Formation and evolution of merging black-hole binaries

Mario Spera ([email protected]) ([email protected]) Postdoc @ Institut für Astro- und Teilchenphysik – Universität Innsbruck

Collaborators: Michela Mapelli, Nicola Giacobbo, Alessandro Trani Alessandro Bressan

GRAvitational-wave Science&technology Symposium (GRASS 2018) March 1 - 2, Palazzo Moroni, Padova (Italy) 1. The GW detections

2. The black hole mass spectrum

3. Formation channels of black hole binaries

4. Conclusions GW150914 GW151226 GW170104 . . = 31.2 . M = 35.4 . M = 14.2 . M . +5 0 +.8 3 +8.4 . 1 = 7.5 M ⊙ 1 = 19.4−6 0 M⊙ 𝑚𝑚1 = 29.8−3.4 M⊙ 𝑚𝑚 −.3 7 𝑚𝑚 . +3 3 +2 3 +5 3 𝑚𝑚2 −4 3 ⊙ 𝑚𝑚2 −2 3 ⊙ 𝑚𝑚2 −5 9 ⊙ GW170608 GW170814

. . = 12.0 . M = 30.5 . M +.7 0 +5 7 1 = 7.0 M ⊙ . 𝑚𝑚 −.2 0 𝑚𝑚1 = 25.3−3.0 M⊙ +2 0 +2 8 𝑚𝑚2 −2 0 ⊙ 𝑚𝑚2 −4 2 ⊙  BINARY black holes exist

 MASSIVE black holes exist > 25 M

⊙  Binary black holes can MERGE within𝑚𝑚 a Hubble time www.ligo.caltech.edu Models from the BSE code (Hurley+ 2002)

Most models of BH mass spectrum were not able to explain GW150914 (progenitors evolution)

Black Hole Binaries (GW emission)

Superova Stellar Explosion Dynamics (Compact object (N-body mass) simulations) Stellar evolution  stellar winds lose mass (Vink+ 2001, 2005, Bressan+ 2012, Tang, Bressan+ 2014, Chen, Bressan+ 2015) Wind from WR WR 124, , HST  Mass-loss depends on mass and metallicity

( [0.5; 0.9])

𝑑𝑑𝑑𝑑 𝛼𝛼 𝛼𝛼 ∈ ∝ 𝑍𝑍  The amount𝑑𝑑𝑑𝑑of mass-loss for massive stars can be conspicuous

Wolf-Rayet star WR 124

30 stellar winds 10

𝑀𝑀initial ≃ 𝑀𝑀⊙ ≃ 𝑀𝑀⊙ explosion BHs form after supernovae Crab , HST (Fryer+ 1999, 2001, 2012, Heger+ 2003, Mapelli+ 2009, 2010)

 Supernova remnant (BH/NS) depends on the final mass of the star

 It depends on the compactness of the star

 It depends on the rapidity of the explosion

 It depends on fallback

 : the SN explosion may fail  direct collapse  massive black hole 𝑀𝑀final ≳ 30𝑀𝑀⊙ The SEVN code Stellar EVolution for N-body

• MS, Mapelli, Bressan, 2015, MNRAS, 451, 4086 • MS & Mapelli, 2017, MNRAS, 470, 4739 Up-to-date Stellar Up-to-date Evolution • Stellar evolution through look-up tables. SN models a. Versatile approach. (5 models) b. Default: tables from the code (Bressan+ 2012, Chen+ 2015)

• Several up-to-date SN explosion models SEVN a. CO-based criteria b. Compactness-based criteria Versatile easy-to-change input Ready for • Already coupled with some N-body codes stellar evolution tables N-body sims Starlab • SEVN is publicly available HiGPUs-RX • https://gitlab.com/mario.spera/SEVN • http://web.pd.astro.it/mapelli/SEVN.tar.gz Massive (> 30M ) stellar black holes: ⊙ Low metallicity + direct collapse

• MS, Mapelli, Bressan, 2015, MNRAS, 451, 4086 • Abbott+ 2016 ApJL, 818, L22 (Fig. 1) Pulsational- and pair-instability SNe are missing in most population-synthesis codes

• MS & Mapelli, 2017, MNRAS, 470, 4739 • MS & Mapelli, 2017, MNRAS, 470, 4739 • MS & Mapelli, 2017, MNRAS, 470, 4739

PPISN PPISN: 0.017

𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ • MS & Mapelli, 2017, MNRAS, 470, 4739

PPISN PPISN: 0.017

𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ PISN: 0.008 0.5

𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ PISN • MS & Mapelli, 2017, MNRAS, 470, 4739

PPISN PPISN: 0.017

𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ PISN: 0.008 0.5

𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ PISN Direct collapse

230 1 𝑍𝑍𝑍𝑍𝑍𝑍𝑍𝑍0.001 ⊙ 𝑀𝑀 ≳ 20𝑀𝑀 200 𝑍𝑍 ≲ ≃ 𝑍𝑍⊙ 𝑀𝑀𝐵𝐵𝐵𝐵 ≳ 𝑀𝑀⊙ Formation channels

Primordial binary Dynamically formed binary

Two stars are bound Two stars become bound since the beginning after one (or more) of their life. gravitational encounters Two stars are bound since the beginning of their life.

Question: can primordial binaries become black-hole binaries?

Not so easy  Binary stellar evolution processes (mass transfer, common envelope, tidal dissipation, supernova kicks)

Question: if a BH binary is formed, can it merge within a Hubble time?

Not so easy  BHs must be very close to each other (tens of )

𝑅𝑅⊙

Question: is there any process that brings the compact remnants close to each other? Two compact objects orbit in a shared envelope Tight BH binary can form Common envelope phase Envelope Black hole BH + Giant star ejected

Roche lobe Energy transferred Evolved overflow to the envelope star Unstable mass transfer BH + envelope Envelope not ejected

Modeled through the fromalism (see Nicola’s talk)

𝛼𝛼𝛼𝛼, = , 𝐺𝐺𝑚𝑚1𝑚𝑚2 𝐺𝐺𝑚𝑚1 core𝑚𝑚2 𝐺𝐺𝑚𝑚1𝑚𝑚1 env See Ivanova et al. 2012 for a review 𝜶𝜶 − − 2𝑎𝑎𝑖𝑖 2𝑎𝑎𝑓𝑓 𝝀𝝀𝑅𝑅1 The effect of binary stellar evolution • MS+ 2018, in preparation

10M binary systems

Power-law initial mass function (Kroupa 2001)

Initial period and eccentricity distributions as in Sana+2012 Young star cluster R136, Large Magellanic Cloud, HST Two stars become bound after one (or more) gravitational encounters Gravitational encounters: are they common?

YES, in dense stellar environments Dense means 10 stars pc 3 −3 𝑛𝑛 ≳ Is it common to find stars in dense environments?

The majority of stars form in dense stellar environments (Lada & Lada 2003) star

BH BH

BHs born from single stars in a star cluster likely acquire a companion thanks to dynamics

Exchanges favour the formation of massive BH-BH binaries, with high eccentricity and misaligned spins

> 90% BH-BH binaries form dynamically in star clusters (Ziosi, Mapelli+ 2014) See also Elisa’s talk See also Elisa’s talk Conclusions

1. Models of BH formation are hampered by uncertainties on stellar winds and supernova explosion (Mapelli+ 2009, 2010)

2. Recent population-synthesis models show that BHs may have mass 20 if progenitor is metal poor (Mapelli+ 2013; MS+ 2015; MS+ 2017) ≫ 𝑀𝑀⊙

3. BH binaries can form both from primordial binaries and from dynamically evolved binaries – there is a strong degeneracy

4. Binary stelar evolution can dramatically affect the BH mass psectrum

5. Dynamics can affect the formation and evolution of BH-BH binaries in many ways a. dynamical exchanges (Ziosi, Mapelli+ 2014) b. Spitzer Instability (MS+ 2016) c. Kozai resonance (Kimpson, MS+ 2016) d. Runaway growth of IMBHs(Mapelli 2016)