initial angular momentum, it is
important to study the angular momentum of filaments where they form. While
theoretical studies on filament rotation have been explored, there exist very
few observational measurements of the specific angular momentum in star-forming
filaments. We present high-resolution N2D+ ALMA observations of the LBS 23
(HH24-HH26) region in Orion B, which provide one of the most reliable
measurements of the specific angular momentum in a star-forming filament. We
find the total specific angular momentum (), the
dependence of the specific angular momentum with radius (j(r) ), and the ratio of rotational energy to gravitational energy
() comparable to those observed in rotating cores with
sizes similar to our filament width ( 0.04 pc) in other star-forming
regions. Our filament angular momentum profile is consistent with rotation
acquired from ambient turbulence and with simulations that show cores and their
host filaments develop simultaneously due to the multi-scale growth of
nonlinear perturbation generated by turbulence.Comment: accepted by ApJ, 2020.12.
CMR exploration I -- filament structure with synthetic observations
In this paper, we carry out a pilot parameter exploration for the
collision-induced magnetic reconnection (CMR) mechanism that forms filamentary
molecular clouds. Following Kong et al. (2021), we utilize Athena++ to model
CMR in the context of resistive magnetohydrodynamics (MHD), considering the
effect from seven physical conditions, including the Ohmic resistivity
(), the magnetic field (), the cloud density (), the cloud
radius , the isothermal temperature , the collision velocity , and
the shear velocity . Compared to their fiducial model, we consider a
higher and a lower value for each one of the seven parameters. We quantify the
exploration results with five metrics, including the density probability
distribution function (-PDF), the filament morphology (250 m dust
emission), the - relation, the dominant fiber width, and the ringiness
that describes the significance of the ring-like sub-structures. The
exploration forms straight and curved CMR-filaments with rich sub-structures
that are highly variable in space and time. The variation translates to
fluctuation in all the five metrics, reflecting the chaotic nature of magnetic
reconnection in CMR. A temporary relation is noticeable during
the first 0.6 Myr. Overall, the exploration provides useful initial insights to
the CMR mechanism.Comment: 31 pages, 20 figures, 1 tabl
SMA and Spitzer Observations of Bok Glouble CB17: A Candidate First Hydrostatic Core?
We present high angular resolution SMA and Spitzer observations toward the
Bok globule CB17. SMA 1.3mm dust continuum images reveal within CB17 two
sources with an angular separation of about 21" (about 5250 AU at a distance of
250 pc). The northwestern continuum source, referred to as CB17 IRS, dominates
the infrared emission in the Spitzer images, drives a bipolar outflow extending
in the northwest-southeast direction, and is classified as a low luminosity
Class0/I transition object (L_bol ~ 0.5 L_sun). The southeastern continuum
source, referred to as CB17 MMS, has faint dust continuum emission in the SMA
1.3mm observations (about 6 sigma detection; ~3.8 mJy), but is not detected in
the deep Spitzer infrared images at wavelengths from 3.6 to 70 micron. Its
bolometric luminosity and temperature, estimated from its spectral energy
distribution, are less than 0.04 L_sun and 16 K, respectively. The SMA CO(2-1)
observations suggest that CB17 MMS may drive a low-velocity molecular outflow
(about 2.5 km/s), extending in the east-west direction. Comparisons with
prestellar cores and Class0 protostars suggest that CB17 MMS is more evolved
than prestellar cores but less evolved than Class0 protostars. The observed
characteristics of CB17 MMS are consistent with the theoretical predictions
from radiative/magneto hydrodynamical simulations of a first hydrostatic core,
but there is also the possibility that CB17 MMS is an extremely low luminosity
protostar deeply embedded in an edge-on circumstellar disk. Further
observations are needed to study the properties of CB17 MMS and to address more
precisely its evolutionary stage.Comment: 33 pages, 11 figures, to be published by Ap
Core Emergence in a Massive Infrared Dark Cloud: A Comparison Between Mid-IR Extinction and 1.3 mm Emission
Stars are born from dense cores in molecular clouds. Observationally, it is
crucial to capture the formation of cores in order to understand the necessary
conditions and rate of the star formation process. The {\it Atacama Large
Mm/sub-mm Array} (ALMA) is extremely powerful for identifying dense gas
structures, including cores, at mm wavelengths via their dust continuum
emission. Here we use ALMA to carry out a survey of dense gas and cores in the
central region of the massive () Infrared Dark Cloud (IRDC)
G28.37+0.07. The observation consists of a mosaic of 86 pointings of the
12m-array and produces an unprecedented view of the densest structures of this
IRDC. In this first paper about this data set, we focus on a comparison between
the 1.3 mm continuum emission and a mid-infrared (MIR) extinction map of the
IRDC. This allows estimation of the "dense gas" detection probability function
(DPF), i.e., as a function of the local mass surface density, , for
various choices of thresholds of mm continuum emission to define "dense gas".
We then estimate the dense gas mass fraction, , in the central
region of the IRDC and, via extrapolation with the DPF and the known
probability distribution function, to the larger-scale surrounding regions,
finding values of about 5\% to 15\% for the fiducial choice of threshold. We
argue that this observed dense gas is a good tracer of the protostellar core
population and, in this context, estimate a star formation efficiency per
free-fall time in the central IRDC region of 10\%, with
approximately a factor of two systematic uncertainties.Comment: 11 pages, 4 figures, 1 table, accepted by ApJL, comments welcom
Widespread Molecular Outflows in the Infrared Dark Cloud G28.37+0.07: Indications of Orthogonal Outflow-Filament Alignment
We present ALMA CO(2-1) observations toward a massive infrared dark cloud
G28.37+0.07. The ALMA data reveal numerous molecular (CO) outflows with a wide
range of sizes throughout the cloud. Sixty-two 1.3 mm continuum cores were
identified to be driving molecular outflows. We have determined the position
angle in the plane-of-sky of 120 CO outflow lobes and studied their
distribution. We find that the distribution of the plane-of-sky outflow
position angles peaks at about 100 degree, corresponding to a concentration of
outflows with an approximately east-west direction. For most outflows, we have
been able to estimate the plane-of-sky angle between the outflow axis and the
filament that harbors the protostar that powers the outflow. Statistical tests
strongly indicate that the distribution of outflow-filament orientations is
consistent with most outflow axes being mostly orthogonal to their parent
filament in 3D. Such alignment may result from filament fragmentation or
continuous mass transportation from filament to the embedded protostellar core.
The latter is suggested by recent numerical studies with moderately strong
magnetic fields.Comment: 4 figures, 1 table, accepted by Ap
A Bubbling Nearby Molecular Cloud: COMPLETE Shells in Perseus
We present a study on the shells (and bubbles) in the Perseus molecular cloud
using the COMPLETE survey large-scale 12CO(1-0) and 13CO(1-0) maps. The twelve
shells reported here are spread throughout most of the Perseus cloud and have
circular or arc-like morphologies with a range in radius of about 0.1 to 3 pc.
Most of them have not been detected before most likely as maps of the region
lacked the coverage and resolution needed to distinguish them. The majority of
the shells are coincident with infrared nebulosity of similar shape and have a
candidate powering source near the center. We suggest they are formed by the
interaction of spherical or very wide-angle winds powered by young stars inside
or near the Perseus molecular cloud -a cloud that is commonly considered to be
mostly forming low-mass stars. Two of the twelve shells are powered by
high-mass stars close to the cloud, while the others appear to be powered by
low or intermediate mass stars in the cloud. We argue that winds with a mass
loss rate of about 10^-8 to 10^-6 M_sun/yr are required to produce the observed
shells. Our estimates indicate that the energy input rate from these stellar
winds is similar to the turbulence dissipation rate. We conclude that in
Perseus the total energy input from both collimated protostellar outflows and
powerful spherical winds from young stars is sufficient to maintain the
turbulence in the molecular cloud. Large scale molecular line and IR continuum
maps of a sample of clouds will help determine the frequency of this phenomenon
in other star forming regions.Comment: 48 pages in total: 16 pages of text and references; 2 pages of
tables; 30 figures (one page per figure). Accepted for publication in the
Astrophysical Journa
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