Superflare G and K Stars and the Lithium abundance

We analyzed here the connection of superflares and the lithium abundance in G and K stars based on Li abundance determinations conducted with the echelle spectra of a full set of 280 stars obtained with the ELODIE spectrograph. For high-active stars we show a definite correlation between $\log A(Li)$ and the chromosphere activity. We show that sets of stars with high Li abundance and having superflares possess common properties. It relates, firstly, to stars with activity saturation. We consider the X-ray data for G, K, and M stars separately, and show that transition from a saturation mode to solar-type activity takes place at values of rotation periods 1.1, 3.3, and 7.2 days for G2, K4 and M3 spectral types, respectively. We discuss bimodal distribution of a number of G and K main-sequence stars versus an axial rotation and location of superflare stars with respect to other Kepler stars. We conclude that superflare G and K stars are mainly fast rotating young objects, but some of them belong to stars with solar-type activity. At the same time, we found a group of G stars with high Li content $(\log A(Li) = 1.5 - 3)$, but being slower rotators with rotation periods > 10 days, which are characterized by low chromospheric activity. This agrees with a large spread in Li abundances in superflare stars. A mechanism leading to this effect is discussed.Comment: 6 pages, 8 figures. The 19th Cambridge Workshop on Cool Stars, Stellar Systems, and the Su

Solar-type Stars Observed by LAMOST and Kepler

Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar-stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (i.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could not be measured due to the low amplitude of the photometric variability and highly irregular temporal profile of light curves, 254 stars were further identified as having near-solar rotation periods. We show that stars with near-solar rotation periods have chromospheric activities that are systematically higher than stars with undetected rotation periods. Furthermore, while the solar level of photospheric and chromospheric activity appears to be typical for stars with undetected rotation periods, the Sun appears to be less active than most stars with near-solar rotation periods (both in terms of photospheric and chromospheric activity).Comment: 7 pages, 6 figure

An XMM-Newton observation of the young open cluster NGC 2547: coronal activity at 30 Myr

We report XMM-Newton observations of the young open cluster NGC 2547 which allow us to characterise coronal activity in solar-type stars at an age of 30 Myr. X-ray emission peaks among G-stars at luminosities (0.3-3keV) of Lx~10^{30.5} erg/s and declines to Lx<=10^{29.0} erg/s among M-stars. Coronal spectra show evidence for multi-temperature differential emission measures and low coronal metal abundances (Z~0.3). The G- and K-type stars follow the same relationship between X-ray activity and Rossby number established in older clusters and field stars, although most solar-type stars in NGC 2547 exhibit saturated/super-saturated X-ray activity levels. Median levels of Lx and Lx/Lbol in the solar-type stars of NGC 2547 are similar to T-Tauri stars of the Orion Nebula cluster (ONC), but an order of magnitude higher than in the older Pleiades. The spread in X-ray activity levels among solar-type stars in NGC 2547 is much smaller than in older or younger clusters. Coronal temperatures increase with Lx, Lx/Lbol and surface X-ray flux. Active solar-type stars in NGC 2547 have coronal temperatures between those in the ONC and the most active older ZAMS stars. A flaring rate (for total flare energies [0.3-3keV] >10^{34} erg) of 1 every 350^{+350}_{-120} ks was found for solar-type stars, similar to rates found in the ONC and Pleiades. Comparison with ROSAT HRI data taken 7 years previously reveals that only 10-15 percent of solar-type stars or stars with Lx>3x10^{29} erg/s exhibit X-ray variability by more than a factor of two. The similar levels of X-ray activity and rate of occurrence for large flares in NGC 2547 and the ONC demonstrate that the X-ray radiation environment around young solar-type stars remains relatively constant over their first 30 Myr (abridged).Comment: Accepted for publication in MNRAS. Electronic tables available from the autho

Investigating magnetic activity of F stars with the it Kepler mission

The dynamo process is believed to drive the magnetic activity of stars like the Sun that have an outer convection zone. Large spectroscopic surveys showed that there is a relation between the rotation periods and the cycle periods: the longer the rotation period is, the longer the magnetic activity cycle period will be. We present the analysis of F stars observed by Kepler for which individual p modes have been measure and with surface rotation periods shorter than 12 days. We defined magnetic indicators and proxies based on photometric observations to help characterise the activity levels of the stars. With the Kepler data, we investigate the existence of stars with cycles (regular or not), stars with a modulation that could be related to magnetic activity, and stars that seem to show a flat behaviour.Comment: 2 pages, 1 figure, proceedings of IAU Symposium 302 'Magnetic fields through stellar evolution', 25-30 August 2013, Biarritz, Franc

Is it possible to detect planets around young active G and K dwarfs?

Theoretical predictions suggest that the distribution of planets in very young stars could be very different to that typically observed in Gyr old systems that are the current focus of radial velocity surveys. However, the detection of planets around young stars is hampered by the increased stellar activity associated with young stars, the signatures of which can bias the detection of planets. In this paper, we place realistic limitations on the possibilities for detecting planets around young active G and K dwarfs. The models of stellar activity based on tomographic imaging of the G dwarf HD 141943 and the K1 dwarf AB Dor also include contributions from plage and many small random starspots. Our results show that the increased stellar activity levels present on young solar-type stars strongly impacts the detection of Earth-mass and Jupiter-mass planets and that the degree of activity jitter is directly correlated with stellar v sin i. We also show that for G and K dwarfs, the distribution of activity in individual stars is more important than the differences in induced radial velocities as a function of spectral type. We conclude that Jupiter-mass planets can be detected close-in around fast-rotating young active stars, Neptune-mass planets around moderate rotators and that Super-Earths are only detectable around very slowly rotating stars. The effects of an increase in stellar activity jitter by observing younger stars can be compensated for by extending the observational base-line to at least 100 epochs.Peer reviewe

Searching for Planets in the Hyades II: Some Implications of Stellar Magnetic Activity

The Hyades constitute a homogeneous sample of stars ideal for investigating the dependence of planet formation on the mass of the central star. Due to their youth, Hyades members are much more chromospherically active than stars traditionally surveyed for planets using high precision radial velocity (RV) techniques. Therefore, we have conducted a detailed investigation of whether magnetic activity of our Hyades target stars will interfere with our ability to make precise RV searches for substellar companions. We measure chromospheric activity (which we take as a proxy for magnetic activity) by computing the equivalent of the R'HK activity index from the Ca II K line. is not constant in the Hyades: we confirm that it decreases with increasing temperature in the F stars, and also find it decreases for stars cooler than mid-K. We examine correlations between simultaneously measured R'HK and RV using both a classical statistical test and a Bayesian odds ratio test. We find that there is a significant correlation between R'HK and the RV in only 5 of the 82 stars in this sample. Thus, simple Rprime HK-RV correlations will generally not be effective in correcting the measured RV values for the effects of magnetic activity in the Hyades. We argue that this implies long timescale activity variations (of order a few years; i.e., magnetic cycles or growth and decay of plage regions) will not significantly hinder our search for planets in the Hyades if the stars are closely monitored for chromospheric activity. The trends in the RV scatter (sigma'_v) with , vsini, and P_rot for our stars is generally consistent with those found in field stars in the Lick planet search data, with the notable exception of a shallower dependence of sigma'_v on for F stars.Comment: 15 pages, 7 figures, 3 tables; To appear in the July 2002 issue of The Astronomical Journa

Evolution of emission line activity in intermediate mass young stars

We present optical spectra of 45 intermediate mass Herbig Ae/Be stars. Together with the multi-epoch spectroscopic and photometric data compiled for a large sample of these stars and ages estimated for individual stars by using pre-main sequence evolutionary tracks, we have studied the evolution of emission line activity in them. We find that, on average, the H_alpha emission line strength decreases with increasing stellar age in HAeBe stars, indicating that the accretion activity gradually declines during the PMS phase. This would hint at a relatively long-lived (a few Myr) process being responsible for the cessation of accretion in Herbig Ae/Be stars. We also find that the accretion activity in these stars drops substantially by ~ 3 Myr. This is comparable to the timescale in which most intermediate mass stars are thought to lose their inner disks, suggesting that inner disks in intermediate mass stars are dissipated rapidly after the accretion activity has fallen below a certain level. We, further find a relatively tight correlation between strength of the emission line and near-infrared excess due to inner disks in HAeBe stars, indicating that the disks around Herbig Ae/Be stars cannot be entirely passive. We suggest that this correlation can be understood within the frame work of the puffed-up inner rim disk models if the radiation from the accretion shock is also responsible for the disk heating.Comment: 39 pages, accepted for publication in Ap