Supplementary southern standards for UBV(RI)C photometry

© 1998 RAS

Mon. Not. R. Astron. Soc. 294, 93–104 (1998)

Supplementary southern standards for UBV(RI)C photometry

D. Kilkenny, F. van Wyk, G. Roberts, F. Marang and D. Cooper

South African Astronomical Observatory, PO Box 9, Observatory 7935, South Africa

Accepted 1997 September 12. Received 1997 July 17

A B STR ACTWe present UBV(RI)C photometry for 80 southern red and blue stars for use asadditional standards. The data are tied to the Johnson UBV and Cousins (RI)C

systems and extend the range of the available stars for colour equationdetermination, especially in (UÐB) for blue stars and (VÐR) and (VÐI) for redstars. Comparisons with published data are made and particularly good agreementis found with Bessell for the red (Gliese) stars.

Key words: techniques: photometric – stars: fundamental parameters.

1 INTRODUCTION

Stars in the nine Harvard ‘E-regions’ centred at declinationÐ45° provide fundamental standards for UBV(RI)C photo-metry in the southern hemisphere. The development ofthese standards began 50 years ago with a need to haveaccurate photometric standards to calibrate photographicmagnitudes in the Cape Photographic Catalogue (see Jack-son & Stoy 1954). The existing standards were found to beinadequate – inconsistent from region to region and lessinternally precise than the stars they were required to cali-brate – and so various programmes aimed at improving thissituation were begun at the Royal Observatory, Cape ofGood Hope [now the South African Astronomical Observa-tory (SAAO)]. Between 1947 and 1960, 24 ‘series’ of obser-vations, first photographic, then photoelectric, were carriedout. The resulting V, (BÐV), (UÐB)C and preliminaryJohnson (UÐB) were collated by Cousins & Stoy (1962).These results formed the basis of all Cape and a great dealof other southern hemisphere photometry.

Much of the photographic (Fabry) photometry and alsothe photoelectric photometry was obtained by Dr A. W. J.Cousins, who continued to work on the standard stars,improving the (UÐB) results (e.g. Cousins 1967), definingthe zero-points and scales using only photoelectric data forthe brighter E-region stars (Cousins 1973) and adding in Rand I bands (Cousins 1976, 1978, 1980, for example). Thelatter are on the ‘Kron–Cousins’ system, nowadays usuallycalled the ‘Cousins’ system and written (RI)C or RCIC.Fainter UBV standards were added (Menzies & Laing 1980;Cousins 1983), and a review of the results from 10 years ofphotometry at the Sutherland site of the SAAO (Menzies &Laing 1985) showed that the UBV(RI)C system set up inCape Town had been well reproduced at Sutherland. Thisenabled the standards to be somewhat improved (particu-

larly the fainter ones) without changing the system signifi-cantly. The current list of E-region standards contains 570stars in the magnitude range 2sVs11 (Menzies et al.1989) and is not likely to be significantly changed, althoughwork in progress on the second decade of Sutherland photo-metry has winkled out a small number of low-amplitudevariables.

The above description is a very brief outline of thedevelopment of the Southern standard system; more detailsof the early work on photoelectric standards at the Cape, ofthe work of Dr Alan Cousins and of the evolution of the E-region standard stars can be found in Stoy (1956, 1993),Kilkenny (1993) and Menzies (1993a). It is also worth not-ing that work at SAAO on the E-region UBV(RI)C stand-ards has established them as secondary standards for uvbyphotometry (Cousins 1987; Kilkenny & Laing 1992), for Hbphotometry (Cousins 1990), for JHKL photometry (Carter1990) and most recently for DDO photometry (Cousins1994; Cousins & Caldwell 1996).

At a conference on photometry held in Cape Town inearly 1993 (Kilkenny, Lastovica & Menzies 1993) to honourDr Cousins in his 90th year, a list of desiderata for futurework on standard stars was given by Menzies (1993b).Amongst these was the suggestion that the provision ofstandards at more extreme colours than are currently avail-able in the E-regions would be useful. In this paper, wepresent UBV(RI)C photoelectric photometry for such stars,attempt to transfer these to the standard system and com-pare the data with some published results.

2 STA R SELECTION

Candidate blue and red standards were selected from Laing(1989), Menzies, Marang & Westerhuys (1990) and Bessell(1990). All three sources have the advantage of being inter-

nally homogeneous sets of data rather than compilations, sothat the internal precisions are likely to be high. Goodagreement between different observations of the same indi-vidual stars (with allowance for any overall systematic dif-ferences of zero-point or colour equation) will argue infavour of reasonable long-term stability of the stars. It must,however, be noted that apart from transformation difficul-ties with stars of extreme colour, there are significant prob-lems in creating standards from such stars.

The hot stars tend to be either very bright (the bluest E-region stars are too bright to be observed with pulse-count-ing systems on even quite modest-sized telescopes) orfainter, but reddened, which could also cause transforma-tion problems. In addition, the hottest massive stars willinclude b Cephei stars, emission-line variables, supergiantvariables and so on. There are very few E-region standardswith (UÐB)sÐ0.75 and so candidate standards bluerthan (UÐB)1Ð0.65 were selected from Menzies, Marang& Westerhuys (1990). Data from this source generally covertwo seasons, so that there is some evidence for long-termstability. In addition, some of the stars have been tested forshort-term variability (a time-scale of hr) by these authors.Included in our selection are some hot subdwarf stars, asthese tend to be very blue and of low reddening.

The coolest stars are quite likely to be variable – it is well-known that M dwarfs exhibit a range of magnetically drivenvariability, such as flares and spots, which can cause varia-tions on time-scales from a few tens of seconds to years –and the dwarfs are almost invariably of quite faint apparentmagnitude because of their intrinsic faintness. (Beinggenerally very ‘nearby’ stars, some of them have a tendencyto wander irresponsibly from their catalogued positions; aless than desirable trait in a standard star.) In order tocreate standards redder than those already in the E-regionsample, however, it is necessary to select M dwarfs. Thesehave been taken from published photometry by Laing(1989) and Bessell (1990). Only the brighter stars wereselected, since an M dwarf that has V110 mag can be113th magnitude at U, making precise photoelectricmeasurement more difficult with the telescope/photometercombination that was available for this project. Any starsthat showed strong disagreement between the Laing andBessell results were excluded.

Initially, red and blue stars were selected with declina-tions in the range Ð30°sdsÐ60°, as the E-regions arecentred at d\Ð45° and it was considered desirable to mini-mize the angular separation of E-region and programmestars. This restriction had to be substantially relaxed toobtain a reasonable number of red and blue stars well distri-buted around the southern sky.

3 OB SERVATION A ND R EDUCTION

All observations were made with the 0.5-m telescope andmodular photometer (Kilkenny et al. 1988b) at the Suther-land site of the SAAO during 1993 August–1995 July. Thephotometer employs a Hamamatsu R943-02 (GaAs) photo-multiplier and UBV(RI)C filter set, composed of the follow-ing filters (all Schott, except the CuSO4):

U 1-mm UG1+5-mm (solid) CuSO4;B 2-mm GG385+1-mm BG12+1-mm BG38;

V 2-mm GG495+1 mm BG18;RC 2-mm OG570+2-mm KG3;IC 3-mm RGN9.

The programme stars were observed as a ‘background’project to the 0.5-m general programme of variable starwork, although sometimes more intensive observationswere made. In all cases, the programme stars were inter-spersed with E-region standards from Menzies et al. (1989)and all observations were made at low air mass (typicallyXs1.2).

All data were reduced using standard SAAO procedures(see Menzies 1993a and the Appendix to this paper) except,of course, that for essentially all of the programme stars,significant extrapolation of the colour equations was neces-sary. In order to keep the results as internally precise aspossible, the colour equations were changed as infrequentlyas possible, consistent with accurate transformation withinthe E-region colour ranges (residuals are inspected regu-larly to check for inconsistencies). In the end, three verysimilar colour equations were used during the period ofobservation, the change in each case being small but neces-sitated by mirror re-aluminization. Care was taken to obtaingood overlap among all three sets of data and the first andlast sets were forced to fit the middle set, which was also thelargest. It was possible to effect these fits with rather small(s0.005 mag) zero-point shifts or, in two cases, with small(s2 per cent) colour equations, so that we believe it hasbeen possible to maintain homogeneity in the data.

The ‘extrapolated’ results are listed in Table 1, whichcontains the star name [Gliese (1969) catalogue for the redstars, HD or Durchmusterung number for the blue], meanmagnitudes and colours, the number of observations in themean and the respective standard deviations in the mean foreach quantity (in units of 0.001 mag). The final columncontains some comments: ‘JH’ and ‘C’ refer to the Johnson& Harris (1954) and Cousins (1971, 1984) stars discussedbelow, and three known variable stars are noted. Althoughit is somewhat embarrassing that these slipped into theprogramme, face is partly saved by the fact they all showhigher standard deviations in V than the other stars, and sowould be suspect from our data alone. Note that for the redstars, the (UÐB) standard deviations are conspicuouslylarger than for the other colours – a direct consequence ofthe much lower total counts obtained at U for these stars,despite longer integration times. The poorer precisionmakes these data less useful.

Included in the programme of observation describedabove were some stars to tie the programme stars to thestandard system, in so far as this is possible. The originalJohnson standards are mainly in the north and aretypically much too bright for the pulse-counting system onthe 0.5-m telescope (the GaAs photomultiplier respondsnon-linearly to high count rates, over and above the usual‘dead time’ or pulse coincidence losses, so it is necessary torestrict count rates). At the suggestion of Dr Cousins, anumber of Johnson & Harris (1954) standards wereobserved. Some of these were already included in the pro-gramme (GL 273\BD+5° 1668, GL 352\BDÐ12° 2918,GL 628\BDÐ12° 4523 and GL 908\BD+1° 4774) butmost of the Johnson & Harris stars are also too bright forour equipment. To cover the (RI)C system, a number of stars

94 D. Kilkenny et al.

© 1998 RAS, MNRAS 294, 93–104

Supplementary standards for UBV(RI)C photometry 95

© 1998 RAS, MNRAS 294, 93–104

Table 1. ‘Extrapolated’ UBV(RI)C results for the programme stars.

from Cousins (1971, 1984) were measured; some of theseare Johnson & Harris (1954) stars and some were alreadyincluded in the programme (GL 1, 54, 191, 229, 273, 628,832 and 908). Again, the reddest and bluest stars tend to betoo bright. In an attempt to circumvent this problem, a smallnumber of experimental measurements were made of someof these bright stars using the front-end mask for the 0.5-mtelescope, which was used to make observations of SN1987aat its brightest. This mask has two 110-cm holes, thusreducing the light by a factor of 10 or more. Observationswere made during one night with both holes open and onanother night with one hole covered, and the data were tiedto E-region standards in the usual way. As there seemed tobe no obvious problem with the E-region standards usingthe mask, the bright star data are listed (along with somefainter stars observed without the mask) in Table 2, whichhas the same format as Table 1.

4 TR A NSFOR M ATION TO THE STA NDA RD SYSTEMS

The Table 2 results for the Johnson & Harris (JH) andCousins (C) stars and the Table 1 overlap with those authorswere used to transform the Table 1 data to the UBV and(RI)C systems as described below.

4.1 Blue stars

In Figs 1 and 2, small crosses represent residuals (E-regionstandard minus observed) from the blue end of a typicalexample of the colour equations runs used during this pro-gramme. The JH stars are represented by asterisks and theC stars by open circles. The corrections actually applied tothe Table 1 data are the solid lines which are eye fits to theJH and C star residuals, forced to be zero at the blue end ofthe E-region residuals. It can be seen that the (BÐV),


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Table 1 – continued

Table 2. ‘Extrapolated’ UBV(RI)C results for some Johnson & Harris and Cousins stars.

(VÐR) and (VÐI) corrections are quite well defined; the Vand (UÐB) corrections less well so, as might be expected.

4.2 Red stars

Figs 3 and 4 show the residuals for the red end of the colourequations. In the DV/(BÐV), D(BÐV)/(BÐV), D(UÐB)/(BÐV) and D(UÐB)/(UÐB) diagrams, the programmestars lie within the range defined by the E-region stars;however, because the programme stars are dwarfs whereasthe E-region standards are almost exclusively giants, there isa clear separation between red dwarfs and giants in the two-colour diagram. There is obviously a transformation prob-lem in Fig. 3, even though the ‘programme’ stars lie withinthe colour range of the E-region stars.

In Fig. 5, D(BÐV) and D(UÐB) are plotted as functionsof (VÐI) and, although the scatter is larger than one mightlike, there is a clear trend in each diagram; as before, thesolid lines are the corrections applied to the Table 1 data.Figs 3 and 5 are a reminder that luminosity is a factor thatmust be taken into account when transforming the very redstars to the standard system.

In Fig. 4, it can be seen that the DV/(VÐI) plot indicatesthat little or no colour equation correction is necessary tothe V data in Table 1. The corrections to (VÐR) and (VÐI)are not very well defined because of the small numbers ofstars available but have been applied as shown and can bejustified somewhat by the comparisons in the next section.

4.3 The ‘final’ results

After application of the corrections described in the pre-vious section and sketched in Figs 1, 2, 4 and 5, we derive aset of values for the magnitudes and colours of the red stars(Table 3) and blue stars (Table 4) that we believe are asclose to the standard system as we can get with the materialcurrently available to us. Given that the standard systemsare not as well-defined in these extreme regimes and accept-ing that other problems such as incipient variability exist, itis hoped that the data in Tables 3 and 4 will provide usefuladditional colour equation standards for UBV(RI)C photo-metry.

In Table 3, the (UÐB) results are only quoted to thenearest 0.01 mag when the standard deviations (Table 1) aregreater than 0.04 mag. For a typical star, this corresponds toa standard error of 10.01 mag or greater. The (UÐB) forGL 375 is so poorly defined that it is omitted from thetable.


© 1998 RAS, MNRAS 294, 93–104

Figure 1. UBV residuals (standard minus this paper) for the bluestars. The small crosses represent the blue end of a typical colourequation run using E-region standards (Menzies et al. 1989). Opencircles and asterisks represent comparisons with Cousins (1971,1984) and Johnson & Harris (1954) data, respectively. The solidlines indicate corrections to be applied to the Table 1 data.

Figure 2. VRI residuals (standard minus this paper) for the bluestars. Comments are as for Fig. 1.

Both Tables 3 and 4 include the best coordinates we couldfind; where possible, these are taken from the HipparcosInput Catalogue (HIC) (Turon et al. 1992) but only quotedto the nearest 0.1 s of time in RA and the nearest arcsec inDec. The HIC positions are particularly useful as theyinclude proper motion corrections; observers should notethat most finding chart programmes do not contain thesecorrections, which can make identification of the correctstar difficult. Where HIC data could not be found, we haveused Durchmusterung data, which are less accurate andwhich have been flagged with a colon in the tables. Spectraltypes have also been taken from the HIC, except for the hotsubdwarfs where spectral types are from the Kilkenny,Heber & Drilling (1988a) catalogue.

The HIC also includes some useful cross-identifications,which compound the embarrassment admitted in Section 3by revealing two more known variable stars in Table 3,GL 729 (\V1216 Sgr) and GL 867A (\FK Aqr). Both areflare stars (of BY Dra and UV ceti type, respectively: Kho-lopov et al. 1985) although there is little evidence in Table 1,or in comparison with Laing (1989) and Bessell (1990), forvariability. Since flares can be both infrequently occurringand rapidly transient events, we have left both stars in thetable, but the faint-hearted observer might do well to avoidthem.

More disquieting, though hardly surprising, is the numberof ‘NSV’ or ‘suspected’ variables in Table 3. Bessell (1990)

noted that Weis has suggested (in a private communication)that most late M dwarfs are variable in magnitude at least,and thus it is an unsafe practice to rely on a small number ofsuch objects to define the red end of a colour equation (it is,


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Figure 3. UBV residuals (standard minus this paper) for the redstars. Comments are as for Fig. 1.

Figure 4. VRI residuals (standard minus this paper) for the redstars. Comments are as for Fig. 1.

Figure 5. (BÐV) and (UÐB) residuals (standard minus thispaper) plotted as a function of (VÐI). Open circles and asterisksrepresent comparisons with Cousins (1971, 1984) and Johnson &Harris (1954) data, respectively. The solid lines indicate cor-rections to be applied to the Table 1 data.

of course, not good practice to use small numbers of stand-ards to define any colour equation). The evidence in Table3 indicates that many early M dwarfs are also likely to varyand that observers should use as many different stars aspossible to define the red end of transformations.

5 COMPA R ISONS WITH PUBLISHED WORK

In this section, the results in Tables 3 and 4 are comparedwith the photometry in the sources from which the pro-gramme stars were selected.

5.1 Red stars – comparison with Bessell (1990)

Fig. 6 shows a comparison between the Table 3 data andBessell (1990), with the residuals in the sense (Table 3minus Bessell). Given that both we and Bessell made someeffort to reproduce the original standard systems, it isgratifying and unusual to see the good agreement betweenthe two sets of data. The filled circles in Fig. 6 representstars that Bessell (1990) has labelled as secondary standards(‘STD’ in his table 1). Although not explicitly stated, it is

reasonable to assume that these are the best observed starswith the smallest internal scatter. Note that the agreementbetween our data and Bessell is extremely good for thesestars in (VÐR) and (VÐI) and acceptable in V and(BÐV). The other stars in the comparisons show morescatter, but little in the way of systematic effects. Table 5lists mean differences between Table 3 and Bessell (‘STD’and ‘other’ stars); the STD stars are in better agreement andhave smaller scatter, except in V. The larger V differenceresults from the small number of stars in the comparisonand the fact that one star (GL 402) has a difference of 0.041mag. If this star is omitted, the mean difference of+0.008¹0.016 reduces to +0.003¹0.011. The goodagreement between our results and those of Bessell rein-forces the suggestion that substantial numbers of stars fromthe Bessell list can be used to help define red colour equa-tions, in addition to the proposed secondary standards fromboth Bessell (1990) and our Table 3.

5.2 Red stars – comparison with Laing (1989)

Fig. 7 shows the results for our Table 3 data minus Laing(1989). The differences are small, but at first sight a little


© 1998 RAS, MNRAS 294, 93–104

Table 3. Final UBV(RI)C results for red stars.

disappointing, since they are essentially from the samesource as the current data. However, note that the agree-ment is good at the ‘blue’ ends of the comparisons [near(VÐI)\2], where the E-region standards anchor the col-our equations, and that the scatter in the residuals is rela-tively small, suggesting that small colour equationdifferences resulting from extrapolation are probably thecause. Table 5 contains the means of the differencesbetween our Table 3 data and Laing. From a small overlapwith Cousins, Laing (1989) finds differences with the samesign and comparable size to those found here, so that thedifferences (this paper minus Laing) are consistent with theTable 3 data being close to the standard system.

5.3 Blue stars – comparison with Menzies et al. (1990)

Figs 8 and 9 show the differences Table 4 minus Menzies etal. (1990) for UBV and V(RI)C respectively. The solid linesin the figures are reproduced from Figs 1 and 2 for compari-son and show that the Menzies et al. data require verysimilar corrections to the standard system to our Table 1data [except in (VÐI)]. In other words, except for smallcolour equation (extrapolation) differences, our uncorrec-

ted data are in very good agreement with Menzies et al., asmight be expected, since both data sets were acquired withthe same instrumentation and reduced in the same way.

6 ADDITION A L STA NDA RDS

In Section 5.1, the suggestion was reiterated that observa-tions of Bessell (1990) stars would be useful for defining thered end of colour equations. In Table 6, we list six Bessell(1990) ‘STD’ stars that are not included in our Table 3 andthat are significantly redder than the available E-regionstandards. These additional stars could be particularly use-ful for colour equations, especially GL 406 and 866 whichhave (VÐI)14 and (VÐR)a1.6 and are thus conspicu-ously redder than any of the Table 3 stars. Note that GL 644is a known variable (V1054 Oph) of UV Ceti type (a flarestar) and GL 654 is the suspected variable NSV 8176. Thecaveats mentioned in Section 4.3, with reference to Table 3,obviously apply.

In Table 6 we also list six stars from Menzies et al. (1990)as proposed additional blue standards; some of them areextremely blue. Accurate coordinates for the UVO starshave been kindly provided by Dr M. Bessell (private com-


© 1998 RAS, MNRAS 294, 93–104

Table 4. Final UBV(RI)C results for blue stars.

munication). The photometric data for these stars havebeen transformed to the standard system using the compari-sons shown in Figs 8 and 9 (the data points, not the lineswhich are from Figs 1 and 2). The corrections to be appliedto the Menzies et al. data to force agreement with ourresults can be derived to good accuracy with the followingsimple equations:

for (UÐB)MsÐ0.73, D(UÐB)\0.052(UÐB)M+0.038,

for (BÐV)MsÐ0.18, D(BÐV)\+0.080(BÐBÐV)M

+0.014,

for (VÐI)MsÐ0.16, DV\Ð0.037Ð0.23(VÐI)M,

for (VÐR)Ms0.00, D(VÐR)\+0.003,

for (VÐI)MsÐ0.10, D(VÐI)\Ð0.010.

Since the corrections are well-defined, these blue starsshould be transformed to the standard system with compar-able accuracy to the Table 4 stars. The Table 6 blue starswere selected on the basis of substantial numbers of obser-vations and small standard deviations for all five quanti-ties.


© 1998 RAS, MNRAS 294, 93–104

Figure 6. BVRI residuals for the corrected data (Table 3) minusBessell (1990). Filled circles are Bessell data labelled ‘STD’ in hispaper; open circles are other stars from the same source. Threelarge residuals (a0.04) have escaped from the diagram because ofthe scale, but do not affect the general trends.

Figure 7. BVRI residuals for the corrected data (Table 3) minusLaing (1989). Filled and open circles serve only to separate thecolours.

Table 5. Mean differences (SAAO minus other) for the Table 3 stars.

7 NOTE ON THE EC SURVEY PHOTOMETRY

A catalogue of results from the first zone of the Edinburgh–Cape (EC) blue object survey has recently been published(Kilkenny et al. 1997) and contains UBV photometry fornearly 1000 stars, all of which were transformed to the E-region standard system, but many of which are bluer thanthe bluest accessible E-region standards. To control trans-formation of the very blue stars, the EC survey used asample of the Menzies et al. (1990) stars as additional

‘standards’ (because the survey work was started longbefore this work). If it is accepted that the Table 4 data areon the standard system and therefore that the Menzies et al.data require the corrections given in the previous section,then clearly the EC zone 1 data will also require correction.Since (VÐI) colours were not obtained in the EC photo-

metry, we use Fig. 8 to derive a correction to V as a functionof (BÐV), namely

for (BÐV)MsÐ0.17, DV\Ð0.039Ð0.23(BÐV)M,

and the (BÐV) and (UÐB) corrections are as given inSection 6. Note that for the very bluest stars, the maximum


© 1998 RAS, MNRAS 294, 93–104

Figure 8. UBV residuals for the corrected data (Table 4) minusMenzies et al. (1990). Solid lines indicate the corrections applied toour Table 1 blue star data (see Fig. 1).

Figure 9. VRI residuals for the corrected data (Table 4) minusMenzies et al. (1990). Solid lines indicate the corrections applied toour Table 1 blue star data (see Fig. 2).

Table 6. Suggested additional standards from Bessell (1990) and (‘corrected’) Menzies et al.(1990).

corrections will be only 1+0.03, Ð0.01 and Ð0.03 in V,(BÐV) and (UÐB) respectively.

8 DESIDER ATA

Tables 3, 4 and 6 list over 90 stars that should be suitable fordetermining colour transformations for redder and bluercolours than are currently represented in the E-region UBV-(RI)C system. The internal precision of the observationsappears to be high and it is believed that transformation tothe standard UBV and (RI)C has been done with someaccuracy.

It would be desirable to test these proposed standards byrepeated observation and we plan to include these in ourregular programmes of observation on the SAAO 0.5-mtelescope, so that tests for long-term stability and improve-ments to the internal precision might be made, in a similarfashion to the Menzies & Laing (1985) ‘ten years’ study. Itwould also be useful to add further stars to the list, both toincrease the total number of available ‘extreme colour’standards and to extend the red range. The latter might bevery difficult because of the high incidence of variabilityamongst the reddest stars and because of their low intrinsicluminosity. Above all, it would be extremely valuable tohave further checks on the transformation of the red andblue stars to the standard systems; this is not a trivial taskbecause of the problems described in Sections 2 and 3.

Ultimately, high transformation accuracy might not bepossible because of the inherent scatter in the original UBVstandard system, the difficulties in transforming over largeangles (equator to E-regions, for example) and the lack ofaccessible standards (too bright, too far north, etc.). In thiscase, it will be necessary for southern observers to decide ona system that hopefully matches the ‘original’ UBV and(RI)C systems. (In fact, since the extreme colours are notwell-defined, if that system is adopted, then it will effectivelydefine the system.) Bessell (1990) has put considerableeffort into matching and transforming the southern andequatorial observations of the SAAO (Laing 1989), MtStromlo and Siding Spring Observatories (Bessell 1990) andLandolt (1983, 1992). The data and intercomparisons pre-sented here suggest that for the very red stars (i.e. redderthan defined by existing E-region stars) the closest approxi-mation to the standard system might be Bessell (1990) plusthe stars in our Table 3; the agreement is good and bothsources have tried to tie to the original systems rather thanto extrapolate colour equations. It is axiomatic that, inestablishing red star transformations, observers should useas many stars as possible to counter the pandemic varia-bility; this point cannot be made too frequently or forci-bly.

ACKNOWLEDGMENTS

We are grateful to Drs Mike Bessell, Alan Cousins and JohnMenzies for constructive comments on drafts of this paper.We thank Mike Bessell for communicating accurate coordi-nates for the UVO stars in Table 6.


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S. Afr. Astron. Obs., Circ., 13, 1Menzies J. W., Marang F., Westerhuys J. E., 1990, S. Afr. Astron.

Obs., Circ., 14, 33Stoy R. H., 1956, Vistas Astron., 2, 1099Stoy R. H., 1993, in Kilkenny D., Lastovica E., Menzies J. W., eds,

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APPENDI X

After kindly reading a draft of this paper, Dr Mike Bessellnoted that the paper presented by Menzies (1993a) at the‘Precision Photometry’ conference (Kilkenny et al. 1993)contains useful information on photometry reduction pro-cedures, but has had a limited circulation because the con-ference proceedings were published ‘in house’ at theSAAO. It was suggested therefore that we reproduce table2 and fig. 2 from the Menzies (1993a) paper and expand alittle on our colour equation determinations and theirextrapolation.

The normal procedure for fixing the colour equations forthe SAAO 0.5-m telescope (used for this project) is to spendone night observing E-region standards at low air mass (tominimize extinction effects). Typically, of the order of 100

standards will be observed, depending on the length of thenight. The differences between the standard and ‘natural’system magnitudes and colours are first fitted with straightlines to remove gross differrences and then the resulting(standard minus corrected natural) differences are plottedagainst various colours to determine any residual non-linearcorrections. These are removed by up to five piecewiselinear fits. The data are then re-reduced with linear andnon-linear corrections, atmospheric extinction corrections,and removal of any time-dependent ‘drift’. The process isrepeated iteratively until no obvious trends remain.

In the colour equation examples quoted by Menzies(1993, table 2), for 1992 February 8–9, the linear fits were

V\v+0.013(vÐi),

V\v+0.012(bÐv),

(BÐV)\1.015(bÐv),

(UÐB)\0.968(uÐb),

(VÐR)C\1.037(vÐr),

(VÐI)C\1.006(vÐi),

where upper and lower case indicate standard and naturalsystem quantities respectively. Note that the deviationsfrom unity in the ‘scale factors’ are all less than about 3 percent, indicating that the filters are probably close tostandard. These colour equations are typical of the resultsobtained for the 0.5-m telescope/photometer and filtercombination noted in Section 3 and do not seem to changemuch, even after minor re-aluminization (indicating thatthe mirrors get dusty rather than discoloured). A colourequation run some nine months later derived essentially thesame linear equations and only small changes in the non-linear equations (see Fig. A1), illustrating the stability of thesystem.

Examples of the non-linear corrections are shown in Fig.A1, which is a reproduction of fig. 2 from Menzies (1993a).The colour equations are well-determined in the rangecovered by the E-regions, as care is taken to observe thereddest and bluest standards available, as well as a gooddistribution in between. However, extrapolation is neces-sary at both ends if programme stars of extreme colour areto be transformed to standard. There are, for example, onlyfive E-region stars with (VÐI)C greater than 2.0 and onlythree with (VÐI)C greater than 2.4, and all of these will not

be observable at any given time. Consequently, the very redand blue ends of the colour equations will not be as accurateas the intermediate colours, and extrapolations of these willbe worse, particularly where it is clear that there are sub-stantial changes in slope (‘non-linear’ effects) of the colourequations at the extrema. With peculiar stars, differencescan occur at extreme colours. For example, Menzies (1993a,his fig. 4) has shown that, for very red stars, the mainsequence and giants show a large change in the slope of thecolour equation in (VÐR)C, whereas carbon stars lie alonga linear extrapolation of the colour equation for hotterstars.


© 1998 RAS, MNRAS 294, 93–104

Figure A1. Non-linear corrections required (1992 February 8–9)to transform to the E-region standard system, after removal of thelinear terms noted in the Appendix. Broken lines indicate thedifferences between the colour equation of 1992 February 8–9 anda colour equation run some nine months later. Figure reproduced(with permission) from Menzies (1993a).

Supplementary southern standards for UBV(RI)C photometry

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