Hunting pre-stellar cores with APEX: Corona Australis 151, the densest pre-stellar core or the youngest protostar?

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstrasse 1, 85748 Garching, Germany
³ Department of Physics, University of Helsinki, PO Box 64, 00014 Helsinki, Finland
⁴ National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
⁵ Max-Planck-Institut für Radioastronomie, Auf dem Hügel, 69, 53121 Bonn, Germany

^★ Corresponding author; elena.redaelli@eso.org

Received: 11 October 2024
Accepted: 19 February 2025

Abstract

Context. Pre-stellar cores are the birthplaces of Sun-like stars and represent the initial conditions for the assembly of protoplanetary systems. Due to their short lifespans, they are rare. As part of recent efforts to increase the number of such sources identified in the solar neighbourhood, we have selected a sample of 40 starless cores from the publicly available core catalogues of the Herschel Gould Belt survey. In this work, we focus on a source that stands out for its high central density: Corona Australis 151.

Aims. We used molecular lines that trace dense gas (n ≳ 10⁶ cm⁻³) to confirm the exceptionally high density of this object, study its physical structure, and understand its evolutionary stage.

Methods. We detected the N₂H⁺ 3 − 2 and 5 − 4 transitions and the N₂D⁺ 3 − 2, 4 − 3, and 6 – 5 lines with the Atacama Pathfinder EXperiment (APEX) telescope. We used the Herschel continuum data to infer a spherically symmetric model of the core’s density and temperature. This was used as input to perform a non-local-thermodynamic-equilibrium radiative transfer to fit the five observed lines.

Results. Our analysis confirms that this core is characterised by very high densities (a few × 10⁷ cm⁻³ at the centre) and cold temperatures (8 − 12 K). We inferred a high deuteration level of N₂D⁺/N₂H⁺ = 0.50, indicative of an advanced evolutionary stage. In the large bandwidth covered by the APEX data, we detected several other deuterated species, including CHD₂OH, D₂CO, and ND₃. We also detected multiple sulphurated species that present broader lines with signs of high-velocity wings.

Conclusions. High-angular resolution observations will be necessary to unveil the evolutionary stage of Cra 151. The detection of a compact emission at 70 μm does not exclude that the source is a first hydrostatic core or in a very early stage of the protostellar phase. The observation of high-velocity wings and the fact that the linewidths of N₂H⁺ and N₂D⁺ become larger with increasing frequency can be interpreted as either an indication of supersonic infall motions developing in the central parts of a very evolved pre-stellar core or the signature of outflows from a very low luminosity object.