Large-scale magnetic field of the G8 dwarf ξ Bootis A

We investigate the magnetic geometry of the active G8 dwarf ξ Bootis A (ξ Boo A), from spectropolarimetric observations obtained in 2003 with the MuSiCoS échelle spectropolarimeter at the Télescope Bernard Lyot (Observatoire du Pic du Midi, France). We repeatedly detect a photospheric magnetic field, with periodic variations consistent with rotational modulation. Circularly polarized (Stokes V) line profiles present a systematic asymmetry, showing up as an excess in amplitude and area of the blue lobe of the profiles. Direct modelling of Stokes V profiles suggests that the global magnetic field is composed of two main components, with an inclined dipole and a large-scale toroidal field. We derive a dipole intensity of about 40 G, with an inclination of 35° of the dipole with respect to the rotation axis. The toroidal field strength is of the order of 120 G. A noticeable evolution of the field geometry is observed over the 40 nights of our observation window and results in an increase in field strength and dipole inclination.
This study is the first step of a long-term monitoring of ξ Boo A and other active solar-type stars, with the aim of investigating secular fluctuations of stellar magnetic geometries induced by activity cycles.     

The chromospheric emission of solar-type stars in the young open clusters IC 2391 and IC 2602

In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High-resolution spectroscopic data were obtained for over 50 F, G and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid rotators  ( v  sin  i > 20 km s⁻¹)  with five of the stars being classified as ultra-rapid rotators  ( v  sin  i > 100 km s⁻¹)  . The emission levels in the calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against the Rossby number ( N _R), the star's chromospheric emission levels show a plateau in the emission for  log( N _R) ≲−1.1  indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars shows little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators.     

Doin' the twist: secular changes in the surface differential rotation on AB Doradus

We present measurements of the rotation rates of individual starspots on the rapidly rotating young K0 dwarf AB Doradus, at six epochs between 1988 December and 1996 December. The equatorial rotation period of the star decreased from 0.5137 to 0.5129 d between 1988 December and 1992 January. It then increased steadily, attaining a value of 0.5133 d by 1996 December. The latitude dependence of the rotation rate mirrored the changes in the equatorial rotation rate. The beat period between the equatorial and polar rotation periods dropped from 140 to 70 d initially, then rose steadily. The most rigid rotation, in 1988 December, occurred when the starspot coverage was at a maximum. The time-dependent part of the differential rotation is found to have     , which should alter the oblateness of the star enough to explain the period changes observed in several close binaries via the Applegate mechanism.     

Search for Microflaring Activity in the Magnetic Network

We analyze the temporal behavior of network bright points (NBPs) searching for low-atmosphere signatures of flares occurring on the magnetic network. We make use of a set of data acquired during coordinated observations between ground-based observatories (NSO/Sacramento Peak) and the MDI instrument on board SOHO. Light curves in chromospheric spectral lines show only small-amplitude temporal variations, without any sudden intensity enhancement that could suggest the presence of a transient phenomenon such as a (micro)flare. Only one NBP shows spikes of downward velocity, of the order of 2–4 km s^–1, considered as signals of compression associated with a (micro)flare occurrence. For this same NBP, we also find a peculiar relationship between the magnetic and velocity fields fluctuations, as measured by MDI. Only for this point the B–V fluctuations are well correlated, suggesting the presence of magneto-acoustic waves propagating along the magnetic structure. This correlation is lost during the compression episodes and resumes afterward. An A6 GOES soft X-ray burst is temporally associated with the downward velocity episodes, suggesting that this NBP is the footpoint of a flaring loop. This event has a total thermal energy content of about 10²⁸ erg, and, hence, belongs to the microflare class.     

MULTISPECTRAL OBSERVATIONS OF AN ERUPTIVE FLARE

We analyze the pre-flare and impulsive phase of an eruptive (two-ribbon) flare at several wavelengths. The total energy (mechanical plus radiative) released by the flare is 8 x 10³⁰ erg, about a factor 6 higher than the free magnetic energy (1.3 10³⁰ erg) estimated from the non-potentiality of the magnetic field configuration in the flare area. During the impulsive phase, we find a very good time coincidence between the hard X-ray light curve and the light curves for 2 small areas ( 4 in size) in the red wing of the H line and in the He-D₃ line center. This temporal coincidence is compatible with the interpretation that hard X-ray emission is produced by bremsstrahlung of accelerated electron beams striking these dense areas. For the other regions of the H ribbons we find more gradual light curves, suggesting a different energy transport mechanism such as conduction.