Isolating Dust and Free-Free Emission in ONC Proplyds with ALMA Band 3 Observations

TL;DR

This study uses high-resolution ALMA Band 3 observations to spatially separate dust and free-free emissions in ONC proplyds, revealing their small, optically thick disks and estimating their photoevaporative mass-loss rates, addressing the proplyd lifetime problem.

Contribution

First spatially isolates dust and free-free emissions in ONC proplyds at 3.1 mm, providing new insights into disk properties and photoevaporation processes.

Findings

Disks are small (6.4–38 au) and likely truncated by photoevaporation.

Disks exhibit low spectral indices, indicating optically thick dust emission.

Mass-loss rates range from 0.6 to 18.4 × 10^{-7} M_sun/yr, shorter than ONC age.

Abstract

The Orion Nebula Cluster (ONC) hosts protoplanetary disks experiencing external photoevaporation by the cluster's intense UV field. These ``proplyds" are comprised of a disk surrounded by an ionization front. We present ALMA Band 3 (3.1 mm) continuum observations of 12 proplyds. Thermal emission from the dust disks and free-free emission from the ionization fronts are both detected, and the high-resolution (0.057") of the observations allows us to spatially isolate these two components. The morphology is unique compared to images at shorter (sub)millimeter wavelengths, which only detect the disks, and images at longer centimeter wavelengths, which only detect the ionization fronts. The disks are small ($r_d$ = 6.4--38 au), likely due to truncation by ongoing photoevaporation. They have low spectral indices ($\alpha \lesssim 2.1$) measured between Bands 7 and 3, suggesting the dust emission is optically thick. They harbor tens of Earth masses of dust as computed from the millimeter flux using the standard method, although their true masses may be larger due to the high optical depth. We derive their photoevaporative mass-loss rates in two ways: first, by invoking ionization equilibrium, and second using the brightness of the free-free emission to compute the density of the outflow. We find decent agreement between these measurements and $\dot M$ = 0.6--18.4 $\times$ 10$^{-7}$ $M_\odot$ yr$^{-1}$. The photoevaporation timescales are generally shorter than the $\sim$1 Myr age of the ONC, underscoring the known ``proplyd lifetime problem." Disk masses that are underestimated due to being optically thick remains one explanation to ease this discrepancy.