Accretion Disc Atmospheres

Details Accretion Disc Atmospheres Accretion Disc Atmospheres

Nov 1990

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1990mnras.247...43m&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Vol. 247, NO. 1/NOV1, P. 43, 1990

Astronomy and Astrophysics

Astronomy

28

Scientific paper

The thermal structure of accretion discs in dwarf novae and soft X-ray transients is studied using frequency-dependent opacities. First, we calculate thermal equilibrium curves using the one-zone approximation for the vertical structure of the disc. Three thermally stable branches are obtained: the normal (or higher ionization) branch (Tc ≥ 10000 K and Te ≥-8000 K) the neutral branch (Tc ≤ 5000 K and Te ≤ 5000 K), and the lower (partial) ionization branch (Tc ˜6000 K and Te <1000 K), where Te and Te denote the effective temperature and the central temperature, respectively. These stable branches are connected by two unstable branches. When the radius is small, e.g., log R(cm) ≤ 7.0, or viscosity parameter, α, is high, e.g., α > 1, the neutral branch disappears.
Next we calculate the vertical structure of optically thin atmospheres above the discs. We find that, even in the absence of external heating sources, hydrogen can be partially ionized in the atmosphere due to the viscous heating there. For constant α vertically in the disc, cool (Te ≤ 5000 K) discs are thus covered by chromospheres with temperatures of ˜6000 K, with a temperature inversion in the atmospheres. In addition, the disc will have a hot (T ≥ 106 K) corona or wind. We conclude that emission lines from dwarf novae and the soft X-ray transient, A0620-00, in quiescence could be generated in disc chromospheres and coronae, and further suggest the possibility that our mechanism may play a certain role in the broad-line region (BLR) of active galactic nuclei (AGN) and classical T Tauri stars.

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