Magnetic fields in O-, B- and A-type stars on the main sequence
aa r X i v : . [ a s t r o - ph . S R ] N ov Magnetic fields in O-, B- and A-type stars on the mainsequence
Maryline Briquet , , a , b1 Institut d’Astrophysique et de G´eophysique, Universit´e de Li`ege, All´ee du 6 Aoˆut 17, Bˆat B5c, 4000,Li`ege, Belgium LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Univ. Paris Diderot, 5 place Jules Janssen,92195, Meudon Cedex, France
Abstract.
In this review, the latest observational results on magnetic fields in main-sequence stars with radiative envelopes are summarised together with the theoretical worksaimed at explaining them.
Magnetic fields have first been discovered in the Sun [1] and in the chemically peculiar A-type star78 Virginis [2]. These two stars are representative of two major groups of magnetic stars, which aredirectly related to the dominant mechanism of heat transport in the outer layers of the stars. For theSun and low-mass stars on the main sequence, the magnetic fields have complex surface structuresand they are variable over time on various time scales. It is understood that those fields are generatedby an ongoing solar-type dynamo, involving di ff erential rotation and turbulence [3]. For Ap stars, thefields are simple fields of globally dipole structure and they are extremely stable over time on longtime scales. For these stars with radiative envelopes, it is proven that the fields are remnants of an earlyphase of the star-life and one speaks of fossil fields (see Sect. 3.1.3).Over the past decade important progress has been achieved in the area of stellar magnetism. It ismainly due to the development of a new generation of high-performance spectropolarimeters, amongwhich FORS 1 / ff ect, i.e. the splitting and polarisation of spectrallines produced by a magnetic field. By measuring the circular polarisation produced by the longitu-dinal component of the stellar field, it is now possible to detect fields down to a few Gauss in certaintypes of stars, for instance, in bright main-sequence A-type stars with sharp lines (see Sect. 2.1.1).Recent reviews on magnetic fields across the H-R diagram are given in [4] and [5]. In what fol-lows, an overview of our current observational and theoretical knowledge on magnetic main-sequenceintermediate- and high-mass stars is presented. a e-mail: [email protected] b F.R.S.-FNRS Postdoctoral Researcher, Belgium
PJ Web of Conferences
It has been known for a long time that Ap / Bp stars are usually found to possess strong large-scaledorganized magnetic fields. Current spectropolarimetric data give us new insights about main-sequenceA-type stars. First, it is found that all Ap stars are magnetic [6]. The observed field is always higherthan a limit of 100 G for the longitudinal field, which corresponds to a polar field of about 300 G. Inaddition, such strong fields are not observed in the A-type non Ap stars down to a few Gauss, whichproves that the non-detection is not due to an instrumental limitation [7]. Finally, sub-Gauss fields areconvincingly detected in the bright A-type star Vega and in a few other A-type stars as well [8], [9].Therefore, among intermediate-mass stars, there are two classes of magnetism: the Ap-like “strong”magnetism and the Vega-like “ultra-weak” magnetism, and between them, there is a “magnetic desert”[10].
For several years, intensive magnetic surveys of massive stars have been performed by di ff erent col-laborations (e.g., Magori collaboration, [11], [12]) and international consortia (e.g., MiMeS project –Magnetism in Massive Stars [13], BOB collaboration – B-fields in OB stars [14], BinaMIcs project– Binarity and Magnetic Interactions in various classes of Stars [15]). These e ff orts provide us withthe first observational view of the magnetic properties of hot stars. Both MiMeS and BOB surveysconclude to an incidence of magnetic detection in massive stars similar to that of Ap stars. A fractionbetween 5 to 10 percent of main-sequence stars with radiative envelopes hosts detectable fields, re-gardless of the spectral type. The topologies of magnetic massive stars are also usually similar to thoseof Ap stars. They are oblique dipole very stable over years but complex structures are also observed[16], [17]. Among magnetic massive stars, there are very diverse objects: slowly rotating and rapidlyrotating stars, strong fields and weaker fields, stars with surface chemical peculiarity, and others withnormal chemical abundances. Magnetics fields are detected in non-pulsating stars and also in pulsatingstars [18], [19]. Moreover, there is no correlation between the observed stellar properties. In particular,no correlation is found between the stellar rotation rate and the stellar field strength, contrary to whatis observed in main-sequence stars with convective envelopes. Among O and B-type stars, there is the extension of the Ap stars, i.e. stars with a typical polar strengthof the order of kiloGauss (“strong” fields). In addition, evidence for fields with polar field of theorder of hundred Gauss (“weak” fields) is increasingly being observed. So far, such weak fields seemto be observed in HD 37742 ( ζ OriAa) [20], τ Sco [16], ǫ CMa [21], β CMa [21], and ζ Cas [22].The case of the pulsating β Cep star β CMa illustrates that magnetic fields in massive stars might bemore ubiquitous than currently known. The first magnetic analyses by independent teams using twodi ff erent instruments and di ff erent methods for data analysis led to a non-detection in this star [23],[24]. However, the typical signature in Stokes V of a weak ( <
30 G in absolute value) longitudinalmagnetic field has recently been revealed by HARPSpol data [21]. The polar field strength of this staris found to be about 100 G, which is much weaker than what is usually determined for other massivestars, and also lower than what is found in the typical Ap-type stars. This indicates a possible lack ofa “magnetic desert” in massive stars, contrary to what is observed in main-sequence A-type stars.onference Title, to be filled
A way to investigate the e ff ects of magnetic fields on stellar structure and evolution is to performasteroseismic studies of magnetic objects. The first asteroseismic modelling of a magnetic massivestar was performed on the B-type pulsator β Cephei [25]. This work dates back to more than a decadeago and was the only one available until recently. With the aim to perform an asteroseismic modellingof the magnetic β Cep star V2052 Oph, intensive multisite photometric and spectroscopic campaignswere set up [26], [27]. The outcome of the modelling shows that the magnetic field observed in thestar inhibits mixing in its radiative zone. The field strength observed in this star [28] is 6 to 10 higherthan the critical field limit needed to inhibit mixing as determined from theory [29], [30].Thanks to photometric data assembled by the CoRoT satellite complemented by ground-basedspectroscopy and spectropolarimetry, HD 43317 was discovered to be a single magnetic B-type hy-brid pulsator with a wealth of observational information that can be used to calibrate models of hotmagnetic stars [31], [32]. As detailed in [33], current opportunities to perform such studies are beingprovided by the BRITE and K2 satellites.
First, it is presently believed that the observed surface magnetic fields cannot be core dynamo fields.Numerical simulations show that dynamo activity in the core is taking place but the time needed forthe field to be visible at the surface would be longer than the lifetime of the star [34], [35]. Moreover,such fields do not have the observed topology. Second, it has been proposed that a dynamo processcould operate in radiative envelopes and a toroidal field could be generated by di ff erential rotation [36].However, another study concluded that it is not possible to excite / maintain a Tayler-Spruit dynamo ina radiative envelope [37]. If such a dynamo process in radiative envelopes would exist, it would implya correlation between stellar rotation and field strength that is not observed. For this reason, it can beconcluded that such fields are not those observed at the surface of massive stars. Massive stars have thin sub-surface convective layers that are induced by the iron opacity bump nearthe surface. Therefore, it is plausible that a dynamo process can operate in the sub-surface convectionzones. Moreover, these fields could reach the stellar surface and it is shown that increasing strengthsare found for increasing masses and towards the end of the main sequence [38]. The potential e ff ects ofthese fields would be larger for the hottest stars. Such fields would produce small-scaled field structureand when emerging at the surface would produce hot bright spots at the surface of massive stars. Sofar there are no direct observations of these fields because they lead to small-scaled structure of weakstrengths. However, there is increasing evidence of indirect observations of such fields in hot starsthanks to space-based photometry. CoRoT and Kepler light curves have revealed rotational modulationin several hot OB stars that might be due to such bright spots [39], [40]. Recently, a convincing caseof co-rotating bright spots on an O-type star has also been reported thanks to data taken by the MOSTsatellite [41]. Again, the current opportunities to study this kind of magnetic activity in massive starsare the K2 and BRITE satellites. Such studies are potentially important as these fields induced by sub-surface convective layers are very likely to exist and they may directly a ff ect the evolution of the moremassive stars by altering the stellar wind mass-loss and enhancing the loss of angular momentum.PJ Web of Conferences An explanation of the observed fields of Ap stars already proposed a long time ago is the fossil ori-gin. The fossil origin suggests that magnetic fields reside inside the star without being continuouslyrenewed. Therefore, these fields have been formed during an early phase of the life of the star. Thestrong observational argument supporting this view is that the observed large-scaled organised fieldsare extremely stable over time. This hypothesis was very plausible but di ffi cult to prove. Early ana-lytical works could show that certain field configurations are unstable, for instance all axisymmetricfields which are either purely poloidal or purely toroidal are unstable. Then, it was postulated that aconfiguration with both a torodial component and a poloidal component (a mixed configuration) wouldbe stable. In recent years, semi-analytical works and numerical simulations have definitely proven thatmixed fields can be stable inside radiative envelopes [42], [43]. Numerical simulations show that aninitial arbitrary field in a star evolves towards a stable mixed configuration over relatively short timescales and afterwards it continues to evolve on very long timescales. Moreover, these stable axisym-metric configurations can reproduce those observed at the stellar surface where they are seen as dipole.Other simulations show that non-axisymmetric configurations can also be stable and could explain themore complex structure observed in some massive stars [44]. Several scenarios to explain the Vega-like magnetism (sub-Gauss fields in main-sequence A-type stars)have been proposed but more observational and numerical works are needed in order to discriminatebetween the di ff erent suggestions. A first possibility is that these fields are fossil fields but, becauseof the weakness of the field, the time needed to reach a dipolar equilibrium is longer than the currentage of the star. Therefore, we would observe fields that are still evolving towards an equilibriumstate. Such fields have been called “failed fossils” [45]. Another suggestion is that the field is tooweak to freeze possible di ff erential rotation. As a consequence a strong toroidal field is produced andthe configuration becomes unstable. In this scenario, the lower limit for Ap-like magnetism wouldcorrespond to the critical dipolar field which separates the stable from the unstable configurations[10]. A third explanation would be that sub-surface convective layers induced by the helium opacitybump could generate a magnetic field. An observational way to test whether magnetic fields on the main sequence are generated during anearly phase of the star-life is to investigate the incidence and properties of pre-main sequence stars.With the aim to test the fossil origin of magnetic intermediate-mass main-sequence stars, surveysdedicated to the study of the magnetic properties of Herbig stars have been performed over the lastyears. About 6 percent of the Herbig stars are magnetic. The topologies are simple and stable overyears. The properties of the magnetic fields of the studied Herbig stars, which are fully radiative stars,indicate that the fields observed on the main sequence are already present during the Herbig phase[46]. Consequently, the fields must have been shaped before the Herbig phase, at an earlier stage ofevolution.In conclusion, the fossil field hypothesis for intermediate- and high-mass stars on the main se-quence has been proven both theoretically and observationally. What remains speculative and debatedis the exact way in which such fields are formed. Several scenarios have been proposed but furtherobservational e ff orts are needed to test them. The magnetic field could be the capture of the magneticfield of the interstellar medium from which the star contracts [47], or it could be generated by dynamoaction during a pre-main sequence stage during which convection is active [48], or it could also be theresult of the merging of components of a close binary [49], [50].onference Title, to be filled References
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