Monitoring orbital period variations in eclipsing white dwarf binaries

Transcription

1 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 RAS Meeting 1 / 15

2 Outline 1 Motivation and targets 2 Observations Liverpool Telescope + RISE ULTRACAM 3 Observed orbital period variations 4 Possible causes Applegate s mechanism Third companions 5 Conclusions Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 2 / 15

3 White dwarfs in eclipsing binaries Our targets: white dwarf primary low-mass / white dwarf secondary typically P orb = hr In the last 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) Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 3 / 15

4 Why monitor many eclipsing white dwarf binaries? Observed minus calculated (O-C) diagram. Calculation is based on a constant orbital period: T = T 0 + P orb E. Already decades ago certain eclipsing white dwarf binaries were known to show variations in their orbital period. Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 4 / 15

5 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. Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 5 / 15

6 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 Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 6 / 15

7 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 RAS Meeting 7 / 15

8 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

9 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

10 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

11 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) Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 11 / 15

12 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 Madelon Bours (Warwick) Monitoring eclipsing white dwarf binaries RAS Meeting 12 / 15

13 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 13 / 15

14 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

15 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 15 / 15