Upload
others
View
2
Download
0
Embed Size (px)
Citation preview
White Dwarfs: The most interes2ng boring
objects in the universe
F.M. Walter 3 March 2017
Sirius (α CMa), the brightest star in the sky, is a main sequence A star. Visual magnitude -‐1.5
The Discovery of Sirius B ñ
The Discovery of Sirius B
• 1844: Friedrich Bessel notes “wobble” in proper mo2on, deduces unseen companion
• 1862: Alvan Clark sees Sirius B while tes2ng 18.5” telescope for Dearborn Observatory. Visual magnitude 8.4
Sirius A and B have a common proper mo2on
The Discovery of Sirius B
HST image
Aside: Magnitudes
Sirius A is 9.9 magnitudes brighter than Sirius B. What does this mean? • Magnitudes are propor2onal to the logarithm of the brightness
• 1 magnitude is a factor of 2.512 in brightness • 5 magnitudes -‐> a factor of 100 in brightness • 10 magnitudes -‐> a factor of 10,000 (104)
Sirius A is 9100 :mes brighter than Sirius B.
What determines the brightness of a
star? • How fast they fuse Hydrogen into Helium
• The Sun: – Absolute magnitude: 4.8
– Luminosity: 4 x 1033 erg/s
• 1915: Walter Adams obtains spectrum with Mt Wilson 60”; shows it is spectra type A, like Sirius A
• The stars have about the same temperature – about twice a hot as the Sun
Sirius B
A White Dwarf
¢
¢
How Big is Sirius B? • Stars can be approximated as black bodies: L = 4πr2σT4 – r: stellar radius; 4πr2 is surface area – T: stellar temperature – σ: Stefan-‐Boltzmann constant
How Big is Sirius B? Stars can be approximated as black bodies: L = 4πr2σT4
(rA/rB)2 = (LA/LB)(TB/TA)4 Plugging in: • TB/TA ~ 1 • LA/LB ~ 104
• è rA/rB ~ 100 • rA ~ 2 R¤, so rB ~ 0.02 R¤, or 2 Earth radii
• More accurately, rB = 0.0084 R¤, or 0.92 R⊕
Mass of Sirius B
MA/MB = aA/aB = 2.07; a is semi-‐major axis of orbit è MB = 0.98 M¤
Evolu2on of White Dwarfs
Evolu2on of White Dwarfs
Exposed core of a low mass star (<8 M¤)
Ring Nebula - M 57
Catseye Nebula
Hourglass Nebula MyCn 18
Evolu2on of White Dwarfs
Exposed core of a low mass star (<8 M¤) • Composition:
– C + O (if solar mass) – O + Ne + Mg (if more massive) – He (if less massive)
• Inert: no nuclear fusion • Cool with time • Crystalize at T~4000K
40 Eri
Source: University of Alabama/SARA
Density of Sirius B • Density (ρ) = mass/volume • ρ = 4.7 x 106 gm/cm3
• ρ confirmed by gravita2onal redshii in 1925
• Reference densi2es (gm/cm3): – Water: 1.0 – Sun: 1.4 – Rock: 3.3 – Earth: 5.5 – Osmium: 22.6
Theory of White Dwarfs. I.
• Normal gas: PV=nkT (or P ~ ρT) – Pressure from thermal energy (kT)
• Gas law breaks down at high pressure – High pressure dislodges electrons from atoms: a sea of ions and electrons
• Pauli Exclusion Principle: no two iden2cal fermions can have exactly the same posi2on and momentum. – Sets a minimum pressure independent of temperture
• degenerate electron pressure
Theory of White Dwarfs. II. • Non-‐rela2vis2c electrons:
– Radius ~ mass -‐1/3
– As mass increases, star gets smaller! • Rela2vis2c electrons:
– Radius independent of mass. – Implies maximum mass ~ 1.4 M¤ – This is called the Chandrasekhar limit
Single White Dwarfs
• Generally boring • Stra2fied atmospheres (heavy elements sink) • They make good calibrators
White Dwarfs in the Field
20 WDs show debris disks/evidence of accre2ng asteroids
White Dwarfs in Binary Systems
V471 Tau: K2V + DA Period = 12.5 hours
V471 Tau: Egress 13 March 1998
White Dwarfs in Binary Systems
WDs have a strong gravita2onal poten2al Dropping something onto the surface releases a lot of energy! • E = GMm/R • 184 g (0.4lb) falling on
– Sirius B: 1 kiloTon; – Earth: 0.000003 ktons
WDs in Binaries: I. Cataclysmic Variables
• Dwarf novae
SS Cygni – a dwarf nova Disk accre:on • Maper inspirals • Under certain
condi2ons disk becomes unstable, and collapses onto star
• Rapid release of gravita2onal poten2al energy heats maper and disk brightens
EF Eri – a Polar • Accre2on directly onto surface. • Accre2on flow channeled by strong magne2c field – no disk forms
• Outbursts from instabili2es in donor star?
WDs in Binaries: II. Classical Novae
What happens to the maper that reaches the surface of the WD? – It is ini2ally non-‐degenerate – It is in hydrosta2c equilibrium – Pressure and temperature build up at the base of the accreted layer – The base of the layer becomes degenerate
• Degenerate H is unstable to exploding – Nuclear reac2on rates are propor2onal to Tn, where 4 < n < 16 – Reac2on release energy hea2ng the material and causing T to rise – A thermo-‐nuclear runaway ensues
WDs in Binaries: II. Classical Novae
What happens to the maper that reaches the surface of the WD? – It is ini2ally non-‐degenerate – It is in hydrosta2c equilibrium – Pressure and temperature build up at the base of the accreted layer – The base of the layer becomes degenerate
• Degenerate H is unstable to exploding – Nuclear reac2on rates are propor2onal to Tn, where 4 < n < 16 – Reac2on release energy hea2ng the material and causing T to rise – A thermo-‐nuclear runaway ensues
Finally! An explosion
The Nova Phenomenon
• Thermonuclear runaway at base of ejected layer
• Overlying gas ejected at veloci2es up to 5000 km/s (16% of c)
• Remaining H fuses at LEdd (1038 erg/s; 105 L¤) • Ejecta cools, dust forms
V1369 Cen He I λ7065
High velocity ouslows in Helium, up to 4500 km/s by day 40
Velocity (km/s)
Time (days) è
V1369 Cen Sodium D Time (days) è
Velocity (km/s)
Op2cal Dips • Due to dust absorp2on on line of sight • Cause suggested by McLaughlin in 1935 • Occur in 18% of light curves (Strope et al. 2010)
A Big Dust Dip
GK Per (N Per 1901)
Source: WIYN
T Pyx (N Pyx 1890)
Source: HST
Galac2c Novae
• Are a surface phenomenon • Do not destroy the star • Do not disrupt the binary star system
• Implica2on: Novae can recur…
WDs in Binaries: III. Symbio2c and Recurrent Novae
U Scorpii
Recurrent Novae
All novae repeat • Recurrence 2mes: • ~ 1 year (M31N 2008-‐12a) • ~ 1 decade (U Sco; first seen in 1863) • ~ 25 years (T Pyx) • ~ 75 years (YY Dor) • …
Nova Theory
Fujimoto 1982 ApJ, 257, 767
Recurrent Novae
Do white dwarfs gain mass during a nova cycle? • Mass ejected ~ mass accreted • If Mej > Macc then WD shrinks • If Mej < Macc then WD gain mass What happens when MWD approaches the Chandrasekhar mass?
WDs in Binaries: IV. Type Ia Supernovae
As the mass approaches the Chandrasekhar mass, increasing temperature can cause carbon can ignite. • CO core detonates • Carbon fusion releases about 1051 ergs, enough to unbind the star.
Complete destruc2on of star – no compact remnant
SNe IA
• Recycle about 1 M¤ of Fe and Ni into the interstellar medium.
SNe IA • A standardizable candle useful for determining
cosmological distances.
SNe Ia
SN 2005cs in M51
SNe Ia
SN 2011dh in M51
Faint SNe from HST
SNe Ia
Standardizable candle: Observed light curves can be corrected to a standard template because width depends on brightness
SNe Ia
All because white dwarfs are the only stars that can explode