A Comparative Study of Flaring Loops in Active Stars

Dynamo activity in stars of different types is expected to generate magnetic fields with different characteristics. As a result, a differential study of the characteristics of magnetic loops in a broad sample of stars may yield information about dynamo systematics. In the absence of direct imaging, certain physical parameters of a stellar magnetic loop can be extracted if a flare occurs in that loop. In this paper we employ a simple nonhydrodynamic approach introduced by Haisch, to analyze a homogeneous sample of all of the flares we could identify in the EUVE DS database: a total of 134 flares that occurred on 44 stars ranging in spectral type from F to M and in luminosity class from V to III. All of the flare light curves that have been used in the present study were obtained by a single instrument (EUVE DS). For each flare, we have applied Haisch's simplified approach (HSA) in order to determine loop length, temperature, electron density, and magnetic field. For each of our target stars, a literature survey has been performed to determine quantitatively the extent to which our results are consistent with independent studies. The results obtained by HSA are found to be well supported by results obtained by other methods. Our survey suggests that, on the main sequence, short loops (with lengths ,0.5R sub(*)) may be found in stars of all classes, while the largest loops (with lengths up to 2R sub(*)) appear to be confined to M dwarfs. Based on EUVE data, the transition from small to large loops on the main sequence appears to occur between spectral types K2 and M0. We discuss the implications of this result for dynamo theories.