2,435 research outputs found
Molecular line profiles as diagnostics of protostellar collapse: modelling the `blue asymmetry' in inside-out infall
The evolution of star-forming core analogues undergoing inside-out collapse
is studied with a multi-point chemodynamical model which self-consistently
computes the abundance distribution of chemical species in the core. For
several collapse periods the output chemistry of infall tracer species such as
HCO+, CS, and N2H+, is then coupled to an accelerated Lambda-iteration
radiative transfer code, which predicts the emerging molecular line profiles
using two different input gas/dust temperature distributions. We investigate
the sensitivity of the predicted spectral line profiles and line asymmetry
ratios to the core temperature distribution, the time-dependent model
chemistry, as well as to ad hoc abundance distributions. The line asymmetry is
found to be strongly dependent on the adopted chemical abundance distribution.
In general, models with a warm central region show higher values of blue
asymmetry in optically thick HCO+ and CS lines than models with a starless core
temperature profile. We find that in the formal context of Shu-type inside-out
infall, and in the absence of rotation or outflows, the relative blue asymmetry
of certain HCO+ and CS transitions is a function of time and, subject to the
foregoing caveats, can act as a collapse chronometer. The sensitivity of
simulated HCO+ line profiles to linear radial variations, subsonic or
supersonic, of the internal turbulence field is investigated in the separate
case of static cores.Comment: Accepted to MNRAS; 20 pages, 13 fig
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Comparing the rates of absorption and weight loss during a desorption test using near infrared spectroscopy
The importance of determining skin hydration has over the years prompt the development of many instruments and methods, specifically designed to assess this parameter or water contents especially in the stratum corneum, and have greatly matured to suit different anatomical sites and measure multiple attributes. Of those, Near Infrared Spectroscopy (NIRS) has gained wide interest as a precise, safe, fast and noninvasive technique for determining skin hydration due to its high sensitivity to hydrogen bonding and ability to measure the amount of water in skin directly using the intensities of overtone and combination bands of OH and HOH water bonds occurring in the NIR region, that are good indicators of the state of skin hydration. This paper reports near infrared spectrophotometric measurements using a highly sophisticated spectrophotometer in the region of 1000-2500 nm to study the water uptake and dehydration properties of skin in vitro using samples of porcine skin. Initial results of pure liquid water and skin samples have clearly displayed the prominent bands associated with water content, and desorption tests have been able to verify changes in these bands associated with water content, although a clear correlation between the rates of weight loss and absorbance loss at various hydration periods has not yet been established. These preliminary results are expected to further explain the relationship between water and skin, and its role within, in hope to aid the future development of a portable instrument based on near infrared spectroscopy that would be capable of directly measuring skin hydration and/or water content in a fast and noninvasive manner
On Star Formation and the Non-Existence of Dark Galaxies
We investigate whether a baryonic dark galaxy or `galaxy without stars' could
persist indefinitely in the local universe, while remaining stable against star
formation. To this end, a simple model has been constructed to determine the
equilibrium distribution and composition of a gaseous protogalactic disk.
Specifically, we determine the amount of gas that will transit to a Toomre
unstable cold phase via the H2 cooling channel in the presence of a UV--X-ray
cosmic background radiation field.
All but one of the models are predicted to become unstable to star formation.
Moreover, we find that all our model objects would be detectable via HI line
emission, even in the case that star formation is potentially avoided. These
results are consistent with the non-detection of isolated extragalactic HI
clouds with no optical counterpart (galaxies without stars) by HIPASS.
Additionally, where star formation is predicted to occur, we determine the
minimum interstellar radiation field required to restore gravothermal
stability, which we then relate to a minimum global star formation rate. This
leads to the prediction of a previously undocumented relation between HI mass
and star formation rate that is observed for a wide variety of dwarf galaxies
in the HI mass range 10^8--10^10 M_sun. The existence of such a relation
strongly supports the notion that the well observed population of dwarf
galaxies represent the minimum rates of self-regulating star formation in the
universe. (Barely abridged)Comment: 19 pages, 8 figures, TeX using emulateapj.cls, v2 accepted for
publication in ApJ (16/8/5) with one figure deleted and a number of minor
clarifying revision
Sensitive Limits on the Water Abundance in Cold Low Mass Molecular Cores
We present SWAS observations of water vapor in two cold star-less clouds, B68
and Core D in rho Ophiuchus. Sensitive non-detections of the 1(10)-1(01)
transition of o-H2O are reported for each source. Both molecular cores have
been previously examined by detailed observations that have characterized the
physical structure. Using these rather well defined physical properties and a
Monte-Carlo radiation transfer model we have removed one of the largest
uncertainties from the abundance calculation and set the lowest water abundance
limit to date in cold low-mass molecular cores. These limits are < 3 x 10^{-8}
(relative to H2) and < 8 x 10^{-9} in B68 and rho Oph D, respectively. Such low
abundances confirm the general lack of ortho-water vapor in cold (T < 20 K)
cores. Provided that the ortho/para ratio of water is not near zero, these
limits are well below theoretical predictions and appear to support the
suggestion that most of the water in dense low-mass cores is frozen onto the
surfaces of cold dust grains.Comment: 12 pages, 3 figures, accepted by Astrophysical Journal Letter
Silicon isotopic abundance toward evolved stars and its application for presolar grains
Galactic chemical evolution (GCE) is important for understanding the
composition of the present-day interstellar medium (ISM) and of our solar
system. In this paper, we aim to track the GCE by using the 29Si/30Si ratios in
evolved stars and tentatively relate this to presolar grain composition. We
used the APEX telescope to detect thermal SiO isotopologue emission toward four
oxygen-rich M-type stars. Together with the data retrieved from the Herschel
science archive and from the literature, we were able to obtain the 29Si/30Si
ratios for a total of 15 evolved stars inferred from their optically thin 29SiO
and 30SiO emission. These stars cover a range of masses and ages, and because
they do not significantly alter 29Si/30Si during their lifetimes, they provide
excellent probes of the ISM metallicity (or 29Si/30Si ratio) as a function of
time. The 29Si/30Si ratios inferred from the thermal SiO emission tend to be
lower toward low-mass oxygen-rich stars (e.g., down to about unity for W Hya),
and close to an interstellar or solar value of 1.5 for the higher-mass carbon
star IRC+10216 and two red supergiants. There is a tentative correlation
between the 29Si/30Si ratios and the mass-loss rates of evolved stars, where we
take the mass-loss rate as a proxy for the initial stellar mass or current
stellar age. This is consistent with the different abundance ratios found in
presolar grains. We found that older objects (up to possibly 10 Gyr old) in our
sample trace a previous, lower 29Si/30Si value of about 1. Material with this
isotopic ratio is present in two subclasses of presolar grains, providing
independent evidence of the lower ratio. Therefore, the 29Si/30Si ratio derived
from the SiO emission of evolved stars is a useful diagnostic tool for the
study of the GCE and presolar grains.Comment: 7 pages, 4 figure
Radio galaxy evolution: what you can learn from a Brief Encounter
We describe the pitfalls encountered in deducing from classical double radio
source observables (luminosity, spectral index, redshift and linear size) the
essential nature of how these objects evolve. We discuss the key role played by
hotspots in governing the energy distribution of the lobes they feed, and
subsequent spectral evolution. We present images obtained using the new 74 MHz
receivers on the VLA and discuss constraints which these enforce on models of
the backflow and ages in classical doubles.Comment: invited talk at `Lifecycles of Radio Galaxies' workshop; eds John
Biretta et a
The under-explored radio-loudness of quasars and the possibility of radio-source--environment interactions
I demonstrate that radio observations in the literature to date of
optically-selected quasars are largely inadequate to reveal the full extent of
their jet-activity. I discuss a recent example of an optically-powerful quasar,
which is radio-quiet according to all the standard classifications, which
Blundell & Rawlings discovered to have a >100 kpc jet, and show that other than
being the first FRI quasar to be identified, there is no reason to presume it
is exceptional. I also discuss a possible new probe of accounting for the
interactions of radio sources with their environments. This tool could help to
avoid over-estimating magnetic fields strengths within cluster gas. I briefly
describe recent analyses by Rudnick & Blundell which confront claims in the
literature of cluster gas B-fields > 10 micro-G.Comment: invited talk at "The Physics of Relativistic Jets in the CHANDRA and
XMM Era", proceedings edited by G. Brunetti, D.E. Harris, R.M. Sambruna, and
G. Setti, to be published in New Astronomy Review
Deep GMRT 150 MHz observations of the LBDS-Lynx region: Ultra-Steep Spectrum Radio Sources
It has been known for nearly three decades that high redshift radio galaxies
exhibit steep radio spectra, and hence ultra-steep spectrum radio sources
provide candidates for high-redshift radio galaxies. Nearly all radio galaxies
with z > 3 have been found using this redshift-spectral index correlation. We
have started a programme with GMRT to exploit this correlation at flux density
levels about 10 to 100 times deeper than the known high-redshift radio galaxies
which were identified primarily using the already available radio catalogues.
In our programme, we have obtained deep, high resolution radio observations at
150 MHz with GMRT for several 'deep' fields which are well studied at higher
radio frequencies and in other bands of the electromagnetic spectrum, with an
aim to detect candidate high redshift radio galaxies. In this paper we present
results from the deep 150 MHz observations of LBDS-Lynx field, which has been
already imaged at 327, 610 and 1412 MHz with the WSRT and at 1400 and 4860 MHz
with the VLA. The 150 MHz image made with GMRT has a rms noise of ~0.7 mJy/beam
and a resolution of ~19" X 15". It is the deepest low frequency image of the
LBDS-Lynx field. The source catalog of this field at 150 MHz has about 765
sources down to ~20% of the primary beam response, covering an area of about 15
degree. Spectral index was estimated by cross correlating each source
detected at 150 MHz with the available observations at 327, 610, 1400 and 4860
MHz and also using available radio surveys such as WENSS at 327 MHz and NVSS
and FIRST at 1400 MHz. We find about 150 radio sources with spectra steeper
than 1. About two-third of these are not detected in SDSS, hence are strong
candidate high-redshift radio galaxies, which need to be further explored with
deep infra-red imaging and spectroscopy to estimate the redshift.Comment: Accepted for publication in MNRAS, 24 pages (including 12 pages
online material), 9 Figures, 5 Table
Characterisation of a mobilisable plasmid conferring florfenicol and chloramphenicol resistance in Actinobacillus pleuropneumoniae
The complete nucleotide sequence of a 7.7 kb mobilisable plasmid (pM3446F), isolated from a florfenicol resistant isolate of Actinobacillus pleuropneumoniae, showed extended similarity to plasmids found in other members of the Pasteurellaceae containing the floR gene as well as replication and mobilisation genes. Mobilisation into other Pasteurellaceae species confirmed that this plasmid can be transferred horizontally
A Sample of Ultra Steep Spectrum Sources Selected from the Westerbork In the Southern Hemisphere (WISH) survey
The 352 MHz Westerbork In the Southern Hemisphere (WISH) survey is the
southern extension of the WENSS, covering 1.60 sr between -9 < DEC < -26 to a
limiting flux density of ~18 mJy (5sigma). Due to the very low elevation of the
observations, the survey has a much lower resolution in declination than in
right ascension (54" x 54"cosec(DEC)). A correlation with the 1.4 GHz NVSS
shows that the positional accuracy is less constrained in declination than in
right ascension, but there is no significant systematic error. We present a
source list containing 73570 sources.
We correlate this WISH catalogue with the NVSS to construct a sample of faint
Ultra Steep Spectrum (USS) sources, which is accessible for follow-up studies
with large optical telescopes in the southern hemisphere. This sample is aimed
at increasing the number of known high redshift radio galaxies to allow
detailed follow-up studies of these massive galaxies and their environments in
the early Universe.Comment: 12 Pages, including 5 PostScript figures. Accepted for publication in
Astronomy & Astrophysics. The full WISH catalog with 73570 sources is
available from http://www.strw.leidenuniv.nl/wenss
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