On the Galactic disc age–metallicity relation

A comparison is made between the age–metallicity relations obtained from four different types of studies: F and G stars in the solar neighbourhood, analysis of open clusters, galactic structure studies with the stellar population synthesis technique and chemical evolution models. Metallicities of open clusters are corrected for the effects of the radial gradient, which we find to be −0.09 dex kpc⁻¹ and most likely constant in time. We do not correct for the vertical gradient, because its existence and value are not firmly established.
Stars and clusters trace a similar age–metallicity relation, showing an excess of rather metal-rich objects in the age range 5–9 Gyr. Galactic structure studies tend to give a more metal-poor relation than chemical evolution models. Neither relation explains the presence of old, relatively metal-rich stars and clusters. This might be caused by uncertainties in the ages of the local stars, or pre-enrichment of the disc with material from the bulge, possibly as a result of a merger event in the early phases of the formation of our Galaxy.     

Morphological analysis of open clusters’ properties: II. Relationships projected onto the galactic plane

A morphological analysis study of open clusters’ properties has been achieved for a sample of 160 UBVCCD open star clusters of approximately 128,000 stars near the galactic plane. The data was obtained and reduced from Tadross (2001) using the same reduction procedures, which makes this catalogue the largest homogeneous source of open clusters’ parameters.     

The trouble with the Local Bubble

The model of a Local Hot Bubble has been widely accepted as providing a framework that can explain the ubiquitous presence of the soft X-ray background diffuse emission. We summarize the current knowledge on this local interstellar region, paying particular reference to observations that sample emission from the presumed local million degree K hot plasma. However, we have listed numerous observations that are seemingly in conflict with the concept of a hot Local Bubble. In particular, the discovery of solar wind charge exchange that can generate an appreciable soft X-ray background signal within the heliosphere, has led to a re-assessment of the generally accepted model that requires a hot local plasma. In order to explain the majority of observations of the local plasma, we forward two new speculative models that describe the physical state of the local interstellar gas. One possible scenario is similar to the present widely accepted model of the Local Hot Bubble, except that it accounts for only 50% of the soft X-ray emission currently detected in the galactic plane, has a lower thermal pressure than previously thought, and its hot plasma is not as hot as previously believed. Although such a model can solve several difficulties with the traditional hot Local Bubble model, a heating mechanism for the dimmer and cooler gas remains to be found. The second possible explanation is that of the ‘Hot Top’ model, in which the Local Cavity is an old supernova remnant in which no (or very little) million degree local plasma is presently required. Instead, the cavity is now thought to be filled with partially ionized cloudlets of temperature ∼7000 K that are surrounded by lower density envelopes of photo-ionized gas of temperature ∼20,000 K. Although this new scenario provides a natural explanation for many of the observations that were in conflict with the Local Hot Bubble model, we cannot (as yet) provide a satisfactory explanation or the emission levels observed in the B and Be ultra-soft X-ray bands.     

Weak Elliptical Distortion of the Milky Way Potential traced by Open Clusters

From photometric observations and star counts, the existence of a bar in the cen-tral few kpc of the Galaxy is suggested. It is generally thought that our Galaxy is surrounded by a massive invisible halo. The gravitational potential of the Galaxy is therefore made non-axisymmetric generated by the central tfiaxial bar, by the outer triaxial halo, and/or by the spiral structures. Selecting nearly 300 open clusters with complete spatial velocity measure-ments and ages, we were able to construct the rotation curve of the Milky Way within a range of 3 kpc of the Sun. Using a dynamic model for an assumed elliptical disk, a clear weak el-liptical potential of the disk with ellipticity of ε(R0) = 0.060 ± 0.012 is detected, the Sun is found to be near the minor axis, displaced by 30°± 3°. The motion of the clusters is suggested to be on an oval orbit rather than on a circular one.     

Possible Streams of the Globular Clusters in the Galaxy

1 INTRODUCTION The globular cluster (GC hereafter), as the oldest star group in the universe, has been a target that astro- physics has paid close attention to all the time. The near-field (Galaxy) cosmology makes contacts with the far-field cosmology by …     

The environment of formation as a second parameter for globular cluster classification

We perform an evolutionary multivariate analysis of a sample of 54 Galactic globular clusters with high-quality colour–magnitude diagrams and well-determined ages. The four parameters adopted for the analysis are: metallicity, age, maximum temperature on the horizontal branch and absolute V magnitude. Our cladistic analysis breaks the sample into three novel groups. An a posteriori kinematical analysis puts groups 1 and 2 in the halo, and group 3 in the thick disc. The halo and disc clusters separately follow a luminosity–metallicity relation of much weaker slope than galaxies. This property is used to propose a new criterion for distinguishing halo and disc clusters. A comparison of the distinct properties of the two halo groups with those of Galactic halo field stars indicates that the clusters of group 1 originated in the inner halo, while those of group 2 formed in the outer halo of the Galaxy. The inner halo clusters were presumably initially the most massive one, which allowed the formation of more strongly helium-enriched second generation stars, thus explaining the presence of Cepheids and of very hot horizontal-branch stars exclusively in this group. We thus conclude that the 'second parameter' is linked to the environment in which globular clusters form, the inner halo favouring the formation of the most massive clusters which subsequently become more strongly self-enriched than their counterparts of the galactic outer halo and disc.     

A Monte Carlo Study of the Evolution of the Scale Height of Normal Pulsars in the Galaxy

Based on the undisturbed, finite thickness disk gravitational potential, we carried out 3-D Monte Carlo simulations of normal pulsars. We find that their scale height evolves in a similar way for different velocity dispersions (σv): it first increases linearly with time, reaches a peak, then gradually decreases, and finally approaches a stable asymptotic value. The initial velocity dispersion has a very large influence on the scale height. The time evolution of the scale height is studied. When the magnetic decay age is used as the time variable, the observed scale height has a similar trend as the simulated results in the linear stage, from which we derive velocity dispersions in the range 70- 178km s-1, which are near the statistical result of 90 - 270km s-1 for 92 pulsars with known transverse velocities. If the characteristic age is used as the time variable, then the observed and theoretical curves roughly agree for t > 108 yr only if σv < 25km s-1.     

Detecting the effect of globular cluster impacts on the disc of the Milky Way

The crossing of the Galactic disc by a globular cluster (GC) could produce star formation due to gravitational focusing or compression of disc material. We report on simulations of the effect on disc material which reveal that the crossing can sometimes cause local gravitational focusing of disc material. We also present the salient points of a little-known paper by Levy, which shows that strong compression can result from the shock wave generated by GC disc crossing. The main thrust of our paper is a search for remnants of disc crossings by GCs. Using the gravitational potential of the Galaxy to locate the position of the most recent crossings of a subset of fifty-four GCs reveals that systematic errors and uncertainties in initial conditions limit the scope for unequivocal identification. From the subset of fifty-four, six possible search sites with the best constraints are retained for further scrutiny. Three of the six potentially promising search areas in the disc are from GCs NGC 3201, 6397 and 6838, for which we cannot rule out some observed star associations observed nearby as being remnants. The other three of the six areas are too large to provide meaningful identification of remnants. Also, a possible remnant (open cluster NGC 6231) is shown not to be due to GC impact, contrary to a previous report. In a more wide-ranging screening of 155 GCs, we identify which GCs are compatible with being responsible for the formation of any of the Galaxy's five most prominent star superclusters.