No abstract

No abstract

The interstellar medium is enriched primarily by matter ejected from old, evolved stars. The outflows from these stars create spherical envelopes, which foster gas-phase chemistry. The chemical complexity in circumstellar shells was originally thought to be dominated by the elemental carbon to oxygen ratio. Observations have suggested that envelopes with more carbon than oxygen have a significantly greater abundance of molecules than their oxygen-rich analogues. Here we report observations of molecules in the oxygen-rich shell of the red supergiant star VY Canis Majoris (VY CMa). A variety of unexpected chemical compounds have been identified, including NaCl, PN, HNC and HCO+. From the spectral line profiles, the molecules can be distinguished as arising from three distinct kinematic regions: a spherical outflow, a tightly collimated, blue-shifted expansion, and a directed, red-shifted flow. Certain species (SiO, PN and NaCl) exclusively trace the spherical flow, whereas HNC and sulphur-bearing molecules (amongst others) are selectively created in the two expansions, perhaps arising from shock waves. CO, HCN, CS and HCO+ exist in all three components. Despite the oxygen-rich environment, HCN seems to be as abundant as CO. These results suggest that oxygen-rich shells may be as chemically diverse as their carbon counterparts.

No abstract

No abstract

Abstract. We present a detailed study of the gas depletion in L183 (= L134N) for a set of important species, namely, CO, CS, SO, N 2 H + and NH 3 . We show that all these species are depleted at some level. This level seems to depend mostly on a density threshold rather than on dust opacity. Therefore UV shielding would not be a main factor in the triggering of depletion. Our data suggest that CO, CS and SO depletion happen at densities of ∼3 × 10 4 cm −3 , while N 2 H + and NH 3 seem to deplete at densities close to 10 6 cm −3 . The latter result is consistent with the Bergin & Langer (1997, ApJ, 486, 316) polar (H 2 O) ice case but not with the more recent models of Aikawa et al. (2003, ApJ, 593, 906). CS depletion occurs much below its (J:2-1) critical density, (7 × 10 5 cm −3 ) and therefore makes this species unsuitable to study the density structure of many dark cloud cores.

No abstract

Seven rotational transitions of SiC 3 , a planar, highly polar ring molecule, have been detected in the millimeterwave band in the expanding envelope of the evolved carbon star IRC ϩ10216. On the assumption that SiC 3 has the same 40Љ diameter shell as SiCC, the mean column density of SiC 3 is cm . The rotational 12 Ϫ24.3 # 10 excitation of SiC 3 is similar to that of SiCC, with a low rotational temperature within the K-stacks of 10-20 K and a high rotational temperature across the K-stacks of roughly 50 K, which is probably close to the kinetic temperature of the shell.Subject headings: ISM: molecules -line: identification -molecular data -molecular processesradio lines: ISMThe most stable molecule with the elemental formula SiC 3 has been calculated to be the rhomboidal isomer with a transannular bond, as shown in Figure 1 (Alberts, Grev, & Schaefer 1990;Gomei et al. 1997). It has recently been detected for the first time in this laboratory, and its rotational spectrum has been measured to high precision in both the centimeterand millimeter-wave bands Apponi et al. 1999). Observing at the laboratory frequencies, we have now detected, with the NRAO 3 12 m telescope on Kitt Peak, seven transitions of this unusual molecular ring in the rich molecular envelope of the evolved carbon star IRC ϩ10216, which is a source where three silicon-carbon molecules have already been detected: the diatomic radical SiC al. 1997], the only stable ring so far detected).The observations are summarized in Table 1, and four of the lines are shown in Figure 2. The uncertainties in our rest frequencies are only 0.3 km s or less and, therefore, negligible Ϫ1 relative to both the width of the lines in the molecular shell of IRC ϩ10216 (28 km s ) and the spectral resolution of our Ϫ1 observations (3-4 km s ). Lines of nearly all molecules in Ϫ1 this source lie at a radial velocity of Ϫ km s , which, Ϫ126 ‫ע‬ 1 as Table 1 shows, to within the observational uncertainties, is the velocity of all the lines attributed to SiC 3 . Because the average interval between unidentified lines as strong as 5 mK in this part of the spectrum of IRC ϩ10216 is at least 30 MHz in frequency, or 100 km s in velocity, the probability that Ϫ1 any one of our seven lines is misidentified is a few percent at most, and the joint probability that all are misidentified is extremely small. In spite of its fairly weak lines, is one of the more securely identified molecules in IRC ϩ10216.The observations were done with the 12 m NRAO telescope on Kitt Peak during 1998 December when the sky was clear and the zenith opacity of water vapor at 225 GHz was generally 0.2 or less. The dual polarization SIS receiver used for the observations had a single-sideband system temperature of 200 K or less, atmospheric emission included. Pointing was checked every few hours against Saturn or 3C 273 and was found to be good to 10Љ. Spectra were taken by beam switching with the telescope's nutating subreflector at 1.25 Hz, with an angular throw of 4Ј. Two filter banks, each of 256...