astr377: a six week marathon through the firmament
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ASTR377: A six week marathon through the firmament. by Orsola De Marco [email protected] Office: E7A 316 Phone: 9850 4241. Week 5, May 17-20, 2009. Overview of the course. Where and what are the stars. How we perceive them, how we measure them. - PowerPoint PPT PresentationTRANSCRIPT
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ASTR377:A six week marathon
through the firmament
by
Orsola De [email protected]
Office: E7A 316Phone: 9850 4241
Week 5, May 17-20, 2009
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Overview of the course
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What death becomes her?
• Depending on the main sequence mass, a star will end in a different way.
• M<0.08Mo no H burning – BD [L or T dwarfs].• 0.08Mo < M < 0.5Mo no He burning. Stars become
He WDs.• 0.5Mo < M < 5Mo no C burning: stars become CO
WDs.• 5Mo < M < 7Mo yes C burning to Ne and Mg: stars
become ONeMg WDs.• M > 7Mo burn all the way to Fe: these stars go
through a type II SN and become NSs or BHs.
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What happens after the AGB?• Spectra of planetary nebulae indicate that the
“shell” is expanding with speeds of 20-30 km/s. The central star is often visible inside the PN.
Jacoby, De Marco & Sawyer 1998
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What happens after the AGB?
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Angular size and distance
34
76
km ?
D
Base
Hypotenuse= sin
Moon diameter
D= sin
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The PN clock
• The central stars of PN on the HRD: they sit in a locus and have increasing ages along the red arrow.
• Conclusion: the star evolves to the blue in a very short time.
AGBlogTeff=3.5
PN “ignition”: log Teff ~ 4.4
Oldest PNe
Youngest PNe
Bob O’Dell American (alive)
O’Dell 1968
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At the end of the AGB
• Core mass growing because of the He shell burning. This generates an increase in L.
• Two shell sources, H (out) and He (in). Every time a star has multiple shell sources, all burning outwards through the fresh fuel supply, there is instability.
• The relative speed at which they burn out can create an instability where the helium shell L increases with no release of pressure, leading to even more L.
• Eventually the L pushes the envelope out, the H shell extinguished and the He burning rate decreases, L gets out, then the entire star returns to equilibrium.
• At the end of each thermal shell flash the envelope convection zone extends downwards and dredges up the results of He burning: C and O. This is the third and last dredge up chance a star has and makes of AGB star the C factories of the Universe.
http://outreach.atnf.csiro.au/education/senior/astrophysics/stellarevolution_postmain.html
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Departing the AGB
• When the AGB H envelope mass drops below a few times 10-4 Mo the star loses its equilibrium and contracts.
• The contraction and the increased transparency of the envelope result in a fast increase of the effective temperature.
• All the while the H or the He shell source are still burning.
• Eventually the burning stops and the star cools on the WD cooling track.
Vassiliadis & Wood 1994
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The PN phase• As the star heats a fast
but this time tenuous wind sweeps AGB wind material up and creates a shells.
• When the photosphere of the heating star passes the ~25,000-K mark the swept up shell is ionized and can be seen in forbidden lines.
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Animation from the Space Telescope Science Institute
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What do Planetary Nebulae What do Planetary Nebulae look like in the skylook like in the sky
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3 arcmin
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Planetary Nebula shapes: round….
Abell 39; WIYN image; G. Jacoby
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Planetary Nebula shapes: “elliptical”….
The Helix nebula; Spitzer image; K. Su
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Planetary Nebula shapes: “elliptical++”….
The Cat’s Eye nebula; HST image
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OH321.8+4.2; Bujarrabal; HST
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PN shapes and shaping
• Young PN and pre-PN (shining from shocks not from radiative ionization) are always non-round.
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NGC6543 HST/NOT [OIII]/[NII]/Ha. (P. Harrington, R. Corradi)
2.5 pc
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3.4
Empirically shown to happen, theoretically unexplained
AGB
WD
post-AGB
CSPN
Planetary nebulae as we teach themPlanetary nebulae as we teach themHow do PN form?How do PN form?
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Envelope mass < 10-3 to -4 Mo: departure from the AGB.Fast thin post-AGB wind compresses the super-wind.
3.4
Kwok 1982; Balick 1987Kwok 1982; Balick 1987
AGB
WD
post-AGB
CSPN
Planetary nebulae as we teach themPlanetary nebulae as we teach themHow do PN form?How do PN form?
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Post-AGB star heats up T>25,000K: ionized PNA39: a well behaved PN
AGB
WD
post-AGB
CSPN
Planetary nebulae as we teach themPlanetary nebulae as we teach themHow do PN form?How do PN form?
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… … but how do PN acquire their shapes?but how do PN acquire their shapes?
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Relatively-fast rotation during AGB super-wind …
Garcia-Segura et al. 2003Garcia-Segura et al. 2003
… … but how do PN acquire their shapes?but how do PN acquire their shapes?
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Garcia-Segura et al. 2003Garcia-Segura et al. 2003
… … but how do PN acquire their shapes?but how do PN acquire their shapes?
… and/or magnetic fields
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… … but how do PN acquire their shapes?but how do PN acquire their shapes?
… result in circumstellar material with an equatorial enhancement
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… … but how do PN acquire their shapes?but how do PN acquire their shapes?
When the star heats up, on its way to becoming a white dwarf, a fast wind rums into the previously-ejected gas.
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… … but how do PN acquire their shapes?but how do PN acquire their shapes?
Let’s zoom out …
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… but how do PN acquire their shapes?
Let’s zoom out …
… the lobes perpendicular to the plane of the disk continue to expand.
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… but how do PN acquire their shapes?
When the star heats up
the gas “shines” and we see the bipolar PN
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… … but how do PN acquire their shapes?but how do PN acquire their shapes?
The problem: giant stars do not rotate fast enough.
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How can a companion spin up a giant?How can a companion spin up a giant?
Animation from the Space Telescope Science Institute
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White dwarfs: key properties
• Small – about Earth radius.
• Not (all) white: some are very cool (~4000K).
• Super dense.
Srius A and B; HST/FGS image
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History of WDs• 1844 Friedrich Bessel
discovers that Sirius changes position and the change is not due to annual parallax…. There must be an unseen companion.
• In 1864 the companion was found by Alvan Graham Clark. It was 25,000K bit only 10,000th the luminosity of the Sun. What does this mean?
• A better solution of the orbit gave us Sirius B’s mass (0.9Mo) This meant that this star was much denser.
Friedrich Wilhelm Bessel German 1784-1846
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History of WDs• A better solution of the orbit
gave us Sirius B’s mass (0.9Mo) This meant that this star was much denser.
• In 1917 Adriaan van den Maaren discovered another WD (a single one).
• It was not till quantum mechanics that Ralph Fowler determined that degeneracy pressure was supporting the WDs.
• Eventually Chandrasekhar determined the mass limit above which WDs cannot exist (among many many other things that got him the Nobel Prize)
Ralph H. Fowler, UK 1889-1944
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WD density and pressure
• You can determine the WD density from a value of its mass and radius (which for Sirius B come from observations).
• You can then determine a value for the central pressure of the WD using the equation of hydrostatic equilibrium.
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The mass-radius relation for WDsaka: the more you have the smaller it is!
€
R ≈ 0.01 R•
M
0.7M•
⎛
⎝ ⎜
⎞
⎠ ⎟
−1/ 3
(No demonstration)
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The Chandrasekhar limit for WDs• As you increase the mass of a WD the radius decreases,
the density increases and eventually the electrons speeds approach the speed of light, i.e., they become relativistic.
• By equating the core pressure for a star in hydrostatic equilibrium, to the pressure for relativistic electrons we see that there is no radius for which the star will be in equilibrium: the star just collapses.
• From the same equation we can derive the mass of such star. For masses larger than the Chandrasekhar mass limit, the star collapses.
€
M ≤1.4 M•
for stability
(Board demonstration)
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Neutron Stars: formation
• Si-burning adds Fe to the core which increases in mass. And contracts under its own weight.
• Electron degeneracy provides the pressure but when the electrons become relativistic at the Chandrasekhar limit the star collapses in a free-fall time (<1 second).
• Protons and electrons combine to form neutrons and neutrinos (the neutrinos take energy out).
• Collapse is halted by neutron degeneracy pressure. • We have a neutron star.• A neutron star is not technically a star because it is not
gaseous and it is not powered by thermonuclear fusion….
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Neutron stars: radius
• The mass radius relations for WD is the same as that for neutron star except that the neutron mass is in place of the electron mass.
€
RNS
RWD
≈ me
mn
MWD
MNS
⎛
⎝ ⎜
⎞
⎠ ⎟
1/ 3
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Neutron stars: escape velocity
• The escape velocity from a NS is about ½ of the speed of light.
€
vesc ≈ 2GM
R
⎛
⎝ ⎜ ⎜
⎞
⎠ ⎟ ⎟
1/ 2
≈ 2 ×108 m s-1 M
1.4M•
⎛
⎝ ⎜
⎞
⎠ ⎟
2 / 3
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Neutron stars: luminosity
• When first formed a NS has T~106K. So, despite their small radius the luminosity is not terribly low.
• Using Wien’s law we see this spectrum peaks at ~30A or 400 eV in the X-ray range.
• But there is another type of radiation from NS.
€
LNS ≈ RNSR•
⎛
⎝ ⎜
⎞
⎠ ⎟
2TNST•
⎛
⎝ ⎜
⎞
⎠ ⎟
4
L• ≈ 0.2 L•
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Neutron stars: pulsars
• As the star collapses conservation of angular momentum makes it spin at about 0.1c, implying a rotation period of the order of milliseconds.
• Magnetic flux is also conserved such that the surface B fields is intensified.
• The rotating B field creates an E field that rips charged particles from the surface of the star, which later get beamed by the B field and ejected at the poles.
• They were discovered during a radio survey of the Galaxy and the first one was named LGM-1.
• There are about 500 known pulsars and considering the selection effects, there must be a lot more.
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Neutron stars: pulsars
• Why are pulsars not WDs?• If the periodicity was
due to pulsations, the material pulsating would have to be very dense indeed, much denser than a WD can be.
• If it were a rotating WD, the rotation speed at the periphery of the WD would have to be 100c.
• Pulsars are often seen in the middle of SN remnants.
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BH History
• Postulated by John Mitchell 200 years ago as objects with such high g that not even light could escape (he called them black stars).
• Calculated by Schwarzschild in 1916, as soon as GR was invented.
• Died shortly after at the front of some disease– very sad.
• John Wheeler called them Black Holes
John Wheeler American 1911-2008
Karl Schwarzschild German 1873-1916
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Black Holes: the Oppenheimer-Volkov limit
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Black Holes: the Schwarzshild radius
• By equating the escape velocity to the speed of light you see the size of a BH’s event horizon.
• Nothing going inside can emerge, not particles, not light, not information.
€
RSch ≈ 2GM
c2
⎛
⎝ ⎜
⎞
⎠ ⎟= 3 km
M
M•
⎛
⎝ ⎜
⎞
⎠ ⎟
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BH: tidal ripping
• At what distance can a BH rip a person approaching feet first? (assuming that a force that can rip a person is about 16 tonnes)
• For a 2000 Mo BH you will be ripped at exactly the Schwarzshild radius.
€
ΔF ≈ GMm
R3
⎛
⎝ ⎜
⎞
⎠ ⎟l
Rrip ≈ 435 M
M•
⎛
⎝ ⎜
⎞
⎠ ⎟
1
3
km
compare with
RSch = 3 km M
M•
⎛
⎝ ⎜
⎞
⎠ ⎟
RripRSch
≈ 160 M
M•
⎛
⎝ ⎜
⎞
⎠ ⎟
−2
3
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Observations of BHs
• V404 Cyg – binary system where a K0 star donates mass to a dark object via an accretion disk.
• The BH is revealed by the orbital motion of the companion or the luminosity of the accretion disk.