1,723,737 research outputs found
A Chandra look at the X-ray faint millisecond pulsars in the globular cluster NGC 6752
No full textWe combine new and archival Chandra observations of the globular cluster NGC 6752 to create a deeper X-ray source list, and study the faint radio millisecond pulsars (MSPs) of this cluster. We detect four of the five MSPs in NGC 6752, and present evidence for emission from the fifth. The X-rays from these MSPs are consistent with thermal emission from the neutron star surfaces, with significantly higher fitted blackbody temperatures than other globular cluster MSPs (though we cannot rule out contamination by non-thermal emission or other X-ray sources). NGC 6752 E is one of the lowest-LX MSPs known, with LX(0.3–8 keV) = 1.0+0.9−0.5×1030 erg s−1. We check for optical counterparts of the three isolated MSPs in the core using new Hubble Space Telescope Advanced Camera for Surveys images, finding no plausible counterparts, which is consistent with their lack of binary companions. We compile measurements of LX and spin-down power for radio MSPs from the literature, including errors where feasible. We find no evidence that isolated MSPs have lower LX than MSPs in binary systems, omitting binary MSPs showing emission from intrabinary wind shocks. We find weak evidence for an inverse correlation between the estimated temperature of the MSP X-rays and the known MSP spin period, consistent with the predicted shrinking of the MSP polar cap size with increasing spin period
Spectroscopic study of red giants in the globular cluster ngc 6752.
No full textUp until recently, globular clusters were believed to be simple stellar populations, but recent studies have revealed a notable variety in chemical composition of cluster stars, thus giving rise to the search for the origins of multiple stellar populations. Several scenarios have been developed to explain the formation of the two stellar populations, however, due to inconsistencies in their predictions with real stellar observations and the lack of research itself, the origins of the populations and the sources of the chemical elements have not yet been reliably determined. The aim of this work is to determine the abundance of various chemical elements in the stars NGC 6752-24 and NGC 6752-23 belonging to different populations and to evaluate the probable sources of the cluster NGC 6752 nucleosynthesis responsible for the formation of the two stellar populations. The atmospheric parameters of the studied stars were also determined and the results were compared with the works of other authors. The archival spectra of stars NGC 6752-23 and NGC 6752-24 were used in the work, and the abundances were determined using two methods - by measuring spectral line equivalent widths for lighter elements, and by calculating synthetic spectra for heavier elements and Li. The general trends of the identified abundances suggest that the AGB stellar scenario would be more suitable for the cluster NGC 6752, but accurate conclusions are limited by uncertainties of the results of this work and the probable sources of nucleosynthesis responsible for the creation of two stellar populations cannot be reliably determined
Slowly Cooling White Dwarfs in NGC 6752
Get PDFRecently, a new class of white dwarfs (“slowly cooling WDs”) has been identified in the globular cluster M13. The cooling time of these stars is increased by stable thermonuclear hydrogen burning in their residual envelope. These WDs are thought to be originated by horizontal branch (HB) stars populating the HB blue tail that skipped the asymptotic giant branch phase. To further explore this phenomenon, we took advantage of deep photometric data acquired with the Hubble Space Telescope in the near-ultraviolet and investigate the bright portion of the WD cooling sequence in NGC 6752, another Galactic globular cluster with a metallicity, age, and HB morphology similar to M13. The normalized WD luminosity function derived in NGC 6752 turns out to be impressively similar to that observed in M13, in agreement with the fact that the stellar mass distribution along the HB of these two systems is almost identical. As in the case of M13, the comparison with theoretical predictions is consistent with ∼70% of the investigated WDs evolving at slower rates than standard, purely cooling WDs. Thanks to its relatively short distance from Earth, NGC 6752 photometry reaches a luminosity 1 order of a magnitude fainter than the case of M13, allowing us to sample a regime where the cooling time delay, with respect to standard WD models, reaches ∼300 Myr. The results presented in this paper provide new evidence for the existence of slowly cooling WDs and further support to the scenario proposing a direct causal connection between this phenomenon and the HB morphology of the host stellar cluster
On the missing second generation AGB stars in NGC 6752
No full textIn recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, it is now thought that basically all globular clusters host multiple stellar populations, each with its own chemical abundance pattern and colour–magnitude diagram sequence. Recent spectroscopic observations of asymptotic giant branch stars in the globular cluster NGC 6752 have disclosed a low [Na/Fe] abundance for the whole sample, suggesting that they are all first generation stars, and that all second generation stars fail to reach the AGB in this cluster. A scenario proposed to explain these observations invokes strong mass loss in second generation horizontal branch stars – all located at the hot side of the blue and extended horizontal branch of this cluster – possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the asymptotic giant branch phase, thus explaining at the same time the low value of the ratio between horizontal branch and asymptotic giant branch stars (the R2 parameter) observed in NGC 6752. We have critically discussed this mass-loss scenario, finding that the required mass-loss rates are of the order of 10-9 M⊙ yr-1, significantly higher than current theoretical and empirical constraints. By making use of synthetic horizontal branch simulations, we demonstrate that our modelling correctly predicts the R2 parameter for NGC 6752, without the need to invoke very efficient mass loss during the core He-burning stage. As a test of our stellar models we show that we can reproduce the observed value of R2 for both M 3, a cluster of approximately the same metallicity and with a redder horizontal branch morphology, and M 13, a cluster with a horizontal branch very similar to NGC 6752. However, our simulations for the NGC 6752 horizontal branch predict however the presence of a significant fraction of second generation stars (about 50%) along the cluster asymptotic giant branch. We conclude that there is no simple explanation for the lack of second generation stars in the spectroscopically surveyed sample, although the interplay between mass loss (with low rates) and radiative levitation may play a role in explaining this puzzle
The HST Large Programme on NGC 6752 - II. Multiple populations at the bottom of the main sequence probed in NIR
Get PDFHistorically, multiple populations in globular clusters (GCs) have been mostly studied from ultraviolet and optical filters down to stars that are more massive than ∼0.6 M⊙. Here, we exploit deep near-infrared (NIR) photometry from the Hubble Space Telescope to investigate multiple populations among M-dwarfs in the GC NGC 6752. We discovered that the three main populations (A, B, and C), previously observed in the brightest part of the colour-magnitude diagram (CMD), define three distinct sequences that run from the main-sequence (MS) knee towards the bottom of the MS (∼0.15 M⊙). These results, together with similar findings on NGC 2808, M 4, and ω Centauri, demonstrate that multiple sequences of M-dwarfs are common features of the CMDs of GCs. The three sequences of low-mass stars in NGC 6752 are consistent with stellar populations with different oxygen abundances. The range of [O/Fe] needed to reproduce the NIR CMD of NGC 6752 is similar to the oxygen spread inferred from high-resolution spectroscopy of red giant branch (RGB) stars. The relative numbers of stars in the three populations of M-dwarfs are similar to those derived among RGB and MS stars more massive than ∼0.6 M⊙. As a consequence, the evidence that the properties of multiple populations do not depend on stellar mass is a constraint for the formation scenarios. © 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical Society
IUE observations of blue horizontal branch stars in the globular clusters M 3 and NGC 6752
Get PDFIUE observations of 3 blue horizontal branch (BHB) stars in M 3 and 4 stars in NGC 6752 are presented. In addition, unpublished IUE archive data have been used for 5 more BHB stars in NGC 6752. Using the most recent model atmospheres (Kurucz 1992) the physical parameters of these stars have been derived, and compared with the ZAHB evolutionary tracks by Dorman et al. (1993). All the stars, including those that appear to be evolved off the ZAHB, are consistent with the theoretical predictions. The present results are compared with those obtained in previous similar analyses: the possible presence of some low-gravity stars is confirmed in M 3, while no clear indication is found of multiple stellar populations on the HB of NGC 6752. However different populations, if any, would be difficult to identify in the logg-logT_e_ plane because of the simultaneous effect of various parameters and the poor accuracy of the gravity estimates from IUE data
Estudio del cúmulo globular NGC 6752 y sus variables
Get PDFSe construyó un diagrama color-magnitud en NGC 6752 derivándose los parámetros geométricos, físicos y fotométricos. De las dos variables que posee este cúmulo descubiertas en Córdoba, se estudió la Nº 1 insertándola en el diagrama color-magnitud.Asociación Argentina de Astronomí
Rubidium and Lead Abundances in Giant Stars of the Globular Clusters M13 and NGC 6752
No full textWe present measurements of the neutron-capture elements Rb and Pb in five giant stars of the globular cluster NGC 6752 and Pb measurements in four giants of the globular cluster M13. The abundances were derived by comparing synthetic spectra with high-resolution, high signal-to-noise ratio spectra obtained using HDS on the Subaru telescope and MIKE on the Magellan telescope. The program stars span the range of the O-Al abundance variation. In NGC 6752, the mean abundances are [Rb/Fe] = -0.17 ± 0.06 (σ = 0.14), [Rb/Zr] = -0.12 ± 0.06 (σ = 0.13), and [Pb/Fe] = -0.17 ± 0.04 (σ = 0.08). In M13 the mean abundance is [Pb/Fe] = -0.28 ± 0.03 (σ = 0.06). Within the measurement uncertainties, we find no evidence for star-to-star variation for either Rb or Pb within these clusters. None of the abundance ratios [Rb/Fe], [Rb/Zr], or [Pb/Fe] are correlated with the Al abundance. NGC 6752 may have slightly lower abundances of [Rb/Fe] and [Rb/Zr] compared to the small sample of field stars at the same metallicity. For M13 and NGC 6752 the Pb abundances are in accord with predictions from a Galactic chemical evolution model. If metal-poor intermediate-mass asymptotic giant branch stars did produce the globular cluster abundance anomalies, then such stars do not synthesize significant quantities of Rb or Pb. Alternatively, if such stars do synthesize large amounts of Rb or Pb, then they are not responsible for the abundance anomalies seen in globular clusters
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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