6,788 research outputs found
Detection of CO+ in the nucleus of M82
We present the detection of the reactive ion CO+ towards the prototypical
starburst galaxy M82. This is the first secure detection of this short-lived
ion in an external galaxy. Values of [CO+]/[HCO+]>0.04 are measured across the
inner 650pc of the nuclear disk of M82. Such high values of the [CO+]/[HCO+]
ratio had only been previously measured towards the atomic peak in the
reflection nebula NGC7023. This detection corroborates that the molecular gas
reservoir in the M82 disk is heavily affected by the UV radiation from the
recently formed stars. Comparing the column densities measured in M82 with
those found in prototypical Galactic photon-dominated regions (PDRs), we need
\~20 clouds along the line of sight to explain our observations. We have
completed our model of the molecular gas chemistry in the M82 nucleus. Our PDR
chemical model successfully explains the [CO+]/[HCO+] ratios measured in the
M~82 nucleus but fails by one order of magnitude to explain the large measured
CO+ column densities (~1--4x10^{13} cm^{-2}). We explore possible routes to
reconcile the chemical model and the observations.Comment: 12 pages, 2 figure
Detection of CO+ toward the reflection nebula NGC 7023
We have detected CO+ toward the photon-dominated region (PDR) associated with
the reflection nebula NGC 7023. This is the first detection of CO+ in the
vicinity of a Be star. A CO+ column density of ~ 3E11 cm-2 has been derived
toward the PDR peak. We have, however, not detected CO+ in a well shielded
clump of the adjacent molecular cloud, where the CO+/HCO+ abundance ratio is at
least 100 times lower than in the PDR. Our results show, for the first time,
that CO^+ column densities as large as ~ 3E11 cm-2 can be produced in regions
with incident UV fields of just a few 1E3 (in units of Habing field) and
densities of ~ 1E5 cm-3. Furthermore, since the ionization potential of CO is
larger than 13.6 eV, our data rule out the direct photoionization of CO as a
significant CO+ formation mechanism.Comment: 14 pages, 1 Postscript figure, uses aasms4.sty. to appear in
Astrophysical Journal Letter
Molecular line probes of activity in galaxies
The use of specific tracers of the dense molecular gas phase can help to
explore the feedback of activity on the interstellar medium (ISM) in galaxies.
This information is a key to any quantitative assessment of the efficiency of
the star formation process in galaxies. We present the results of a survey
devoted to probe the feedback of activity through the study of the excitation
and chemistry of the dense molecular gas in a sample of local universe
starbursts and active galactic nuclei (AGNs). Our sample includes also 17
luminous and ultraluminous infrared galaxies (LIRGs and ULIRGs). From the
analysis of the LIRGs/ULIRGs subsample, published in Gracia-Carpio et al.(2007)
we find the first clear observational evidence that the star formation
efficiency of the dense gas, measured by the L_FIR/L_HCN ratio, is
significantly higher in LIRGs and ULIRGs than in normal galaxies. Mounting
evidence of overabundant HCN in active environments would even reinforce the
reported trend, pointing to a significant turn upward in the Kennicutt-Schmidt
law around L_FIR=10^11 L_sun. This result has major implications for the use of
HCN as a tracer of the dense gas in local and high-redshift luminous infrared
galaxies.Comment: 4 pages, 2 figures, contributed paper to Far-Infrared Workshop 07
(FIR 2007
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
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 Foggy Disks Surrounding Herbig Ae Stars: a Theoretical Study of the H2O Line Spectra
Water is a key species in many astrophysical environments, but it is
particularly important in proto-planetary disks. So far,observations of water
in these objects have been scarce, but the situation should soon change thanks
to the Herschel satellite. We report here a theoretical study of the water line
spectrum of a proto-planetary disk surrounding Ae stars. We show that several
lines will be observable with the HIFI instrument onboard the Herschel Space
Observatory. We predict that some maser lines could also be observable with
ground telescopes and we discuss how the predictions depend not only on the
adopted physical and chemical model but also on the set of collisional
coefficients used and on the H2 ortho to para ratio through its effect on
collisional excitation. This makes the water lines observations a powerful, but
dangerous -if misused- diagnostic tool.Comment: Accepted for publication in ApJ Letter
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