An Evolutionary Diagram for Prestellar Cores

June 11, 2011 — Leave a comment

Yesterday saw my latest research paper posted on to the arXiv. It has been accepted for publication in Monthly Notices of the Royal Astronomical society. Here’s the abstract:

We propose an evolutionary path for prestellar cores on the radius-mass diagram, which is analogous to stellar evolutionary paths on the Hertzsprung-Russell Diagram. Using James Clerk Maxwell Telescope (JCMT) observations of L1688 in the Ophiuchus star-forming complex, we analyse the HCO+ (J=43) spectral line profiles of prestellar cores. We find that of the 58 cores observed, 14 show signs of infall in the form of a blue-asymmetric double-peaked line profile. These 14 cores all lie beyond the Jeans mass line for the region on a radius-mass plot. Furthermore another 10 cores showing tentative signs of infall, in their spectral line profile shapes, appear on or just over the Jeans mass line. We therefore propose the manner in which a prestellar core evolves across this diagram. We hypothesise that a core is formed in the low-mass, low-radius region of the plot. It then accretes quasistatically, increasing in both mass and radius. When it crosses the limit of gravitational instability it begins to collapse, decreasing in radius, towards the region of the diagram where protostellar cores are seen.

If you’re interested in such things, why not take a look? It all comes down to trying to see if there is evidence for an idea that Doug Johnstone had, regarding a sort of evolutionary diagram for prestellar cores (the predecessors of protostars).

We looked at the Ophiuchus star-forming region using the HARP instrument on the JCMT in Hawaii

…inspected the spectra of all the starless cores, looking for the blue-shifted asymmetric double-peaked profile that indicates that they are collapsing…

…and used this to establish that, in Ophiuchus at least, there is tentative evidence that prestellar cores follow the evolutionary path shown in our final figure:

The dark squares show cores with a strong infall signature, white boxes have a weak infall signature. The rest do not show infall. The dotted arcs are examples of a known mathematical model for the way a core can grow by accreting material under constant pressure from the cloud. The diagonal line is the ‘Jeans limit’ for this region: the point at which gravitational attraction exceeds internal pressure for a core, causing it to collapse. Cores progress up the left hand side until they reach the Jeans limit and then collapse more or less vertically. This is our proposed evolutionary path through this parameter space.

The details are of course in the paper itself. It would be interesting to see if this proposed model for the evolution of prestellar cores has evidence elsewhere, in other star forming parts of our galaxy. Herschel and SCUBA-2 present new instruments that could observe objects deep into the left hand side of the graph and perhaps shed more (submillimetre) light on this idea.

This paper was, in large part, a chunk of my PhD thesis and was written with my supervisor Derek Ward-Thompson and the super-helpful submillimetre guru Dave Nutter, with help from Anthony Whitworth and Doug Johnstone.

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