Polarimetry of stellar active regions and flares

Abstract

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. Polarxmetric 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.

Discovery of the Sub-second Linearly Polarized Spikes of Synchrotron Origin in the UV Ceti Giant Optical Flare

“…The synchrotron mechanism for optical flaring emission from red dwarfs had been initially proposed by Gordon (1954), who noted that the detection of polarized flares might be the proof for it. However, the sparse and irregular (see Section 1) polarimetric observations of UV Ceti-type stars have been fruitless until now (Karpen et al 1977;Eritsian 1978;Tuominen et al 1989;Alekseev et al 1994). It seems that the majority of flares, especially the longer ones studied in these works, are dominated by thermal emission.…”

“…Numerous attempts to detect the polarization in flares of YZ CMi, AD Leo, EV Lac and YY Gem, observed with various telescopes (Karpen et al 1977;Eritsian 1978;Tuominen et al 1989), were also unsuccessful. Most reliable data have been acquired during EV Lac study using 1.25-m telescope of the Crimean Astrophysical Observatory (Alekseev et al 1994).…”

A multiwavelength study of the M dwarf binary YY Geminorum

“…Optical spectra covering the Hα region were obtained with the coude Spectrograph on the 2.6 m Shajn telescope at the Crimean Astrophysical Observatory on 5 and 6 March 1988 with ten spectra obtained on the first night and nine on the second (see Tuominen et al 1989). The Hα emission at various phases close to and out of eclipse are shown in Figures 5 and 6.…”

“…Our general aim is to bring together results of some work, previously reported (e.g. , Butler, et al 1994, 1995, Budding et al 1996, Tuominen et al 1989) with contemporaneous satellite and radio observations thereby allowing an overview of the campaign. One specific intention concerns the various light curves and their analyses in terms of standard eclipsing binary models that include photospheric inhomogeneities.…”

“…Figure5. Hα line profiles of YY Gem obtained on the 5 and 6 March 1988 at the Crimean Astrophysical Observatory (seeTuominen et al 1989). Fluxes are normalised to the continuum.…”