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Galactic Astronomy. 5. Metallicities of GCs 6. The 3 rd parameter. Dong-hyun Lee 2007/05/14. Metallicities of GCs. Variation b/w shapes of sequence in globular CMD – due to metallicites Narrowness ← chemically homogeneous Particular evolution point : metallicities broadening

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galactic astronomy

Galactic Astronomy

5. Metallicities of GCs

6. The 3rd parameter

Dong-hyun Lee


metallicities of gcs
Metallicities of GCs
  • Variation b/w shapes of sequence in globular CMD – due to metallicites
  • Narrowness ← chemically homogeneous
  • Particular evolution point : metallicities broadening
    • Notable exceptions : fraction of O rel. to Fe in RGB → vary more than a factor of ten & abundance variation correlate with location on the RGB (lower O : ↑)
    • O role in CNO cycle : some mixing process dredge up to the surface
    • Tendency for O depletion : convective envelope have longer to redistribute O-depleted cores into envelope
metallicities of gcs1
Metallicities of GCs
    • Compare b/w data & model calculations : [O/Fe] varies significantly from star to star, [C+N+O/Fe] is approx. const. for all the stars in a single cluster
    • However, there is scatter in this relation, with stars from same cluster at the same location in CMD : different O
  • Abundance of Na : anomalies
    • Peterson(1980) - Na anti-correlated with O
    • Norris et al.(1995) – Al anti-correlated with O

⇒ proton capture enhance Na & Al in cores

→ dredge up O-deficient material bring Na- & Al- enhanced material to the surf. of star

metallicities of gcs2
Metallicities of GCs
  • Mixing process ??
    • Convective zone predicted by stellar struc. calc. → not reach deep enough into the stars’ interiors
    • Mixing process must be hypothesized if observed chemical abun. ← explained by nuclear process within present generation of stars
  • Omega Cen : the most luminous cluster in MW
    • RGB has a large intrinsic width than usual CMD → might arise from a spread in chemical composition amongst its RG stars : direct confirmation – observation : blue side of GB have Fe, a factor of ten lower than on red side
metallicities of gcs3
Metallicities of GCs
    • Abundances of heavy elements all track one another , the relation are not always linear : Fe varies by a factor of ten within omega Cen, Ba varies by a factor of a hundred
    • These can not be explained by the self-enrichment and mixing scenario
    • Chemical mix in omega Cen results from a combination of a primordial distribution and self-enrichment within the current generation.
  • Why omega Cen is so different from the other GCs?
    • Perhaps explanation lies in this cluster’s other unusual properties : large size & high rotation rate (Kinematics)
the 3 rd parameter problem
The 3rd parameter problem
  • Variation b/w cluster CMDs : [Fe/H] & t
    • NGC288 & NGC32 : [Fe/H]= -1.2+-0.1 while former very blue HB and latter very red HB
    • Age difference : near the turnoff deduce 3Gyr, while the HB morphologies compatible with delta t only if the clusters are both implausibly young(~10Gyr)
  • Variation in He abundance
    • He abundance of GCs are not agreeable to direct spectroscopic measurement : He lines only excited at high temp.s , whereas GCs - old, cool stars
    • Ratio – time on HB / time on RGB depends on He
the 3 rd parameter problem1
The 3rd parameter problem
  • Number of stars at stage i of evolution : N_i = dot n_i tau_i , where dot n_i the rate at stars enter & leave
  • Star leaving RGB evolve directly on to HB : dot n_RGB = dot n_HB , so R=N_HB / N_RGB
  • Mean He: Y=0.23, with a dispersion from cluster to cluster of sigma_Y = 0.02 ⇒ directly comparable to the errors, observations are entirely consistent
  • These indirect methods do not provide very accurate measurements of Y, still possible that unobservable variations in He may have significant impact on the properties of GCs
the 3 rd parameter problem2
The 3rd parameter problem
  • Variation in other element : C, N, O
    • Increasing the abundance of these → increase the efficiency of H burning
    • Mixture of these in atmosphere affects opacity
    • varying C, N, O, rel to Fe : alter appearance in CMDs
    • [C+N+O / Fe] may vary from cluster to cluster
    • However, spectroscopic observation : not good
  • Other candidates : delta M, dynamical factor
    • Rotation : faster-rotating stars easy to loss mass
    • Concentration :Fusi Pecci et al.(1993) – centrally(blue)
    • Cluster properties depend on external factors such as location within the galaxy