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 find 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. Similarly 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 clusters' MSs. We suggest that variable stars, binary interactions, and stellar rotation affect the eMSTO morphology of these very young clusters.

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 sites1,2 (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 astrophysics3–5. 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.

Although previous searches for star clusters have been very successful, many clusters are likely still omitted, especially at high-Galactic-latitude regions. In this work, based on the astrometry of Gaia EDR3, we searched nearby (ϖ > 0.8 mas) all-sky regions, obtaining 886 star clusters, of which 270 candidates have not been cataloged before. At the same time, we have presented the physical parameters of the clusters by fitting theoretical isochrones to their optical magnitudes. More halo members and expanding structures in many star clusters were also found. Most of the new objects are young clusters that are less than 100 million years old. Our work greatly increased the sample size and physical parameters of star clusters in the solar neighborhood, in particular, 46 clusters are newly found with ∣b∣ > 20°, which represents a nearly threefold increase in the number of clusters at high-Galactic-latitude regions.

We report the discovery of two new pulsating extremely low-mass pre-white dwarf (pre-ELMV) candidates in the Transiting Exoplanet Survey Satellite (TESS) eclipsing binaries, TIC 149160359 and TIC 416264037. Their light curves show a typical feature of EL CVn-type binaries. The light-curve modeling indicates that they are both detached systems with very low-mass ratios (q ≃ 0.1). Based on the photometric solutions, the masses and radii of the two main-sequence primary components are estimated, and those of the secondaries are deduced. The results show that the less-massive components of the two binaries are both probably thermally bloated, pre-ELMVs. Apart from the eclipsing light changes, short-period light variations are clearly shown in their residual light curves. We have made the Fourier analysis of their light-curve residuals with the Period04 program. TIC 149160359 was found to pulsate in 21 independent frequencies, 17 of which are between 21 and 35 day−1 and the others are between 63 and 77 day−1. The Fourier amplitude spectrum of TIC 416264037 also shows two frequency concentration ranges. Out of nine independent frequencies, seven reside within the low-frequency range of 12.5–19.9 day−1. Two pulsating signals, f 4 = 122.2698 day−1 and f 10 = 112.3603 day−1, were detected in the high-frequency region. These low-frequency signals that are detected on TIC 149160359 and TIC 416264037 are probably due to the intrinsic pulsations of their δ Sct-type primary components. However, the high-frequency signals are likely to come from the pulsations of the pre-ELM WD components. This brings the number of pre-ELMV candidates to 12.

No abstract

No abstract

We report the discovery of a new eclipsing EL CVn-type binary, consisting of a δ Sct-type pulsator and a thermally bloated low-mass pre-He white dwarf (WD). Spectroscopy and time-series BV photometry of V1224 Cas were carried out. The spectroscopy reveals a spectral type of A3 for the star. Light-curve modelling indicate that V1224 Cas is a short-period detached system containing a possible low-mass WD with an effective temperature of about 9516 K. Based on the effective temperature and the surface gravity of the A-type primary star from the spectroscopic results, the absolute parameters of the components were estimated as:We therefore introduce V1224 Cas as a new EL CVn-type binary candidate. The light curves in both filters all show multi-periodic pulsations, superimposed on binary effects. We performed a preliminary frequency analysis of the light residuals after removing the synthetic eclipsing curve from the original observational data. The results suggest that the rapid light variations among the light curves could be attributed to the δ Sct-type primary component. We therefore conclude that V1224 Cas is very likely a WD+δ Sct binary.

New photometric observations of a W UMa system, V410 Aur, were carried out over 10 nights from 2014 December 19 to 2015 February 8, from which four sets of light curves were obtained. The light curves show many unusual behavioral features, including changing occultation depths, transit minima, and asymmetric maxima. The four sets of light curves have been separately analyzed with the Wilson–Devinney method. The results suggest a totally eclipsing contact configuration for the system. Over a surprisingly short time span of only 52 days, the dominant spot distortion phase jumped twice between phases 0.0 and 0.5. The light-curve variations can be interpreted by the presence of two cool spots on the massive component. Based on our analysis, it is further suggested that the peculiar behavioral patterns are probably caused by the presence of two permanent, active large spots separated in longitude by about 180°, whose locations remain almost unchanged throughout. Our study demonstrates that the system has been undergoing typical flip-flop activity. We therefore conclude that V410 Aur is a W UMa-type system exhibiting flip-flop activity.