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Register a new userFlare stars in nearby Galactic open clusters based on {it TESS} data
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The study is devoted to search for flare stars among confirmed members of Galactic open clusters using high-cadence photometry from {it TESS} mission. We analyzed 957 high-cadence light curves of members from 136 open clusters. As a result, 56 flare stars were found, among them 8 hot B-A type objects. Of all flares, 63% were detected in a sample of cool stars ($T_{rm eff}<5000$~K), and 29% -- in stars of spectral type G, while 23% in K-type stars and approximately 34% of all detected flares are in M-type stars. Using the FLATWRM (FLAre deTection With Ransac Method) flare finding algorithm, we estimated parameters of flares and rotation period of detected flare stars. The flare with the largest amplitude appears on the M3 type EQ,Cha star. Statistical analysis did not reveal any direct correlation between ages, rotation periods and flaring activity.
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From sector 1--40 {em TESS} observations, 20 new roAp stars, 97 ostensibly non-peculiar stars with roAp-like frequencies (the roA variables) and 617 $delta$~Scuti stars with independent frequencies typical of roAp stars were found. There is no criterion that can distinguish roAp/roA stars from $delta$~Sct stars. For expediency, an arbitrary low frequency of 60,d$^{-1}$ was chosen as the boundary between the $delta$~Sct and roAp/roA classes. Because an unknown mode selection process is clearly present in $delta$~Sct stars, the roAp/roA stars may be considered as $delta$~Sct stars in which high frequencies are preferentially selected. This interpretation is supported by the fact that the combined proportion of $delta$~Sct and roAp stars among Ap stars is the same as among non-Ap stars. Contrary to models, observations show that low frequencies in Ap stars are not suppressed. One of the most puzzling aspects of roAp stars is the large fraction which have short mode lifetimes. The failure of current models to explain these results may be due to an incorrect treatment of the outer layers of these stars.
All-sky photometric time-series missions have allowed for the monitoring of thousands of young ($t_{rm age} < 800$Myr) to understand the evolution of stellar activity. Here we developed a convolutional neural network (CNN), $texttt{stella}$, specifically trained to find flares in $textit{Transiting Exoplanet Survey Satellite}$ ($textit{TESS}$) short-cadence data. We applied the network to 3200 young stars to evaluate flare rates as a function of age and spectral type. The CNN takes a few seconds to identify flares on a single light curve. We also measured rotation periods for 1500 of our targets and find that flares of all amplitudes are present across all spot phases, suggesting high spot coverage across the entire surface. Additionally, flare rates and amplitudes decrease for stars $t_{rm age} > 50$Myr across all temperatures $T_{rm eff} geq 4000$K, while stars from $2300 leq T_{rm eff} < 4000$K show no evolution across 800 Myr. Stars of $T_{rm eff} leq 4000$K also show higher flare rates and amplitudes across all ages. We investigate the effects of high flare rates on photoevaporative atmospheric mass loss for young planets. In the presence of flares, planets lose 4-7% more atmosphere over the first 1 Gyr. $texttt{stella}$ is an open-source Python tool-kit hosted on GitHub and PyPI.
Blue straggler stars are exotic objects present in all stellar environments whose nature and formation channels are still partially unclear. They seem to be particularly abundant in open clusters (OCs), thus offering a unique chance to tackle these problems statistically.We aim to build up a new and homogeneous catalogue of blue straggler stars (BSS) in Galactic OCs using Gaia to provide a more solid assessment of the membership of these stars. We also aim to explore possible relationships of the straggler abundance with the parent clusters structural and dynamical parameters. As a by-product, we also search for possible yellow straggler stars (YSS), which are believed to be stragglers in a more advanced evolution stage. We employed photometry, proper motions, and parallaxes extracted from Gaia DR2 for 408 Galactic star clusters and searched for stragglers within them after performing a careful membership analysis. The number of BBS emerging from our more stringent, selection criteria turns out to be significantly smaller than in previo
Superflares on solar-type stars has been a rapidly developing field ever since the launch of $it Kepler$. Over the years, there have been several studies investigating the statistics of these explosive events. In this study, we present a statistical analysis of stellar flares on solar-type stars made using photometric data in 2-min cadence from $it TESS$ of the whole southern hemisphere (sectors 1 - 13). We derive rotational periods for all stars in our sample from rotational modulations present in the lightcurve as a result of large starspot(s) on the surface. We identify 1980 stellar flares from 209 solar-type stars with energies in the range of $10^{31} - 10^{36}$erg (using the solar flare classification, this corresponds to X1 - X100,000) and conduct an analysis into their properties. We investigate the rotational phase of the flares and find no preference for any phase suggesting the flares are randomly distributed. As a benchmark, we use GOES data of solar flares to detail the close relationship between solar flares and sunspots. In addition, we also calculate approximate spot areas for each of our stars and compare this to flare number, rotational phase, and flare energy. Additionally, two of our stars were observed in the continuous viewing zone with lightcurves spanning one year, as a result, we examine the stellar variability of these stars in more detail.
The Transiting Exoplanet Survey Satellite (TESS) is providing precise time-series photometry for most star clusters in the solar neighborhood. Using the TESS images, we have begun a Cluster Difference Imaging Photometric Survey (CDIPS), in which we are focusing both on stars that are candidate cluster members, and on stars that show indications of youth. Our aims are to discover giant transiting planets with known ages, and to provide light curves suitable for studies in stellar astrophysics. For this work, we made 159,343 light curves of candidate young stars, across 596 distinct clusters. Each light curve represents between 20 and 25 days of observations of a star brighter than $G_{rm Rp}=16$, with 30-minute sampling. We describe the image subtraction and time-series analysis techniques we used to create the light curves, which have noise properties that agree with theoretical expectations. We also comment on the possible utility of the light curve sample for studies of stellar rotation evolution, and binary eccentricity damping. The light curves, which cover about one sixth of the galactic plane, are available as a MAST High Level Science Product at https://doi.org/10.17909/t9-ayd0-k727 .
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