1,371 research outputs found
A new infrared band in the Interstellar and Circumstellar Clouds: C_4 or C_4H?
We report on the detection with the Infrared Space Observatory (ISO) of a
molecular band at 57.5 microns (174 cm^{-1}) in carbon-rich evolved stars and
in Sgr B2. Taking into account the chemistry of these objects the most
likelihood carrier is a carbon chain. We tentatively assign the band to the
nu_5 bending mode of C_4 for which a wavenumber of 170-172.4 cm^{-1} has been
derived in matrix experiments (Withey et al. 1991). An alternate carrier might
be C_4H, although the frequency of its lowest energy vibrational bending mode,
nu_7, is poorly known (130-226 cm^{-1}). If the carrier is C_4, the derived
maximum abundance is nearly similar to that found for C_3 in the interstellar
and circumstellar media by Cernicharo, Goicoechea & Caux (2000). Hence,
tetra-atomic carbon could be one of the most abundant carbon chain molecules in
these media.Comment: 11 pages, 1 figure, accepted in ApJ Letter
Molecular Carbon Chains and Rings in TMC-1
We present mapping results in several rotational transitions of HC3N, C6H,
both cyclic and linear C3H2 and C3H, towards the cyanopolyyne peak of the
filamentary dense cloud TMC-1 using the IRAM 30m and MPIfR 100m telescopes. The
spatial distribution of the cumulene carbon chain propadienylidene H2C3
(hereafter l-C3H2) is found to deviate significantly from the distributions of
the cyclic isomer c-C3H2, HC3N, and C6H which in turn look very similar. The
cyclic over linear abundance ratio of C3H2 increases by a factor of 3 across
the filament, with a value of 28 at the cyanopolyyne peak. This abundance ratio
is an order of magnitude larger than the range (3 to 5) we observed in the
diffuse interstellar medium. The cyclic over linear abundance ratio of C3H also
varies by ~2.5 in TMC-1, reaching a maximum value (13) close to the
cyanopolyyne peak. These behaviors might be related to competitive processes
between ion-neutral and neutral-neutral reactions for cyclic and linear
species.Comment: Accepted for publication in The Astrophysical Journal, part I. 24
pages, including 4 tables, 7 figures, and figure caption
The abundances of polyacetylenes towards CRL618
We present a mid-infrared high spectral resolution spectrum of CRL618 in the
frequency ranges 778-784 and 1227-1249 cm^-1 (8.01-8.15 and 12.75-12.85 um)
taken with the Texas Echelon-cross-Echelle Spectrograph (TEXES) and the
Infrared Telescope Facility (IRTF). We have identified more than 170
ro-vibrational lines arising from C2H2, HCN, C4H2, and C6H2. We have found no
unmistakable trace of C8H2. The line profiles display a complex structure
suggesting the presence of polyacetylenes in several components of the
circumstellar envelope (CSE). We derive total column densities of 2.5 10^17,
3.1 10^17, 2.1 10^17, 9.3 10^16 cm^-2, and < 5 10^16 cm^-2 for HCN, C2H2, C4H2,
C6H2, and C8H2, respectively. The observations indicate that both the
rotational and vibrational temperatures in the innermost CSE depend on the
molecule, varying from 100 to 350 K for the rotational temperatures and 100 to
500 K for the vibrational temperatures. Our results support a chemistry in the
innermost CSE based on radical-neutral reactions triggered by the intense UV
radiation field.Comment: 9 pages, 4 figures, 1 table; accepted for publication in The
Astrophysical Journa
Non--local radiative transfer in strongly inverted masers
Maser transitions are commonly observed in media exhibiting a large range of
densities and temperatures. They can be used to obtain information on the
dynamics and physical conditions of the observed regions. In order to obtain
reliable constraints on the physical conditions prevailing in the masing
regions, it is necessary to model the excitation mechanisms of the energy
levels of the observed molecules. We present a numerical method that enables us
to obtain self-consistent solutions for both the statistical equilibrium and
radiative transfer equations. Using the standard maser theory, the method of
Short Characteristics is extended to obtain the solution of the
integro-differential radiative transfer equation, appropriate to the case of
intense masing lines. We have applied our method to the maser lines of the H2O
molecule and we compare with the results obtained with a less accurate
approach. In the regime of large maser opacities we find large differences in
the intensity of the maser lines that could be as high as several orders of
magnitude. The comparison between the two methods shows, however, that the
effect on the thermal lines is modest. Finally, the effect introduced by rate
coefficients on the prediction of H2O masing lines and opacities is discussed,
making use of various sets of rate coefficients involving He, o-H2 and p-H2. We
find that the masing nature of a line is not affected by the selected
collisional rates. However, from one set to the other the modelled line
opacities and intensities can vary by up to a factor ~2 and ~10 respectively
Observational evidence of the formation of cyanopolyynes in CRL618 through the polimerization of HCN
The abundance ratio of consecutive members of the cyanopolyynes family has
been explored in CRL618 using data acquired in a complete line survey covering
the frequency range 81-356 GHz. The Jup range explored for the different
molecules is the following: 1 to 4 for HCN and HNC, 9 to 39 for HC3N, 31 to 133
for HC5N, and 72 to 85 for HC7N (not detected beyond Jup=85). The lowest
vibrationally excited state of HC7N (nu_15 at 62 cm^-1) has been tentatively
detected. Data analysis has been performed by extending our previous
geometrical and radiative transfer model of the slowly expanding envelope (SEE)
surrounding the compact central continuum source of CRL 618, that was
established from the study of rotational lines in several vibrationally excited
states of HC_3N. The new lines analyzed here require to model the high velocity
wind (HVW) component and the colder circumstellar gas, remnant of the AGB phase
of CRL618. The derived HC3N/HC5N and HC5N/HC7N abundance ratios from this set
of uniformly calibrated lines are between 3 and 6 in the different regions,
similar to standard values in the CSM and ISM, and consistent with previous
estimates obtained from ISO observations and chemical models. However, the
abundance ratios of HC3N, HC5N and HC7N with respect to HCN are at least two
orders of magnitude larger than those typical for AGB C-rich stars, such as
IRC+10216. This fact indicates that, in the short transition toward the
Planetary Nebula phase, HCN is quickly reprocessed into longer cyanopolyyne
chains. A similar behavior was previously found in this object for the
polyacetylenic chains (C(2n)H2).Comment: 8 figures, accepted in ApJ main journa
Detection of the linear radical HC4N in IRC+10216
We report the detection of the linear radical HC4N in the C-rich envelope of
IRC+10216. After HCCN, HC4N is the second member of the allenic chain family
HC_(2n)N observed in space. The column density of HC4N is found to be 1.5
10**12 cm**(-2). The abundance ratio HC2N/HC4N is 9, a factor of two larger
than the decrement observed for the cyanopolyynes HC$_(2n+1)N/HC_(2n+3)N.
Linear HC_4N has a 3-Sigma electronic ground state and is one of the 3
low-energy isomeric forms of this molecule. We have searched for the bent and
ringed HC4N isomers, but could only derive an upper limit to their column
densities of about 3 10**(12) cm**(-2).Comment: Preprint of 10 page
The slowly expanding envelope of CRL618 probed with HC3N rotational ladders
Lines from HC3N and isotopic substituted species in ground and vibrationally
excited states produce crowded millimeter and submillimeter wave spectra in the
C-rich protoplanetary nebula CRL618. The complete sequence of HC3N rotational
lines from J=9-8 to J=30-29 has been observed with the IRAM 30m telescope
toward this object. Lines from a total of 15 different vibrational states
(including the fundamental), with energies up to 1100 cm^-1, have been detected
for the main HC3N isotopomer. In addition, the CSO telescope has been used to
complement this study in the range J=31-30 to J=39-38, with detections in five
of these states, all of them below 700 cm^-1. Vibrationally excited HC3N
rotational lines exhibit P-Cygni profiles at 3 mm, evolving to pure emission
lineshapes at shorter wavelengths. This evolution of the line profile shows
little dependence on the vibrational state from which they rotational lines
arise. The absorption features are formed against the continuum emission, which
has been successfully characterized in this work due to the large frequency
coverage. The HC3N column density in front of the the continuum source has been
determined by comparing the output of an array of models to the data. The best
fits are obtained for column densities in the range 2.0-3.5 10$^17 cm^-2,
consistent with previous estimates from ISO data, and TK in the range 250 to
275 K, in very good agreement with estimates made from the same ISO data.Comment: 24 pages, 7 figures, accepted in ApJ part I (Jul. 8, 2004
Star Formation in the Trifid Nebula
We have obtained maps of the 1.25mm thermal dust emission and the molecular
gas emission over a region of 20' by 10' arcmin around the Trifid Nebula (M20),
with the IRAM 30m and the CSO telescopes as well as in the mid-infrared
wavelength with ISO and SPITZER. Our survey is sensitive to features down to
N(H2) \sim 10^{22} cm-2 in column density. The cloud material is distributed in
fragmented dense gas filaments (n(H2) \sim 1000 cm-3) with sizes ranging from 1
to 10 pc. A massive filament, WF, with properties typical of Infra Red Dark
Clouds, connects M20 to the W28 supernova remnant. These filaments pre-exist
the formation of the Trifid and were originally self-gravitating. The fragments
produced are very massive (100 Msun or more) and are the progenitors of the
cometary globules observed at the border of the HII region. We could identify
33 cores, 16 of which are currently forming stars. They are usually
gravitationally unbound and have low masses of a few Msun. The densest starless
cores (several 10^5 cm-3) may be the site for the next generation of stars. The
physical gas and dust properties of the cometary globules have been studied in
detail and have been found very similar. They all are forming stars. Several
intermediate-mass protostars have been detected in the cometary globules and in
the deeply embedded cores. Evidence of clustering has been found in the shocked
massive cores TC3-TC4-TC5. M20 is a good example of massive-star forming region
in a turbulent, filamentary molecular cloud. Photoionization appears to play a
minor role in the formation of the cores. The observed fragmentation is well
explained by MHD-driven instabilities and is usually not related to M20. We
propose that the nearby supernova remnant W28 could have triggered the
formation of protostellar clusters in nearby dense cores of the Trifid.Comment: 16 pages, 24 figures, 5 Tables To appear in Astronomy and
Astrophysic
Far-Infrared detection of H2D+ toward Sgr B2
We report on the first far-IR detection of H2D+, using the Infrared Space
Observatory, in the line of sight toward Sgr B2 in the galactic center. The
transition at 126.853 um connecting the ground level of o-H2D+, 1_1,1 with the
the 2_1,2 level at 113 K, is observed in absorption against the continuum
emission of the cold dust of the source. The line is broad, with a total
absorption covering 350 km s^-1, i.e., similar to that observed in the
fundamental transitions of H2O, OH and CH at ~179, 119 and 149 um respectively.
For the physical conditions of the different absorbing clouds the H2D+ column
density ranges from 2 to 5x10^13 cm^-2, i.e., near an order of magnitude below
the upper limits obtained from ground based submillimeter telescopes. The
derived H2D+ abundance is of a few 10^-10, which agrees with chemical models
predictions for a gas at a kinetic temperature of ~20K.Comment: Accepted in ApJ letters. Non edite
A spectroscopic survey of Orion KL between 41.5 and 50 GHz
Orion KL is one of the most frequently observed sources in the Galaxy, and
the site where many molecular species have been discovered for the first time.
With the availability of powerful wideband backends, it is nowadays possible to
complete spectral surveys in the entire mm-range to obtain a spectroscopically
unbiased chemical picture of the region. In this paper we present a sensitive
spectral survey of Orion KL, made with one of the 34m antennas of the Madrid
Deep Space Communications Complex in Robledo de Chavela, Spain. The spectral
range surveyed is from 41.5 to 50 GHz, with a frequency spacing of 180 kHz
(equivalent to about 1.2 km/s, depending on the exact frequency). The rms
achieved ranges from 8 to 12 mK. The spectrum is dominated by the J=1-0 SiO
maser lines and by radio recombination lines (RRLs), which were detected up to
Delta_n=11. Above a 3-sigma level, we identified 66 RRLs and 161 molecular
lines corresponding to 39 isotopologues from 20 molecules; a total of 18 lines
remain unidentified, two of them above a 5-sigma level. Results of radiative
modelling of the detected molecular lines (excluding masers) are presented. At
this frequency range, this is the most sensitive survey and also the one with
the widest band. Although some complex molecules like CH_3CH_2CN and CH_2CHCN
arise from the hot core, most of the detected molecules originate from the low
temperature components in Orion KL.Comment: Accepted for Astronomy and Astrophysics. 29 pages, 5 tables, 6
figure
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