Article

Discovery of an SX Phoenicis Type Pulsating Component in the Algol-Type Semidetached Eclipsing Binary QU Sagittae in M71

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Abstract

We report the discovery of an SX Phoenicis type pulsating component in the Algol-type semidetached eclipsing binary QU Sge, in the metal-rich globular cluster M71. QU Sge is only about 80'' from the center of M71 and is located in the blue straggler region in the color-magnitude diagram of M71. It is considered to be a probable member of M71, with a membership probability greater than 60% deduced from a proper-motion study in the literature. From time-series CCD photometry, we find that QU Sge has an orbital period of 3.790818 days and a primary minimum depth of ΔV = 1.333 mag. The eclipsing light curve solution shows that QU Sge has a semidetached binary configuration with the secondary component fully filling its Roche lobe. After subtracting the eclipses from the light curve, we discover an SX Phoenicis type pulsation feature. It is found to have a short period of about 0.03 days and a small amplitude of about 0.024 mag. This is the first eclipsing binary system in a globular cluster to exhibit a pulsating feature. This result supports the model in which the origin of some blue stragglers in globular clusters is mass transfer between two components in the primordial binary systems.

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The radial velocity behavior and chemical compositions of sixty-two blue metal-poor (BMP) stars have been established from more than 1200 echelle spectra obtained at Las Campanas Observatory from 1992 through 1999. Analysis of survey spectra provides abundances for this sample, which we use to calibrate the K line versus B-V relation. Forty-four of the stars have [Fe/H] < -1, while eighteen lie on -1 < [Fe/H] < 0. One star, the SX Phe variable CS 22966-043, appears to be the most extreme example of a rare abundance class characterized by α-element deficiencies, high [Cr/Fe], [Mn/Fe], and [Ti/Fe], and extremely low [Sr/Fe] and [Ba/Fe]. Of the 62 stars, 17 appear to have constant radial velocities, while 42 are definite or probable members of binary systems. The binary fraction of BMP stars appears to be independent of chemical composition. The high binary fraction fBMP ~ 0.6 of BMP stars compared with that found for the F- and G-type stars near the Sun, the systematically low mass functions of these binaries, and the paucity of double-lined binaries among them lead us to suggest that at least half of the BMP binaries are blue stragglers and that these blue stragglers are formed by McCrea mass transfer rather than by the various merger processes that are currently believed to produce most blue stragglers in globular clusters. This conclusion is supported by the abnormally high proportion of BMP binaries with long periods and small orbital eccentricities, properties these binaries share with McClure's carbon star binaries. The great majority of field blue stragglers (BSs) probably are created by Roche-lobe overflow during red giant branch evolution. Primaries of more widely separated binaries that survive this phase of stellar evolution may engage in mass transfer during subsequent asymptotic giant branch evolution to form s-process abundance enhanced carbon stars. Our result requires a major downward revision of the fraction of BMP stars attributed to a captured intermediate-age population of metal-poor field stars. The high original estimate of the size of this component probably arose from improper use of the globular cluster BS specific frequency, SBS = n(BS)/n(HB)~1, to estimate the halo BS space density. We use a simple model to calculate the specific frequency of BSs produced by McCrea mass transfer in an old metal-poor population with a given primordial binary fraction fB. Our model calculations return values of SBS ~ 5 for fB = 0.15, much more like our value for the field blue stragglers. We suggest that globular clusters either destroy the primordial binaries that produce long period BS binaries like those in the Galactic field reported here, or they never possessed them.
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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.
Article
Through time-series CCD photometry of the globular cluster NGC 5466, we have detected nine SX Phoenicis stars, including three new ones. All the SX Phoenicis stars are located in the blue straggler region in the color-magnitude diagram of NGC 5466. Five of them clearly show double-radial mode features, the periods of which are consistent with the theoretical ratio of the first-overtone mode to the fundamental mode (P1H/PF). Normally, it has not been easy to secure a P-L relation for the SX Phoenicis stars because their pulsational mode has been difficult to determine. The existence of five SX Phoenicis stars in NGC 5466 with double-radial modes allows us to reliably derive a P-L relation for the fundamental mode of SX Phoenicis stars. Using seven SX Phoenicis stars, including five stars with double-radial modes, we derive a P-L relation for the fundamental mode in NGC 5466, V = -3.25(±0.46) log P + 14.70(±0.06),(σ = ±0.04), corresponding to MV = -3.25(±0.46) log P - 1.30(±0.06) for an adopted distance modulus of (m - M)0 = 16.00 and zero reddening.
Article
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.
Article
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.
Article
Blue stragglers (BSs) are important objects in cluster populations because of their peculiar properties. The colours and magnitudes of these objects are critical parameters in the population synthesis of the host cluster and may depend remarkably on the surface composition of BSs. Observations show that some BSs are short-orbital-period binaries, which may be accounted for by mass transfer in low-mass binaries. We have therefore studied the effects of surface composition and thermohaline mixing, caused by secular instability, on the accreting components for low-mass binaries, and we have applied the results to a short-orbital-period BS F190 in the old cluster M67. We examine thermohaline mixing in a low-mass accreting main-sequence star and find that, except for the redistribution of composition under the surface, the mixing affects the accretor very little during Roche lobe overflow unless thermohaline mixing is treated as an instantaneous process. A series of calculations is then carried out for low-mass binaries under different assumptions. The results indicate no distinction in surface composition between the models with and without thermohaline mixing during Roche lobe overflow, but we still see the divergences of evolutionary tracks on the Hertzsprung–Russell and colour–magnitude diagrams. The change of surface composition makes the gainer bluer and smaller than those with original surface composition, while thermohaline mixing lessens the effect slightly. If thermohaline mixing were to act instantaneously, the effect would be lessened more. Our calculation shows that case A and case B mass transfer may produce BSs in short- or relatively short-orbital-period binaries (including Algol systems), and that CNO abundance abnormalities could be observed in these products. This is consistent with the results of Monte Carlo simulations by previous studies. Our simulation of F190 shows that the primary's mass M1i of the appropriate models is located in the range of 1.40–1.45 M⊙ with initial mass ratio qi= 1.5 and initial orbital period Pi= 0.8 d, indicating that case A is a more likely evolutionary channel than case B to form this object. The simulation also shows that it is very likely that F190 is still in a slow stage of mass transfer. As a consequence, obvious CNO abundance abnormalities should be observed for the object.
Article
We present the first results of a photometric and astrometric investigation of stars in the region of the globular cluster M 71 (NGC 6838). Using wide field CCD observations we determined $B$ and $V$ magnitudes of 4450 stars up to a limiting magnitude of $V$ $\leq$ 18.5 mag. Relative proper motions were used to derive membership probabilities for stars with $R<3\arcmin$ around the centre of M 71. Our colour-magnitude diagram (CMD) of 320 probable cluster members reaches down to $V$ = 18. A well defined red giant bump - for the first time seen in M 71 - could be detected in our CMD. From isochrone fitting we find M 71 metal poorer and older than previously assumed. Four faint variable stars were confirmed to be members of M 71. Moreover, we detected 13 blue stragglers among our cluster members.
Article
We report radial velocity studies of five candidate metal-poor field blue stragglers, all known to be deficient in lithium. Four of the five stars are single-lined spectroscopic binaries, with periods ranging from 302 to 840 days, and low orbital eccentricities, in agreement with similar behavior found for other blue straggler candidates by Preston & Sneden (2000) and Carney et al. (2001). The limited data available for lithium abundances indicate that all blue straggler candidates have depleted lithium abundances. Our results show higher values of v(rot) sin i for the binary stars than comparable temperature constant-velocity stars. The orbital periods are too long for tidal effects to now be important, implying that spin-up during mass transfer when the orbital separations and periods were smaller is that cause of the enhanced rotation. Comment: To appear in Astronomical Journal (January 2005 issue)
Article
Recent HST observations of a large sample of globular clusters reveal that every cluster contains between 40 and 400 blue stragglers. The population does not correlate with either stellar collision rate (as would be expected if all blue stragglers were formed via collisions) or total mass (as would be expected if all blue stragglers were formed via the unhindered evolution of a subset of the stellar population). In this paper, we support the idea that blue stragglers are made through both channels. The number produced via collisions tends to increase with cluster mass. In this paper we show how the current population produced from primordial binaries decreases with increasing cluster mass; exchange encounters with third, single, stars in the most massive clusters tend to reduce the fraction of binaries containing a primary close to the current turn-off mass. Rather their primaries tend to be somewhat more massive (~1-3 M_sun) and have evolved off the main sequence, filling their Roche lobes in the past, often converting their secondaries into blue stragglers (but more than 1 Gyr or so ago and thus they are no longer visible as blue stragglers). We show that this decline in the primordial blue straggler population is likely to be offset by the increase in the number of blue stragglers produced via collisions. The predicted total blue straggler population is therefore relatively independent of cluster mass, thus matching the observed population. This result does not depend on any particular assumed blue straggler lifetime. Comment: 7 pages, 6 figures; MNRAS in press
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