A Reappraisal of the Solar Photospheric C/O Ratio

Accurate determination of photospheric solar abundances requires detailed modeling of the solar granulation and accounting for departures from local thermodynamical equilibrium (LTE). We argue that the forbidden C I line at 8727 A is largely immune to departures from LTE, and can be realistically modeled using LTE radiative transfer in a time-dependent three-dimensional simulation of solar surface convection. We analyze the [C I] line in the solar flux spectrum to derive the abundance log epsilon(C)= 8.39 +/- 0.04 dex. Combining this result with our parallel analysis of the [O I] 6300 A line, we find C/O=0.50 +/- 0.07, in agreement with the ratios measured in the solar corona from gamma-ray spectroscopy and solar energetic particles.Comment: 5 pages, 2 figures, to appear in ApJL July 1

Conformance Verification of Normative Specifications using C-O Diagrams

C-O Diagrams have been introduced as a means to have a visual representation of normative texts and electronic contracts, where it is possible to represent the obligations, permissions and prohibitions of the different signatories, as well as what are the penalties in case of not fulfillment of their obligations and prohibitions. In such diagrams we are also able to represent absolute and relative timing constrains. In this paper we consider a formal semantics for C-O Diagrams based on a network of timed automata and we present several relations to check the consistency of a contract in terms of realizability, to analyze whether an implementation satisfies the requirements defined on its contract, and to compare several implementations using the executed permissions as criteria.Comment: In Proceedings FLACOS 2012, arXiv:1209.169

Detonations in Sub-Chandrasekhar Mass C+O White Dwarfs

Explosions of sub-Chandrasekhar-mass white dwarfs are one alternative to the standard Chandrasekhar-mass model of Type Ia supernovae. They are interesting since binary systems with sub-Chandrasekhar-mass primary white dwarfs should be common and this scenario would suggest a simple physical parameter which determines the explosion brightness, namely the mass of the exploding white dwarf. Here we perform one-dimensional hydrodynamical simulations, associated post-processing nucleosynthesis and multi-wavelength radiation transport calculations for pure detonations of carbon-oxygen white dwarfs. The light curves and spectra we obtain from these simulations are in good agreement with observed properties of Type Ia supernovae. In particular, for white dwarf masses from 0.97 - 1.15 Msun we obtain 56Ni masses between 0.3 and 0.8 Msun, sufficient to capture almost the complete range of Type Ia supernova brightnesses. Our optical light curve rise times, peak colours and decline timescales display trends which are generally consistent with observed characteristics although the range of B-band decline timescales displayed by our current set of models is somewhat too narrow. In agreement with observations, the maximum light spectra of the models show clear features associated with intermediate mass elements and reproduce the sense of the observed correlation between explosion luminosity and the ratio of the Si II lines at 6355 and 5972 Angstroms. We therefore suggest that sub-Chandrasekhar mass explosions are a viable model for Type Ia supernovae for any binary evolution scenario leading to explosions in which the optical display is dominated by the material produced in a detonation of the primary white dwarf.Comment: 6 pages, 4 figures. Accepted for publication by ApJ Letters

Geographic variation in U. S. populations of the tiger beetle Cicindela obsoleta Say (Coleoptera: Cicindelidae)

Geographic variation and subspecific taxonomy of United States populations of the tiger beetle Cicindela obsoleta Say are reviewed. Study of primary types and 1,424 museum specimens indicates that four subspecific entities are present in the U.S., for which the valid names are C. o. obsoleta Say, C. o. santaclarae Bates, C. o. vulturina LeConte, and C. o. neojuvenilis Vogt. All four subspecies are illustrated, including many color variants. ArcView Geographic Information System (GIS) computer software is used to study the distribution of these subspecies in the United States. Cicindela o. obsoleta and C. o. santaclarae are only partially allopatric, with extensive areas in Arizona, New Mexico, and Texas where their distributions overlap. Specimens intermediate in their elytral markings between C. o. obsoleta and C. o. santaclarae are reported from localities in New Mexico and Texas where these two subspecies co-occur. In contrast, C. o. vulturina and C. o. neojuvenilis are largely allopatric and show little intergradation with the C. o. obsoleta - C. o. santaclarae complex. It has been suggested recently that disjunct (but as yet unnamed) populations of C. o. vulturina in Missouri and Arkansas may represent a separate subspecies of C. obsoleta. However, the color and elytral pattern characteristics which have been interpreted as diagnostic features of these disjunct populations are also found in many Texas populations of C. o. vulturina, suggesting that the Arkansas and Missouri populations do not merit recognition as a separate subspecies on the basis of these characters alone

C/O ratio as a Dimension for Characterizing Exoplanetary Atmospheres

Until recently, infrared observations of exoplanetary atmospheres have typically been interpreted using models that assumed solar elemental abundances. With the chemical composition fixed, attempts have been made to classify hot Jupiter atmospheres on the basis of stellar irradiation. However, recent observations have revealed deviations from predictions based on such classification schemes, and chemical compositions retrieved from some datasets have also indicated non-solar abundances. In hot hydrogen-dominated atmospheres, the C/O ratio critically influences the relative concentrations of several spectroscopically dominant molecules. Between a C/O of 0.5 (solar value) and 2, the H2O and CH4 abundances can vary by several orders of magnitude in the observable atmosphere, and new hydrocarbon species such as HCN and C2H2 become prominent for C/O >= 1, while the CO abundance remains almost unchanged. Furthermore, a C/O >= 1 can preclude a strong thermal inversion due to TiO and VO in a hot Jupiter atmosphere, since TiO and VO are naturally under-abundant for C/O >= 1. We, therefore, suggest a new two-dimensional classification scheme for hydrogen-dominated exoplanetary atmospheres with irradiation (or temperature) and C/O ratio as the two dimensions. We define four classes in this 2-D space (O1, O2, C1, and C2) with distinct chemical, thermal and spectral properties. We characterize the thermal structures and C/O ratios of six hot Jupiters (XO-1b, CoRoT-2b, WASP-14b, WASP-19b, WASP-33b, and WASP-12b) in the framework of our proposed 2D classification scheme. While the data for several systems in our sample are consistent with carbon-rich atmospheres (i.e. C/O >= 1), new observations are required to conclusively constrain the C/O ratios in their atmospheres. We discuss how observations using existing and forthcoming facilities can constrain C/O ratios in exoplanetary atmospheres.Comment: Accepted for publication in ApJ, 22 pages in emulate ApJ, 15 figure

Very low mass white dwarfs with a C-O core

The lower limit for the mass of white dwarfs (WDs) with C-O core is commonly assumed to be roughly 0.5 Msun. As a consequence, WDs of lower masses are usually identified as He-core remnants. However, when the initial mass of the progenitor star is in between 1.8 and 3 Msun, which corresponds to the so called red giant (RGB) phase transition, the mass of the H-exhausted core at the tip of the RGB is 0.3 < M_H/Msun < 0.5. Prompted by this well known result of stellar evolution theory, we investigate the possibility to form C-O WDs with mass M < 0.5 Msun. The pre-WD evolution of stars with initial mass of about 2.3 Msun, undergoing anomalous mass-loss episodes during the RGB phase and leading to the formation of WDs with He-rich or CO-rich cores have been computed. The cooling sequences of the resulting WDs are also described. We show that the minimum mass for a C-O WD is about 0.33 Msun, so that both He and C-O core WDs can exist in the mass range 0.33-0.5 Msun. The models computed for the present paper provide the theoretical tools to indentify the observational counterpart of very low mass remnants with a C-O core among those commonly ascribed to the He-core WD population in the progressively growing sample of observed WDs of low mass. Moreover, we show that the central He-burning phase of the stripped progeny of the 2.3 Msun star lasts longer and longer as the total mass decreases. In particular, the M= 0.33 Msun model takes about 800 Myr to exhausts its central helium, which is more than three time longer than the value of the standard 2.3 Msun star: it is, by far, the longest core-He burning lifetime. Finally, we find the occurrence of gravonuclear instabilities during the He-burning shell phase.Comment: Accepted for publication in Astronomy and Astrophysics, 9 page

The carbon-to-oxygen ratio in stars with planets

In some recent works, the C/O abundance ratio in high-metallicity stars with planets is found to vary from 0.4 to about 1.0. This has led to discussions about the existence of terrestrial planets with a carbon-dominated composition that is very different from the composition of the Earth. The C/O values were obtained by determining carbon abundances from high-excitation CI lines and oxygen abundances from the forbidden [OI] line at 6300 A. This weak line is, however, strongly affected by a nickel blend at high metallicities. Aiming for more precise C/O ratios, oxygen abundances in this paper are derived from the high-excitation OI triplet at 7774 A and carbon abundances from the CI lines at 5052 and 5380 A using MARCS model atmospheres and including non-LTE corrections. The results do not confirm the high C/O ratios previously found. C/O shows a tight, slightly increasing dependence on metallicity from C/O=0.58 at [Fe/H]=0.0 to C/O=0.70 at [Fe/H] =0.4 with an rms scatter of only 0.06. Assuming that the composition of a proto-planetary disk is the same as that of the host star, the C/O values found in this paper lend no support to the existence of carbon-rich planets. The small scatter of C/O among thin-disk stars suggests that the nucleosynthesis products of Type II supernovae and low- to intermediate-mass stars are well mixed in the interstellar medium.Comment: 10 pages, 11 figures, accepted for publication in A&

Akari Observations of Brown Dwarfs. II CO2 as Probe of Carbon and Oxygen Abundances in Brown Dwarfs

Recent observations with the infrared astronomical satellite AKARI have shown that the CO2 bands at 4.2 micron in three brown dwarfs are much stronger than expected from the unified cloudy model (UCM) based on recent solar C & O abundances. This result has been a puzzle, but we now find that this is simply an abundance effect: We show that these strong CO2 bands can be explained with the UCMs based on the classical C & O abundances (log Ac and log Ao), which are about 0.2 dex larger compared to the recent values. Since three other brown dwarfs could be well interpreted with the recent solar C & O abundances, we require at least two model sequences based on the different chemical compositions to interpret all the AKARI spectra. The reason for this is that the CO2 band is especially sensitive to C & O abundances, since the CO2 abundance depends approximately on AcAo^2 --- the cube of C & O abundances. For this reason, even low resolution spectra of very cool dwarfs, especially of CO2 cannot be understood unless a model with proper abundances is applied. For the same reason, CO2 is an excellent indicator of C & O abundances, and we can now estimate C & O abundances of brown dwarfs: Three out of six brown dwarfs observed with AKARI should have high C & O abundances similar to the classical solar values (e.g. logAc = 8.60 and logAo = 8.92), but the other three may have low C & O abundances similar to the recent solar values (e.g. logAc = 8.39 and logAo = 8.69). This result implies that three out of six brown dwarfs are highly metal rich relative to the Sun if the recent solar C & O abundances are correct.Comment: 12 pages, 6 figures, To appear in ApJ June 20 issu