Colloquia Schedule

Anna Sajina (Tufts University)

Sept. 10, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Darby Dyar (Mt. Holyoke College)

Sept. 17, 2015, 3:45 p.m. — 1033 LGRT
Title: Laser-Induced Breakdown Spectroscopy on Mars: The Long Road towa

Abstract: Laser-induced breakdown spectroscopy (LIBS) is being used almost daily on Mars by the ChemCam instrument on the Curiosity rover. The technique uses UV, VIS, and VNIR atomic emission spectra of surface rocks and soils to quantify elemental abundances. Because only 67 standards were run on the flight instrument before launch, calibration efforts are still ongoing. LIBS is different from other types of spectroscopy because atoms interact in the plasma, such that the ratio of intensity or area of an emission line to the abundance of the element producing that line is not a direct correlation. Thus LIBS quantitative analysis is complicated by chemical matrix effects related to abundances of neutral and ionized species in the plasma, collisional interactions within plasma, laser-to-sample coupling efficiency, and self-absorption. Atmospheric composition and pressure also influence plasma intensity. This talk will detail our group's contributions to calibrating Chemcam data, both through dramatic expansion of the laboratory calibration database and development of advanced machine learning tools for their analysis. We find that many previously-unappreciated steps in spectral processing, such as baseline/continuum removal, wavelength calibration, and normalization, have dramatic effects on prediction accuracy; many of these new insights have broader applications to other types of spectroscopy as well.

Lorenzo Sironi (Harvard University)

Sept. 24, 2015, 3:45 p.m. — 1033 LGRT
Title: The Journey of High-Energy Photons in Blazar Jets

Abstract: We investigate the origin and the fate of high-energy photons in blazar jets, by means of analytical theory and first-principles particle-in-cell (PIC) kinetic simulations. In magnetically-dominated jets, magnetic reconnection is often invoked as a mechanism to transfer the jet magnetic energy to the emitting particles, thus powering the observed non-thermal emission. With 2D and 3D PIC simulations, we show that magnetic reconnection in blazar jets satisfies all the basic conditions for the emission: extended non-thermal particle distributions (with power-law slope between -2 and -1), efficient dissipation and rough equipartition between particles and magnetic field in the emitting region.

TeV photons emitted by the highest energy electrons accelerated by reconnection will interact in the intergalactic medium (IGM) with the extragalactic background light, producing a dilute beam of ultra-relativistic pairs. It is a matter of recent debate whether the energy of the pair beam is lost due to inverse Compton scattering off the CMB -- resulting in ~10-100 GeV photons -- or heats the IGM via collective plasma instabilities. The astrophysical stakes are very high because of the large amount of energy and extensive cosmic volume involved in this process. We study the relaxation of blazar-induced beams in the IGM, by means of 2D and 3D PIC simulations. We find that at most 10% of the beam energy is deposited into the IGM plasma, so that at least 90% of the beam energy will be ultimately re-processed in the multi-GeV band.

Fred C. Adams (University of Michigan)

Oct. 1, 2015, 3:45 a.m. — 1033 LGRT
Title: Effects of Clusters on their Constituent Solar Systems and Possi

Abstract: Most stars -- and most solar systems -- form within groups and clusters. One objective of this work is to explore how these star forming environments affect the solar systems forming within them via three channels: dynamical interactions, elevated radiation fields, and increased particle fluxes. The discussion starts with the dynamical simulations, which are used to study how cluster evolution depends on system size and initial conditions. Multiple realizations of equivalent cases are used to build up a statistical description of these systems, e.g., distributions of closest approaches and radial locations. These results provide a framework from which to assess the effects of clusters on solar system formation. Distributions of radial positions are used in conjunction with UV luminosity distributions to estimate the radiation exposure of circumstellar disks. Photoevaporation models determine the efficacy of radiation in removing disk gas and compromising planet formation. The distributions of closest approaches are used in conjunction with scattering cross sections to determine probabilities for solar system disruption. Finally, we determine the distributions of radioactive nuclei that are provided to circumstellar disks, where they enhance ionization and heating. This work provides a quantitative determination of the effects of clusters on forming solar systems. In the second part of the talk, these results are used to place constraints on the possible birth environments for our own solar system.

Dan McIntosh (UMKC)

Oct. 8, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Rosalba Perna (Stony Brook)

Oct. 15, 2015, 3:45 p.m. — 1033 LGRT
Title: Highly magnetic neutron stars: bewildering astrophysical laborat

Abstract: Anomalous X-ray Pulsars and Soft Gamma-Ray Repeaters (SGRs) are young neutron stars characterized by high X-ray quiescent luminosities, outbursts, and, in the case of SGRs, sporadic giant flares. They are believed to be magnetars, that is neutron stars powered by ultra-strong magnetic fields. However, the diversity of their behaviours, and, especially, the observation of magnetar-like bursts from 'low-field' neutron stars, has been a theoretical puzzle. In the first part of the talk, I will discuss results of long-term MHD simulations which, by following the evolution of magnetic stresses within the neutron star crust, have allowed to relate the observed magnetar phenomenology to the physical properties of the neutron stars, and in particular to their age and magnetic field strength and topology. The dichotomy of 'high-B' field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field neutron stars are predicted. In the second part of the talk, I will discuss how observations of highly magnetized neutron stars can be handy tools in the cosmological domain, and in particular as a way to set constraints on the hypothetical particle 'axion'.

Jeyhan Kartaltepe (RIT)

Oct. 22, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Francesca Civano (Yale University)

Oct. 29, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Cara Battersby (CfA)

Nov. 5, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Daniela Calzetti (University of Massachusetts Amherst)

Nov. 12, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Nicolas Bouche (IRAP)

Nov. 19, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Jo Bovy (University of Toronto)

Dec. 3, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA

Juna Kollmeier (Carnegie Observatory)

Dec. 10, 2015, 3:45 p.m. — 1033 LGRT
Title: TBA

Abstract: TBA