Download Monitoring orbital period variations in eclipsing white dwarf binaries

Monitoring orbital period variations
in eclipsing white dwarf binaries
Madelon Bours
Tom Marsh, Steven Parsons
Astronomy & Astrophysics Group - University of Warwick - UK
RAS Meeting
London, January 11, 2013
Madelon Bours (Warwick)
Monitoring eclipsing white dwarf binaries
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Outline
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Motivation and targets
Observations
Liverpool Telescope + RISE
ULTRACAM
Observed orbital period variations
Possible causes
Applegate’s mechanism
Third companions
Conclusions
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Monitoring eclipsing white dwarf binaries
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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 the
number of known eclipsing
white dwarf binaries has
grown enormously!
detached
semi-detached
Ritter H., Kolb U. 2003, A& A, 404, 301 (update RKcat7.18, 2012)
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Monitoring eclipsing white dwarf binaries
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Why monitor many eclipsing white dwarf binaries?
Observed minus calculated (O-C) diagram. Calculation is based on a
constant orbital period: T = T0 + Porb · E .
Already decades ago certain eclipsing white dwarf binaries were known to
show variations in their orbital period.
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Monitoring eclipsing white dwarf binaries
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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. Minimum
exposure times are of the order of 1 second.
To monitor short period variations we aim to observe one eclipse for each
binary 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 arms
simultaneously. We mostly use the
SDSS u’, g’ and r’ filters.
Visitor instrument on
WHT - 8.2m
VLT - 4.2m
NTT - 3.6m
Minimum exposure times can be as
short as 0.1 seconds. We can observe
1-2 eclipses per target per year.
Dhillon et al. 2007
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Monitoring eclipsing white dwarf binaries
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Some example light curves
Deep and sharp eclipse features allow measurements of eclipse times with
accuracies 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
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Huge orbital period variations
33% of the well monitored targets show huge period variations.
Madelon Bours (Warwick)
Monitoring eclipsing white dwarf binaries
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Small orbital period variations
Another 33% of these targets show small but significant deviations.
Madelon Bours (Warwick)
Monitoring eclipsing white dwarf binaries
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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
Madelon Bours (Warwick)
Monitoring eclipsing white dwarf binaries
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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!
WD
companion
WD
companion
Applegate (1992)
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Monitoring eclipsing white dwarf binaries
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Applegate’s mechanism
Strength of Applegate’s mechanism correlates with
companion’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 period
variations expected for
double white dwarf
binaries.
Double white dwarf CSS41177 data:
ULTRACAM, LT, Backhaus et al. 2012
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Monitoring eclipsing white dwarf binaries
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Third companions
The additional mass shifts
the system’s center of mass.
One or more companions in
wide circumbinary orbits.
Can generate any
quasi-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
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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+RISE
data enables us to detect any period variations in a large number of
eclipsing white dwarf binaries.
Most of the well enough studied binaries show some sort of variation
in 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
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