Gravita'onal Wave Sources: Binary Stellar Evolu'on

Transcription

1 Gravita'onal Wave Sources: Binary Stellar Evolu'on seeing hearing hearing seeing Samaya Nissanke Radboud University, Nijmegen, the Netherlands

2 Gravita'onal radia'on opens up an en'rely new window onto the Universe 1

3 Gravita'onal radia'on opens up an en'rely new window onto the Universe f GW 2f orb 1 s GM r

4 GW astrophysical sources Low Frequency GWs High Frequency GWs Neutron Star/Black Hole Binary Mergers Supermassive Black Hole Binary Mergers Credit: NASA Pulsar Credit: Bode, GIT Supernova Credit: NASA AM CVn (mass- transferring White Dwarfs ) or Detached White Dwarfs Credit: NASA Credit: NASA Credit; Ott (CIT) Strong quadrupole moment, compact stars, rela'vis'c speeds 2

5 GW astrophysical sources Low Frequency GWs High Frequency GWs Neutron Star/Black Hole Binary Mergers Supermassive Black Hole Binary Mergers Credit: NASA Pulsar Credit: Bode, GIT Supernova Credit: NASA AM CVn (mass- transferring White Dwarfs ) or Detached White Dwarfs Credit: NASA Credit: NASA Credit; Ott (CIT) Strong quadrupole moment, compact stars, rela'vis'c speeds 2

6 The astrophysics of compact object mergers Neutron Stars (NS), Black Holes (BH) Credit: NASA NS or BH Gravita'onal Wave (GW) emission: ergs/s (final orbits) 10 mins pre- merger 1 erg ~ 10-7 J ~ ev ~ Solar luminosity ~ the visible Universe s galac'c luminosity 10 3

7 EM radia'on probes the microphysics at play in extreme dynamical space'mes NS Credit: NASA NS or BH EM emission?? 10s pre- merger 10ms post- merger 3

8 How did the binary form? NS Credit: NASA NS or BH Binary evolu'on t ~ yr r - 1 AU ~ m (214 solar radii) 3

9 Our aim: understanding GW merger rates Credit: NASA [Abadie et al. 2010; ] LIGO- Virgo merger rates: Theore'cal (NS- BH/BH- BH) derived from binary stellar evolu'on Observa'onal (NS- NS) from Galac'c radio pulsars 4

10 Lecture Plan in 1h 15 mins! Part 1: Stellar Evolu'on 101 Part 2: Stellar Binary Evolu'on: Binary interac'ons, 'mescales,? Part 3: GW merger rates [References: Tauris & van der Heuvel, 2006; Pringle and Wade; lecture notes of Verbunt & Nelemans (2015) & van der Sluys (2012), Yungelson et al. 2015, Podsiadlowski 2006]

11 Part I: Stellar Evolu'on 101 (a compact object s perspec've!)

12 Stellar Remnants from Massive Stars [Tauris and van der Heuvel 2006] Evolu'on: self- gravita'ng gas in hydrosta'c equilibrium (virial theorem) radia've loss of energy causes it to contract and hence, due to release of gravita'onal poten'al energy, T. Nega've heat capacity: while the star tries to cool itself by radia'ng away energy from its surface, it gets homer instead of cooler. Unstable virial theorem: the more it radiates to cool itself, contract, T and the more it is forced to go on radia'ng. 5

13 Massive Star Evolu'on: three 'mescales Dynamical 'mescale: when the hydrosta'c equilibrium of a star is disturbed Kelvin- Helmholtz 'mescale: when the thermal equilibrium of a star is disturbed, 'me taken to emit all of its thermal energy content at its present luminosity Nuclear 'mescale 'me needed for the star to exhaust its nuclear fuel reserve ( M), at its present fuel consump'on rate ( L ) 6

14 Massive Stellar Evolu'on: Hertzsprung Russell Diagram 7

15 Important Evolu'onary Stages 5 M ZAMS 1- >2. long- las'ng phase of core H burning (nuclear 'mescale). 3. H ignites in a shell around the He core. For massive stars, the en're star briefly contracts causing its central temperature to rise. 4. When the central temperature reaches 10 8 K, core He ignites - > red giant, with a dense core and a very large radius. During He burning, we have a loop in the H- R diagram. 2- >4. thermal 'mescale; helium- burning loop on a (helium) nuclear 'mescale. 5. During He shell burning, the outer radius expands again and at C igni'on the star has become a red supergiant on the asympto'c giant branch (AGB) e - degenerate C core. 48

16 Important Evolu'onary Stages > 10 ZAMS Massive stars con'nue to burn nuclear fuel beyond H and He burning and ul'mately form an Fe core. Alterna'on of nuclear burning and contrac'on phases. carbon burning (T ~ K) oxygen burning (T ~ 10 9 K) silicon burning: photodisintegra'on of complex nuclei, hundreds of reac'ons iron form iron core iron is the most 'ghtly bound nucleus no further energy from nuclear fusion iron core surrounded by onion- like shell structure 9

17 Part II: Binary Evolu'on

18 10 Past surprise: PSR and low mass X- ray binaries Cumula've shiw of Periastron 'me (s) Year Hulse- Taylor Binary (Nobel Prize 1993) Confirms General Rela'vity predic'on to 0.4% Orbital period: 7.75 hr Eccentricity: 0.617

19 Past surprise: PSR and low mass X- ray binaries Cumula've shiw of Periastron 'me (s) Year Hulse- Taylor Binary (Nobel Prize 1993) Confirms General Rela'vity predic'on to 0.4% Orbital period: 7.75 hr Eccentricity: Tight orbits: a) Supernova - > unbound? b) 100s days - > hours? 10

20 Past surprise: PSR evolu'on [Tauris and van der Heuvel 2006] 11

21 Angular Momentum Driven Past surprise: PSR evolu'on [Tauris and van der Heuvel 2006] [SNe kicks 400 km/s; see Prof. Om talk: metallicity; see de Mink and Belcynski 2015] 11

22 12 Stellar Remnants from Binary Stars [Tauris and van der Heuvel 2006] Mass loss (stellar winds), SNe kicks and mass transfer

23 Why are binary stars important? Most stars are members of binary or mul'ple systems - orbital period distribu'on: P orb = 11 min to ~ 10 6 yr Majority of binaries are wide and do not interact strongly About 30-50% binaries are close (with P orb < ~10 yr) & can transfer mass - > changes structure and subsequent evolu'on Approx. period distribu'on: f (log P) ~ const: (rule of thumb: 10% of systems in each decade of log P from 10-3 to 10 7 yr) Large scamer in distribu'on of eccentrici'es Systems with eccentrici'es with P < 10 d tend to be circular - > evidence for 'dal circulariza'on 13

24 14 Observa'onal Systems visual binaries periodic wobbling of two stars in the sky (e.g. Sirius A and B); if the mo'on of only one star is seen: astrometric binary spectroscopic binaries periodic Doppler shiws of spectral lines - single- lined - double- lined photometric binaries periodic varia'on of fluxes, colours, etc. are observed eclipsing binaries

25 Roche poten'al Restricted three- body problem: [Tauris and van der Heuvel 2006] determine the mo'on of a test par'cle in the field of two masses M1 and M2 in a circular orbit about each other. Equa'on of mo'on of the par'cle in a co- rota'ng frame : where the effec've poten'al is given by: Lagrangian points five sta'onary points of the Roche poten'al U eff (i.e. where the effec've gravity U eff = 0) 3 saddle points L 1, L 2, L 3 15

26 Roche poten'al Roche lobe: equipoten'al surface passing through the inner Lagrangian point L 1 ( connects the gravita'onal fields of the two stars) Roche radius: radius of sphere which has the same volume has Roche lobe where q = M1/M2 is the mass ra'o, A is the orbital separa'on. [Eggleton 1983] 16

27 Classifica'on of close binaries Detached binaries: - - both stars underfill their Roche lobes, i.e. the photospheres of both stars lie beneath their respec've Roche lobes gravita'onal interac'ons only (e.g. 'dal interac'on) Semidetached binaries: - - one star fills its Roche lobe the Roche- lobe filling component transfers mamer to the detached component mass- transferring binaries - Contact binaries: - - both stars fill or overfill their Roche lobes forma'on of a common photosphere surrounding both components: common envelope Semidetached binaries 17

28 17 Binary Mass Transfer 30-50% of all stars experience mass transfer by Roche- lobe overflow during their life'mes (generally in late evolu'onary phases) 1. (quasi- )conserva've mass transfer - mass loss + mass accre'on - the mass loser tends to lose most of its envelope forma'on of helium stars - - the accretor tends to be rejuvenated orbit generally widens 2. dynamical mass transfer common- envelope and spiral- in phase (mass loser is usually a red giant and thermal 'mescale) accre'ng component also fills its Roche lobe mass donor (primary) engulfs secondary spiral- in of the core of the primary and the secondary immersed in a common envelope if envelope ejected very close binary (compact core + secondary) otherwise: complete merger of the binary components forma'on of a single, rapidly rota'ng star

29 18 Common envelope If mass transfer is too rapid, the accre'ng star is unable to accept mass at the rate provided by the donor forma'on of a hot envelope around the accretor. Drag: rela've mo'on of inspiralling star and envelope conversion of orbital into thermal energy.

30 Common envelope If mass transfer is too rapid, the accre'ng star is unable to accept mass at the rate provided by the donor forma'on of a hot envelope around the accretor. Drag: rela've mo'on of inspiralling star and envelope conversion of orbital into thermal energy. [Paczynski, 1976; Webbink, 1984] 19

31 Common envelope: poor understanding 1) Efficiency parameter into conver'ng orbital energy into unbinding the envelope: 2) Numerous factors affec'ng α CE : - convec've envelope (energy maybe radiated to the surface faster than τ decay ) α CE. - pulsa'ons, winds driven by induced rota'on, enhanced nuclear burning α CE. 3) Inspiral not necessary: angular momentum cons. [see e.g. Fryer] [Nelemans et al. 2000, Van der Sluys 2006] 4) Common envelopes and ejec'ons occur much faster than nuclear evolu'on, hence: - core mass does not grow during envelope ejec'on - no accre'on by companion during envelope ejec'on 20

32 Other forms of mass transfer Extrinsic Changes: Loss of Angular Momentum 1) gravita'onal waves 2) magne'c braking Rota'ng stars can have magne'c fields Evolved stars can have strong winds Stellar wind follows magne'c- field lines Star loses angular momentum efficiently Tidal coupling causes orbit to shrink in case of a binary 3) mass loss from the system 4) 'dal dissipa'on Intrinsic Changes: Donor 1) dynamical instability of the donor 2) thermal evolu'on of the donor 3) nuclear evolu'on of the donor 21

33 Part III: GW merger rates

34 Known NS- NS binary systems 6 known systems that will merge within a Hubble 'me (10 billion yr), M If d binary neutron star systems i, each of total life'me τ (i), are detected in surveys j which could have detected pulsar i in a volume V max (i) = Σ j,max V(i), the merger rate in the Galaxy can be es'mated as: where V Gal is the volume of the Galaxy. [Phinney 1991] [Postnov and Yungleson 2014] 22

35 Known NS- NS binary systems 6 known systems that will merge within a Hubble 'me (10 billion yr), M [Postnov and Yungleson 2014] 22

36 Extrapolated & Pop Synthesis NS- NS binary systems Popula'on synthesis: prac'ce of simula'ng a large number of objects/systems of interest, simula'ng observa'ons of them, comparing that to what you do see, and inferring something about the proper inputs to your model. Extrapolated rates: beaming, selec'on effects of surveys [Abadie et al. 2010] 23

37 Conclusions Credit: NASA [Abadie et al ] binary evolu'on of massive stars is complex many unknown stages: common envelope, SNe kicks, metallicity only NS- NS systems have been observed low number sta's'cs: large uncertainty in rates future is loud and bright: next step: GW- EM popula'on of binaries and how this can constrain our understanding. 24

38 Extract source informa'on from GWs h(t): 9-16 dimensions + Masses + Spins + NS radii + Geometric proper'es: - Inclina'on angle - Source Posi'on - Luminosity distance [see e.g. Cutler and Flanagan 1994, Poisson and Will 1996 ] 38

39 GWs constraining popula'ons models [Dominik et al ]