Neutron star retention and millisecond pulsar production in globular clusters

We investigate the conditions by which neutron star retention in globular clusters is favoured. We find that neutron stars formed in massive binaries are far more likely to be retained. Such binaries are likely to then evolve into contact before encountering other stars, possibly producing a single neutron star after a common envelope phase. A large fraction of the single neutron stars in globular clusters are then likely to exchange into binaries containing moderate-mass main-sequence stars, replacing the lower-mass components of the original systems. These binaries will become intermediate-mass X-ray binaries (IMXBs), once the moderate-mass star evolves off the main sequence, as mass is transferred on to the neutron star, possibly spinning it up in the process. Such systems may be responsible for the population of millisecond pulsars (MSPs) that has been observed in globular clusters. Additionally, the period of mass-transfer (and thus X-ray visibility) in the vast majority of such systems will have occurred 5–10 Gyr ago, thus explaining the observed relative paucity of X-ray binaries today, given the MSP population.     

On the number ratio of horizontal branch stars to red giant stars in globular clusters

The number ratios of horizontal branch stars to red giant stars were obtained for globular clusters and Draco dwarf galaxy and the helium abundance was estimated using model results without semiconvection zone (SCZ) and with fully developed one. The analysis was confined to the four clusters (M4, M5, M13, and 47 Tuc) and the Draco galaxy, for which fairly precise star counts had been carried out. The effect of the difference in radial distribution between horizontal and red giant branch stars were taken into account, if necessary. The statistically significant difference inR exists among these objects. The cause may be the difference in the helium abundance and/or in the development of the SCZ. In the case of the fully developed SCZ, the helium abundance for M5 and Draco is appreciably smaller than the value given by the big-bang cosmology. It may be taken as an evidence against the full development of the SCZ for the horizontal branch stars in these objects.     

On the orbits of globular clusters of stars in the galaxy

The study of the possible shapes of orbits of globular clusters of stars is closely connected with problems of studying the processes of formation and evolution of stars. The main difficulty in studying the orbits of these objects is the incompleteness of the information about the initial conditions of the motion, in particular the nearly complete absence of observational material on the proper motions of globular clusters. For that reason the study of the motion of these objects by direct dynamic methods has not yet succeeded. In such conditions it is necessary to resort to statistical methods developed by Edmondson and Von Hoerner, which make it possible to draw certain conclusions about the possible shapes of orbits of globular clusters on the basis of the Newtonian point-mass model.The purpose of this paper is to test the results of investigations of other authors against the latest observational data on globular clusters of stars and to develop the method itself further.Translated fromAstrofizika, Vol. 37, No. 4, 1994.     

High-resolution simulations of stellar collisions between equal-mass main-sequence stars in globular clusters

We performed high-resolution simulations of two stellar collisions relevant for stars in globular clusters. We considered one head-on collision and one off-axis collision between two 0.6-M_⊙ main-sequence stars. We show that a resolution of about 100 000 particles is sufficient for most studies of the structure and evolution of blue stragglers. We demonstrate conclusively that collision products between main-sequence stars in globular clusters do not have surface convection zones larger than 0.004 M_⊙ after the collision, nor do they develop convection zones during the 'pre-main-sequence' thermal relaxation phase of their post-collision evolution. Therefore, any mechanism which requires a surface convection zone (i.e. chemical mixing or angular momentum loss via a magnetic wind) cannot operate in these stars. We show that no disc of material surrounding the collision product is produced in off-axis collisions. The lack of both a convection zone and a disc proves a continuing problem for the angular momentum evolution of blue stragglers in globular clusters.     

Self-enrichment by asymptotic giant branch stars in globular clusters: comparison between intermediate and high metallicities

We present theoretical evolutionary sequences of intermediate-mass stars  ( M = 3 − 6.5 M_⊙)  with metallicity   Z = 0.004  . Our goal is to test whether the self-enrichment scenario by massive asymptotic giant branch stars may work for the high-metallicity globular clusters, after previous works by the same group showed that the theoretical yields by this class of objects can reproduce the observed trends among the abundances of some elements, namely the O–Al and O–Na anticorrelations, at intermediate metallicities, i.e.  [Fe/H]=−1.3  . We find that the increase in the metallicity favours only a modest decrease of the luminosity and the temperature at the bottom of the envelope for the same core mass, and also the efficiency of the third dredge-up is scarcely altered. On the contrary, differences are found in the yields, due to the different impact that processes with the same efficiency have on the overall abundance of envelopes with different metallicities. We expect the same qualitative patterns as in the intermediate-metallicity case, but the slopes of some of the relationships among the abundances of some elements are different. We compare the sodium–oxygen anticorrelation for clusters of intermediate metallicity ( Z ≈ 10⁻³) and clusters of metallicity large as in these new models. Although the observational data are still too scarce, the models are consistent with the observed trends, provided that only stars of   M ≳ 5 M_⊙  contribute to self-enrichment.     

Discovery of infrared stars in globular clusters in the Magellanic Clouds and their light variations

A systematic near-infrared survey was made for globular clusters in the Magellanic Clouds, and two previously unknown infrared stars were discovered in NGC419 and NGC1783. Light variations of these IR stars were revealed.     

The fractions of post-binary-interaction stars and evolved blue straggler stars on the red giant branch of globular clusters

The red giant branch(RGB) of globular clusters(GCs) is home to some exotic stars,which may provide clues on the formation of multiple stellar populations in GCs.It is well known that binary interactions are responsible for many exotic stars.Thus,it is important to understand what fraction of stars on the RGB of GCs is the result of binary interactions.In this paper,we performed a binary population synthesis study to track the number of post-binary-interaction(post-BI) stars that appear on the RGB,with particular emphasis on the evolved blue straggler stars(E-BSSs).Assuming an initial binary fraction of nearly 50%,we find that about half of the objects on the RGB(called giants) underwent the binary interactions,and that E-BSSs account for around 10% of the giants in our standard simulation.We also compare the properties of post-BI giants that evolved from different channels.We find that the initial orbital period and mass ratio distributions significantly affect the fraction of post-BI giants.Our results imply that the non-standard stars from binary interactions provide a non-negligible contribution to the RGB stars in GCs,which should be considered in future investigations of the origin of multiple stellar populations.     

What we would like to know about extreme horizontal branch stars in globular clusters

In this contribution, we explore some open questions about Extreme Horizontal Branch (EHB) stars in globular clusters. In particular, we present the current status of the search for rapid pulsators, He-depleted stars, and close binaries, three kind of objects very common among field EHB stars, but which first surveys failed to detect in clusters. We also analyze how the lack of cluster EHB binaries can reconcile with theoretical expectations. We give special attention to the first close EHB binary discovered in a globular cluster, whose characteristics point to a very rare or even unique object. Finally, we analyze some recent puzzling results about spectroscopically derived masses, that could point to the presence of two distinct families of EHB’s in clusters. For all these topics, we present the recent advancement in knowledge, the results requiring more investigation, and what has still to be done to fix the unsolved problems, showing the main points of our studies, the aims of our works, and what we expect to obtain from them.     

Colour gradients in globular clusters

Radial colour gradients in the globular clusters NGC 2808, NGC 1851, NGC 5139, NGC 5139, NGC 6388, NGC 6441, NGC 6541, NGC 6723, and NGC 7099 were searched byB-V and DDO photoelectric photometry, using a set of circular diaphragms. Two of the observed globular clusters, NGC 5139 (Cen) and NGC 7099 (M30), showedB-V and C(45–48) colour gradients; the colour C(42–45) is essentially constant in these clusters, which means that the physical reason for those gradients cannot be attributed to chemical composition variations, since C(42–45) is a good metal abundance indicator.B-V colours in the external regions of NGC 5139 and NGC 7099 were obtained by synthesizing the observed HR diagrams of these clusters. The ages and the relaxation times for NGC 5139 and NGC 7099 indicate that mass segregation must have taken place in both clusters, which is probably related to the observed colour gradients.     

Mass segregation in globular clusters

Photoelectric aperture-photometry ofω Cen inU, B, V, R andI bands has established that the cluster is bluer between 2 arcmin and 4 arcmin from the centre, than it is elsewhere. The difference inB - I colour between the centre and this blue zone is ≃ 0.45 mag. The core radius is found to be dependant on the wavelength band chosen for observation, the smallest core radius being for theI band. Equidensitometry ofω Cen inB, V and infrared bands shows a wavelength dependence with the cluster being nearly spherical in the infrared band. It shows a maximum ellipticity around 3 arcmin from the cluster centre. The blue contribution in this zone comes from both a diffuse background of unresolved stars and an increase in the relative abundance of horizontal branch (HB) stars. The similarity between the diffuse background and the HB stars has been demonstrated. A photographic subtraction technique is used to study the distribution of HB stars in the cluster. Results of equidensitometry of the cluster 47 Tuc, obtained in the present study, are compared with the earlier results of photoelectric photometry. Here too an increase in ellipticity is associated with an increase in the blueness of the cluster. All globular clusters studied so far for ellipticity show a similarity in the dependence of the ellipticity on the distance from the centre. The ellipticity has small values near the centre and in the outer regions, with the maximum value in between. We suggest that the red stars in globular clusters have a nearly spherical distribution. The blue stars form a bulge around the core with a more elliptical distribution and a different orientation. A similarity between the ellipticity aspects of both the globular clusters and rotation in the nucleus of M 31 is pointed out; the M 31 nucleus may thus show a bluer colour and smaller UV excess around the region where the rotation curve shows a peak. An erratum to this article is available at .     

Neutron stars:a relativistic study

We study the inner structure of a neutron star from a theoretical point of view and the outcome results are compared with observed data. We propose a stiff equation of state relating pressure with matter density. From our study we calculate mass(M),compactness(u) and surface redshift(Zs) for two binary millisecond pulsars,namely PSR J1614–2230 and PSR J1903+327,and four X-ray binaries,namely Cen X-3,SMC X-1,Vela X-1 and Her X-1,and compare them with recent observational data.Finally,we examine the stability for such a type of theoretical structure.     

Expected planets in globular clusters

We argue that all transient searches for planets in globular clusters have a very low detection probability. Planets of low-metallicity stars typically do not reside at small orbital separations. The dependence of planetary system properties on metallicity is clearly seen when the quantity   I _e ≡ M _p[ a (1 − e )]²  is considered;   M _p, a   and e are the planet mass, semimajor axis and eccentricity, respectively. In high-metallicity systems, there is a concentration of systems at high and low values of I _e , with a low-populated gap near   I _e ∼ 0.3 M _J au²  , where M _J is Jupiter's mass. In low-metallicity systems, the concentration is only at the higher range of I _e , with a tail to low values of I _e . Therefore, it is still possible that planets exist around main-sequence stars in globular clusters, although at small numbers because of the low metallicity, and at orbital periods of ≳10 d. We discuss the implications of our conclusions on the role that companions can play in the evolution of their parent stars in globular clusters, for example, influencing the distribution of horizontal branch stars on the Hertzsprung–Russell diagram of some globular clusters, and in forming low-mass white dwarfs.     

On the existence of differences in luminosity between horizontal branch stars in globular clusters and in the field

The discrepancy between a long distance-scale derived from Hipparcos -based distances to globular clusters via main-sequence fitting to local subdwarfs, and a short distance-scale derived from the absolute magnitude of field RR Lyraes via statistical parallaxes and the Baade–Wesselink method could be accounted for whether an intrinsic difference of about ∼0.1–0.2 mag was found to exist between horizontal branch (HB) stars populating the sparse general field and the dense globular clusters. In this paper we discuss the possible existence of such a systematic difference comparing the period-shifts observed for field and cluster RR Lyraes. Various approaches based on different parameters and data sets for both cluster and field variables were used in order to establish the size of such a hypothetical difference, if any. We find that on the whole very small not significant differences exist between the period–metallicity distributions of field and cluster RR Lyraes, thus confirming with a more quantitative approach, the qualitative conclusions by Catelan . This observational evidence translates into a very small difference between the horizontal branch luminosity of field and cluster stars, unless RR Lyraes in globular clusters are about 0.06 M_⊙ more massive than field RR Lyraes at same metallicity, which is to be proven.