White Dwarf Spectra and Atmospheres Tala Monroe A540 Stellar Atmospheres Apr. 6, 2005

Outline HistoryHistory Current Classification SchemeCurrent Classification Scheme SpectraSpectra AtmospheresAtmospheres Spectral EvolutionSpectral Evolution Future WorkFuture Work

History Bessell (1844)-variability in proper motions of Sirius and Procyon  dark companionsBessell (1844)-variability in proper motions of Sirius and Procyon  dark companions Clark (1861) visually sighted Sirius BClark (1861) visually sighted Sirius B Schaeberle (1896) Lick Obs. announced Procyon’s companionSchaeberle (1896) Lick Obs. announced Procyon’s companion 40 Eri (faint white and red stars)40 Eri (faint white and red stars) –Class A0, Russell dismissed when 1 st Russell diagram published –Adams confirmed A-type Adams (1915)-Sirius B spectrum  Type A0Adams (1915)-Sirius B spectrum  Type A0 Eddington (1924) Mass-Luminosity RelationshipEddington (1924) Mass-Luminosity Relationship –Coined “white dwarfs” for 1 st time –Deduced mass and radius of Sirius B  density=53,000x water Fowler (1926) WDs supported by electron degeneracy pressure, not thermal gas pressureFowler (1926) WDs supported by electron degeneracy pressure, not thermal gas pressure Chandrasekhar (early 1930s) worked out details of white dwarf structure, predicted upper mass limit of 1.44 M sun, & found mass- radius relationChandrasekhar (early 1930s) worked out details of white dwarf structure, predicted upper mass limit of 1.44 M sun, & found mass- radius relation

Early Classifications Kuiper (mid-1930s, Lick Obs.) WDs found in increasing numbersKuiper (mid-1930s, Lick Obs.) WDs found in increasing numbers –1941 introduced 1 st WD classification scheme w in front of spectral type and Con starsw in front of spectral type and Con stars Luyten (1921) proper motion studies from faint blue star surveysLuyten (1921) proper motion studies from faint blue star surveys –1952 presented new scheme for 44 WDs D for true degeneracy, followed by A, B, C, or FD for true degeneracy, followed by A, B, C, or F Greenstein (1958) introduced new schemeGreenstein (1958) introduced new scheme –9 types

Current Classifications Sion (et al. 1983) ~2200 WDs w/in ~500 pc of Sun~2200 WDs w/in ~500 pc of Sun D=degenerateD=degenerate Second Letter-primary spectroscopic signature in opticalSecond Letter-primary spectroscopic signature in optical –DA-Hydrogen lines (5000K<T eff <80000K) –DB-He I lines (T eff <30000K) –DC-Continuous spectrum (T eff <11,000K) –DZ-Metal lines (Mg, Ca, Fe) –DQ-Atomic/Molecular carbon features –DO-He II lines (T eff >45,000K) Additional letters indicate increasingly weaker or secondary features, e.g. DAZ, DQABAdditional letters indicate increasingly weaker or secondary features, e.g. DAZ, DQAB –P-polarized magnetic, H-non-polarized magnetic, V-variable T eff indicated by digit at end; 50,400/T eff, e.g. DA4.5T eff indicated by digit at end; 50,400/T eff, e.g. DA4.5 New class T eff <4000K, IR absorption for CIA by H 2New class T eff <4000K, IR absorption for CIA by H 2

DA Spectra Rapid settling of elements heavier than H in high log g DB Spectra

DQ Stars & Spectra Helium-rich stars, generally characterized by C 2 -Swan bandsHelium-rich stars, generally characterized by C 2 -Swan bands Hotter DQs have C IHotter DQs have C I

PG 1159 Spectra Features due to CNO ions, T eff >100,000KFeatures due to CNO ions, T eff >100,000K Absence of H or He I features; He II, C IV, O VIAbsence of H or He I features; He II, C IV, O VI ZZ Ceti

Magnetic WDs About 5% of field white dwarfs display strong magnetismAbout 5% of field white dwarfs display strong magnetism 3 classes of H- atmosphere MWDs based on field strength3 classes of H- atmosphere MWDs based on field strength He-atmosphere MWDs have unique featuresHe-atmosphere MWDs have unique features

Basic Picture 75% DA, 25% non-DA75% DA, 25% non-DA Spectral classification provides info about principal constituent, with some T infoSpectral classification provides info about principal constituent, with some T info Progenitors: Post-AGB stars, central stars of planetary nebulae (CSPN), hot subdwarfsProgenitors: Post-AGB stars, central stars of planetary nebulae (CSPN), hot subdwarfs Expected structure-stratified object with ~0.6M sunExpected structure-stratified object with ~0.6M sun –C-O core, He-rich envelope, H-rich shell O-Ne cores-most massiveO-Ne cores-most massive –Atmosphere contains < M Many WDs have pure H or He atmospheresMany WDs have pure H or He atmospheres Thicknesses of H and HeThicknesses of H and He

Mechanisms in Atmosphere Gravitational diffusionGravitational diffusion ConvectionConvection Radiative levitationRadiative levitation MagnetismMagnetism AccretionAccretion Wind-lossWind-loss T-sensitive  T determines chemical abundancesT-sensitive  T determines chemical abundances

Effects of Mechanisms Diffusion & SettlingDiffusion & Settling –Gravitational separation leads to pure envelope of lightest element t<10 8 yr But, observations show traces of heavier elementsBut, observations show traces of heavier elements –radiative levitation –Cooler WDs result of recent accretion event Radiative Levitation T>40kKRadiative Levitation T>40kK –Radiative acceleration on heavy elements Convection for T<12kKConvection for T<12kK –Convection zone forms and increases inward as star cools –For He envelopes, convection begins at high T –Mixing changes surface composition –Need to couple models of atmospheres and interiors

Statistics T>45kK DA far outnumber DOT>45kK DA far outnumber DO –Ratio increases to about 30kK (diffusion) DB gap in 45k-30kK rangeDB gap in 45k-30kK range –Float up of H Always enough H to form atmosphere?Always enough H to form atmosphere? –Dredge up of He T<30kK He convection zone massive engulfs outer H layer if thinT<30kK He convection zone massive engulfs outer H layer if thin –30kK-12kK 25% stars revert to DB spectral type (edge of ZZ Ceti Strip) –Convection zone increases as T decreases. At T~11kK, numbers of DAs and non-DAs are ~equal (ZZ Ceti Strip) ‘Non-DA gap’ for K dearth of He atmospheres‘Non-DA gap’ for K dearth of He atmospheres

Spectral Evolution Gaps  individual WDs undergo spectral evolutionGaps  individual WDs undergo spectral evolution –Compositions change, DA  DB  DA, as T changes Evolution of convection zone? Accretion?Evolution of convection zone? Accretion? Explanation of ‘non-DA gap’-opacity? Bergeron et al.Explanation of ‘non-DA gap’-opacity? Bergeron et al. –Low opacity of He I means small amounts of H dominates opacity –H - atomic energy levels destroyed when H added to dense atmosphere-reduces H opacity contribution –Must accrete a lot of H to make difference in photospheric conditions  DA (fixes 6000K edge) –Re-appearance of DBs at 5000K b/c convection zone grows, H is diluted with additional He –This fails! Destruction of H - bound level produces free e -, which provide opacity

ZZ Ceti Cooling Evolution CSPN Hot DAZs (T>40kK) Radiative leviation makes Z No Z cooler than 35kK ZZ Ceti w/ variable H layers …………………10 -4 Msun He-Rich DA (0.01<He/H<20) Some DC, DZ Pure DA (He/H<0.01) Cool DAs Some w/ T<5kK

Model Atmospheres Plane-parallel geometryPlane-parallel geometry Hydrostatic equilibrium (mass loss rates)Hydrostatic equilibrium (mass loss rates) NLTENLTE Stratisfied AtmospheresStratisfied Atmospheres –Parameters: degree of ionization, intensity of radiation field Make radiative cross sections of each element depth dependentMake radiative cross sections of each element depth dependent ConvectionConvection –Parameters of Mixing Length theory

Future/Active Work Exact masses of H and He layersExact masses of H and He layers –Thin or Thick Envelopes Explanations for DB-gapExplanations for DB-gap Explanations for ‘non-DA gap’Explanations for ‘non-DA gap’ DAs outnumber He-rich WDs, yet progenitor PNN have ~equal numbers of H- and He-rich stars. What rids degenerates of He?DAs outnumber He-rich WDs, yet progenitor PNN have ~equal numbers of H- and He-rich stars. What rids degenerates of He? Couple core & atmosphere modelsCouple core & atmosphere models

References Dreizler, S. 1999, RvMA, 12, 255DDreizler, S. 1999, RvMA, 12, 255D Fontaine et al. 2001, PASP, 113, 409Fontaine et al. 2001, PASP, 113, 409 Hansen, B. 2004, Physics Reports, 399, 1Hansen, B. 2004, Physics Reports, 399, 1 Hansen, B & Liebert, J ARA&A, 41, 465Hansen, B & Liebert, J ARA&A, 41, 465 Hearnshaw, J.B. 1986, The Analysis of Starlight.Hearnshaw, J.B. 1986, The Analysis of Starlight. Koester, D. & Chanmugam, G. 1990, RPPh, 53, 837KKoester, D. & Chanmugam, G. 1990, RPPh, 53, 837K Shipman, H. 1997, White Dwarfs, p KluwerShipman, H. 1997, White Dwarfs, p Kluwer Wesemael et al. 1993, PASP, 105, 761Wesemael et al. 1993, PASP, 105, 761

WR Central Stars PG 1159 stars DO stars Via float-up, at 45 kK DA stars w/ then H layers Via dredge-up, at 30 kK DB Stars Accretion DZ  (from DAs) Accretion DBADredge-up DQ

DA Stars 5,000-80,000K5,000-80,000K Heavily broadened Balmer linesHeavily broadened Balmer lines –Strongest near 12,000K at log g~8 (DA4) No other features in optical spectrumNo other features in optical spectrum –Rapid settling of elements heavier than H in high log g –Underabundances of elements by at least 1/100 Higher dispersion revealed traces of helium in a few-DAO (HeII) and DAB (HeI)Higher dispersion revealed traces of helium in a few-DAO (HeII) and DAB (HeI)

DO Stars Spectra dominated by He IISpectra dominated by He II T eff >45,000 KT eff >45,000 K 2 subclasses2 subclasses –Cool (T eff ~45-70,000K), very strong 4686, also He I features –Hot (T eff >80,000K), only 4686 At T eff <30,000K, He II can no longer be detected, only see He IAt T eff <30,000K, He II can no longer be detected, only see He I

DO Spectra

DB Stars Classical DB stars have rich spectra of He I in optical, with nothing elseClassical DB stars have rich spectra of He I in optical, with nothing else Coolest DB stars merge with He-rich DQ starsCoolest DB stars merge with He-rich DQ stars Many DBs have H, metals (Ca II), and carbon (C I and C 2 )Many DBs have H, metals (Ca II), and carbon (C I and C 2 )

DZ Stars & Spectra He-rich stars to cool to show He I, below (DB K) still show metal featuresHe-rich stars to cool to show He I, below (DB K) still show metal features Ca I, Ca II H and K, Mg I, Fe I, Na ICa I, Ca II H and K, Mg I, Fe I, Na I

PG 1159 Stars Features due to CNO ions, T eff >100,000KFeatures due to CNO ions, T eff >100,000K Absence of H or He I features; He II, C IV, O VIAbsence of H or He I features; He II, C IV, O VI 3 groups3 groups –A: Cooler T eff ~100,000K, He II, C IV, O VI –E: T eff ~140,000K, emission cores, He II, C IV, O VI, some have N V (DOQZ1) –lgE: Low g central stars of planetary nuclei Characteristic emission cores, narrower absorption featuresCharacteristic emission cores, narrower absorption features

DC Stars & Spectra Featureless, no line deeper than 5% of continuumFeatureless, no line deeper than 5% of continuum Higher resolution reveals weak featuresHigher resolution reveals weak features Many reclassified as DB or DAMany reclassified as DB or DA True DCs remain, among coolest WDs, T eff < 11,000 KTrue DCs remain, among coolest WDs, T eff < 11,000 K