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Accretion Processes in Binaries of White Dwarfs Hee-Won Lee Dept. of Astronomy Astrophysical Research Center for the Structure and Evolution of the Cosmos Sejong University Contents • Accretion in Astrophysics • Binary Systems of White Dwarfs • Cataclysmic Variables Non-magnetic CV Polars : AM Her Stars Intermediate Polars : DQ Her • Symbiotic Stars • Raman Scattering in Symbiotic Stars • Discussion and Summary Energy Production in the Universe • Nuclear interaction – Stellar Radiation 0.007 mp c2 per proton • Gravitational interactionAccretion onto compact objects Accretion in the Universe I • • Gravitational Potential Energy GMm/R = 0.5mc2 (RSch/R) Eddington Luminosity = Maximally allowed energy output for steady spherical accretion Importance of Accretion 1. Formation of Planetary Rings, Solar System and Stars 2. Mysteries of Quasars IGM, galaxy evolution 3. Various high energy processes around compact objects Accretion in the Universe II • • • • • Super Massive Black Holes (Quasars) Protostars Stellar Black Holes Neutron Stars White Dwarfs Roche Geometry • Effective Potential in the Co-rotating Frame • 5 Lagrangian Points where the gradient of the effective potential vanishes. L2 • Through L1, matter may overflow from one onto the other • Stellar matter may reach L1 by filling the lobe by the star or by stellar wind – Roche Lobe Overflow or Wind Accretion L4 L3 L5 Formation of Accretion Disk 1 .Through the L1 point, the stream has an angular momentum that is hard to eliminate. 2. Energy may be dissipated via stream collision. 3. Eventually the mass stream will settle into the orbit with minimum energy for given angular momentum Circularization of the orbit and the accretion ring is formed. Angular Momentum Transport • Viscosity is responsible for the angular momentum transport from inner region to outer region. • High accretion rate is possible in the presence of high viscosity. • Accretion flow is stable, when mass is transferred from the lighter star to the heavier star. • Magnetic field for slowly rotating systems and gravitational wave radiation may operate for fast rotating systems Binaries of White Dwarfs • Non-interacting binaries – e.g. Sirius A, B • Roche Lobe Overflowing Systems -Cataclysmic Variables • Stellar Wind Accretion Systems – Symbiotic Stars Sirius B In optical Sirius B In X-ray Cataclysmic Variables • Ideal systems for investigation of accretion processes • Quasars are too far away. • Protostars are severely obscured by dust. • Neutron star and black hole systems emit most energy in X-rays. Introduction of CV • Consists of an accreting white dwarf (primary) and a low-mass main sequence star (secondary) filling its Roche lobe • Orbital periods : 90 min – 14 hr • Accretion Luminosity – several times of the solar luminosity (mainly in UV through X-ray) Classification of CVs • Non-magnetic CV (1) Dwarf Novae (U Gem stars) (2) Z Cam Stars (3) Nova-like Variable • Magnetic CV (1) Intermediate Polar (DQ Her)-partially disrupted accretion disk by magnetic field (2) Polar (AM Her) – totally disrupted accretion disk U Gem (or SS Cyg) Stars • Dwarf Nova Outbursts are semi-regular (repeating quiescence and outburst state) • Brightening by 2-5 Mag • Orbital period > 3h • Angular momentum loss via magnetic field Disk Thermal Instability Model • • • • • • • Y. Osaki proposed that Dwarf nova outbursts occur due to Thermal Instability Viscosity may change abruptly. There are two branches of equation of state between the surface mass density of disk and temperature (or accretion rate). High temperature state corresponds to high viscosity state. In cold disk state, material piles up in the disk, because accretion is inefficient and secondary keeps dumping mass. In hot disk state, accretion rate exceeds the mass transfer rate to become cooler. Instability due to B Hot disk High viscosity Cold disk Low viscosity Z Cam Stars 1. Light curves are characterized by standstills. 2. Brightness remains at a constant level before outbursts resume. 3. Mass transfer rate is high, close to the lower limit of the upper branch of Osaki’s model Novalike Variables • Classical Novae : historical record • Novalike variables: without a historical record but similar observational characters • Quasi-steady light curves with high mass transfer rate SU UMa Stars • Superoutbursts • Longer duration • Small mass ratio q=M2/M1 • Superhumps • A few percent longer than the orbital period • Eccentric Disk : precession and orbital (3:1) resonance Superhumps of SU UMa • The potential is not exactly Keplerian (due to the presence of the secondary), and the orbit is not closed. • Prograde precession of an eccentric accretion disk • Beating phenomena between the precession period and the binary orbital period Polars : AM Her Stars • Strong magnetic fields (B ~ 107-8 Gauss) completely disrupt the accretion flow, preventing the formation of an accretion disk. • Accretion column forms at the polar regions, where charged particles falls almost freely. • X-ray pulses with the half-period that of the primary spin. Polars : Accretion Column • Accretion column forms at the polar regions, where charged particles fall almost freely. • Hard X-rays are emitted at the shock front. • Soft X-rays and UV photons are emitted at the photosphere. X-ray Observation of AM Her Star • Quasi-sinusoidal hard X-ray light curve • Square wavelike soft X-ray light curve out of phase with hard X-ray • Existence of two accretion poles with one pole dominating in soft X-rays and the other in hard X-rays Intermediate Polar : DQ Her • With intermediate strength of B field (1067 Gauss), accretion flow is partially disrupted inside the accretion disk. • Asynchronous rotation : Weaker magnetic field is not sufficient to lock the primary spin to the binary orbital rotation X-ray Observations of IP • Low Energy Dip before Eclipse • No or Little Dip is seen in hard X-ray light curve • Dip is characterized by high hardness ratio • Absorption by accretion disk for soft X-ray AE Aqr : Propeller System Intermediate polar Known as a magnetic propeller Very rapidly rotating WD P_spin = 33.08s, P_orbit = 9.88 hr Spin-down power = 1034 erg s-1 Luminosity several orders less than spin-down power -Propeller system : most material is ejected by the primary Doppler Tomography • Keplerian disk : symmetric double peak profile • Isovelocity contour: regions giving rise to the same velocity component w.r.t. the observer’s line of sight • Bright spot contributes significantly to the line profile, destroying the symmetry and exhibits variations as the binary phase. Doppler Tomography • Useful tool to investigate the accretion flow and the bright spot Polarimetry for Magnetic Stars • Cyclotron emission is generally circularly polarized. • = eB/(2 mc)=3x1014 B8 Hz • Higher harmonics are observed in the optical region • Zeeman effect can be used to derive the strength of B field. Evolution of CV • Period minimum at 78 min and period gap between 2 and 3 h • Reaching the period minimum, the secondary red dwarf becomes degenerate with the size increasing as mass decreases. • Binary expands and angular momentum transport is less efficient leading to faint accretion luminosity. Symbiotic Stars • Wide binary systems of a giant and a hot white dwarf • TiO absorption band, Prominent Emission Lines • Detached system (underfills the Roche lobe) • S(stellar) type and D(dusty) type • D type symbiotics have Mira and OH/IR sources as the giant component. Symbiotic Stars • Often accompany bipolar nebulae • 10 percent of planetary nebulae are bipolar Binarity of the central star system? • Severe mass loss rate through slow stellar wind from the giant. • Fast stellar wind from the white dwarf component • Collision of slow stellar wind and fast wind Wind Accretion in Symbiotics 1. Mass losing giant may shed a large amount of mass in the form of the slow stellar wind 2. SPH simulations show that steady accretion disk can be formed via gravitational capture of slow stellar wind Raman Scattering in Symbiotic Stars I • Very broad emission features at 6830 and 7088 are only known to exist in about a half of symbiotic stars. • They were identified in 1989 by Schmid as Raman scattered features of O VI 1032 and 1038 Raman Scattering in Symbiotics II • O VI 1032 and 1038 are less energetic than Ly beta 1025. • If Ly beta 1025 is absorbed by a hydrogen atom, Balmer alpha can be generated with the deexcitation into 2s state. • The hydrogen atom is excited by an incident O VI photon and may de-excite into 2s state with an re-emission of an optical photon redward of Balmer alpha. Raman Scattering in Symbiotics III • The scattering cross section can be computed from the 2nd order perturbation theory in quantum mechanics. σ=10-22 cm2 for O VI σ=10-20 cm2 for He II Raman Scattering in Symbiotics IV • O VI 1032 and 1038 are prominently emitted near white dwarf. • Hydrogen atoms are Giant abundantly found around the giant, forming an extended atmosphere or a part of slow stellar wind • The condition of operation of Raman scattering is ideally met only in symbiotic stars. H I Scatering • So far 6830 and 7088 Region features have never been observed other than in symbiotic stars Stellar Wind Stream OVI 1032 Dis k White Dwarf Hot spot O VI Emission Region Spectroscopic and Polarimetric Properties • Double or Triply Peaked Profile • Strongly polarized • Polarization flip is shown in the red wing. • 6830 and 7088 exhibits different profiles Characteristic of wind accretion disk emission in symbiotic stars Wind Accretion Disk Model • Lee & Park (1999) proposed to interpret Raman O VI adopting the wind accretion disk emission model. • Refer to the next Talk by Suna Kang Giant Stellar Wind Stream OVI 1032 Dis k White Dwarf Hot spot H I Scatering O VI Emission Region Region BOES Observation of Symbiotic Stars BOES Spectrograph H HeII4860 Raman 4850 Mass Loss Rate and He II Raman Spectroscopy • He II Raman scattered feature has flux proportional to the extent of H I region. • He II optical emission lines allow one to estimate exactly the Raman scattering efficiency. • 3.6X10-7 M/yr for V1016 Cyg (determined by Yang Chan Jung) Summary • Accretion processes take place from a small scale of planetary systems to a huge galactic scale involving spiral galaxies. • Photometric and spectroscopic, polarimetric observations from radio to Xray can all contribute to understanding accretion processes. • White dwarf system is particularly interesting to investigate various aspect of accretion processes. Sciences for the 2.4m Telescope • High Resolution Spectroscopy of CV, Symbiotic Stars will be very interesting. • Doppler Tomography is a useful tool to investigate the accretion processes. • Raman Spectroscopy provides unique opportunities to look into the wind accretion processes in symbiotic stars. Thank you