Heavy Metal in Ancient Superstars
Old stars in the Milky Way Galaxy record the history of nucleosynthesis in our part of the Universe stretching back almost to the beginning of time. From studies of the compositions of low mass stars formed more than 10 billion years ago, Pilachowski and collaborators D. Burris, T. Armandroff, J. Cowan, C. Sneden, and H. Roe were able to trace the origin of the heavy, neutron-capture elements in the early Milky Way. The origin of heavy metals at the earliest times (at metallicities of 0.01% of the metal content of the Sun, [Fe/H]=-4.0) in the history of the Galaxy remains uncertain because few stars are known at this very low metallicity, and even fewer have been studied. Such stars appear to have an excess of the lighter neutron capture element strontium, perhaps produced by the weak s-process operating in massive stars (25 M(Sun)), or by some other alternative neutron-capture process operating in similarly massive stars.
Their results suggest that significant production of r-process elements such as europium and dysprosium began at a metallicity of [Fe/H]=-2.9 (about 0.1% of the solar metallicity) in the early Galaxy, although small amounts were produced at even earlier times. This onset is consistent with the suggestion that Type II supernovae from 8-10 M(Sun) stars are responsible for the r-process. Until a metallicity of [Fe/H]=-2.4, the abundance pattern of the heavy (Z>56) n-capture elements in most metal-poor giant stars is well matched to a dominant r-process nucleosynthesis pattern, rather than the traditional mix of r- and s-process products.
The onset of r-process production was followed by the production of s-process elements (barium, lanthanum, and other lanthanides) at a metallicity near [Fe/H]=-2.4 (0.4% of solar), but with some spread in the metallicity at which s-process elements first appeared. Contributions from the s-process can first be seen in some stars with metallicities as low as [Fe/H]=-2.75, and are present in most stars with metallicities [Fe/H]>-2.3. The delay in the appearance of s-process elements reflects the longer stellar evolutionary timescale of the (low-mass) s-process nucleosynthesis sites.
The abundances of the heavy metals in
these metal poor stars show clear evidence for a large star-to-star
dispersion in the heavy element-to-iron ratios. This condition
must have arisen from individual nucleosynthetic events in
rapidly evolving halo progenitors that injected newly manufactured
n-capture elements into an inhomogeneous early Galactic halo
interstellar medium. Chemical enrichment occurred in local regions
of the proto-Galaxy on timescales shorter than the time needed
for widespread mixing of newly produce
Compositions of Globular Clusters
Pilachowski, in collaboration with C. Sneden, R. P. Kraft, and others, is continuing to investigate abundance variations in globular clusters. The evolution of stars can be examined in detail through changes in the abundances of elements at the stellar surface from nucleosynthesis and mixing inside a star. These changes tell us about the physical conditions inside the star and about processes that create and bring new elements to the surface. Much of her work has involved exploration of abundance changes in low mass, red giant stars to improve our understanding of stellar evolution.
Pilachowski's previous work on giants in the M13 globular cluster established the role that nucleosynthesis and mixing play during evolution. In collaboration with Sneden, R. Kraft, and E. Langer, she has demonstrated that proton-capture nucleosynthesis, including not only the CN and ON cycles, but also the NeNa and MgAl cycles of hydrogen burning, is occurring in M13 giants. Similar studies have been carried out in M15 and M92, and are underway in M3 and other clusters.
In collaboration with C. Deliyannis, C. Sneden, R. P. Kraft, and others, Pilachowski continues to investigate abundance variations in globular clusters. With the Hydra multi-fiber spectrographs on WIYN and the Blanco 4-m telescopes, she has accumulated spectra of more than 700 giants in more than a dozen globular clusters to survey lithium, H-alpha emission, and several critical abundances (e.g. barium, europium, aluminum). Preliminary analysis of the Li I observations suggest that Li-rich cluste r giants fall into two groups. Stars near Log L/L(Sun)=1.8 have lithium abundances similar to field giants, consistent with dilution from an initial main sequence lithium abundance of Log (Li)=2.3. d elements. Lithium does not appear to be present in other cluster giants at the same luminosity, but might be detected in spectra of higher signal-to-noise ratio and resolution.
At somewhat higher luminosity (Log L/L(Sun)=2.8) are giants with higher
lithium abundances than are seen among typical field giants at the same
Approximately 6% of cluster giants brighter than M(V)=0.75 show measurabl
lithium. Lithium may be brought to the surface of these
giants as the unstable isotope 7Be, which decays to 7Li,
through mixing induced by differential internal rotation.
Pilachowski, in collaboration with A. Quillen, is investigating the role of falling evaporative bodies in the dynamical evolution of solar systems, using observations from the WIYN telescope of the Ca II K line in young A-type stars in clusters.
Valenti and Pilachowski, in collaboration with B. Chaboyer and solar astronomers from NSO, are attempting to detect acoustic oscillations in solar type stars using HST.
Saha and Pilachowski are conducting Baade-Wesselink analyses of an ensemble of RR Lyrae variables in the globular cluster M3, using data from the WIYN and the 0.9-m telescopes.
Pilachowski serves as Deputy Project Scientist for the U.S. Gemini Program, with particular responsibility for telescope proposals, user support, and community outreach. She also supports the Public Access program providing observing time to the community on the new 6.5-m telescope of the MMT Observatory and on the Hobby-Eberly Telescope, as well as collaborative programs with the SIRTF and Chandra Observatories and the Hubble Space Telescope.
Pilachowski is also involved with numerous educational outreach projects and serves on the local ASP/Project Astro Coalition. She assists students interested in gaining research experience in astronomy and participates with the University of Arizona's Women in Science and Engineering Program. She serves as a Shapley Lecturer for the American Astronomical Society.
Pilachowski has recently been elected to serve as President of the American Astronomical Society, and she serves on the Nominating Committees of Astronomical Society of the Pacific and of the Astronomy Section of the American Association for the Advancement of Science. She is also a member of the U. S. National Committee for the Internationa l Astronomical Union.