Magnetism of Herbig Ae/Be stars

📝 Abstract

Observations of magnetic fields of stars at the pre-main sequence phase can provide important new insights into the complex physics of the late stages of star formation. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. Recent circularly-polarised spectroscopic observations of pre-main sequence Herbig Ae/Be stars have revealed the presence of organised magnetic fields in the photospheres of a small fraction of these objects. To date, 9 magnetic HAeBe stars have been detected, and those detections confirmed by repeated observations. The morphology and variability of their Stokes V signatures indicates that their magnetic fields have important dipole components of kG strength, and that the dipole is stable on timescales ofat least years. These magnetic stars exhibit a large range of stellar mass, from about 2-13 solar masses, and diverse rotational properties, with vsini from a few km/s to 200 km/s. Most magnetic HAeBe stars show approximately solar abundances; they clearly do not generally exhibit the strong and systematic peculiarities of the magnetic main sequence A and B type stars (the Ap/Bp stars). The observed fractional bulk incidence of magnetic HAeBe stars is about 7%, a value compatible with the incidence of magnetic intermediate-mass stars on the main sequence. This low incidence is at odds with formation scenarios generally involving magnetically-mediated accretion. The similarily between the magnetic properties of the pre-main sequence and main sequence intermediate-mass stars appears compatible with the hypothesis of a fossil origin of magnetism in these objects.

💡 Analysis

Observations of magnetic fields of stars at the pre-main sequence phase can provide important new insights into the complex physics of the late stages of star formation. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. Recent circularly-polarised spectroscopic observations of pre-main sequence Herbig Ae/Be stars have revealed the presence of organised magnetic fields in the photospheres of a small fraction of these objects. To date, 9 magnetic HAeBe stars have been detected, and those detections confirmed by repeated observations. The morphology and variability of their Stokes V signatures indicates that their magnetic fields have important dipole components of kG strength, and that the dipole is stable on timescales ofat least years. These magnetic stars exhibit a large range of stellar mass, from about 2-13 solar masses, and diverse rotational properties, with vsini from a few km/s to 200 km/s. Most magnetic HAeBe stars show approximately solar abundances; they clearly do not generally exhibit the strong and systematic peculiarities of the magnetic main sequence A and B type stars (the Ap/Bp stars). The observed fractional bulk incidence of magnetic HAeBe stars is about 7%, a value compatible with the incidence of magnetic intermediate-mass stars on the main sequence. This low incidence is at odds with formation scenarios generally involving magnetically-mediated accretion. The similarily between the magnetic properties of the pre-main sequence and main sequence intermediate-mass stars appears compatible with the hypothesis of a fossil origin of magnetism in these objects.

📄 Content

arXiv:0901.0347v1 [astro-ph.SR] 4 Jan 2009 Astronomical Polarimetry 2008: Science from Small to Large Telescopes ASP Conference Series, Vol. 4**, 2009 Bastien and Manset Magnetism of Herbig Ae/Be stars G.A. Wade, E. Alecian, J. Grunhut Royal Military College of Canada C. Catala Observatoire de Paris (LESIA) S. Bagnulo, C.P. Folsom Armagh Observatory J.D. Landstreet University of Western Ontario Abstract. Observations of magnetic fields of stars at the pre-main sequence phase can provide important new insights into the complex physics of the late stages of star formation. This is especially true at intermediate stellar masses, where magnetic fields are strong and globally organised, and therefore most amenable to direct study. Recent circularly-polarised spectroscopic observations of pre-main sequence Herbig Ae/Be stars have revealed the presence of organised magnetic fields in the photospheres of a small fraction of these objects. To date, 9 magnetic HAeBe stars have been detected, and those detections confirmed by repeated observations. The morphology and variability of their Stokes V signatures indicates that their magnetic fields have important dipole components of ∼kG strength, and that the dipole is stable on timescales ofat least years. These magnetic stars exhibit a large range of stellar mass, from ∼2 −13 M⊙, and diverse rotational properties, with v sin i from a few km/s to ∼200 km/s. Most magnetic HAeBe stars show approximately solar abundances; they clearly do not generally exhibit the strong and systematic peculiarities of the magnetic main sequence A and B type stars (the Ap/Bp stars). The observed fractional bulk incidence of magnetic HAeBe stars is about 7%, a value compatible with the incidence of magnetic intermediate-mass stars on the main sequence. This low incidence is at odds with formation scenarios generally involving magnetically- mediated accretion. The similarily between the magnetic properties of the pre- main sequence and main sequence intermediate-mass stars appears compatible with the hypothesis of a fossil origin of magnetism in these objects. 1. Introduction Herbig Ae/Be (HAeBe) stars are pre-main sequence (PMS) stars of intermedi- ate mass (Herbig 1960; Hillenbrand et al. 1992), characterised by spectral types A and B with strong emission lines. They often exhibit infrared excess and are frequently located within dust-obscured regions and associated with nebulae (Waters & Waelkens 1998). According to stellar evolution theory, HAeBe stars 1 2 should not posses deep outer convection zones which generate the important quantities of outward-flowing mechanical energy required to power an MHD dy- namo. Rather, these stars are expected to have convective cores surrounded by primarily radiative sub-photospheric envelopes (Iben 1965; Gilliland 1986). However, since 1980, repeated observations (e.g. Praderie et al. 1982; Catala et al. 1986; Hamann & Persson 1992; Pogodin et al. 2005) have shown that many HAeBe stars are intensely active. In particular, some stars display characteris- tics often associated with magnetic activity and the presence of chromospheres or coronae (e.g. Skinner & Yamauchi 1996). These properties have been pro- posed as indicators that many of these stars or their circumstellar envelopes are intensely magnetically active. This proposal has important implications for our picture of how intermediate- mass stars form. In lower-mass pre-main sequence T Tauri stars, it is now gen- erally supposed that accretion is mediated by the presence of strong, large-scale magnetic fields (e.g. discussion by Johns-Krull et al. (1999) and references therein). Some authors (e.g. Muzzerole et al. 2004) have suggested that similar “magnetospheric accretion” may occur in intermediate-mass PMS stars as well. The magnetic properties of the well-studied main sequence Ap and Bp stars provide a useful context for discussion of the magnetism in HAeBe stars. A few percent (e.g. Wolff1968, Auri`ere et al. 2007, Power et al. 2008) of all A and B type main sequence stars exhibit organised magnetic fields with strengths ranging from a few hundred to a few tens of thousands of gauss (e.g. Borra & Landstreet 1980; Mathys et al. 1997). Because, like their pre-main sequence progenitors, main sequence A and B type stars have radiative envelopes, these magnetic fields are not believed to result from a contemporaneous MHD dynamo. Rather, they are thought to be fossil fields: the passively-decaying remnants of magnetic fields produced at an earlier convective evolutionary stage, or swept up during star formation. The presence of these fields has important conse- quences for the structure of the atmospheres of these Ap/Bp stars, suppressing large-scale mixing and leading to strong atmospheric chemical peculiarities and abundance patches (e.g. Folsom et al. 2006; Lueftinger et al. 2003). Although the separation and mixing processes leading to these chemical peculiarities and abundance structures are not well under

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