A Search for Variable Stars in the Globular Cluster M4 with K2

Abstract

We extract light curves for 4554 objects with 9<G<19 in the K2 superstamp observations of the globular cluster M4, including 3784 cluster members, and search for variability. Among cluster member objects, we detect 66 variables, of which 52 are new discoveries. Among objects not belonging to the cluster, we detect 24 variables, of which 20 are new discoveries. We additionally discover 57 cluster-member suspected variables, 10 cluster-non-member suspected variables, and four variables with ambiguous cluster membership. Our light curves reach sub-millimagnitude precision for the cluster horizontal branch, permitting us to detect asteroseismic activity in six horizontal branch stars outside the instability strip and one inside the strip but with only ∼1 mmag amplitude variability. 19 additional stars along the red giant branch also have detected asteroseismic variability. Several eclipsing binaries are found in the cluster, including a 4.6-day detached eclipsing binary and an EWclass eclipsing binary, as well as an EW with uncertain cluster membership and three other candidate EWs. A 22-day detached eclipsing binary is also found outside the cluster. We identify a candidate X-ray binary that is a cluster member with quiescent and periodic ∼20 mmag optical variability. We also obtain high-precision light curves for ten of the previously known RR Lyrae variables in the cluster and identify one as a candidate Blazhko variable with a Blazhko period in excess of 78 days. We make our light curves publicly available.

A PSF-based Approach to TESS High quality data Of Stellar clusters (PATHOS) – II. Search for exoplanets in open clusters of the Southern ecliptic hemisphere and their frequency

“…In the last years, many approaches have been developed for the extraction of high precision light curves of stars in crowded regions, based on the use of the Point Spread Functions (PSFs, Nardiello et al 2015a;Libralato et al 2016a) or the Difference Imaging Analysis (Alard & Lupton 1998;Chang et al 2015;Soares-Furtado et al 2017;Bouma et al 2019;Wallace et al 2019;Soares-Furtado et al 2020). In the first paper of this series we presented our project "A PSF-based Approach to TESS High Quality data Of Stellar clusters" (PATHOS, Nardiello et al 2019, hereafter Paper I), aimed at the extraction of high precision light curves for stars in stellar clusters (open and globular clusters) and young associations.…”

A Search for Transiting Planets in the Globular Cluster M4 with K2: Candidates and Occurrence Limits

“…Despite this, information on this and six other of our most promising candidates are presented. The light curves are publicly available at Wallace et al (2019b).…”

“…In this work, all objects are referred to by the identifiers assigned them in Wallace et al (2019a); see table 1 in that work. All of the raw and processed light curves are available at Wallace et al (2019b) 1 .…”

The Kepler IRIS Catalog: Image subtraction light curves for 9,150 stars in and around the open clusters NGC 6791 and NGC 6819

“…However, various image subtraction algorithms have been developed for use on Kepler and TESS pixel data. The most common is the FITSH package (Pál 2012), which includes utilities that have been used by Soares-Furtado et al ( 2017) and Wallace et al (2019) on K2 cluster data, and by Bouma et al (2019) in the search for exoplanets in TESS open clusters. There is also the DIA pipeline (Oelkers & Stassun 2018), which is designed to perform image subtraction photometry on TESS data.…”