Observations of Neutron-Capture Elements in the Early Galaxy Chris Sneden University of Texas at Austin

Involving the Efforts of Many People, Including : John Cowan Jim Truran Scott Burles Tim Beers Jim Lawler Inese Ivans Jennifer Simmerer Caty Pilachowski Andy McWilliam George Preston Debra Burris Bernd Pfeiffer Karl-Ludwig Kratz Francesca Primas Rica French Taft Armandroff

Talk outline Reminder of solar r- and s-process breakdown General n-capture trends in the Galactic halo Star-to-star scatter Shift to r-process dominance Detailed abundance distributions in a few stars Elemental Isotopic Radioactive element observations There is more to halo star life than the r-process Summary, future questions

A detailed view of part of the n-capture synthesis paths Ba La Cs PPs,r s r r p s ss r-process path s-process path Xe

ELEMENTAL r- and s-process solar-system abundances Data from Burris et al. (2000)

General halo n-capture “bulk” abundance trends: LARGE scatter Large-sample surveys are needed to show this: Gilroy et al. (1988), McWilliam et al. (1995); Ryan et al. (1996); Burris et al. (2000); Johnson & Bolte (2001) Obvious from simple spectrum comparisons σ[n-capture/Fe] > 1 dex local nucleosynthesis events occurring in a poorly mixed early Galactic halo

Stellar Spectroscopic Definitions [A/B] = log 10 (N A /N B ) star – log 10 (N A /N B ) Sun log  (A) = log 10 (N A /N H ) Atmospheric parameters: T eff, log g, v t, [Fe/H] Metallicity [Fe/H] Metal-poor halo star [Fe/H] < -1.5 Very metal-poor star [Fe/H] < -2.5

Sr II line strength variations at lowest metallicities McWilliam et al. (1995) All three stars have similar atmospheric parameters and [Fe/H] ~ -3.4

Strontium abundance scatter at lowest metallicities McWilliam et al. (1995): filled circles Gratton & Sneden (1994): open squares

n-capture/Fe variations are obvious even in spectra of “higher” metallicity stars These two metal-poor ([Fe/H]=-2.3) giants have similar atmospheric parameters Burris et al. (2000)

n-capture abundance variations do not occur at random Comparison with an ordinary metal Comparison with nearby n-capture element Dy Burris et al. (2000)

General halo n-capture abundance ratios: trend toward pure r-process Not considered here: carbon-rich stars with/without s-process overabundances Usual comparison: [Ba/Eu] Ba solar-system > 90% s-process Eu solar-system > 90% r-process [Ba/Eu] ~ -0.9 ~ pure r-process value at [Fe/H] ~ -3.0 Scatter is higher than desirable: blame the Ba abundances?

The decline of Ba/Eu at lowest metallicities The solar-system r-process-only ratio

An alternative: La/Eu La also sensitive to s-process (70% s-process in solar system) Both elements have several useful lines at accessible ’s Atomic parameters of Eu, La lines very well known Can determine La/Eu with higher accuracy than Ba/Eu Can use same transitions over 3 dex metallicity range

Previous lanthanum work Burris et al. (2000),magenta points; Johnson & Bolte (2001), black points The La/Eu (e.g, the s-/r-) ratio is constant???

La II lines in the solar spectrum: synthetic spectra fits with new atomic data hyperfine structure pattern Green line is the solar observed spectrum Lawler et al. (2001)

La/Eu at low metallicity The Ba/Eu (e.g, the s-/r-) ratio is NOT constant Simmerer et al. (2002)

A better idea: employ abundances of more elements than just Ba and Eu Johnson & Bolte (2001) Four stars, with mean abundance levels scaled to the solar-system curves by their average Ba, La, Ce, Sm, and Eu abundances

Detailed elemental abundance distributions in a few very low metallicity stars Stars with # of n-capture abundances > 15: CS (Sneden et al. 2000); HD (Westin et al. 2000); BD+17 o 3248 (Cowan et al. 2002); CS (Hill et al. 2002) Rare earths: “perfect” agreement with solar- system r-process-only abundances Heaviest stable elements: must use HST Z < 56: need for another r-process?

A small spectral interval of a metal- poor but n-capture-rich star Sneden et al. (2000)

First example: BD+17 o 3248 Most “metal-rich” of n-capture-enhanced stars: [Fe/H] = -2.1 A warmer giant by about 500K than other examples Extensive high res, high S/N HST data in hand First metal-poor star with gold detection Takes advantage of large sets of new atomic data La II (Lawler et al. 2001); Ce II (Palmeri et al. 2000); Pr II (Ivarsson et al. 2001); Tb II (Lawler et al. 2001); Eu II (Lawler et al. 2002)

Detection of n-capture elements in HST STIS spectra HD is n-capture-poor; BD+17 o 3248 is n-capture-rich Cowan et al. (2002)

Discovery of gold in a metal-poor star Cowan et al. (2002)

n-capture abundances in BD+17 o 3248: 1 st solar-system comparison Scaled solar-system r-process curve: Burris et al. (2000) Cowan et al. (2002)

The BD+17 o 3248 abundances are not compatible with s-process synthesis Scaled solar-system s-process curve: Burris et al. (2000) Cowan et al. (2002)

Second example: CS First metal-poor star discovered with extreme r-process: [Fe/H] = -3.1 [Eu/Fe] = +1.6 One puzzle: also carbon-rich: [C/Fe] = +1.0 Good high res, high S/N ground-based spectra and lower quality HST data in hand Even more exploration of atomic data (Mo, Yb, Lu, Ga, Ge, Sn, etc.) Abundances or significant upper limits for 57 elements

Abundance Summary Colors identify different element groups Sneden et al. (2002), in preparation Li and Be values are w.r.t. to unevolved stars of similar metallicity

Terbium in the Sun and CS Relative Flux Sun This is the cleanest Tb II feature in the solar spectrum n-capture-rich metal-poor stars are good “laboratories” for these lines CS

Summary of the latest n-capture abundances for CS Sneden et al. (2003), in preparation

Z  56 stable n-capture elements: excellent match to solar r-process Sneden et al. (2003), in preparation

Z<56 n-capture elements: some deviations, some questions The upper limits for Sn and especially for Ga, Ge are significant Ga and Ge share the metal poverty of Fe-peak and lighter elements Sneden et al. (2003), in preparation

Comparison with CS abundances Note difference of HD : real or needing better data? Perfect agreement with CS would be a horizontal line

Some attempts to get isotopic abundances Need large hyperfine and/or isotopic splitting Rare-earth lines provide best opportunity Some elements have only one stable isotope Barium and now europium have been studied in metal-poor stars See Ivans et al. poster at this meeting

An example of Eu II syntheses: the A line The Eu abundance is altered by 0.2 dex for each synthesis

Eu isotopic fractions are very similar to solar-system values %( 151 Eu): %( 153 Eu) = %( 151 Eu) Solar system: %( 151 Eu) = 47.8 %( 153 Eu) = 52.2 Sneden et al. (2002)

Barium isotopic mixes s & rs only s onlys & r %0.0% %20.4%53.9% yesno noyesno %2.4% %11.2%71.8% 136 synthesis cause solar system abundances r-process abundances hyperfine splitting? odd isotopes 18% odd isotopes 46% odd isotopes are only11% of solar system s-process material

Barium Isotopic Abundances in HD Lambert & Allende Prieto (2002) odd isotopes: 10% 31% 52% Solar system: total = 18% r-only = 46% s-only = 11% 31% is best fit

Radioactive cosmochronometry for metal-poor stars Galactic chemical evolution effects do not matter for radioactive elements Th and U “frozen” into metal-poor stars born near the start of the Galaxy. ? Daughter product Pb is also a direct n- capture synthesis product Rolfs & Rodney (1988)

Best Th II and U II lines Cowan et al. (2002)Cayrel et al. (2001) BD +17 o 3248 CS

Age computations for halo stars  1/2 (Th) = 14.0 Gyr;  1/2 (U) = 4.5 Gyr So for thorium: N Th,now /N Th,start = exp(-t/  mean )= exp(-t/20.3Gyr) Cannot know N Th,start assume N Th,start /N Eu and compare that to N Th,observed /N Eu IF solar-system r-process abundances can be assumed to extend to U, then can use [Th observed /Eu ] as a measure of Th decay = / (  = 0.07, # = 10) = /-1.0 Gyr (  ~ 3.6 Gyr) But in CS the [Th/Eu] ratio is much larger: [Th/U] t = 12.5 Gyr [Th/Eu] t = 4 to 5 Gyr

Thorium-to-europium ratios in some halo stars Open circles: new data Filled squares: Johnson & Bolte (2001)

The curious chemical composition of CS M68 [M68 diagram from Walker 1994] It is like a “blue straggler” It is a binary (companion undetected) Preston & Sneden ( 2000)

CS is another example of lead-enriched metal-poor stars These are s-process enrichments! Log  (Pb) solar system = 1.9 All data for CS point to mass transfer from former AGB companion

Summary, future work Large star-to-star scatter in n-capture levels below [Fe/H] ~ -2: established but not well interpreted Switch from r,s-process contributions to r-only abundances is seen in many low metallicity stars Th, U radioactive element chronometry is in its nfancy, but is a promising technique Extreme s-process stars may be understood? Do [Th/Eu] ratios always yield “same” ages? Are there more U detections be had? Can the abundances of Z<56 n-capture elements be understood?

Total r- and s-process synthesis paths The r-process alone makes radioactive chronometer elements Th and U Bi is the end of the s-process Rolfs & Rodney (1988)

What are s-/r- trends in the Galactic disk? Woolf et al. (1995) derived [Eu/Fe] in disk dwarf stars with [Fe/H] > -1 Woolf spectra also contain 4123 Å La II line One La II and one Eu II line used to derive La/Eu for “disk” metallicity stars Complements Mashonkina & Gehren study of Ba/Eu

Europium in Galactic disk stars Woolf et al Results confirmed by Koch & Evardsson (2002)

La/Eu at high metallicity Does La/Eu have a break at [Fe/H] -0.4 ? Simmerer et al. (2002)

La/Eu and space velocity The s-/r- process abundance ratio correlates with space velocity as much as (more than?) [Fe/H] Simmerer et al. (2002) s.s. r-process s.s. total