LAMOST Time-Domain survey:first results of four K2 plates

From Oct.2019 to Apr.2020,LAMOST performed a time-domain(TD)spectroscopic survey of four K2 plates with both low-and medium-resolution observations.The low-resolution spectroscopic survey acquired 282 exposures(≈46.6 h)over 25 nights,yielding a total of about 767000 spectra,and the medium-resolution survey took 177 exposures(≈49.1 h)over 27 nights,collecting about 478000 spectra.More than 70%/50%of low-resolution/medium-resolution spectra have signal-to-noise ratio higher than 10.We determine stellar parameters(e.g.,Teff,log g,[Fe/H])and radial velocity(RV)with different methods,including LASP,DD-Payne and SLAM.In general,these parameter estimations from different methods show good agreement,and the stellar parameter values are consistent with those of APOGEE.We use the Gaia DR2 RV values to calculate a median RV zero point(RVZP)for each spectrograph exposure by exposure,and the RVZP-corrected RVs agree well with the APOGEE data.The stellar evolutionary and spectroscopic masses are estimated based on the stellar parameters,multi-band magnitudes,distances and extinction values.Finally,we construct a binary catalog including about 2700 candidates by analyzing their light curves,fitting the RV data,calculating the binarity parameters from medium-resolution spectra and cross-matching the spatially resolved binary catalog from Gaia EDR3.The LAMOST TD survey is expected to represent a breakthrough in various scientific topics,such as binary systems,stellar activity,stellar pulsation,etc.

Stellar activity cycles as revealed by long-term beat-like patterns from light curves of Kepler

Stellar activity cycles have been found on many stars through different methods.Although in debate,an empirical relation has been found where a period of stellar activity cycle Pcyc is positively correlated with rotation period Prot along some segregated branches.In this work,we built a systematic process to search for the Kepler objects that are analogous to the long-term solar brightness variations and investigate their possible origins.After a rigorous selection,we obtained 43 objects that all exhibit clear long-term beat-like patterns and their variability(amplitude of rotational modulation)periods and rotation periods were derived.Due to the 4-year duration of the Kepler mission,our targets are located in the region where the cycle periods are longer than~1 yr and shorter than~2.5 yr.However,all of our targets exhibit clear and notable quasi-periodic variations in variability,yet we cannot confirm any linear trends that were predicted by previous research independently.This may suggest that in the Kepler realm of the Pcyc-Prot diagram,these relations would mix with each other or the beat-like patterns may not be related to the stellar cycle unless other evidence is provided.