Evolution of the Accretion Rate of Young Intermediate Mass Stars: Implications for Disk Evolution and Planet Formation

TL;DR

Pre-main-sequence intermediate-mass stars (IMTTSs) are found to have a much lower median accretion rate than Herbig stars, challenging simple disk evolution models that predict declining accretion with age. The authors assemble a uniform IMTTS sample (1.5-4.0 $M_\odot$; $T_{eff}<7200$ K) and derive $\dot{M}$ from $L_{H\alpha}$ (and alternative tracers) via the $L_{acc}$–$L_{H\alpha}$ relation, converting to $\dot{M}=L_{acc}R_*/(G M_*)$. They find a median $\dot{M}$ of $1.2\times10^{-8}$ $M_\odot$ yr$^{-1}$ for IMTTSs, about an order of magnitude below Herbig stars' median of $1.9\times10^{-7}$ $M_\odot$ yr$^{-1}$, and propose that increasing $T_{eff}$ toward the ZAMS boosts far-ultraviolet (FUV) radiation to drive higher accretion. This FUV-driven accretion scenario addresses the Herbig disk lifetime problem, implies a population of non-accreting A stars, and has implications for interpreting disk morphologies as signposts of embedded gas giant planets.

Abstract

This work presents a study of the evolution of the stellar accretion rates of pre-main-sequence intermediate-mass stars. We compare the accretion rate of the younger intermediate-mass T Tauri stars (IMTTSs) with the older Herbig stars into which they evolve. We find that the median accretion rate of IMTTSs (1.2$\times$10$^{-8}$ M$_{\odot}$ yr$^{-1}$) is significantly lower than that of Herbig stars (1.9$\times$10$^{-7}$ M$_{\odot}$ yr$^{-1}$). This increase stands in stark contrast with canonical models of disk evolution that predict that the stellar accretion rate declines with age. We put forward a physically plausible scenario that accounts for the systematic increase of stellar accretion based on the increase of the effective temperature of the stars as they evolve towards the zero-age main sequence. For example, the temperature of a 2M$_{\odot}$ star will increase from 4900~K in the IMTTS phase to 9100~K during the Herbig phase. Thus, the luminosity of the far ultraviolet (FUV) radiation will increase by orders of magnitude. We propose that this increase drives a higher stellar accretion rate. The scenario we propose to account for the increase in the stellar accretion rate solves the lifetime problem for Herbig disks because the increasing stellar accretion rates require lower initial disk masses to account for present-day disk masses. This work highlights the importance of the role FUV radiation has in driving the accretion rate, predicts a large population of pre-main-sequence non-accreting A stars, and has implications for interpreting disk morphologies that may serve as signposts of embedded gas giant planets in Herbig disks.