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Henyey track

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Path taken by pre-main-sequence stars in the Hertzsprung–Russell diagram

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An HR Diagram of PMS stars with different masses. The Hayashi Track is depicted as vertical lines, while the Henyey are horizontal. Higher mass stars spend very little time on the Hayashi Track, while the lowest mass stars never reach the Henyey Track, with a gradient seen of time spent on each track as the mass increases.^[1]

TheHenyey track is a path taken bypre-main-sequence stars with masses greater than 0.5solar masses in theHertzsprung–Russell diagram after the end of theHayashi track. The astronomerLouis G. Henyey and his colleagues in the 1950s showed that the pre-main-sequence star can remain inradiative equilibrium throughout some period of its contraction to the main sequence.

The Henyey track is characterized by a slow collapse in nearhydrostatic equilibrium, approaching the main sequence almost horizontally in the Hertzsprung–Russell diagram (i.e. theluminosity remains almost constant).^[2]

Deviation from Hayashi Track

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${\begin{aligned}{\frac {dT}{dt}}&={\frac {-3\rho \kappa l(r)}{64\pi \sigma _{sb}T^{3}r^{2}}}\\[4pt]\end{aligned}}$

The equation for radiative heat transfer tells us the relation ofopacity (κ) and temperature gradient T. Stars with high opacity will beconvective, while low opacity will beradiative for heat transfer.

Protostars on the Hayashi track are fully convective and due to the large presence ofH- ions, are optically thick. These stars will continue to contract, until the central core reaches a certain temperature threshold, where the H- ions will break apart, causing a decrease in opacity.

What determines when and how long a star moves from theHayashi track to the Henyey track is heavily dependent on its initial mass. Stars that are massive enough (0.6 solar mass) will deviate onto the Henyey Track, depicted as a near-horizontal line on an HR diagram. A core that becomes sufficiently hot enough will become less opaque, making convection inefficient.^[3] The core will instead become fullyradiative to transfer its thermal energy. During this phase theluminosity stays constant or gradually increases, with the temperature increasing as the core undergoes radiative contraction.^[4] At the end of the track, the star will undergonuclear burning, however, and will experience a dip in luminosity, until it reaches the main sequence.

Larger mass stars will evolve quickly from the Hayashi track, while lower mass stars will enter later. Stars that are not sufficiently massive on the other hand will never develop a radiative core, as the core does not become hot enough, and instead, will remain on the Hayashi track until it reaches the main sequence.^[1]

References

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^^a ^bIben, Icko, Jr. (1965-04-01)."Stellar Evolution. I. The Approach to the Main Sequence".The Astrophysical Journal.141: 993.Bibcode:1965ApJ...141..993I.doi:10.1086/148193.ISSN 0004-637X.{{cite journal}}: CS1 maint: multiple names: authors list (link)
^Fang, Herczeg, Rizzuto (2017)."Age Spreads and the Temperature Dependence of Age Estimates in Upper Sco".The Astrophysical Journal.842 (2): 123.arXiv:1705.08612.Bibcode:2017ApJ...842..123F.doi:10.3847/1538-4357/aa74ca.S2CID 119087788.
^D'Antona, Francesca; Mazzitelli, Italo (1994-01-01)."New Pre--Main-Sequence Tracks for M".The Astrophysical Journal Supplement Series.90: 467.Bibcode:1994ApJS...90..467D.doi:10.1086/191867.ISSN 0067-0049.
^Jensen, Sigurd S.; Haugbølle, Troels (2017-11-02)."Explaining the luminosity spread in young clusters: proto and pre-main sequence stellar evolution in a molecular cloud environment".Monthly Notices of the Royal Astronomical Society.474 (1):1176–1193.arXiv:1710.00823.doi:10.1093/mnras/stx2844.ISSN 0035-8711.

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