The Optical Gravitational Lensing Experiment. The OGLE-III Catalog of Variable Stars. XI. RR Lyrae Stars in the Galactic Bulge
I. Soszynski, W.A. Dziembowski, A. Udalski, R. Poleski, M.K. Szymanski, M. Kubiak, G. Pietrzynski, L. Wyrzykowski, K. Ulaczyk, S. Kozlowski, P. Pietrukowicz
aa r X i v : . [ a s t r o - ph . S R ] M a y ACTA ASTRONOMICA
Vol. (2011) pp. 1–23 The Optical Gravitational Lensing Experiment.The OGLE-III Catalog of Variable Stars.XI. RR Lyrae Stars in the Galactic Bulge ∗ I. S o s z y ´n s k i , W. A. D z i e m b o w s k i , A. U d a l s k i , R. P o l e s k i ,M. K. S z y m a ´n s k i , M. K u b i a k , G. P i e t r z y ´n s k i , ,Ł. W y r z y k o w s k i , , K. U l a c z y k , S. K o z ł o w s k i and P. P i e t r u k o w i c z Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Polande-mail:(soszynsk,wd,udalski,rpoleski,msz,mk,pietrzyn,kulaczyk,simkoz,pietruk)@astrouw.edu.pl Universidad de Concepción, Departamento de Astronomia, Casilla 160–C, Concepción,Chile Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB30HA, UKe-mail: [email protected]
Received March 18, 2011
ABSTRACTThe eleventh part of the OGLE-III Catalog of Variable Stars (OIII-CVS) contains 16 836 RR Lyrstars detected in the OGLE fields toward the Galactic bulge. The total sample is composed of 11 756RR Lyr stars pulsating in the fundamental mode (RRab), 4989 overtone pulsators (RRc), and 91double-mode (RRd) stars. About 400 RR Lyr stars are members of the Sagittarius Dwarf SpheroidalGalaxy. The catalog includes the time-series photometry collected in the course of the OGLE survey,basic parameters of the stars, finding charts, and cross-identifications with other catalogs of RR Lyrstars toward the Milky Way center.We notice that some RRd stars in the Galactic bulge show unusually short periods and smallratio of periods, down to P F ≈ .
35 days and P / P F ≈ . ≈ − . Key words:
Stars: variables: RR Lyrae – Stars: oscillations (including pulsations) – Stars: Popu-lation II – Galaxy: center – Galaxies: individual: Sagittarius Dwarf Spheroidal Galaxy ∗ Based on observations obtained with the 1.3-m Warsaw telescope at the Las Campanas Observa-tory of the Carnegie Institution of Washington.
A. A.1. Introduction
RR Lyrae stars are of particular interest to astronomers for several reasons.First, they are useful indicators of old, metal-poor population of stars. Second, theyare standard candles, enabling an estimate of their distances to be made. Third,they are very numerous and bright enough that they can be easily observed in ourand nearby galaxies. Thus, RR Lyr stars play an essential role in our understandingof the formation and evolution of the Galaxy, as well as the internal constitutionand evolution of stars. RR Lyr stars in the Galactic bulge are an important sourceof information on the distance to the center of the Milky Way, the geometry of thebar and the bulge and the interstellar extinction in these regions. The properties ofthese stars give us an insight into the earliest history of our Galaxy.First significant sample of RR Lyr stars close to the central regions of the MilkyWay was discovered by van Gent (1932, 1933). He noticed that cluster type vari-ables (the historical name of RR Lyr stars) strongly concentrate toward the Galaxycenter. The fields located closer to the Galactic center were observed under asurvey conducted by the Harvard Observatory ( e.g. , Swope 1936, 1938). Baade(1946, 1951) observed the relatively unobscured area today called Baade’s Win-dow † which yielded over 100 newly identified RR Lyr stars (Gaposchkin 1956).Each of the many other efforts to detect variable stars in the Galactic bulge( e.g. , Plaut 1948, 1973, Fokker 1951, Oosterhoff and Horikx 1952, Oosterhoff etal. et al. et al. (1994, 1995ab,1996, 1997) published a catalog of over 3000 periodic variable stars in the Galac-tic bulge detected in the fields covered by the first phase of the Optical Gravita-tional Lensing Experiment (OGLE-I). In total, 215 of these stars were classified asRR Lyr variables. The next stage of the OGLE project (OGLE-II) resulted in muchlarger samples of RR Lyr stars in the central regions of the Milky Way. Mizerski(2003) detected and analyzed over 2700 RR Lyr stars in the bulge. He noticed veryhigh incident rate of Blazhko stars, and very low percentage of RRd stars. TheOGLE-II data were also used by Collinge et al. (2006) to prepare a catalog of 1888fundamental-mode RR Lyr stars (RRab).Also the MACHO microlensing project observed a numerous sample of RR Lyrstars toward the Galactic center (Alcock et al. et al. (2008) onthe basis of the MACHO database. Their sample contains 3525 RR Lyr stars of abtype. † It is interesting to note that Baade called this region “van Tulder’s pole”. ol. 61 et al. et al.
2. Observational Data
Our observations of the Galactic bulge were obtained at Las Campanas Ob-servatory with the 1.3-m Warsaw telescope. The observatory is operated by theCarnegie Institution of Washington. During the OGLE-III project (2001–2009),the Warsaw telescope was equipped with an eight-chip mosaic camera covering ap-proximately 35 ×
35 arcmin in the sky with the scale of 0.26 arcsec/pixel. Detailsof the instrumentation setup can be found in the paper by Udalski (2003).The time coverage, as well as a number of points obtained by the OGLE projectin the bulge varies considerably from field to field. Some fields have been moni-tored since 1992 and for these fields up to several thousand points per star havebeen collected until now. Other fields were observed for only one or two seasonsand only several dozen observations were collected for them. In this study we usedonly those OGLE-II and OGLE-III fields for which at least 30 epochs were gath-ered. These fields range in the Galactic coordinates within approximately | l | < ◦ and | b | < ◦ and cover an area of 68.7 square degrees.Observations were obtained through the I and V filters closely resembling theJohnson-Cousins system. The accuracy of the transformations from the instrumen-tal to the standard magnitudes is better than 0.02 mag (Udalski et al. I -band filter, while in the V -band we obtained from a fewto several dozen points.The time-series photometry attached to this catalog was compiled from theOGLE-II and OGLE-III observations, so it covers up to 13 years (from March 1997to May 2009). For individual stars both datasets were tied by shifting the OGLE-IIphotometry to agree with the OGLE-III light curves. For 279 RR Lyr stars exclu- A. A. sively the OGLE-II photometry is available. We also combined the photometry ofstars observed in the overlapping regions of two or more adjacent fields.The photometry was obtained with the standard OGLE data reduction pipeline(Udalski et al. I -band DIAphotometry in the OGLE database, due to saturation or location close to other brightstars. For these objects we provide the photometry measured with the D O P HOT package (Schechter et al.
3. Selection and Classification of RR Lyr Stars
A massive periodicity search was performed for all 3 × stars monitoredby the OGLE-III survey in the Galactic bulge. To perform this time-consumingtask, we used supercomputers assembled at the Interdisciplinary Centre for Math-ematical and Computational Modelling (ICM) of the University of Warsaw. Theperiod-search code F NPEAKS (by Z. Kołaczkowski – private communication) wasrun on each I -band light curve with more than 30 points. Ten the highest peaksin the periodogram were selected and archived with the corresponding amplitudesand signal-to-noise ratios. Then, each light curve was prewhitened with the primaryperiod and the procedure of the period search was repeated on the residual data.Before we began selection and classification of variable stars, all light curveswere fitted with a series of Fourier cosine functions, and the Fourier coefficients R , f , R , f (Simon and Lee 1981) were calculated. We used the positionsof stars in the period–Fourier coefficient planes (Fig. 1) to provisionally dividethe sample into pulsating and other stars. However, the main selection procedurewas based on the visual inspection of the light curves. We inspected all stars withperiods between 0.2 and 1.0 day and amplitudes larger than a limit that dependedon the average brightness of the star. For the brightest objects the amplitude limitreached 0.01 mag, which allowed us to select a number of RR Lyr variables blendedby other stars.The selection and classification of variable stars based primarily on the mor-phology of light curves. Short-period variable stars were divided into two groups:pulsating stars and much more numerous group of eclipsing and ellipsoidal bina-ries, which will be published in a future part of the OIII-CVS. The vast majorityof pulsating stars were categorized as RR Lyr stars, only a small fraction was clas-sified as Cepheids and d Sct stars due to their characteristic light curve shapes orperiod ratios in double-mode pulsators. Note, that our catalog may still contain asmall fraction of d Sct stars, which are difficult to distinguish from short-periodRR Lyr variables, when their absolute magnitudes are not a priori known.It was relatively easy to discriminate RRab stars from overtone pulsators andother types of variable stars, since fundamental-mode RR Lyr stars have character- ol. 61 Fig. 1. Parameters R , f , R and f of the Fourier light curve decomposition (Simon and Lee1981) plotted against the logarithm of periods for RR Lyr stars from our catalog. Blue dots representRRab variables, red are RRc stars while green dots show the first overtone mode of RRd stars. istic, asymmetric light curves. The correct classification was more problematic inthe case of the overtone pulsators (RRc stars), as their light curves are much moresinusoidal and may be confused with W UMa, ellipsoidal, rotating, etc. variablestars. In this catalog we classified as RRc stars only those objects, which reveal de-tectable asymmetry of their light curves. This affects the completeness of the RRclist among the fainter stars. In difficult cases we took into account the position ofa star in the period–Fourier coefficients (Fig. 1), period–amplitude (Fig. 2), color–magnitude diagrams (Fig. 3), and a ratio of amplitudes in the V- and I -bands (whenthe number of observing points in the V band was high enough to determine theamplitude in this band). However, the classification of about one hundred objectsin our catalog remains uncertain. Information about these stars can be found in theremarks of the catalog.In contrast to the OGLE-III catalogs of RR Lyr stars in the Magellanic Clouds(Papers I and II), we have not distinguished between RRc and RRe stars, i.e. , the A. A.
Fig. 2. Period–amplitude diagram for RR Lyr stars toward the Galactic bulge. Different colorsrepresent the same type of stars as in Fig. 1.Fig. 3. Color–magnitude diagram for RR Lyr stars toward the Galactic bulge. Different colors corre-spond to types of stars as shown in Fig. 1. RR Lyr stars below the black line have been recognized asmembers of the Sgr dSph. ol. 61
RR Lyr stars pulsating simultaneously in two radial modes (RRd stars) are veryrare in the Galactic bulge (Moskalik and Poretti 2003, Mizerski 2003). Only fiveobjects of this type have been known to date in this region of the Milky Way. Oursearch for multiperiodic RR Lyr stars has been carried out in two ways. First, weused the database of periods measured for all stars observed by OGLE in the bulge.We selected and visually inspected light curves with periods and period ratios char-acteristic for the previously known RRd stars, i.e. , with longer periods in the range0.42–0.6 days and the shorter-to-longer period ratios between 0.74 and 0.75. Sec-ond, we performed a search for secondary periods in the previously selected setof RR Lyr stars. Each light curve was fitted with the Fourier sum with the num-ber of elements that minimizes the c per degree of freedom. Then, the functionwas subtracted from the light curve and the search for additional periodicities wasperformed on the residual data.The latter method revealed, somewhat surprisingly, that the well known se-quence in the Petersen diagram ( i.e. , the diagram of the period ratios vs. the longerperiods) has its continuation toward shorter periods and smaller period ratios. Fig. 4shows the Petersen diagram for RRd stars in the bulge. For comparison we plot-ted 986 RRd stars detected in the LMC (Paper I). Though the total number of theRRd in the Galactic bulge is by two orders of magnitude lower than in the LMC,yet the range of the period ratios is considerably wider. This appears strange butin part may be explained by the difference in metal abundance between these twoenvironments. Selected results of our calculations shown as the segments in Fig. 4demonstrate that models of high metal abundance account for the low values of theperiod ratios in the bulge RRd stars. In Section 6.2 we discuss application of RRdstars as a probe of metallicity.In total, we identified 91 RRd stars (0.5% of the whole sample of RR Lyr stars),confirming very low incident rate of these stars in the bulge. In the LMC (Paper I)RRd stars constitute almost 4% of the total sample of RR Lyr stars, while in theSMC (Paper II) more than 10% of RR Lyr stars are double-mode pulsators. Tenof the 91 detected RRd stars in the bulge are brighter than typical RR Lyr stars inthe Galactic center, so they are likely located in front of the bulge. Further 20 RRdstars are distinctly fainter than bulge RR Lyr variables, so they are located behindthe bulge. Among them, 11 RRd stars most likely belong to the Sgr dSph.In the Petersen diagram one should notice a compact group of 16 RRd starsaround P / P F ≈ .
74 and log P F ≈− .
36. All these objects have the overtone modemuch stronger than the fundamental one, with the amplitude ratio A / A F > . A. A.
Fig. 4. Petersen diagram for RRd stars toward the Galactic bulge. Red symbols represent RRd starsin the Sgr dSph. Small grey dots show RRd stars from the LMC (Paper I). The short segmentsdepict calculated values for selected models covering central part of the instability strip for a mass of M ≈ . ⊙ . The metal abundance parameter, Z , and luminosity, L , are given in the legend. We believe that the similarity of these stars is not by accident, and probably theseobjects are relicts of a disrupted dwarf galaxy or stellar cluster.During the search for double-mode RR Lyr stars we found a significant numberof objects with the secondary periods very close to the primary periods. Such abehavior may be related to the Blazhko effect (Blažko 1907) or changes of theprimary period. Long-term OGLE photometry offers an opportunity to study bothphenomena. Using the methods described by Poleski (2008), we initially selectedRR Lyr stars with detectable rates of period change. We performed this searchonly among objects with high quality photometry (brighter than 16.5 mag in I )covering a time baseline longer than 2000 days. As a result we obtained incidentrates of RR Lyr stars with variable periods. RRab stars that change their periodsare relatively rare and constitute less than 4% of the total population. Variable ol. 61 Fig. 5. Light curves of four RR Lyr stars with the Blazhko effect.
Left panels : unfolded OGLE-II(if available) and OGLE-III I -band light curves. Right panels : the same light curves folded with thepulsation periods.
Despite many years of efforts, there is not even one case of RR Lyr star ina binary system known today. During the search for the secondary periods wepaid particular attention to the eclipsing variations superimposed on the pulsationlight curves. In the bulge we detected one promising candidate for an RR Lyr star0
A. A. in an eclipsing binary system. The light curve of OGLE-BLG-RRLYR-02792 isplotted in Fig. 6. The original I -band photometry folded with the pulsation periodis shown in the left panel, while the right panel shows the eclipsing light curveafter subtracting the RR Lyr component. Further spectroscopic observations wouldconfirm or exclude the possibility that we detected an RR Lyr star being a memberof the binary system. It is interesting to note that we found very similar (in the senseof the pulsation and orbital periods and the light curve shapes) case of an RR Lyrstar with eclipsing modulation in the LMC (Paper I). Besides, we identified in theGalactic bulge three additional RR Lyr stars (OGLE-BLG-RRLYR-03539, -09197,-11361) that exhibited one eclipsing-like fading during the whole time span coveredby the OGLE-III observations. These objects will be monitored during the OGLE-IV phase. Fig. 6. Light curve of the RR Lyr star with additional eclipsing variability.
Left panel : the originalphotometric data folded with the pulsation period.
Right panel : eclipsing light curve after subtractingthe RR Lyr component.
4. Catalog of RR Lyr Stars Toward the Galactic Bulge
The OGLE-III Catalog of RR Lyr Stars in the Galactic Bulge consists of 16 836objects, of which 11 756 have been classified as RRab, 4989 as RRc and 91 as RRdstars. 394 objects in our catalog (343 RRab, 40 RRc and 11 RRd stars) likelybelong to the Sgr dSph. The list of all stars, their identifications with the previouslypublished catalogs, basic parameters, time-series I- and V -band photometry andfinding charts are available only in electronic form via FTP site or WWW interface: http://ogle.astrouw.edu.pl/ftp://ftp.astrouw.edu.pl/ogle/ogle3/OIII-CVS/blg/rrlyr/
The file ident.dat at the FTP site lists all RR Lyr stars with their coordinates andidentifications in various databases. Designations of objects in this catalog followthe scheme presented in the previous parts of the OIII-CVS – stars are named with ol. 61 Fig. 7.
Upper panel : spatial distribution of RR Lyr stars toward the Galactic bulge. The backgroundimage of the bulge originates from the Axel Mellinger’s Milky Way Panorama (Mellinger 2009).Yellow and blue contours show OGLE-II and OGLE-III fields with the number of observations ex-ceeding 30.
Lower panel : surface density map of RR Lyr stars toward the Galactic bulge obtained byblurring the upper map with the Gaussian function. White circles show positions of globular clusterswhich contain RR Lyr stars. A. A. the symbols OGLE-BLG-RRLYR-NNNNN, where NNNNN is a five-digit consec-utive number. Objects are arranged according to increasing right ascension. Thesubsequent columns in the file ident.dat give: star designation, OGLE-III field andinternal database number (consistent with the photometric maps of the bulge bySzyma´nski et al. in preparation), mode of pulsation (RRab, RRc, RRd), J2000.0right ascension and declination, cross-identifications with the OGLE-II photomet-ric database (Szyma´nski 2005), cross-identifications with the MACHO catalog ofRR Lyr stars in the bulge (Kunder et al. et al. I and V mag-nitudes, periods with uncertainties (derived with the T ATRY code of Schwarzenberg-Czerny 1996), peak-to-peak I -band amplitudes, epoch of maximum light and Fourierparameters R , f , R , f (Simon and Lee 1981) derived for I -band lightcurves – are provided in the files RRab.dat , RRc.dat , and
RRd.dat . The latter filegives relevant information about both periodicities of the double-mode stars. Whenthe number of observing points in the V -band was less than 20, we derived mean V magnitude by fitting a template light curve, which was obtained from scaled andshifted I -band light curve. Additional information on some objects ( e.g. , additionalperiods, uncertain classification, proper motion, etc.) can be found in the file re-marks.txt . The OGLE-II and OGLE-III multi-epoch VI photometry can be down-loaded from the directory phot/ . Finding charts for each star are stored in the direc-tory fcharts/ . These are 60 ′′ × ′′ subframes of the I -band DIA reference images,oriented with N up and E to the left.A spatial distribution of RR Lyr stars from our catalog is presented in Fig. 7.The upper panel shows individual stars plotted on the background image originatedfrom the Axel Mellinger’s Milky Way Panorama (Mellinger 2009). Contours ofthe OGLE-II and OGLE-III fields (only those with the number of observing pointslarger than 30) are also plotted in Fig. 7. The bottom panel in Fig. 7 presents asurface density map obtained by the convolution of the upper distribution with theGaussian function. The strong concentration of RR Lyr stars toward the Galaxycenter is well visible.
5. Completeness of the Catalog
The RR Lyr stars in our catalog cover practically the entire range of magnitudes(13 < I < . ol. 61 et al. (2008) compiled from the MACHO pho-tometry. Among 2114 MACHO RR Lyr stars that are covered by the OGLE fields,we found counterparts for 2087 (98.7%) objects in the preliminary version of ourcatalog. This result may be regarded as the upper limit for our catalog complete-ness, and it is valid only for brighter fundamental-mode RR Lyr stars. We carefullychecked the missing 27 objects and noticed that 13 of them were located close tobright, saturated stars and were masked during the reduction process. We includedthese RR Lyr stars in the final version of the catalog providing their D O P HOT pho-tometry. Most of the remaining missing RR Lyr stars were affected by a smallnumber ( <
30) of observing points, usually due to their location at the edge ofthe OGLE fields. When it was possible, we supplemented our catalog with theseobjects.We also cross-identified our catalog with the list of 1888 RRab stars detected byCollinge et al. (2006) in the OGLE-II fields. We missed only one object – a blendedstar, and thus with reduced amplitude. An independent test of the completenessof our catalog was the search for RR Lyr stars carried out by us in the OGLE-IIfields using generally the same methods as in the OGLE-III fields. In this waywe extended our catalog by more than 400 RR Lyr stars, mostly in the regionsmonitored by the OGLE-II survey, and not covered by the OGLE-III fields, or withnumber of points collected during the OGLE-III phase smaller than 30. Five ofthese newly detected RR Lyr stars could potentially be identified on the basis ofthe OGLE-III data only, but were overlooked at the first stage of the search. Mostof them were faint RRc variables with nearly sinusoidal light curves and initiallywere categorized as close binaries.Among stars classified as RR Lyr variables in the GCVS (Kholopov et al. I -band magnitudesof about 20.5 mag. For this reason the spatial density of the RR Lyr is underes-timated in the narrow area close to the Galaxy center (see Fig. 7). These regionswill be observed in the near-infrared domain by the VISTA Variables in the Via4 A. A.
BULGELMCSMC
Fig. 8. Period distribution of RR Lyr stars in the Galactic bulge, LMC and SMC. Each color representsdifferent type of pulsators. Blue regions show RRab stars, red – RRc (+RRe) stars and green – thefirst-overtone period of RRd stars. The width of bins is 0.01 day. ol. 61 et al.
6. Discussion
The distribution of periods of RR Lyr stars is a powerful tool for studying prop-erties of the oldest stellar population. It is well known that average periods arecorrelated with the metallicity of RR Lyr stars, or more specifically, longer-periodvariables are generally more metal-poor. Fig. 8 displays the histograms of periodsof RR Lyr stars from the bulge, LMC (Paper I) and SMC (Paper II). Each bin wasproportionally divided among different modes of pulsation and presented in differ-ent colors. RRc and RRe stars from the Magellanic Clouds were combined andmarked with the same (red) color in Fig. 8.It is clear that RR Lyr stars in the Galactic bulge have on average shorter pe-riods than in the Magellanic Clouds. The mean period of RRab stars in the bulgeis 0.556 days, which is exactly 0.02 days shorter than in the LMC (0.576 days)and 0.04 days shorter than in the SMC (0.596 days). The difference between theseRRab populations is larger, when comparing the most frequent (modal) periods:0.54, 0.58 and 0.62 days for the bulge, LMC and SMC, respectively. Also the over-tone RR Lyr variables have shorter mean periods in the more metal-rich environ-ments: 0.310 days (mode: 0.32 days) in the bulge, 0.323 days (mode: 0.34 days) inthe LMC (merging together RRc and RRe stars), and 0.338 days (mode: 0.37 days)in the SMC.The existence of the second-overtone pulsators among RR Lyr stars (RRe) is amatter of debate. There is no doubt that the overtone RR Lyr variables in the Galaxycenter show two maxima in the period distribution, although the short-period peakis not as prominent as in the LMC and SMC. Moreover, the “RRe peak” does notfollow the rule defined by the “RRab” and “RRc peaks”, i.e. , the bulge RRe starsdo not have shorter periods than the LMC ones. The local maximum in the perioddistribution for the short-period overtone RR Lyr stars is at 0.29 days for the bulge,0.28 days for the LMC and 0.31 days for the SMC. The origin of this additionalpeak in the period distribution of RR Lyr stars remains a mystery.
The segments shown in the Petersen diagram (Fig. 4), were selected from oursurvey of the linear pulsation of stellar envelope models in the relevant range ofparameters. More results from this survey is shown in Fig. 9. We considered mod-els with masses and luminosities appropriate for horizontal branch stars. In theadopted effective temperature range, which is about 300 K wide, first two radialmodes are unstable. Comparing these two figures, we note that with the adopted6
A. A.
Fig. 9. RR Lyr star models in the Petersen diagram. At each four indicated values of the metallicityparameter, Z = . − . − . − .
67, and − .
36) there are four lines. The solid lines correspond to M = . ⊙ and dashed to M = .
55 M ⊙ . The cyan and red colors correspond to the hot and cool boundaries, respectively, of theadopted effective temperature range. Luminosity varies along these lines from log ( L / L ⊙ ) = .
42 tolog ( L / L ⊙ ) = .
75. The short black segments show the loci of the frequencies n = . ( n F + n ) within the temperature range. range of Z , almost the whole observed range is covered. Only some stars lyingin the upper right corner may need Z < . Z > . P F , the period ratio mainly depends on Z . A slight decreasewith increasing T eff is seen in the difference between red and cyan lines. The solidand dashed lines represent different masses. Calculated numbers depend somewhaton the adopted heavy element mixture and source of opacity data, which is moresignificant (Buchler 2008). In models calculated for Fig. 9 we used the OPALopacity data.In any case, to explain the existence of the short period RRd stars in the Galacticbulge we need to postulate that these objects have metal abundance much closer ol. 61 Z values.Kunder and Chaboyer (2008), who based their assessment of the RR star metal-licity in the Galactic bulge on light curve data, found a broad range of the [Fe/H]values extending up to − .
15 dex. Our result provides an independent evidencefor existence of high metallicity RR Lyr stars in the Galactic bulge.The high metallicity RR Lyr stars in the Galactic field have been known forlong time, but still their existence presents a challenge for stellar evolution theory.There are no satisfactory evolutionary models starting from ZAMS for metal richhorizontal branch stars. In particular, even with enhanced mass loss BaSTI tracks(Pietrinferni et al. Z & .
004 enter the instability strip duringthe helium phase only, if the initial mass is less than 0.9 M ⊙ . However, it takestime longer than the Universe age for such objects to reach this phase of evolution.Still larger mass loss in the red giant phase than adopted in the BaSTI tracks isneeded. These issues has been contemplated by various authors (see e.g. , Catelan2009). The question why it is more likely to happen in the bulge than in otherenvironments remains to be answered.We also do not have explanation for the large disparity in the incident rate ofdouble mode pulsation between the LMC and the Galactic bulge RR Lyr stars, dueto insufficient understanding of how such a form of pulsation arises. This problemin the context of Cepheid pulsation was discussed recently by Smolec and Moskalik(2010). One effect that they identify as a possible source of such a pulsationalbehavior is the w = ( w F + w ) resonance. It may also play a role in oursample of RRd stars. The segments in Fig. 9 mark positions where the resonancecondition is satisfied exactly. For other acceptable models the condition is nearlysatisfied. However, only nonlinear modeling may provide an answer whether thisis the actual cause of the double mode pulsation. Since RR Lyr stars have approximately the same absolute magnitudes and col-ors they are excellent indicators of the interstellar extinction, in the sense of mea-suring both – the amount of extinction and the extinction law. The study of theextinction toward the Milky Way center were undertaken by Kunder et al. (2008)using RRab stars identified in the MACHO fields. The map of the interstellar ex-tinction on the basis of our catalog will be prepared elsewhere. In this paper wepresent only the map of the mean apparent ( V − I ) colors of RR Lyr stars in thebulge (Fig. 10). Very large reddening toward the Milky Way center changes the ap-parent colors of RR Lyr stars up to ( V − I ) > V -band,and only the I -band light curves are available. In Fig. 10 the lines of constant mean8 A. A.
Fig. 10. Spatial distribution of the mean apparent ( V − I ) colors of RR Lyr stars toward the Galacticbulge. colors are roughly parallel to equatorial plane of the Galaxy, which is expectedwhen the absorbing medium is located in the thin disk in front of the bulge. Thedeviation from this symmetry visible in Fig. 10 (Baade’s Window) may be relatedto the inclined barred structure of the Galaxy center. Sagittarius Dwarf Spheroidal Galaxy is a substantially tidally disrupted satelliteof the Milky Way. The galaxy is distributed across much of the celestial sphere.First RR Lyr stars in Sgr dSph were discovered by Mateo et al. (1995) as a part ofthe first phase of the OGLE project. During the next years, the population of knownRR Lyr stars in Sgr dSph grew significantly thanks to the studies by Alard (1996),Alcock et al. (1997), Cseresnjes (2001), Kunder and Chaboyer (2009).The OGLE-III fields are located at angular distances from 7.6 to 23 degreesfrom the globular cluster M 54, which is believed to be the center of Sgr dSph.So, our catalog is suitable to study only the outer parts of this galaxy. The color–magnitude diagram (Fig. 3) clearly shows the sequence of faint RR Lyr stars thatbelong to Sgr dSph. We separated the Sgr dSph members from other RR Lyr starsby adopting a somewhat arbitrary condition: I > . ( V − I ) + . ol. 61 Fig. 11. Spatial distribution of RR Lyr stars from the Sagittarius Dwarf Galaxy. agram (Fig. 3) clearly shows that Sgr dSph RR Lyr stars with apparent colors V − I > . The OGLE fields in the Galactic bulge cover ten globular clusters ‡ . We se-lected RR Lyr stars which lay inside the area outlined by the angular radii of theseclusters. To estimate the number of field RR Lyr stars, which may be present bychance within the cluster radii, we counted RR Lyr stars in the rings around theclusters (from 1.5 to 2.5 of the cluster radii, but we checked also other values) andrescaled the number of detected stars to the area covered in the sky by a cluster. Weemphasize that our survey is not able to detect variable stars in the very cores of theglobular clusters.Seven globular cluster which may host RR Lyr stars are listed in Table 1. NoRR Lyr stars were found in the following globular clusters: NGC 6355, NGC 6528 ‡ A. A.
Fig. 12. Period–amplitude ( upper panel ) and color–magnitude ( lower panel ) diagrams for RR Lyrstars in the globular cluster NGC 6441 (red points). Background grey dots represent other RR Lyrstars toward the Galactic bulge. ol. 61 T a b l e 1
Globular clusters in the OGLE fields containing RR Lyr starsCluster RA Dec Cluster N RR N fieldRR name (J2000) (J2000) radius [ ′ ] (estimated)NGC 6304 17 h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ h m s − ◦ ′ ′′ and NGC 6553. Each of the further 3 clusters: NGC 6304, NGC 6453, NGC 6540,may host up to four RR Lyr stars, but it cannot be ruled out that all of the detectedpulsators are field variables. Other four globular clusters observed by OGLE in thebulge – NGC 6441, Djorg 2, NGC6522 and NGC 6558 – contain RR Lyr stars,although usually only several objects.An exceptionally rich cluster is NGC 6441, which hosts around 40 RR Lyr starsoutside its core. RR Lyr variables in this cluster also have exceptionally long peri-ods, actually the longest mean periods from all known globular clusters. Moreover,NGC 6441 together with another globular cluster, NGC 6388, violates the rule thatmore metal-rich clusters host shorter-period RR Lyr stars. Pritzl et al. (2000) sug-gested that NGC 6441 and NGC 6388 represent a new, third Oosterhoff group ofglobular clusters.Fig. 12 shows the period–amplitude and color–magnitude diagrams for RR Lyrstars in NGC 6441 overplotted on other RR Lyr stars from our catalog. The log P ofthe NGC 6441 members is shifted toward longer periods by about 0.15 compared tothe field bulge RRab variables. RR Lyr stars in NGC 6441 are significantly fainterthan field variables surrounding the cluster in the sky, confirming the backgroundlocation of the cluster with respect to the bulge. A more detailed description ofRR Lyr stars in globular clusters will be presented in a separate paper.
7. Conclusions
We presented the largest catalog of RR Lyr stars toward the Galactic bulge pub-lished so far. Our sample is about five times larger than the largest set of RR Lyrstars identified in the bulge before. A huge number of objects distributed over a rel-atively large area in the central parts of the Galaxy, high completeness (especiallyfor RRab stars), and excellent multi-epoch standard photometry gives an opportu-nity to map a 3D structure of the bulge, to test the presence of barred distribution2
A. A. among the oldest population of stars, to explore the earliest history of the Galaxyformation, and to determine an accurate distance to the Milky Way center.
Acknowledgements.
We are grateful to Z. Kołaczkowski, A. Schwarzenberg-Czerny and J. Skowron for providing software which enabled us to prepare thisstudy.The research leading to these results has received funding from the EuropeanResearch Council under the European Community’s Seventh Framework Program-me (FP7/2007-2013)/ERC grant agreement no. 246678. This work has been sup-ported by the MNiSW grant no. IP2010 031570 (the Iuventus Plus program)to P. Pietrukowicz. The massive period search was performed at the Interdisci-plinary Centre for Mathematical and Computational Modeling of Warsaw Univer-sity (ICM), project no. G32-3. We wish to thank M. Cytowski for his skilled sup-port. REFERENCES
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