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Published August 2016 | Published + Submitted
Journal Article Open

Many new variable stars discovered in the core of the globular cluster NGC 6715 (M 54) with EMCCD observations

Abstract

Context. We show the benefits of using electron-multiplying CCDs and the shift-and-add technique as a tool to minimise the effects of atmospheric turbulence, such as blending between stars in crowded fields, and to avoid saturated stars in the fields observed. We intend to complete, or improve on, the census of the variable star population in globular cluster NGC 6715. Aims. Our aim is to obtain high-precision time-series photometry of the very crowded central region of this stellar system via the collection of better angular resolution images than has been previously achieved with conventional CCDs on ground-based telescopes. Methods. Observations were carried out using the Danish 1.54-m telescope at the ESO La Silla observatory in Chile. The telescope is equipped with an electron-multiplying CCD that enables short-exposure-time images to be obtained (ten images per second) that were stacked using the shift-and-add technique to produce the normal-exposure-time images (minutes). The high precision photometry was performed via difference image analysis employing the DanDIA pipeline. We attempted automatic detection of variable stars in the field. Results. We statistically analysed the light curves of 1405 stars in the crowded central region of NGC 6715 to automatically identify the variable stars present in this cluster. We found light curves for 17 previously known variable stars near the edges of our reference image (16 RR Lyrae and 1 semi-regular) and we discovered 67 new variables (30 RR Lyrae, 21 irregular (long-period type), 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11 unclassified). Photometric measurements for these stars are available in electronic form through the Strasbourg Astronomical Data Centre.

Additional Information

© 2016 ESO. Received: 6 May 2016. Accepted: 27 May 2016. Published online: 09 August 2016. Our thanks go to Christine Clement for clarifying the known variable star content in NGC 6715 and the numbering systems of the variable stars while we were working on these clusters. This support to the astronomical community is very much appreciated. The Danish 1.54 m telescope is operated based on a grant from the Danish Natural Science Foundation (FNU). This publication was made possible by NPRP grant # X-019-1-006 from the Qatar National Research Fund (a member of Qatar Foundation). The statements made herein are solely the responsibility of the authors. K.H. acknowledges support from STFC grant ST/M001296/1. G.D. acknowledges Regione Campania for support from POR-FSE Campania 2014-2020. D.F.E. is funded by the UK Science and Technology Facilities Council. T.H. is supported by a Sapere Aude Starting Grant from the Danish Council for Independent Research. Research at Centre for Star and Planet Formation is funded by the Danish National Research Foundation. T.C.H. acknowledges support from the Korea Research Council of Fundamental Science & Technology (KRCF) via the KRCF Young Scientist Research Fellowship. Programme and for financial support from KASI travel grant number 2013-9-400-00, 2014-1-400-06 & 2015-1-850-04. N.P. acknowledges funding by the Gemini-Conicyt Fund, allocated to project No. 32120036 and by the Portuguese FCT – Foundation for Science and Technology and the European Social Fund (ref: SFRH/BGCT/113686/2015). CITEUC is funded by National Funds through FCT – Foundation for Science and Technology (project: UID/Multi/00611/2013) and FEDER – European Regional Development Fund through COMPETE 2020 – Operational Programme Competitiveness and Internationalisation (project: POCI-01-0145-FEDER-006922). OW and J. Surdej acknowledge support from the Communauté française de Belgique – Actions de recherche concertées – Académie Wallonie-Europe. This work has made extensive use of the ADS and SIMBAD services, for which we are thankful.

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Published - aa28864-16.pdf

Submitted - 1605.06141v1.pdf

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Created:
August 20, 2023
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October 23, 2023