orbital evolution of compact black-hole binaries and white dwarf binaries wencong chen...
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Orbital evolution of compact Black-hole binaries and white dwarf binaries
Wencong Chen
Astro-ph/0511760
Astro-ph/0510331
Astro-ph/0511760
1.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
2 2 62
6
1
2 8 8s dB B R
vr
24windM v r
2 d
d
GMv
R
1/ 4
1/81/ 2 13/8windm s d dr B R M GM
1/ 2
1/ 42 13/ 4wind windMB d m d s d dJ r M B R M GM
Loss rate of AM due to MB:
2. Assumptions and derivations:
Assume wind corotates out to magnetospheric radius
11
2RLOFMB d
d BH
J a M M
J a M M
1/ 2RLOFMB totJ M GM a
2 2wind RLOFmM r M a
2
2 5/ 2 4d
wind RLOF
s d L
GMM M
B R r
/d La R r
1/ 2 1/ 2 1/ 2BH d TJ G M M M a
2.1 estimate of the required wind loss rate
Mass conservative
If adot<0
For a typical mass ratio
1/3Lr Mass –radius relation of donor star
4/5dM R
23/ 2
131 2 1
15 10
wind RLOFd
s
M M M
M yr B M M yr
10 1~ 4 10windM M yr
1000sB G
8 110RLOFM M yr
5dM M
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 windM
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: 3,4,5dM M7BHM M
, 1orb iP day
3.1 long and short period: population statistics
Ap stars ~5% in A stars
Zero magnetic field: 0.2-0.4Gyr
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/0510331
1.introduction:
CVs: white dwarf primary
low mass main sequence secondary
Main period distribution: 1.3-10 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
0.0043 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