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Monitoring orbital period variationsin eclipsing white dwarf binaries
Madelon BoursTom Marsh, Steven Parsons
Astronomy & Astrophysics Group - University of Warwick - UK
RAS MeetingLondon, January 11, 2013
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 1 / 15
Outline
1 Motivation and targets2 Observations
Liverpool Telescope + RISEULTRACAM
3 Observed orbital period variations4 Possible causes
Applegate’s mechanismThird companions
5 Conclusions
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 2 / 15
White dwarfs in eclipsing binaries
Our targets:
white dwarf primary
low-mass / white dwarf secondary
typically Porb = 1.5 - 12 hr
In the last 10-20 years thenumber of known eclipsingwhite dwarf binaries hasgrown enormously!
detached
semi-detached
Ritter H., Kolb U. 2003, A& A, 404, 301 (update RKcat7.18, 2012)
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 3 / 15
Why monitor many eclipsing white dwarf binaries?
Observed minus calculated (O-C) diagram. Calculation is based on aconstant orbital period: T = T0 + Porb · E .
Already decades ago certain eclipsing white dwarf binaries were known toshow variations in their orbital period.
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 4 / 15
Liverpool Telescope + RISE camera
The Liverpool Telescope (LT) is a 2m fully robotic telescope on La Palma.
RISE is a fast-readout camera with a single ‘V+R’ filter. Minimumexposure times are of the order of 1 second.
To monitor short period variations we aim to observe one eclipse for eachbinary every 4-8 weeks, depending on the target’s priority.
http://telescope.livjm.ac.uk/
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 5 / 15
ULTRACAM
High-speed frame transfer CCD.Takes images in three armssimultaneously. We mostly use theSDSS u’, g’ and r’ filters.
Visitor instrument on
WHT - 8.2m
VLT - 4.2m
NTT - 3.6m
Minimum exposure times can be asshort as 0.1 seconds. We can observe1-2 eclipses per target per year.
Dhillon et al. 2007
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 6 / 15
Some example light curves
Deep and sharp eclipse features allow measurements of eclipse times withaccuracies of less than 0.1 seconds.
←− LT+RISE
ULTRACAM −→
We currently monitor ∼50 binaries, of which ∼20 are recent additions.
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 7 / 15
Huge orbital period variations
33% of the well monitored targets show huge period variations.
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 8 / 15
Small orbital period variations
Another 33% of these targets show small but significant deviations.
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 9 / 15
Unknown
The last 33% do not (yet) show significant orbital period variations:
gaps in the data
only monitored for a short period of time
simply no observed variations
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 10 / 15
Applegate’s mechanism
magnetic cycles in the companion star
variations in gravitational quadrupole moment
changing gravitational attraction
balanced by centrifugal acceleration/deceleration
semi-periodic variations in orbital speed and distance
Requires energy!
companionWDcompanionWD
Applegate (1992)
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 11 / 15
Applegate’s mechanism
Strength of Applegate’s mechanism correlates withcompanion’s spectral type→ stronger magnetic cycles for younger companions
binary’s orbital period→ effect is too weak for long period binaries
Monitoring many binaries may reveal such a trend.
No orbital periodvariations expected fordouble white dwarfbinaries.
Double white dwarf CSS41177 data:
ULTRACAM, LT, Backhaus et al. 2012
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 12 / 15
Third companions
The additional mass shiftsthe system’s center of mass.
One or more companions inwide circumbinary orbits.
Can generate anyquasi-sinusoidal variation.
Circumbinary planets do exist! Some have already been directly detected.
Doyle et al. 2011, Welsh et al. 2012, Orosz et al. 2012a, 2012b
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 13 / 15
Third companions
Sensitive to very low mass planets.→ Jupiter causes the Sun to move by 2 light seconds
We expect to find binaries without circumbinary planets→ no orbital period variations
Fitting planetary systems to the data:→ correctly predict future timings→ dynamical stability
UZ Fornacis:
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 14 / 15
Conclusions
Combining precise ULTRACAM eclipse times with regular LT+RISEdata enables us to detect any period variations in a large number ofeclipsing white dwarf binaries.
Most of the well enough studied binaries show some sort of variationin their orbital periods.
If Applegate’s mechanism is the dominant cause we expect:
→ variations to correlate with companion’s spectral type→ variations to correlate with binary’s orbital period→ no variations in double white dwarf binaries
If circumbinary planets are the main cause we expect:
→ planetary fits to be able to predict future timings→ to find some systems without variations / planets→ planetary models to be dynamically stable
Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 15 / 15