The first determination of the actinide Th abundance for a red giant of the Ursa Minor dwarf galaxy

The Thorium abundance for the red giant COS82 in the Ursa Minor dwarf spheroidal galaxy is determined based on a high resolution spectrum. This is the first detection of actinides in an extra Galactic object. A detailed abundance pattern is determined for 12 other neutron-capture elements from the atomic number 39 to 68. These elements are significantly over-abundant with respect to other metals like Fe (> 1 dex) and their abundance pattern agrees well with those of the r-process-enhanced, very metal-poor stars known in the Galactic halo, while the metallicity of this object ([Fe/H] ~ -1.5) is much higher than these field stars ([Fe/H] ~ -3.0). The results indicate that the mechanism and the astrophysical site that are responsible for neutron-capture elements in COS82 is similar to that for field r-process-enhanced stars, while the condition of low mass star formation is quite different. An estimate of the age of this object based on the Th abundance ratio is discussed.Comment: 5 pages, 2 figures, 1 table, to appear in PAS

Roles of SNIa and SNII in ICM Enrichment

Based on ASCA observations Mushotzky et al. (1996, ApJ 466, 686) have recently derived the relative-abundance ratios of $\alpha$-elements to iron, [\alpha/Fe] \simeq 0.2-0.3$, for four rich clusters, and have suggested that the origin of metals in an intra-cluster medium (ICM) is not a type-Ia supernovae (SNIa), but a type-II supernovae (SNII). However, these authors used the solar photospheric iron abundance for ASCA data reduction, while the meteoritic iron abundance is usually adopted in chemical-evolution studies. It is true that although the photospheric and meteoritic solar abundances are consistent for most of the elements, a serious discrepancy is known to exist for iron; indeed, the photospheric abundance of iron is$N_{Fe}/N_H = 4.68 10^{-5}$by number, while the meteoritic value is$3.24 10^{-5}$. The argument concerning the relative roles of SNIa and SNII in ICM enrichment is quite sensitive to the precise values of [\alpha/Fe], and one should use an identical solar iron abundance in data reduction as well as in theoretical arguments. We therefore adopt the meteoritic iron abundance, which is consistent with chemical-evolution studies, and shift Mushotzky et al.'s ASCA data by$\Delta[\alpha/Fe] \simeq -0.16$ dex. By comparing the corrected [\alpha/Fe] values with theoretical nucleosynthesis prescriptions of SNIa and SNII, we reach a conclusion that an SNIa iron contribution of 50% or higher in the ICM enrichment could not be ruled out, and might indeed be favoured based on the ASCA spectra.Comment: 8 pages, 2 figures, requires PASJ LaTeX macros. To appear in PAS

Secondary Episode of Star Formation in Elliptical Galaxies

We put upper limits of secondary burst of star formation in elliptical galaxies of the Gonzalez's (1993) sample, most of which locates in small groups, based on the colour dispersion around the U-V versus central velocity dispersion relation, and the equivalent width of Hbeta absorption. There are significant number of Hbeta strong galaxies which have EW(Hbeta)>2A, however they do not always have bluer colours in U-V. To be consistent with small colour dispersion of U-V, the mass fraction of secondary burst to the total mass should be less than only 10% at the maximum within recent 2Gyr. This result suggests that even if recent galaxy merging produce some ellipticals, it should not have been accompanied by an intensive star burst, and hence it could not involve large gas-rich systems. A capture of dwarf galaxy is more likely to explain dynamical disturbances observed in some elliptical galaxies. The above analysis based on the U-V is not compatible with the one based on the line indices, which requires that more than 10% of mass is present in a 2Gyr old star burst to cover the full range of the observed Hbeta (de Jong & Davies 1997). The discrepancy might be partly explained by the internal extinction localized at the region where young stars form. However, considering that the Hbeta index might have great uncertainties both in models and in observational data, we basically rely on U-V analysis.Comment: 8 pages, 8 figures, Accepted for publication in MNRA

The most Distant [OIII]-emitting Quasar PKS 1937-101 at redshift 3.8

We report the discovery of a high-z quasar with unambiguous [OIII]5007 emission; PKS 1937-101 at redshift 3.8. This quasar, however, shows little evidence for rest-frame ultraviolet and optical FeII emission. It is thus shown that PKS 1937-101 does not belong to a class of super iron-rich high-z quasars reported by Elston, Thompson, & Hill (1994). The epoch of major star formation in the host galaxy is discussed briefly.Comment: 10 Latex (AASTeX) pages, 4 PostScript figures. To appear in Publications of the Astronomical Society of Japa

Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Bo\"otes I

Ultra-faint dwarf galaxies recently discovered around the Milky Way (MW) contain extremely metal-poor stars, and might represent the building blocks of low-metallicity components of the MW. Among them, the Bo\"otes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population. We determine chemical compositions of six red giant stars in Bo\"otes I, based on the high-resolution spectra obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12 elements, including C, Na, alpha, Fe-peak, and neutron capture elements, were determined for the sample stars. The abundance results were compared to those in field MW halo stars previously obtained using an abundance analysis technique similar to the present study. We confirm the low metallicity of Boo-094 ([Fe/H]=-3.4). Except for this star, the abundance ratios ([X/Fe]) of elements lighter than zinc are generally homogeneous with small scatter around the mean values in the metallicities spanned by the other five stars (-2.7-2.7 show no significant enhancement of carbon. The [Mg/Fe] and [Ca/Fe] ratios are almost constant with a modest decreasing trend with increasing [Fe/H] and are slightly lower than the field halo stars. The [Sr/Fe] and [Sr/Ba] ratios also tend to be lower in the Bo\"otes I stars than in the halo stars. Our results of small scatter in the [X/Fe] ratios for elements lighter than zinc suggest that these abundances were homogeneous among the ejecta of prior generation(s) of stars in this galaxy.Comment: 16 pages, 12 figures. Accepted to A&A, language correcte

On the Models of Early Main Sequence Evolution of Moderately Massive Stars with growing Inhomogeneous Convection Zone

The models similar to those proposed by Sakashita and Hayashi to represent the main sequence evolution of massive stars have been studied for the evolution of moderately massive star. Each of the models consists of the inhomogeneous convection zone developing between the outer radiative envelope of initial chemical composition and the homogeneous convection core of the diminishing hydrogen content. In actual computation, such an inhomogeneous convection zone is approximated by the treatment of Sakashita and Hayashi\u27s semiconvection zone. The models are examined for two cases of simplified opacity law. Along the evolutionary sequence of such models, the intermediate inhomogeneous convection zone extends steadily inward and chemically homogeneous central core shrinks without changing the outermost boundary of the convection core. Though it has not been made sure exactly yet, the central homogeneous core disappears when the central hydrogen content becomes a critical value which is still appreciably high and depends on the adopted opacity law and initial chemical composition. In general, it may be said that the central hydrogen content at such a critical stage should be smaller for the star of larger mass. Until arriving at the critical stage defined here, the present scheme of evolution does not bring about any appreciable change in stellar luminosity, radius and the internal run of physical quantities such as mass, pressure and density except the temperature which increases in a way of just compensating the increase of the mean molecular weight in the equation of state for pressure. Some speculations have been briefly made of the model characters of the subsequent evolution

Age and Dust Degeneracy for Starburst Galaxies Solved?

A spectral evolution model of galaxies that includes both stellar and dust effects is newly built. xApplying the model to 22 nearby starburst galaxies, we have shown that far infrared luminosity of galaxies helps to break the age-dustiness degeneracy. We have derived a unique solution of age and the dustiness for each starburst galaxy. The resulting starburst ages and optical depths are in the range $10 \le t (Myr) \le 500$ and $0.5 \le \tau_{V} \le 5.0$, respectively. The result is robust and is almost independent of model assumptions such as dust distributions, extinction curves, and burst strengths. With the rapidly growing sensitivity of submillimeter detectors, it should become possible in the near future to determine the age and $\tau_{V}$ of star-forming galaxies at redshifts $z \simeq 3$ and beyond. Accurate estimates of $\tau_{V}$ for Lyman-break galaxies and high-z galaxies might require a substantial revision of the previously claimed picture of star formation history over the Hubble time.Comment: Latex (aas2pp4) 15 pages, 1 table, 6 figures. Accepted for Ap

On the Origin of the Colour-Magnitude Relation in the Virgo Cluster

We explore the origin of the colour-magnitude relation (CMR) of early type galaxies in the Virgo cluster using spectra of very high S/N ratio for six elliptical galaxies selected along the CMR. The data are analysed using a new evolutionary stellar population synthesis model to generate galaxy spectra at the resolution given by their velocity dispersions. In particular we use a new age indicator that is virtually free of the effects of metallicity. We find that the luminosity weighted mean ages of Virgo ellipticals are greater than ~8 Gyr, and show no clear trend with galaxy luminosity. We also find a positive correlation of metallicity with luminosity, colour and velocity dispersion. We conclude that the CMR is driven primarily by a luminosity-metallicity correlation. However, not all elements increase equally with the total metallicity and we speculate that the CMR may be driven by both a total metallicity increase and by a systematic departure from solar abundance ratios of some elements along the CMR. A full understanding of the role played by the total metallicity, abundance ratios and age in generating the CMR requires the analysis of spectra of very high quality, such as those reported here, for a larger number of galaxies in Virgo and other clusters.Comment: To appear in The Astrophysical Journal Letters, 2001 April 20 (551, number 2). 5 pages and 4 figure