Abundances, Stellar Parameters, and Spectra From the SDSS-III/APOGEE Survey (1501.04110v1)

Abstract: The SDSS-III/APOGEE survey operated from 2011-2014 using the APOGEE spectrograph, which collects high-resolution (R~22,500), near-IR (1.51-1.70 microns) spectra with a multiplexing (300 fiber-fed objects) capability. We describe the survey data products that are publicly available, which include catalogs with radial velocity, stellar parameters, and 15 elemental abundances for over 150,000 stars, as well as the more than 500,000 spectra from which these quantities are derived. Calibration relations for the stellar parameters (Teff, log g, [M/H], [alpha/M]) and abundances (C, N, O, Na, Mg, Al, Si, S, K, Ca, Ti, V, Mn, Fe, Ni) are presented and discussed. The internal scatter of the abundances within clusters indicates that abundance precision is generally between 0.05 and 0.09 dex across a broad temperature range; within more limited ranges and at high S/N, it is smaller for some elemental abundances. We assess the accuracy of the abundances using comparison of mean cluster metallicities with literature values, APOGEE observations of the solar spectrum and of Arcturus, comparison of individual star abundances with other measurements, and consideration of the locus of derived parameters and abundances of the entire sample, and find that it is challenging to determine the absolute abundance scale; external accuracy may be good to 0.1-0.2 dex. Uncertainties may be larger at cooler temperatures (Teff<4000K). Access to the public data release and data products is described, and some guidance for using the data products is provided.

• The paper presents a detailed methodology for obtaining high-resolution near-IR spectra from over 150,000 stars to derive radial velocities and elemental abundances.
• It calibrates stellar parameters using photometric and asteroseismic data, revealing systematic offsets such as a 90 K correction in effective temperature.
• The study emphasizes public data accessibility and offers insights to refine spectroscopic models, advancing Galactic archaeology and stellar physics research.

An Expert Review of the SDSS-III/APOGEE Survey: Abundances, Stellar Parameters, and Spectra

The paper "Abundances, stellar parameters, and spectra from the SDSS-III/APOGEE Survey" offers a comprehensive overview of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) within the third phase of the Sloan Digital Sky Survey (SDSS-III). Conducted from 2011 to 2014, the APOGEE project aimed to map the kinematics and chemical compositions of the Milky Way using a sophisticated near-infrared (near-IR) spectrograph. This analysis focused on obtaining high-resolution spectral data from over 150,000 stars to measure radial velocities, stellar parameters, and elemental abundances.

The APOGEE spectrograph, capable of collecting 300 simultaneous spectra, operates in the H-band (1.51-1.70 μm) at a resolution of ~22,500. The spectrograph's design, including a multi-fiber system and a volume phase holographic grating, facilitated detailed observations, albeit the initial data revealed projection-related astigmatisms remedied during early survey operations.

Key Features of the APOGEE Data Release

1. Data Products and Spectroscopic Analysis:
   • The data release includes radial velocity measurements and chemical abundances for 15 elements across a wide range of stellar temperatures.
   • Internal consistency within clusters indicates abundance precision ranging between 0.05 and 0.09 dex, though cooler stars (T < 4000K) present greater uncertainty due to line features.
2. Calibration and Validation:
   • Stellar parameters, including effective temperature (Teff), surface gravity (log g), and microturbulence velocity (vmicro), were calibrated against photometric surveys and asteroseismic data, finding offsets needing systematic correction—Teff, for example, required calibration to be around 90 K higher than derived spectroscopically.
   • Elemental abundances presented in cluster calibration show a temperature-dependent spread, necessitating empirical correction for better homogeneity.
3. Assessments and Public Data Accessibility:
   • The analysis emphasizes the importance of spectroscopic modeling, particularly in handling non-LTE effects and stellar rotation, which hampers dwarf star parameterizations.
   • Extensive datasets are distributed via the Science Archive Server (SAS) and the Catalog Archive Server (CAS), with accompanying software for comprehensive data access and analysis.

Strong Numerical Results and Implications

APOGEE's data release reports numerous reliable measurements, like distributions of [$\alpha$/Fe] ratios across stellar populations, showing distinct Galactic chemical history threads. However, it also identifies systematic deviations at lower metallicities, particularly for Ti, revealing limitations in current synthetic spectral models.

Implications and Future Directions in Stellar Physics and Spectroscopy

The APOGEE results, while precise, underscore the challenge of accurate external abundance scaling, especially at extremes of metallicity. The survey's observations and data products are set to influence extensive theoretical models and practical methods within the astronomical community, pointing toward further refinement in atmospheric models (e.g., extensions with MARCS model atmospheres for cooler stars).

Given its lineage and meticulous detail, the APOGEE survey not only lays groundwork for expansive Milky Way science but also forms a cornerstone for follow-up investigations in the SDSS-IV and future incarnations of stellar and Galactic surveys. The continuous advancement in spectroscopic techniques and data calibration, as outlined in this survey, represents a substantial effort in the field of Galactic archaeology and high-precision stellar characterization.