299 research outputs found
Triggered/sequential star formation? A multi-phase ISM study around the prominent IRDC G18.93-0.03
G18.93-0.03 is a prominent dust complex within an 0.8deg long filament, with
the molecular clump G18.93/m being IR dark from near IR wavelength up to 160mu.
Spitzer composite images show an IR bubble spatially associated with G18.93. We
use GRS 13CO and IRAM 30m H13CO+ data to disentangle the spatial structure of
the region. From ATLASGAL submm data we calculate the gas mass, while we use
the H13CO+ line width to estimate its virial mass. Using HERSCHEL data we
produce temperature maps from fitting the SED. With the MAGPIS 20cm and
SuperCOSMOS Halpha data we trace the ionized gas, and the VGPS HI survey
provides information on the atomic hydrogen gas. We show that the bubble is
spatially associated with G18.93, located at a kinematic near distance of
3.6kpc. With 280Msun, the most massive clump within G18.93 is G18.93/m. The
virial analysis shows that it may be gravitationally bound and has neither
Spitzer young stellar objects nor mid-IR point sources within. Fitting the SED
reveals a temperature distribution that decreases towards its center, but
heating from the ionizing source puts it above the general ISM temperature. We
find that the bubble is filled by HII gas, ionized by an O8.5 star. Between the
ionizing source and the IR dark clump G18.93/m we find a layered structure,
from ionized to atomic to molecular hydrogen, revealing a PDR. Furthermore, we
identify an additional velocity component within the bubble's 8mu emission rim
at the edge of the infrared dark cloud and speculate that it might be shock
induced by the expanding HII region. While the elevated temperature allows for
the build-up of larger fragments, and the shock induced velocity component may
lead to additional turbulent support, we do not find conclusive evidence that
the massive clump G18.93/m is prone to collapse because of the expanding HII
region.Comment: Accepted for publication in A&
SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51
Observations of W51 with the Submillimeter Wave Astronomy Satellite (SWAS)
have yielded the first detection of water vapor in a diffuse molecular cloud.
The water vapor lies in a foreground cloud that gives rise to an absorption
feature at an LSR velocity of 6 km/s. The inferred H2O column density is
2.5E+13 cm-2. Observations with the Arecibo radio telescope of hydroxyl
molecules at ten positions in W51 imply an OH column density of 8E+13 cm-2 in
the same diffuse cloud. The observed H2O/OH ratio of ~ 0.3 is significantly
larger than an upper limit derived previously from ultraviolet observations of
the similar diffuse molecular cloud lying in front of HD 154368. The observed
variation in H2O/OH likely points to the presence in one or both of these
clouds of a warm (T > 400) gas component in which neutral-neutral reactions are
important sources of OH and/or H2O.Comment: 15 pages (AASTeX) including 4 (eps) figures. To appear in the
Astrophysical Journa
Effects of Genotype and Child Abuse on DNA Methylation and Gene Expression at the Serotonin Transporter
Altered regulation of the serotonin transporter (SLC6A4) is hypothesized to be a key event in many forms of neuropsychiatric illness, yet our understanding of the molecular mechanisms through which changes in gene function could lead to illness remains incomplete. In prior studies, we and others have demonstrated that methylation of CpG residues in the promoter associated CpG island alters SLC6A4 gene expression, that the extent of that DNA methylation in child abuse is genotype dependent, and that adverse childhood experiences such as child sex abuse are related to methylation. However, we have not examined whether these effects are splice variant specific, whether the association of methylation to gene expression varies as a function of genotype, and whether methylation in other SLC6A4 gene regions are more likely candidates for GxE effects. In the current investigation we measured methylation in lymphoblast DNA from 158 female subjects in the Iowa Adoption Studies at 16 CpG residues spread across the SLC6A4 locus, and analyzed their relationship to gene expression for two SLC6A4 splice variants. Methylation of two CpG residues in the shore of the CpG island (cg22584138 and cg05951817), a location immediately upstream from exon 1A, predicted gene expression for the splice variant containing Exon 1A + 1B. Methylation at two residues in the CpG island itself (cg 25769822 and cg05016953) was associated with total SLC6A4 expression. Examination of these four CpG residues indicated that methylation of cg22584138 was influenced by both genotype and sex abuse, whereas methylation of cg05016953 was influenced only by sex abuse history. Factors influencing methylation at other CpG dinucleotide pairs were not identified. We conclude that methylation effects on transcription may vary as a function of underlying gene motif and splice variant, and that the shore of CpG islands, upstream of TSS, may be of particular interest in examining environmental effects on methylation
Irreversible and reversible modes of operation of deterministic ratchets
We discuss a problem of optimization of the energetic efficiency of a simple
rocked ratchet. We concentrate on a low-temperature case in which the
particle's motion in a ratchet potential is deterministic. We show that the
energetic efficiency of a ratchet working adiabatically is bounded from above
by a value depending on the form of ratchet potential. The ratchets with
strongly asymmetric potentials can achieve ideal efficiency of unity without
approaching reversibility. On the other hand we show that for any form of the
ratchet potential a set of time-protocols of the outer force exist under which
the operation is reversible and the ideal value of efficiency is also achieved.
The mode of operation of the ratchet is still quasistatic but not adiabatic.
The high values of efficiency can be preserved even under elevated
temperatures
Extended warm and dense gas towards W49A: starburst conditions in our Galaxy?
The star formation rates in starburst galaxies are orders of magnitude higher
than in local star-forming regions, and the origin of this difference is not
well understood. We use sub-mm spectral line maps to characterize the physical
conditions of the molecular gas in the luminous Galactic star-forming region
W49A and compare them with the conditions in starburst galaxies. We probe the
temperature and density structure of W49A using H_2CO and HCN line ratios over
a 2'x2' (6.6x6.6 pc) field with an angular resolution of 15" (~0.8 pc) provided
by the JCMT Spectral Legacy Survey. We analyze the rotation diagrams of lines
with multiple transitions with corrections for optical depth and beam dilution,
and estimate excitation temperatures and column densities. Comparing the
observed line intensity ratios with non-LTE radiative transfer models, our
results reveal an extended region (about 1'x1', equivalent to ~3x3 pc at the
distance of W49A) of warm (> 100 K) and dense (>10^5 cm^-3) molecular gas, with
a mass of 2x10^4 - 2x10^5 M_Sun (by applying abundances derived for other
regions of massive star-formation). These temperatures and densities in W49A
are comparable to those found in clouds near the center of the Milky Way and in
starburst galaxies. The highly excited gas is likely to be heated via shocks
from the stellar winds of embedded, O-type stars or alternatively due to UV
irradiation, or possibly a combination of these two processes. Cosmic rays,
X-ray irradiation and gas-grain collisional heating are less likely to be the
source of the heating in the case of W49A.Comment: Accepted for publication in A&A; 11 pages, 9 figure
CO in HI Self-Absorbed Clouds in Perseus
We have observed 12CO J = 2-1 and J = 1-0, and 13CO J = 1-0 emission in two
regions of HI Self-Absorption (HISA) in Perseus: a small, isolated HISA feature
called the globule and a more extended HISA cloud called the complex. Using
both Large Velocity Gradient and Monte Carlo radiative transfer codes we found
that, in the globule, N(12CO) < 6.0x10^15 cm-2 which, using PDR models, implies
that N(H_2) < 9.9x10^20 cm-2. In the complex we found that the H_2 column
densities ranged from 1.2 - 2.2 x 10^21 cm-2. By comparing the HISA and CO
observations we are able to constrain the physical conditions and atomic gas
fraction (f). In the globule, 8 K < T_spin < 22 K and 0.02 < f < 0.2 depending
on whether the (unknown) gas density is 10^2, 10^3, or 10^4 cm-3. In the
complex, 12 K < T_spin < 24 K, 0.02 < f < 0.05, and we were also able to
constrain the gas density (100 < n < 1200 cm-3). These results imply that the
gas in the HISA clouds is colder and denser than that usually associated with
the atomic ISM and, indeed, is similar to that seen in molecular clouds. The
small atomic gas fractions also imply that there is a significant molecular
component in these HISA clouds, even when little or no 12CO is detected. The
level of 12CO detected and the visual extinction due to dust is consistent with
the idea that these HISA clouds are undergoing a transition from the atomic to
molecular phase.Comment: 25 pages, 5 figures, tentatively scheduled for the ApJ 1 October
2005, v 631, 2 issu
The Ionization Fraction in Dense Molecular Gas II: Massive Cores
We present an observational and theoretical study of the ionization fraction
in several massive cores located in regions that are currently forming stellar
clusters. Maps of the emission from the J = 1-> O transitions of C18O, DCO+,
N2H+, and H13CO+, as well as the J = 2 -> 1 and J = 3 -> 2 transitions of CS,
were obtained for each core. Core densities are determined via a large velocity
gradient analysis with values typically 10^5 cm^-3. With the use of
observations to constrain variables in the chemical calculations we derive
electron fractions for our overall sample of 5 cores directly associated with
star formation and 2 apparently starless cores. The electron abundances are
found to lie within a small range, -6.9 < log10(x_e) < -7.3, and are consistent
with previous work. We find no difference in the amount of ionization fraction
between cores with and without associated star formation activity, nor is any
difference found in electron abundances between the edge and center of the
emission region. Thus our models are in agreement with the standard picture of
cosmic rays as the primary source of ionization for molecular ions. With the
addition of previously determined electron abundances for low mass cores, and
even more massive cores associated with O and B clusters, we systematically
examine the ionization fraction as a function of star formation activity. This
analysis demonstrates that the most massive sources stand out as having the
lowest electron abundances (x_e < 10^-8).Comment: 35 pages (8 figures), using aaspp4.sty, to be published in
Astrophysical Journa
The IC1396N proto-cluster at a scale of 250 AU
We investigate the mm-morphology of IC1396N with unprecedented spatial
resolution to analyze its dust and molecular gas properties, and draw
comparisons with objects of similar mass. We have carried out sensitive
observations in the most extended configurations of the IRAM Plateau de Bure
interferometer, to map the thermal dust emission at 3.3 and 1.3mm, and the
emission from the =13 hyperfine transitions of methyl cyanide
(CHCN). We unveil the existence of a sub-cluster of hot cores in IC1396N,
distributed in a direction perpendicular to the emanating outflow. The cores
are embedded in a common envelope of extended and diffuse dust emission. We
find striking differences in the dust properties of the cores ( 0)
and the surrounding envelope ( 1), very likely testifying to
differences in the formation and processing of dust material. The CHCN
emission peaks towards the most massive hot core and is marginally extended in
the outflow direction
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