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

ArticleinThe Astrophysical Journal 636(2):L129 · December 2008with 27 Reads 
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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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    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.
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    Recent observations of the blue straggler population in 47 Tucanae give the first detailed characterization of their spatial distribution in the cluster over its entire volume. Relative to the light distribution, blue stragglers appear to be overabundant in the core and at large radii. The observed surface density profile shows a central peak, a zone of avoidance, and a rise beyond 20 core radii. In light of these findings we explored the evolution of blue stragglers mimicking their dynamics in a multimass King model for 47 Tucanae. We find that the observed spatial distribution cannot be explained within a purely collisional scenario in which blue stragglers are generated exclusively in the core through direct mergers. An excellent fit is obtained if we require that a sizable fraction of blue stragglers is generated in the peripheral regions of the cluster inside primordial binaries that evolve in isolation experiencing mass transfer.
  • 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
    We studied the effects of surface composition and thermohaline mixing caused by secular instability on the accreting components for low--mass binaries and applied the results on a short-orbital-period BS F190 in the old cluster M67. 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 HRD and CMD. The change of surface composition makes the gainer bluer and smaller than the ones 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. Our simulation of F190 shows that the primary's mass $M_{\rm 1i}$ of the appropriate models is located in the range of 1.40 to 1.45$M_{\odot}$ with initial mass ratio $q_{\rm i}=1.5$ and initial orbital period $P_{\rm i}=0.8$ days, 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.
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    The first multisite photometric campaign devoted to the rapidly oscillating mass-accreting (primary) component of the Algol-type eclipsing binary system AS Eri has confirmed the presence of rapid pulsations with frequency 59.03116 d-1, and revealed the second and third oscillation modes with frequencies 62.5631 d-1 and 61.6743 d-1, respectively. These modes are related to the 5-6 overtone oscillations and are among the shortest periods excited in non-magnetic MS A-F stars. The nearly equator-on visibility of eclipsing binaries help to narrow the range of possible mode identifications for the detectable modes as radial or (l, m) = (1, ±1), (l, m) = (2, ±2) and (l, m) = (2, ±0). We checked the high-order pulsation-to-orbital synchronization (POS) using the trial mode identification and the Doppler effect correction for frequencies of non-radial pulsation. We found that (l, m, n) = (1, 1, 5) or (2, 2, 5) and (l, m, n) = (2, -2, 6) identifications for f1 and f2 modes respectively satisfied the high-order POS. These mode identifications are in agreement with the range of modes visible in disk integrated light of an equator-on visible pulsating component. The wavelength distribution of pulsation amplitudes in AS Eri is largest in the Strömgren u filter and decreases toward longer wavelengths. We place AS Eri and other known mass-accreting pulsating components of Algols on HR-diagram. They are located inside the instability strip on the Main Sequence. We also discuss the peculiar evolutionary status of primary components in Algols and stress that they are not normal δ Scuti stars, but form a separate group of pulsators. Finally, we discuss proximity and eclipse effects, and have simulated the effect of primary minimum data gaps that may produce the 1/Porb alias sidelobes in DFT analysis of eclipsing binary data. Aliases from gaps in primary minimum observations seem to be the principal limitation on spectral window functions in asteroseismic studies of eclipsing binaries.
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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
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    We compute theoretical evolutionary tracks of blue stragglers created by mergers. Two formation scenarios are considered: mergers of primordial binaries, and stellar collisions. These two scenarios predict strikingly different luminosity functions, which are potentially distinguishable observationally. Tabulated theoretical luminosity functions and lifetimes are presented for blue stragglers formed under a variety of input conditions. We compare our results with observations of the blue straggler sequences in 47 Tucanae and M3. In the case of 47 Tuc, the luminosity function and the formation rate are compatible with the hypothesis that the blue stragglers formed through the collision of single stars. Mergers of primordial binaries are only marginally cosistent with the data, and a significant enhancement of the collision cross section by binary-single-star encounters appears to be ruled out. In the case of M3, we find that the innermost blue stragglers have a luminosity function significantly different from that of the outer stragglers, thus confirming earlier suggestions that there are two distinct populations of blue stragglers in this cluster. The inner stragglers are preferentially brighter and bluer, as would be expected if they were made by collisions, but there are so many of them that the collision rate would need to be enhanced by interactions involving wide binaries. The luminosity function of the outer stragglers is almost identical to the predictions of mergers from primordial binaries and is inconsistent with the collision hypothesis.
  • Article
    Full-text available
    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. Comment: A&A accepted, 11 figures
  • Article
    Full-text available
    Using Hubble Space Telescope WFPC2 observations in two ultraviolet (UV) filters (F225W and F336W) of the central region of the high-density Galactic globular cluster (GGC) M80, we have identified 305 blue straggler stars (BSS), which represents the largest and most concentrated population of BSS ever observed in a GGC. We also identify the largest clean sample of evolved BSS yet found. The high stellar density alone cannot explain the BSS, and we suggest that in M80 we are witnessing a transient dynamical state, during which stellar interactions are delaying the core-collapse process leading to an exceptionally large population of collisional BSS.
  • 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