Searching for Links Between Magnetic Field and Stellar. Evolution: a Survey of Magnetic Stars in Open Clusters with FORS1 at the VLT. S. Bagnulo, E. Mason, ...
Astronomical Polarimetry: Current Status and Future Directions ASP Conference Series, Vol. 343, 2005 Adamson, Aspin, Davis, and Fujiyoshi
Searching for Links Between Magnetic Field and Stellar Evolution: a Survey of Magnetic Stars in Open Clusters with FORS1 at the VLT S. Bagnulo, E. Mason, and T. Szeifert European Southern Observatory, Casilla 19001, Santiago 19, Chile J.D. Landstreet Physics & Astronomy Department, The University of Western Ontario, London, Ontario, Canada N6A 3K7 G.A. Wade Department of Physics, Royal Military College of Canada, P.O. Box 17000, Station ‘Forces’ Kingston, Ontario, Canada K7K 7B4 V. Andretta Istituto Nazionale di Astrofisica/Osservatorio Astronomico di Capodimonte, Salita Moiariello, 16, 80131 Napoli, Italy Abstract. We outline a diagnostic technique for stellar magnetic fields based on spectropolarimetry of H Balmer lines. We present preliminary results of a survey of magnetic stars in open clusters carried out with FORS1 at the ESO VLT, aimed at examining the characteristics of the magnetic fields of intermediatemass stars as they evolve onto and across the main sequence.
1.
Introduction
One of the most interesting applications of polarimetric techniques in stellar astrophysics is the observation and study of magnetic fields through the analysis of the Zeeman effect on the Stokes profiles of spectral lines. Generally speaking, the linear polarization is sensitive to the components of the magnetic field perpendicular to the line of sight averaged over the visible stellar disk, and the circular polarization is sensitive to the component of the magnetic field vector along the line of sight, averaged over the visible stellar disk. Measurements of this latter quantity, often referred to as the mean longitudinal magnetic field, represent the most common diagnostic technique for stellar magnetic fields (see, e.g., Borra & Landstreet 1980, Mathys 1994). Among the various targets for studies of stellar magnetism, chemically peculiar stars of the upper main sequence (Ap stars) are of special interest. On the one hand, the typical magnetic field of Ap stars has a relatively smooth morphology, and a strength of a few hundreds up to few tens of thousands of gauss. These characteristics make it relatively easy to detect the field through spectropolarimetric techniques. On the other hand, Ap stars represent a challenge for theoreticians, as the origin of their magnetic fields is far from understood. 369
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FORS1 is a multi-purpose instrument of the ESO VLT, capable of doing both optical imaging and spectroscopy, equipped with polarimetric optics. In spectropolarimetric mode it allows one to obtain IQUV Stokes profiles with a spectral resolution up to about 2000. The instrument has a multi-object mode which permits one to observe Stokes profiles of up to nine stars at once, within a 6.8 × 6.8 arcmin field of view. We decided to use this instrument for a spectropolarimetric survey of magnetic Ap stars in open clusters, to search for links between magnetic fields and stellar evolution. The polarimetric mode of FORS1 was originally planned to be used for observations of white dwarfs, that are generally characterised by very strong magnetic fields (up to several hundred MG). Before starting our survey we performed a feasibility study to check if FORS1 could also be used for observations of weaker fields such as those of magnetic Ap stars. This study has been presented by Bagnulo et al. (2002). In the context of a conference dedicated to astronomical polarimetry, it appears appropriate to outline again the adopted observing technique. Section 2 is dedicated to a description of the method used to measure stellar magnetic fields with FORS1. In Section 3 we describe the scientific rationale and present some preliminary results of a study of open cluster magnetic stars. 2.
The Observing Technique
In Ap stars, the mean longitudinal magnetic field is usually determined by measuring of the splitting of metal lines observed in right and left circular polarization. This technique has been used for the first time in a star other than the Sun by Babcock (1947; see also Babcock 1958), and further developed and extensively applied by several authors (see, e.g., Mathys 1994, and Wade et al. 2000). An alternative approach for the interpretation of the observed polarized spectra is given in terms of the formula V 1 dI = −cz geff λ2 hBz i , I I dλ
(1)
A−1 , geff is the effective Land´e factor, V and I are the where cz = 4.67 × 10−13 ˚ Stokes V and I parameters (both functions of wavelength), λ is the wavelength expressed in ˚ A, hBz i is the mean longitudinal field expressed in G. Using narrow band photopolarimetry centred at the wings of the H Balmer lines, Angel & Landstreet (1970) have developed a technique for the measurement of stellar magnetic field based on Eq. (1) (see also Borra & Landstreet 1980). Equation (1) can also be applied to spectro-polarimetric observations of magnetic stars. A least-square technique is used to derive the longitudinal field via Eq. (1), allowing also for a constant contribution related to the instrumental polarization (i.e., as a zero-order term in a first-order polynomial interpolation). One minimises the expression χ2 =
X (yi − hBz i xi − b)2 i
σi2
(2)
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Figure 1. Left panel: observed Stokes I (thin solid line) and Stokes V (thick solid line) of Hβ of the star HD 45583. Right panel: the slope of the solid line (obtained via a least-square fit to the observed data) gives the star’s mean longitudinal field. For the newly discovered magnetic star HD 45583 we measured hBz i = −1510 ± 50 G.
where, for each spectral point i, yi = (V /I)i , xi = −cz geff λ2i (1/I × dI/dλ)i , and b is a constant term. Figure 1 illustrates this technique. The left panel shows the observed Stokes I and V profiles of Hβ for the Ap star HD 45583. The right panel shows the results of the least-square technique. For this star (previously not known as magnetic) we have measured a mean longitudinal field of −1510 ± 50 G (to determine this value we have used all Balmer lines from Hβ down to H16). More details of the diagnostic technique are given by Bagnulo et al. (2002). 3.
A Spectropolarimetric Survey of Stars in Open Clusters
The magnetic field of Ap stars is probably a fossil remnant, either of a field swept up by the star during formation, or possibly generated during pre-main sequence evolution (e.g., Moss 2001). Many basic questions concerning these fields and their effects on the stars where they reside are not yet answered. Very little information is available even on the fundamental problem of whether fields evolve during the main sequence lifetime and if so, by how much. Two studies attempting to address this question using the newly available Hipparcos parallaxes have come to quite contradictory conclusions. Gomez et al. (1998) found that the magnetic type of chemically peculiar stars are scattered from the ZAMS to the TAMS, whereas Hubrig et al. (2000) conclude that, for stars with mass < 3 M⊙ , magnetic fields become detectable only when the star has already spent 30 % of its time on the main sequence. One very important limitation of the observational data available in the literature is that hardly any of the 200+ known magnetic stars have secure ages. This is a result of the perturbation of the energy distribution by the atmospheric anomalies, which lead to substantial uncertainties in both the effective temperature and the bolometric magnitude. When the bolometric magnitudes are con-
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verted to luminosities using accurate parallaxes, and then to ages by comparison with theoretical models such as those of Schaller et al. (1992), one cannot realistically do much better than classify a field magnetic Ap star as evolutionarily “young” or “old”. To obtain a qualitatively new kind of information with which to address the problems discussed above, our group has begun a survey of magnetic fields in cluster Ap stars. FORS1 at the VLT is an ideal instrument because, thanks to its multi-object capabilities, it allows one to observe more than one star at once. Furthermore, the large collecting area of the telescope allows us to reach a large number of open clusters, and hence to do a full sampling in stellar age and mass. The data we have already obtained provide the first magnetic measurements in stars whose age is known to within a factor of two or better, and thus for the first time allow us to place stars reasonably accurately on their evolution paths. (In parallel we have been obtaining UVES spectroscopy to enable us to study the evolution of chemistry with movement from the ZAMS to the TAMS.) During the observations carried out so far, we have discovered a few especially interesting objects. In particular, we have discovered and studied two very young magnetic objects: HD 66319 is a 2.1 M⊙ magnetic star belonging to the open cluster NGC 2516 that has spent 16 ± 5 % of its life on the main sequence (Bagnulo et al. 2003). NGC 2244 334 is a very young 4 M⊙ star belonging to the Rosette Nebula cluster that has spent about 2 % of its life on the main sequence (Bagnulo et al. 2004). In this star, we measured a −9 kG longitudinal field, which is the second strongest value measured so far in a main sequence star. Detection of a magnetic field in HD 45583 is reported in this paper for the first time. From previous studies we know that HD 45583 is an almost unevolved 4 M⊙ star with R = 2.3 R⊙ (North 1987). These results suggest that magnetic field exists in the stellar photosphere since the very moment at which the star reaches the ZAMS. Figure 2 shows the position in the Hertzsprung-Russell diagram of a number of Ap stars that are open cluster members. Different symbols are used to indicate: stars for which no magnetic field measurements exist so far (diamonds) and that are likely to be future targets of our survey; stars already known to be magnetic (crosses); stars that have been observed (mostly for the first time) during our spectropolarimetric survey (squares). The positions of the stars in the HR diagram have been calculated (after an estimate of the stellar temperature and of the clusters’ ages) by interpolating the stellar temperature and an interpolation of the evolutionary tracks by Schaller et al. (1992) and related papers. It should be noted that, although some of the points in Figure 2 are well established, a lot of work is still needed in order to accurately establish temperature and cluster membership of our targets. Furthermore, several targets are still missing from Figure 2 because no temperature estimate has been calculated. Finally, further searching is still needed in the literature to identify additional targets for our study. Therefore Figure 2 should be regarded as only roughly representative of our survey. Yet it appears that our observations are currently biased toward very young objects, and that data are not yet sufficient to trace the evolution of the magnetic field as the stars evolve along the main sequence. There is obviously a need for further data, and work in this direction is currently in progress.
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Figure 2. HR diagram of a (still incomplete) sample of Ap stars belonging to open clusters with known age. The Figure is explained in the text. References Angel, J.R.P., & Landstreet, J.D. 1970, ApJ, 160, L147 Babcock H.W. 1947, ApJ, 105, 105 Babcock H.W. 1958, ApJS, 3, 141 Bagnulo, S., Wade, G.A., Szeifert, T., Landstreet, J.D., & Mathys, G. 2002, A&A, 389, 191 Bagnulo, S., Hensberge,H., Landstreet, J.D., Szeifert, T., & Wade, G.A. 2004, A&A, 416, 1149 Bagnulo, S., Landstreet, J.D., Lo Curto, G., Szeifert, T., & Wade, G.A. 2003, A&A, 403, 645 Borra, E.F., & Landstreet, J.D. 1980, ApJS, 42, 421 Gomez, A.E., et al. 1998, A&A, 336, 953 Hubrig, S., North, P., & Mathys,G. 2000, ApJ, 539, 352 Mathys, G. 1994, A&AS, 108, 547 Moss, D. 2001, ASP Conf. Series No. 248, p. 305 North, P. 1987, A&AS, 69, 371 Schaller, G., Schaerer, D., Meynet, G. & Maeder, A. 1992, A&AS, 96, 269 Wade G.A., Donati J.-F., Landstreet J.D., & Shorlin S.L.S. 2000, MNRAS, 313, 851
