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Magnetic Stars, 2011, pp. 220 ­ 223

A Low­Resolution Sp ectrop olarimeter for Zeeman Measurements of Stellar Magnetic Fields
Hiriart D.1 , Valyavin G.1 , Plachinda S.2 , Ramirez J.1 , Valdez J.,1 , Quiros F.1 , Mart´ inez B.1
1

Observatorio Astron´ omico Nacional SPM, Instituto de Astronom´a, Universidad Nacional Aut´ i onoma de M´ exico, Ensenada, BC, M´ exico 2 Crimean Astrophysical Observatory, Nauchny, Crimea, Ukraine

Abstract. We present a new spectropolarimeter based on the low­resolution slit spectrograph Boller & Chivens of the National Institute of Astronomy (Mexico) 2.1 m telescope. The instrument is intended to measure stellar longitudinal magnetic fields of stars from 8m to 16m with a characteristic accuracy from 0.5 kG to 10 kG . Key words: instrumentation: polarimetry ­ magnetic fields ­ stars: individual (WD1658­441) ­ stars: magnetic fields ­ stars: white dwarfs

1

Intro duction

Low­resolution spectropolarimeters are intended mainly for spectropolarimetric studies of faint ob jects. A significant part of these studies is related to their magnetism. The following studies are among the most important to be mentioned: · 1. Magnetism of white dwarfs. The investigation of white dwarf stars (WDs) is of fundamental importance for the understanding of stellar and galactic evolution, as WDs represent the final evolutionary stage of more than 90 % of all stars. Nowadays, we believe that the general properties and evolution of WDs are understood fairly well. However, there are several important problems that still need to be properly addressed, especially those connected with the group of about two hundred isolated magnetic white dwarfs (MWDs) (Angel et al., 1981; Schmidt &Smith, 1995; Liebert et al., 2003; Valyavin et al., 2003; Aznar Cuadrado et al., 2004) the origin of which is still not quite well understood. · 2. Magnetism of hot subdwarf stars The presence of kilogauss magnetic tively recently (Elkin, 1996; O'Toole to the theory of stellar evolution is tionary channels to the white dwarf al., 1994, Williams et al., 2001a,b). fields has been reported in hot subdwarf stars comparaet al., 2005). The importance of hot subdwarf star studies clear due to the fact that they exhibit a variety of evolustage (Greenstein & Sargent, 1974; Heber, 1986; Saffer et

· 3. Magnetic fields in cataclysmic systems A large number of great historical results in the studies of cataclysmic binary systems has come from the broad­band polarimetry and low­resolution spectropolarimetry. One of the


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most important of these studies is related to the discovery of polars. To review these results see, for instance, Wickramasinghe (1995)

This brief presentation illustrates the importance of low-resolution spectropolarimetry in the studies of cataclysmic and degenerate stars. Here we introduce a new spectropolarimeter mounted at the 2.1-m telescope of the National Astronomical Observatory (Ensenada, Mexico).

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Sp ectrop olarimeter: Design

The spectropolarimeter is based on the Cassegrain focus spectrograph B&Ch at the 2.1 m telescope of National Institute of Astronomy (Ensenada, Mexico). The instrument is intended for classic long­slit, low­resolution spectroscopy with several modes of resolution from R 500 to R 4000. The polarimetric analyzer that we use in the spectropolarimetric mode consists of a rotatable quarter­wave plate for polarimetric modulation of circular polarization, and a Savart plate as a beam­splitter. In the present design (the design is very similar to that presented by Naidenov et al. (2002)) we decided to use the polymer quarter­wave plate (Samoylov et al., 2004) instead of the frequently used Quartz/MgF2 crystal wave plate or Fresnel rhombs. The used polymer QWP gives the wave retardation of 0.25 ± 0.007 in the 4000 8000 ° range (Samoylov et al., 2004), A and ripple below 0.1 %. Outside of this region magnetic observations are also possible, but with some depolarization factor which should be measured and taken into account in the observations of standard stars.

Figure 1: Magnetic sensitivity k over wavelengths

In order to estimate the depolarization factor (sensitivity of the instrument in the mode of circular polarization registration as a function of wavelength ) we carried out observations of different magnetic stars with a well-known behaviour of their longitudinal magnetic fields. Obtaining circular polarizations, and measuring the longitudinal magnetic fields Bl at each of the Balmer lines Bl ( i ) we estimated k () as a relationship Bl (defacto) , where Bl (i ) is a result of the measurement of the field at an individual spectral line i, and Bl (def acto) is a "defacto" result taken from other studies. The depolarization function in the system polarization optics + spectrograph + telescope is presented in Fig. 1. As can be seen, in the wavelength region between 3900 ° and 7000 ° k characteristically A A varies from 0.8 to 1 what is an acceptable result.


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HIRIART ET AL.

Figure 2: Spectrum (upper plot) and circular polarization (lower plot) of the star HD 215441

3

Tests
test the instrument we carried out the observations of two well­known magnetic stars: (the Babcock's star, mV 9m ), and a strong magnetic white dwarf WD 1658+441 , Bl 800 kG). In the tests the B&Ch was used to derive the Stokes V profiles from of magnetic stars obtained with the grating 400 gv/mm that provides the interval from ° to 7000 ° With a slit width of 1 arcsec, the spectral resolving power was about 1000. A A.

In order to HD 215441 (mV 15m the spectra about 3600

Figure 3: Spectrum (upper plot) and circular polarization (lower plot) of the magnetic white dwarf WD 1658+441

The derived intensity and circular polarization spectra of the studied stars are presented in Fig. 2 and Fig. 3. The spectra of a zero polarization standard are presented in Fig. 4. The Zeeman circular polarization in the observations of the star HD 215441 was registered once in the phase of its


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Figure 4: Spectrum (upper plot) and circular polarization (lower plot) of the zero polarization standard

maximal rotationally modulated longitudinal magnetic field. The measurements of the field using the Balmer lines yielded the result Bl = +20.2 ± 0.6 kG. Measurements of the field of the magnetic white dwarf WD 1658+441 provide Bl = +824 ± 12 kG, what is also a canonical result for this star (Shtol' et al., 1997).

References
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