The $\dot{M}$--$M_{\rm{disk}}$ relationship for Herbig Ae/Be stars: a lifetime problem for disks with low masses?

TL;DR

This study examines the relationship between accretion rates and disk masses in Herbig Ae/Be stars, revealing a largely flat trend with some outliers indicating very short disk lifetimes, possibly due to rapid dispersal processes.

Contribution

It provides the first detailed comparison of the $ m{ ext{M}}$--$ m{ ext{disk}}$ relationship in Herbig Ae/Be stars versus T Tauri stars, highlighting potential rapid disk dispersal in a subset of intermediate-mass stars.

Findings

Most Herbig Ae/Be stars follow the T Tauri accretion trend.

A subset shows high accretion rates with low disk masses, implying very short disk lifetimes.

Outliers may be experiencing rapid disk dispersal due to radial drift, photoevaporation, or truncation.

Abstract

The accretion of material from protoplanetary disks onto their central stars is a fundamental process in the evolution of these systems and a key diagnostic in constraining the disk lifetime. We analyze the relationship between the stellar accretion rate and the disk mass in 32 intermediate-mass Herbig Ae/Be systems and compare them to their lower-mass counterparts, T Tauri stars. We find that the $\dot{M}$--$M_{\rm{disk}}$ relationship for Herbig Ae/Be stars is largely flat at $\sim$10$^{-7}$ M$_{\odot}$ yr$^{-1}$ across over three orders of magnitude in dust mass. While most of the sample follows the T Tauri trend, a subset of objects with high accretion rates and low dust masses are identified. These outliers (12 out of 32 sources) have an inferred disk lifetime of less than 0.01 Myr and are dominated by objects with low infrared excess. This outlier sample is likely identified in part by the bias in classifying Herbig Ae/Be stars, which requires evidence of accretion that can only be reliably measured above a rate of $\sim$10$^{-9}$ M$_{\odot}$ yr$^{-1}$ for these spectral types. If the disk masses are not underestimated and the accretion rates are not overestimated, this implies that these disks may be on the verge of dispersal, which may be due to efficient radial drift of material or outer disk depletion by photoevaporation and/or truncation by companions. This outlier sample likely represents a small subset of the larger young, intermediate-mass stellar population, the majority of which would have already stopped accreting and cleared their disks.