14 research outputs found
The distribution of H13CN in the circumstellar envelope around IRC+10216
H13CN J=8-7 sub-millimetre line emission produced in the circumstellar
envelope around the extreme carbon star IRC+10216 has been imaged at
sub-arcsecond angular resolution using the SMA. Supplemented by a detailed
excitation analysis the average fractional abundance of H13CN in the inner wind
(< 5E15 cm) is estimated to be about 4E-7, translating into a total HCN
fractional abundance of 2E-5 using the isotopic ratio 12C/13C=50.
Multi-transitional single-dish observations further requires the H13CN
fractional abundance to remain more or less constant in the envelope out to a
radius of about 4E16 cm, where the HCN molecules are effectively destroyed,
most probably, by photodissociation. The large amount of HCN present in the
inner wind provides effective line cooling that can dominate over that
generated from CO line emission. It is also shown that great care needs to be
taken in the radiative transfer modelling where non-local, and non-LTE, effects
are important and where the radiation field from thermal dust grains plays a
major role in exciting the HCN molecules. The amount of HCN present in the
circumstellar envelope around IRC+10216 is consistent with predicted
photospheric values based on equilibrium chemical models and indicates that any
non-equilibrium chemistry occurring in the extended pulsating atmosphere has no
drastic net effect on the fractional abundance of HCN molecules that enters the
outer envelope. It further suggests that few HCN molecules are incorporated
into dust grains.Comment: Accepted for publication in ApJ. 20 pages, 7 figure
PROSAC: A Submillimeter Array Survey of Low-Mass Protostars. I. Overview of Program: Envelopes, Disks, Outflows and Hot Cores
This paper presents a large spectral line and continuum survey of 8 deeply
embedded, low-mass protostellar cores using the Submillimeter Array. Each
source was observed in high excitation lines of some of the most common
molecular species, CO, HCO+, CS, SO, H2CO, CH3OH and SiO. Line emission from 11
species originating from warm and dense gas have been imaged at high angular
resolution (1-3"; typically 200-600 AU) together with continuum emission at 230
GHz (1.3 mm) and 345 GHz (0.8 mm). Compact continuum emission is observed for
all sources which likely originates in marginally optically thick circumstellar
disks, with typical lower limits to their masses of 0.1 M_sun (1-10% of the
masses of their envelopes) and having a dust opacity law with beta
approximately 1. Prominent outflows are present in CO 2-1 observations in all
sources: the most diffuse outflows are found in the sources with the lowest
ratios of disk-to-envelope mass, and it is suggested that these sources are in
a phase where accretion of matter from the envelope has almost finished and the
remainder of the envelope material is being dispersed by the outflows. Other
characteristic dynamical signatures are found with inverse P Cygni profiles
indicative of infalling motions seen in the 13CO 2-1 lines toward
NGC1333-IRAS4A and -IRAS4B. Outflow-induced shocks are present on all scales in
the protostellar environments and are most clearly traced by the emission of
CH3OH in NGC1333-IRAS4A and -IRAS4B. These observations suggest that the
emission of CH3OH and H2CO from these proposed "hot corinos" are related to the
shocks caused by the protostellar outflows. Only one source, NGC1333-IRAS2A,
has evidence for hot, compact CH3OH emission coincident with the embedded
protostar.Comment: Accepted for publication in ApJ (52 pages; 9 figures). Abstract
abridge
Modelling CO emission from Mira's wind
We have modelled the circumstellar envelope of {\it o} Ceti (Mira) using new
observational constraints. These are obtained from photospheric light scattered
in near-IR vibrational-rotational lines of circumstellar CO molecules at 4.6
micron: absolute fluxes, the radial dependence of the scattered intensity, and
two line ratios. Further observational constraints are provided by ISO
observations of far-IR emission lines from highly excited rotational states of
the ground vibrational state of CO, and radio observations of lines from
rotational levels of low excitation of CO. A code based on the Monte-Carlo
technique is used to model the circumstellar line emission.
We find that it is possible to model the radio and ISO fluxes, as well as the
highly asymmetric radio-line profiles, reasonably well with a spherically
symmetric and smooth stellar wind model. However, it is not possible to
reproduce the observed NIR line fluxes consistently with a `standard model' of
the stellar wind. This is probably due to incorrectly specified conditions of
the inner regions of the wind model, since the stellar flux needs to be larger
than what is obtained from the standard model at the point of scattering, i.e.,
the intermediate regions at approximately 100-400 stellar radii (2"-7") away
from the star. Thus, the optical depth in the vibrational-rotational lines from
the star to the point of scattering has to be decreased. This can be
accomplished in several ways. For instance, the gas close to the star (within
approximately 2") could be in such a form that light is able to pass through,
either due to the medium being clumpy or by the matter being in radial
structures (which, further out, developes into more smooth or shell-like
structures).Comment: 18 pages, 3 figures, accepted for publication in Ap
The Kr85 s-process Branching and the Mass of Carbon Stars
We present new spectroscopic observations for a sample of C(N)-type red
giants. These objects belong to the class of Asymptotic Giant Branch stars,
experiencing thermal instabilities in the He-burning shell (thermal pulses).
Mixing episodes called third dredge-up enrich the photosphere with newly
synthesized C12 in the He-rich zone, and this is the source of the high
observed ratio between carbon and oxygen (C/O > 1 by number). Our spectroscopic
abundance estimates confirm that, in agreement with the general understanding
of the late evolutionary stages of low and intermediate mass stars, carbon
enrichment is accompanied by the appearance of s-process elements in the
photosphere. We discuss the details of the observations and of the derived
abundances, focusing in particular on rubidium, a neutron-density sensitive
element, and on the s-elements Sr, Y and Zr belonging to the first s-peak. The
critical reaction branching at Kr85, which determines the relative enrichment
of the studied species, is discussed. Subsequently, we compare our data with
recent models for s-processing in Thermally Pulsing Asymptotic Giant Branch
stars, at metallicities relevant for our sample. A remarkable agreement between
model predictions and observations is found. Thanks to the different neutron
density prevailing in low and intermediate mass stars, comparison with the
models allows us to conclude that most C(N) stars are of low mass (M < 3Mo). We
also analyze the C12/C13 ratios measured, showing that most of them cannot be
explained by canonical stellar models. We discuss how this fact would require
the operation of an ad hoc additional mixing, currently called Cool Bottom
Process, operating only in low mass stars during the first ascent of the red
giant branch and, perhaps, also during the asymptotic giant branch.Comment: 54 pages + 6 figures + 6 tables. ApJ accepte
Spitzer Space Telescope Spectroscopy of Ices toward Low Mass Embedded Protostars
Sensitive 5-38 um Spitzer Space Telescope (SST) and ground based 3-5 um
spectra of the embedded low mass protostars B5 IRS1 and HH46 IRS show deep ice
absorption bands superposed on steeply rising mid-infrared continua. The ices
likely originate in the circumstellar envelopes. The CO2 bending mode at 15 um
is a particularly powerful tracer of the ice composition and processing
history. Toward these protostars, this band shows little evidence for thermal
processing at temperatures above 50 K. Signatures of lower temperature
processing are present in the CO and OCN- bands, however. The observed CO2
profile indicates an intimate mixture with H2O, but not necessarily with CH3OH,
in contrast to some high mass protostars. This is consistent with the low CH3OH
abundance derived from the ground based L band spectra. The CO2/H2O column
density ratios are high in both B5 IRS1 and HH46 IRS (~35%). Clearly, the SST
spectra are essential to study ice evolution in low mass protostellar
environments, and to eventually determine the relation between interstellar and
solar system ices.Comment: Scheduled for publication in ApJS Sept. 2004, v154 [Spitzer Special
issue]. 5 pages, 1 b&w figure, 1 color figur
The H2CO abundance in the inner warm regions of low mass protostellar envelopes
We present a survey of the formaldehyde emission in a sample of eight Class 0 protostars obtained with the IRAM and JCMT millimeter telescopes. The range of energies of the observed transitions allows us to probe the physical and chemical conditions across the protostellar envelopes. The data have been analyzed with three different methods with increasing level of sophistication. We first analyze the observed emission in the LTE approximation, and derive rotational temperatures between 11 and 40 K, and column densities between 1 and 20 x 10{13} cm -2. Second, we use a LVG code and derive higher kinetic temperatures, between 30 and 90 K, consistent with subthermally populated levels and densities from 1 to 6 x 10{5} cm -3. The column densities from the LVG modeling are within a factor of 10 with respect to those derived in the LTE approximation. Finally, we analyze the observations based upon detailed models for the envelopes surrounding the protostars, using temperature and density profiles previously derived from continuum observations. We approximate the formaldehyde abundance across the envelope with a jump function, the jump occurring when the dust temperature reaches 100 K, the evaporation temperature of the grain mantles. The observed formaldehyde emission is well reproduced only if there is a jump of more than two orders of magnitude, in four sources. In the remaining four sources the data are consistent with a formaldehyde abundance jump, but the evidence is more marginal ( ${\leq} 2 ~ \sigma ). The inferred inner H 2CO abundance varies between 1 x 10{-8} and 6 x 10{-6}. The absolute values of the jump in the H 2CO abundance are uncertain by about one order of magnitude, because of the uncertainties in the density, ortho to para ratio, temperature and velocity profiles of the inner region, as well as the evaporation temperature of the ices. We discuss the implications of these jumps for our understanding of the origin and evolution of ices in low mass star forming regions. Finally, we give predictions for the submillimeter H 2CO lines, which are particularly sensitive to the abundance jumps
Observations of the circumstellar water 110 → 110 and ammonia 110 → 00 lines in IRC +10216 by the Odin satellite
Submillimeter lines of H 2O and NH 3 have been detected in the carbon star IRC +10216 (CW Leo) with the Odin submillimeter satellite. The detection of the J K-, K+ = 1 10 → 1 10 557 GHz line of ortho-H 2O confirms the earlier detection in the same source with SWAS. The detection of the J K = 1 0 rarr; 0 0 572 GHz line represents the first observation of the ground-state rotational transition of NH 3 in a stellar envelope. By fitting a molecular line transfer model to the observed lines, we derive an ortho-H 20 abundance of 2.4 × 10 -6, which is consistent with estimates from the SWAS observation. The derived ortho-NH 3 abundance of 1 × 10 -6 relative to H 2 is significantly higher than those derived from 24 GHz inversion transitions and is slightly higher than those from vibrational transitions in the infrared band. The high H 2O and NH 3 abundances in the carbon-rich star IRC+10216 underscore shortcomings in the conventional gas-phase LTE and non-LTE chemical models. © 2006. The American Astronomical Society. All rights reserved.published_or_final_versio
Evolutionary Conservation of Infection-Induced Cell Death Inhibition among Chlamydiales
Control of host cell death is of paramount importance for the survival and replication of obligate intracellular bacteria. Among these, human pathogenic Chlamydia induces the inhibition of apoptosis in a variety of different host cells by directly interfering with cell death signaling. However, the evolutionary conservation of cell death regulation has not been investigated in the order Chlamydiales, which also includes Chlamydia-like organisms with a broader host spectrum. Here, we investigated the apoptotic response of human cells infected with the Chlamydia-like organism Simkania negevensis (Sn). Simkania infected cells exhibited strong resistance to apoptosis induced by intrinsic stress or by the activation of cell death receptors. Apoptotic signaling was blocked upstream of mitochondria since Bax translocation, Bax and Bak oligomerisation and cytochrome c release were absent in these cells. Infected cells turned on pro-survival pathways like cellular Inhibitor of Apoptosis Protein 2 (cIAP-2) and the Akt/PI3K pathway. Blocking any of these inhibitory pathways sensitized infected host cell towards apoptosis induction, demonstrating their role in infection-induced apoptosis resistance. Our data support the hypothesis of evolutionary conserved signaling pathways to apoptosis resistance as common denominators in the order Chlamydiales