Short Gas Dissipation Timescales: Diskless Stars in Taurus and Chamaeleon I

TL;DR

This study investigates the rapid dissipation of gas in the inner regions of disks around young stars, revealing that gas depletion can occur as early as 1-3 million years, influenced by initial conditions and various physical mechanisms.

Contribution

It provides new FUV observations of gas depletion in diskless stars and links initial cloud angular momentum to early disk evolution, highlighting factors beyond standard dispersal processes.

Findings

Gas is depleted from inner disks in stars as young as 1-3 Myr.

A significant fraction of cloud cores have low angular momentum, leading to rapid disk formation and accretion.

FUV luminosity contribution from chromospheric activity saturates at a specific ratio.

Abstract

We present an Advanced Camera for Surveys/ Solar Blind Channel far-ultraviolet (FUV) study of \h2 gas in 12 weak T Tauri stars in nearby star-forming regions. The sample consists of sources which have no evidence of inner disk dust. Our new FUV spectra show that in addition to the dust, the gas is depleted from the inner disk. This sample is combined with a larger FUV sample of accretors and non-accretors with ages between 1 and 100 Myr, showing that as early as 1--3 Myr, systems both with and without gas are found. Possible mechanisms for depleting gas quickly include viscous evolution, planet formation and photoevaporation by stellar radiation fields. Since these mechanisms alone cannot account for the lack of gas at 1--3 Myr, it is likely that the initial conditions (e.g. initial disk mass or core angular momentum) contribute to the variety of disks observed at any age. We estimate the angular momentum of a cloud needed for most of the mass to fall very close to the central object and compare this to models of the expected distribution of angular momenta. Up to 20% of cloud cores have low enough angular momenta to form disks with the mass close to the star, which would then accrete quickly; this percentage is similar to the fraction of diskless stars in the youngest star forming regions. With our sample, we characterize the chromospheric contribution to the FUV luminosity and find that $L_{FUV}/L_{bol}$ saturates at $\sim10^{-4.1}$.