Calibrations of Atmospheric Parameters Obtained from the First Year of SDSS-III APOGEE Observations

TL;DR

This paper presents empirical calibrations of stellar parameters from SDSS-III APOGEE data, improving accuracy for temperature and metallicity, based on cluster and Kepler star observations, with implications for Galactic studies.

Contribution

It introduces calibration relations for APOGEE stellar parameters that enhance the accuracy of temperature, gravity, and metallicity measurements using cluster and asteroseismic data.

Findings

ASPCAP achieves good accuracy for temperature and metallicity.

Systematic errors in surface gravity are identified and corrected.

Metallicity precision is better than 0.1 dex, temperature better than 150 K.

Abstract

The SDSS-III Apache Point Observatory Galactic Evolution Experiment (APOGEE) is a three year survey that is collecting 100,000 high-resolution spectra in the near-IR across multiple Galactic populations. To derive stellar parameters and chemical compositions from this massive data set, the APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) has been developed. Here, we describe empirical calibrations of stellar parameters presented in the first SDSS-III APOGEE data release (DR10). These calibrations were enabled by observations of 559 stars in 20 globular and open clusters. The cluster observations were supplemented by observations of stars in NASA's Kepler field that have well determined surface gravities from asteroseismic analysis. We discuss the accuracy and precision of the derived stellar parameters, considering especially effective temperature, surface gravity, and metallicity; we also briefly discuss the derived results for the abundances of the alpha-elements, carbon, and nitrogen. Overall, we find that ASPCAP achieves reasonably accurate results for temperature and metallicity, but suffers from systematic errors in surface gravity. We derive calibration relations that bring the raw ASPCAP results into better agreement with independently determined stellar parameters. The internal scatter of ASPCAP parameters within clusters suggests that, metallicities are measured with a precision better than 0.1 dex, effective temperatures better than 150 K, and surface gravities better than 0.2 dex. The understanding provided by the clusters and Kepler giants on the current accuracy and precision will be invaluable for future improvements of the pipeline.