Presentation #553.03 in the session “Hidden and Active Binaries”.
The symbiotic binary, MWC 560, contains an M5 III red-giant and a white dwarf embedded in an accretion disk with a jet oriented along our line of sight. This unusual geometry makes MWC 560 extremely valuable for modeling jet/disk systems. It also allows us to probe the absorption in the jet and to deduce the velocity and ionization structure along this jet. Four HST/STIS spectra with the E140M (R~46,000) and four E230M (R~30,000) gratings were obtained on 23 October 2019, well into the current prolonged outburst starting in 2016. This provided good S/N from 124.0 to 171.0 nm and from 200.0 to 271.0 nm. Previous IUE UV spectra with usable S/N had a resolution of R~200. Although earlier ground-based and IUE observations of prior outbursts had found short-term variability on timescales as short as ~17 minutes, our HST spectra acquired over 3.8 hours indicate no variability greater than our S/N of ~ 1% over that time-span. These spectra reveal extensive veiling from shortward-displaced absorption in transitions for abundant ionic species arising from the ground configurations and low-lying metastable levels. These features are completely consistent with a low-density absorbing gas in front of the white dwarf. The exact spectral shapes, velocity extents and quantitative analysis are often difficult because much of the absorption of the species exhibiting negative velocity-shifted features are saturated and blended, where the flux levels are at, or near, zero flux over large ranges of wavelength. The most notable observed species are Si II-IV, C II, Fe II, S II, S II, Fe II, and Al II. Many resonance multiplets show saturated absorption with zero fluxes extending from near 0 km/s up to -500 and sometimes -800 km/s as is the case for C II 133.4 and C I 133.5 nm. Some spectral features, including C II show indications of discrete absorption components up to -1100 km/s. The highest ionization observed is that of Si IV at 139.3, 140.2 nm, with displaced unsaturated absorption seen up to -750 km/s. Superposed on the broad displaced Si IV features are discrete components at -500 and -120 km/s. The strongest spectral features, such as C II 133.4 133.5 nm, and Fe II 259.9 nm indicate P Cygni profiles with emission lobes roughly 20% above the local continuum extending to approximately +200 km/s. Spectral modeling of these data is continuing. To present a more complete picture of the physical state of the MWC 560 system, we also present ground-based spectra by AAVSO observers which are contemporaneous with our HST/STIS observations.