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Estimating the Parameters of Globular Cluster M 30 (NGC 7099) from Time-Series Photometry

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Estimating the Parameters of Globular Cluster M 30 (NGC 7099) from Time-Series Photometry Astronomy & Astrophysics manuscript no. ngc7099 c ESO 2018 September 23, 2018 Estimating the parameters of globular cluster M 30 (NGC 7099) from time-series photometry⋆,⋆⋆ N. Kains1, D. M. Bramich1, A. Arellano Ferro2, R. Figuera Jaimes1,3, U. G. Jørgensen4,5, S. Giridhar6 and K.A. Alsubai7, J. M. Andersen8,5, V. Bozza9,10, P. Browne3, M. Burgdorf11, S. Calchi Novati9,12, Y. Damerdji13, C. Diehl14,15, P. Dodds3, M. Dominik3,⋆⋆⋆, A. Elyiv13,16, X.-S. Fang17,18, E. Giannini14, S.-H. Gu17,18, S. Hardis4, K. Harpsøe4,5, T.C. Hinse19,4, A. Hornstrup20, M. Hundertmark3, J. Jessen-Hansen21, D. Juncher4,5, E. Kerins22, H. Kjeldsen21, H. Korhonen4,5, C. Liebig3, M. N. Lund21, M. Lundkvist21, L. Mancini23, R. Martin24, M. Mathiasen4, M.T. Penny25, M. Rabus26, S. Rahvar27,28, D. Ricci29,13, K. Sahu30, G. Scarpetta9,31, J. Skottfelt4,5, C. Snodgrass32, J. Southworth33, J. Surdej13, J. Tregloan-Reed33, C. Vilela33, O. Wertz13, and A. Williams24 (The MiNDSTEp consortium) (Affiliations can be found after the references) Received ... ; accepted ... Abstract Aims. We present the analysis of 26 nights of V and I time-series observations from 2011 and 2012 of the globular cluster M 30 (NGC 7099). We used our data to search for variable stars in this cluster and refine the periods of known variables; we then used our variable star light curves to derive values for the cluster’s parameters. Methods. We used difference image analysis to reduce our data to obtain high-precision light curves of variable stars. We then estimated the cluster parameters by performing a Fourier decomposition of the light curves of RR Lyrae stars for which a good period estimate was possible. We also derive an estimate for the age of the cluster by fitting theoretical isochrones to our colour- magnitude diagram (CMD). Results. Out of 13 stars previously catalogued as variables, we find that only 4 are bona fide variables. We detect two new RR Lyrae variables, and confirm two additional RR Lyrae candidates from the literature. We also detect four other new variables, including an eclipsing blue straggler system, and an SX Phoenicis star. This amounts to a total number of confirmed variable stars in M 30 of 12. We perform Fourier decomposition of the light curves of the RR Lyrae stars to derive cluster parameters using empirical relations. We find a cluster metallicity [Fe/H]ZW = −2.01 ± 0.04, or [Fe/H]UVES = −2.11 ± 0.06, and a distance of 8.32 ± 0.20 kpc (using RR0 variables), 8.10 kpc (using one RR1 variable), and 8.35 ± 0.42 kpc (using our SX Phoenicis star detection in M 30). Fitting isochrones to the CMD, we estimate an age of 13.0 ± 1.0 Gyr for M 30. Key words. globular clusters – RR Lyrae – variable stars 1. Introduction (Forbes & Bridges 2010). M 30 is also thought to have un- dergone core collapse (e.g. Djorgovski & King 1986). Here In this paper we analyse time-series observations of M we detect and classify already reported, as well as new vari- 30 (NGC 7099, or C2137-234 in the IAU nomenclature; h m s ◦ ′ ′′ ables, and use Fourier decomposition to derive properties α = 21 40 22 ,δ = −23 10 47.5 at J2000.0), one of the of the RR Lyrae stars in this cluster. We then estimate the most metal-poor globular clusters known, with [Fe/H] ∼ - arXiv:1305.3606v1 [astro-ph.SR] 15 May 2013 cluster parameters of M 30, providing us with an indepen- 2.1, located at a distance of ∼ 8 kpc. This cluster is thought dent estimate of the metallicity, distance and age of this to be of extra-Galactic origin, due to its retrograde or- cluster, and the first using this method. bit (Allen et al. 2006), pointing to its possible accretion by the Milky Way following an encounter with a satel- lite, as well as its position on the age-metallicity diagram ⋆ The full light curves presented in this paper (Table 2) are only available in electronic form at the Centre de donn´ees astronomiques de Strasbourg (CDS) via anony- Our observations are detailed in Sec. 2; we discuss the mous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via variables in M 30 in Sec. 3, and in Sec. 4 we derive prop- http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/ erties of the RR Lyrae stars in this cluster using Fourier ⋆⋆ This work is based on data collected by MiNDSTEp with decomposition and empirical relations. We use this to de- the Danish 1.54m telescope at the ESO La Silla Observatory. rive properties of the cluster in Sec. 5 and summarise our ⋆⋆⋆ Royal Society University Research Fellow findings. 1 N. Kains et al.: Estimating the parameters of globular cluster M 30 (NGC 7099) from time-series photometry Date NV tV (s) NI tI (s) After preprocessing (bias level correction and flat- − − 20110728 48 10 fielding), our images were blurred with a Gaussian of full- 20110730 − − 27 15-20 − − width half-maximum (FWHM) of 3 pixels to avoid under- 20110803 12 30 sampling, which is detrimental for determining the kernel 20110804 − − 4 30 in DIA. We then produce a reference image for each filter, 20110806 6 30 14 30 ′′ 20110809 − − 16 30-40 stacking images within 10% of the best seeing (∼ 1.2 ), also 20110810 1 30 12 30 taking care to avoid including images with many saturated 20110811 − − 12 30 stars. Our reference image consists of 5 stacked images, 20110813 6 40 6 30 with a combined exposure time of 100 s, and a FWHM of 20110814 − − 10 27 the point-spread function (PSF) of 3.88 pixels (∼ 1.5′′) in 20110815 − − 6 27 V , while in I the combined exposure time is 80 s from 4 − − ′′ 20110817 6 30 images, and the PSF FWHM is 3.31 pixels (∼ 1.3 ). For 20110820 − − 16 30 − − each filter, the source positions and reference fluxes were 20110821 16 30 then extracted from the reference image. Images were then 20110822 − − 7 30 20110828 − − 8 30 registered with the reference, and the reference was con- 20110830 6 20 11 30 volved with the kernel solution and subtracted from each 20110831 − − 16 30 image, producing a set of difference images. Finally, dif- 20120807 37 10-20 36 10-20 ference fluxes were measured from each difference image 20120810 38 20 38 20 for each source, which we used to construct a set of light 20120811 34 20 41 20 curves. In Table 2, we outline the format of the data as it 20120819 6 100 4 100 is provided in electronic format with the online version of 20120820 24 100 22 100 this paper. 20120821 38 70-100 38 70-100 As was noted in previous papers, blending due to the 20120828 65 50-100 65 40-50 20120919 40 50 39 40 presence of other objects near a source may lead to the Total 301 530 reference flux being overestimated. However, non-variable sources are still fully subtracted in the difference images, Table 1. Numbers of images and exposure times for the V while variable objects have the amplitude of their vari- and I band observations of M 30. When varying exposure ability underestimated if their reference flux is affected by times were used, a range is given. blending, although the shape of their light curve, crucially, will remain unaffected. 2. Observations and reductions 2.3. Photometric calibration 2.1. Observations 2.3.1. Self-calibration We obtained Johnson V - and I-band data with the Danish Although all photometry is affected by systematic errors, Faint Object Spectrograph and Camera (DFOSC) imaging steps can be taken to correct for this, in particular with camera at the Danish 1.54m telescope at La Silla in Chile, time-series photometry, for which we can carry out a self- as part of a programme with the MiNDSTEp consortium. calibration of the data (e.g. Padmanabhan et al. 2008). To In this programme we are making use of parts of the night do this, we use the method of Bramich & Freudling (2012) when the MiNDSTEp primary-science microlensing obser- to derive magnitude offsets to be applied to each photo- vations towards the Galactic Bulge are not possible. The metric measurement. In practice, this equates to correcting data were obtained in 2011 and 2012, with the 2011 data for errors in the fitted value of the photometric scale fac- consisting mostly of I-band observations, while the 2012 ob- tor p. We find magnitude offsets of the order of 2-3 mmag servations are evenly split between V and I-band images. for most data points, which leads to small difference in the The observations are summarised in Table 1. resulting light curves. The imaging CCD is 2147 × 2101 pixel, with a scale of 0.396 arcsec per pixel, giving a field of view of 14.2 × 13.9 2.3.2. Photometric standards arcmin2. We converted the instrumental magnitudes from our data reduction pipeline to standard Johnson-Kron-Cousins mag- 2.2. Difference image analysis nitudes by carrying out a linear fit to the relation between As in our previous studies of variables in globular clusters our light curve magnitudes and those of the photometric (Figuera et al. 2013, Kains et al. 2012; Arellano Ferro et al. standards of Stetson (2000) in the field of this cluster. The 2011), we used the DanDIA1 pipeline (Bramich et al. 2013; relations, shown in Fig. 1, were used to obtain light curves Bramich 2008) to reduce our observations.
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