Core Contraction powers RGB on HRD ?

[Widdekind]

Core-Contraction powers GB luminosities ?

 

"Gravitational Luminosity", the power provided by core contraction, could account, for observed post-MS luminosities, with modest core-contraction infall velocities:

 

[math]U_g \approx -\frac{3}{5} \frac{G M_c^2}{R_c}[/math]

 

[math]L_g \equiv \frac{dU_g}{dt} \approx \frac{3}{5} \frac{G M_c^2}{R_c^2} \dot{R_c} \approx \left( 10^8 L_{\odot} \right) \left( \frac{M_c}{M_{\odot}} \right)^2 \left( \frac{R_c}{0.1 R_{\odot}} \right)^{-2} \dot{R_c}[/math]

Thus, as a sun-like star evolves off the MS, core-contraction infall velocities of ~10 nm / s could account, for observed proto-RGB luminosities. And, even the peak RGB luminosity [math]\approx 10^4 L_{\odot}[/math], when the He-core has contracted to WD size [math]\approx 10^{-2} R_{\odot}[/math], at the RGB-to-HB "bounce" on the HRD, core-contraction infall velocities of ~1000 nm / s could still account, for observed peak RGB emission power:

 

After the "bounce", He-core ignition occurs, and the now-WD-sized, He-fusing-to-C core stabilizes the star, onto the HB. Then, once core He is depleted, the core-contraction process repeats, and the now-WD-sized core is (partially) "trash-compacted" yet again. And again, similar rates of core-contraction could account, for observed AGB luminosities.

 

 

 

every GB trajectory represents core-contraction & "trash-compaction" ? every "bounce" back towards MS (e.g., HB) represents core-re-ignition ?

 

Massive stars traverse "to-and-fro" on HRD, (1) evolving away from MS, perhaps powered by core-contraction; and then (2) "bouncing" back towards MS, perhaps powered by a stabilized-and-re-ignited core, until the next core "flare-out-and-failure" -- at which point the cycle may repeat. Thus, could the cyclical migrations, of massive stars, "back-and-forth" across the HRD, be "counted like tree-rings", to infer the internal "core compaction state", and commensurate core-fusion processes ?

 

At the present epoch, most massive stars SN in their red-giant phase, well off-and-to-the-right of the MS, on the HRD (Mazure. Exploding Superstars, p.49; Wheeler. Cosmic Catastrophes, p.261):

 

Yet, long long ago, most massive stars, of much lower metallicity, may have SN'd as blue-giants, near the MS, like SN1987a:

 

(source:

SN1987a

)

Could it be, that red-giant phase SN occur at a core-re-ignition "bounce" event, and so represent Thermo-Nuclear SN (e.g., SNIa) ? And/or, that blue-giant phase SN occur at a core-failure-and-flare-out event, and so represent a true neutronizing-and-neutrino-generating Core-Collapse SN (e.g., SNII) ?

 

AGB phase, of middle-mass stars, represents repeated "bounces", of periodic C-core fusion flashes, whose prodigious power exceeds stars' gravitational confinement capacity, so expelling envelope, into burgeoning PNe ??

 

AGB 'thermal pulses' typically last 10-100 kyr, begun by few kyr of C-core fusion 'flash', whose power cannot be contained by over-lying envelope (cf. Tokamaks), which is blown back out towards space. Pressure drop quenches C-core fusion, and envelope ballistically expands, cools, and re-collapses, re-initiating subsequent cycle. However, with each fusion 'flash', some overlying envelope is completely ejected, and remainder, having been blown out to ever higher altitudes, comes "crashing back down" ever harder, compressing the core ever more, thereby exaggerating & escalating subsequent C-core fusion flashes. These un-harnessed fusion flashes also blow C-core material out into ejected envelope:

 

Due to these pulses, which only last a few thousand years, material from the core region is mixed into the outer layers, changing its composition, a process referred to as dredge-up. Because of this dredge-up, AGB stars may show [Carbon and] S-process elements in their spectra. Subsequent dredge-ups can lead to the formation of Carbon stars. AGB stars are typically long period variables, and suffer large mass loss in the form of a stellar wind. A star may lose 50 to 70% of its mass during the AGB phase.

Is this an accurate understanding, of the AGB thermal pulse process ?? What would be a good-yet-accessible book, on star fusion processes ??

 

the carbon core contracts, and Temperature increases, but not enough to ignite carbon or oxygen...[fusion] flashes, in a shell surrounding the core, ejecting an envelope

(

uvic

)

 

Thermal Pulses of AGB stars

(

mnsu

). Note that the initial luminosity 'spikes' resemble that of

SN1987a

.