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

Abstract

Context. Herbig Ae/Be stars are fully radiative and not expected to support dynamo action analogous to their convective lower-mass counterparts, the T Tauri stars. Alternative X-ray production mechanisms, related to stellar winds or star-disk magnetospheres have been proposed, but their X-ray emission has remained a mystery. Aims. A study of Herbig Ae/Be stars' global X-ray properties (such as detection rate, luminosity, temperature, variability), helps to constrain 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 A-and B-type stars, and with respect to model predictions. Methods. We performed a systematic search for Chandra archival observations of Herbig Ae/Be stars. The superior spatial resolution of this satellite with respect to previous X-ray instrumentation has allowed us to also examine the possible role of late-type companions in generating the observed X-rays. Results. In the total sample of 17 Herbig Ae/Be stars, 8 are resolved from X-ray emitting faint companions or other unrelated X-ray bright objects within 10 . The detection fraction of Herbig Ae/Be stars is 76%, but decreases to 35% if all emission is attributed to further known and unresolved companions. The spectral analysis confirms the high X-ray temperatures (∼20 MK) and large range of fractional X-ray luminosities (log L x /L * ) of this class derived from earlier studies of individual objects. Conclusions. Radiative winds are ruled out as an emission mechanism on the basis of the high temperatures. 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), nor from other young late-type stars used for comparison. Therefore, either a similar kind of process takes place in both classes of objects, or there must be as yet undiscovered companion stars.

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

“…Warm H 2 gas is reported by Thi et al (2001). Similar to the case of HD 144668, resolved Chandra images show that most of the Xray emission comes from the T Tauri companion, but weak emission is attributed to the Herbig star (Stelzer et al 2006). HD 176386: a magnetic field is definitely present: we measure B z = −119 ± 33 G using the full spectrum and B z = −121 ± 35 G using hydrogen lines.…”

“…Surprisingly, HD 176386 is one of the few Herbig Ae/Be stars confirmed to be X-ray dark (log L X [erg/s] < 28.6). Stelzer et al (2006) showed on basis of Chandra imaging that apparent X-ray detections of this star in previous lower-resolution observations are to be attributed to the companion at 4 separation.…”

“…Certainly, the advantage of using high resolution spectropolarimeters such as ESPaDOnS at the Canada-France-Hawaii Bernacca & Perinotto (1970), c Grady et al (1996), d van den Ancker et al (1998), e Acke & Waelkens (2004), f Hamidouche et al (2008), g this work, h Dunkin et al (1997), i Hubrig et al (2007b), j Alencar et al (2003), k Royer et al (2007), l Mora et al (2001), m Slettebak (1982), n de la Reza & Pinzón (2004), o Beskrovnaya et al (2004), p Alecian et al (2008), q Landstreet et al (2008) Pontoppidan et al (2007b), aa Siebenmorgen et al (2000), bb Kessler-Silacci et al (2005), cc Okamoto et al (2004), dd Schütz et al (2005b), ee Dommanget & Nys (1994), f f Baines et al (2006), gg Corporon & Lagrange (1999), hh Stelzer et al (2006), ii Chen et al (2006a), j j Augereau et al (2001), kk Carmona et al (2007), ll Stecklum et al (1995), mm Reipurth & Zinnecker (1993), nn Roberge et al (2002), oo Grady et al (2007), pp Stauffer et al (1995), qq SIMBAD, rr Eisner et al (2004), ss Grady et al (2004), tt Pirzkal et al (1997), uu Schnerr et al (2007), vv Blondel et al (2006), ww van den Ancker et al (1997), xx Wade et al (2007), yy …”

“…Highly excited H 2 gas was reported by Martin-Zaïdi et al (2008), who estimate that the warm H 2 is located within about 1.5 AU from the star. The star is detected in X-rays (Feigelson et al 2003;Stelzer et al 2006) with an L X typical of Herbig Ae stars. No jet is seen in HST imagery in the optical (Grady et al 2001;Ardila et al 2007) or in Lyα (Grady et al 2005a).…”

“…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.…”