Discovery of new TESS pulsating hot subdwarfs

This work is dedicated to a search for new pulsating hot subdwarfs in TESS photometric data which could have been missed in previous searches. By matching catalogues of hot subdwarfs with TESS targets and using luminosities from Gaia parallaxes, a list of 1389 candidate hot subdwarfs observed by TESS was created. The periodograms of these stars were inspected, and the stars were classified according to variability type. An updated catalogue of all known pulsating hot subdwarfs is presented. A number of probable pulsating binaries have been identified, which might prove useful for verifying the asteroseismic masses. The mean masses of p- and g-mode pulsators are estimated from the stellar parameters. A list of 63 previously unknown pulsating hot subdwarfs observed by TESS is presented. More than half of the stars previously identified as pure p-mode pulsators are found to have frequencies in the g-mode region as well. As a result, hybrid p- and g-mode pulsators occur over the whole instability strip

Short-period line profile and light variations in the Be star λ Eridani

We present three seasons of photometric observations and one season of intensive high-dispersion spectroscopic observations of the Be star λ Eridani. We show that only one period, P=0.70173 d, is present in the photometry, although there are large light amplitude variations from season to season. We confirm a suspicion that light outbursts repeat at intervals of about 475 d. A total of 348 echelle spectra of the star were obtained over a 2-week observing run. We show that the periodic variations are present in the emission wings of the helium lines, in the emission wings of the Hα line and in the absorption cores of Hβ and Hγ. Together with the fact that the periodic variations appear outside the projected rotational velocity limit, this indicates that they are associated with circumstellar material immediately above the photosphere and supports the idea of corotating gas clouds. We present evidence in support of a true rotational period of 2P=1.40346 d and suggest that the mass loss in Be stars is caused by centrifugal magnetic acceleratio

Abundances in HD27411 and the helium problem in Am stars

We analyze a high-resolution spectrum of the A3m star HD27411. We compare abundances derived from ATLAS9 model atmospheres with those using the more computationally-intensive ATLAS12 code. We found very little differences in the abundances, suggesting that ATLAS9 can be used for moderate chemical peculiarity. Our abundances agree well with the predictions of diffusion theory, though for some elements it was necessary to calculate line profiles in non-thermodynamic equilibrium to obtain agreement. We investigate the effective temperatures and luminosities of Am/Fm stars using synthetic Stromgren indices derived from calculated spectra with the atmospheric abundances of HD27411. We find that the effective temperatures of Am/Fm stars derived from Stromgren photometry are reliable, but the luminosities are probably too low. Caution is required when deriving the reddening of these stars owing to line blanketing effects. A comparison of the relative proportions of pulsating and non-pulsating Am stars with delta Scuti stars shows quite clearly that there is no significant decrease of helium in the driving zone, contrary to current models of diffusion.Comment: 10 pages, 8 figures, 6 tables, accepted for publication in MNRA

Oscillations in stellar superflares

Two different mechanisms may act to induce quasi-periodic pulsations (QPP) in whole-disk observations of stellar flares. One mechanism may be magneto-hydromagnetic (MHD) forces and other processes acting on flare loops as seen in the Sun. The other mechanism may be forced local acoustic oscillations due to the high-energy particle impulse generated by the flare (known as `sunquakes' in the Sun). We analyze short-cadence Kepler data of 257 flares in 75 stars to search for QPP in the flare decay branch or post-flare oscillations which may be attributed to either of these two mechanisms. About 18 percent of stellar flares show a distinct bump in the flare decay branch of unknown origin. The bump does not seem to be a highly-damped global oscillation because the periods of the bumps derived from wavelet analysis do not correlate with any stellar parameter. We detected damped oscillations covering several cycles (QPP), in seven flares on five stars. The periods of these oscillations also do not correlate with any stellar parameter, suggesting that these may be a due to flare loop oscillations. We searched for forced global oscillations which might result after a strong flare. To this end, we investigated the behaviour of the amplitudes of solar-like oscillations in eight stars before and after a flare. However, no clear amplitude change could be detected. We also analyzed the amplitudes of the self-excited pulsations in two delta Scuti stars and one gamma Doradus star before and after a flare. Again, no clear amplitude changes were found. Our conclusions are that a new process needs to be found to explain the high incidence of bumps in stellar flare light curves, that flare loop oscillations may have been detected in a few stars and that no conclusive evidence exists as yet for flare induced global acoustic oscillations (starquakes).Comment: 13 pages, 14 figures, 3 table