The accretion disk paradigm for young stars

Bertout, C.

doi:10.1017/s1743921307009362

Cited by 4 publications

(2 citation statements)

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“…More specific historical approaches on accretion disks in TT stars and on how our understanding of accretion in these sources has evolved over time can be found in the literature (e.g., [43,44]), as well as detailed reviews on MA mainly devoted to low-mass stars [22,45].…”

Section: Accretion In T Tauri Starsmentioning

confidence: 99%

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Mendigutía

2020

Galaxies

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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.

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Section: Accretion In T Tauri Starsmentioning

confidence: 99%

On the Mass Accretion Rates of Herbig Ae/Be Stars. Magnetospheric Accretion or Boundary Layer?

Mendigutía

2020

Galaxies

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“…With protoplanetary disks, accretion, and outflows in the form of winds and jets recognized as significant processes for CTTSs, essentially all major features of CTTS systems were identified [27] and required observational support/constraints with the ". .…”

Section: The First Fuv Observations Of Young Accreting Starsmentioning

confidence: 99%

The UV Perspective of Low-Mass Star Formation

Schneider¹,

Günther²

2020

Galaxies

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The formation of low-mass (M 2 M ) stars in molecular clouds involves accretion disks and jets, which are of broad astrophysical interest. Accreting stars represent the closest examples of these phenomena. Star and planet formation are also intimately connected, setting the starting point for planetary systems like our own. The ultraviolet (UV) spectral range is particularly suited to study star formation, because virtually all relevant processes radiate at temperatures associated with UV emission processes or have strong observational signatures in the UV. In this review, we describe how UV observations provide unique diagnostics for the accretion process, the physical properties of the protoplanetary disk, and jets and outflows.CTTSs were initially identified as a new class of objects due their optical variability. Very early on, it was realized that they also show signs of enhanced chromospheric activity, i.e., "emission lines resembling those of the solar chromosphere" [2]. These emission lines are superposed on a photospheric spectrum similar to main sequence stars of late spectral type. Initially, the source of emission in excess of a normal photosphere was speculated to be circumstellar or chromospheric in origin [3].The discovery that CTTSs are above and to the right of the main sequence in an HR-diagram demonstrated in conjunction with theoretical work [4], that these objects must be young and, thus, the progenitors of the large main sequence population [5,6]. This notion is corroborated by the presence of strong Li absorption, which requires ∼ 100× the Li abundance of the Sun [e.g. 7]. Because Li is depleted quickly in stellar interiors, the amount of surface lithium decreases with time in convective stars. Therefore, strong Li absorption can only be found in young stars and thus CTTS must be young [e.g. review by 8].Meanwhile the number of peculiarities found in CTTSs grew, but their established youth alone was insufficient to explain those peculiarities and the "mysteries" of CTTSs remained. Many features, like the origin of the "chromospheric emission" remained unexplained, until the early 1980s when a large variety of models were proposed to explain features of CTTSs including envelopes, outflows, infall, and dust disks.Specifically, the "ultraviolet excess" (measured around 3700 Å) and the "blue continuum", which go along with a weakening of photosperic absorption lines due to veiling [9,10], were shown to correlate with the strength of Hα [11,12]. Such a correlation indicates a common origin of both features as observed for chromospheric emission on the Sun. Thus, it was natural to ascribe the excess continuum emission at short wavelengths to a Balmer continuum, i.e., emission following the capture of a free electron by an ionized hydrogen atom. While that finding pinpoints neither the physical origin nor location of the emitting material, it paved the way for explanations involving a suitably tuned chromosphere for producing the observed (Balmer & Ca II H+K) line fluxes in CTTSs. Quantitative mod...

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