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

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Outline

1 Motivation and targets2 Observations

Liverpool Telescope + RISEULTRACAM

3 Observed orbital period variations4 Possible causes

Applegate’s mechanismThird companions

5 Conclusions

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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)

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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.

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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/

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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

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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.

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Huge orbital period variations

33% of the well monitored targets show huge period variations.

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Small orbital period variations

Another 33% of these targets show small but significant deviations.

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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

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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)

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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

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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

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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:

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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

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