Baltic Astronomy, vol. 10, 375-384, 1998.
SEVEN-COLOR PHOTOELECTRIC PHOTOMETRY OF THE OMICRON VELORUM CLUSTER
M. C. Forbes1-2, R. J. Dodd2 and D. J. Sullivan1
1 Victoria University of Wellington, P. 0. Box 600, Wellington, New Zealand 2 Carter Observatory, P. 0. Box 2909, Wellington, New Zealand Received March 9, 1998; revised May 10, 2000.
Abstract. Photoelectric observations in the Vilnius seven-color photometric system of 36 stars in the magnitude range 3.6< V<10.7 in the open cluster around the star o Velorum are presented. Pho- tometric spectral and luminosity classes are determined for each star from which the mean distance modulus of the cluster is found to be 5.94±0.02 mag and the mean color excess Ey~v = 0.00±0.02 mag. The membership of the cluster stars is discussed. Key words: stars: fundamental parameters - clusters: individual: Omicron Velorum cluster
1. INTRODUCTION
In conjunction with an observational program to establish a southern hemisphere network of standard stars in the seven-color Vilnius photometric system (Dodd, Forbes, Sullivan & Zdanavicius 1996), a photoelectric study of selected star clusters was begun. This paper describes the reduction, analysis and interpretation of obser- vations of the cluster known as Omicron Velorum, after the bright star near the cluster center. It is also known as IC2391 or C0838- 528. The star o Vel is located at «(2000) = 8h40m18s; ¿(2000) = —52°55.3'. A full review of previously published literature on the cluster is given by Forbes (1996). The main characteristics of the cluster are given in Table 1. 376 M. C. Forbes, R. J. Dodd, D. J. Sullivan
Table 1. Review of the main characteristics of the Omicron Velorum cluster.
Author Distance Reddening Age 6 (PC) EB-V 10 yr
Hogg (1960) 153±3 0.004Ì0.003 25-53 . Lynga (1961) 160 0.017 <20-40 Buscombe (1965) 210Ì10 Graham (1967) 226Ì40 0.00 Perry & Hill (1969) 148Ì4 (UBV) 0.00±0.01 <30 158Ì4 (uvby) Eggen (1972) 151 (UBV) 0.02 Eggen (1983) 166Ì22 (uvby) O.OOliO.007 30 Lynga Sz Wramdemark (1984) 140±11 0.1±0.1 36 Balona & Shobbrook (1984) 167 0.022Ì0.011 Levato & Malaroda (1984) 150±6 0.02 Koester & Reimers (1985) 57-80 Maitzen & Catalano (1986) 154Ì6 0.00 Average 154 0.010 30
2. PHOTOMETRIC OBSERVATIONS
Observations of the stars in and around the cluster, shown and numbered by Lynga's (1959) notation in Fig. 1, were made using either the 61 cm Boiler & Chivens telescope or the 61 cm Optical Craftsmen telescope at the Mount John Observatory of the Univer- sity of Canterbury in New Zealand. The star No. 1 is o Vel itself. It is a Cephei-type variable, spectroscopic binary. Its spectral class is B3 IV, V magnitude range 3.56-3.67 mag and B-V = -0.18. A thermo-electrically cooled EMI 9558 photomultiplier operating at 1700 V was used. It has a trialkali S20 photocathode with a QE exceeding 10% from 300 nm to 650 nm. A discriminator/amplifier drives a pulse counter with the integrated count being logged by a computer (initially a PET microcomputer and more recently an IBM PC 386 clone). The filter set, supplied by the Institute of Theoretical Physics and Astronomy (Vilnius), closely matches that used for the stan- dard system (Forbes, Dodd & Sullivan 1993). The diameter of the Photometry of the Omicron Velorum cluster 377
Fig. 1. Identification chart of the Omicron Velorum cluster stars. 378 M. C. Forbes, R. J. Dodd, D. J. Sullivan most commonly used aperture was 32 arcsec. The form of observing follows that described by Straizys (1992), with the extinction and control stars being observed in addition to the cluster stars. The normal filter observing sequence was: UPXYZVS for the star, check star centering, again UPXYZVS for the star and UPXYZVS for the sky, though for faint stars the sky was measured both before and after the star. Integration times for the PXYZV filters ranged from 5 s for stars brighter than V = 7 mag to 40 s for the faintest stars. For the U and S filters the integration time was twice as long. Each integra- tion was repeated three times per filter. Detailed descriptions of the reduction methods used (Bouguer and Nikonov) are given in Forbes et al. (1993) and Zdanavicius, Dodd, Forbes & Sullivan (1995). A total of 36 stars was observed at least twice in the cluster. The mean magnitudes and colors derived and are listed in Table 2. The standard deviations about the mean values are~0.01 mag for the stars down to 9.5 mag and ~0.02 mag for fainter stars. At the precision level of this study (±0.02 mag for a single ob- servation) none of the observed stars was found to be variable. As a further check, the standard deviation of the mean for each star was compared with the expected deviation for a star with n observations using the inequality given by Forbes (1996):
I ^residuals | < 0.025n"1/2 + 0.16e"o-21n.
No stars were found to be variable using this criterion. The Vilnius V magnitudes compare well with the broad-band V magnitudes found in the literature (see Table 1), some 50% agree within ±0.01 mag and ~90 % within ±0.02 mag.
3. SPECTRAL AND LUMINOSITY CLASSES
The spectral and luminosity classes of each star were derived in three ways: the Vilnius stellar box method, Q vs. spectra' di- agrams and Q, Q-diagrams, respectively (Straizys 1992), using the variations given by Forbes (1996). The average results are listed in Table 3. These spectral types were compared with previously pub- lished values. Some 55% agree within ±1 spectral subclass and all agree within ±3 spectral subclasses. Three stars have been classified for the first time: No. 37 (G2 V), No. 40 (Gl V) and No. 41 (B6 V). Photometry of the Omicron Velorum cluster 379
Table 2. Mean magnitudes and color indices for the stars in the Omicron Velorum cluster, n is the number of independent observations.
Star V_ U-P P-X X-Y Y-Z Z-V V-S n 1. 3.621 0.235 0.359 0.109 0.072 0.027 0.088 IF 2. 4.858 0.194 0.356 0.133 0.047 0.033 0.044 2 3. 5.206 0.249 0.415 0.158 0.069 0.039 0.087 11 4. 5.501 0.248 0.426 0.149 0.045 0.048 0.051 2 5. 5.545 0.254 0.442 0.169 0.040 0.045 0.069 2 6. 5.579 0.317 0.471 0.153 0.079 0.029 0.107 28 7. 6.470 0.397 0.604 0.189 0.079 0.041 0.096 6 8. 7.683 0.563 0.846 0.373 0.114 0.077 0.151 4 9. 7.074 0.322 0.509 0.167 0.059 0.053 0.077 6 10. 7.258 0.481 0.726 0.278 0.064 0.061 0.143 6 11. 7.298 0.491 0.792 0.289 0.092 0.064 0.125 4 12. 7.344 0.579 0,740 0.495 0.184 0.137 0.306 9 13. 7.402 0.511 0.834 0.316 0.108 0.058 0.133 2 14. 7.561 0.536 0.851 0.369 0.154 0.052 0.186 2 15. 7.585 0.524 0.766 0.506 0.218 0.081 0.321 2 16. 7.682 0.505 0.844 0.364 0.105 0.102 0.163 2 17. 7.773 0.591 0.824 0.482 0.160 0.099 0.262 4 18. 7.819 0.412 0.652 0.876 0.297 0.260 0.583 4 20. 7.943 0.349 0.473 0.227 0.130 0.083 0.203 4 21. 8.155 0.606 0.812 0.472 0.186 0.082 0.293 2 23. 8.523 0.545 0.742 0.524 0.174 0.152 0.281 2 24. 8.619 0.611 0.855 0.434 0.150 0.092 0.229 3 25. 8.668 0.551 0.804 0.491 0.145 0.117 0.234 4 27. 8.745 0.489 0.603 0.555 0.231 0.163 0.392 4 0.461 0.590 0.639 0.252 0.201 0.439 28. 8.884 2 0.486 0.622 0.584 0.251 0.151 0.414 29. 9.029 6 0.603 0.927 1.257 0:468 0.299 0.767 30. 9.243 2 0.470 0.581 0.627 0.271 0.146 0.456 32. 9.407 2 0.443 0.565 0.652 0.273 0.179 0.479 33. 9.663 2 0.394 0.578 0.638 0.253 0.206 0.484 34. 9.516 4 0.426 0.982 1.422 0.464 0.408 0.855 35. 9.592 6 0.436 0.546 0.640 0.310 0.142 0.492 36. 9.890 2 0.310 0.611 0.852 0.305 0.260 0.570 37. 9.974 2 0.693 0.848 0.390 0.175 0.054 0.265 38. 10.196 2 0.414 0.642 0.864 0.324 0.280 0.564 40. 10.265 2 0.381 0.563 0.331 0.149 0.141 0.240 41. 10.653 2 380 M. C. Forbes, R. J. Dodd, D. J. Sullivan
Table 3. Spectral classifications of stars in the Omicron Velorum cluster from Vilnius photometry.
Star Sp Star Sp Star Sp 1. B4 IV 13. A1 V 28. F7 V 2. B4 IV 14. A3 V 29. F4 V 3. B5 V 15. A6 V 30. K0 IV 4. B5 V 16. A4 V 32. F5 V 5. B6 V 17. A6 V 33. F7 V 6. B6 V 18. G4 V 34. F8 V 7. B8 V 20. B5 V 35. K2 V 8. A4 V 21. A4 V 36. F7 V 9. B6 V 23. A7 V 37. G2 V 10. A1 V 24. A4 V 38. A3 V 11. A1 V 25. A5 V 40. G1 V 12. A7 V 27. F3 V 41. B6 V
4. REDDENING AND DISTANCE MODULUS
Using the average spectral type from Table 3, the mean redden- ing and distance modulus for the cluster were found as follows: (1) using the spectral type and luminosity class for each star the intrinsic magnitude Vo and color index (F-V)o were obtained from Appendix I and Table 73 in Straizys (1992); (2) the color excess was calculated for each star from the apparent and intrinsic colors: Ey-v = (^-V^F-V^o; (3) the distance modulus VQ — My — V — My — REy-v for each star was calculated using the extinction to color excess ratio (R) from Table 74 in Straizys (1992); the distance r in parsecs is then found by the equation
R _ 10(VO-MV+5)/5;
(4) the mean distance to the cluster < r> and the mean distance modulus
(7) after that we return back to step (2) and repeat the calculations until the iterated mean distance modulus converges. Following this procedure, we obtain the mean distance modulus of the cluster as 5.94±0.02 mag and the mean color excess Ey-v as 0.00±0.02 mag. This corresponds to a distance of 154 pc, ex- actly in accordance to the mean value of Table 1. Assuming the cluster to be spherical, its measured angular diameter of 45' at the determined mean distance would lead to an expected variation of the distance modulus about the mean value by ±0.03 mag. Thus it seems reasonable to assume that the entire spread in values is due to the difference in distance moduli of individual cluster stars rather than to interstellar reddening along the line of sight to the cluster.
5. CLUSTER MEMBERSHIP
Once the cluster's distance modulus and color excess are deter- mined, the distances and color excesses of individual stars can be used to decide on the star's cluster membership. A histogram of distance moduli for zero color excess was examined. Stars which lay outside the main distribution were considered as non-members and stars in the tails of the distribution were considered as "maybe" members. The results are listed in Table 4. The first column gives the star number in the Lynga (1959) catalogue, the second column gives the cluster membership based on the distance modulus and color excesses from the photometry and the third column lists the average memberships from the literature. Only two stars show complete disagreement: No. 37, for which the non-membership was assessed from the radial velocity and proper motion studies, and No. 17, for which one of three authors in the literature considers a positive membership. Fig. 2 shows the HR diagram for the Omicron Velorum cluster obtained with the mean distance modulus and zero reddening. The upper plot shows all the observed stars with the error bars indicating the size of the standard deviation of color indices. The lower plot shows only the true and probable cluster members. The smoothed zero-age main-sequence line is taken from Straizys (1992). 382 M. C. Forbes, R. J. Dodd, D. J. Sullivan
Table 4. Membership of the Omicron Velorum cluster stars from Vilnius photometry. The second column is based on the distance mod- ulus and color excess analysis. The third column is the average memberships from the literature.
Star Vilnius Literature Star Vilnius Literature No. Vo-Mv (average) No. Vq-MV (average) 1. yes yes 20. no no 2. yes yes 21. yes yes 3. yes yes 23. yes yes 4. yes yes 24. yes yes 5. yes yes 25. yes maybe 6. yes yes 27. yes maybe 7. yes yes 28. yes maybe 8. yes yes 29. yes yes 9. yes maybe 30. no (no) 10. maybe yes 32. yes yes 11. yes yes 33. yes yes 12. maybe maybe 34. yes yes 13. yes yes 35. no no 14. maybe yes 36. yes yes 15. maybe maybe 37. yes (no) 16. yes yes 38. no no 17. yes no 40. maybe yes 18. no no 41. no (no)
6. CONCLUSIONS
This study of open cluster Omicron Velorum using seven-color Vilnius photometry gives new values for distance, reddening and membership which agree well with data obtained earlier using the U,B, V and the Stromgren medium-band photometry. A distance modulus of 5.94±0.02 and a mean color excess of 0.00±0.02 are found. Photometric membership obtained in this paper and the lit- erature data agree within ~80%.
ACKNOWLEDGMENTS. We wish to thank the University of Canterbury for the generous allocation of telescope time at the Photometry of the Omicron Velorum cluster 383
My
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2 -
4 -
My Legend * • 01 B4 IV o 02 B4 rv -2 « 03 BS V o 04 BS V * 05 B8 V a e 06 B8 V \ * 0• 079 BB8 8V V * 11 A1 V • 13 A1 V \p> « 08 A4 V T • 18 A4 V \ A 17 A8 V n V D 21 A4 V ^ E 24 A4 V • * A T 25 AS V G 23 A7 V X D H 27 F3 V I 29 F4 V K 32 F5 V L L 28 F7 V "••v. r. M 33 F7 V J^f P0 348 F178 V Q Q 37 G2 V S S 40 G1 V
0.0 0.2 0.4 0.6 0.8 1.0 (Y- V)o
Fig. 2. HR diagram of the Omicron Velorum cluster from Vilnius photometry. For explanations see the text. 384 M. C. Forbes, R. J. Dodd, D. J. Sullivan
Mount John Observatory, the Marsden Fund of the Royal Society of New Zealand for partial funding, Prof. V. Straizys of the Institute of Theoretical Physics and Astronomy, Vilnius for his enthusiastic sup- port of the southern hemisphere Vilnius photometric programme.
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