Oosterhoff Type Research Articles

The MACHO Project Large Magellanic Cloud Variable-Star Inventory. XIII. Fourier Parameters for the First-Overtone RR Lyrae Variables and the LMC Distance

Shapes of RR Lyrae light curves can be described in terms of Fourier coefficients that past research has linked with physical characteristics such as luminosity, mass, and temperature. Fourier coefficients have been derived for the V and R light curves of 785 overtone RR Lyrae variables in 16 MACHO fields near the bar of the LMC. In general, the Fourier phase differences 21, 31, and 41 increase and the amplitude ratio R21 decreases with increasing period. The coefficients for both the V and R magnitudes follow these patterns, but the phase differences for the R curves are on average slightly greater, and their amplitudes are about 20% smaller, than the ones for the V curves. The 31 and R21 coefficients have been compared with those of the first-overtone RR Lyrae variables in the Galactic globular clusters NGC 6441, M107, M5, M3, M2, ω Centauri, and M68. The results indicate that many of the LMC variables have properties similar to the ones in M2, M3, M5, and the Oosterhoff type I variables in ω Cen, but they are different from the Oosterhoff type II variables in ω Cen. Equations derived from hydrodynamic pulsation models have been used to calculate the luminosity and temperature for the 330 bona fide first-overtone variables. The results indicate that they have log L in the range 1.6–1.8 L⊙ and log Teff between 3.85 and 3.87. Based on these temperatures, a mean color excess E(V-R) = 0.08 mag, equivalent to E(B-V) = 0.14 mag, has been estimated for these 330 stars. The 80 M5-like variables (selected according to their location in the 31–log P plot) are used to determine an LMC distance. After correcting for the effects of extinction and crowding, a mean apparent magnitude V0 = 18.99 ± 0.02 (statistical) ±0.16 (systematic) has been estimated for these 80 stars. Combining this with a mean absolute magnitude MV = 0.56 ± 0.06 for M5-like stars derived from Baade-Wesselink analyses, main-sequence fitting, Fourier parameters, and the trigonometric parallax of RR Lyrae, we derive an LMC distance modulus μ = 18.43 ± 0.06 (statistical) ±0.16 (systematic) mag. The large systematic error arises from the difficulties of correcting for interstellar extinction and for crowding.

The Evolutionary Status of M3 RR Lyrae Variable Stars: Breakdown of the Canonical Framework?

In order to test the prevailing paradigm of horizontal-branch (HB) stellar evolution, we use the large databases of measured RR Lyrae parameters for the globular cluster M3 (NGC 5272) recently provided by Bakos et al. and Corwin & Carney. We compare the observed distribution of fundamentalized periods against the predictions of synthetic HBs. The observed distribution shows a sharp peak at Pf ≈ 0.55 days, which is primarily due to the RRab variables, whereas the model predictions instead indicate that the distribution should be more uniform in Pf, with a buildup of variables with shorter periods (Pf < 0.5 days). Detailed statistical tests show, for the first time, that the observed and predicted distributions are incompatible with one another at a high significance level. This indicates either that canonical HB models are inappropriate, or that M3 is a pathological case that cannot be considered representative of the Oosterhoff type I (OoI) class. In this sense, we show that the OoI cluster with the next largest number of RR Lyrae variables, M5 (NGC 5904), presents a similar, although less dramatic, challenge to the models. We show that the sharp peak in the M3 period distribution receives a significant contribution from the Blazhko variables in the cluster. We also show that M15 (NGC 7078) and M68 (NGC 4590) show similar peaks in their Pf distributions, which in spite of being located at a Pf value similar to that of M3, can, however, be primarily ascribed to the RRc variables. Again similar to M3, a demise of RRc variables toward the blue edge of the instability strip is also identified in these two globulars. This is again in sharp contrast to the evolutionary scenario, which also foresees a strong buildup of RRc variables with short periods in OoII globulars. We speculate that in OoI systems RRab variables may somehow get close to the transition line between RRab and RRc pulsators as they evolve to the blue in the H-R diagram, whereas in OoII systems it is the RRc variables that may get similarly trapped instead, as they evolve to the red, before changing their pulsation mode to RRab. Such a scenario is supported by the available CMDs and Bailey diagrams for M3, M15, and M68.

Hubble Space TelescopeSnapshot Study of Variable Stars in Globular Clusters: The Inner Region of NGC 6441

We present the results of a Hubble Space Telescope snapshot program to survey the inner region of the metal-rich globular cluster NGC 6441 for its variable stars. A total of 57 variable stars were found, including 38 RR Lyrae stars, six Population II Cepheids, and 12 long-period variables. Twenty-four of the RR Lyrae stars and all of the Population II Cepheids were previously undiscovered in ground-based surveys. Of the RR Lyrae stars observed in this survey, 26 are pulsating in the fundamental mode with a mean period of 0.753 days and 12 are first-overtone–mode pulsators with a mean period of 0.365 days. These values match up very well with those found in ground-based surveys. Combining all the available data for NGC 6441, we find mean periods of 0.759 and 0.375 days for the RRab and RRc stars, respectively. We also find that the RR Lyrae stars in this survey are located in the same regions of a period-amplitude diagram as those found in ground-based surveys. The overall ratio of RRc to total RR Lyrae stars is 0.33. Although NGC 6441 is a metal-rich globular cluster and would, on that ground, be expected either to have few RR Lyrae stars or to be an Oosterhoff type I system, its RR Lyrae stars more closely resemble those in Oosterhoff type II globular clusters. However, even compared with typical Oosterhoff type II systems, the mean period of its RRab stars is unusually long. We also derived I-band period-luminosity relations for the RR Lyrae stars. Of the six Population II Cepheids, five are of W Virginis type and one is a BL Herculis variable star. This makes NGC 6441, along with NGC 6388, the most metal-rich globular cluster known to contain these types of variable stars. Another variable, V118, may also be a Population II Cepheid, given its long period and its separation in magnitude from the RR Lyrae stars. We examine the period-luminosity relation for these Population II Cepheids and compare it with those in other globular clusters and in the Large Magellanic Cloud. We argue that there does not appear to be a change in the period-luminosity relation slope between the BL Herculis and W Virginis stars, but that a change of slope does occur when the RV Tauri stars are added to the period-luminosity relation.

The Carina Project. I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy. Current data cover an area of about 0.3 square degrees around the center of the galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae stars, 15 are bona fide Anomalous Cepheids, one might be a Galactic field RR Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply for the first time accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitude and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae and Anomalous Cepheids are new identifications. Among the RR Lyrae sample 6 objects are new candidate double-mode variables. On the basis of their pulsation properties we estimated that two variables (V152, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of two more massive than fundamental mode RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He burning phases. The data for Carina, together with data available in the literature, strongly support the conclusion that dSph galaxies can barely be classified into the classical Oosterhoff dichotomy. The mean period of fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first overtones and total number of RR Lyrae is quite similar to that found in Oosterhoff type I clusters.

M75, A Globular Cluster with a Trimodal Horizontal Branch. II.BVPhotometry of the RR Lyrae Variables

We present new BV CCD photometry, light curves, and ephemerides for nine previously known, 29 newly detected RR Lyrae variables, and one newly detected variable of an unknown type in the globular cluster M75. The photometry used for the detection of the additional variables was obtained with the image subtraction package ISIS. The data were acquired on an observing run in 1999 July and range over seven observing nights. Estimates of fundamental photometric parameters are presented including intensity- and magnitude-averaged B and V magnitudes, magnitude-averaged colors, pulsation periods, and pulsation amplitudes. The mean period of the RRab variables, Pab = 0.5868 days, and the number fraction of RRc stars, Nc/NRR = 0.342, are both large for an Oosterhoff type I (OoI) globular cluster, suggesting that M75 may be Oosterhoff-intermediate. Possible conflicts between Oosterhoff-type determination based on the AV–log P and AB–log P diagrams are discussed. The physical parameters of the RRc and RRab variables, as obtained from Fourier decomposition of their light curves, do not show any clear deviation from normal OoI behavior.

Variable stars in the globular cluster M 13

Results of a search for variable stars in the central region o f the globular cluster M 13 are presented. Prior to this study, 36 variable and suspected variable stars were known in this cluster (Osborn 2000; Clement et al. 2001). Of these stars, five were not observed by us. We find v3, v4, v10, v12, and v13 to b e constant in light. Surprisingly, only two out of the ten variable star candidates of Kadla et al. (1980) appear to be variable. Both are RRc variables. Additionally, three RR Lyrae stars and one SX Phoenicis variable are discovered. Three close frequencies are detected for an RRc star v36. It appears that this variable is another multi-periodic RR Lyrae star pulsating in non-radial modes. Light curves of the three known BL Herculis stars and all known RR Lyrae stars are presented. The total number of known RR Lyrae stars in M 13 is now nine. Only one is an RRab star. The mean period of RRc variables amounts to 0.36± 0.05 d, suggesting that M 13 should be included in the group of Oosterhoff type II globular clusters. Mean V magnitudes and ranges of variation are derived for seven RR Lyrae and three BL Herculis variables. Almost all observed bright giants show some degree of variability. In particular, we confir m the variability of two red giants announced to be variable by Osborn (2000) and in addition find five new cases.

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda VI

We have surveyed Andromeda VI, a dwarf spheroidal galaxy companion to M31, for variable stars by using F450W and F555W observations obtained with the Hubble Space Telescope. A total of 118 variables were found, including 111 RR Lyrae stars, six anomalous Cepheids, and one variable that we were unable to classify. We find that the Andromeda VI anomalous Cepheids have properties consistent with those of anomalous Cepheids in other dwarf spheroidal galaxies. We revise the existing period-luminosity relations for these variables. Further, using these and other available data, we show that there is no clear difference between fundamental and first-overtone anomalous Cepheids in a period-amplitude diagram at shorter periods, unlike the RR Lyrae stars. For the Andromeda VI RR Lyrae stars, we find that they lie close to the Oosterhoff type I Galactic globular clusters in the period-amplitude diagram, although the mean period of the RRab stars, ⟨Pab⟩ = 0.588 days, is slightly longer than that of the typical Oosterhoff type I cluster. The mean V magnitude of the RR Lyrae stars in Andromeda VI is 25.29 ± 0.03, resulting in a distance 815 ± 25 kpc on the Lee, Demarque, & Zinn distance scale. This is consistent with the distance derived from the I magnitude of the tip of the red giant branch. Similarly, the properties of the RR Lyrae stars indicate a mean abundance for Andromeda VI that is consistent with that derived from the mean red giant branch color.

Variable Stars in the Unusual, Metal-rich Globular Cluster NGC 6388

We have undertaken a search for variable stars in the metal-rich globular cluster NGC 6388, using time-series BV photometry. Twenty-eight new variables were found in this survey, increasing the total number of variables found near NGC 6388 to ∼57. A significant number of the variables are RR Lyrae (∼14), most of which are probable cluster members. The periods of the fundamental-mode RR Lyrae are shown to be unusually long compared with metal-rich field stars. The existence of these long-period RRab stars suggests that the horizontal branch of NGC 6388 is unusually bright. This implies that the metallicity-luminosity relationship for RR Lyrae stars is not universal if the RR Lyrae in NGC 6388 are indeed metal-rich. We consider the alternative possibility that the stars in NGC 6388 may span a range in [Fe/H]. Four candidate Population II Cepheids were also found. If they are members of the cluster, NGC 6388 would be the most metal-rich globular cluster to contain Population II Cepheids. The mean V magnitude of the RR Lyrae is found to be 16.85 ± 0.05, resulting in a distance of 9.0 to 10.3 kpc for a range of assumed values of ⟨MV⟩ for RR Lyrae. We determine the reddening of the cluster to be E(B-V) = 0.40 ± 0.03 mag, with differential reddening across the face of the cluster. We discuss the difficulty in determining the Oosterhoff classification of NGC 6388 and NGC 6441 due to the unusual nature of their RR Lyrae and address evolutionary constraints on a recent suggestion that they are of Oosterhoff type II.

RR Lyrae Variable Stars in the Dwarf Spheroidal Galaxy Leo I

We report the discovery of a significant population of RR Lyrae variable stars in the dwarf spheroidal galaxy Leo I. Based on 40 V and 22 B images of the galaxy taken using the ESO Wide-Field Imager, we have so far identified 74 candidate RR Lyrae stars in two CCDs hosting the main body of the galaxy. Full coverage of the light variations and pulsation periods have been obtained for 54 of them, 47 of which are Bailey ab-type RR Lyrae (RRab) stars and seven are c-type RR Lyrae (RRc) stars. The period distribution of the presently confirmed sample of RRab stars peaks at P = 0.60 days, with a minimum period of 0.54 days. The pulsational properties indicate an intermediate Oosterhoff type for Leo I, similar to other dwarf galaxies in the Local Group and the LMC. However, the rather minimum period of the ab-type variable stars, and the significant number of RRab stars with period and large amplitude, suggest that the bulk of the old population in Leo I is more like the Oosterhoff type II globular clusters. The most straightforward interpretation is that a range in metallicity is present among the RR Lyrae stars of Leo I, with a significant population of very metal-poor stars. Alternatively, these Oosterhoff type II variable stars could be more evolved. The average apparent magnitude of the RR Lyrae stars across the full cycle is V(RR) = 22.60 ± 0.12 mag, yielding a distance modulus (m-M)V, 0 = 22.04 ± 0.14 mag for Leo I on the long distance scale.

Variable Stars in Galactic Globular Clusters

Based on a search of the literature up to 2001 May, the number of known variable stars in Galactic globular clusters is approximately 3000. Of these, more than 2200 have known periods and the majority (approximately 1800) are of the RR Lyrae type. In addition to the RR Lyrae population, there are approximately 100 eclipsing binaries, 120 SX Phoenicis variables, 60 Cepheids (including Population II Cepheids, anomalous Cepheids and RV Tauri), and 120 SR/red variables. The mean period of the fundamental mode RR Lyrae variables is 0.585 days, for the overtone variables it is 0.342 days (0.349 days for the first-overtone pulsators and 0.296 days for the second-overtone pulsators) and approximately 30% are overtone pulsators. These numbers indicate that about 65% of RR Lyrae variables in Galactic globular clusters belong to Oosterhoff type I systems. The mean period of the RR Lyrae variables in the Oosterhoff type I clusters seems to be correlated with metal abundance in the sense that the periods are longer in the more metal poor clusters. Such a correlation does not exist for the Oosterhoff type II clusters. Most of the Cepheids are in clusters with blue horizontal branches.

CCD Observations of the RR Lyrae Variables in the Globular Cluster NGC 5897

CCD observations for 10 (six previously known and four newly discovered) of the 11 RR Lyrae variables in NGC 5897 have been analyzed. The period-luminosity and period-amplitude plots indicate that the population of RR Lyrae variables in NGC 5897 includes three fundamental mode, four first-overtone and four second-overtone variables with mean periods 0.828, 0.459, and 0.343 days, respectively. The variables have properties similar to some of the Oosterhoff type II variables in ω Centauri. Two of the new variables (V11 and V13) were previously considered to be possible nonvariables that lie within or near the instability strip on the horizontal branch; both have V amplitudes less than 0.2 mag. There is a chance that one or two of the variables (V10 and possibly V3) might be anomalous Cepheids and not RR Lyrae stars.

Metallicities for Double-Mode RR Lyrae Stars in the Large Magellanic Cloud

Metallicities for six double-mode RR Lyrae stars (RRd's) in the Large Magellanic Cloud have been estimated using the ΔS method. The derived [Fe/H] values are in the range [Fe/H] = -1.09 to -1.78 (or -0.95 to -1.58, adopting a different calibration of [Fe/H] versus ΔS). Two stars in our sample are at the very metal-rich limit of all RRd's for which metal abundance has been estimated, either by direct measure (for field objects) or on the basis of the hosting system (for objects in globular clusters or external galaxies). These metal abundances, coupled with mass determinations from pulsational models and the Petersen diagram, are used to compare the mass-metallicity distribution of field and cluster RR Lyrae variables. We find that field and cluster RRd's seem to follow the same mass-metallicity distribution, within the observational errors, strengthening the case for uniformity of properties between field and cluster variables. At odds to what is usually assumed, we find no significant difference in mass for RR Lyrae variables in globular clusters of different metallicity and Oosterhoff types, or there may even be a difference contrary to the commonly accepted one, depending on the metallicity scale adopted to derive the masses. This unusual result for the mass-metallicity relation is probably due, at least in part, to the inclusion of updated opacity tables in the computation of metal-dependent pulsation models.

The Globular Cluster ω Centauri and the Oosterhoff Dichotomy

CCD observations obtained by the OGLE team for 128 RR Lyrae variables in ? Centauri have been analyzed. The period-luminosity and period-amplitude plots indicate that, in addition to fundamental (RRab) and first overtone (RRc) pulsators, the ? Centauri RR Lyrae population seems to include second overtone (RRe) and possibly third-overtone pulsators. The mean period for the 59 RRab stars is 0649, for the 48 RRc stars, it is 0383, and for the 21 RRe stars, it is 0304. The mean periods derived for the RRab and RRc stars are typical values for an Oosterhoff type II (OoII) cluster. Nevertheless, the period-amplitude plot also shows that some of the RR Lyrae variables have Oosterhoff type I (OoI) characteristics. Most of the second-overtone variables exhibit nonradial pulsations similar to those recently detected in some of the RR Lyrae variables in the clusters M55 and M5, in the Galactic bulge, and in the LMC. Relative luminosities derived for the RRc variables from Fourier coefficients correlate with the observed apparent magnitudes. Masses for the RRc stars have been calculated from Fourier coefficients. A comparison of the derived masses for RRc stars in the four OoII clusters ? Cen, M15, M55, and M68 indicates that the masses of the RRc stars in M15 and M68 are almost 0.2 M? greater than those in the other two. Since M15 and M68 have a high frequency of RRd stars among their first-overtone pulsators, while none have been identified in ? Cen or M55, this suggests that the double-mode pulsation phenomenon may be associated with mass. Among the RRc variables in ? Cen, the OoII variables have lower derived masses and higher luminosities than the OoI variables. An application of the period-density law to pairs of OoI and OoII RRab stars selected according to their position in the period-amplitude plot also indicates that the OoII variables in general have lower masses and higher luminosities. These findings support the hypothesis that the RR Lyrae variables in OoII systems are evolved horizontal-branch stars that spend their zero-age horizontal-branch phase on the blue side of the instability strip.

The outer-halo globular cluster NGC 6229 - IV. Variable stars

We present time-series B, V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the candidate list of Borissova et al. located very close to the cluster centre. Two stars suspected to be variables by Carney, Fullton and Trammell are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates of Borissova et al. are confirmed variables – three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one an eclipsing binary (most likely an Algol system) and one a bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard has been employed which, together with the Welch–Stetson technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRabs, whereas the other two are RRcs. As originally suggested by Mayer, NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are contrasted with those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.

RR Lyrae variables in the globular cluster M 5

We present V-band CCD photometry of 65 RR Lyr variables from the globular cluster M5. We have estimated the basic physical parameters for 16 RRc stars and 26 RRab stars using a Fourier decomposition of the light curves of the variables. The mean values of mass, luminosity, effective temperature and relative helium abundance for the RRc stars are measured to be ${\cal M}=0.54{\cal M}_\odot$, log L = 1.69 T_{eff}=7353 K and Y=0.28, respectively. For the RRab variables the derived mean values of absolute magnitude, metallicity and effective temperature are: M_V=0.81, [Fe/H]=-1.23 and T_{eff}=6465 K. We find that the V amplitude of an RRab star for a given period is a function of metal abundance rather than Oosterhoff type. We find significant problems with the calibration of both the zero point and the scale of the luminosities measured with the Fourier technique. The apparent distance modulus derived from RRc stars is equal to $14.47\pm0.11$ and it is in good agreement with recent determinations. On the other hand distance modulus obtained from the sample of RRab stars (calibrated by the Baade-Wesselink observations of field RR Lyr variables) is significantly smaller and equal to $14.27\pm0.04$

The NGC 6426 RR Lyrae Variables and Horizontal-Branch Morphology

We present BVRI CCD photometry for 12 RR Lyrae variables, including three newly discovered ones in the Oosterhoff type II globular cluster NGC 6426. New light curves and ephemerides are presented. The mean periods of the RRab and RRc variables whose light curves are analyzed in this work are Pab = 0.70 ± 0.02 days and Pc = 0.34 ± 0.03 days, respectively. The number ratio of the RRc type variables to the total number of RR Lyrae type variables is n(c)/n(ab + c) = 0.36. The period-amplitude relation for fundamental-mode RR Lyrae variables (RRab) in NGC 6426 supports the recent conclusion of Clement & Shelton that this relation is not a function of metal abundance. Fourier decomposition of the light curves has been used to determine the mass, luminosity, and temperature for the RRc stars. Application of the formula of Jurcsik & Kovacs, which relates Fourier parameters of RRab stars to [Fe/H], yielded the value [Fe/H]= -2.16 ± 0.13 dex, which is compatible (given the errors) with the value -2.33 ± 0.15 dex, derived from the red giant branch color index developed by Hatzidimitriou et al. From star counts along the horizontal branch (HB), we obtain the Lee et al. HB color distribution index, (B-R)/(B+V+R) = 0.58 ± 0.18. The HB of NGC 6426 resembles that of NGC 5053 and M68. We find almost no faint blue stars analogous to the ones constituting the extended HB in M15.

The Distance to the Galactic Center

ABSTRACTThe Optical Gravitational Lensing Experiment (OGLE) data on high‐amplitude δ Scuti stars (HADS) and RR Lyrae stars have been analyzed to determine the distance to the Galactic bulge. Individual color excesses and V absorption for each individual variable star have been computed, which lead to accurate 〈V0〉 magnitudes. The period‐luminosity relation of the HADS is utilized to calculate distance moduli. We find that the metal‐strong HADS are concentrated strongly to the bulge. The best distance modulus to the bulge from the HADS data is 14.49 ± 0.06 mag. The dependence of Mv on period for the bulge RR Lyrae stars can be found from the reddening‐free magnitude VV - I. The zero point of the Mv–log P relation is set with the horizontal branches of Oosterhoff type I globular clusters. Mv≅0.44 mag (long‐distance scale) in the log P interval −0.29 to −0.22 as inferred by main‐sequence fitting of the clusters to local subdwarfs and utilizing short‐period HADS in the clusters. The best distance modulus to the bulge from the RR Lyrae (ab) stars is 14.45 mag, which compares favorably with the distance modulus of 14.49 mag found from the HADS. We adopt the mean value 14.47 ± 0.08 mag for the bulge distance modulus. A small additive correction leads to 7.9 ± 0.3 kpc as the best distance to the Galactic center.

New CCD Observations of the RR Lyrae Variables in the Oosterhoff Type IICluster M9

CCD observations have been used to study 16 of the previously known variables in the Oosterhoff type II globular cluster M9. The properties of the RR Lyrae variables in M9 are compared with those in the Oosterhoff type I cluster M3. Methods devised by Simon, Jurcsik, & Kovacs have been used to derive physical parameters for the stars from Fourier analysis. The mean metal abundance so derived for M9, [Fe/H] = -1.77, is in good agreement with the metal abundance derived from spectroscopic observations. All methods for calculating the absolute magnitudes of RR Lyrae variables from Fourier coefficients indicate that the RR Lyrae variables in M9 are more luminous than those in M3. However, there are three RR Lyrae stars in M3 that may be as bright as the M9 stars. These three are thought to be in a more advanced evolutionary state than the other M3 RR Lyrae stars. Absolute magnitudes have also been derived for the RR Lyrae stars in M68 and M107. The different methods predict that RR Lyrae stars in metal-poor clusters are more luminous than those in more metal-rich clusters, but there are inconsistencies in the relative luminosities derived by the different methods.

The RR Lyrae Period-Amplitude Relation as a Clueto the Origin of the Oosterhoff Dichotomy

An examination of the period-V amplitude relation for RRab stars (fundamental mode pulsators) with "normal" light curves in the Oosterhoff type I clusters M3 and M107 and in the Oosterhoff type II clusters M9 and M68 reveals that the V amplitude for a given period is not a function of metal abundance. Rather, it is a function of the Oosterhoff type. This result is confirmed by published observations of RRab stars in M4, M5, and M92. A method devised by Jurcsik & Kovács has been used to determine whether the light curve of an RRab star is "normal" or "peculiar." Although M3 is considered to belong to the Oosterhoff type I group, it has three bright RRab stars that seem to fit the period-amplitude relation for Oosterhoff type II RRab stars. There is evidence that these bright stars are in a more advanced evolutionary state than the other RRab stars in M3, thus leading to the conclusion that the Oosterhoff dichotomy is due to evolution. Our result gives support to the Lee, Demarque, & Zinn hypothesis that most RR Lyrae variables in Oosterhoff type I clusters are zero-age horizontal branch (ZAHB) objects, while those in the Oosterhoff type II clusters are more evolved. This may have important implications for the derived ages of Oosterhoff type II clusters. If their RR Lyrae variables have all evolved away from the ZAHB, then their ages have been overestimated in studies that assume they are ZAHB objects.

The Mode Change of the RR Lyrae Variable V79 in M3

The RR Lyrae variable V79 in M3 has been studied on approximately 1000 photographs in the Sternberg Observatory plate collection and on 126 CCD frames. The photographs were obtained over the interval 1949-1992, and the CCD observations were made in 1998. A comparison of our data with observations made in 1992, 1993, and 1996 indicates that V79 changed its pulsation mode in 1992. Before 1992, it was a fundamental mode pulsator (RRab star) with an irregular light curve, but since then it has been a double-mode pulsator (RRd star) with the first overtone the dominant mode. Prior to the mode change, there were sporadic episodes when pulsation in the first overtone mode occurred, for example, for a few days in 1976 and again in 1989. In each case, however, the fundamental mode pulsation resumed afterward. During the first two weeks in April of 1989, the star appeared to stop varying, and when it began again, the period of the fundamental mode pulsation had decreased substantially, from 0.483 to 0.480 days. Although the first overtone has been the dominant mode of pulsation for V79 since 1992 when it became an RRd star, our 1998 observations show that the amplitude of the fundamental mode is greater than it was in 1996. Previously published investigations of the Oosterhoff type II cluster M15 indicate that some of its RRd stars have exhibited irregular behavior, but no mode switches have been observed. In addition, there are some short period RRab stars in other Oosterhoff type I clusters that have changed their light curve amplitude and shape on a timescale of years, but up to now, none of these have been shown to be RRd stars.