555 research outputs found
The "Nessie" Nebula: Cluster Formation in a Filamentary Infrared Dark Cloud
The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large
aspect ratio of over 150:1 (1.5 degrees x 0.01 degrees, or 80 pc x 0.5 pc at a
kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense
molecular gas, made with the Australia Telescope National Facility Mopra
telescope, show an excellent morphological match to the mid-IR extinction.
Moreover, because the molecular line emission from the entire nebula has the
same radial velocity to within +/- 3.4 km/s, the nebula is a single, coherent
cloud and not the chance alignment of multiple unrelated clouds along the line
of sight.
The Nessie Nebula contains a number of compact, dense molecular cores which
have a characteristic projected spacing of ~ 4.5 pc along the filament. The
theory of gravitationally bound gaseous cylinders predicts the existence of
such cores, which, due to the "sausage" or "varicose" fluid instability,
fragment from the cylinder at a characteristic length scale. If turbulent
pressure dominates over thermal pressure in Nessie, then the observed core
spacing matches theoretical predictions. We speculate that the formation of
high-mass stars and massive star clusters arises from the fragmentation of
filamentary IRDCs caused by the "sausage" fluid instability that leads to the
formation of massive, dense molecular cores. The filamentary molecular gas
clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334,
etc.) may represent a later stage of IRDC evolution.Comment: 5 pages, 2 figures, accepted for publication in The Astrophysical
Journal Letter
Kinematic Distances to Molecular Clouds identified in the Galactic Ring Survey
Kinematic distances to 750 molecular clouds identified in the 13CO J=1-0
Boston University-Five College Radio Astronomy Observatory Galactic Ring Survey
(BU-FCRAO GRS) are derived assuming the Clemens rotation curve of the Galaxy.
The kinematic distance ambiguity is resolved by examining the presence of HI
self-absorption toward the 13CO emission peak of each cloud using the Very
Large Array Galactic Plane Survey (VGPS). We also identify 21 cm continuum
sources embedded in the GRS clouds in order to use absorption features in the
HI 21 cm continuum to distinguish between near and far kinematic distances. The
Galactic distribution of GRS clouds is consistent with a four-arm model of the
Milky Way. The locations of the Scutum-Crux and Perseus arms traced by GRS
clouds match star count data from the Galactic Legacy Infrared Mid-Plane Survey
Extraordinaire (GLIMPSE) star-count data. We conclude that molecular clouds
must form in spiral arms and be short-lived (lifetimes < 10 Myr) in order to
explain the absence of massive, 13CO bright molecular clouds in the inter-arm
space
A Mid-Infrared Census of Star Formation Activity in Bolocam Galactic Plane Survey Sources
We present the results of a search for mid-infrared signs of star formation
activity in the 1.1 mm sources in the Bolocam Galactic Plane Survey (BGPS). We
have correlated the BGPS catalog with available mid-IR Galactic plane catalogs
based on the Spitzer Space Telescope GLIMPSE legacy survey and the Midcourse
Space Experiment (MSX) Galactic plane survey. We find that 44% (3,712 of 8,358)
of the BGPS sources contain at least one mid-IR source, including 2,457 of
5,067 (49%) within the area where all surveys overlap (10 deg < l < 65 deg).
Accounting for chance alignments between the BGPS and mid-IR sources, we
conservatively estimate that 20% of the BPGS sources within the area where all
surveys overlap show signs of active star formation. We separate the BGPS
sources into four groups based on their probability of star formation activity.
Extended Green Objects (EGOs) and Red MSX Sources (RMS) make up the highest
probability group, while the lowest probability group is comprised of
"starless" BGPS sources which were not matched to any mid-IR sources. The mean
1.1 mm flux of each group increases with increasing probability of active star
formation. We also find that the "starless" BGPS sources are the most compact,
while the sources with the highest probability of star formation activity are
on average more extended with large skirts of emission. A subsample of 280 BGPS
sources with known distances demonstrates that mass and mean H_2 column density
also increase with probability of star formation activity.Comment: 20 pages, 12 figures, 3 tables. Accepted for publication in ApJ. Full
Table 2 will be available online through Ap
Protostellar Outflow Heating in a Growing Massive Protocluster
The dense molecular clump P1 in the infrared dark cloud (IRDC) complex
G28.34+0.06 harbors a massive protostellar cluster at its extreme youth. Our
previous Submillimeter Array (SMA) observations revealed several jet-like CO
outflows emanating from the protostars, indicative of intense accretion and
potential interaction with ambient natal materials. Here we present the
Expanded Very Large Array (EVLA) spectral line observations toward P1 in the
NH3 (J,K) = (1,1), (2,2), (3,3) lines, as well as H2O and class I CH3OH masers.
Multiple NH3 transitions reveal the heated gas widely spread in the 1 pc clump.
The temperature distribution is highly structured; the heated gas is offset
from the protostars, and morphologically matches the outflows very well. Hot
spots of spatially compact, spectrally broad NH3 (3,3) emission are also found
coincident with the outflows. A weak NH3 (3,3) maser is discovered at the
interface between an outflow jet and the ambient gas. These findings suggest
that protostellar heating may not be effective in suppressing fragmentation
during the formation of massive cores.Comment: 15 pages, 4 figures, 1 table, accepted to ApJ Letter
The southern dust pillars of the Carina Nebula
We present preliminary results from a detailed study towards four previously
detected bright mid-infrared sources in the southern part of the Carina Nebula:
G287.73--0.92, G287.84--0.82, G287.93--0.99 and G288.07--0.80. All of these
sources are located at the heads of giant dust pillars that point towards the
nearby massive star cluster, Trumpler 16. It is unclear if these pillars are
the prime sites for a new generation of triggered star formation or if instead
they are the only remaining parts of the nebula where ongoing star fromation
can take place.Comment: 2 pages, to appear in the proceedings of "Hot Star Workshop III: The
Earliest Phases of Massive Star Birth" (ed. P.A. Crowther
Multiple low-turbulence starless cores associated with intermediate- to high-mass star formation
To characterize the initial conditions for intermediate- to high-mass star
formation, we observed two Infrared Dark Clouds (IRDCs) that remain absorption
features up to 70mum wavelength, with the PdBI in the 3.23mm dust continuum as
well as the N2H+(1--0) and 13CS(2-1) line emission. While IRDC19175-4 is
clearly detected in the 3.23mm continuum, the second source in the field,
IRDC19175-5, is only barely observable above the 3sigma continuum detection
threshold. However, the N2H+(1-0) observations reveal 17 separate sub-sources
in the vicinity of the two IRDCs. Most of them exhibit low levels of turbulence
(dv \leq 1km/s), indicating that the fragmentation process in these cores may
be dominated by the interplay of thermal pressure and gravity, but not so much
by turbulence. Combining the small line widths with the non-detection up to
70mum and the absence of other signs of star formation activity, most of these
17 cores with masses between sub-solar to ~10M_sun are likely still in a
starless phase. Furthermore, we find a large CS depletion factor of the order
100. Although the strongest line and continuum peak is close to virial
equilibrium, its slightly broader line width compared to the other cores is
consistent with it being in a contraction phase potentially at the verge of
star formation. The relative peak velocities between neighboring cores are
usually below 1km/s, and we do not identify streaming motions along the
filamentary structures. Average densities are between 10^5 and 10^6cm^{-3} (one
to two orders of magnitude larger than for example in the Pipe nebula) implying
relatively small Jeans-lengths that are consistent with the observed core
separations of the order 5000AU. The quest for high-mass starless cores prior
to any star formation activity remains open.Comment: 10 pages, 8 figures. Accepted by Astronomy and Astrophysic
An Infrared through Radio Study of the Properties and Evolution of IRDC Clumps
We examine the physical properties and evolutionary stages of a sample of 17
clumps within 8 Infrared Dark Clouds (IRDCs) by combining existing infrared,
millimeter, and radio data with new Bolocam Galactic Plane Survey (BGPS) 1.1 mm
data, VLA radio continuum data, and HHT dense gas (HCO+ and N2H+) spectroscopic
data. We combine literature studies of star formation tracers and dust
temperatures within IRDCs with our search for ultra-compact (UC) HII regions to
discuss a possible evolutionary sequence for IRDC clumps. In addition, we
perform an analysis of mass tracers in IRDCs and find that 8 micron extinction
masses and 1.1 mm Bolocam Galactic Plane Survey (BGPS) masses are complementary
mass tracers in IRDCs except for the most active clumps (notably those
containing UCHII regions), for which both mass tracers suffer biases. We find
that the measured virial masses in IRDC clumps are uniformly higher than the
measured dust continuum masses on the scale of ~1 pc. We use 13CO, HCO+, and
N2H+ to study the molecular gas properties of IRDCs and do not see any evidence
of chemical differentiation between hot and cold clumps on the scale of ~1 pc.
However, both HCO+ and N2H+ are brighter in active clumps, due to an increase
in temperature and/or density. We report the identification of four UCHII
regions embedded within IRDC clumps and find that UCHII regions are associated
with bright (>1 Jy) 24 micron point sources, and that the brightest UCHII
regions are associated with "diffuse red clumps" (an extended enhancement at 8
micron). The broad stages of the discussed evolutionary sequence (from a
quiescent clump to an embedded HII region) are supported by literature dust
temperature estimates; however, no sequential nature can be inferred between
the individual star formation tracers.Comment: 33 pages, 26 figures, 6 tables, accepted for publication in ApJ. Full
resolution version available here:
http://casa.colorado.edu/~battersb/Publications.htm
- …