Transcript Evolved Massive Stars - University of Arizona

Evolved Massive Stars
Wolf-Rayet Stars
• Classification
• WNL - weak H, strong
He, NIII,IV
• WN2-9 - He, N III,IV,V
earliest types have
highest excitation
• WC4-9 - He, C II,III,IV,
O III,IV,V
• WO1-4 - C III,IV O
IV,V,VI
• WN most common,
WO least
Wolf-Rayet Stars
• log L/L > 5.5
• log Teff > 4.7 (but ill
defined - photosphere
Ýat different radii and
is
M
Teff for different )
•
~ 10-6 - 10-4 M yr-1
• vwind ~ 1-4x103 km s-1
• ~ 1/2 of kinetic energy
in ISM within 3 kpc of
sun is from WR winds
• Wind energy
comparable to SN
Wolf-Rayet Stars
• Have lost H envelope - M > 40 M or binary with envelope ejection
• WNL WNWCWO is an evolutionary sequence and a mass
sequence
• Mass loss first exposes CNO burning products - mostly He,N
• Next partial 3 burning - He, C, some O
• finally CO rich material
• Lowest mass stars end as WN, only most massive become WO
• Surrounded by ionized, low density wind-blown bubble
• Metallicity dependence for occurrence of WRs
– in Galaxy observed min mass for WR ~ 35 M
– in SMC min mass ~ 70 M
– WOs found only in metal-rich systems
Wolf-Rayet Stars
• High luminosities result in supereddington luminosities in opacity
bumps produced by Fe peak elements at ~70,000K and 250,000K
• Without H envelope these temperatures occur near surface
• Radiative acceleration out to sonic point of wind
• Wind driven by continuum opacity instead of line opacity
• Photosphere lies in optically thick wind
Advanced Burning Stages
• No observations - these stages are so short that they are
completed faster than the thermal adjustment time of the star the stellar surface doesn’t know what’s happening in the interior
• Hydrodynamics may render the previous statement untrue
• For stars >~ 8 M C ignition occurs before thermal pulse-like
double shell burning
– limits s-process to producing elements with A < 90
• C burning and later (T > 5e8 K) dominated are neutrino cooled energy carried by , not photons
• Near minimum mass C ignition is degenerate and often offcenter since  cooling starting in core - maximum T occurs
outside core
Advanced Burning Stages
• C burning and later (T > 5e8 K) dominated are neutrino cooled energy carried by , not photons
• When does  cooling take over?
– at low T, energy loss rate ≈1.1x107T98 erg g-1 s-1 for T9 < 6 &
 < 3x105 g cm-3
–  = L/M ~ 3.1x104S/R erg g-1 s-1 after H burning
– set  = 
– rates equal for S /R = 1 at T9 = 0.62; S /R = 0.1 at T9 = 0.46
 cooling
• photons must diffuse, so rate of energy loss  2T
– ’s must traverse star, interacting with and depositing energy
in material
–  ~ R2N/c ~ 1/3M2/3
• ’s are ~ free streaming; even in stellar material
interaction cross sections are small
– cooling is local - ’s don’t interact with star to depositi energy
before escaping
– since ’s don’t interact, they provide no pressure support
• Homework: What does this imply about late burning
stages?
 cooling
• several paths for neutrino creation
e  e     1020 of e  e 2 

•
•
•
•
plasmon decay - plasma excitation decays into  pair
photoneutrino process -  pair replaces  in -e- interaction
neutrino-nuclear bremsstrahlung - ’s of breaking radiation
replaced by  pairs
At low T photoneutrino dominates, cooling/g independent
of 
At higher T e-e+ annihilation dominates, suppressed w/
increasing 
At high , low T e- degeneracy inhibits pair formation &
plasmon rate dominates
Overall rate increases w/ T
 cooling
 cooling
 cooling
• The URCA process - generating changes in neutron
excess and thereby heating & cooling through mass
movements of material undergoing weak interactions
• rate of emission of energy by escaping
neutrinos/mole
dE dP

 emiss
dt dt
dE
dV
dS
dY
P
T
  N A i i
dt
dt
dt
dt
i
A
T
dS
dY
 emiss   N A i i
dt
dt
i
A

• If A = 0 entropy decreases & there is cooling
• A = 0 if there is no composition change
 cooling
• If composition is changing
dYZ dYe
dY
dY

  Z 1   
dt
dt
dt
dt
i  ui  mic 2 chem icalpotential

• for e- capture and  decay w/ energy release Q
N 
A
i

dYi
dYZ

u

u

u

Q


i
Z
Z 1
e
dt
dt
affinity
• if affinity is positive, e- capture (ec) is driven to
completion & dYZ/dt is negative - generates entropy
• if affinity is negative,  decay is driven to completion
& dYZ/dt is positive - also generates entropy
 cooling
• If conversion is slow, process is reversible and no
heat generated
• If fast, degeneracy energy transferred into ’s
inefficient & heat generated
• depending on rate of  cooling, heating or cooling
can occur
• For fluid with mass motions (convection)
T
dS
Y
 emiss   N A ui i  T(v  S)   N A ui (v  Yi )
dt
t
i
i
advectionof entropy & material

 cooling
dS
Yi
T
 emiss   N A ui
 T(v  S)   N A ui (v  Yi )
dt

t
i
i
•
•
•
•

advectionof entropy & material
affinity will change with T, as fluid moves, as will S
More complications from nuclear excited states
De-excitation releases ’s which heat material
In convection or waves ’s may be deposited in
different place from capture or decay - net energy
transport
net  

dYZ
(uZ  uZ 1  ue  Q) Ycd mec 2  c  d
dt
where the Urca pair are nuclei c & d and c & d are
the rates of energy emission as antineutrinos from 
decay of c and as neutrinos from e- capture on d,
respectively