Review: Low mass stars evolution off M-S

1. Review: Low mass stars evolution off M-S • Red dwarfs (<0.4Msun) are “fully convective” (see Tf19-3) and so mix their He ash throughout star; no He WD until all the H is burned, which requires ~1/0.42.5 ~10X Sun’s lifetime, or 100Gy! • ~0.8Msun stars just now leaving M-S if born ~12Gy ago when fewer “metals” (C,N, O, etc.) in their primordial GMCs. These are Pop II stars, in globular clusters (and galactic halo) with ~1% fraction of metals vs. Sun and “Pop I” stars, born in galactic disk. • The Sun will evolve off the M-S as a sub-giant (nascent He WD starting to form in core), then over ~300My evolve to a more luminous Red Giant, then ignite He burning in the He flash and evolve down thru instability strip to become Cepheid type (actually RR Lyra) variable, then back up the Asymptotic Giant Branch as an AGB star, then it blows off its envelope due to radiation pressure to produce a Planetary Nebula, which in ~105 years disperses leaving behind a rapidly cooling Carbon-Oxygen White Dwarf Nov. 29, 2007

2. Post AGB: a Planetary Nebula then a C-O WD… which slowly cools and lives forever, as a lurking dark hulk… • At tip of AGB branch, the RG is so luminous, it literally blows off its envelope. Radiation exerts pressure, or a force/unit area, on the envelope and blows it off in a stellar wind • What’s produced? A luminous nebula, pure emission lines (H, He, C, O) (think of Kirchoff’s Laws…), that was the stellar envelope, surrounding the very hot exposed core, a white dwarf! • WD has R ~104km ~1/70Rsun and T ~105 K ~20Tsun, so L ~30Lsun (derive!) – but this slowly cools to (now) ~3000K since no more internal L… Nov. 29, 2007 Planetary Neb. in globular! A PN in visible and another in IR

3. The remnant WDs…. • The C-O WD is “visible” in the H-R diagram on a cooling track which is below and to left of M-S. Temp. (color) measures age • WDs, as degenerate matter, are smaller if more massive, and thus are less luminous and bluer for given luminosity. WD mass weakly prop. to progenitor giant mass (e.g. 3Msun giant  1.2Msun WD) • WD cooling tracks measured in globular clusters; ~30% of a GC in WDs! Nov. 29, 2007

4. Massive star (>4Msun) evolution off M-S • Massive stars live life in fastlane; short lifetime on M-S since central pressure and temp. much larger and so H, He burning much more efficient and thus faster • Burning of C to Ne, Na, Mg is possible while still on M-S which forms onion skin layers of more massive ash • Still more massive stars (>8Msun) can burn the Ne to (eventually) Si Nov. 29, 2007

5. Fate of the massive core… • As nuclear burning proceeds to, finally, burning Silicon (Si) into iron (Fe), catastrophe looms ahead… • The Fe core “grows” to approach a limiting mass allowed for degenerate matter – the Chandrasekhar limit, 1.4Msun. More massive than this, and degeneracy pressure upwards can no longer withstand gravitational pressure downwards • Within ~0.1sec, the core begins to collapse. As the density and temperature rise, the protons (p) and electrons (e) in the Fe nuclei “fuse” into neutrons: p + e  n + neutrino, where the neutrino is a sub-atomic particle with virtually no interaction probability, so it streams out of the dense core, releasing further pressure and allowing the core to collapse to a • NEUTRON STAR! Nov. 29, 2007

6. And a Type II supernova results! • The enormous outrush of neutrinos combined with a strong shock wave from the collapse of material onto the NS, exerts pressure on the overlying stellar envelope (the orig. ~10Msun star – 1.4Msun core), blowing it off at velocity ~10,000 km/s Nov. 29, 2007