A Dynamic Radio View of the Orion Nebula Cluster: Protostellar Nonthermal Variability at Centimeter and Millimeter Wavelengths

Abstract

This thesis contains a comprehensive observational study of the radio time domain of young stars in the search for high-energy (HE) processes. This requires high spatiotemporal-resolution observations and thus the use of the most capable radio interferometers in the world such as the Karl G. Jansky Very Large Array (VLA), Atacama Large Millimeter/submillimeter Array (ALMA) and the Very Long Baseline Array (VLBA). Young Stellar Objects (YSOs) present high levels of magnetic activity and related HE processes including strong flares that are orders of magnitude more luminous than that of their main-sequence counterparts. Both X-ray and radio wavelength observations provide evidence of these processes. Despite extensive X-ray studies, HE processes are still poorly understood, and only with the improved capabilities of radio facilities in the last decade have we gained access to radio emission associated with HE processes at unprecedented time resolution. YSO radio flaring variability at cm- and mm-wavelengths is associated with nonthermal (gyro-)synchrotron emission from magnetospheric activity with electrons gyrating along magnetic field lines. I present a radio variability study for an unprecedented sample of YSOs at cm- and mm-wavelengths at high spatiotemporal resolution with the VLA, ALMA, and the VLBA. I first present an enlarged census of compact cm-wavelength radio sources towards the Orion Nebula Cluster (ONC) using the VLA. We find evidence of strong variability by up to a factor of five on timescales of minutes to hours towards several sources, and order-of-magnitude variability on a timescale of 4 years in a few sources. These findings lead to a mean time between extreme radio variability events of 2482±1433 h. I also report the discovery of high proper motions from non-stellar radio sources (up to ∼373 km s−1). Motivated by the widespread variability found in our VLA study and the few serendipitous discoveries of strong millimetre flares reported to date, I performed the first systematic search for such variability at mm-wavelengths in the ONC using ALMA. This was achieved for a large number of YSOs at high-time resolution. We find widespread mm-wavelength variability including the discovery of an order of magnitude mm-flare from a known YSO. I also present an assessment of systematic effects from interferometric imaging making use of simulated ALMA observations. Finally, I present a follow-up study of a multi-epoch VLBA survey for nonthermal radio emission for a large sample of sources in the ONC where, contrary to the VLA and ALMA data of the ONC, lightcurves can be produced directly from the visibilities allowing us to efficiently generate lightcurves at high-time resolution under given assumptions (unresolved and isolated sources) and allowing us to also explore different Stokes parameters following the same approach.