A. Collier Cameron

A new method for estimating the distance of young open clusters

We present a new technique for estimating the distance to young open clusters. The method requires accurate measurement of the axial rotation period of late-type members of the cluster: rotation periods are first combined with projected rotation velocities and an estimate of the angular diameter for each star - obtained using the Barnes-Evans relation between colour and surface brightness. A best cluster distance estimate is then determined using standard techniques from the theory of order statistics, which are in common use in the general statistics literature. It is hoped that this new method will prove a useful adjoint to more traditional distance methods, and will provide a better determination of the distance scale within the solar neighbourhood

Riass Observations of AB Doradus (HD 36705)

We present the analysis of the data acquired in the fall of 1990 during coordinated multiwavelength observations of the K2 IVp rapidly rotating, very active, single star AB Doradus (HD 36705). These observations did involve, within the RIASS program, the IUE and ROSAT satellites, and various ground based Observatories. Table I summarizes the observation dates, the telescopes and instruments we used.

Magnetostatic equilibria for coronal loops on rotating stars

We have investigated magnetostatic equilibria for coronal loops embedded in a potential magnetic field on a rotating star. We find that for any given star, there is a maximum value of the plasma pressure inside a single loop, above which no equilibrium exists. This maximum internal pressure depends on the ratio of the temperatures inside and outside the loop, and on the ratio of the plasma pressure to the magnetic pressure at the base of the external field. Thus, any loop of a large-scale field which is heated or cooled to a different temperature from its immediate surroundings, or which experiences a change in its internal pressure may eventually lose equilbrium. For some values of the base pressure and temperature ratio the relation between summit height and footpoint separation is double-valued. As the summit height of a loop is increased, its footpoint separation increases to a critical value, then decreases to zero at the maximum possible summit height. At the critical footpoint separation the slope of the loop height-footpoint separation relation becomes infinite, and no equilibrium solution exists for greater footpoint separations.We find also that the strength and scale of the field external to the flux tube is the most important factor in determining its maximum height. The effects of varying the stellar rotation rate - and, hence, the variation in pressure with height - are comparatively unimportant, even for very high rotation rates at which the point of balance between gravitational and centrifugal forces lies close to the stellar surface. In this case it is possible to find equilibrium loop solutions whose summits lie outside the centrifugal balance point.We have also investigated the effects of varying the stellar surface gravity. For stellar of fixed mass and rotation rate, the loop dimensions scale approximately linearly with the stellar radius.

THEORY OF MAGNETIC BRAKING OF LATE-TYPE STARS

We review the basic theory of angular momentum loss via a magnetically channelled stellar wind. Various forms of power-law dependence of the instantaneous braking rate on the stellar rotation rate are derived. Observational constraints on the rotational evolution of young solar-type stars in open clusters and the field are used to calibrate and test the braking laws at rotation rates up to 100 times the solar rate.

X-ray spectral diagnostics for coronal loop sizes and heating rates in the active K dwarf AB Doradus

Abstract We present a set of theoretical X-ray spectra derived from models of coronal loops in thermal and hydrostatic equilibrium on the rapidly-rotating K0 dwarf AB Doradus. This star is a typical representative of active X-ray bright stars. The coronal loop models are based on EXOSAT and IUE observations of AB Dor. The resulting synthetic spectra are folded through the expected responses of the XMM and XSPECT missions. Useful constraints on the DEM(T) distribution could be obtained for a star as bright as AB Dor with the grating spectrometer of XMM in 10 3 to 10 4 s exposure times. The ratio of the He-type (O VII) forbidden and intercombination lines at 0.56 keV will provide density diagnostics sufficient to determine whether, for example, the X-rays originate mainly in compact, dense flare loops or in more extended loops of lower density. The strong O VIII line at 0.65 keV can be observed with the Bragg crystal spectrometers of both missions, in 10 4 s, and may be used for studies of rotational modulation and of flow velocities in flaring loops. At the Iron 6.7 keV lines, the crystal spectrometers and the lower-resolution CCD and GSPC instruments can be used to set constraints on the hottest gas at the loop summits.

Coronal Mass Ejections from a Rapidly-Rotating K0 Dwarf Star

We present a new set of time-resolved Hα spectra of the rapidly-rotating southern K0 dwarf star AB Doradus (= HD 36705). These data show that clouds of neutral hydrogen are forming in the outer corona of the star at the rate of about two per day, and are subsequently being ejected from the system. We propose a cloud formation scenario analogous to solar quiescent prominence formation, and estimate the likely importance of these cloud ejections as a magnetic braking mechanism in young low-mass stars. The magnetic braking timescale resulting from this process alone could be as short as 107 years.