Spokes cluster: The search for the quiescent gas

Context. Understanding the role of turbulent and thermal fragmentation is one of the most important current questions of star formation. To better understand the process of star and cluster formation, we need to study in detail the physical structure and properties of the parental molecular cloud. In particular, it is important to understand the fragmentation process itself; this may be regulated by thermal pressure, magnetic fields, and/or turbulence. The targeted region, the Spokes cluster, or NGC 2264-D, is a rich protostellar cluster where previous N2H+ (1-0) observations of its dense cores presented linewidths consistent with supersonic turbulence. However, the fragmentation of the most massive of these cores appears to have a scale length consistent with that of the thermal Jeans length, suggesting that turbulence was not dominant. Aims: These two results (derived from N2H+ (1-0) observations and measurements of the spatial separations of the protostars) probe different density regimes. Our aim is to determine if there is subsonic or less-turbulent gas (than previously reported) in the Spokes cluster when probing higher densities, which would reconcile both previous observational results. To study denser gas it is necessary to carry out observations using transitions with a higher critical density to directly measure its kinematics. Methods: We present APEX N2H+ (3-2) and N2D+ (3-2) observations of the NGC 2264-D region to measure the linewidths and the deuteration fraction of the higher density gas. The critical densities of the selected transitions are more than an order of magnitude higher than that of N2H+ (1-0). Results: We find that the N2H+ (3-2) and N2D+ (3-2) emission present significantly narrower linewidths than the emission from N2H+ (1-0) for most cores. In two of the spectra, the nonthermal component is close (within 1-σ) to the sound speed. In addition, we find that the three spatially segregated cores for which no protostar had been confirmed show the highest levels of deuteration. Conclusions: These results show that the higher density gas, probed with N2H+ and N2D+ (3-2), reveals more quiescent gas in the Spokes cluster than previously reported. More high-angular resolution interferometric observations using high-density tracers are needed to truly assess the kinematics and substructure within NGC 2264-D. This publication is based on data acquired with the Atacama Pathfinder Experiment (APEX) under programme 088.F-9322A. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory.