425 research outputs found
Imaging diffuse clouds: Bright and dark gas mapped in CO
We wish to relate the degree scale structure of galactic diffuse clouds to
sub-arcsecond atomic and molecular absorption spectra obtained against
extragalactic continuum background sources. To do this, we used the ARO 12m
telescope to map J=1-0 CO emission at 1' resolution over 30' fields around the
positions of 11 background sources occulted by 20 molecular absorption line
components, of which 11 had CO emission counterparts. We compare maps of CO
emission to sub-arcsec atomic and molecular absorption spectra and to the
large-scale distribution of interstellar reddening. The main results are: 1)
Typical covering factors of individual features at the 1 K.km/s level were 20%.
2) CO-H2 conversion factors as much as 4-5 times below the mean value N(H2)/Wco
= 2e20 H2 cm^-2 /(K.km/s) are required to explain the luminosity of CO emission
at/above the level of 1 K.km/s. Small conversion factors and sharp variability
of the conversion factor on arcminute scales are due primarily to CO chemistry
and need not represent unresolved variations in reddening or total column
density. Hence, like FERMI and PLANCK we see some gas that is dark in CO and
other gas in which CO is overluminous per H2. A standard CO-H2 conversion
factor applies overall owing to balance between the luminosities per H2 and
surface covering factors of bright and dark CO., but with wide variations.Comment: 23 pages, 22 PostScript figures. Accepted for publication in
Astronomy \& Astrophysics. Uses aa LaTeX macro
Revisiting the theory of interferometric wide-field synthesis
After several generations of interferometers in radioastronomy, wide-field
imaging at high angular resolution is today a major goal for trying to match
optical wide-field performances. All the radio-interferometric, wide-field
imaging methods currently belong to the mosaicking family. Based on a 30 years
old, original idea from Ekers & Rots, we aim at proposing an alternate
formalism. Starting from their ideal case, we successively evaluate the impact
of the standard ingredients of interferometric imaging. A comparison with
standard nonlinear mosaicking shows that both processing schemes are not
mathematically equivalent, though they both recover the sky brightness. In
particular, the weighting scheme is very different in both methods. Moreover,
the proposed scheme naturally processes the short spacings from both
single-dish antennas and heterogeneous arrays. Finally, the sky gridding of the
measured visibilities, required by the proposed scheme, may potentially save
large amounts of hard-disk space and cpu processing power over mosaicking when
handling data sets acquired with the on-the-fly observing mode. We propose to
call this promising family of imaging methods wide-field synthesis because it
explicitly synthesizes visibilities at a much finer spatial frequency
resolution than the one set by the diameter of the interferometer antennas.Comment: 22 pages, 6 PostScript figures. Accepted for publication in Astronomy
& Astrophysics. Uses aa LaTeX macros
Limits on chemical complexity in diffuse clouds: search for CH3OH and HC5N absorption
Context: An unexpectedly complex polyatomic chemistry exists in diffuse
clouds, allowing detection of species such as C2H, C3H2, H2CO and NH3 which
have relative abundances that are strikingly similar to those inferred toward
the dark cloud TMC-1
Aims: We probe the limits of complexity of diffuse cloud polyatomic
chemistry.
Methods: We used the IRAM Plateau de Bure Interferometer to search for
galactic absorption from low-lying J=2-1 rotational transitions of A- and
E-CH3OH near 96.740 GHz and used the VLA to search for the J=8-7 transition of
HC5N at 21.3 GHz.
Results: Neither CH3OH nor HC5N were detected at column densities well below
those of all polyatomics known in diffuse clouds and somewhat below the levels
expected from comparison with TMC-1. The HCN/HC5N ratio is at least 3-10 times
higher in diffuse gas than toward TMC-1.
Conclusions: It is possible to go to the well once (or more) too ofte
Intermittency of interstellar turbulence: extreme velocity-shears and CO emission on milliparsec scale
The condensation of diffuse gas into molecular clouds occurs at a rate driven
largely by turbulent dissipation. This process still has to be caught in action
and characterized. A mosaic of 13 fields was observed in the CO(1-0) line with
the IRAM-PdB interferometer in the translucent environment of two low-mass
dense cores. The large size of the mosaic compared to the resolution (4 arcsec)
is unprecedented in the study of the small-scale structure of diffuse molecular
gas. Eight weak and elongated structures of thicknesses as small as 3 mpc (600
AU) and lengths up to 70mpc are found. These are not filaments because once
merged with short-spacing data, they appear as the sharp edges of larger-scale
structures. Six out of eight form quasi-parallel pairs at different velocities
and different position angles. This cannot be the result of chance alignment.
The velocity-shears estimated for the three pairs include the highest ever
measured far from star forming regions (780 km/s/pc). Because the large scale
structures have sharp edges, with little or no overlap, they have to be thin
CO-layers. Their edges mark a sharp transition between a CO-rich component and
a gas undetected in the CO line because of its low CO abundance, presumably the
cold neutral medium. We propose that these sharp edges are the first
directly-detected manifestations of the intermittency of interstellar
turbulence. The large velocity-shears reveal an intense straining field,
responsible for a local dissipation rate several orders of magnitude above
average, possibly at the origin of the thin CO-layers.Comment: 16 pages, 11 figures, Accepted for publication in Astronomy and
Astrophysic
HCO, c-C3H and CF+ : three new molecules in diffuse, translucent and "spiral-arm'' clouds
%methods {We used the EMIR receiver and FTS spectrometer at the IRAM 30m to
construct absorption spectra toward bright extra-galactic background sources at
195 kHz spectral resolution ( 0.6 \kms). We used the IRAM Plateau de
Bure interferometer to synthesize absorption spectra of \hthcop\ and HCO toward
the galactic HII region W49.} %results {HCO, \cc3h\ and CF\p\ were detected
toward the blazars \bll\ and 3C111 having \EBV\ = 0.32 and 1.65 mag. HCO was
observed in absorption from ``spiral-arm'' clouds in the galactic plane
occulting W49. The complement of detectable molecular species in the 85 - 110
GHz absorption spectrum of diffuse/translucent gas is now fully determined at
rms noise level at \EBV\ = 0.32 mag (\AV\ = 1 mag)
and /\EBV\ mag overall.} %conclusions {As
with OH, \hcop\ and \cch, the relative abundance of \cc3h\ varies little
between diffuse and dense molecular gas, with N(\cc3h)/N({\it o-c}-\c3h2)
0.1. We find N(CF\p)/N(HCO\p) , N(CF\p)/N(\cch)
0.005-0.01 and because N(CF\p) increases with \EBV\ and with the
column densities of other molecules we infer that fluorine remains in the gas
phase as HF well beyond \AV\ = 1 mag. We find N(HCO)/N(HCO\p) = 16
toward \bll, 3C111 and the 40 km/s spiral arm cloud toward W49, implying X(HCO)
, about 10 times higher than in dark clouds. The behaviour of
HCO is consistent with previous suggestions that it forms from C\p\ and \HH,
even when \AV\ is well above 1 mag. The survey can be used to place useful
upper limits on some species, for instance N(\hhco)/N(\HH CS) 32 toward
3C111, compared to 7 toward TMC-1, confirming the possibility of a gas phase
formation route to \hhco.}Comment: A\%A in pres
Complex organic molecules in strongly UV-irradiated gas
We investigate the presence of COMs in strongly UV-irradiated interstellar
molecular gas. We have carried out a complete millimetre line survey using the
IRAM30m telescope towards the edge of the Orion Bar photodissociation region
(PDR), close to the H2 dissociation front, a position irradiated by a very
intense far-UV (FUV) radiation field. These observations have been complemented
with 8.5 arcsec resolution maps of the H2CO 5(1,5)-4(1,4) and C18O 3-2 emission
at 0.9 mm. Despite being a harsh environment, we detect more than 250 lines
from COMs and related precursors: H2CO, CH3OH, HCO, H2CCO, CH3CHO, H2CS, HCOOH,
CH3CN, CH2NH, HNCO, H13-2CO, and HC3N (in decreasing order of abundance). For
each species, the large number of detected lines allowed us to accurately
constrain their rotational temperatures (Trot) and column densities (N). Owing
to subthermal excitation and intricate spectroscopy of some COMs (symmetric-
and asymmetric-top molecules such as CH3CN and H2CO, respectively), a correct
determination of N and Trot requires building rotational population diagrams of
their rotational ladders separately. We also provide accurate upper limit
abundances for chemically related molecules that might have been expected, but
are not conclusively detected at the edge of the PDR (HDCO, CH3O, CH3NC,
CH3CCH, CH3OCH3, HCOOCH3, CH3CH2OH, CH3CH2CN, and CH2CHCN). A non-LTE LVG
excitation analysis for molecules with known collisional rate coefficients,
suggests that some COMs arise from different PDR layers but we cannot resolve
them spatially. In particular, H2CO and CH3CN survive in the extended gas
directly exposed to the strong FUV flux (Tk = 150-250 K and Td > 60 K), whereas
CH3OH only arises from denser and cooler gas clumps in the more shielded PDR
interior (Tk = 40-50 K). We find a HCO/H2CO/CH3OH = 1/5/3 abundance ratio.
These ratios are different from those inferred in hot cores and shocks.Comment: 29 pages, 22 figures, 17 tables. Accepted for publication in A&A
(abstract abridged
The CO-H2 conversion factor of diffuse ISM: Bright 12CO emission also traces diffuse gas
We show that the XCO factor, which converts the CO luminosity into the column
density of molecular hydrogen has similar values for dense, fully molecular gas
and for diffuse, partially molecular gas. We discuss the reasons of this
coincidence and the consequences for the understanding of the interstellar
medium.Comment: 5 pages, 1 PostScript figure. To be published in the proceedings of
the Zermatt 2010 conference: "Conditions and impact of star formation: New
results with Herschel and beyond". Uses EAS LaTeX macro
Nascent bipolar outflows associated with the first hydrostatic core candidates Barnard 1b-N and 1b-S
In the theory of star formation, the first hydrostatic core (FHSC) phase is a
critical step in which a condensed object emerges from a prestellar core. This
step lasts about one thousand years, a very short time compared with the
lifetime of prestellar cores, and therefore is hard to detect unambiguously.
We present IRAM Plateau de Bure observations of the Barnard 1b dense
molecular core, combining detections of H2CO and CH3OH spectral lines and dust
continuum at 2.3" resolution (~ 500 AU). The two compact cores B1b-N and B1b-S
are detected in the dust continuum at 2mm, with fluxes that agree with their
spectral energy distribution. Molecular outflows associated with both cores are
detected. They are inclined relative to the direction of the magnetic field, in
agreement with predictions of collapse in turbulent and magnetized gas with a
ratio of mass to magnetic flux somewhat higher than the critical value, \mu ~ 2
- 7. The outflow associated with B1b-S presents sharp spatial structures, with
ejection velocities of up to ~ 7 kms from the mean velocity. Its dynamical age
is estimated to be ~2000 yrs. The B1b-N outflow is smaller and slower, with a
short dynamical age of ~1000 yrs. The B1b-N outflow mass, mass-loss rate, and
mechanical luminosity agree well with theoretical predictions of FHSC. These
observations confirm the early evolutionary stage of B1b-N and the slightly
more evolved stage of B1b-S.Comment: 6 pages, 3 figure
The ionization fraction gradient across the Horsehead edge: An archetype for molecular clouds
The ionization fraction plays a key role in the chemistry and dynamics of
molecular clouds. We study the H13CO+, DCO+ and HOC+ line emission towards the
Horsehead, from the shielded core to the UV irradiated cloud edge, i.e., the
Photodissociation Region (PDR), as a template to investigate the ionization
fraction gradient in molecular clouds. We analyze a PdBI map of the H13CO+
J=1-0 line, complemented with IRAM-30m H13CO+ and DCO+ higher-J line maps and
new HOC+ and CO+ observations. We compare self-consistently the observed
spatial distribution and line intensities with detailed depth-dependent
predictions of a PDR model coupled with a nonlocal radiative transfer
calculation. The chemical network includes deuterated species, 13C
fractionation reactions and HCO+/HOC+ isomerization reactions. The role of
neutral and charged PAHs in the cloud chemistry and ionization balance is
investigated. The detection of HOC+ reactive ion towards the Horsehead PDR
proves the high ionization fraction of the outer UV irradiated regions, where
we derive a low [HCO+]/[HOC+]~75-200 abundance ratio. In the absence of PAHs,
we reproduce the observations with gas-phase metal abundances, [Fe+Mg+...],
lower than 4x10(-9) (with respect to H) and a cosmic-rays ionization rate of
zeta=(5+/-3)x10(-17) s(-1). The inclusion of PAHs modifies the ionization
fraction gradient and increases the required metal abundance. The ionization
fraction in the Horsehead edge follows a steep gradient, with a scale length of
~0.05 pc (or ~25''), from [e-]~10(-4) (or n_e ~ 1-5 cm(-3)) in the PDR to a few
times ~10(-9) in the core. PAH^- anions play a role in the charge balance of
the cold and neutral gas if substantial amounts of free PAHs are present ([PAH]
>10(-8)).Comment: 13 pages, 7 figures, 6 tables. Accepted for publication in A&A
(english not edited
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