Publications of the MPIfR
Optical & Infrared
Interferometry Group
Men'shchikov, A.B. and Miroshnichenko,
A.S.
Properties of galactic B[e] supergiants. V.
Two-dimensional radiative transfer model of RY Sct and its dusty disc.
A&A, 443, Issue 1, pg.211-222 (2005)
Abstract
We present results of the first two-dimensional radiative transfer
modelling of the eclipsing binary RY Sct and its
dusty disc. Assuming an effective temperature Tstar = 27 000
K for both components and the distance D = 1.8 kpc, we derive the total
luminosity Lstar = 4.2 × 105
L⊙. The optically thin dusty disc (τV
≈ 0.04 in the equatorial plane) extends from its inner boundary at
R1 = 60 AU to the distances of R2 ≈
105 AU, where it blends into the interstellar medium. The
very high energy output of the supergiants heats up the interstellar
dust, well beyond the outer boundary, to temperatures of 100 K. It is
the large interstellar extinction towards RY Sct
(AV = 4.5 mag) that defines its spectral energy distribution
in the ultraviolet, optical and near infrared. The disc has a full
opening angle {ψ} = 26° and we observe it at a viewing angle
θv = 14° from its midplane (inclination i =
76°). There is a strong density enhancement in the disc within a
narrow ring at r = 1500 AU, that emits most of the infrared flux and is
prominent in Keck telescope images (Gehrz et al. 2001, ApJ, 559, 395).
The dust mass contained in the disc within 1́́ from the star
(r < 1800 AU) is md = 3.2 × 10-7
M⊙, by a factor of 3 lower than in previous estimates.
However, in our model there is 30 times more mass in the surroundings
of the binary system than within the dense ring. As much as 95% of the
total dust mass Md = 9 × 10-6
M⊙ and gas mass M = 0.017 M⊙ of the
circumbinary material is contained in the outer, old wind at 1800 <
r
< 105 AU. Presumably the dense ring has been created by a
fast wind that swept out and compressed the previously lost material in
the older and slower stellar wind. Based on the new Keck data, our
model
predicts that presently there is a relatively large number of small,
hot
dust grains in the dust formation zone, whose emission substantially
changes the shape of the SED of RY Sct in the near
infrared. This suggests a higher mass-loss rate or dust-to-gas mass
ratio or lower wind velocity, or a combination of these factors.
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