Characterization of the magnetic field of the Herbig Be star HD 200775★

Abstract

The origin of the magnetic fields observed in some intermediate‐mass and high‐mass main‐sequence stars is still a matter of vigorous debate. The favoured hypothesis is a fossil field origin, in which the observed fields are the condensed remnants of magnetic fields present in the original molecular cloud from which the stars formed. According to this theory a few per cent of the pre‐main‐sequence (PMS) Herbig Ae/Be star should be magnetic with a magnetic topology similar to that of main‐sequence intermediate‐mass stars. After our recent discovery of four magnetic Herbig stars, we have decided to study in detail one of them, HD 200775, to determine if its magnetic topology is similar to that of the main‐sequence magnetic stars. With this aim, we monitored this star in Stokes I and V over more than 2 yr, using the new spectropolarimeters ESPaDOnS at Canada–France–Hawaii Telescope (CFHT), and Narval at Bernard Lyot Telescope (TBL). By analysing the intensity spectrum we find that HD 200775 is a double‐lined spectroscopic binary system, whose secondary seems similar, in temperature, to the primary. We have carefully compared the observed spectrum to a synthetic one, and we found no evidence of abundance anomalies in its spectrum. We infer the luminosity ratio of the components from the Stokes I profiles. Then, using the temperature and luminosity of HD 200775 found in the literature, we estimate the age, the mass and the radius of both components from their HR diagram positions. From our measurements of the radial velocities of both stars we determine the ephemeris and the orbital parameters of the system. A Stokes V Zeeman signature is clearly visible in most of the least‐squares deconvolution profiles and varies on a time‐scale on the order of 1 d. We have fitted the 30 profiles simultaneously, using a χ2 minimization method, with a centred and a decentred‐dipole model. The best‐fitting model is obtained with a reduced χ2= 1.0 and provides a rotation period of 4.3281 ± 0.0010 d, an inclination angle of 60°± 11° and a magnetic obliquity angle β= 125°± 8°. The polar strength of the magnetic dipole field is 1000 ± 150 G, which is decentred by 0.05 ± 0.04 R* from the centre of the star. The derived magnetic field model is qualitatively identical to those commonly observed in the Ap/Bp stars. Our determination of the inclination of the rotation axis leads to a radius of the primary which is smaller than that derived from the HR diagram position. This can be explained by a larger intrinsic luminosity of the secondary relative to the primary, due to a larger circumstellar extinction of the secondary relative to the primary.

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

“…The magnetic field for HD 31648 was detected by Hubrig et al (2006b), while V380 Ori and BF Ori were studied by Wade et al (2005). The detection of a weak magnetic field in HD 104237 was reported by Donati et al (1997) and the study of the magnetic field of HD 200775 was carried out by Alecian et al (2008). We note, however, that, as was shown by Hubrig et al (2007b), the detected magnetic field of HD 31648 is predominantly circumstellar (CS) and that the strength of the photospheric magnetic field remains unknown.…”

“…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 …”

The circumstellar envelopes of the Cepheids $\mathsf{\ell}$ Carinae and RS Puppis

“…The central object of this nebula is a binary with a composite spectrum B2Ve. The primary component has a mass of 11 M and the secondary component 9 M , with a system age of only ≈100 000 years (Alecian et al 2008). Considering their masses, the two stars will cross the instability strip in the HR diagram and become Cepheids within a few tens of million years.…”

Discovery of new magnetic early-B stars within the MiMeS HARPSpol survey

“…Moss 2001). Studies performed in recent years have brought a variety of new evidence in favour of this theory and in the generation and history of such fossil fields (Wade et al 2005(Wade et al , 2007Braithwaite & Nordlund 2006;Folsom et al 2008;Alecian et al 2008aAlecian et al ,b, 2009Alecian et al 2013).…”