1 Introduction

Mass loss is ubiquitous among highly luminous OBA stars. The last decade has seen an enormous increase in the number and the quality of the observational studies of stellar winds. At the same time the theory of stellar winds has been improved dramatically. However most of this work has been limited to O and B stars. To date, very few studies have been devoted to A-type supergiants. These stars occupy a region of the HR diagram where evolution is rapid and therefore they are few in number. Furthermore, the indicators of stellar winds are significantly weaker in A-supergiants than in OB supergiants. Nevertheless it is known from the few A supergiants studied that the structure of their stellar wind is unique in some way that is not yet understood.

There are no comprehensive studies of the line profiles formed in the winds of these stars, of which only a few have been studied in detail. In the optical, a photographic survey of H$\alpha$ emission in luminous O9 to A5 stars was undertaken by Rosendhal (1973) showing the strong influence of luminosity upon the H$\alpha$ profile in late-B and A supergiants. The emission at H$\alpha$ disappears for stars fainter than absolute visual magnitude -6.8 to -7.0, while at the highest luminosities emission dominates over absorption. In the ultraviolet, where most of the indicators of mass loss are observed, the UV P Cygni profiles for O and early B stars have been studied by a large number of authors. However the UV spectra of A-supergiants have scarcely been examined (e.g. Lamers et al. 1995; Lamers et al. 1978; Praderie et al. 1980; Underhill & Doazan 1982; Hensberge et al. 1982); the most extensive study was performed by Talavera & Gómez de Castro (1987) from the high resolution data available in the International Ultraviolet Explorer (IUE) satellite archive. They found two different groups of A-supergiants: stars showing spectral lines characteristic of mass outflow and stars which do not show any sign of stellar winds in their spectrum. The mass-losing stars showed absorption shortward shifted components in the Mg II, Fe II, C II, Si II and Al II lines. Moreover these components are variable, as well as the terminal velocity measured from the violet edge of the resonance Mg II lines. Contrary to the relation found by Abbot (1978) for radiatively driven winds in OB supergiants, Talavera & Gómez de Castro (1987) found that the measured terminal velocity in A-supergiants decreases as the escape velocity increases. Therefore the radiation driven wind theory seems to be not applicable to A-supergiants. However, the latest theoretical progress on this point (Achmad et al. 1997; McCarthy et al. 1997; Kudritzki et al. 1997) has begun to clarify such discrepancies in the radiatively driven wind theory frame.

Mass loss from early type stars is due to radiation pressure on UV lines, although a detailed comparison of the predicted and observed mass loss rates of OB stars shows systematic differences. Such a comparison has not been made for A-supergiants. Moreover for the few studied, the mass loss rates derived from the UV lines are significantly lower, by a factor 10^-1 to 10^-2, than the rates derived from H$\alpha$ (Praderie et al. 1980; Hensberge et al. 1982; Kunasz & Morrison 1982). Another source of discrepancy is the problem of the superionization of the wind of early-type stars (Lamers & Snow 1978; Cassinelli et al. 1978; Groenewegen & Lamers 1991; Owocki 1992). There is as yet no study of the presence or absence of superionization in the winds of A supergiants.

We have selected 41 A-type supergiants to carry out a systematic study of stellar wind indicators in these stars in the visible and ultraviolet. In this atlas we show the profiles of the H$\alpha$, H$\beta$, H$\gamma$, H$\delta$, H$\epsilon$ Balmer lines, the Ca II H and K lines, the Na I D lines, Mg II₄₄₈₁, Mg II [uv1] and Fe II [uv1, uv2, uv3, uv62, uv63, uv161] for the program stars, as well as some relevant examples of the variations in the profiles.

The A-supergiants sample is presented in Sect. 2. The observations and the data reduction techniques are described in Sect. 3. The spectral atlas is introduced in Sect. 4 and a brief summary is provided in Sect. 5.

A detailed analysis of these data and their implications for the understanding of A-type supergiants will be published in a series of forthcoming papers (Verdugo et al. 1999).