Radio activity reloaded: Young Stellar Objects as seen by VLA, VLBA, and ALMA

In recent years, high-energy processes in young stellar objects have largely been studied using X-ray observations. While it has been known for some time that radio observations provide complementary information on coronal activity, high-energy irradiation of both protoplanetary disks and planets, mass accretion, and jet formation, it is only now, with unprecedented observational capabilities, that we can systematically obtain this information. To highlight these capabilities, I will present results of radio surveys targeting the Orion Nebula Cluster (ONC), using the upgraded VLA and VLBA as well as ALMA, partly with simultaneous Chandra observations. Our deep VLA and Chandra observations have enlarged the sample of known radio sources in the ONC by a factor of more than 7, enabling detailed comparisons of X-ray and radio YSO populations while providing the first systematic set of simultaneous YSO radio and X-ray lightcurves. With these data, we can look into the detailed correlation of X-ray and radio flares from YSOs, including radio spectral index information. Improved VLBA capabilities now allow us to follow up hundreds of sources in an astrometric monitoring program that is producing both a definitive census of nonthermal emission and precision astrometry of embedded YSOs that remain out of reach for Gaia, thus complementing the Gaia mission by producing a proper motion survey of embedded YSOs and a rare cross-check of absolute astrometric measurements. Last but not least, we have now used ALMA to extend our scope of high-energy phenomena from gyrosynchrotron radiation in the centimeter wavelength regime to synchrotron radiation by obtaining a first systematic set of millimeter-wavelength lightcurves of Orion YSOs. Overall, these results highlight a new perspective on high-energy processes in YSOs while serving as a stepping stone for future surveys with the SKA and ngVLA