Young starless cores embedded in the magnetically dominated Pipe nebula. II. Extended dataset

Abstract

The Pipe nebula is a massive, nearby, filamentary dark molecular cloud with a low star formation efficiency threaded by a uniform magnetic field perpendicular to its main axis. It harbors more than a hundred, mostly quiescent, very chemically young starless cores. The cloud is therefore a good laboratory to study the earliest stages of the star formation process. We aim to investigate the primordial conditions and the relation among physical, chemical, and magnetic properties in the evolution of low-mass starless cores. We used the IRAM 30 m telescope to map the 1.2 mm dust continuum emission of five new starless cores, which are in good agreement with previous visual extinction maps. For the sample of nine cores, which includes the four cores studied in a previous work, we derived an A(V) to N-H2 factor of (1.27 +/- 0.12) x 10(-21) mag cm(2) and a background visual extinction of similar to 6.7 mag possibly arising from the cloud material. We derived an average core diameter of similar to 0.08 pc, density of similar to 10(5) cm(-3), and mass of similar to 1.7 M-circle dot. Several trends seem to exist related to increasing core density: (1) the diameter seems to shrink, (2) the mass seems to increase, and (3) the chemistry tends to be richer. No correlation is found between the direction of the surrounding diffuse medium magnetic field and the projected orientation of the cores, suggesting that large-scale magnetic fields seem to play a secondary role in shaping the cores. We also used the IRAM 30 m telescope to extend the previous molecular survey at 1 and 3 mm of early- and late-time molecules toward the same five new Pipe nebula starless cores, and analyzed the normalized intensities of the detected molecular transitions. We confirmed the chemical differentiation toward the sample and increased the number of molecular transitions of the "diffuse" (e.g., the " ubiquitous" CO, C2H, and CS), "oxo-sulfurated" (e.g., SO and CH3OH), and "deuterated" (e.g., N2H+, CN, and HCN) starless core groups. The chemically defined core groups seem to be related to different evolutionary stages: "diffuse" cores present the cloud chemistry and are less dense, while "deuterated" cores are the densest and present a chemistry typical of evolved dense cores. "Oxo-sulfurated" cores might be in a transitional stage exhibiting intermediate properties and a very characteristic chemistry.