Astronomy 218 The Spectacular Deaths of Massive End of a Massive By the time the helium shell burning ignites, the core is effectively decoupled from the envelope. The star’s remaining life is less than the Kelvin-Helmholtz timescale, τ ~ U/L ~ GM2/RL ~ 104 years. The visible luminosity is largely provided by the helium shell. 0.01 R 1000 R In the core, Tc > 500 MK lead ☉ ☉ to neutrino-pair production, allowing the luminosity of the core to escape primarily as neutrinos. Successive burning stages proceed rapidly to form an iron core with lighter element shells. High Energy Photons γ-rays are photons with wavelengths λ < 10−11 m, carrying energies of 100 keV or more. This corresponds to kT for a temperature of 1.1 GK. At temperatures of 0.5 GK or more, the tail of the photon distribution has sufficient energy (1.02 MeV) to transform into a electron-positron pair. Usually the positron will annihilate with an electron, producing γ-rays, but rarely a neutrino-antineutrino pair is produced. These radiate freely from the core, cooling it. For T > 1.2 GK the tail of the photon field is sufficient to photo-disintegrate neon, 20Ne + γ → 16O + α, igniting neon burning. This occurs at a lower temperature than the T > 1.5 GK required for oxygen burning. Inside a Massive Star This diagram shows the history of a 22 M☉ star. The green and red shadings show convective regions. Blue marks nuclear Rauscher, Heger, Hoffman & Woosley 2002 energy generation. Purple marks nuclear energy loss. Carbon, Neon, Oxygen and Silicon burning, leave a core of Iron. Nuclear burning stages

The nuclear burning stages in the life of a 20 M☉ star.

Stage Fuel Products Tc Lumin. Duration (100 MK) (100 L☉) H Burning H He 0.3 600 10 Myr

He Burning He C 2 1000 1 Myr

C Burning C O, Ne, Na, Mg 9 1400 103 yr

Ne Burning Ne O, Mg 16 1500 200 days

O Burning O Mg to S 20 1500 500 days

Si Burning Si Fe peak 30 1500 10 days

Collapse up to Th >30 0.3 s Why Stop at Iron?

Fission Fusion For T > 3 GK, the photon field is sufficient to photodisintegrate most nuclei. This causes silicon burning. Photodisintegrations balance captures resulting in Nuclear Statistical Equilibrium (NSE). This favors iron, the most tightly bound nucleus. Iron core ⇒ no fusion to generate energy to balance gravitational collapse. !

Sk 202-69 SN 1987a

© Anglo-Australian Observatory Supernova Taxonomy Observationally, there are 2 types (7 subtypes) based on their spectra and light curves. Physically, there are 2 3 4 mechanisms, thermonuclear Cappellaro & Turatto () 2001 core collapse (massive star) collapsar (very massive star) pair production (very very massive star) 6 from 1, 1 from another

The core collapse Super Luminous Supernova mechanism results in Ia White Dwarf supernovae with quite II-L Red Supergiant varied spectra and light II-P Blue Supergiant curves. Ib Wolf-Rayet Star Each CCSN is unique because of variations in the massive star’s mass and envelope which surrounds the central engine. In contrast, the Type Ia SN are remarkable similar, suggesting a mechanism with little variation. Supernova at 320 Years The supernova : Type IIb SN deposits 1044 J (the equivalent of 1028 MTons TNT) of Kinetic Energy into the ISM. X-ray (NASA/CXC/SAO) Optical (MDM Obs.) Supernovae are ~ 10 pc a major heating source in the ISM, responsible for coronal gas and star formation. Radio (VLA) Infrared (ISO) Old Supernovae Supernova remnants sweep up the interstellar medium as they spread, which gradually slows the ejecta. Thus old remnants lose their spherical shape as they encounter irregularities in the interstellar medium, including other supernova remnants. Vela SNR is Vela 8° on the sky. supernova The Vela remnant sits at the center of the Vela SNR, marking this as the grave of Exploded ~ a massive star. 9000 BCE Pulsar Winds In many supernova remnants that result from core collapse supernovae, the initial 1044 J carried by the blast wave is augmented over the course of thousands of years by 1043 J of pulsar magnetic dipole radiation and pulsar winds. Most CCSN SNRs combine shell and PW nebulae. The is an example where the shell remnant is missing leaving only the PW nebula. Central Source Even with complex morphology of supernova remnants it is generally possible to reconstruct their origin. Positive and negative photographs of the Crab Nebula 14 years apart show the the gaseous filaments moving away from the site of the explosion. Invisible Supernova As bright as supernova are, you might think they can be seen whenever they occur nearby, but this is untrue. As young stars, O & B stars occur predominantly in the Galactic Disk. In the disk, the UV and optical light that carries much of their light is heavily extinguished by interstellar dust, making some supernova invisible. The young G1.9+0.3A lies in the direction of the Galactic Center, in the Sagittarius. From X-ray and radio observations, we know its angular expansion rate and size, we can calculate an age of 140 years, but no SN was seen. Wind Interactions Supernovae begin interacting with their environment almost immediately. The initial interactions are with their own stellar wind. In the case of SN1987A, the first interaction was a light echo illuminating a hourglass shaped wind. The UV flash of the supernova ionized gas in the waist of the hourglass and along both funnels. The supernova shock is now striking the central ring, which was ejected 20,000 years earlier. Textbook Supernova A Core-Collapse Supernova is the inevitable death knell of a massive star. E = 1046 J comes from the creation of a . Neutrinos carry away the binding energy of the neutron star, heating the stellar envelope and driving the Hillebrandt, Janka, explosion. Müller/Sci. Am. Observing Neutrinos We are now able to observe neutrinos using a number of different methods. These detectors will allow an amazing view of the next galactic supernova.

SN 1987a

1045 W Failed Supernova?

2 Because there is a maximum mass for a 1.8 ]

NS, we expect collapse . O of a massive enough star to lead instead to a 1.6 . 1.4 Without the neutron 12-WH07 D-Series 15-WH07 star radiating neutrinos, Proto-NS Mass [M 1.2 B-Series 20-WH07 the core collapse 25-WH07 mechanism fails. 1 0 500 1000 1500 2000 Mass loss & metallicity Time After Bounce [ms] are important to this limit. and very Massive stars may lose much of their envelope leaving the He or C/O core visible, re-enabling CCSN. Collapsar About once per day, a bright burst of highly beamed γ-rays comes to the Earth. Observations have recently tied some of these GRBs to peculiar, hyper- energetic SN Ic. One proposed model is a collapsar, where an accretion disk forms around a newly formed black hole in a failed supernova (M > 30 M☉), producing a jet which we see as the GRB. Heavy Element Ejecta

Hughes, Rakowski, Burrows & Slane 2000 The ejecta from core-collapse supernovae are rich in many elements from oxygen to calcium and iron/cobalt/nickel. They may also be responsible for half of the elements heavier than iron. Fresh Nuclei Observations of core collapse supernovae reveal freshly made nuclear species. γ-ray telescopes reveal the characteristic γ-ray lines of 56 57 Ni (t½ = 6 days), Ni (t½ 56 = 36 hrs), Co (t½ = 77 57 days), Co (t½ = 272 days) 44 COMPTEL/NASA and Ti (t½ = 60 yrs). NuSTAR NASA/JPL-Caltech/CXC/SAO The supernova’s light curve in the later, linear phase is 56 56 powered by the conversion of ~0.1 M☉ of Ni to Co and ultimately 56Fe. Almost half of terrestrial iron was made in this way, a direct product of a star’s death. rich in heavy elements Not just iron, but many elements are made in these explosions. Others, made during stellar evolution, are released into the interstellar medium.

Umeda & Nomoto 2003

Rauscher, Heger, Hoffman & Woosley 2002 Accretion Power White dwarves, neutron stars and black holes that are part of binary star systems exhibit a range of afterlives impossible for their solitary counterparts. The unifying power source for this range of objects is matter accreted from a binary companion. The wind of a companion can be the accretion source. Companions that fill their Roche lobes provide the largest & steadiest accretion rates. Loading a White Dwarf The result of accretion onto a white dwarf Nomoto, Saio, Kato & Hachisu 2007 depends on the accretion rate and WD mass. For high accretion rates, a Red Giant-like envelope re-forms.

For the right accretion Novae rate, accreted H and He burn steadily to C & O, causing the WD to grow. For lesser accretion rates, a layer of H builds on the surface. As accretion continues, the pressure at the base of the accreted layer grows until it ignites. Finding Novae

T. Credner 12/99

About 40 novae occur each year in our galaxy and they are frequently discovered by amateurs. Novae are classified as cataclysmic variables. The binary systems that make up cataclysmic variables are typically very tight (a ~ R☉) with periods of < 1 day. Classical Nova

The ignition of the degenerate hydrogen layer causes a thermonuclear runaway, a 1038 J (1022 Megaton) Hydrogen bomb. The star to rapidly brighten as much as a million-fold.

S2006 S1998 P1995

Starrfield, Iliadis, Hix, … 2009

The enhanced luminosity fades 2-3 magnitudes in < 25 days for a fast nova while a slow nova may take more than 80 days to decline. Nova Nuclear Reactions Ar As a result of the degenerate Cl ignition, hydrogen burning S in novae occurs at higher 36 P temperatures Si (T < 400 MK) 32 Al than in main Mg sequence stars. 26 Na

Ne 22 The Hot CNO cycle F involves an expanded

O 18 series of catalytic N reactions on pre-existing C 14 CNO nuclei, as well as NeNa and MgAl cycles at higher mass. The net result is the conversion of 4 1H→4He. Nova Ejecta

Nova Cygni

+1 year +1.5 year Observations shows shells of ejected material expanding away from a star after a nova which form dust grains. Spectroscopy reveals variations of material across the explosion, including white dwarf material. T Pyxidis Types of Novae Observationally novae are categorized into 3 types based on recurrence timescale; dwarf (0.04-1 years), recurrent (~10-100 years) & classical (>103 years). Dwarf and some recurrent seem to be due to an accretion disk instability. Others are thermonuclear explosion with recurrence time Nova Z Cam (NASA) related to WD mass and accretion rate (massive WD recur quickly). White Dwarf Supernova For the correct range in accretion rate, the accreting material burns steadily to C & O, growing the mass of the white dwarf.

As a white dwarf’s mass approaches Mch, electron degeneracy can no longer keep the core from collapsing. In a C-O white dwarf, increasing density leads to carbon fusion in the core.

For an ONe white dwarf, M→Mch leads to electron capture and collapse to a neutron star. Thermonuclear SN Mechanism

Double Single Degenerate Degenerate

Several ways to destroy a White Dwarf.

R. Diehl Deflagration Burning starts as simmering in the center. Because of degeneracy, carbon burning leads to oxygen & silicon burning. F.X. Timmes The thermonuclear flame is initially subsonic, called a deflagration. It may eventually transition to a detonation, a supersonic flame. Nuclear Lightcurve

Producing ~1 M☉ of iron & nickel, much of it radioactive 56 Ni and ~ 0.3 M☉ of Si-S-Ar- Ca, thermonuclear SN are important element foundries.

The supernova light curve reflects the decay of 56Ni → 56Co → 56Fe. Neutron Star Binaries

NASA/CXC

Accretion from a companion can also occur on a neutron star. The accretion energy of 0.1 mc2 heats the neutron star to 106 K. Such accreting neutron stars, called low (LMXB) or high (HMXB) mass X-ray binaries, account for many observed X-ray sources. X-ray Bursts

X-ray Luminosity

Some X-ray binaries periodically produce bursts of X-rays (~1031 W) that last for seconds but can recur hourly. These Type 1 X-ray bursts are due unstable H-He burning on the neutron star surface, making proton-rich nuclei up to tin (Z=50) and tellurium (Z=52). Black Hole Binaries Some of the observed x-ray binaries contain a dark object too massive to be a neutron star. Cygnus X-1 is one example.

The visible companion HDE226868 is B0Ib supergiant with a mass of roughly 25 M☉.

The systems total mass is ~35 M☉, so the X-ray source must be more than 7 solar masses, even with uncertainties in the masses of HDE226868. Short time-scale variations indicate that the source is very small (< 100 km), but 10 solar masses is too large to be a neutron star, thus Cygnus X-1 must contain a black hole. Pulsar Spin-up The accreted material delivers not just thermal energy to the surface of the neutron star (or white dwarf), but also angular momentum. This “spins up” the compact object, decreasing its period. In the case of a neutron star, this may rekindle a pulsar. Most have periods between 30 and 200 ms, but spun- up neutron stars have periods as small as 1.4 ms. This fast rotation allows even old neutron stars, with low magnetic fields (B ~ 106 tesla), to become pulsars. Gamma-Ray Bursts Intense bursts of gamma-rays are also observed lasting seconds and occurring roughly once per day. The bursts can be divided into short and long duration bursts (t ~ 0.3 s and ~ 30 s, respectively). Maps of the burst’s distribution on the sky show no “clumping” of bursts anywhere, particularly not within the Milky Way. Therefore, the bursts must originate from outside our Galaxy. Extra-Galactic Bursts Distance measurements of optical counterparts of some gamma bursts have revealed coincidence with distant galaxies billions of light years away. Occasional spectral lines in the burst confirm the large redshift.

This requires tremendous luminosity, 1044 J, but confined to a narrow beam a few degrees wide. Gamma-Ray Jets In both cases, the γ rays originate as relativistic jets created by accretion disks around newly formed black holes. For the short bursts, binary neutron stars merge as a result of gravitational radiation, producing a black hole and jet. Long bursts result when the black hole is the result of a failed supernova. The jets punch through the envelope of the star, later a disk wind drives of the envelope, causing a very bright supernova. Nuclear Source

122 The elements that H Solar s- and r- Process Abundances 10 Solar Abundances He 1

make up our planet ) ) 6 6

= Ba i = 8 and ourselves have i N S O Te/I Pb S ( C

( +

Ne e 0 + e Fe their origins in c Si c 6 Os/Ir/Pt

n S n a + a Ca d

cosmic events, d n + n 4 + u

u -1 b distant in time and b A

A

g

g 2 o space. o l L -2 Ba Te/I Pb The Big Bang 0 Os/Ir/Pt

--23 produced H, He & 0 80 20 4100 60 12080 110400 120160140 116800 180200200 the light elements. AAtotommici cM Masasss AGB Stars are responsible for 12C, 14N and the s-process. Novae for 13C, 15N, 17-18O; Type Ia SN for ~½ of Fe/Co/Ni. The rest comes from massive stars in supernovae. Neutron Capture Because fusion above Iron is energetically unfavorable, the heavier elements are made as side effects of violent outbursts. In both cases, captures of neutrons drive the progress. For the s-process, which occurs when AGB thermal pulses mix the burning shells, the captures are slow enough that the new nucleus decays before the next capture. In the r-process a rapid series of neutron captures builds nuclei up to uranium in a matter of seconds.