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Flares are frequent energetic explosions in the stellar atmosphere, and are thought to occur by impulsive releases of magnetic energy stored around starspots. Large flares (so called “superflares”) generate strong high energy emissions and coronal mass ejections (CMEs), which can greatly affect the planetary environments. For haracterizing stellar CMEs, understanding of their sources (e.g., flares and erupting prominences) is important. Recent Kepler/TESS photometric data have revealed the statistical properties of superflares on G, K, M-type stars. Superflare stars are well characterized by the existence of large starspots on the surface, and their magnetic fluxes can explain well superflare energies. Flare frequency/energy depends on stellar rotation period and stellar temperature. Young rapidly-rotating stars and cooler stars tend to have frequent flares. Recent spectroscopic observation campaigns have detected some possible candidates of stellar prominence eruptions from G-M stars using blueshifts of chromospheric lines (e.g., H-alpha). They could be the start point for quantitatively discussing physical properties and time evolution of stellar CMEs, and their effects on the environment and habitability of various planets. In the early part of the talk, I briefly overview the recent statistical results of superflares from Kepler/TESS data. Then in the latter part, I discuss the recent reports of stellar prominence eruptions and how these results can help our understanding of stellar CMEs., Invited talk at the Splinter Session "Solar and stellar coronal mass ejections"

The Sun and Sun-like stars commonly host the multi-million-K corona and the 10,000-K chromosphere. Because these extremely hot gases generate X-ray and extreme ultraviolet (XUV) emissions that may impact the erosion and chemistry of (exo)planetary atmospheres, influencing the climate and conditions for habitability, it is of crucial importance in solar and stellar physics to understand the responsible heating mechanisms. While, for the Sun, the magnetic field is thought to play a pivotal role in driving and transporting the energy from the surface upwards, it is not clear whether such a magnetically driven heating is commonly at work on other stars. Here we present the analysis of 10 years of multi-wavelength synoptic observations of the Sun and comparison with stellar data, providing the critical clues to the common nature of the heating mechanisms of coronae, transition regions, and chromospheres of the Sun and Sun-like stars. First, by analyzing the sequences of solar images of sunspot transit events, we derive the means to understand the magnetic and thermal environments of starspot magnetic fields that cannot be spatially resolved. Specifically, it is found that the surface magnetic flux can be inferred from the chromospheric light curves and that the thermal structures around the starspots can be inspected from the sub-MK UV light curves. Second, by investigating the power-law relationships between the surface magnetic flux and the luminosity of various emission lines with the formation temperatures from the corona to the chromosphere of the Sun, we discover that, in any temperature ranges, the solar scaling laws can be extended to the Sun-like stars with the ages of 50 Myr to 4.5 Gyr. This suggests that the magnetically-driven heating of the atmospheres is universal among the Sun and Sun-like stars, regardless of age or activity. Furthermore, by deriving the scaling laws between the magnetic flux and XUV spectrum, it is now possible to empirically reproduce the XUV spectra for various solar-type stars given the observationally-obtained magnetic fluxes. It is expected that the reproduced XUV spectra are used for photochemical calculations of (exo)planets of various solar-type stars, including the young Sun, and estimation of atmospheric formation and habitability of surface life., {"references":["Toriumi, S. & Airapetian, V. S. (2022), ApJ, 927, 179","Toriumi, S. et al. (2022), ApJS, 262, 46","Toriumi, S. & Wang, H. (2019), LRSP, 16, 3"]}

Transit observations in the helium lines near 10830 Angstrom are a new successful tool to study exoplanetary atmospheres and their mass loss. Forming those lines requires ionization and recombination of helium in the exoplanetary atmosphere. This ionization is caused by stellar photons in the extreme UV (EUV); however, no currently active telescopes can observe this part of the stellar spectrum. The stellar spectrum close to the helium ionization threshold consists of individual emission lines, many of which are formed by iron at coronal temperatures. Coronal elemental abundances exhibit distinct patterns related to the first ionization potential (FIP) of those elements, with elements like iron being strongly depleted for high-activity low-mass stars. I show that stars with high versus low coronal iron abundances follow different scaling laws that tie together their X-ray emission and the EUV flux close to the helium ionization threshold. I also show that the currently observed large scatter in the relationship of EUV irradiation with exoplanetary helium transit depths can be reduced by taking coronal iron abundances into account, allowing us to target exoplanets with well-observable helium signatures with much higher confidence., {"references":["Wood et al. (2018), The Astrophysical Journal, Volume 862, Issue 1, article id. 66, arXiv:1806.05111","Oklopcic and Hirata (2018), The Astrophysical Journal Letters, Volume 855, Issue 1, article id. L11, arXiv:1711.05269","Spake et al. (2018), Nature, Volume 557, Issue 7703, p.68-70, arXiv:1805.01298"]}

Activity cycles are commonly found among late type stars through the chromospheric Ca II emission. Their coronal counterpart, however, remains elusive in most cases, despite of the clear X-ray cycle observed in the solar corona, spanning as much as 1.7 dex in L x . The recent discovery of a Ca II cycle in iota Hor of just 1.6 yr, the shortest to date, offered us a unique opportunity to monitor its X-ray counterpart in short time. The star offers also two more interesting properties: a planet of ~1.9 MJ orbits the star at 0.9 a.u., and with an age of only ~600 Myr and spectral type F8V, ι Hor represents a young solar analog, so its cycle might be the paradigm of the first activity cycles in the life of a solar-like star. Our XMM-Newton observations show the first coronal cycle in a single star. In good agreement with Ca II contemporaneous observations, the long term XMM-Newton light curve suggests also a long-term trend that seems to modulate the 1.6 yr cycle. Iota Hor may offer us the unique possibility to observe for the first time a double coronal cycle similar to those observed in the chromosphere of other stars.

We use a 440.5 ks Chandra observation of the $\approx$500-Myr-old open cluster M37 to derive the X-ray luminosity functions of its $\leq1.2$ $M_{\odot}$ stars. Combining detections of 162 M37 members with upper limits for 160 non-detections, we find that its G, K, and M stars have a similar median (0.5$-$7 keV) X-ray luminosity L$_X =10^{29.0}$ erg/s, whereas the L$_X$-to-bolometric-luminosity ratio (L$_X$/L$_{bol}$) indicates that M stars are more active than G and K stars by $\approx$1 order of magnitude at 500 Myr. To characterize the evolution of magnetic activity in low-mass stars over their first $\approx$600 Myr, we consolidate X-ray and optical data from the literature for stars in six other open clusters: from youngest to oldest, the Orion Nebula Cluster (ONC), NGC 2547, NGC 2516, the Pleiades, NGC 6475, and the Hyades. For these, we homogenize the conversion of instrumental count rates to L$_X$ by applying the same one-temperature emission model as for M37, and obtain masses using the same empirical mass-absolute magnitude relation (except for the ONC). We find that for G and K stars X-ray activity decreases $\approx$2 orders of magnitude over their first 600 Myr, and for M stars, $\approx$1.5. The decay rate of the median L$_X$ follows the relation L$_X \propto~t^b$, where $b=-0.61\pm0.12$ for G, $-0.82\pm0.16$ for K, and $-0.40\pm0.17$ for M stars. In L$_X$/L$_{bol}$ space, the slopes are $-0.68\pm0.12$, $-0.81\pm0.19$, and $-0.61\pm0.12$, respectively. These results suggest that for low-mass stars the age-activity relation steepens after $\approx$625 Myr, consistent with the faster decay in activity observed in solar analogs at $t>1$ Gyr., Comment: 2 machine readable tables