Wako Aoki

A Subaru/High Dispersion Spectrograph Study of Lead (Pb) Abundances in Eight s-Process Element-rich, Metal-poor Stars

We report the abundances of neutron-capture elements in eight carbon-rich, metal-poor (� 2:7 �½ Fe=H ��� 1:9) stars observed with the Subaru Telescope High Dispersion Spectrograph. The derived abundance patterns indicate that the neutron-capture elements in these objects primarily originated from s-process nucleosynthesis, although the [Ba/Eu] abundance ratios in some objects are lower than that of the solar system s-process component. The present analysis has yielded the Pb abundances for seven objects as well as an upper limit for one object from use of the Pb i � 4057 and � 3683 lines. The values of [Pb/ Ba] in these objects cover a wide range, between � 0.3 and 1.2. Theoretical studies of s-process nucleosynthesis at low metallicity are required to explain this large dispersion of the [Pb/Ba] values. Variations in radial velocity have been found for two of the eight objects, suggesting that, at least in these instances, the observed excess of s-process elements is due to the transfer of material across a binary system including an AGB star. Comparisons with predictions of AGB nucleosynthesis models are discussed. Subject headings: nuclear reactions, nucleosynthesis, abundances — stars: abundances — stars: AGB and post-AGB — stars: carbon — stars: Population II On-line material: machine-readable table

Neutron Capture Elements in s -Process-rich, Very Metal-poor Stars

We report abundance estimates for neutron capture elements, including lead (Pb), and nucleosynthesis models for their origin, in two carbon-rich, very metal-poor stars, LP 625-44 and LP 706-7. These stars are subgiants whose surface abundances are likely to have been strongly aUected by mass transfer from companion asymptotic giant branch (AGB) stars that have since evolved to white dwarfs. The detections of Pb, which forms the —nal abundance peak of the s-process, enable a comparison of the abundance patterns from Sr (Z 38) to Pb (Z 82) with predictions of AGB models. The derived chemical compositions provide strong constraints on the AGB stellar models, as well as on s-process nucleosynthesis at low metallicity. The present paper reports details of the abundance analysis for 16 neutron capture elements in LP 625-44, including the eUects of hyper—ne splitting and isotope shifts of spectral lines for some elements. A Pb abundance is also derived for LP 706-7 by a reanalysis of a previously observed spectrum. We investigate the characteristics of the nucleosynthesis pathway that produces the abundance ratios of these objects using a parametric model of the s-process without adopting any speci—c stellar model. The neutron exposure q is estimated to be about 0.7 mbarn~1, signi—cantly larger than that which best —ts solar system material, but consistent with the values predicted by models of moderately metal-poor AGB stars. This value is strictly limited by the Pb abundance, in addition to those of Sr and Ba. We also —nd that the observed abundance pattern can be explained by a few recurrent neutron exposures and that the overlap of the material that is processed in two subsequent exposures is small (the overlap factor r D 0.1).

The Chemical Composition of Carbon-rich, Very Metal Poor Stars: A New Class of Mildly Carbon Rich Objects without Excess of Neutron-Capture Elements

We report on an analysis of the chemical composition of —ve carbon-rich, very metal poor stars based on high-resolution spectra. One star, CS 22948-027, exhibits very large overabundances of carbon, nitrogen, and the neutron-capture elements, as found in the previous study of Hill et al. This result can be interpreted as a consequence of mass transfer from a binary companion that previously evolved through the asymptotic giant branch stage. By way of contrast, the other four stars we investigate exhibit no overabundances of barium ([Ba/Fe] 0), while three of them have mildly enhanced carbon and/or nitrogen ([C ] N] D ]1). We have been unable to determine accurate carbon and nitrogen abundances for the remaining star (CS 30312-100). These stars are rather similar to the carbon-rich, neutron-capture

Warm Dust in the Cool Brown Dwarf Gliese 229B and Spectroscopic Diagnosis of Dusty Photospheres

Dust should be formed in the photosphere of cool brown dwarfs, but it is not clear in what form the dust exists. We propose a model in which a rather warm dust layer exists deep in the photosphere. At relatively high temperature there, dust grains are in detailed balance with the gas and the homogeneous dust-gas mixture works as an efficient source of opacity. The warm dust layer, which can be optically thick in the optical region, effectively blocks the optical radiation and emits like a blackbody of rather high temperature. The dust layer, however, can be optically thin in the infrared and has little effect on the infrared radiation. In the cool upper region, dust grows too large to be sustained with the gas and segregates from the gaseous atmosphere. Dust no longer works as an opacity source, and volatile molecules such as CH4 and H2O take the role. Also, nonrefractory elements such as alkali metals remain in gaseous form, and the neutral atoms contribute significantly to block the optical radiation (e.g., K I doublet). We show that such a hybrid model can be constructed without any ad hoc assumption except that the transition from the dust-gas detailed balance regime to the dust-gas segregation phase takes place at a certain transition temperature Ttr. By a sudden increase of opacity due to dust at Ttr, an outer convective zone appears, followed by intermediate radiative and inner deep convective zones. The emergent spectrum is also a hybrid of the dust- and gas-dominated cases and offers a natural explanation of the observed spectrum of Gliese 229B through the optical to the infrared.

Measurement of the Europium Isotope Ratio for the Extremely Metal poor, r-Process-enhanced Star CS 31082-001

We report the first measurement of the isotope fraction of europium (151Eu and 153Eu) for the extremely metal poor, r-process-enhanced star CS 31082-001, based on high-resolution spectra obtained with the Subaru Telescope High Dispersion Spectrograph. We have also obtained new measurements of this ratio for two similar stars with previous europium isotope measurements, CS 22892-052 and HD 115444. The measurements were made using observations of the Eu lines in these spectra that are most significantly affected by isotope shifts and hyperfine splitting. The fractions of 151Eu derived for CS 31082-001, CS 22892-052, and HD 115444 are 0.44, 0.51, and 0.46, respectively, with uncertainties of about ±0.1. CS 31082-001, the first star with a meaningful measurement of U outside of the solar system, is known to exhibit peculiar abundance ratios between the actinide and rare earth elements (e.g., Th/Eu), ratios that are significantly different from those for other stars with large excesses of r-process elements, such as our two comparison objects. Nevertheless, our analysis indicates that the Eu isotope ratio of CS 31082-001 agrees, within the errors, with those of other r-process-enhanced objects and with that of solar system material.

High-dispersion spectrograph (HDS) for Subaru Telescope

This paper describes a high dispersion spectrograph (HDS) now under construction towards the first-light phase of Subaru Telescope. It is located at a Nasmyth focus. HDS is an echelle spectrograph with grating as a post-disperser. The collimated beam size is 272 mm, and the echelle is 300 mm by 840 mm in total size (31.6 gr/mm, R-2.8). The overall throughput (resolution X slit width product) achieved is 38,000 arcseconds. HDS has two cross-dispersers (400 gr/mm, 250 gr/mm) which are optimized for blue- and red-wavelength regions, respectively. The camera is of catadioptric type system, consisting of three correctors and a mirror. It spans the entire chromatic range from 0.30 to 2.0 micrometer. It delivers 10-micrometer images on average within a flat 60 mm- diameter focal plane area in the wavelength range from 0.3 to 1.1 micrometer, without refocusing. This image quality corresponds to typical limiting spectral resolutions well above 300,000 though the resolution will generally be limited to less than this by the entrance slit width and finite CCD pixel sizes. The detector will be a 2 X 1 mosaic of 2k X 4k CCDs with 15-micrometer pixels. Typical limiting spectral resolution will be 100,000.

Spectra and Colours of Brown Dwarfs: Predictions by Model Atmospheres

One useful approach to help in the identification of yet undiscovered brown dwarfs may be to use model atmospheres to predict major observable properties. As a simplest possible approach, we applied the methodology of model stellar atmospheres to substellar objects, and we constructed a unified grid of model atmospheres for stellar and substellar objects covering the Teff range between 1000 and 4000K (log g = 5. 0 and v micro = 1. 0 km s-1 throughout) with metallicities (Z′s) of the disk and halo (Tsuji and Ohnaka 1994).

7Li/6Li Ratio in Distant Interstellar Media and Primordial 7Li—Implementation in Big‐Bang Cosmology and Galactic Chemical Evolution—

We have detected the isotopic abundance ratio of 7Li/6Li in the interstellar media (ISMs) along the line of sight to HD169454 and HD250290 using High Dispersion Spectrograph equipped with the Subaru Telescope. We also observed ζ Oph for the comparison purpose with the previous data. The first background star HD169454 is at the distance 1.7 kpc inward from the solar sysytem in the Galactic plane, and the absorbing clouds are estimated to locate at 0.125u2009kpc⩽d⩽0.7u2009kpc and/or 1u2009kpc⩽d⩽1.7u2009kpc. If the observed 7Li/6Li ratio is for the gas cloud at the larger distance 1u2009kpc⩽d⩽1.7u2009kpc, this could be the first observation of the ISM 7Li/6Li ratio beyond the solar neighborhood. As for the second background star HD250290, the distance from the solar system is uncertain, 2.2±0.2u2009kpc or 0.439−0.147+0.446u2009kpc. The observed abundance ratios turn out to be 7Li/6Liu2009=u20098.1−1.8+3.6, and 6.3−1.7+3.0 for HD169454 and HD250290, respectively. These values are in reasonable agreement with those observed previously in the ISMs of solar neighborhood within ±1σ error bars and also consistent with our measurement of 7Li/6Liu2009=u20097.1−1.6+2.9 for nearby cloud along the line of sight to ζ Oph. This suggests a piece of evidence for homogeneous mixing and instantaneous recycling of gas component in the Galactic disk. We cannot claim a slope of the 7Li/6Li ratio within ±1σ error bars as a function of Galactocentric distance RG. Decomposing the observed ISM 7Li/6Li ratio into several possible source of 7Li, we discuss three different contributions to the 7Li from the Galactic cosmic‐ray interactions, the stellar nucleosynthesis, and the Big‐Bang nucleosynthesis.We have detected the isotopic abundance ratio of 7Li/6Li in the interstellar media (ISMs) along the line of sight to HD169454 and HD250290 using High Dispersion Spectrograph equipped with the Subaru Telescope. We also observed ζ Oph for the comparison purpose with the previous data. The first background star HD169454 is at the distance 1.7 kpc inward from the solar sysytem in the Galactic plane, and the absorbing clouds are estimated to locate at 0.125u2009kpc⩽d⩽0.7u2009kpc and/or 1u2009kpc⩽d⩽1.7u2009kpc. If the observed 7Li/6Li ratio is for the gas cloud at the larger distance 1u2009kpc⩽d⩽1.7u2009kpc, this could be the first observation of the ISM 7Li/6Li ratio beyond the solar neighborhood. As for the second background star HD250290, the distance from the solar system is uncertain, 2.2±0.2u2009kpc or 0.439−0.147+0.446u2009kpc. The observed abundance ratios turn out to be 7Li/6Liu2009=u20098.1−1.8+3.6, and 6.3−1.7+3.0 for HD169454 and HD250290, respectively. These values are in reasonable agreement with those observed previously in the ISMs of...

Nitrogen Abundance in Oxygen-Rich Giants through K to late M

Theanalysesofhighresolutioninfraredspectrahave beendoneforCNlinesinoxygen-richcoolevolvedstarsinclud- ing 2 K giants, 20 M giants and 1 S-type star. Since CN lines analyzed in the present work are weak and resolved well, they are appropriate for quantitative analyses. CN lines of v = 2 and 1 sequences (red system) which are in the K- and the H-window regions, respectively, give the consistent nitrogen abundance for each star. The analyses of NH lines in the L- window region have been done for 5 late M giants for which CN lines have been also analyzed. Although the triplet structure ofNHlinescannotbefullyresolved,theyarepreferablebecause determination of nitrogen abundance is almost independent of other elemental abundances while nitrogen abundance based on CN depends on carbon abundance. The nitrogen abundances derived from NH for late M giants agree well with those from CN for which we adopt 7.75eV as the dissociation energy in the analysis. The results show that the nitrogen abundances in late M giants are larger than those in early M giants while decrease of thecarbonabundancewasfoundinlateMgiantsbyourprevious work (Tsuji 1991). These variations of abundances can not be explained by the rst dredge-up model but require additional processing by the CN cycle and mixing after the rst dredge- up. However, there is no obvious evidence of other processes such as the 3-process and subsequent hot bottom burning in our program stars. Such variation of the carbon and nitrogen abundances is not well understood by the present evolutionary models of low-mass and intermediate-mass stars.

Analysis of Infrared Spectra of Carbon Stars Observed by the ISO SWS

Infrared spectra of carbon stars are of great importance in the understanding the stellar evolution at the asymptotic giant branch (AGB). The analysis of elemental abundances of carbon, nitrogen, and oxygen, which is indispensable to understand the nuclear processes in AGB stars, can be done based on high-resolution infrared spectra. In addition, infrared spectra of carbon stars bring us information on the physics and chemistry in the circumstellar envelope.