The spectroscopic indistinguishability of red giant branch and red clump stars

Stellar spectroscopy provides useful information on the physical properties of stars such as effective temperature, metallicity and surface gravity (log g). However, those photospheric characteristics are often hampered by systematic uncertainties. The joint spectro-seismo project (APOKASC) of field red giants has revealed a puzzling offset between the log g determined spectroscopically and those determined using asteroseismology, which is largely dependent on the stellar evolutionary status. Therefore, in this letter, we aim to shed light on the spectroscopic source of the offset using the APOKASC sample. We analyse the log g discrepancy as a function of stellar mass and evolutionary status and discuss the impact of He and carbon isotopic ratio. We first show that for stars at the bottom of the red giant branch, the discrepancy between spectroscopic and asteroseismic log g depends on stellar mass. This indicates that the discrepancy is related to CN cycling. We demonstrate that the C isotopic ratio ($\rm ^{12}C/^{13}C$) has the largest impact on the stellar spectrum. We find that this log g discrepancy shows a similar trend in mass as the $\rm ^{12}C/^{13}C$ ratios expected by stellar evolution theory. Although we do not detect a direct signature of $\rm ^{13}C$, the data suggests that the discrepancy is tightly correlated to the production of $\rm ^{13}C$. Moreover, by running a data-driven algorithm (the Cannon) on a synthetic grid trained on the APOGEE data, we quantitatively evaluate the impact of various $\rm ^{12}C/^{13}C$ ratios. While we have demonstrated that $\rm ^{13}C$ impacts all parameters, the size of the impact is smaller than the observed offset in log g. If further tests confirm that $\rm ^{13}C$ is not the main element responsible of the log g problem, the number of spectroscopic effects remaining to be investigated is now relatively limited. [Abridged]Comment: 4 Pages, 6 Figures. Accepted for publication in A&

Carbon, nitrogen and α\alpha-element abundances determine the formation sequence of the Galactic thick and thin disks

Using the DR12 public release of APOGEE data, we show that thin and thick disk separate very well in the space defined by [$\alpha$/Fe], [Fe/H] and [C/N]. Thick disk giants have both higher [C/N] and higher [$\alpha$/Fe] than do thin disk stars with similar [Fe/H]. We deduce that the thick disk is composed of lower mass stars than the thin disk. Considering the fact that at a given metallicity there is a one-to-one relation between stellar mass and age, we are then able to infer the chronology of disk formation. Both the thick and the thin disks - defined by [$\alpha$/Fe] -- converge in their dependance on [C/N] and [C+N/Fe] at [Fe/H]$\approx$-0.7. We conclude that 1) the majority of thick disk stars formed earlier than did the thin disk stars 2) the formation histories of the thin and thick disks diverged early on, even when the [Fe/H] abundances are similar 3) that the star formation rate in the thin disk has been lower than in the thick disk, at all metallicities. Although these general conclusions remain robust, we also show that current stellar evolution models cannot reproduce the observed C/N ratios for thick disk stars. Unexpectedly, reduced or inhibited canonical extra-mixing is very common in field stars. While subject to abundance calibration zeropoint uncertainties, this implies a strong dependence of non canonical extra-mixing along the red giant branch on the initial composition of the star and in particular on the $\alpha$ elemental abundance.Comment: 12 pages, 14 figures, accepted in MNRA

IP Eri: A surprising long-period binary system hosting a He white dwarf

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M$_\odot$, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with He-burning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are $T_{\rm eff} = 4960$ K, $\log{g} = 3.3$, [Fe/H] = 0.09 and $\xi = 1.5$ km/s. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper (Siess et al. 2014).Comment: 14 pages, 10 figures, 4 tables, accepted for publication in A&A (Update of Table 3, Fig. 8 and text in Sect. 5.1, 5.3 and 6 due to minor corrections on N and Y II

A holistic approach to carbon-enhanced metal-poor stars

By considering the various CEMP subclasses separately, we try to derive, from the specific signatures imprinted on the abundances, parameters (such as metallicity, mass, temperature, and neutron source) characterizing AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion.To put CEMP stars in a broad context, we collect abundances for about 180 stars of various metallicities, luminosity classes, and abundance patterns, from our own sample and from literature. First, we show that there are CEMP stars which share the properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This strongly points at the operation of the 13C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s- and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs stars exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. Finally, we argue that most CEMP-no stars (with no overabundances for the neutron-capture elements) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2). This trend is not predicted by any of the current AGB models.Comment: 27 pages, 24 figures, accepted for publication in A&