Detached WD/BD binaries as progenitors of CVs

1. Detached WD/BD binaries as progenitors of CVs Matt Burleigh with Paul Steele, Francesca Faedi, Paul Dobbie (AAO) and Boris Gaensicke (Warwick)

2. Motivation • Investigate the known deficit of BD companions to main sequence stars • McCarthy & Zuckerman (2004), Grether & Lineweaver (2006) • WD gives an age constraint: benchmark BDs for testing evolutionary and atmospheric models • Pinfield et al. (2006) • In close systems, irradiated BD provides a laboratory for testing models of heated substellar atmospheres (hot Jupiters) • WD0137-349 • Post-CE systems may provide another channel for CV evolution • Politano (2004) • Previous searches by e.g. Probst (1983), Zuckerman & Becklin (1987), Green, Ali & Naperwotzki (2000), Farihi, Becklin & Zuckerman (2005), and using 2MASS: Wachter et al. (2003), Wellhouse et al. (2005), Holberg & Magargal (2005), Tremblay & Bergeron (2007) and Hoard et al. (2007)

3. Irradiation of WD0137-349B Green - K (+/-14%) Red - H (+/- 8%) White - J (+/- 3%) • AAT + IRIS2 in July 2007

4. Post-CE binaries with BD secondaries • Constrain models of common envelope evolution • What is the lowest mass object that can survive a CE? • What fraction of CVs are born with a BD secondary? • Growing number of CVs with confirmed BD secondaries • Including examples thought to have formed directly from a detached WD+BD progenitor • e.g. SDSS J150722.30+523039.8, Littlefair et al. (2007)

5. Known detached WD+BD binaries • Rare: only three confirmed systems • GD165 • 120AU separation, L4 secondary • Becklin & Zuckerman (1988) • WD0137-349 • P=116 mins, L8 secondary (0.053Msun) • Maxted et al. (2006); Burleigh et al. (2006) • GD1400 • P=9.98 hours, L6/7 secondary • Farihi & Christopher (2004), Burleigh et al. (2009) • Fraction of L dwarf companions <0.5% • (Farihi et al. 2005, survey of >300 WDs) • One companion on M/L border found among ~150 WDs observed by Spitzer • PG1234+482 (Mullally et al. 2007; Steele et al. 2007) • A handful of other WD + ultracool dwarf binaries known

6. UKIDSS • UKIRT Infrared Deep Sky Survey • LAS (Large Area Survey) - 4000 sq. deg • (~20% of northern sky, coincident with SDSS) • Limiting magnitudes - Y=20.2, J=19.6, H=18.8 & K=18.2 (5 sigma) • 2-3 mags deeper than 2MASS

7. Searching for unresolved WD+BD binaries in UKIDSS • Cross-correlate UKIDSS LAS DR4 release (1060 sq. deg) with • SDSS DR4 spectroscopically id-ed WDs (Eisenstein et al. 2006) • McCook and Sion WDs • J-H v H-K plot provides an initial selection • Better method: model each WD SED, first with BBs and finally with model atmospheres (DAs only) • Identify WDs with >3 sigma excesses at H and K

8. Results • 922 SDSS WDs in UKIDSS DR4 • + a further 84 brighter WDs from McCook & Sion • But DR4 incomplete: many stars missing photometry at one of J, H or K • After eliminating known WD+dMs, non-DAs and DAs missing photometry at H or K: • 254 DAs with photometry at both H and K • 8 with late M dwarf (M7-9) companions • 5 candidate L dwarf companions • 1 candidate T dwarf companion

9. WD+L4 WD+L6 WD+L8

10. PHL5038 = SDSS J222030.68-004107.3 K • B=18, UKIDSS K=16.7 • Follow-up imaging and spectroscopy with Gemini-North + NIRI • Resolved at 0.92” (=55AU projected) • L8 companion (T~1600K, M~0.055Msun @ 2-3Gyr) • WD T=8000+/-100K, M=0.72+/-0.15Msun, d=64+/-10pc • 4th known WD+BD detached system, 2nd wide • Steele et al. 2009

11. Frequency of close, detached WD+BD binaries • PHL5038 is flagged as partially resolved in UKIDSS • From evolutionary considerations, WD+BD binaries should either be wide (> few AU) or close (post-CE) • Substellar companion either spirals inwards in CE, or orbit expands outwards as Red Giant loses mass • Farihi, Hoard & Wachter (2006) • 2 of 5 L dwarf candidate companions classed as partially resolved (i.e. wide) • Single T dwarf candidate classed as “stellar” i.e. more likely to be a close binary

12. Sensitive to companions L0 or later in 247 DAs • Hence limit on frequency of close L-dwarf companions is < 1.2% • c.f. Farihi et al. (2005) <0.5%, at all separations…. • Sensitive to companions T0 or later in 99 DAs • Frequency of close T-dwarf companions is < 1% • Caveat: this assumes all the unresolved candidate binaries are real and are post-CE

13. Next stage • Need to spectroscopically confirm remaining substellar candidates • Determine periods and secondary masses for candidate close binaries • Optical/IR photometric variability c.f. WD0137-349 • Radial velocities of brown dwarfs from Halpha emission (WD0137-349) or near-IR spectroscopy (GD1400) • Follow-up further candidates from future UKIDSS data releases

14. Magnetic WDs in UKIDSS • 13 MWDs in list of Kawka et al. (2007) have UKIDSS photometry • One interesting system….

15. Eclipsing WD binaries in WASP Average number of points per star V magnitude 12.8 WASP-1 Orbital Period = 2.52 days X-ray and observational Astronomy X-ray and observational Astronomy ● Wide Angle Search for Planets Search for eclipses and transits of WDs by unseen very low mass companions (from BDs to Moon-size!) FoV 7.8 x 7.8 degrees Cadence 8 minutes Limit: V=13 (transiting planets) V=15 (eclipses of WDs) Instrument characteristics: - 8 Canon lenses 200mm f/1.8 - CCD 2048 x 2048 pixels - Field of View 7.8 x 7.8 degrees (61 sq. degrees) - magnitude limit =15 V mag. - photometric accuracy of 1% down to 1 15 new transiting planets discovered so far

16. Simulations white noise Simulated WASP light curve of WD0137-349 (WD + 0.053Msun BD, P=116 mins) if it was an eclipsing binary X-ray and observational Astronomy X-ray and observational Astronomy

17. 194 white dwarfs V<15 in WASP fields • Searched for eclipsing systems with periods from 2hrs - 15 days • No eclipsing binaries discovered • After full consideration of detection limits, conclude <1% of WDs have undetected substellar companions P<3hrs (0.125 days)