TY - JOUR
T1 - Why circumstellar disks are so faint in scattered light
T2 - The case of HD 100546
AU - Mulders, G. D.
AU - Min, M.
AU - Dominik, C.
AU - Debes, J. H.
AU - Schneider, G.
N1 - Funding Information:
This research project is financially supported by a joint grant from the Netherlands Research School for Astronomy (NOVA) and the Netherlands Institute for Space Research (SRON). The research is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy (AURA), Inc., under NASA contract NAS5-26555. These observations are associated with program #s 10167, 9295, and 9987. Support for program # 10167 was provided by NASA through a grant from STScI.
PY - 2013
Y1 - 2013
N2 - Context. Scattered light images of circumstellar disks play an important role in characterizing the planet forming environments around young stars. The characteristic size of the scattering dust grains can be estimated from the observed brightness asymmetry between the near and far side of the disk, for example using standard Mie theory. Such models, however, often overpredict the brightness of the disk by one or two orders of magnitude, and have difficulty explaining very red disk colors. Aims. We aim to develop a dust model that simultaneously explains the observed disk surface brightness, colors, and asymmetry in scattered light, focusing on constraining grain sizes. Methods. We use the 2D radiative transfer code MCMax with anisotropic scattering to explore the effects of grain size on synthetic scattered light images of circumstellar disks. We compare the results with surface brightness profiles of the protoplanetary disk HD 100546 in scattered light at wavelengths from 0.4 to 2.2 microns. Results. We find that extreme forward scattering by micron-sized particles lowers the effective dust albedo and creates a faint, red disk that appears only slightly forward scattering. For the outer (3;100 AU) disk of HD 100546 we derive a minimum grain size of 2.5 microns, likely present in the form of aggregates. Intermediate-sized grains are too bright, whereas smaller grains are faint and scatter more isotropically, but also produce disk colors that are too blue. Conclusions. Observed surface brightness asymmetries alone are not sufficient to constrain the grain size in circumstellar disks. Additional information, such as the brightness and colors of the disk, are needed to provide additional constraints.
AB - Context. Scattered light images of circumstellar disks play an important role in characterizing the planet forming environments around young stars. The characteristic size of the scattering dust grains can be estimated from the observed brightness asymmetry between the near and far side of the disk, for example using standard Mie theory. Such models, however, often overpredict the brightness of the disk by one or two orders of magnitude, and have difficulty explaining very red disk colors. Aims. We aim to develop a dust model that simultaneously explains the observed disk surface brightness, colors, and asymmetry in scattered light, focusing on constraining grain sizes. Methods. We use the 2D radiative transfer code MCMax with anisotropic scattering to explore the effects of grain size on synthetic scattered light images of circumstellar disks. We compare the results with surface brightness profiles of the protoplanetary disk HD 100546 in scattered light at wavelengths from 0.4 to 2.2 microns. Results. We find that extreme forward scattering by micron-sized particles lowers the effective dust albedo and creates a faint, red disk that appears only slightly forward scattering. For the outer (3;100 AU) disk of HD 100546 we derive a minimum grain size of 2.5 microns, likely present in the form of aggregates. Intermediate-sized grains are too bright, whereas smaller grains are faint and scatter more isotropically, but also produce disk colors that are too blue. Conclusions. Observed surface brightness asymmetries alone are not sufficient to constrain the grain size in circumstellar disks. Additional information, such as the brightness and colors of the disk, are needed to provide additional constraints.
KW - Circumstellar matter
KW - Protoplanetary disks
KW - Radiative transfer
KW - Scattering
KW - Stars: individual: HD 100546
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U2 - 10.1051/0004-6361/201219522
DO - 10.1051/0004-6361/201219522
M3 - Article
AN - SCOPUS:84872068049
SN - 0004-6361
VL - 549
JO - Astronomy and astrophysics
JF - Astronomy and astrophysics
M1 - A112
ER -