An Ao-Assisted Variability Study of Four Globular Clusters*

Abstract

The image subtraction technique applied to study variable stars in globular clusters represented a leap in the number of new detections, with the drawback that many of these new light curves could not be transformed to magnitudes due to the severe crowding. In this paper we present observations of four Galactic globular clusters, M 2 (NGC 7089), M 10 (NGC 6254), M 80 (NGC 6093) and NGC 1261, taken with the ground-layer adaptive optics module at the SOAR Telescope, SAM. We show that the higher image quality provided by SAM allows the calibration of the light curves of the great majority of the variables near the cores of these clusters as well as the detection of new variables even in clusters where image-subtraction searches were already conducted. We report the discovery of 15 new variables in M 2 (12 RR Lyrae stars and 3 SX Phe stars), 12 new variables in M 10 (11 SX Phe and one long-period variable) and one new W UMa-type variable in NGC 1261. No new detections are found in M 80, but previous uncertain detections are confirmed and the corresponding light curves are calibrated into magnitudes. Additionally, based on the number of detected variables and new HST /UVIS photometry, we revisit a previous suggestion that M 80 may be the globular cluster with the richest population of blue stragglers in our Galaxy.

A likely runaway star cluster in the outer disc of the Large Magellanic Cloud

“…Salinas et al 2015). c) Nearly 40 bright and reasonably isolated point sources were selected in the image with the best image quality, in order to model the point-spread-function (PSF) as a 1 1 https://www.astro.uni-bonn.de/theli/gui/background.html quadratically varying Moffat function as performed in previous studies with the same instrument (Fraga et al 2013;Salinas et al 2016). The resulting PSF subtracted images revealed no significant increase of the residuals as function of radius across the whole small FOV (3 × 3 arcmin 2 ).…”

Short period variability in the globular cluster NGC 419 and the SMC field

“…All subtracted images are stacked in a variance image, which is then visually inspected for residuals consistent with intrinsically varying sources (e.g. Salinas et al 2016a).…”

“…Even though variability in star clusters in the Milky Way have been extensively studied (e.g. Clement et al 2001;Salinas et al 2016a), in the Magellanic Clouds (MCs), the large variability surveys like OGLE (Udalski et al 2015) and SuperMACHO (Rest et al 2005) have only scratched the surface of variability in dense star clusters given the difficulties posed by crowding at these distances; and very few clusters have been targeted individually for variability outside these surveys (e.g. Kuehn et al 2013).…”

OUP accepted manuscript

“…Due to the large 14-arcmin FOV of the proposed GLAO system and the large telescope aperture, there is still much work that needs to be further studied. First, although we have defined different metrics for the GLAO performance evaluation of different scientific observations, it still needs improvement: we plan to consider different scientific observations (Basden, Evans & Morris 2014;Salinas et al 2016;Chun et al 2016;Mendes de Oliveira et al 2017;Gavel 2017) and carry out end-to-end observation simulations to generate real observation data for further performance analysis. We will also use some clustering-based PSF evaluation methods (Jia et al 2017) to better evaluate the GLAO performance.…”