E. D. Tenenbaum

CARBON CHEMISTRY IN THE ENVELOPE OF VY CANIS MAJORIS: IMPLICATIONS FOR OXYGEN-RICH EVOLVED STARS

Observations of the carbon-bearing molecules CO, HCN, CS, HNC, CN, and HCO+ have been conducted toward the circumstellar envelope of the oxygen-rich red supergiant star, VY Canis Majoris (VY CMa), using the Arizona Radio Observatory (ARO). CO and HCN were also observed toward the O-rich shells of NML Cyg, TX Cam, IK Tau, and W Hya. Rotational transitions of these species at 1 mm, 0.8 mm, and 0.4 mm were measured with the ARO Submillimeter Telescope, including the J = 6 → 5 line of CO at 691 GHz toward TX Cam and W Hya. The ARO 12 m was used for 2 mm and 3 mm observations. Four transitions were observed for HCO+ in VY CMa, the first definitive identification of this ion in a circumstellar envelope. Molecular line profiles from VY CMa are complex, indicating three separate outflows: a roughly spherical flow and separate red- and blueshifted winds, as suggested by earlier observations. Spectra from the other sources appear to trace a single outflow component. The line data were modeled with a radiative transfer code to establish molecular abundances relative to H2 and source distributions. Abundances for CO derived for these objects vary over an order of magnitude, f ~ 0.4-5 × 10–4, with the lower values corresponding to the supergiants. For HCN, a similar range in abundance is found (f ~ 0.9-9 × 10–6), with no obvious dependence on the mass-loss rate. In VY CMa, HCO+ is present in all three outflows with f ~ 0.4-1.6 × 10–8 and a spatial extent similar to that of CO. HNC is found only in the red- and blueshifted components with [HCN]/[HNC] ~ 150-190, while [CN]/[HCN] ~ 0.01 in the spherical flow. All three velocity components are traced in CS, which has a confined spatial distribution and f ~ 2-6 × 10–7. These observations suggest that carbon-bearing molecules in O-rich shells are produced by a combination of photospheric shocks and photochemistry. Shocks may play a more prominent role in the supergiants because of their macroturbulent velocities.

A search for phosphine in circumstellar envelopes: PH3 in IRC +10216 and CRL 2688?

We present the results of a search for the JK = 10→ 00 transition of PH3 (phosphine) at 267 GHz toward several circumstellar envelopes using the Arizona Radio Observatory 10 m Submillimeter Telescope (SMT). In the carbon-rich shells of IRC +10216 and CRL 2688, we have detected emission lines exactly at the PH3 frequency. Toward the oxygen-rich supergiant VY Canis Majoris, only an upper limit was obtained, while in the evolved carbon-rich proto-planetary nebula CRL 618, the transition is contaminated by vibrationally excited HC3N (ν7 = 4). The line shape in IRC +10216 appears to consist of two distinct components: a flat-topped profile with a width of ~28 km s−1, as is typical for this source, and a narrower feature approximately 4 km s−1 wide. The narrow component likely arises from the inner envelope (r < 8R*) where the gas has not reached the terminal expansion velocity, or it is nonthermal emission. Based on the broader component, the abundance of PH3 with respect to H2 is estimated to be 5 × 10−8 in a region with a radius of r < 150R*. If the narrower component is thermal, it implies a phosphine abundance of ~5 × 10−7 close to the stellar photosphere (r < 8R*). In CRL 2688, the PH3 abundance is less constrained, with plausible values ranging from 3 × 10−8 to 4 × 10−7, assuming a spherical distribution. Phosphine appears to be present in large concentrations in the inner envelope of C-rich AGB stars, and thus may function as a parent molecule for other phosphorus species.

An Extremely Large Excess of 18O in the Hydrogen-deficient Carbon Star HD 137613

We report the discovery of a uniquely large excess of 18O in the hydrogen-deficient carbon (HdC) star HD 137613 based on a spectrum of the first-overtone bands of CO at 2.3-2.4 μm in which three strong absorption bands of 12C18O are clearly present. Bands of 12C16O also are present, but no bands of 13C16O or 12C17O are seen. We estimate an isotopic ratio 16O/18O 1. The solar value of this ratio is ~500. Neither He-core burning nor He-shell flash burning can produce the isotopic ratios of oxygen and carbon observed in HD 137613. However, a remarkable similarity exists between the observed abundances and those found in the outer layers of the broad He shell of early-asymptotic giant branch (AGB) stars, soon after the end of He-core burning. It is not known how the outer envelope down to the He shell could be lost, but some mechanism of enhanced mass loss must be involved. HD 137613 may be a post-early-AGB star with the outer layers of the former He-burning shell as its photosphere. The unusual elemental abundances of the HdC stars resemble those of the R Coronae Borealis (RCB) stars, but HdC stars do not produce clouds of dust that produce declines in brightness. None of the other RCB or HdC stars observed show significant 18O.

Detection of C3O in IRC +10216: Oxygen-Carbon Chain Chemistry in the Outer Envelope

The oxygen-bearing species C3O has been identified in the circumstellar envelope of the carbon star IRC 10216. The , , , , and transitions were detected at 2 and 3 mm using the Arizona J p 8 r 79 r 81 0r 91 4r 13 15 r 14 Radio Observatory’s 12 m telescope. Measurements of the , , and lines were simultaJ p 9 r 81 0r 91 2r 11 neously conducted at the IRAM 30 m telescope. The line profiles of C 3O are roughly U-shaped, indicating an extended shell distribution for this molecule in IRC 10216. The total column density derived for C3 Oi s 1.2# cm , at least an order of magnitude higher than that predicted by current chemical models. However, a revised 12 2 10 model that includes reactions of atomic oxygen with carbon-chain radicals, such as l-C 3H and C4, can reproduce the observed abundance. This model also predicts that C3O arises from a shell source with an outer radius near , consistent with the observations. These results suggest that gas phase neutral-neutral chemistry may be �� r ∼ 30 producing the oxygen-bearing molecules present in the outer envelope of IRC 10216. Subject headings: astrochemistry — circumstellar matter — ISM: molecules — radio lines: stars — stars: carbon — stars: individual (IRC 10216)

Detection of Near-Infrared CO Absorption Bands in R Coronae Borealis Stars

R Coronae Borealis (RCB) stars are hydrogen-deficient, carbon-rich, pulsating, post–asymptotic giant branch stars that experience massive irregular declines in brightness caused by circumstellar dust formation. The mechanism of dust formation around RCB stars is not well understood. It has been proposed that CO molecules play an important role in cooling the circumstellar gas so that dust may form. We report on a survey for CO in a sample of RCB stars. We obtained H- and K-band spectra including the first- and second-overtone CO bands for eight RCB stars, the RCB-like star DY Per, and the final-helium-flash star FG Sge. The first- and second-overtone CO bands were detected in the cooler (Teff 6000 K) RCB stars, R CrB, RY Sgr, SU Tau, and XX Cam. In addition, first-overtone bands are seen in FG Sge, a final-helium-flash star that is in an RCB-like phase at present. Effective temperatures of the eight RCB stars range from 4000 to 7250 K. The observed photospheric CO absorption bands were compared to line-blanketed model spectra of RCB stars. As predicted by the models, the CO bands are strongest in the coolest RCB stars and not present in the warmest. No correlation was found between the presence or strength of the CO bands and dust formation activity in the stars.

The 1 mm spectrum of VY Canis Majoris: Chemistry in an O-rich envelope

We present preliminary results of an unbiased spectral survey at 1 mm of the oxygen-rich supergiant, VY CMa. A number of exotic molecules have been detected, including NaC1 and PO, and a relatively rich organic chemistry is observed. Results of the survey will be compared with carbon-rich stars.