Stellar Evolution in AGN Disks

Harvard, ITC Colloquium

Abstract: I will discuss the exotic evolution of stars embedded in AGN disks, showing that in sufficiently dense and cold regions rapid accretion can lead to the formation of massive and very massive objects. These stars undergo core-collapse, leave behind compact remnants and contribute to polluting the disk with heavy elements. I will show that AGN stars can have a profound impact on the evolution of AGN metallicities, as well as the production of gravitational waves sources observed by LIGO-Virgo. AGN stars can also lead to the formation of short and Long GRBs, and I will discuss the electromagnetic signature produced by relativistic explosions in AGN disks.

Transcript

1. Stellar Evolution in AGN Disks Matteo Cantiello - CCA & Princeton
2. A Growing Team Alex Dittmann (CCA & U Maryland) Adam Jermyn (CCA) Doug Lin (UCSC) Rosalba Perna (CCA & Stony Brook)
3. What is this talk about Stars can be captured by or born in AGN disks AGN disks give stars weird boundary conditions This dramatically changes stellar evolution How can we observations tell?
4. Sustained cosmic beacons powered by release of gravitational energy. Accretion onto a SMBH (MBH ~106-10 MSun ) Surrounded by geometrically-thin accretion disks, a BLR region, and a pu ed-up torus Active AGN phases ranging from ~106 - 108 yrs (e.g. King & Nixon 2015) Inner regions seem to be alpha- enhanced compared to solar value, with values increasing with MBH but independent on redshift (e.g. Xu et al. 2018) Iron abundance might also be enhanced (e.g. Tanaka et al. 1995, Nandra et al. 1997) Active Galactic Nuclei
5. Supermassive Black Hole (SMBH) ! ∼ 10!…#$!⊙ Centers of 1-10% of galaxies AGN Active phase ~10!…##$ (e.g. King & Nixon 2015) Densities of 10!"#…!%##/%&& Temperatures of 10%…'' Sound speeds of 10#…((&/) Range over 100pc from SMBH e.g. Thompson, Quataert & Murray 2005
6. Why Studying Stars in AGNs? AGNs are relatively common (~1-10% of galaxies) The immediate surroundings of galactic centers are densely populated with stars (NSC). ~107 stars within 1 pc of our Galactic Center (Do et al. 2009, Genzel+2010) Stars can form in AGN disk (Goodman & Tan 2004, Dittmann & Miller 2020)
7. Nuclear Star Cluster AGN Disk Supermassive Star Massive Star Low-Mass Star Compact Remnants Stellar trapping relies on hydrodynamical drag, as well as the excitation of resonant density waves and bending waves (Artymowicz+1993, MacLeod & Lin 2020, Fabj+2020)
8. Canonical Stellar Evolution Stars are usually evolved in a relatively cold, ~empty ISM Surface boundary conditions account for low external temperature and pressure Densities in AGN disks can be ~105…10 times typical ISM Typical sound speeds of ~10 km/s We expect AGN stars to accrete at large rates
9. Modeling AGN Stars
10. AGN Stars - 3 main ingredients ✴ Atmosphere Boundary Conditions ✴ Accretion ✴ Mass Loss
13. Bondi Radius Shock Radius
14. Bondi Radius Shock Radius Stellar Evolution Calculation
15. Bondi Radius Shock Radius Stellar Evolution Calculation Need P,T, · M
16. Boundary Conditions Cantiello, Jermyn & Lin (CJL, 2020) R S = R * (ρ a , c s,a ) We derive the structure of the accretion stream, and hence of the surface of stellar model, by making the following assumptions and approximations: 1. The stream is spherically symmetric 2. The stream is in steady state 3. The stream is not pressure supported 4. The stream primarily transports heat via radiative di usion 5. The luminosity is constant in the stream 6. The mass of the stream is small compared with M* 7. The opacity of the stream is constant
17. Boundary Conditions R B /R * ∼ 103
18. Eddington Luminosity When both large accretion and mass loss expected. This is likely a complex multi-dimensional problem. We assume accretion is reduced · M B, Γ = · M B ( 1 − L * LEdd ) 2 L * → L Edd Dittmann, Cantiello & Jermyn In Prep. Cantiello, Jermyn & Lin (CJL, 2020)
19. Eddington Luminosity When both large accretion and mass loss expected. This is likely a complex multi-dimensional problem. We assume accretion is reduced. L * → L Edd We also assume mass loss is enhanced via a super-Eddington wind
20. AGN Stars - 3 main ingredients ✴ Atmosphere Boundary Conditions ✴ Accretion ✴ Mass Loss
21. Internal Mixing Stars become massive and radiation dominated. They are marginally stable. Also likely rapidly rotating. We model the e ect of mixing by adding a compositional di usivity D [cm2/s] that increases with stellar luminosity. The form of this additional di usivity is set to be of order the convective di usivity were the region convectively unstable
22. AGN Stars Evolution
23. Initial Conditions ✴ We use to include the extra physics required to simulate AGN stars (modified boundary conditions, Bondi Accretion, super-Eddington Massloss, extra mixing) ✴ We take a solar metallicity 1MSun model and embed it in an AGN with ✴ We assume constant composition of accreted material ✴ For now we neglect rotation (but see Adam Jermyn’s follow-up work) (ρ a , c s,a )
24. AGN Stars Evolution Cantiello, Jermyn & Lin (CJL 2020) · M B ∝ M2 * ρ AGN c3 s,AGN
25. Started as a 1Msun, evolves to ~200MSun in 73Myr, loses ~400MSun of He-rich, CNO-processed material, leaves behind ~10MSun BH Mass Budget
26. Mass Accretion and Loss CJL 2020 · M B ∝ M2 * ρ AGN c3 s,AGN
27. Results: three regimes ✴ Slow Accretion: or ✴ Intermediate Accretion: ✴ Runaway Accretion: τ Acc > τ Nuc τ Acc > τ AGN τ Nuc ≲ τ Acc < τ AGN τ Acc ≪ τ Nuc < τ AGN
28. “Immortal” Massive Stars Because and since the star is well-mixed, the star is constantly fed fresh fuel from the disk. Massive stars can stay on the main sequence burning H as long as accretion proceeds τ Acc ≪ τ Nuc
29. Results: three regimes ✴ Slow Accretion: or ✴ Intermediate Accretion: ✴ Runaway Accretion: τ Acc > τ Nuc τ Acc > τ AGN τ Nuc ≲ τ Acc < τ AGN τ Acc ≪ τ Nuc < τ AGN
30. A few next steps
31. Modifications to Bondi Accretion Dittmann, Cantiello & Jermyn In Prep. SMBH RB H RB RH Ω (R+R B ) RB Ω (R-R B ) A. Rarefication C. Tides B. Shear Reduction of Bondi accretion in AGN disks. Better mapping of evolutionary outcomes in realistic disk conditions
32. Runaway Accretion Region Intermediate Accretion Region Slow Accretion Region SMBH AGN Disk Supermassive Star (M ≫100 M⊙ ) Massive Star (M ≈ 10-100 M⊙ ) Low-Mass Star (M < 8 M⊙ ) Compact Remnants Massive & Very Massive Stars More realistic mapping to AGN models Dittmann, Cantiello & Jermyn In Prep.
33. AGN Stars Rotational Evolution Jermyn, Cantiello, Dittmann & Perna In Prep. Stars accrete mass but also angular momentum. They rotate very rapidly by the end of their lives. 1) Rapidly spinning compact remnants 2) Relativistic explosions (LGRBs)
34. Observational Signatures?
35. The Galactic Center Observations of stellar populations and stellar remnants are directly available Might have experienced AGN-like phase ~6-7Myr ago ESO/Gillessen et al. NASA/Goddard
36. The Galactic Center ~200 young massive stars within ~1pc. Presence of so many young stars in the immediate vicinity of the central BH unexpected (Ghez et al. 2003b; Alexander 2005) Density of massive O/WR stars in the inner 1pc region raises steeply within ~0.5pc. No O/WR stars beyond 0.5pc (Paumard et al. 2006; Bartko et al. 2010) Top-heavy present day mass function within 0.5pc (Genzel 2010) Stellar spectroscopy shows that some of these stars might be He- rich (Martins et al. 2008; Habibi et al. 2017; Do et al. 2018) Genzel 2010
37. The Galactic Center Missing stellar cusp (Bahcall&Wolf 1976). Possible depletion of low- mass stars in the inner regions 1pc thermal non-thermal CHANDRA Do et al. 2009
38. The Galactic Center LMXB candidates identified by Hailey+ 2018 are found only within ≈ 1 pc. 1pc thermal non-thermal CHANDRA
39. Possible Implications / Predictions ✴ Population of massive & very massive * in AGN disks ✴ AGN Disk composition evolution ✴ AGN Feedback (from winds / SNe / GRBs …) ✴ Mergers of Compact Objects (LIGO/Virgo, LISA) ✴ Galactic Center stellar populations and abundances (e.g. Levin 2003, Levin & Beloborodov 2003) (McKernan et al. 2012, 2014; Bellovary et al. 2016, Bartos et al. 2017; Stone et al. 2017; Leigh et al. 2018, Fragione et al. 2019;Secunda et al. 2019; Tagawa et al. 2020; Yang et al. 2020; Gröbner et al. 2020; Ishibashi & Gröbner 2020; Graham et al. 2020, Abbott et al. 2020ab) (e.g. Goodman & Tan 2004, Dittmann & Miller 2020) (Perna, Lazzati & Cantiello 2020; Jermyn, Cantiello, Dittmann & Perna 2021 In Prep.) (Cantiello, Jermyn, Lin 2020)
40. (Embedded) Relativistic Explosions? Perna, Lazzati & Cantiello 2021 AGN Stars evolve chemically homogeneously. They also spin rapidly (Jermyn, Cantiello, Dittmann, Perna In prep.) -> Relativistic explosions in AGNs (Long and short GRBs)
41. (Embedded) Relativistic Explosions? Perna, Lazzati & Cantiello 2021 Possible impact on feedback and AGN variability e.g. Graham+2017
42. What was this talk about 1. Stars can be born in, or captured by, AGN disks 2. Their evolution is radically altered 3. We modified the typical assumptions of stellar evolution to model these objects 4. Most important processes: Bondi accretion, internal mixing, mass loss at L~LEdd 5. Stars in AGN disks rapidly become massive and very massive 6. Impact for the production of compact remnants (LMXBs/HMXBs, LIGO/Virgo sources etc.) and relativistic explosions (SN, GRBs) 7. Impact on AGN composition (metal enrichment) 8. Feedback on AGN structure and evolution
43. Thanks!