the role of laboratory astrophysics in studies of fe-group nucleosynthesis in the early universe

The Role of Laboratory Astrophysicsin studies of Fe-group nucleosynthesis

in the early Universe

Betsy Den HartogUniv. of Wisconsin

Jim Lawler, Mike Wood, (U Wisc)Chris Sneden (U TX-Austin) John Cowan (U OK-Norman) Jennifer Sobeck (U Chicago)

+ other collaborators

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!

figure from:C. Sneden et al. ApJS 182:80 (2009)

last decade: n-capture abundances were dramatically improved with new log(gf) values.

figure from:J E Lawler et al ApJS 162:227 (2006)

Tightly defined r-process abundance pattern will constrain future modeling efforts.(Tens of person-years work underlie this plot.)

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)

Fe-group abundance patterns are not well understood at low metallicity.

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

u

2

3

4

1

1/ττττu = ∑∑∑∑ Aui

Au1

BFuk = Auk / ∑∑∑∑ Aui

Auk = BFuk / ττττ u

Au2

Au3

Au4

Transition probabilities are determined by combining radiative lifetimes and branching fractions.

Radiative Lifetimes are measured using time-resolved laser-induced fluorescence (LIF) on an atomic beam.

Lifetime Experiment Apparatus

Aligning the laser with summer research student Ms. Allie Fittante

Sample LIF radiative lifetime data for Mn I

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

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

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.

HD 84937 Teff = 6275 K log(g) = 4.00 [Fe/H] = -2.10 Dwarf Star, Metal poor

Mn I linesMn II lines

Initial application of lab data (LTE/1D) shows interesting trend with excitation potential χχχχ.

HD 115444 Teff = 4575 K log(g) = 1.25 [Fe/H] = -2.90Giant star, Metal poor

Mn II lines

Mn I lines

The trend with excitation potential χχχχ is even more pronounced at lower gravity.

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

BF measurements of weak lines will be tackled using an upgraded

Echelle spectrometer.

• 3m focal length, vacuum compatible echellespectrograph 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

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

Sample FTS data Ti II 3261.62 Åhollow cathode lamp 770 mA

Sample echelle data Ti II 3261.62 Åhollow cathode lamp 10 mA

Sample echelle data Ti II 3261.62 Åhollow cathode lamp 10 mA

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