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Giant Branch Stars (giant + branch_star)
Kinds of Giant Branch Stars Selected AbstractsA search for circumstellar material around B-type stars in the Galactic haloMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2002H. R. M. Magee Abstract 19 B-type stars, selected from the Palomar,Green Survey, have been observed at infrared wavelengths to search for possible infrared excesses, as part of an ongoing programme to investigate the nature of early-type stars at high Galactic latitudes. The resulting infrared fluxes, along with Strömgren photometry, are compared with theoretical flux profiles to determine whether any of the targets show evidence of circumstellar material, which may be indicative of post-main-sequence evolution. Eighteen of the targets have flux distributions in good agreement with theoretical predictions. However, one star, PG 2120+062, shows a small near-infrared excess, which may be due either to a cool companion of spectral type F5,F7, or to circumstellar material, indicating that it may be an evolved object such as a post-asymptotic giant branch star, in the transition region between the asymptotic giant branch and planetary nebula phase, with the infrared excess due to recent mass loss during giant branch evolution. [source] Stardust in Antarctic micrometeoritesMETEORITICS & PLANETARY SCIENCE, Issue 8 2008Toru YADA The oxygen isotopic compositions of the eighteen presolar silicate (and one oxide) grains found are similar those observed previously in primitive meteorites and interplanetary dust particles, and indicate origins in oxygen-rich red giant or asymptotic giant branch stars, or in supernovae. Four grains with anomalous C isotopic compositions were also detected. 12C/13C as well as Si ratios are similar to those of mainstream SiC grains; the N isotopic composition of one grain is also consistent with a mainstream SiC classification. Presolar silicate grains were found in three of the seven AMMs studied, and are heterogeneously distributed within these micrometeorites. Fourteen of the 18 presolar silicate grains and 3 of the 4 C-anomalous grains were found within one AMM, T98G8. Presolar silicate-bearing micrometeorites contain crystalline silicates that give sharp X-ray diffractions and do not contain magnesiowüstite, which forms mainly through the decomposition of phyllosilicates and carbonates. The occurrence of this mineral in AMMs without presolar silicates suggests that secondary parent body processes probably determine the presence or absence of presolar silicates in Antarctic micrometeorites. [source] The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009M. Rejkuba ABSTRACT We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope, covering three fields in the north-east side of the edge-on disc galaxy NGC 891. The observed fields span a wide range of galactocentric distances along the eastern minor axis, extending from the plane of the disc to 12 kpc, and out to ,25 kpc along the major axis. The photometry of individual stars reaches ,2.5 mag below the tip of the red giant branch. We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disc and spheroid of NGC 891. The colour,magnitude diagrams of thick disc and spheroid population are dominated by old red giant branch stars with a wide range of metallicities, from the sparsely populated metal-poor tail at [Fe/H],,2.4 dex, up to about half-solar metallicity. The peak of the metallicity distribution function of the thick disc is at ,0.9 dex. The inner parts of the thick disc, within ,14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of ,(V,I)0/,|Z| = 0.1 ± 0.05 kpc,1 or less than 0.1 dex kpc,1 is detected, with bluer colours or more metal-poor stars at larger distances from the plane. This gradient is, however, accounted for by the mixing with the metal-poor halo stars. No metallicity gradient along the major axis is present for thick-disc stars, but strong variations of about 0.35 dex around the mean of [Fe/H]=,1.13 dex are found. The properties of the asymmetric metallicity distribution functions of the thick-disc stars show no significant changes in both the radial and the vertical directions. The stellar populations situated within the solar-cylinder-like distances show strikingly different properties from those of the Galaxy populating similar distances. This suggests that the accretion histories of both galaxies have been different. The spheroid population, composed of the inner spheroid and the halo, shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about ,1.15 dex in the inner parts to ,1.27 dex at 24 kpc distance from the centre, corresponding to ,13reff. Similar to the thick-disc stars, large variations around the mean relation are present. [source] A spectroscopic survey of EC4, an extended cluster in Andromeda's halo,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009M. L. M. Collins ABSTRACT We present a spectroscopic survey of candidate red giant branch stars in the extended star cluster, EC4, discovered in the halo of M31 from our Canada,France,Hawaii Telescope/MegaCam survey, overlapping the tidal streams, Streams ,Cp' and ,Cr'. These observations used the DEep Imaging Multi-Object Spectrograph mounted on the Keck II telescope to obtain spectra around the Caii triplet region with ,1.3 Å resolution. Six stars lying on the red giant branch within two core radii of the centre of EC4 are found to have an average vr=,287.9+1.9,2.4 km s,1 and ,v,corr = 2.7+4.2,2.7 km s,1, taking instrumental errors into account. The resulting mass-to-light ratio for EC4 is M/L = 6.7+15,6.7 M,/L,, a value that is consistent with a globular cluster within the 1, errors we derive. From the summed spectra of our member stars, we find EC4 to be metal-poor, with [Fe/H]=,1.6 ± 0.15. We discuss several formation and evolution scenarios which could account for our kinematic and metallicity constraints on EC4, and conclude that EC4 is most comparable with an extended globular cluster. We also compare the kinematics and metallicity of EC4 with Streams ,Cp' and ,Cr', and find that EC4 bears a striking resemblance to Stream ,Cp' in terms of velocity, and that the two structures are identical in terms of both their spectroscopic and photometric metallicities. From this, we conclude that EC4 is likely related to Stream ,Cp'. [source] Dust mass-loss rates from asymptotic giant branch stars in the Fornax and Sagittarius dwarf spheroidal galaxiesMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Eric Lagadec ABSTRACT To study the effect of metallicity on the mass-loss rate of asymptotic giant branch (AGB) stars, we have conducted mid-infrared photometric measurements of such stars in the Sagittarius and Fornax dwarf spheroidal galaxies with the 10-,m camera VISIR at the Very Large Telescope. We derive mass-loss rates for 29 AGB stars in Sgr dSph and two in Fornax. The dust mass-loss rates are estimated from the K,[9] and K,[11] colours. Radiative transfer models are used to check the consistency of the method. Published IRAS and Spitzer data confirm that the same tight correlation between K,[12] colour and dust mass-loss rates is observed for AGB stars from galaxies with different metallicities, i.e., the Galaxy, the Large Magellanic Clouds and the Small Magellanic Clouds. The derived dust mass-loss rates are in the range 5 × 10,10 to 3 × 10,8 M, yr,1 for the observed AGB stars in Sgr dSph and around 5 × 10,9 M, yr,1 for those in Fornax; while values obtained with the two different methods are of the same order of magnitude. The mass-loss rates for these stars are higher than the nuclear burning rates, so they will terminate their AGB phase by the depletion of their stellar mantles before their core can grow significantly. Some observed stars have lower mass-loss rates than the minimum value predicted by theoretical models. [source] Carbon-rich extremely metal poor stars: signatures of Population III asymptotic giant branch stars in binary systemsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007Herbert H. B. Lau ABSTRACT We use the Cambridge stellar evolution code stars to model the evolution and nucleosynthesis of zero-metallicity intermediate-mass stars. We investigate the effect of duplicity on the nucleosynthesis output of these systems and the potential abundances of the secondaries. The surfaces of zero-metallicity stars are enriched in CNO elements after second dredge-up. During binary interaction, such as Roche lobe overflow or wind accretion, metals can be released from these stars and the secondaries enriched in CNO isotopes. We investigate the formation of the two most metal poor stars known, HE 0107,5240 and HE 1327,2326. The observed carbon and nitrogen abundances of HE 0107,5240 can be reproduced by accretion of material from the companion-enhanced wind of a 7-M, star after second dredge-up, though oxygen and sodium are underproduced. We speculate that HE 1327,2326, which is richer in nitrogen and strontium, may similarly be formed by wind accretion in a later asymptotic giant branch phase after third dredge-up. [source] Mass loss and yield uncertainty in low-mass asymptotic giant branch starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007Richard J. Stancliffe ABSTRACT We investigate the uncertainty in surface abundances and yields of asymptotic giant branch (AGB) stars. We apply three different mass-loss laws to a 1.5-M, star of metallicity Z= 0.008 at the beginning of the thermally pulsing-asymptotic giant branch (TP-AGB) phase. Efficient third dredge-up is found even at very low envelope mass, contrary to previous simulations with other evolution codes. We find that the yield of carbon is uncertain by about 15 per cent and for most other light elements the yield is uncertain at the level of 20,80 per cent. For iron group elements, the uncertainty varies from around 30 per cent for the more-abundant species to over a factor of 2 for the less-abundant radioactive species, like 60Fe. The post-AGB surface abundances for this mass and metallicity are much more uncertain due to the dilution of dredged-up material in differing envelope masses in the later stages of the models. Our results are compared to known planetary nebula and post-AGB abundances. We find that the models are mostly consistent with observations but we are unable to reproduce observations of some of the isotopes. [source] Variability in red supergiant stars: pulsations, long secondary periods and convection noiseMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006L. L. Kiss ABSTRACT We study the brightness variations of galactic red supergiant stars using long-term visual light curves collected by the American Association of Variable Star Observers over the last century. The full sample contains 48 red semiregular or irregular variable stars, with a mean time-span of observations of 61 yr. We determine periods and period variability from analyses of power density spectra and time,frequency distributions. We find two significant periods in 18 stars. Most of these periods fall into two distinct groups, ranging from a few hundred to a few thousand days. Theoretical models imply fundamental, first and possibly second overtone mode pulsations for the shorter periods. Periods greater than 1000 d form a parallel period,luminosity relation that is similar to the long secondary periods of the asymptotic giant branch stars. A number of individual power spectra shows a single mode resolved into multiple peaks under a Lorentzian envelope, which we interpret as evidence for stochastic oscillations, presumably caused by the interplay of convection and pulsations. We find a strong 1/f noise component in the power spectra that is remarkably similar in almost all stars of the sample. This behaviour fits the picture of irregular photometric variability caused by large convection cells, analogous to the granulation background seen in the Sun. [source] Integrated-light VRI imaging photometry of globular clusters in the Magellanic CloudsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Paul Goudfrooij ABSTRACT We present accurate integrated-light photometry in Johnson/Cousins V, R and I for a sample of 28 globular clusters in the Magellanic Clouds. The majority of the clusters in our sample have reliable age and metallicity estimates available in the literature. The sample encompasses ages between 50 Myr and 7 Gyr, and metallicities ([Fe/H]) between ,1.5 and 0.0 dex. The sample is dominated by clusters of ages between roughly 0.5 and 2 Gyr, an age range during which the bolometric luminosity of simple stellar populations is dominated by evolved red giant branch stars and thermally pulsing asymptotic giant branch (TP-AGB) stars whose theoretical colours are rather uncertain. The VRI colours presented in this paper have been used to calibrate stellar population synthesis model predictions. [source] | ||||||||||