Analysis of the first overtone bands of isotopologues of CO and SiO in stellar spectra

Context. This study is based on models of the first overtone (Δv = 2) bands of the monosubstituted isotopologues of CO at 2.3 μm in the spectrum of Arcturus (K2 III) and of the monosubstituted isotopologues of SiO at 4 μm in the spectrum of the red giant HD 196610 (M6 III). Aims. We aim to investigate problems involving the computation of the first overtone bands of isotopologues of CO and SiO in the spectra of late-type stars and to determine isotopic abundances. Methods. We used fits of theoretical synthetic spectra to the observed stellar molecular bands of CO and SiO for determining the abundances for isotopes of C, O, and Si. Results. Fits of synthetic spectra of the 12C16O first overtone bands at 2.3 μm computed with three available line lists to the observed spectrum of Arcturus provide the same carbon abundance [C] = − 0.6 and isotopic ratio of carbon 12C/13C = 10 ± 2. However, the quality of fits to the observed spectrum differ for three line lists used. Furthermore, the derived oxygen isotopic ratio 16O/18O = 2000 ± 500 is larger than that known in the solar system, where 16O/18O = 500. The silicon isotopic ratio in the atmosphere of the red giant HD 196610 has been revised. Using the ExoMol SiO line list with appropriate statistical weights for the SiO isotopologues, the “non-solar” ratio 28Si:29Si:30Si = 0.86 ± 0.03:0.12 ± 0.02:0.02 ± 0.01 is obtained. Conclusions. We find that: (a) the computed isotopic carbon and silicon ratios determined by the fits to the observed spectrum depend on the adopted abundance of C and Si, respectively; and (b) Correct treatment of the nuclear spin degeneracies parameter is of crucial importance for today’s application of HITRAN and ExoMol line lists in the astrophysical computations.

Collisional erosion and the non-chondritic composition of the terrestrial planets

The compositional variations among the chondrites inform us about cosmochemical fractionation processes during condensation and aggregation of solid matter from the solar nebula. These fractionations include: (i) variable Mg–Si–RLE ratios (RLE: refractory lithophile element), (ii) depletions in elements more volatile than Mg, (iii) a cosmochemical metal–silicate fractionation, and (iv) variations in oxidation state. Moon- to Mars-sized planetary bodies, formed by rapid accretion of chondrite-like planetesimals in local feeding zones within 10 6 years, may exhibit some of these chemical variations. However, the next stage of planetary accretion is the growth of the terrestrial planets from approximately 10 2 embryos sourced across wide heliocentric distances, involving energetic collisions, in which material may be lost from a growing planet as well as gained. While this may result in averaging out of the ‘chondritic’ fractionations, it introduces two non-chondritic chemical fractionation processes: post-nebular volatilization and preferential collisional erosion. In the latter, geochemically enriched crust formed previously is preferentially lost. That post-nebular volatilization was widespread is demonstrated by the non-chondritic Mn/Na ratio in all the small, differentiated, rocky bodies for which we have basaltic samples, including the Moon and Mars. The bulk silicate Earth (BSE) has chondritic Mn/Na, but shows several other compositional features in its pattern of depletion of volatile elements suggestive of non-chondritic fractionation. The whole-Earth Fe/Mg ratio is 2.1±0.1, significantly greater than the solar ratio of 1.9±0.1, implying net collisional erosion of approximately 10 per cent silicate relative to metal during the Earth's accretion. If this collisional erosion preferentially removed differentiated crust, the assumption of chondritic ratios among all RLEs in the BSE would not be valid, with the BSE depleted in elements according to their geochemical incompatibility. In the extreme case, the Earth would only have half the chondritic abundances of the highly incompatible, heat-producing elements Th, U and K. Such an Earth model resolves several geochemical paradoxes: the depleted mantle occupies the whole mantle, is completely outgassed in 40 Ar and produces the observed 4 He flux through the ocean basins. But the lower radiogenic heat production exacerbates the discrepancy with heat loss.

Solar Composition of Icy Planetesimals: A New Source For Comet Nuclei?

The enrichment of heavy elements on Jupiter appears to require the existence of a new class of icy planetesimal that exhibits solar relative abundances.Prior to the Galileo probe mission, observations of methane in Jupiter’s atmosphere had revealed that C/H was approximately three times the solar ratio. This enrichment was thought to be the result of the delivery of heavy elements by icy planetesimals, which were assumed to be essentially identical to comets. However, comets are notoriously deficient in nitrogen (e.g., Geiss 1987; Krankowsky 1991) and recent upper limits on argon in three comets (Weaver et al. 2002) indicate that this element is also sub-solar relative to O. Hence it was assumed that Jupiter would exhibit the same deficiency in argon and nitrogen relative to carbon (Pollack and Bodenheimer 1989; Owen & Bar-Nun 1995). Yet the mass spectrometer on the Galileo Probe clearly showed that Ar, Kr, Xe, N, C, and S are all enriched in Jupiter’s atmosphere by the same factor of 3 ± 1 (Niemann et al. 1998; Owen et al. 1999).

The Abundance of Boron in Disk-Metallicity Stars

Although the behavior of boron versus metallicity has been probed in a fairly large sample of halo dwarfs with HST, it is only very recently that boron abundances have been derived systematically in solar metallicity dwarfs. This effort began with a re-analysis of the solar spectrum with modern atomic data and model atmospheres so that the Sun could be adopted as a standard for the calibration of a line list in the region of the B I transition at 2497 Â. The solar analysis indicates that boron is not depleted in the solar photosphere. From a subsequent study of a sample of 14 field F/G-dwarfs with roughly solar metallicities, it is found that the behavior of boron versus [Fe/H] follows the linear trend that is observed for the halo stars. The average B/Be obtained for solar metallicity stars is 27±5 compared to the solar ratio of 23. The determination of boron abundances in the young B-type and G-type stars of the Orion association reveals a behavior of boron and oxygen in Orion that is opposite of the positive correlation which is observed for the field stars: the boron and oxygen abundances are anticorrelated.

On the difference in formation history between bulges and ellipticals

On the diagram (Fe5270, Mg5175) ellipticals and bulges of disk galaxies (from S0 to Sc) maintain a very different position: E's seem to be overabundant in Mg, bulges seem to have a solar ratio Mg/Fe, except several ones who are overabundant in Fe.

5. Low-Temperature Condensates in Comets

Recent observational data on the volatile fraction of comets are confronted with a model based on the fractional condensation, in the 80-100 °K range, of a higher-temperature equilibrium obtained from a solar mixture, more or less depleted in oxygen and in hydrogen. It is possible to almost duplicate the observational data, only by assuming that the solar ratio of C/0 is at least as large as 0.66 and that the hydrogen was drastically depleted by an unknown process in the primitive solar nebula. Although none of these two assumptions is at variance with present knowledge, the latter is sufficiently exotic to propose a simpler explanation, namely that comets could be made of interstellar grains relatively unprocessed by heat.