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Hot HB stars in globular clusters - Physical parameters and consequences for theory. V. Radiative levitation versus helium mixing  [PDF]
S. Moehler,A. V. Sweigart,W. B. Landsman,U. Heber
Physics , 2000,
Abstract: Atmospheric parameters (effective temperature Teff, surface gravity log g), masses and helium abundances are derived for 42 hot horizontal branch (HB) stars in the globular cluster NGC 6752. For 19 stars we derive magnesium and iron abundances as well and find that iron is enriched by a factor of 50 on average with respect to the cluster abundance whereas the magnesium abundances are consistent with the cluster abundance. Radiation pressure may levitate heavy elements like iron to the surface of the star in a diffusive process. Taking into account the enrichment of heavy elements in our spectroscopic analyses we find that high iron abundances can explain part, but not all, of the problem of anomalously low gravities along the blue HB. The blue HB stars cooler than about 15,100 K and the sdB stars (Teff > 20,000 K) agree well with canonical theory when analysed with metal-rich ([M/H] = +0.5) model atmospheres, but the stars in between these two groups remain offset towards lower gravities and masses. Deep Mixing in the red giant progenitor phase is discussed as another mechanism that may influence the position of the blue HB stars in the (Teff, log g)-plane but not their masses.
Mixing, Enhanced Helium and Blue Tails in Globular Clusters  [PDF]
V. Caloi
Physics , 2000, DOI: 10.1051/0004-6361:20000210
Abstract: We investigate the consequences of an increase in the envelope helium abundance of pre-helium flash red giants in globular clusters. Comparing predictions with the CM diagrams of a few crucial GC, one finds no evidence for a substantial increase in the surface helium content of HB members of these clusters, at least for objects in the RR Lyrae region or close to it. The possibility that the most peculiar giants belong to the asymptotic giant branch is discussed. The consequences of a delay in the helium flash are briefly examined.
Can globular clusters be as young as 10 Gyr?--The Uncertainties in the Age of Globular Clusters from Their Helium Abundance and Mass Loss  [PDF]
Xiangdong Shi
Physics , 1994,
Abstract: The age of globular clusters inferred from observations depends sensitively on assumptions such as the initial helium abundance and the mass loss rate. A high helium abundance (e.g., $Y\approx$0.28), as well as an inclusion of helium diffusion and oxygen-enhancement in stellar models, can lower the current age estimate for metal-poor globular clusters from 14$\pm 1.5$ Gyr to about 11$\pm 1$ Gyr, significantly relaxing the constraints on the Hubble constant, allowing values as high as 60km/sec/Mpc for a universe with the critical density and 90km/sec/Mpc for a baryon-only universe. The uncertainties of a high helium abundance and an instability strip induced mass loss near the turn-off in globular clusters are discussed. Ages lower than 10 Gyr are not possible even with the operation of both of these mechanisms unless the initial helium abundance in globular clusters is $>0.28$, which can hardly be accomodated by indirect inferences of helium abundances in globular clusters.
Helium Mixing in Globular Cluster Stars  [PDF]
A. V. Sweigart
Physics , 1997,
Abstract: The observed abundance variations in globular cluster red giants indicate that these stars may be mixing helium from the hydrogen shell outward into the envelope, presumably as a result of internal rotation. We have investigated the implications of such helium mixing for both the red- giant-branch (RGB) and horizontal-branch (HB) phases by computing a number of noncanonical evolutionary sequences for different assumed mixing depths and mass loss rates. We find that the helium-mixed models evolve to higher luminosities during the RGB phase and consequently lose more mass than their canonical counterparts. This enhanced mass loss together with the higher envelope helium abundances of the helium-mixed models produces a markedly bluer and somewhat brighter HB morphology. As a result, helium mixing can mimic age as a 2nd parameter and can reduce the ages of the metal-poor globular clusters derived from the luminosity difference between the HB and the main-sequence turnoff. Helium mixing might also lead to a larger RR Lyrae period shift and to a steeper slope for the RR Lyrae luminosity - metallicity relation if the mixing is more extensive at low metallicities, as suggested by the observed abundance variations. We discuss the implications of helium mixing for a number of other topics including the low gravities of the blue HB stars, the origin of the extreme HB stars, and the evolutionary status of the sdO stars. A variety of observational tests are presented to test this helium-mixing scenario.
A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752  [PDF]
A. P. Milone,A. F. Marino,G. Piotto,L. R. Bedin,J. Anderson,A. Aparicio,A. Bellini,S. Cassisi,F. D'Antona,F. Grundahl,M. Monelli,D. Yong
Physics , 2013, DOI: 10.1088/0004-637X/767/2/120
Abstract: Multi-band Hubble Space Telescope photometry reveals that the main sequence, sub-giant, and the red giant branch of the globular cluster NGC6752 splits into three main components in close analogy with the three distinct segments along its horizontal branch stars. These triple sequences are consistent with three stellar groups: a stellar population with a chemical composition similar to field halo stars (population a), a population (c) with enhanced sodium and nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y ~0.03), and a population (b) with an intermediate (between population a and c) chemical composition and slightly helium enhanced (Delta Y ~0.01). These components contain ~25% (population a), ~45% (population b), and ~30% (population c) of the stars. No radial gradient for the relative numbers of the three populations has been identified out to about 2.5 half mass radii.
On the missing second generation AGB stars in NGC6752  [PDF]
Santi Cassisi,Maurizio Salaris,Adriano Pietrinferni,Jorick S. Vink,Matteo Monelli
Physics , 2014, DOI: 10.1051/0004-6361/201424540
Abstract: [Abridged] In recent years the view of Galactic globular clusters as simple stellar populations has changed dramatically, as it is now thought that basically all GCs 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 GC NGC6752 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 possibly induced by the metal enhancement associated to radiative levitation. This enhanced mass loss would prevent second generation stars from reaching the AGB phase, thus explaining at the same time the low value of the ratio between HB and AGB stars (the R_2 parameter) observed in NGC6752. We have critically discussed this scenario, finding that the required mass-loss rates are of the order of 10^{-9} Mo/yr, significantly higher than current theoretical and empirical constraints. By making use of synthetic HB simulations, we demonstrate that our modelling predicts correctly the R_2 parameter for NGC6752, without the need to invoke very efficient mass loss during the core He-burning stage. Our simulations for NGC6752 HB predict however the presence of a significant fraction - at the level of about 50% - second generation stars along the cluster AGB. 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.
A Catalog of Helium Abundance Indicators from Globular Cluster Photometry  [PDF]
Eric L. Sandquist
Physics , 1999, DOI: 10.1046/j.1365-8711.2000.03262.x
Abstract: For each of the three indicators used, we have conducted a thorough error analysis, and identified systematic errors in the computational procedures. For the population ratio R = N_HB / N_RGB, we find that there is no evidence of a trend with metallicity, although there appears to be real scatter in the values derived. Although this indicator is the one best able to provide useful absolute helium abundances, the mean value is Y approximately 0.20, indicating the probable presence of additional systematic error. For the magnitude difference from the horizontal branch to the main sequence Delta and the RR Lyrae mass-luminosity exponent A, it is only possible to reliably determine relative helium abundances. This is due to continuing uncertainties in the absolute metallicity scale for Delta, and uncertainty in the RR Lyrae temperature scale for A. Both indicators imply that the helium abundance is approximately constant as a function of [Fe/H]. According to the A indicator, both Oosterhoff I and II group clusters have constant values independent of [Fe/H] and horizontal branch type. In addition, the two groups have slopes d log P / d[Fe/H] that are consistent with each other, but significantly smaller than the slope for the combined sample.
A lack of close binaries among hot horizontal branch stars in globular clusters - M80 and NGC5986  [PDF]
C. Moni Bidin,S. Moehler,G. Piotto,Y. Momany,A. Recio-Blanco
Physics , 2009, DOI: 10.1051/0004-6361/200810579
Abstract: Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. The present paper extends our study of NGC6752 to M80 and NGC5986. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC5986 and 31 in M80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations, temperatures, gravities, and helium abundances. By means of a statistical analysis, we detected one EHB close binary candidate per cluster. In M80, the best estimate of the close binary EHB fraction is f=12%, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC5986, although our best estimate is rather low (f=25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC6752, our analysis of M80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature. Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations.
Helium and Multiple Populations in the Massive Globular Cluster NGC6266 (M62)  [PDF]
A. P. Milone
Physics , 2014, DOI: 10.1093/mnras/stu2198
Abstract: Recent studies suggest that the helium content of multiple stellar populations in globular clusters (GCs) is not uniform. The range of helium varies from cluster to cluster with more massive GCs having, preferentially, large helium spread. GCs with large helium variations also show extended-blue horizontal branch (HB). I exploit Hubble Space Telescope photometry to investigate multiple stellar populations in NGC6266 and infer their relative helium abundance. This cluster is an ideal target to investigate the possible connection between helium, cluster mass, and HB morphology, as it exhibits an extended HB and is among the ten more luminous GCs in the Milky Way. The analysis of color-magnitude diagrams from multi-wavelength photometry reveals that also NGC6266, similarly to other massive GCs, hosts a double main sequence (MS), with the red and the blue component made up of the 79+-1% and the 21+-1% of stars, respectively. The red MS is consistent with a stellar population with primordial helium while the blue MS is highly helium-enhanced by Delta Y=0.08+-0.01. Furthermore, the red MS exhibits an intrinsic broadening that can not be attributed to photometric errors only and is consistent with a spread in helium of ~0.025 dex. The comparison between NGC6266 and other GCs hosting helium-enriched stellar populations supports the presence of a correlation among helium variations, cluster mass, and HB extension.
The Second-Parameter Effect in Metal-Rich Globular Clusters  [PDF]
A. V. Sweigart,M. Catelan
Physics , 1998, DOI: 10.1086/311443
Abstract: Recent Hubble Space Telescope observations have found that the horizontal branches (HB's) in the metal-rich globular clusters NGC 6388 and NGC 6441 slope upward with decreasing B-V. Such a slope is not predicted by canonical HB models and cannot be produced by either a greater cluster age or enhanced mass loss along the red-giant branch (RGB). The peculiar HB morphology in these clusters may provide an important clue for understanding the second-parameter effect. We have carried out extensive evolutionary calculations and numerical simulations in order to explore three non-canonical scenarios for explaining the sloped HB's in NGC 6388 and NGC 6441: i) A high cluster helium abundance scenario, where the HB evolution is characterized by long blue loops; ii) A rotation scenario, where internal rotation during the RGB phase increases the HB core mass; iii) A helium-mixing scenario, where deep mixing on the RGB enhances the envelope helium abundance. All three of these scenarios predict sloped HB's with anomalously bright RR Lyrae variables. We compare this prediction with the properties of the two known RR Lyrae variables in NGC 6388. Other possible observational tests are suggested.
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