Secondary Calibrators for Scuba: 1998-2001

THIS IS AN OLD VERSION OF THIS DOCUMENT. The most recent version can be found here.

Use the following numbers with caution; this is a work in progress! For further information see the notes following the Table by clicking on highlighted source names.

Here is a link to the flux densities found using UKT14 and presented in Sandell's 1994 paper (1994, MNRAS, 271, 75).

Often planets are unavailable for calibration of SCUBA data. We have therefore selected a sub-set of calibators used for UKT14 (Sandell, 1994, MNRAS, 271, 75). This subset is chosen from sources that are compact, non-variable, and located in regions free from confusing background emission. We also added HL Tau as a new secondary calibrator, because the star is unresolved and isolated, i.e. it has no surrounding cloud emission. (However there is low-lying extended emission. See below for details.)

However, because these selection criteria left us without calibrators in the whole LST range from 9 - 17 hours, we have added IRC+10216 and 16293-2422 to the list. IRC+10216 is variable (period ~ 635 days) and also extended at 850 micron, because of its unsually large and bright CO envelope, but can be observed without problems with a chop throw of 60" or larger. 16293-2422 is a young bright protostellar binary in Ophiuchus, surrounded by rather extended dust emission and a second cloud core about 80" west. The source is elliptical, especially at 450 micron, and should not be used as a pointing source. (See below.) Note: if used for photometry and map calibration, I recommend that 16293-2422 be observed with a chop of 60" or larger at a position angle of 30 degrees (a north-south, i.e. chop in Declination is acceptable but 30 degrees preferred).

The Table above lists the results of SCUBA observations that we have obtained so far. Note that calibration with SCUBA has only been attempted at 850/450 micron, and some data are now available at 1350 micron. For other filters we still have to rely on old UKT14 data. In the Table I have kept photometry and map calibration separate, but for these sources the results should be the same, which is indeed supported by the results in the table. For the larger data sets I believe the true errors may be close to the quoted errors (one standard deviation from the mean). For small data sets the quoted errors are simply too optimistic, and should perhaps be doubled. All flux densities in the table above are given in Jy/beam. The number of data points used for the mean and rms of each flux density is given in parenthesis. The column headed ``Pointing maps'' refers to calibrated pointing observations (16-point jiggle maps), which are expected to be identical to other calibrated maps.

Fluxes of these sources and others at ~1.2mm have been published by IRAM.

Secondary Calibrators - Morphology

Our telescope is very temperature sensitive. We see large changes in the gain and error lobe structure in the early evening until 8 or 9pm and also in late morning, i.e. any time after 8 am. Furthermore, even during stable night time conditions the error beam is by no means constant from night to night. Changes in the absolute temperature causes structural changes in the dish, which unless the dish is left unadjusted shows up as seasonal changes in the error beam pattern. Any error in keeping the telescope focussed will also affect the telescope beam pattern.

It is therefore extremely difficult to separate faint extended emission from the error beam, which we need to be able to do, if we want to know whether our secondary calibrators are extended or surrounded by faint extended emission. I have gone through most of the data I have calibrated last year to try to create the most reliable maps of each source. Except for CRL2688 (see below), I have only used data in stable night time conditions. Each data set has been throroughly despiked, pointing corrected and coadded by weighting with rms noise. Any suspicious data set (poorly collimated, exceptionally warm night etc) has been removed from the final averages. I have also permutated the data sets in different ways to see how large variations I do see over a period of a month.

Ideally we want to compare these maps with planet maps taken in the same conditions and the same nights, but that has not been possible. CRL618, the source for which I have most data, was observed together with Uranus in September, but an eaqually large set of maps for December lack planet data, because Uranus was setting early and there were no planets for the stable part of the night.

CRL618

This source appears unresolved both at 850 and 450 micron. The September data give FWHM of 14.7"/8.1" at 850/450 micron, compared to HPBWs derived from Uranus of 14.8"/8.3". Note that the CRL618 data appear to give a sharper beam size than Uranus, almost certainly because it is difficult to make an accurate Gaussian fit to the convolved beam of a Gaussian + cylinder, which is not really Gaussian in shape. The December data of CRL618 give a FWHM of 14.7"/7.9" to be compared with HPBWs from early evenming data of Uranus, which give 14.8" and 8.1" at 850 and 450 micron, respectively. Both CRL618 data sets include large numbers of maps (from 10 - 18 maps), and therefore it is no doubt that CRL618 is unresolved. However, CRL618 can still be associated with faint extended emission at a level about or below the error beam. By comparing integrated intensities over 60" and 90" apertures of CRL618 to what we would expect from Uranus, we find an excess of 0.2 - 0.3 Jy within a 30" radius at 850 micron. At 450 micron this is even more difficult to assess, because of larger variations in the error beam level from night to night, but I estimate the amount of faint extended emission to be about 0.8 - 3 Jy in the same area. No extended emission is seen outside a 30" radius, and the excess seen is almost certainly much more compact.

HL Tau

HL Tau has a compact unresolved core, but the star is associated with faint extended emission, clearly visible above the error beam both at 850 and 450 micron (see Fig 1c vs Fig 1b). The measured FWHM is 14.8" at 850 micron and 8.0" at 450 micron, while the corresponding values for CRL618 in the same time period are 14.7" and 7.9" for 850 and 450 micron, respectively. At 850 micron I estimate that HL Tau contributes to about 430 mJy in a 30" radius and about 960 mJy in a 60" radius surrounding the star. For 450 micron the amount of extended emission is more uncertain, because examining data over December show variations of the integrated error lobe of the order of 15 - 20 % for benign conditions. However, the corresponding estimates are about 2.3 Jy and 3.6 Jy for 60 and 120" apertures, respectively. Most of the extended emission is relatively compact and at low level, but I would recommend not to observe HL Tau with chop throws shorter than 60". Note that when I examined data taken with HL Tau setting, I seemed to be chopping onto emission with a 120" Azimuth chop. This shows up as negative emission in the SW part of jiggle-maps, and is not likely to affect the flux of HL Tau. However, I still need to verify whether this emission is real or not.

IRC+10216

It is well known that IRC+10216 is extended. Yet I have had difficulties in finding maps obtained with a 120" chop throw. The results below are based on 3 maps from December. At 850 micron I find a deconvolved FWHM of 8.9" in good agreement with old UKT14 results. We can also clearly see fainter, halo type emission. A rough estimate of the halo emission predicts 3.4 Jy within a 30" radius and 6 Jy within a 60" radius. At 450 micron the corresponding values are 10.0 Jy and 14.5 Jy repspectively. The dust emission is more compact at 450 micron with a deconvolved FWHM of about 4.1". For mid December I therefore predict an integrated 850 micron flux (core + halo) of 11.9 Jy and at 450 micron an integrated flux of 25.8 Jy.

IRC+10216 is variable (period ~ 635 days) and also extended at 850 micron, because of it's unsually large and bright CO envelope, but can be observed without problems with a chop throw of 60" or larger.

CRL2688

I have had difficulties to determine whether CRL2688 is a point source or not. Some data suggest it is extended, others predict it to be unresolved. My concusion is that it is likely to be a point source, but I cannot exclude the possibility of a source size of 2 - 3" at 850 micron. ith the assumption that CRL2688 is unresolved I estimate an additional contribution of 0.5 Jy at 850 micron within a 30" radius. The 850 micron map in Fig 2 is from Apr 98, when the beam from Uranus observations were determined to be ~ 15.2" (CRL2688 gives 15.4"). For 450 micron I show data from early eveing in mid December, which show a substantial error lobe, but suggest that the core is unresoved.

OH231.8+4.2

I don't have much data for OH231.8. All datasets analyzed here are from my own observing runs in February 1998. Similarly to CRL2688, OH231.8 could be marginally extended. At 450 micron I obtain a FWHM of 8.7", but I don't have planet data to compare with. A target object observed during one of the nights give a FWHM of 8.1", while a single map of CRL618 gives 8.5". I would therefore classify it as compact until we get more accurate maps. Examination of integrated fluxes suggest excess emission of < 200 mJy in a 30" radius at 850 micron and less than 600 mJy at 450 micron, which argues for OH231.8 being a point source.

16293-2422

I have not analyzed any additional data since my first report on secondary calibrators. However, since I have noticed that observers still tend to use it for pointing and photometry calibration with a 60" chop, I include two jiggle maps in Fig 3, which shows that if 16293 is used for calibration, one should use a fixed chop with a 30 degree position angle.