StarEvolution
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Caution!! Although I am happy to share my research notes below with all visitors to my webpages, these pages are mainly designed for my own use and subject to change without warning. I do not guarantee the correctness of all contents as well.

Stellar evolution
  • Nuclear reactions
    • A web version of ARA&A review on the r-, s- and p-processes in nucleosynthesis (here).
    • Nuclear reaction rate from SSRN for stellar nuclear synthesis: link
    • A web note on Neutron Capture and the Production of Heavy Elements (here). 
    • 26Al is mainly formed in the H burning shell in an AGB star during the long interpulse phase from pre-existing 25Mg. (from the introduction section of Guelin et al., 1995A&A...297..183G)
    • In massive AGB stars (with initial mass > 4 Msun), 26Al is can be made from 24Mg in the hot bottom burning (HBB) process through MgAl chain as well.  HBB usually proceed through CN cycle, or even NeNa and MgAl chains. The HBB products is directly mixed into the convective layer and affects the surface abundances. (from the introduction section of Guelin et al., 1995A&A...297..183G)
    • (from Karakas, 2003, PhD thesis: see link here)

      -TDU: (only occurs when M>2Msun, if Z=0.02, or M>1.25Msun, if z=0.008 and 0.004)
      --H shell burning: (proton capture)
      ---CNO cycle
      ----13C, 14N increase
      ----12C, 15N destroyed
      ----16O, 18O destroyed
      ----17O might increase, depends on uncertain rate
      ---Ne-Na chain
      ----22Ne destroyed
      ----23Na increase
      ----20Ne,21Ne depends on T
      ---Mg-Al chain
      ----25Mg destroyed
      ----26Al increase
      (conclusion: H burning products is not as important as He burning and HBB upon the effect to surface abundances.)
      --He shell burning (alpha capture)
      ---12C increase
      ---22Ne, 25Mg, 26Mg, 19F increase
      ---13C, 14N destroyed
      ---26Al destroyed by neutron capture: 26Al(n,p)26Mg and 26Al(n,alpha)23Na in intershell region (s-process)
      (25Mg,22Ne,12C may be destroyed a little when they are pushed into outer H burning shell before TDU.)
      (13C(alpha,n)16O is active in low mass stars, 22Ne(alpha,n)25Mg is active only in more massive stars)
      (massive AGB stars should have less s-process products, due to short inter-pulse period.)
      --HBB: (M>4Msun, CNO cycles, but Ne-Na and Mg-Al chains as well for higher temperatures)
      ---12C destroyed
      ---13C, 14N increase
      ---7Li increase->destroyed (made from 7Be, destroyed by proton capture at high temperature)
      ---15N destroyed (by proton capture)
      ---19F destroyed
      ---16O increase->destroyed (surface, decrease a little)
      ---18O destroyed much (16O/18O rises up)
      ---22Ne, 24Mg destroyed
      ---23Na, 25Mg, 26Mg, 26Al increase
      (convective turnover timescase=1year, very efficient in producing elements and very strong effects to surface abundances.)

      Early AGB: 12C/13C = 20, CNO equilibrium: 12C/13C = 4.
      2nd Dredge Up: 14N/15N = ~10^3, HBB: 14N/15N = 10^4~10^5.

    • With observed low metallicity of Z=0.008, the third dredge-up occurs early in lower mass AGB stars in the stellar evolution model, an so produce enough low mass carbon stars (1-3 Msun) to reproduce the observed carbon star luminosity function in LMC. However, their model still can not reproduce the carbon star luminosity function in the SMC with observed very low metallicity of Z=0.004.  (from Stancliffe et al., 2005MNRAS.356L...1S)
    • Cold-bottom processing (CBP) for stars with M<2Msun: Wasserburg et al., 1995, ApJ, 447, L37
  • Stellar evolution theory and database
    • A web based class note on stellar evolution theory (here, many pdf files for download there.)
    • Dartmouth stellar evolution database -- 0.1~4 Msun
    • A review of the evolution of the First Star by Dr. Simon Campbell (here)
    • Lifetime of a star: t~10(M/Msun)^-3 billion years. Eg., a star of 10 Msun has lifetime of ~ 10^7 years
      H burning time is the longest, the second is He (at around 10^8 K), C burning only lasts ~ 170 year, Ne burning lasts for 1 year, the O and Si burning has a total time of ~ 1 year, Fe can not burn. (from Imamura's website)
    • Enhanced Extra Mixing in RGB
      • For the first time they proposed a real physical mechanism for the extra mixing in upper RGB stars -- Zahn's mechanism. The joint operation of meridional circulation and turbulent diffusion cause extra mixing in the radiative zone of a rotating star. The mechanism requies the top of the radiative zone to be rotating much faster than the stellar surface. (from Denissenkov & Tout, 2000MNRAS.316..395D)
      • They  showed that the extra mixing between H and He burning shells during TPAGB can be induced by inner gravity waves (IGWs). This mixing allows the formation of 13C pocket after the extinguishment of H burning (due to He flash), which further allow s-process to occur (produce new heavier elements beyond Fe peak via beta decay after the absorption of one neutron).  (from Denissenkov & Tout, 2003MNRAS.340..722D)
      • They defined "cannonical extra mixing" and "enhanced extra mixing" for upper RGB stars. The cannonical extra mixing is a kind of non-convective mixing is caused by diffusion and ubiquitous among low mass RGB stars. The enhanced extra mixing is caused by rotation-induced turbulent diffusion and is not only ~100 times faster but also deeper than the cannonical extra mixing. The enhanced extra mixing requires ~10 times faster stellar rotation than cannonical extra mixing. (from Denissenkov & van den Berg, 2003ApJ...593..509D)
      • The observed CNO abundance variation in globular clusters can not be explained by hot bottom burning (HBB) in intermediate mass AGB stars, but can be explained by enhanced extra mixing in the upper RGB of low mass stars. (from Denissenkov & Weiss, 2004ApJ...603..119D)
      • The cannoical extra mixing that is believed to operate when most low mass RGB stars reach the bump luminosity can not explain the observed high 7Li abundance. An additional extra mixing caused by differential rotation that is spun-up by tidal interaction in a binary system can reproduce the enhancement of 7Li. (from Denissenkov & Herwig, 2004ApJ...612.1081D)
      • Vertical turbulent diffusion in the Zahn model is able to cause extra mixing in massive (>7~10 Msun) main sequence stars and hence enhance the surface Na abundance. (from Denissenkov, 2005ApJ...622.1058D)
      • In binaries where an RGB star may have experienced spin-up and have shear mixing effects in its interior. This rotational mixing will occur on the upper RGB and alter the surface elemental abundances of H burning involved elements such as 12C, 13C, N, O, and Na. (from Denissenkov et al., 2006ApJ...641.1087D)
      • Enhanced extra mixing in low mass RGB stars can result in a variation F abundance that is correlated with other H burning involved elements like C, N, O, and Na. This has the potential to explain the observed F abundance variation in globular cluster M4. (from Denissenkov et al., 2006ApJ...651..438D)
      • They present new models of TPAGB evolution of low mass stars (2 Msun). (from Cristallo et al., arXiv:0902.0243)

    • S-Processes

      • .... (to be added)
    • Population sythesis with AGB stars: Marigo, 2007ASPC..378..392M.
  • Globular clusters (GC) is a collection of spherical stellar clusters that are extremely interesting for stellar evolution study. Here I list some facts from Wiki page.
    • Main catalogues of globular clusters: Messier catalogue and NGC (New General Catalogue)
    • Currently 150 GCs are known in the Milky Way galaxy.
    •  number density of stars in a GC is typically 0.4 star/pc^3, but can be as high as 100-1000stars/pc^3.
    • Metalicity: GCs are usually composed of metal poor stars. But there are two type of GC: Oosternhoff type I (OoI, metal rich) and Oosternhoff type II (OoII, metal poor) GCs. In our Milky Way galaxy, type I clusters are associated with the halo while type II clusters with the bulge.
    • Exotic components: blue stragglers, millisecond pulsars, low mass X-ray binaries, black holes(?).
    • On the HR diagram, all member stars are usually assumed to have the same age.
    • GCs can be spherical or with slight ellipticity due tidal interactions with massive objects outside.
    • Different characteristic radii: core radius (rc) -- a radius at with the apparent surface luminosity drops to one half of the maximum at the center; half-light radius (rh) -- a radius within which the integrated luminosity is one half of the total luminosity of the GC; tidal radius (rt) -- the radius beyond which the gravitational force of the galaxy dominates over that the cluster itself (gravitational bound radius); half-mass radius (rm) -- the radius from the core that contains half of the total cluster mass.
    • mass segregation effect: When massive member stars close-encounter with lower mass member stars, low mass stars will aquire kinetic energy and  speed up, while the heavier stars will slow down and settle towards the cluster core. As the consequence, the core owns more massive stars.
    • N-body simulation of GC is very time consuming (computing time scale with N^3). Such simulation with binaries or tidal force from outwards considered is more difficult.
    •  BH 176 in the south part of the Milky Way galaxy is found to be an intermediate type between globular cluster and open cluster.
    • Tidal encounter: When a GC encounter a massive objects such as the galaxy center or galaxy plain, the outer part of stars will be stripped off the GC and form a star stream along the cluster's orbital. The star stream can be as large as several arc min. Palomar 5 is just such an example in Milky Way.
  • Second Parameter problem of Horizontal Branch Morphologies (from Zhenxin Lei's BPS group meeting talk given on Apr. 16, 2009 at YNAO)
    • The morphologies of the horizontal branch (HB)of a globular cluster (GC) is partly described by some parameters: (B, V, R, B2 are number of stars in the blue HB stars, RR Lyrae variables, red HB stars, and extreme blue HB stars with B-V<-0.02)
      • B/(B+R) ~ 0.63
      • (B-R)/(B+V+R) ~ 0.24 (sometimes also called HBR)
      • (B2-R)/(B+V+R) ~ 0.08
      • B:V:R ~ 0.57:0.097:0.333
    • The major parameter that determine the HB morphologies is the metallicity. The candidate seconde parameters include
      • rotation that produce circulation in the envelope of the star to enhance the He abundance in the stellar surface (deep mixing), so as to raise the temperature;
      • self-pollution by the stellar winds of the first generation of AGB stars in the cluster (however, the model require more than one half of the current cluster members to be the second generation of polluted stars which are not able to be produced by the steller wind of the first generation stars.)
      • age -- older GC tends to have smaller stellar masses on the HB and thus bluer colors.
  • Structural Evolution & Chemical Pollution from the First and Second Stars (Colloquium talk by Dr. Simon Campbell on 2007, Aug. 10 at ASIAA).
    • Two ways to specify metallicity: (1) use mass fraction Z = sum(Xi), (2) neuclei number ratio [Fe/H] = lg(NFe/NH)star - lg(NFe/NH)sun. The solar value of metallicity is Zsun = 0.02 or [Fe/H] = 0.
    • Three star populations in our Milky Way Galaxy: population I stars are young massive and metal rich stars, population II stars are old low mass metal poor stars, while population III are premitive stars (the oldest halo stars that could exist but not found up to now).
    • IMF: premitive IMF is a top heavy IMF that is dominated by metal poor high mass stars; current IMF is saltpeter IMF that is dominated by metal rich low mass stars; simulation by Nakamura and Umemura (2001ASPC..222...39N) gave a bimodal IMF for imagined populatioin III stars.
    • In the first generation of stars (extreme metal poor stars (EMP) or pop III stars), nuclear reaction is dominated by p-p chain reaction, CNO reaction is not active due to the lack of catalysts C.
    • Among the nuclear reactions from H to He and all the way to 35S, the fastest steps of reaction is that of 4He -> 12C and 13C -> 16O.
    • Simon's code includes nuclear reactions from H to 35S, produces chemical yields for 74 nuclear species. The computed abundances and isotopic ratios can be compared to observations. His work was devoted to the simulation of first generation stars and carbon- rich EMP (CEMP).
    • Simon's code considers time dependent mixing in convective regions, which is necessary to simulate the violent Helium and Hydrogen flash. It is found that dual core flash (DCF) will happen in stars with [Fe/H] < -5, while dual shell flash (DSF) will happen in metal poor stars with [Fe/H] > -5.
    • Several example of known CEMP stars: HE 1327-2326, HE 0107-5240, ...
  • AGB pollution and chemical evolution of Galactic globular clusters (a 30 year-old problem that is still unsolved)  by Simon Campbell at the Late Star Group meeting on Sep. 12, 2007.
    • In the globular cluster NGC 6752, member stars show in HR diagram a nice distribution from main sequence to RGB to horizontal branch to AGB. Ananlysis on a group of AGB stars with the same metalicity in their stellar surface showed interesting anticorrelations: C-N abundance anticorrelation, C-L (abundance-luminosity) anticorrelation, Na-O and Al-O abundance anticorrelations. 
    •  Possible hyperthesis:  C-N and C-L anticorrelation could be explained by enhanced CN cycle that is can be expected by assuming deeper mixing on RGB stage. CN cycle consumes C and produces N, while luminosity increases with time. The Na-O and Al-O anticorrelation could be interpreted by simultaneous ON and NeNa cycles that happen with high temperature in the hot bottom burning (HBB) processes (H burning) during AGB phase. 
    • Failure of the current models: The detailed stellar structure and evolution model calculations that take into account above cycles, however, can not reproduce the observed anti-correlations! Nobody know why!
 

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