4. Paschen lines of hydrogen
The infrared spectral region is distorted by strong lines of the hydrogen Paschen series. A spectrum of Vega (Babel, private communication) recorded at OHP with the Aurélie spectrograph (Gillet et al. 1994) at a resolution of 30 000 covers the four Paschen lines P13 to P16. It allows a detailed comparison to be made with calculated line profiles.
In an earlier work (Rentzsch-Holm 1996) the opacity of the Paschen lines was calculated using the theory implemented in ATLAS9 (Kurucz 1970). This theory is valid only for the wings of high-series members and should not be used to calculate detailed line profiles for comparison with observation. A comparison of the synthetic spectrum computed with an ATLAS9 model atmosphere (9500, 3.90, -0.5) with the observed spectrum (Fig. 1 ) shows that the theoretical profiles (dashed line) are too strong.
The `unified theory' of line broadening generates normalized Stark profiles for hydrogen lines (Vidal et al. 1970, 1971). Using their program (Vidal et al. 1973) Lemke calculated tables (analogous to the VCS tables of the Balmer lines) for the first 17 Paschen lines of hydrogen covering electron densities from to and temperatures between 2500 and 80000 K. A synthetic spectrum computed by linearly interpolation in those tables fits the observational data (Fig. 1 , dashed-dotted line) adequately. The remaining differences in the line cores vanish if the gravity is reduced by 0.10 dex.
A detailed comparison between the profiles of Vidal et al. (1973), and the recently calculated Stark profiles of Stehlé (1994) using the Model Microfield Method, reveals significant differences in the line cores which diminish or disappear when Doppler broadening dominates (Clausset et al. 1994). Profiles for the higher members of the Paschen series have not yet been given in the literature.
To synthesize P12 and P13 the Paschen lines needed in this work, Stark profiles are taken from the tables of Lemke. For all program stars synthetic spectra are computed assuming solar nitrogen and sulphur abundances, 8.05 (Rentzsch-Holm 1996) and 7.21 (Anders & Grevesse 1989) respectively. The observed spectra are then normalized to the synthetic ones to obtain the correct continuum placement, as shown in Fig. 2 .
© European Southern Observatory (ESO) 1997