Fundamental Mode RR Lyrae Stars Research Articles

The Dwarf Spheroidal Companions to M31: Variable Stars in Andromeda I and Andromeda III

We present the results of variable star searches of the M31 dwarf spheroidal (dSph) companions Andromeda I and Andromeda III using the Hubble Space Telescope. A total of 100 variable stars were found in And I, while 56 were found in And III. One variable found in And I and another in And III may be type II Cepheids. In addition to this variable in And III, another four variables are anomalous Cepheids (ACs). So far, no definite ACs have been discovered in And I. We discuss the properties of these variables with respect to those found in the other dSph galaxies and revisit the AC period-luminosity relations. We found 72 fundamental mode RR Lyrae (RRab) stars and 26 first-overtone mode RR Lyrae (RRc) stars in And I giving mean periods of 0.575 and 0.388 days, respectively. One likely RR Lyrae star in And I remains unclassified because of a lack of F555W data. For And III, 39 RR Lyrae stars are pulsating in the fundamental mode with a mean period of 0.657 days, and 12 are in the first-overtone mode with a mean period of 0.402 days. Using the mean metal abundances derived from the red giant branch colors, the mean RRab period for And I is consistent with the mean period-metallicity relation seen in the RR Lyrae populations of Galactic globular clusters, while And III is not, having too large a mean RRab period for its abundance. In And I, we found two RR Lyrae stars that are noticeably fainter than the horizontal branch. We discuss the possibility that these stars are associated with the recently discovered stellar stream in the halo of M31. Using various methods, we estimate the mean metallicity of the RR Lyrae stars to be [Fe/H] ≈ -1.5 for And I and ≈-1.8 for And III. These estimates match well with other mean metallicity estimates for the galaxies. Interestingly, a comparison of the period-amplitude diagrams for these two galaxies with other dSph galaxies shows that And III is lacking in shorter period, higher amplitude RR Lyrae stars. This may be a consequence of the very red horizontal branch morphology in this dSph. Not including the two faint RR Lyrae stars, we find VRR = 25.14 ± 0.04 mag for And I, resulting in a distance of 765 ± 25 kpc. For And III, VRR = 25.01 ± 0.04 mag, giving a distance of 740 ± 20 kpc. These distance estimates are consistent with those previously found for these galaxies. We discuss the relation between the specific frequency of the ACs in dSph galaxies and the mean metallicity of the galaxy, finding that the M31 dSph galaxies follow the same relations as the Galactic dSph galaxies. We also find that the period-luminosity relations of ACs and short-period Cepheids are distinct, with the short-period Cepheids having higher luminosities at a given period.

Exploring the Upper Red Giant and Asymptotic Giant Branches: The Globular Cluster M5

We have tabulated lists of upper red giant, horizontal-, and asymptotic giant branch (RGB, HB, and AGB) stars in the globular cluster M5 that are complete to over 10' from the core for the RGB and AGB samples, and 8' for the HB sample. The large samples give us the most precise value of R2 = NAGB/NHB to date for a single globular cluster (0.176 ± 0.018). This is incompatible with theoretical calculations using the most recent physical inputs. The discrepancy can probably be attributed to the dependence of observed R2 values on HB morphology. We identify the cluster M55 as another possible example of this effect. Samples of HB and AGB stars in populous clusters may provide a means of calibrating the masses of HB stars in globular clusters. The cumulative luminosity function of the upper RGB shows an apparent deficit of observed stars near the tip of the branch. This feature has less than a 2% chance of being due to statistical fluctuations. The slope of the cumulative luminosity function for AGB stars is consistent with the theoretically predicted value from stellar models when measurement bias is taken into account. We also introduce a new diagnostic Rclump that reflects the fraction of the AGB lifetime that a star spends in the AGB clump. For M5 we find Rclump = 0.42 ± 0.05, in marginal disagreement with theoretical predictions. Finally, we note that the blue half of M5's instability strip (where first-overtone RR Lyrae stars reside) is underpopulated, based on the large numbers of fundamental mode RR Lyrae stars and on the nonvariable stars at the blue end of the instability strip. This fact may imply that the evolutionary tracks (and particularly the colors) of stars in the instability strip are affected by pulsations.

The MACHO Project Large Magellanic Cloud Variable Star Inventory. XI. Frequency Analysis of the Fundamental‐Mode RR Lyrae Stars

We have frequency-analyzed 6391 variables classified earlier as fundamental-mode RR Lyrae (RR0) stars in the MACHO database on the Large Magellanic Cloud (LMC). The overwhelming majority (i.e., 96%) of these variables have been proved to be indeed RR0 stars, whereas the remaining ones have fallen into one of the following categories: single- and double-mode Cepheids, binaries, first-overtone and double-mode RR Lyrae stars, and nonclassified variables. Special attention has been paid to the properties of the amplitude- and phase-modulated RR0 stars (the Blazhko stars). We found altogether 731 Blazhko variables showing either a doublet or an equidistant triplet pattern at the main pulsation component in their frequency spectra. This sample overwhelmingly exceeds the number of Blazhko stars known in all other systems combined. The incidence rate of the Blazhko variables among the RR0 stars in the LMC is 11.9%, which is 3 times higher than their rate among the first-overtone RR Lyrae stars. No difference is found in the average brightness between the single-mode and Blazhko variables. However, the latter ones show a somewhat lower degree of skewness in their average light curves and a concomitant lower total amplitude in their modulation-free light curves. From the frequency spectra we found that variables with larger modulation amplitudes at the higher frequency side of the main pulsation component are 3 times more common than the ones showing the opposite amplitude pattern. A search for a modulation component with the Blazhko period in the average brightness of the individual variables showed the existence of such a modulation with an overall amplitude of ≈0.006 mag. On the other hand, a similar search for quadruple modulation patterns around the main pulsation component has failed to clearly detect such components at the ≈0.004 mag level. This means that the amplitudes of the quadruple components (if they exist) should be, on average, at least 10 times smaller than those of the triplet components. This finding and the existence of Blazhko variables with highly asymmetric modulation amplitudes not only question the validity of the magnetic oblique rotator model but also put stringent constraints on models based on mode-coupling theories.

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.

Frequency Analysis of a Subset of Fundamental Mode RR Lyrae Stars from the MACHO Project Large Magellanic Cloud Database

AbstractWe present the preliminary results of a frequency analysis of 1457 fundamental mode RR Lyrae (RR0) stars in the Large Magellanic Cloud (LMC) from MACHO Project photometry. We find the same classes of pulsational behavior as were found in our earlier survey of first overtone RR Lyrae (RR1) stars. Variables whose prewhitened power spectra contain one or two peaks close to the main frequency component in the original power spectra are commonly known as Blazhko-type variables. The present analysis shows the overall frequency of Blazhko-type stars in the total RR0 population analysed to date to be ≈ 10%. This is lower than the often cited Galactic field/globular rate of 20-30% (Szeidl, 1988).The incidence rate of Blazhko-type variability in the LMC appears to be about three times higher in RR0 stars than in RR1 stars. This puts important constraints on possible models of the Blazhko effect.

Empirical relations for cluster RR Lyrae stars revisited

Our former study on the empirical relations between the Fourier parameters of the light curves of the fundamental mode RR Lyrae stars and their basic stellar parameters has been extended to considerably larger data sets. The most significant contribution to the absolute magnitude MV comes from the period P and from the first Fourier amplitude A1, but there are statistically significant contributions also from additional higher order components, most importantly from A3 and in a lesser degree from the Fourier phase . When different colors are combined in reddening-free quantities, we obtain basically period-luminosity-color relations. Due to the relation from stellar atmosphere models, we would expect some dependence also on . Unfortunately, the data are still not extensive and accurate enough to decipher clearly the small effect of this Fourier phase. However, with the aid of more accurate multicolor data on field variables, we show that this Fourier phase should be present either in or in or in both. From the standard deviations of the various regressions, an upper limit can be obtained on the overall inhomogeneity of the reddening in the individual clusters. This yields mag, which also implies an average minimum observational error of mag.