Magnetic cycles of Sun-like stars with different levels of coronal and chromospheric activity——comparison with the Sun

The atmospheric activity of the Sun and Sun-like stars is analyzed involving observations from the HK-project at the Mount Wilson Observatory, the California and Carnegie Planet Search Program at the Keck and Lick Observatories and the Magellan Planet Search Program at the Las Campanas Observatory. We show that for stars of E G and K spectral classes, the cyclic activity, similar to the 11-yr solar cycle, is different： it becomes more prominent in K-stars. Comparative study of Sun-like stars with different levels of chromospheric and coronal activity confirms that the Sun belongs to stars with a low level of chromospheric activity and stands apart among these stars by its minimum level of coronal radiation and minimum level of variations in photospheric flux.

Multiple cycles of magnetic activity in the Sun and Sun-like stars and their evolution

The wavelet transform method for high-quality time-frequency analysis is applied to sets of observations of relative sunspot numbers and stellar chromosphere fluxes of 10 Sun-like stars. Wavelet analysis of solar data shows that in a certain interval of time there are several cycles of activity with pe- riods of duration which vary considerably from each other： from quasi-biennial cycles to lO0-yr cycles. Cyclic activity was detected in almost all Sun-like stars that we examined, even those that previously were not considered as stars with cyclic activity according to analysis using a Scargle periodogram. The durations of solar and stellar cycles significantly change during the observation period.

Chromospheric activity on the late-type star V1355 Ori using Lijiang 1.8-m and 2.4-m telescopes

We obtained new high-resolution spectra using the Lijiang 1.8-m and 2.4-m telescopes to inves- tigate the chromospheric activities of V1355 Ori as indicated in the behaviors of Ca n H＆K, Hγ HT, H/3, Na1 D1, D2, Ha and Ca II infrared triplet （IRT） lines. The observed spectra show obvious emissions above the continuum in Ca II H＆K lines, absorptions in the Hδ, Hγ, H/3 and NaI D1, D2 lines, variable behavior （filled-in absorption, partial emission with a core absorption component or emission above the continuum） in the Ha line, and weak self-reversal emissions in the strong filled-in absorptions of the Ca II IRT lines. We used a spectral subtraction technique to analyze our data. The results show no excess emission in the H6 and H7 lines, very weak excess emissions in the Na I D1, D2 lines, excess emission in the H/3 line, clear excess emission in the Ha line, and excess emissions in the Ca II IRT lines. The value of the ratio of EWs542/EWsa9s is in the range 0.9 to 1.7, which implies that chromospheric activity might have been caused by plage events. The value of the ratio EHδ/EHβ is above 3, indicating that the Balmer lines would arise from prominence-like material. We also found time variations in light curves associated with equiva- lent widths of chromospheric activity lines in the Na I D1, phenomena can be explained by plage events, which are activity indicators. D2, Ca II IRT and Ha lines in particular. These consistent with the behavior of chromospheric

Radio stars observed in the LAMOST spectral survey

Radio stars have attracted astronomers＇ attention for several decades. To better understand the physics behind stellar radio emissions, it is important to study their optical behaviors. The LAMOST survey provides a large database for researching stellar spectroscopic properties of radio stars. In this work, we concentrate on their spectroscopic properties and infer physical properties from their spectra, such as stellar activity and variability. We mined big data from the LAMOST spectral survey Data Release 2 （DR2）, published on 2016 June 30, by cross-matching them with radio stars from FIRST and other surveys. We obtained 783 good stellar spectra with high signal to noise ratio for 659 stars. The criteria for selection were positional coincidence within 1.5＂ and LAMOST objects classified as stars. We calculated the equivalent widths （EWs） of the Ca II H＆K, HS, H＇7, H/3, Ha and Ca II IRT lines by integrating the line profiles. Using the EWs of the Ha line, we detected 147 active stellar spectra of 89 objects having emissions above the Ha continuum. There were also 36 objects with repeated spectra, 28 of which showed chromospheric activity variability. Furthermore, we found 14 radio stars emitting noticeably in the Ca II IRT lines. The low value of the EWs542/EWsags ratio for these 14 radio stars possibly alludes to chromospheric plage regions.

Magnetic activity of the spotted dwarf AP149 in the α Persei open cluster

The simultaneous photometric and spectroscopic observations of the spotted G dwarf AP149 in the young open cluster α Persei are analyzed here. We reconstruct the observed light curves with a two-starspot model by means of a light curve modeling technique, and find that the active regions shift oppositely along longitude on a time scale of one day. Combining with the observational data obtained by other groups, we discuss the evolution of spotted regions in the photosphere, and find that its starspots evolve not only on a short time scale but also on a long time scale. The pure chromospheric emissions for Ca IIHK and Hβ lines are derived by using the spectral subtraction technique. The variation of Ca IIHK lines＇ excess emission is spatially correlated to the starspot regions. There is no clear rotational modulation for the Hβ line＇s excess emission, probably due to the contamination of prominence emission.

The current status of galaxy formation

Understanding galaxy formation is one of the most pressing issues in cos- mology. We review the current status of galaxy formation from both an observational and a theoretical perspective, and summarize the prospects for future advances.

Three X-ray flares near primary eclipse of the RS CVn binary XY UMa

We report on an archival X-ray observation of the eclipsing RS CVn binary XY UMa （Porb≈0.48 d）. In two Chandra ACIS observations spanning 200 ks and almost five orbital periods, three flares occurred. We find no evidence for eclipses in the X-ray flux. The flares took place around times of primary eclipse, with one flare occurring shortly （〈 0.125 Porb） after a primary eclipse, and the other two happening shortly （〈 0.05/9orb） before a primary eclipse. Two flares occurred within roughly one orbital period （△Ф≈ 1.024 Porb） of each other. We analyze the light curve and spectra of the system, and investigate coronal length scales during both quiescence and flares, as well as the timing of the flares. We explore the possibility that the flares are orbit-induced by introducing a small orbital eccentricity, which is quite challenging for this close binary.