Properties of galactic B[e] supergiants V. Two-dimensional radiative transfer model of RY Sct and its dusty disc

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 T[subscript *] = 27 000 K for both components and the distance D = 1.8 kpc, we derive the total luminosity L[subscript *] = 4.2 × 10[superscript 5] L[subscript circled dot]. The optically thin dusty disc ([tau][subscript V] [approximately equal to] 0.04 in the equatorial plane) extends from its inner boundary at R[subscript 1] = 60 AU to the distances of R[subscript 2] [approximately equal to] 10[superscript 5] 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 (A[subscript V] = 4.5 mag) that defines its spectral energy distribution in the ultraviolet, optical and near infrared. The disc has a full opening angle [psi] = 26[degrees symbol] and we observe it at a viewing angle [theta][subscript v] = 14[degrees symbol] from its midplane (inclination i = 76[degrees symbol]). 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 1'' from the star (r < 1800 AU) is m[subscript d] = 3.2 × 10[superscript −7] M[subscript circled dot], 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 M[subscript d] = 9 × 10[superscript −6] M[subscript circled dot] and gas mass M = 0.017 M[subscript circled dot] of the circumbinary material is contained in the outer, old wind at 1800 < r < 10[superscript 5] 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.