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Physics of Magnetic Stars, 2007, pp. 207­213

First registration of CS magnetic fields in classical Be stars with FORS1 at 8m-VLT
M.A. Pogo din1 , R.V. Yudin1 , S. Hubrig2 , M. SchЁ oller
1 2

2

Central Astronomical Observatory at Pulkovo, Saint-Petersburg, Russia. European Southern Observatory, Santiago, Chile.

Abstract. We have investigated the magnetic fields of 15 classical Be stars. The investigation is based on the low-resolution sp ectrop olarimetric data collected in 2005 at the ESO (Chile) with the multi-mo de instrument FORS1 at the 8m-VLT. We have used the data analysis, the same as in the case of young Herbig Ae/Be stars (Pogo din et al., these Pro ceedings). Significant p olarization features have b een revealed in CS comp onents of the CaI I K and H lines in the sp ectra of 5 Be stars. These features are likely to b e formed in the equatorial gaseous disks surrounding the stars. Besides, a clear Stokes V signature is observed in a redshifted absorption comp onent of the H line profile in the sp ectrum of HD 58011. We conclude that it is an indicator of a magnetized gaseous flow infalling from the disk onto the star. We should like to emphasize that the presented results concern only the first stage of our investigation. The next step of our study will b e a search for short-term magnetic field variability of Be stars which may result from the rotational mo dulation of p olarization sp ectra by hyp othetical lo cal magnetic features in the CS envelop es of Be stars.

1

Intro duction

We present results of our sp ectrop olarimetric investigation of the second group of early-typ e stars with circumstellar envelop es -- the classical Be stars. The first group of our programme ob jects is the young Herbig Ae/Be stars. Their magnetic prop erties are discussed in Pogo din et al. (these Pro ceedings). Similar to the Herbig Ae/Be stars, the classical Be stars are surrounded by CS envelop es containing an equatorial gaseous/dusty disk and a stellar wind zone at higher latitudes. But in contrast to the Herbig Ae/Be stars, the classical Be stars are not pre-main sequence ob jects. Their evolutionary status remains to b e the matter of debate. But, in any case, they are likely to b e evolved ob jects at a p ost-main sequence stage. Their gaseous disks are not relict but are formed from the wind matter accumulated in the equatorial plane. Their wind matter is much less dense and sufficiently hotter than the matter in the disk. Episo dically the disk can dissipate, and the star is transformed into a typical B-typ e star. But then a new disk starts accumulate again. As a whole, the general picture of observed active phenomena in the classical Be stars is not less puzzling than in young Herbig Ae/Be stars. Two main typ es of physical pro cesses are considered now to b e resp onsible for the Be phenomenon: pulsation and magnetic activity. Non-radial pulsations can b e resp onsible for a p erio dic sp ectral and photometric variability observed on short-term scales (less than 1 day). Interpretation of more long-time variability and the mechanism of disk formation requires the presence of magnetic fields on the stellar surface. The well-known Wind-Compressed Disk Mo del by Bjorkman 207


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Table 1: The list of classical Be stars -- ob jects of the observing programme in 2005 Ob ject HD HD HD HD HD HD HD HD HD HD HD HD HD HD HD 53367 56014 56139 57219 58011 58050 79351 105435 117357 137432 148259 148251 153261 155806 181615 Sp ectral class B01ve B3I I Ip+sh B2.5Ve B2Vne B1Vnep B2Ve B2.5e B2Ine B0.5I I Ine B4Ve B1.5Vp e B2Ve B2IVe O8Ve B2Ve V sin i (km/s) 30 144 109 117 18 123 35 207 78 127 139 86 184 116 69 CS magnetic field + + + + +

and Cassinelli (1993) do es not lead to an equatorial disk with sufficient density without invoking magnetic field. Magnetic disconnections of the field lines are assumed to interpret the app earance of flaring events and discrete ejections of matter into circumstellar (CS) environment. Balona (2003) and his colleagues have develop ed a scenario of formation of co-rotating gaseous clouds accumulated in closed magnetic lo ops. Such lo ops can b e resp onsible for p erio dic light and line profiles variations. According to mo dern theoretical predictions, magnetic fields of ab out 100 G are sufficient to explain the Be phenomenon (Maheswaren 2003). The develop ed mo dels do not require the necessary presence of a large-scale organized field across the entire stellar surface. They assume also the existence of different typ es of lo cal magnetized features.

2

Observations

Up to now, all previous attempts to measure directly magnetic fields in the classical Be stars have not lead to a definite p ositive result. In all cases, either the accuracy was not go o d enough (Neiner et al. 2003), or the ob jects of investigation were not typical Be stars (Donati et al. 2001). In 2005 a sample of 15 classical Be stars b ecame ob jects of our sp ectrop olarimetric study. We tried to investigate magnetic prop erties of this class of ob jects. The sp ectrop olarimeter FORS1 mounted at the 8m-VLT and the same observing programme were used as for young Herbig Ae/Be stars (Pogo din et al., these Pro ceedings). All observations were carried out with R=4000, and only Balmer lines were chosen for the field measurements. The list of all programme stars is presented in Table 1. The standard metho d of data analysis describ ed in Hubrig et al. (2006) led to a contradictory result. We tried to re-analyze our data following the same metho d as used for young Herbig Ae/Be stars (Pogo din et al., these Pro ceedings).


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Figure 1: Normalized and p olarization sp ectra of HD 58050 (B2Ve) near the CaI I K and H lines. The synthetic sp ectrum is given for comparison by dotted line.

3
3.1

Results
CaI I K and H lines

As a result of our analysis, we have found no obvious traces of atmospheric magnetic field in all 15 ob jects of our programme. However, significant p olarization features indicating the presence of a field in the lines of the resonance CaI I doublet were revealed in 5 Be stars. No doubt that these lines are of circumstellar (CS) origin. A comparison of the observed normalized sp ectra of these ob jects with synthetic shows that strong CaI I absorption lines are observed, and these absorptions are much wider and deep er than the theoretical predictions for these photospheric lines (see Figs. 1-5). The observed CaI I absorptions are likely to b e formed in the rotating equatorial disk, which is much co oler than the stellar surface of B2-O8-typ e stars. No p olarization signatures are observed in other sp ectral regions (the only exception -- a CS H comp onent in the sp ectrum of HD 58011). It should b e noted that in the normalized sp ectra of all programme stars the Balmer lines demonstrate intense emission profiles. In our opinion, these ob jects can b e included in group I I I (Pogo din et al., these Pro ceedings), and there is no p ossibility of diagnosing their stellar magnetic fields. Figs. 1­5 illustrate p olarization sp ectra of 5 Be stars with Stokes V signatures in the CaI I K and H lines.

3.2

CS H comp onent

The only programme ob ject demonstrating p olarization features not only in ionized calcium lines but also in H is HD 58011. The significant feature corresp onds to a narrow redshifted absorption comp onent of the line p ossessing an inverse P Cyg-typ e profile (Fig. 6). Profiles of such a typ e are indicators of the presence of matter infall from the CS disk onto the star. Therefore, we can assume


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Figure 2: The same as Fig.1, but for HD 117357 (B0.5I I Ine).

Figure 3: The same as Fig.1, but for HD 155806 (O8Ve).


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Figure 4: The same as Fig.1, but for HD 181615 (B2Vp e).

Figure 5: The same as Fig.1, but for HD 58011 (B1Vnep).


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Figure 6: The same as Fig.5, but for the H line.

that the infalling flow in the CS envelop e of HD 58011 is magnetized. It should b e noted, that the list of our programme ob jects contains several stars with the inverse P Cyg-typ e profile of H . But all of them (except HD 58011) demonstrate no p olarization signatures in this line. It is remarkable that the profiles of p olarization features observed in the CaI I and in the H lines in the sp ectrum of HD 58011, are not similar (Fig. 6). This fact indicates that the CS regions of their formation are likely to b e different.

4

Conclusions

We have presented now only the results of the first stage of our investigation of magnetic fields in classical Be stars. For the time b eing, we make no conclusion as to the origin and physical prop erties of the revealed magnetic field signatures. The next step of our study is to obtain long-term series of sp ectrop olarimetric observations for a numb er of Be stars from an extended list of programme ob jects on a time scale of several hours. That is comparable with times of rotation p erio ds of classical Be stars. These data will allow us to reveal traces of the rotational mo dulation of p olarization sp ectra connected with the existence of lo cal magnetic fields in the CS media around these ob jects.


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References
Balona L., 2003, ASP Conf.Ser., 305, 263. Bjorkman J., Cassinelli J.P., 1993, Astrophys. J., 409, 429. Donati J.F., Wade G.A., Babel J., et al., 2001, Mon. Not. R. Astron. Soc., 326, 658. Hubrig S., Yudin R.V., SchЁ oller M., Pogodin M.A., 2006, Astron. Astrophys., 446, 1089. Maheswaran M., 2003, Astrophys. J., 592, 1156. Neiner C., Hubert A.-M., Fremat Y., 2003, Astron. Astrophys., 409, 275. Pogodin M.A., Yudin R.V., Hubrig S., SchЁ oller M., 2007, Proc. of Intern. Conf. "Physics of magnetic stars" (this issue), Special Astrophysical Observatory, Nizhnij Arkhyz, p. 196