857 research outputs found
Molecular Line Observations of Infrared Dark Clouds: Seeking the Precursors to Intermediate and Massive Star Formation
We have identified 41 infrared dark clouds from the 8 micron maps of the
Midcourse Space Experiment (MSX), selected to be found within one square degree
areas centered on known ultracompact HII regions. We have mapped these infrared
dark clouds in N2H+(1-0), CS(2-1) and C18O(1-0) emission using the Five College
Radio Astronomy Observatory. The maps of the different species often show
striking differences in morphologies, indicating differences in evolutionary
state and/or the presence of undetected, deeply embedded protostars. We derive
an average mass for these clouds using N2H+ column densities of ~2500 solar
masses, a value comparable to that found in previous studies of high mass star
forming cores using other mass tracers. The linewidths of these clouds are
typically ~2.0 - 2.9 km/s. Based on the fact that they are dark at 8 micron,
compact, massive, and have large velocity dispersions, we suggest that these
clouds may be the precursor sites of intermediate and high mass star formation.Comment: Accepted to ApJS, 22 pages, 10 pages of figures. For full-resolution
images, see http://www.astro.lsa.umich.edu/~seragan/pubs/fcrao/figures.tar.g
Spectral Energy Distributions of 6.7 GHz methanol masers
The 6.7 GHz maser transition of methanol has been found exclusively towards
massive star forming regions. A majority of the masers have been found to lack
the presence of any associated radio continuum. This could be due to the maser
emission originating prior to the formation of an HII region around the central
star, or from the central object being too cool to produce a HII region. One
way to distinguish between the two scenarios is to determine and model the
spectral energy distributions (SEDs) of the masers. We observed a sample of 20
6.7 GHz methanol masers selected from the blind Arecibo survey, from centimeter
to submillimeter wavelengths. We combined our observations with existing data
from various Galactic plane surveys to determine SEDs from centimeter to
near-infrared wavelengths. We find that 70% of the masers do not have any
associated radio continuum, with the rest of the sources being associated with
hypercompact and ultracompact HII regions. Modeling the SEDs shows them to be
consistent with rapidly accreting massive stars, with accretion rates well
above 10^{-3} M_sun/yr. The upper limits on the radio continuum are also
consistent with any ionized region being confined close to the stellar surface.
This confirms the paradigm of 6.7 GHz methanol masers being signposts of early
phases of massive star formation, mostly prior to the formation of a
hypercompact HII region.Comment: 15 pages, 4 figures; Accepted by A&
On the Identification of High Mass Star Forming Regions using IRAS: Contamination by Low-Mass Protostars
We present the results of a survey of a small sample (14) of low-mass
protostars (L_IR < 10^3 Lsun) for 6.7 GHz methanol maser emission performed
using the ATNF Parkes radio telescope. No new masers were discovered. We find
that the lower luminosity limit for maser emission is near 10^3 Lsun, by
comparison of the sources in our sample with previously detected methanol maser
sources. We examine the IRAS properties of our sample and compare them with
sources previously observed for methanol maser emission, almost all of which
satisfy the Wood & Churchwell criterion for selecting candidate UCHII regions.
We find that about half of our sample satisfy this criterion, and in addition
almost all of this subgroup have integrated fluxes between 25 and 60 microns
that are similar to sources with detectable methanol maser emission. By
identifying a number of low-mass protostars in this work and from the
literature that satisfy the Wood & Churchwell criterion for candidate UCHII
regions, we show conclusively for the first time that the fainter flux end of
their sample is contaminated by lower-mass non-ionizing sources, confirming the
suggestion by van der Walt and Ramesh & Sridharan.Comment: 8 pages with 2 figures. Accepted by Ap
Discovery of 6.035GHz Hydroxyl Maser Flares in IRAS18566+0408
We report the discovery of 6.035GHz hydroxyl (OH) maser flares toward the
massive star forming region IRAS18566+0408 (G37.55+0.20), which is the only
region known to show periodic formaldehyde (4.8 GHz H2CO) and methanol (6.7 GHz
CH3OH) maser flares. The observations were conducted between October 2008 and
January 2010 with the 305m Arecibo Telescope in Puerto Rico. We detected two
flare events, one in March 2009, and one in September to November 2009. The OH
maser flares are not simultaneous with the H2CO flares, but may be correlated
with CH3OH flares from a component at corresponding velocities. A possible
correlated variability of OH and CH3OH masers in IRAS18566+0408 is consistent
with a common excitation mechanism (IR pumping) as predicted by theory.Comment: Accepted for publication in the Astrophysical Journa
Nutritional, immune, micronutrient and health status of HIV-infected children in care centres in Mangaung
Aim: To assess the nutritional, immune, micronutrient and health status of antiretroviral-naïve HIV-infected children.Method: A cross-sectional descriptive study was undertaken between September 2004 and March 2006 amongst HIV-infected children of which none received antiretroviral therapy, in care centres in Mangaung, Free State.Results: The study included 37 clinically stable and food-secure HIV-infected children. Their median age was 5.4 years (range 1.2–10.2 years). Fifteen children (41%) were underweight, 30 (81%) were stunted and one (3%) was wasted. The most commonly observed clinical features were lymphadenopathy (84%), skin rashes (51%), hepatomegaly (32%) and pallor (41%). Eight per cent of children had features of TB, while 19% had a lower respiratory tract infection. The median viral load of the group (n = 35) was 117 000 copies/ml, the median CD4+ cell count was 477 cells/mm3 and the median CD4 percentage was 22.5%. A significant negative correlation could be demonstrated between viral load and nutritional indicators. Children had deficient serum levels relative to normal reference values for glutathione (91% of children), albumin (78%), vitamin A (63%), vitamin D (44%), zinc (38%) and vitamin E (13%). Sixty per cent of the children were anaemic and 30% were iron deficient.Conclusion: A high prevalence of acute and chronic malnutrition and micronutrient deficiencies occurred among HIV-infected children residing in care centres. The study highlights the need to investigate early initiation of antiretroviral therapy and nutrition interventions, including aggressive supplementation, in order to improve the prognosis of these children
High-Mass Proto-Stellar Candidates - I : The Sample and Initial Results
We describe a systematic program aimed at identifying and characterizing
candidate high-mass proto-stellar objects (HMPOs). Our candidate sample
consists of 69 objects selected by criteria based on those established by
Ramesh & Sridharan (1997) using far-infrared, radio-continuum and molecular
line data. Infrared-Astronomical-Satellite (IRAS) and
Midcourse-Space-Experiment (MSX) data were used to study the larger scale
environments of the candidate sources and to determine their total luminosities
and dust temperatures. To derive the physical and chemical properties of our
target regions, we observed continuum and spectral line radiation at millimeter
and radio wavelengths. We imaged the free-free and dust continuum emission at
wavelengths of 3.6 cm and 1.2 mm, respectively, searched for H2O and CH3OH
maser emission and observed the CO 2-1 and several NH3 lines toward all sources
in our sample. Other molecular tracers were observed in a subsample. The
presented results indicate that a substantial fraction of our sample harbors
HMPOs in a pre-UCHII region phase, the earliest known stage in the high-mass
star formation process.Comment: 16 pages, 11 eps-figures. Astrophysical Journal, in pres
Time evolution of the Rabi Hamiltonian from the unexcited vacuum
The Rabi Hamiltonian describes a single mode of electromagnetic radiation
interacting with a two-level atom. Using the coupled cluster method, we
investigate the time evolution of this system from an initially empty field
mode and an unexcited atom. We give results for the atomic inversion and field
occupation, and find that the virtual processes cause the field to be squeezed.
No anti-bunching occurs.Comment: 25 pages, 8 figures, RevTe
Polarized point sources in the LOFAR Two-meter Sky Survey: A preliminary catalog
The polarization properties of radio sources at very low frequencies (h45m–15h30m right ascension, 45°–57° declination, 570 square degrees). We have produced a catalog of 92 polarized radio sources at 150 MHz at 4.′3 resolution and 1 mJy rms sensitivity, which is the largest catalog of polarized sources at such low frequencies. We estimate a lower limit to the polarized source surface density at 150 MHz, with our resolution and sensitivity, of 1 source per 6.2 square degrees. We find that our Faraday depth measurements are in agreement with previous measurements and have significantly smaller errors. Most of our sources show significant depolarization compared to 1.4 GHz, but there is a small population of sources with low depolarization indicating that their polarized emission is highly localized in Faraday depth. We predict that an extension of this work to the full LOTSS data would detect at least 3400 polarized sources using the same methods, and probably considerably more with improved data processing
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