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Physics of Magnetic Stars, 2007, pp. 197­206

Measurements of multi-component magnetic fields in Herbig Ae/Be stars with FORS1 at 8m-VLT
M.A. Pogo din1 , R.V. Yudin1 , S. Hubrig2 , M. Sch¨ 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 several Herbig Ae/Be stars separately for the stellar and circumstellar (CS) field comp onents. The investigation is based on the low-resolution sp ectrop olarimetric data collected in 2003­2005 at the ESO (Chile) with the multi-mo de instrument FORS1 installed at the 8m-VLT. We have shown that the results of sp ectrop olarimetry dep end heavily on the level of CS contribution to stellar sp ectra. Stellar magnetic fields of ab out 100 G have b een found in some programme ob jects at an accuracy level of 7 , when they were at a low-state stage of their CS activity. We have established that at high state of this activity all p olarization indications of magnetic field relate mainly to the CS media. An analysis of the CS field comp onent has confirmed that the magnetic field centrifuge is likely to b e a principle mechanism of the stellar wind acceleration in Herbig Ae/Be stars.

1

Intro duction

The Herbig Ae/Be stars (Herbig 1960) are now recognized to b e pre-main sequence ob jects of intermediate mass (from 2 to 10 M ). According to common views they are surrounded by CS envelop es of a complex structure. The envelop es contain an equatorial gaseous/dusty disk and a stellar wind zone at higher latitudes. The matter outflow from the Herbig Ae/Be stars is timevariable and spatially inhomogeneous, and signatures of a non-stable matter infall onto the star are frequently observed. Presently, our knowledge of the relation b etween these different pro cesses is clearly lacking. Numerous theoretical works predict the existence of a global magnetic field of a complex configuration formed as a result of interaction b etween the star and the accretion disk. Namely, such a field is very likely to b e resp onsible for all observational p eculiarities of the Herbig Ae/Be stars. For this reason, an investigation of the strength and the top ology of the magnetic field can provide imp ortant information on the origin of activity in these ob jects. Over an extended p erio d all attempts to measure directly the magnetic field in the Herbig Ae/Be stars have b een rather unsuccessful. On the one hand, this could b e explained by the weakness of the magnetic field in these ob jects with strengths considerably lower than those in magnetic Atyp e stars or low-mass young T Tau-typ e stars. The accuracy of all previous measurements was not high enough to measure such weak fields. On the other hand, the observational strategy allowed determination of only the average value of magnetic field using a numb er of different sp ectral lines. Due to the complex spatial structure of the field, the measured value must dep end strongly on individual lines used for the field determination, which are formed in different spatial regions from the stellar surface to remote CS regions. Undoubtedly, any attempt to detect and to study fields 197


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in the Herbig Ae/Be stars must aim at: a) considerably b etter accuracy of measurements, and b) applying a sp ecific observational strategy. The first direct recording of the magnetic field in the Herbig star HD 104237 (A4­A7e) was carried out by Donati at al. (1997) ten yeas ago. Using high-resolution sp ectrop olarimetry with the AAT/UCLES they found in metal lines of the ob ject sp ectrum a marginal circular p olarization signature indicating the existence of a weak longitudinal magnetic field of ab out 50 G. But somewhat later it was shown that HD 104237 is a multiple system containing a Herbig Ae star and additionally four T Tau-typ e comp onents (Feigelson et al., 2003; B¨ ohm et al., (2004). The most massive T Tautyp e companion has a b olometric luminosity that is only a factor of 10 less than a Herbig Ae primary. It is well-known that T Tau-typ e G-M stars can p ossess their own magnetic fields of order of 1 kG. Therefore, the p ossibility that the detected field is actually related to the T Tau-typ e comp onents cannot b e ruled out.

2

Observations and data analysis

Sp ectrop olarimetric low-resolution observations of 7 Herbig Ae stars were carried out in 2003­2005 with the sp ectrop olarimeter FORS1 installed at the 8m-VLT in Chile. Observational data were obtained with the sp ectral resolving p ower R = 2000 and 4000 in the sp ectral regions 3700­ 5900 ° and 3900­5000 ° resp ectively. The technique applied for measuring stellar magnetic A A fields was develop ed by Angel and Landstreet (1970). Exclusively Balmer lines were used for the field determination. A detailed description of the programme is given by Hubrig et al. (2004, 2006). Definite evidence for the presence of magnetic field (100±30 G) was presented for four Herbig Ae stars of 7 observed. But just after the publication, these results b ecame a matter of discussion. Wade et al. (2005) made a re-analysis of the observational data for the programme ob ject HD 139614 (A7Ve) obtained by Hubrig at al. in 2003. They confirmed the detection of a mean longitudinal field in Balmer lines Bz equal to -320±75 G, which is similar to our former estimation B z = -450±93 G. But the magnetic field was not detected when they included numerous metal lines in their analysis. The presence of the weak magnetic field in HD 139614 was confirmed by our more recent observations in 2005 (B z = -116±34 G, Hubrig et al. 2006). But 3 weeks after our observations were made, Wade et al. (2005) used ESPaDOnS at the CFHT to obtain two additional high-resolution sp ectrop olarimetric observations of this ob ject and found no significant magnetic fields. We tried to clarify this situation and assumed that an imp ortant factor having a strong effect on the results of the field measurement could b e the CS contribution to the stellar sp ectrum. The CS influence can differ for different sp ectral lines and b e time-dep endent. Starting from this assumption, we re-analyzed our sp ectrop olarimetric data for 7 programme stars separating all atmospheric and CS comp onents of sp ectral lines. After that, we re-measured the stellar magnetic fields for the programme ob jects using the entire sp ectrum with the lo cal wavelength regions affected by the CS matter excluded. Separation of CS line comp onents was carried out by comparison of observational and synthetic sp ectra which were calculated using the computer co de SYNTH+ROTATE (Piskunov 1992). Because of the rather low resolution of our sp ectra, the fitting was applied only to the slop es of wide Stark broadened wings of the Balmer lines. We carried out numerical detections of B z only for the stellar comp onent of the field. Standard Landstreet's technique for measuring magnetic fields in stellar atmospheres can hardly b e applied to the CS fields for the following reasons: a) top ology of CS fields may b e strongly dissimilar to that of stellar surface fields, b) profiles of CS lines may b e asymmetric and contain absorption as well as emission comp onents, and c) spatial regions where CS lines form may b e rather extended, and the magnetic field may b e significantly varying in such large volumes. Nevertheless, CS p olarization signatures observed at wavelengths corresp onding to clearly


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Table 1: Measurements of magnetic field in HD 139614 in 2003 and in 2005 using different metho ds: Bz (old) ­ Hubrig et al. (2006), only hydrogen lines are used; B z (new) ­ this pap er, regions of CS Balmer lines comp onents are excluded Season MJD R Bz (old) Bz (new) Group Magnetic field 2003 52904.04 2000 -450±93 G -112±36 G I I-I I I CS 2005 53405.37 4000 -116±34 G -93±14 G I Stellar

identified CS comp onents allow some conclusions concerning the prop erties of magnetic fields in the line formation region to b e made.

3

Results

According to our analysis, all the observations can b e divided into three groups. The first group contains the ob jects with the sp ectra demonstrating small CS contribution. The stellar magnetic fields of these ob jects can b e measured numerically with rather high accuracy if sp ectral regions distorted by the CS influence are excluded from calculations. The ob jects of group I I exhibit p olarization features in numerous lines. But these features are likely to result from different combinations of the atmospheric and CS comp onents of the magnetic field. We have no p ossibility of separating the stellar wind comp onent of these ob jects. Ob jects with clear circular p olarization signatures mainly of CS origin can b e distinguished as a third group. We can analyze the CS magnetic field comp onent of these ob jects only qualitatively, and can conclude nothing ab out their atmospheric field comp onent.

3.1

HD 139614

Fig.1 displays the low-resolution normalized (I /I c) sp ectra of HD 139614 in the region of the CaI I doublet observed in 2003 and 2005 in comparison with the synthetic sp ectrum; the corresp onding p olarization (V /I ) sp ectra are also presented. In the season of 2005 the resolution was higher, and we could see that the CS contribution to the stellar sp ectrum was rather small. Polarization features typical of stellar surface magnetic fields are clearly seen in b oth K and H comp onents of the CaI I doublet and in the FeI blend also (Fig.1, b ottom). In Balmer lines H - H the centers of wide photospheric lines are overlapp ed by CS emission, and no traces of p olarization are observed. We used the entire sp ectrum for the stellar magnetic field determination excluding 3 lo cal sp ectral regions distorted by CS emission in Balmer lines. As a result, in 2005 we obtained the value of B z with an accuracy of ±7 instead of our previous estimation, ±3 (see Table 1). In 2003 the sp ectral resolution was twice lower comparing to that in 2005, and we cannot exactly estimate the CS contribution to the stellar sp ectrum. But the p olarization sp ectrum demonstrate features in b oth K and H CaI I lines, which are not similar to those observed in 2005 (Fig.1, top). They have a complex two-comp onent profile. We b elieve that they are formed by comp osition of the stellar and CS field comp onents. In our opinion, the CS contribution to the stellar sp ectrum


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Figure 1: Normalized and p olarization sp ectra of HD 139614 near the CaI I doublet observed in 2003 (top) and 2005 (b ottom). The synthetic sp ectrum is given for comparison by thin line.


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Table 2: Measurements of magnetic field in HD 144432 in 2003 and in 2005 using different metho ds: Bz (old) ­ Hubrig et al. (2006), only hydrogen lines are used; B z (new) ­ this pap er, regions of CS Balmer and CaI I K and H lines comp onents are excluded Season MJD R Bz (old) Bz (new) Group Magnetic field 2003 52900.99 2000 -94±60 G -32±37 G I I-I I I CS 2005 53447.35 4000 -119±38 G -111±16 G I Stellar

was much higher in 2003 than in 2005. And a high value of B z (of order of 400 G) obtained using Balmer lines relates exclusively to CS magnetic field and is only nominal (see Table 1).

3.2

HD 144432

A similar situation takes place in the case of the Herbig A9Ve star HD 144432. The normalized and p olarization sp ectra of the ob ject near the H , H and CaI I K and H lines observed in 2005 are presented in Fig.2 (b ottom). These sp ectra show the presence of a dense stellar wind. Blueshifted absorption comp onents are clearly seen in these lines. Sufficient p olarization features at the wavelengths of these comp onents indicate that outflowing gas is magnetized. As in the case of HD 139614, we determined the stellar surface magnetic field of HD 144432 in 2005 with an accuracy of 7 , using the entire sp ectrum but excluding the lo cal regions of the hydrogen and CaI I CS comp onents. Our former estimation was made at a level of only 3 (Table 2). The upp er panel of Fig.2 shows a notable difference in the H profiles observed in 2003 and 2005. In 2003 the CS blueshifted absorption was significantly deep er. We b elieve that in this season the CS contribution to the stellar sp ectrum was much larger. It is p ossible that a larger amount of dense outflowing gas was accumulated close to the line-of-sight. As a consequence, the p olarization signatures of the stellar magnetic field, which were clearly visible in 2005, were likely distorted by CS matter. And no magnetic field has b een found in this p erio d.

3.3

Ob jects of group I I

The p olarization sp ectra of two programme ob jects HD 38238 (A7I I Ie) and HD 144668 (A7IVe) from group I I are presented in Fig.3. The most significant p olarization features are observed in the regions where CS comp onents are clearly seen. But the ma jority of these features probably represent a combination of the stellar atmospheric and CS magnetic field signatures. No significant stellar magnetic field has b een revealed in these ob jects.

3.4

Ob jects of group I I I

Three ob jects of group I I I: HD 163296 (A1Ve), HD 31648 (A3p esh) and HD 190073 (A2I I Ie) are the most early-typ e targets of our programme. Also, they demonstrate the most prominent CS line comp onents in comparison with other programme stars. All our attempts to determine the stellar magnetic field comp onent of these ob jects, excluding different sp ectral bands containing CS line comp onents, were unsuccessful.


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Figure 2: Normalized sp ectra of HD 144432 near H observed in 2003 and in 2005 (top) and normalized and p olarization sp ectra of the ob ject near H , H and CaI I doublet observed in 2005 (b ottom). The synthetic sp ectra are given for comparison by thin lines.


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Figure 3: Normalized and p olarization sp ectra of ob jects of group I I.

Therefore, different p olarization features, clearly observed in sp ectra of these stars, are exclusively of CS origin (Fig.4). In our last pap er (Hubrig et al. 2006) the values of magnetic fields are given for HD 31648 and HD 190073 determined with an accuracy of 3 . Now we conclude that these estimations are only nominal and are not related to the stellar surface magnetic field. It is remarkable that in Balmer lines notable p olarization features are observed only in one ob ject of the three -- HD 31648. This star demonstrates very deep blueshifted absorption comp onents of H and H formed in the stellar wind along the line-of-sight b etween the star and the observer. In sp ectra of the other two stars such absorptions are also present, but they are not so deep. It indicates that the density of the outflowing gas crossing the line-of-sight in these stars is likely to b e lower than in HD 31648. No p olarization signatures are observed in Balmer lines of HD 163296, and only a weak feature is seen in the sp ectrum of HD 190073 near H . However in the CaI I K and H lines p olarization signatures are present very clearly in all three ob jects. The profile of the CaI I K line in the sp ectra of HD 163296 and HD 31648 is much wider than that predicted by the synthetic sp ectra. They display a two-comp onent structure containing blueshifted and redshifted absorptions. The CaI I lines in the sp ectra of these stars are very likely to b e formed at the base of the stellar wind and in the accretion gaseous flows. Therefore the p olarization sp ectrum near the CaI I resonance doublet allows the CS magnetic field in the region of interaction of the star with the accretion disk to b e diagnosed.


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Figure 4: Normalized and p olarization sp ectra of ob jects of group I I I.


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3.5

HD 190073

The most interesting result has b een obtained for the ob ject of group I I I, HD 190073. Its very prominent p eculiarity is a complex multi-comp onent structure of the CaI I K and H absorption profiles (Pogo din et al. 2005). Actually, it consists of ab out 20 blueshifted comp onents of various width and depth, but our low-resolution sp ectrop olarimetry allowed us to distinguish only three main comp onents (Fig.4, b ottom). It is remarkable, that three distinctive p olarization signatures are clearly visible and their p ositions coincide with the p ositions of three blueshifted comp onents of the K and H line profiles in the normalized sp ectrum. This picture leads us to the conclusion that there is a direct relation b etween the sp ecific structure of these line profiles and the CS magnetic field.

4

Conclusions

As a result of our magnetic field investigation of seven Herbig Ae stars, we made the following advances: · We conclude that previous discrepancies in B z estimations in some Herbig Ae/Be stars observed on different dates by different investigators are likely to b e a result of a variable CS contribution to the stellar sp ectrum. · The use of a sp ecific metho d of data analysis allowed us to improve considerably the accuracy of the stellar magnetic field determination (for ob jects of group I), namely from the 3 to 7 level. · We deduced that our formerly registered magnetic fields of HD 31648 ( B z = +87±22 G) and HD 190073 (Bz = +84±30 G), as it is rep orted by Hubrig et al. (2006), are of CS but not of atmospheric origin. · We found that p olarization signatures of the CS magnetic field in Balmer lines in the sp ectra of the Herbig Ae/Be stars are mainly observed in deep absorption comp onents formed in the stellar wind. · We also established that the most sensitive indicator for the CS magnetic field in Herbig Ae/Be stars in the region of the interaction b etween the star and the accretion disk is the resonance CaI I (K & H) doublet. · The results of our investigation evidence in favour of the hyp othesis that the well-known puzzling multi-comp onent structure of the absorption CaI I lines in the sp ectrum of HD 190073 is caused by a complex top ology of the CS magnetic field of the ob ject (Pogo din et al. 2005). We would like to emphasize that the most effective strategy for future studies of magnetic fields in the Herbig Ae/Be stars will b e to carry out sp ectrop olarimetric monitoring of these stars followed by close control over the state of their CS contribution to the stellar sp ectrum. In the state of minimum CS influence the stellar magnetic field can b e relatively easy detected applying our metho d presented ab ove. When the CS sp ectrum b ecomes the most prominent, one has a go o d chance for studying the field in the CS environment. We also note that high-resolution sp ectrop olarimetric observations would allow using for the field determination a considerably larger sample of sp ectral lines originating at different levels in the stellar atmosphere and the CS envelop e. This should provide an opp ortunity to reconstruct the magnetic field top ology from the stellar surface to the external CS regions. The first results of high-resolution sp ectrop olarimetric investigations of several Herbig Ae/Be stars are presented in Wade et al. (these Pro ceedings).


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