Overview of APOGEE Data Releases 13 and 14
This paper comprehensively elucidates the methodologies and improvements implemented in the Apache Point Observatory Galactic Evolution Experiment (APOGEE) within the Sloan Digital Sky Survey (SDSS) Data Releases 13 and 14. It outlines the enhancements made since DR12, specifically in data reduction, stellar parameter estimation, and abundance determination processes. The primary aim is to obtain and improve high-resolution infrared spectra of red giants to better map the kinematic and chemical structure of stars across the Milky Way.
Data Reduction and Improvements
Several advancements in data reduction techniques are highlighted, including the enhanced handling of telluric absorption and persistence. The paper describes the deployment of a persistence correction algorithm for both the "blue" and "green" chips of the spectrograph, aiming to mitigate the undesirable effects observed in DR12 due to persistence. The telluric correction is notably improved by adopting a more precise line spread function (LSF) characterization, which directly impacts the accuracy of derived spectra. These improvements are critical, especially when dealing with spectra dominated by atmospheric CO2 absorption.
Stellar Parameter and Abundance Pipeline Enhancements
The APOGEE Stellar Parameters and Chemical Abundances Pipeline (ASPCAP) receives substantial refinement characterized by the inclusion of cooler spectral grids and adoption of Turbospectrum for spectral synthesis. This update addresses limitations in DR12 related to model atmospheres, line lists, and LSFs. It also involves calibrating microturbulent and macroturbulent velocities to reflect more accurate spectral fits. The introduction of MARCS stellar atmospheres for stars cooler than 3500 K offers a more cohesive modeling strategy moving forward.
Calibration and Uncertainty Assessment
For calibrated effective temperatures, DR14 introduces a quadratic correction function derived from comparisons with photometric scales, addressing the systematic metallicity-dependent discrepancies seen in uncalibrated results. Surface gravity assessments capitalize on comparisons with asteroseismic data, although challenges remain in calibrating surface gravities for dwarf stars due to the lack of stellar models and calibrators. Elemental abundance calibrations are handled using cluster photometric effective temperatures, ensuring robust internal consistency across the range of observed stellar spectra.
Introduction of The Cannon as a Complementary Technique
A novel approach involved the application of The Cannon, a data-driven model that can supplement ASPCAP by exploiting all spectral information available. While promising higher precision in parameter determination, censoring of wavelength regions is introduced to mitigate correlations in untrained regions, primarily showcasing improvements in elements with spectral lines that The Cannon models accurately. However, the significance of uncensored versus censored mode in terms of reliability calls for careful future investigation.
Implications and Future Developments
The findings presented in this paper signify remarkable progress in deriving reliable astrophysical measurements from APOGEE spectra while acknowledging the necessity for continued refinement. As future data releases aim for unified model atmospheres and enhanced line lists, APOGEE's capability in mapping Galactic evolution continues to enhance. The dual approach of combining refined ASPCAP methodologies with data-driven techniques like The Cannon highlights the evolution toward exploiting comprehensive datasets in astronomical spectroscopic surveys.
In conclusion, while substantial improvements have been made, the study advocates for ongoing calibration and methodological advancement to maintain precision and accuracy in spectral analyses crucial for Galactic archaeology.