FSR 1716: A new Milky Way Globular Cluster confirmed using VVV RR Lyrae stars
Dante Minniti, Tali Palma, Istvan Dékány, Maren Hempel, Marina Rejkuba, Joyce Pullen, Javier Alonso-García, Rodolfo Barbá, Beatriz Barbuy, Eduardo Bica, Charles Bonatto, Jura Borissova, Marcio Catelan, Julio A. Carballo-Bello, Andre Nicolas Chene, Juan J. Clariá, Roger E. Cohen, Rodrigo Contreras-Ramos, Bruno Dias, Jim Emerson, Dirk Froebrich, Anne S.M. Buckner, Douglas Geisler, Oscar A. Gonzalez, Felipe Gran, Gergely Hagdu, Mike Irwin, Valentin D. Ivanov, Radostin Kurtev, Philip W. Lucas, Daniel Majaess, Francesco Mauro, Christian Moni-Bidin, Camila Navarrete, Sebastian Ramírez Alegría, Roberto K. Saito, Elena Valenti, Manuela Zoccali
DDraft version October 16, 2018
Preprint typeset using L A TEX style AASTeX6 v. 1.0
FSR 1716: A NEW MILKY WAY GLOBULAR CLUSTER CONFIRMED USING VVV RR LYRAE STARS
Dante Minniti , Tali Palma , Istvan D´ek´any , Maren Hempel , Marina Rejkuba , Joyce Pullen , JavierAlonso-Garc´ıa , Rodolfo Barb´a , Beatriz Barbuy , Eduardo Bica , Charles Bonatto , Jura Borissova
12, 2 ,Marcio Catelan , Julio A. Carballo-Bello , Andre Nicolas Chene , Juan Jos´e Clari´a , Roger E. Cohen ,Rodrigo Contreras-Ramos , Bruno Dias , Jim Emerson , Dirk Froebrich , Anne S. M. Buckner , DouglasGeisler , Oscar A. Gonzalez , Felipe Gran , Gergely Hagdu , Mike Irwin , Valentin D. Ivanov , RadostinKurtev
12, 2 , Philip W. Lucas , Daniel Majaess , Francesco Mauro , Christian Moni-Bidin , CamilaNavarrete , Sebastian Ram´ırez Alegr´ıa , Roberto K. Saito , Elena Valenti , Manuela Zoccali Departamento de F´ısica, Facultad de Ciencias Exactas, Universidad Andr´es Bello, Av. Fernandez Concha 700, Las Condes, Santiago, Chile Instituto Milenio de Astrof´ısica, Santiago, Chile Vatican Observatory, V00120 Vatican City State, Italy Astronomisches Rechen-Institut, Zentrum fuer Astronomie der Universitaet Heidelberg, Moenchhofstr. 12-14, D-69120 Heidelberg, Germany Pontificia Universidad Cat´olica de Chile, Instituto de Astrof´ısica, Av. Vicu˜na Mackenna 4860, Santiago, Chile European Southern Observatory, Karl-Schwarszchild-Str. 2, D85748 Garching bei Muenchen, Germany Excellence Cluster Universe, Boltzmann-Str. 2, D85748 Garching bei Muenchen, Germany Unidad de Astronom´ıa, Facultad Cs. B´asicas, Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile. Departamento de F´sica y Astronom´a, Universidad de la Serena, Av. Juan Cisternas 1200 Norte, La Serena, Chile Dept. of Astronomy, University of Sao Paulo, Sao Paulo, Brazil Universidade Federal do Rio Grande do Sul, Brazil Instituto de F´ısica y Astronom´ıa, Universidad de Valpara´ıso, Av. Gran Breta˜na 1111, Playa Ancha, Casilla 5030, Valpara´ıso, Chile Gemini Observatory, Northern Operations Center, 670 North A’ohoku Place, Hilo, HI 96720, USA Observatorio Astron´omico, Universidad Nacional de C´ordoba, Laprida 854, C´ordoba, Argentina Dept. of Astronomy, University of Concepci´on, Concepci´on, Chile European Southern Observatory, Alonso de Cordova 3107, Vitacura, Santiago, Chile Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, Mile End Road, London, E1 4NS, UK Centre for Astrophysics and Planetary Science, University of Kent, Canterbury CT2 7NH, UK School of Physics and Astrophysics, University of Leeds, Leeds LS2 9JT, UK UK Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh EH9 3HJ, UK Institute of Astronomy, Cambridge University, Cambridge CB3 0HA, UK Dept. of Astronomy, University of Hertfordshire, Hertfordshire, UK Mount Saint Vincent University, Halifax, Nova Scotia, Canada Saint Mary’s University, Halifax, Nova Scotia, Canada Instituto de Astronom´ıa, Universidad Cat´olica del Norte, Av. Angamos 0610, Antofagasta, Chile Departamento de F´ısica, Universidade Federal de Santa Catarina, Trindade 88040-900, Florian´opolis, SC, Brazil
ABSTRACTWe use deep multi-epoch near-IR images of the VISTA Variables in the V´ıa L´actea (VVV) Surveyto search for RR Lyrae stars towards the Southern Galactic plane. Here we report the discovery ofa group of RR Lyrae stars close together in VVV tile d025. Inspection of the VVV images and PSFphotometry reveals that most of these stars are likely to belong to a globular cluster, that matchesthe position of the previously known star cluster FSR 1716. The stellar density map of the fieldyields a >
100 sigma detection for this candidate globular cluster, that is centered at equatorialcoordinates RA J =16:10:30.0, DEC J = − l =329.77812, b = − K s = 13 . ± .
05, and J − K s = 1 . ± .
05. We present the clusterRR Lyrae positions, magnitudes, colors, periods and amplitudes. The presence of RR Lyrae indicatesan old globular cluster, with age >
10 Gyr. We classify this object as an Oosterhoff type I globularcluster, based on the mean period of its RR Lyrae type ab, < P > = 0 .
540 days, and argue that this isa relatively metal-poor cluster with [
F e/H ] = − . ± . a r X i v : . [ a s t r o - ph . GA ] M a r this cluster are A K s = 0 . ± .
02, and E ( J − K s ) = 0 . ± .
02 mag, respectively, as measured fromthe RR Lyrae colors and the near-IR color-magnitude diagram. We also measure the cluster distanceusing the RR Lyrae type ab stars. The cluster mean distance modulus is ( m − M ) = 14 . ± . D = 7 . ± . R G = 4 . INTRODUCTIONRR Lyrae variable stars are distance indicators that can be used to detect substructures in the Milky Way halo(Baker & Wilman 2015). Indeed, they have been recently used to find streams far out in the Milky Way halo (e.g.Ivezic et al. 2004, Keller et al. 2008, Sesar et al. 2010, Drake et al. 2014, Duffau et al. 2014, Munari et al. 2014,Torrealba et al. 2015) avoiding the Galactic plane regions. Because of high extinction and stellar crowding, manyglobular clusters may remain undetected towards the Galactic plane (Ivanov et al. 2005). The VISTA Variables in theV´ıa L´actea (VVV) Survey could detect some of them as well as measuring their astrophysical parameters in the near-IR(Minniti, et al. 2010). Our previous cluster searches were based on the identification (visually or automatically) offield stellar over-densities, successfully yielding new open and globular clusters (Moni-Bidin et al. 2011, Minniti et al.2011, Borissova et al. 2011, 2014, Barb´a et al. 2015). Our new globular cluster search concentrated in the Galacticplane. The places where suspected incompleteness and where the last few globular clusters were found are deep in thebulge or far out in the halo (e.g Ivanov et al. 2005, Baker & Willman 2015). Applying the idea of Baker & Wilman(2015) to the inner Galaxy and approaching it in a complementary way, we use the RR Lyrae stars as tracers to markthe location of old and metal-poor globular clusters hidden behind regions of large extinction in the Galactic plane.Indeed, the present globular cluster search found a new globular cluster embedded in the middle of the disk. Ourdiscovery underscores the need to search also the Milky Way disk for missing globular clusters.We have used the VVV Survey data to search for RR Lyrae type ab (hereafter RRab) in extremely reddenedenvironments of the Milky Way, including the Galactic disk, bulge and center (e.g. D´ek´any et al. 2013, Gran et al.2016, Minniti et al. 2016, 2017). We have found hundreds of RRab stars located in a thin strip across the Galacticdisk (Minniti et al. 2017), at Galactic latitudes − . < b < − .
05 deg, and Galactic longitudes (295 < l < ω Cen have 0 . < ( J − K s ) < .
35 (Navarrete et al. 2015). Therefore,it can be assumed an intrinsic (unreddened) color ( J − K s ) = 0 . ± .
05 for any individual (unblended) RR Lyrae.The reddenings in the Galactic disk fields explored here range from E ( J − K s ) = 0 . . σ = 0 .
05 mag) is comparatively negligible.We then searched our RR Lyrae database with these ideas in mind. Indeed, the RR Lyrae maps of the Galactic diskshowed overdensities, some of which can be just random groupings. However, we found that a few of these groupsare real (Minniti et al. 2017). For example, there are 5 RR Lyrae stars located at the same distance centered in theVVV tile d031 at l = 339 . , b = − . l = 329 . , b = − . RR LYRAE SELECTIONThe search for RR Lyrae type ab in the disk of the Milky Way (Minniti et al. 2017) revealed a few groups ofRR Lyrae type ab located at the same distance in the same fields. The group considered here (listed in Table 1) isone of the largest over densities outside the bulge, consisting of about a dozen RR Lyrae stars, located in a smallregion near the edge of the VVV tile d025, at l = 329 . b = − . ∼
15 arcmin of the cluster center, witheight of them (5 RRab plus 3 RRc) being most likely cluster members because they are more tightly packed at theposition of the cluster. The remaining RR Lyrae are more distant in the sky, and proper motions are needed in orderto establish cluster membership. In addition, there are a few other candidate RR Lyrae variable stars in the field, forwhich additional epochs of observation are needed in order to confirm them as bonafide RR Lyrae stars.We have initially concentrated on the search for fundamental mode pulsators (RR Lyrae type ab stars) that haveasymmetric light curves, in order to avoid contamination from eclipsing binaries (Minniti et al. 2017). Furtherinspection of the VVV light curves revealed four candidate RR Lyrae type c in this region. Although these are alsolisted in Table 1, they were not used to determine the cluster parameters (reddening, distance, metallicity) because ofthe possibility of contamination for eclipsing binaries. Table 1 lists the Galactic ( l, b ) coordinates, K s -band amplitudes,periods (in days), mean near-IR magnitudes and colors, and types for the RR Lyrae sample. Figure 1 shows the lightcurves of the RR Lyrae that are well classified (listed without a question mark in Table 1). VVV IMAGES AND COLOR-MAGNITUDE DIAGRAMSA close inspection of the deep VVV images of tile d025 where the RR Lyrae group is located clearly reveals abonafide star cluster that we initially called VVV-GC05 (Figure 2). This turns out to be close to the position of thepreviously known cluster FSR 1716 (Froebrich et al. 2007). This cluster can also be seen in the GLIMPSE infraredimages (Benjamin et al. 2005). The cluster FSR 1716 was classified as an ”open-globular cluster” on the basis of NTTphotometry (Froebrich et al. 2008). This cluster was also listed by Kharchenko et al. (2013), Buckner & Froebrich(2013, 2014), arguing for an open cluster nature as well, but not much more is known about this object. Bonatto& Bica (2008) studied the color-magnitude diagram (CMD) from 2MASS, and argued that FSR 1716 is an old opencluster ( ∼ D = 0 . ± . D = 2 . ± . Av = 6 . ± . RA J = 16 : 10 : 30 . , DEC J = −
53 : 44 : 56,and Galactic coordinates l = 329 . , b = − . b/a ∼ .
7) and a total extension of a ∼
500 pix( ∼ ∼ . D = 8 kpc, consistent with thesizes of known globular clusters.The CMD centered on the new globular cluster (Figure 4) is different from that of the surrounding region. This CMDreveals a populated red giant branch (RGB), with a prominent red clump (core-He burning stars). Because the field isvery crowded and with variable reddening, we decontaminate the cluster CMD following the procedures adapted forthe VVV images as described by Palma et al. (2016). Briefly, we took a small region with radius 3.0 arcmin centredon FSR 1716, and four equivalent background area regions located 15 arcmin away from the cluster. We tried a fewbackground areas which exhibited similar apparent reddenings. The decontamination was done by eliminating thestars in the cluster CMD that appear as the closest neighbours in the background CMDs. After a few iterations, webuilt the decontaminated cluster CMD shown in the third panel of Figure 4. This CMD clearly exhibits the clusterRGB and red clump at K s = 13 . ± .
05 and J − K s = 1 . ± .
05. The luminosity function (rightmost panel ofFigure 4) also shows the red clump. However, still some contaminating stars belonging to the Galactic disk remain(blue stars located in the left region of the CMD with J − K s < . REDDENING AND EXTINCTIONThe reddening towards low latitude fields in the Galactic plane is large and non-uniform. There are previousestimates for the reddening in the field of FSR 1716, which show a significant spread, and since there was no generalagreement, the field extinction still was uncertain. Froebrich et al. (2008) obtained a reddening value E ( J − K s ) = 0 . A K s = 0 . A V = 6 . ± . A K s = 0 . A K s = 0 .
86, and 1 .
01 mag, respectively (in the UKIRT system which shouldbe similar to the VISTA K s system), for this region of tile d025. These extinctions are equivalent to A V = 7 . . J − K s = 0 . ± .
05. The mean intrinsic (unreddened) color of RR Lyraetype ab should be J − K s = 0 . ± .
05, from which we derive a cluster reddening value of E ( J − K s ) = 0 . ± . A K s = 0 .
39, using the extinction ratio A K s /E ( J − K s ) = 0 .
528 of Nishiyama etal. (2009), adopted for this work. The choice of a different extinction ratio gives a measure of the external uncertaintythat will be used in estimating the distance uncertainties in section 5. For example, Cardelli et al. (1989) gives A K s /E ( J − K s ) = 0 .
72, and Alonso-Garcia et al. (2015) give A K s /E ( J − K s ) = 0 . ± .
03, yielding A K s = 0 .
53, and0 .
33, respectively.We also used for this field the reddening maps of Irwin et al. (2016, private communication), to obtain E ( J − K s ) =0 .
75 and A Ks = 0 .
40. These maps for the VVV disk fields were made using the field red clump stars, following theprocedure of Gonzalez et al. (2012). Considering the wide range of values published in the literature, this value isconsistent with the previous determinations.In order to obtain a reddening based on the red clump, we adopted a red clump mean intrinsic color ( J − K s ) =0 . ± .
01, following Alves et al. (2002), Pietrzynski et al. (2002), Grocholski & Sarajedini (2002), and Minniti etal. (2011). The observed mean red clump color is J − K s = 1 . ± .
05 (Figure 4), yielding E ( J − K s ) = 0 .
71, and A K s = 0 .
38 mag, in excellent agreement with the RR Lyrae determination.We adopted the mean reddening and extinction values E ( J − K s ) = 0 .
72 and A K s = 0 .
38 mag, determined fromthe position of the red clump in the CMD, and from the RR Lyrae type ab stars. This is equivalent to A V = 3 . DISTANCE, METALLICITY AND AGEWe can perform two independent distance measurements for this new globular cluster using: (i) RR Lyrae stars,(ii) the clump giants, expecting the first method to be more accurate. We used the Period-Luminosity relation forGalactic RR Lyrae type ab: M K s = − . × log ( P ) − .
95 from Muraveva et al. (2015) to compute individual distances.Adopting A K s = 0 .
39, the mean distance modulus for the 8 RRab from Table 1 is ( m − M ) = 14 .
40, equivalent to D = 7 . ± . D = 7 . ± . M K s = − . ± .
03 and J − K s = 0 . ± .
01, following Alves et al. (2002), Pietrzynski et al. (2002), Grochol-ski & Sarajedini 2002, and Minniti et al. (2011). Figure 4 shows that the globular cluster RGB is well defined.We are able to measure the location of the red clump from the statistically decontaminated CMD of Figure 4 at K s = 13 . ± .
05 and J − K s = 1 . ± .
05. The red clump gives E ( J − K s ) = 0 .
70 and A K s = 0 .
38 mag from above.This extinction yields a clump giant distance modulus ( m − M ) = 14 .
62, equivalent to a distance of D = 8 . ± . A K s = 0 .
40 (from the reddening maps), the distance modulus should be: ( m − M ) = 14 . K -band luminosity function, one at K = 13 . K = 13 . D = 7 kpc for an age of ∼ D = 5 kpc, if the age is >
10 Gyr (for their adopted extinction A K = 0 . D = 7 . ± . R G = 4 . < P > = 0 .
607 days, we classified FSR 1716 as anOosterhoff type I globular cluster (even though it is at the edge of the Oosterhoff intermediate clusters, Catelan 2004).We applied the period–amplitude–metallicity relations of Alcock et al. (2000), Yang et al. (2010), and Feast et al.(2010) to find a mean metallicity [
F e/H ] = − . ± . F e/H ] = − . ± . K s vs J − K s plane. It is possible to measure the photometric indices along the RGB in the [ M K s , ( J − K s ) ]absolute plane, namely the magnitude at fixed color, the colors at fixed magnitudes and the slope of the RGB. Acomparison of the unreddened RGB of FSR 1716 with these fiducial globular cluster RGBs reveals that our clustershould be relatively metal-poor. Adopting a mean reddening E ( J − K s ) = 0 .
72, the RGB fits well the fiducial sequencesfor the globular clusters ω Cen at [
F e/H ] ∼ − . F e/H ] ∼ − . F e/H ] = − . − . F e/H ] = − . ± . V ∼
18 mag.The cluster age would have been difficult to constrain, if it had not contained RR Lyrae stars. The presence of thesevariables indicates that this is an old ( >
10 Gyr) globular cluster, ruling out a young or intermediate-age star cluster.As mentioned before, the CMD is well fit by the fiducial line of the globular cluster ω Cen. We have also fitted 10 Gyrold isochrones from Bressan et al. (2012) for the appropriate metallicity (see Figure 4). However, the globular clusterturn-off region is just beyond the limit of the photometry, and the isochrones can only be used to rule out younger ages. CONCLUSIONSWe have discovered a compact group of RR Lyrae type ab stars towards the Milky Way southern plane, located ata common distance in the direction of VVV tile d025. These stars are centered at l = 329 . , b = − . RA J = 16 : 10 : 30 . , DEC J = −
53 : 44 : 56, and appear to belong to the cluster FSR 1716, which is thereforeidentified as a new Galactic globular cluster. The CMD is consistent with that of a typical globular cluster, and thered clump is clearly seen in the cluster RGB at K s = 13 . ± .
05 and J − K s = 1 . ± .
05. We found 8 type aband 4 type c RR Lyrae in total within the field of this new globular cluster. We present accurate positions, near-IRmagnitudes, colors, periods and amplitudes for these stars. The high quality of the VVV Survey near-IR photometryallow us to measure the parameters for this cluster, like reddening, distance, metallicity and age.We estimated the mean reddening ( E ( J − K s ) = 0 .
74 mag) and distance ( D = 7 . ± . > F e/H ] = − . ± . REFERENCES
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Table 1 . Photometric Observations of RR Lyrae in the field of FSR 1716
Tile-Identification Gal. longitude Gal. latitude Amplitude a Period (d) a K sa < J − K s > a < H − K s > Type b d025-0024383 329.7272551 -1.595908408 0.29 0.561578 14.497 1.026 0.346 RRabd025-0049819 329.7627501 -1.623391291 0.25 0.678234 14.326 0.928 0.307 RRabd025-0064017 329.7812822 -1.562397002 0.34 0.602790 14.432 0.998 0.329 RRabd025-0082743 329.8081587 -1.584147587 0.37 0.504263 14.634 0.890 0.250 RRabd025-0094906 329.8245256 -1.582463034 0.29 0.688511 14.243 1.059 0.334 RRabd025-0114911 329.8517936 -1.413703166 0.22 0.409830 14.802 0.929 0.222 RRab?d025-0157039 329.9120381 -1.376730367 0.40 0.400766 14.956 1.085 0.359 RRabd025-0332556 330.1635175 -1.600583516 0.39 0.471127 14.666 0.893 0.313 RRabd025-0047454 329.75932157 -1.59434742 0.37 0.367261 14.678 0.777 0.171 RRcd025-0065945 329.78466411 -1.58232102 0.31 0.345701 14.692 0.702 0.145 RRc?d025-0083265 329.80844225 -1.60445726 0.28 0.330532 14.699 0.888 0.284 RRcd025-0175388 329.93871016 -1.65044743 0.23 0.379827 14.664 0.651 0.193 RRc? a Typical photometric errors are σ K s = 0 .
01 mag, and σ J,H = 0 .
03 mag. Periods are accurate to 10 − days, and K s -band amplitudeerrors are of the order σ A = 0 .
02 mag. b Variables with uncertain classification are labelled with a question mark, and their light curves are omitted from Figure 1. K s [ m ag .] phase phase phase K s [ m ag .] K s [ m ag .] Figure 1 . Phased light curves for the candidate RR Lyrae type ab of the new globular cluster FSR 1716 that areclassified as certain RRab or RRc. The more dubious variables listed with a question mark in Table 1 are not shown.
Figure 2 . Illustration of the cluster stellar field. Deep Ks-band image of the field of the new globular cluster FSR 1716located at the far right in order to illustrate the field density. This image covers 13 × of VVV tile d025 andis oriented along Galactic coordinates l, b . Figure 3 . Density map of the VVV-GC005 surrounding region. There is a clear maximum of stars at pixels (4200,1000), marking the position of FSR 1716 (white area). The significance scale on the right illustrates that the cluster’ssignificance above the background is > Figure 4 . First panel: VVV PSF near-IR CMD for a 3 arcmin field centered on FSR1716. The position of the globularcluster red clump is marked at K s = 13 .
35 and J − K s = 1 .
31. The location of the globular cluster RR Lyrae type abfound here is indicated with large black crosses. Second panel: CMD of the surrounding comparison field. The mainsequence of the Galactic disk is seen at J − K s ∼ .
5. Third panel: Decontaminated VVV PSF near-IR CMD forVVV-GC005. The points are the globular cluster stars within 3 arcmin of the cluster center, while the number of fieldstars has been minimized by the statistical decontamination procedure. The globular cluster RGB is well populatedand the red clump is well defined. The main sequence turn off is located just below the faintest magnitudes. A 10 Gyrisochrone for [
F e/H ] = − . J − K s < ..