[en] We report the relative abundances of the three stable isotopes of silicon, ²⁸Si, ²⁹Si, and ³⁰Si, across the Galaxy using the $v=0,J=1\to <!--\; ???\; -->0$ transition of silicon monoxide. The chosen sources represent a range in Galactocentric radii ($R_{\mathrm{G}\mathrm{C}}$) from 0 to 9.8 kpc. The high spectral resolution and sensitivity afforded by the Green Bank Telescope permit isotope ratios to be corrected for optical depths. The optical-depth-corrected data indicate that the secondary-to-primary silicon isotope ratios ${}^{29}\mathrm{S}\mathrm{i}{/}^{28}\mathrm{S}\mathrm{i}$ and ${}^{30}\mathrm{S}\mathrm{i}{/}^{28}\mathrm{S}\mathrm{i}$ vary much less than predicted on the basis of other stable isotope ratio gradients across the Galaxy. Indeed, there is no detectable variation in Si isotope ratios with $R_{\mathrm{G}\mathrm{C}}$. This lack of an isotope ratio gradient stands in stark contrast to the monotonically decreasing trend with $R_{\mathrm{G}\mathrm{C}}$ exhibited by published secondary-to-primary oxygen isotope ratios. These results, when considered in the context of the expectations for chemical evolution, suggest that the reported oxygen isotope ratio trends, and perhaps those for carbon as well, require further investigation. The methods developed in this study for SiO isotopologue ratio measurements are equally applicable to Galactic oxygen, carbon, and nitrogen isotope ratio measurements, and should prove useful for future observations of these isotope systems.

[en] The presence of excesses of short-lived radionuclides in the early solar system evidenced in meteorites has been taken as testament to close encounters with exotic nucleosynthetic sources, including supernovae or AGB stars. An analysis of the likelihoods associated with different sources of these extinct nuclides in the early solar system indicates that, rather than being exotic, their abundances were typical of star-forming regions like those observed today in the Galaxy. The radiochemistry of the early solar system is therefore unexceptional, being the consequence of extensive averaging of solids from molecular clouds.

[en] We present evidence that excesses in Be in polluted white dwarfs (WDs) are the result of accretion of icy exomoons that formed in the radiation belts of giant exoplanets. Here we use excess Be in the white dwarf GALEX J2339–0424 as an example. We constrain the parent body abundances of rock-forming elements in GALEX J2339–0424 and show that the overabundance of beryllium in this WD cannot be accounted for by differences in diffusive fluxes through the WD outer envelope nor by chemical fractionations during typical rock-forming processes. We argue instead that the Be was produced by energetic proton irradiation of ice mixed with rock. We demonstrate that the MeV proton fluence required to form the high Be/O ratio in the accreted parent body is consistent with irradiation of ice in the rings of a giant planet within its radiation belt, followed by accretion of the ices to form a moon that is later accreted by the WD. The icy moons of Saturn serve as useful analogs. Our results provide an estimate of spallogenic nuclide excesses in icy moons formed by rings around giant planets in general, including those in the solar system. While excesses in Be have been detected in two polluted WDs to date, including the WD described here, we predict that excesses in the other spallogenic elements Li and B, although more difficult to detect, should also be observed, and that such detections would also indicate pollution by icy exomoons formed in the ring systems of giant planets.

[en] This study reports an unusual heterogeneity in ["1"2C"1"6O]/["1"3C"1"6O] 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 ["1"2C"1"6O]/["1"3C"1"6O] 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 "1"2C/"1"3C 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 ["1"2C"1"6O]/["1"3C"1"6O] ratios observed in this study. Comparison of the solid CO against gas-phase ["1"2C"1"6O]/["1"3C"1"6O] suggests that interactions between CO ice and gas reservoirs need to be further investigated as at least a partial explanation for the unusually high ["1"2C"1"6O]/["1"3C"1"6O] observed

[en] Using very high resolution (λ/Δλ ∼ 95 000) 4.7 μm fundamental and 2.3 μm 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.

[en] The element beryllium is detected for the first time in white dwarf stars. This discovery in the spectra of two helium-atmosphere white dwarfs was made possible only because of the remarkable overabundance of Be relative to all other elements, heavier than He, observed in these stars. The measured Be abundances, relative to chondritic, are by far the largest ever seen in any astronomical object. We anticipate that the Be in these accreted planetary bodies was produced by spallation of one or more of O, C, and N in a region of high fluence of particles of MeV or greater energy.

[en] 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 [¹⁸O]/[¹⁷O] 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.