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

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

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

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

Spectroscopic parameters for silacyclopropynylidene, SiC2_2, from extensive astronomical observations toward CW Leo (IRC +10216) with the Herschel satellite

A molecular line survey has been carried out toward the carbon-rich asymptotic giant branch star CW Leo employing the HIFI instrument on board of the Herschel satellite. Numerous features from 480 GHz to beyond 1100 GHz could be assigned unambiguously to the fairly floppy SiC$_2$ molecule. However, predictions from laboratory data exhibited large deviations from the observed frequencies even after some lower frequency data from this survey were incorporated into a fit. Therefore, we present a combined fit of all available laboratory data together with data from radio-astronomical observations.Comment: 7 pages, 1 figure, J. Mol. Spectrosc., appeared; CDMS links corrected (version 2; current version: 3; may be updated later this year

Electronic spectra of linear HC5_5H and cumulene carbene H2_2C5_5

The $1 ^3\Sigma_u^- \leftarrow X^3\Sigma_g^-$ transition of linear HC$_5$H (A) has been observed in a neon matrix and gas phase. The assignment is based on mass-selective experiments, extrapolation of previous results of the longer HC$_{2n+1}$H homologues, and density functional and multi-state CASPT2 theoretical methods. Another band system starting at 303 nm in neon is assigned as the $1 ^1 A_1 \leftarrow X ^1 A_1$ transition of the cumulene carbene pentatetraenylidene H$_2$C$_5$ (B).Comment: 7 pages, 4 figures, 5 table

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

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