SX Phoenicis Stars Research Articles

High‐Amplitude δ Scuti and SX Phoenicis Stars: The Effects of Chemical Composition on Pulsations and the Period‐Luminosity Relation

We present a theoretical study of the radially pulsating δ Scuti and SX Phoenicis variables, concentrating on the blue straggler SX Phoenicis variables found in globular clusters. We have evolved a grid of stellar models with the metal abundance of the globular cluster M55, including models with alpha-enhanced metal abundances, and tested these models for radial pulsations observed in the high-amplitude δ Scuti and SX Phoenicis stars. Our grid includes models with globally enriched helium content to simulate the effects of stellar collisions and global mixing possible in blue stragglers. We find that global enrichment of helium strongly affects the temperature and luminosity of a given star, but the location of the instability strip blue edge and the slope of the period-luminosity (PL) relation are unchanged. This suggests that the PL relation is not affected by blue straggler formation if blue stragglers are fully mixed stellar mergers. Our blue edges and PL relations are in agreement with other theoretical determinations and also with the observational PL relation of M55, but they are not in agreement with the PL relation previously derived for high-amplitude δ Scuti stars in the field. Analysis of the double-mode variable, V41, suggests either that the star may not be pulsating in the first and second overtones as claimed or that normal stellar models may not be accurate models of blue straggler stars.

Preliminary Results of a Search for Variable Stars in the Globular Cluster M 13

AbstractResults of a search for variable stars in the globular cluster M 13 are presented. Comments on the variability of all observed variable or suspected variable stars are given. Five new RR Lyrae stars and one SX Phoenicis star are identified.

Time Series CCD Photometry of the Globular Cluster M 53

AbstractThrough time-series CCD photometry of the globular cluster M 53, we have discovered eight new SX Phoenicis type stars. All the new SX Phoenicis stars are located in the blue straggler star region in the color-magnitude diagram of M 53. In addition, we have obtained light curves for 45 known RR Lyrae stars.

A New SX Phoenicis Star in the Globular Cluster M15

A new SX Phoenicis star (labeled SXP 1) found from BV CCD photometry is the first to be discovered in the globular cluster M15. It is a blue straggler and is located 1028 north and 2856 west of the center of M15. The mean magnitudes of SXP 1 are B = 18.671 and V = 18.445. The amplitude of variability of SXP 1 is measured to be ΔV ≈ 0.15. From multiple-frequency analysis based on the Fourier decomposition method, we detect two very closely separated pulsation frequencies: the primary frequency at f1 = 24.630 cycles day-1 for both B and V bands, and the secondary frequency at f2 = 24.338 cycles day-1 for the B band and 24.343 cycles day-1 for the V band. This star is the second among known SX Phoenicis stars found to pulsate with very closely separated frequencies (f2/f1 ≥ 0.95). These frequencies may be explained by excitation of nonradial modes; however, we have an incomplete understanding of this phenomenon in the case of SX Phoenicis stars with relatively high amplitudes. The relations between metallicity and period and between the variability amplitude and period for SXP 1 are found to be consistent with those for SX Phoenicis stars in other globular clusters.

Pulsating Blue Metal-poor Stars

The blue metal-poor (BMP) star CS 22966-043 is an SX Phoenicis star and the primary of a spectroscopic binary with a provisional orbital period of ~430 days. Radial velocity and UBV photometric observations of this star made in 1998 require downward revision of the orbital period to 319 days. The BMP star CS 29499-057 also appears to be an SX Phoenicis star with small amplitude (ΔV ~ 0.04 mag) and short period (P = 0.0417 days), on the basis of photometric and radial velocity observations obtained in 1998. There is some indication that it too may be the primary of a spectroscopic binary. Three other BMP stars have radial velocity standard deviations greater than those of 17 BMP radial velocity standards. We suggest that they may be small-amplitude SX Phoenicis stars. Finally, the BMP star CS 29497-017 is shown to be a short-period velocity variable (P = 0.302 days) on the basis of observations accumulated over an interval of 2200 days, but we were unable to detect a light variation in 1998 July. Therefore, the nature of the velocity variation of this object remains uncertain.

Multiple frequency analysis of the large-amplitude SX Phoenicis star BL Camelopardalis

A multifrequency analysis of the SX Phoenicis star BL Camelopardalis is presented on the basis of new high-speed photometry, along with fitting a total of 136 maxima. BL Cam is a multiple periodic pulsator. We find f0=25.5768, f1=25.2982, f2=25.8622, f3=31.5912, f4=25.1065, f5=25.5147 and f6=25.6188 cycle d−1 together with the harmonics 51.1513 and 76.7268 cycle d−1 and combination frequencies f0+f1, f0+f2 and f0+f3. The new frequency solution represents the light curves of BL Cam quite well. The observed minus calculated (O-C) analysis indicates that the fundamental frequency is in good agreement with the results of Fourier analysis.

CCD Photometry of Faint Variable Stars in the Globular Cluster NGC 6752

We present the results of a photometric survey for variable stars in the field of the nearby globular cluster NGC 6752. The cluster was monitored in 1996 and 1997 for a total of 54 hr with three different CCD cameras mounted on the 1.0 m Swope Telescope. Eleven new variables were identified: three SX Phoenicis stars, seven contact binaries, and one candidate detached eclipsing binary. All three SX Phoenicis variables are likely members of the cluster, while only one out of the seven contact binaries is a potential cluster member. As a by-product of our survey, we obtained UBV photometry for a large sample of stars in the cluster field. Two stars with U-B ≈ -1.0 and V = 19.3 and V = 20.6 were identified. They lie along the extended horizontal branch of the cluster and are likely to be faint sdB stars from NGC 6752.

The duplicity of large amplitude SX Phoenicis star CY Aquarii

New photoelectric times of maximum light of the large amplitude SX Phoenicis star CY Aquarii are analyzed together with times of maximum light in the literature. The general shape of the fitted curve of the observed minus calculated (O-C) times of maximum light suggest the duplicity of the variable star, which causes long-term period variations in the light-time. The period changes of CY Aqr are probably consequences of a continuously increasing period combined with the light-time effect in a binary system. The secondary companion is less massive. Some spectroscopic orbital elements are derived from the O-C diagram.

CS 22966−043: A Bright New Field SX Phoenicis Star Similar to Those in NGC 5053

CS 22966-043 is an ultra–short-period pulsating star with high velocity (RV = -266 km s-1) discovered during the course of a search for spectroscopic binaries among blue metal-poor field stars, in progress since 1992. With respect to period (0.0374 days), mean color (B - V = 0.24), and metal abundance ([Fe/H] ≈ -2.4), it closely resembles the SX Phoenicis stars found among the blue stragglers in NGC 5053. CS 22966-043 also is the primary of a spectroscopic binary with (probable) period of 430 days. Light-travel time across the projected orbit, as large as 0.0037 days, must be added to the times of observation to combine data obtained in different years with minimal phase dispersion. If CS 22966-043 is, indeed, a blue straggler formed by binary interaction as is now generally believed, then it seems most probable that the interaction was one of mass transfer from the present-day secondary during its post–main-sequence evolution rather than merger of a close binary. The latter option would require that this rare field star was, in addition, a member of a primordial triple system.

TIME-SERIES ENSEMBLE PHOTOMETRY OF SX PHOENICIS STARS. II. AE URSAE MAJORIS

ABSTRACT We have examined the short period, double-mode variable star AE Ursae Majoris. Using time series ensemble photometry we have determined six new times of maximum light. We also applied a Fourier decomposition to the data to determine the component frequencies of the light curve. Using our data, along with archival data, we find that the period of AE UMa is continually decreasing at a rate of -1.14x 10-10 d d-1 . However, from the Fourier decomposition we find that the period ratio P1/P0 has remained constant at 0.773. We also find that AE UMa is incorrectly classified as a SX Phoenicis star.

Structural Properties of Pulsating Star Light Curves Through Fuzzy Divisive Hierarchical Clustering

The aim of this paper is to explore the possibility of using a fuzzy divisive clustering technique to the classification of pulsating stars according to the shape of their light curves. The considered sample contains Cepheides, RR Lyrae stars, and high amplitude δ Scuti and SX Phoenicis stars. Our investigations proved the ability of this method to identify morphological groups in a given class of pulsating stars. The method is also useful for establishing the membership of an unknown variable star in a given class.

TIME-SERIES ENSEMBLE PHOTOMETRY OF SX PHOENICIS STARS. I. BL CAMELOPARDALIS

ABSTRACT We present an analysis of the multiperiodic SX Phoenicis star BL Camelopardalis (GD 428). Along with 24 times of maximum light from archival data, six previously unpublished times of maximum light from photomultiplier observations and 39 new CCD observations of maximum light are reported. The new CCD observations indicate that BL Cam is a double-mode variable with a primary period of 0.0391 day, a secondary period of 0.0306 day, and a π1/ π0 ratio of 0.783. The relation between metallicity and period ratio for large amplitude δ Scuti variables is examined in detail. Finally, evidence is presented that the fundamental period π0 has increased by 0.009 seconds in the last 20 years.

Period-luminosity relations of SX Phoenicis stars

view Abstract Citations (54) References (23) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Period-Luminosity Relations of SX Phoenicis Stars McNamara, D. H. Abstract We propose the following criteria to distinguish between fundamental (F) and first-overtone (H) pulsating SX Phoenicis variables: the (H) pulsators are SX Phe stars with nearly symmetrical light curves and light amplitudes (total range) <= 0.20^m^ in V; the (F) pulsating stars have light amplitudes >= 0.25^m^ in V and asymmetrical light curves. These criteria provide a separation of the stars into two groups that lead to two parallel P-L relations that are offset by 0.37^m^. This offset is in excellent agreement with the offset, 0.36^m^, predicted from the double- mode pulsating variable SX Phe. The (H) pulsating variables have shorter periods on average than the (F) pulsators as predicted by P_1_/P_0_ = Q_1_/Q_0_ ~ 0.778. The mean periods of the SX Phe stars in globular clusters are shown to be correlated with the metallicity [Fe/H] in the sense that the shortest period variables are found in the most metal-poor clusters. The fundamental-mode SX Phe stars in the Carina dwarf galaxy give a distance modulus of 20.0 and [Fe/H]= - 1.5. Publication: The Astronomical Journal Pub Date: April 1995 DOI: 10.1086/117405 Bibcode: 1995AJ....109.1751M Keywords: Dwarf Galaxies; Galactic Clusters; Periodic Variations; Stellar Luminosity; Variable Stars; Correlation; Distance; Harmonics; Light Curve; Metallicity; Stellar Oscillations; Astrophysics; STARS: POPULATION II; STARS: OSCILLATIONS; STARS: VARIABLES: OTHER full text sources ADS | data products SIMBAD (38) NED (1)

Radial velocities with CORAVEL: results on stellar variability and duplicity

The complete radial velocity curve has been determined with CORAVEL for many pulsating stars of various classes: cepheid stars in the Galaxy and in the Magellanic Clouds, RR Lyrae, δ Scuti and SX Phoenicis stars. These measurements allow the determination of the radius variation and of the surface acceleration of these stars. In addition, the mean stellar radius of many of these stars has been determined by applying the Baade-Wesselink method.Systematic surveys of definite groups of binary or multiple stars are in progress with CORAVEL in order to determine the distribution functions of the orbital parameters. The eccentricity distributions for the binaries in the open clusters Pleiades, Praesepe, Coma Ber and Hyades are presented and their dependence on the physical processes (star formation mechanisms, mass exchange, tidal circularization, dynamical evolution) is briefly discussed.