Carbon and Oxygen in Nearby Stars: Keys to Protoplanetary Disk Chemistry (original) (raw)
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The Astrophysical Journal, 2013
The super-Earth exoplanet 55 Cnc e, the smallest member of a five-planet system, has recently been observed to transit its host star. The radius estimates from transit observations, coupled with spectroscopic determinations of mass, provide constraints on its interior composition. The composition of exoplanetary interiors and atmospheres are particularly sensitive to elemental C/O ratio, which to first order can be estimated from the host stars. Results from a recent spectroscopic study analyzing the 6300Å [O I] line and two C I lines suggest that 55 Cnc has a carbon-rich composition (C/O=1.12±0.09). However oxygen abundances derived using the 6300Å [O I] line are highly sensitive to a Ni I blend, particularly in metal-rich stars such as 55 Cnc ([Fe/H]=0.34±0.18). Here, we further investigate 55 Cnc's composition by deriving the carbon and oxygen abundances from these and additional C and O absorption features. We find that the measured C/O ratio depends on the oxygen lines used. The C/O ratio that we derive based on the 6300Å [O I] line alone is consistent with the previous value. Yet, our investigation of additional abundance indicators results in a mean C/O ratio of 0.78±0.08. The lower C/O ratio of 55 Cnc determined here may place this system at the sensitive boundary between protoplanetary disk compositions giving rise to planets with high (>0.8) versus low (<0.8) C/O ratios. This study illustrates the caution that must applied when determining planet host star C/O ratios, particularly in cool, metal-rich stars. Recently Madhusudhan et al. (2012) suggest an alternative and carbon-rich composition of 55 Cnc e, garnering the super-Earth popular attention as "the diamond planet." Measurements of the carbon and oxygen abundances from two C I lines (5052Å, 5135Å) and one forbidden [O I] line (6300Å) indicate a C/O 1 ratio of 1.12±0.19 (Delgado Mena et al. 2010), i.e., a highly carbon-rich star compared to the solar C/O∼0.50 (Asplund et al. 2005). If the disk shared the host star's composition, and the host star is carbon-rich, then the planetesimals accreted during the formation of 55 Cnc e were likely Fe-and C-rich (Bond et al. 2010; Madhusudhan et al. 2012). To investigate the composition of the possibly carbon-rich exoplanet, Madhusudhan et al. (2012) consider two families of carbon-rich interior models of 55 Cnc e, consisting of layers, from inner to outer, of Fe-SiC-C and Fe-MgSiO 3 -C. Included in their carbon equation of state (EOS) are the graphite EOS at low pressures, the phase transition to diamond between 10 GPa<P<1000 GPa, and the Thomas-Fermi-Dirac EOS at high pressures. Madhusudhan et al. (2012) find a wide range of compositions are possible, including extreme combinations like (Fe, SiC, C) = (33%, 0%, 67%)
The recent inference of a carbon-rich atmosphere, with C/O ≥ 1, in the hot Jupiter WASP-12b motivates the exotic new class of carbon-rich planets (CRPs). We report a detailed study of the atmospheric chemistry and spectroscopic signatures of carbon-rich giant planets (CRGs), the possibility of thermal inversions in their atmospheres, the compositions of icy planetesimals required for their formation via core accretion, and the apportionment of ices, rock, and volatiles in their envelopes. Our results show that CRG atmospheres probe a unique region in composition space, especially at high temperature (T ). For atmospheres with C/O ≥ 1, and T 1400 K in the observable atmosphere, most of the oxygen is bound up in CO, while H 2 O is depleted and CH 4 is enhanced by up to two or three orders of magnitude each, compared to equilibrium compositions with solar abundances (C/O = 0.54). These differences in the spectroscopically dominant species for the different C/O ratios cause equally distinct observable signatures in the spectra. As such, highly irradiated transiting giant exoplanets form ideal candidates to estimate atmospheric C/O ratios and to search for CRPs. We also find that the C/O ratio strongly affects the abundances of TiO and VO, which have been suggested to cause thermal inversions in highly irradiated hot Jupiter atmospheres. A C/O = 1 yields TiO and VO abundances of ∼100 times lower than those obtained with equilibrium chemistry assuming solar abundances, at P ∼ 1 bar. Such a depletion is adequate to rule out thermal inversions due to TiO/VO even in the most highly irradiated hot Jupiters, such as WASP-12b. We estimate the compositions of the protoplanetary disk, the planetesimals, and the envelope of WASP-12b, and the mass of ices dissolved in the envelope, based on the observed atmospheric abundances. Adopting stellar abundances (C/O = 0.44) for the primordial disk composition and low-temperature formation conditions (T 30 K) for WASP-12b leads to a C/O ratio of 0.27 in accreted planetesimals, and, consequently, in the planet's envelope. In contrast, a C/O ratio of 1 in the envelope of WASP-12b requires a substantial depletion of oxygen in the disk, i.e. by a factor of ∼ 0.41 for the same formation conditions. This scenario also satisfies the constraints on the C/H and O/H ratios reported for WASP-12b. If, alternatively, hotter conditions prevailed in a stellar composition disk such that only H 2 O is condensed, the remaining gas can potentially have a C/O ∼ 1. However, a high C/O in WASP-12b caused predominantly by gas accretion would preclude super-stellar C/H ratios which also fit the data. Subject headings: planetary systems -planets and satellites: general -planets and satellites: individual (WASP-12b)
Searching for the signatures of terrestrial planets in F-, G-type main-sequence stars
Astronomy & Astrophysics, 2013
Context. Detailed chemical abundances of volatile and refractory elements have been discussed in the context of terrestrial-planet formation during in past years. Aims. The HARPS-GTO high-precision planet-search program has provided an extensive database of stellar spectra, which we have inspected in order to select the best-quality spectra available for late type stars. We study the volatile-to-refractory abundance ratios to investigate their possible relation with the low-mass planetary formation. Methods. We present a fully differential chemical abundance analysis using high-quality HARPS and UVES spectra of 61 late F-and early G-type main-sequence stars, where 29 are planet hosts and 32 are stars without detected planets. Results. As for the previous sample of solar analogs, these stars slightly hotter than the Sun also provide very accurate Galactic chemical abundance trends in the metallicity range −0.3 < [Fe/H] < 0.4. Stars with and without planets show similar mean abundance ratios. Moreover, when removing the Galactic chemical evolution effects, these mean abundance ratios, ∆[X/Fe] SUN−STARS , against condensation temperature tend to exhibit less steep trends with nearly zero or slightly negative slopes. We have also analyzed a subsample of 26 metal-rich stars, 13 with and 13 without known planets, with spectra at S/N ∼ 850, on average, in the narrow metallicity range 0.04 < [Fe/H] < 0.19. We find the similar, although not equal, abundance pattern with negative slopes for both samples of stars with and without planets. Using stars at S/N ≥ 550 provides equally steep abundance trends with negative slopes for stars both with and without planets. We revisit the sample of solar analogs to study the abundance patterns of these stars, in particular, 8 stars hosting super-Earth-like planets. Among these stars having very low-mass planets, only four of them reveal clear increasing abundance trends versus condensation temperature. Conclusions. Finally, we compared these observed slopes with those predicted using a simple model that enables us to compute the mass of rocks that have formed terrestrial planets in each planetary system. We do not find any evidence supporting the conclusion that the volatile-to-refractory abundance ratio is related to the presence of rocky planets.
Present-Day Carbon Abundances of Early-Type Stars
2006
Carbon is one of the most abundant metals in the universe because of its synthesis in the fundamental triple alpha reaction. The knowledge of carbon abundances in different environments is one key ingredient to our understanding of stellar and galactochemical evolution. Studies of luminous OB-type stars allow us to address both topics even in galaxies beyond our own. Unfortunately the history of carbon abundance determinations from these objects in the last three decades is one of limited success. Analyses of the strong and weak line spectra of C II as well as C III tend to be largely discrepant. We present results of quantitative spectral analyses based on a sophisticated model atom for non-LTE line-formation calculations of C II-IV. As a first application, carbon abundances in a sample of B-type dwarfs and giants in nearby associations and in the field are determined. Consistency is finally achieved for all measurable lines (up to 40) from the three ionization stages. This include...
Carbon abundances of early B-type stars in the solar vicinity
Astronomy and Astrophysics, 2008
Context. Precise determinations of the chemical composition in early B-type stars constitute fundamental observational constraints on stellar and galactochemical evolution. Carbon, in particular, is one of the most abundant metals in the Universe but analyses in earlytype stars are known to show inconclusive results. Large discrepancies between analyses of different lines in C ii, a failure to establish the C ii/iii ionization balance and the derivation of systematically lower abundances than from other indicators like H ii regions and young FG-type stars pose long-standing problems. Aims. We discuss improvements to the non-LTE modelling of the visual line spectrum and to the spectral analysis of early B-type stars, as well as their consequences for stellar parameter and abundance derivations. The most relevant sources of systematic uncertainies and their effects on the analysis are investigated. Consequences for the present-day carbon abundance in the solar vicinity are discussed. Methods. We present a comprehensive and robust C ii/iii/iv model for non-LTE line-formation calculations based on carefully selected atomic data. The model is calibrated with high-S/N spectra of six apparently slow-rotating early B-type dwarfs and giants, which cover a wide parameter range and are randomly distributed in the solar neighbourhood. A self-consistent quantitative spectrum analysis is performed using an extensive iteration scheme to determine stellar atmospheric parameters and to select the appropriate atomic data used for the derivation of chemical abundances. Results. We establish the carbon ionization balance for all sample stars based on a unique set of input atomic data. Consistency is achieved for all modelled carbon lines of the sample stars. Highly accurate atmospheric parameters and a homogeneous carbon abundance of log (C/H) + 12 = 8.32 ± 0.04 with reduced systematic errors are derived. Present evolution models for massive stars indicate that this value may require only a small adjustment because of the effects of rotational mixing, by < +0.05 dex per sample star. This results in a present-day stellar carbon abundance in the solar neighbourhood, which is in good agreement with recent determinations of the solar value and with the gas-phase abundance of the Orion H ii region. Our finding of a homogeneous presentday carbon abundance also conforms with predictions of chemical-evolution models for the Galaxy. Moreover, the present approach allows us to constrain the effects of systematic errors on fundamental parameters and abundances. This suggests that most of the difficulties found in previous work may be related to large systematic effects in the atmospheric parameter determination and/or inaccuracies in the atomic data.
Carbon Abundance of Stars in the LAMOST-Kepler Field
The Astronomical Journal
The correlation between host star iron abundance and the exoplanet occurrence rate is well established and arrived at in several studies. Similar correlations may be present for the most abundant elements, such as carbon and oxygen, which also control the dust chemistry of the protoplanetary disk. In this paper, using a large number of stars in the Kepler field observed by the LAMOST survey, it has been possible to estimate the planet occurrence rate with respect to the host star carbon abundance. Carbon abundances are derived using synthetic spectra fit of the CH- G-band region in the LAMOST spectra. The carbon abundance trend with metallicity is consistent with the previous studies and follows the Galactic chemical evolution (GCE). Similar to [Fe/H], we find that the [C/H] values are higher among giant-planet hosts. The trend between [C/Fe] and [Fe/H] in planet hosts and single stars is similar; however, there is a preference for giant planets around host stars with a subsolar [C/...
C/O Ratios of Stars with Transiting Hot Jupiter Exoplanets
The Astrophysical Journal, 2014
The relative abundances of carbon and oxygen have long been recognized as fundamental diagnostics of stellar chemical evolution. Now, the growing number of exoplanet observations enable estimation of these elements in exoplanetary atmospheres. In hot Jupiters, the C/O ratio affects the partitioning of carbon in the major observable molecules, making these elements diagnostic of temperature structure and composition.
Galactic Chemical Evolution of Exoplanet Hosting Stars: Are High-mass Planetary Systems Young?
The Astronomical Journal
The imprints of stellar nucleosynthesis and chemical evolution of the galaxy can be seen in different stellar populations, with older generation stars showing higher α-element abundances and the later generations becoming enriched with iron-peak elements. The evolutionary connections and chemical characteristics of circumstellar disks, stars, and their planetary companions can be inferred by studying the interdependence of planetary and host star properties. Numerous studies in the past have confirmed that high-mass giant planets are commonly found around metal-rich stars, while the stellar hosts of low-mass planets have a wide range of metallicity. In this work, we analyzed the detailed chemical abundances for a sample of >900 exoplanet hosting stars drawn from different radial velocity and transit surveys. We correlate the stellar abundance trends for α- and iron-peak elements with the planets’ mass. We find the planet mass–abundance correlation to be primarily negative for α-e...
2010
We present a determination of photospheric parameters and C abundances for a sample of 172 G and K dwarfs, subgiants, and giants with and without detected planets in the solar neighbourhood. The analysis was based on high S/N and high resolution spectra observed with the ELODIE spectrograph, and for which the observational data was publicly available. We intend to contribute precise and homogeneous C abundances in studies that compare the behaviour of light elements in stars, hosting planets or not. This will bring new arguments to the discussion of possible anomalies that have been suggested and will contribute to a better understanding of different planetary formation process. The photospheric parameters were computed through the excitation potential, equivalent widths, and ionisation equilibrium of Fe lines selected in the spectra. C abundances were derived from spectral synthesis applied to prominent molecular head bands of C_2 Swan (5128 and 5165) and to a C atomic line (5380.3). The distribution of [C/Fe] vs. [Fe/H] shows no difference in the behaviour of planet-host stars in comparison with stars for which no planet was detected, for both dwarf and giant subsamples. This result is in agreement with the hypothesis of primordial origin for the chemical abundances presently observed instead of self-enrichment during the planetary system formation and evolution. Additionally, giants are clearly depleted in [C/Fe] (~0.14 dex) when compared with dwarfs, which is probably related to evolution-induced mixing of H-burning products in the envelope of evolved stars. Subgiants, although in small number, seems to follow the same C abundance distribution as dwarfs. We also analysed the kinematics of the sample stars that, in majority, are members of the Galaxy's thin disc. Finally, comparisons with other analogue studies were performed and, within the uncertainties, showed good agreement.