Houten, C.J. van

Leiden University Scholary Publications

A&A 369, 527–529 (2001)DOI: 10.1051/0004-6361:20000470c© ESO 2001Astronomy&AstrophysicsObservations of OB-stars at the former Leiden Southern Station?C. J. van Houten??Sterrewacht Leiden, PO Box 9513, 2300 RA Leiden, The NetherlandsReceived 25 February 2000 / Accepted 4 December 2000Abstract. About 700 stars, mostly OB-stars, were observed by the author at the former Leiden Southern Stationat Hartebeespoortdam, South Africa, in the observing seasons 1965, 1968, 1969, 1974, 1977, 1978. Observationswere made in the five channels of the Walraven photometric system. Due to weathering of the telescope mirrorthe W channel gave no reliable results for the faintest stars (m = 11 mag); in these cases the U −W colour indexis not given. The change in sensitivity in the V channel, supposedly having occurred in 1968, was not recognised.Key words. techniques: photometric – stars: fundamental parameters1. IntroductionThe first program carried out with the 90 cm light-collector and Walraven 5-colour photometer after the in-stallation of the telescope in South-Africa was a photome-try of all then known O-and early B-stars in the southernhemisphere. This program was carried out in 1959 and1960 by Th. Walraven (van Houten et al. 1999). In themeantime the number of known OB-stars has increasedconsiderably so that it appeared worth while to augmentthe older survey with a new, deeper survey in selectedparts of the southern milky way. This program was startedin 1965 by the present author using the list of Lyng̊a (1964)and an unpublished list by Dr. E. H. Geyer, then of theHeidelberg Observatory. These lists were later augmentedby selected stars from the catalogue of Stephenson &Sanduleak (1971). In total about 700 stars were observed.This material is published here, containing the results ofthe observing campaigns of the years 1965, 1968, 1969,1974, 1977 and 1978. Almost all selected stars were ob-served three times, in three different nights, according tothe advice given by Walraven & Walraven (1960).2. The phometric system, comparisonwith Lub & Pel (1977)The photometric system is defined by the photometric val-ues of a number of stars, which will be denoted here asstandard stars. They number 10 in the present publica-tion and are listed in Table 1 together with their adoptedcolours. Since the photometry yields only differences of? Table 5 is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/369/527?? e-mail: vhouten@strw.leidenuniv.nlTable 1. The standard starsHD V V −B B − U U −W B − L61068 0.472 −0.057 −0.009 −0.020 −0.01474575 1.289 −0.053 −0.018 −0.007 −0.02288015 0.202 −0.044 0.095 0.005 0.021104337 0.652 −0.060 −0.007 −0.014 −0.016122980 1.018 −0.062 0.026 −0.013 0.002144470 1.174 0.002 0.001 0.001 0.002151515 −0.121 0.081 −0.009 0.031 0.014164402 0.450 0.009 −0.037 0.002 −0.017191091 −0.159 −0.005 0.322 0.066 0.111210934 0.575 −0.038 0.225 0.044 0.062brightnesses, a zero point for each colour must be adopted.The zero point adopted here is both the brightness andcolours of standard star HD 144470 as given in Walravenet al. (1964, 1977). As usual in the Walraven photomet-ric system, the brightnesses are expressed in logarithms ofintensities.Eight of the ten standard stars were also used asstandards by Lub & Pel (1977). The differences Lub &Pel minus van Houten are listed in Table 3. It is seen thatthe average differences are 0.01 mag or less.In 1965 part of these standard stars were not used,but instead of them three other stars were used as stan-dard stars. They are listed in Table 2. The other standardstars used in 1965 were HD 88015, 104337, 151515, 191091and 210934. It appeared that the photometric system wassomewhat different in 1965, which is explained as due tosmall changes in efffective wavelengths. For that reasonstars observed in 1965 are denoted by 65 in the remarkscolumn of the main table (Table 5).528 C. J. van Houten: OB star observationsTable 2. Additional standards used in 1965HD V V −B B − U U −W B − L123980 0.010 0.060 0.443 0.137 0.214164806 0.023 −0.012 0.149 0.021 0.015178175 0.519 −0.009 0.076 0.004 0.0133. ExtinctionThe extinction coefficients for the different channels weredetermined in the usual way, by observing stars nearzenith and stars observed through large airmass, selectedfrom the list of standard stars. It was shown by Walraven& Walraven (1960) that those five extinction coefficientscan be expressed by one variable parameter,K = K0 + q.K1 (1)K being the extinction coefficient, K0 and K1 constantsfor each channel, and q the variable parameter. The valuesfor K0 an K1 as given by Walraven & Walraven (1960)were corrected using the results of Lub & Pel (1977).Ideally the five channels should yield the same value of q;in practice this was not the case. These differences aredue to inaccuracies of the measurements, such as by vari-able extinction. The method used was to take an averagevalue of the five parameters q, and with this value recom-pute the extinction coefficients. By means of the extinc-tions so obtained the brightnesses of the standard starswere extrapolated to the zenith. The differences with thecatalogued brightnesses were practically constant for eachchannel and average values were used for the reduction ofthe program stars. It turned out that the extinction at theobservatory site was very variable, so that for each extinc-tion measurement different q-values were found. The ex-tinction values actually used for the reduction of the pro-gram stars were obtained by interpolating between theseq-values.4. V -colour variationLub & Pel (1977) discovered that the colour dependence oftheir V channel was different from that found by Walraven& Walraven (1977); its cause was ascribed to a high volt-age accident with the power supply of the amplifier in1968. Since the observations presented here were partlymade before that date, and partly thereafter, this factgives the opportunity to check this assumption. In thepresent material 16 stars were observed both in 1965 andlater than 1968, and 10 stars both in 1968, but beforethe calamity and later than 1968. Comparison betweenthe 1965 and 1968 values with those obtained later shouldgive clarity of this issue. First it should be realised thatthe choice of the zero point of the photometry, the samefor pre-calamity and post-calamity, causes the constantterm in the Lub & Pel relation to vanish. The relation tobe tested thus becomes:(V −B)new − (V −B)old = −0.026(V −B)old (2)Table 3. Differences: Lub & Pel minus van Houten forstandard starsHD V V −B B − U U −W B − L61068 0.000 0.000 0.000 −0.001 0.00074575 −0.003 −0.003 0.004 −0.010 0.00188015 0.002 −0.003 −0.002 0.001 −0.004104337 −0.005 −0.006 0.005 −0.007 0.001122980 −0.001 −0.004 0.007 −0.005 0.003144470 0.002 −0.005 0.004 −0.003 0.002164402 0.005 −0.006 0.007 −0.001 0.005210934 −0.002 −0.002 0.006 −0.002 0.004average 0.000 −0.004 0.004 −0.004 0.002and a similar relation for V . Here “new” means: after,and “old” before calamity. The V brightness, however,might possibly be influenced by small variations which donot influence its V − B index; therefore only V − B wascompared. The 16 stars observed in 1965 yielded: ∆(V −B) = −0.003 ± 0.001 (∆ defined as in (2)) and the 10stars observed in 1968: ∆(V −B) = −0.001± 0.003. Bothvalues are, within their limits of accuracy, virtually equalto zero. Therefore it was decided to omit transformation(2) for the 1965 and 1968 observations and to give allobservations in one list, re-calculated to the Lub & Pelvalues for the standards.5. PrecisionWith three observations per star the V brightnesses canbe averaged, as well as the four colour indices. This yieldsthree differences with respect to this average value, andan average difference without regard to sign; this lattervalue will be denoted here as the precision of the obser-vation. The results of such comparisons are summarizedin Table 4. Here “brightest stars” means: stars of about7 mag, and “faintest stars” means: stars of about mag 11.It can be seen that the U −W colour index of the fainteststars is very uncertain, which is caused by the increasingoxidation of the telescope mirrors compared to 1959-1960.In Table 5 values for U −W are only given when the pre-cision (as defined above) is better than 0.016. Even thislimit is not sufficient for a successful use of the spectralclassification scheme developed by Walraven & Walraven(1960). It is advised to omit the U −W colour index al-together, except for the brightest stars. An attempt wasmade to partly counteract this dependence of accuracy onmagnitude by increasing the integration time of one mea-surement for fainter stars. The usual integration time was30 s, but, depending on the brightness, it was increased to2 min for the faintest stars.6. The main tableTable 5 contains the measurements of the OB-stars ob-served in the years 1965, 1968, 1969, 1974, 1977 and 1978.C. J. van Houten: OB star observations 529Table 4. Accuracy of observations (∆ log I)brightness or colour brightest stars faintest starsV 0.004 0.005V −B 0.002 0.004B − U 0.003 0.012U −W 0.005 0.040B − L 0.002 0.008Some stars are included which are not OB-stars; this ismostly a result of misidentifications. The listed valuesare mostly an average of three observations, if this num-ber is different, it is mentioned in the column: obs. Thestars are identified by their HD or CPD number, or, forthose stars that do not have an HD or CPD number,by their number in the Lyng̊a (1964) and Stephenson &Sanduleak (1971) catalogue. The latter are denoted withtheir Lyng̊a or Stephenson & Sanduleak number, precededwith L or S respectively. As usual in the Walraven photo-metric system, the brighnesses are expressed in log I.Acknowledgements. Many thanks are due to Dr. E. H. Geyerfor making a list of OB-stars available to the author, which wasa very welcome addition to the lists of Lyng̊a and Stephenson& Sanduleak. Mr. W. van Vliet and Dr. R. A. Windhorstare thanked for assistance in these observations. Mr. D. F.Stevenson took good care for a proper functioning of theinstruments, which is hereby gratefully acknowledged.Referencesvan Houten, C. J., Walraven, Th., & Walraven, J. H. 1999,A&AS, 137, 113Lub, J., & Pel, J. W. 1977, A&A, 54, 137Lyng̊a, G. 1964, Lund Medd. Ser. II, 141Stephenson, C. B., & Sanduleak, N. 1971, Publ. Warner andSwasey Obs., 1, 1Walraven, T., & Walraven, J. H. 1960, Bull. Astr. Inst. Neth.,15, 67Walraven, T., & Walraven, J. H. 1977, A&AS, 30, 245Walraven, J. H., Tinbergen, J., & Walraven, T. 1964, Bull.Astr. Inst. Neth., 17, 520

Observations of OB-stars at the former Leiden Southern Station

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Observations of OB-stars at the former Leiden Southern Station

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