Miho N. Ishigaki

CHEMICAL ABUNDANCES OF THE MILKY WAY THICK DISK AND STELLAR HALO. II. SODIUM, IRON-PEAK, AND NEUTRON-CAPTURE ELEMENTS

We present chemical abundance analyses of sodium, iron-peak, and neutron-capture elements for 97 kinematically selected thick disk, inner halo, and outer halo stars with metallicities ?3.3 ?1.5 support the hypothesis that these components formed through different mechanisms. In particular, our results favor the scenario that the inner and outer halo components formed through an assembly of multiple progenitor systems that experienced various degrees of chemical enrichments, while the thick disk formed through rapid star formation with an efficient mixing of chemical elements. The lower [Na/Fe] and [Zn/Fe] observed in stars with the outer halo kinematics may further suggest that progenitors with longer star formation timescales contributed to the buildup of the relatively metal-rich part of stellar halos.

CHEMICAL ABUNDANCES OF THE MILKY WAY THICK DISK AND STELLAR HALO. I. IMPLICATIONS OF [α/Fe] FOR STAR FORMATION HISTORIES IN THEIR PROGENITORS

We present the abundance analysis of 97 nearby metal-poor (–3.3 –2. These results favor the scenarios that the MW thick disk formed through rapid chemical enrichment primarily through Type II supernovae of massive stars, while the stellar halo has formed at least in part via accretion of progenitor stellar systems having been chemically enriched with different timescales.

Chemical compositions of six metal-poor stars in the ultra-faint dwarf spheroidal galaxy Boötes I

Context. 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 Bootes I dwarf spheroidal galaxy is of particular interest because of its exclusively old stellar population. Detailed chemical compositions of individual stars in this galaxy are a key to understanding formation and chemical evolution in the oldest galaxies in the Universe and their roles in building up the MW halo. Aims. Previous studies of the chemical abundances of Bootes I show discrepancies in elemental abundances between different authors, and thus a consistent picture of its chemical enrichment history has not yet been established. In the present work, we independently determine chemical compositions of six red giant stars in Bootes I, some of which overlap with those analyzed in the previous studies. Based on the derived abundances, we re-examine trends and scatters in elemental abundances and make comparisons with MW field halo stars and other dwarf spheroidal galaxies in the MW. Methods. High-resolution spectra of a sample of stars were obtained with the High Dispersion Spectrograph mounted on the Subaru Telescope. Abundances of 12 elements, including C, Na, α, 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. Results. 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 Bootes I stars than in the halo stars. Conclusions. 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. The lower mean [Mg/Fe] and [Ca/Fe] ratios relative to the field halo stars and the similarity in these abundance ratios with some of the more luminous dwarf spheroidal galaxies at metallicities [Fe/H] < −2 can be interpreted as star formation in Bootes I having lasted at least until Type Ia supernovae started to contribute to the chemical enrichment in this galaxy.

FAINT POPULATION III SUPERNOVAE AS THE ORIGIN OF THE MOST IRON-POOR STARS

The most iron-poor stars in the Milky Way provide important observational clues to the astrophysical objects that enriched the primordial gas with heavy elements. Among them, the recently discovered iron-deficient star SMSS J031300.36–670839.3 shows a remarkable chemical composition with a non-detection of iron ([Fe/H] <–7.1) and large enhancement of carbon and magnesium relative to calcium. We investigate supernova yields of metal-free (Population III) stars to interpret the abundance pattern observed in this star. We report that the high [C/Ca] and [C/Mg] ratios and upper limits of other elemental abundances are well reproduced with the yields of core-collapse supernovae (which have normal kinetic energies of explosion E of E 51 = E/1051 erg =1) and hypernovae (E 51 ≥ 10) of Population III 25 M ☉ or 40 M ☉ stars. The best-fit models assume that the explosions undergo extensive matter mixing and fallback, leaving behind a black hole remnant. In these models, Ca is produced by static/explosive O burning and incomplete Si burning in the Population III supernova/hypernova, in contrast to the suggestion that Ca is originated from the hot-CNO cycle during pre-supernova evolution. Chemical abundances of four carbon-rich iron-poor stars with [Fe/H] <–4.5, including SMSS J031300.36–670839.3, are consistently explained by faint supernova models with ejected masses of 56Ni less than 10–3 M ☉.

Chemical abundances of the Milky Way thick disk and stellar halo

We report the chemical abundances of 97 metal-poor stars likely belonging to the Milky Way thick disk and inner/outer stellar halo. All of the spectra of the sample stars have been obtained with the High Dispersion Spectrograph (HDS) mounted on the Subaru Telescope and have analyzed in a homogeneous manner. We show that the thick disk stars show higher [Na/Fe], [Mg/Fe] and [Si/Fe] ratios with small scatter, while the inner and outer halo stars show lower average abundance ratios of these elements with larger scatter in [Fe/H]>−1.5 [1]. The abundance ratios of many of the neutron-capture elements are more enhanced in the halo stars than the thick disk stars. These results suggest that the thick disk and halo stars were formed under the influences of different degrees of Type II/Type Ia supernovae (SNe) enrichments. The larger scatter in the abundances for the halo stars may further indicate that a sizable fraction of the halo stars have formed within different star forming clumps, that may have various che...

TYPE IIb SUPERNOVA 2013df ENTERING INTO AN INTERACTION PHASE: A LINK BETWEEN THE PROGENITOR AND THE MASS LOSS

We report the late-time evolution of Type IIb Supernova (SN IIb) 2013df. SN 2013df showed a dramatic change in its spectral features at ~1 year after the explosion. Early on it showed typical characteristics shared by SNe IIb/Ib/Ic dominated by metal emission lines, while later on it was dominated by broad and flat-topped Halpha and He I emissions. The late-time spectra are strikingly similar to SN IIb 1993J, which is the only previous example clearly showing the same transition. This late-time evolution is fully explained by a change in the energy input from the

Dark matter annihilation and decay from non-spherical dark halos in galactic dwarf satellites

^{56}

A NEW MILKY WAY SATELLITE DISCOVERED IN THE SUBARU/HYPER SUPRIME-CAM SURVEY

Co decay to the interaction between the SN ejecta and dense circumstellar matter (CSM). The mass loss rate is derived to be ~(5.4 +- 3.2) x 10^{-5} Msun/yr (for the wind velocity of ~20 km/s), similar to SN 1993J but larger than SN IIb 2011dh by an order of magnitude. The striking similarity between SNe 2013df and 1993J in the (candidate) progenitors and the CSM environments, and the contrast in these natures to SN 2011dh, infer that there is a link between the natures of the progenitor and the mass loss: SNe IIb with a more extended progenitor have experienced a much stronger mass loss in the final centuries toward the explosion. It might indicate that SNe IIb from a more extended progenitor are the explosions during a strong binary interaction phase, while those from a less extended progenitor have a delay between the strong binary interaction and the explosion.

THE ORIGIN OF LOW [α/Fe] RATIOS IN EXTREMELY METAL-POOR STARS

The dwarf spheroidal galaxies (dSphs) in the Milky Way are the primary targets in the indirect searches for particle dark matter. To set robust constraints on candidate of dark matter particles, understanding the dark halo structure of these systems is of substantial importance. In this paper, we rst evaluate the astrophysical factors for dark matter annihilation and decay for 24 dSphs, with taking into account a non-spherical dark halo, using generalized axisymmetric mass models based on axisymmetric Jeans equations. First, from a tting analysis of the most recent kinematic data available, our axisymmetric mass models are a much better

Chemical feature of Eu abundance in the Draco dwarf spheroidal galaxy

We report the discovery of a new ultra-faint dwarf satellite companion of the Milky Way based on the early survey data from the Hyper Suprime-Cam Subaru Strategic Program. This new satellite, Virgo I, which is located in the constellation of Virgo, has been identified as a statistically significant (5.5 sigma) spatial overdensity of star-like objects with a well-defined main sequence and red giant branch in their color-magnitude diagram. The significance of this overdensity increases to 10.8 sigma when the relevant isochrone filter is adopted for the search. Based on the distribution of the stars around the likely main sequence turn-off at r ~ 24 mag, the distance to Virgo I is estimated as 87 kpc, and its most likely absolute magnitude calculated from a Monte Carlo analysis is M_V = -0.8 +/- 0.9 mag. This stellar system has an extended spatial distribution with a half-light radius of 38 +12/-11 pc, which clearly distinguishes it from a globular cluster with comparable luminosity. Thus, Virgo I is one of the faintest dwarf satellites known and is located beyond the reach of the Sloan Digital Sky Survey. This demonstrates the power of this survey program to identify very faint dwarf satellites. This discovery of VirgoI is based only on about 100 square degrees of data, thus a large number of faint dwarf satellites are likely to exist in the outer halo of the Milky Way.