The First MMT Science Symposium

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The First MMT Science Symposium In recognition of
the service of J.T. Williams
The MMTs Role in Understanding Magnetic
Accretion Binaries June 15, 2006 Collaborators
P. Smith, P. Szkody
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A textbook cataclysmic variable (CV)
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• White dwarf primary
• Sub-solar mass secondary
• Period few HOURS, Size 1 solar RADIUS!
• Accretion stream
• Viscous disk
• Mass transfer at 1011 108 M yr1 via
  Roche-lobe overflow

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Accretion funnels replace disks in magnetic CVs
(Polars)
magnetically-channelled accretion funnel
coupling region
free-falling stream
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CCD Spectropolarimeter (SPOL)

• 3900 8500Å, Dl 5 15Å
• Cosmetically-perfect, 1200x800 pixel, 2e noise
  CCD (Steward ITL)
• Linear or circular spectropolarimetry or imaging
  polarimetry
• Total throughput (opticsCCD) 40
• 6.5m MMT SPOL Keck 10m LRIS (pol)

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Polars are X-ray factories
Infall energy per nucleon is A magnetic white
dwarf will be a strong X-ray emitter (e.g., 3U
180950 AM Herculis) if m gt 0.1 g cm-2 s-1.
X-rays originate in a radial shock.
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stand-off accretion shock
h uff tcool /4
white dwarf photosphere
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A new variation the Low Accretion Rate Polar
(LARP)
LARPs accrete at rates lt1 of Roche lobe overflow!
Even this is 6 megatons/s at 0.02c!
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Radiating the accretion energy
100 10 1 0.1 0.01 0.001
Eddington specific accretion rate
polars
Shock Brems cooling (X-rays)
Shock Cyclotron (optical/IR)
g cm2 s1
No shock (bombardment regime)
LARPs
Lamb Masters (1979)
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Accretion in the bombardment regime
Incoming ions lose
energy to atmospheric
electrons by grazing
collisions, with
local cooling.
No shock forms!
Woelk Beuermann (1992)
p e transfers 0.2 of KE / collision, so kTe
0.002 x 100 keV 200 eV.
white dwarf photosphere
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Spectrum of optically-thin cyclotron emission

• Orbits are quantized in Landau levels
• wc eB/gmec

Emissivity scales as m7-12, depending on
temperature
Landau levels for uniform energy electrons
wc
2wc
3wc
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Spectrum of optically-thickish cyclotron emission

• Cyclotron harmonics are broadened by
• magnetic field spread
• Doppler effect in Maxwellian distribution
• broadened/shifted according to relativistic mass
• Emission at high m is dominated by high g
  electrons. So, high-m harmonics develop long-l
  tails.

B 30 MG
m7
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Rayleigh-Jeans for T 15 keV
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Wickramasinge Ferrario (2000)
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Polarization of cyclotron emission
Wickramasinge Ferrario (2000)
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One orbit of a Low Accretion Rate Polar from the
MMT
m4 m3
Szkody et al (2002)
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A variety of views of low-M accretion

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Undersized secondaries in LARPs
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If you dont accrete, you cant compete!
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
LARP
Earth-Sun
Li, Wickramasinghe, Wu
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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

50 MG 20 MG 10 MG
NO OPEN FIELD LINES
M6 M5 M4 M3 M2
Li, Wickramasinghe, Wu
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Were measuring low-mass wind rates!

• Theory has ranged over gt4 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 lt10-12 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.

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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
M3.5
M4.5
M5
M4
M6
M5
M3
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A synopsis of pre-CV evolution
0.3M
few M
P mos - yrs a 1-10 AU
period evolution via angular momentum losses
0.6M
0.3M
P hrs a 1 R
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Evolution of pre-Polars to Polars
pre-Polars
Polars
Roche-lobe contact for all but the smallest
secondary stars
TIME
Inefficient magnetic siphon in relatively weak
magnetic field
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By the numbers

• 6 pre-Polars have been discovered in the portion
  of the sky covered thus far by the SDSS (Þ 50
  over entire sky). All but one have B gt 50 MG.
• 80 known Polars, which is dominated by RASS
  X-ray catalog.
• But! Less than 20 of white dwarfs have B gt 50
  MG, so pre-Polars probably outnumber
  traditional Polars!

SDSS coverage through release 4

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Evolutionary status

• Underfilling of Roche lobe Þ pre-Polar.
• In fact, Twd lt 10,000K for SDSS15535516
  requires
• á M ñ lt 3x10-12 M/yr over last 106 yr!
  (Townsley Bildsten 2004)

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For M5V secondary, D 100 pc. Then, observed
4000Å flux implies WD Mass Max WD Temp
Cooling Age 0.6 M 7,500 K
4GY By contrast, life of typical Polar is
1-2 GY
SDSS15535516