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PRE-MAIN-SEQUENCE STELLAR EVOLUTIONARY TRACKS

PRE-MAIN-SEQUENCE STELLAR EVOLUTIONARY TRACKS. The paths followed by a newly-formed star as it approaches the main sequence on the HR diagram: “Vertical”: Hayashi track “Horizontal”: Henyey track. From Protostars to Stars. Star emerges from the enshrouding dust cocoon.

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PRE-MAIN-SEQUENCE STELLAR EVOLUTIONARY TRACKS

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  1. PRE-MAIN-SEQUENCE STELLAR EVOLUTIONARY TRACKS The paths followed by a newly-formed star as it approaches the main sequence on the HR diagram: “Vertical”: Hayashi track “Horizontal”: Henyey track

  2. From Protostars to Stars Star emerges from the enshrouding dust cocoon Ignition of H He fusion processes

  3. HAYASHI TRACK PMS begin the evolution at the upper right of the HR diagram: Low T Large R (and L) Luminosity is supplied by contraction (i.e. from Eg, see “virial theor.”) At the beginning, isothermal collapse: released Eg, freely radiated away Later on: hydrostatic core+envelope of increasing density: released Eg increasingly hard to be radiated outside: Luminosity decreases Temperature at the outer edge, roughly constant HAYASHI TRACK IS ROUGHLY VERTICALLY DOWNWARDS ON THE HR DIAGRAM Hydrostatic core contracts quasi-statically: Kelvin-Helmholtz contraction Kelvin-Helmholtz time scale tkh Eg/L GM2/RL

  4. Henyey tracks Contraction at nearly constant T stops when the star develops a radiative core The star moves to the MS along nearly horizontal paths in the HR diagram: Henyey tracks Radiative transfer of energy: diffusion equation: L(r) = -(64psr2)/3kr dT4/dr (1/kr) dT4/dr ~ 1/<kr> T4/R* ~ L */(R *)2 From: Hydro- equilibrium: GM/R = kT/mmH --------------- <T> ~ M*/ R * ; <r> ~ M */(R *)3 Results: (M *)3 <k> -1 ~ L *

  5. High-mass stars: (electron scattering dominated) <k> ~ cte L * ~ (M *)3 Low-mass stars <k> (b-b,b-f transitions) L * ~(M *)3 <k> -1 As a PMS becomes increasingly radiative, this mass-luminosity scaling becomes more relevant, and contraction to the MS occurs at nearly constant luminosity

  6. THEORETICAL PREDICTIONS OF HOW PROTOSTAR EVOLVES FROM CORE TO MAIN SEQUENCE IN HR DIAGRAM Ex. Evolutionary tracks calculations  D´Antona & Mazzitelli 1994, ApJS, 90, 467

  7. Ex.: For M= 0.8 Msol; R=2Rsol and L=1Lsol tk-h 4.3 106 yr  Hayashi track ~ 1.4 106 yr (Evolutionary tracks for different masses, separated in Teff partially due to different gravities affect to the outer atmospheric opacities.)

  8. THE BIRTH LINE: THEROY VS. OBSERVATIONS Line on the HR diagram beyond which no PMS are observed. This BIRTH LINE cut across the Hayashi track FROM CALVET (2004)

  9. The birth line: stars are not observed in the top right-hand side of the HR diagram, where the theory predicts the start of Hayashy track. HR diagram is normally plotted for optical colours  the start of Hayashi track corresponds to the youngest PMS  surrounded by gas and dust optical/nir radiation cannot escape  undetectable

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