The Coronae of Low-Mass Dwarf Stars

We report the results of our analysis of pointed X-ray observations of nearby dMe and dM stars using the position sensitive proportional counter (PSPC) on board the ROSA T satellite (Roentgensatellit). In the cases of those M dwarf stars where PSPC pulse-height distributions of sufficient quality for spectral fitting were obtained, we derive key coronal plasma parameters in order to investigate stellar coronal structure in more detail. In particular, we utilize temperatures and emission measures inferred for one or more distinct components as constraints for the development of semiempirical magnetic loop models as representations of the coronae of low-mass stars. The consistency of these static models as adequate descriptions of the coronae of M dwarfs is then examined. <P />We find that the coronae of low-mass dwarfs consist of two distinct thermal components: a "soft" component with T ∼ 2-4 x 10<SUP>6</SUP> K and a "hard" component with T ∼ 10<SUP>7</SUP> K. We find that the pulse- height spectra are systematically fitted better with "depleted" abundances compared to solar; the high- temperature emission component on dMe stars appears to contribute a systematically larger fraction of the total flux than the corresponding component in dM stars; and the high-temperature emission component on dMe stars is responsible for most of the observed variation in the count rate. <P />We have modeled the observed temperature components with hydrostatic coronal loop models, and find that: the low-temperature components can be modeled with loops of small size (l ≪ R<SUB>*</SUB>) and high pressure (Po ); and the high-temperature components require solutions with either small filling factors ( 0.1), large loops (1 &gt; R*), and high base pressure (P<SUB>0</SUB> &amp;#8819 P<SUB>0</SUB><SUB>sun</SUB>), or very small filling factors (∼0.1), small loops (1 &amp;#8819 R<SUB>*</SUB>), and very high pressure (P<SUB>0</SUB> ≫ P<SUB>0</SUB><SUB>sun</SUB>)). Based on these observational and model results, we conclude that coronal emission in dMe stars can be interpreted as arising from quiescent active regions (a quiescent, low-temperature component) and compact flaring structures (variable, high- temperature component). <P />Our conclusion that the coronal geometry for low-mass dwarf stars is dominated by a combination of relatively compact, quiescent loop configurations and an unstable flaring component has implications for both stellar dynamo theory and for our understanding of stellar angular momentum evolution. With regard to rotation in late-type stars, which has a direct bearing on dynamo action, we know from observations that the lowest mass stars spin down (via magnetic braking) more slowly than the more nearly solar-type stars. The compact loops we find for the low-temperature component suggests a natural explanation for the observed mass dependence of angular momentum evolution in late-type, main-sequence stars.