Abundance Determinations from the IUE Spectra of α Draconis. I. Carbon and Aluminum

Using coadded IUE high resolution spectra, abundances are derived for carbon and aluminum, which have few lines in the optical spectrum of the bright spectroscopic binary α Draconis. The derived abundances agree well with those derived from the synthesis of a high resolution high signal-to-noise SOPHIE spectrum in the optical range and reinforce the metal-poor status of α Dra.

The bright northern spectroscopic binary α Dra (HR 5291, A0 III C, V = 3.68) has been extensively studied. α Dra has been observed on several occasions with the International Ultraviolet Explorer but most spectra have remained unpublished. A survey of these spectra has recently been published by Monier (2021) and indicates that α Dra might be variable at wavelengths shorter than 2000 Å. The most recent published abundance determination for α Dra was carried out by Adelman et al. (2001) from optical spectra. They found slightly subsolar abundances for most elements in the atmosphere of α Dra.

In the temperature regime of the late B-type stars, a few chemical elements only have strong lines in the ultraviolet range. This is in particular true for aluminum, phosphorus and gallium. The light elements C, N and O also have strong transitions in the far-ultraviolet at wavelengths shorter than 2000 Å.

The purpose of this note is to analyze all the spectra of α Dra taken with the IUE and derive elemental abundances of aluminum and carbon which have few lines in the optical range. The ultraviolet lines of C i and Al ii analyzed here are low-lying levels transitions and therefore are strong easily recognizable lines. Smith & Dworetsky (1993) have shown that reliable abundances can be derived from coadded IUE high resolution spectra.

A mean far-UV spectrum of α Dra has been constructed using all high resolution SWP (Short Wavelength Prime camera observing from 1200 up to 2000 Å at a resolving power of 20000) and shifting them onto a common wavelength scale. The far-ultraviolet spectrum of α Dra is crowded with absorption lines, abundances can only be derived using spectral synthesis. I am using the following fundamental parameters: T_eff = 9970 ± 250 K and $\mathrm{log}\ g$ = 3.60 ± 0.25 dex for α Dra taken from Monier (2021) for the spectrum synthesis. They agree well with those chosen by Adelman et al. (2001). A model atmosphere was then computed for these parameters using ATLAS9 (Kurucz 1992) assuming local thermodynamical equilibrium, hydrostatic equilibrium and radiative equilibrium. The adjustment of a series of synthetic spectra conputed with SYNSPEC49 (Hubeny & Lanz 1992) to the observed coadded spectrum yields the abundance of aluminum and carbon for α Draconis. The linelist used is gfall08Oct17.dat retrieved fom B. Kurucz's site, ¹ it includes the the hyperfine structure of the various isotopes for several chemical elements (Kurucz 2018). The Al ii lines modeled are the lines of the UV Multiplet 6 at 1719.44 Å and the resonance line of Al ii at 1670.79 Å and the low excitation multiplet of C i between 1656 Å and 1658 Å. The initial abundances for the line synthesis are those derived from an unpublished synthesis of unblended optical lines in a SOPHIE spectrum ² of α Dra (the instrument paper of SOPHIE is Perruchot et al. 2011) . This analysis reveals underabundances for most metals which reinforces the metal-poor status of α Dra proposed by Adelman et al. (2001).

In Figure 1, the mean IUE high resolution spectrum of α Dra is compared with the final synthetic spectrum providing the best fit to the low excitation multiplet of C i and the resonance line of Al ii at 1670.79 Å. The measured aluminum abundance is about solar with an uncertainty of ±0.20 dex, for this line and the other Al ii lines. The carbon abundance is slightly sub-solar, about 70% of the solar value which agrees well with the carbon abundance measured from optical high resolution high signal-to-noise SOPHIE spectra using only one line of C ii at 4663.06 Å.

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The abundances derived from the co-added IUE spectrum confirm the solar abundance of aluminum and the underabundance of carbon of α Dra derived from recent SOPHIE spectra. These new determinations differ slightly from those of Adelman et al. (2001) who found an underabundance of aluminum. The abundances of iron and chromium which have several lines in the interval analyzed here are also slightly subsolar (about 70% the solar value). They reinforce the metal-poor status of α Dra.