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Astron. Astrophys. 350, 517-528 (1999)

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3. The UV spectrum of FO Aqr

The grand average UV spectra of FO Aqr as observed with FOS and IUE are shown in Fig. 1 (upper panel), where the above flux difference is apparent.

The UV luminosity in the 1150-3200 Å IUE range is [FORMULA] ergs s-1 assuming a distance of 325 pc (Paper 1), a factor 1.3 larger than during previous UV observations in 1990. The optical photometry also indicates a brightening of 0.17 mag between the two epochs, indicating long term luminosity variations (see also Sect. 8.5)

The UV spectrum of FO Aqr is typical of magnetic CVs (Chiappetti et al., 1989; de Martino 1995) with strong emissions of N V [FORMULA], Si IV [FORMULA], C IV [FORMULA], He II [FORMULA] and Mg II [FORMULA]. The weaker Si IV with respect to N V emission, classes the IP nature (de Martino 1995). Weaker emissions from lower ionization states of different species such as C III [FORMULA], the blend of Si III [FORMULA] multiplet, Si II[FORMULA] and geocoronal O I[FORMULA], Si III [FORMULA], Si II [FORMULA], N IV [FORMULA], N III [FORMULA], Al III [FORMULA] and Al II [FORMULA], as well as He II [FORMULA], possibly blended with C III [FORMULA], and He II [FORMULA], are identified in the higher quality FOS spectrum. Also weak oxygen lines of O IV [FORMULA]1343 and O V [FORMULA]1371 are detected. Some of these lines are also observed in the HST/FOS spectra of AE Aqr (Eracleous & Horne 1994), DQ Her (Silber et al. 1996) and PQ Gem (Stavroyiannopoulos et al. 1997). The presence of high ionization species together with extremely weak emissions (E.W. [FORMULA] 1 Å) of lower ionization species are characteristic of a higher ionization efficiency in IPs with respect to Polars (de Martino 1998). The line ratios N V/Si IV and N V/He II, when compared with photoionization models developed by Mauche et al. (1997) are close to the predicted values for an ionizing blackbody spectrum at 30 eV.

In contrast to the IUE spectra, the FOS data allow us to finally detect the intrinsic [FORMULA] [FORMULA]1216 line. This appears to be composed of a relatively deep (E.W.=5.4[FORMULA]0.1 Å) absorption and a weak emission (E.W.=1.4[FORMULA]0.1 Å). The center wavelength of the absorption feature is however red-shifted by [FORMULA]4 Å with respect to rest wavelength and other emission line positions, while the weak emission is blue-shifted at 1206 Å in the grand average spectrum. Discussion on the nature of this feature is left until Sects. 5 and 7, however the [FORMULA] absorption provides an upper limit to the hydrogen column density along the line of sight to FO Aqr.

A pure damping Lorentzian profile (Bohlin 1975) convolved with a 7 Å FWHM Gaussian has then been fitted to the [FORMULA] absorption line (Fig. 1, bottom panel). The resulting neutral hydrogen column density is [FORMULA]. The residual from the fit shows an emission line with maximum flux at [FORMULA] Å , probably geocoronal or intrinsic, with an excess of flux in the blue wing possibly due to emission of Si III [FORMULA]1206.

The derived value for [FORMULA] is consistent with the total interstellar column density in the direction of FO Aqr as derived from Dickey & Lockman (1990) and with the upper limit estimated from X-rays (Mukai et al. 1994). Assuming an average gas-to-dust ratio (Shull & van Steenberg 1985), this upper limit corresponds to a reddening of [FORMULA]. Although FO Aqr was already known to be negligibly reddened from IUE observations (Chiappetti et al. 1989), an upper limit [FORMULA] 0.005 is derived from the absence of the 2200 Å absorption in the FOS data. This indicates that, despite the coincidence, most of the neutral absorption is unrelated to the interstellar dust and hence it is likely located within the binary system.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999