A Rotational and Variability Study of a Large Sample of PMS Stars in NGC 2264

Abstract. We present the results of an extensive search for periodic and irregular variable pre-main sequence (PMS) stars in the young (2–4 Myr) open cluster NGC 2264, based on photometric monitoring using the Wide Field Imager (WFI) on the 2.2 m telescope on La Silla (Chile). In total, about 10600 stars with IC magnitudes between 9.8 mag and 21 mag have been monitored in our 34 ′ ×33 ′ field. Time series data were obtained in the IC band in 44 nights between Dec. 2000 and March 2001; altogether we obtained 88 data points per star. Using two different time series analysis techniques (Scargle periodogram and CLEAN) we found 543 periodic variable stars with periods between 0.2 days and 15 days. Also, 484 irregular variable stars were identified using a χ 2-test. In addition we have carried out nearly simultaneous observations in V, RC and a narrow-band Hα filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the IC vs (RC − IC) colour-magnitude and (RC − Hα) vs (RC − IC) colour-colour diagrams. We identified 405 periodic variable and 184 irregular variable PMS stars as cluster members using these two different tests. In addition 35 PMS stars for which no significant variability were detected could be identified as members using an Hα emission index criterion. This yields a total of 624 PMS stars in NGC 2264, of which only 182 were previously known. Most of the newly found PMS stars are fainter than IC � 15 mag and of late spectral type (> ∼M2). We find that the periodic variables, as a group, have a smaller degree of variability and smaller Hα index than the irregular variables. This suggests that the sample of periodic variables is biased towards weak-line T Tauri stars (WTTSs) while most of the irregular variables are probably classical T Tauri stars

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