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Evidence for planets orbiting the post-common envelope binary

NN Serpentis

Stefan DreizlerInstitut für Astrophysik Göttingen

Based onTwo planets orbiting the recently formed post-common envelope

binary NN SerpentisA&A submitted

K. Beuermann1, F. V. Hessman1 , S. Dreizler1, T. R. Marsh2, S.G. Parsons2, D.E. Winget3, G. F. Miller3,

M. R. Schreiber4, W. Kley5, V. S. Dhillon6, S. P. Littlefair6, C.M. Copperwheat2, J. J. Hermes3

1) Göttingen, 2) Warwick, 3) Austin, 4) Valparaiso, 5) Tübingen, 6)

Sheffield

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Exoplanets

• ≤ 500 exoplantes

• Various methods

• Growing number of planets around evolved stars

• ≈ 10% in binaries

• Recently: circum-binary planets

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A school project …

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NN Serpentis

• PG1550+131 (Wilson et al. 1986)• Porb = 3.12 hr (Haefner et al. 1989; MCCP)

• VLT imaging, spectroscopy (Haefner et al. 2004)

Haefner et al. 1989

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isOrbital Parameters

Parsons et al. 2010a

a = 0.934 R TWD = 57,000 K

MWD = 0.535 M D = 512 pc

Msec = 0.111 M Age of WD ~ 106 yr

i = 89.6°

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Eclipse Timing ResidualsQian et al. 2009

MCCP

VLT

Lijiang

UltraCam

Bialkow

P = 7.6 years, a < 3.3 A.U., M = 11 MJupiter

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isEclipse Timing Residuals

Parsons et al. 2010b

• Planetary solution rejected• No satisfactory fit with linear ephemeris

• VLT point suspicious

MCCP

VLT

UltraCam

Bialkow

Lijiang

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Revisiting the VLT Observations

• Trailed FORS images (Haefner et al. 2004)

1125.7462 secs

(±0.2 secs !)

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MONET/North Observations

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Timing Residuals

MCCP

VLT

Lijiang

UltraCam

MONET

Bialkow

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What the timing variations are not

• Not due to complicated eclipse profile

• Not due to stellar activity• Not due to Applegate’s mechanism

– Spin-orbit coupling due to magnetic cycles and radius changes within the secondary

– Time scale on decades or longer– Needs too much energy (Chen 2009)

• Not due to apsidal motion– Precession of periastron due to tides

– Amplitude t = Pbin ebin = 3577 s ebin OK with ebin~0.01

– Variation of the FWHM not seen– Period would be ~0.4 years

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McDonald Observations

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UltraCam Observations

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Model #1 : 3rd Body

P = 22.6 years, e > 0.65

a = 6.9 A.U., M = 8.4 MJupiter

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Model #2 : 2 Bodies

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The 2+2-Body Solutions

• Binary– Pdot < -10-13 (GR angular momentum loss OK)

• Two stable 2+2 solutions (grid search)– Pb:Pc ≈ 2:1 ≈ 5:2 (±15%)– Reduced 2 0.90 0.91

• NN Ser b– eb 0 0 – Pb [years] 15.50±0.45 16.73±0.26– ab [A.U.] 5.38±0.20 5.65±0.06– Mb sin i [MJupiter] 6.89±0.54 5.93±0.40

• NN Ser c– ec 0.20±0.02 0.22±0.02– Pc [years] 7.75±0.35 6.69±0.40– ac [A.U.] 3.39±0.10 3.07±0.13– Mc sin i [MJupiter] 2.24±0.38 1.61±0.27

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0.7 A.U.

0.9 A.U.

Orbital Histories of NN Ser A,B

Red Giant Envelope

Common Envelope Ejection

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Orbital History of NN Ser b,c

• Binary Star System– ~2.1 Msun A star + M dwarf at ~1 A.U. (CE=0.25)

– RGB expansion causes CE ejection ~1 million years ago

– Planets around NN Ser A absorbed

• 1st Generation (circumbinary):– NN Ser b,c at > ~3 A.U.– Drift outwards/near escape due to loss of 1.5 M from NN Ser A

– Differential drift inwards due to frictional drag (gravitational), tidal forces

– Dynamical evolution stops at radii ~3 & 5-6 A.U. with resonance condition between b & c

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A Primitive Evolution Simulation

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Orbital History of NN Ser b,c

• 2nd Generation (circumbinary) :– Original planets at a < ~1 A.U. lost in RGB – Formation of planets in the metal rich and massive CE with 1.5 M

– NN Ser b,c come into resonance as very young planets

• Mixed :– Original planets at a < ~1 A.U. lost in RGB– Less massive planets at a ~ 2-6 A.U. survive CE and accretes from CE

– 1st genaration plate might trigger planet formation in CE

– NN Ser b,c come into resonance as rejuvinated/young planets

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Conclusions

• The task of observing the variations and constraining the origin of the timing variations is very difficult

• It helps to have lots of access to 1m and occational access to 2-3m telescopes

• The eclipse time variations in NN Ser A/B are most simply explained as the timing effect due to two massive, circumbinary planets

• The planets could either be 1st or 2nd generation (or both), depending upon the details of their interaction with the CE and the CE’s long-term evolution

• Potentially many more circum-binary, post-common envelope planets to come

http://solar-flux.forumandco.com/worlds-f12/edasich-s-work-t337.htm

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Common Envelope