# Using APOGEE Stellar Parameters

This page has important information on the DR16 APOGEE stellar parameters. We recommend all users interested in using these data read this page. A proper understanding of how the quality of the supplied parameters is flagged using bitmasks is useful to identify spectra/stars that likely are inaccurately analyzed by the pipeline. If you, after exploration, find that you would like to use the DR16 stellar parameters in a publication, also take the time to read Jönsson et al. (in prep.), which supplies even more information and quality assessment.

The APOGEE Stellar Parameters and Abundances Pipeline (ASPCAP) works in two steps. First, it determines stellar parameters using a global fit over the entire spectral range. Second, it sequentially fits each elemental abundance using limited spectral windows and assuming the initially derived parameters. For more information, see the pages on ASPCAP and read Jönsson et al. (in prep.). The fundamental parameters in ASPCAP include both traditional parameters (e.g., effective temperature, surface gravity, and overall metallicity) but also some initial estimates of abundances.

The basic stellar parameters -- effective temperature and surface gravity -- are scientifically important (e.g., in the determination of distances), but the global-fit abundance parameters may also be of utility, as they provide an average abundance of multiple elements as determined from the entire spectrum.

As described below, calibration relations have been applied to some of the ASPCAP parameters. We provide uncalibrated parameters for all stars and calibrated parameters for a subset. In the allStar file, the uncalibrated --spectroscopic -- parameters are stored in an array called FPARAM, while the calibrated parameters are stored in a PARAM array. The order of the parameters in these arrays are [effective temperature: Teff, surface gravity: log g, microturbulence: vmicro, overall metal abundance: [M/H], carbon abundance: [C/M], nitrogen abundance: [N/M], & alpha-element abundance: [α/M], vsini/vmacro]. For convenience, the data in these arrays have been duplicated in more specific arrays for many stellar parameters, see the sections below and/or the allStar data model.

It is important to note that the calibration of stellar parameters is performed in a post-process phase. So, similar to the previous data releases, the spectroscopic stellar parameters are used in the derivation of the 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 accurate element abundance determination from blended spectral features (a phenomenon common in the H-band with its many molecular lines). Alternatively, the use of the calibrated stellar parameters could potentially improve the accuracy of abundances derived from spectral transitions very sensitive to Teff or log g.

While we characterize the spectra using eight parameters (Teff, log g, vmicro, [M/H], [C/M], [N/M], [α/M], vsini/vmacro), there are likely to be degeneracies between some parameters in certain regions of parameter space. For example, some warmer stars (where fewer lines can be seen), may masquerade as cooler, metal-poor stars, and vice versa.

### Data Access

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

For users with only interest in part of the data, the ASPCAP output for all stars in a given field (i.e., location in the sky) is stored in a single aspcapField file. Results for each star are stored in aspcapStar files.

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

## Effective Temperature, Teff

Spectroscopic Teff for all stars have been calibrated using a calibration to photometric Teff determined from low reddening stars. The calibrated values are provided in the TEFF (and PARAM array), while the spectroscopic -- uncalibrated -- values are provided in the TEFF_SPEC (and FPARAM array) in the allStar file. Uncertainties are estimated using the scatter around the calibration relations, parameterized as a function of Teff, [M/H], and S/N, and are presented in TEFF_ERR.

## Surface gravity (log $g$)

Stellar surface gravities have been calibrated for giants using asteroseismic values from 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 (and PARAM array), while spectroscopic values can be found in the LOGG_SPEC (and FPARAM array). Uncertainties are estimated from the scatter around the calibration relations, parameterized as a function of Teff, [M/H], and S/N, and are presented in LOGG_ERR.

## The Abundance Parameters

At the stellar parameter determination step, ASPCAP fits for the abundances [M/H], [α/M], [C/M], and [N/M], using a global fit over the entire spectrum. After the parameters are determined, abundances of individual elements (including C and N) are fit using portions (windows) of the spectrum with maximum sensitivity to the abundance of each element. In general, if you are interested in abundances, we recommend using the abundance from this second run that uses the windows, which is described in Using APOGEE Stellar Abundances.

We have found that [M/H] is closely correlated with [Fe/H]. For an overall metallicity, users may choose either one; however, note that [M/H] determination may include information from different elements in different temperature ranges.

Different α-elements likely influence the [α/M] ratio at different temperatures. As an average over multiple elements, [α/M] may produce higher precision abundances (as judged from the internal scatter within clusters) than the abundances of any individual α-element.

For carbon and nitrogen, the global and window fits are well correlated. Still, we recommend the use of the window fits since they concentrate on areas of the spectrum most sensitive to the carbon and nitrogen abundances.

Information about potential issues with the ASPCAP parameters is stored as a set of bitmasks. The ASPCAPFLAG bitmask is used to flag potential issues with the star and/or with specific parameters for that star. In addition, there is a separate bitmask for each parameter that flags possible conditions for that parameter. In the APOGEE data files, these parameter bitmasks are stored in a PARAMFLAG bitmask array; in the CAS, each parameter has its own named PARAMFLAG bitmask (TEFF_FLAG, LOGG_FLAG, M_H_FLAG, PARAM_C_M_FLAG, PARAM_N_M_FLAG, ALPHA_M_FLAG). Users need to be sure to consult the bitmasks and employ the derived parameters accordingly. For individual element abundances, there is an additional ELEMFLAG that should be consulted as described in Using APOGEE Stellar Abundances.

Various conditions trigger the setting of individual bits. For most quantities, there is a WARN condition and a BAD condition, where the latter means that the result is unreliable and the former means that it should be used with caution. To facilitate the judicious use of the data, we provide summary STAR_BAD and STAR_WARN bits in addition to individual bits for various parameters and different conditions (as discussed further below). The STAR_BAD bit flags unreliable results: it is set if any critical individual bits in ASPCAPFLAG are set to BAD; more specifically, if any of TEFF_BAD, LOGG_BAD, CHI2_BAD, COLORTE_BAD, ROTATION_BAD, or SN_BAD bits are set, or any individual parameter is near a grid edge, then BAD will be set. The STAR_WARN bit is set if the WARN bits are set for Teff (TEFF) or log g (LOGG) (but not for [M/H], [α/M], [C/M], or [N/M]) or for WARN bits related to CHI2, ROTATION, SN, or COLORTE.

In addition to the ASPCAPFLAG bits, several star level flag bits may also be relevant, as encoded in the STARFLAG bitmask. In particular, users may want to beware of stars with significant radial velocity variation (as indicated by VSCATTER), which may be binary stars (although most cases of binarity are not expected to lead to stellar parameter issues, e.g., if the luminosity ratio is large). Users may also want to be more cautious about stars that are flagged as having potential issues from persistence in the detectors (PERSIST_HIGH, etc. bits in the STARFLAG mask).

Details of different bits in the ASPCAPFLAG are discussed below.

### Poor Matches to Synthetic Spectra (CHI2)

The synthetic spectra do not always provide good matches to real spectra because of possible issues with the line list or model atmospheres adopted or due to the star having an atypical chemical composition (e.g., carbon stars, S-type stars, etc.). The quality of the match is characterized by the ASPCAP_CHI2 value. Cooler and more metal-rich stars have worse fits than warmer or more metal-poor stars, which is not unexpected given the larger number and greater strength of many absorption features at cooler temperatures.  The residuals at cooler temperatures are likely to be dominated by systematic uncertainties. Consequently, results at these temperatures may be less reliable, judging from the quality of the matches to the synthetic spectra.  If systematic effects are the dominant source of uncertainty, ASPCAP_CHI2 by itself is not a good quantity to discriminate, especially bad fits, because stars with higher signal-to-noise will have higher χ2. We calculate ASPCAP_CHI2/(SNR/100)2; if this value is greater than 50, the star with CHI2_BAD and we set CHI2_WARN if it exceeds 30.

### Signal-to-Noise (SN)

APOGEE's goal is to achieve S/N=100 per half-resolution element for all APOGEE stars. In the combined spectrum, this goal per half-resolution element corresponds to S/N~80 per pixel, because the combined (apStar) spectra are sampled at roughly three pixels per resolution element. Both the summary data files and CAS report the S/N per pixel. While the S/N goal was mostly met, there are a few stars that have lower S/N. Because the quality of the parameters and abundances depends on S/N, we set the SN_BAD bit in ASPCAPFLAG when the combined spectrum S/N<50 and the SN_WARN when S/N<70, based on analysis of calibration stars with a range of S/N.

Please see the Caveats page for the best S/N parameter to use.

### Color-Temperature Relation (COLORTE)

There is a relation between the dereddened color of a star and its effective temperature. For each star, we calculate the photometric temperature from the relations of  González Hernández and Bonifacio (2009), the observed J-K color, and the expected reddening determined 2MASS and 4.5 micron photometry (using the RJCE method, Majewski et al. (2011). If the photometric temperature differs by more than 1000 K from the ASPCAP spectroscopic temperature, the COLORTE_BAD bit in ASPCAPFLAG is set; if it deviates by more than 500 K than then COLORTE_WARN bit is set.

The technique used by ASPCAP — matching against a template library of synthetic spectra — is only valid to the degree to which stars are represented in the library. Currently, the library of giant spectra for giants does not include rotation, and so, any giants with significant rotation will not be well-matched. The rotation is estimated during the RV determination by comparing correlation widths. More specifically, the width of the cross-correlation peak of the science spectrum against its best-matching template is compared to the width of the autocorrelation of the best-matching template. When the ratio of the cross-correlation width to the autocorrelation width is greater than 2, the ROTATION_BAD bit in ASPCAPFLAG is set; when it is greater than 1.5, the ROTATION_WARN bit is set.
Each parameter has an associated WARN and BAD bit in ASPCAPFLAG. These are set to BAD if the derived parameter is within 1/8 of a grid spacing from a grid edge, and to WARN if the derived parameter is within 1/2 the grid spacing from a grid edge.