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Astron. Astrophys. 364, L101-L104 (2000)

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1. Introduction

A study of molecular lines in sunspots is of particular interest because of their high temperature and pressure sensitivity. Many of them are also magnetically sensitive, but this has not yet been investigated in detail.

Earlier investigations of molecular lines in sunspots generally concentrated on line identification, determination of temperatures (rotational and vibrational) and pressure as well as elemental and isotopic abundances (e.g. Sotirovski 1971, Boyer 1978). Wöhl (1969) demonstrated that molecular lines in spectra of sunspots show the Zeeman effect. He found that the magnetic field strength derived from molecular lines was smaller than that derived from iron lines. Only few reports of polarimetric observations of molecular lines in sunspots can be found in the literature (e.g. Harvey 1973, 1985; Rüedi et al. 1995). They reported a puzzling reversal of the circular polarization and unconventional Stokes V profile shapes of some molecular lines compared to atomic or other molecular lines. No in-depth analysis of those observations has been carried out as yet.

The problem of the molecular Zeeman and Paschen-Back effects was addressed by Schadee (1978). In spite of the major advance which Schadee's work represents, there seems to have been only a single attempt to utilize this theory, namely to explain the broad-band circular polarization in sunspots (Illing 1981). However, only a part of the effect could be attributed to molecular bands, and asymmetries of atomic Stokes profiles turned out to be a more suitable explanation for the effect (e.g. Sánches Almeida & Lites 1992; Solanki & Montavon 1993; Martínez Pillet 2000). Hence the theory of the Zeeman splitting of molecular lines of interest for the study of solar and stellar magnetism has remained largely untested.

In this paper we present calculations of splitting and the Stokes parameters for the green system of MgH and TiO [FORMULA]-system, which are the most studied band systems in sunspot spectra. We present the first spectral synthesis calculations and, also for the first time, confront synthetic molecular Stokes profiles with low-noise observations. Here we demonstrate that the synthetic spectra successfully reproduce the observations for reasonable atmospheric parameters. This opens the door to use these lines to recover the thermal, magnetic and dynamic properties of sunspots, and eventually for novel investigations of stellar spots.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001