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 Physics , 2012, Abstract: We compiled a list of about 250 SX Phoenicis stars known in Galactic globular clusters in order to study period-luminosity relation for this type of variable. The absolute magnitudes of these stars are derived using metallicity-luminosity calibration for RR Lyrae stars. The mixture of different radial and non-radial modes present in SX Phoenicis stars and the lack of unique method of mode identification cause the difficulties in defining strict period-luminosity relation. As a solution we propose to use confirmed double-mode radial pulsators.
 Physics , 2001, DOI: 10.1051/0004-6361:20010369 Abstract: We present new results on five of six known SX Phoenicis stars in the core of the globular cluster 47 Tucanae. We give interpretations of the light curves in the V and I bands from 8.3 days of observations with the Hubble Space Telescope near the core of 47 Tuc. The most evolved SX Phe star in the cluster is a double-mode pulsator (V2) and we determine its mass to be 1.54+/-0.05 solar masses from its position in the Hertzsprung-Russell diagram and by comparing observed periods with current theoretical pulsation models. For V14 we do not detect any pulsation signal. For the double-mode pulsators V3, V15, and V16 we cannot give a safe identification of the modes. We also describe the photometric techniques we have used to extract the light curves of stars in the crowded core. Some of the SX Phoenicis are saturated and we demonstrate that even for stars that show signs of a bleeding signal we can obtain a point-to-point accuracy of 1-3%.
 Physics , 2011, DOI: 10.1111/j.1365-2966.2011.19201.x Abstract: We report CCD V and I time series photometry of the globular cluster NGC 5024 (M53). The technique of difference image analysis has been used which enables photometric precisions better than 10 mmag for stars brighter than V 18.5mag even in the crowded central regions of the cluster. The high photometric precision has resulted in the discovery of two new RR1 stars and thirteen SX Phe stars. A detailed identification chart is given for all the variable stars in the field of our images of the cluster. Periodicities were calculated for all RR Lyraes and SX Phe stars and a critical comparison is made with previous determinations. Out of four probable SX Phe variables reported by Dekany & Kovacs (2009), the SX Phe nature is confirmed only for V80, V81 is an unlikely case while V82 and V83 remain as dubious cases. Previous misidentifications of three variables are corrected. Astrometric positions with an uncertainty of ~ 0.3 arcsec are provided for all variables. The light curve Fourier decomposition of RR0 and RR1 is discussed, we find a mean metallicity of [Fe/H]=-1.92+-0.06 in the scale of Zinn & West from 19 RR0 stars. The true distance moduli 16.36+-0.05 and 16.28+-0.07 and the corresponding distances 18.7+-0.4 and 18.0+-0.5 kpc are found from the RR0 and RR1 stars respectively. These values are in agreement with the theoretical period-luminosity relations for RR Lyrae stars in the I band and with recent luminosity determinations for the RR Lyrae stars in the LMC. The age of 13.25+-0.50 Gyr (Dotter et al. 2010), for NGC 5024, E(B-V)=0.02 and the above physical parameters of the cluster, as indicated from the RR0 stars, produce a good isochrone fitting to the observed CMD. The PL relation for SX Phe stars in NGC 5024 in the V and I bands is discussed in the light of the 13 newly found SX Phe stars and their pulsation mode is identified in most cases.
 Physics , 2003, DOI: 10.1086/375323 Abstract: Through time-series CCD photometry of the metal-poor globular cluster M53, we have discovered eight new SX Phoenicis type stars (labeled from SXP1 to SXP8). All the new SX Phoenicis stars are located in the blue straggler star region of a color-magnitude diagram of M53. One of these stars (SXP2) is found to have very closely separated pulsation frequencies: $f_1/f_2 = 0.9595$ where $f_1$ and $f_2$ are primary and secondary frequencies. This may be due to excitation of non-radial modes. Six of these SX Phoenicis stars are considered to be pulsating in the fundamental mode. They show a tight linear correlation between the period and luminosity. We derive a period - luminosity relation for the fundamental mode for the period range of $-1.36 < Log P[d]< -1.15$ : $=-3.010(\pm0.262)Log P + 15.310(\pm0.048)$ with an rms scatter of 0.038, corresponding to $=-3.010 Log P - 1.070$ for an adopted distance modulus of $(m-M)_V=16.38$ (Harris 1996).
 Physics , 2012, Abstract: We report the discovery of 6 SX Phoenicis stars in the southern globular cluster NGC 4833. Images were obtained from January through June 2011 with the Southeastern Association for Research in Astronomy 0.6 meter telescope located at Cerro Tololo Interamerican Observatory. The image subtraction method of Alard & Lupton (1998)was used to search for variable stars in the cluster. We confirmed 17 previously cataloged variables by Demers & Wehlau(1977). In addition to the previously known variables we have identified 10 new variables. Of the total number of confirmed variables in our 10x10 arcmin^2 field, we classified 10 RRab variables, with a mean period of 0.69591 days, 7 RRc, with a mean period of 0.39555 days, 2 possible RRe variables with a mean period of 0.30950 days, a W Ursae Majoris contact binary, an Algol-type binary, and the 6 SX Phoenicis stars with a mean period of 0.05847 days. The periods, relative numbers of RRab and RRc variables, and Bailey diagram are indicative of the cluster being of the Oosterhoff type II. We present the phased-light curves, periods of previously known variables and the periods and classifications of the newly discovered variables, and their location on the color-magnitude diagram.
 Physics , 2013, Abstract: Using V images taken in May and June 2012 with the SARA Consortium's 0.9-meter telescope located at Kitt Peak National Observatory, we searched for variable stars in the globular cluster M107 (NGC 6171). The search was accomplished using the ISIS v2.2 image subtraction software. We refined the positions of the previously known variables and confirmed the 21 RR Lyrae variables from Clement's Catalog of Variable Stars in Globular Clusters (Clement et al. 2001). We also discovered a previously unknown variable which is likely an SX Phoenicis star. For this SX Phoenicis star we measured a fundamental pulsation frequency 19.0122/day (P=0.05257 days) and a mean amplitude of 0.046 magnitudes in the V band. This variable had an average V-band magnitude of 17.72, nearly 2 magnitudes dimmer than the horizontal branch of M107, typical of SX Phoenicis stars and blue stragglers lying just beyond the main sequence turn-off in globular clusters.
 Physics , 2001, DOI: 10.1086/321408 Abstract: We present a detailed linear, nonadiabatic pulsational scenario for oscillating Blue Stragglers (BSs)/SX Phoenicis (SX Phe) in Galactic Globular Clusters (GGCs) and in Local Group (LG) dwarf galaxies. The sequences of models were constructed by adopting a wide range of input parameters and properly cover the region of the HR diagram in which these objects are expected to be pulsationally unstable. Current calculations together with more metal-rich models already presented by Gilliland et al. suggest that the pulsation properties of SX Phe are partially affected by metal content. Our calculations also suggest that the secular period change seems to be a good observable to identify the pulsation mode of cooler SX Phe variables. The dependence of pulsation properties on the helium envelope content is discussed and we find that the secular period change for He-enhanced models is approximately a factor of two larger than for canonical ones. According to this evidence we suggest that the pulsation properties of SX Phe can be soundly adopted to constrain the evolutionary history of BSs, and in turn to single out the physical mechanisms which trigger their formation.
 Physics , 2003, DOI: 10.1046/j.1365-8711.2003.06361.x Abstract: We report on the discovery of eleven multimode SX Phoenicis--type blue stragglers in the field of the southern globular cluster NGC 3201. In these variables both radial and non-radial modes are excited. For three variables the derived period ratio is close to that observed in SX Phoenicis itself, suggesting that these stars are pulsating in the fundamental and the first-overtone radial modes. Using the McNamara (1997) period-luminosity relation we have estimated the apparent distance modulus to NGC 3201 to be 14.08$\pm0.06\pm0.1$mag.
 Physics , 2001, DOI: 10.1051/0004-6361:20000349 Abstract: We present CCD photometry of SX Phe variables in the field of the globular cluster M 55. We have discovered 27 variables, three of which are probable members of the Sagittarius dwarf galaxy. All of the SX Phe stars in M 55 lie in the blue straggler region of the cluster color-magnitude diagram. Using period ratio information we have identified the radial pulsation modes for one of the observed variables. Inspection of the period-luminosity distribution permits the probable identifications of the pulsation modes for most of the rest of the stars in the sample. We have determined the slope of the period-luminosity relation for SX Phe stars in M 55 pulsating in the fundamental mode. Using this relation and the HIPPARCOS data for SX Phe itself, we have estimated the apparent distance modulus to M 55 to be (m-M)_V=13.86 +- 0.25 mag.
 Physics , 2013, Abstract: Time-series photometric observations were made for the SX Phoenicis star AE UMa between 2009 and 2012 at the Xinglong Station of National Astronomical Observatories of China (NAOC). No additional frequencies were detected. With the frequency analysis of the light curves, we confirmed the fundamental and the first overtone frequencies as $f_{0} = 11.62560\ cd^{-1}$ and $f_{1} = 15.03124\ cd^{-1}$. Additionally, we also got 37 frequencies which are the harmoics and the combinations of the fundamental and first overtone frequency. 25 of these frequencies are newly detected. The O-C diagram, produced from 84 newly determined times of maximum light combined with those derived from the literature, reveals a continuous period variation with the rate of $(1/P_{0})(dP_{0}/dt) = 5.36(84) \times 10^{-9}\ yr^{-1}$ and $(1/P_{1})(dP_{1}/dt) = -2.78(37) \times 10^{-8}\ yr^{-1}$. Theoretical rates of period change due to the stellar evolution were calculated with a modeling code. Constraint of the variation of $P_{1}$ allows us to determine the star's location on the H-R diagram precisely. The result shows that the observed rate of period change is fully consistant with period change caused by the evolutionary behavior predicted by standard theoretical model.
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