Presentation on theme: "The First MMT Science Symposium In recognition of the service of J.T. Williams The MMT’s Role in Understanding Magnetic Accretion Binaries June 15, 2006."— Presentation transcript:
The First MMT Science Symposium In recognition of the service of J.T. Williams The MMT’s Role in Understanding Magnetic Accretion Binaries June 15, 2006 Collaborators: P. Smith, P. Szkody
A textbook cataclysmic variable (CV). White dwarf primary Sub-solar mass secondary Period = few HOURS, Size = ~1 solar RADIUS! Accretion stream Viscous disk Mass transfer at 10 –11 – 10 –8 M yr –1 via Roche-lobe overflow
free-falling stream magnetically-channelled accretion “funnel” Accretion funnels replace disks in magnetic CVs (Polars) coupling region
3900 – 8500Å, = 5 – 15Å Cosmetically-perfect, 1200 x 800 pixel, 2e – noise CCD (Steward ITL) Linear or circular spectropolarimetry or imaging polarimetry Total throughput (optics+CCD) = 40% 6.5m MMT + SPOL Keck 10m + LRIS (pol) CCD Spectropolarimeter (SPOL)
Polars are X-ray factories Infall energy per nucleon is A magnetic white dwarf will be a strong X-ray emitter (e.g., 3U = AM Herculis) if m > 0.1 g cm 2 s 1. X-rays originate in a radial shock. stand-off accretion shock white dwarf photosphere h ~ ff t cool /4. ~
A new variation: the Low Accretion Rate Polar (LARP) LARPs accrete at rates <1% of Roche lobe overflow! Even this is ~6 megatons/s at 0.02c!
Radiating the accretion energy No shock (bombardment regime) Shock: Cyclotron (optical/IR) polars LARPs Eddington specific accretion rate g cm –2 s –1 Lamb & Masters (1979) Shock: Brems cooling (X-rays)
Accretion in the “bombardment” regime Incoming ions lose energy to atmospheric electrons by grazing collisions, with local cooling. No shock forms! white dwarf photosphere Woelk & Beuermann (1992) p + * e – transfers 0.2% of KE / collision, so kT e ~ x 100 keV ~ 200 eV.
Spectrum of optically-thin cyclotron emission Orbits are quantized in Landau levels: c eB m e c cc 2c2c 3c3c Emissivity scales as m 7-12, depending on temperature Landau levels for uniform energy electrons
Spectrum of optically-thickish cyclotron emission Cyclotron harmonics are broadened by 1) magnetic field spread 2) Doppler effect in Maxwellian distribution 3) broadened/shifted according to relativistic mass Emission at high m is dominated by high electrons. So, high-m harmonics develop long- tails. Rayleigh-Jeans for T ~ 15 keV m=7 6 5 B = 30 MG Wickramasinge & Ferrario (2000) 4
Polarization of cyclotron emission Wickramasinge & Ferrario (2000)
m=4 m=3 One orbit of a Low Accretion Rate Polar from the MMT Szkody et al (2002)
A variety of views of low-M accretion –.
Object P (hr) D (pc) B (MG) X-rays (cgs) M (M /yr) Emiss lines Primary (K) Secon- dary SDSS ?~2e-13 H wk 9,000 M6 (M6) SDSS <1e29 ~1e 13 H v.wk <7,500 M6 (M4.5) HS e27 <3e 13 H wk ~13,000 M4.5 (M4) SDSS > ?~2e-13No<8,500M4 SDSS / Not in RASS 2e 13 H wk <14,000 M5 (M3.5) HS <6e30 ~3e 13 No<10,000 M3.5 (M2.5) SDSS :? ~5e 14 H v.wk <7,500 M3 (M2.5) SDSS e28 6e 14 H v.wk <10,000 M5 (M2). Undersized secondaries in LARPs Underfilling of Roche lobe implies pre- magnetic CV, or pre-polar
If you don’t accrete, you can’t compete! Li, Wickramasinghe, & Wu In contrast to the Sun/Earth system, the strong field of a magnetic white dwarf can couple its field lines directly onto those of the companion star. Called “magnetic siphon” Earth-Sun LARP
M6 M5 M4 M3 M2 50 MG 20 MG 10 MG NO OPEN FIELD LINES Li, Wickramasinghe, & Wu Siphon can be nearly perfect for sufficiently high fields Some amount of wind accretion occurs in all “detached” systems – magnetic or not. White dwarf captures most or all of the wind in strongly magnetic systems. Enhanced wind capture rate plus narrow cyclotron features facilitates detection - only for strongly magnetic systems! Fields detected thus far: 42 68 MG
We’re measuring low-mass wind rates! Theory has ranged over >4 orders of magnitude: ~1 x 10 15 M /yr for M5 (Reimers 1975) ~3 x 10 11 M /yr (Mestel & Spruit 1976) Observations generally provided only limits: 3 x 10 −13 − 4 x 10 −10 M /yr for V471 Tau (Mullan et al. 1989) Some dMe stars ~few x 10 −10 M /yr (Mullan et al. 1992) But! dMe rates <10 −12 M /yr ! (Lim & White 1996; van den Oord & Doyle 1997) Importance: Stellar evolution, mass-loss mechanisms, coronal cooling, element dispersal, kinetic heating & ionization of ISM, stellar spin-down. Winds might even sweep out gas and dust ejected by red giants in globular clusters.
Stellar winds at the bottom of the main sequence Accretion rates measured in LARPs are the first realistic measures of wind mass loss rates from stars with M3-M6. (Allowance must be made for heating and forced rotation) M6 M4.5 M4 M5 M3.5 M3 M5
A synopsis of pre-CV evolution P ~ mos - yrs a ~ 1-10 AU ~0.3M few M ~0.3M ~0.6M P ~ hrs a ~1 R period evolution via angular momentum losses
pre-Polars Evolution of pre-Polars to Polars Polars Roche-lobe contact for all but the smallest secondary stars Inefficient magnetic siphon in relatively weak magnetic field TIME
. By the numbers… 6 pre-Polars have been discovered in the portion of the sky covered thus far by the SDSS ( ~50 over entire sky). All but one have B > 50 MG. 80 known Polars, which is dominated by RASS X-ray catalog. But! Less than 20% of white dwarfs have B > 50 MG, so pre-Polars probably outnumber traditional Polars! ~ SDSS coverage through release #4
Underfilling of Roche lobe pre-Polar. In fact, T wd < 10,000K for SDSS requires M < 3 x 10 12 M /yr over last 10 6 yr! (Townsley & Bildsten 2004) Evolutionary status. For M5V secondary, D ~ 100 pc. Then, observed 4000Å flux implies: WD Mass Max WD Temp Cooling Age 0.6 M 7,500 K 4+GY By contrast, life of typical Polar is 1-2 GY SDSS