A comparative study of the ultraviolet spectrum of subdwarfs

The high and low resolution IUE spectra of four O subdwarfs (BD +75°325, BD +48°1777, BD +37°1977, BD +37°442) are analyzed. Line identification is given for the most abundant ions in the far ultraviolet. Fe V is by far the ion with the largest number of lines. Broad P Cygni profiles of the N V and C IV resonance doublets are observed in BD +37°442 and BD +37°1977, with terminal velocities of 2200 and 2300 km s-1 respectively. The N IV 1718 Å line presents an extended blue wing also arising in the winds of these stars. A low interstellar extinction (E(B-V) ≤ 0.05) is derived from the weakness of the 2200 Å i.s. band, and from the Lyα profile. Mean radial velocities are given for the photospheric and interstellar atomic species, and a high stellar velocity is found for the four stars, in agreement with the Population II nature of these objects.