1. A Test for the Disruption of Magnetic Braking in Cataclysmic Variable Evolution P. Davis 1, U. Kolb 1, B. Willems 2, B. T. Gänsicke 3 1 Department of Physics & Astronomy, Open University, Walton Hall, Milton Keynes, MK7 6AA, UK 2 Department of Physics & Astronomy, Northwestern University, 2131 Tech Drive, Evanston, Illinois, USA 3 Department of Physics, University of Warwick, Coventry, UK MNRAS, 2008, 389, 1563-1576

2. Talk Overview ■ The period gap ■ The disrupted magnetic braking hypothesis ■ Our method ■ Summary of results ■ Future work: the SDSS ■ Conclusions

3. The Period Gap & The Disrupted Magnetic Braking Hypothesis

4. Ritter & Kolb (2003), Edition 7.9 (2008)

5. Donor fully convective → magnetic braking ceases System becomes “dCV” Mass transfer resumes ~ 2 h Evolution driven by gravitational radiation. Rappaport, Verbunt & Joss (1983) Spruit & Ritter (1983) Magnetic braking drives rapid mass transfer  donor star swells

6. Method

7. ■ Calculate present day populations of: ● “detached CVs” (dCVs) ● “gap post-common envelope binaries” (gPCEBs)  0.17 < M 2 / M sun < 0.36 ■ BiSEPS (Binary System Evolution and Population Synthesis) ● Stellar evolution package: Hurley, Pols & Tout 2000 ● Binary Evolution based on Hurley, Tout & Pols 2002 ● Open University developed code (Willems & Kolb 2002, 2004) ● Significant modifications: □ Realistic treatment of mass transfer in CVs □ Reaction of donor star due to mass loss □ Evolution of dCV across period gap

8. Primary mass ? Initial Mass Ratio Distribution Common Envelope ejection efficiency, α CE α CE = constant = 0.1 – 5.0 (e.g. Willems & Kolb 2004) α CE = (M 2 /M sun ) p, p = 0.5, 1, 2 (Politano & Weiler 2007) Magnetic Braking Strength + Hurley, Pols & Tout (2002) Rappaport, Verbunt & Joss (1983) R 3 Ω 3 (M env /M) R 2 Ω 3 M R 4 Ω 3 M Angular momentum loss rate Calibrate strength  ~10 -9 M sun yr -1 at 3 hr (e.g. McDermot & Taam 1989) Disrupting Magnetic Braking ■ Gap width of ~ 1 hour ■ R 2 ~ 1.3R MS at 3 hr ■ Disrupt magnetic braking once M 2 = 0.17M sun ■  lower edge at ~ 2 hr

9. Results

10. Excess of dCVs over PCEBs in the period gap → “Mirror Gap” ■ Flat initial mass ratio distribution (Goldberg, Lazeh & Latham 2003 ■ α CE = 1 gPCEB dCV Total

11. “Mirror Gap” α CE = 0.6 α CE = 0.1 ■ Flat initial mass ratio distribution (Goldberg, Lazeh & Latham 2003) ■ Significant Mirror Gap. Ratio dCV/gPCEB in gap: ● ~ 13 for α CE = 0.1 ● ~ 4 for α CE = 0.6 Iben & Livio 1993

12. ■ The ratio dCV:gPCEB  indicator of size of mirror gap

13. How about… ■ Different Magnetic braking strengths? ■ Narrower period gap? From a weaker magnetic braking law? (Ivanova & Taam 2003) Gap width of ½ hr  dCV:gPCEB ~ 2.1  mirror gap still expected. ■ CVs from thermal timescale mass transfer Contribute an extra ~40% to calculated dCV population (Kolb & Willems 2005) dCV:gPCEB =3.5 dCV:gPCEB =5.5 dCV:gPCEB =6.0  Still obtain a mirror gap with a significant peak height, irrespective of MB law

14. ■ ~ 50 PCEBs identified with determined orbital periods. ~10 from SDSS (Rebassa-Mansergas et al 2008, Schreiber et al. 2008) ■ 3 dCV candidates so far identified. □ At 164.2, 129.5 and 130 minutes ■ Require few hundred white dwarf-main sequence binaries to adequately resolve mirror gap. SDSS

15. Conclusions ■ Dearth of CVs with P orb ≈ 2 and 3 hours. ■ Standard explanation  disrupted magnetic explanation… ■ Test: Orbital period distribution of gPCEB and dCV population  “Mirror Gap”  excess of dCV over gPCEBs there. ■ Expect dCV:gPCEB ~ 4 to 13  mirror gap with a significant peak height ■ Observationally feasible  SDSS

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