Exploring the contribution of AGB stars to the Galactic chemical inventory using isotopic ratios

Transcription

1 Exploring the contribution of AGB stars to the Galactic chemical inventory using isotopic ratios Aruna Goswami Indian Institute of Astrophysics Bangalore

2 Setting the stage Understanding GCE still remains an outstanding problem. Major Issues Chemical composition of stars (Disk, Halo) GCE model: Scope and adequacy GCE model parameters: Stellar yields, SFR, IMF, Infall, ouflow, Mixing etc. Methodology Theoretical yields from an appropriate GCE model Comparison of computed abundances with the observed abundances. Elements,i.e., Mg and its isotopic ratios are useful indicators.

3 Measurement of isotope ratios: Observational challenges Isotopic shifts are small at optical wavelengths HR stellar spectra are required to measure isotopic ratios. R > 90,000 and S/N > 200 In cool stars Mg isotopic ratios can be estimated from MgH band. Locations of 24 MgH, 25 MgH, and 26 MgH are indicated (dashed line) in the spectra showing A MgH band. Shows substantial asymmetries for M71 A4, M13 L70, and M13 L973. Profile of Fe I at Å is generally symmetric (Yong et al. 2006).

4 Isotope ratios open up a new window into Nucleosynthesis, GCE Mixing within stars and stellar evolution. Such data when available significantly improves our understanding of nuclear processes in various astrophysical sites.

5 Exploration with Mg isotope ratios 24 Mg, 25 Mg, 26 Mg : stable isotopes Alpha nucleus 24 Mg : primary isotope (massive stars) Neutron-rich 25,26 Mg : secondary isotopes ( IM-AGB stars) Isotope ratio 25,26 Mg/ 24 Mg should increase with the onset of AGB stars. In young Galaxy there was not enough time for AGB stars to contribute. CC SNe started exploding quickly after massive stars first formed. After the formation of the first generation of stars the enrichment occurs ~ a billion years later due to the long MS life-time of the first low-mass stars before passing through the AGB phase. Mg isotope ratios in the halo stars could be used to constrain the rise of AGB stars in our Galaxy.

6 Early studies Evolution of the isotopic abundance ratios of Mg as a function of metallicity ( [Fe/H] ~ -1.8 ) Observed isotopic ratios : black dots. In both the panels the solid line corresponds to the disk model, the dashed line corresponds to halo model (Goswami & Prantzos).

7 Mg isotopic ratios in Metal-poor cool dwarfs ([Fe/H] < -2.5) Wavelength regions , and Å are usually employed. Features fitted are indicated by the black arrows. Positions of 24MgH, 25MgH and 26MgH: dashed lines Filled circles represent the observed spectra., the best fit is shown by the solid line. MP stars show relatively high 25,26 Mg/ 24 Mg rations ( [Fe/H] < -2.5, T eff < 5000 K), suggest an important contribution from IM-AGB stars at low [Fe/H] < (Yong et al. 2006)

8 Spectrum synthesis fits for the Å region in the stars HD and G Observed spectra: filled circles; Synthetic spectra: solid lines. The calculation were performed for 25,26 Mg/ 24 Mg ratios 0-5%. The relative variations of the X 2 fits are shown as a function of the isotopic abundance. (Melendez & Cohen 2007).

9 Mg isotopic ratios: comparison with model predictions Low isotopic ratios can be explained mostly by massive stars. No need to invoke the contribution of intermediate- mass AGB stars at metallicities [Fe/H] ~ HD lies slightly above the Fenner 2003 predicted curve of massive star nucleosynthesis. Probably an indication that the contribution from AGB stars begin at [Fe/H] > -1.5 (Melendez & Cohen 2007).

10 In Summary, Further, high spectral resolution, high S/N observation of a larger sample will help constrain the rise of AGB stars in the Galaxy, including the formation timescale of the Galactic halo. Measurement of isotopic abundances and isotopic ratios will provide new insight into nuclear processes in various astrophysical sites, GCE, stellar mixing processes and stellar evolution. Many cool halo dwarfs are either not accessible or requires long integration time with the current observational facilities (8-10 m telescopes). Future large telescopes of 25-40m size when equipped with high resolution spectrograph will enable Shorter integration time Detection and follow-up HR spectroscopic studies of more halo dwarfs Leading to better understanding of GCE.

11 Thank you

12 Limiting distances for spectroscopic observations of point sources Assumes 4 hr integrations at λ/δλ = 5000, 50000, and for WFOS, HROS, NIRES. The corresponding central WLs are 0.5 μm, 0.5μm, and 1.2μm,. SNR, 100, 100, 60 ( J Cohen, CIT) HROS WL: μm Slit length: 5 ; R > NIRES WL: 1 5 μm Slit length : 2, R ~ Local Group Observable with TMT