Physical characteristics of a dark cloud in an early stage of star formation toward NGC 7538 ^*,**

An outer Galaxy infrared dark cloud?

¹ Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, The Netherlands e-mail: frieswyk@astro.rug.nl
² SRON, National Institute for Space Research, PO Box 800, 9700 AV Groningen, The Netherlands
³ ESAC, European Space Astronomy Centre, Urb. Villafrancadel Castillo, PO Box 50727, Madrid 28080, Spain
⁴ IRAM, Institut de Radio Astronomie Millimétrique, 300 rue de la Piscine, St-Martin d'Hères, France

Received: 22 January 2007
Accepted: 22 August 2007

Abstract

Context.In the inner parts of the Galaxy the Infrared Dark Clouds (IRDCs) are presently believed to be the progenitors of massive stars and star clusters. Many of them are predominantly devoid of active star formation and for now they represent the earliest observed stages of massive star formation. Their Outer Galaxy counterparts, if present, are not easily identified because of a low or absent mid-IR background.

Aims.We characterize the ambient conditions by determining physical parameters in the Outer Galaxy IRDC candidate G111.80+0.58, a relatively quiescent molecular core complex in the vicinity of active star forming regions such as NGC 7538 and S159.

Methods.We conduct molecular line observations on a number of dense cores in G111.80+0.58. We analyze the data in terms of excitation temperature, column and volume density, mass and stability.

Results.The temperatures we find (15–20 K) are higher than expected from only cosmic ray heating, but are comparable to those found in massive cores, such as IRDCs. Star forming activity could be present in some cores, as indicated by the presence of warm gas (NH₃, ¹³CO self-absorption) and Young Stellar Object candidates. The observed super-thermal line-widths are typical for star forming regions. The velocity dispersion is consistent with a turbulent energy cascade over the observed size scales of the complex. We do not find a correlation between the gas temperature and the line-width. The LTE masses we derive are much larger than the thermal Jeans mass. Therefore, fragmentation is expected and may have occurred already, in which case the observed lines represent the combined emission of multiple unresolved components.

Conclusions.We conclude that G111.80+0.58 is a molecular core complex with bulk properties very similar to IRDCs in an early, but not pristine, star forming state. The individual cores are close to virial equilibrium and some contain sufficient material to form massive stars and star clusters. The ambient conditions suggest that turbulence is involved in supporting the cores against gravitational collapse, at least down to the observed sizes. Additional high resolution data are necessary to resolve and analyze the smaller scale properties.