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Discovery of an SX Phoenicis Type Pulsating Component in the Algol-Type Semidetached Eclipsing Binary QU Sagittae in M71
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.
We analysed photometric time series of the active, semidetached Algol-type system RZ Cas obtained in 1999–2009, in order to search for seasonal and short-term variations in the oscillation spectrum of RZ Cas A. The orbital period shows ±1 s cyclic variations on time-scales of 6–9 years. We detected six low-degree p-mode oscillations with periods between 22.3 and 26.22 min and obtained safe mode identifications using the periodic spatial filter method. The amplitudes and frequencies of all modes vary.
We tested and confirm the hypothesis that rapid variations in the pulsation spectrum of the mass-accreting component and rapid increases in the orbital period are driven by high mass transfer and accretion outbursts caused by the cyclic magnetic activity of the Roche lobe-filling donor star. Two rapid pulsation-amplitude decays observed in 2001 and 2009 can be explained by high-mass transfer events separated by the duration of the last 9-yr long magnetic cycle. We also tested and confirm the hypothesis of an acceleration of the outer envelope of the pulsating component. We discovered synchronous, modal m-dependent variations in the frequencies of three identified modes, in good agreement with results of our mode identification using the periodic spatial filter method. We suggest that m-dependent pulsation frequency variations are caused via the Doppler-effect by variations of the rotation speed of the outer envelope of the pulsating gainer. With this method, we obtained the first asteroseismic detection and accurate measurement of the accretion driven acceleration of the outer envelope of the mass-accreting component of an Algol-type star.
The results of extensive time-series photometry of a new variable star, 2MASS 06451725 + 4122158 , are presented. The , observations (over 4000 individual measurements) were made over 23 nights between 2006 and 2007. To investigate the star’s physical characteristics additional Strömgren photometric observations ( ) were made on another four nights in 2007 and 2009. Thirteen frequencies were found from a multifrequency analysis of the and photometry. All but two of the frequencies, 19.99716 cycles day (corresponding to the pulsation period of 0.0500071 day) and 25.84662 cycles day (0.0386898 day), have been identified as combination frequencies. The ratio of the two frequencies, / = 0.77369 , and their pulsation constants, () = 0.034 and () = 0.026 , are well matched with theoretical values of the fundamental and the first-overtone radial modes. But the phase shift of was estimated to be a negative value, in contrast to that of to be positive, suggesting that may be a nonradial mode. The period versus amplitude diagram shows that the star has pulsational properties similar to those of high-amplitude SX Phœnicis stars. We obtained its photometric properties from the Strömgren indices: a metal abundance of [Fe/H] = -0.6 dex , an absolute magnitude of = 2.7 mag , a distance of ∼1.6 kpc in the direction of the Galactic anticenter from the Sun, and a vertical distance from the Galactic plane of ∼0.45 kpc . The tangential velocity of 78 km sec , which was estimated from the proper motion and the distance, gives us a hint that the star may have high-space motion. Considering these observational properties, we suggest that this new variable is a field SX Phœnicis-type star, although its metallicity value seems not to be low enough for matching the metallicity versus period relation. Future spectroscopic studies would be required to obtain the metallicity more accurately.
We present the results of a high-cadence photometric survey of an 11 arcmin × 11 arcmin field centred on the globular cluster
M71, with the Near-Infrared Transiting ExoplanetS Telescope. The aim of our survey is to search for stellar variability and
giant transiting exoplanets. This survey differs from previous photometric surveys of M71 in that it is more sensitive to
lower amplitude (ΔM ≤ 0.02 mag) and longer period (P > 2 d) variability than previous work on this cluster. We have discovered 17 new variable stars towards M71 and confirm the
nature of 13 previously known objects, for which the orbital periods of 7 are refined or newly determined. Given the photometric
precision of our high-cadence survey on the horizontal branch of M71, we confirm that the cluster is devoid of RR Lyrae variable
stars within the area surveyed. We present new B- and V-band photometry of the stars in our sample from which we estimate spectral types of the variable objects. We also search
our survey data for transiting hot Jupiters and present simulations of the expected number of detections. Approximately 1000
stars were observed on the main sequence of M71 with sufficient photometric accuracy to detect a transiting hot Jupiter; however,
none were found.
This study is aimed at investigating the period–luminosity relation of SX Phoenicis (SX Phe) pulsators in Galactic globular
clusters (GGCs) and Local Group dwarf galaxies. We verify isochrone-fitting distances of 46 GGCs by fitting their main sequences
to a carefully chosen set of nearby, unevolved subdwarfs. We find that the difference between cluster distances obtained via
isochrone fitting by Dotter et al. and those resulting from our subdwarf fits has a mean of 0.094 mag and a standard deviation
of 0.098 mag. The cluster distances from Dotter et al. are used to calibrate an SX Phe period--luminosity relation based on
radial double mode pulsators. The resulting empirical period–luminosity relation, which is insensitive to the inclusion of
colour and/or metallicity terms, generally agrees well with previous empirical relations as well as theoretical predictions
based on single-star pulsational and evolutionary models. However, there is a subset of ‘subluminous’ variables identified
most notably in Fornax, Carina, NGC 2419 and Omega Centauri. We explore the possibility that, at least in GGCs, they represent
blue stragglers which have enhanced helium content that was either inherited from second-generation progenitors or gained
as a result of the blue straggler formation process.
We present the results of a three-continent multisite photometric campaign carried out on the Algol-type eclipsing binary system Y Cam, in which the primary component is a multiperiodic δ Sct-type pulsator. The observations consist of 86 nights and more than 450 h of useful data collected mainly during the Northern winter 2002–2003. This means that this is the most extensive time series for such kind of systems obtained so far. These observations were collected mostly in the Johnson V filter, but they also include, for the first time, nearly complete binary light curves in simultaneous Strömgren uvby filters together with a few Crawford Hβ data obtained around the orbital phase of the first quadrature. A detailed photometric analysis is presented for both binarity and pulsation. The results indicate a semidetached system with the secondary filling its Roche lobe. No significant contribution from a third body is found. The residuals from the computed binary solution were then used to investigate the pulsational content of the primary component. The frequency analysis of the out-of-primary-eclipse data leads to a set of eight significant and independent pulsational peaks in a well-defined region of the frequency domain. This means that this is the largest set of excited modes discovered so far in the pulsating component of such kind of systems. The possibility of aliasing problems during the present run or short-term time-scale amplitude variations in some of them was investigated with null results. Indeed the results indicate that f1 and f3 form a frequency doublet with a beat period of Pbeat= 17.065 d. Our results confirm the frequencies already detected by earlier authors and show the presence of some additional significant peaks. The observed amplitudes during the present run are also consistent with those derived from older data sets. We perform a preliminary mode identification for most of the frequencies on the basis of the collected multicolour photometry, the observed frequency spacings and the mode visibility in eclipsing binaries.
Discrete Scale Relativity proposes a new symmetry principle called discrete
cosmological self-similarity which relates each class of systems and phenomena
on a given Scale of nature's discrete cosmological hierarchy to the equivalent
class of analogue systems and phenomena on any other Scale. The new symmetry
principle can be understood in terms of discrete scale invariance involving the
spatial, temporal and dynamic parameters of all systems and phenomena. This new
paradigm predicts a rigorous discrete self-similarity between Stellar Scale
variable stars and Atomic Scale excited atoms undergoing energy-level
transitions and sub-threshold oscillations. Previously, methods for
demonstrating and testing the proposed symmetry principle have been applied to
RR Lyrae, Delta Scuti and ZZ Ceti variable stars. In the present paper we apply
the same analytical methods and diagnostic tests to a new class of variable
stars: SX Phoenicis variables. Double-mode pulsators are shown to provide an
especially useful means of testing the uniqueness and rigor of the conceptual
principles and discrete self-similar scaling of Discrete Scale Relativity.
Blue straggler stars (BSSs) are stars observed to be hotter and bluer than
other stars with the same luminosity in their environment. As such they appear
to be much younger than the rest of the stellar population. Two main channels
have been suggested to produce such stars: (1) collisions between stars in
clusters or (2) mass transfer between, or merger of, the components of
primordial short-period binaries. Here we suggest a third scenario, in which
the progenitor of BSSs are formed in primordial (or dynamically formed)
hierarchical triple stars. In such configurations the dynamical evolution of
the triples through the Kozai mechanism and tidal friction can induce the
formation of very close inner binaries. Angular momentum loss in a magnetized
wind or stellar evolution could then lead to the merger of these binaries (or
to mass transfer between them) and produce BSSs in binary (or triple) systems.
We study this mechanism and its implications and show that it could naturally
explain many of the characteristics of the BSS population in clusters, most
notably the large binary fraction of long period BSS binaries; their unique
period-eccentricity distribution (with typical periods > 700 days); and the
typical location of these BSSs in the color-magnitude diagram, far from the
cluster turn-off point of their host clusters. We suggest that this scenario
has a major (possibly dominant) role in the formation of BSSs in open clusters
and give specific predictions for the the BSSs population formed in this
manner. We also note that triple systems may be the progenitors of the
brightest planetary nebulae in old elliptical galaxies, which possibly evolved
from BSSs.
We present the first results of a deep WFPC2 photometric survey of the loose galactic globular cluster NGC 288. The fraction of binary systems is estimated from the color distribution of objects near the main sequence (MS) with a method analogous to that introduced by Rubenstein & Bailyn. We have unequivocally detected a significant population of binary systems with a radial distribution that has been significantly influenced by mass segregation. In the inner region of the cluster (r=1rh), fb must be less than 0.10, and the most likely value is 0.0, independently of the adopted F(q). The detected population of binaries is dominated by primordial systems. The specific frequency of blue stragglers (BSs) is exceptionally high, suggesting that the BS production mechanism via binary evolution can be very efficient. A large population of BSs is possible even in low-density environments if a sufficient reservoir of primordial binaries is available. The observed distribution of BSs in the color-magnitude diagram is not compatible with a rate of BS production that has been constant in time, if it is assumed that all the BSs are formed by the merging of two stars. Based on observations made with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute (STScI), which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with proposal GO-6804.
We give a brief review on the present status of our research of A-F pulsating components of semi-detached Algol-type systems. We suggest a new asteroseismic approach for estimating the evolutionary stage of the mass-accreting components of Algols during rapid and slow mass-transfer phases, asynchronization and differential rotation.
Sixteen photometric variable stars with mean V magnitudes between 15.2 and 20.3 have been discovered in a 167 × 167 field centered on the metal-rich globular cluster M71. The identifications are the result of a systematic time-series CCD imaging survey of the cluster using the Dominion Astrophysical Observatory 1.8 m telescope. Light curves and finding charts are presented for the newly identified stars (eight of which are definite variables, three probable, and five possible). These 16 stars, when combined with the seven faint variable stars discovered recently by Hodder et al. and Yan & Mateo, bring to 23 the number of faint variables now known in M71. The variable stars are of two basic types: W UMa– or Algol-like eclipsing binaries and SX Phoenicis radially pulsating variable blue straggler stars. Additional photometry is required to confirm the nature of the variability of several of the stars.
We discuss the main properties of the Galactic globular cluster (GC) blue straggler stars (BSSs), as inferred from our new catalog containing nearly 3000 BSSs. The catalog has been extracted from the photometrically homogeneous V versus (B-V) color-magnitude diagrams (CMDs) of 56 GCs, based on Wide Field Planetary Camera 2 images of their central cores. In our analysis, we used consistent relative distances based on the same photometry and calibration. The number of BSSs has been normalized to obtain relative frequencies (FBSS) and specific densities (NS) using different stellar populations extracted from the CMD. The cluster FBSS is significantly smaller than the relative frequency of field BSSs. We find a significant anticorrelation between the BSS relative frequency in a cluster and its total absolute luminosity (mass). There is no statistically significant trend between the BSS frequency and the expected collision rate. The value of FBSS does not depend on other cluster parameters, apart from a mild dependence on the central density. Post-core-collapse clusters act like normal clusters as far as the BSS frequency is concerned. We also show that the BSS luminosity function for the most luminous clusters is significantly different, with a brighter peak and extending to brighter luminosities than in the less luminous clusters. These results imply that the efficiency of BSS production mechanisms and their relative importance vary with the cluster mass.
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.
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.
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.
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
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.
A primary production mechanism for blue stragglers in globular clusters is thought to be collisionally induced mergers, perhaps mediated by dynamical encounters involving binary stars. We model the formation and evolution of such blue stragglers and produce theoretical distributions of them in the colormagnitude diagram. We use a crude representation of cluster dynamics and detailed binary-single star encounter simulations to produce cross sections and rates for a variety of collisions. The results of the collisions are determined on the basis of smoothed-particle hydrodynamics simulations of realistic star models. The evolution of the collision products are then followed in detail using the Yale stellar evolution code. We present our results in the form of distributions in the observed color-magnitude diagram. We use our results to explore the effects of a variety of input assumptions on the number and kind of blue stragglers created by collisions. In particular, we describe the changes in the blue straggler distribution that result from using realistic collision products rather than the "fully mixed" assumption, and from changes in assumptions about the number and orbital-period distribution of the primordial binary population. We then apply our models to existing data from the core of M3, where the large blue straggler population is thought to be dominated by collision products. We find that we have difficulty successfully modeling the observed blue stragglers under a single consistent set of assumptions. However, if three particularly bright blue stragglers are considered to be part of a different observed population (as has been suggested previously for other clusters) and left out of the population, the remainder can be successfully modeled using realistic encounter products and assuming a 20% binary fraction with a plausible period distribution. Finally, we suggest a Variety of routes toward a more comprehensive understanding of the blue straggler phenomenon.
The GCVS is the only reference source on all known variable stars. This version contains all of the electronically-readable version as distributed by the Sternberg Astronomical Institute and Institute of Astronomy (Russian Acad.Sci.), Moscow. It includes the catalog of variable stars, updated and completed with the seven Name-Lists of Variables Stars Nos.67-73 (see IBVS No.2681,1985; No.3058,1987; No.3323,1989; No.3530,1990; No.3840,1993; No.4140,1995; No.4471,1997), a catalog of suspected variables, a cross-index of variable star names, a catalog of extragalactic variables, and a catalog of supernovae. (10 data files).
New photographic photometry and proper motions have been obtained for over 350 stars with V less than about 16 in the region of the globular cluster M71 (NGC 6838). The photometry has been calibrated via new photoelectric observations which resolve the zero-point discrepancy of previous sequences. The proper motions have been used to derive cluster membership probabilities. The horizontal branch and lower giant branch very closely resemble those of 47 Tuc, but the upper part of the giant branch is slightly brighter and bluer in M71. Three probable UV-bright stars are likely members. Four variable stars near the tip of the giant branch are probable members, as are most of the stars observed spectroscopically by various authors. The internal velocity dispersion has been detected. The cluster space velocity is well determined and is characteristic of a disk-population object: about 50 km/s.
There are several observed phenomena in globular clusters that are
thought to be the result of dynamical processes or binary star
evolution. This review examines these manifestations of the interaction
between globular cluster dynamics and stellar evolution. Blue stragglers
may be formed by the evolution of primordial binaries or by collisions.
Current evidence suggests that both processes are likely to occur, and
that the observed blue straggler sequences can place dynamically
interesting limits on rates. Color gradients in globular clusters are
thought to becaused by the stripping of giants by collisions, although
the creation of blue subdwarfs by the same process may also be required
to explain the observations. The observed X-ray sources and radio
pulsars are apparently also made by a variety of dynamical processes
that are still not fully understood.
Radial velocity measures are presented for the eclipsing binary V3 in
NGC 6838 (M7 1) and the RR Lyrae variables V56 and V 168 in NGC 5139
(ω Cen). The velocities obtained preclude cluster membership for
the stars.
Two sets of observations of the metal-rich globular cluster M71 are
presented. The first uses deep CCD exposures to derive a fiducial in V,
B - V and compares this with a cluster of similar metallicity, 47 Tuc.
Comparison with oxygen enhanced theoretical isochrones leads to age
estimates for M71 of from about 14 +/- 2 Gyr to about 16 +/- 2 Gyr.
Within the limits imposed by the quality of the data, no significant age
difference between the two clusters is found. The second set of 73
frames is used to search an overlapping 67 x 104 arcsec area of the
cluster for variable stars.
A technique for obtaining CCD photometry of a large ensemble of stars,
with precision limited by atmospheric scintillation and photon
statistics is presented. Under bright-sky conditions with a 0.9-m
telescope, a precision of about 0.0015 mag relative to an ensemble
average standard was obtained for 12th-13th mag stars in M 67 with
exposure times of 1 min. The increase in noise level due to variable
cirrus clouds is minimal. Effective noise levels for the detection of
coherent oscillations with periods of 5-20 min could thus be reduced to
about 30 micromag over ten nights of observing for stars of this
brightness. A much larger ensemble of faint stars could be followed at
lower precision with sky background and photon statistics as dominant
error sources. Four probable-cluster DA white dwarfs were detected in
the old galactic cluster M 67, using B, V calibration photometry.
We report a detection of a short-periodic pulsating component in the
Algol-type eclipsing binary system AO Ser
A database of parameters for globular star clusters in the Milky Way is
described which is available in electronic form through the
WorldWideWeb. The information in the catalog includes up-to-date
measurements for cluster distance, reddening, luminosity, colors and
spectral types, velocity, structural and dynamical parameters,
horizontal branch morphology, metallicity, and other quantities. This
catalog will be updated regularly and maintained in electronic form for
widest possible accessibility.
The hypothesis of this paper is that considerable mass-transfer can occur between the components of close binary stars, a possibility that has been widely discussed in the literature. The purpose here is to study the consequences for cluster HR diagrams. In a cluster that contains close binaries and that has evolved far enough to produce red giants, we expect cases to occur of mass-transfer from an incipient giant to a secondary companion. The latter thus becomes a star on or near a continuation of the main-sequence beyond its normal turn-off point for the cluster. The extent of this continuation is predicted to be about 2.5 magnitudes. At any one epoch, the number of stars constituting this continuation would be relatively small; the theory relates this number to the number of stars in a certain interval of the giant branch of the cluster diagram. Such predictions are in good qualitative accord with well known features particularly of the clusters M3 and M67. Under certain conditions, reversal of the mass-transfer may subsequently occur. In that case, or in some cases of the original mass-transfer, the material involved would be turned inside-out; the result would be a star whose outer part is poor in hydrogen and rich in helium. There are known cases of such stars. Further attention to these phenomena might provide novel ways for observational study of stellar evolution.
Through time-series CCD photometry of the metal-poor globular cluster M53 we have discovered eight new SX Phoenicis–type stars (labeled "SXP1" to "SXP8"). All the new SX Phoenicis stars are located in the blue straggler star region of the color-magnitude diagram of M53. One of these stars (SXP2) is found to have very closely separated pulsation frequencies: f1/f2 = 0.9595, where f1 and f2 are the primary and secondary frequencies, respectively. This may be due to excitation of nonradial 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(days) < -1.15: V = -3.010(±0.262) log P + 15.310(±0.048) with an rms scatter of 0.038, corresponding to MV = -3.010 log P - 1.070 for an adopted distance modulus of (m - M)V = 16.38.
We summarize the results from a program of monitoring the radial velocities of 10 metal-poor, high-velocity field stars whose colors are 0.01 to 0.13 mag bluer than main-sequence turnoffs of comparable-metallicity globular clusters. Two of the candidate halo blue stragglers (BD +72 94 and BD +40 1166) show no signs of velocity variability, one (HD 84937) shows only weak signs of variability, one (BD +25 1981) appears to be a very long-period binary, and six (BD -12 2669, HD 97916, HD 106516, BD +51 1817, G66-30, and G202-65) are single-lined spectroscopic binaries, with periods ranging from 167 to 844 days. Velocity coverage for the four candidates without orbital solutions ranges from 15.9 to 19.0 years. The orbital eccentricities are all low, e < 0.30 and e = 0.11. Five of the six binary orbits have very low eccentricities, with e = 0.07. We have reanalyzed the velocity data from Preston & Sneden and have derived orbital solutions similar to theirs for 10 of the spectroscopic binaries among their "blue metal-poor" stars with [Fe/H] ≤ -0.6. We confirm their conclusion that the binary frequency is high; we find 47 ± 10% if we include only the definite binaries with [Fe/H] ≤ -0.6. Our orbital solutions for the seven binaries with periods longer than 20 days all have low eccentricities, with e ≤ 0.26 and e = 0.11. These orbital characteristics are very similar to the Ba II, CH, subgiant CH, and dwarf carbon stars, suggesting that mass transfer has been involved in their formation. Of the five binary stars in our program with published abundances of lithium, all have been found to be deficient (and one in beryllium as well). In contrast, two of the three apparently single stars have published lithium abundances and show no deficiency. The mass functions for the six binaries in our program and seven similar systems studied by Preston & Sneden are consistent with their unseen companions all being white dwarfs with M ≈ 0.55 M and random orbital inclinations. Taking all of our observations and those of others together, we argue that the results are consistent with all field blue stragglers being binary systems with long periods and low eccentricities, the primary stars being deficient in lithium and the secondary stars being normal-mass white dwarfs. All these properties are suggestive of a blue-straggler formation model that involves mass transfer. For six of the 13 stars in the two programs for which s-process elemental abundances are available, no signs of enhancement are discernible, suggesting that the donor star was a first-ascent red giant. For the star with the longest orbital period (1307 days), CS 22956-028, s-process abundance enhancements have been reported. This star may be a precursor to the subgiant CH class, as suggested by Luck & Bond.
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.
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.
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.
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.
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.
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.
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.
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)
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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