Making Carbon in Stars:  (Falk Herwig, LANL, Sam Austin MSU)

An important fraction of carbon in the cosmos is produced by  Asymptotic Giant Branch (AGB) stars.  These stars have exhausted first the hydrogen and then the helium in their cores and are burning hydrogen and helium in shells surrounding the carbon + oxygen core.  During a late phase of their evolution these stars undergo instabilities in the helium burning shell and undergo violent pulse which both produce carbon and later bring it to the stellar envelope.  This envelop is convective so the  carbon produced is transported to the surface of the star where strong stellar winds blow it into the interstellar medium.  A mystery, the carbon star mystery,  is that light, 1-2 solar mass stars, are observed to have carbon rich surfaces  but simulations fail to produce carbon rich envelopes.  We have investigated whether uncertainties in the reaction rates could result in greater carbon production.  We find that varying either the 14N(p,gamma)   or the triple alpha reaction rates to their lower or upper limits respectively, could explain the observed carbon abundance.

Helium Burning In Stars:  (MSU:Sam Austin, Ken Yoneda, Chris Starosta; WMU (Alan Wuosmaa)). 

Although the triple-alpha reaction plays a central role in the production of ¹²C in stars, it is only recently that the range of its effects and the required precision of its reaction rate has become clear.  The best known example is in SNII explosions, where a precision in the ratio of the triple-alpha and ¹²C(a,g) rates is required to reproduce the abundances of medium mass elements [1]. It has also been shown that the production of ¹²C in Asymptotic Giant Branch stars varies by a factor of two or more if one changes the triple-alpha rate within its uncertainty range [2].  The reaction is also important in understanding the early evolution of the first (initially zero metallicity) stars.

Recent measurements [3] have shown that the rate of the triple-alpha reaction over a wide range of temperatures, from 10⁷ to 10¹⁰ K,  is well reproduced by the  contributions of the Hoyle state, the 0⁺ state at 7.65 MeV in ¹²C.  Improving the accuracy of the rate then depends essentially on improving our knowledge of the 0⁺ state, and in particular its radiative width G_rad.  Presently G_rad is known to about 13% which dominates the triple-alpha uncertainty; the position of  the resonance is sufficiently well known. G_rad is determined from:                                                                                                                                                                   

  where the three factors on the right are known to ±2.7%, ±9.2% and ±6.4%.  The pair width, determined from electron scattering, and the pair branch dominate the uncertainty.  A new analysis  [4] of a range of data indicate that the uncertainty in the pair width can be halved, leaving the pair branch as the dominant uncertainty. 

A Western Michigan University--Michigan State University collaboration is undertaking a new measurement of the pair branch that is expected to reduce its uncertainty to about 5%.  The principle is simple—see Fig. 1.  The 7.65 MeV state is excited by inelastic proton scattering, taking advantage of a strong resonance at an excitation energy of 10.6 MeV and 135 degrees in the lab. The proton beam is provided by the newly refurbished EN Tandem Accelerator at Western Michigan University. The pair branch is given essentially by the ratio of the number of positron-electron pairs detected in the plastic scintillators to the number of scattered protons from the 7.65 MeV state.  In order to reduce the gamma ray background a coincidence is required between a thin cylindrical scintillator and a large plastic scintillator surrounding it.   This will little affect the number of detected pairs, but will strongly discriminate against gamma rays, which have small probability of interacting in the thin scintillator.  An investigation of the systematic uncertainties in an earlier experiment [5] indicates that an accuracy of  ±5% is achievable. 

The present state of the experiment is as follows:  A GEANT simulation of the detector has been carried out to fix the detector design parameters, a detailed design is out for bid, photomultipliers and silicon detectors have been obtained, and the scattering chamber has been expanded and cleaned to reduce contaminant background.  First data should be taken late in summer, 2005

References

[1]  S. E. Woosley, A. Heger, T. Rauscher, and R. D. Hoffman,  Nucl. Phys. A 718,    c3 (2003).

[2]  F. Herwig and S. M. Austin, Astrophys. J. Lett, 613, L81 (1994).  F. Herwig, S. M. Austin and J.C. Lattanzio, Phys Rev. C, submitted

[3]  H. O. U. Fynbo, et al., Nature 433, 136 (2005).

[4]  H. Crannell,  et al.,  Nucl. Phys. A, to be published

[5]  R. G. H. Robertson, R. A. Warner and S. M. Austin, Phys. Rev. C, 15 1072 (1977).

Electron Capture  for stable nuclei via (t, ³He)  reactions and Supernovae Evolution: ( With  R. Zegers, B. Sherrill, NSCL A1200 Group, Osaka). 

    Electron capture (EC) on stable and radioactive elements governs the electron abundance in pre-supernova stars and hence the size of the collapsing core in Type II supernovae. It is also important in determining  the distribution of iron-like elements produced in Type IA supernovae. We plan to study EC strength on stable nuclei using the (t, ³He) reaction. Secondary triton beams and the use of the S800 spectrograph in a dispersion matched mode should provide resolutions 5-10 times better than that obtained with (n,p) reactions at TRIUMF, permitting a much more detailed understanding of the EC strength.  The experiments and the theoretical analysis both appear to be much simpler than for the (d, ²He) reaction studied at KVI. Preliminary results for¹²C and ⁵⁸Ni obtained in collaboration with the Osaka group shoed great promise--see figure.  

    We have obtained an enhanced tritium yield (> 10⁷/ sec) using the ¹⁶O + ⁹Be as a source reaction and have performed new  measurements on  ²H, ¹²C, ^24,26Mg, ⁶³Cu, and ⁹⁴Mo  to calibrate the reaction and to determine GT strength in the Fe region and in the Z , N > 40 region where shell closures strongly reduce GT strength in the naive Shell Model.   Use of more realistic wave functions (Langanke, et al. PRL 90, 241102 (2003), shows that these heavier nuclei dominate electron capture and  significantly affect supernova properties during the collapse phase. Unfortunately , there have been very few detailed studies to validate the approximations used in these model calculations.  The present  experiments are intended to remedy this situation and provide the tests of the calculations that are necessary to put the theoretical description on a firmer basis.

Eventually, we hope to build a high efficiency tritium ion source to make possible experiments with primary beams.  The resulting resolution is expected to be well under 100 keV, and will permit rigorous tests of model calculations in the mass range near and above Fe. 

Electron Capture on key radioactive nuclei using inverse kinematics reactions (with A. Cole. R. Zegers, B. Sherrill et al.).  Radioactive nuclei play an important role in the evolution of a supernova's pre-collapse core.  In order to develop techniques for studying these reactions in inverse kinematics, we have measured cross sections for the ⁷Li (⁵⁶Ni, ⁵⁶Co)⁷Be(gamma) reaction.  The gamma ray labels the GT transitions leading to the first excited  1/2⁺ state of the ⁷Be ejectile .  This reaction appears to have the most promise for studies in inverse kinematics. The data is under analysis. 

Nucleosynthesis by Cosmic Rays in the Early Galaxy: (with D. Mercer).

Galactic evolution  models indicate that a + a reactions play an important role in the synthesis of  ^6,7Li in the early galaxy; however, the cross sections leading to ⁶Li are poorly known, making it difficult to check the models. As more data on the ⁶Li abundance in very metal poor stars become available, this has become an important problem, because ⁶Li can be used to limit also the astration of ⁷Li in these stars. The energies needed for these measurements are uniquely available at the NSCL. We developed a new technique which made the measurements possible and have obtained sufficiently accurate date up to a bombarding energy of 150 MeV/nucleon. In the graph the total cross section for production of ⁶Li is shown as a function of the bombarding energy.  The small points show the standard cross sections (Read and Viola) previously used for cosmic ray studies, and the larger points are from the present measurements at the higher energies and from a re-analysis of earlier data at the lower energies.  The dashed curve is an exponential fit to the data.  These results allow a much better founded prediction of the production of ^6,7Li by cosmic ray alpha + alpha collisions in the early galaxy.  The resulting ⁶Li production, calculated for a typical cosmic ray spectrum,  is about a factor of two smaller than for the Read-Viola cross sections normally used.  In many models the calculated production of ⁶Li is at most that observed, greatly limiting possible astration of ⁶Li.