VIEWING THE SUN AS AN X-RAY STAR.pdf

VIEWING THE SUN AS AN X-RAY STAR S. Orlando 1 G. Peres 2 and F. Reale 21 INAF - Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy2 Dip. di Scienze Fisiche ed Astronomiche - Sez. di Astronomia - Univ. di Palermo, Piazza del Parlamento 1, I-90134 Palermo, Italy ABSTRACT The Sun is the late-type star we can study with the highest level of detail. In the interpretation of stellar data, therefore, it is often assumed that the physical processes of the coronae of late-type stars are similar to those of the solar corona, i.e. the “solar-stellar analogy”. In order to investigate the validity of this assumption, we have started a program to study systematically the Sun as an X-ray star. Our program aims to explore how far the solar model can be applied to other stars. In this paper we review the results obtained from these studies and, in particular, we discuss the variability of a star identical to the Sun during its cycle, the contribution of different coronal regions to the whole corona X-ray spectrum, and the role of flares. We find that the non-flaring Sun during its cycle fits well in the stellar scenario as a low-activity star. Our results suggest that the coronae of stars from low to intermediate activity may be explained as the effect of structures similar to those present on the Sun, changing the relative weight of the different kinds of non- flaring coronal regions; the coronae of very active stars may be explained if a continuous flaring activity characterizes these stars. Possible implications in stellar data interpretation are discussed. INTRODUCTION Solar observations give us a unique opportunity to study in detail an example of a late type star and, in particular, to examine the spatial structure of the source regions of its X-ray emission. They provide us a very detailed physical scenario of the phenomena occurring in corona. It is not surprising, therefore, the strong interest in comparing such a detailed physical scenario