New X-ray detections of Herbig stars

Abstract

Context. The interpretation of X-ray detections from Herbig Ae/Be stars is disputed as it is not clear whether these intermediate-mass pre-main sequence stars are able to drive a dynamo and ensuing phenomena of magnetic activity. Alternative X-ray production mechanisms, related to stellar winds, star-disk magnetospheres, or unresolved late-type T Tauri star companions have been proposed. Aims. The companion hypothesis can be tested by resolving Herbig stars in X-rays from their known visual secondaries. Furthermore, their global X-ray properties (such as detection rate, luminosity, temperature, variability) may give clues to the emission mechanism by comparison to other types of stars, e.g. similar-age but lower-mass T Tauri stars, similar-mass but more evolved main-sequence Aand B-type stars, and with respect to model predictions. Methods. In a series of papers we have been investigating high-resolution X-ray Chandra images of Herbig Ae/Be and main-sequence B-type stars where known close visual companions are spatially separated from the primaries. Results. Here we report on six as yet unpublished Chandra exposures from our X-ray survey of Herbig stars. The target list comprises six Herbig stars with known cool companions, and three other A/B-type stars that are serendipitously in the Chandra field-of-view. In this sample we record a detection rate of 100%; i.e. all A/B-type stars display X-ray emission at levels of log (L x /L bol ) ∼ −5... − 7. The analysis of hardness ratios confirms that HAeBes have hotter and/or more absorbed X-ray emitting plasma than more evolved B-type stars. Conclusions. Radiative winds are ruled out as an exclusive emission mechanism on the basis of the high X-ray temperatures. Confirming earlier results, the X-ray properties of Herbig Ae/Be stars are not vastly different from those of their late-type companion stars (if such are known). The diagnostics provided by the presently available data leave it open whether the hard X-ray emission of Herbig stars is due to young age or indicative of further coronally active low-mass companion stars. In the latter case, our detection statistics imply a high fraction of higher order multiple systems among Herbig stars.

Searching for a link between the magnetic nature and other observed properties of Herbig Ae/Be stars and stars with debris disks

“…Carmona et al (2007) reported the detection of Hα emission and Li absorption in the K5Ve T Tauri companion. The companion is an X-ray source, and weaker X-ray emission is also detected from the position of the Herbig Ae/Be star (Stelzer et al 2009). The 10 μm silicate emission band shows rather pristine dust, while the SED points towards a flat disk, as reported by Meeus et al (2001).…”

“…Preibisch et al (2006) derived an inclination of 58 • from MIDI/VLTI observations and found that the disk is very compact with an outer edge of ∼2.6 AU, potentially caused by a further, even closer, yet undetected companion. Recently, HD 144668 was for the first time resolved from its T Tauri companion in X-rays (Stelzer et al 2009). This observation showed that the bulk of the X-ray emission comes from the companion, but a weak source is associated with HD 144668 (or the hypothesized sub-arcsecond companion).…”

“…In the absence of a theoretical model for X-ray production in intermediate-mass stars, the detections have often been ascribed to known or assumed late-type T Tauri star companions. However, recent Chandra imaging studies have resolved Herbig Ae/Be stars from most known companions and still came up with very high detection rates (Stelzer et al 2006(Stelzer et al , 2009.…”

“…Due to the probable role of magnetic fields in launching and collimating jets, this implies that a significant fraction of Herbig Ae stars should have measurable magnetic fields, which could persist through much, if not all, of the star's PMS lifetime. X-ray activity is known for a number of Herbig Ae stars (Hamaguchi et al 2005;Feigelson et al 2003;Swartz et al 2005;Stelzer et al 2006Stelzer et al , 2009Skinner et al 2004), suggesting the presence of magnetic fields. Recent advances in instrumentation have resulted in the first magnetic field measurements for Herbig Ae stars, derived from spectropolarimetry.…”

New X-ray observations of IQ Aurigae andα2 Canum Venaticorum

“…However, especially peculiar Ap/Bp stars and young Herbig AeBe stars are prime candidates for being intrinsic X-ray emitters (e.g. Babel & Montmerle 1997;Stelzer et al 2009). In the Ap/Bp stars discussed here an interplay between stellar winds and large scale magnetic fields is thought to play the major role in the X-ray generation.…”