ASPCAP

One main objective of the APOGEE survey is to extract the chemical abundances of multiple elements for the entire stellar sample. This is achieved by comparing APOGEE observations to a large library of synthetic spectra and determining the best matching synthetic spectrum using the code FERRE (Allende Prieto et al. 2006), which additionally allows for interpolation within the library.

ASPCAP employs a two-step process to extract abundances: first, the stellar atmospheric parameters are determined by fitting the entire APOGEE spectrum; and second, these parameters are then used to derive an individual element abundance by fitting over limited-wavelength regions of the spectrum (windows) that are dominated by spectral features associated with that element. The H-band wavelength regime covered by the APOGEE spectra encompasses a vast number of atomic transitions for a variety of elements. Molecular features, predominantly from CN, CO, and OH, can also be very prominent, which is especially true for the cooler stars that comprise the bulk of the APOGEE sample. The ASPCAP global (full-wavelength) fit takes into account that variations in certain elemental abundance ratios can significantly effect the equation of state (e.g. through CO formation or contributing free electrons) or the opacity. Accordingly, the stellar parameters portion of the ASPCAP pipeline has the potential to allow for variations in eight parameters: effective temperature (T_eff), surface gravity (log g), microturbulence (v_micro), macroturbulence/rotation (v_macro/vsini), overall metal abundance ([M/H]), relative α-element abundance ([α/M]), carbon abundance ([C/M]), and nitrogen abundance ([N/M]). The possible values for the different parameters vary over the HR-diagram because of how the spectral grids are constructed; the detailed composition of the grid is given in Jönsson et al. (2020).

The model atmospheres used in DR12 are described in Meszaros et al. (2012) and the spectral libraries in Zamora et al. (2015). Updates to the model atmosphere and synthetic spectral grids for DR13 and DR14 are described in Holtzman et al. (2018) and DR16 modifications are detailed in Jönsson et al. (2020). The atomic and line list data used for synthesizing the spectra in DR12-14 is reported in Shetrone et al. (2015), while the data used in DR16 is described in V. Smith et al. (in prep.).

As in earlier data releases, two sets of values for T_eff and log g are provided for each target star: one set is directly from the FERRE-determined best-fit synthetic spectrum and another set that have been slightly modified -- calibrated -- to better align with independent measurements of T_eff and log g. The exact methodology for these calibrations has changed over time. While a short description of the DR16 calibrations is provided here, users should consult Jönsson et al. (2020) for further details.

Stellar effective temperatures, T_eff, for all stars have been calibrated using a relation derived from the photometric T_eff for low-reddened stars. The calibrated values are provided in the TEFF tag (and PARAM array), while the spectroscopic -- uncalibrated -- values are provided in the TEFF_SPEC (and FPARAM array) in the allStar file. Uncertainties are are presented in TEFF_ERR tag; these are estimated from the scatter around the calibration relations and are parameterized as a function of T_eff, [M/H], and S/N.

Stellar surface gravities, log g, have been calibrated for giants using asteroseismic values for stars in the Kepler field and for dwarfs using a combination of asteroseismic values (for warmer dwarfs) and isochrones (for cooler dwarfs). Calibrated surface gravities are presented in the LOGG tag (and PARAM array), while the uncalibrated, spectroscopic values can be found in the LOGG_SPEC tag (and FPARAM array). Uncertainties are presented in the LOGG_ERR tag; these are estimated from scatter around the calibration relations, parameterized as a function of T_eff, [M/H], and S/N.

Note that the calibration of the stellar parameters is a post-processing step (i.e., implemented after the ASCAP derivations are performed). Similar to prior data releases, the DR16 spectroscopic stellar parameters (i.e., the FPARAM parameter array) are employed in the determination of the individual element abundances. As
the spectroscopic stellar parameters achieve the best fit between the synthetic and observed spectrum, this in turn, can allow for a more robust element abundance derivation from blended spectral features. Alternatively, the use of the calibrated stellar parameters could potentially improve the accuracy of abundances derived from spectral transitions very sensitive to T_eff or log g.

ASPCAP employs the same eight-dimensional grid of synthetic stellar spectra for the derivation of the stellar abundances as was used for the determination of the stellar parameters. Now, for the individual element abundances, the fit is performed only in the spectral windows encompassing the transitions corresponding to the element of interest. Additionally, the spectral fitting process entails the variation of the appropriate abundance dimension -- [M/H], [α/M], [C/M] or [N/M] -- depending on the element. Some of the element abundance windows have significant levels of blending/line contamination (which are not taken into consideration by ASPCAP). Consequently, these windows are assigned a lower weight in the abundance determination. For elements with few available transitions, this could potentially lead to the determination of an element abundance from a single line.

In DR16, the abundance determination of 26 species is attempted; C, C I, N, O, Na, Mg, Al, Si, P, S, K, Ca, Ti, Ti II, V, Cr, Mn, Fe, Co, Ni, Cu, Ge, Rb, Ce, Nd, and Yb. The accuracy of an individual element varies by the element and with the type of star; certain parts of the element-star parameter space have been deemed impossible to explore with the APOGEE data. Nevertheless, the ASPCAP-derived element abundances are stored in the FELEM array and may be employed by some users for exploratory purposes. Calibrated element abundance values are supplied in the X_H and X_M arrays. Specifically, the calibration of these element abundances is described as follows: a zero-point shift is applied such that the median of the elemental abundances in stars near solar metallicity in the solar neighborhood are adjusted to have [X/M] = 0. Finally, the relative element abundances ([X/Fe]) are presented in the "named" tags, e.g. C_FE, N_FE, MG_FE, etc. These are derived from the X_M and M_H values, but are not populated for stars and/or elements that were not well fit (see Jönsson et al. (2020) for details). This means that the named tags are the most conservative sets of abundances and are recommended for most purposes. However, it should be noted that certain abundances in the named tags may still suffer from systematic errors, in particular for cooler stars.

For more information on this and the APOGEE abundance scale, see Using APOGEE Stellar abundances and Jönsson et al. (2020). For more information on the spectral lines used for determining the neutron-capture elements Nd and Ce, see Hasselquist et al. (2016) and Cunha et al. (2017), respectively.

Empirical uncertainties for the abundances are derived from the scatter in abundances from stars in the sample that are observed in two or more different fields as ASPCAP analyzes each field separately. These uncertainties are parameterized as a function of T_eff, [M/H], and S/N, with different relations for giants and dwarfs. The empirical uncertainties are presented in the X_H_ERR and X_M_ERR arrays, as well as in the named tags, e.g.,C_FE_ERR, N_FE_ERR, MG_FE_ERR. The current ASPCAP methodology does not intrinsically provide upper limits.

ASPCAP is generally run separately for each APOGEE field (i.e. location in the sky). The ASPCAP output for all stars in the field is stored in a single aspcapField file (data model). Results for each individual star are stored in aspcapStar files ( data model). See the links for a full description of the data in these files.

All of the derived parameters and abundances are stored in the master allStar file with contents described in the data model.

Data Access also contains a full description of files provided for the user.