1 Introduction

Accurate oscillator strengths are needed for the determination of chemical abundances in the atmospheres of astrophysical objects. Elemental abundance studies are used to address problems related to stellar evolution and chemically peculiar stars, to the interstellar medium, and to cosmology. The need for accurate oscillator strengths is particularly acute in ultraviolet astronomy, because of the dramatic increase in observational capabilities provided by orbiting observatories such as the Hubble Space Telescope. Most of the lines seen in spectra from the interstellar medium and stellar photospheres result from neutral and lowly ionized atoms. In the present work we study various transitions occurring in neutral arsenic, As I, and in singly- and doubly-ionized arsenic, As II and As III.

The first detection of arsenic in any star, including the sun, was made with the high-resolution Echelle spectrometer aboard the Hubble Space Telescope ([Leckrone et al. 1991]). The 1937.6 Å line of As I was detected for the first time in the ultraviolet spectrum of the bright, ultra-sharp-lined B-peculiar star, chi Lupi. This line results from the transition 4p³(⁴S $^{\circ }_{3/2}$) $\rightarrow$4p²(³P)5s(⁴P_3/2), which is discussed in Sect. 3 of the present work. Arsenic is known to be present in meteorites ([Anders & Grevesse 1989]). A knowledge of the oscillator strengths of As I is also important for an understanding of the mechanisms involved in arsenic lasers ([Fowles et al. 1974]).

The first detection of As II in the interstellar medium was made by the Goddard High Resolution Spectrograph aboard the Hubble Space Telescope ([Cardelli et al. 1993]). Oscillator strengths for various ultraviolet transitions in As II are discussed in Sect. 4 of this work. In Sect. 5 we present results for As III.

 

Table 1: Experimental and computed energy levels of As III. Units are in Rydbergs

Level	Experimental	Computed
4p	2.0813	2.0511
5s	1.1091	1.0920
6s	0.5970	0.5947
5p	0.8790	0.8807
4d	1.0087	1.0509
5d	0.5716	0.5762
4f	0.5859	0.5768
5g	0.3599	0.3605