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Register a new userCarbon, isotopic ratio $^{12}$C/$^{13}$C and nitrogen in solar twins: constraints for the chemical evolution of the local disc
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Abundances of light elements in dwarf stars of different ages are important constraints for stellar yields, Galactic chemical evolution and exoplanet chemical composition studies. We have measured C and N abundances and $^{12}$C/$^{13}$C ratios for a sample of 63 solar twins spanning a wide range in age, based on spectral synthesis of a comprehensive list of CH,A-X and CN,B-X features using HARPS spectra. The analysis of 55 thin disc solar twins confirms the dependences of [C/Fe] and [N/Fe] on [Fe/H]. [N/Fe] is investigated as a function of [Fe/H] and age for the first time for these stars. Our derived correlation [C/Fe]-age agrees with works for solar-type stars and solar twins, but the [N/Fe]-age correlation does not. The relations [C,N/Fe]-[Fe/H] and [C,N/Fe]-age for the solar twins lay under-solar. $^{12}$C/$^{13}$C is found correlated with [Fe/H] and seems to have decreased along the evolution of the local thin disc. Predictions from chemical evolution models for the solar vicinity corroborate the relations [C,N/Fe]-[Fe/H], $^{12}$C/$^{13}$C-age and [N/O]-[O/H], but do not for the $^{12}$C/$^{13}$C-[Fe/H] and [C/O]-[O/H] relations. The N/O ratio in the Sun is placed at the high end of the homogeneous distribution of solar twins, which suggests uniformity in the N-O budget for the formation of icy planetesimals, watery super-earths and giant planets. C and N had different nucleosynthetic origins along the thin disc evolution, as shown by the relations of [C/N], [C/O] and [N/O] against [O/H] and age. [C/N] and [C/O] are particularly observed increasing in time for solar twins younger than the Sun.
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We derive molecular-gas-phase $^{12}$C/$^{13}$C isotope ratios for the central few 100 pc of the three nearby starburst galaxies NGC 253, NGC 1068, and NGC 4945 making use of the $lambda$ $sim$ 3 mm $^{12}$CN and $^{13}$CN $N$ = 1--0 lines in the ALMA Band 3. The $^{12}$C/$^{13}$C isotopic ratios derived from the ratios of these lines range from 30 to 67 with an average of 41.6 $pm$ 0.2 in NGC 253, from 24 to 62 with an average of 38.3 $pm$ 0.4 in NGC 1068, and from 6 to 44 with an average of 16.9 $pm$ 0.3 in NGC 4945. The highest $^{12}$C/$^{13}$C isotopic ratios are determined in some of the outskirts of the nuclear regions of the three starburst galaxies. The lowest ratios are associated with the northeastern and southwestern molecular peaks of NGC 253, the northeastern and southwestern edge of the mapped region in NGC 1068, and the very center of NGC 4945. In case of NGC 1068, the measured ratios suggest inflow from the outer part of NGC 1068 into the circum-nuclear disk through both the halo and the bar. Low $^{12}$C/$^{13}$C isotopic ratios in the central regions of these starburst galaxies indicate the presence of highly processed material.
LP 876-10 is a nearby active M4 dwarf in Aquarius at a distance of 7.6 pc. The star is a new addition to the 10-pc census, with a parallax measured via the Research Consortium on Nearby Stars (RECONS) astrometric survey on the Small & Moderate Aperture Research Telescope Systems (SMARTS) 0.9-m telescope. We demonstrate that the astrometry, radial velocity, and photometric data for LP 876-10 are consistent with the star being a third, bound, stellar component to the Fomalhaut multiple system, despite the star lying nearly 6 degrees away from Fomalhaut A in the sky. The 3D separation of LP 876-10 from Fomalhaut is only 0.77+-0.01 pc, and 0.987+-0.006 pc from TW PsA (Fomalhaut B), well within the estimated tidal radius of the Fomalhaut system (1.9 pc). LP 876-10 shares the motion of Fomalhaut within ~1 km/s, and we estimate an interloper probability of ~10^{-5}. Neither our echelle spectroscopy nor astrometry are able to confirm the close companion to LP 876-10 reported in the Washington Double Star Catalog (WSI 138). We argue that the Castor Moving Group to which the Fomalhaut system purportedly belongs, is likely to be a dynamical stream, and hence membership to the group does not provide useful age constraints for group members. LP 876-10 (Fomalhaut C) has now risen from obscurity to become a rare example of a field M dwarf with well-constrained age (440+-40 Myr) and metallicity. Besides harboring a debris disk system and candidate planet, Fomalhaut now has two of the widest known stellar companions.
[ABRIDGED]We study the carbon abundances with a twofold objective. On the one hand, we want to evaluate the behaviour of carbon in the context of Galactic chemical evolution. On the other hand, we focus on the possible dependence of carbon abundances on the presence of planets and on the impact of various factors (such as different oxygen lines) on the determination of C/O elemental ratios. We derived chemical abundances of carbon from two atomic lines for 757 FGK stars in the HARPS-GTO sample. The abundances were derived with the code MOOG using automatically measured EWs and a grid of Kurucz ATLAS9 atmospheres. Oxygen abundances, derived using different lines, were taken from previous papers in this series and updated with the new stellar parameters. We find that thick- and thin-disk stars are chemically disjunct for [C/Fe] across the full metallicity range that they have in common. Moreover, the population of high-$alpha$ metal-rich stars also presents higher and clearly separated [C/Fe] ratios than thin-disk stars up to [Fe/H],$sim$,0.2,dex. The [C/O] ratios present a general flat trend as a function of [O/H] but this trend becomes negative when considering stars of similar metallicity. We find tentative evidence that stars with low-mass planets at lower metallicities have higher [C/Fe] ratios than stars without planets at the same metallicity, in the same way as has previously been found for $alpha$ elements. Finally, the elemental C/O ratios for the vast majority of our stars are below 0.8 when using the oxygen line at 6158A however, the forbidden oxygen line at 6300A provides systematically higher C/O values. Moreover, by using different atmosphere models the C/O ratios can have a non negligible difference for cool stars. Therefore, C/O ratios should be scaled to a common solar reference in order to correctly evaluate its behaviour.
The lithium abundances in a few percent of giants exceed the value predicted by the standard stellar evolution models, and the mechanisms of Li enhancement are still under debate. The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) survey has obtained over six million spectra in the past five years, and thus provides a great opportunity to search these rare objects and to more clearly understand the mechanisms of Li enhancement. Based on the high-resolution spectrum we obtained the stellar parameters ($T_mathrm{eff}$, $log g$, [Fe/H]), and determined the elemental abundances of Li, C, N, $alpha$, Fe-peak, r-process, s-process elements, and the projected rotational velocity. For a better understanding of the effect of mixing processes, we also derived the $^{12}rm{C}$ to $^{13}rm{C}$ ratio, and constrained the evolutionary status of TYC,3251-581-1 based on the BaSTI stellar isochrones. The super Li-rich giant TYC,3251-581-1 has $rm{A(Li)} = 3.51$, the average abundance of two lithium lines at $lambda = 6708$ AA and 6104 AA based on the non-local thermodynamic equilibrium (NLTE) analysis. The atmospheric parameters show that our target locates on the luminosity function bump. The low carbon isotopic ratio ($^{12}rm{C}/^{13}rm{C} = 9.0 $), a slow rotational velocity $vsin i = 2.2 rm{km,s}^{-1}$, and no sign of IR excess suggest that additional mixing after first dredge up (FDU) should occur to bring internal synthesized Li to the surface. The low carbon ($[rm{C}/rm{Fe}] sim -0.34$ ) and enhanced nitrogen ($[rm{N}/rm{Fe}] sim 0.33$) are also consistent with the sign of mixing. Given the evolutionary stage of TYC,3251-581-1 with the relatively low $^{12}rm{C}/^{13}rm{C}$, the internal production which replenishes Li in the outer layer is the most likely origin of Li enhancement for this star.
The rotational spectral lines of c-C$_3$H$_2$ and two kinds of the $^{13}$C isotopic species, c-$^{13}$CCCH$_2$ ($C_{2v}$ symmetry) and c-CC$^{13}$CH$_2$ ($C_s$ symmetry) have been observed in the 1-3 mm band toward the low-mass star-forming region L1527. We have detected 7, 3, and 6 lines of c-C$_3$H$_2$, c-$^{13}$CCCH$_2$ , and c-CC$^{13}$CH$_2$, respectively, with the Nobeyama 45 m telescope, and 34, 6, and 13 lines, respectively, with the IRAM 30 m telescope, where 7, 2, and 2 transitions, respectively, are observed with the both telescopes. With these data, we have evaluated the column densities of the normal and $^{13}$C isotopic species. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] ratio is determined to be $310pm80$, while the [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratio is determined to be $61pm11$. The [c-C$_3$H$_2$]/[c-$^{13}$CCCH$_2$] and [c-C$_3$H$_2$]/[c-CC$^{13}$CH$_2$] ratios expected from the elemental $^{12}$C/$^{13}$C ratio are 60-70 and 30-35, respectively, where the latter takes into account the statistical factor of 2 for the two equivalent carbon atoms in c-C$_3$H$_2$. Hence, this observation further confirms the dilution of the $^{13}$C species in carbon-chain molecules and their related molecules, which are thought to originate from the dilution of $^{13}$C$^+$ in the gas-phase C$^+$ due to the isotope exchange reaction: $mathrm{^{13}C^++COrightarrow{}^{13}CO+C^+}$. Moreover, the abundances of the two $^{13}$C isotopic species are different from each other. The ratio of c-$mathrm{^{13}CCCH_2}$ species relative to c-$mathrm{CC^{13}CH_2}$ is determined to be $0.20pm0.05$. If $^{13}$C were randomly substituted for the three carbon atoms, the [c-$mathrm{^{13}CCCH_2}$]/[c-$mathrm{CC^{13}CH_2}$] ratio would be 0.5. Hence, the observed ratio indicates that c-$mathrm{CC^{13}CH_2}$ exists more favorably. Possible origins of the different abundances are discussed.
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