Background Science

The observed line profiles of giant molecular clouds hold a number of mysteries.  They display line widths much larger than the expected thermal velocity given their expected ambient temperature.  They are also extremely smooth and very centrally peaked.  Observed line profiles displayed below are taken from Phillips et. al. 1979 and you may also see Wolfire et. al. 1993 for a nice description of this phenomenon.

Screen Shot 2013-04-22 at 10.18.26 PMScreen Shot 2013-04-22 at 10.42.32 PMScreen Shot 2013-04-22 at 10.19.19 PM

A number of theories have been put forward regarding possible explanations for these wide, smooth profiles.  The entire cloud could be undergoing microturbulence, in which the scale size of the turbulence is much smaller than the photon mean free path.  The velocity dispersion of the microturbulence would then need to be on order of the observed line width.  One detraction from this theory is that the modeled profile shapes tend to vary greatly with the cloud mass, while observed line profiles are remarkably similar (Wolfire et. al. 1993).  Furthermore, microturbulent models also predict CO to be far more self-absorbed, thus producing a flat top profile rather than the observed central peak (Leung and Liszt 1976).  Finally, models predict the 12CO and 13CO shapes to be dissimilar, contrary to observation (Kwan 1978).

A competing, and currently more accepted, model to explain these line profile shapes is macroturbulence.  This supposes that the cloud is composed of individual clumps, each with an inartistic line width similar to the cloud’s thermal line width.  The clumps then have some velocity dispersion much greater than their intrinsic line width.  Macroturbulence has been modeled on very large scales by Kwan and Sanders 1986 and on smaller clump scales by Wolfire et. al. 1993.

This project attempts to create an interactive model of macroturbulence within GMCs, allowing the user to quickly change parameters of the cloud and see their affects on the observed line profile. The radiative transfer methods used in this code are by no means rigorous, and therefore should not be used to extract parameters from observed line profiles a posteriori.  This is simply meant as a learning technique to explore the relation of various clump and cloud properties on observed line widths.


Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s