Presentation on theme: "History of White Dwarfs"— Presentation transcript:
1 History of White Dwarfs Bessell (1844)Proper motions of Sirius and Procyon wobbleSuggested they orbited “dark stars”Alvan Clark (1862)Found Sirius B at Northwestern’s Dearborn ObservatoryProcyon B found in 1895 at LickWas it a star that had cooled and dimmed?Spectrum of 40 Eri B observed – an A star!It must be hotMust have small radius to be so faintThe first “white dwarf”Adams found Sirius B is also an A star in 1915From luminosity, R~ 2 x Earth (actually ¾)From orbit, about 1 solar massDensity 105 x water (actually 106)
2 20th Century History Eddington R. H. Fowler (1926) Gas must be fully ionized so that nuclei could be compacted togetherConundrum – as the white dwarf cools, the atoms should recombine, but they can’t because the star can’t swell against gravityR. H. Fowler (1926)Recognized the role of degeneracy pressure in supporting the starChandrasekhar (1935)Upper limit to mass supported by electron degeneracy pressure due to limit of velocity of light (1.4 solar masses)Zwicky (1930’s) - Degenerate Neutron StarsSchatzman (1958) – chemical diffusion in strong gravity (plus radiative levitation, winds and mass loss, convective mixing, accretion)Greenstein and Trimble (1967) - Gravitational redshiftHewish and Bell (1967) - Pulsars
3 Interiors in a Nutshell Upper mass limit for white dwarf formation is somewhere between 5-9 solar masses – “Inside every red giant is a white dwarf waiting to get out” (Warner)Most have C-O cores, most massive may have O-Ne coresIn hot, pre-white dwarfs, neutrinos dominate energy lossWhen nuclear burning stops, photon cooling dominatesinterior becomes strongly electron degenerate, mechanical and thermal states decouple, ions are a classical ideal gasIons eventually crystallize but we still have no empirical evidence for thisCrystallization releases latent heat and carbon and oxygen may undergo a phase separation on crystallization may also provide heat which would prolong cooling timesafter crystallization, heat capacity drops, cooling times shortenInterplay of gravitational settling of heavier species and turbulent energy transport (convection) may affect surface abundancesAs the degeneracy boundary moves outward, it eventually halts the convectionAt cool enough temperatures H2 forms, and possibly even He2
4 Masses of White Dwarfs Methodology orbital solutions or binary starsmeasurements of surface gravity (with a mass-radius relation)model atmospheres with photometry, parallaxesgravitational redshiftsasterseismology<M> = 0.58 – 0.59 solar massesAbout 1/6 of (presumed) single white dwarfs show radial velocity variations
5 White Dwarfs White Dwarfs – DO, DB, DA, DF, DG, DM, DC Classifications NOT analogous to MS – reflect compositions, not temperatureDA – hydrogen lines (no other lines, pure H atmosphere)DB – neutral He lines (no hydrogen at all, pure He)DO – ionized He lines (no hydrogen at all, hotter DBs)DC – continuous spectrum, no linesDF, DG, DM (can’t discriminate DA or DB)Heavier atoms sink in gravitational fieldAbove 15,000 K, 15% are non-DA, below 15,000 K, half are non-DA. How do the stars do that?NO DB stars between 30,000 and 45,000 K
6 Surface Compositions DA (80% of WDs) and non-DA Most WDs have pure or nearly pure H or He atmospheresDAs found from hottest to coolestNon-DAs start with hot starsDOs for Teff > 45,000K with He II or He I and He IIDBs for Teff < 30,000 with He I onlyDCs (featureless) for Teff < 11,000NO He-rich WDs between 45,000 and 30,000K
7 Why the DB Gap?Simple picture of parallel sequences of H and He-rich objects doesn’t workAccelerated evolution of DBs between 45,000 and 30,000K doesn’t make senseChange in ratio of DAs and DBs around 10-15,000K also hard to explainMean masses of DAs and DBs are the sameTheory of spectral evolution – Fontaine and WesemaelAll WDs have a common origin (PNN) with some hydrogen, upper limit of 10-4 solar masses to solar masses of hydrogen (recall that 10-4 is the limit where H burning stops)Only about is needed to produce an optically thick H layer at the surfaceDiffusion brings H to surface; by Teff=45,000 K, all WDs have hydrogen atmospheres, so there are no DBsAt 30,000 K, the formation of an He ionization zone creates turbulence which mixes the H with He, and leads to He stars (stars with more than H have too much H to form a sufficient convection zone, and they remain DAs)Change in DA/non-DA ratio at 11,000 K results from onset of convection from H ionization zone, increases mixing, and more DBs appearBut this model doesn’t work
8 Spectral Evolution Model What’s wrong with the spectral evolution model?model suggests DAs should have a wide range of hydrogen layers, from 10-4 solar masses to solar masses of hydrogenAsteroseismology results suggest all DAs have thick hydrogen layersThe model also predicts trace amounts of H in the hottest DB stars (just at the cool edge of the DB gap)H was found with GHRS on HST but at a level way to low (<10-18 solar masses) to have ever permitted this DB to have been a DA in the DB gapThe WDs are fed by other sources than PNNsubdwarf O and B stars (whose origin is still not clear)IBWDs (interacting-binary white dwarfs)Both of these enter the cooling curve somewhere along the spectral sequenceMaybe the DBs come from the IBWDs, and all the DOs become DAs at 45,000 K (and stay that way)
9 Variable White DwarfsAsteroseismology with the Whole Earth Telescope (WET)Determine masses, hydrogen masses, rotation rates, magnetic fieldsZZ Ceti Stars – extension of Cepheid instability stripHydrogen ionization zone below the photosphereTemperature range from 10,500 to 13,000 KAmplitudes of 0.01 to 0.3 magPeriods of 3-20 minutesDB pulsators from He ionization zoneT ~ 20,000KPG 1159 stars – also pulsatorsT ~ 130,000Oxygen ionization zone drives pulsationsPeriods of minutes
10 Rotation Measuring rotation rates (vsini) shapes of hydrogen line coresrotational variation of polarization in magnetic white dwarfsasteroseismologyvsini measurements suggest rotation rates < 8-40 km/sec – very slow! (where does the angular momentum go?)Periodic changes in polarization gives two groups, those with rotation periods of a few days, and those with periods >100 yearsAsteroseismology also gives slow ratesClass Problem – What is the approximate rotational velocity of a star with a rotational period of 2 days? (assume we are observing it in its equatorial plane)
11 Why Is Slow Rotation a Problem? Assume a solar rotation period of 30 days, conserve angular momentum, and estimate the rotation rate if the Sun were shrunk to the radius of the Earth...
12 Magnetic Fields Broadband circular polarization detects fields > 50 MGaussCircular spectropolarimetrylimits to 1-50 kGDetected fields range from 3 kG to 1 GGasteroseismology suggests fields of 1 kGMagnetic fields detected at all temperatuers, but more and stronger fields in cool WDs (<16,000K)Does a dynamo form when convection starts?Oblique rotators again?
13 Neutron Star Oddities The non-pulsar neutron star (Geminga) discovered from x-ray brightnessimaged by HST in 1998 (V~25)distance <~400 pc (in front of IS cloud)Teff > 106 KProbably lots of these aroundThe Black Widow PulsarEclipsing double, companion R=0.2 RSun, mass of 0.02 MSunMass transfer spins up pulsarPulsar is eroding away the companionThe MagnetarMagnetic field of 1014 Gfield cracked pulsar’s crust, producing gamma and x-ray burstburst partially ionized the upper atmosphere of EarthQuark Stars?
14 Ages from White Dwarfs Age of the disk – from the coolest WDs found Liebert, Dahn, and Monet (1988, etc) used a sample from the Luyten Half-Second catalogOswalt from common proper motion binariesObservational problemscompletenessundetected binariessmall sample statistics, especially for the coolest, faintest white dwarfs. Need larger samples!Many remaining issues in WD cooling physicsC/O ratio in core, phase separation at crystallizationSettling of heavier species (22Ne, 56Fe)depth of He layerAge estimated around 10 GyrAge of the halo and globular clusters still to be done
15 Degenerate Binaries Novae 10 magnitudes or more increase in brightness over a day or twoDrop typically 3 mags in a month or twoBack to original brightness after a few years or decadesWhite dwarf – low mass main sequence binaryBegan as wider binary, then common envelope evolution tightens the binaryRecurrent novae, dwarf novaeSymbiotic Stars – binary separation sufficient that stars don’t interact until companion becomes a giantSpectrum is a cool star + hot accretion diskMass loss from giants feeds an accretion disk around the white dwarfNova-like eruptions – due to white dwarf mass accretion or to instabilities in the accretion diskX-ray binaries – neutron star + companion
16 R Corona Borealis (and other) Stars RCorBor Stars & Extreme Helium StarsA to G-type supergiantsOccasionally dim by ~8 magnitudesRecovery can take a yearVeiling by carbon dust from mass lossHighly deficient in hydrogenHelium dominatesCarbon greatly enrichedCepheid-like pulsationsHeating by 30K per year, shrinkingPost-AGB stars?Coalesced white dwarf binaries?
17 White Dwarf Merger Scenario The camera aspect remains the same, but moves back to keep the star in shot as it expands. After the star reaches 0.1 solar radii, an octal is cut away to reveal the surviving disk and white dwarf core. The red caption (x) is a nominal time counter since merger. A rod of length initially 0.1 and later 1 solar radius is shown just in front of the star.