SDSS-IV MaStar -- A Large and Comprehensive Empirical Stellar Spectral
  Library: First Release
  We present the first release of the MaNGA Stellar Library (MaStar), which is
a large, well-calibrated, high-quality empirical library covering the
wavelength range of 3,622-10,354A at a resolving power of R~1800. The spectra
were obtained using the same instrument as used by the Mapping Nearby Galaxies
at Apache Point Observatory (MaNGA) project, by piggybacking on the
SDSS-IV/APOGEE-2N observations. Compared to previous empirical libraries, the
MaStar library will have a higher number of stars and a more comprehensive
stellar-parameter coverage, especially of cool dwarfs, low-metallicity stars,
and stars with different [alpha/Fe], achieved by a sophisticated target
selection strategy that takes advantage of stellar-parameter catalogs from the
literature. This empirical library will provide a new basis for stellar
population synthesis, and is particularly well-suited for stellar-population
analysis of MaNGA galaxies. The first version of the library contains 8646
high-quality per-visit spectra for 3321 unique stars. Compared to photometry,
the relative flux calibration of the library is accurate to 3.9% in g-r, 2.7%
in r-i, and 2.2% in i-z. The data are released as part of Sloan Digital Sky
Survey Data Release 15. We expect the final release of the library to contain
more than 10,000 stars.
