We have used the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST) low-resolution and medium-resolution spectroscopic surveys to explore the chromospheric statistical properties and variations of M stars. There are 738,477 LAMOST low-resolution spectra of 622,523 M stars. We have calculated equivalent widths of the Ca ii H&K, Hα, Hβ, Hγ, Hδ, He i D3, and Ca ii infrared triplet lines. Based on the Hα EWs, we found that 33,374 objects showed Hα emissions. We revised the relationship between the active fraction and spectral subtype, and confirmed that the active fractions increased from M0 to M3, and possibly declined from M4 to subsequent types. Among 86,005 objects with repeated spectra, 19,422 objects showed Hα variations. The variation fraction of stars with M4 and subsequent types with a fully convective envelope was higher than those of M0–M3 with a partially convective envelope. By combining the distances from Gaia, we have determined the positions of the M dwarfs in the catalog for M stars. The active fraction decreased rapidly in the height range of 0–400 pc above and below the Galactic plane, and maintained a steady trend in 400–1000 pc. We cross-matched the M catalog of the LAMOST low-resolution survey with LAMOST medium-resolution spectra. We obtained 272,181 spectra of 13,355 M objects and calculated the Hα EWs. Among them, 3813 stars show variations, which might be caused by chromospheric activity evolution with orbital phase or time. The Hα emission for 972 objects shows variations over short and long timescales, especially on short timescales of 20 minutes.
Aims. A stellar flare is a strong energy burst event. Understanding stellar flares is important for determining the properties of stellar magnetic activities. We aim to perform detailed statistical analyses on stellar flare characteristics in terms of the different stellar parameters using the light curves of the Transiting Exoplanet Survey Satellite (TESS) survey from July 2018 to October 2020. Methods. By using repeated fitting to differentiate stellar background light curves from flare events, we precisely detected 60 810 flare events on 13 478 stars from sectors 1–30 of the TESS data and determined the flare parameters. After cross-matching our samples with the Gaia survey, we obtained more stellar parameters to determine the relationship between flare parameters and stellar parameters. Results. The fraction of flaring stars decreases as stellar temperatures increase in the region of 2500–6500 K and stellar mass increases from 0.08 to 1.4 M⊙. The flare energies increase as the stellar temperature and the stellar mass decrease. There is a weak relationship between flare amplitude and Ro. We also confirm that M-type stars produce flares more frequently than F-, G-, and K-type stars. We find that the proportion of flaring stars increases from M0 to M5, and decreases from M5 to M7. We obtain a power-law index of 1.83 ± 0.15 for the cumulative flare frequency distribution. The index shows a decreasing trend from F- to K-type stars, and M-type stars rise for dwarf samples. The fraction of flaring stars decreases as the vertical absolute height increases in the range 0–200 pc but this fraction is stable in the range 200–800 pc. We find that eclipsing binary stars show more frequent flares than single stars.
Stellar parameters and magnetic activity are important for the understanding of stellar physics and dynamo theory. We have used LAMOST spectroscopic surveys to study stellar parameters and magnetic activity properties of TESS objects. There are 46,684 spectra of 31,385 TESS objects in the LAMOST low-resolution spectral survey. We have calculated equivalent widths of Ca ii H and K, and Hα lines, and discussed the relationship of chromospheric activity with stellar temperature. There are 25,710 spectra of 10,404 stars with repeated low-resolution observations. We found that there are 694 objects with variations of the Hα line and 1406 objects with varying radial velocities. Moreover, there are 14,099 spectra of 5834 TESS objects in the LAMOST medium-resolution survey, of which there are 2666 objects with Hα variations and 1480 objects with variation in radial velocities. By using the light curves of the TESS objects in the LAMOST field, we detected 4269 flare events and determined their flare parameters. We have confirmed that the fraction of the flare stars increases as stellar temperature decreases. There is also an observed relationship between the flare energy and chromospheric activity. We have calculated the power-law index between the flare energy and decay time using TESS data to be 0.27(0.01), which can be explained by the magnetic reconnection theory. We confirmed that R H α ′ is stable for Rossby number R < 0.1 and decreases with the increase of Rossby number R > 0.1. There is a positive correlation between the brightness variation amplitude due to star spots from TESS and the Hα intensity from LAMOST.
Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) low- and medium-resolution spectroscopic surveys are important for determination of the orbital parameters and chromospheric activity of extrasolar planet systems. We crossmatched the exoplanet catalog confirmed before 2021 March 11 with the LAMOST DR7 survey to study their properties. There are 1026 targets with exoplanets observed in the LAMOST DR7 low-resolution spectroscopic survey and 158 targets in the medium-resolution spectroscopic survey. We have calculated the equivalent width of the Hα line and determined their stellar activity. The Hα and flare intensities are almost constant for the Rossby number Ro ≤ 0.12 in the saturated regime and decrease with increasing Ro in the unsaturated regime. In addition, we searched the flare events of all stars with exoplanets in the Transiting Exoplanet Survey Satellite (TESS), Kepler, and K2 surveys. Among the 733 extrasolar planetary systems observed by TESS, we found 481 flares from 57 stars. For Kepler data, we obtained the light curve of 1699 stars and found 1886 flares from 417 stars. For K2 data, we obtained the light curves of 347 stars and found 467 flares from 89 stars. There were light curves of 361 objects with obvious eclipse observed from the TESS survey. We have fitted their light curves with a high signal-to-noise ratio using the JKTEBOP program, and we reobtained the orbital parameters, such as inclination, radius, and period. In the end, we made a judgment on the habitability of exoplanets of stars with flares.
In this paper, we present four sets of photometric V RI light curves, and several LAMOST low and medium resolution spectra of contact binary CC Com. We revised the orbital parameters by simultaneously combining with previously published radial velocity measurements using the Wilson-Devinney program. We used light curves at different observational times to obtain the starspot parameters. The values of the starspot radius are variable in short- and long-term scales, and their longitudes are stable. We updated the orbital period change of CC Com, and analyzed the periodic variation. The period of CC Com decreases at a rate of 4.66 (±0.20) × 10−11 d yr−1, which may be due to mass transfer from the secondary component to the primary component. The oscillation of its orbital period with a period of 17.18(0.08) years and amplitude of 0.0018(1) d may be caused by the light time effect (LITE) via a third body of 0.06 M ⊙ dwarf or magnetic activity cycle. Furthermore, we obtained one optical spectrum from the LAMOST survey, which gives the spectral type of CC Com as K7±2V. Strong emissions exist in the Hα, and Ca ii H&K lines in the observed spectrum, indicating strong chromospheric activity on CC Com. In the 12 LAMOST medium-resolution spectra, the EWs of Hα line are variable along the phase and time, which may be a plage or flare event.
RS Ser, V449 Per, MR Del, V593 Cen, and V1095 Her are short-period eclipsing binaries. We made photometric observations on 38 nights using four 1-m-class telescopes and plotted eight light curves. We determined the spectral type of V449 Per as K0(±2)V using low-resolution spectra from the Lijiang 2.4-m telescope. We found cyclic variation in the orbital periods for RS Ser and V1095 Her, and confirmed the cyclic variation of MR Del. The periods of the hypothetical third bodies are close to the duration of observation, and the detected cycles are questionable. For V593 Cen, we followed the previously published suggestion that it probably has a black hole with a minimum mass of 3.68 M⊙. We used the orbital period decreasing at a rate of 8.2(0.1) × 10−8 d yr–1 to explain it. There are two alternative interpretations, and hence the black hole candidate of V593 Cen remains questionable because the minimum points are concentrated in four clusters. The period of V449 Per increases continuously at a rate of 9.5 × 10−8 d yr–1, which can be attributed to mass transfer from the less massive component to the more massive component. For MR Del, we used a new light curve and the published radial velocity to revise its absolute parameters. Furthermore, we revised the photometric solution of V593 Cen and confirmed it as an early-type contact binary with a higher contact factor. We obtained preliminary photometric parameters for RS Ser, V1095 Her, and V449 Per.
The eclipsing binary 2MASS J04100497+2931023 (J04100497+2931023) is classified its spectral type of M0 ± 2V on basis of a low-resolution spectral survey by the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST). The low-resolution spectra exhibit strong single-peak emission in the Hα line. We obtained the radial velocities of this binary by means of the Cross-Correlation Function method from the LAMOST medium-resolution spectra. Both components of J04100497+2931023 indicate strong emissions in the Hα line. We performed follow-up photometric observations of J04100497+2931023 using the Xinglong 85 cm telescope of National Astronomical Observatories, Chinese Academy of Sciences. We obtained its full light curve in VRI filters. We first determined their absolute parameters from simultaneously radial velocity and light curves by the Wilson-Devinney program. From our new light curves, we detected three flares for the first time, including one convective flare. The amplitudes, durations, energies, and spectral indices of three flares were also determined. J04100497+2931023 was monitored for approximately 29 h, which indicates that the flare rate is 0.1 flare per hour. We conclude that J04100497+2931023 is a low-mass detached eclipsing binary with strong magnetic activity.
A catalogue of M stars has been published from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope data release 7 (LAMOST DR7). We cross-matched the LAMOST M catalogue with {\it Kepler}, {\it Kepler} 2 (K2) and Transiting Exoplanet Survey Satellite (TESS) surveys. We obtained the light curves from the {\it Kepler} and Tess surveys. We detected 20,047 flare events from 4053 M stars and calculated their durations, amplitudes, and energies. We analyzed the distribution of the flare durations and energies. The distributions of flare duration for {\it Kepler}, K2 and TESS peak at 3-3.5 hr, 4-6 hr and 1-1.5 hr, respectively. This may be the result of the different cadences for the three data sets. The highest regions of the flare energies of {\it Kepler}, K2 and TESS are 32-32.5, 32-32.5 and 31-31.5 erg in Log format, respectively. A linear relationship between flare duration and energy emerges from our analysis. The ratio of flare duration to total observational time is approximately 0.2\% -0.3 \% for the {\it Kepler}, K2 and TESS surveys. The occurrence rate of a star with a flare event increases from the M0 to M4 subtypes. We also determined the spatial distribution of the flare rate of M stars in the Milky Way. It seems that the flare rate decreases as the vertical height increases. The power index of the flare energies is in the region of 1.53-2.32, which is similar to previous result for solar type star (2.0). Further, we examined the relationship between the flare amplitude and chromospheric intensity. The flare activity increases rapidly with the increase in the H$\alpha$ EW at the lower values (less approximately 2 \AA) and it increases slowly at the higher values.
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