Variability & Rotation in Magnetic White Dwarfs

1. Variability & Rotation in Magnetic White Dwarfs Katherine Lawrie Matt Burleigh, Carolyn Brinkworth (Spitzer Science Center) & Tom Marsh (Warwick) 17th European White Dwarf Workshop August 17, 2010

2. Outline • Brief introduction on rotation periods • Previous work by Carolyn Brinkworth & Motivation • Results from LT • Current work on SDSS MWDs with INT Magnetic White Dwarfs Katherine Lawrie

3. Introduction Photometric Variability • 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). Magnetic White Dwarfs Katherine Lawrie

4. Introduction Rotation Periods • 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. Magnetic White Dwarfs Katherine Lawrie

5. 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 Magnetic White Dwarfs Katherine Lawrie

6. 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 Magnetic White Dwarfs Katherine Lawrie

7. Previous Work & Motivation • G240-72 • B 100 MG, T=5590 K • Suggested to have • P 100 years • (Angel, Landstreet & Borra 1981) Normalised Differential Flux HJD-52000 (day) Courtesy C. Brinkworth Magnetic White Dwarfs Katherine Lawrie

8. Introduction Correlations with physical parameters Weak negative correlation between known periods & magnetic field strength. No correlation found between periods and temperature, mass or age. Magnetic White Dwarfs Katherine Lawrie

9. 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 http://www.schoolsobservatory.org.uk/obs/lt/ Robotic Liverpool Telescope Magnetic White Dwarfs Katherine Lawrie

10. Results Grw+70 8247 • 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. Magnetic White Dwarfs Katherine Lawrie

11. Results G240-72 target/comp1+3 • 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 comp1/comp3 ~4.5% variability ~1.5% variability Magnetic White Dwarfs Katherine Lawrie

12. Results G240-72 56.3 d • 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 CLEAN 16.3 d Scargle Magnetic White Dwarfs Katherine Lawrie

13. Results G240-72 • Period of 56±2 days • Sinusoid fit • FAP=0.04±0.02 • Period of 16.3±0.2days • Sinusoid fit Magnetic White Dwarfs Katherine Lawrie

14. Results G240-72 fake light curve simulations • Variety of periods with varying amplitudes recovered Flat + noise added P=56 d 56 d 20 d Scargle 16.3 d Magnetic White Dwarfs Katherine Lawrie

15. Results G227-28 ~3% variability target/comp3+4 • 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. comp3/comp4 ~2% variability Magnetic White Dwarfs Katherine Lawrie

16. Results G227-28 • 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 CLEAN 16 d? 16 d 67 d 34 d Scargle Magnetic White Dwarfs Katherine Lawrie

17. Results G227-28 • 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 CLEAN 16 d? Folded on 16d 16 d 67 d 34 d Scargle Magnetic White Dwarfs Katherine Lawrie

18. Results G227-28 • 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 CLEAN 16 d? Folded on 16d Folded on 67d Magnetic White Dwarfs Katherine Lawrie

19. Next… MWDs from SDSS • 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. WD0003-103 light curve in r-band taken at INT Oct 2009 Magnetic White Dwarfs Katherine Lawrie

20. Results 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 Magnetic White Dwarfs Katherine Lawrie

21. Summary Correlations with physical parameters Weak negative correlation between period & magnetic field strength. Magnetic White Dwarfs Katherine Lawrie

22. 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). Magnetic White Dwarfs Katherine Lawrie