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Register a new userSearch for flares and associated CMEs on late-type main-sequence stars in optical SDSS spectra
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This work aims to detect and classify stellar flares and potential stellar coronal mass ejection (CME) signatures in optical spectra provided by the Sloan Digital Sky Survey (SDSS) data release 14. The sample is constrained to all F, G, K, and M main-sequence type stars, resulting in more than 630,000 stars. This work makes use of the individual spectral exposures provided by the SDSS. An automatic flare search was performed by detecting significant amplitude changes in the $Halpha$ and $Hbeta$ spectral lines after a Gaussian profile was fit to the line core. CMEs were searched for by identifying asymmetries in the Balmer lines caused by the Doppler effect of plasma motions in the line of sight. We identified 281 flares on late-type stars (spectral types K3 to M9). We identified six possible CME candidates showing excess flux in Balmer line wings. Flare energies in $Halpha$ were calculated and masses of the CME candidates were estimated. The derived $Halpha$ flare energies range from $3 times 10^{28} - 2 times 10^{33}$ erg. The $Halpha$ flare energy increases with earlier types, while the fraction of flaring times increases with later types. Mass estimates for the CME candidates are in the range of $6 times 10^{16} - 6 times 10^{18}$ g, and the highest projected velocities are $sim300 - 700:$ km/s. The low detection rate of CMEs we obtained agrees with previous studies, suggesting that for late-type main-sequence stars the CME occurrence rate that can be detected with optical spectroscopy is low.
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About 22000 Kepler stars and nearly 60000 TESS stars from sectors 1-24 have been classified according to variability type. A large proportion of stars of all spectral types appear to have periods consistent with the expected rotation periods. A previous analysis of A and late B stars strongly suggests that these stars are indeed rotational variables. In this paper we have accumulated sufficient data to show that rotational modulation is present even among the early B stars. A search for flares in TESS A and B stars resulted in the detection of 110 flares in 68 stars. The flare energies exceed those of typical K and M dwarfs by at least two orders of magnitude. These results, together with severe difficulties of current models to explain stellar pulsations in A and B stars, suggest a need for revision of our current understanding of the outer layers of stars with radiative envelopes.
Opacity enhancements for stellar interior conditions have been explored to explain observed pulsation frequencies and to extend the pulsation instability region for B-type main-sequence variable stars. For these stars, the pulsations are driven in the region of the opacity bump of Fe-group elements at $sim$200,000 K in the stellar envelope. Here we explore effects of opacity enhancements for the somewhat cooler main-sequence A-type stars, in which $p$-mode pulsations are driven instead in the second helium ionization region at $sim$50,000 K. We compare models using the new LANL OPLIB vs. LLNL OPAL opacities for the AGSS09 solar mixture. For models of 2 solar masses and effective temperature 7600 K, opacity enhancements have only a mild effect on pulsations, shifting mode frequencies and/or slightly changing kinetic-energy growth rates. Increased opacity near the bump at 200,000 K can induce convection that may alter composition gradients created by diffusive settling and radiative levitation. Opacity increases around the hydrogen and 1st He ionization region (13,000 K) can cause additional higher-frequency $p$ modes to be excited, raising the possibility that improved treatment of these layers may result in prediction of new modes that could be tested by observations. New or wider convective zones and higher convective velocities produced by opacity increases could also affect angular momentum transport during evolution. More work needs to be done to quantify the effects of opacity on the boundaries of the pulsation instability regions for A-type stars.
We have statistically analyzed a set of 115 low frequency (Deca- Hectometer wavelengths range) type II and type III bursts associated with major Solar Energetic Particle (SEP: Ep > 10 MeV) events and their solar causes such as solar flares and coronal mass ejections (CMEs) observed from 1997 to 2014. We classified them into two sets of events based on the duration of the associated solar flares:75 impulsive flares (duration < 60 min) and 40 gradual flares (duration > 60 min).The impulsive flare-associated SEP events (Rt = 989.23 min: 2.86 days) are short lived and they quickly reach their peak intensity (shorter rise time) when compared with gradual flares associated events (Rt =1275.45 min: 3.34 days). We found a good correlation between the logarithmic peak intensity of all SEPs and properties of CMEs (space speed: cc = 0.52, SEcc = 0.083), and solar flares (log integrated flux: cc = 0.44, SEcc = 0.083). This particular result gives no clear cut distinction between flare-related and CME-related SEP events for this set of major SEP events. We derived the peak intensity, integrated intensity, duration and slope of these bursts from the radio dynamic spectra observed by Wind/WAVES. Most of the properties (peak intensity, integrated intensity and starting frequency) of DH type II bursts associated with impulsive and gradual flare events are found to be similar in magnitudes. In addition, we also found a significant correlation between the properties of SEPs and key parameters of DH type III bursts. This result shows a closer association of peak intensity of the SEPs with the properties of DH type III radio bursts than with the properties DH type II radio bursts, at least for this set of 115 major SEP events.
Getman et al. (2021) reports the discovery, energetics, frequencies, and effects on environs of $>1000$ X-ray super-flares with X-ray energies $E_X sim 10^{34}-10^{38}$~erg from pre-main sequence (PMS) stars identified in the $Chandra$ MYStIX and SFiNCs surveys. Here we perform detailed plasma evolution modeling of $55$ bright MYStIX/SFiNCs super-flares from these events. They constitute a large sample of the most powerful stellar flares analyzed in a uniform fashion. They are compared with published X-ray super-flares from young stars in the Orion Nebula Cluster, older active stars, and the Sun. Several results emerge. First, the properties of PMS X-ray super-flares are independent of the presence or absence of protoplanetary disks inferred from infrared photometry, supporting the solar-type model of PMS flaring magnetic loops with both footpoints anchored in the stellar surface. Second, most PMS super-flares resemble solar long duration events (LDEs) that are associated with coronal mass ejections. Slow rise PMS super-flares are an interesting exception. Third, strong correlations of super-flare peak emission measure and plasma temperature with the stellar mass are similar to established correlations for the PMS X-ray emission composed of numerous smaller flares. Fourth, a new correlation of loop geometry is linked to stellar mass; more massive stars appear to have thicker flaring loops. Finally, the slope of a long-standing relationship between the X-ray luminosity and magnetic flux of various solar-stellar magnetic elements appears steeper in PMS super-flares than for solar events.
We study the sodium D lines (D1: 5895.92 AA; D2: 5889.95 AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D) as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.
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