Variability & Rotation in Magnetic White Dwarfs Matt Burleigh, Carolyn Brinkworth (Spitzer Science Center) & Tom Marsh (Warwick) Katherine Lawrie 17 th European White Dwarf Workshop August 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 Lawrie Magnetic 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 Lawrie Magnetic 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 Lawrie Magnetic White Dwarfs Rotation Periods

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 Lawrie Magnetic 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 Lawrie Magnetic White Dwarfs

Previous Work & Motivation Katherine Lawrie Magnetic White Dwarfs Courtesy C. Brinkworth HJD (day) Normalised Differential Flux G B 100 MG, T=5590 K Suggested to have P 100 years (Angel, Landstreet & Borra 1981)

Introduction Katherine Lawrie Magnetic 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 Searched for long term photometric variability in: – suspected long period variables (months - years) from Brinkworth et al – suggested long term variables (years - decades) from literature Katherine Lawrie Magnetic White Dwarfs 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 Lawrie Magnetic White Dwarfs Grw

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 Lawrie Magnetic White Dwarfs G target/comp1+3 ~4.5% variability comp1/comp3 ~1.5% variability

Results Two peaks detected in CLEAN periodogram d & 16.3 d Peaks also detected in Scargle periodogram Neither peak in analysis of comparison stars Katherine Lawrie Magnetic White Dwarfs G d 56.3 d CLEAN Scargle

Results Period of 56±2 days Sinusoid fit FAP=0.04±0.02 Katherine Lawrie Magnetic White Dwarfs G Period of 16.3±0.2 days Sinusoid fit

Results Katherine Lawrie Magnetic White Dwarfs G fake light curve simulations Variety of periods with varying amplitudes recovered Flat + noiseadded P=56 d Scargle 56 d 20 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 Lawrie Magnetic White Dwarfs G ~3% variability target/comp3+4 comp3/comp4 ~2% variability

Results Best fitting period of 16±0.3 days Sinusoid fit FAP=0.31± d period not in CLEAN Also strong peak at 67±7 d Katherine Lawrie Magnetic White Dwarfs G CLEAN Scargle 16 d 67 d 34 d 16 d?

Results Best fitting period of 16±0.3 days Sinusoid fit FAP=0.31± d period not in CLEAN Also strong peak at 67±7 d Katherine Lawrie Magnetic White Dwarfs G CLEAN Scargle 16 d 67 d 34 d 16 d? Folded on 16d

Results Best fitting period of 16±0.3 days Sinusoid fit FAP=0.31± d period not in CLEAN Also strong peak at 67±7 d Katherine Lawrie Magnetic White Dwarfs G 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 Targets for follow-up to determine periods. Katherine Lawrie Magnetic White Dwarfs MWDs from SDSS WD light curve in r-band taken at INT Oct 2009

Results Katherine Lawrie Magnetic 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

Summary Katherine Lawrie Magnetic 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 - G & G 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 Lawrie Magnetic White Dwarfs