An Analysis of Variable Objects in the Globular Cluster M4 Using Observations from the NASA K2 Mission

Abstract

This dissertation presents an analysis of data from the NASA K2 mission for the globular cluster (GC) M4, representing the longest continuous observation of a GC. The image reduction and variable object identification pipeline is described. Light curves for 4554 objects are extracted and made publicly available. Among cluster-member stars, 66 variables are found, including 52 new discoveries. Among field stars, 24 variables are found, including 20 new discoveries. Additionally, 57 cluster-member suspected variables, 10 cluster-nonmember suspected variables, and four variables with ambiguous cluster membership are discovered. Two objects appear to be representative of a new variable subclass, which are dubbed "millimagnitude RR Lyrae" (mmRR). Asteroseismic activity is detected in seven horizontal branch stars, as well as 19 stars along the red giant branch. A previously known cluster-member RR Lyrae variable is identified as a candidate Blazhko variable, with a Blazhko period in excess of 78 days. A search for transiting planets in M4 is also performed. Previous surveys have at best been sensitive to planets with periods P < 16 days and, at the shortest periods, planets with radii R_p > 0.8 R_J---this search is sensitive to larger periods (P < 35 days) and, at short periods, smaller planet radii (R_p > 0.3 R_J) than any previous survey. Seven planet candidates are presented which, if any are confirmed, would be the first transiting planet known in a GC. An analysis of the systematic noise reveals that the false alarm probabilities for these candidates are high. Upper limits are placed on planet occurrence and compared to previous results. In the new GC parameter space explored by the results, I calculate 3-sigma occurrence rate upper limits of 6.6% for 0.71--2 R_J planets with 1--36 day periods and 17% for 0.36--0.71 R_J planets with 1--10 day periods. A theoretical motivation for the planet search, examining collisional fragmentation during the formation of short-period rocky planets, is presented, as is the M4 proper motion membership catalog I compiled, which is the most complete assembled for M4 to date.