The spectroscopic indistinguishability of red giant branch and red clump stars

Abstract

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 (${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$) has the largest impact on the stellar spectrum. We find that this log g discrepancy shows a similar trend in mass as the ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratios expected by stellar evolution theory. Although we do not detect a direct signature of ${}^{13}\mathrm{C}$, the data suggests that the discrepancy is tightly correlated to the production of ${}^{13}\mathrm{C}$. 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 ${}^{12}\mathrm{C}{/}^{13}\mathrm{C}$ ratios. While we have demonstrated that ${}^{13}\mathrm{C}$ impacts all parameters, the size of the impact is smaller than the observed offset in log g. If further tests confirm that ${}^{13}\mathrm{C}$ 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]

arXiv comment4 Pages, 6 Figures. Accepted for publication in A&A

References in corpus (16)

• Gaia Data Release 1. Summary of the astrometric, photometric, and survey properties
• The GALAH Survey: Scientific Motivation
• ASPCAP: The Apogee Stellar Parameter and Chemical Abundances Pipeline
• Gaia Data Release 1: Astrometry - one billion positions, two million proper motions and parallaxes
• Gaia broad band photometry
• Abundances, Stellar Parameters, and Spectra From the SDSS-III/APOGEE Survey
• The Cannon: A data-driven approach to stellar label determination
• The APOKASC Catalog: An Asteroseismic and Spectroscopic Joint Survey of Targets in the Kepler Fields
• Gaia FGK Benchmark Stars: Effective temperatures and surface gravities
• Thermohaline instability and rotation-induced mixing. III - Grid of stellar models and asymptotic asteroseismic quantities from the pre-main sequence up to the AGB for low- and intermediate-mass stars at various metallicities
• Gaia-ESO Survey: The analysis of high-resolution UVES spectra of FGK-type stars
• Testing the asteroseismic scaling relations for Red Giants with Eclipsing Binaries Observed by Kepler
• Calibrations of Atmospheric Parameters Obtained from the First Year of SDSS-III APOGEE Observations
• An accurate and self-consistent chemical abundance catalogue for the APOGEE/Kepler sample
• Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
• Nitrogen depletion in field red giants: mixing during the He flash?

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