Determining the flux value for protostellar sources using Miriad
(original text by Demerese Salter --- 11/15/2011)
(edited by Shaye Storm --- 11/16/2011, Feb 2012)
- For point sources, use imfit to find the centroid of the emission, in order to get the correct phase center (solving for the position down to a couple of decimal points). As a general guideline, it is good to have a lambda/2 positional accuracy. Then use uvamp or uvfit with the offset= and center= keywords, respectively, in order to calculate the flux of the point source.
- For a Gaussian object, using imfit and uvfit is also reasonable.
- For a fairly circular extended source, sometimes consisting of an unresolved point source as well as extended emission that can be represented by a Gaussian, you can use uvfit on a subset of baselines. First, determine the point source flux by fitting the longer baselines. Then, take the flux on the shortest baselines to be the total estimated flux. Subtracting the two numbers gives you the flux of the extended emission. You can also use a model to fit the shorter baselines, but this involves its own errors. And without single-dish (or zero-spacing) data, you cannot know the total flux more precisely.
- For asymmetric extended sources, the process is a little more complicated. Since routines like uvamp bin the data in symmetric annuli according to uv distance, asymmetries of the source can cause non zero phases that will yield to flux loss upon binning. You can use ellint to sum up the flux, but this is best if the minor and major axes of the object are within a factor 2 of one another. With ellint, you want to plot the summed flux value for annuli of increasing radii until the cummulative flux flattens out. However, this ellint method has its own dependences on beam size and the noise level.
- Finally, using imlist with options=stat for a region drawn around the object (e.g. a box or polygon) will give a total flux. imstat provides a similar summed flux calculation, but doesn't take into account the integrated beam size.
- John C advice for determining the flux of extended protostellar objects is to just use imlist with options=statistics. Otherwise it's more complicated. He says that if you believe your source is Gaussian, then you can use uvfit with object=Gaussian. He does not recommended ellint because of the dependence on your beam size and the noise level. You are probing different size scales and you can get different fluxes this way in different array configurations depending on the Fourier components that your array is sensitive to and that your source possesses.