4 Line intensities

  The measured intensity of each line (in ergcm^-2s^-1) is listed in Col. (3) of Table 2. These intensities were obtained over the wavelength range 3490$\leq$$\lambda$$\leq$5530Å using the methods described in McKenna et al. ([1997]). The error in each line intensity listed in Table 2 is estimated at <15%, except where indicated by the footnotes. Outside the range 3490$\leq$$\lambda$$\leq$5530Å, the intensities were measured using recombination line intensity ratios in HeII (Hummer & Storey [1987]; Storey & Hummer [1995]). Ionised helium lines at $\lambda\lambda$3203.16, 5952.96, 5977.05, 6036.81, 6074.22, 6170.72, 6233.84, 6406.41, 6527.12 and 6683.22Å were compared to the n=4$\rightarrow$3 transition at 4685.68Å. However, the intensities that this method yields are not as accurate as those found using the low resolution spectrum, and hence errors of $\sim$20% in the intensities of the lines listed in Table 3 are expected.

The line intensities were corrected for interstellar extinction (see Col. (4) in Table 2) using the coefficients listed in Cardelli et al. ([1989]) for the U, B, V and R spectral regions. E(B-V) for RR Tel is well quantified as equal to 0.08 (Jordan et al. [1994]).

McKenna et al. ([1997]) were the first to produce an absolutely calibrated line list of the optical spectrum of RR Tel. However, Aller et al. ([1973]) list line intensities relative to H$\beta$. Thus, it was possible to carry out an approximate comparision between the intensity of RR Tel's lines in 1968 (Aller et al. [1973]), 1993 (McKenna et al. [1997]) and 1996, by scaling the line intensities in this study relative to H$\beta$. It was found that the strengths of the lines for low ionisation species (for example, HeI, [OI], FeII and OIII) are decreasing, while the line intensities of species of higher degrees of ionisation (for example, [FeVII]) are increasing (see Table 1). This is the expected result, as RR Tel's emission spectrum has been well documented as advancing towards higher excitation (Kenyon et al. [1993]; Allen [1980]).