Intermediate Mass Stars

The above-demonstrated cool star paradigm states that all stars with outer convection zones possess hot X-ray emitting coronae. Main-sequence stars of spectral type A are devoid of outer convective zones and should hence not display any coronal emission. A stars and late-B stars also have weak stellar winds or no winds at all, such that the X-ray emission mechanism thought to operate in hot stars (see Sect. 10.5) does not work either.

Indeed, the prototypical nearby A star Vega, one of the brightest stars in the sky, could not be detected as an X-ray source [41]. The upper limit to its X-ray luminosity of Lx ~ 5.5 x 1025 erg s^1 places Vega at the very bottom of the distribution for cool stars in terms of activity. However, contrary to expectations, the detection rate of A stars in the RASS data is not zero, but about 15% (cf., Fig. 10.8). The standard hypothesis to explain the observed but unexpected X-ray emission from A stars is to attribute it to (unknown) optically faint, late-type companions. Obviously, such a cool companion star can rather easily escape detection because of the much brighter optical emission of the A star.

In a few cases, the companion hypothesis can be tested by an analysis of the X-ray lightcurve. For example, in the eclipsing binary a CrB the late-type secondary (spectral type G5 V) is occulted by the early-type primary (spectral type A0 V), and in X-rays a total eclipse is seen at the time of optical secondary minimum (see Fig. 10.9 and Sect. 10.1.3).

Further support for the companion hypothesis comes from imaging observations in the infrared using the technique of adaptive optics. Such observations have

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O.Ol . i j_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_i_

Fig. 10.8 Detection rate of BSC stars in the RASS shown as a function of B —V color, a proxy for spectral type. The transition from A to F spectral types is located at B —V ~0.3 mag, and coincident with a sharp increase of the X-ray detection rate o q_

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Fig. 10.9 ROSAT PSPC lightcurve of the eclipsing binary a CrB at optical secondary eclipse (A star in front of G star); note the totality of the eclipse

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Fig. 10.9 ROSAT PSPC lightcurve of the eclipsing binary a CrB at optical secondary eclipse (A star in front of G star); note the totality of the eclipse

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Fig. 10.10 Chandra images of main-sequence B stars (labeled "A" because they represent the primary in the stellar multiple) and their faint companions (labeled "B," "C," and "D") discovered in high angular resolution infrared observations. For the case shown on the left the Chandra image shows that all X-ray emission must be attributed to the companions; in the example on the right both the companion and the B star are detected with Chandra. Figure 1 from [48]

recently identified a substantial number of faint objects at separations between and 15" from apparently X-ray emitting B stars. The imaging instruments onboard the Chandra satellite provide a spatial resolution comparable to adaptive optics in the infrared (< 1"), such that the newly identified systems can now be studied in X-rays. However, investigations with Chandra did not unambiguously solve the question of whether the X-ray flux is to be attributed to the early-type primary or the late-type secondary component, as X-ray emission is detected from more than half of the B stars even after resolving them spatially from the new faint infrared objects [48]. An example of this is shown in Fig. 10.10 (right panel), where X-ray emission is clearly detected from both the optical primary and from the secondary identified on recent infrared images. Whether these (primary) stars have additional even closer secondary companions or whether they are intrinsic X-ray emitters by themselves has remained unknown to date. Thus, the old saying "Absence of Evidence is not Evidence of Absence" still holds. Clearly the physics of coronal formation in those stars - if coronae exist at all - must be very different.

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