Mass Loss and Massive Star Evolution

Nathan Smith University of Arizona/Steward Observatory

OUTLINE §  Massive star evolution: Wind vs. binary mass loss.

Impact of mass loss on star’s evolution and fate. Revised main sequence mass loss and implications.

§  Post-MS mass loss: scary stuff. RSGs, LBVs, WRs. Processed winds, dust. Uncertainties and influence on late evolution.

§  Supernovae and Pre-Supernovae: more scary stuff.

End results of mass loss, binary interaction. Explosions or not? Uncertain end phases.

( see review Smith 2014, ARAA, 52, 487 )

§  Rotation-induced QCHE: Sung-Chul Yoon

Enhances ionizing UV.

§  Rotating Single Stars: Emily Levesque Enhances ionizing UV.

§  Binary pop-synth: Elizabeth Stanway/JJ Eldridge/Selma de Mink

Enhances ionizing UV (including He-ionizing, longer time)

§  Low metallicity single stars: Jose Groh Enhances ionizing UV.

§  Reduced mass-loss rates: Me

Enhances ionizing UV.

UV Output

Erad = fSFE fUV f* 0.007 Mgas c2 f* is fraction of stellar H that gets burned

All these act to increase f* and may trick us into thinking that there is a flatter IMF.

120

20

M/M¤

t = 0 2.5-3 Myr

WR

MS clumped

MASS LOSS AND STELLAR EVOLUTION (reduced mass loss rates)

120

20

M/M¤

t = 0 2.5-3 Myr

WR

Convective core

Convective core

More massive stars have a larger convective core. Less mass loss means that you burn more of the star, end MS with a more massive He core.

Evolution Models With Error Bars?

30 M¤

RSG

LBV

“wolf-rayet’’

Wolf-Rayet YSG BSG

YHG LBV

Stellar mergers

§  Chosen mass loss inputs determine the outcome

§  Problem: mass-loss prescriptions are not only uncertain, but almost certainly wrong.

§  1960s: As with low-mass stars, binary evolution drives mass

stripping through Roche lobe overflow. (e.g., Paczynski 1967) Winds probably too weak (Sobolev)

§  Late 60s/1970s: advent of UV astronomy with rockets & satellites.

P Cyg lines = strong hot star winds (Morton 1967). Line-driven wind theory (Lucy; CAK75, etc.), Z-dependent. “Conti scenario” = WR stars are the result of wind mass loss

§  1980s/90s: Single-star evolution models with mass loss.

Show that Conti scenario works (e.g., Maeder+94,00). Models tuned match observed stellar populations (Massey). Models can also produce main SN subtypes (e.g. Heger+03).

§  Last 10 years: Problems creep in:

Line-driven winds are weaker than we thought, SN fractions & MZAMS don’t work. Binary fraction is measured more reliably, and it is high (e.g. Sana+12).

HISTORY

The central issue in Massive Star Evolution: SHEDDING THE HYDROGEN ENVELOPE

Massive stars are born as H-rich O-type stars on the main sequence, and they die as:

H-rich RSGs Type II SNe

and/or:

H-free Wolf-Rayet Stars and

Type Ibc SNe

Also: Geneva Models Maeder & Meynet et al.

RSG WN WC

Adapted from Heger et al. 2003

CLUMPING IN LINE-DRIVEN WINDS

Observational mass-loss rates come mostly from Hα emission and IR/radio free-free. Both are sensitive to ρ2. If winds are highly clumped (FC>>1) . Then M derived from Hα and free-free is much lower. •  Fullerton et al. (2006); factors of 10-20 reduction in Mdot. •  Bouret et al. (2005); factors of >3. •  Puls et al. (2006); median of 5, but as much as 10x lower •  see also Crowther et al. 2003; Hillier et al. 2003; Massa et al. 2003; Evans et al. 2004.

fc =< ρ2 >< ρ >2

Stellar Winds. Weaker than you think.

Most studies require fc of at least 10, or reduction of wind density by at least 3

fc =< ρ2 >< ρ >2

Hillier 1991

recombination F ~ ρ2

e- scattering F ~ ρ

In very dense winds (WR stars, LBVs), electron scattering wings are strong enough to measure clumping with optical lines. Known since 1990s.

For O stars, we need unsaturated P Cyg profiles in UV resonance lines (also F ~ ρ), plus sophisticated non-LTE etc models that fit many lines simultaneously to measure clumping. These indicate reduced mass-loss rates of factors of 3-10 compared to those derived from Hα and radio.

Winds of O-type stars: Weaker than we used to think b/c of clumping

Massive Star Mass Loss Smith (2014, ARAA, 52, 487)

30 M¤

Clumping corrected (reduced) mass loss rates and “weak-wind problem” are not included in any (?) models yet.

Metallicity dependence §  Line-driven winds are pushed mostly by Fe lines and scale

with metallcity.

§  If mass-loss rates are actually 3x lower (clumping), then Z¤ stars actually evolve more like SMC stars. i.e. basically we get no WR stars from massive single stars at Z¤.

m = 0.5 (Kudritski & Puls) m = 0.69 (Vink et al. 2001) m = 0.83 (Mokiem et al. 2007)

Only tested observationally to Z = 1/5 Z¤, so extrapolate with caution.

There are no published grid of models that use these lower mass-loss rates for the right Z.

§  Less opacity in wind: more H ionizing photons escape.

§  End MS with more massive He core (much like rotating models): higher post-MS Luminosity as RSG or BSG/LBV (more N-rich Mdot, dust) higher UV flux from WR star (whether envelope stripped by RSG, LBV, binary). changes inferred MZAMS for SN progenitor

§  Much harder to make WR stars from single-star population (like SMC)

§  Less angular momentum loss, more rotation, more mixing. §  End fate may change

harder to remove H envelope, SN types more massive He and CO core: NS or BH? Explosion or dud? PISN? GRB?

§  Yields: more N in post-MS winds, possibly more O from SN (if they explode)

§  Less mechanical wind feedback in H II regions (low anyway)

What happens when the assumed O star mass-loss rates are lowered?

Single-star mass-loss (STELLAR WINDS )

Binary-star mass-transfer (ROCHE LOBE OVERFLOW)

MASS LOSS and the EVOLUTION of MASSIVE STARS:

With reduced mass loss rates, O star winds become mostly irrelevant for shedding H envelope. Burden then falls on evolved phases to remove H envelope (RSG, LBV) or binaries, or WNH stars at highest masses. These don’t have the same simple Z dependence.

Heger et al. Woosley et al. Maeder & Meynet

Paczynski et al. 67; Podsiadlowski et al. 92; Langer 2012, Vanbeveren et al. 1998 Heger et al. 2003

Mass donor Mass gainer

Massive Star Mass Loss Smith (2014, ARAA, 52, 487)

WNH winds. Strong & fast, 1 Myr, most massive stars.

30 M¤

WNH stars: The most massive, most luminous hot stars. These are not “real” WR stars (still have H). Basically O stars on steroids. Near LEdd, these are partly continuum driven (Graefner, Vink, Owock 2012) - End of MS of most massive stars? - Mergers/mass gainers?

10-20 M¤

30-35 M¤

60 M¤

120 M¤ LBV

Wolf-Rayet

RSG

YHG

G.I.G.O.

Single-Star Evolution Models

BSG

BSG

WNH

WNH star winds might be strong enough to shed H envelope if phase lasts for most of MS. …but only for ≥100 M¤ stars (rare). Can’t account for observed population of WR stars.

Luminous Blue Variable eruptions, binary interaction events.

Massive Star Mass Loss Smith (2014, ARAA, 52, 487)

30 M¤

Early O type è LBV è WN è WC è SN Ibc

THIS LONG-HELD PARADIGM IS WRONG:

Distribution of separations to nearest O-type stars in LMC/SMC for various classes of stars:

O stars behave as expected. More massive ones are more clustered. WR stars are evolved and skewed to right – but too dispersed to come from the most massive stars. LBVs don’t behave as expected. They should be in between O stars & WN… but they are as dispersed (or moreso) than WC stars!

Smith &Tombleson (2015, MNRAS, 447, 602)

RSGs & OH/IR winds

Massive Star Mass Loss Smith (2014, ARAA, 52, 487)

30 M¤

Regular RSGs ★ Betelgeuse

We (still) have no viable quantitative theory for RSG winds, so we use empirical prescriptions: Derived from IR excess and assumed gas:dust mass ratio. de Jager: normal RSGs van Loon: very strong rates for self-obscured “cocoon” stars. Not normal RSGs.

RSG mass-loss rates probably ramp up in last ~1000 yr of evolution due to high L/M ratio, but some models use high Mdot for entire RSG phase.

Davies et al. (2008)

We don’t know the Z dependence of RSG mass loss. Could be limited by dust, or by pulsations.

30-35 M¤

60 M¤

120 M¤ LBV

Wolf-Rayet YHG

LBVs and their role in single-star evolution

BSG

BSG

WNH

Without LBVs, WR stars would need to come from RSG mass loss (maybe) or binaries (of course). This means that WR stars are older than one expects from single star models.

Also: Geneva Models Maeder & Meynet et al.

RSG WN WC

Adapted from Heger et al. 2003

Smith et al. (2011) MNRAS, 412, 1522

Large galaxies, roughly Z¤

SN subtype fractions from the LOSS (Li et al. 2011)

Type: Ic Ib

IIb

(IIn)

II-L II-P

. M determines SN type (initial mass, binarity)

§  Supernovae: SN types & initial mass.

MWR

Smith et al. (2011) MNRAS, 412, 1522

Large galaxies, roughly Z¤

. M determines SN type (initial mass, binarity)

§  Supernovae: SN types & initial mass.

SN subtype fractions from the LOSS (Li et al. 2011)

Type: Ic Ib

IIb

(IIn)

II-L II-P

Smith et al. (2011) MNRAS, 412, 1522

SN subtype fractions

Sana et al. 2012

Sana et al. 2012, de Mink et al. 2013 (see also Kiminki et al. 2007,2014; Kobulnicky & Fryer, etc.):

•  Binary interaction must dominate the evolution of massive stars •  Roughly 2/3 of massive stars will interact & exchange mass or merge

71%

CONSTRAINTS FROM SUPERNOVA PROGENITOR STARS

Type II-P RSGs with initial mass 8.5 – 20 M¤ (~12) Type Ibc Maybe 1 detection, (15 upper limits) Type IIb 13-17 M¤ binary (3) Type II-L 18-25 M¤ (2) Type IIn >30-100 M¤ (5)

II-P IIn

IIb

II-L

Most common. Single stars (or wide binaries) of low-ish mass. Binary channel: mass donors in RLOF. Might favor locations in clusters. Could be 8-100 M¤ Like II-P, but a little more massive (?). LBV-like. Some very massive stars, but weird & poorly understood.

Smith et al. (2011)

** Also: SN ejecta masses of SNe Ibc & IIb are small (Dessart et al.; Haschinger et al.)

Missing high-mass progenitors >20 M¤ (Smartt 2015)

Salpeter IMF 9.5 M¤ - 30 M¤

Salpeter IMF 9.5 M¤ - 16.5 M¤

If this is true, it has a huge impact on SN yields and SN feedback at high z…

Missing high-mass progenitors >20 M¤ (Smartt 2015)

If this is true, it has a huge impact on SN yields and SN feedback at high z…

§  Stellar evolution models are hypothetical.

Mass loss dominates the evolution of massive stars, and mass loss is highly uncertain (although almost certainly too high in all current models at most phases).

§  Scaling with Z is dangerous Z-dependent winds do little at Z¤. Strongest winds are WNH and LBVs, which are partly or wholly continuum driven (at or above Eddington)… and these are not included in any models. Most important mass loss across the board is binary RLOF.

§  Binary pop-synth: Elizabeth Stanway/JJ Eldridge/Selma de Mink

Enhances ionizing UV (including He-ionizing, longer time) §  SN is maybe the most uncertain: Binaries dominate SN types. We

don’t yet know if 1/3 of them explode or not.

Summary