Completing the Census of (Bright) Variable Stars in Galactic Globular Clusters
a r X i v :a s t r o -p h /0509631v 1 21 S e p 2005
Mem.S.A.It.V ol.75,282
c
SAIt 2004Memorie
della
M.Catelan,1H.A.Smith,2B.J.Pritzl,3J.Borissova,4C.Cacciari,5R.Contreras,1T.M.Corwin,6N.De Lee,2M.E.Escobar,https://www.wendangku.net/doc/b610178089.html,yden,7C.Navarro,1G.Prieto,1R.
Salinas,1,8P.B.Stetson,9A.V .Sweigart,10E.Vidal,1M.Zoccali,1and M.Zorotovic 1
1
Ponti?cia Universidad Cat′o lica de Chile,Departamento de Astronom′?a y Astrof′?sica,Av.Vicu?n a Mackenna 4860,782-0436Macul,Santiago,Chile;e-mail:mcatelan@astro.puc.cl
2Dept.of Physics and Astronomy,Michigan State Univ.,East Lansing,MI 48824,USA 3Macalester College,1600Grand Avenue,Saint Paul,MN 55105,USA 4European Southern Observatory,Av.Alonso de C′o rdova 3107,763-0581Vitacura,Santiago,Chile
5INAF –Osservatorio Astronomico di Bologna,via Ranzani 1,I-40127Bologna,Italy 6Dept.of Physics,University of North Carolina at Charlotte,Charlotte,NC 28223,USA 7Dept.of Physics and Astronomy,104Overman Hall,Bowling Green State University,Bowling Green,OH 43403,USA 8Grupo de Astronom′?a,Facultad de Ciencias F′?sicas y Matem′a ticas,Universidad de Concepci′o n,Concepci′o n,Chile
9Dominion Astrophysical Observatory,Herzberg Institute of Astrophysics,National Research Council,5071West Saanich Road,Victoria,BC V9E 2E7,Canada
10
NASA Goddard Space Flight Center,Exploration of the Universe Division,Code 667,Greenbelt,MD 20771,USA
Abstract.We present a long-term project aimed at completing the census of (bright)vari-
able stars in Galactic globular clusters.While our main aim is to obtain a reliable assessment of the populations of RR Lyrae and type II Cepheid stars in the Galactic globular cluster system,due attention is also being paid to other types of variables,including SX Phoenicis stars,long-period variables,and eclipsing binaries.
Key words.Stars:Population II –Galaxy:globular clusters –stars:variables:RR Lyr
1.Introduction
Variable stars remain one of the most impor-tant types of object in astronomy.They are widely used in a variety of di ?erent areas,from tests of stellar structure and evolution theory to
Catelan et al.:Variable Stars in Globular Clusters283
same distance from us,with a common age, and(nearly)the same chemical composition.
By the early-1990’s,it was widely per-ceived that“most variables that are in[globu-lar]clusters have by now become discovered”(Suntze?et al.1991).In the Suntze?et al. compilation,only a few“notable excep-tions”were mentioned.More speci?cally, Suntze?et al.(1991)estimate that“only6% of the cluster[RR Lyrae]variables remain to be discovered.”Has this assessment withstood the test of time?
Unfortunately,not quite:some15years later,it is now clear that not only do sev-eral of the“notable exceptions”mentioned by Suntze?et al.(1991)remain poorly stud-ied in terms of variability,but also,and im-portantly,many—if not all—globular clusters whose variable star populations were consid-ered exhaustively known by the early1990’s are now known to be a?ected by severe incom-pleteness in their reported numbers of even the brighter variable stars(such as the RR Lyrae).
Consider,as an example,the case of M3 (NGC5272),?rst studied by Bailey(1913), and subsequently investigated in detail by Carretta et al.(1998),Kaluzny et al.(1998), Corwin&Carney(2001),and Strader et al. (2002),among others.According to the com-pilation of variable stars in globular clusters by Clement et al.(2001),by the end of the 1990’s M3had a total of182catalogued RR Lyrae variables.By the year2004,about230 RR Lyrae variables were already known in the cluster(Bakos et al.2000;Clementini et al. 2004)—thus representing an increment of26% with respect to the Clement et https://www.wendangku.net/doc/b610178089.html,pilation.
There are two main reasons for the some-what unexpected increase in the numbers of (bright)variable stars in globular clusters. First,the pre-1990’s studies were primarily based on photographic photometry,which in many cases appears not to have been precise enough to tell small-amplitude variables from noise.Second,it was only very recently that an image-subtraction technique was developed which is capable of quickly,e?ciently and au-tomatically subtracting out the constant infor-mation from one CCD frame of a cluster to the next,also matching the di?erences in see-ing between the frames in the process.This re-markable achievement is due primarily to tools created by Alard&Lupton(1998)and Alard (2000),which have become accessible to the astronomical community in the form of the ISIS image-subtraction program.
While very e?cient in detecting variable stars in crowded?elds,ISIS presents the draw-back of providing light curves in?ux values relative to a reference image only.For this reason,ISIS does not provide light curves in standard magnitudes,and the reference im-age has to be processed independently for this purpose.While DAOPHOT/ALLFRAME (Stetson1994)o?er excellent tools to perform absolute photometry in the crowded regions found in globular clusters,it is still often the case that the variable stars cannot have their ab-solute?uxes reliably measured in the ISIS ref-erence image,making it very di?cult to con-vert the ISIS relative-?ux light curves into cali-brated ones without additional images obtained with higher spatial resolution.
In the present paper,we present our long-term project to complete the census of(bright) variable stars in Galactic globular clusters.The tools adopted to analyze our time-series pho-tometry are ISIS(detection,light curves in relative?uxes for period and variability type determination)and DAOPHOT/ALLFRAME (light curves in standard magnitudes,position-ing of the variables in the color-magnitude di-agram,determination of variability types,and determination of physical properties on the ba-sis of the Fourier decomposition parameters).
2.Observational Data
The data upon which this project is based comes primarily from small(i.e.,2m class or less)telescopes,such as the Danish1.54m,the CTIO0.9m and1.3m,the LCO Warsaw1.3m, and the Rozhen2m telescopes.Data available in public archives are also being used.The ob-servational data,generally consisting of sets of B,V,and(less often)I images,are usu-ally collected during observing runs split in blocks of several days,spread over several months,which helps ensure proper coverage of the periods of both RR Lyrae stars and type II
284Catelan et al.:Variable Stars in Globular
Clusters
Fig.1.Light curve in B for a newly discovered RRab Lyrae variable in
M69.
Fig.2.Light curve in B for a newly discovered RRc Lyrae variable in NGC 5286.
Cepheids.In the case of the CTIO data,most of our data has been obtained in service mode,with one or more datapoints being collected per night over a period of several months.
In what follows,we describe some of the latest results of this project,which have not yet appeared in the refereed literature.
3.Some Recent Results
3.1.The Metal-Rich Globular Cluster
M69(NGC 6637)
This project is based on data collected at LCO with the Warsaw 1.3m telescope.In Figure 1is shown a light curve of an RR Lyrae star that
P = 59.862 d
P = 59.878 d
0.0
0.5
1.0
1.5
2.0
I S I S R e l a t i v e F l u x (B )
Phase
Fig.3.Light curves for two newly discovered type II Cepheids in M28.
P = 0.0598 d
I S I S R e l a t i v e F l u x (B )
0.00.5 1.0 1.5 2.0
Phase
Fig.4.Light curves for a newly discovered SX Phe candidate in NGC 2808.
was discovered in the ?eld of the cluster in the course of our study.Its period is relatively long for a metal-rich globular cluster,thus bringing to mind the cases of V9in 47Tucanae and the many long-period RR Lyrae variables in NGC 6388and NGC 6441.While membership status is unclear at present,the star does ap-pear to lie su ?ciently close to the cluster’s hor-izontal branch (HB)in the color-magnitude di-agram as to merit further analysis of this possi-bility.De Lee et al.(these proceedings)discuss the possibility that NGC 6304may provide yet
Catelan et al.:Variable Stars in Globular Clusters285
another example of a metal-rich globular clus-ter with long-period RR Lyrae variables.
3.2.NGC5286:an RR Lyrae-Rich
Globular in the Canis Major Dwarf
Spheroidal Galaxy(CMa dSph)? This project is also based on data collected at the LCO with the Warsaw 1.3m tele-scope.NGC5286is a particularly interest-ing cluster due to its suggested association with the CMa dSph(Frinchaboy et al.2004; Martin et al.2004).We have found a rich har-vest of RR Lyrae stars in this cluster,increasing quite substantially the number of known vari-ables.Figure2shows a light curve for a newly discovered RRc star.
3.3.Type II Cepheids in the Blue HB
Globular Cluster M28(NGC6626) The data used in this project were collected us-ing the CTIO1.3m telescope in service mode.
A fairly large number of variable stars was dis-covered,including short-period(P<0.3d) RRc variables;long-period(P>0.7d)RRab variables;type II Cepheids(Fig.3),including four stars with periods very close to60d;and long-period or semi-regular variables with pe-riods longer than50d.
3.4.SX Phoenicis Variables in
NGC2808
Until recently,NGC2808had been though to be essentially devoid of RR Lyrae variables. In the course of the present project,however, we were able to discover a sizeable number of RR Lyrae stars in the cluster,as described in Corwin et al.(2004).Recently,we have also found that the cluster contains a signi?cant number of SX Phe variables,and a sample light curve is provided in Figure4.
4.Conclusions
This project is still in its beginning,and it is already clear that there is a long road before we can consider the variable star populations in globular clusters as well known.In addi-tion,while our attention(and observing strate-gies)have mainly focused on the brighter RR Lyrae and type II Cepheid variables,it is be-coming increasingly clear that there is also a rich harvest of SX Phe and eclipsing variable stars in globular clusters waiting to be found. It is our hope that the astronomical commu-nity will continue to support the operation of 1m-class telescopes throughout the world,so that the study of variable stars—upon which so much of modern astrophysics relies—can truly become a well-established enterprise. Acknowledgements.Support for M.C.,M.E.E., R.C.,C.N.,G.P.,and R.S.was provided by Fondecyt #1030954;for H.A.S.,by NSF grant AST-0205813; and for B.J.P.,by CAREER award AST99-84073. References
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