148 research outputs found
Discovery of Hydrogen Fluoride in the Cloverleaf Quasar at z = 2.56
We report the first detection of hydrogen fluoride (HF) toward a high
redshift quasar. Using the Caltech Submillimeter Observatory (CSO) we detect
the HF J = 1 - 0 transition in absorption toward the Cloverleaf, a broad
absorption line (BAL) quasi-stellar object (QSO) at z=2.56. The detection is
statistically significant at the ~ 6 sigma level. We estimate a lower limit of
4 \times 1014 cm-2 for the HF column density and using a previous estimate of
the hydrogen column density, we obtain a lower limit of 1.7 \times 10-9 for the
HF abundance. This value suggests that, assuming a Galactic N(HF)/NH ratio, HF
accounts for at least ~10% of the fluorine in the gas phase along the line of
sight to the Cloverleaf quasar. This observation corroborates the prediction
that HF should be a good probe of the molecular gas at high redshift.
Measurements of the HF abundance as a function of redshift are urgently needed
to better constrain the fluorine nucleosynthesis mechanism(s)
Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
Aims. To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions.
Methods. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN.
Results. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H^(13)CN). This is most evident in the HCN 12–11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part.
Conclusions. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important
Hydrogen Fluoride in High-Mass Star-forming Regions
Hydrogen fluoride has been established to be an excellent tracer of molecular
hydrogen in diffuse clouds. In denser environments, however, the HF abundance
has been shown to be approximately two orders of magnitude lower. We present
Herschel/HIFI observations of HF J=1-0 toward two high-mass star formation
sites, NGC6334 I and AFGL 2591. In NGC6334 I the HF line is seen in absorption
in foreground clouds and the source itself, while in AFGL 2591 HF is partially
in emission. We find an HF abundance with respect to H2 of 1.5e-8 in the
diffuse foreground clouds, whereas in the denser parts of NGC6334 I, we derive
a lower limit on the HF abundance of 5e-10. Lower HF abundances in dense clouds
are most likely caused by freeze out of HF molecules onto dust grains in
high-density gas. In AFGL 2591, the view of the hot core is obstructed by
absorption in the massive outflow, in which HF is also very abundant 3.6e-8)
due to the desorption by sputtering. These observations provide further
evidence that the chemistry of interstellar fluorine is controlled by freeze
out onto gas grains.Comment: accepted in Ap
The NH2D/NH3 ratio toward pre-protostellar cores around the UCHII region in IRAS 20293+3952
The deuterium fractionation, Dfrac, has been proposed as an evolutionary
indicator in pre-protostellar and protostellar cores of low-mass star-forming
regions. We investigate Dfrac, with high angular resolution, in the cluster
environment surrounding the UCHII region IRAS 20293+3952. We performed high
angular resolution observations with the IRAM Plateau de Bure Interferometer
(PdBI) of the ortho-NH2D 1_{11}-1_{01} line at 85.926 GHz and compared them
with previously reported VLA NH3 data. We detected strong NH2D emission toward
the pre-protostellar cores identified in NH3 and dust emission, all located in
the vicinity of the UCHII region IRAS 20293+3952. We found high values of
Dfrac~0.1-0.8 in all the pre-protostellar cores and low values, Dfrac<0.1,
associated with young stellar objects. The high values of Dfrac in
pre-protostellar cores could be indicative of evolution, although outflow
interactions and UV radiation could also play a role.Comment: 5 pages, 3 figures. Accepted for publication in Astronomy and
Astrophysics Letter
[12CII] and [13CII] 158 mum emission from NGC 2024: Large column densities of ionized carbon
Context: We analyze the NGC 2024 HII region and molecular cloud interface
using [12CII] and [13CII] observations. Aims: We attempt to gain insight into
the physical structure of the interface layer between the molecular cloud and
the HII region. Methods. Observations of [12CII] and [13CII] emission at 158
{\mu}m with high spatial and spectral resolution allow us to study the detailed
structure of the ionization front and estimate the column densities and
temperatures of the ionized carbon layer in the PDR. Results: The [12CII]
emission closely follows the distribution of the 8 mum continuum. Across most
of the source, the spectral lines have two velocity peaks similar to lines of
rare CO isotopes. The [13CII] emission is detected near the edge-on ionization
front. It has only a single velocity component, which implies that the [12CII]
line shape is caused by self-absorption. An anomalous hyperfine line-intensity
ratio observed in [13CII] cannot yet be explained. Conclusions: Our analysis of
the two isotopes results in a total column density of N(H)~1.6\times10^23 cm^-2
in the gas emitting the [CII] line. A large fraction of this gas has to be at a
temperature of several hundred K. The self-absorption is caused by a cooler
(T<=100 K) foreground component containing a column density of N(H)~10^22
cm^-2
Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the SMA
We aim at deriving the molecular abundances and temperatures of the hot
molecular cores in the high-mass star-forming region NGC 6334I and consequently
deriving their physical and astrochemical conditions. In the framework of the
Herschel guaranteed time key program CHESS, NGC 6334I is investigated by using
HIFI aboard the Herschel Space Observatory. A spectral line survey is carried
out in the frequency range 480-1907 GHz, and auxiliary interferometric data
from the SMA in the 230 GHz band provide spatial information for disentangling
the different physical components contributing to the HIFI spectrum. The
spectral lines are identified with the aid of former surveys and spectral line
catalogs. The observed spectrum is then compared to a simulated synthetic
spectrum with XCLASS, assuming local thermal equilibrium, and best fit
parameters are derived using the model optimization package MAGIX. A total of
46 molecules are identified, with 31 isotopologues, resulting in about 4300
emission and absorption lines. High- energy levels of the dominant emitter
methanol and vibrationally excited HCN are detected. The number of unidentified
lines remains low with 75, or less than 2 percent of the lines detected. The
modeling suggests that several spectral features need two or more components to
be fitted properly. Other components could be assigned to cold foreground
clouds or to outflows, most visible in the SiO emission. A chemical variation
between the two embedded hot cores is found, with more N-bearing molecules
identified in SMA1 and O-bearing molecules in SMA2. Spectral line surveys give
powerful insights into the study of the interstellar medium. Different
molecules trace different physical conditions like the inner hot core, the
envelope, the outflows or the cold foreground clouds. The derived molecular
abundances provide further constraints for astrochemical models.Comment: 30 pages including appendix, 49 figures, accepted for publication in
Astronomy and Astrophysic
Molecular line survey of the high-mass star-forming region NGC 6334I with Herschel/HIFI and the Submillimeter Array
Aims. We aim at deriving the molecular abundances and temperatures of the hot molecular cores in the high-mass star-forming region NGC 6334I and consequently deriving their physical and astrochemical conditions.
Methods. In the framework of the Herschel guaranteed time key program CHESS (Chemical HErschel Surveys of Star forming regions), NGC 6334I is investigated by using the Heterodyne Instrument for the Far-Infrared (HIFI) aboard the Herschel Space Observatory. A spectral line survey is carried out in the frequency range 480–1907 GHz, and further auxiliary interferometric data from the Submillimeter Array (SMA) in the 230 GHz band provide spatial information for disentangling the different physical components contributing to the HIFI spectrum. The spectral lines in the processed Herschel data are identified with the aid of former surveys and spectral line catalogs. The observed spectrum is then compared to a simulated synthetic spectrum, assuming local thermal equilibrium, and best fit parameters are derived using a model optimization package.
Results. A total of 46 molecules are identified, with 31 isotopologues, resulting in about 4300 emission and absorption lines. High-energy levels (E_u > 1000 K) of the dominant emitter methanol and vibrationally excited HCN (ν_2 = 1) are detected. The number of unidentified lines remains low with 75, or <2% of the lines detected. The modeling suggests that several spectral features need two or more components to be fitted properly. Other components could be assigned to cold foreground clouds or to outflows, most visible in the SiO and H_(2)O emission. A chemical variation between the two embedded hot cores is found, with more N-bearing molecules identified in SMA1 and O-bearing molecules in SMA2.
Conclusions. Spectral line surveys give powerful insights into the study of the interstellar medium. Different molecules trace different physical conditions like the inner hot core, the envelope, the outflows or the cold foreground clouds. The derived molecular abundances provide further constraints for astrochemical models
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