Testing the companion hypothesis for the origin of the X-ray emission from intermediate-mass main-sequence stars

Abstract

Context. The X-ray emission from B-type main-sequence stars is a longstanding mystery in stellar coronal research. Since there is no theory at hand that explains intrinsic X-ray emission from intermediate-mass main-sequence stars, the observations have often been interpreted in terms of (unknown) late-type magnetically active companion stars. Aims. Resolving the hypothesized companions requires high spatial resolution observations in the infrared and in X-rays. We use Chandra imaging observations to spatially resolve a sample of main-sequence B-type stars with recently discovered companions at arcsecond separation. Methods. Our strategy is to search for X-ray emission at the position of both the B-type primary and the faint companion. Results. We find that all spatially resolved companions are X-ray emitters, but seven out of eleven intermediate-mass stars are also X-ray sources. If this emission is interpreted in terms of additional sub-arcsecond or spectroscopic companions, this implies a high multiplicity of B-type stars. Firm results on B star multiplicity pending, the alternative, that B stars produce intrinsic X-rays, cannot be discarded. An appropriate scenario would be a magnetically confined wind, as suggested for the X-ray emission of the magnetic Ap star IQ Aur. However, the only Ap star in the Chandra sample is not detected in X-rays, and therefore does not support this picture.

On the origin of the X-ray emission from Herbig Ae/Be stars

“…Similar conclusions on the detection fractions were reached for MS B-type stars observed with Chandra (Stelzer et al 2006). However, a direct comparison with the detection fractions of HAeBe stars is impeded by the different criteria for the sample selection.…”

“…A further hypothesis is that of unknown/unresolved T Tauri like companions that would be responsible for the X-ray emission. This idea is the most favored explanation for the observed X-rays from more evolved A-and B-type stars on the MS. Stelzer et al (2003) and Stelzer et al (2006) have checked the companion hypothesis by resolving a sample of B-type stars with Chandra from their close visual companions. The result was ambiguous because more than half of the B-type stars were detected with Chandra even after being resolved from all known visual companions.…”

Chemical spots in the absence of magnetic field in the binary HgMn star 66 Eridani

“…In the later study Hubrig et al (2001) detected the third, A&A 529, A160 (2011) low-mass companion, which is likely to be the actual source of the X-ray radiation coming from the system. This was directly confirmed with the Chandra X-ray imaging by Stelzer et al (2006).…”

“…We redetermined the fundamental parameters of 66 Eri using the Padova evolutionary tracks (Girardi et al 2000) and taking A160, page 4 of 12 advantage of the improved orbital parameters determined in our study and an accurate Hipparcos parallax, π = 10.56 ± 0.34 mas, available for the system (van Leeuwen & Fantino 2005). The primary component mass and the age of the system were found by interpolating in the set of evolutionary tracks for Z = 0.018 to match the total Hipparcos luminosity, L/L = 98.2 ± 6.3, and the mean effective temperature, T eff = 11 000 K, determined by Yushchenko et al (1999) from the photometric properties and spectral energy distribution of 66 Eri.…”

New X-ray observations of IQ Aurigae andα2 Canum Venaticorum

“…However, their contribution is likely minor, because even the overall fraction of close, i.e., spectroscopic, binaries in Ap stars of CP2/Si type is with ∼20% significantly lower than those in non-magnetic stars (Abt & Snowden 1973). The comparison sample is specified in Table 4; some of these objects have already been presented, albeit with a different focus in Stelzer et al (2006); Czesla & Schmitt (2007). Most stars are Si-stars, but some of the hotter ones are also classified as He-weak, moreover all stars have detected magnetic fields of a few hundred G up to several kG (Bychkov et al 2003).…”