309 research outputs found
Methanol detection in M82
We present a multilevel study of the emission of methanol, detected for the
first time in this galaxy, and discuss the origin of its emission. The high
observed methanol abundance of a few 10^-9 can only be explained if injection
of methanol from dust grains is taken into account. While the overall
[CH3OH]/[NH3] ratio is much larger than observed towards other starbursts, the
dense high excitation component shows a similar value to that found in NGC 253
and Maffei 2. Our observations suggest the molecular material in M 82 to be
formed by dense warm cores, shielded from the UV radiation and similar to the
molecular clouds in other starbursts, surrounded by a less dense
photodissociated halo. The dense warm cores are likely the location of recent
and future star formation within M 82.Comment: Accepted for publication in A&A Letter
A Molecular Counterpart to the Herbig-Haro 1-2 Flow
We present high angular resolution (12"-24") and high sensitivity 12CO and
13CO J=2-1 and J=1-0 observations of the HH 1-2 outflow. The observations show
the molecular counterpart, moving with a velocity of approx. 30 km/s, of the
optical bipolar system driven by the VLA 1 embedded source. Along the optical
jet there are certain regions where the molecular gas reaches deprojected
velocities of 100-200 km/s, and that we interpret as the molecular jet. The
bipolar CO outflow has a length of approx. 260" with a curved morphology
towards the North where it extends beyond the HH 1 object (approx. 120") .
Two new molecular outflows have been detected, one arising from IRAS
05339-0647 which excites the HH 147 optical flow and another powered by VLA 2
which drives the HH 144 optical outflow. The molecular outflow driven by the
VLA 3 source is also clearly detected and spatially resolved from the VLA 1
main outflow.Comment: 14 pages, 4 figures, accepted ApJLet
Organic Molecules in the Galactic Center. Hot Core Chemistry without Hot Cores
We study the origin of large abundances of complex organic molecules in the
Galactic center (GC). We carried out a systematic study of the complex organic
molecules CH3OH, C2H5OH, (CH3)2O, HCOOCH3, HCOOH, CH3COOH, H2CO, and CS toward
40 GC molecular clouds. Using the LTE approximation, we derived the physical
properties of GC molecular clouds and the abundances of the complex
molecules.The CH3OH abundance between clouds varies by nearly two orders of
magnitude from 2.4x10^{-8} to 1.1x10^{-6}. The abundance of the other complex
organic molecules relative to that of CH3OH is basically independent of the
CH3OH abundance, with variations of only a factor 4-8. The abundances of
complex organic molecules in the GC are compared with those measured in hot
cores and hot corinos, in which these complex molecules are also abundant. We
find that both the abundance and the abundance ratios of the complex molecules
relative to CH3OH in hot cores are similar to those found in the GC clouds.
However, hot corinos show different abundance ratios than observed in hot cores
and in GC clouds. The rather constant abundance of all the complex molecules
relative to CH3OH suggests that all complex molecules are ejected from grain
mantles by shocks. Frequent (similar 10^{5}years) shocks with velocities >6km/s
are required to explain the high abundances in gas phase of complex organic
molecules in the GC molecular clouds. The rather uniform abundance ratios in
the GC clouds and in Galactic hot cores indicate a similar average composition
of grain mantles in both kinds of regions. The Sickle and the Thermal Radio
Arches, affected by UV radiation, show different relative abundances in the
complex organic molecules due to the differentially photodissociation of these
molecules.Comment: 18 pages, 10 Postscript figures, uses aa.cls, aa.bst, 10pt.rtx,
natbib.sty, revsymb.sty revtex4.cls, aps.rtx and aalongtabl.sty. Accepted in
A&A 2006. version 2. relocated figures and tables. Language editor
suggestions. added reference
Coupling the dynamics and the molecular chemistry in the Galactic center
The physical conditions of the Galactic center (GC) clouds moving with
non-circular velocities are not well-known. We have studied the physical
conditions of these clouds with the aim of better understanding the origin of
the outstanding physical conditions of the GC molecular gas and the possible
effect of the large scale dynamics on these physical conditions.Using published
CO(1-0) data, we have selected a set of clouds belonging to all the kinematical
components seen in the longitude-velocity diagram of the GC. We have done a
survey of dense gas in all the components using the J=2-1 lines of CS and SiO
as tracers of high density gas and shock chemistry. We have detected CS and SiO
emission in all the kinematical components. The gas density and the SiO
abundance of the clouds in non-circular orbits are similar those in the nuclear
ring (GCR). Therefore, in all the kinematical components there are dense clouds
that can withstand the tidal shear. However, there is no evidence of star
formation outside the GCR. The high relative velocity and shear expected in the
dust-lanes along the bar major axis could inhibit the star formation process,
as observed in other galaxies. The high SiO abundances derived in the
non-circular velocity clouds are likely due to the large-scale shocks that
created the dust lanesComment: One figure as an independent PDF file. Accepted by A&
On the history of the interplay between HD 56925 and NGC 2359
NGC 2359 is an optical nebula excited by the powerful wind and the radiation of the Wolf-Rayet star HD 56925. We have investigated the interaction between this massive star and the surrounding neutral gas by analyzing the large-scale 21cm-HI emission and by mapping the nebula in the J = 1-0 and the J = 2-1 lines of CO. We found a conspicuous (70 x 37 pc) HI shell, expanding at 12 km/s, likely produced during the main-sequence phase of the star. The molecular gas towards NGC 2359 shows three velocity components. Two of these components, A1 and A2, have narrow linewidths (1-2 km/s) and radial velocities of 35-38 and 64-68 km/s, respectively. The third component is detected at radial velocities between 50 and 58 km/s and has a broader profile (up to 5.5 km/s). Furthermore, this component is morphologicaly related with the nebula and has a velocity gradient of a few km/s. We have also estimated the physical parameters of the molecular gas by means of a LVG modelling of the CO emission. The gas projected onto the southern HII region of the nebula has low CO column density and is rather hot, probably up to 80 K. Several profiles of the 13CO J = 1-0 line near the peak of the emission, together with a weak emission bridge between the broad and one of the narrow components (component A2), suggest the presence of a shock front acting in the southern part of the nebula. This shock was likely produced in a previous RSG/LBV phase of HD 56925
High-resolution study of a star-forming cluster in the Cep-A HW2 region
Due to its relatively small distance (725 pc), the Cepheus A East
star-forming region is an ideal laboratory to study massive star formation
processes. Based on its morphology, it has been suggested that the flattened
molecular gas distribution around the YSO HW2 may be a 350-AU-radius massive
protostellar disk. Goal of our work is to ascertain the nature of this
structure. We have employed the Plateau de Bure Interferometer to acquire
(sub-)arcsecond-resolution imaging of high-density and shock tracers, such as
methyl cyanide (CH3CN) and silicon monoxide (SiO), towards the HW2 position. On
the 1-arcsecond (about 725 AU) scale, the flattened distribution of molecular
gas around HW2 appears to be due to the projected superposition, on the plane
of the sky, of at least three protostellar objects, of which at least one is
powering a molecular outflow at a small angle with respect to the line of
sight. The presence of a protostellar disk around HW2 is not ruled out, but
such structure is likely to be detected on a smaller spatial scale, or using
different molecular tracers.Comment: 6 pages, 5 figures, accepted for publication in Astronomy &
Astrophysic
Photodissociation chemistry footprints in the Starburst galaxy NGC 253
We report the first detection of PDR molecular tracers, namely HOC+, and CO+,
and confirm the detection of the also PDR tracer HCO towards the starburst
galaxy NGC 253, claimed to be mainly dominated by shock heating and in an
earlier stage of evolution than M 82, the prototypical extragalactic PDR. Our
CO+ detection suffers from significant blending to a group of transitions of
13CH3OH, tentatively detected for the first time in the extragalactic
interstellar medium. These species are efficiently formed in the highly UV
irradiated outer layers of molecular clouds, as observed in the late stage
nuclear starburst in M 82. The molecular abundance ratios we derive for these
molecules are very similar to those found in M 82. This strongly supports the
idea that these molecules are tracing the PDR component associated with the
starburst in the nuclear region of NGC 253. A comparison with the predictions
of chemical models for PDRs shows that the observed molecular ratios are
tracing the outer layers of UV illuminated clouds up to two magnitudes of
visual extinction. Chemical models, which include grain formation and
photodissociation of HNCO, support the scenario of a photo-dominated chemistry
as an explanation to the abundances of the observed species. From this
comparison we conclude that the molecular clouds in NGC 253 are more massive
and with larger column densities than those in M 82, as expected from the
evolutionary stage of the starbursts in both galaxies.Comment: 32 pages, 4 figures, Published in Ap
On the chemical ladder of esters. Detection and formation of ethyl formate in the W51 e2 hot molecular core
The detection of organic molecules with increasing complexity and potential
biological relevance is opening the possibility to understand the formation of
the building blocks of life in the interstellar medium. One of the families of
molecules with astrobiological interest are the esters, whose simplest member,
methyl formate, is rather abundant in star-forming regions. The next step in
the chemical complexity of esters is ethyl formate, CHOCHO. Only two
detections of this species have been reported so far, which strongly limits our
understanding of how complex molecules are formed in the interstellar medium.
We have searched for ethyl formate towards the W51 e2 hot molecular core, one
of the most chemically rich sources in the Galaxy and one of the most promising
regions to study prebiotic chemistry, especially after the recent discovery of
the PO bond, key in the formation of DNA. We have analyzed a spectral line
survey towards the W51 e2 hot molecular core, which covers 44 GHz in the 1, 2
and 3 mm bands, carried out with the IRAM 30m telescope. We report the
detection of the trans and gauche conformers of ethyl formate. A Local
Thermodynamic Equilibrium analysis indicates that the excitation temperature is
7810 K and that the two conformers have similar source-averaged column
densities of (2.00.3)10 cm and an abundance of
10. We compare the observed molecular abundances of ethyl formate
with different competing chemical models based on grain surface and gas-phase
chemistry. We propose that grain-surface chemistry may have a dominant role in
the formation of ethyl formate (and other complex organic molecules) in hot
molecular cores, rather than reactions in the gas phase.Comment: Accepted in A&A; 11 pages, 6 figures, 7 Table
First detections of extragalactic SO2, NS and NO
We report the first detections of SO_2, NS and NO in an extragalactic source,
the nucleus of the starburst galaxy NGC 253. Five SO_2 transitions, three
groups of hyperfine components of NO and five of NS were detected. All three
species show large abundances averaged over the inner 200 pc of NGC 253. With a
relative abundance of a few 10^-7, the emission of the NO molecule is similar
or even larger than that found in Galactic star forming regions. The derived
relative molecular abundances for each molecule have been compared with those
of prototypical Galactic molecular clouds. These results seem to confirm that
large scale shocks dominate the chemistry of these molecules in the nucleus of
NGC 253, ruling out a chemistry dominated by PDRs for the bulk of the gas.Comment: Accepted by A&A, 4 pages, 6 figure
Warm H2 in the Galactic center region
We present ISO observations of several H2 pure-rotational lines (from S(0) to
S(5)) towards a sample of 16 molecular clouds distributed along the central ~
500 pc of the Galaxy. We also present C18O and 13CO J=1->0 and J=2->1
observations of these sources made with the IRAM-30m telescope. With the CO
data we derive H2 densities of 10e(3.5-4.0) cm-3 and H2 column densities of a
few 10e22 cm-2. We have corrected the H2 data for ~ 30 magnitudes of visual
extinction using a self-consistent method. In every source, we find that the H2
emission exhibits a large temperature gradient. The S(0) and S(1) lines trace
temperatures (T) of ~150 K while the S(4) and S(5) lines indicate temperatures
of ~ 600K. The warm H2 column density is typically ~1-2 x 10e22 cm-2, and is
predominantly gas with T=150 K. This is the first direct estimate of the total
column density of the warm molecular gas in the Galactic center region. These
warm H2 column densities represent a fraction of ~ 30 % of the gas traced by
the CO isotopes emission. The cooling by H2 in the warm component is comparable
to that by CO. Comparing our H2 and CO data with available ammonia NH3
observations from literature one obtains relatively high NH3 abundances of a
few 10e(-7) in both the warm and the cold gas. A single shock or
Photo-Dissociation Region (PDR) cannot explain all the observed H2 lines.
Alternatives for the heating mechanisms are discussed.Comment: 14 pages including figures, to be published in A&
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