Campus, PAB 102/103
Zoom info: https://stanford.zoom.us/j/98604058568
The starlight polarization produced by the dust extinction of background stars reveals the properties of the Galactic magnetic field (GMF) projected on the plane of the sky. The polarization is, thus, tied not only to the properties of the GMF but also to the properties of the magnetized dust grains in the interstellar medium (ISM). This research aims to use optical linear polarization observations from the Interstellar Polarization Survey (IPS) to probe the magnetized, turbulent ISM. We study variations of the polarization degree per reddening unit with Galactic coordinates that contain information on the mechanisms that reduce polarization efficiency. Polarization efficiency increases with Galactic latitude, and its variations with Galactic longitude broadly agree with the change in orientation of the regular GMF about the line-of-sight for different pitch angles. Intermediate Galactic latitude (|b|<7°.5) IPS regions have polarization efficiencies above the maximum value proposed by Planck Collaboration. These regions have only one polarizing screen in the foreground of the stars at d < 400 pc. Structure functions of polarization angle showed small field fluctuations above sub-parsec scales at d ~ 150 pc. We used the David-Chandrasekhar-Fermi method and the multi-phase neutral ISM properties to find the average plane-of-sky magnetic field strengths. The values found between ~6 μG and ~8 μG are consistent with previous estimations in the local interstellar medium. Finally, optical and thermal dust polarization demonstrate a highly regular large-scale magnetic field orientation on the plane-of-sky unfazed by diffuse small structures observed at 12 μm in the nearby ISM, even though the diffuse dust is consistent with the thermal dust optical depth at 353 GHz.
The origin of the heaviest elements in the Universe remains an open question in nuclear astrophysics. In this talk, I will focus on a scenario in which the heaviest elements are produced during the merger of a neutron star (NS) with the core of a giant star. When a star at least few times more massive than the sun and a NS coexist in a close binary system, the immense swelling of the star as it evolves into a red supergiant might destroy the harmony and result in the engulfment of the NS that keeps orbiting inside the envelope of its giant predator. During this common envelope evolution phase, dynamical friction transfers energy from the orbit of the NS to the envelope leading to a substantial decrease in the orbital separation. If the NS reaches the core of the red supergiant it tidally destroys it, forming a thick accretion disk from its dense matter. The accretion disk disposes of excessive angular momentum by launching gaseous jets and the extreme condition within the accretion disk and jets lead protons and electrons to merge into neutrons, which are then captured by iron nuclei producing heavy elements. I explored this novel scenario for heavy elements nucleosynthesis using various state of the art numerical methods. I will discuss the feasibility of this nucleosynthesis channel based on results from my detailed stellar evolution models and its contribution to heavy elements formation estimated from binary population synthesis and galactic chemical evolution simulations. Then, I will discuss potential observational signatures including electromagnetic measurements, gravitational waves, and neutrinos.