A summary of the search for new Submillimetre Secondary Calibrators
Vicki Barnard, June 2005

This document is a summary of the E&C project (ec05) for locating and monitoring potential new Secondary Calibrators for SCUBA. Unfortunately SCUBA's poor performance and large downtime over the last two years has hampered the collection of data for this project, and so this document is a progress report rather than a conclusion.

The sources in this report are largely grouped into two categories: those that still appear useful for SCUBA and SCUBA-2, and those that have been considered at one time or another but rejected for some reason, either in this project or by other studies. I will discuss the first group in some depth, and then list the second group with comments regarding the reasons for their rejection.

1. Sources worth further study

• V883 Ori

  RA: 05:38:19, Dec: -07:02.2(J2000)

  Average 850 µm flux: (1.34 ± 0.01) Jy
  Average 450 µm flux: (7.28 ± 0.07) Jy

  Also known as V* V883 Ori, this is an FU Orionis star. This was suggested originally by (1) as a potential calibrator after imaging by (9) revealed its compact nature at both 450 and 850 µm. Whilst its RA is similar to that of CRL618, our preferred secondary calibrator, it has the important difference of being in the South. Despite the identification of this source as an FU Ori based on its broad CO overtone band absorption at 2.3 µm, there is little evidence of ongoing outflow, and (9) suggest that it has broken free from its surrounding accretion cloud.

  This interpretation would seem to be in agreement with its apparently steady 850 µm flux, as shown in the plots below. The 450 µm flux does appear to move around more over the approximately 1.5 year timescale, but the standard devition of these measurements is still less than 1% of the average flux. Furthermore, there is clearly a wide variety in the quality of the 450 µm data from observation to observation. There also appears to be some extension in the 450 µm image, but this is only visible in the 7% contour level and may be caused by noise. There is no clear extension in the 850 µm image.

  450 µm, 24 integrations	850 µm, 30 integrations
  Contours at 7, 6.5, ... 0.5 Jy	Contours at 1.3, 1.2, ... 0.1 Jy

• Alph Ori

  RA: 05 55 10.31, Dec:+07 24 25.4 (J2000)

  Average 850 µm flux: (0.629 ± 0.008) Jy
  Average 450 µm flux: (1.39 ± 0.04) Jy

  This is of course also known as Betelgeuse, and is regarded as a semi-pulsating regular star. Whilst always too faint as a calibrator for SCUBA, it would be a realistic option for SCUBA-2, although its R.A. means it would be in competition with V883 Ori, above.

  Observations of Alph Ori appear to show that it is compact at both wavelengths. There is a strange kink to the East of the 14% of peak contour at 450 µm but this may not be real. Whilst there are not many data, the time plots appear to show a pretty steady flux at both wavelengths, although of course this star is a known variable at other wavelengths. This object would need careful study with SCUBA-2 but may be useful.

  450 µm, 25 integrations	850 µm, 25 integrations
  Contours at 1.3, 1.2, ... 0.2 Jy	Contours at 0.6, 0.55, ... 0.05 Jy

• TW Hydrae

  RA: 11 01 51.91 Dec: -34 42 17.0 (J2000)

  Average 850 µm flux: (1.37 ± 0.01) Jy
  Average 450 µm flux: (3.9 ± 0.7) Jy

  Although quite far south, the R.A. of this object makes it an extremely interesting prospect for helping to fill in the current gap in calibrators in the R.A. range 10 - 16 hours. It was first suggested by (1) as a potential calibrator. It is a classical and nearby T Tauri star, with a circumstellar disk which has recently been imaged by (10) using the Submillimeter Array. Whilst the CO line emission appears to have a radius of perhaps 5", we do not see strong evidence of extension in the SCUBA continuum images.

  TW Hydrae is in the JCMT pointing catalogue. This has lead to a lot of 850 µm data, which indicates that while the flux of TW Hydrae is fairly stable at this wavelength, there may be some systematic variation, with a minimum around 2002.0 and maxima at 2000.0 and 2004.0? Unfortunately the source is sufficiently faint at 450 µm that we only were able to detect it once in the E&C monitoring campaign with SCUBA, producing a very noisy map. However it appears compact at both wavelengths. It should be bright enough for SCUBA-2, and observations with the new instrument will be vital to measure any extension at the shorter wavelength.

  450 µm, 3 integrations	850 µm, 20 integrations
  Contours at 3.9, 3.6, ... 0.9 Jy	Contours at 1.3, 1.2, ... 0.1 Jy

• Arp 220

  RA: 15 34 57.21, Dec: +23 30 09.5 (J2000)

  Average 850 µm flux: (0.688 ± 0.007) Jy
  Average 450 µm flux: (2.77 ± 0.06) Jy

  Another famous object and another source with a potentially very useful R.A. This was suggested by (1) as a potential SCUBA calibrator but its flux makes it a much more likely prospect for SCUBA-2.

  The images suggest that it is compact at both wavelengths, with only the 7% of peak contours showing extension - at 450 µm this may well just represent noise. Furthermore some of the data included in the co-adds below used a 180"e; chop throw, which can lead to extended images (see here for more discussion of this effect). There are no clear trends in the time plots at either wavelength, although of course as a Seyfert galaxy/ULIRG we might expect Arp 220 to have an irregularly variable flux. The extent of this in the submillimetre has not been considered to my knowledge.

  450 µm, 34 integrations	850 µm, 34 integrations
  Contours at 2.6, 2.2, ... 0.2 Jy	Contours at 0.6, 0.55, ... 0.05 Jy

2. Rejected Sources

Source name	RA (J2000)	Dec (J2000)	JCMT pointing source?	Comments
w3(OH)	02 27 03.831	+ 61 52 24.77		Ultracompact HII region. Bright and reasonably stable at ~ 40 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
gl490	03 27 38.842	+ 58 47 00.51		Protostar. Fairly bright and reasonably stable at ~ 9 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
Elias 1	04 18 40	+ 28 19 16		Originally suggested by (1), but its close proximity to CRL 618, an established secondary calibrator, and far fainter fluxes rule out this object from being worthy of further study. Monitoring by JCMT over 5 years suggests an 850 µm flux of (0.605 ± 0.011) Jy (image, contours at 0.6, 0.55, ... 0.05Jy) and a 450 µm flux of (1.0 ± 0.1) Jy (image, contours at 1.0, 0.9, ... 0.1 Jy). Compact appearance in the submillimetre was also found in the 1.3 map made by (3) using IRAM with a resolution of 11".
dgTau	04 27 04.7	+ 26 06 17		Source is too variable at 850 µm for consideration.
l1551-irs5	04 31 34.140	+ 18 08 05.13		Protostar. Bright and reasonably stable at ~ 7 Jy (850 µm), but shows clear extension in images in (2).
OMC1 IRc2	05 35 14.373	- 05 22 32.35		Protostar in an Ultracompact HII region. Rejected by (5) as too extended.
n2071ir	05 47 04.851	+ 00 21 47.10		Protostar. Bright and reasonably stable at ~ 18 Jy (850 µm), but rejected by (1) as extended.
VYCma	07 22 58.33	- 25 46 03.2		Long period variable. Source is too variable at 850 µm for consideration.
CIT6	10 16 02.27	+ 30 34 18.6		Source is too variable at 850 µm for consideration.
RXBoo	14 24 11.61	+ 25 42 14.1		Source is too variable at 850 µm for consideration.
g343.0	16 58 17.136	- 42 52 06.61		Ultracompact HII region. Bright and reasonably stable at ~ 35 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
ngc6334i	17 20 53.445	- 35 47 01.67		Ultracompact HII region. Bright and reasonably stable at ~ 90Jy (850 µm) in pointing observations, but rejected by (1) as extended.
BVP1	17 43 10.32	-29:51:43.5		Located by (7) in the Galactic centre maps of (8), this source seems to be an isolated pre-stellar fragment. It was studied as part of the E&C program since its position would make it a useful calibrator for Galactic programs. However, the deeper images (at 850 µm, contours at 1.2, 1.1, ... 0.1 Jy, and 450 µm, contours at 7, 6.5, ... 0.5 Jy) show that it is clearly extended at around the 20% of peak contour at 450 µm, and that there are nearby but separate structures to the South at both wavelengths that would complicate the use of this object as a calibrator.
HD 163296	17 56 21.29	- 21 57 21.9		Pre-main sequence Herbig Ae/Be star. Appeared compact in the 1.3 map made by (3) using IRAM with a resolution of 11", but appears extended in the NW-SE direction in the 450 µm image and perhaps also slightly in the 850 µm image, which use both data taken as part of the E&C campaign and as science observations in 1997. (Contours are at 7, 6, ... 1, 0.8, 0.6, 0.4 Jy in the 450 µm image and at 1.7, 1.5, ... 0.1 Jy in the 850 µm image.)
g5.89	18 00 30.376	- 24 04 00.48		Ultracompact HII region. Bright and reasonably stable at ~ 50Jy (850 µm) in pointing observations, but rejected by (1) as extended.
g10.62	18 10 28.661	-19 55 49.76		Ultracompact HII region. Bright and reasonably stable at ~ 55 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
IRAS18089-1732	18 11 51.46	-17 31 28.8		Protostellar object. Considered by (6), but strong extended component, also seen in archival SCUBA maps.
g10.841-2.592	18 19 12.12	-20 47 31.3		Ultracompact HII region. Found to contain multiple sources at 850 µm by (6)
g15.04-0.68	18 20 24.82	-16 11 35.0		Ultracompact HII region. Rejected by (6) due to extended emision at 850 µm.
IRAS18264-1152	18 29 14.42	-11 50 23.5		Protostellar object. Considered by (6), but clearly extended in archival SCUBA maps.
g34.3	18 53 18.569	+ 01 14 58.26		Ultracompact HII region. Bright and reasonably stable at ~ 90 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
g45.1	19 13 22.079	+ 10 50 53.42		Ultracompact HII region. Bright and reasonably stable at ~ 12 Jy (850 µm) in pointing observations, but rejected by (1) as extended, confirmed by (6).
IRAS19217+1651	19 23 58.79	+16 57 41.5		Protostellar object. Significant extended emission found by (6) at 850 µm.
k3-50	20 01 45.689	+ 33 32 43.52		Source is too variable at 850 µm for consideration.
on-1	20 10 09.146	+ 31 31 37.67		Ultracompact HII region. Bright and reasonably stable at ~ 20 Jy (850 µm) in pointing observations, but rejected by (1) as extended. Found to have multiple components at 850 µm by (6)
gl2591	20 29 24.719	+ 40 11 18.87		Protostar? Bright and reasonably stable at ~ 14 Jy (850 µm) in pointing observations. However, very close to CRL2688, an established SCUBA Secondary Calibrator.
mwc349	20 32 45.6	+ 40 39 37		Also known as v* v1478 Cyg. Evolved emission line star. Reasonably stable but not too bright at ~ 2.8 Jy (850 µm) in pointing observations. Compact in 1.3 mm observations by (4). Rejected by (1) as too faint at 450 µm (~ 3 Jy) but also very close to and fainter than CRL2688, an established SCUBA Secondary Calibrator.
g80.865+0.420	20 36 52.19	+41 36 24.3		Ultracompact HII region. Found by (6) to be < 1 Jy at 850 µm, also very close to CRL2688, an established SCUBA Secondary Calibrator.
w75n	20 38 36.433	+ 42 37 34.49		Ultracompact HII region. Bright and reasonably stable at ~ 45 Jy (850 µm) in pointing observations. However, rejected by (1) as extended, and also very close to CRL2688, an established SCUBA Secondary Calibrator.
PVCep	20 45 54.39	+ 67 57 38.8		Also known as v* PVCep. YSO. Quite faint but reasonably stable at ~ 1.4 Jy (850 µm) in pointing observations. However close to and fainter than CRL2688, an established SCUBA Secondary Calibrator.
ngc7027	21 07 01.593	+ 42 14 10.18		Evolved star. Quite compact at 1.3 mm (4) and fairly bright and steady at ~ 6 Jy at 850 µm in pointing observations, but extremely close to CRL2688, an established SCUBA Secondary Calibrator.
lkha234	21 43 06.170	+ 66 06 56.09		Also known as v* v373 Cep. Protostar. Fairly bright and reasonably stable at ~ 5.5 Jy (850 µm) in pointing observations, but extended in 1.3 mm observations by (3) and (4).
CepA	22 56 17.87	+ 62 01 49.79		Clearly extended at 1.3 mm in (4).
n7538irs1	23 13 45.346	+ 61 28 10.32		Ultracompact HII region. Bright and reasonably stable at ~ 35 Jy (850 µm) in pointing observations, but rejected by (1) as extended.
gl3068	23 19 12.39	+ 17 11 35.4		Source is too variable at 850 µm for consideration.

Notes:

• Other Galactic sources which are in the JCMT pointing catalogue but not mentioned in either section above do not have sufficient data from which to draw conclusions.
• Blazars in the JCMT pointing catalogue have not generally been considered as candidates.
• The other class of object which may come to the forefront of submillimetre calibration, especially for other instruments and experiments, are asteroids such as Pallas and Ceres. These are certainly bright enough objects and some data was taken on these objects in both E&C time and in PATT projects over the last few years. However the current data set is insufficient in both accuracy and frequency to address the inherent variability of these objects, and so I have not included them in either section above for now.

References

1. Sandell G., "Submillimeter Calibration - Experience From Ground Based Observations", in "The Calibration Legacy of the ISO Mission", proceedings of a conference held Feb 5-9, 2001. Edited by L. Metcalfe, A. Salama, S.B. Peschke and M.F. Kessler. Published as ESA Publications Series, ESA SP-481. European Space Agency, p. 439, 2003
2. Chandler C. J., and Richer J. S., "The Structure of Protostellar Envelopes Derived from Submillimetre Continuum Images", ApJ, 530, p. 851, 2000
3. Henning Th., Burkert A., Launhardt R., Leinert Ch., Stecklum B., "Infrared Imaging and Millimetre Continuum Mapping of Herbig Ae/Be and FU Orionis Stars", A&A, 336, p. 565, 1998
4. Lisenfeld U., Thum C., Neri R., Sievers A., "Secondary Calibrators for Continuum Measurements in the 1.3 mm Window at the IRAM 30m Telescope", in IRAM Newsletter 46, available at the IRAM document website
5. Sandell G., "Secondary Calibrators at Submillimetre Wavelengths", MNRAS, 271, p. 75, 1994 (note that this article should always be read with its update, (1) above)
6. Su Y.-N. et al., "Search for Calibrators for the Submillimeter Array. I. High-Mass Star-forming Regions", ApJ, 616, p. L39, 2004
7. Barnard V. E. et al., "The Very Bright SCUBA Galaxy Count: Looking for SCUBA Galaxies with the Mexican Hat Wavelet", MNRAS, 352, p. 961, 2004
8. Pierce-Price D. P. I. et al., "A Deep Submillimeter Survey of the Galactic Center", ApJ, 545, p. L121, 2000
9. Sandell G. and Weintraub D. A., "On the Similarity of FU Orionis Stars to Class I Protostars: Evidence from the Submillimetre", ApJSS, 134, p. 115, 2001
10. Qi C. et al., "Imaging the Disk Around TW Hydrae with the Submillimeter Array", ApJ, 616, p. L11, 2004