Summary As one of the worst invasive weeds in the world, Mikania micrantha H. B. K. spread to southern China after 1910 but without any natural enemies. Since the 1980s it has started to spread and invade widely. The weed has caused significant damage to many ecosystems in Guangdong province in recent years. This paper reviews the research in China on this weed, including its description, history of introduction together with information from morphological and physiological studies. The distribution of this invasive plant in China has been mapped and is presented together with damage being caused and analyses of soil types infested. The measures taken by the Chinese government for prevention and elimination are also discussed. Chinese scientists are now looking for effective biocontrol options to deal with this deleterious plant. Several native insects, mites and fungi have been reported to be able to control the reproduction of this weed in China, but no natural control has been achieved. Cuscuta sp., a native autoecious vine has recently been shown to suppress the growth of M. micrantha. Three major causes for the massive spread of this weed in southern China are presented together with recommendations for future control activities.

Aims. We present the new high-resolution echelle spectra of SZ Psc, obtained in Nov. 2004 and Sep.−Dec. 2006, and study its chromospheric activity. Results. All indicators show that the chromospheric activity of the system is associated with the cooler component. We find that the values of EW 8542 /EW 8498 are in the range 1−3, which indicates optically thick emission in plage-like regions. The 2006 data suggest the presence of active longitude phenomena. For the Ca ii 8542 and 8662 and the H α lines, it seems that the excess emission is stronger near the two quadratures of system. This may be anti-correlated with the behavior of the Na i D 1 line. The absorption features are detected in the subtracted H α lines, which could be explained by prominence-like extended material seen on the stellar disk or by mass transfer from the cooler component to the hotter one.

We performed a statistical study of magnetic activities of M-type stars by combining the spectra of LAMOST DR5 with light curves from the Kepler and K2 missions. We mainly want to study the relationship between chromospheric activity and flares, and their relations of magnetic activity and rotation period. We have obtained the maximum catalog of 516,688 M-type stellar spectra of 480,912 M stars from LAMOST DR5 and calculated their equivalent widths of chromospheric activity indicators (Hα, Hβ, Hγ, Hδ, Ca ii H&K, and He i D3). Using the Hα indicator, 40,464 spectra of 38,417 M stars show chromospheric activity, and 1791 of these 5499 M-type stars with repeated observations have Hα variability. We used an automatic detection plus visual inspection method to detect 17,432 flares on 8964 M-type stars from the catalog by cross-matching LAMOST DR5 and the Kepler and K2 databases. We used the Lomb–Scargle method to calculate their rotation periods. We find that the flare frequency is consistent with the ratio of activities of these chromospheric activity indicators as a function of spectral type in M0–M3. We find the equivalent widths of Hα and Ca ii H have a significant statistical correlation with the flare amplitude in M-type stars. We confirm that the stellar flare is affected by both the stellar magnetic activity and the rotation period. Finally, using the Hα equivalent width equal to 0.75 Å and using the rotation period equal to 10 days as the threshold for the M-type stellar flare time frequency are almost equivalent.

Stellar magnetic activity is an interesting phenomenon in late-type stars. We use the medium-resolution spectroscopic observations of 406 069 late-type stars from LAMOST to explore their properties. We perform a statistical analysis on the magnetic activity of the stars and their associated physical parameters. Our samples are cross-matched with other data bases (LAMOST DR5, VSX, and SDSS DR12) to obtain additional observational properties. The equivalent widths (EWs) of Hα lines, an indicator of chromospheric activity, are calculated. According to the EWs of the Hα line, we detect 8816 spectra with apparent Hα emission for a total of 2115 stars among the 2 108 565 spectra analysed. Furthermore, 1521 of these stars show variability in their Hα lines. In addition, we detect 2132 flare events associated with 41 542 stars from the catalogue by cross-matching our LAMOST medium-resolution samples and the Kepler and K2 data bases. We also confirm a weak relationship of the flare amplitude with increasing Rossby number. There is a clear decease in the strength of chromospheric activity (LHα/Lbol) with increasing rotation period. Moreover, the ratio of the flare energy and stellar luminosity is found to decrease with the stellar mass.

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.

We present a catalog of 75,867 periodic variable stars from data release 7 of the Large Sky Area Multi-Object Fiber Spectroscopic Telescope. We calculated the chromospheric activity index, equivalent widths for 201,349 spectra, and excess fractional luminosity ( R H α ′ ) for 130,796 spectra. Based on the intensity of the Hα line, we detected 30,719 F-, G-, and K-type variable stars with Hα fill-in or emission and 2521 M-type variable stars exhibiting Hα emission. Furthermore, 14,938 out of 30,573 periodic variable stars showed Hα variability. Analysis of samples with different spectral types confirmed that the activity fraction of stars with later spectral type clearly decreases as the absolute Galactic height increases away from the Galactic plane. The results also show that at the same absolute Galactic height, the activity fractions in eclipsing binaries are higher than those in single stars. In this study, we present the activity–rotation relations for F-, G-, K-, and M-type variable stars. The results of analysis show that rapid rotators keep a saturated value, and the absolute value of the power-law index exhibits an increasing trend from F- to K-type stars in the unsaturated regime. Moreover, we confirm that the angular momentum and magnetic momentum are positively correlated, which indicates that the α–ω dynamo may be working. In addition, we confirmed the correlations among the Hα, Hβ, Hγ, Hδ, Ca ii H&K, and Ca ii infrared triplet lines.

We present the results from a systematic study of temporal variation of stellar activity in young late-type stars. We used multi-epoch LAMOST (Large sky Area Multi-Object fiber Spectroscopic Telescope) low-resolution spectra of over 300 member candidates in three young open clusters: Pleiades, Praesepe, and Hyades. The spectral measurements of TiO band strength near 7050 Å (TiO2) and equivalent width of H α line (EWH α) are used as the tracers of cool spot coverage and chromospheric emission strength, respectively. The analysis of time-variation patterns of these two tracers suggested that there exist detectable variabilities in TiO2 and EWH α, and their time-scales are in the wide range from days to years. Results showed that more active stars, younger and fast rotators, tend to have larger activity variations. There is a tendency of anticorrelation between temporal variations in TiO2 and EWH α. Also, appreciable anticorrelation in the rotational phase between H α emission and K2 brightness is detected in some M dwarfs, indicating spatial co-location of the plages with cool star-spots; however, cool stars do not always show such co-location features. Furthermore, spot coverage and H α emission were evident at all rotational phases of several M dwarfs, indicating a basal level of activity, perhaps due to many small and randomly located active regions in the atmosphere.

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.