A Search for Variable Stars and Planetary Occultations in NGC 2301. II. Variability

We obtained R‐band time‐series observations of the young, metal‐rich open cluster NGC 2301 for 12 nights in 2004 February. B‐band images were also obtained, and color‐magnitude diagrams with limits of R = 19.5 and B = 21.5 were produced. Only a small effort was made to determine cluster membership, as our magnitude limits are far deeper than previously published values. Our photometric precision for the brightest 5 mag of sources is 1–2 mmag. We determine that for the ∼4000 stars that have time‐series data, 56% are variable, and of these, approximately 13% are observed to exhibit periodic light curves ranging from tens of minutes to days. We present some examples of the light curves obtained, and produce cuts in variability space based on parameters such as color and amplitude. The percentage of variability is approximately equal across all colors, with the majority of variables having amplitudes of 0.15 mag or less. In general, redder stars show larger variability amplitudes. We find a smooth decline in the number of periodic variables toward longer period. This decline is probably due to a transition from intrinsic to extrinsic variability, and also due in part to our limited observing period of 12 nights. Essentially all the A and F main‐sequence stars in our sample are variable (∼2 mmag and larger), and most present complex light curves containing multiple periods that are suggestive of their inclusion in the δ Scuti and γ Doradus classes. A variable non–cluster member giant and two variable white dwarf candidates are discussed. Our equational description of variability is shown to be an excellent predictive tool for determining the cumulative fraction of variables that will be observed in a photometric survey. Our entire data set is available electronically.