MaStar Stellar Parameters

Overview of the Stellar Parameter Value-Added Catalog

Accurate stellar parameters are essential for applications of the stellar library. Although some stars have stellar parameters derived by other surveys, such as APOGEE, LAMOST, and SEGUE, the sources are heterogeneous and they could have systematics relative to each other. In addition, some stars were selected based on photometry and do not have existing stellar parameters. Therefore, we want to derive stellar parameters uniformly for the whole sample using the MaStar spectra, which have high signal-to-noise ratio and excellent flux calibration. We provide here a value-added catalog that contains multiple sets of stellar parameter measurements derived based on MaStar spectra using different methods, and a master set of stellar parameters made by taking the median among those based on different methods. The catalog will be described in full detail by Yan et al. (in prep) along with the papers on the individual efforts: Hill et al. (in press), Imig et al. (in press), Chen et al. (in prep), and Lazarz et al. (in prep). Please cite all these papers when using this catalog.

Algorithm Details

Four different methods were used to derive the parameters. These are labelled as DL (led by Daniel Lazarz at University of Kentucky), JI (led by Julie Imig at New Mexico State University), LH (led by Lewis Hill at University of Portsmouth), and YC (led by Yanping Chen at New York University Abu Dhabi). The common properties derived by all methods include the effective temperature (T_eff), the surface gravity (log g), the Iron-to-Hydrogen ratio ([Fe/H]), and the Alpha-to-Iron ratio ([alpha/Fe]). Some methods derive additional properties for each star, which are detailed below.

The methods are:

  1. JI: Derived using a neural network which models flux as a function of parameters and is trained on a combination of empirical MaStar spectra with ASPCAP parameters and the model spectra produced by Allende Prieto et al. (2018). This method also derives the microturbulence parameter. (Imig et al. in press)
  2. DL: Full-spectrum fitting with an Markov Chain Monte Carlo (MCMC) sampler using interpolated BOSZ model spectra with continuum shape information included in the chi-square calculation. Extinction is fitted as a by-product (see the Caveats section below). No photometry priors is used.
  3. LH: Full-spectrum, single-template, pPXF fitting with an MCMC sampler, using interpolated BOSZ and MARCS model spectra, with a flat prior based on Gaia color-magnitude diagram. The continuum is modeled with a multiplicative polynomial. (Hill et al. in press)
  4. YC: Full-spectrum fitting using both the BOSZ and MARCS model spectra without interpolation, with the result produced by Bayesian average and a flat prior based on Gaia color-magnitude diagram. The continuum is modeled with a multiplicative polynomial. The [alpha/Fe] parameters provided currently by this method are considered not reliable and are not included in deriving the master set of parameter.

The master set of parameters is derived by taking the median of the parameters of the four methods when they are available and are considered valid. Each set of parameter measurements has an associated validity flag to indicate whether the parameters of that method is valid for each star or each visit. For the median set, we provide a few columns (INPUT_GROUPS and INPUT_GROUPS_NAME) to indicate results from which methods are included in the median calculation. The median for [alpha/Fe] is treated differently from the other three properties. Additional quality filtering is done on [alpha/Fe] beyond that indicated by the validity flag associated with each method. The columns, INPUT_ALPHA_GROUPS and INPUT_ALPHA_GROUPS_NAME, indicate which sets of [alpha/Fe] parameters were included in the median [alpha/Fe] calculation.

The parameter derivations are done for each visit spectrum individually and only for those good quality visits. For those stars with multiple good visits, multiple measurements are generated which could be used to assess the internal uncertainty. We provide two tables (files) in this VAC. One has one entry per visit giving the per-visit measurements. The other has one entry per star which gives the median measurements among the many good visits of the star. The master set for the per-star table is done by first taking median among multiple visits and then taking median among multiple methods.

The metallicity measurement is critical for assigning spectra to different metallicity bins when building stellar population models. We applied a calibration on 3 of the 4 sets of [Fe/H] measurements using the overlap sample with APOGEE and the parameters derived by ASPCAP. The median of the calibrated [Fe/H] is provided in this VAC along with the median of the original [Fe/H] measurements. For the parameters from each method, we only provide the original [Fe/H] but not the calibrated ones. The calibration formula are all quadratic in the following form with the coefficients given in the table below.

[Fe/H]cal = a[Fe/H]2+b[Fe/H]+c

Method Abbrv. a b c
JI 0 1 0
DL -0.1161 0.7702 0.0755
LH -0.2317 0.7882 0.1337
YC -0.2418 0.9382 0.1260

The VAC also provides the total metallicity ([Z/H]) in addition to the iron-to-hydrogen ratio ([Fe/H]). This is derived by combining [Fe/H] and [alpha/Fe] measurements to compute the total number of atoms heavier than Helium, relative to the number of Hydrogen atoms. We assume the abundance pattern among the alpha-elements and that among the other elements respectively follow those in the solar abundance computed by Asplund (2005).

Data Access

The per-visit file, mastar-goodvisits-v3_1_1-v1_7_7-params-v1.fits, can be downloaded at this link with the data model here. The table is also available on SciServer under the DR17 context with the name "mastar-goodvisits-param". It can be queried using either the SQL Search tool or the CasJobs data access tool.

The per-star file, mastar-goodstars-v3_1_1-v1_7_7-params-v1.fits can be downloaded at this link with the data model here. The table is also available on the SkyServer under the DR17 context with the name "mastar-goodstars-param". It can be queried using either the SQL Search tool or the CasJobs data access tool.

Caveats

There is a minor bug in assigning the validity flag for the extinction estimates from DL. We accidentally flagged those without extinction estimates as valid as well, which have both AV_DL and AV_ERR_DL set to -999. This can be easily fixed by applying additional selection. To select those entries with valid extinction estimates, choose entries that satisfy both AV_VALID_DL equal to 1 and AV_ERR_DL greater than 0.

The stellar parameter estimates presented here are our best effort at the moment. There can still be quality issues in the parameter estimates for some stars. Use with caution. We are continuing to improve them, and plan to provide an update of the catalog in the future.