Volume 522, November 2010
|Number of page(s)||10|
|Section||Interstellar and circumstellar matter|
|Published online||29 October 2010|
Radiative transfer modeling of the dust disk of the Herbig Be star R Monocerotis
Max-Planck-Institut für Radioastronomie,
Auf dem Hügel 69,
Accepted: 8 July 2010
Our previous near-infrared (NIR), high-resolution polarimetric images of the Herbig Be star R Mon show that the polarization disk has a large, nearly constant angular size of ~4″ in the JHK bands, low polarizations of 1.1–1.5% in the H and K bands, and somewhat centro-symmetrical vector alignment. To derive the disk geometry parameters and grain sizes that explain this result, we modeled R Mon’s dust disk by combining it with previously obtained spectral energy distribution and gas phase observations. Our model assumes an inner disk and outer envelope structure, dust of different populations for the disk and envelope, and a power-law size distribution of with 0.005 μm ≤ a ≤ amax μm. We obtained an outer disk radius of 3000 AU, a ratio of the height to the radius of the disk of H0 = 0.4, a V-band optical depth of 4.2 in the disk midplane, and an amax of 1000.0 μm for the disk. Such a large extension of the disk is similar to the ones of the central feature detected in previous observations in the CO emission lines (1500 AU) and the 2.7 mm continuum (3000 AU). The shape and angular size of the polarization disk in our model fit the observations and approximately trace the projected appearance of the real disk. Since the estimated dust disk height has an intermediate value and is lower than the gas phase (CO) disk height (H0 = 1.0), it is likely that the dust settling is ongoing. Our selected model with a large dust size with an amax of 1000.0 μm in the inner part of the disk and a disk mass of 0.02 M⊙ fit the near-infrared polarization towards the disk and the mid-infrared to millimeter fluxes well. Our estimated disk mass roughly agrees with a previous gas phase modeling of ~0.01 M⊙. Such a large grain model reproduces low NIR polarizations (1.6%), which is consistent with our observation. Not detecting the vector alignment is explained with this large grain model and an intermediate optical depth of the disk. We expect that R Mon’s disk contrasts to the typical T Tauri and Herbig Ae disks in terms of an expected large outer disk radius and an intermediate optical depth. Evidence of the dust settling and large grains suggests that the dust in R Mon’s disk grows faster than the disk accretion advances, which is theoretically predicted in typical T Tauri disks.
Key words: circumstellar matter / reflection nebulae / infrared: ISM
© ESO, 2010
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