Abstract
Combining 3D smoothed-particle hydrodynamics and Monte Carlo radiative transfer calculations, we examine the morphology of spiral density waves induced by gravitational instability (GI) in protoplanetary disks, as they would appear in direct images at near-infrared (NIR) wavelengths. We find that systems with disk-to-star-mass ratios q=M_disk/M_star that are ~0.25 or more may produce prominent spiral arms in NIR imaging, remarkably resembling features observed in the MWC 758 and SAO 206462 systems. The contrast of GI-induced arms at NIR wavelengths can reach a factor of ~3, and their pitch angles are about 10-15 degree. The dominant azimuthal wavenumber of GI-induced spiral arms roughly obeys m~1/q in the range 2<~1/q<~8. In particular, a massive disk with q~0.5 can exhibit grand-design m=2 spirals. GI-induced arms are in approximate corotation with the local disk, and may therefore trap dust particles by pressure drag. Although GI can produce NIR spiral arms with morphologies, contrasts, and pitch angles similar to those reported in recent observations, it also makes other demands that may or may not be satisfied in any given system. A GI origin requires that the spirals be relatively compact, on scales <~100 AU; that the disk be massive, q>~0.25; and that the accretion rate Mdot be high, on the order of 1e-6 solar mass per year.
Highlights
In the past few years, direct imaging at near-infrared (NIR) wavelengths has revealed a panoply of fine features in gaseous protoplanetary disks, thanks to the commissioning of several major high contrast, high angular resolution, adaptive optics imaging instruments onboard 8-m class telescopes (e.g., Subaru/HiCIAO, Suzuki et al 2010; Very Large Telescope (VLT)/SPHERE, Beuzit et al 2008; Gemini/GPI, Macintosh et al 2008)
We focus on the observational signatures of gravitational instability (GI)-induced density waves in NIR imaging observations, as predicted by 3D smoothed-particle hydrodynamics (SPH) + Monte Carlo radiative transfer (MCRT) simulations
We ask whether GI-induced features can resemble the observed spiral arms. This general topic has been explored by Dipierro et al (2015a), who studied the evolution of dust in a fixed gas disk background obtained from the Lodato & Rice (2004) SPH GI simulations
Summary
In the past few years, direct imaging at near-infrared (NIR) wavelengths has revealed a panoply of fine features in gaseous protoplanetary disks, thanks to the commissioning of several major high contrast, high angular resolution, adaptive optics imaging instruments onboard 8-m class telescopes (e.g., Subaru/HiCIAO, Suzuki et al 2010; Very Large Telescope (VLT)/SPHERE, Beuzit et al 2008; Gemini/GPI, Macintosh et al 2008). We ask whether GI-induced features can resemble the observed spiral arms This general topic has been explored by Dipierro et al (2015a), who studied the evolution of dust in a fixed gas disk background obtained from the Lodato & Rice (2004) SPH GI simulations. They calculated 2D dust surface density distributions, which were puffed up in the vertical direction by a parametrized gaussian profile and fed into a 3D MCRT code to produce synthetic images. All simulations are run for about 3500 years, or ∼10 orbits at 50 AU where the spiral arms are most prominent
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