High-precision time-domain astrophysics in crowded star-fields with ground based telescopes : globular clusters and the mitigation of the atmospheric turbulence
We carried out a three year (2013-2015) observational campaign at the Danish 1.54-m Telescope at the ESO observatory at La Silla in Chile in which we obtained ~1000 astronomical images in the field of 11 Galactic
globular clusters. The selection of these stellar systems was focused mainly on the visibility of the targets and
their relevant physical properties available in the catalogues, among them were considered the density, variable
stars known, colour-magnitude diagrams, and luminosity.
The telescope was equipped with an electron-multiplying CCD (EMCCD) with the aim of taking very short exposure-time
images. The camera was configured to take 10 frames per second. Due to this, the brighter stars
observed were not affected by saturation, it helped to give higher signal to noise ratio to the fainter stars and, importantly,
it minimised the effects of the atmospheric turbulence such as blending between stars in the crowded
fields. To produce normal-exposure-time images (minutes) we implemented the shift-and-add technique that
also enabled us to produce images with better angular resolution than previously achieved with conventional
CCDs on ground-based telescopes, and even enabled us to produce images with angular resolution close to that
obtained with space telescopes.
The detection of the stars in each of the globular clusters and the photometry was performed via difference
image analysis by using the DanDIA pipeline whose procedures and mathematical techniques have been demonstrated
to produce high-precision time-series photometry of very crowded stellar regions.
We produced time-series photometry for ~15000 stars in the fields observed which were statistically analysed
in order to automatically extract variable stars. Our aim is to complete, or improve, the census of the
variable star population in the globular clusters. In NGC 6715, 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 long-period irregular, 3 semi-regular, 1 W Virginis, 1 eclipsing binary, and 11
unclassified). This cluster was particularly interesting because apart from the results obtained, it shows the benefits
of using the EMCCD cameras and the shift-and-add technique. It is a cluster studied several times including
data obtained with the OGLE survey and also with the Hubble Space Telescope and our discoveries were still new. Our new RR Lyrae star discoveries help confirm that NGC 6715 is of intermediate Oosterhoff type.
In the other 10 globular clusters, we obtained light curves for 31 previously known variable stars (3 L, 2
SR, 20 RR Lyrae, 1 SX Phe, 3 cataclysmic variables, 1 EW and 1 NC) and we discovered 30 new variables
(16 L, 7 SR, 4 RR Lyrae, 1 SX Phe and 2 NC). In NGC 6093, we analysed the famous case of the 1860 Nova, for which no observations of the Nova in outburst have been made until the present study.
Ephemerides and photometric measurements for the variable stars are available in electronic form through
the Strasbourg Astronomical Data Centre