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Astron. Astrophys. 338, 933-946 (1998)

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1. Introduction

In AM Herculis stars (see Warner 1995 for a monograph), three main sources of ultraviolet (UV) emission are present: the white dwarf photosphere, the illuminated accretion stream, and the heated secondary star. While the white dwarf contributes mostly in the continuum, the accretion stream and the heated atmosphere of the secondary star are sources of strong emission lines. Phase-resolved UV spectroscopy can, therefore, reveal details of the temperature structure on the white dwarf surface as well as kinematic information of the various emission regions within the binary system.

In a previous paper, we have reported phase-resolved UV observations of AM Her, obtained with IUE during both low and high states (Gänsicke et al. 1995; hereafter Paper 1). During both states, a modulation of the UV continuum flux, peaking at the phase of maximum hard X-ray flux, was detected. In the low state, this flux modulation is accompanied by an orbital variation of the broad [FORMULA] absorption profile. The modulation of both continuum flux and [FORMULA] absorption width can be explained with a moderately hot spot near the main accretion pole on the white dwarf. The spot temperatures estimated from the IUE data were [FORMULA] K and [FORMULA] K in the low state and the high state, respectively, with the spot covering [FORMULA] of the white dwarf surface. The unheated regions of the white dwarf have [FORMULA] K (Heise & Verbunt 1988; Paper 1). Considering that the sum of the observed hard X-ray flux and cyclotron emission roughly equals the UV excess flux of the spot, we concluded that irradiation by emission from the hot post-shock plasma is the most probable cause for the heating of the spot. A puzzling result from ORFEUS-I FUV observations of AM Her in high state was the absence of [FORMULA] and [FORMULA] absorption lines (Raymond et al. 1995). The poor resolution of the IUE data gave only limited evidence for the presence of a [FORMULA] absorption line from the white dwarf photosphere during the high state, so that a full test of the hot spot-hypothesis had to await dedicated HST observations.

A general spectroscopic characteristic of polars in their high states are complexly structured emission lines. At least two components, a broad and a narrow one, can be identified. The common belief is that the broad component originates in the stream while the narrow component arises from the irradiated face of the secondary (e.g. Liebert & Stockman 1985). A beautiful example where three different line components can be discerned and identified with the secondary star, and the free-fall and magnetic coupled parts of the accretion stream is HU Aqr (Schwope et al. 1997). Doppler tomography reveals that the narrow line emission from the secondary in HU Aqr is asymmetric , probably due to shielding of the leading hemisphere by the accretion stream/curtain. In AM Her, broad and narrow components have been detected in various optical emission lines. Discussion has been stimulated by the fact that the individual lines differ in their radial velocity amplitudes: e.g. [FORMULA] km s-1 in He II, [FORMULA] km s-1 in He I (Greenstein et al. 1979) and [FORMULA] km s-1 in Ca I (Young & Schneider 1979). Absorption lines show even larger velocities: [FORMULA] km s-1 in Na I (Southwell et al. 1995). Hence, the narrow emission lines and the absorption lines originate on different parts of the secondary star, and interpreting their radial velocities as the [FORMULA] velocity amplitude of the secondary star is ambiguous.

In this paper, we report the first high-resolution phase-resolved UV spectra of AM Herculis obtained with the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope (HST).

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998