The Chinese Space Station Telescope (CSST) spectroscopic survey aims to deliver high-quality low-resolution (R > 200) slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag, distributed within a large survey area (17 500 deg2) and covering a wide wavelength range (255–1000 nm by three bands GU, GV, and GI). As slitless spectroscopy precludes the usage of wavelength calibration lamps, wavelength calibration is one of the most challenging issues in the reduction of slitless spectra, yet it plays a key role in measuring precise radial velocities of stars and redshifts of galaxies. In this work, we propose a star-based method that can monitor and correct for possible errors in the CSST wavelength calibration using normal scientific observations, taking advantage of the facts that (i) there are about ten million stars with reliable radial velocities now available thanks to spectroscopic surveys like LAMOST, (ii) the large field of view of CSST enables efficient observations of such stars in a short period of time, and (iii) radial velocities of such stars can be reliably measured using only a narrow segment of CSST spectra. We demonstrate that it is possible to achieve a wavelength calibration precision of a few km s−1 for the GU band, and about 10 to 20 km s−1 for the GV and GI bands, with only a few hundred velocity standard stars. Implementations of the method to other surveys are also discussed.

Early-type stars are short lived and scarce in comparison with other types. Based on the recently released catalogs of early type stars from the largest LAMOST spectroscopic survey, the intrinsic colors of the stars with effective temperature up to 32,000 K are determined for the bands from ultraviolet to infrared by using the blue-edge method. Analytic relations are derived for the intrinsic color index with the effective temperature for the WISE, 2MASS, Gaia, APASS, SDSS, Pan-STARRS1, and GALEX bands. The results are generally consistent with previous works. In addition, the intrinsic colors of O-type dwarfs and OB supergiants are roughly estimated.

The Chinese Space Station Telescope (CSST) spectroscopic survey plans to deliver high-quality low-resolution (R > 200) slitless spectra for hundreds of millions of targets down to a limiting magnitude of about 21 mag, covering a large survey area (17 500 deg2) and a wide wavelength range (255–1000 nm by three bands GU, GV, and GI). In this work, we use empirical spectra of the Next Generation Spectral Library to simulate the CSST stellar spectra at R = 250, and investigate their capabilities in measuring radial velocities. We find that velocity uncertainties depend strongly on effective temperature, weakly on metallicity for only FGK stars, and hardly on surface gravity. It is possible to deliver stellar radial velocities to a precision of about 3 km s−1 for AFGKM stars, and about 10 km s−1 for OB stars, at signal-to-noise ratio (SNR) of 100. Velocity uncertainties using single GU/GV/GI band spectra are also explored. Given the same SNR, the GU band performs best, the GV band the second best, and then the GI band. The effects of spectral normalization and the imperfect template on velocity measurements are investigated and found to be very weak. The uncertainties caused by wavelength calibration are considered and found to be moderate. Given the possible precision of radial velocities, the CSST spectroscopic survey can enable interesting science such as searching for hyper-velocity stars. Limitations of our results are also discussed.

The infrared (IR) excess from OB stars is commonly considered to be a contribution from ionized stellar wind or circumstellar dust. With the newly published Large Sky Area Multi-Object Fiber Spectroscopy Telescope (LAMOST)-OB catalog and Galactic O-Star Spectroscopic Survey data, this work steps further on understanding the IR excess of OB stars. Based on a forward-modeling approach comparing the spectral slope of observational spectral energy distributions and photospheric models, 1147 stars are found to have IR excess out of 7818 stars with good-quality photometric data. After removing the objects in the sightline of dark clouds, 532 (∼7%) B-type stars and 118 (∼23%) O-type stars are identified to be true OB stars with circumstellar IR excess emission. The ionized stellar wind model and the circumstellar dust model are adopted to explain the IR excess, and Bayes factors are computed to quantitatively compare the two. It is shown that the IR excess can be accounted for by the stellar wind for about 65% cases, of which 33% by free–free emission and 32% by synchrotron radiation. Other 30% sources could have and 4% should have a dust component or other mechanisms to explain the sharp increase in flux at λ > 10 μm. The parameters of the dust model indicate a large-scale circumstellar halo structure, which implies the origin of the dust from the birthplace of the OB stars. A statistical study suggests that the proportion with IR excess in OB stars increases with the stellar effective temperature and luminosity, and that there is no systematic change in the mechanism for IR emission with stellar parameters.