Galactic Astronomy

1. Galactic Astronomy 5. Metallicities of GCs 6. The 3rd parameter Dong-hyun Lee 2007/05/14

2. 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

3. 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

4. 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

5. 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)

6. 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

7. 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

8. 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