920 research outputs found
A high resolution study of complex organic molecules in hot cores
We present the results of a line identification analysis using data from the
IRAM Plateau de Bure Inferferometer, focusing on six massive star-forming hot
cores: G31.41+0.31, G29.96-0.02, G19.61-0.23, G10.62-0.38, G24.78+0.08A1 and
G24.78+0.08A2. We identify several transitions of vibrationally excited methyl
formate (HCOOCH) for the first time in these objects as well as transitions
of other complex molecules, including ethyl cyanide (CHCN), and
isocyanic acid (HNCO). We also postulate a detection of one transition of
glycolaldehyde (CH(OH)CHO) in two new hot cores. We find G29.96-0.02,
G19.61-0.23, G24.78+0.08A1 and 24.78+0.08A2 to be chemically very similar.
G31.41+0.31, however, is chemically different: it manifests a larger chemical
inventory and has significantly larger column densities. We suggest that it may
represent a different evolutionary stage to the other hot cores in the sample,
or it may surround a star with a higher mass. We derive column densities for
methyl formate in G31.41+0.31, using the rotation diagram method, of
10 cm and a T of 170 K. For G29.96-0.02,
G24.78+0.08A1 and G24.78+0.08A2, glycolaldehyde, methyl formate and methyl
cyanide all seem to trace the same material and peak at roughly the same
position towards the dust emission peak. For G31.41+0.31, however,
glycolaldehyde shows a different distribution to methyl formate and methyl
cyanide and seems to trace the densest, most compact inner part of hot cores.Comment: Accepted to MNRA
First ALMA maps of HCO, an important precursor of complex organic molecules, towards IRAS 16293-2422
The formyl radical HCO has been proposed as the basic precursor of many
complex organic molecules such as methanol (CHOH) or glycolaldehyde
(CHOHCHO). Using ALMA, we have mapped, for the first time at high angular
resolution (1, 140 au), HCO towards the Solar-type
protostellar binary IRAS 162932422, where numerous complex organic molecules
have been previously detected. We also detected several lines of the chemically
related species HCO, CHOH and CHOHCHO. The observations revealed
compact HCO emission arising from the two protostars. The line profiles also
show redshifted absorption produced by foreground material of the circumbinary
envelope that is infalling towards the protostars. Additionally, IRAM 30m
single-dish data revealed a more extended HCO component arising from the common
circumbinary envelope. The comparison between the observed molecular abundances
and our chemical model suggests that whereas the extended HCO from the envelope
can be formed via gas-phase reactions during the cold collapse of the natal
core, the HCO in the hot corinos surrounding the protostars is predominantly
formed by the hydrogenation of CO on the surface of dust grains and subsequent
thermal desorption during the protostellar phase. The derived abundance of HCO
in the dust grains is high enough to produce efficiently more complex species
such as HCO, CHOH, and CHOHCHO by surface chemistry. We found that
the main formation route of CHOHCHO is the reaction between HCO and
CHOH.Comment: Accepted in Monthly Notices of the Royal Astronomical Society; 19
pages, 12 figures, 7 table
Extended warm gas in Orion KL as probed by methyl cyanide
In order to study the temperature distribution of the extended gas within the
Orion Kleinmann-Low nebula, we have mapped the emission by methyl cyanide
(CH3CN) in its J=6_K-5_K, J=12_K-11_K, J=13_K-12_K, and J=14_K-13_K transitions
at an average angular resolution of ~10 arcsec (22 arcsec for the 6_K-5_K
lines), as part of a new 2D line survey of this region using the IRAM 30m
telescope. These fully sampled maps show extended emission from warm gas to the
northeast of IRc2 and the distinct kinematic signatures of the hot core and
compact ridge source components. We have constructed population diagrams for
the four sets of K-ladder emission lines at each position in the maps and have
derived rotational excitation temperatures and total beam-averaged column
densities from the fitted slopes. In addition, we have fitted LVG model spectra
to the observations to determine best-fit physical parameters at each map
position, yielding the distribution of kinetic temperatures across the region.
The resulting temperature maps reveal a region of hot (T > 350 K) material
surrounding the northeastern edge of the hot core, whereas the column density
distribution is more uniform and peaks near the position of IRc2. We attribute
this region of hot gas to shock heating caused by the impact of outflowing
material from active star formation in the region, as indicated by the presence
of broad CH3CN lines. This scenario is consistent with predictions from C-shock
chemical models that suggest that gas-phase methyl cyanide survives in the
post-shock gas and can be somewhat enhanced due to sputtering of grain mantles
in the passing shock front.Comment: 24 pages, 20 figures, accepted for publication in A&
3D-PDR: a new three-dimensional astrochemistry code for treating photodissociation regions
Photodissociation regions (PDRs) define the transition zone between an ionized and a dark molecular region. They consist of neutral gas which interacts with far-ultraviolet radiation and are characterized by strong infrared line emission. Various numerical codes treating one-dimensional PDRs have been developed in the past, simulating the complexity of chemical reactions occurring and providing a better understanding of the structure of a PDR. In this paper we present the three-dimensional code, 3D-PDR, which can treat PDRs of arbitrary density distribution. The code solves the chemistry and the thermal balance self-consistently within a given three-dimensional cloud. It calculates the total heating and cooling functions at any point in a given PDR by adopting an escape probability method. It uses a HEALPIx-based ray tracing scheme to evaluate the attenuation of the far-ultraviolet radiation in the PDR and the propagation of the far-infrared/submm line emission out of the PDR. We present benchmarking results and apply 3D-PDR to (i) a uniform-density spherical cloud interacting with a plane-parallel external radiation field, (ii) a uniform-density spherical cloud interacting with a two-component external radiation field and (iii) a cometary globule interacting with a plane-parallel external radiation field. We find that the code is able to reproduce the benchmarking results of various other one-dimensional numerical codes treating PDRs. We also find that the accurate treatment of the radiation field in the fully three-dimensional treatment of PDRs can in some cases leads to different results when compared to a standard one-dimensional treatment
The influence of cosmic rays in the circumnuclear molecular gas of NGC1068
We surveyed the circumnuclear disk of the Seyfert galaxy NGC1068 between the
frequencies 86.2 GHz and 115.6 GHz, and identified 17 different molecules.
Using a time and depth dependent chemical model we reproduced the observational
results, and show that the column densities of most of the species are better
reproduced if the molecular gas is heavily pervaded by a high cosmic ray
ionization rate of about 1000 times that of the Milky Way. We discuss how
molecules in the NGC1068 nucleus may be influenced by this external radiation,
as well as by UV radiation fields.Comment: 6 pages. Conference proceeding for the workshop on "Cosmic-ray
induced phenomenology in star-forming environments" held in Sant Cugat,
Spain, on April 16-19, 201
N2H+ depletion in the massive protostellar cluster AFGL 5142
We aim at investigating with high angular resolution the NH3/N2H+ ratio
toward the high-mass star-forming region AFGL 5142 in order to study whether
this ratio behaves similarly to the low-mass case, for which the ratio
decreases from starless cores to cores associated with YSOs. CARMA was used to
observe the 3.2 mm continuum and N2H+(1-0) emission. We used NH3(1,1) and
(2,2), HCO+(1-0) and H13CO+(1-0) data from the literature and we performed a
time-dependent chemical modeling of the region. The 3.2 mm continuum emission
reveals a dust condensation of ~23 Msun associated with the massive YSOs,
deeply embedded in the strongest NH3 core (hereafter central core). The N2H+
emission reveals two main cores, the western and eastern core, located to the
west and to the east of the mm condensation, and surrounded by a more extended
and complex structure of ~0.5 pc. Toward the central core the N2H+ emission
drops significantly, indicating a clear chemical differentiation in the region.
We found low values of the NH3/N2H+ ratio ~50-100 toward the western/eastern
cores, and high values up to 1000 in the central core. The chemical model
indicates that density, and in particular temperature, are key parameters in
determining the NH3/N2H+ ratio. The high density and temperature reached in the
central core allow molecules like CO to evaporate from grain mantles. The CO
desorption causes a significant destruction of N2H+, favoring the formation of
HCO+. This result is supported by our observations, which show that N2H+ and
HCO+ are anticorrelated in the central core. The observed values of the
NH3/N2H+ ratio in the central core can be reproduced by our model for times
t~4.5-5.3x10^5 yr (central) and t~10^4-3x10^6 yr (western/eastern). The
NH3/N2H+ ratio in AFGL 5142 does not follow the same trend as in regions of
low-mass star formation mainly due to the high temperature reached in hot
cores.Comment: Accepted for publication in A&A. 14 pages, 9 Figures, 5 Table
The clumpy structure of the chemically active L1157 outflow
We present high spatial resolution maps, obtained with the Plateau de Bure
Interferometer, of the blue lobe of the L1157 outflow. We observed four lines
at 3 mm, namely CH3OH (2_K-1_K), HC3N (11-10), HCN (1-0) and OCS (7-6).
Moreover, the bright B1 clump has also been observed at better spatial
resolution in CS (2-1), CH3OH (2_1-1_1)A-, and 34SO (3_2-2_1). These high
spatial resolution observations show a very rich structure in all the tracers,
revealing a clumpy structure of the gas superimposed to an extended emission.
In fact, the three clumps detected by previous IRAM-30m single dish
observations have been resolved into several sub-clumps and new clumps have
been detected in the outflow. The clumps are associated with the two cavities
created by two shock episodes driven by the precessing jet. In particular, the
clumps nearest the protostar are located at the walls of the younger cavity
with a clear arch-shape form while the farthest clumps have slightly different
observational characteristics indicating that they are associated to the older
shock episode. The emission of the observed species peaks in different part of
the lobe: the east clumps are brighter in HC3N (11-10), HCN (1-0) and CS (2-1)
while the west clumps are brighter in CH3OH(2_K-1_K), OCS (7-6) and 34SO
(3_2-2_1). This peak displacement in the line emission suggests a variation of
the physical conditions and/or the chemical composition along the lobe of the
outflow at small scale, likely related to the shock activity and the precession
of the outflow. In particular, we observe the decoupling of the silicon
monoxide and methanol emission, common shock tracers, in the B1 clump located
at the apex of the bow shock produced by the second shock episode.Comment: 11 pages, 8 figures, accepted for publication in the MNRA
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