Multicolour Photometry of the Stars in NGC 2264*

Abstract

The young cluster NGC 2264 contains early type and non-emission stars, as well as T Tauri-like variables which populate the elevated main sequence (Walker 1956). The astrometric study of Vasilevskis et al . (1965) indicates a high probability of these variable stars being cluster members. The generally accepted view is that the stars which lie above the zero-age main sequence are still undergoing contraction. They exhibit infra-red, blue and ultra-violet excesses (Mendoza 1968, Smak 1964), and they are characterized by H α emission, the intensity of which is correlated with ultra-violet excess (Kuhi 1966). In the (U–B, B–V) diagram these stars occupy the same region as would the heavily reddened OB stars. It is necessary to distinguish between them and this can be achieved by the application of multi-colour (UBVRI) photometry. In the present paper UBV photometry of the stars of NGC 2264 has been extended to R and I magnitudes in Johnson's (1966) system, and the infra-red and ultra-violet excesses of the T Tauri-like stars have been studied. The UBV data have been taken from Walker (1956). The plates were taken with the 40/60/150 cm Schmidt telescope of the Royal Observatory, Edinburgh's outstation at Monte Porzio during the period from October 1968 to January 1969. For R and I magnitudes the emulsion/filter combinations used are Kodak IaE + Ilford colour filter 204 and Kodak IN+ Ilford colour filter 207 respectively. The photographic calibration has been achieved by using a pair of doubly refracting calcite plates in conjunction with a polaroid (Bruck et al . 1969). The limiting magnitudes are R = 14 m .0 and I = 13 m .0. No attempt has been made to search for variables. Variability of the known or suspected variables has been studied over the period of observations, and mean magnitudes have been derived from plates taken in the same interval of time. The standard error of photographic magnitudes due to different sources of errors including the variability of stars is ± 0 m .14 The separation of T Tauri-like stars from background early type stars which are heavily reddened can be achieved in the following way: The (U–V) and (V–I) colours can be predicted for the early type stars from the parameter Q defined as Q = (U–B)–0.70 (B–V) (Nandy 1968), which determines the spectral type and colour excesses E B–V , E V–R , E V–I . In the case of reddened early type stars the predicted colours would agree with the observed (V–R) and (V–I) colours. On the other hand, if the difference between the predicted and observed colours is much larger than the errors of observation, the stars are likely to be of later types with anomalous colours characteristic of T Tauri stars. For these stars, an additional parameter (V–R) 0 , which is the observed (V–R) colour corrected for reddening has been found necessary to estimate their spectral types, and infrared and ultraviolet excesses can be directly computed. The variables, in particular the H α emission stars in NGC 2264, exhibit infra-red and ultra-violet excesses, and there appears to be a positive correlation between them. These stars may be more luminous than indicated by their colour magnitude diagrams, and following Mendoza (1968) the mean ratio L total /L cm has been assumed to be 3. Masses and ages of these variables as faint as V = 15 m .0 have been computed on this assumption. It is found that their average age is 10 6 years, in agreement with the estimated age of the B stars of the cluster, and their masses range from one to two solar masses.