958 research outputs found
CH abundance gradient in TMC-1
We observed the 9-cm Lambda-doubling lines of CH along the dense filament of
TMC-1. The CH column densities were compared with the total H2 column densities
derived using the 2MASS NIR data and previously published SCUBA maps and with
OH column densities derived using previous observations with Effelsberg. We
also modelled the chemical evolution of TMC-1 adopting physical conditions
typical of dark clouds using the UMIST Database for Astrochemistry gas-phase
reaction network to aid the interpretation of the observed OH/CH abundance
ratios. The CH column density has a clear peak in the vicinity of the
cyanopolyyne maximum of TMC-1. The fractional CH abundance relative to H2
increases steadily from the northwestern end of the filament where it lies
around 1.0e-8, to the southeast where it reaches a value of 2.0e-8. The OH and
CH column densities are well correlated, and we obtained OH/CH abundance ratios
of ~ 16 - 20. These values are clearly larger than what has been measured
recently in diffuse interstellar gas and is likely to be related to C to CO
conversion at higher densities. The good correlation between CH and OH can be
explained by similar production and destruction pathways. We suggest that the
observed CH and OH abundance gradients are mainly due to enhanced abundances in
a low-density envelope which becomes more prominent in the southeastern part
and seems to continue beyond the dense filament. An extensive envelope probably
signifies an early stage of dynamical evolution, and conforms with the
detection of a large CH abundance in the southeastern part of the cloud. The
implied presence of other simple forms of carbon in the gas phase provides a
natural explanation for the observation of "early-type" molecules in this
region.Comment: 12 pages, 16 figure
SMA observations of C2H in High-Mass Star Forming Regions
CH is a representative hydrocarbon that is abundant and ubiquitous in the
interstellar medium (ISM). To study its chemical properties, we present
Submillimeter Array (SMA) observations of the CH and HCN
transitions and the 1.1 mm continuum emission toward four OB
cluster-forming regions, AFGL 490, ON 1, W33 Main, and G10.6-0.4, which cover a
bolometric luminosity range of 10--10 . We found that
on large scales, the CH emission traces the dense molecular envelope.
However, for all observed sources, the peaks of CH emission are offset by
several times times 10 AU from the peaks of 1.1 mm continuum emission,
where the most luminous stars are located. By comparing the distribution and
profiles of CH hyperfine lines and the 1.1 mm continuum emission, we find
that the CH column density (and abundance) around the 1.1 mm continuum
peaks is lower than those in the ambient gas envelope. Chemical models suggest
that CH might be transformed to other species owing to increased
temperature and density; thus, its reduced abundance could be the signpost of
the heated molecular gas in the 10 AU vicinity around the embedded
high-mass stars. Our results support such theoretical prediction for centrally
embedded -- OB star-forming cores, while future
higher-resolution observations are required to examine the CH
transformation around the localized sites of high-mass star formation.Comment: 10 pages, 6 figures. ApJ accepted. Comments welcom
A-type stars: evolution, rotation and binarity
We discuss the internal structure of stars in the mass range 1.5 to 4 M_sun
from the PMS to the subgiant phase with a particular emphasis on the convective
core and the convective superficial layers. Different physical aspects are
considered such as overshooting, treatment of convection, microscopic diffusion
and rotation. Their influence on the internal structure and on the photospheric
chemical abundances is briefly described. The role of binarity in determining
the observed properties and as a tool to constrain the internal structure is
also introduced and the current limits of theories of orbital evolution and of
available binary data--sets are discussed. keywords{stars: evolution, stars:
binaries: general, stars: rotation}Comment: 11 pages, 7 figures, conference: The A-star Puzzle, IAU Simp. 224,
200
arrostii roots
A new acylated and triterpenoidal saponin, named GS1, was isolated from the roots of Gypsophila arrostii Guss. On the basis of acid hydrolysis, comprehensive spectroscopic analyses and comparison with spectral data of known compounds, its structure was established as 3-O--D-xylopyranosyl-(12)-[-D-xylopyranosyl-(13)]--D-glucopyranosyl-{21-O-[(E)-3,4,5trimethoxycinnamoyl]}21-hydroxygypsogenin 28-O--D-glucopyranosyl-(12)- [-D-arabinopyranosyl-(13)]--D-xylopyranosyl-(13]--L-rhamnopyranosyl ester. This article deals with the isolation and structural elucidation of new acylated and oleanane-type saponin
Molecular abundances in OMC-1: The chemical composition of interstellar molecular clouds and the influence of massive star formation
We present here an investigation of the chemical composition of the various regions in the core of the
Orion molecular cloud (OMC-1) based on results from the Caltech Owens Valley Radio Observatory (OVRO) millimeter-wave spectral line survey (Sutton et al.; Blake et al.). This survey covered a 55 GHz interval in the
1.3 mm (230 GHz) atmospheric window and contained emission from over 800 resolved spectral features. Of the 29 identified species 14 have a sufficient number of detected transitions to be investigated with an LTE "rotation diagram" technique, in which large numbers of lines are used to estimate both the rotational excitation
and the overall abundance. The rotational temperatures and column densities resulting from these fits have then been used to model the emission from those remaining species which either have too few lines or which are too weak to be so analyzed. When different kinematic sources of emission are blended to produce a single feature, Gaussian fits have been used to derive the individual contributions to the total line profile. The uniformly calibrated data in the unique and extensive Caltech spectral line survey lead to accurate estimates of the chemical and physical parameters of the Orion molecular cloud, and place significant constraints on models of interstellar chemistry.
A global analysis of the observed abundances shows that the markedly different chemical compositions of
the kinematically and spatially distinct Orion subsources may be interpreted in the framework of an evolving,
initially quiescent, gas-phase chemistry influenced by the process of massive star formation. The chemical composition
of the extended Orion cloud complex is similar to that found in a number of other objects, but the central regions of OMC-1 have had their chemistry selectively altered by the radiation and high-velocity outflow from the young stars embedded deep within the interior of the molecular cloud. Specifically, the extended ridge clouds are inferred to have a low (subsolar) gas-phase oxygen content from the prevalence of reactive carbon-rich species like CN, CCH, and C_3H_2 also found in more truly quiescent objects such as TMC-1. The similar abundances of these and other simple species in clouds like OMC-1, Sgr B2, and TMC-1 lend support to gas-phase ion-molecule models of interstellar chemistry, but grain processes may also play a significant role in maintaining the overall chemical balance in such regions through selective depletion mechanisms and grain mantle processing. In contrast, the chemical compositions of the more turbulent plateau and hot core components of OMC-1 are dominated by high-temperature, shock-induced gas and grain surface neutral-neutral reaction processes. The high silicon/sulfur oxide and water content of the plateau gas is best modeled by fast shock disruption of smaller grain cores to release the more refractory elements followed by a predominantly neutral chemistry in the cooling postshock regions, while a more passive release of grain mantle products driven toward kinetic equilibrium most naturally explains the prominence of fully hydrogenated
N-containing species like HCN, NH_3 , CH_3CN, and C_2H_5CN in the hot core. The clumpy nature of the outflow is illustrated by the high-velocity emission observed from easily decomposed molecules such as H_2CO. Areas immediately adjacent to the shocked core in which the cooler, ion-rich gas of the surrounding molecular cloud is mixed with water/oxygen rich gas from the plateau source are proposed to give rise to the enhanced abundances of complex internal rotors such as CH_30H, HCOOCH_3 , and CH_30CH_3 whose line widths are similar to carbon-rich species such as CN and CCH found in the extended ridge, but whose rotational temperatures are somewhat higher and whose spatial extents are much more compact
Infalling-Rotating Motion and Associated Chemical Change in the Envelope of IRAS 16293-2422 Source A Studied with ALMA
We have analyzed rotational spectral line emission of OCS, CH3OH, HCOOCH3,
and H2CS observed toward the low-mass Class 0 protostellar source IRAS
16293-2422 Source A at a sub-arcsecond resolution (~0".6 x 0".5) with ALMA.
Significant chemical differentiation is found at a 50 AU scale. The OCS line is
found to well trace the infalling-rotating envelope in this source. On the
other hand, the CH3OH and HCOOCH3 distributions are found to be concentrated
around the inner part of the infalling-rotating envelope. With a simple
ballistic model of the infalling-rotating envelope, the radius of the
centrifugal barrier (a half of the centrifugal radius) and the protostellar
mass are evaluated from the OCS data to be from 40 to 60 AU and from 0.5 to 1.0
Msun, respectively, assuming the inclination angle of the envelope/disk
structure to be 60 degrees (90 degrees for the edge-on configuration). Although
the protostellar mass is correlated with the inclination angle, the radius of
the centrifugal barrier is not. This is the first indication of the centrifugal
barrier of the infalling-rotating envelope in a hot corino source. CH3OH and
HCOOCH3 may be liberated from ice mantles due to weak accretion shocks around
the centrifugal barrier, and/or due to protostellar heating. The H2CS emission
seems to come from the disk component inside the centrifugal barrier in
addition to the envelope component. The centrifugal barrier plays a central
role not only in the formation of a rotationally-supported disk but also in the
chemical evolution from the envelope to the protoplanetary disk
Exploring the molecular chemistry and excitation in obscured luminous infrared galaxies: An ALMA mm-wave spectral scan of NGC 4418
We obtained an ALMA Cycle 0 spectral scan of the dusty LIRG NGC 4418,
spanning a total of 70.7 GHz in bands 3, 6, and 7. We use a combined local
thermal equilibrium (LTE) and non-LTE (NLTE) fit of the spectrum in order to
identify the molecular species and derive column densities and excitation
temperatures. We derive molecular abundances and compare them with other
Galactic and extragalactic sources by means of a principal component analysis.
We detect 317 emission lines from a total of 45 molecular species, including 15
isotopic substitutions and six vibrationally excited variants. Our LTE/NLTE fit
find kinetic temperatures from 20 to 350 K, and densities between 10 and
10 cm. The spectrum is dominated by vibrationally excited HCN,
HCN, and HNC, with vibrational temperatures from 300 to 450 K. We find high
abundances of HCN, SiO, HS, and c-HCCCH and a low CHOH abundance. A
principal component analysis shows that NGC 4418 and Arp 220 share very similar
molecular abundances and excitation, which clearly set them apart from other
Galactic and extragalactic environments. The similar molecular abundances
observed towards NCG 4418 and Arp 220 are consistent with a hot gas-phase
chemistry, with the relative abundances of SiO and CHOH being regulated by
shocks and X-ray driven dissociation. The bright emission from vibrationally
excited species confirms the presence of a compact IR source, with an effective
diameter 350 K. The molecular abundances
and the vibrationally excited spectrum are consistent with a young
AGN/starburst system. We suggest that NGC 4418 may be a template for a new kind
of chemistry and excitation, typical of compact obscured nuclei (CON). Because
of the narrow line widths and bright molecular emission, NGC 4418 is the ideal
target for further studies of the chemistry in CONs.Comment: accepted by A&A on 29/06/201
Time-dependent models of dense PDRs with complex molecules
We present a study of the chemistry of a dense photon-dominated region (PDR)
using a time-dependent chemical model. Our major interest is to study the
spatial distribution of complex molecules such as hydrocarbons and
cyanopolyynes in the cool dense material bordering regions where star formation
has taken place. Our standard model uses a homogeneous cloud of density 2x10e4
cm-3 and temperature T=40 K, which is irradiated by a far-ultraviolet radiation
field of intermediate intensity, given by X=100. We find that over a range of
times unsaturated hydrocarbons (e.g., C2H, C4H, C3H2) have relatively high
fractional abundances in the more external layers of the PDR, whereas their
abundances in the innermost layers are several orders of magnitudes lower. On
the other hand, molecules that are typical of late-time chemistry are usually
more abundant in the inner parts of the PDR. We also present results for models
with different density, temperature, intensity of the radiation field and
initial fractional abundances. Our results are compared with both high- and
moderate-angular resolution observations of the Horsehead nebula. Our standard
model is partially successful in reproducing the observations. Additional
models run with different physical parameters are able to reproduce the
abundance of many of the observed molecules, but we do not find a single model
that fits all the observations at the same time. We discuss the suitability of
a time-dependent model of a dense PDR such as ours as an estimator of the age
of a PDR, provided that enough observational data exist.Comment: 15 pages, 12 figures, 8 tables, to be published in MNRA
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