Is the solar spectrum latitude-dependent?

An investigation with SST/TRIPPEL

¹ The Institute for Solar Physics of the Royal Swedish Academy of Sciences, AlbaNova University Centre, 10691 Stockholm, Sweden
e-mail: dan@astro.su.se
² Stockholm Observatory, Department of Astronomy, Stockholm University, AlbaNova University Centre, 10691 Stockholm, Sweden
³ Research School of Astronomy and Astrophysics, Australian National University, Cotter Rd., Weston, ACT 2611, Australia
⁴ Department of Physics and Astronomy, Uppsala University, Box 515, 75120 Uppsala, Sweden
⁵ Nordita, AlbaNova University Centre, 10691 Stockholm, Sweden
⁶ Max-Planck-Institut für Astrophysik, Postfach 1317, 85741 Garching b. München, Germany
⁷ Departamento de Astronomia do IAG/USP, Universidade de São Paulo, Rua Matão 1226, São Paulo, 05508-900, SP, Brazil

Received: 23 June 2011
Accepted: 23 August 2011

Abstract

Context. In studies of the solar spectrum compared to spectra of solar twin stars, it has been found that the chemical composition of the Sun seems to depart systematically from those of the twins. One possible explanation could be that the effect is caused by the special aspect angle of the Sun when observed from Earth compared with the aspect angles of the twins. This means that a latitude dependence of the solar spectrum, even with the heliocentric angle constant, could lead to the observed effects.

Aims. We explore a possible variation in the strength of certain spectral lines that are used in the comparisons between the composition of the Sun and the twins at loci on the solar disk with different latitudes but at constant heliocentric angle.

Methods. We use the TRIPPEL spectrograph at the Swedish 1-m Solar Telescope on La Palma to record spectra in five spectral regions to compare different locations on the solar disk at a heliocentric angle of 45°. Equivalent widths and other parameters are measured for fifteen different lines representing nine atomic species. Spectra acquired at different times are used in averaging the line parameters for each line and observing position.

Results. The relative variations in equivalent widths at the equator and at solar latitude  ~45° are found to be less than 1.5% for all spectral lines studied. Translated into elemental abundances as they would be measured from a terrestrial and a hypothetical pole-on observer, the difference is estimated to be within 0.005 dex in all cases.

Conclusions. It is very unlikely that latitude effects could cause the reported abundance difference between the Sun and the solar twins. The accuracy obtainable in measurements of small differences in spectral line strengths between different solar disk positions is very high, and can be exploited in studies of, e.g. weak magnetic fields or effects of solar activity on atmospheric structure.