X-Ray Behavior of the Sun and Stars

Tom Ayres (CASA)

An important piece of Solar Twin-ology is the perspective that stellar studies bring to understanding the Sun's magnetic activity. Of special contemporary interest is the appearance of the dynamo cycle at high energies, owing to the photochemical and erosive impacts of ionizing radiations on planetary atmospheres (relevant to the present-day solar system, its early evolution including the pre-solar nebula, and young exoplanets around other stars).

Even with our privileged view of the high-energy Sun, however, a debate still simmers over key XUV properties. Estimates of cycle amplitudes, for example, range from factors of hundreds to just a few. Even worse, our two decade XUV record of the Sun represents just a glimpse of what that variability might be like over centuries or millennia (or billions of years, for that matter).

One is tempted, then, to turn to the stars for insight; but one finds disappointment instead. Long term stellar variability is well documented in low-energy tracers like Ca HK, but comparable dedicated campaigns on the high-energy side are rare. As a result, discouragingly little is known about stellar X-ray cycles (making a review, in principle at least, rather brief). The best examples, in terms of sample sizes, are X-ray mosaics of young clusters like the Hyades and Pleiades, repeated a decade or two apart. Long term variability generally is not evident in these records, although transient flares are conspicuous and common. Only a handful of older solar-like field stars have been studied long enough and persistently enough to reveal XUV cycles on the anticipated decadal time scales.

This review describes what (little) is known from these examples. Special attention is paid to one important case: Alpha Centauri AB, and its X-ray time series over the past 15 years. The other best solar-age example is the Sun itself. Even here, broad-band measurements comparable to those delivered by cosmic observatories for the stars generally are lacking, and must be reconstructed from proxies. This makes solar-stellar comparisons especially tricky. The main lessons learned so far are: (1) low-activity sources like the Sun and Alpha Cen A have much of their coronal emissions at longer wavelengths than efficiently captured by modern `dark side' instruments; and (2) coronal variability increases rapidly with increasing energy. As a result, the perception of cycle amplitude depends very much on instrumental bandpass; and discovery of exceptional variability might be inconsequential if the high-energy measurement is not particularly "bolometric."

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