NGTS clusters survey – II. White-light flares from the youngest stars in Orion

ABSTRACT We present the detection of high-energy white-light flares from pre-main-sequence stars associated with the Orion Complex, observed as part of the Next Generation Transit Survey (NGTS). With energies up to 5.2 × 1035 erg these flares are some of the most energetic white-light flare events seen to date. We have used the NGTS observations of flaring and non-flaring stars to measure the average flare occurrence rate for 4 Myr M0–M3 stars. We have also combined our results with those from previous studies to predict average rates for flares above 1 × 1035 erg for early M stars in nearby young associations.

Polarimetry of Stellar Active Regions and Flares

Observations of regular and irregular polarimetric variability in late-type stars are reviewed, and the related physical and geometrical effects are discussed. There are indications that the irregular part of the variability could be caused by transient events, possibly associated with flares. Polarimetric observations during flares are reviewed, and preliminary results of new observations of a well-known flare star, YY Geminorum, are presented. The results show that the small flare in YY Gem did not cause any significant variations in linear polarization, while the binary eclipse evidently causes an enhancement in the polarization. The reasons for the difficulties in stellar flare polarimetry are discussed. Finally, future prospects for the observations of flaring stars and for the utilization of linear polarimetry as a complementary method to other techniques of surface imaging of stellar activity and flares are presented.

The Physics of Flare Stars

This paper reports the researches of the Texas flare star group. High time resolution and simultaneous photometric, spectroscopic and, if possible, radio observations of flaring stars are essential. Fast and slow flares are distinguished and their distinct optical, spectroscopic, and probably, radio effects are noted. The starspot model satisfactorily accounts for many optical and spectroscopic flare star properties, including the precursor phenomenon and quasi periodicity of flaring found by some observers. The basic mechanism of flares is thought to be magnetic field collapse on a scale vastly greater than in the Sun, but further observations are essential, even though there are theoretical grounds for support. No analogue of the solar cycle has yet been detected. The problem of duplicity remains a puzzle.