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Register a new userA multi-color and Fourier study of RR Lyrae variables in the globular cluster NGC 5272 (M3)
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We have performed a detailed study of the pulsational and evolutionary characteristics of 133 RR Lyrae stars in the globular cluster NGC5272 (M3) using highly accurate BVI data taken on 5 separate epochs. M3 seems to contain no less than ~32% of Blazhko stars, and the occurrence and characteristics of the Blazhko effect have been analyzed in detail. We have identified a good number (~ 14%) of overluminous RR Lyrae stars that are likely in a more advanced evolutionary stage off the Zero Age Horizontal Branch (ZAHB). Physical parameters (i.e. temperature, luminosity, mass) have been derived from (B--V) colors and accurate color-temperature calibration, and compared with Horizontal Branch evolutionary models and with the requirements of stellar pulsation theory. Additional analysis by means of Fourier decomposition of the V light curves confirms, as expected, that no metallicity spread is present in M3. Evolution off the ZAHB does not affect [Fe/H] determinations, whereas Blazhko stars at low amplitude phase do affect [Fe/H] distributions as they appear more metal-rich. Absolute magnitudes derived from Fourier coefficients might provide useful average estimates for groups of stars, if applicable, but do not give reliable {em individual} values. Intrinsic colors derived from Fourier coefficients show significant discrepancies with the observed ones, hence the resulting temperatures and temperature-related parameters are unreliable.
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We present predicted relations connecting pulsational and evolutionary parameters, as based on a wide set of convective pulsating models of RR Lyrae stars with Z=0.001, Y=0.24 and mass and luminosity suitable for the ``old (age $>$ 8 Gyr) variables observed in globular clusters. The relations are collated with sound constraints on the mass of pulsators, as inferred from up-to-date evolutionary models of horizontal branch stars, in order to provide a self-consistent theoretical framework for the analysis of observed variables. The theoretical predictions are tested through a detailed comparison with measurements of RR Lyrae stars in the globular cluster M3. We show that the predicted relations satisfy a variety of observed data, thus providing a pulsational route to the determination of accurate distances to RR Lyrae-rich globular clusters with intermediate metal content. We discuss the effect of different physical inputs in evolutionary computations, as well as of different bolometric corrections adopted to convert bolometric luminosity into absolute magnitude. We show that the constraints inferred by pulsation theory support the large value of the mixing-length parameter adopted to fit observed RGB, but, at the same time, they would yield that the luminosity of updated HB models is too bright. Such discrepancy is significantly reduced if element diffusion is properly taken into account.
It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium-enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of > 0.02 in virtually all blue HB stars. Using high-precision Stroemgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the clusters blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature.
NGC 362 is a bright southern globular cluster for which no extensive variability survey has ever been done. Time-series CCD photometric observations have been obtained. Light curves have been derived with both profile fitting photometry and image subtraction. We developed a simple method to convert flux phase curves to magnitudes, which allows the use of empirical light curve shape vs. physical parameters calibrations. Using the RR Lyrae metallicity and luminosity calibrations, we have determined the relative iron abundances and absolute magnitudes of the stars. The color-magnitude diagram has been fitted with Yale-Yonsei isochrones to determine reddening and distance independently. For five RR Lyrae stars we obtained radial velocity measurements from optical spectra. We found 45 RR Lyr stars, of which the majority are new discoveries. About half of the RR Lyraes exhibit light curve changes (Blazhko effect). The RR Lyrae-based metallicity of the cluster is [Fe/H]=-1.16 +/- 0.25, the mean absolute magnitude of the RR Lyrae stars is M_V=0.82 +/- 0.04 mag implying a distance of 7.9 +/- 0.6 kpc. The mean period of RRab stars is 0.585 +/- 0.081 days. These properties place NGC 362 among the Oosterhoff type I globular clusters. The isochrone fit implies a slightly larger distance of 9.2 +/- 0.5 kpc and an age of 11 +/- 1 Gyr. We also found 11 eclipsing binaries, 14 pulsating stars of other types, including classical Cepheids in the SMC and 15 variable stars with no firm classification.
Recently, Kundu et al (2019) reported that the globular cluster NGC 5024 (M53) possesses five extra-tidal RR Lyrae. In fact, four of them were instead known members of a nearby globular cluster NGC 5053. The status of the remaining extra-tidal RR Lyrae is controversial depending on the adopted tidal radius of NGC 5024. We have also searched for additional extra-tidal RR Lyrae within an area of $sim8$~deg$^2$ covering both globular clusters. This includes other known RR Lyrae within the search area, as well as stars that fall within the expected range of magnitudes and colors for RR Lyrae (and yet outside the cutoff of 2/3 of the tidal radii of each globular clusters for something to be called extra-tidal) if they were extra-tidal RR Lyrae candidates for NGC 5024 or NGC 5053. Based on the the proper motion information and their locations on the color-magnitude diagram, none of the known RR Lyrae belong to the extra-tidal RR Lyrae of either globular clusters. In the cases where the stars satisfied the magnitude and color ranges of RR Lyrae, analysis of time series data taken from the Zwicky Transient Facility do not reveal periodicities, suggesting that none of these stars are RR Lyrae. We conclude that there are no extra-tidal RR Lyrae associated with either NGC 5024 or NGC 5053 located within our search area.
We have applied the image subtraction method to the M3 dataset previously analyzed by Corwin & Carney (2001; CC01). The new analysis produced light curves and periods for 15 variables, bringing to 222 the total number of RR Lyrae stars in CC01 M3 dataset. We have identified three new candidate double-mode (RRd) variables (V13, V200, and V251) in M3. Of the newly discovered RRds V13 is unusual in that it has the fundamental as the dominant pulsation mode. Two of the new candidate RRds (V13 and V200) have period ratios as low as 0.738-0.739. They lie separated from all previously known RRds in the Petersen diagram, in positions implying a large spread in mass and/or, less likely, in heavy element mass fraction, among the M3 horizontal branch (HB) stars. We explore mass transfer and helium enhancement as possible explanations for the apparent spread in HB masses. We also note that the masses derived from the RRd analyses now favor little mass loss on the red giant branch. V200 has changed its dominant pulsation mode from fundamental to first overtone, while V251 has changed its dominant mode from first overtone to fundamental in the interval 1992 to 1993. Together with M3-V166 this is the first time that RRd variables are observed to switch their dominant pulsation modes while remaining RRds. The phenomenon is found to occur in a one year time-span thus suggesting that these stars are undergoing a rapid evolutionary phase, and that both redward and blueward evolution may take place among the HB stars in M3. The unusual behavior of the M3 RRds is discussed and compared to that of the RRds identified so far in globular clusters and in the field of our and other Local Group galaxies. We find lack of correlation between the presence of RRd variables and any of the cluster structural parameters.
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