the first 10 million years of a helium burning star · ‘young’ (< 5 myrs, or 1 in 10) ones!...

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

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Hydrogen/Helium Transition Layer in Mass Coordinates

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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

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MESA + ADIPLS 4 Myrs after Flash

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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?)