Multiple low-turbulence starless cores associated with intermediate- to high-mass star formation^*

Max-Planck-Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [beuther;henning]@mpia.de

Received: 10 March 2009
Accepted: 6 July 2009

Abstract

Aims. Characterizing the gas and dust properties prior to and in the neighborhood of active intermediate- to high-mass star formation.

Methods. Two Infrared Dark Clouds (IRDCs) – IRDC 19175-4 and IRDC 19175-5 – that are located in the vicinity of the luminous massive star-forming region IRAS 19175+1357, but that remain absorption features up to 70 μm wavelength, were observed with the Plateau de Bure Interferometer in the 3.23 mm dust continuum as well as the N₂H⁺(1–0) and ¹³CS(2–1) line emission.

Results. While IRDC 19175-4 is clearly detected in the 3.23 mm continuum, the second source in the field, IRDC 19175-5, is only barely observable above the continuum detection threshold. However, the N₂H⁺(1–0) observations reveal 17 separate sub-sources in the vicinity of the two IRDCs. Most of them exhibit low levels of turbulence ( km s^-1), indicating that the fragmentation process in these cores may be dominated by the interplay of thermal pressure and gravity, but not so much by turbulence. Combining the small line widths with the non-detection up to 70 μm and the absence of other signs of star formation activity, most of these 17 cores with masses between sub-solar to ~10  are likely still in a starless phase. The N₂H⁺ column density analysis indicates significant abundance variations between the cores. Furthermore, we find a large CS depletion factor of the order 100. Although the strongest line and continuum peak is close to virial equilibrium, its slightly broader line width compared to the other cores is consistent with it being in a contraction phase potentially at the verge of star formation. Based on the 3.23 mm upper limits, the other cores may be gravitationally stable or even transient structures. The relative peak velocities between neighboring cores are usually below 1 km s^-1, and we do not identify streaming motions along the filamentary structures. Average densities are between 10⁵ and 10⁶ cm^-3 (one to two orders of magnitude larger than for example in the Pipe nebula) implying relatively small Jeans-lengths that are consistent with the observed core separations of the order 5000 AU. Environmental reasons potentially determining these values are discussed.

Conclusions. These observations show that multiple low- to intermediate-mass low-turbulence starless cores can exist in the proximity of more turbulent active intermediate- to high-mass star-forming regions. While masses and levels of turbulence are consistent with low-mass starless core regions, other parameters like the densities or Jeans-lengths differ considerably. This may be due to environmental effects. The quest for high-mass starless cores prior to any star formation activity remains open.