How Bright are Planet-Induced Spiral Arms in Scattered Light?

How Bright are Planet-Induced Spiral Arms in Scattered Light?

Authors:

Dong et al

Abstract:

Recently, high angular resolution imaging instruments such as SPHERE and GPI have discovered many spiral-arm-like features in near-infrared scattered light images of protoplanetary disks. Theory and simulations have suggested that these arms are most likely excited by planets forming in the disks; however, a quantitative relation between the arm-to-disk brightness contrast and planet mass is still missing. Using 3D hydrodynamics and radiative transfer simulations, we examine the morphology and contrast of planet-induced arms in disks. We find a power-law relation for the face-on arm contrast (δmax) as a function of planet mass (Mp) and disk aspect ratio (h/r): δmax≈((Mp/MJ)/(h/r)1.38)0.22. With current observational capability, at a 30 AU separation, the minimum planet mass for driving detectable arms in a disk around a 1 Myr 1M⊙ star at 140 pc at low inclinations is around Saturn mass. For planets more massive than Neptune masses, they typically drive multiple arms. Therefore in observed disks with spirals, it is unlikely that each spiral arm originates from a different planet. We also find only massive perturbers with at least multi-Jupiter masses are capable of driving bright arms with δmax≳2 as found in SAO 206462, MWC 758, and LkHα~330, and these arms do not follow linear wave propagation theory. Additionally, we find the morphology and contrast of the primary and secondary arms are largely unaffected by a modest level of viscosity with α≲0.01. Finally, the contrast of the arms in the SAO 206462 disk suggests that the perturber SAO 206462 b at ∼100 AU is about 5−10MJ in mass.\end{abstract}

SAO 206462/HD 135344B Appears to Have a SuperJupiter Orbiting at 100 to 120 AU

PLANETARY SIGNATURES IN THE SAO 206462 (HD 135344B) DISK: A SPIRAL ARM PASSING THROUGH VORTEX?

Authors:

Bae et al

Abstract:

The disk surrounding SAO 206462, an 8 Myr old Herbig Ae star, has recently been reported to exhibit spiral arms, an asymmetric dust continuum, and a dust-depleted inner cavity. By carrying out two-dimensional, two-fluid hydrodynamic calculations, we find that a planetary-mass companion located at the outer disk could be responsible for these observed structures. In this model, the planet excites primary and secondary arms interior to its orbit. It also carves a gap and generates a local pressure bump at the inner gap edge where a vortex forms through Rossby wave instability. The vortex traps radially drifting dust particles, forming a dust-depleted cavity in the inner disk. We propose that the vortex is responsible for the brightest southwestern peak seen in infrared scattered light and sub-millimeter dust continuum emission. In particular, it is possible that the scattered light is boosted as one of the spiral arms passes through the high density vortex region, although the vortex alone may be able to explain the peak. We suggest that a planetary companion with a mass of 10–15 ${M}_{J}$ is orbiting SAO 206462 at 100–120 au. Monitoring of the brightest peak over the next few years will help reveal its origin because the spiral arms and vortex will show distinguishable displacement.

Inferring Planet Mass from Spiral Structures in Protoplanetary Disks

Inferring Planet Mass from Spiral Structures in Protoplanetary Disks

Authors:

Fung et al

Abstract:

Recent observations of protoplanetary disk have reported spiral structures that are potential signatures of embedded planets, and modeling efforts have shown that a single planet can excite multiple spiral arms, in contrast to conventional disk-planet interaction theory. Using two and three-dimensional hydrodynamics simulations to perform a systematic parameter survey, we confirm the existence of multiple spiral arms in disks with a single planet, and discover a scaling relation between the azimuthal separation of the primary and secondary arm, ϕsep, and the planet-to-star mass ratio q: ϕsep=102∘(q/0.001)0.2 for companions between Neptune mass and 16 Jupiter masses around a 1 solar mass star, and ϕsep=180∘ for brown dwarf mass companions. This relation is independent of the disk's temperature, and can be used to infer a planet's mass to within an accuracy of about 30% given only the morphology of a face-on disk. Combining hydrodynamics and Monte-Carlo radiative transfer calculations, we verify that our numerical measurements of ϕsep are accurate representations of what would be measured in near-infrared scattered light images, such as those expected to be taken by Gemini/GPI, VLT/SPHERE, or Subaru/SCExAO in the future. Finally, we are able to infer, using our scaling relation, that the planet responsible for the spiral structure in SAO 206462 has a mass of about 6 Jupiter masses.

Spiral Arms Observed in MWC 758 and SAO 206462 Systems' Protoplanetary Disks

Spiral Arms in Gravitationally Unstable Protoplanetary Disks as Imaged in Scattered Light

Authors:

Dong et al

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 2les sthan approximately 1/q less than approximately 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 less than approximately 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.