137 research outputs found
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 -elements to iron,
[\alpha/Fe] \simeq 0.2-0.3N_{Fe}/N_H = 4.68
10^{-5}3.24 10^{-5}\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 and , 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 of
star-forming galaxies at redshifts and beyond. Accurate estimates
of 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
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