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Physics of Magnetic Stars, 2007, pp. 189­196

Investigating the pre-main sequence magnetic chemically peculiar system HD 72106
C.P. Folsom1,2 , G.A. Wade1 , D.A. Hanes2 , C. Catala3 , E. Alecian3 , S. Bagnulo4 , T. Bo ehm5 , J.-C. Bouret6 , J.-F. Donati5 , J.D. Landstreet7
Dept. of Physics, Royal Military College of Canada, PO Box 17000, Stn Forces, Kingston, Ontario, Canada, K7K 7B4 2 Dept. of Physics, Engineering Physics & Astronomy, Queen's University, Kingston, Ontario, Canada, K7L 3N6 3 Observatoire de Paris, LESIA, 5 place Jules Janssen, 92190 Meudon CEDEX 4 European Southern Observatory, Casilla 19001, Santiago 19, Chile 5 Observatoire de Midi-Pyr´ ´ 14 Av. E. Belin, F-31400 Toulouse, France enes, 6 Laboratoire d'Astrophysique de Marseille, Traverse du Siphon, BP 8, 13376 Marseille Cedex 12, France 7 Physics & Astronomy Department, University of Western Ontario, London, Ontario, Canada, N6A 3K7
1

Abstract. The origin of the strong magnetic fields observed in chemically p eculiar Ap and Bp stars has long b een debated. The recent discovery of magnetic fields in the intermediate mass pre-main sequence Herbig Ae and Be stars links them to Ap and Bp stars, providing vital clues ab out Ap and Bp stars and the origin and evolution of magnetic fields in intermediate and high mass stars. A detailed study of one young magnetic B star, HD 72106A, is presented. This star app ears to b e in a binary system with an apparently normal Herbig Ae star. A maximum longitudinal magnetic field strength of +391 ± 65 G is found in HD 72106A, as are strong chemical p eculiarities, with photospheric abundances of some elements ranging up to 100x ab ove solar.

1

Intro duction: HAeBe and Ap stars

A few p ercent of main sequence A and B stars display strong, globally ordered magnetic fields; these are the so-called Ap and Bp stars. The origin and evolution of these magnetic fields is currently the sub ject of intensive research. An imp ortant avenue of this research is the investigation the progenitors of Ap and Bp stars. Herbig Ae and Be (HAeBe) stars are the pre-main sequence progenitors of the main sequence A and B typ e stars. As such, HAeBe stars have intermediate masses and display emission, infrared excess, and tend to b e asso ciated with dust and nebulosity. It has b een long suggested that some HAeBe stars may evolve into magnetic main sequence Ap and Bp stars. If this were the case, it was hop ed that there would b e some distinguishing observable feature in HAeBe stars linking them to Ap stars. Pioneering observations in 2005 and 2006 successfully detected magnetic fields in HAeBe stars for the first time (Wade et al. 2005, Catala et al. 2006). The detected fields app ear to display similar intensities and geometries when compared to Ap stars (Alecian et al. 2006). The longitudinal field strengths detected are on the order of hundreds of gauss, and the geometries are predominantly dip olar. Additionally, the incidence of magnetic fields in HAeBe stars is similar to that of the magnetic Ap and Bp stars. This strongly supp orts the idea that magnetic HAeBe stars do in fact represent the progenitors of main sequence magnetic Ap and Bp stars. 189


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In this pap er we investigate one particularly interesting young magnetic B star, HD 72106A, in detail.

2

HD 72106

HD 72106 is a visual double system, with a 0.8 separation. The primary comp onent has a definitely detected magnetic field (Wade et al. 2005), and the secondary is clearly a Herbig Ae/Be (HAeBe) star (Vieira et al. 2003). The secondary shows strong, variable emission in H, as shown in Fig. 1. The system has a very large prop er motion in : -5.18 ± 1.08 mas/yr, and in : 9.73 ± 1.36 mas y-1 but no relative motion b etween comp onents is observed (Hartkopf et. al. 1996 & Hipparcos data, HÜg et al. 2000). The radial velo cities of b oth comp onents are identical to within ±1 km s -1 , at 22 km s-1 . As well, the Hipparcos parallax solution for the system places the two stars at the same distance: 288(+202/ - 84) p c. This indicates that the two stars are travelling in tandem, and strongly suggests that the system is in fact a binary. This is critical b ecause, if the system is truly binary, then b oth comp onents likely have the same age and initial formation conditions. Wade et al. (2006) determine, for the primary, an effective temp erature of 11000 ± 1000 K, a log g of 4.0 ± 0.5, and a mass of 2.4 ± 0.4 M . For the secondary they derive an effective temp erature of 8000 ± 500 K, a log g of 4.25 ± 0.25, and a mass of 1.75 ± 0.25 M . The two stars can b e placed on an H-R diagram (Wade et al. 2006), and have consistent ages of ab out 10 Myr, near the zero-age main sequence, when compared with iso chrones from Palla & Stahler (1993). This is consistent with the comp onents b eing co-eval. Both the H-R diagram p osition and the asso ciation of HD 72106A with the HAeBe secondary provide evidence that the primary is a pre-main sequence or very early zero-age main sequence star. The H-R diagram of b oth stars is shown in Fig. 2.

1.2

Normalized Intensity

1

0.8

0.6 6520 6540 6560 6580 Wavelength (A) 6600 6620

Figure 1: Observed H profiles for HD 72106B (the apparently non-magnetic secondary) from 11 Jan. 2006 (dark/black) and 11 Feb. 2006 (light/green). Strong and variable emission is observed, indicating the presence of an emitting circumstellar envelop e.


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Figure 2: The pre-main sequence H-R diagram of the HD 72106 system. The solid lines are the mo del evolutionary tracks of Palla and Stahler (1993), with masses as lab eled, and the dot-dashed lines are iso chrones, with ages lab eled. The birthline (which assumes an accretion rate of 10 -5 M yr-1 ) and zero age main sequence (ZAMS) are shown as dashed lines.

3

Sp ectrum Reconstruction

Ten Stokes V sp ectra of HD 72106 were obtained b etween February 2005 and February 2006 with ESPaDOnS, the high-resolution echelle sp ectrop olarimeter mounted at CFHT. The sp ectra cover a wavelength range from 3700 °to just past 1 µm. Problematically, the ESPaDOnS pinhole is 1.6 A in diameter, whereas the two comp onents of HD 72106 are separated by 0.8 . With go o d seeing and careful guiding it is p ossible to isolate individual comp onents. This was accomplished on one o ccasion. However, the ma jority of our sp ectra are of combined light from the system. Observations were conducted using atmospheric disp ersion correction, with careful attention to keeping the system centroid centred in the pinhole. In order to isolate the sp ectrum of the primary star, we subtracted the individual sp ectrum of the secondary (obtained on a night of excellent seeing) from the combined sp ectrum, weighted by its luminosity contribution. Assuming the stars are lo cated at the same distance, their apparent magnitudes, suitably b olometric-corrected, yield their relative luminosities. The relative apparent magnitudes of the comp onents of HD 72106 are well known (eg. Fabricius & Makarov 2000), and the Hipparcos parallax solution puts b oth comp onents at the same distance. The subtraction pro cess also assumes that the sp ectrum of the secondary is non-variable, which is a reasonable assumption for metallic lines of a non-magnetic late A star (excluding H). Thus, with the relative apparent magnitude, we can p erform the subtraction effectively. To check this metho d, Balmer lines (excluding H), as well as several deep, apparently nonvariable metallic lines, were compared in the recovered sp ectra and in the individual observed sp ectrum of the primary. A very go o d agreement was found. Presented in Fig. 3 is a typical observed sp ectrum of the combined system, compared with a sp ectrum of just the secondary, and the reconstructed sp ectrum of the primary, resulting from the subtraction pro cess, compared with a sp ectrum of just the primary.


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1

Intensity

0.95 0.9 0.85 0.8 526 527 528
Secondary Observed

1

Intensity

0.95 0.9 0.85 0.8 526 527 Wavelength (nm) 528
Primary Recovered

Figure 3: The top frame shows a sample of the sp ectrum from the secondary (in black) and an observed sp ectrum of the combined light from the system (in gray). The b ottom frame compares the sp ectrum reconstructed sp ectrum of the primary (in gray), resulting from the weighted subtraction pro cess, with an observation of just the primary (in black). A very go o d corresp ondence is obtained (although some differences are exp ected due to the line profile variability of the primary).

4

Magnetic Field Measurements

Least Squares Deconvolution (LSD) (Donati et al. 1997) was p erformed on the reconstructed sp ectra of the primary. LSD is a metho d of effectively averaging over many lines in a sp ectrum in order to improve the signal to noise ratio of the line profile. A line mask for a 10000 K Bp star, and sp ectra with the contribution of the secondary removed, were used. Significant Stokes V signatures are detected at all observed phases. Longitudinal magnetic field measurements were obtained from the LSD Stokes V profile for each observation. The largest longitudinal field found was +391 ± 65 G. Stokes V profiles with larger amplitudes were observed, but with crossover signatures, corresp onding to weaker longitudinal fields. The Stokes I and V LSD profiles corresp onding to the 391 G measurement are shown in Fig. 4, and are lab eled 0.4015 in the phase column.

5

Perio d Searching

Clear variability, presumably due to rotation, can b e seen in the sp ectrum of HD 72106A, with an apparent p erio d around two days. To investigate this further we first searched our measured longitudinal magnetic field data for a p erio d. This was done using a mo dified Lomb-Scargle technique, fitting sinusoids through the data to pro duce a p erio dogram, and lo oking for a minimum in 2 . Unfortunately, due to the small numb er of longitudinal field observations, a unique solution could not b e found. In an attempt to improve our p erio d determination, a more sophisticated technique was used.


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Figure 4: The Stokes I and V profiles obtained for HD 72106A, phased according to a 1.953 day p erio d. The Stokes V profiles, detected significantly at all phases, have b een multiplied by 20 times relative to the I profiles, and shifted vertically for clarity. The profiles are lab eled by phase, with an arbitrary zero p oint. Clear variations of the Stokes I profile are apparent. This metho d is still based on fitting a sinusoid through a time series of data p oints to pro duce a p erio dogram. However, now the set of measurements for one pixel in the LSD profile is used as the time series of data p oints. The p erio dogram for one data p oint may b e heavily affected by noise, but if one averages the p erio dograms of all the pixels in the LSD line profile, pro ducing an average p erio dogram, the effects of noise are substantially reduced and the true p erio d is found. To verify the technique, p erio ds were determined for a numb er of known Ap stars (Auriere et al., in preparation), as well as synthetic test sp ectra, with go o d results. While this improved the situation significantly by reducing the numb er of p ossible p erio ds, it


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still did not pro duce a unique p erio d solution for HD 72106A. Examining phased LSD profiles by eye allowed us to reject several more p erio ds as un-physical, but a few p ossibilities still remain around 1.8 and 1.9 days. A set of LSD profiles phased with one of the b etter candidate p erio ds is presented in Fig. 4.

6

Sp ectrum Synthesis

Preliminary sp ectrum mo deling has b een p erformed for b oth the primary and the secondary, for a single phase only, using observations of the individual comp onents. The Zeeman2 sp ectrum synthesis co de was used, with ATLAS9 mo del atmospheres corresp onding to the temp eratures and gravities deduced by Wade et al. (2006). In the primary, a dip ole magnetic field of 1 kG was assumed, oriented along the line of sight, with no microturbulence. A homogeneous abundance distribution b oth vertically and horizontally was assumed. In the secondary, a homogeneous vertical and horizontal abundance distribution was used, with no magnetic field. Microturbulence was left as a free parameter in the fitting pro cedure. We find that the primary displays clear chemical p eculiarities. The abundance pattern is typical of those of Ap/Bp stars, although the abundance enhancements are remarkably strong. In particular, strong overabundances of Cr, Fe, Si, and Ti are found. In contrast, the secondary displays solar abundance for all elements detected. Clear structure and variability can b e seen in the line profile of the primary, due to abundance sp ots on the surface of the star. These app ear most clearly in lines of Fe and Cr, but can also b e observed in some Ti and Si lines. Complex, variable line profiles of this typ e are typical of Ap stars, and are explained by the presence of abundance non-uniformities in the stellar photosphere. The secondary displays smo oth line profiles with no visible asymmetries (although the S/N of this star is relatively p o or, and additional observations are required to confirm this). A sample sp ectrum of the primary, with a b est fit synthetic sp ectrum, is shown in Fig. 5. Best fit abundances and v sin i for the primary are shown in Table 1. For the secondary, b est fit abundances, v sin i, and microturbulence can also b e seen in Table 1, with sample observed and synthetic sp ectra in Fig. 6 . Element Al Si Ca Ti Cr Fe Ba v sin i Primary -0.2 ± 0.4 +1.0 ± 0.2 +1.0 ± 0.3 +1.9 ± 0.1 +0.9 ± 0.1 41 ± 3 km s-1 0 km s-1 -0.4 -0.1 +0.1 -0.1 -0.3 ± ± ± ± ± 0.4 0.4 0.3 0.3 0.5 Secondary

50 ± 4 km s-1 2.5 ± 0.5 km s-1

Table 1: Preliminary b est-fit abundances, v sin i, and microturbulence values for HD 72106A and HD 72106B. Abundances are based on the 4500-4700 ° region. A


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1

Normalized Intensity

0.95

Fe
0.9

Fe

Fe Cr Si Fe Fe Cr Fe

Fe

0.85

Cr
0.8

Cr

4615

4620

4630 4625 Wavelength (A)

4635

4640

Figure 5: Observed (black) and b est fit synthetic (gray) sp ectra for HD 72106A. Ma jor contributers to each feature have b een lab eled.

1

Normalized Intensity

0.9

Fe Cr
0.8

Fe Ba Fe Cr

Ti Ti Cr

Ti Fe

0.7 4540

Fe Ti

Wavelength (A)

4550

4560

4570

Figure 6: Observed (black) and b est fit synthetic (gray) sp ectra for HD 72106B. Ma jor contributers to each feature have b een lab eled.

7

Conclusions

These results demonstrate that strong chemical p eculiarities, of the typ e seen in Ap/Bp stars, together with magnetic fields, can exist in late B-typ e stars at the pre-main sequence or very early zero-age main sequence phase. A maximum longitudinal magnetic field of +391 ± 65 G was found in HD 72106A. Overabundances of up to 2 dex ab ove solar were also found, along with a complex absorption line structure, suggesting an inhomogeneous surface abundance distribution of elements. These prop erties are all characteristic of Ap stars. In contrast, HD 72106B which presumably formed at the same p oint in time, under the same conditions as the primary, displays solar abundances and


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no magnetic field. Thus the HD 72106 system provides a unique link b etween Ap stars and their pre-main sequence progenitors. Further circular p olarisation observations of the HD 72106 system are currently planned. This will allow us to b etter sample the sp ectroscopic and p olarimetric cycle of the primary to determine a unique rotational p erio d. With additional observations we can mo del the magnetic field geometry and map the surface distributions of elements. Additional observations will also allow us to test the stability of the sp ectrum of the secondary and further test the sp ectrum reconstruction technique. With these new observations we will b e able to more completely describ e this unique and remarkable system.

References
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