Wetensch. publicatieFaculteit der Wiskunde en Natuurwetenschappe

Houten, C.J. van

Walraven, T.

Walraven-Terlinden, J.H.

Leiden University Scholary Publications

ASTRONOMY & ASTROPHYSICS MAY II 1999, PAGE 113SUPPLEMENT SERIESAstron. Astrophys. Suppl. Ser. 137, 113–116 (1999)Five-colour photometry of OB-stars in the Southern Hemisphere?C.J. van Houten, Th. Walraven, and J.H. WalravenSterrewacht Leiden, P.O. Box 9513, 2300 RA Leiden, the NetherlandsReceived September 10, 1997; accepted December 11, 1998Abstract. Observations of OB-stars, made in 1959and 1960 at the Leiden Southern Station nearHartebeespoortdam, South Africa, with the VBLUWphotometer attached to the 90 cm light-collector, aregiven in this paper. They are compared with photometryobtained by Graham (1968), Walraven & Walraven(1977), Lub & Pel (1977) and Van Genderen et al.(1984). Formulae for the transformation of the presentobservations to those of Walraven & Walraven (1977) andLub & Pel (1977) are given.Key words: stars, early type — stars, fundamentalparameters (colours)1. IntroductionThe 90 cm light-collector of the Leiden Observatory, sit-uated at the Leiden Southern Station near Hartebees-poortdam, South Africa, from 1958 till 1978, has beenused almost exclusively in combination with the simulta-neous five-colour photometer, designed by the second au-thor of this paper. A description of this photometer can befound in Walraven & Walraven (1960), and a discussionof the photometry based on observations with this pho-tometer is given by Lub & Pel (1977). Many programmescarried out with this instrument have been published, butthe first observational programme made with this pho-tometer, the OB-star programme observed by the secondauthor in 1959 and 1960, is still unpublished. The 1959observations were reduced completely, shortly after theywere made, by the second author.The 1960 observations,as far as they are available in Leiden, were reduced by thefirst author, who also wrote the text of this publication.Send offprint requests to: C.J. van Houten,e-mail: vhouten@strw.leidenuniv.nl? Table 4 is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html2. ReductionThe reduction of the 1959 material was made by means ofthe electronic computer of the central computing instituteof the Leiden University. In this part of the reduction theauthors were assisted by Mr. L. Maitimo. The 1960 obser-vations were reduced by hand. Comparison with the 1959material showed, for most of the 1960 material, systematicdifferences in the V colour, depending on the V −B colourindex. This was caused by the use of a photomultiplier forthe V channel with a colour sensitivity different from theone used in 1959. From comparisons between stars ob-served both in 1959 and in 1960 the following relationbetween V (1959) and V (1960) could be derived:V (1959) = V (1960)− 0.254{V (1960)−B(1960)}. (1)The Walraven VBLUW colour system is constructed suchthat it is independent of the photomultipliers used, exceptfor the V channel, where the red edge is determined by thephotomultiplier used. Expression (1) was used to transferthe 1960 material to the 1959 photometric system. Afterapplying this correction, it turned out that still system-atic differences between 1959 and 1960 existed. In orderto combine the 1959 and the 1960 results the latter werereduced to the 1959 system by applying corrections to theobservations made in 1960. It turned out that a still betteragreement with the 1959 observations could be obtained ifthese systematic differences were determined from and ap-plied to the individual nights. In this way the values listedin Table 4 were obtained. To give an impression of the sys-tematic corrections applied to the 1960 observations, thecorrection applied to the B−U colour index is −0.007 andto the U −W colour index amounts to +0.012. The othercorrections were all considerably smaller; they amount to0.002 on the average. These corrections to B − U andU −W are found again in a comparison of OB star ob-servations made in 1965 and 1968 by the first author (vanHouten, unpublished) with the 1959 values given here. Itsorigin is unknown.It was assumed that the photomultiplier used in the Vchannel in 1959 was the same as the one used since 1965.114 C.J. van Houten et al.: OB star photometryHowever, comparisons of the values given in this paperwith a compilation by Dr. E. Brinks (unpublished) of ob-servations in the Lub & Pel (1977) system clearly showeda colour term in the V − B colour indices. The relationfound is(V −B)st = (V −B)59 − 0.076(V −B)59 − 0.003. (2)Here (V − B)st refers to the Lub & Pel colour systemand (V − B)59 to the system used in this publication. Acorresponding formula is valid for the V channel. It canbe expected that a similar transformation can be foundfrom a comparison of the present material with Walraven& Walraven (1977). The relation actually found is slightlydifferent:(V −B)77 = (V −B)59 − 0.055(V −B)59 + 0.001. (3)Here (V − B)77 refers to the observations published inWalraven & Walraven (1977). The cause of the differencebetween (2) and (3) is probably that Walraven & Walraven(1977) was not corrected for the transformation found byLub & Pel (1977):(V −B)st = (V −B)77 − 0.026(V −B)77 − 0.002. (4)If this is true, (4) can be derived from (2) and (3). Adding(3) and (4) yields:(V −B)st = (V −B)59 − 0.079(V −B)59which is, within the limits of accuracy, equal to (2). Itfollows that for a transformation of the observations pre-sented here to the standard system of Lub & Pel (1977)Eq. (2) should be used. Transformation of Walraven &Walraven (1977) to the same standard system needsEq. (4).In Table 4 the original 1959 reduction is listed in fourdecimals. This is more than the observational accuracywarrants, but the reduction was made in this way and itwas judged an unnecessary loss of time to reduce themto a three-decimal value. All additions and corrections tothis list were made with three-decimal values. Thus three-decimal values in Table 4 mean that an error in the origi-nal table has been corrected, that the observations in 1959were combined with observations in 1960 or that the starwas observed in 1960 only. These remarks are meant toexplain why some entries in Table 4 have four decimalsand other have only three. They are not meant to indi-cate which observations were made in 1959 and which in1960. Readers who want to obtain this information canrequest them from the first author.3. The standard starsThe values for the standard stars, derived in this investi-gation, are collected in Table 1, together with their (in-ternal) mean errors. The average value of the standarddeviation of one observation is 0.008 for V and 0.004 forthe colour indices. All quantities here are expressed in log-arithms of intensity, in conformity with the usual practicein the Walraven VBLUW colour system. In Table 1 thesymbol n denotes the number of observations used; theother column headings are self-explanatory. Comparisonswith standard star values given in other publications showreasonable agreement, with the exception that the derivedintensity of HD 210934 in the U channel is approximately0.015 too bright in 1959.4. Comparisons with other VBLUW observationsTo judge the accuracy of the photometry given here it isimportant to compare the present values with lists of ob-servations published earlier, and which have stars in com-mon with our Table 4. Four of such lists were compared:1. Walraven & Walraven (1977) (denoted here as WW77)published a list of VBLUW measurements on southernstars. They are not all OB-stars. This list has 80 starsin common with the present material. The average dif-ferences, defined as: Table 4 minus WW77, are given inTable 3. The V − B values of 1959 were corrected bymeans of Eq. (3). The material in W77 was observed in(approximately) 1961-1962 and is thus closest in time tothe material presented here.2. Graham (1968) published the results of an extensiveinvestigation of OB-stars in Carina, observed in 1964.Average differences: Table 4 minus Graham (1968) aregiven in Table 3.3. Pel (1976) and Lub & Pel (1977) have 23 stars in com-mon with our material. Their material was collected in1971 and 1972. The average differences are again shownin Table 3; the sign convention is as for the comparisons 1and 2. For this comparison our V and V −B values werecorrected with expression (2).4. Van Genderen et al. (1984), observing the Sco OB1 as-sociation, have 51 stars in common with the present ma-terial. Comparison with the 1959 values at first showedrather large differences. Van Genderen et al. used the starHD 151515 as a local substandard for their photometry.This star was used as a standard star by the first au-thor in his (unpublished) survey of southern OB-stars,extending from 1965 to 1978. The average of the calibra-tions of HD 151515 made by the first author in 1969, 1974and 1977 is entered in Table 2, together with the valuesused by Van Genderen et al. (1984). Using the 1969-1977calibration instead of the one used by Van Genderen et al.considerably reduced the difference with the 1959 mate-rial. It is the difference with the latter values which is listedin Table 3. Inspection of Table 3 shows that the compar-ison of our material with Walraven & Walraven (1977),Pel (1976) and Lub & Pel (1977) and Van Genderen et al.(1984), after applying the correction of these data, men-tioned above, all show about the same pattern, whereasC.J. van Houten et al.: OB star photometry 115Table 1. Adopted values for the primary standards, with their mean errorsHD n V V −B B − U U −W B − L104337 220 +0.6430 −0.0645 −0.0084 −0.0147 −0.0197±8 ±3 ±3 ±5 ±3144470 400 +1.1735 +0.0014 −0.0010 +0.0002 −0.0003±3 ±2 ±2 ±3 ±2164402 328 +0.4553 +0.0078 −0.0346 +0.0008 −0.0151±3 ±2 ±2 ±3 ±2178175 162 +0.5322 −0.0134 +0.0701 +0.0212 +0.0169±6 ±3 ±3 ±3 ±3210934 109 +0.5786 −0.0386 +0.2154 +0.0528 +0.0662±7 ±4 ±5 ±8 ±5Table 2. Calibration of HD 151515 by Van Genderen et al. (1984) and Van Houten (unpublished) and the corrections appliedto the photometry of Van Genderen et al. (1984)V V −B B − U U −W B − LVan Houten −0.119 0.079 −0.001 0.032 0.014Van Genderen et al. −0.126 0.069 0.003 0.029 0.018correction terms +0.007 +0.010 −0.004 +0.003 −0.004Table 3. Comparison of the average differences between the present material and four other surveys; in the sense this surveyminus others. The internal accuracy of each entry is about 0.001∆V ∆(V −B) ∆(B − U) ∆(U −W ) ∆(B − L)Walraven & Walraven (1977) −0.003 +0.001 −0.009 +0.005 −0.005Pel (1976), Lub & Pel (1977) −0.005 +0.000 −0.012 +0.010 +0.001Van Genderen et al. (1984) 0.000 0.000 −0.009 +0.007 −0.001average −0.002 +0.001 −0.009 +0.007 −0.002Graham (1968) +0.002 +0.001 +0.003 +0.022 +0.007the comparison with Graham (1968) is different, especiallyin the ultraviolet. The differences can be interpreted suchthat in the 1959 material the U channel intensity wassystematically 0.008 too bright. This is the same effectas found for the standard star HD 210934, although inthe latter case the excess is larger. This suggests that theadoption of an incorrect value of the U channel intensityof one of the standard stars introduced a systematic effectin the colours of the program stars. If this is correct thesystematic error can be expected to be dependent on theright ascension of the stars, since for stars with a right as-cension very different from that of HD 210934, the lattercan have played no part in their photometry. The compar-ison with WW77, which extends over all right ascensions,shows this indeed to be the case. For stars which have2h < α < 10h no systematic effect is present, whereas theother stars have their 1959 brightness in the U channelabout 0.010 too bright. Using the spectral classificationschemes published by Walraven & Walraven (1960) it isseen that this effect will introduce errors in classificationof one tenth of a spectral class, and of about half a magni-tude in Mv. Obviously it would have been better to reducethe 1959 material anew, with improved values of the stan-dard stars, but the material is so large that the amount ofwork involved is prohibitive, and if the systematic effectderived above is taken into account the data publishedhere are thought to be still useful.The values in Table 4 were not corrected by means ofEq. (2) or otherwise. In all comparisons of Table 3 thevalues of the primary standard stars from Table 1 arenot included. The standard deviation of one difference inTable 3 is, on the average, ∆ log I = 0.011. If it is sup-posed that the accuracy of all series is equal, then thisaccuracy amounts to ∆ log I = 0.008, or 0.02 magnitudes.5. The main tableTable 4 contains the result of the 1959 and part of the1960 campaigns. The stars are denoted by their HD num-ber, or BD number if north of δ = −23◦, or by their Co Dnumber if south of this limit. For the close visual binariesHD 36861/36862, HD 120709/120710 andHD 147934/147935 the intensities and magnitudesgiven are the combined values for the pair.Following a suggestion by the referee, and with a viewtowards making Table 4 more useful, Dr. J. Lub at LeidenObservatory added in the third column a simple code indi-cating whether the data were taken in 1959, 1960 or bothyears: 10, 01 or 11 respectively. He also provided a further116 C.J. van Houten et al.: OB star photometrytwo columns in which the V and (B − V ) colours on theJohnson system are given, based upon the transformationspublished by WW77.Acknowledgements. The authors acknowledge with thanksMr. L. Maitimo’s share in the reduction of the observations.Many thanks are due to Dr. A.M. van Genderen for severaluseful remarks on an earlier version of this paper. Manythanks are due to an unknown refeee. His remarks led to aconsiderable improvement of this paper.During the preparation of this paper Mrs. J.H. Walravendied. It seems here a fitting place to emphasize how essentialher contribution was to the many observational and instru-mental projects of her husband, the second author of thispaper. She took an active part in the construction of opticalinstruments by grinding and polishing mirrors and lenses, sheassisted in the observations and took a major share in their re-ductions. Without her such a large production of observationswould not have been possible.Referencesvan Genderen A.M., Bijleveld W., van Groningen E., 1984,A&AS 58, 537Graham J.A., 1968, Bull Astr. Inst. Neth. Suppl. 2, 397Lub J., Pel J.W., 1977, A&A 54, 137Pel J.W., 1976, A&AS 24, 413Walraven T., Walraven J.H., 1960, Bull. Astr. Inst. Neth. 15,67Walraven T., Walraven J.H., 1977, A&AS 30, 245

Five-colour photometry of OB-stars in the Southern Hemisphere

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Five-colour photometry of OB-stars in the Southern Hemisphere

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