Sep 22

"Constraining Cosmological and Inflationary Parameters with 21cm Tomography"
Yi Mao, University of Texas at Austin

Three-dimensional neutral hydrogen mapping using the redshifted 21cm line has recently emerged as a promising cosmological probe. We investigate how various assumptions will affect the cosmological constraints, and find that with realistic assumptions, a future square-kilometer array (FFTT) optimized for 21cm tomography could improve the sensitivity to spatial curvature and neutrino masses to Delta Omega_k ~ 0.0002 and Delta m_nu ~ 0.007 eV, and give 4-sigma detection of the spectral index running predicted by the simplest inflation models. We will also discuss how 21cm tomography can improve the constraints on the slow roll parameters which can be used to reconstruct the inflationary potentials.

Sep 29

"Compulsory Mixing of 3He and CNO Isotopes in the Envelopes of low-mass Red Giants"
David S. P. Dearborn, Lawrence Livermore National Laboratory

Three-dimensional stellar modeling enabled us to identify a mixing mechanism that must operate in all low mass giants (Egglton 2006a). This mixing process is not optional, and is driven by a molecular weight inversion created by the 3He(3He,2p)4He reaction. We characterize the behavior of this mixing, and study its impact on the envelope abundances. It not only eliminates the problem of 3He overproduction, reconciling stellar and big bang nucleosynthesis, with observations, but solves the discrepancy between observed and calculated CNO isotope ratios in low mass giants, a problem of more than 3 decades standing. This mixing mechanism operates rapidly once the hydrogen burning shell approaches the material homogenized by the surface convection zone. In agreement with observations, pop 1 stars between 0.8 and 2.0 solar masses develop C12/C13 ratios of 14.5 ± 1.5, while Pop II stars process the carbon to ratios of 4.0 ± 0.5. In stars less than 1.25 solar masses, this mechanism also destroys 90% to 95% of the of the 3He produced on the main sequence.

Oct 06

"The First Stars"
Christopher F. McKee, University of California at Berkeley

The first stars formed out of pristine gas with initial conditions determined by the Big Bang. These stars produced the first heavy elements, began the reionization of the universe, and may have produced the first stellar-mass black holes. These effects all depend critically upon the mass of the first stars. The conditions under which these stars form is believed to be determined by the physics of the H2 molecule. Large-scale numerical simulations indicate that the mass of gas out of which these stars form is thousands of times greater than the mass of the Sun. The final mass of these stars is much less than this and is most likely set by feedback from the stars on their environment, including photodissociation of H2 by ultraviolet radiation, radiation pressure associated with Lyman-alpha photons, photoionization of the accreting gas, and photoevaporation of the protostellar accretion disks. Rotation of the gas accreting onto these stars governs the effectiveness of these feedback processes. Theoretical estimates place the typical mass of the first stars at about 100 times the mass of the Sun. Such stars have a short lifetime, but they leave a fossil record of their properties in the elemental abundances in the oldest stars in the Galaxy, which are less massive than the Sun.

Oct 27

"Special Seminar: The M31 Halo: New Perspectives"
R. Michael Rich, University of California, Los Angeles

The M31 halo is revealed to be complex, with age dispersions, Solar metallicity stars at 30 kpc, and a steep abundance gradient to 100 kpc. We report on HST/ACS imaging and analysis of deep fields in the disk and halo, and Keck/Deimos spectroscopy of giants reaching 150 kpc from the nucleus. We find a steep abundance gradient, employing a new Ca triplet coaddition method, and find spectroscopic evidence that the halos of M31 and M33 overlap at R>120 kpc from the M31 nucleus. The M31 halo is complex, both from the perspective of abundances and kinematics, due to (evidently) multiple accretion events, including the impact of a massive galaxy responsible for the Giant Stream. We also report spectroscopy for giants in the radial arc structures at 70 kpc; kinematic coherence is shown for at least one of the structures.