Supermassive Dark Stars:stellar evolution

Tanja Rindler-Daller & Katherine Freese

Dep.of Physics & Michigan Center for Theoretical Physics

University of Michigan, Ann Arbor

in collaboration with Mike Montgomery & Don Winget

Dep.of Astronomy & McDonald Observatory, UT Austin

27th Texas Symposium on Relativistic Astrophysics, Dallas, TX 2013

Prerequisite: self-annihilating dark matter

• Self-annihilating dark matter (DM):

(DM particle is its own anti-particle)

-) WIMPs (lightest SUSY partners to W,Z,Higgs; Kaluza-Klein particles, sterile neutrinos)

→ self-annihilation gives correct relic density of DM

→ possibly already indirect detection signatures

(FERMI-LAT, AMS) • Self-annihilation produces heat

→ affecting stellar evolution

.) present-day stars: constrained by observations

.) first stars: yet unconstrained

First Stars

• First stars form in the high-DM density peaks of primordial (mini-)halos with 105-6 Msun at high redshift

z ~ 25-15

• Those halos arose from the merging of smaller structures as overdense regions in the Universe assemble hierarchically into ever larger halos

• Pristine atomic gas of hydrogen and helium

• Baryonic matter cools and collapses via molecular hydrogen cooling into a single small protostar at the center of the halo

The Dark Star Proposal: Spolyar, Freese & Gondolo (2008)

The effect of DM heating is more pronounced at the time when first stars form:

• DM density scales as (1+z)3

• Protostar forms in the center of minihalo

• Upon collapse, baryons pull in more DM via adiabatic contraction

• For high-enough WIMP-nucleon scattering cross section, more DM gets captured in the central regions

Enhanced DM density due to adiabatic contraction

DM heating

Note: fQ = 2/3, i.e. DM heating is 67% efficient, instead of

0.07% for hydrogen fusion

The Dark Star Proposal: Spolyar, Freese & Gondolo (2008)

Critical temperature Tc(n) below which DM heating dominates over

all cooling mechanism (H2 cooling, H line cooling, Compton cooling)

at a given gas core density n

The Dark Star Proposal: Spolyar, Freese & Gondolo (2008)

Equations of stellar structure in 1D

Supermassive Dark Stars: stellar evolution using polytropes

(Freese, Ilie, Spolyar,Valluri,Bodenheimer, 2010)

Assume that DS can be described using polytropic law

P = Kρ1+1/n in hydrostatic equilibrium:

grow DS and calculate new equilibria iteratively during its

evolution

→ established that DS can grow to supermassive size

with luminosities L ~ 109 – 1011 Lsun

→ great prospects of observing them with JWST

(→ see Cosmin Ilie's talk this session)

Dark Star evolution: improved models

Use 1D fully-fledged stellar evolution code

MESA

(Modules for Experiments in Stellar Astrophysics)

http://mesa.sourceforge.net/

→ improve upon polytropic models

→ study pulsations of dark stars

→ other features in stellar evolution ('flashes')

DS forms in minihalo of 106 Msun

• z = 20

• primordial He/H gas: 0.76

• baryon-to-DM ratio: 0.15

• c = 3.5

• dM/dt = 10-3 Msun /yr

for mDM = 10, 100, 1000 GeV

DS forms in minihalo of 106 Msun

Tracks in the Hertzsprung-Russell Diagram

DS forms in minihalo of 106 Msun

• mDM = 100 GeV

• z = 20

• primordial He/H gas: 0.76

• baryon-to-DM ratio: 0.15

• c = 3.5

• dM/dt = 10-3 Msun /yr

DS forms in minihalo of 106 Msun

DS forms in minihalo of 106 Msun

DS forms in minihalo of 106 Msun

DS forms in minihalo of 106 Msun

DS forms in minihalo of 106 Msun

MESA dark stars with 105 Msun are

• brighter ~ 2x

• hotter ~ 1.5x (for Teff and Tc)

• smaller ~ 1.6x

• denser ~ 3.5x

than polytropic DS models

DS forms in minihalo of 108 Msun

• mDM = 100 GeV

• z = 15

• primordial He/H gas: 0.76

• baryon-to-DM ratio: 0.15

• c = 3.5

• MESA: dM/dt = 10-2 Msun /yr

• Polytropes: dM/dt = 10-1 Msun /yr

DS forms in minihalo of 108 Msun

DS forms in minihalo of 108 Msun

DS forms in minihalo of 108 Msun

DS forms in minihalo of 108 Msun

DS forms in minihalo of 108 Msun

MESA dark stars with 107 Msun are

• brighter ~ 2.7x

• hotter ~ 1.8x (for Teff and Tc)

• smaller ~ 1.9x

• denser ~ 7x

than polytropic DS models

Next steps:

• implement mass-dependent accretion rates

• implement dark matter capture

• calculate pulsation modes of dark stars

• study dark star 'flash'

in luminosity when

DM runs out