1111.6970 (Andrew Gould)
Andrew Gould
I show that after the "helium flash" abruptly ends its first ascent red giant
evolution, a solar-type star is powered primarily by gravitational contraction
of its helium core, rather than by nuclear fusion. Because this energy is
released in the core rather than the envelope, the overall structure of the
star, and so its luminosity, is driven toward that of a red clump star from its
initial position at the tip of the red giant branch (TRGB). This occurs on a
first (and well recognized) Kelvin-Helmholtz timescale t_{KH,1} ~ E_env/L_TRGB
~ 1e4 yrs., where E_env is the thermal energy stored in the envelope and L_TRGB
is the luminosity at the TRGB. However, once the star assumes the approximate
structure of a clump star, it remains powered primarily by contraction for a
{\it second} Kelvin-Helmholtz timescale t_{KH,2} ~ E_core/L_clump ~ 1e6 yrs,
where E_core is the thermal energy stored in the core and L_clump is the
luminosity of a clump star. It is this second Kelvin-Helmholtz timescale that
determines the overall pace of the moderately violent processes by which the
star returns to nuclear-power generation as a full-fledged clump star. The
reservoir of gravitational energy acts as ultimate regulator, providing
whatever supplemental energy is needed to power L_clump and occasionally
absorbing the large momentary excesses from helium mini-flashes. As this
reservoir is gradually exhausted, helium fusion approaches the level of
steady-state clump stars.
View original:
http://arxiv.org/abs/1111.6970
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