Hubble space telescope STIS observations of the Wolf-Rayet star HD 5980 in the small magellanic cloud. II. The interstellar medium components

Abstract

Observations of the interstellar and circumstellar absorption components obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph (STIS) along the line of sight toward the Wolf-Rayet-luminous blue variable (LBV) system HD 5980 in the Small Magellanic Cloud are analyzed. Velocity components from C I, C I*, C II, C II*, C IV, N I, N V, O I, Mg II, Al II, Si II, Si II*, Si III, Si IV, S II, S III, Fe II, Ni II, Be I, Cl I, and CO are identified, and column densities estimated. The principal velocity systems in our data are (1) interstellar medium (ISM) components in the Galactic disk and halo (Vhel = 1.1 ± 3, 9 ± 2 km s-1); (2) ISM components in the SMC (Vhel = + 87 ± 6, + 110 ± 6, + 132 ± 6, + 158 ± 8, + 203 ± 15 km s-1); (3) SMC supernova remnant SNR 0057-7226 components (Vhel = + 312 ± 3, + 343 ± 3, + 33, +64 km s-1); (4) circumstellar (CS) velocity systems (Vhel = -1020, -840, -630, -530, -300 km s-1); and (5) a possible system at -53 ± 5 km s-1 (seen only in some of the Si II lines and marginally in Fe II) of uncertain origin. The supernova remnant SNR 0057-7226 has a systemic velocity of + 188 km s-1, suggesting that its progenitor was a member of the NGC 346 cluster. Our data allow estimates to be made of Te ∼ 40,000 K, ne ∼ 100 cm-3, N(H) ∼ (4-12) × 1018 cm-2 and a total mass between 400 and 1000 M⊙ for the supernova remnant (SNR) shell. We detect C I absorption lines primarily in the + 132 and + 158 km s-1 SMC velocity systems. As a result of the LBV-type eruptions in HD 5980, a fast-wind/slow-wind circumstellar interaction region has appeared, constituting the earliest formation stages of a windblown H II bubble surrounding this system. Variations over a timescale of 1 year in this circumstellar structure are detected.