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1.
[Fe/H]–φ31– P relations are found for c-type RR Lyrae stars in globular clusters. The relations are analogous to that found by Jurcsik & Kovács for field ab-type RR Lyrae stars, where a longer period correlates with lower metallicity values for similar values of the Fourier coefficient φ31. The relations obtained here are used to determine the metallicity of field c-type RR Lyrae stars, those within ω Cen, the Large Magellanic Cloud and toward the galactic bulge. The results are found to compare favourably to metallicity values obtained elsewhere.  相似文献   

2.
We review and discuss horizontal branch (HB) stars in a broad astrophysical context, including both variable and non-variable stars. A reassessment of the Oosterhoff dichotomy is presented, which provides unprecedented detail regarding its origin and systematics. We show that the Oosterhoff dichotomy and the distribution of globular clusters in the HB morphology-metallicity plane both exclude, with high statistical significance, the possibility that the Galactic halo may have formed from the accretion of dwarf galaxies resembling present-day Milky Way satellites such as Fornax, Sagittarius, and the LMC—an argument which, due to its strong reliance on the ancient RR Lyrae stars, is essentially independent of the chemical evolution of these systems after the very earliest epochs in the Galaxy’s history. Convenient analytical fits to isochrones in the HB type–[Fe/H] plane are also provided. In this sense, a rediscussion of the second-parameter problem is also presented, focusing on the cases of NGC 288/NGC 362, M13/M3, the extreme outer-halo globular clusters with predominantly red HBs, and the metal-rich globular clusters NGC 6388 and NGC 6441. The recently revived possibility that the helium abundance may play an important role as a second parameter is also addressed, and possible constraints on this scenario discussed. We critically discuss the possibility that the observed properties of HB stars in NGC 6388 and NGC 6441 might be accounted for if these clusters possess a relatively minor population of helium-enriched stars. A technique is proposed to estimate the HB types of extragalactic globular clusters on the basis of integrated far-UV photometry. The importance of bright type II Cepheids as tracers of faint blue HB stars in distant systems is also emphasized. The relationship between the absolute V magnitude of the HB at the RR Lyrae level and metallicity, as obtained on the basis of trigonometric parallax measurements for the star RR Lyr, is also revisited. Taking into due account the evolutionary status of RR Lyr, the derived relation implies a true distance modulus to the LMC of (m–M)0=18.44±0.11. Techniques providing discrepant slopes and zero points for the M V (RRL)–[Fe/H] relation are briefly discussed. We provide a convenient analytical fit to theoretical model predictions for the period change rates of RR Lyrae stars in globular clusters, and compare the model results with the available data. Finally, the conductive opacities used in evolutionary calculations of low-mass stars are also investigated. M. Catelan is John Simon Guggenheim Memorial Foundation Fellow.  相似文献   

3.
We obtained a series of more than two hundred R-band CCD images for the crowded central (115″×77″) region of the metal-poor globular cluster M 15 with an angular resolution of \(0\mathop .\limits^{''} 5 - 0\mathop .\limits^{''} 9\) in most images. Optimal image subtraction was used to identify variable stars. Brightness variations were found in 83 stars, 55 of which were identified with known cluster variables and the remaining 28 are candidates for new variables. Two of them are most likely SX Phe variables. The variability type of two more stars is uncertain. The remaining stars were tentatively classified as RR Lyrae variables. A preliminary analysis of published data and our results shows that the characteristics of RR Lyrae variables in the densest part (r<35″) of the cluster probably change. More specifically, the maximum of the period distribution of first-and second-overtone (RR1, RR2) pulsating stars shifts toward shorter periods; i.e., there is an increase in the fraction of stars pulsating with periods \( < 0\mathop .\limits^d 3\) and a deficiency of stars with \(0\mathop .\limits^d 35 - 0\mathop .\limits^d 40\). The ratio of the number of these short-period RR Lyrae variables to the number of fundamental-tone (RR0) pulsating variables changes appreciably. We found and corrected the error of transforming the coordinates of variables V128–155 in M 15 into the coordinate system used in the catalog of variable stars in globular clusters.  相似文献   

4.
Population synthesis is used to model the number of neutron stars in globular clusters that are observed as low-mass X-ray sources and millisecond radio pulsars. The dynamical interactions between binary and single stars in a cluster are assumed to take place only with a continuously replenished “background” of single stars whose properties keep track of the variations in parameters of the cluster as a whole and the evolution of single stars. We use the hypothesis that the neutron stars forming in binary systems from components with initial masses of ～8–12 M ⊙ during the collapse of degenerate O-Ne-Mg cores through electron captures do not acquire a high space velocity. The remaining neutron stars (from single stars with masses >8 M ⊙ or from binary components with masses >12 M ⊙) are assumed to be born with high space velocities. According to this hypothesis, a sizeable fraction of the forming neutron stars remain in globular clusters (about 1000 stars in a cluster with a mass of 5 × 105 M ⊙). The number of millisecond radio pulsars forming in such a cluster in the case of accretion-driven spinup in binary systems is found to be ～10, in agreement with observations. Our modeling also reproduces the observed shape of the X-ray luminosity function for accreting neutron stars in binary systems with normal and degenerate components and the distribution of spin periods for millisecond pulsars.  相似文献   

5.
A study of the mean photographic magnitudes of RR Lyrae variables in galactic globular clusters has shown that the luminosity of RR-ab variables decreases with period. It is found that RR-ab variables, in the cluster Centauri, whose distances are greater than 8 arc min from the centre of the cluster, are less luminous than such variables seen in the inner region. The possibility of two transition periods for RR-ab variables, one near 0 . d 4 and the other near 0 . d 9, is suggested to explain the sharp boundaries of the instability gap and of the periods of RR Lyrae variables. It is also shown that the inner RR Lyrae variables in Centauri are more massive than the outer ones.  相似文献   

6.
B and V data of Population II Cepheids in M2, M5, M10, M13, M80 and ω Cen are combined in order to obtain period-luminosity and period-colour relations. The mean absolute magnitudes are determined adopting as distance modulus the quantily [V(RR)-M v(RR)], whereV(RR) is the apparent mean magnitude of RR Lyrae andM v(RR) is the absolute magnitude derived from the transistion periodP tr. It is shown that the scatter of the points around the (P-L) and (P-C) relations depends also from the width of the instability strip. In fact the position of the variables in the instability strip may be correlated with their amplitude in the sense that the short period variables (logP<1) which have largest amplitudes are brightest and bluest while the long period variables (logP>1) show a reversed relation. Finally, after correcting the observed colours for the amplitude effect, period-colour-amplitude relations are derived for the both short and long period variables.  相似文献   

7.
We have analyzed the space velocities of stars with the proper motions and trigonometric parallaxes from the Gaia TGAS catalogue in combination with the line-of-sight velocities from the RAVE5 catalogue. In the V, \(\sqrt {{U^2} + 2{V^2}} \) velocity plane we have identified three clumps, BB17-1, BB17-2, and BB17-3, in the region of large velocities (V<?150 km s?1). The stars of the BB17-1 and BB17-2 clumps are associated with the kinematic groups VelHel-6 and VelHel-7 detected previously by Helmi et al. We give the greatest attention to the BB17-3 clump. The latter is shown to be most closely linked with the debris of the globular cluster ω Cen. In the BB17-3 clump we have identified 28 stars with a low velocity dispersion with respect to the center of their distribution. All these stars have very close individual age estimates: log t ≈ 10. The distribution of metallicity indices in this sample is typical for the stars of the globular cluster ω Cen. In our opinion, the BB17-3 clump can be described as a homogeneous stream in the debris of the cluster ω Cen.  相似文献   

8.
We present the first results of an observational campaign aimed at detecting rapid extreme horizontal branch (EHB) pulsators in globular clusters. So far, we have observed multi-frequency luminosity variations for three EHB stars in ω Cen, with typical periods in the 100–120 s range. This is towards the short end, but comparable to, the periodicities measured for rapidly pulsating subdwarf B (sdB) stars in the field. Given that the effective temperatures of the variables discovered seem to be compatible with the instability strip for fast sdB pulsators, we believe we have uncovered the first such variables in a globular cluster.  相似文献   

9.
We use the results from recent computations of updated non-linear convective pulsating models to constrain the distance modulus of Galactic globular clusters through the observed periods of first-overtone (RR c ) pulsators. The resulting relation between the mean absolute magnitude of RR Lyrae stars 〈 M V (RR)〉 and the heavy element content [Fe/H] appears well in the range of several previous empirical calibrations, but with a non-linear dependence on [Fe/H] so that the slope of the relation increases when moving towards larger metallicities. On this ground, our results suggest that metal-poor ([Fe/H]<−1.5) and metal-rich ([Fe/H]>−1.5) variables follow two different linear 〈 M V (RR)〉−[Fe/H] relations. Application to RR Lyrae stars in the metal-poor globular clusters of the Large Magellanic Cloud (LMC) provides an LMC distance modulus of the order of 18.6 mag, thus supporting the 'long' distance scale. The comparison with recent predictions based on updated stellar evolution theory is briefly presented and discussed.  相似文献   

10.
We describe the public ESO near-IR variability survey (VVV) scanning the Milky Way bulge and an adjacent section of the mid-plane where star formation activity is high. The survey will take 1929 h of observations with the 4-m VISTA telescope during 5 years (2010–2014), covering ～109 point sources across an area of 520 deg2, including 33 known globular clusters and ～350 open clusters. The final product will be a deep near-IR atlas in five passbands (0.9–2.5 μm) and a catalogue of more than 106 variable point sources. Unlike single-epoch surveys that, in most cases, only produce 2-D maps, the VVV variable star survey will enable the construction of a 3-D map of the surveyed region using well-understood distance indicators such as RR Lyrae stars, and Cepheids. It will yield important information on the ages of the populations. The observations will be combined with data from MACHO, OGLE, EROS, VST, Spitzer, HST, Chandra, INTEGRAL, WISE, Fermi LAT, XMM-Newton, GAIA and ALMA for a complete understanding of the variable sources in the inner Milky Way. This public survey will provide data available to the whole community and therefore will enable further studies of the history of the Milky Way, its globular cluster evolution, and the population census of the Galactic Bulge and center, as well as the investigations of the star forming regions in the disk. The combined variable star catalogues will have important implications for theoretical investigations of pulsation properties of stars.  相似文献   

11.
By adopting the same approach outlined by De Santis & Cassisi, we evaluate the absolute bolometric magnitude of the zero-age horizontal branch (ZAHB) at the level of the RR Lyrae variable instability strip in selected Galactic globular clusters. This allows us to estimate the ZAHB absolute visual magnitude for these clusters and to investigate its dependence on the cluster metallicity. The derived M V (ZAHB)–[Fe/H] relation, corrected in order to account for the luminosity difference between the ZAHB and the mean RR Lyrae magnitude, has been compared with some of the most recent empirical determinations in this field, such as the one provided by Baade–Wesselink analyses, RR Lyrae periods, Hipparcos data for field variables and main-sequence fitting based on Hipparcos parallaxes for field subdwarfs. As a result, our relation provides a clear support to the 'long' distance scale. We discuss also another method for measuring the distance to Galactic globular clusters. This method is quite similar to the one adopted for estimating the absolute bolometric magnitude of the ZAHB but it relies only on the pulsational properties of the Lyrae variables in each cluster. The reliability and accuracy of this method have been tested by applying it to a sample of globular clusters for which, owing to the morphology of their horizontal branch (HB), the use of the commonly adopted ZAHB fitting is a risky procedure. We notice that the two approaches for deriving the cluster distance modulus provide consistent results when applied to globular clusters, the RR Lyrae instability strip is well populated. As the adopted method relies on theoretical predictions on both the fundamental pulsational equation and the allowed mass range for fundamental pulsators, we give an estimate of the error affecting present results, owing to systematic uncertainties in the adopted theoretical framework.  相似文献   

12.
The grid of evolutionary tracks of population II stars with initial masses 0.81 M⊙ ≤ MZAMS ≤ 0.85 M⊙ and chemical composition of the globular cluster M3 is computed. Selected models of horizontal branch stars were used as initial conditions for solution of the equations of radiation hydrodynamics and time–dependent convection describing radial stellar oscillations. The boundaries of the instability strip on the Herztsprung–Russel diagram were determined using ≈100 hydrodynamic models of RR Lyr pulsating variables. For each evolutionary track crossing the instability strip the pulsation period was determined as a function of evolutinary time. The rate of period change of most variables is shown to range within ?0.02 ≤ \(\dot{\Pi}\) ≤ 0.05 day/106 yr. Theoretical estimate of the mean period change rate obtained by the population synthesis method is 〈\(\dot{\Pi}\)〉 = 6.0 × 10?3 day/106 yr and agrees well with observations of RR Lyr variables of the globular cluster M3.  相似文献   

13.
The current knowledge of the evolution of Population II stars, as observed in galactic halos and globular clusters, is outlined. The recent theoretical results provided by an improved physical understanding of the stars are reported, with a particular emphasis upon those stellar evolutionary phases which are the keystones of the interpretation of globular cluster stars. Within the up–to–date theoretical scenario, the luminosity of the turn–off stars and horizontal branch stars, which are of fundamental importance for distance and age determinations, turns out to be fainter and brighter, respectively, in comparison with previous theoretical computations. The predicted absolute magnitude of RR Lyrae stars is consistent with the bright values suggested from the long distance scale (Sandage 1993), but the slope of the relation between and metallicity ([Fe/H]) agrees with that suggested from the short distance scale (see, e.g., Carney et al. 1992, Clementini et al. 1995), at least with [Fe/H]-1.3. As to the globular cluster ages, the new computations provide younger ages ( 10 Gyr–13 Gyr), weakening the conflict with current cosmological estimates. The recent results derived from the fitting of HIPPARCOS parallaxes of field subdwarfs to the fiducial main sequence of globular clusters are consistent with the up–to–date theoretical models. However, the direct determinations of , which are based on the HIPPARCOS proper motions and trigonometric parallaxes of field RR Lyrae stars, give fainter absolute magnitudes than those obtained from the improved evolutionary models. This seems to suggest that the problem of the globular cluster distance and age is still open. Received 6 April 1998  相似文献   

14.
The Galway/DIAS Image Sharpening Camera, TRIFFID, has been used to make observations in two colours of the centre of the post-core-collapse globular cluster M15. We present here our analysis of the photometry in B over two seasons. We have combined the complementary qualities of the HST 's high astrometric precision and TRIFFID's extended coverage and photometric precision, to perform crowded-field photometry in the innermost region of M15. Our technique virtually eliminates the problem of extreme crowding which has hitherto hampered studies of the variable star populations in globular cluster cores, and thereby provides an extension of the HST 's capability. Candidate variables detected with the HST have been confirmed and monitored over longer periods. We show that most of these are RR Lyrae stars, and that one is a short-period Type II Cepheid (the third to be discovered in M15). Our photometric study also produced evidence of a similar number of new variables. These also appear to be RR Lyrae stars, except for a possible eclipsing system. Further data from an upgraded version of TRIFFID have recently been obtained to help to refine the light curves of all these objects.  相似文献   

15.
Evolutionary model sequences for (X, Z)=(0.7,4×10–3) are constructed by using the same input physics and programming code as those of Saioet al. (1977). From these results the ages of globular clusters are estimated under the assumption of constant helium abundance (Y=0.3). The results suggest that there is a correlation between age and metal abundance for the globular clusters and that metal enrichment in the Galaxy slowly proceeded in several billion years from the value of the extreme Population II stars to that of the Population I stars. Comparison with some models of chemical evolution of galaxies is briefly made.  相似文献   

16.
The expected characteristics of stars with zero metal content are investigated for a large range of masses and in varying the original helium-content. It is shown that, in such an extreme case (Z=0), the Main Sequence locus is much less sensitive than expected to the He-content;Y=0 models are found to ignite 3α reactions in the Zero-Age Main Sequence at masses larger by ～ 5M ⊙ than in theY=0.2 case. A comparison with previous results discloses the M.S. slope to be a sensitive function of the metal content only. Evolutionary computations confirm that possible survivors of the zero population can develop a giant branch, akin to that of the well-known Population II stars.  相似文献   

17.
We present time-series B , V photometry of NGC 6229, obtained with the purpose of providing the first extensive CCD variability study of this cluster. As a result, we were able to obtain periods for all NGC 6229 variables, with the exception of five stars from the candidate list of Borissova et al. located very close to the cluster centre. Two stars suspected to be variables by Carney, Fullton and Trammell are first-overtone RR Lyrae (RRc) stars, whereas seven of the 12 candidates of Borissova et al. are confirmed variables – three of them being fundamental RR Lyrae (RRab) pulsators, two first-overtone pulsators, one an eclipsing binary (most likely an Algol system) and one a bright star whose variability status could not be satisfactorily determined. A new image subtraction method (ISM) suggested by Alard has been employed which, together with the Welch–Stetson technique, has allowed us to discover twelve new RR Lyrae variables in the cluster, for which ephemerides are provided. Ten of these are RRabs, whereas the other two are RRcs. As originally suggested by Mayer, NGC 6229 is clearly an Oosterhoff type I globular cluster. We also confirm that V8 is a Population II Cepheid of the W Virginis type, and suspect that this is the case for V22 as well. The physical properties of the NGC 6229 RR Lyrae population are contrasted with those of M3 (NGC 5272) using several different methods, including a standard period-shift analysis. Possible differences between these two clusters are discussed.  相似文献   

18.
《New Astronomy》2007,12(2):117-123
Both V701 Sco and BH Cen are two early-type short-period overcontact systems (P = 0.d762 and P = 0.d792, respectively). V701 Sco is a member of the young galactic cluster NGC 6383, while BH Cen is a component of a younger galactic cluster IC 2944 where star formation is in process. They provide good opportunity to understand the formation and evolution of binary stars. In the present paper, orbital period changes of the two binaries are investigated. It is discovered that the orbital period of BH Cen shows a long-term increase with a rate of dP/dt = +1.70(±0.39) × 10−7 days/year while it undergoes a cyclic oscillation with a period of 44.6 years and an amplitude of A3 = 0.d0216. For V701 Sco, its O-C curve reveals a periodic change with a period of 41.2 years and amplitude of A3 = 0.d0158. The mass ratio of BH Cen is 0.84, but V701 Sco contains twin B1-1.5V type stars with a mass ratio of unit. The continuous period increase of BH Cen is caused by the mass transfer from the less massive component to the more massive one at a rate of dM2/dt = 3.5 × 10−6 days/year.The cyclic period changes of both systems can be plausibly explained as the results of light-travel time effects suggesting that they are triple systems. The astrophysical parameters of the unseen tertiary components in the two systems have been determined. We think that the invisible tertiary components in both binaries played an important role in the formations and evolutions of the overcontact configurations by bringing angular momentum out from the central systems. For BH Cen, this process created the initial short period and will support its evolution into an overcontact configuration via a Case A mass transfer within the life time of the extremely young cluster IC 2944. For V701 Sco, two identical zero-age main-sequence components in an overcontact configuration suggest that it may have been formed by fission, possibly by the fission of the third body. The fact that no long-term continuous period variations were found for V701 Sco may suggest that an overcontact binary with the mass ratio of unity can be in an equilibrium revealing that the original configuration of the binary was overcontact as is its present state. It has been reported that faint stars in the two extremely young clusters are relatively scare. From the present study, it is shown that faint stars in young clusters are usually formed as companions of OB stars (including binaries). It is very difficult to detect them because of their low luminosity when compared with the more luminous OB stars.  相似文献   

19.
In the light of our present knowledge on stellar evolution, it is shown that red ZAHB will not develop for very low metal Globular Clusters ifY is appreciably greater than 0.1.Recent determinations of the mass-sizes of the He-cores at the time of the He-flash support independently conclusion that, if no increase in surface He-abundance occurs between the turn-off evolutionary phase and the H.B. stages,Y-values lower than 0.2 imply ages of the clusters larger than 20 b.y. By comparing evolutionary paths with the observed populations of the Horizontal Branches we find evidence for mass loss in blue H.B., and evidence for mass dispersion in red H.B. not depopulated in RR Lyrae. A few clusters (M3, M15) have their H.B. dominated by mass dispersion. Finally the expected correlations between mass loss and spatial distribution of H.B. stars are briefly discussed in order to point out some observational possibilities.  相似文献   

20.
One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the red giant branch (RGB) and is followed by a series of secondary flashes before the star settles into the zero-age horizontal branch (ZAHB). Yet, no stars have been positively identified in this key evolutionary phase, mainly for two reasons: first, this pre-ZAHB phase is very short compared to other major evolutionary phases in the life of a star; and second, these pre-ZAHB stars are expected to overlap the loci occupied by asymptotic giant branch (AGB), HB, and RGB stars observed in the color-magnitude diagram (CMD). We investigate the possibility of detecting these stars through stellar pulsations, since some of them are expected to rapidly cross the Cepheid/RR Lyrae instability strip in their route from the RGB tip to the ZAHB, thus becoming pulsating stars along the way. As a consequence of their very high evolutionary speed, some of these stars may present anomalously large period change rates. We constructed an extensive grid of stellar models and produced pre-ZAHB Monte Carlo simulations appropriate for the case of the Galactic globular cluster M3 (NGC 5272), where a number of RR Lyrae stars with high period change rates are found. Our results suggest that some—but certainly not all—of the RR Lyrae stars in M3 with large period change rates are in fact pre-ZAHB pulsators.  相似文献   

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