The magnetic Bp star 36 Lyncis II . A spectroscopic analysis of its co - rotating disk

We report on the physical properties of the disk-like structure of B8 IIIp star 36 Lyncis from line syntheses of phase-resolved, high resolution spectra obtained from the International Ultraviolet Explorer archives and from newly obtained ground-based spectra of the Hα absorption profile. This disk is highly inclined to the rotational axis and betrays its existence every half rotation cycle as one of two opposing sectors pass in front of the star. Although the disk absorption spectrum is at least ten times too weak to be visible in optical iron lines during these occultations, its properties can be readily examined in a large number of UV " iron curtain " lines because of their higher opacities. The UV Fe II and Fe III lines in particular permit a determination of the disk temperature: 7,500±500 K and a column density of 3−1.5 +3 ×10 20 cm −2. The analysis of the variations of the UV resonance lines brings out some interesting details about the radiative properties of the disks: (1) they are optically thick in the C IV and Si IV doublets, (2) the range of excitation of the UV resonance lines is larger at the primary occultation (φ = 0.00) than at the secondary one, and (3) the relative strengths of the absorption peaks for the two occultations varies substantially from line to line. We have modeled the absorptions of the UV C IV resonance and Hα absorptions by means of a simulated disk with opaque and translucent components. Our simulations suggest that a gap separates the star and the inner edge of the disk. The disk extends radially out to ≥10 R *. The disk scale height perpendicular to the plane is ≈1R *. However, the sector causing the primary occultation is about four times thicker than the opposite sector. The C IV scattering region extends to a larger height than the Hα region does, probably because it results from shock heating far from the cooler disk plane.