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

¹ SUPA, School of Physics and Astronomy, University of St. Andrews, North Haugh, St Andrews, KY16 9SS, UK
e-mail: robertofiguera@gmail.com
² European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching bei München, Germany
³ Qatar Environment and Energy Research Institute (QEERI), HBKU, Qatar Foundation, Doha, Qatar
⁴ Centre for Electronic Imaging, Dept. of Physical Sciences, The Open University, Milton Keynes MK7 6AA, UK
⁵ Niels Bohr Institute and Centre for Star and Planet Formation, University of Copenhagen, Øster Voldgade 5, 1350 Copenhagen K, Denmark
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Dipartimento di Fisica “E. R. Caianiello”, Università di Salerno, via Giovanni Paolo II 132, 84084 Fisciano, Italy
⁸ Istituto Nazionale di Fisica Nucleare, Sezione di Napoli, 80126 Napoli, Italy
⁹ NASA Exoplanet Science Institute, MS 100-22, California Institute of Technology, Pasadena CA 91125, USA
¹⁰ Istituto Internazionale per gli Alti Studi Scientifici (IIASS), 84019 Vietri Sul Mare, Italy
¹¹ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹² Yunnan Observatories, Chinese Academy of Sciences, 650011 Kunming, PR China
¹³ Key Laboratory for the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, 650011 Kunming, PR China
¹⁴ Korea Astronomy and Space Science Institute, Daejeon 305-348, Republic of Korea
¹⁵ Finnish Centre for Astronomy with ESO (FINCA), University of Turku, Väisäläntie 20, 21500 Piikkiö, Finland
¹⁶ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, 7820436 Macul, Santiago, Chile
¹⁷ Department of Physics, Sharif University of Technology, PO Box 11155-9161 Tehran, Iran
¹⁸ Astronomisches Rechen-Institut, Zentrum für Astronomie der Universität Heidelberg, Mönchhofstr. 12-14, 69120 Heidelberg, Germany
¹⁹ Planetary and Space Sciences, Department of Physical Sciences, The Open University, Milton Keynes, MK7 6AA, UK
²⁰ Max-Planck-Institute for Solar System Research, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
²¹ Astrophysics Group, Keele University, Staffordshire, ST5 5BG, UK
²² Las Cumbres Observatory Global Telescope Network, 6740 Cortona Drive,Suite 102, Goleta, CA 93117, USA
²³ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 9c, 4000 Liège, Belgium
²⁴ Dark Cosmology Centre, Niels Bohr Institute, University of Copenhagen, Juliane Maries vej 30, 2100 Copenhagen Ø, Denmark
²⁵ Department of Astronomy, Stockholm University, AlbaNova University Center, 106 91 Stockholm, Sweden
²⁶ Meteorologisches Institut, Universität Hamburg, Bundesstraße 55, 20146 Hamburg, Germany
²⁷ Universidad de Antofagasta, Unidad de Astronomía, Facultad Cs. Básicas, Av. U. de Antofagasta, 02800 Antofagasta, Chile
²⁸ 20 CITEUC – Centre for Earth and Space Science Research of the University of Coimbra, Observatório Astronómico da Universidade de Coimbra, 3040-004 Coimbra, Portugal
²⁹ Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Oxford Road, Manchester M139 PL, UK
³⁰ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark

Received: 6 May 2016
Accepted: 27 May 2016

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.