Open AccessUV spectroscopy of the peculiar binary V Sagittae in a low state
Author(s) -
Wood Janet H.,
Lockley J. J.
Publication year - 2000
Publication title -
monthly notices of the royal astronomical society
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.058
H-Index - 383
eISSN - 1365-2966
pISSN - 0035-8711
DOI - 10.1046/j.1365-8711.2000.03276.x
Subject(s) - physics , astrophysics , eclipse , emission spectrum , spectral line , orbital period , equivalent width , line (geometry) , spectroscopy , absorption spectroscopy , light curve , orbital plane , radial velocity , astronomy , stars , optics , geometry , mathematics
In this paper we present the first phase‐resolved, high‐resolution, UV spectroscopy of V Sge. HST GHRS observations were obtained covering 1.5 orbital cycles of V Sge in a low state. We covered the He ii 1640 line and the C iv 1550 doublet alternately, and also obtained one spectrum covering Mg ii 2800. We create continuum and line flux light curves for the He ii and C iv spectra. A secular decrease in continuum and line strength occurs throughout our observation. The continuum light curves show a deep primary eclipse but no obvious secondary eclipse. The line fluxes show no significant changes during primary or secondary eclipses. The He ii line shows dramatic orbital variations in shape, but it is always purely an emission line, i.e., it shows no sign of a P Cygni profile. The C iv doublet shows a P Cygni profile in which the absorption vanishes at primary mid‐eclipse. This locates the source of the P Cygni line in the primary star wind. The maximum velocity of absorption in the P Cygni profile is 700 km s −1 , much smaller than the HWZI of the C iv emission which is 1500 km s −1 . We attribute this to the presence of a broad C iv emission component underlying the primary star P Cygni profile. The C iv line also varies with orbital phase. In particular, the velocity at the blue edge of the absorption varies between −700 and −250 km s −1 as a function of orbital phase. The red and blue peaks of the profile also vary around the orbit, with the red peak at maximum strength near phases 0.5–0.7. The blue peak is at maximum strength near phases 0.1–0.2. We can explain the detailed behaviour of the He ii and C iv line profiles in the context of a colliding wind model. Here the winds from each star collide, forming a shock which, due to the rapid rotation of the binary, is wrapped around both stars. The C iv P Cygni line comes from the primary star wind, which has a terminal velocity of ∼700 km s −1 . We attribute the He ii line and the broad C iv emission to a fast, optically thin wind from the secondary star. This wind has a terminal velocity of ∼1500 km s −1 . Interstellar lines due to C i , C i *, C iv , Mg ii and P ii are detected. Column densities are measured for C i and P ii . The C i column density is used to estimate the hydrogen column density. We find that n H is of order 10 19 atom cm −2 .