Tuesday, September 16, 2014

Cold Gas in the Pipe Nebula

One portion of the vast dark cloud of interstellar dust called the Pipe Nebula (shown here is the object Barnard 59). The Pipe Nebula is known for being massive, and so a likely candidate for young stars, yet it is cold and dark with few signs of star formation. Astronomers have observed fifty-two dense cores in the Pipe in six key interstellar molecules using millimeter telescopes, and find clump temperatures as low as thirteen kelvin. Credit: ESO

The Pipe Nebula is a prominent dark molecular cloud located about 430 light-years from us. It contains about ten thousand solar-masses of material, making it one of the closest giant molecular complexes to us, and with dimensions of about 10 by 46 light-years, one of the largest. These properties should make it an excellent place to study star formation up close, except for one problem: there is very little star formation underway there. Instead, the Pipe Nebula has become one of the prime cases for testing ideas about star formation, since apparently an abundance of material is not enough by itself to produce new stars.

The Pipe Nebula is not uniform in density; it contains a large population of dense, low-mass cores, about 134 distinct objects. In other molecular clouds these cores evolve into young stars, but in the Pipe they remain quiescent. Astronomers have been using radio astronomy techniques to study the density and temperatures of particular molecular species in these cold cores, for example species of carbon monoxide and ammonia, whose relative line strengths can quantify these parameters.

CfA astronomers Jan Frobrich, Karin Oberg, and Charlie Lada, together with four colleagues, have now completed a study of fifty-two of the Pipe’s cores in the light from six additional molecules that are particularly useful in characterizing star formation activity, and also complemented that data with infrared images from Herschel that show the cold dust distribution. They report that the dense cores in the Pipe are very cold indeed, between 13 and 19 kelvin,with the coldest ones containing the most obscuring dust. The astronomers find that one molecule in particular, N2H+, is the only species to exclusively trace the very densest and coldest cores, making this molecule a key diagnostic for future studies to figure out why these dense cores do not form stars.


"Some Like It Cold: Molecular Emission and Effective Dust Temperatures of Dense Cores in the Pipe Nebula," Jan Forbrich, Karin Öberg, Charles J. Lada, Marco Lombardi, Alvaro Hacar, João Alves, and Jill M. Rathborne, A&A, 568, A27, 2014.