J. L. Edwards

CIRCUMSTELLAR ION-MOLECULE CHEMISTRY: OBSERVATIONS OF HCO+ IN THE ENVELOPES OF O-RICH STARS AND IRC+10216

Millimeter-wave observations of HCO+ have been conducted toward oxygen-rich circumstellar envelopes, as well as IRC+10216, using the facilities of the Arizona Radio Observatory (ARO). The J = 1 → 0 and 2 → 1 transitions of this molecule were measured with the ARO 12 m antenna, while the J = 3 → 2 and 4 → 3 lines were observed using the ARO Sub-Millimeter Telescope. HCO+ was detected toward the supergiant NML Cyg and the asymptotic giant branch (AGB) stars IK Tau, TX Cam, and W Hya in at least two transitions. The J = 2 → 1 and 3 → 2 lines of this ion were also detected toward IRC+10216, confirming the identification of HCO+ in this object. The line profiles measured for HCO+ toward NML Cyg consist of red- and blueshifted components, suggesting a non-spherical shell. Based on a radiative transfer analysis, the abundances in the O-rich envelopes were f(HCO+/H2) ~ 0.15-1.3 × 10–7, with the AGB stars typically exhibiting the higher values. In IRC+10216, f(HCO+/H2) ~ 4.1 × 10–9, lower than the O-rich counterparts. The abundances of HCO+ were also found to peak at considerable distances from the star, indicative of an outer envelope molecule. Comparison with H2O and CO, the main precursor species, suggests that HCO+ is more prevalent in envelopes that have substantial water, but CO also plays a role in its formation. The abundance of HCO+ appears to increase inversely with mass-loss rate, provided the rate is >10–6 M ☉ yr–1. The common appearance of HCO+ in circumstellar gas indicates that, at some level, ion-molecule reactions influence the chemistry of evolved stellar envelopes.

SULFUR- AND SILICON-BEARING MOLECULES IN PLANETARY NEBULAE: THE CASE OF M2-48

Molecular-line observations of the bipolar planetary nebula (PN) M2-48 have been conducted using the Sub-Millimeter Telescope and the 12 m antenna of the Arizona Radio Observatory at 1, 2, and 3 mm. M2-48 is estimated to be ~4800 yr old, midway through the PN evolutionary track. SiO and SO2 were detected in this source—the first identification of either molecule in a PN. CN, HCN, HNC, CS, SO, HCO+, N2H+, and several 13C isotopologues such as 13CN, H13CN, and H13CO+ were also observed toward this object. A radiative transfer analysis of multiple SiO transitions indicates a gas kinetic temperature of T K ~ 55 K and a density of n(H2) ~ 9 × 105 cm–3 in M2-48, in agreement with previous CS and CO modeling. After CO, CN, and SO were found to be the most prevalent molecules in this nebula, with fractional abundances, relative to H2, of f ~ 3.8 × 10–7 and 2.4 × 10–7, respectively. SO2 and HCN are also abundant, with f ~ 1.2 × 10–7, indicating an [SO]/[SO2] ratio of ~2. Relatively high ion abundances were measured in M2-48 as well, with f ~ 10–7 for both HCO+ and N2H+. An [HCN]/[HNC] ratio of ~2 was determined, as typically observed in other PNe, independent of age. The high abundances of SO and SO2, along with the presence of SiO with f ~ 2.9 × 10–8, suggest O/C > 1 in this source; furthermore, the prevalence of CN and N2H+ indicates nitrogen enrichment. The 12C/13C ratio of ~3 in the nebula was also established. These factors indicate hot-bottom burning occurred in the progenitor star of M2-48, suggesting an initial mass > 4 M ☉.

Prebiotic Chemical Evolution in the Astrophysical Context

An ever increasing amount of molecular material is being discovered in the interstellar medium, associated with the birth and death of stars and planetary systems. Radio and millimeter-wave astronomical observations, made possible by high-resolution laboratory spectroscopy, uniquely trace the history of gas-phase molecules with biogenic elements. Using a combination of both disciplines, the full extent of the cycling of molecular matter, from circumstellar ejecta of dying stars – objects which expel large amounts of carbon - to nascent solar systems, has been investigated. Such stellar ejecta have been found to exhibit a rich and varied chemical content. Observations demonstrate that this molecular material is passed onto planetary nebulae, the final phase of stellar evolution. Here the star sheds almost its entire original mass, becoming an ultraviolet-emitting white dwarf. Molecules such as H2CO, HCN, HCO+, and CCH are present in significant concentrations across the entire age span of such nebulae. These data suggest that gas-phase polyatomic, carbon-containing molecules survive the planetary nebula phase and subsequently are transported into the interstellar medium, seeding the chemistry of diffuse and then dense clouds. The extent of the chemical complexity in dense clouds is unknown, hindered by the high spectral line density. Organic species such as acetamide and methyl amine are present in such objects, and NH2CHO has a wide Galactic distribution. However, organophosphorus compounds have not yet been detected in dense clouds. Based on carbon and nitrogen isotope ratios, molecular material from the ISM appears to become incorporated into solar system planetesimals. It is therefore likely that interstellar synthesis influences prebiotic chemistry on planet surfaces.