112 research outputs found
The Lifetimes of Phases in High-Mass Star-Forming Regions
High-mass stars form within star clusters from dense, molecular regions, but
is the process of cluster formation slow and hydrostatic or quick and dynamic?
We link the physical properties of high-mass star-forming regions with their
evolutionary stage in a systematic way, using Herschel and Spitzer data. In
order to produce a robust estimate of the relative lifetimes of these regions,
we compare the fraction of dense, molecular regions above a column density
associated with high-mass star formation, N(H2) > 0.4-2.5 x 10^22 cm^-2, in the
'starless (no signature of stars > 10 Msun forming) and star-forming phases in
a 2x2 degree region of the Galactic Plane centered at l=30deg. Of regions
capable of forming high-mass stars on ~1 pc scales, the starless (or embedded
beyond detection) phase occupies about 60-70% of the dense, molecular region
lifetime and the star-forming phase occupies about 30-40%. These relative
lifetimes are robust over a wide range of thresholds. We outline a method by
which relative lifetimes can be anchored to absolute lifetimes from large-scale
surveys of methanol masers and UCHII regions. A simplistic application of this
method estimates the absolute lifetimes of the starless phase to be 0.2-1.7 Myr
(about 0.6-4.1 fiducial cloud free-fall times) and the star-forming phase to be
0.1-0.7 Myr (about 0.4-2.4 free-fall times), but these are highly uncertain.
This work uniquely investigates the star-forming nature of high-column density
gas pixel-by-pixel and our results demonstrate that the majority of high-column
density gas is in a starless or embedded phase.Comment: 10 pages, accepted to Ap
The Origins Space Telescope
The Origins Space Telescope, one of four large Mission Concept Studies sponsored by NASA for review in the 2020 US Astrophysics Decadal Survey, will open unprecedented discovery space in the infrared, unveiling our cosmic origins
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The Bolocam Galactic Plane Survey. XI. Temperatures and Substructure of Galactic Clumps Based on 350 ?M Observations
We present 107 maps of continuum emission at 350 mu m from Galactic molecular clumps. Observed sources were mainly selected from the Bolocam Galactic Plane Survey (BGPS) catalog, with three additional maps covering star-forming regions in the outer Galaxy. The higher resolution of the SHARC-II images (8."5 beam) compared with the 1.1 mm images from BGPS (33"beam) allowed us to identify a large population of smaller substructures within the clumps. A catalog is presented for the 1386 sources extracted from the 350 mu m maps. The color temperature distribution of clumps based on the two wavelengths has a median of 13.3 K and mean of 16.3 +/- 0.4 K, assuming an opacity law index of 1.7. For the structures with good determination of color temperatures, the mean ratio of gas temperature, determined from NH3 observations, to dust color temperature is 0.88 and the median ratio is 0.76. About half the clumps have more than 2 substructures and 22 clumps have more than 10. The fraction of the mass in dense substructures seen at 350 mu m compared to the mass of their parental clump is similar to 0.19, and the surface densities of these substructures are, on average, 2.2 times those seen in the clumps identified at 1.1 mm. For a well-characterized sample, 88 structures (31%) exceed a surface density of 0.2 g cm(-2), and 18 (6%) exceed 1.0 g cm(-2), thresholds for massive star Formation suggested by theorists.National Science Foundation through NSF grant AST-0708403NRAONSF AST-9980846, AST-0206158, AST-1109116Fulbright FellowshipASI, AgenziaSpaziale Italiana I/038/080/0, I/029/12/0national funding agency: CSA (Canada)national funding agency: NAOC (China)national funding agency: CEA (France)national funding agency: ASI (Italy)national funding agency: MCINN (Spain)national funding agency: Stockholm Observatory (Sweden)national funding agency: STFC (UK)national funding agency: NASA (USA)national funding agency: CNES (France)national funding agency: CNRS (France)Astronom
The Bolocam Galactic Plane Survey. XII. Distance Catalog Expansion Using Kinematic Isolation of Dense Molecular Cloud Structures With 13CO(1-0)
We present an expanded distance catalog for 1,710 molecular cloud structures
identified in the Bolocam Galactic Plane Survey (BGPS) version 2, representing
a nearly threefold increase over the previous BGPS distance catalog. We
additionally present a new method for incorporating extant data sets into our
Bayesian distance probability density function (DPDF) methodology. To augment
the dense-gas tracers (e.g., HCO+(3-2), NH3(1,1)) used to derive line-of-sight
velocities for kinematic distances, we utilize the Galactic Ring Survey
13CO(1-0) data to morphologically extract velocities for BGPS sources. The
outline of a BGPS source is used to select a region of the GRS 13CO data, along
with a reference region to subtract enveloping diffuse emission, to produce a
line profile of 13CO matched to the BGPS source. For objects with a HCO+(3-2)
velocity, \approx 95% of the new 13CO(1-0) velocities agree with that of the
dense gas. A new prior DPDF for kinematic distance ambiguity (KDA) resolution,
based on a validated formalism for associating molecular cloud structures with
known objects from the literature, is presented. We demonstrate this prior
using catalogs of masers with trigonometric parallaxes and HII regions with
robust KDA resolutions. The distance catalog presented here contains
well-constrained distance estimates for 20% of BGPS V2 sources, with typical
distance uncertainties \lesssim 0.5 kpc. Approximately 75% of the
well-constrained sources lie within 6 kpc of the Sun, concentrated in the
Scutum-Centarus arm. Galactocentric positions of objects additionally trace out
portions of the Sagittarius, Perseus, and Outer arms in the first and second
Galactic quadrants, and we also find evidence for significant regions of
interarm dense gas.Comment: 28 pages, 19 figures. Accepted for publication in ApJ.
Distance-Omnibus code available at https://github.com/BGPS/distance-omnibu
The Bolocam Galactic Plane Survey. XIII. Physical Properties and Mass Functions of Dense Molecular Cloud Structures
We use the distance probability density function (DPDF) formalism of
Ellsworth-Bowers et al. (2013, 2015) to derive physical properties for the
collection of 1,710 Bolocam Galactic Plane Survey (BGPS) version 2 sources with
well-constrained distance estimates. To account for Malmquist bias, we estimate
that the present sample of BGPS sources is 90% complete above 400 and
50% complete above 70 . The mass distributions for the entire sample
and astrophysically motivated subsets are generally fitted well by a lognormal
function, with approximately power-law distributions at high mass. Power-law
behavior emerges more clearly when the sample population is narrowed in
heliocentric distance (power-law index for sources nearer
than 6.5 kpc and for objects between 2 kpc and 10 kpc).
The high-mass power-law indices are generally for
various subsamples of sources, intermediate between that of giant molecular
clouds and the stellar initial mass function. The fit to the entire sample
yields a high-mass power-law . Physical
properties of BGPS sources are consistent with large molecular cloud clumps or
small molecular clouds, but the fractal nature of the dense interstellar medium
makes difficult the mapping of observational categories to the dominant
physical processes driving the observed structure. The face-on map of the
Galactic disk's mass surface density based on BGPS dense molecular cloud
structures reveals the high-mass star-forming regions W43, W49, and W51 as
prominent mass concentrations in the first quadrant. Furthermore, we present a
0.25-kpc resolution map of the dense gas mass fraction across the Galactic disk
that peaks around 5%.Comment: Accepted for publication in ApJ; 32 pages, 21 figure
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