The winner of the third edition of the A&A award for a paper by a PhD student was announced at the annual meeting of the European Astronomical Society in Krakow, Poland.
PhD prize
Raphael Skalidis
Raphael Skalidis, now a postdoctoral researcher a the California Institute of Technology, did both his undergraduate and graduate studies at the University of Crete in Heraklion, and obtained the A&A prize for his paper entitled "Why take the square root? An assessment of interstellar magnetic field strength estimation methods."
The magnetic field that permeates the interstellar medium is an essential ingredient of its dynamics, yet it is very hard to observationally constrain. The celebrated Davis-Chandrasekhar-Fermi (DCF) method measures the magnetic field from the morphology of dust polarization maps, using the spread of the polarization angles to evaluate the ratio of the smooth large scales and the fluctuating small scales of the field. Assuming equipartition between the turbulent kinetic energy and the fluctuating magnetic energy then provides the normalization. The DCF method however assumes incompressible magnetohydrodynamics (MHD), when the interstellar medium is in fact highly compressible. A previous A&A paper by Skalidis and Tassis showed analytically that the appropriate scaling for the magnetic field in compressible MHD turbulence is with sqrt(sigma(theta)) rather than with sigma(theta) in the classical DCF formulation. The present paper evaluates the two options on synthetic polarized emission maps generated from an extensive set of compressible MHD numerical simulations for a wide range of Alfven and sonic Mach numbers. It finds that the Skalidis & Tassis formulation recovers the (known) input magnetic field to within factors of a few, where the classical DCF method can be off by orders of magnitude. This represents a true breakthrough in our ability to measure interstellar magnetic fields and therefore to understand the dynamics of the interstellar medium and the first stages of star formation.