Abstract
Understanding how young stars gain their masses through disk-to-star accretion is of paramount importance in astrophysics. It affects our knowledge about the early stellar evolution, the disk lifetime and dissipation processes, the way the planets form on the smallest scales, or the connection to macroscopic parameters characterizing star-forming regions on the largest ones, among others. In turn, mass accretion rate estimates depend on the accretion paradigm assumed. For low-mass T Tauri stars with strong magnetic fields there is consensus that magnetospheric accretion (MA) is the driving mechanism, but the transfer of mass in massive young stellar objects with weak or negligible magnetic fields probably occurs directly from the disk to the star through a hot boundary layer (BL). The intermediate-mass Herbig Ae/Be (HAeBe) stars bridge the gap between both previous regimes and are still optically visible during the pre-main sequence phase, thus constituting a unique opportunity to test a possible change of accretion mode from MA to BL. This review deals with our estimates of accretion rates in HAeBes, critically discussing the different accretion paradigms. It shows that although mounting evidence supports that MA may extend to late-type HAes but not to early-type HBes, there is not yet a consensus on the validity of this scenario versus the BL one. Based on MA and BL shock modeling, it is argued that the ultraviolet regime could significantly contribute in the future to discriminating between these competing accretion scenarios.
Highlights
The evolution of young stars comprises several stages from the initial collapse in a molecular cloud until they enter the main sequence (MS), when the central objects reach enough temperature to burn hydrogen [1]
This review focuses on our estimates of accretion rates in Herbig Ae/Be stars (HAeBe) stars, discussing the way that disk-to-star accretion may proceed in these sources
This results from the fact that the shape of the initial mass function favors the formation of low-mass objects, and because the pre-main sequence (PMS) phase is shorter as the stellar mass increases
Summary
The evolution of young stars comprises several stages from the initial collapse in a molecular cloud until they enter the main sequence (MS), when the central objects reach enough temperature to burn hydrogen [1]. For CTTs there is consensus that accretion is magnetically driven according to the magnetospheric accretion scenario (MA [11,12,13]), while for more massive stars without magnetic fields accretion may proceed directly from the disk to the star through a boundary layer (BL [14]) In this respect, HAeBes represent a fundamental regime that bridges the gap between the accretion properties of low-mass CTTs and those of MYSOs. early-type Herbig Be stars (HBes) are the most massive stars for which direct accretion signatures can still be observed, given that MYSOs embedded in their natal clouds are opaque to robust accretion tracers that emit in the optical and ultraviolet (UV).
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