L. Deng

The blue stragglers formed via mass transfer in old open clusters

In this paper, we present the simulations for the primordial blue stragglers in the old open cluster M 67 based on detailed modelling of the evolutionary processes. The principal aim is to discuss the contribution of mass transfer between the components of close binaries to the blue straggler population in M 67. Methods. First, we followed the evolution of a binary of 1.4 M-circle dot + 0.9 M-circle dot. The synthetic evolutionary track of the binary system revealed that a primordial blue straggler had a long lifetime in the observed blue straggler region of color-magnitude diagram. Second, a grid of models for close binary systems experiencing mass exchange were computed from 1 Gyr to 6 Gyr in order to account for primordial blue-straggler formation in a time sequence. Based on such a grid, Monte- Carlo simulations were applied for the old open cluster M 67. Results. Adopting appropriate orbital parameters, 4 primordial blue stragglers were predicted by our simulations. This was consistent with the observational fact that only a few blue stragglers in M 67 were binaries with short orbital periods. An upper boundary of the primordial blue stragglers in the color-magnitude diagram (CMD) was defined and could be used to distinguish blue stragglers that were not formed via mass exchange. Using the grid of binary models, the orbital periods of the primordial BSs could be predicted. Conclusions. Compared with the observations, it is clear that the mechanism discussed in this work alone cannot fully predict the blue straggler population in M 67. There must be several other processes also involved in the formation of the observed blue stragglers in M 67.

Blue Stragglers in Galactic Open Clusters and Integrated Spectral Energy Distributions

Synthetic integrated spectral properties of the old Galactic open clusters are studies in this work, in which 27 Galactic open clusters of ages ≥1 Gyr are selected as the working sample. Based on the photometric observations of these open clusters, a synthetic integrated spectrum has been made for the stellar population of each cluster. The effects of blue straggler (BS) stars on the conventional simple stellar population (SSP) model are analyzed on an individual cluster basis. It is shown that the BSs, whose positions in the color-magnitude diagrams cannot be predicted by the current single-star evolution theory, require significant modifications to the integrated properties of theoretical SSP model. The synthesized integrated spectral energy distributions (ISEDs) of our sample clusters are dramatically different from those of SSPs based on an isochrone only. The BS-corrected ISEDs of stellar populations show systematic enhancements toward shorter wavelengths in the spectra. When measured with broadband colors in unresolvable conditions, the age of a stellar population can be seriously underestimated by the conventional SSP model. Therefore, considering the common existence of BS components in real stellar populations, we should expect considerable alterations of the conventional ISEDs when we apply the technique of evolutionary population synthesis to more complicated stellar systems.

The Blue Stragglers in M67 and Single-Population Synthesis

Based on the photometry of the classical open cluster M67 and the thorough membership survey, we made a color-magnitude diagram (CMD) of high-membership stars for the cluster. Apart from the stars fitted by a conventional isochrone (single-star evolutionary model) scheme, we noticed a number of features on the CMD that are usually ignored when constructing the corresponding single stellar population (SSP), especially the large number of blue stragglers. As for the study of the integrated properties of SSPs, we argue that the contributions from all ingredients in a single population of stars, such as the luminous blue stragglers, should be included. These bright blue objects may modify remarkably the spectral energy distribution (SED) in the blue and ultraviolet. In this paper we investigate their effects on the integrated light of single stellar populations. We will show in this work that, by using the observation of M67 in a multicolor intermediate-band system, we can assign adequate theoretical spectra to individual stars. The integrated spectrum of this cluster is dramatically different from the SSP built using isochrones only, which shows increasing enhancement toward shorter wavelengths (starting from about 5000 A) of the spectrum. Taking this as a general case for SSPs, we can expect a considerable amount of modification to the integrated SEDs, which might cast new light on the analysis of the stellar content in the complex stellar systems.

Blue Straggler Stars in Galactic Open Clusters and the Simple Stellar Population Model

The presence of blue straggler stars (BSs) as secure members of Galactic open clusters (OCs) poses a major challenge to the conventional picture of simple stellar population (SSP) models. These are based on the stellar evolution theory of single stars, whereas the major formation mechanisms of BSs are all correlated with stellar interactions. We have illustrated this in a previous study based on a small sample of old (age >= 1 Gyr) Galactic OCs. However, for the purpose of demonstrating the contributions of BSs to the conventional SSP models statistically and systematically, a large database with sufficient coverage of age and metallicity is definitely needed. The working sample now includes 100 Galactic OCs with ages ranging from 0.1 to 10 Gyr. The contributions of BSs to the integrated light of their host clusters are calculated on an individual cluster basis. The general existence of BSs in our star cluster sample dramatically alters the predictions of conventional SSP models in terms of their integrated properties. Neglecting the consequences of nonstandard evolutionary products, such as BSs, in stellar populations, very large uncertainties can be made in analyzing their integrated spectral energy distributions at unresolvable conditions. The current work strongly suggests that when evolutionary population synthesis technique is used to study the properties of unresolved stellar populations in galaxies, the contributions of BSs should be taken into account.

Simple stellar population models as probed by the Large Magellanic Cloud star cluster ESO 121-SC03

The presence of blue straggler stars (BSs) in star clusters has proven a challenge to conventional simple stellar population (SSP) models. Conventional SSP models are based on the evolution theory of single stars. Meanwhile, the typical locations of BSs in the colour-magnitude diagram of a cluster are brighter and bluer than the main-sequence turn-off point. Such loci cannot be predicted by single-star evolution theory. However, stars with such properties contribute significantly to the integrated light of the cluster. In this paper, we reconstruct the integrated properties of the Large Magellanic Cloud cluster European Southern Observatory (ESO) 121-SC03, the only cluster populating the well-known age gap in the cluster age distribution, based on a detailed exploration of the individual cluster stars, and with particular emphasis on the clusters BSs. We find that the integrated light properties of ESO 121-SC03 are dramatically modified by its BS component. The integrated spectral energy distribution (ISED) flux level is significantly enhanced towards shorter wavelengths, and all broad-band colours become bluer. When fitting the fully integrated ISED of this cluster based on conventional SSP models, the best-fitting values of age and metallicity are significantly underestimated compared to the true cluster parameters. The age underestimate is similar to 40 per cent if we only include the BSs within the clusters half-light radius and similar to 60 per cent if all BSs are included. The corresponding underestimates of the clusters metallicity are similar to 30 and similar to 60 per cent, respectively. The populous star clusters in the Magellanic Clouds are ideal objects to explore the potential importance of BSs for the integrated light properties of more distant unresolved star clusters in a statistically robust manner, since they cover a large range in age and metallicity.

The asymmetric structure of the Galactic halo

Using the stellar photometry catalogue based on the latest data release (DR4) of the Sloan Digital Sky Survey (SDSS), a study of the Galactic structure using star counts is carried out for selected areas of the sky. The sample areas are selected along a circle at a Galactic latitude of +60 ◦ , and 10 strips of high Galactic latitude along different longitudes. Direct statistics of the data show that the surface densities of l from 180 ◦ to 360 ◦ are systematically higher than those of l from 0 ◦ to 180 ◦ , defining a region of overdensity (in the direction of Virgo) and another one of underdensity (in the direction of Ursa Major) with respect to an axisymmetric model. It is shown by comparing the results from star counts in the (g r) colour that the density deviations are due to an asymmetry of the stellar density in the halo. Theoretical models for the surface density profile are built and star counts are performed using a triaxial halo of which the parameters are constrained by observational data. Two possible reasons for the asymmetric structure are discussed.

The structure of the Galactic halo: SDSS versus SuperCOSMOS

The halo structure at high Galactic latitudes near both the north and south poles is studied using Sloan Digital Sky Survey (SDSS) and SuperCOSMOS data. For the south cap halo, the archive of the SuperCOSMOS photographic photometry sky survey is used. The coincident source rate between SuperCOSMOS data in B J band from 16.5 to 20.5 mag and SDSS data is about 92 per cent, in a common sky area in the south. While that in the R F band is about 85 per cent from 16.5 to 19.5 mag. Transformed to the SuperCOSMOS system and downgraded to the limiting magnitudes of SuperCOSMOS, the star counts in the North Galactic Cap from SDSS show up to an 16.9±6.3 per cent asymmetric ratio (defined as relative fluctuations over the rotational symmetry structure) in the B J band, and up to 13.5 ± 6.7 per cent asymmetric ratio in the R F band. From SuperCOSMOS Bj and R F bands, the structure of the Southern Galactic hemisphere does not show the same obvious asymmetric structures as the northern sky does in both the original and downgraded SDSS star counts. An axisymmetric halo model with n = 2.8 and q = 0.7 can fit the projected number density from SuperCOSMOS fairly well, with an average error of about 9.17 per cent. By careful analysis of the difference of star counts between the downgraded SDSS northern halo data and SuperCOSMOS southern halo data, it is shown that no asymmetry can be detected in the South Galactic Cap at the accuracy of SuperCOSMOS, and the Virgo overdensity is likely a foreign component in the Galactic halo.

Stellar evolution with turbulent diffusion

In this paper we briefly report on two recent studies by Deng, Bressan, & Chiosi (1944a,b) in which a new formulation of diffusive mixing in stellar interiors is presented. In particular, the analysis is devoted to cast light on the kind of mixing that should take place in the so‐called overshoot regions surrounding the inner fully convective cores. Key points of the analysis are the inclusion of intermittence and stirring efficiency and the concept of scale length most effective for mixing, by means of which the diffusion coefficient is formulated. Depending on the value of the diffusion coefficient that holds good in the overshoot region, we go from the case of virtually no mixing (semiconvective like structures) to that of full mixing over there (standard overshoot models). With the aid of this formalism, we find that stellar models of massive and intermediate‐mass stars calculated with mild efficiency of mixing in the region of overshoot (in our notation 0.4≤Pdif≤0.8) possess at the same time evolutio...

NEW MODELS FOR WOLF-RAYET STARS: GLOBAL DIFFUSION

We present a scenario for the evolution of massive stars in which a new mixing mechanism (named global diffusion) is taken into account. This type of mixing stands on the critical Reynolds number and radiative viscosity (Schatzman 1977) and allows mixing of material to take place between the core and the surface during the whole evolution on a very slow time scale. The physical processes triggering global diffusion deserve further study. We find that stellar models of massive stars calculated with global diffusion offer interesting clues to understanding the properties of Wolf-Rayet stars and their location in the HRD.