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Distributions of Neutron Exposures in AGB Stars and the Galaxy 总被引:1,自引:0,他引:1
Wen-Yuan Cui Feng-Hua Zhang Wei-Juan Zhang Lu Zhang Bo Zhang National Astronomical Observatories Chinese Academy of Sciences Beijing Department of Physics Hebei Normal University Shijiazhuang Department of Modern Physics University of Science Technology of China Hefei Graduate School of Chinese Academy of Sciences Bejing 《中国天文和天体物理学报》2007,7(2):224-234
Based on the s-process nucleosynthesis model with the 13C(α,n)16O reaction oc- curring under radiative conditions in the interpulse phases, we investigate the characteristics of the distribution of neutron exposure in low-mass Asymptotic Giant Branch (AGB) stars. We introduce a new concept, the distribution of neutron exposures of the Galaxy (NEG), to study the chemical evolution characteristics of the Galaxy for s-process elements. Using a chemical evolution model of the Galaxy, we develop a model for the NEG and obtain the evolution results of the NEG in different epochs. The present results appear to reasonably re- produce the distribution of neutron exposures of the solar system (hereafter NES). The main component and the strong component in the NES are built up in different epochs. The strong component of the s-process is mainly synthesised in the low-mass and metal-poor AGB stars, and the main component is produced by the s-process in the low-mass AGB stars with higher metallicities. 相似文献
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S. Casassus P.F. Roche D.K. Aitken C.H. Smith 《Monthly notices of the Royal Astronomical Society》2001,327(3):744-750
A sample of 25 infrared-bright planetary nebulae (PNe) towards the Galactic bulge is analysed through 8–13 μm spectroscopy. The classification of the warm dust emission features provides a measure of the C/O chemical balance, and represents the first C/O estimates for bulge PNe. Out of 13 PNe with identified dust types, four PNe have emission features associated with C-based grains, while the remaining 9 have O-rich dust signatures. The low fraction of C-rich PNe, ≲ 30 per cent, contrasts with that for local PNe, around ∼ 80 per cent, although it follows the trend for a decreasing frequency of C-rich PNe with galactocentric radius (Paper I). We investigate whether the PNe discussed here are linked to the bulge stellar population (similar to type IV, or halo, PNe) or the inner Galactic disc (a young and super-metal-rich population). Although 60 per cent of the PNe with warm dust are convincing bulge members, none of the C-rich PNe satisfies our criteria, and they are probably linked to the inner Galactic disc. In the framework of single star evolution, the available information on bulge PNe points towards a progenitor population similar in age to that of local PNe (type I PNe are found in similar proportions), but super-metal-rich (to account for the scarcity of C-rich objects). Yet the metallicities of bulge PNe, as inferred from [O/H], fail to reach the required values – except for the C-rich objects. It is likely that the sample discussed here is derived from a mixed disc/bulge progenitor population and dominated by type IV PNe, as suggested by Peimbert. The much higher fraction of O-rich PNe in this sample than in the solar neighbourhood should result in a proportionally greater injection of silicate grains into the inner Galactic medium. 相似文献
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I. S. Glass S. Matsumoto B. S. Carter K. Sekiguchi 《Monthly notices of the Royal Astronomical Society》2001,321(1):77-95
We report here the results of a 4-yr K -band (2.2 μm) survey for large-amplitude variable stars in a area centred on the Galactic Centre. A total of 409 likely long-period variables (LPVs) were detected, for which positions, amplitudes, average magnitudes and periods were obtained whenever possible. The surface density of LPVs is more than ten times greater than in the Sgr I Baade window at
The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction which affects the probability of detecting variables, occurs in less than 25 per cent of the area.
Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K -band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters. 相似文献
The limits of completeness arising from interstellar and circumstellar absorption are discussed. Most of the area suffers interstellar extinction of The shorter-period LPVs are less luminous than the longer-period ones and may be slightly under-represented in the data. Extremely heavy extinction which affects the probability of detecting variables, occurs in less than 25 per cent of the area.
Almost all of the LPVs are Miras or OH/IR stars, with periods ranging from 150 d to about 800 d. K -band counterparts have been found for 59 per cent of the 109 known OH sources in the field. The average period of the variables found is 427 d, while that of the OH/IR stars is 524 d. For comparison, the average period in the Sgr I window, which contains no known OH/IR stars, is 333 d and only two stars are detected with The survey field also contains a number of long-period, large-amplitude variables that are not OH emitters. 相似文献
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Kenneth J.Mitchell Rex A.Saffer Steve B.Howell Thomas M.Brown 《Monthly notices of the Royal Astronomical Society》1998,295(1):225-239
A new sample of 31 faint B and A0 stars is reported, 30 of which comprise a complete sample within the limits ( U − V )<0 and 10.0< B ≲18.0. The sample is based on low- and intermediate-resolution spectrophotometry of colour-excess objects selected in the US survey. Atmospheric parameters for the stars are derived through the use of synthetic colours, Balmer-line strengths, and model-atmosphere fitting. The atmospheric parameters and preliminary metallicity estimates indicate that most of the stars are distributed along the blue horizontal branch, with low metallicities ([Fe/H]∼−1.0) and with both the first and second Newell gaps present. However, nine of the B/A0 stars can be identified as candidate main-sequence stars, based on evidence of high metallicities ([Fe/H]∼0) and/or derived effective temperatures and surface gravities which place them close to the main-sequence relation. The completeness characteristics of the sample are discussed, and its surface density is compared to that of other recently isolated B-star samples. The sample exhibits a shallow integral number-count slope. This new sample will help provide increased statistical coverage of the B-star population in the Galactic halo through its relatively faint magnitude-completeness limits and its relatively red colour-completeness limit. 相似文献
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Noriyuki Matsunaga † Takahiro Kawadu Shogo Nishiyama † Takahiro Nagayama Hirofumi Hatano † Motohide Tamura Ian S. Glass Tetsuya Nagata 《Monthly notices of the Royal Astronomical Society》2009,399(4):1709-1729
We report the results of a near-infrared survey for long-period variables in a field of view of 20× 30 arcmin2 towards the Galactic Centre (GC). We have detected 1364 variables, of which 348 are identified with those reported in Glass et al. We present a catalogue and photometric measurements for the detected variables and discuss their nature. We also establish a method for the simultaneous estimation of distances and extinctions using the period–luminosity relations for the JHK s bands. Our method is applicable to Miras with periods in the range 100–350 d and mean magnitudes available in two or more filter bands. While J band means are often unavailable for our objects because of the large extinction, we estimated distances and extinctions for 143 Miras whose H - and K s -band mean magnitudes are obtained. We find that most are located at the same distance to within our accuracy. Assuming that the barycentre of these Miras corresponds to the GC, we estimate its distance modulus to be 14.58 ± 0.02 (stat.) ± 0.11 (syst.) mag , corresponding to 8.24 ± 0.08 (stat.) ± 0.42 (syst.) kpc . We have assumed the distance modulus to the Large Magellanic Cloud to be 18.45 mag, and the uncertainty in this quantity is included in the above systematic error. We also discuss the large and highly variable extinction. Its value ranges from 1.5 mag to larger than 4 mag in except towards the thicker dark nebulae and it varies in a complicated way with the line of sight. We have identified mid-infrared counterparts in the Spitzer /IRAC catalogue of Ramírez et al. for most of our variables and find that they follow rather narrow period–luminosity relations in the 3.6–8.0 μm wavelength range. 相似文献
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The origin of the lead-rich stars in the Galactic halo: investigation of model parameters for the s-process 总被引:1,自引:0,他引:1
Several stars at the low-metallicity extreme of the Galactic halo show large spreads of lead and associated 'heavy' s-process elements ([Pb/hs]). Theoretically, an s-process pattern should be obtained from an AGB star with a fixed metallicity and initial mass. For the third dredge-up and the s-process model, several important properties depend primarily on the core mass of AGB stars. Zijlstra reported that the initial-to-final mass relation steepens at low metallicity, due to low mass-loss efficiency. This might affect the model parameters of the AGB stars, e.g. the overlap factor and the neutron irradiation time, in particular at low metallicity. The calculated results do indeed show that the overlap factor and the neutron irradiation time are significantly small at low metallicities, especially for 3.0 M⊙ AGB stars. The scatter of [Pb/hs] found in low metallicities can therefore be explained naturally when varying the initial mass of the low-mass AGB stars. 相似文献
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Abundance Gradient from Open Clusters and Implications for the Galactic Disk Evolution 总被引:3,自引:0,他引:3
Jin-Liang Hou * Rui-Xiang Chang Li Chen Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai National Astronomical Observatories Chinese Academy of Sciences Beijing Galactic structure dynamics Received Jun 《中国天文和天体物理学报》2002,2(1):17-32
1 INTRODUCTIONRadial abundance gradient along the Galactic disk constitutes one of the most importantobservational constraillts fOr models of the evolution of the Galactic disk. The existence of sucha gradient is now well established, through radio and optical observations of HII regions, diskstars, pIanetary nebulae (see Henry and Worthey 1999 for a detailed review) and open clusters(Friel 1995, 1999). An average gradiellt of dlog(X/H)/dR ~ --0.06 dex kpc--' is observed inthe Milky … 相似文献
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C. Travaglio S. Randich D. Galli C. Abia J. Lattanzio 《Astrophysics and Space Science》2002,281(1-2):219-220
Different stellar sources may have contributed to the 7Li enrichmentof the Galaxy: type-II supernovae, novae, and AGB stars. In the latter case, the interplay between the Hot Bottom
Burning (HBB) process (via the Cameron-Fowler mechanism) and a very high mass-loss rate before the evolution off the AGB (the
so-called ‘superwind’ phase), can lead to a significant production of 7Li from low- and intermediate-mass AGB stars (Travaglio et al., 2001). We have now undertaken an observational campaign aimed
at constraining our stellar and Galactic models, with a twofold goal: (i) to assemble a compilation of high-resolution spectra of Galactic, unevolved (i.e. dwarfs), warm(spectral type F) stars,
in a selected metallicity range (-1.0 ≤>[Fe/H] ≤ -0.3), using the ESO 1.5m telescope and the FEROS spectrograph; (ii) to carry out a Li survey among a sample of selected AGB stars, to investigate the possible correlation between7Li abundance (when detected) and mass-loss rate.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
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High signal-to-noise ratio spectra were obtained of 10 high-proper-motion stars having −1 ≲[Fe/H] < 0 , and a comparable number of disc stars. All but two of the high-proper-motion stars were confirmed to have [Fe/H] > −1.0 , some approaching solar metallicity, but, even so, earlier measurements overestimated the metallicities and velocities of some of these stars. Models of stellar populations were used to assign membership probabilities to the Galactic components to which the high-velocity stars might belong. Many were found to be more probably thick-disc than halo objects, despite their large space motions, and two might be associated with the inner Galaxy. It may be necessary to reassess contamination of previous halo samples, such as those used to define the metallicity distribution, to account for contamination by high-velocity thick-disc stars, and to consider possible subcomponents of the halo.
The change in [α/Fe] ratios at [Fe/H]≃−1.0 is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. [Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-[Ti/Fe] halo stars exist; likewise high- and low-[Ti/Fe] thick-disc stars exist. We conclude that the [Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the [α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of [Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al. 相似文献
The change in [α/Fe] ratios at [Fe/H]≃−1.0 is often used to constrain the degree and timing of Type Ia supernova nucleosynthesis in Galactic chemical-evolution models. [Ti/Fe] values were measured for eight of the high-velocity stars. Both high- and low-[Ti/Fe] halo stars exist; likewise high- and low-[Ti/Fe] thick-disc stars exist. We conclude that the [Ti/Fe]'break' is not well defined for a given population; nor is there a simple, continuous evolutionary sequence through the break. Implications for the interpretation of the [α/Fe] break in terms of SN Ia time-scales and progenitors are discussed. The range of [Ti/Fe] found for high -velocity (low rotation) thick-disc stars contrasts with that for the low -velocity (high rotation) thick-disc sample studied by Prochaska et al. 相似文献