Session T3: Invited Session: New Stars in Astrophysics

Hans A. Bethe Prize Lecture: The Primordial Helium Abundance

It is generally accepted that the production of the light elements (He, D, and Li) during the early stages of the expansion of the Universe is one of the three pillars of the Big Bang theory. The main results obtained from the observational determination of the primordial helium abundance, Yp, and its comparison with the value predicted by Big Bang Nucleosynthesis will be presented, in particular: a) the recognition that galaxies form with Yp in the 0.24 to 0.26 range, b) that Yp was produced during the Big Bang, c) that Yp is fundamental as a critical test for cosmological theories and the baryonic content of the Universe, and d) that the value of Yp provides an observational constraint on the number of light neutrino species, which is smaller than four and probably equal to three. In addition, the present status of the observationally determined Yp value based on extragalactic H II regions will be discussed.   

A quest for sources of ultrahigh energy cosmic rays

The origin of ultrahigh energy cosmic rays (UHECRs, particles arriving on the Earth with energy $10^{17}- 10^{21}$ eV) is still a mystery. I will review the experimental and theoretical efforts that are being deployed by the community to solve this long-standing enigma, including the recent results from the Auger Observatory. I will discuss the observable signatures that help narrow down the list of possible candidate sources, namely the distribution of the arrival directions of UHECRs in the sky, their energy spectrum, their chemical composition, and their multi-messenger signatures (in neutrinos, gamma-rays and gravitational waves). I will focus in particular on one candidate source that has been little discussed in the literature: young rotation-powered pulsars. The production of UHECRs in these objects could give a picture that is surprisingly consistent with the latest data measured with the Auger Observatory.   

Uncovering the Explosions of Supernova Remnants

Supernova remnants (SNRs), the objects leftover hundreds to thousands of years after supernovae, are excellent laboratories to study the debris of these explosions and its interaction with the interstellar medium. Nearly 300 SNRs have been identified in the Milky Way galaxy, and more than 100 of these have been observed with modern X-ray telescopes. However, the complex and heterogeneous nature of SNRs has hindered the characterization of their properties, precluding comparison of observations with theoretical predictions. In this talk, I will highlight results from the first large-scale, systematic study of X-ray observations of SNRs. This work provides insights regarding the nature of explosions, the efficiency of hydrodynamical instabilities in ejecta mixing, and the role of environment in SNR evolution.