Photoevaporation and spatial variation of grain sizes in Orion 114-426

TL;DR

This study uses high-resolution HST images to analyze the shape, dust grain size distribution, and photoevaporative processes of the Orion 114-426 proplyd, revealing a gradient in grain size and a rapid disk dissipation timescale.

Contribution

It provides the first detailed spatial mapping of dust grain sizes and morphology of Orion 114-426, highlighting the effects of photoevaporation and environmental shielding.

Findings

Dust grain size decreases with distance from the disk center.

The disk is rapidly dissipating with an estimated timescale of about 10,000 years.

The system is on the verge of destruction due to photoevaporation.

Abstract

Deep HST broad-band images taken with ACS and WFPC2 of the giant ($\sim 1000$ AU diameter) dark silhouette proplyd 114-426 in the Orion Nebula show that this system is tilted, asymmetric, warped and photoevaporated. The exquisite angular resolution of ACS allows us to map the distribution of dust grains at the northern translucent edge of the disk, dominated by the photoevaporative flow. Using the Mie theory for standard circumstellar disk grains, we find evidence for a spatial gradient in grain size. The typical dust radius, $\simeq 0.2-0.7 \mu$m (less than what reported by previous studies) becomes smaller as the distance from the disk center increases, consistent with the expectations for the dynamic of dust entrained in a gaseous photoevaporative wind. Our analysis of the disk morphology and location within the nebula indicates that this system is photoevaporated by the diffuse radiation field of the Orion Nebula, while being shielded from the radiation coming directly from the central Trapezium stars. We estimate the mass-loss rate from the disk surface and the time-scale for total disk dissipation, which turns out to be of the order of $10^4$yr. Such a short time, of the order of 1/100 of the cluster age, indicates that this system is seen on the verge of destruction. This is compatible with the exceptional nature of the disk, namely its combination of huge size and low mass. Finally, we briefly discuss the viability of possible mechanisms that may lead to the peculiar morphology of this system: external UV flux, binary star and past close encounter.