An Old Disk That Can Still Form a Planetary System

TL;DR

This paper reports the detection of hydrogen deuteride (HD) in the TW Hya disk, providing a more accurate estimate of its mass, which is sufficient for forming a planetary system similar to our own.

Contribution

It introduces the use of HD emission as a reliable tracer for disk mass, resolving uncertainties from previous methods.

Findings

HD emission indicates a disk mass >0.05 solar masses

The disk has enough material to form a planetary system like ours

Previous estimates varied widely, but HD provides a more precise measurement.

Abstract

From the masses of planets orbiting our Sun, and relative elemental abundances, it is estimated that at birth our Solar System required a minimum disk mass of ~0.01 solar masses within ~100 AU of the star. The main constituent, gaseous molecular hydrogen, does not emit from the disk mass reservoir, so the most common measure of the disk mass is dust thermal emission and lines of gaseous carbon monoxide. Carbon monoxide emission generally probes the disk surface, while the conversion from dust emission to gas mass requires knowledge of the grain properties and gas-to-dust mass ratio, which likely differ from their interstellar values. Thus, mass estimates vary by orders of magnitude, as exemplified by the relatively old (3--10 Myr) star TW Hya, with estimates ranging from 0.0005 to 0.06 solar masses. Here we report the detection the fundamental rotational transition of hydrogen deuteride, HD, toward TW Hya. HD is a good tracer of disk gas because it follows the distribution of molecular hydrogen and its emission is sensitive to the total mass. The HD detection, combined with existing observations and detailed models, implies a disk mass >0.05 solar masses, enough to form a planetary system like our own.