Rachel L. Smith

High-Precision C17O, C18O, and C16O Measurements in Young Stellar Objects: Analogues for Co Self-shielding in the Early Solar System

Using very high resolution (?/?? 95 000) 4.7 ?m fundamental and 2.3 ?m overtone rovibrational CO absorption spectra obtained with the Cryogenic Infrared Echelle Spectrograph infrared spectrometer on the Very Large Telescope (VLT), we report detections of four CO isotopologues?C16O, 13CO, C18O, and the rare species, C17O?in the circumstellar environment of two young protostars: VV CrA, a binary T Tauri star in the Corona Australis molecular cloud, and Reipurth 50, an intermediate-mass FU Ori star in the Orion Molecular Cloud. We argue that the observed CO absorption lines probe a protoplanetary disk in VV CrA, and a protostellar envelope in Reipurth 50. All CO line profiles are spectrally resolved, with intrinsic line widths of 3-4 km s?1 (FWHM), permitting direct calculation of CO oxygen isotopologue ratios with 5%-10% accuracy. The rovibrational level populations for all species can be reproduced by assuming that CO absorption arises in two temperature regimes. In the higher temperature regime, in which the column densities are best determined, the derived oxygen isotope ratios in VV CrA are: [C16O]/[C18O] =690 ? 30; [C16O]/[C17O] =2800 ? 300, and [C18O]/[C17O]=4.1 ? 0.4. For Reipurth 50, we find [C16O]/[C18O] =490 ? 30; [C16O]/[C17O] =2200 ? 150, [C18O]/[C17O] = 4.4 ? 0.2. For both objects, 12C/13C are on the order of 100, nearly twice the expected interstellar medium (ISM) ratio. The derived oxygen abundance ratios for the VV CrA disk show a significant mass-independent deficit of C17O and C18O relative to C16O compared to ISM baseline abundances. The Reipurth 50 envelope shows no clear differences in oxygen CO isotopologue ratios compared with the local ISM. A mass-independent fractionation can be interpreted as being due to selective photodissociation of CO in the disk surface due to self-shielding. The deficits in C17 O and C18 O in the VV CrA protoplanetary disk are consistent with an analogous origin of the 16O variability in the solar system by isotope selective photodissociation, confirmation of which may be obtained via study of additional sources. The higher fractionation observed for the VV CrA disk compared with the Reipurth 50 envelope is likely due to a combination of disk geometry, grain growth, and vertical mixing processes.

Astronomical Oxygen Isotopic Evidence for Supernova Enrichment of the Solar System Birth Environment by Propagating Star Formation

New infrared absorption measurements of oxygen isotope ratios in CO gas from individual young stellar objects confirm that the solar system is anomalously high in its [^(18)O]/[^(17)O] ratio compared with extrasolar oxygen in the Galaxy. We show that this difference in oxygen isotope ratios is best explained by ~1% enrichment of the protosolar molecular cloud by ejecta from Type II supernovae from a cluster having of order a few hundred stars that predated the Sun by at least 10-20 Myr. The likely source of exogenous oxygen was the explosion of one or more B stars during a process of propagating star formation.

Heterogeneity in

© 2015. The American Astronomical Society. All rights reserved.. This study reports an unusual heterogeneity in [12C16O]/[13C16O] abundance ratios of carbon monoxide observed in the gas phase toward seven ∼solar-mass young stellar objects (YSOs) and three dense foreground clouds in the nearby star-forming regions, Ophiuchus, Corona Australis, Orion, and Vela, and an isolated core, L43. Robust isotope ratios were derived using infrared absorption spectroscopy of the 4.7 μm fundamental and 2.3 μm overtone rovibrational bands of CO at very high spectral resolution (λ/Δλ ≈ 95,000), observed with the Cryogenic Infrared Echelle Spectrograph (CRIRES) on the Very Large Telescope. We find [12C16O]/[13C16O] values ranging from ∼85 to 165, significantly higher than those of the local interstellar medium (ISM) (∼65-69). These observations are evidence for isotopic heterogeneity in carbon reservoirs in solar-type YSO environments, and encourage the need for refined galactic chemical evolution models to explain the 12C/13C discrepancy between the solar system and local ISM. The oxygen isotope ratios are consistent with isotopologue-specific photodissociation by CO self-shielding toward the disks, VV CrA N and HL Tau, further substantiating models predicting CO self-shielding on disk surfaces. However, we find that CO self-shielding is an unlikely general explanation for the high [12C16O]/[13C16O] ratios observed in this study. Comparison of the solid CO against gas-phase [12C16O]/[13C16O] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high [12C16O]/[13C16O] observed.

^{12}

© 2015. The American Astronomical Society. All rights reserved.. This study reports an unusual heterogeneity in [12C16O]/[13C16O] abundance ratios of carbon monoxide observed in the gas phase toward seven ∼solar-mass young stellar objects (YSOs) and three dense foreground clouds in the nearby star-forming regions, Ophiuchus, Corona Australis, Orion, and Vela, and an isolated core, L43. Robust isotope ratios were derived using infrared absorption spectroscopy of the 4.7 μm fundamental and 2.3 μm overtone rovibrational bands of CO at very high spectral resolution (λ/Δλ ≈ 95,000), observed with the Cryogenic Infrared Echelle Spectrograph (CRIRES) on the Very Large Telescope. We find [12C16O]/[13C16O] values ranging from ∼85 to 165, significantly higher than those of the local interstellar medium (ISM) (∼65-69). These observations are evidence for isotopic heterogeneity in carbon reservoirs in solar-type YSO environments, and encourage the need for refined galactic chemical evolution models to explain the 12C/13C discrepancy between the solar system and local ISM. The oxygen isotope ratios are consistent with isotopologue-specific photodissociation by CO self-shielding toward the disks, VV CrA N and HL Tau, further substantiating models predicting CO self-shielding on disk surfaces. However, we find that CO self-shielding is an unlikely general explanation for the high [12C16O]/[13C16O] ratios observed in this study. Comparison of the solid CO against gas-phase [12C16O]/[13C16O] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high [12C16O]/[13C16O] observed.