Are stellar embryos in Perseus radio-synchrotron emitters?

Statistical data analysis with Herschel and LOFAR paving the way for the SKA

¹ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
² Laboratoire de Physique de l’Ecole Normale Supérieure, ENS, Université PSL, CNRS, Sorbonne Université, Université de Paris, 75005 Paris, France
³ Istituto di Astrofisica e Planetologia Spaziali (IAPS), INAF, Via Fosso del Cavaliere 100, 00133 Roma, Italy
⁴ Thüringer Landessternwarte, Sternwarte 5, 07778 Tautenburg, Germany

^★ Corresponding author; andrea.bracco@inaf.it

Received: 27 August 2024
Accepted: 29 November 2024

Abstract

Cosmic rays (CRs) are crucial to the chemistry and physics of star-forming regions. By controlling the ionization rate of molecular gas, they mediate the interaction between matter flows and interstellar magnetic fields, thereby regulating the entire star formation process, from the diffuse interstellar medium to the formation of stellar embryos, or cores. The electronic GeV component of CRs is expected to generate nonthermal synchrotron radiation, which should be detectable at radio frequencies across multiple physical scales. However, synchrotron emission from star-forming regions in the Milky Way has barely been observed to date. In this work, we present the first attempt to statistically detect synchrotron emission with the LOw Frequency ARray (LOFAR) at 144 MHz from the nearby Perseus molecular cloud (at a distance of ~300 pc). We perform median stacking over 353 prestellar and 132 protostellar cores derived from the Herschel Gould Belt Survey. Using data from the LOFAR two meter sky survey (LoTSS) with an angular resolution of 20″, we identify 18 potential protostellar candidates and 5 prestellar ones. However, we interpret these as extragalactic contamination in the Herschel catalog. Our statistical analysis of the remaining cores does not reveal any significant radio counterpart of prestellar and protostellar cores at levels of 5 μJy beam⁻¹ and 8 μJy beam⁻¹ in the stacked maps, respectively. We discuss our non-detections in two ways. For protostellar cores, we believe that strong extinction mechanisms of radio emission, such as free-free absorption and the Razin–Tsytovich effect, may be at play. For prestellar cores, using analytical models of magnetostatic–isothermal cores, the lack of detection with LOFAR helps us constrain the maximum ordered magnetic-field strength statistically attainable by these objects, on the order of 100 μJG. We predict that the statistical emission of the prestellar-core sample in Perseus seen by LoTSS should be detectable in only 9 hours and 4 hours with the Square Kilometre Array-Low (SKA-Low) array assemblies AA* and AA4, respectively.