The SEGUE Stellar Parameter Pipeline (SSPP) implements a
"multi-method" approach, using a combination of techniques to
extract reliable estimates of stellar parameters from the SEGUE
Details of the SSPP can be found in two recent publications:
Stellar Parameter Estimation
The SEGUE Stellar Parameter
Pipeline (SSPP) makes use of multiple techniques in order to estimate the
fundamental stellar atmospheric parameters:
- effective temperature →Teff
- surface gravity → log g
- metallicity → parameterized by [Fe/H]
for stars with spectra and ugriz
photometry obtained during the course
of the original Sloan Digital Sky Survey (SDSS-I) and its first extension
The use of multiple approaches allows for an empirical
determination of the internal errors for each derived parameter, based on the
range of the reported values from each method. Based on about 120,000 spectra
for stars obtained during SDSS-I and SEGUE that satisfy S/N ≥ 10/1 (average
value over the spectra range 3,850-6,000Å, and have derived temperatures
in the range 4500K ≤ T
eff ≤ 7,500K, typical internal errors
obtained by the SSPP are:
- σ(Teff) ≤ 75K
- σ(log g) ≤ 0.20 dex
- σ([Fe/H]) ≤ 0.10 dex
A comparison with an analysis of high-resolution spectra for over 150
SDSS-I/SEGUE stars suggests that the SSPP is able to estimate
Teff, log g, and [Fe/H] to an external uncertainty
(random plus systematic errors ) of
117K, 0.26 dex, and 0.22 dex, respectively, in the temperature range
4500K ≤ Teff ≤ 7,500K.
These errors apply for the very highest S/N
spectra obtained from SDSS (S/N > 50/1), as only quite bright stars were
targeted for high-resolution observations. Outside of the quoted temperature
range, we presently to do not have sufficient high-resolution spectra to fully
test the parameters obtained by the SSPP.
An analysis of likely member stars in a handful of Galactic open and globular
clusters indicates that the SSPP slightly over-estimates [Fe/H]
(by on the order of 0.15 dex) for stars with [Fe/H] < -2.0, and
underestimates [Fe/H] for stars with near solar metallicities by ~ 0.3 dex.
Determinations of Teff
and log g exhibit no obvious offsets
relative to expectations.
Despite these small remaining uncertainties, the SSPP determines stellar
atmospheric parameters with sufficient accuracy and precision, in the
temperature range 4500K ≤ T
≤ 7,500K, to carry out
detailed explorations of the chemical compositions and kinematics of the
thick-disk and halo populations of the Milky Way.
SSPP parameter definitions.
The initial step in calculation of line indices for SDSS spectra is to
the wavelength scale of the original SDSS spectrum over to an air-based (rather
than vacuum-based) wavelength scale, and to linearly rebin the spectrum to
1 Å bins in the blue (3,800-6,000 Å), and 1.5 bins in the red
(6,000-9,000 Å). Then, based on the adopted radial velocity described
above, the wavelength scale is shifted to zero rest wavelength.
The SSPP measures 77 atomic and molecular lines. Line index
calculations are performed in two modes; one is to use a global
continuum fit over the entire wavelength range (3,850-9,000 Å),
the other is to obtain a local continuum around the line bands of
interest. The choice between which mode is used depends on the line
depth and width of the feature under consideration. Local continua
are employed for the determinations of stellar atmospheric
parameters based on techniques that depend on individual line
indices. Other techniques, such as the neural network, spectral
matching, and autocorrelation methods, require wider ranges to be
considered; for these the global continuum is used. We make use of
the errors in the fluxes reported by the SDSS spectroscopic
reduction pipeline in order to obtain estimates in the uncertainties
in the line indices.
DR7 'sppLines' Table
Improvements between DR6 and DR7
There have been several improvements made in the SSPP since the
release of DR6. In particular, the SSPP averages no longer suffer
underestimates of metallicities (by about 0.3 dex) for stars approaching
This improvement was accomplished by adding synthetic
spectra with super-solar metallicities to two of the synthetic grid
matching techniques (
NGS2), as well as
through a recalibration of the
ANNRR methods. See Table 5 in
Lee et al. 2008a
for the naming convention for each technique.
CaIIT, have been
recalibrated to the "native"
g-r system, instead of making use
of calibration on
B-V, which required application of an uncertain
transformation in color space. The
ANNRR approach, which also
tended to underestimate metallicity for near-solar metallicity stars, has been
re-trained on the SDSS/SEGUE spectra with improved stellar parameters, resulting
in a better determination of the metallicity for metal-rich stars.
Moreover, a neural network approach, based solely on noise-added synthetic
spectra, has also been introduced. There remains a tendency for the SSPP to
assign slightly higher metallicities for stars with [Fe/H] < -2.7. This offset
is presently being calibrated out, and will be corrected in SEGUE-2 (part of
SDSS-III). For more detailed descriptions of the individual methods of the SSPP,
see Lee et al. 2008a.
Additionally, the pipeline now identifies cool main sequence stars of low
metallicity (late-K and M subdwarfs). The stars are assigned metallicity classes
and spectral subtypes following the classification system of
Lépine et al. 2007.
Cool and ultra-cool subdwarfs are classified as subdwarfs (sdK, sdM), extreme
subdwarfs (esdK, esdM), and ultrasubdwarfs (usdK, usdM) in order of decreasing metal content.
The classification is based on the absolute and relative values of the TiO and CaH molecular
bandstrengths, and derived from fits to K-M dwarf and K-M subdwarf spectral templates.
A number of open and globular clusters have been observed photometrically and
spectroscopically with the SDSS instruments to evaluate the performance of the SSPP
(Lee et al. 2008b). In addition,
high-resolution spectra have been obtained for about 100 field stars included in the SDSS, and
used to expand the SSPPP checks over a wider parameter space
(Allende Prieto et al. 2008).
These high-resolution observations have also been used to assess the precision of the SDSS radial
velocities at the bright end of the SDSS spectroscopic range, and to set the zero-point of the
stellar radial velocity pipeline.