Searching for coronal radio emission from protostars using very-long-baseline interferometry

Aims. In order to directly study the role of magnetic fields in the immediate vicinity of protostars, we use Very-Long-Baseline Interferometry ( VLBI), aiming at the detection of non-thermal centimetric radio emission. This is technically the only possibility to study coronal emission at sub-AU resolution. Methods. We performed VLBI observations of the four nearby protostars HL Tau, LDN 1551 IRS5, EC 95, and YLW 15 in order to look for compact non-thermal centimetric radio emission. For maximum sensitivity, we used the High Sensitivity Array ( HSA) where possible, involving the Very Long Baseline Array ( VLBA), the phased Very Large Array ( VLA), as well as the Arecibo, Green Bank, and Effelsberg radio telescopes. Results. While all four protostars were detected in VLA-only data, only one source ( YLW15VLA2) was detected in the VLBI data. The possibility of non-detections due to free-free absorption, possibly depending on source geometry, is considered.