The abundance of S- and Si-bearing molecules in O-rich circumstellar envelopes of AGB stars.

AIMS: We aim to determine the abundances of SiO, CS, SiS, SO, and SO2 in a large sample of oxygen-rich asymptotic giant branch (AGB) envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in these environments. METHODS: We surveyed a sample of 30 oxygen-rich AGB stars in the λ 2 mm band using the IRAM 30m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. RESULTS: We detected SiO in all 30 targeted envelopes, as well as CS, SiS, SO, and SO2 in 18, 13, 26, and 19 sources, respectively. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10-6 M⊙ yr-1, whereas it is detected in all envelopes with mass loss rates above that threshold. From a comparison with a previous, similar study on C-rich sources, it becomes evident that the fractional abundances of CS and SiS show a marked differentiation between C-rich and O-rich sources, being two orders of magnitude and one order of magnitude more abundant in C-rich sources, respectively, while the fractional abundance of SiO turns out to be insensitive to the C/O ratio. The abundance of SiO in O-rich envelopes behaves similarly to C-rich sources, that is, the denser the envelope the lower its abundance. A similar trend, albeit less clear than for SiO, is observed for SO in O-rich sources. CONCLUSIONS: The marked dependence of CS and SiS abundances on the C/O ratio indicates that these two molecules form more efficiently in C- than O-rich envelopes. The decline in the abundance of SiO with increasing envelope density and the tentative one for SO indicate that SiO and possibly SO act as gas-phase precursors of dust in circumstellar envelopes around O-rich AGB stars.

Study of CS, SiO, and SiS abundances in carbon star envelopes: Assessing their role as gas-phase precursors of dust.

AIMS: We aim to determine the abundances of CS, SiO, and SiS in a large sample of carbon star envelopes covering a wide range of mass loss rates to investigate the potential role that these molecules could play in the formation of dust in the surroundings of the central AGB star. METHODS: We surveyed a sample of 25 carbon-rich AGB stars in the λ 2 mm band, more concretely in the J = 3 - 2 line of CS and SiO, and in the J = 7 - 6 and J = 8 - 7 lines of SiS, using the IRAM 30 m telescope. We performed excitation and radiative transfer calculations based on the large velocity gradient (LVG) method to model the observed lines of the molecules and to derive their fractional abundances in the observed envelopes. We also assessed the effect of infrared pumping in the excitation of the molecules. RESULTS: We detected CS in all 25 targeted envelopes, SiO in 24 of them, and SiS in 17 sources. Remarkably, SiS is not detected in any envelope with a mass loss rate below 10-6 M⊙ yr-1 while it is detected in all envelopes with mass loss rates above that threshold. We found that CS and SiS have similar abundances in carbon star envelopes, while SiO is present with a lower abundance. We also found a strong correlation in which the denser the envelope, the less abundant are CS and SiO. The trend is however only tentatively seen for SiS in the range of high mass loss rates. Furthermore, we found a relation in which the integrated flux of the MgS dust feature at 30 µm increases as the fractional abundance of CS decreases. CONCLUSIONS: The decline in the fractional abundance of CS with increasing density could be due to gas-phase chemistry in the inner envelope or to adsorption onto dust grains. The latter possibility is favored by a correlation between the CS fractional abundance and the 30 µm feature, which suggests that CS is efficiently incorporated onto MgS dust around C-rich AGB stars. In the case of SiO, the observed abundance depletion with increasing density is most likely caused by an efficient incorporation onto dust grains. We conclude that CS, SiO (very likely), and SiS (tentatively) are good candidates to act as gas-phase precursors of dust in C-rich AGB envelopes.

IRC +10216 as a spectroscopic laboratory: improved rotational constants for SiC2, its isotopologues, and Si2C.

This work presents a detailed analysis of the laboratory and astrophysical spectral data available for 28SiC2, 29SiC2,30SiC2, Si13CC, and Si2C. New data on the rotational lines of these species between 70 and 350 GHz have been obtained with high spectral resolution (195 kHz) with the IRAM 30m telescope in the direction of the circumstellar envelope IRC +10216. Frequency measurements can reach an accuracy of 50 kHz for features observed with a good signal to noise ratio. From the observed astrophysical lines and the available laboratory data new rotational and centrifugal distortion constants have been derived for all the isotopologues of SiC2, allowing to predict their spectrum with high accuracy in the millimeter and submillimeter domains. Improved rotational and centrifugal distortion constants have also been obtained for disilicon carbide, Si2C. This work shows that observations of IRC +10216 taken with the IRAM 30m telescope, with a spectral resolution of 195 kHz, can be used for any molecular species detected in this source to derive, or improve, its rotational constants. Hence, IRC +10216 in addition to be one the richest sources in molecular species in the sky, can also be used as a state-of-the-art spectroscopy laboratory in the millimeter and submillimeter domains.

Time-dependent molecular emission in IRC+10216.

CONTEXT: The variability in IRC+10216, the envelope of the asymptotic giant branch (AGB) star CW Leo, has attracted increasing attention in recent years. Studying the details of this variability in the molecular emission required a systematic observation program. AIMS: We aim to reveal and characterize the periodical variability of the rotational lines from several molecules and radicals in IRC+10216, and to compare it with previously reported IR variability. METHODS: We carried out systematic monitoring within the ~80 to 116 GHz frequency range with the IRAM 30m telescope. RESULTS: We report on the periodical variability in IRC+10216 of several rotational lines from the following molecules and radicals: HC3N, HC5N, CCH, C4H, C5H, and CN. The analysis of the variable molecular lines provides periods that are consistent with previously reported IR variability, and interesting phase lags are revealed that point toward radiative transfer and pumping, rather than chemical effects. CONCLUSIONS: This study indicates that observations of several lines of a given molecule have to be performed simultaneously or at least at the same phase in order to avoid erroneous interpretation of the data. In particular, merging ALMA data from different epochs may prove to be difficult, as shown by the example of the variability we studied here. Moreover, radiative transfer codes have to incorporate the effect of population variability in the rotational levels in CW Leo.

Clues to NaCN formation.

CONTEXT: ALMA is providing us essential information on where certain molecules form. Observing where these molecules emission arises from, the physical conditions of the gas, and how this relates with the presence of other species allows us to understand the formation of many species, and to significantly improve our knowledge of the chemistry that occurs in the space. AIMS: We studied the molecular distribution of NaCN around IRC +10216, a molecule detected previously, but whose origin is not clear. High angular resolution maps allow us to model the abundance distribution of this molecule and check suggested formation paths. METHODS: We modeled the emission of NaCN assuming local thermal equilibrium (LTE) conditions. These profiles were fitted to azimuthal averaged intensity profiles to obtain an abundance distribution of NaCN. RESULTS: We found that the presence of NaCN seems compatible with the presence of CN, probably as a result of the photodissociation of HCN, in the inner layers of the ejecta of IRC +10216. However, similar as for CH3CN, current photochemical models fail to reproduce this CN reservoir. We also found that the abundance peak of NaCN appears at a radius of 3 × 1015cm, approximately where the abundance of NaCl, suggested to be the parent species, starts to decay. However, the abundance ratio shows that the NaCl abundance is lower than that obtained for NaCN. We expect that the LTE assumption might result in NaCN abundances higher than the real ones. Updated photochemical models, collisional rates, and reaction rates are essential to determine the possible paths of the NaCN formation.

Discovery of methyl silane and confirmation of silyl cyanide in IRC +10216.

We report the discovery in space of methyl silane, CH3SiH3, from observations of ten rotational transitions between 80 and 350 GHz (Ju from 4 to 16) with the IRAM 30 m radio telescope. The molecule was observed in the envelope of the C-star IRC +10216. The observed profiles and our models for the expected emission of methyl silane suggest that the it is formed in the inner zones of the circumstellar envelope, 1-40 R*, with an abundance of (0.5-1) × 10-8 relative to H2. We also observed several rotational transitions of silyl cyanide (SiH3CN), confirming its presence in IRC +10216 in particular, and in space in general. Our models indicate that silyl cyanide is also formed in the inner regions of the envelope, around 20 R*, with an abundance relative to H2 of 6×10-10. The possible formation mechanisms of both species are discussed. We also searched for related chemical species but only upper limits could be obtained.