Astron. Astrophys. 356, 146-156 (2000)

1. Introduction

HR 1094 (=HD 22316) has been classified as a B9p star (Hoffleit & Jaschek 1982) with strong spectral lines of chromium (Cowley et al. 1969) and silicon (Palmer et al. 1968). Its spectrum also displays more pronounced characteristics such as strong Co, Cl and Hg lines. In an earlier analysis (Sadakane 1992) HR 1094 was investigated with respect to its chemical composition, with results revealing the star to have an abnormally high abundance of Co and Cl, while other elements such as Si, Mg and Ca were found to be underabundant relative to the sun. This analysis was limited to a small number of elements, mostly from the iron group.

HR 1094 was proposed to be an Ap star by Hill & Blake (1995), based on polarisation measurements in the line wings, however, the magnetic field is not large enough to produce resolved Zeeman components. Hill & Blake determined the observed longitudinal magnetic field to be time variable with an amplitude of 2.0 kG and a period of approximately 3 days. The angle between the rotational and the magnetic axes, , was estimated to be 76^o. These results were based on a stellar atmosphere model from Sadakane, including a value for the rotational velocity (). Since the signal-to-noise ratio (S/N) of Sadakane's photographic spectrum was rather low, Hill & Blake stressed the importance of obtaining spectra of higher quality in order to determine more accurately.

The influence of a magnetic field has been typically neglected in stellar spectrum analyses for chemical abundances. In the absence of observed resolved Zeeman line splitting, slightly broadened lines analyzed under the assumption of negligible magnetic field will lead to an overestimate of chemical abundances. One approach to accounting for magnetically increased line equivalent width is by an appropriate increase in the turbulent velocity. This approach may produce the desired increase in absorption but assumes that all atoms/ions are affected in a similar manner by the local magnetic field. More accurate line profile modelling techniques take into consideration the sensitivity of individual spectral lines to a magnetic field through the Landé factors for the transition upper and lower level. Computer codes have been formulated to various degrees that account for polarized radiation transport through model atmospheres for application to magnetic field work in both warm (Landstreet et al. 1989) as well as solar and cooler (e.g. Basri & Marcy 1988; Saar 1988; Valenti et al. 1995) stars. Our synthetic spectrum analysis addresses the individual Zeeman line broadening in a simpler manner, with each component treated as an individual spectral line undergoing unpolarized radiative transport. This approach is similar to that one employs for individual hyperfine or isotopic components of a line and allows us to explore the gross nature of the magnetic field and its effect upon spectral line profiles.

Our main reasons for conducting further investigation of HR 1094 were to confirm the curious abundance enhancement for Co and Cl, and to extend the abundance analysis to additional elements, for example the rare-earth elements (REE) and the heavy elements of the platinum group to better characterize a potentially different elemental abundance distribution. This initial line of inquiry lead to other paths as we realised that the magnetic field had a greater influence upon the line widths than we had anticipated.

In this paper we investigate elemental abundances for the magnetic, chemically peculiar (CP) star HR 1094 and incorporate an approximated treatment of the line structure for selected lines in order to explore the nature of the magnetic field. Since our ground-based spectra are of higher S/N and spectral resolution than that used by Sadakane, it was also possible to determine a more accurate value of .

The southern magnetic star HR 5049 (=HD 116458, spectral type A0p (Hoffleit & Jaschek 1982)), has similar characteristics as HR 1094 and is therefore presented for comparison. Both stars show an enhancement of cobalt and have a measurable magnetic field. HR 5049 has sharper spectral features and a stronger field than observed for HR 1094, consequently the effects of some characteristics are more distinct in the spectrum of HR 5049.

The A1V (Hoffleit & Jaschek 1982) star HR 3383 (=HD 72660) is also used for comparison in the rotation velocity analysis since it is a sharp-lined star with a more solar-like chemical composition.

© European Southern Observatory (ESO) 2000

Online publication: March 28, 2000
helpdesk.link@springer.de