7) CARINAE AND THE TRUMPLER 16 CLUSTER. Alejandro Feinstein* Observatorio Astronómico Universidad Nacional de La Plata, Argentina

Size: px

Start display at page:

Download "7) CARINAE AND THE TRUMPLER 16 CLUSTER. Alejandro Feinstein* Observatorio Astronómico Universidad Nacional de La Plata, Argentina"

Irene Daniels
6 years ago
Views:

1 7) CARINAE AND THE TRUMPLER 6 CLUSTER Alejandro Feinstein* Observatorio Astronómico Universidad Nacional de La Plata, Argentina A large number of investigations, many of them quoted and discussed by Burbidge, have been carried out on rj Carinae and the surrounding nebulosity NGC 7. The group of stars known as Trumpler 6 around rj Carinae has also been studied; photo- graphic photometry of this cluster has been published by Becker. When the new photoelectric photometer was put into opera- tion we began observations of southern stars with peculiar charac- teristics, among them rj Carinae and some of the brighter stars of Trumpler 6. INSTRUMENTATION The photometer consists of a P photomultiplier enclosed in a sealed container and mounted inside a box of tergopol, a material made of cellulose and air which gives exceptionally good insulation. The box holds about one kilogram of dry ice. Evapo*- ration is so low that after eight hours nearly 80% of the original amount of dry ice remains. The output current of the photomultiplier is fed into a General Radio 0-A amplifier and a Brown recording potentiometer. A fine attenuator with three steps and a time constant switch were added to the amplifier. The high-voltage supply was made in the Comisión Nacional de la Energía Atómica in Buenos Aires. The photometer is attached to the -inch reflector of the La Plata Observatory. The color filters are those recommended by Johnson: mm Schott UG for U, mm BG + mm GG for B, and mm GG for V. A radium-paint standard source is used to check the photometer sensitivity. The focal-plane diaphragms are.0,.5, and.9 mm in diameter corresponding to angular diameters of ", 0", and 5". * Member of the Carrera del Investigador Científico, Consejo Nacional de Investigaciones Científicas y Técnicas, Argentina. 9

2 7] CARINAE AND TRUMPLER 6 9 OBSERVATIONS The observations, all obtained during June 96, were made in the usual way: by measuring the brightness of the star in blue, visual, and ultraviolet light, then in reverse order. Measurements of the sky were included when necessary. At intervals of five to ten stars we measured the deflection of the radium source. The standard stars were those suggested by Johnson and Harris. Mean extinction coefficients resulted from the values obtained on dif - ferent nights. Table I lists the results for the stars indicated on the finding chart, Figure, and the next to the last three columns of Table I give the magnitudes and color indices of the nebulosity measured approximately 0" south of each star. The sky surrounding the stars appears very bright in comparison to other parts of the sky and also with the zenith due to the fact that the sky in the y Carinae region is the sum of normal sky brightness and the nebulosity. The observations of the stars were corrected for the measured sky (equal to zenith sky plus nebulosity). Mean magnitudes per square second of arc of the sky at La Plata were V =.0 with color indices B V and U B = 0^0. Star No. Y] Car * 6 TABLE I Magnitudes and Color Indices of Stars in Trumpler 6 AND OF THE BACKGROUND NEBULOSITY Nebulosity 0" South of Stars each Star CPD HD 908 HD * Perhaps variable in V. V B-V U B n (mag/ ") B V U B n

3 9 ALEJANDRO FEINSTEIN Fig.. Finding chart of the y Carinae region. North is at the top, east to the left. M v B-V Fig.. Color-magnitude diagram of yj Carinae and the cluster Trumpler 6. Filled circles: stars of the cluster ; crosses: main sequence stars of age zero.

4 7] CARINAE AND TRUMPLER 6 95 Fig.. U B, B V diagram of tj Carinae and the cluster Trumpler 6 with reddening lines indicated. TRUMPLER 6 Figure is a plot of B V against V, and Figure is a plot of B V against U B for all the stars measured. The two brightest, Nos. 7 and are, according to the B V, U B diagram, of very early spectral type (O-BO). Thackeray has stated that stars numbers 7 and... are of type O. Star No. 7 is listed as type Oe5 in the Cape Photographic Catalogue and as 06 by Beer. 5 Let us assume here a spectral type of 07. Star No TABLE II Results for Stars of Trumpler 6 Spec. Type (diagram) (B F) 0 E B _ V tn 0 m 0 M 07 0* 0* BO B B Bl Bl Bl Bl B

5 96 ALEJANDRO FEINSTEIN On this assumption we fitted the (B V) 0, M v sequence given by Johnson and Hiltner 6 and Hiltner 7 to the observations in Figure. Table II gives for each star the spectral type suggested by Figures and, the intrinsic color index, the color excess, the magnitude corrected for absorption (assuming A r = E B -v) and the distance modulus based on the absolute magnitudes of John- son and Hiltner. 6 The true modulus of the cluster, m 0 M, then is. mag., corresponding to a distance of 800 parsecs.* This value of the distance is greater than that commonly ac- cepted for 7) Carinae. To compute the distance it was assumed that all stars belong to the main sequence, a suggestion that is supported by the evidence in Figures and. From these dia- grams it appears that the cluster is very young, of the order of x 0 6 years. 7} CARINAE The star tj Carinae requires special discussion. Is it a mem- ber or not of the cluster Trumpler 6? Its location in Figure suggests that perhaps it is. If so, the visual absolute magnitude would be around 7, and the intrinsic color indices would be (B V)o = 0^8 and (U B) 0 = 0^77. Then, assuming that the value of B V is the true color index of the star, we find it would be a supergiant of type F with a very strong ultraviolet excess (around 0^8). Perhaps this ultraviolet excess is related to the very large nebulous halo surrounding the star. No other star belonging to the known young clusters occupies a place like tj Carinae in the color-magnitude diagram of Figure. In h and x Persei 6 the reddest star of the group just evolving from the main sequence appears, however, to be not far from this position. If we assume that tj Carinae belongs to the cluster, it would be in the helium-burning phase according to the computations of Hayashi and Cameron, 8 supergiant region. and evolving very fast to the red The present magnitude B = 6.8 of tj Carinae is around 0^ brighter than the photographic magnitude observed by O Connell * Note added in press: In agreement with the value of 500 parsecs derived Dy D. J. Faulkner {Pub. ASJ*. f 75, 76,96).

6 r CARINAE AND TRUMPLER 6 97 in If we take into account the enormous changes in magnitude displayed by this star during the last century, our results suggest that its brightness is now relatively stable. Despite the evidence presented here, the problem of the distance of y Carinae is obviously far from solution. The group of Trumpler 6 stars shows cluster characteristics, but its relation o r Carinae, if any, is not yet established. The writer is grateful to the many persons who helped to put the photoelectric photometer into operation, especially to Dr. G. E. Kron who made valuable suggestions to improve the equip- ment, and to Mr. O. Gonzales Ferro who was in charge of all the electronic details. The National Science Foundation gave generous support to Dr. C. Jaschek to put the photometer into operation. It is also a pleasure to thank Dr. Jaschek for helpful suggestions in preparing the manuscript. G. R. Burbidge, Ap. J., 6, 0, 96. W. Becker, Zs. f. Ap., 5, 9, 960. H. L. Johnson and D. L. Harris HE, Ap. J., 0,96,95. A. D. Thackeray, The Observatory, 76, 0, A. Beer, MJSUL.AS.,, 0, H. L. Johnson and W. A. Hiltner, Ap. J.,, 67, W. A. Hiltner, Ap. J. Supplements,,6,956 (No. ). 8 C. Hayashi and R. C. Cameron, Ap. J., 6, 66, D. J. K. O Connell, in Vistas in Astronomy, Vol., A. Beer, ed. (London and New York: Pergamon, 956), p. 65.

1. Introduction. 2. Observations and reductions

1. Introduction. 2. Observations and reductions J. Astrophys. Astr. (1985) 6, 61 70 A Study of the Open Cluster NGC 2374 G. S. D. Babu Indian Institute of Astrophysics, Bangalore 560034 Received 1984 September 25; accepted 1985 January 11 Abstract.