A search for magnetic fields in cool sdB stars
G. Mathys, S. Hubrig, E. Mason, G. Michaud, M. Schoeller, F. Wesemael
aa r X i v : . [ a s t r o - ph . S R ] O c t Astron. Nachr. / AN , No. 88, 1 – 4 (2010) /
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A search for magnetic fields in cool sdB stars ⋆ G. Mathys , ,⋆⋆ , S. Hubrig , E. Mason , G. Michaud , M. Sch¨oller , and F. Wesemael , ◦ Joint ALMA Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile European Southern Observatory, Casilla 19001, Santiago 19, Chile Leibniz-Institut f¨ur Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA Depart´ement de Physique, Universit´e de Montr´eal, Montr´eal, PQ, H3C 3J7, Canada European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, GermanyReceived date / Accepted date
Key words stars: stars: horizontal-branch — stars: magnetic field — stars: kinematics and dynamics — X-rays: stars— stars: individual: Feige 86Hot cluster Horizontal Branch (HB) stars and field subdwarf B (sdB) stars are core helium burning stars that exhibitabundance anomalies that are believed to be due to atomic diffusion. Diffusion can be effective in these stars becausethey are slowly rotating. In particular, the slow rotation of the hot HB stars ( T eff > , K), which show abundanceanomalies, contrasts with the fast rotation of the cool HB stars, where the observed abundances are consistent with thoseof red giants belonging to the same cluster. The reason why sdB stars and hot HB stars are rotating slowly is unknown.In order to assess the possible role of magnetic fields on abundances and rotation, we investigated the occurrence of suchfields in sdB stars with T eff < , K, whose temperatures overlap with those of the hot HB stars. We conclude thatlarge-scale organised magnetic fields of kG order are not generally present in these stars but at the achieved accuracy, thepossibility that they have fields of a few hundred Gauss remains open. We report the marginal detection of such a field inSB 290; further observations are needed to confirm it. c (cid:13) Horizontal branch (HB) stars of clusters and their fieldequivalents, the subdwarf B and O (sdB and sdO) stars, rep-resent a challenge for stellar evolution theory. While theyare all known to burn He in their centre, the evolutionaryscenario leading to sdB and sdO stars is not well estab-lished. It is not well understood either why there are suchrelative differences as observed in the number of blue or ex-tremely blue HB stars from one cluster to the other. Whilemetallicity is part of the explanation, it cannot account forthe diversity of HB branch morphologies observed. Re-views of typical characteristics of HB stars in clusters arepresented in the works of Moehler (2001) and Moehler &Sweigart (2006). In those globular clusters where HB starswith T eff > , K are present, they have been observedto have 180metal abundances very different from those ofthe red giant (RG) branch stars of the same clusters, whilethe HB stars with T eff < , K have the same abun-dances as RG branch stars. For instance, 6 of the 7 HBstars of M15 with T eff > , K observed by Behr etal. (1999) (see their Fig. [1]) have [Fe/H] larger by a fac-tor of 50 than all cooler HB stars and the RG branch stars. ⋆ Based on observations obtained at the European Southern Obser-vatory, Paranal, Chile (ESO programmes 083.D-0007(A) and 087.D-0049(A)). ⋆⋆ Corresponding author: e-mail: [email protected] ◦ Deceased on September 28, 2011.
Similar results were obtained in many other clusters (Behret al. 2000a; Behr 2003; Moehler et al. 2000; Fabbian et al.2005; Pace et al. 2006). Furthermore, the hotter HB starsrotate more slowly than the cooler ones that show no abun-dance anomalies (Behr et al. 2000a,2000b; Recio-Blanco etal. 2002)The abundance anomalies in those hot HB stars are be-lieved to be caused by atomic diffusion, radiative acceler-ations leading for instance to the observed Fe overabun-dances (Michaud et al. 2008). The link with atomic diffu-sion is strengthened by the observed slow rotation, a featurewhich is also a characteristic of Ap and HgMn stars and isrequired to allow the slow diffusion processes to be effec-tive. Similar statements can be made for the sdB stars, forwhich both abundance anomalies of heavy elements associ-ated with diffusion processes (Geier et al. 2010; Michaudet al. 2011) and very slow rotation velocities are usuallythe norm (Edelmann 2003). However the reason why veryblue HB and sdB stars rotate slowly is not known. Somesuggestions have been made but are not generally accepted.Magnetic fields are believed to be responsible for the slowrotation of Ap stars, but the origin of the slow rotation ofHgMn stars is also unknown. Could the presence of a mag-netic field differentiate slowly rotating blue HB stars with T eff > , K and abundance anomalies from the redHB stars with the same composition as the RG stars? MostsdBs and sdOs are also believed to have abundance anoma-lies caused by atomic diffusion. Gravitational settling of He c (cid:13) G. Mathys et al.: Magnetic field in cool sdB stars N o r m a li ze d F l ux ˚ Fig. 1
Stokes I spectra of the ten sdB stars andFeige 86 in the spectral region from 4200 to 5000 ˚A. Fromthe bottom to the top we present: Feige 86, EC 15327-1341, EC 19490-7708, EC 19579-4259, GD 1110, LB 1516,LB 1559, SB 290, SB 410, SB 459, and SB 815. Note thesimilarity between most sdB spectra and the spectrum ofFeige 86.is important in most of them except for some of the sdOswhere, in some cases, He is more abundant than H.In the past, magnetic fields of up to 1450 G were de-tected at significance levels ranging from 4 to 12 σ in foursdB and two sdO stars by O’Toole et al. (2005). A vari-able magnetic field (from ∼ − − Fig. 2
The FORS 2 measurements of the mean longi-tudinal magnetic field of the Ap star HD 142070 that arereported here ( open squares ) are plotted together with theCASPEC measurements of Mathys et al. (in preparation; dots ) against phase, assuming a rotation period of . d and a phase origin MJD = 49877 . . The solid curve is thebest fit of the data by a sinusoid.fields have been suggested to do for the solar radiative inte-rior (Charbonneau & MacGregor 1993). To test the hypoth-esis that the observed abundance anomalies of sdOs, sdBsand hot HB stars reflect the presence of magnetic fields inthese stars, we conducted a systematic search for magneticfields in field sdB stars with T eff < , K. We obtained FORS 2 longitudinal magnetic field measure-ments of ten sdB stars over three consecutive nights, from28 to 31 August 2009. Measurements of each star werebased on a sequence of observations with the following po-sitions of the retarder waveplate: + − + −
45, etc.We used grism 600B and a slit width of 0 . ′′ R ≈ . The observationswere performed using the readout mode (100kHz,high,1x1).Most of the stars in our sample were observed on each nightto check the magnetic field variability. More details on theobserving technique with FORS 1 can be found elsewhere(e.g., Hubrig et al. 2004a, 2004b, and references therein).The mean longitudinal magnetic field, h B z i , was derivedusing VI = − g eff eλ πm e c I d I d λ h B z i , (1)where V is the Stokes parameter that measures the circularpolarisation, I is the intensity in the unpolarised spectrum, g eff is the effective Land´e factor, e is the electron charge, λ is the wavelength, m e the electron mass, c the speed oflight, and d I/ d λ is the derivative of Stokes I . c (cid:13) stron. Nachr. / AN (2010) 3 Table 1
Longitudinal magnetic field measurements of ten sdB stars.
Object name MJD h B z i all h B z i all ( χ /n ) all h B z i hyd h B z i hyd ( χ /n ) hyd [G] [G] [G] [G]HD 142070 55071.9949 296 ±
144 376 29.2 311 ±
228 312 6.0HD 142070 55072.9935 80 ±
48 104 ± ±
64 431 ± ±
152 136 0.9 232 ±
166 140 0.7EC15327 55073.0209 57 ±
124 65 ± − ± − ± − ±
132 103 0.6 − ±
144 140 0.9EC19490 55073.0881 112 ±
125 162 ± − ± − ± ±
120 215 3.3 317 ±
133 226 3.0EC19579 55073.0485 30 ±
65 32 ± ±
118 229 ± ±
180 249 1.7 294 ±
205 355 2.7GD1110 55073.1538 216 ±
211 408 ± − ±
182 402 4.8 − ±
212 455 4.8LB1516 55073.2173 − ± − ± − ±
202 224 1.2 − ±
220 298 1.8LB1559 55073.3097 63 ±
178 80 ± − ± − ± ±
200 389 4.7 118 ±
208 549 8.1SB290 55073.2561 − ± − ± − ± − ± − ±
182 339 3.5 − ±
210 369 3.1SB410 55073.3526 − ± − ± ±
180 290 2.1 144 ±
198 321 2.1SB459 55073.3965 − ± − ± ±
188 229 ± − ±
204 261 1.6 − ±
212 352 2.3SB815 55072.4024 417 ±
208 563 ± − ± − ± ±
210 301 ± The longitudinal magnetic field was measured in twoways: using only the absorption hydrogen Balmer lines orusing the entire spectrum including all available absorptionlines. In Fig. 1 we present FORS 2 integral spectra for allobserved targets together with the well-studied Pop II haloB-type star Feige 86 with T eff = 16 430 K, in which wesearched for a magnetic field in May 2011. As we mentionabove, only upper limits have been obtained for Feige 86 byBorra et al. (1983). It is however possible that Feige 86 ex-hibits more similarity with HgMn stars than with sdB stars,as it shows He and Hg isotopic anomalies (Hubrig et al.2009).Our measurements of magnetic fields in ten sdB starstogether with the observations of the classical Ap starHD 142070 (used as a standard star) are shown in Table 1.The first two columns list the object name and the modi-fied Julian date of mid-exposure, followed by the measuredlongitudinal magnetic field h B z i all using the whole spec-trum. In columns 4 and 5 we give the rms field h B z i all andthe reduced χ . Columns 6 to 8 list h B z i hyd , h B z i hyd , and ( χ /n ) hyd for the measurements using hydrogen lines. In order to minimize the risk of apparent non-detectionin some of the targets of our sample, due to fortuitous nullobservations of the longitudinal field close to the phaseswhere it reverses its sign, stars were observed at two to fourdifferent epochs. The rms field is defined as: h B z i = n n X i =1 h B z i i ! / , (2)and the reduced χ as χ /n = 1 n n X i =1 (cid:18) h B z i i σ i (cid:19) , (3)where n is the number of measurements of the consideredstar, h B z i is the i -th such measurement and σ i is its uncer-tainty.Figure 2 shows the three measurements of the mean lon-gitudinal magnetic field of the Ap star HD 142070 that weobtained from consideration of its whole spectrum, togetherwith the measurements of Mathys et al. (in preparation),based on CASPEC spectropolarimetric observations. Thegood agreement between the two datasets confirms the qual-ity of the longitudinal field determinations achieved from c (cid:13) G. Mathys et al.: Magnetic field in cool sdB stars
FORS 2 observations and indicates that the order of magni-tude of their quoted uncertainties is correct.
In no star do our measurements reveal the presence of 1-2 kG fields. Our ability to detect weaker fields is limited bythe accuracy of the measurements, as a result of the faint-ness of the studied stars and of the readout mode that wasused. In only one case, SB 290, the longitudinal field de-termined at one of three epochs is formally significant at alevel greater than 3 σ . For this star, the reduced χ of thethree measurements that were performed also supports thereality of a detection at a confidence level greater than 99%.However, this conclusion depends critically on the correct-ness of the adopted measurement uncertainty; if the latterwas only slightly underestimated (by 20% for the measure-ments based on all absorption lines), it would be invalidated.Thus measurements at more epochs and with better accu-racy are needed to confirm the presence of a magnetic fieldin SB 290.On the other hand, no significant longitudinal field wasdetected in Feige 86 in our recent spectropolarimetric ob-servations of May 2011. The measurements resulted in h B z i all = 55 ± G.In conclusion, this study shows that large-scale organ-ised magnetic fields of kG order are not generally presentin sdB stars with T eff <
30 000
K. Yet it leaves open thepossibility that these stars may have fields of a few hundredGauss, with in particular a tantalising, although marginal,detection in one of them, SB 290. A firmer conclusion willrequire additional observations of higher quality.
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