Lars Bildsten Kavli Institute for Theoretical
Physics University of California Santa
Barbara
The First 10 Million Years of a Helium
Burning Star
The ability to seismically probe Red Giants and Clump Stars has opened up a new window on aspects of stellar evolution that were hard to previously query. I will emphasize here the transition that every <2 M star experiences: The Core Flash and arrival onto the He core burning clump.
Joergen Christensen-Dalsgaard (Aarhus Univ.), Phil Macias (UCSB=>UCSC), Chris Mankovich
(UCSB=>UCSC), Kevin Moore (UCSB=>UCSC), Bill Paxton (KITP), Dennis Stello (U. Sydney) &
Rich Townsend (U. Wisconsin)
Seismic Collaborators:
Outline • He Core Flash Initiation and First 10,000
years; quenches the H burning and creates the Relic Layer
• Seismic signatures of the subsequent He Flashes
• The importance of the “Relic Layer” from H burning at the tip of the Red Giant Branch in splitting the g-mode cavity.
• First 10 Million years on the clump and the construction of the stable burning H shell.
Reminder: Red Giant Branch and Clump Stars
• M< 2 M develop degenerate Helium cores that increase in mass with time until ignition in a flash => lifting degeneracy => stable He burning in core
Paxton et al. ‘11
Clump stars
Here is the ‘Textbook’ view of the Helium Core Flash
Binney and Merrifield “Galactic Astronomy”, pg 342: “This explosive phenomena causes an almost instantaneous mass loss and a re-arrangement of the structure of the star, which we have no hope of modeling in detail. It is thus not possible to follow the evolution of a star from the RGB on to the HB where it settles down to core helium burning.”
• Thomas (1967) calculated the Helium core flash, finding no explosions or dynamics. Though exciting, it remains hydrostatic
• BUT, there is substantial evidence for mass lost somewhere between leaving the main sequence and arriving to the model with stable He core burning, especially in globular clusters (e.g. HB)
• Any way to get an observational probe would be fantastic!
Red Giant Branch Evolution from 1967
Degenerate He core
2.4 My
11 My
M=1.3M
600 y decline
Thomas ’67 Thomas ’67
Timescales can be Short, but not dynamic !
MESA is open source: anyone (over 600 users!) can download the source code, compile it, and run it for their own research or education purposes.
Bill Paxton, Father of MESA
Second “Instrument Paper” just Appeared
http://mesa.sourceforge.net Third MESA Summer School at UCSB
August 11-15, 2014 Lecturers: Pascale Garaud, Eliot Quataert, Dean Townsley
Degenerate Core => Burning Core • Time spent on the Red Giant Branch (RGB) at L>30L is comparable to that spent on the Red Clump.
• Kelvin Helmholtz contraction happens rapidly after the first flash due to extinction of the H burning shell.
• Most time during flash is spent on/near the clump.
Bildsten et al. ‘12
MESA Results on He Core Flash • No need for ad-
hoc ‘transition’ from RGB to HB or clump.
• Seismic models during flash and as the clump star ‘comes to equilibrium’ can be simply made and tested.
Paxton et al. ‘11
Temperature Evolution of First Flash Macias, Moore, LB & Paxton, in preparation
Macias et al. 2013
Temperature Evolution of First Flash
H Burning Layer
Macias et al. 2013
Outer Envelope Undergoes Kelvin Helmholtz Contraction
• The radial expansion leads to adiabatic T decline in the H burning layer, shutting it off.
• Leads to rapid KH contraction of the giant
Macias et al. 2013
Propagation Diagrams and Mixed Modes
• Scuflaire ’74; Osaki ’75 and Aizenman et al. ’77 noted that the acousIc waves couple to the non-‐radial g-‐modes, which are uniformly spaced in period at:
• Coupling is strongest for l=1, and many g-‐modes between each successive acousIc mode
• Points are color coded based on their seismically inferred mass. All Kepler data (Stello et al. 2013)
• BoWom panel is theory (MESA), color coded in the same manner.
• Some stars in surprising locaIons
Luminosity increase
Stello et al. 2013
Core Flash Sequence
Bildsten et al. ‘12
Propagation Diagrams
Bildsten et al. ‘12
• ContracIon leads to outer envelope profiles during the flash nearly idenIcal to the red clump
• Coupling to core modes during the flash will be as strong as on clump.
• Core is in an intermediate state => g-‐modes dif’t
Bildsten et al. ‘12
Relic Layer
Bildsten et al. ‘12
• Seismic properIes vary during the 2 Myrs of the Core Flash
• Period spacings are in the WKB limit.
• Coupling is always strong, as it is on the clump
• Should be one in 35 compared to the clump, the number of ‘Unusual’ objects on this diagram is ~ 3-‐5
• ~5000 giants studied by Kepler, so many examples expected
• With a core that has just undergone radius expansion, rotaIon will be interesIng !
Bildsten et al. ‘12
First 10 Million Years as a Clump Star
• Even though core helium burning is established in 2 Myrs, it still takes another 10 Myr for the star to reach a steady-state.
• Main time dependent phenomena is the construction of the new H burning shell appropriate to the lower gravity of the post-flash He Core
• This competes with diffusion of the Relic layer to thicken the shell.
Mankovich et al, 2013, in preparation
Evolution of the Relic Layer into the stable H burning Shell
Man
kovi
ch e
t al,
2013
, in
prep
arat
ion
Hydrogen/Helium Transition Layer in Mass Coordinates
Man
kovi
ch e
t al,
2013
, in
prep
arat
ion
Mode Trapping • Transition layers of thickness << mode
wavelength will split the g-modes into two separate oscillation cavities.
• For the He core flashers with the thin relic layer, this limit is satisfied, splitting the modes into those that are either “above” or “below” the H/He transition region!
• Impact on period spacing measurements well known and measured in white dwarfs and discussed earlier for sdB stars (Hu et al. 2009)
• sdB star model with fixed relic layer
• Periods are shorter than in clump stars due to excitation mechanism
• They did show that diffusion would moderate, but it does not disappear!
Hu et al. 2009
Man
kovi
ch e
t al,
2013
, in
prep
arat
ion
MESA + ADIPLS 4 Myrs after Flash
MESA + ADIPLS M
anko
vich
et a
l, 20
13, i
n pr
epar
atio
n 52 Myrs after Flash
Conclusions • With the large number of Kepler clump stars,
we should be able to identify and study a few ‘young’ (< 5 Myrs, or 1 in 10) ones!
• Mode trapping, especially during the first 2 Myrs, will complicate the search for mixed modes from those actively flashing
• Confirming (or denying) the reality of the thin relic layer may inform us about other mixing mechanisms at such a sharp boundary (e.g. can rotation impact this result?)
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