variability & rotation in magnetic white dwarfs matt burleigh, carolyn brinkworth (spitzer...
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Variability & Rotation in Magnetic White Dwarfs
Matt Burleigh, Carolyn Brinkworth (Spitzer Science Center) & Tom Marsh (Warwick)
Katherine Lawrie
17th European White Dwarf WorkshopAugust 17, 2010
Outline
• Brief introduction on rotation periods
• Previous work by Carolyn Brinkworth & Motivation
• Results from LT
• Current work on SDSS MWDs with INT
Katherine LawrieMagnetic White Dwarfs
Introduction
• Rotation period measured by variable photometric or circular polarization measurements.
• HFMWDs changes in magnetic field & structure with rotation.
• Cool MWDs (T<12000K) variation attributed to star spots (Brinkworth et al. 2004, 2005).
Katherine LawrieMagnetic White Dwarfs
Photometric Variability
Introduction
• Rotation velocities estimated from H line in spectroscopic measurements.
• Generally provides upper limits.
• Berger et al (2005) used the Ca II K line and measured rotation velocities ~<10 km/s . Only possible for rare metal rich DAZ white dwarfs.
• Rotation velocities consistent with rotation periods of MWDs.
Katherine LawrieMagnetic White Dwarfs
Rotation Periods
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Previous Work & Motivation
• Wickramasinghe & Ferrario 2000 noted 16/65 single MWDs with periodic variability between 12 m to 17 d
• Some MWDs suspected to have periods ~decades to ~100y
• Brinkworth et al 2007 searched for short term variability (hrs to weeks) in 34 MWDs and found:– 5 displaying strong variability and clear periods– 12 displaying strong variability but undefined periods– 15 with weak evidence for variability– 2 not varying
Katherine LawrieMagnetic White Dwarfs
Previous Work & Motivation
• Wickramasinghe & Ferrario 2000 noted 16/65 single MWDs with periodic variability between 12 m to 17 d
• Some MWDs suspected to have periods ~decades to ~100y
• Brinkworth et al 2007 searched for short term variability (hrs to weeks) in 34 MWDs and found:– 5 displaying strong variability and clear periods– 12 displaying strong variability but undefined periods– 15 with weak evidence for variability– 2 not varying
Katherine LawrieMagnetic White Dwarfs
Previous Work & Motivation
Katherine LawrieMagnetic White Dwarfs
Courtesy C. Brinkworth
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G240-72
• B 100 MG, T=5590 K• Suggested to have
P 100 years (Angel, Landstreet & Borra
1981)
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Introduction
Katherine LawrieMagnetic White Dwarfs
Correlations with physical parameters
No correlation found between periods and temperature, mass or age.
Weak negative correlation between known periods & magnetic field strength.
Observations
• 10 targets observed with the 2m robotic Liverpool Telescope between March 2005 & January 2007.
• Searched for long term photometric variability in:– suspected long period variables
(months - years) from Brinkworth et al. 2007
– suggested long term variables (years - decades) from literature
Katherine LawrieMagnetic White Dwarfs
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Robotic Liverpool Telescope
Results
• B=320 MG, T=12070 K• Thought to be a very slow rotator with P 100
years
• Constant fit• No variability found
Agrees with very slow rotator
hypothesis.
Katherine LawrieMagnetic White Dwarfs
Grw+70 8247
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χ reduced2 = 0.94 €
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Results• B 100 MG, T=5590 K• Suggested to have a period 100 years (Angel et al
1981)• Brinkworth et al (2007) noted change in flux of ~2.5%
over several months• Constant fit
Katherine LawrieMagnetic White Dwarfs
G240-72
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target/comp1+3 ~4.5%variability
comp1/comp3 ~1.5%
variability
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χ reduced2 =17.50€
≥
Results
• Two peaks detected in CLEAN periodogram - 56.3 d & 16.3 d
• Peaks also detected in Scargle periodogram
• Neither peak in analysis of comparison stars
Katherine LawrieMagnetic White Dwarfs
G240-72
16.3 d
56.3 dCLEAN
Scargle
Results
• Period of 56±2 days• Sinusoid fit • FAP=0.04±0.02
Katherine LawrieMagnetic White Dwarfs
G240-72
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χ reduced2 =10.38
• Period of 16.3±0.2 days
• Sinusoid fit
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χ reduced2 =10.67
Results
Katherine LawrieMagnetic White Dwarfs
G240-72 fake light curve simulations• Variety of periods with varying amplitudes
recovered
Flat + noise added P=56 d Scargle
56 d20 d
16.3 d
Results
• B 0.1 MG, T=4780 K
• 2% difference in flux noted by Brinkworth et al (2007) over several months.
• Constant fit
• Variability in comparisons is quite large.
Katherine LawrieMagnetic White Dwarfs
G227-28~3%variability
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≤target/comp3+4
comp3/comp4
~2%variability
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χ reduced2 = 6.39
Results
• Best fitting period of 16±0.3 days
• Sinusoid fit• FAP=0.31±0.05
• 16 d period not in CLEAN
• Also strong peak at 67±7 d
Katherine LawrieMagnetic White Dwarfs
G227-28
€
χ reduced2 = 2.26
CLEAN
Scargle16 d67 d
34 d
16 d?
Results
• Best fitting period of 16±0.3 days
• Sinusoid fit• FAP=0.31±0.05
• 16 d period not in CLEAN
• Also strong peak at 67±7 d
Katherine LawrieMagnetic White Dwarfs
G227-28
€
χ reduced2 = 2.26
CLEAN
Scargle16 d67 d
34 d
16 d?
Folded on 16d
Results
• Best fitting period of 16±0.3 days
• Sinusoid fit• FAP=0.31±0.05
• 16 d period not in CLEAN
• Also strong peak at 67±7 d
Katherine LawrieMagnetic White Dwarfs
G227-28
€
χ reduced2 = 2.26
CLEAN
16 d?
Folded on 16d
Folded on 67d
Next…
• Search for short term variability (hrs-1wk) in MWDs selected from SDSS.
• 26 Sloan MWDs observed using INT in 3 runs from
March 2009 - 2010. Targets for follow-up
to determine periods.
Katherine LawrieMagnetic White Dwarfs
MWDs from SDSS
WD0003-103 light curve in r-band taken at INT Oct 2009
Results
Katherine LawrieMagnetic White Dwarfs
LHS 5064
• B 0.2 MG, T=6680K• Best-fitting period of 7.7±0.8 d
• Sinusoid fit• Constant fit• FAP=0.02±0.01
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χ reduced2 = 5.75
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χ reduced2 = 67.76
Summary
Katherine LawrieMagnetic White Dwarfs
Correlations with physical parameters
Weak negative correlation between period & magnetic field strength.
Conclusions
• Periodic variability possibly found in 2 out of the 10 targets - G240-72 & G227-28.
• Could not reliably detect below the ±1-2% level.
• Weak negative correlation between period & magnetic field strength, but need more data!
• Using INT data to search for short term variability in SDSS MWDs & follow up (eg. LHS5064).
Katherine LawrieMagnetic White Dwarfs