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The role of laboratory astrophysics in studies of Fe-group nucleosynthesis in the early Universe

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Talk given at the NASA Anuual UV-Vis SR&T Workshop, NASA Head Quarters, 20-21 September 2011.

Talk given at the NASA Anuual UV-Vis SR&T Workshop, NASA Head Quarters, 20-21 September 2011.

Published in: Technology, Education

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  • 1. The Role of Laboratory Astrophysics in studies of Fe-group nucleosynthesis in the early Universe Betsy Den Hartog Univ. of Wisconsin Jim Lawler, Mike Wood, (U Wisc) Chris Sneden (U TX-Austin) John Cowan (U OK-Norman) Jennifer Sobeck (U Chicago) + other collaborators
  • 2. Extended life of HST is an opportunity for studies of Fe-group nucleosynthesis in the early Galaxy • Hubble properties make it ideal for these studies: - access to UV region - high spectral resolving power - good sized primary • UW group - strong collaboration with Chris Sneden (UT-Austin), John Cowan (U OK-Norman),…. • study of metal-poor halo stars sheds light on the early times of galactic history • abundance patterns of many n-capture elements are now better than Fe-group!
  • 3. last decade: n-capture abundances were dramatically improved with new log(gf) values. Tightly defined r-process abundance pattern will constrain future modeling efforts. (Tens of person-years work underlie this plot.)figure from: figure from:J E Lawler et al ApJS 162:227 (2006) C. Sneden et al. ApJS 182:80 (2009)
  • 4. Fe-group abundance patterns are not well understood at low metallicity. Relative Co to Cr abundance [Co/Cr] normalized to the Solar abundance of these elements as a function of metallicity [Fe/H] normalized to the Solar metalicity for a large set of stars. (Plot prepared and provided by Prof. John Cowan and Jason Collier, Univ. of Oklahoma)
  • 5. Fe-group synthesis in the early Universe• Relative Fe-group abundances are not understood!• Is this a non-LTE photospheric effect?• Nuclear physics effect?• Is this an effect from cumulative errors in lab data (f-values) as abundance determinations switch from line-to-line to study lower and lower metallicity stars?• New Fe-group transition probability effort will help shed light on these questions
  • 6. Transition probabilities are determined bycombining radiative lifetimes and branchingfractions. u Au4 Au3 Au2 4 Au1 3 τ 1/τu = ∑ Aui 2 BFuk = Auk / ∑ Aui 1 Auk = BFuk / τ u
  • 7. RadiativeLifetimes aremeasuredusing time-resolved laser-inducedfluorescence Lifetime Experiment Apparatus(LIF) on anatomic beam.
  • 8. Aligning the laser with summer research student Ms. Allie Fittante
  • 9. Sample LIF radiative lifetime data for Mn I
  • 10. Branching Fractions are determined from high-resolution FTS spectra Advantages of an FTS • Very high spectral resolving power • Excellent absolute wavenumber accuracy • Very high data collection rates • Large etendue • Insensitive to source intensity drifts
  • 11. We have recently completed lab work on Mn I and Mn II.• We reported some of the most accurate f-values available for Fe – group species• Multiplets were carefully selected so that branching fraction uncertainties could be minimized• reduced uncertainty of radiative lifetimes using new benchmark lifetimes Mg+, Na to accurately characterize residual systematics• log(gf) ± 0.02 dex with high (2 sigma) confidence
  • 12. Initial application of lab data (LTE/1D) shows interesting trend with excitation potential χ. HD 84937 Teff = 6275 K log(g) = 4.00 [Fe/H] = -2.10 Dwarf Star, Metal poor Mn II lines Mn I lines The lines with excitation potential near 7 eV connect to the ground level of the ion (Mn II resonance lines). Nearly all the photospheric Mn resides in that level and non-LTE effects are negligible.
  • 13. The trend with excitation potential χ is even more pronounced at lower gravity. HD 115444 Teff = 4575 K log(g) = 1.25 [Fe/H] = -2.90 Giant star, Metal poor Mn I lines Mn II lines
  • 14. Choice of transition is critical in abundance determinations in the Fe-group.• UV lines to the ground and low metastable levels of the ion are the most reliable abundance probes - insensitive to non-LTE effects• For Fe–group species, weak lines are the best, insensitive to microturbulance• FTS instruments have many advantages, but are not ideal for weak lines due to multiplex noise: photon noise from every line in a wide spectrum is redistributed evenly throughout the spectrum
  • 15. BF measurements of weak lines will be tackled using an upgraded Echelle spectrometer.• 3m focal length, vacuum compatible echelle spectrograph acquired in the 1990s for NASA work on VUV ion lines used for ISM studies• New grating: 23.2 groove/mm, 63º blaze, 135 x 265 mm2• Custom designed prismatic order separator• Aberration compensated• UV sensitive 4 Mpix CCD, 13.5 micron pix
  • 16. Echelle spectrometer performance• resolving power ~ 100,000• broad UV coverage, 2000 Å - 4000 Å in 3 CCD frames with no gaps• UV sensitivity excellent, low current optically thin lamps give good S/N• no multiplex noise of FTS instruments• main disadvantage compared to FTS: wavelength calibration is not as good
  • 17. Sample FTS data Ti II 3261.62 Å hollow cathode lamp 770 mA
  • 18. Sample echelle data Ti II 3261.62 Å hollow cathode lamp 10 mA
  • 19. Sample echelle data Ti II 3261.62 Å hollow cathode lamp 10 mA
  • 20. Near Term Goals of Wisconsin Laboratory Astrophysics Program• eliminate lab data as major source of uncertainties in the Fe-group abundance patterns of metal poor stars (new and archived HST UV data is crucial)• provide f-values for weak lines connecting to ground state of dominant species - these lines should be reliable abundance probes