Variability & Rotation in Magnetic White Dwarfs

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).


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.


Rotation Periods

€

α

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


Previous Work & Motivation


Courtesy C. Brinkworth

HJD-52000 (day)

Norm

alis

ed D

iffere

nti

al Fl

ux

G240-72

• B 100 MG, T=5590 K• Suggested to have

P 100 years (Angel, Landstreet & Borra

1981)

€

≥

€

≥

Introduction


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


htt

p:/

/ww

w.s

chools

obse

rvato

ry.o

rg.u

k/

obs/

lt/

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.


Grw+70 8247

€

χ reduced2 = 0.94 €

≥

€

o

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


G240-72

€

≥

target/comp1+3 ~4.5%variability

comp1/comp3 ~1.5%

variability

€

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


G240-72

16.3 d

56.3 dCLEAN

Scargle

Results

• Period of 56±2 days• Sinusoid fit • FAP=0.04±0.02


G240-72

€

χ reduced2 =10.38

• Period of 16.3±0.2 days

• Sinusoid fit

€

χ reduced2 =10.67

Results


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.


G227-28~3%variability

€

≤target/comp3+4

comp3/comp4

~2%variability

€

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


G227-28

€

χ reduced2 = 2.26

CLEAN

Scargle16 d67 d

34 d

16 d?

Results






G227-28

€

χ reduced2 = 2.26

CLEAN

Scargle16 d67 d

34 d

16 d?

Folded on 16d

Results






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.


MWDs from SDSS

WD0003-103 light curve in r-band taken at INT Oct 2009

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

€

≤

€

χ reduced2 = 5.75

€

χ reduced2 = 67.76

Summary


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).


Variability & Rotation in Magnetic White Dwarfs

Documents

Transcript of Variability & Rotation in Magnetic White Dwarfs