Using APOGEE Stellar Parameters

Note that the APOGEE data in DR15 are identical to those in DR14, but use the DR15 documentation pages for reference.

The APOGEE Stellar Parameters and Abundances Pipeline (ASPCAP) works in two steps: it first determines stellar parameters using a global fit over the entire spectral range, and then it fits sequentially for individual elemental abundances over limited spectral windows using the initially derived parameters. For more information, see the pages on the ASPCAP pipeline. Note that the fundamental parameters affecting the IR spectra can include both traditional parameters (effective temperature, surface gravity, and overall metallicity) but also some initial estimates of abundances.

The basic stellar parameters -- effective temperature (Teff) and surface gravity (log g) -- 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.

The individual element abundances depend on the stellar parameters, so the accuracy of the latter is important for abundance determination. However, we note that the best-fitting effective temperature and surface gravity may not be the best physical estimates of those quantities, since the nature of the spectral analysis includes some approximations that may not be satisfied, e.g., the use of LTE, 1D plane parallel atmospheres. ASPCAP takes the approach of using the best-fitting parameters to derive abundances.

ASPCAP Parameter Tags/Columns

As described in the ASPCAP page, 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 summary data files (see parameter summary FITS files for information on how to access), the uncalibrated parameters are stored in an array called FPARAM, while calibrated parameters are stored in a PARAM array. For convenience, the summary data files also include explicit columns that restate the calibrated parameters of the PARAM array: TEFF, LOGG, M_H, ALPHA_M.

In the Catalog Archive Server (CAS) (see parameter tables for queries for access information), each parameter value is given its own column name: TEFF, LOGG, M_H, and ALPHA_M for the calibrated parameters, and FPARAM_TEFF, FPARAM_LOGG, FPARAM_M_H, FPARAM_C_M, FPARAM_N_M, and FPARAM_ALPHA_M for the uncalibrated parameters.

Regardless of how you get the parameters, it is important to pay attention to how the data have been flagged, because not all of the values are reliable. This is particularly true for uncalibrated values. For information on the flags, see the bitmask section below.

Quality of the ASPCAP Stellar Parameters

Effective temperature (Teff)

The accuracy of the ASPCAP-determined effective temperatures has been gauged by comparisons to photometrically-derived temperatures for a sample of low-reddened giants. As described on the ASPCAP page, these have been used to provide a calibration relation for effective temperature.

In general, the uncertainties on derived parameters from the FERRE fitting are unrealistically small. To provide more realistic uncertainties, we determine the scatter in the difference between the spectroscopic and photometric temperature for stars with low reddening in bins of Teff, [M/H], and signal-to-noise. We fit a surface through these and it is used to provide uncertainty estimates for effective temperature for the entire sample.

Surface gravity (log $g$)

Surface gravities have been checked/calibrated using observations of asteroseismic stars from the Kepler field, for which independent surface gravities can be obtained. These comparisons suggest that the ASPCAP gravities are still systematically high, although by less than they were for DR12. There continues to be a puzzling difference between stars identified from asteroseismology as core helium burning (red clump) stars and those identified as shell hydrogen (red giant) stars at similar temperatures. The ASPCAP gravities are offset from the corresponding asteroseismic gravities such that the ASPCAP gravities are larger for the core helium burning stars. In addition, the difference between the ASPCAP and the asteroseismic gravities appears to be a function of both surface gravity and metallicity.

We have applied separate surface gravity corrections for red giants (RGB) and red clump (RC) stars (Holtzman et al. 2018, submitted). To do so requires us to separate RGB and RC stars in the full sample for which asteroseismic information is not available. We separate RGB/RC stars on the basis of derived stellar parameters: we use a ridgeline in effective temperature that is a function of metallicity (Bovy et al. 2014) and supplement this with a separation based on the derived C/N abundance, as determined by training on the asteroseismic sample. The details of the calibration relations for RGB and RC stars can be found in the external calibration section of the ASPCAP documentation. We note a caveat regarding the RC stars that requires a recalibration of the log g for the secondary red clump (2RC); see the log g calib section for details.

The scatter of the stars around the calibration relation is relatively small, less than 0.1 dex for stars with [M/H]>-1, but the uncertainties for more metal-poor stars may be considerably larger. We provide an empirical uncertainty estimate for surface gravity busing scatter of the difference between the spectroscopic and asteroseismic gravities and then fitting a surface as a function of effective temperature, [M/H], and signal-to-noise.

For dwarfs, the spectroscopic surface gravities for dwarfs are significantly offset from that expected from stellar isochrones, in the sense that our gravities are too small. Since we do not have independent surface gravities for dwarfs, we do not provide calibrated surface gravities for these stars. However, this does not preclude us from providing calibrated abundances.

Metallicity ([M/H])

Metallicities ([M/H]) have been validated through observations of globular and open clusters. These suggest that the raw ASPCAP metallicities have some small offsets that are characterized as a function of metallicity.

[α/M]

Alpha-element enhancements ([α/M]) have been validated through observations of globular and open clusters. These suggest that the raw ASPCAP [α/M] are generally reasonable. However, at the lowest and highest metallicities, there seems to be some degeneracy between [α/M] and [M/H], such that clusters at these metallicities have a larger spread in both quantities, with a correlation between the two.

[C/M]

Our understanding of the quality of the carbon abundances is limited because we cannot assume that carbon abundances are uniform within clusters, and we do not have a comprehensive, homogeneous calibration sample. Therefore there is no calibrated version of this parameter. Note that, in addition to the parameter level [C/M], we also provide a [C/Fe] measurement from fits to windows sensitive to carbon abundance. As noted elsewhere, the [C/Fe] abundances for dwarfs should not be used because of issues with the methodology.

[N/M]

Our understanding of the quality of the nitrogen abundances is limited, because we cannot make the assumption that nitrogen abundances are uniform within clusters, and we do not have a comprehensive, homogeneous calibration sample. Note that, in addition to the parameter level [N/M], we also provide a [N/Fe] measurement from fits to windows sensitive to nitrogen abundance. As noted elsewhere, the [N/Fe] abundances for dwarfs should not be used because of issues with the methodology.

Use of abundance parameters vs. individual elemental abundances from windows

At the stellar parameter determination step, ASPCAP fits for the parameters [M/H], [α/M] and, for giants, [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) in the spectrum with maximum sensitivity to the abundance of each element. For elements that are fit globally and with windows, both abundances are provided (note that the global fit is the one reported with respect to M).

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 additionally include information from different elements in different temperature ranges. Both [M/H] and [Fe/H] have internal (temperature-dependent) calibrations applied to minimize scatter within clusters.

The [α/M] ratio is likely influenced by different α-elements at different temperatures. As an average over multiple elements, [α/M] may produce higher precision abundances (as judged from internal scatter within clusters) than the abundances of any individual α element. Both [α/M] and the individual elemental [X/Fe] abundances have internal (temperature-dependent) calibrations applied to minimize scatter within clusters.

For carbon and nitrogen, the global and window fits are well correlated; we tend to recommend use of the window fits since they concentrate on areas of the spectrum most sensitive to the carbon and nitrogen abundances. Neither internal nor external calibration is applied to C and N abundances because as noted above, they cannot be assumed to be homogeneous within clusters.

ASPCAP Uncertainties/Errors

Ideally, the true uncertainties in the derived stellar parameters will be well-represented by the parameter uncertainty estimates reported by the pipeline. Unfortunately, the random errors that the parameter pipeline calculates are much smaller than the systematic effects that likely dominate the true uncertainties in derived APOGEE parameters. For effective temperature and surface gravity, we provide empirical uncertainties as derived from the scatter in the calibration relations, as described above.

Degeneracies Between Parameters

While we characterize the spectra using seven parameters (effective temperature, surface gravity, microturbulence, overall metal abundance, $\alpha$-element abundance, carbon abundance, and nitrogen abundance), there are likely to be significant degeneracies between some parameters in certain regions of parameter space. For example, for warmer stars where fewer lines can be seen may masquerade as cooler, metal-poor stars, and vice versa.

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 on the Using abundances page.)

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 RV 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 and/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 and/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 was 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, since the combined (apStar) spectra are sampled at roughly three pixels per resolution element. 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. Since 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.