StarsASCA has observed a number of normal stars, both single and within binary systems, spanning the range of stellar masses from massive O stars to low mass late M dwarfs. These observations have probed the properties of hot X-ray emitting plasma around stars at all evolutionary stages, including the pre-main-sequence, main sequence, and post-main sequence phases. Stellar coronae, present on most cool stars, are strong sources of X-ray continuum and line emission. This emission originates from plasma confined and heated within magnetic structures. Massive O and B stars emit X-rays from shocks generated in their stellar winds.
Coronal Temperature DistributionsASCA is able to measure the temperatures of hot (> 20 MK) coronae more accurately than other previous or existing instruments. As a result ASCA has detected very hot coronal plasmas, that have higher temperatures than measured with lower resolution data from satellites such as Einstein and ROSAT . This sensitivity to high temperature continuum emission and the ability to definitively measure the emission in the Fe K-alpha lines are major observational capabilities of ASCA that contribute immensely to multi-spectral region analyses of coronal thermal distributions. Studies of ASCA spectra show multi-temperature coronal emission measure distributions. Generally, such analyses have taken the form of global fitting of the spectra aginst 1-, 2-, or 3-temperature emissivity models with variable elemental abundances. The coronae of active binary stars, such as RS CVn and Algol binaries, are found to contain very hot plasma. For example, a two temperature parametrization of the Algol spectrum gives temperatures of 2.4 and 0.6 keV (28 and 7 MK; Figure 12; Antunes, Nagase, & White 1994 ApJ 436, L83). The RS CVn binary AR Lac shows a similar effect, requiring temperatures of 2.0 and 0.6 keV (23 and 7 MK; Singh, White, & Drake 1996 ApJ 456, 766). In the latter case the hot component has the larger emission measure, while for Algol the cooler component dominates. Hot emission is also seen from single evolved giants and supergiants, e.g. Beta Dra (G2 Ib-II) at 1.6 and 0.7 keV (19 and 8 MK) (Skinner & Brown 1996) and 31 Com (G0 III) at 20 and 8 MK (Ayres et al. 1996 in preparation). Drake et al. (1994 ApJ 436, L87) found lower temperatures of 0.3-0.4 and 0.7 keV for the G dwarf Pi UMa and the late-G giant Beta Cet.
Figure 12. The SIS spectrum of Algol showing the two temperature components.
Coronal Elemental AbundancesASCA has provoked renewed interest in the question of the elemental abundances present in coronal material. Its higher spectral resolution has allowed abundances to be estimated not only for Fe, where in some cases a clear measurement of the Fe K-alpha feature against the surrounding continuum is possible, but also of Ca, Ar, S, Si, Mg, and Ni. Comparisons between theoretical emissivity models and ASCA spectra of highly active coronae have shown a consistent pattern of Fe abundances that are lower than solar photospheric values by factors that range from 1.5 to 6 (Drake et al. 1994 ApJ 436, L87; Singh et al. 1996 ApJ 456, 766; Mewe et al. A&Ap in press). The other heavy elements, with the exception perhaps of Ni, show similar depletion factors relative to hydrogen compared to their values in the solar photosphere. The solar coronal abundances are generally believed to exhibit an overabundance of easily ionized elements like Fe, Mg, and Si compared to the photosphere, so the opposite effect seen in stellar coronae is puzzling. The difference may be related to the fact that all of the late-type stars ASCA has observed to data have much higher coronal luminosities and temperatures than those typical of the Sun, though there is no real understanding of what the fractionation mechanism might be (either for the Sun or the active stars).
Pre-Main-Sequence StarsASCA observations of star formation regions and pre-main-sequence stars show the presence of unexpected amounts of hard X-ray emission. Pre-main-sequence stars with low absorption columns show very hot coronae with emission from plasma with temperatures of ~2-3 keV (Carkner et al. 1996 ApJ 464, 286; Skinner & Yamauchi 1996 ApJ in press). Observations of the cores of star formation regions show pervasive hard X-ray emission that appears to originate from a number of unresolved point source that are obscured at lower energies (Koyama et al. 1994 PASJ 46, L125; Koyama, Tsuboi, & Ueno 1996 Nagoya meeting; Figure 13). Koyama et al. (1996 PASJ submitted) have detected hard X-ray emission, including a strong flare, from Class I protostellar sources in the R CrA molecular cloud. Such hard X-rays are not expected during the early stages of pre-main-sequence evolution and raise interesting questions about how X-ray emission and the implied magnetic fields influence the star formation process.
Figure 13. The result of searching for point sources in the Rho-Ophiuchi SIS field. A preliminary search of this complex region of patchy absorption, reveals 25 point sources, of which about half are not coincident with cataloged ROSAT sources (Gotthelf, priv.comm.)
X-rays from Massive StarsThe ASCA spectra from hot stars show discrete emission lines and are similar in many respects to those of cool stars. The X-ray emission is basically that of a multi-temperature thermal plasma. Corcoran et al. (1994 ApJ 436, L95) found thermal components at 3 MK dominated the spectra of the O stars Delta and Lambda Ori, with additional emission from hotter plasma, at 7 MK and 26 MK respectively, being present. The very hot high energy tail for Lambda Ori is possibly due to X-rays from nearby pre-main sequence stars. In high mass binaries the shock due to colliding winds from the two stars is a strong source of X-ray emission. The Wolf-Rayet binary HD193793 shows a spectrum dominated by plasma at 3 keV (36 MK) (Koyama et al. 1994 PASJ 46, L93). Massive stars have high mass loss rates and the ASCA spectra of O stars and WR stars show absorption due to these ionized winds and allow quantitative measurement of the hydrogen column density of the winds.
Coronal FlaringObservations of stellar flares have been made by ASCA in coordination with other satellites and ground-based telescopes. Flares have been observed simultaneous by ASCA and EUVE on the young K dwarf AB Dor and the RS CVn binary HR1099. During the HR1099 observation a small (peak L_X ~ 2 x 10^31 ergs/s) flare was seen, with a factor of 2 increase in SIS count rate. The EUVE DS and ASCA SIS light curves are almost identical and the flare increase is due entirely to the gradual cooling phase of the flare. The ASCA spectra were fitted by a 2-temperature model; the cooler 0.85 keV component was constant throughout the observation, while the flare was confined to the hotter component whose temperature increased from 2.1 keV in quiescence to a flare peak of 2.9 keV (Brown & Skinner 1996 Florence Cool Star meeting).
Last modified: Tuesday, 26-Jun-2001 14:22:33 EDT
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