Search

I present an analysis of starspot modulation and flares in the TESS light curves of 112 M dwarfs with a TESS magnitude, {"references":["Stelzer, Beate et al. (2016). A path towards understanding the rotation-activity relation of M dwarfs with K2 mission, X-ray and UV data. MNRAS, 463, 1844","Raetz, Stefanie et al. (2020). Rotation-activity relations and flares of M dwarfs with K2 long- and short-cadence data. A&A, 637, A22","Hawley, Suzanne et al. (2014). Kepler Flares. I. Active and Inactive M Dwarfs. ApJ, 797, 121","Kaltenegger, L. et al. (2019). TESS Habitable Zone Star Catalog . ApJ, 874, L8"]}

The activity-rotation relation of M dwarfs provides observational evidence of the stellar dynamo, which is poorly understood for fully convective stars. It is known that the X-ray activity-rotation relation has a two-regime behavior with saturation for faster rotating stars and with a Lx decrease for slower rotation. However, the number of M dwarfs with sensitive X-ray and rotation measure- ments has for a long time been very limited. New XMM-Newton, Chandra and eROSITA X-ray observations with K2 mission and TESS rotation periods analyzed in my work have substantially increased the number of low activity and fast/slow rotation stars that have been dicult to assess in the past. Combining the new data with a systematic update and homogenization of the literature data has enabled me to examine the activity-rotation relation in different mass bins within the M dwarf regime. Several interesting findings emerged, including a non-constant Lx-level in the saturated regime and a mass-dependence of the slope in the unsaturated regime., {"references":["Magaudda et al. 2020, A&A, 638, A20","Raetz et al. 2020, A&A, 637, A22","Reiners et al. 2015, ApJ, 794"]}

Young stars exhibit short-term photometric variability caused by mass accretion events from circumstellar disks, the presence of dusty warps within the inner disks, starspots that rotate across the stellar surfaces, and flares. Long-term variability also occurs owing to starspot longevity and cycles, and from changes in stellar angular momenta and activity as the stars age. We propose to observe the Carina star-forming region in different bands with a cadence of 30 minutes every night for one week per year to clarify the nature of both the short-term and long-term variability of the thousands of young stars in this region. By obtaining well-sampled multicolor lightcurves of this dense young cluster, LSST would acquire the first statistically significant data on how these objects vary on both short and long timescales. This information will allow us to relate the observed variability to stellar properties such as mass, age, binarity, and to environmental properties such as location within or exterior to the H II region, and to the presence or absence of a circumstellar disk., Comment: Contribution in response of the Call for White Papers on LSST Cadence Optimization

The relation between magnetic activity and rotation periods in late-type stars provides fundamental information on the stellar dynamo and spin evolution. In spite of its importance for stellar physics, homogeneous samples of stars with accurate and sensitive measurement of both rotation period and magnetic activity have been hard to come by. The Kepler mission represents a significant step forward, providing high-cadence optical light curves for thousands of stars, from which the rotation period can be measured observing the brightness modulation due to star spots. A cross-match of the Kepler Input Catalog with the 3XMM-DR5 Catalog and subsequent careful inspection for likely non-stellar sources yields more than 100 late-type stars. We have developed an algorithm which identifies rotation periods and white-light flares in the Kepler light curves. We have calculated the X-ray luminosity from the 3XMM-DR5 count rates, and searched the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project for X-ray flares. Here we discuss the correlation between various measures for coronal and photometric activity (from the XMM-Newton and the Kepler data, respectively) and the Kepler rotation periods and Rossby number.

We present an updated view of the rotation-activity-age relation for (old) field M dwarfs based on K2 rotation periods and flares,archival X-ray and UV data,and dedicated Chandra and XMM-Newton observations. The rotation-activity-age relation can be used as a proxy for magnetic fields-- which are difficult to measure in M dwarfs --and is, therefore, key to studies of(i) the predicted dynamo transition at the fully convective boundary(SpT~M3),(ii) differences in angular momentum loss through magnetized windswith respect to solar-type stars,and (iii) the evaporation of planet atmospheres,especially relevant for M dwarfs because ofthe small separation of their planets' habitable zones wherethey are strongly exposed to the stellar high-energy emission. We use a two-fold approach to obtain tight observational constraints:(1) In our study of the activity-rotation relation of nearby,proper-motion selected M dwarfs using photometric time-series from theK2 mission combined with X-ray and UV data we find strong evidencefor an abrupt change of optical photometric activity (flares, rotationcycle amplitude and residual variability)at P_rot~10d, and an unexpectedly steep decline ofX-ray activity in the unsaturated regime of slow rotators.(2) The age-activity relation is investigated through deep X-ray observationsof a sample of M dwarfs in wide binaries with white dwarfs. Here thewhite dwarf serves as a chronometer for the age of the M dwarf. Our results provide important input foraccurate angular momentum evolution models and planet atmosphere escapecalculations for M dwarfs.&nbsp

We present high-quality, medium resolution X-shooter/VLT spectra in the range 300-2500 nm for a sample of 12 very low-mass stars in the ��Orionis cluster. The sample includes stars with masses ranging from 0.08 to 0.3 M$_\odot$. The aim of this first paper is to investigate the reliability of the many accretion tracers currently used to measure the mass accretion rate in low-mass, young stars. We use our spectra to measure the accretion luminosity from the continuum excess emission in the UV and visual; the derived mass accretion rates range from 10$^{-9}$ M$_{\odot}$ yr$^{-1}$ down to 5$\times10^{-11}$ M$_{\odot}$ yr$^{-1}$, allowing us to investigate the behavior of the accretion-driven emission lines in very-low mass accretion rate regimes. We compute the luminosity of ten accretion-driven emission lines, from the UV to the near-IR, obtained simultaneously. Most of the secondary tracers correlate well with the accretion luminosity derived from the continuum excess emission. We confirm the validity of the correlations between accretion luminosities and line luminosities given in the literature, with the possible exception of H��. When looking at individual objects, we find that the Hydrogen recombination lines, from the UV to the near-IR, give good and consistent measurements of accretion luminosities, often in better agreement than the uncertainties introduced by the adopted correlations. The average accretion luminosity derived from several Hydrogen lines, measured simultaneously, have a much reduced error. This suggests that some of the spread in the literature correlations may be due to the use of non-simultaneous observations of lines and continuum. Three stars in our sample deviate from this behavior, and we discuss them individually., 15 pages, 14 figures

FU Tau A is a young very low mass object in the Taurus star forming region which was previously found to have strong X-ray emission and to be anomalously bright for its spectral type. In this study we discuss these characteristics using new information from quasi-simultaneous photometric and spectroscopic monitoring. From photometric time series obtained with the 2.2m telescope on Calar Alto we measure a period of ~4d for FU Tau A, most likely the rotation period. The short-term variations over a few days are consistent with the rotational modulation of the flux by cool, magnetically induced spots. In contrast, the photometric variability on timescales of weeks and years can only be explained by the presence of hot spots, presumably caused by accretion. The hot spot properties are thus variable on timescales exceeding the rotation period, maybe due to long-term changes in the accretion rate or geometry. The new constraints from the analysis of the variability confirm that FU Tau A is affected by magnetically induced spots and excess luminosity from accretion. However, accretion is not sufficient to explain its anomalous position in the HR diagram. In addition, suppressed convection due to magnetic activity and/or an early evolutionary stage need to be invoked to fully account for the observed properties. These factors cause considerable problems in estimating the mass of FU Tau A and other objects in this mass/age regime, to the extent that it appears questionable if it is feasible to derive the Initial Mass Function for young low-mass stars and brown dwarfs., 10 pages, 7 figures, accepted for publication in MNRAS, 'Note added in proof' included

Studies of disks around young brown dwarfs are of paramount importance to our understanding of the origin, diversity and early evolution of sub-stellar objects. Here we present first results from a systematic search for disk emission in a spectroscopically confirmed sample of young objects near or below the sub-stellar boundary in a variety of star-forming regions. Our VLT and Keck L'-band observations of the sigma Orionis and TW Hydrae associations suggest that if a majority of brown dwarfs are born with disks, at least the inner regions of those disks evolve rapidly, possibly clearing out within a few million years., Comment: to appear in proceedings of the IAU Symposium 211 on "Brown Dwarfs", ASP Conference Series