Dynamical trapping (pile-up) of grains near the sublimation radius Hot Dust around Main Sequence Stars? Observations, Theory and Future Prospects May 20-22, 2015. California Institute of Technology, Pasadena, CA Dynamical trapping (pile-up) of grains near the sublimation radius Hiroshi Kobayashi (Nagoya Univ.) Review: Kobayashi et al. (2009) Icarus 201, 395 Kobayashi et al. (2011) EPS 63, 1067

Hot Debris Disks Interferometric observations found excesses of near infrared emission, which indicate hot disks with T > 1000K (Ciardi et al. 2001; Absil et al. 2006, 2008, 2009; di Folco et al. 2007; Akeson et al. 2009; Defrère et al. 2011, 2012).

Sublimation Temperature H: latent heat μ: molecular weight (Kobayashi+11) The sublimation temperatures of silicates are similar to those of hot debris disks.

Orbital Evolution with Sublimation Stage I, a grain spirals in due to P-R drag. Stage II, sublimation downsize the grain, resulting in stalling drift. Balance between P-R drag and increase of radiation pressure. Stage III, the grain is blown out. (Kobayashi+09)

Pile-up Obsidian, Solar Dust grains are produced in a planetesimal belt. Dust drifts inward due to P-R drag. Sublimation piles up grains. The inner cavity is formed via sublimation of silicates for T > 1000-1500K. ns(r) / ns(rout) (Kobayashi+08,09)

Stellar Luminosity Sublimation pile-up is more effective around luminous stars. Sublimation distance increases with stellar luminosity. Sublimation temperature is independent of stellar types. τ (Kobayashi+09)

Temperature, Distance, & Pile-up (Kobayashi+11) We could analytically derive sublimation temperature, distance, and enhancement factor.

The enhancement factor depends on orbital eccentricity. (Kobayashi+11) The enhancement factor depends on orbital eccentricity. An outer planetesimal belt results in pile-up, while little pile up for an inner dust source.

SED Vega w/o pile-up, olivine pyroxene w/ pile-up, olivine The enhancement due to pile-up does not result in significant difference. If we can arbitrarily chose the amount of drifting grains, sublimation cut-off may account for the SED of hot disks. Pyroxene can better explain the SED of Vega. w/ pile-up, olivine ⚫︎◇ observation (Absil+06) Olivine: 1300K Pyroxene: 1700K (Kobayashi+11)

Amount of drifting grains Sublimation is effective only when sufficient dust grains drift into the sublimation zone. For Vega, τ ~ 4 x 10-4 with pyroxene. However, the amount of grains are limited by collisions: tcol ~1/ τ Ω ~ tPR　(Wyatt 05). For Vega, τ ~ 3 x 10-4 if dust source is located at ~0.5AU. Collisional and orbital evolution with drag is necessary to determine τ accurately.

Summary Refractory dust grains drift inward by P-R drag. They sublime in the vicinity of host stars. Sublimation stalls P-R drift and piles up dust grains. Dust grains are eventually blown out. Debris disks have cavities in the sublimation zone. The cavity is important for SED but the pile-up is not. We need mass and spatial evolution of grains including collisions, P-R drag, & sublimation.