On Earth’s surface, there are only a handful of high-quality astronomical sites that meet the requirements for very large next-generation facilities. In the context of scientific opportunities in time-domain astronomy, a good site on the Tibetan Plateau will bridge the longitudinal gap between the known best sites^1,2 (all in the Western Hemisphere). The Tibetan Plateau is the highest plateau on Earth, with an average elevation of over 4,000 metres, and thus potentially provides very good opportunities for astronomy and particle astrophysics^3–5. Here we report the results of three years of monitoring of testing an area at a local summit on Saishiteng Mountain near Lenghu Town in Qinghai Province. The altitudes of the potential locations are between 4,200 and 4,500 metres. An area of over 100,000 square kilometres surrounding Lenghu Town has a lower altitude of below 3,000 metres, with an extremely arid climate and unusually clear local sky (day and night)⁶. Of the nights at the site, 70 per cent have clear, photometric conditions, with a median seeing of 0.75 arcseconds. The median night temperature variation is only 2.4 degrees Celsius, indicating very stable local surface air. The precipitable water vapour is lower than 2 millimetres for 55 per cent of the night.

Based on the Gaia DR2 catalog of hot subdwarf star candidates, we identified 1587 hot subdwarf stars with spectra in LAMOST DR7. We present atmospheric parameters for these stars by fitting the LAMOST spectra with Tlusty/Synspec non-LTE synthetic spectra. Combining LAMOST radial velocities and Gaia Early Data Release 3 (EDR3) parallaxes and proper motions, we also present the Galactic space positions, velocity vectors, orbital parameters and the Galactic population memberships of the stars. With our He classification scheme, we identify four groups of He rich hot subdwarf stars in the T _eff − log g and T _eff − log(nHe/nH) diagrams. We find two extreme He-rich groups (eHe-1 and eHe-2) for stars with log(nHe/nH) ≥ 0 and two intermediate He-rich groups (iHe-1 and iHe-2) for stars with −1 ≤ log(nHe/nH) < 0. We also find that over half of the stars in Group eHe-1 are thick-disk stars, while over half of the stars in Group eHe-2 correspond to thin-disk stars. The disk population fractions of Group iHe-1 are between those of Group eHe-1 and eHe-2. Almost all of the stars in Group iHe-2 belong to the thin disk. These differences indicate that the four groups probably have very different origins. Comparisons between hot subdwarf stars in the halo and in the Galactic globular cluster ω Cen show that only He-deficient stars with −2.2 ≤ log(nHe/nH) < −1 have similar fractions. Hot subdwarfs with log(nHe/nH) ≥ 0 in ω Cen have no counterparts in the thick-disk and halo populations, but they appear in the thin disk.

Using Gaia Data Release 2 photometry, we report the detection of extended main-sequence turnoff (eMSTO) regions in the color–magnitude diagrams (CMDs) of the ∼14 Myr old double clusters h and χ Persei (NGC 869 and NGC 884). We find that stars with masses below ∼1.3 M _⊙ in both h and χ Persei populate narrow main sequences (MSs), while more massive stars define the eMSTO, closely mimicking observations of young Galactic and Magellanic Cloud clusters (with ages older than ∼30 Myr). Previous studies based on clusters older than ∼30 Myr found that rapidly rotating MS stars are redder than slow rotators of similar luminosity, suggesting that stellar rotation may be the main driver of the eMSTO. By combining photometry and projected rotational velocities from the literature of stars in h and χ Persei, we find no obvious relation between the rotational velocities and colors of non-emission-line eMSTO stars, in contrast with what is observed in older clusters. Similar to what is observed in Magellanic Cloud clusters, most of the extremely rapidly rotating stars, identified by their strong Hα emission lines, are located in the red part of the eMSTOs. This indicates that stellar rotation plays a role in the color and magnitude distribution of MSTO stars. By comparing the observations with simulated CMDs, we find that a simple population composed of coeval stars that span a wide range of rotation rates is unable to reproduce the color spread of the cluster’s MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.