238 research outputs found
Diversity of chemistry and excitation conditions in the high-mass star forming complex W33
The object W33 is a giant molecular cloud that contains star forming regions
at various evolutionary stages from quiescent clumps to developed H II regions.
Since its star forming regions are located at the same distance and the primary
material of the birth clouds is probably similar, we conducted a comparative
chemical study to trace the chemical footprint of the different phases of
evolution. We observed six clumps in W33 with the Atacama Pathfinder Experiment
(APEX) telescope at 280 GHz and the Submillimeter Array (SMA) at 230 GHz. We
detected 27 transitions of 10 different molecules in the APEX data and 52
transitions of 16 different molecules in the SMA data. The chemistry on scales
larger than 0.2 pc, which are traced by the APEX data, becomes more
complex and diverse the more evolved the star forming region is. On smaller
scales traced by the SMA data, the chemical complexity and diversity increase
up to the hot core stage. In the H II region phase, the SMA spectra resemble
the spectra of the protostellar phase. Either these more complex molecules are
destroyed or their emission is not compact enough to be detected with the SMA.
Synthetic spectra modelling of the HCO transitions, as detected with the
APEX telescope, shows that both a warm and a cold component are needed to
obtain a good fit to the emission for all sources except for W33 Main1. The
temperatures and column densities of the two components increase during the
evolution of the star forming regions. The integrated intensity ratios
NH(32)/CS(65) and
NH(32)/HCO(43) show clear trends as a
function of evolutionary stage, luminosity, luminosity-to-mass ratio, and
H peak column density of the clumps and might be usable as chemical
clocks.Comment: 66 pages, 28 figures, 8 tables, accepted for publication at A&
Pre- and Post-burst Radio Observations of the Class 0 Protostar HOPS 383 in Orion
There is increasing evidence that episodic accretion is a common phenomenon
in Young Stellar Objects (YSOs). Recently, the source HOPS 383 in Orion was
reported to have a mid-infrared -- and bolometric -- luminosity
increase between 2004 and 2008, constituting the first clear example of a class
0 YSO (a protostar) with a large accretion burst. The usual assumption that in
YSOs accretion and ejection follow each other in time needs to be tested. Radio
jets at centimeter wavelengths are often the only way of tracing the jets from
embedded protostars. We searched the Very Large Array archive for the available
observations of the radio counterpart of HOPS 383. The data show that the radio
flux of HOPS 383 varies only mildly from January 1998 to December 2014, staying
at the level of to 300 Jy in the X band ( GHz), with a
typical uncertainty of 10 to 20 Jy in each measurement. We interpret the
absence of a radio burst as suggesting that accretion and ejection enhancements
do not follow each other in time, at least not within timescales shorter than a
few years. Time monitoring of more objects and specific predictions from
simulations are needed to clarify the details of the connection betwen
accretion and jets/winds in YSOs.Comment: ApJ Letters, in pres
Time Variability in Simulated Ultracompact and Hypercompact HII Regions
Ultracompact and hypercompact HII regions appear when a star with a mass
larger than about 15 solar masses starts to ionize its own environment. Recent
observations of time variability in these objects are one of the pieces of
evidence that suggest that at least some of them harbor stars that are still
accreting from an infalling neutral accretion flow that becomes ionized in its
innermost part. We present an analysis of the properties of the HII regions
formed in the 3D radiation-hydrodynamic simulations presented by Peters et al.
as a function of time. Flickering of the HII regions is a natural outcome of
this model. The radio-continuum fluxes of the simulated HII regions, as well as
their flux and size variations are in agreement with the available
observations. From the simulations, we estimate that a small but non-negligible
fraction (~ 10 %) of observed HII regions should have detectable flux
variations (larger than 10 %) on timescales of ~ 10 years, with positive
variations being more likely to happen than negative variations. A novel result
of these simulations is that negative flux changes do happen, in contrast to
the simple expectation of ever growing HII regions. We also explore the
temporal correlations between properties that are directly observed (flux and
size) and other quantities like density and ionization rates.Comment: Monthly Notices of the Royal Astronomical Society, in press. The
movie of free-free optical depth can be found at
http://www.ita.uni-heidelberg.de/~tpeters/tau.av
The Origin of OB Clusters: From 10 pc to 0.1 pc
We observe the 1.2 mm continuum emission around the OB cluster forming region
G10.6-0.4, using the IRAM 30m telescope MAMBO-2 bolometer array and the
Submillimeter array. Comparison of the Spitzer 24 m and 8 m images
with our 1.2 mm continuum maps reveals the ionization front of an HII region,
the photon-dominated layer, and several 5 pc scale filaments following the
outer edge of the photon-dominated layer. The filaments, which are resolved in
the MAMBO-2 observations, show regularly spaced parsec-scale molecular clumps,
embedded with a cluster of submillimeter molecular cores as shown in the SMA
0.87 mm observations. Toward the center of the G10.6-0.4 region, the combined
SMA+IRAM 30m continuum image reveals several, parsec-scale protrusions. They
may continue down to within 0.1 pc of the geometric center of a dense 3 pc size
structure, where a 200 M OB cluster resides. The observed filaments
may facilitate mass accretion onto the central cluster--forming region in the
presence of strong radiative and mechanical stellar feedbacks. Their
filamentary geometry may also facilitate fragmentation. We did not detect any
significant polarized emission at 0.87 mm in the inner 1 pc region with the
SMA.Comment: 32 pages, 10 figures, Accepted by ApJ on 2011.October
Flickering of 1.3 cm Sources in Sgr B2: Towards a Solution to the Ultracompact HII Region Lifetime Problem
Accretion flows onto massive stars must transfer mass so quickly that they
are themselves gravitationally unstable, forming dense clumps and filaments.
These density perturbations interact with young massive stars, emitting
ionizing radiation, alternately exposing and confining their HII regions. As a
result, the HII regions are predicted to flicker in flux density over periods
of decades to centuries rather than increasing monotonically in size as
predicted by simple Spitzer solutions. We have recently observed the Sgr B2
region at 1.3 cm with the VLA in its three hybrid configurations (DnC, CnB and
BnA) at a resolution of 0.25''. These observations were made to compare in
detail with matched continuum observations from 1989. At 0.25'' resolution, Sgr
B2 contains 41 UC HII regions, 6 of which are hypercompact. The new
observations of Sgr B2 allow comparison of relative peak flux densites for the
HII regions in Sgr B2 over a 23 year time baseline (1989-2012) in one of the
most source-rich massive star forming regions in the Milky Way. The new 1.3 cm
continuum images indicate that four of the 41 UC HII regions exhibit
significant changes in their peak flux density, with one source (K3) dropping
in peak flux density, and the other 3 sources (F10.303, F1 and F3) increasing
in peak flux density. The results are consistent with statistical predictions
from simulations of high mass star formation, suggesting that they offer a
solution to the lifetime problem for ultracompact HII regions.Comment: 12 pages, 3 figures, Accepted for publication in the Astrophysical
Journal Letter
New Maser Emission from Nonmetastable Ammonia in NGC 7538. II. Green Bank Telescope Observations Including Water Masers
We present new maser emission from ^{14}NH_3 (9,6) in NGC 7538. Our
observations include the known spectral features near v_LSR = -60 km/s and -57
km/s and several more features extending to -46 km/s. In three epochs of
observation spanning two months we do not detect any variability in the ammonia
masers, in contrast to the >10-fold variability observed in other ^{14}NH_3
(9,6) masers in the Galaxy over comparable timescales. We also present
observations of water masers in all three epochs for which emission is observed
over the velocity range -105 km/s < v_LSR < -4 km/s, including the highest
velocity water emission yet observed from NGC 7538. Of the remarkable number of
maser species in IRS 1, H_2O and, now, ^{14}NH_3 are the only masers known to
exhibit emission outside of the velocity range -62 km/s < v_LSR < -51 km/s.
However, we find no significant intensity or velocity correlations between the
water emission and ammonia emission. We also present a non-detection in the
most sensitive search to date toward any source for emission from the CC^{32}S
and CC^{34}S molecules, indicating an age greater than \approx 10^4 yr for IRS
1-3. We discuss these findings in the context of embedded stellar cores and
recent models of the region.Comment: 7 pages, 4 figures, 3 tables; accepted to AJ; color figures only on
arxiv; revised to include references and minor proof change
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