Hot Subdwarf Stars Observed in Gaia DR2 and LAMOST DR5

Abstract

Combing Gaia DR2 with LAMOST DR5, we spectroscopically identified 924 hot subdwarf stars, among which 32 stars exhibit strong double-lined composite spectra. We measured the effective temperature T eff , surface gravity log g, helium abundance y = nHe/nH, and radial velocities of 892 non-composite spectra hot subdwarf stars by fitting LAMOST observations with Tlusty/Synspec non-LTE synthetic spectra. We outlined four different groups in the T eff − log g diagram with our helium abundance classification scheme and two nearly parallel sequences in the T eff − log(y) diagram. 3D Galactic space motions and orbits of 747 hot subdwarf stars with (G BP − G RP ) 0 < −0.36 mag were computed using LAMOST radial velocities and Gaia parallaxes and proper motions. Based on the U − V velocity diagram, J z −eccentricity diagram, and Galactic orbits, we derived Galactic population classifications and the fractional distributions of the four hot subdwarf helium groups in the halo, thin disk and thick disk. Comparisons with the predictions of binary population synthesis calculations (Han 2008) suggest that He-rich hot subdwarf stars with log(y) ≥ 0 are from the double helium white dwarfs merger, He-deficient hot subdwarf stars with −2.2 ≤ log(y) < −1 from the common envelope ejection, and He-deficient hot subdwarf stars with log(y) < −2.2 from the stable Roche lobe overflow channels. The relative number of He-rich hot subdwarf stars with −1 ≤ log(y) < 0 and log(y) ≥ 0 in the halo is more than twice the prediction of Zhang et al. (2017), even more than six times in the thin disk, which implies that the mergers of helium white dwarfs with low mass main sequence stars may not be the main formation channel of He-rich hot subdwarf stars with −1 ≤ log(y) < 0, specially in younger environments.

Observed binary populations reflect the Galactic history

“…7. We also do not expect any noticeable contribution for halo sdB stars in our observed sample, as shown in the previous section, which may be relevant to other types of sdB binaries (Luo et al 2019).…”

Stellar labels for hot stars from low-resolution spectra - I. the HotPayne method and results for 330,000 stars from LAMOST DR6

“…Webbink 1984;Han et al 2002Han et al , 2003Lanz et al 2004;Miller Bertolami et al 2008;Heber 2009;Justham et al 2011;Zhang & Jeffery 2012). Hot subdwarfs have been extensively identified from the LAMOST database by Luo et al (2016Luo et al ( , 2019Luo et al ( , 2020. The positions of the hot subdwarfs in the T eff -log g plane are consistent with Luo et al (2016, e.g.…”

A search for variable subdwarf B stars in TESS full frame images – I. Variable objects in the southern ecliptic hemisphere

“…We detected significant flux variations in 1807 objects, out of which 28 are classified as sdBs, 2 as subdwarfs (sd), 77 as non-sdBs, and 1619 are not spectroscopically classified. To identify sdB objects we used the sdB database Geier (2020), the LAMOST spectroscopic survey (Lei et al 2018(Lei et al , 2019aLuo et al 2019), the Evryscope survey (Ratzloff et al 2020) and the Simbad database (Wenger, M. et al 2000). The large square pixels, 21 arcsec on side, cause serious issues in crowded regions of the sky, since an optimal aperture may contain neighboring objects.…”

“…Two objects, Gaia DR2 3129751228471383808 and 3159937564294110080, show multiple peaks in their amplitude spectra (Figure 1), which we interpret as g-modes. Both objects were first observed by Høg et al (2000) and the spectral classifications were made by Luo et al (2019) and Lei et al (2018).…”