White Dwarfs: The most interes2ng boring objects in the universe. F.M. Walter 3 March 2017

Transcription

1 White Dwarfs: The most interes2ng boring objects in the universe F.M. Walter 3 March 2017

2 The Discovery of Sirius B ñ Sirius (α CMa), the brightest star in the sky, is a main sequence A star. Visual magnitude - 1.5

3 The Discovery of Sirius B 1844: Friedrich Bessel notes wobble in proper mo2on, deduces unseen companion

4 The Discovery of Sirius B 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 HST image

5 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 in brightness 5 magnitudes - > a factor of 100 in brightness 10 magnitudes - > a factor of 10,000 (10 4 ) Sirius A is 9100 :mes brighter than Sirius B.

6 What determines the brightness of a star? How fast they fuse Hydrogen into Helium The Sun: Absolute magnitude: 4.8 Luminosity: 4 x erg/s

7 Sirius B 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

8 A White Dwarf

9 How Big is Sirius B? Stars can be approximated as black bodies: L = 4πr 2 σt 4 r: stellar radius; 4πr 2 is surface area T: stellar temperature σ: Stefan- Boltzmann constant

10 How Big is Sirius B? Stars can be approximated as black bodies: L = 4πr 2 σt 4 (r A /r B ) 2 = (L A /L B )(T B /T A ) 4 Plugging in: T B /T A ~ 1 L A /L B ~ 10 4 è r A /r B ~ 100 r A ~ 2 R, so r B ~ 0.02 R, or 2 Earth radii More accurately, r B = R, or 0.92 R

11 Mass of Sirius B M A /M B = a A /a B = 2.07; a is semi- major axis of orbit è M B = 0.98 M

12 Evolu2on of White Dwarfs

13 Evolu2on of White Dwarfs Exposed core of a low mass star (<8 M )

14 Ring Nebula - M 57

15 Catseye Nebula

16 Hourglass Nebula MyCn 18

17

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

19 40 Eri Source: University of Alabama/SARA

20 Density of Sirius B Density (ρ) = mass/volume ρ = 4.7 x 10 6 gm/cm 3 ρ confirmed by gravita2onal redshii in 1925 Reference densi2es (gm/cm 3 ): Water: 1.0 Sun: 1.4 Rock: 3.3 Earth: 5.5 Osmium: 22.6

21 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

22 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

23 Single White Dwarfs Generally boring Stra2fied atmospheres (heavy elements sink) They make good calibrators

24 White Dwarfs in the Field 20 WDs show debris disks/evidence of accre2ng asteroids

25 White Dwarfs in Binary Systems V471 Tau: K2V + DA Period = 12.5 hours

26 V471 Tau: Egress 13 March 1998

27 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: ktons

28 Dwarf novae WDs in Binaries: I. Cataclysmic Variables

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30 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 SS Cygni a dwarf nova

31 EF Eri a Polar Accre2on directly onto surface. Accre2on flow channeled by strong magne2c field no disk forms Outbursts from instabili2es in donor star?

32 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 T n, where 4 < n < 16 Reac2on release energy hea2ng the material and causing T to rise A thermo- nuclear runaway ensues

33 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 T n, where 4 < n < 16 Reac2on release energy hea2ng the material and causing T to rise A thermo- nuclear runaway ensues Finally! An explosion

34 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 L Edd (10 38 erg/s; 10 5 L ) Ejecta cools, dust forms

35 V1369 Cen He I λ7065 Time (days) è Velocity (km/s) High velocity ouslows in Helium, up to 4500 km/s by day 40

36 V1369 Cen Sodium D Time (days) è Velocity (km/s)

37 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)

38 A Big Dust Dip

39 GK Per (N Per 1901) Source: WIYN

40 T Pyx (N Pyx 1890) Source: HST

41 Galac2c Novae Are a surface phenomenon Do not destroy the star Do not disrupt the binary star system Implica2on: Novae can recur

42 WDs in Binaries: III. Symbio2c and Recurrent Novae

43 U Scorpii

44 Recurrent Novae All novae repeat Recurrence 2mes: ~ 1 year (M31N a) ~ 1 decade (U Sco; first seen in 1863) ~ 25 years (T Pyx) ~ 75 years (YY Dor)

45 Nova Theory Fujimoto 1982 ApJ, 257, 767

46 Recurrent Novae Do white dwarfs gain mass during a nova cycle? Mass ejected ~ mass accreted If M ej > M acc then WD shrinks If M ej < M acc then WD gain mass What happens when M WD approaches the Chandrasekhar mass?

47 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 ergs, enough to unbind the star. Complete destruc2on of star no compact remnant

48 SNe IA Recycle about 1 M of Fe and Ni into the interstellar medium.

49 SNe IA A standardizable candle useful for determining cosmological distances.

50 SNe Ia SN 2005cs in M51

51 SNe Ia SN 2011dh in M51

52 Faint SNe from HST

53 SNe Ia Standardizable candle: Observed light curves can be corrected to a standard template because width depends on brightness

54 SNe Ia

55 All because white dwarfs are the only stars that can explode