اشترك بالحزمة الذهبية واحصل على وصول غير محدود شمرا أكاديميا
تسجيل مستخدم جديدHipparcos open clusters and stellar evolution
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By relying on recently improved Hipparcos parallaxes for the Hyades, Pleiades and Ursa Major clusters we find that stellar models with updated physical inputs nicely reproduce the location in the color magnitude diagram of main sequence stars of different metallicities. Stars in the helium burning phase are also discussed, showing that the luminosity of giants in the Hyades, Praesepe and Ursa Major clusters appears to be in reasonable agreement with theoretical predictions. A short discussion concerning the current evolutionary scenarios closes the paper.
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Base on Gaia Second Data Release and the combination of nonparametric bivariate density estimation with the least square ellipse fitting, we derive the shape parameters of the sample clusters. By analyzing the dislocation of the sample clusters, the
dislocation $d$ is related to the X-axis pointing toward the Galactic center, Y-axis pointing in the direction of Galactic rotation, and the Z-axis (log(|H|/pc)) that is positive toward the Galactic north pole. This finding underlines the important role of the dislocation of clusters in tracking the external environment of the Milky Way. The orientation ($q_{pm}$) of the clusters with $e_{pm}$ $geq$ 0.4 presents an aggregate distribution in the range of -45$degr$ to 45$degr$, about 74% of them. This probably suggests that these clusters tend to deform heavily in the direction of the Galactic plane. NGC 752 is in a slight stage of expansion in the two-dimensional space and will deform itself morphology along the direction perpendicular to the original stretching direction in the future if no other events occur. The relative degree of deformation of the sample clusters in the short-axis direction decreases as their ages increase. On average, the severely distorted sample clusters in each group account for about 26% $pm$ 9%. This possibly implies a uniform external environment in the range of $|$H$|$ $leq$ 300 pc if the sample completeness of each group is not taken into account.
In a companion paper we studied the detailed evolution of stellar collision products that occurred in an $N$-body simulation of the old open cluster M67 and compared our detailed models to simple prescriptions. In this paper we extend this work by st
udying the evolution of the collision products in open clusters as a function of mass and age of the progenitor stars. We calculated a grid of head-on collisions covering the section of parameter space relevant for collisions in open clusters. We create detailed models of the merger remnants using an entropy-sorting algorithm and follow their subsequent evolution during the initial contraction phase, through the main sequence and up to the giant branch with our detailed stellar evolution code. We compare the location of our models in a colour-magnitude diagram to the observed blue straggler population of the old open clusters M67 and NGC 188 and find that they cover the observed blue straggler region of both clusters. For M67, collisions need to have taken place recently. Differences between the evolution tracks of the collision products and normal main sequence stars can be understood quantitatively using a simple analytic model. We present an analytic recipe that can be used in an $N$-body code to transform a precomputed evolution track for a normal star into an evolution track for a collision product.
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D. Barrado y Navascues n MEC/Fulbright Fellow at the Smithsonian Astrophysical Observatory
1998
Randich and Schmitt [1995, A&A 298, 115] found that the coronal activity of solar-type and low mass stars in Praesepe is significantly lower than that of stars in the Hyades cluster. We have carried out several tests in order to find a possible expla
nation for this result. We have measured radial velocities of two groups of Praesepe stars (a dF-dK sample and a dM sample) and have measured H$alpha$ as a chromospheric activity index for the dM sample. We conclude that the Praesepe catalog used in the X-ray analysis does not contain a significant number of non-members. The comparison of the H$alpha$ equivalent widths for the M dwarfs in Praesepe with those in the Hyades indicates that, at least for stars in this mass range, the Praesepe stars are as active or more active than their Hyades counterparts. We have also analyzed a few ROSAT PSPC pointings of Praesepe in order to obtain a new and independent estimate of the X-ray luminosities and upper limits for a small sample of Praesepe members concluding that the small differences between the old and new upper limits are not large enough to explain the dichotomy in the X-ray properties of Praesepe and the Hyades. Therefore, our examination of the available data does not provide a clear reason to explain why the X-ray luminosity functions of the two clusters are different. Part of the explanation could be found in the binaries. Speculatively, these clusters could have different orbital period distributions, with more short period binaries among the Hyades, which would show larger coronal activity.
In this paper we study the evolution of a primordial black hole binary (BHB) in a sample of over 1500 direct-summation $N-$body simulations of small-and intermediate-size isolated star clusters as proxies of galactic open clusters. The BHBs have mass
es in the range of the first LIGO/Virgo detections. Some of our models show a significant hardening of the BHB in a relatively short time. Some of them merge within the cluster, while ejected binaries, typically, have exceedingly long merger timescales. The perturbation of stars around BHB systems is key to induce their coalescence. The BHBs which merge in the cluster could be detected with a delay of a few years between space detectors, as future LISA, and ground-based ones, due to their relatively high eccentricity. Under our assumptions, we estimate a BHB merger rate of $R_{rm mrg} sim 2$ yr$^{-1}$ Gpc$^{-3}$. We see that in many cases the BHB triggers tidal disruption events which, however, are not linked to the GW emission. Open cluster-like systems are, hence, a promising environment for GWs from BHBs and tidal disruptions.
Mixing mechanisms bring the Li from the base of the convective zone to deeper and warmer layers where it is destroyed. These mechanisms are investigated by comparing observations of Li abundances in stellar atmospheres to models of stellar evolution.
Observations in open cluster are especially suitable for this comparison, since their age and metallicity are homogeneous among their members and better determined than in field stars. In this work, we compare the evolution of Li abundances in three different clusters: the Hyades, NGC752, and M67. Our models are calculated with microscopic diffusion and transport of chemicals by meridional circulation, and calibrated on the Sun. These comparisons allow us to follow the evolution of Li abundance as a function of stellar mass in each cluster and as a function of the age by comparing this evolution in each cluster. We evaluate the efficiency of the mixing mechanisms used in the models, and we try to identify the lacking mechanisms to reproduce the observed evolution of Li abundance.
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