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Pre-Main Sequence Star Is Optically Visible, but Has No Central Hydrogen Burning

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Pre-Main Sequence Star Is Optically Visible, but Has No Central Hydrogen Burning Definitions • A protostar is the object that forms in the center of a collapsing molecular cloud core, before it finishes the main accretion phase and becomes “optically visible". • A pre-main sequence star is optically visible, but has no central hydrogen burning. • A main sequence star is the result after subsequent contraction and significant increase in central density and temperature, when the star begins to burn hydrogen in its core. Trueism: Gravity always wins • Free-fall time = time for protostellar collapse 1/ 2 $ 3π ' t ff = & ) % 32Gρ( 7 −1/ 2 t ff = 3.4 ×10 n yrs n = 102 cm-3 => free-fall time = 3 x 106 yrs n = 106 cm-3 => free-fall time = 3 x 104 yrs • Too short!! What else can hold up the cloud? € 2 – Magnetic field threading the cloud has pressure Pmag = B /8π 2 – Turbulent velocities in cloud provide pressure Pturb ~ ρvturb Hosokawa Protostellar Infall dM/dt ~ M / tff core is isothermal (n è ∞ polytrope) until it becomes optically thick to own radiation, increasing central temperature. We have ideal gas at this stage, but not yet polytropic. M is increasing with time as object gains mass from infall. van der Tak * van der Tak Recall E = (4-3 γ)U van der Tak Time Scales and Luminosities (as usual, dynamical timescale is the shortest) 5 (few 10 years for 1 Mo) 7 (few 10 years for 1 Mo) (star evolves normally towards the main sequence) = Hosokawa Hosokawa Hosokawa (shock boundary condition) van der Tak van der Tak Hayashi Limit – The Classic Picture Pols Initial Radius – The Classic Picture 3/10 x x 3/10 (R = 36 Rsun at 1 Msun) However, protostellar accretion geometry and accretion history modifies this simple picture. Pols Hayashi limit = minimum Teff given M, R Hydrostatic equilibrium reached, so radiation can get out. Rather than Rinit = ∞, there is an Rmax(M) also called “the stellar birthline”. Initial Radius – The Modern Picture Hayashi Hayashi track for pre-MS evolution: Stars follow track of declining luminosity at roughly fixed temperature. Area to the right of the track is `forbidden’ in sense of not being in H.S.E. Early-Stage Nuclear Burning Note that these are “classic” tracks with large initial radii, and not “birthline” tracks. The birthline is above, but close to, the deuterium sequence. Paxton et al. (2010) Lithium Burning Lithium Indicates Age (over a limited time) Pre-main sequence evolution is still an open area of research, with different calculations obtaining very different HRD results. Differences are mainly due to: - implementation of mixing length theory - choice of opacities Hillenbrand and White (2004) Evolution of R, g, T with Stellar Mass Evolution of R, g, T with Stellar Mass Age increasing to the left here – sorry! Evolution at the Lowest Masses Paxton et al. (2010) Evolution at the Lowest Masses Paxton et al. (2010) Effects of Composition Hosokawa Effects of Composition Hosokawa Effects of Composition Effects of Composition Clayton.
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