41 research outputs found
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
The Initial mass function of the first stars inferred from extremely metal-poor stars
This is an author-created, un-copyedited version of an article published in The Astrophysical Journal. The Version of Record is available online at https://doi.org/10.3847/1538-4357/aab3de.We compare the elemental abundance patterns of ~200 extremely metal-poor (EMP; [Fe/H] < β3) stars to the supernova yields of metal-free stars, in order to obtain insights into the characteristic masses of the first (Population III or Pop III) stars in the universe. The supernova yields are prepared with nucleosynthesis calculations of metal-free stars with various initial masses (M = 13, 15, 25, 40 and 100 M β) and explosion energies (E 51 = E/1051[erg] = 0.5β60), to include low-energy, normal-energy, and high-energy explosions. We adopt the mixing-fallback model, to take into account possible asymmetry in the supernova explosions, and the yields that best fit the observed abundance patterns of the EMP stars are searched by varying the model parameters. We find that the abundance patterns of the EMP stars are predominantly best-fitted by the supernova yields with initial masses M < 40 M β, and that more than than half of the stars are best-fitted by the M = 25 M β hypernova (E 51 = 10) models. The results also indicate that the majority of the primordial supernovae have ejected 10β2β10β1 M β of 56Ni, leaving behind a compact remnant (either a neutron star or a black hole), with a mass in the range of ~1.5β5 M β. These results suggest that the masses of the first stars responsible for the first metal enrichment are predominantly <40 M β. This implies that the higher-mass first stars were either less abundant, directly collapsed into a black hole without ejecting heavy elements, or a supernova explosion of a higher-mass first star inhibits the formation of the next generation of low-mass stars at [Fe/H] < β3.Peer reviewedFinal Accepted Versio
The mass of our Milky Way
We perform an extensive review of the numerous studies and methods used to
determine the total mass of the Milky Way. We group the various methods into
seven broad classes, including: i) estimating Galactic escape velocity using
high velocity objects; ii) measuring the rotation curve through terminal and
circular velocities; iii) modeling halo stars, globular clusters and satellite
galaxies with the Spherical Jeans equation and iv) with phase-space
distribution functions; v) simulating and modeling the dynamics of stellar
streams and their progenitors; vi) modeling the motion of the Milky Way, M31
and other distant satellites under the framework of Local Group timing
argument; and vii) measurements made by linking the brightest Galactic
satellites to their counterparts in simulations. For each class of methods, we
introduce their theoretical and observational background, the method itself,
the sample of available tracer objects, model assumptions, uncertainties,
limits and the corresponding measurements that have been achieved in the past.
Both the measured total masses within the radial range probed by tracer objects
and the extrapolated virial masses are discussed and quoted. We also discuss
the role of modern numerical simulations in terms of helping to validate model
assumptions, understanding systematic uncertainties and calibrating the
measurements. While measurements in the last two decades show a factor of two
scatters, recent measurements using \textit{Gaia} DR2 data are approaching a
higher precision. We end with a detailed discussion of future developments,
especially as the size and quality of the observational data will increase
tremendously with current and future surveys. In such cases, the systematic
uncertainties will be dominant and thus will necessitate a much more rigorous
testing and characterization of the various mass determination methods.Comment: invited review published by Science China Physics, Mechanics &
Astronom