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A pulsational distance to Omega Centauri based on Near-Infrared Period-Luminosity relations of RR Lyrae stars

We present new Near-Infrared (J,K) magnitudes for 114 RR Lyrae stars in the globular cluster Omega Cen (NGC 5139) which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is presently the largest such sample in any stellar system. We also present updated predictions for J,K-band Period-Luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z=0.0001 to Z=0.004. By adopting for the Omega Cen variables with measured metal abundances an alpha-element enhancement of a factor of 3 (about 0.5 dex) with respect to iron we find a true distance modulus of 13.70 (with a random error of 0.06 and a systematic error of 0.06), corresponding to a distance d=5.5 Kpc (with both random and systematic errors equal to 0.03 Kpc). Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC-17, but differ significantly from the recent distance estimates based on cluster dynamics and on high amplitude Delta Scuti stars.


Introduction
The Period-Luminosity (P L) relation for Classical Cepheids and, in the near-infrared, for RR Lyrae stars is one of the most important methods to estimate distances and to calibrate secondary distance indicators (Bono et al. 2001;Freedman et al. 2001;Saha et al. 2001;Cassisi et al. 2004, hereinafter C04;Catelan, Pritzl, & Smith 2004, hereinafter CPS04;Gieren et al. 2005). Current trigonometric parallaxes from Hipparcos (Feast & Catchpole 1997), HST (Benedict et al. 2002), and from ground based telescopes (Lane et al. 2000;Kervella et al. 2004) have provided a unique possibility to calibrate the zero-point of these relations. Unfortunately, the number of Galactic calibrators with accurate trigonometric parallaxes is very limited, and for RR Lyrae stars it is restricted to a single object, namely RR Lyr itself. Therefore, we still lack a sound knowledge of the slope of the different PL relations based on purely geometrical methods. To overcome this problem, the astronomical community devoted a paramount observational effort to measure the pulsation properties of these robust distance indicators. The ideal laboratories for these studies are the Globular Clusters (GCs) and the dwarf spheroidals (dSphs) in the Local Group as the RR Lyrae stars in a given system are located at largely the same distance.
It has been demonstrated by Longmore et al. (1990) that cluster RR Lyrae follow a well-defined K-band P L relation but we still lack a comprehensive empirical investigation of its properties. For this reason, during the last few years we undertook a detailed pulsation and evolutionary investigation of RR Lyrae stars (Bono et al. 2003 and references therein) and, at the same time, we also started a large observational project aimed at collecting new Near-Infrared (NIR) data for RR Lyrae stars in Galactic (M92: Del Principe et al. 2005) and Large Magellanic Cloud clusters (Reticulum: Dall'Ora et al. 2004).
Although the theoretical and empirical scenarios have recently seen new predictions based on Synthetic Horizontal Branch (SHB) models ); Catelan, Pritzl, & Smith 2004) and new data (Butler 2003;Borissova et al. 2004;Storm 2004), we still lack a detailed NIR photometric analysis of a stellar system which includes a large sample of fundamental (F, RR ab ) and first-overtone (FO, RR c ) RR Lyrae stars (only a dozen Galactic GCs host more than 50 RR Lyrae, see, e.g., Clement et al. 2001) and which spans a broad range in metal abundances. At present, the largest NIR data set includes 74 RR Lyrae stars whose metal abundance, based on 23 objects, ranges from [Fe/H]≈ −2 to [Fe/H]≈ −0.7, as observed by Borissova et al. (2004) in the inner regions of the Large Magellanic Cloud. In this context, the GC ω Cen is a perfect target, since it includes more than 186 RR Lyrae (86 RR ab , 100 RR c ; Kaluzny et al. 2004 Sollima et al. 2006). Moreover, the ω Cen distance has been estimated using different distance indicators such as the First Overtone Blue Edge (FOBE, Caputo et al. 2002), the eclipsing binary OGLEGC-17 (Thompson et al. 2001;Kaluzny et al. 2002), the V -band metallicity relation of RR Lyrae stars (Rey et al. 2000;Catelan 2005), and a dynamical analysis by van de Ven et al. (2006) based on proper motion and radial velocities of a large sample of individual stars.
In this paper, we present new NIR photometry for a substantial number of the RR Lyrae stars in ω Cen and we provide new distance estimates which we compare with determinations already present in the literature.

Observations and data reduction
The present NIR data set includes three different samples of J,K s data either collected with the NIR camera SOFI available at the New Technology Telescope (NTT, ESO, La Silla), or by Longmore et al. (1990), or available from the Two Micron All Sky Survey (2MASS) catalogue 1 .
Near-Infrared J and K s images of ω Cen were collected in two different runs, namely 6,8 February 2001 (UT) and3,4,25, and26 February 2002 (UT). The pixel scale of SOFI is 0.292 ′′ while the field of view is 4.94 ′ × 4.94 ′ . We observed three different fields: Field A is centered on the cluster center, field C at a distance ∼ 10.5 ′ NW away from the center, and field D at a distance ∼ 10.7 ′ SW away from the center (see solid rectangles in Fig. 1). We collected 36 J-and 49 K s -band images of field A, 33 J-and 52 K s -band images of field C, and 12 J-and 18 K s -band images of field D. For J images the exposure time was t = 3 s, while for K s images was t = 12 s. Owing to the crowded nature of the fields the observing time was equally divided between observations on ω Cen and sky observations. Together with these data we retrieved from the ESO archive a mosaic of nine J and nine K s images of ω Cen (Sollima et al. 2004) which cover an area of ∼ 13 ′ × 13 ′ across the cluster center (see dashed rectangles in Fig. 1). Note that our Field A overlaps with the central pointing of this dataset. These images were collected on 13,14 January 2000 (UT), using the same experimental equipment. Seven out of the nine fields were observed once in J,K s bands, while the other two fields were observed twice. The individual exposure times were 54 s and 180 s.
The pre-processing was performed using standard IRAF procedures. To improve the photometric accuracy we did not stack the individual images, but determined the (linearly variable) PSF for each frame based on more than 50 PSF stars uniformly distributed across each image. Photometry for the entire set of J and K s images was then simultaneously performed with DAOPHOT/ALLFRAME (Stetson 1994) together with a few V -and Iband images (Dall'Ora et al. 2004;Del Principe et al. 2005). A more detailed analysis concerning the reduction strategy will be discussed in a forthcoming paper (Del Principe et al. 2006, in preparation).
The absolute calibration in the 2MASS system (Cutri et al. 2003) was performed using a large set of ≈ 400 local stars selected from the 2MASS point-source catalogue. To constrain on a quantitative basis the intrinsic accuracy of both absolute and relative zero-points, we performed an independent calibration using a set of 4 standard stars (Persson et al. 1998) observed during the night of 24 February 2002 at air masses which bracket the observations of fields A, C, and D. The stars selected were S791-C, S867-V, S273-E, and S870-T. Fig. 2 shows the comparison between secondary standards in field A and local 2MASS stars. The observed distribution of magnitude differences has a standard deviation of K 2M −K P = −0.012±0.076 and J 2M − J P = 0.003 ± 0.080, which is a measure of the typical difference for an individual star. Note that the quoted differences agree well, within the errors, with the transformations provided by Carpenter (2001). As a further independent test, we also calibrated the NIR data using standard stars collected during the other observing nights, and we found that the accuracy of the absolute zero-points is ∼ 0.02 mag.
We ended up with a photometric catalogue of ∼ 115, 000 stars with limiting magnitudes of J ≈ 20 and K s ≈ 19.0 mag. Fig. 3 shows the NIR CMD of the stars with a 'separation index' , sep 2 larger than 3.0 and an intrinsic photometric accuracy of σ J,K ≤ 0.08 mag.
We identified 56 catalogued (OGLEII, Kaluzny et al. 2004) RR Lyrae stars (27 RR ab , 29 RR c ) in fields A, C, and D (see circles in Fig. 1) and 58 additional RR Lyrae (25 RR ab , 33 RR c ) variables (see diamonds in Fig. 1) in Sollima's fields. In order to increase the sample of RR Lyrae stars we also included data for the 29 RR Lyrae stars (19 RR ab , 10 RR c ; see squares in Fig. 1) observed by Longmore et al. (1990). We have transformed their mean K-band data from the Anglo Australian Observatory NIR system to the 2MASS K s system using the transformations provided by Carpenter (2001). Additionally, we cross-correlated the catalogues provided by OGLEII and by Clement et al. (2001) with the 2MASS catalogue. In this way we identified 37 more catalogued RR Lyrae stars (10 RR ab , 27 RR c ; see triangles in Fig. 1), and therefore we ended up with a total sample of 180 RR Lyrae (81RR ab , 99RR c ) for which we have at least one J or K s -band measurement.
To obtain the most accurate mean magnitudes for the RR Lyrae, we fit the individual K s phase points measured by ALLFRAME with a template curve (Jones et al. 1996) and compute on the basis of this curve the intensity-averaged mean magnitude. The method requires an accurately known ephemeris of the epoch of maximum for each star as well as the amplitude in the optical V or B bands. These parameters are available from Kaluzny et al. (2004) 3 for 167 of the variables in our sample. For each of the remaining 13 variables, all with single epoch 2MASS measurements, the 2MASS measurement was adopted as the best estimate of the mean magnitude.
The typical accuracy of mean K − band magnitude of RR Lyrae stars either with a good coverage of the light curve or with single epoch measurements, epoch of maxima and luminosity amplitudes is better than 0.02 mag. In order to constrain the typical uncertainty for the remaining thirteen objects (2MASS sample) with only single epoch measurements, we estimated the difference between the mean K − band magnitudes of the RR Lyrae stars for which we have a very good coverage of the light curves (22 RR ab , 22 RR c ) and the mean K − band magnitudes only based on the 2MASS measurements. We found that the individual differences are smaller than 0.2 mag for RR ab and 0.1 for RR c .
The current RR Lyrae sample includes 16 Blazhko RR Lyrae stars (Kaluzny et al. 2004). Ten Blazhko RR Lyrae (eight in our sample and two in the Longmore sample) present a good coverage of the light curves. For four objects (three in the Sollima sample and one in the 2MASS sample) are available single epoch measurements and accurate epochs, and therefore the mean magnitude was determined using the template curve. For only two objects in the 2MASS sample we adopted the single epoch measurements, since recent epochs are not available. The Blazhko effect shows up as a modulation in the luminosity amplitude (Stothers 2006, and references therein) that might affect the accuracy of the mean K − band magnitudes estimated with the template curve. In order to account for the uncertainty in the luminosity amplitude we estimated the mean K − band magnitude by increasing/decreasing the amplitude by a factor of two. The difference in the mean K − band magnitude is of the order of 0.02 mag. The error for the two objects with single epoch J and K − band magnitudes is at most of the order of 0.15-0.20 mag.
Unfortunately, no empirical template is available for the J-band light curves. Instead the mean J-band magnitude of RR Lyrae stars with well-sampled light curves (56 RR Lyrae stars located in fields A,C,D) was determined from a cubic spline fit to the light curves. The fit was performed using spline under tension (Stellingwerf 1978, and references therein). This approach presents several advantages when compared with the classical Fourier fit, in particular for noisy light curves (Del Principe et al. 2006). For the other objects the J-band magnitude is based on a single epoch measurement. We found that the typical difference between the mean J − band magnitudes of the RR Lyrae stars for which we have a very good coverage of the light curves (22 RR ab , 22 RR c ) and the mean J − band magnitudes only based on single epoch 2MASS measurements is systematically smaller than 0.3 mag for RR ab and 0.15 mag for RR c stars.
Open and filled circles in Fig. 3 show the location in the CMD of the entire sample of RR Lyrae stars. This is the largest sample of homogeneous NIR magnitudes for RR Lyrae stars in a stellar system ever collected.

Distance determinations
To determine the distance to ω Cen on the basis of the observed mean dereddened K 0 and J 0 magnitudes we need to adopt a relation between observational quantities and the absolute magnitude. From the theoretical point of view, these relations have been recently investigated (e.g., Bono et al. 2001Bono et al. , 2003; CPS04; C04) on the basis of Synthetic Horizontal-Branch (SHB) models with promising results in good agreement with empirical calibrations. These model grids can be parametrized in different ways, the simplest relation being the log P − M K relation and the more complicated relations taking into account either the global metallicity Z (e.g. Bono et al. 2001Bono et al. , 2003 or the Lee (1990) Horizontal Branch type τ HB 4 (e.g., C04, and CPS04). Bono et al. (2003) showed that metallicity has a significant impact on the K-band magnitude of RR Lyrae stars. C04 showed that using the HB type as a parameter, at fixed metal abundance, also results in tight relations in both J and K. The latter approach is only valid for globular clusters where the HB type can be determined, and it is not directly applicable to field stars. ω Cen is a peculiar object as it is morphologically a globular cluster but the chemical composition and probably the ages of the stars differ significantly, making the population more similar to that of a dwarf galaxy. Consequently, the HB type is not well defined a priori, although the color-magnitude diagram presented in Fig. 3 shows that there are very few HB stars redder than the RR Lyrae stars. This feature suggests that all the sub-populations which contribute to the HB must have a collective HB type close to 1 5 .
In order to determine accurate distance estimates for both F and FO pulsators, we computed a new set of SHB models with selected metal abundances (Z=0.0001, 0.0003, 0.001, 0.004) and a helium content of Y = 0.245. We did this by adopting the detailed set of HB models recently constructed by Pietrinferni et al. (2006) and the procedure already discussed by C04. The pulsation models discussed by Di Criscienzo et al. (2004) were used to fix the boundaries of the RR Lyrae instability strip and to evaluate the periods of the pulsators. On this basis, evolutionary and pulsation predictions were used to constrain the stellar distribution along the HB, and to compute for each individual simulation the HBtype τ HB . The SHB models were constructed by accounting for the hysteresis effect (see, e.g., Bono et al. 2005) and by neglecting the pre-ZAHB evolutionary phases (see, e.g., Piersanti, Tornambe', Castellani 2004).
Theoretical predictions were transformed into the Bessell & Brett (1988) NIR photometric system using the bolometric corrections and color-temperature relations provided by Pietrinferni et al. (2004). They were then transformed into the 2MASS system using the transformations provided by Carpenter (2001) and eventually, for each given SHB, the predicted Period-Luminosity relations, as given in the form M J,K = a J,K + b J,K log P + c J,K τ HB 6 , were derived. The coefficients and the relative errors of the least-squares solutions are listed in Table 1. The zero-points and the coefficients of the above PLZ relations further support the use of independent relations for F and FO pulsators. As a matter of fact, the slopes and the zero-points between the two different groups differ at least at ten σ level for both J and K-band. The referee suggested to check the dependence of current F and FO PL relations on the predicted effective temperature of F blue edge and FO red edge. We performed a few numerical experiments artificially increasing/decreasing by 150 K the effective temperature of FO red edge and of F blue edge. We found that the change in the effective temperature of the F blue edge affects the coefficients of J and K−band FO PL relations by ±0.02 − 0.03 (a) and by ±0.02 − 0.05 (b), respectively. The coefficients of J and K−band F PL relations change by ±0.02 (a) and by ±0.02 − 0.04 (b), respectively. The change in the effective temperature of the FO red edge affects the a, b coefficients of F and FO relations on average by ±0.01. The impact of these changes on the coefficient of the HB type (c) is marginal. Note that the adopted uncertainty on the location of FO red edge and F blue edge is supported by the comparison with RR Lyrae stars in GCs (Di Criscienzo et al. 2004). These findings further support the approach to derive different PL relations for F and FO pulsators. It is also noteworthy, that the use of fundamentalized FO periods causes an increase of a factor of two in the uncertainty of their zero points and slopes.
As a whole, these relations agree quite well with those already presented by C04 (see their Tables 6 and 8). In particular, i) the slopes of the M J,K − log P relations are, at fixed metal content, independent of the HB-type, while they become slightly steeper with increasing Z. ii) The zero-points of the M J,K − log P relations become, at fixed metal content, brighter when moving from blue to red HB populations. This dependence significantly decreases with increasing Z. iii) The zero-points of the M J,K − log P relations, independently of the HB-type, become fainter with increasing Z. An even more striking agreement is found with the relations computed by CPS04. For the relevant mean metallicity model, Z = 0.001, and HB type, τ HB = 0.934, they find for their combined sample of fundamental mode and first-overtone pulsators M K = −2.388 log P F − 1.133 whereas we find for the fundamental mode pulsators M K = −2.37 log P F − 1.13 and for the first-overtone pulsators M K = −2.41 log P F O − 1.20. The referee noted that, according to CPS04, the coefficients a and b of the PL relations present some dependence on both the HB type and the metal abundance. Unfortunately, a global comparison is hampered by the different approach adopted to derive the PL relations, i.e. multilinear least-squares vs. third order polynomials (see their equations. 1 and 2 and their Table 9). Moreover, the metallicity of current SHB models range from Z=0.0001 to Z=0.004, while those from CPS04 range from Z=0.0005 to Z=0.006.
The individual reddening values of RR ab stars have been empirically estimated using a Period-Color(V − K)-Amplitude(A V ) relation (Piersimoni et al. 2002(Piersimoni et al. , 2006, while for RR c stars and RR Lyrae affected by amplitude modulation we used a reddening map based on u,v,b,y Strömgren photometry (Calamida et al. 2005). Accurate mean Vband magnitudes for RR Lyrae in our sample have been measured either by Kaluzny et al. (2004) 7 or by Butler et al. (1978) for the outermost ones. For the remaining objects, we adopted a mean reddening of E(B − V ) = 0.11 ± 0.02. Selective absorptions have been estimated using the reddening law from Cardelli et al. (1989) Data plotted in Fig. 4 show the fit between the dereddened K magnitudes and predicted PL relations at fixed metal abundance and for τ HB = 0.94. In this figure, the asterisks mark the overluminous RR ab stars which are either blended with a fainter close companion (V32, V118, V139, and V150) or have a controversial mode identification (V84), while the open triangle refers to V182 which, together with three other (V168, V181, V183) underluminous variables with K 0 ≥14 mag, presents peculiar J − K colors. By excluding these variables, we estimate a true distance of µ 0 = 13.80 ±0.10 mag and 13.70 ±0.10 mag with Z=0.0001 and Z=0.001, respectively, clearly indicating that individual metallicity estimates are required to improve the distance determination.
Conveniently enough, individual metal abundances on the scale of Zinn & West (1984), as based on the hk index, for a substantial fraction (130) of the RR Lyrae plotted in Fig.  4 have been measured by Rey et al. (2000). In a recent investigation based on medium resolution spectroscopy, Sollima et al. (2006) found, for 74 RR Lyrae in ω Cen , no systematic difference with the metal abundances based on the hk index. To properly use the theoretical predictions, the observed [Fe/H] values have been transformed into "global" Z values by taking into account possible α-element enhancements 8 . Following to Gratton, Sneden, & Caretta (2004), who found [α/Fe]≈ 0.45 as a plausible upper limit to the α-element enhancement, we adopt f = 10 [α/Fe] = 3.
We are now in a position to derive distance moduli of individual RR Lyrae stars with metallicity measurements from Rey et al. (2000). In panel (d) in Fig. 5 we show the resulting values as a function of metallicity, leading to µ 0 = 13.70 ± 0.06 mag. To illustrate the sensitivity of the method to possible errors in the assumed HB type or α-enhancement, we have also computed the distance moduli for τ HB = 0 and/or f = 1 and plotted the results in Fig. 5. We note that the effect on the distance moduli is rather small considering that the estimated error on the HB type, σ τ = 0.2 (see star counts of HB stars in NGC 2808 listed in Table 2 of Castellani et al. 2006), and on the α-enhancement factor, σ f = 0.5, are typically smaller than the values adopted here. Of particular interest is the fact that the effect of an error in the assumed HB-type is to introduce a slope in these diagrams (see panels a and b). According to the relations given in Table 1, this trend can be explained as an empirical evidence that the more metal-poor RR Lyrae are connected with bluer HB populations when compared with the more metal-rich ones.
The K-based distance determinations are fully supported by the J-band measurements. The estimated uncertainty on the mean J-magnitudes depends on the luminosity amplitude, (σ J (RR c ) ≈ 0.1; σ J (RR ab ) ≈ 0.2), and it is less accurate than for the K-band measurements (σ K ≈ 0.015). It is worth mentioning that we still lack empirical template curves for the J − band, and therefore accurate mean J − band magnitudes are only available for 56 out of the 180 RR Lyrae in our sample. As shown in Fig. 6, the true distance moduli based on the relations given in Table 1 agree with the values based on K-magnitudes, but with a larger error. Although, current mean J − band magnitudes present this drawback, this NIR band appears very promising to estimate distances, since it presents several advantages over the K − band and requires much less telescope time to reach similar photometric accuracy.
We can also determine the distance to the individual RR Lyrae stars using the PLZ K relations of Bono et al. (2003). By using both field and cluster RR Lyrae stars they found M K = −0.77 − 2.101 log P F + 0.231[Fe/H] where the fundamentalized logarithmic period was computed as log P F = log P F O +0.127. Combining this relation with the sub-sample of RR Lyrae for which are available metallicity estimates from Rey et al. (2000), we find µ 0 = 13.77±0.07 mag based on a total of 113 RR ab and RR c stars.
In addition to the quoted intrinsic errors we estimate the external, systematic error to be σ sys = 0.06 mag. This is the result of summing in quadrature the contributions from the photometric zero point (σ = 0.02 mag), the uncertainty on the absorption A K (σ = 0.01 mag, based on uncertainties of 0.1 on R K and 0.02 mag on E(B − V )). The uncertainty of the reddening is based on the recent Strömgren photometric investigation by Calamida et al. (2006), while the uncertainty on the selective absorption coefficient is due to current uncertainties in the adopted extinction law (Rieke & Lebofsky 1985;Cardelli et al. 1989, and references therein). The absolute zero point of the metallicity scale is a significant source of uncertainty as discussed by Dall'Ora et al. (2004) due to systematic differences between metallicity scales. They estimate σ = 0.25 dex on the zero point which results in the dominating contribution to the systematic error of σ = 0.25 × 0.231 = 0.06 mag.
As the ω Cen RR Lyrae stars exhibits a broad range of metallicities and all belong to the same cluster, we can empirically constrain the metallicity effect without having to rely on uncertain distance estimates. We have made linear three-parameter fits of the relation in Eq.1 for RR ab and RR c stars separately, using only dereddened apparent magnitudes, and using only the variables with individual hk abundance evaluations. We find where the symbols have their usual meaning. In Fig. 7 we have plotted the observed magnitudes against metallicity after having removed the period dependency. The metallicity effect is weak for the fundamental pulsators and for the first-overtone pulsators the metallicity effect is consistent with only a marginal dependence. In both cases the relations are in reasonable agreement with the predictions from the SHB models. Current K − band PL relations present two substantial differences when compared with the PLZ relation derived by Bono et al. (2003): a) we derived independent PL relations for F and FO pulsators, while Bono et al. only derived semi-empirical "average" PL relations for F and fundamentalized FO pulsators; b) current PL relations only apply to cluster variables for which we can estimate the HB type, while the PLZ relation derived by Bono et al. (2003) applies to individual RR Lyrae stars.

Conclusions and final remarks
In Table 2 we have listed recent empirical distance estimates to ω Cen where we have corrected all the moduli for the same reddening and employed the same reddening law as we have used for the RR Lyrae stars. We note that our new near-IR results are in excellent agreement with the largely geometric distance to the eclipsing binary OGLEGC-17 from Thompson et al. (2001) and Kaluzny et al. (2002), and from the First Overtone Blue Edge method based on pulsation predictions from Caputo et al. (2002). Current distance determinations based on J and K − band measurements and on NIR PL(τ HB ) relations are also in very good agreement with similar estimates based on NIR relations derived by CPS04. In particular, for τ HB = 0.94 and f = 3 we found 13.70 ± 0.06 vs 13.72 ± 0.06 (K − band) and 13.71 ± 0.10 vs 13.76 ± 0.10 (J − band). Note that distance estimates based on CPS04 relations were determined using the same solar metal abundance and we did not include a few very metal-poor RR Lyrae, since CPS04 prediction range from Z=0.0005 to Z=0.006. This finding is also supported by most recent M V −[Fe/H] relations. By adopting the mean V-magnitudes for RR Lyrae stars provided by Kaluzny et al. (2004) and by Butler et al. (1978) together with the calibration given by Bono et al. (2003) we obtain for RR Lyrae with individual metal abundances a distance value of (m − M) 0 = 13.72 ± 0.11. We obtain a similar distance modulus (13.62 ± 0.11) using the same magnitudes and the recent calibration of the M V −[Fe/H] relation provided by Catelan (2005). The slope of such a relation was estimated by Cacciari & Clementini (2003), while the zero-point is based on the trigonometric parallax of RR Lyr itself (Benedict et al. 2002;Bono et al. 2002). Note that the systematic error for distance estimates based on visual magnitudes is σ = 0.08, due to the significant contribution from the absorption correction.
On the other hand, we deviate by more than three σ from the distance evaluation based on High Amplitude δ Scuti (HADS) stars (µ 0 = 14.05 ± 0.02, McNamara 2000) and from the modulus of µ 0 = 13.41 ± 0.13 from van de Ven et al. (2006) based on cluster star dynamics. It is worth noting that current uncertainties on metal abundance and on NIR photometry can not account for such a discrepancy.
The NIR P L relations for RR Lyrae stars provide an accurate approach for determining absolute distances to globular clusters and other stellar systems. We find for ω Cen a distance modulus of 13.70 ± 0.06 ± 0.06 where the error estimates are random and systematic errors, respectively. We note that for populous systems like globulars and dwarf galaxies the dominating source of error are the systematic sources. The accuracy of NIR photometry in crowded regions significantly improved during the last few years and the sample sizes will certainly benefit from the new large field of view NIR cameras available at 4m class telescopes (van der Bliek et al. 2004). New J−band light curve templates for both fundamental and first overtone RR Lyrae are also urgently required to improve the accuracy of their mean magnitudes in this band.
It is a pleasure to thank S. Cassisi and A. Pietrinferni for several helpful discussions and for providing detailed sets of HB models. We also thank M. Marconi for many enlightening suggestions concerning the pulsation properties of RR Lyrae stars. We acknowledge an anonymous referee for his/her positive comments and insights on an early version of this paper. We are very grateful to the ESO support astronomers in La Silla and in Garching for their ongoing support and effort in dealing with series of different Observing Blocks. We also thank the ESO Science Archive for their prompt support. This work was partially supported by Particle Physics and Astronomy Research Council (PPARC) and INAF/PRIN2005. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.