Lecture 13: Binary evolution

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1 Lecture 13: Binary evolution Senior Astrophysics Senior Astrophysics Lecture 13: Binary evolution / 37

2 Outline 1 Conservative mass transfer 2 Non-conservative mass transfer 3 Cataclysmic variables 4 Type Ia supernovae 5 X-ray binaries 6 Recycled pulsars Senior Astrophysics Lecture 13: Binary evolution / 37

3 Consequences of mass transfer We have seen how mass transfer can occur when one (or both) star in a binary system comes in contact with its Roche lobe. We have seen how the material is transferred via either an accretion disk or via stellar wind. In this lecture, we will see the effect this mass transfer has on the properties of the binary. When a pair of stars transfer material, both mass and angular momentum are redistributed in the binary or may even be lost altogether into space, depending on the physical mechanisms at work. When this happens, both the orbital separation and the mass ratio can change, so in general the Roche-lobe radius will change as well. Lecture 13: Binary evolution Conservative mass transfer 3 / 37

4 Binary angular momentum Consider two stars with mass M 1, M 2 (with total mass M = M 1 + M 2 ) in a binary system with semi-major axis a: the system has binary angular velocity G(M1 + M 2 ) ω = total energy E = K 1 + K 2 + Ω = 1 2 M 1v M 2v 2 2 GM 1M 2 a a 3 Lecture 13: Binary evolution Conservative mass transfer 4 / 37

5 Angular momentum The binary angular momentum is J = M 1 a 2 1ω + M 2 a 2 2ω = M 1M 2 M a2 ω Ga = M 1 M 2 M M2 a2 inner Lagrange point a1 + centre of mass M1 Lecture 13: Binary evolution Conservative mass transfer 5 / 37

6 Roche lobe overflow When mass is transferred from one star to the other, or is lost to the system entirely, the angular momentum of the binary changes, which changes orbital parameters like the radius and the period. In particular, if one star fills its Roche lobe, then matter can freely escape from the surface through the inner Lagrange point L 1 and will be captured by the other star. Lecture 13: Binary evolution Conservative mass transfer 6 / 37

7 Roche lobe overflow A star can fill its Roche lobe either due to expansion, e.g. on red giant branch; or when the Roche lobe shrinks. If the binary loses angular momentum, the stars will spiral together, and the Roche lobes will close in on one or both stars. Note that a star can never be bigger than its Roche lobe; otherwise, the excess material would just flow off onto the other star or into space. Let s see how the angular momentum changes when the stars transfer mass. Lecture 13: Binary evolution Conservative mass transfer 7 / 37

8 Change of angular momentum Differentiate the expression for the angular momentum: dj dt = J Ga Ga = Ṁ1M 2 M + M 1Ṁ2 M + M 1 M 2 G M 1 2 a 1 2 ȧ Divide through by J: so J J = ȧ a = 2 J J 2 Ṁ1 + Ṁ2 + ȧ M 1 M 2 2a (Ṁ1 M 1 + Ṁ2 M 2 ) Lecture 13: Binary evolution Conservative mass transfer 8 / 37

9 Conservative mass transfer Now, assume that all the mass lost by one star is gained by the other, so dm dt = Ṁ1 + Ṁ2 = 0. Then Ṁ2 = Ṁ1, so ȧ a = 2 J ( J 2Ṁ1 1 M ) 1 M 1 M 2 Now let s assume that angular momentum is conserved, so J = 0. Then ( ȧ a = 2Ṁ1 1 M ) 1 M 1 M 2 Lecture 13: Binary evolution Conservative mass transfer 9 / 37

10 Conservative mass transfer ( ȧ a = 2Ṁ1 1 M ) 1 M 1 M 2 Assume the primary star (star 1) is losing mass, Ṁ 1 < 0. Then if M 1 < M 2, then ȧ a if M 1 > M 2, then 1 M 1 M 2 i.e. runaway mass transfer > 0, i.e. orbit widens < 0 ȧ a < 0, i.e. orbit shrinks Roche lobe shrinks more mass transferred orbit shrinks more Lecture 13: Binary evolution Conservative mass transfer 10 / 37

11 Unstable mass transfer In the case of runaway mass transfer, we can get a spiral-in. The donor s envelope engulfs both stars, since the Roche lobe of the donor has shrunk so much forms a common envelope around both stars. Both stars feel a frictional drag energy is extracted from orbit and imparted to the envelope. The two stars spiral together, and the envelope is ejected. Angular momentum is no longer conserved Lecture 13: Binary evolution Non-conservative mass transfer 11 / 37

12 Spiral-in The detailed physics of the common-envelope phase is not well understood. Process probably very efficient: a decreases by 10 3 in a very short time ( 1000 y). Final orbital separation will depend on the binding energy of the envelope. If the binding energy is too large, the stars will merge completely (cf. progenitor to SN1987A?) Let s now look at some observed classes of interacting binaries. Lecture 13: Binary evolution Non-conservative mass transfer 12 / 37

13 Cataclysmic variables Cataclysmic variables (CVs) are a class of interacting binary stars consisting of a white dwarf with a low-mass companion star. CVs all have very short orbital periods, 1 12h. System originally consisted of two normal main-sequence stars in a large orbit ( several years). More massive star evolves to become a red giant, and mass transfer starts. Since M donor > M acccretor, orbit shrinks, and a period of common-envelope evolution begins. Stars spiral together until the envelope is ejected; end result is a planetary nebula with a close binary at its centre, consisting of a white dwarf and its companion. The white dwarf is more massive than its companion; typical masses are M wd 0.8M and M 2 0.3M. Lecture 13: Binary evolution Cataclysmic variables 13 / 37

14 Cataclysmic variables Lecture 13: Binary evolution Cataclysmic variables 14 / 37

15 Cataclysmic variables Once the envelope has been ejected (on very short timescales, t th ), binary remains quiescent until the second star comes in contact with its Roche lobe. Then white dwarf begins accreting matter via an accretion disk, and the system becomes visible as a cataclysmic variable. Because M 2 < M 1, the transfer is stable. H-rich gas from outer envelope accumulates on the surface of the white dwarf, until P and ρ reach the point when fusion can begin. Matter is degenerate all H on the surface ignites almost at once in a thermonuclear runaway, which consumes all the accreted material. Lecture 13: Binary evolution Cataclysmic variables 15 / 37

16 Novae These explosions are visible as a massive increase in brightness of the star: by a factor of 50,000 or more. Because the pre-explosion star was essentially anonymous, the star appears as a new star; these are known as novae. Nova Herculis 1934, before and during outburst, when it brightened by a factor of 60,000. Lecture 13: Binary evolution Cataclysmic variables 16 / 37

17 Novae Novae typically brighten in a couple of days, then fade more slowly over a month or more. They brighten by 6 10 magnitudes: there have been seven in the last century which reached magnitude 2 or brighter. The accreted layer is blown off in the explosion, which produces an expanding shell. The process repeats on timescales of thousands of years. Lecture 13: Binary evolution Cataclysmic variables 17 / 37

18 Type Ia supernovae When we were talking about supernovae, I mentioned that there is a class of supernovae that do not show a bump in their lightcurve. A white dwarf, accreting matter in a binary system, accretes enough to push it over the Chandrasekhar limit, so it starts to collapse. However, unlike the iron core of a massive star, the collapsing white dwarf is made up of carbon and oxygen, which ignites during the collapse. The fury of the resulting explosion tears the star completely apart in a second. SN Ia supernovae have leapt into prominence recently with the awarding of the 2011 Nobel Prize to two teams for their discovery of the accelerating universe, based on observations of SNe Ia. Lecture 13: Binary evolution Type Ia supernovae 18 / 37

19 Type Ia supernovae The light curves for SNe Ia are similar enough that it seems likely they all arise in the same manner: the thermonuclear disruption of Chandrasekhar-mass white dwarfs. Unfortunately, the observations do not so far support this hypothesis. I will discuss SNe Ia in the last (ADV) lecture. Standard B light curve, based on observations of 22 supernovae (Branch 1992, after Cadonau 1987) Lecture 13: Binary evolution Type Ia supernovae 19 / 37

20 X-ray binaries X-ray binaries are systems where the accreting star is a neutron star or black hole. Two classes, based on mass of companion (donor) star: the high-mass X-ray binaries (HMXBs), with M 2 10M ; and the low-mass X-ray binaries (LMXBs), with M 2 2M Use the same principles to understand how X-ray binaries evolve. Let s begin with a binary consisting of two massive stars, M 1 25M and M 2 10M. Lecture 13: Binary evolution X-ray binaries 20 / 37

21 Evolution of a massive binary Lecture 13: Binary evolution X-ray binaries 21 / 37

22 Evolution of a massive binary Lecture 13: Binary evolution X-ray binaries 22 / 37

23 Evolution of a massive binary Lecture 13: Binary evolution X-ray binaries 23 / 37

24 Unbinding the binary It is easy to show 1 that if more than half the mass of the system is lost in the supernova explosion, the binary will be unbound. This is a particular problem for low-mass X-ray binaries. In order to form a neutron star, the primary must have M 1 10M, and we observe the secondary to have M 2 0.8M. This means that in the supernova explosion which forms the neutron star, the amount of mass lost is M (10 1.4) M = 8.6 M which is more than half the mass of the initial binary. low-mass X-ray binaries should not exist! 1 see assignment 2 Lecture 13: Binary evolution Recycled pulsars 24 / 37

25 The binary pulsar PSR The end result of the evolution of a HMXB is a pair of neutron stars in a highly eccentric orbit. We can see these as binary pulsars. The first binary pulsar was found by Taylor and Hulse in 1974: a 59 ms pulsar in an elliptical 8 hour orbit with another neutron star (not pulsing). from Hulse & Taylor (1975) Lecture 13: Binary evolution Recycled pulsars 25 / 37

26 PSR J A and B In 2004, the first double pulsar was discovered: a binary where both neutron stars are pulsars. The binary consists of a pulsar with a spin-period of 23 ms the millisecond pulsar orbiting a 2.8 s pulsar in a 2.4 h orbit (a R )). The mean orbital velocity is about 0.1% of the speed of light. Furthermore, the orbit is almost exactly edge-on, so both pulsars are actually eclipsed each orbit. The millisecond pulsar must have initially been the more massive star; the slower pulsar resulted from the second supernova, so is much younger than the other. The chance of finding such a binary is remote, since both pulsars have to be pointing in our direction and the younger pulsar must still be alive. Lecture 13: Binary evolution Recycled pulsars 26 / 37

27 PSR J A and B These binary systems make exquisite laboratories for testing General Relativity in the strong field; the advance of periastron, the decay of the orbit due to gravitational waves, and the Shapiro delay in the pulse arrival times due to the gravitational field of the companion. Lecture 13: Binary evolution Recycled pulsars 27 / 37

28 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 28 / 37

29 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 29 / 37

30 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 30 / 37

31 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 31 / 37

32 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 32 / 37

33 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 33 / 37

34 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 34 / 37

35 Shapiro delay Lecture 13: Binary evolution Recycled pulsars 35 / 37

36 Shapiro delay Measurement of the Shapiro delay for the double pulsar PSR J A. Lecture 13: Binary evolution Recycled pulsars 36 / 37

37 That s all, folks! This is the last ordinary lecture for this module. The last lecture (after the break) is an Advanced-only lecture; I will talk about modern research questions involving binaries, particularly Type Ia supernovae and the binary found using LIGO. Checkpoints for lab 4 due in on Thursday: leave at the Student office or in my pigeonhole. All lab solutions are will be up shortly. There is a repeat computer lab session TODAY at 10am. I will post some review notes on the discussion board. Lecture 13: Binary evolution Recycled pulsars 37 / 37

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