Dec. 6, 2007 1

Review: >8Msun stars become Type II SNe

• 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!

Dec. 6, 2007 2

And a Type II supernova results!

• The enormous outrush of neutrinos combined with a strong shock wave from the collapse of material onto the “hard” NS, exerts pressure on the overlying stellar envelope (the ~10Msun star – 1.4Msun core), blowing it off at velocity ~10,000 km/s

Dec. 6, 2007 3

SN1987A: direct evidence for core collapse!

• On Feb. 23, 1987, a SN-II exploded in the LMC (~55kpc distant satellite galaxy of MW) and was observed as a “new” bright (V ~4) star that night.

• It was also observed in neutrinos! About ~10-15 neutrinos (ν’s) were observed in 2 underground experiments (Japan and Ohio; different neutrinos for each!) and provide direct evidence that a NS was formed: p + e n + ν

• SN87A today observed with HST as “rings” due to swept up gas from SN but still not any direct evidence for the NS that must have formed… or did it then collapse to a black hole?

Dec. 6, 2007 4

• Recall that ~half of all stars have binary companions. Thus a ~1.2Msun C-O WD produced by a ~6Msun star may accrete matter from its binary companion if it fills its “Roche Lobe” as in fig. 19-20b from text

• But if WD then gains only ~0.2Msun, it exceeds its Chandrasekhar limit. The piled-on H, He, C (material from envelope of binary companion) then becomes dense/hot enough to ignite, and the increased pressure in the WD core ignites the C-O. The entire WD becomes a Thermonuclear Bomb and explodes as SN-Ia

Type Ia Supernovae: Explosion of a WD

Dec. 6, 2007 5

Type Ia vs. Type Ib, Ic, II SNe: WD “burns” vs. core collapse

SNIa: no H or He in spectra since burned in WD explosion vs. SNIb, Ic: no H

or H & He since lost from envelope of massive star vs. SNII has H from env.

Dec. 6, 2007 6

Supernova lightcurves & Ia’s as Standard Candles

• The optical (and IR) luminosity of a SN (both Type I and II) is enormous; comparable to entire galaxy

• Energy source in Ia is thermonuclear explosion, whereas in Ib, Ic and II it is core collapse: gravitaional energy released, E = GM/R, where M ~1.4Msun and R ~10km (radius of NS), so E ~1053 ergs, of which 99% is in ν’s

• Optical lightcurves very different: due to radioactive decay of CoFe in SNIa and to decay of different elements in Ib, Ic and II (core collapse SNe)

Since SN-Ia due to “burning” of a fixed mass of 1.4Msun, the peak luminosity of Ia is a fixed value, a “Standard Candle” and thus distance indicator. SNIa’s map the largest structure in the Universe and revealed Dark Energy – stay tuned…

Dec. 6, 2007 7

And now on to Neutron Stars(NSs) and BHs…

• SN-II produce a NS for massive stars in approx. range 8-15Msun; and a black hole for progenitors >20Msun (approx.)

• NSs discovered originally as Pulsars: NS with magnetic fields at their surface ~1012X that at surface of Sun (or Earth), and spinning at ~100X per second (when first born): a gigantic electric dynamo generator, accelerating enormous currents out along its magnetic field axis, and lighting up surrounding Supernova Remnant as best exemplified in Crab Nebula

Dec. 6, 2007 8

What happens when NS remnant has binary companion?

• Just as with SN-Ia when WD has binary companion, a NS with a binary companion will accrete gas from the companion (“normal” star) when the companion becomes a red giant (and fills its Roche Lobe), or when the companion is a lower mass star in very close orbit and then also fills its Roche Lobe.

• NSs with higher mass star companions become High Mass X-ray Binaries (HMXBs) and are often detected as Accretion-powered Pulsars like CenX3

Dec. 6, 2007 9

NSs: X-ray Pulsars vs. X-ray Bursters…

• If the NS has a strong magnetic field, the accretion is funneled onto the magnetic poles of the NS; the very hot poles come in/out of view: pulses of X-rays!

• But if the NS has weak field, which occurs for very old NSs accreting for a long time when field is buried, then matter will “pile-up” on the NS surface and explosively “burn” (He C, O) and produce an X-ray Burst: a ~100X increase in X-ray luminosity over a few seconds while entire surface of the NS “burns” (discovered by JEG…). Such “old NSs” live in globular clusters and in central bulge of our Galaxy and in turn evolve into Millisecond Pulsars….