Photometry of RCrA

R CrA is a nearby (d ~ 100 pc), relatively isolated molecular cloud. Q. Daniel Wang (of Northwestern U.; moving to U. Mass. this Fall) has identified, via X-ray shadowing, a sub-region of R CrA that contains a large amount of cold material not seen using IRAS. This could be a prototype of the kind of cold dust cloud that might account for the excess cold dust in galaxies that has over the last year been reported in external galaxies by investigators using ISO and SCUBA/JCMT. A submillimeter photometric map using SPARO would thus be interesting for studying the likely repositories of cold dust in galaxies. DIRBE did not have sufficient angular resolution to observe this cloud clearly, and yet it is too big to have been observed in the submillimeter with ISO or SCUBA/JCMT.

At the IRAS peak, the flux is 600 MJy/sr at 100 microns, and the color temperature (60/100 microns) is 34 K. Using a 1.5 emissivity exponent to extrapolate to 450 micron fluxes, and using a zenith opacity of 1.5 we find that we can get 10:1 s/n in about 5 hours. Note this is mediocre winter weather. But the brightest submillimeter flux peak is likely to be at the X-ray shadowing peak, and to be brighter than the IRAS peak. It seems likely that the source would be detected in a few hours and that a publishable photometric map could be made in a day or two. Calibration issues are discussed below. IRAS and X-ray maps are shown in Wang, Q.D., 1994 [in AIP Conf. Proc. 313, "The Soft X-ray Cosmos, ROSAT Science Symposium", ed. E.M. Schlegel and R. Petre, (AIP: New York), p. 301]

The table below contains information on R CrA. Coordinates are J2000. Precession from 1950 equinox done by skyview for IRAS, and by Tom for the X-ray coordinates.

Update 3/17/00 Correction to precessed coordinates for the X-Ray RCrA made.

t10 is the time required for a detection of signal-to-noise = 10:1

Source RA Dec Temperature Peak intensity t10
R CrA - IRAS 19 01 53.7 -36 57 07.6 34 K 600 MJy/sr at 100 microns 5 hrs
R CrA - X-ray 19 04 03 -37 20 39 10-20 K (?) Deep shadow so prob. lots of dust 1 hour or less (?)

Photometric calibration

Photometry requires calibration. The only two submillimeter calibrators we have found are Sgr B2 and NGC 6334. Published large-beam submillimeter fluxes are available for both of these sources. By observing both of these calibrators at a time near to when R CrA (or other photometry target) is observed, it will be possible to recover both the gain of SPARO and the zenith tau at 450 microns. It may be useful also to compare with AST/RO skydips and/or CMU tipper skydips done on the same day.

It seems likely that the tempearture of SPARO's cold stage should be monitored for any dependence on dewar orientation, although this is not expected.

Polarimetry of RCrA

Because polarimetry of such diffuse features has not been done, polarimetric observations of R CrA may be of interest as a polarized cosmological foreground. Also, because R CrA is related to the Loop I superbubble, a comparison of magnetic field geometry and superbubble morphology would be interesting. Polarimetry with 10% accuracy is about a factor of two harder than photometry of 10% accuracy, but note that photometry and polarimetry can be done simultaneously (although the calibration observations required for the two kinds of observations are different).