Orbital evolution of compact Black-hole binaries and white dwarf binaries Wencong Chen Astro-ph/ Astro-ph/

Astro-ph/ Introduction: Conventional magnetic braking (MB): radiative envelopes inoperative, Md<1.5 solar mass Author’s suggestion: Compact binaries with Ap & Bp stars Irradiation driven stellar wind Lead to significant MB Magnetic braking of Ap/Bp stars: application to compact BH X-ray binaries

9 of 17 compact BH X-ray binaries: P<1d, Md<1 solar mass Galactic: 1000 short period BH binaries (Wijers, 96; Romani, 98) How to form short period and low donor mass BH binaries? Intermediate mass (IM) star with Strong magnetic field, irradiation driven wind A plausible AM loss mechanism Produce short period low mass BH binaries

Loss rate of AM due to MB: 2. Assumptions and derivations: Assume wind corotates out to magnetospheric radius

2.1 estimate of the required wind loss rate Mass conservative If adot<0

For a typical mass ratio Mass –radius relation of donor star

2.2 Irradiation driven winds Irradiation stellar wind loss rate Stellar wind was driven irradiation in compact binaries ( Ruderman 89) Wind driving parameter maximum

2.3 analytic results for MB torque: Total AM of system Mass conservative and adot=0

2.4 effects on the canonical LMXB population

3. BH binary populations Suggest part of IM star possess strong magnetic field (Ap,Bp stars) New MB in strong field systems via irradiation induced stellar wind Cause a subset of BH binaries to evolve to short periods Assume Bs is a constant, even during mass loss Use an updated version of Eggleton’s code Initial conditions:

3.1 long and short period: population statistics Ap stars ~5% in A stars Zero magnetic field: Gyr Strong field: 10Gyr

3.2 observational test: spectral types

4. Summary and conclusions New MB can cause BH binaries involving Ap/Bp donor stars to evolve to short periods (P<10hr) BH binaries with IM donor star is reasonable than ones with low mass donor star Author’s model is successful at reproducing the short periods and low donor mass Shortcoming: Calculative effective temperatures are significantly higher that for those of the observed donor stars.

Astro-ph/ introduction : CVs: white dwarf primary low mass main sequence secondary Main period distribution: hours Two major features: Period gap 2-3 hours period minimum 1.3 hour Standard model Detection of a period decrease in NN Ser with ULTRACAM: evidence for strong magnetic braking or an unseen companion

In this paper: Measure mid-eclipse timing find period change To calculate AM loss Pdot~5*10^(-4) s /yr Contamination of light curve by accretion process So choose pre-CV NN ser NN ser: WD and M dwarf with ~0.15 solar mass High time resolution of ULTRACAM ~0.15S Deeply eclipsing >4.8mag, strong reflection effect ~0.6mag Orbital period 0.13days

2. Analysis & results: A best fit linear ephemeris A best fit quadratic ephemeris Eclipse time Rate of period decrease

The average rate of period change The current rate of period change

3. discussion- mechanisms for period changes: Applegate’s mechanism (92) Presence of third body in a long orbit around binary A genuine AM loss 3.1 Applegate’s mechanism Gravitational coupling Shape change of secondary Change of quadrupole moment Period change

3.2 third body Light travel time variation leads to period change solar mass < M3 < 0.18 solar mass 30yr < P3 < 285 yr A low mass companion could cause the observed changes in mid-eclipse timings

3.3 AM loss models 1. Gravitational radiation 2. Standard MB (Rappaport, 83)

3. Reduced MB (Sills, 2000)

4. conclusions: Two possible explanations: Presence of a third body Genuine AM loss: standard MB by Rappaport, no cut off Reduced MB underestimate ~ 2 orders of magnitude