Ray Emission from HAeBe Stars

According to evolutionary models, HAeBe stars are on fully radiative tracks, i.e., they are lacking a convection zone. Therefore, they are not expected to drive a solar-like dynamo and exhibit the ensuing phenomena of magnetic activity. Nevertheless, even the relatively low-sensitivity instruments onboard ROSAT and ASCA achieved detection fractions between 30-50 % [17,54]. Indeed, HAeBe stars turned out to be comparatively X-ray luminous objects, with typical emission levels of log Lx [erg/s]^30. ..31. Figure 10.19 shows the position of a sample of HAeBe stars in the HR diagram. The individual X-ray luminosities are indicated by the size of the plotting symbols. The X-ray brightest stars are also the most massive ones, and there is some trend toward lower Lx with increasing age.

The plasma temperatures inferred from modeling of low-resolution CCD spectra reach up to several kiloelectronVolts (^50 MK), similar to the X-ray temperatures of young low-mass stars, and significantly higher than found in high- and intermediate-mass stars on the main sequence. In some cases, strong Fe Ka emission at 6.7keV is seen, a tracer of extremely high temperature.

Two cases of X-ray flares from HAeBe stars have been reported in the literature [11,16]. Flares arise from confined plasmas heated by magnetic reconnection events. Whether the magnetic structures are anchored on the stellar surface or connecting star and disk is difficult to settle on basis of X-ray observations. Furthermore, in both cases mentioned earlier the identification of the flaring source with the HAeBe star is not unambiguous.

Temperature log (Teff) (K)

Fig. 10.19 X-ray luminosity of young intermediate-mass stars in the HR diagram. The size of the plotting symbols is scaled to X-ray luminosity. Black and grey denote ASCA and ROSAT observations, respectively. Open circles represent sources in the Orion cloud. The diagram is overlaid by evolutionary models of [33] (Fig. 16 from [17])

Temperature log (Teff) (K)

Fig. 10.19 X-ray luminosity of young intermediate-mass stars in the HR diagram. The size of the plotting symbols is scaled to X-ray luminosity. Black and grey denote ASCA and ROSAT observations, respectively. Open circles represent sources in the Orion cloud. The diagram is overlaid by evolutionary models of [33] (Fig. 16 from [17])

Similar to the B and A stars on the main sequence, late-type T Tauri like companions could be the cause for the observed X-ray emission from HAeBe stars. The high X-ray luminosities of many HAeBe stars have often been cited against the companion hypothesis, but recent studies showed that their range of X-ray luminosities is compatible with the typical emission level of a late-type premain sequence star. The brightness ratios of known HAeBe binaries suggest that most companions are of significantly lower mass than the primaries, i.e., they are T Tauri stars, and must be strong X-ray emitters by their nature. So far no systematic studies have attempted to resolve the HAeBe stars from their companions in the X-ray regime.

Other proposals to explain intrinsic X-ray emission from HAeBe stars include a nonsolar dynamo operating in the absence of convection [50]. In this model, the energy related to differential rotation between the stars' center and its outer edge is converted into magnetic energy. The timescale on which the shear energy is exhausted according to the theory is on the order of 106 yrs, and is in line with observational hints for a decrease of X-ray emission with age in HAeBe stars. However, the predicted X-ray luminosities tend to underestimate the observed luminosities.

Finally, a number of X-ray production scenarios involve stellar winds. Winds of A and B stars on the main sequence are too weak to generate shocks that reach X-ray emitting temperatures, but calculations show that in the lower gravity HAeBe stars strong winds exist. Shocks, which provide the heating mechanism for X-ray production, may be related to instabilities in the winds themselves or to collision of the wind with the circumstellar material of the HAeBe star. Furthermore, the magnetically confined wind shock model, developed to explain the X-ray emission from magnetic Ap and Bp stars on the main-sequence (see Sect. 10.6), may be applicable in the case of HAeBe stars as well.

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