EDP Sciences
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Volume 417, Number 2, April II 2004
Page(s) 557 - 581
Section Galactic structure, stellar clusters and populations
DOI http://dx.doi.org/10.1051/0004-6361:20035588

A&A 417, 557-581 (2004)
DOI: 10.1051/0004-6361:20035588

A rotational and variability study of a large sample of PMS stars in NGC 2264

M. H. Lamm1, C. A. L. Bailer-Jones1, R. Mundt1, W. Herbst2 and A. Scholz3

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2  Van Vleck Obs., Wesleyan Univ., Middletown, CT 06459, USA
3  Thüringer Landessternwarte Tautenburg, Sternwarte 5, 07778 Tautenburg, Germany

( Received 30 June 2003 / Accepted 19 November 2003 )

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 10 600 stars with $I_{{\rm C}}$ magnitudes between 9.8 mag and 21 mag have been monitored in our $34\arcmin$  $\times$ $33\arcmin$ field. Time series data were obtained in the $I_{{\rm C}}$ 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 $\chi^2$-test. In addition we have carried out nearly simultaneous observations in  V, $R_{{\rm C}}$ and a narrow-band H $\alpha$ filter. The photometric data enable us to reject background and foreground stars from our sample of variable stars according to their location in the $I_{{\rm C}}$ vs. $(R_{{\rm C}}-I_{{\rm C}})$ colour-magnitude and $(R_{{\rm C}}-{\rm H}\alpha)$ vs. $(R_{{\rm C}}-I_{{\rm C}})$ 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 $\rm H\alpha$ 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 $I_{{\rm C}} \simeq 15~{\rm mag}$ and of late spectral type ( $\ga$M 2). We find that the periodic variables, as a group, have a smaller degree of variability and smaller $\rm H\alpha$ 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 (CTTSs). We have quantified this bias and estimated that the expected fraction of WTTSs among PMS stars in the cluster is $77\%$. This is relatively close to the fraction of WTTSs among the periodic variables which is $85\%$. We also estimated the total fraction of variables in the cluster using only two well selected concentrations of PMS stars called NGC 2264 N & S in which we can easily estimate the total number of PMS stars. We find that at least $74\%$ of the PMS stars in the cluster with $I_{{\rm C}}\leq 18.0~{\rm mag}$ were found to be variable (either periodic or irregular) by our study. This number shows that our search for PMS stars in NGC 2264 through extensive and accurate photometric monitoring is very efficient in detecting most PMS stars down to at least $I_{{\rm C}}=18.0~{\rm mag}$.

Key words: galaxy: open clusters and associations: individual: NGC 2264 -- stars: pre-main sequence -- stars: rotation -- stars: activity -- stars: starspots -- methods: data analysis

Offprint request: R. Mundt, mundt@mpia.de

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