392 research outputs found
New Results From Bright Metal-Poor Stars In The Hamburg/Eso Survey
We present an abundance analysis of BE 1327-2326, currently the most iron-poor star known, based on a newly acquired VLT spectrum. The ID abundance pattern is corrected for 3D effects. The 3D iron abundance is [Fe/H] = -5.9, while the CNO elements of the star are extremely overabundant [CNO[Fe] similar to 3 to 4). The cosmologically important element Li is still not detected; the new upper limit is A (Li) < 0.6. A new analysis of the medium-resolution data of the sample of bright metal-poor stars from the Hamburg/ESO Survey (HES) was carried out. We are using this sample to obtain clues to the chemical nature of the early Universe by investigating the kinematic properties of the sample. Based on estimated [Fe/H] and [C/Fe], we are also able to use the sample to test a formation mechanism for low-mass metal-poor stars.Astronom
The stellar content of the Hamburg/ESO survey. IV. Selection of candidate metal-poor stars
We present the quantitative methods used for selecting candidate metal-poor
stars in the Hamburg/ESO objective-prism survey (HES). The selection is based
on the strength of the Ca II K line, B-V colors (both measured directly from
the digital HES spectra), as well as J-K colors from the 2 Micron All Sky
Survey. The KP index for Ca II K can be measured from the HES spectra with an
accuracy of 1.0 Angstrom, and a calibration of the HES B-V colors, using CCD
photometry, yields a 1-sigma uncertainty of 0.07 mag for stars in the color
range 0.3 < B-V < 1.4. These accuracies make it possible to reliably reject
stars with [Fe/H] > -2.0 without sacrificing completeness at the lowest
metallicities. A test of the selection using 1121 stars of the HK survey of
Beers, Preston, and Shectman present on HES plates suggests that the
completeness at [Fe/H] < -3.5 is close to 100% and that, at the same time, the
contamination of the candidate sample with false positives is low: 50% of all
stars with [Fe/H] > -2.5 and 97% of all stars with [Fe/H] > -2.0 are rejected.
The selection was applied to 379 HES fields, covering a nominal area of 8853
square degrees of the southern high Galactic latitude sky. The candidate sample
consists of 20,271 stars in the magnitude range 10 < B < 18. A comparison of
the magnitude distribution with that of the HK survey shows that the magnitude
limit of the HES sample is about 2 mag fainter. Taking the overlap of the sky
areas covered by both surveys into account, it follows that the survey volume
for metal-poor stars has been increased by the HES by about a factor of 10 with
respect to the HK survey. We have already identified several very rare objects
with the HES, including, e.g., the three most heavy-element deficient stars
currently known.Comment: 11 pages, 10 figures, accepted for publication in A&
Linking dwarf galaxies to halo building blocks with the most metal-poor star in Sculptor
Current cosmological models indicate that the Milky Way's stellar halo was
assembled from many smaller systems. Based on the apparent absence of the most
metal-poor stars in present-day dwarf galaxies, recent studies claimed that the
true Galactic building blocks must have been vastly different from the
surviving dwarfs. The discovery of an extremely iron-poor star (S1020549) in
the Sculptor dwarf galaxy based on a medium-resolution spectrum cast some doubt
on this conclusion. However, verification of the iron-deficiency and
measurements of additional elements, such as the alpha-element Mg, are
mandatory for demonstrating that the same type of stars produced the metals
found in dwarf galaxies and the Galactic halo. Only then can dwarf galaxy stars
be conclusively linked to early stellar halo assembly. Here we report
high-resolution spectroscopic abundances for 11 elements in S1020549,
confirming the iron abundance of less than 1/4000th that of the Sun, and
showing that the overall abundance pattern mirrors that seen in low-metallicity
halo stars, including the alpha-elements. Such chemical similarity indicates
that the systems destroyed to form the halo billions of years ago were not
fundamentally different from the progenitors of present-day dwarfs, and
suggests that the early chemical enrichment of all galaxies may be nearly
identical.Comment: 16 pages, including 2 figures. Accepted for publication in Nature. It
is embargoed for discussion in the press until formal publication in Natur
Enhanced tidal stripping of satellites in the galactic halo from dark matter self-interactions
We investigate the effects of self-interacting dark matter (SIDM) on the
tidal stripping and evaporation of satellite galaxies in a Milky Way-like host.
We use a suite of five zoom-in, dark-matter-only simulations, two with
velocity-independent SIDM cross sections, two with velocity-dependent SIDM
cross sections, and one cold dark matter simulation for comparison. After
carefully assigning stellar mass to satellites at infall, we find that stars
are stripped at a higher rate in SIDM than in CDM. In contrast, the total bound
dark matter mass loss rate is minimally affected, with subhalo evaporation
having negligible effects on satellites for viable SIDM models. Centrally
located stars in SIDM haloes disperse out to larger radii as cores grow.
Consequently, the half-light radius of satellites increases, stars become more
vulnerable to tidal stripping, and the stellar mass function is suppressed. We
find that the ratio of core radius to tidal radius accurately predicts the
relative strength of enhanced SIDM stellar stripping. Velocity-independent SIDM
models show a modest increase in the stellar stripping effect with satellite
mass, whereas velocity-dependent SIDM models show a large increase in this
effect towards lower masses, making observations of ultra-faint dwarfs prime
targets for distinguishing between and constraining SIDM models. Due to small
cores in the largest satellites of velocity-dependent SIDM, no identifiable
imprint is left on the all-sky properties of the stellar halo. While our
results focus on SIDM, the main physical mechanism of enhanced tidal stripping
of stars apply similarly to satellites with cores formed via other means.Comment: 19 pages, 18 figures, Accepted by MNRA
HE 1327-2326, An Unevolved Star With Fe/H <-5.0. III. Does Its Atmosphere Reflect Its Natal Composition?
Based on spectroscopic constraints derived from nonlocal thermodynamic equilibrium line formation, we explore the likely range of stellar parameters (T-eff and log g) for the hyper-metal-poor (HMP) star HE 1327-2326. Combining the constraints from Balmer line profiles and the Ca I/II ionization equilibrium, a subgiant stage of evolution is indicated. This result is further supported by spectrophotometric observations of the Balmer jump. If a higher T-eff value was used (as favored by some photometric calibrations), the spectroscopic analysis would indicate a turnoff-point stage of evolution. Using a stellar-structure code that treats the effects of atomic diffusion throughout the star in detail, we evolve a low-mass model star to reach the Hertzsprung-Russell-diagram position of HE 1327-2326 after roughly 13 Gyr. While the surface abundances are modified significantly (by more than 1 dex for the case of uninhibited diffusion), such corrections cannot resolve the discrepancy between the abundance inferred from the nondetection of the Li I resonance line at 6707 angstrom and the Wilkinson Microwave Anisotropy Probe based primordial lithium abundance. As there are numerous processes that can destroy lithium, any cosmological interpretation of a lower-than-expected lithium abundance at the lowest metallicities will have to await sample sizes of unevolved HMP stars that are 1 order of magnitude larger. The situation remains equally inconclusive concerning atomic-diffusion corrections. Here, attempts have to be made to better constrain internal mixing processes, both observationally and by means of sophisticated modeling. With constraints on additional mixing processes taken from a recent globular-cluster study, the likeliest scenario is that HE 1327-2326's surface abundances have undergone mild depletion (of order 0.2 dex).Swedish Research Council (Vetenskapradet)Presidium RAS Programme "Origin and evolution of stars and the Galaxy"Deutsche Forschungsgemeinschaft GE 490/34-1W. J. McDonald Fellowship of the McDonald ObservatoryJSPS 18104003McDonald Observator
Exploring the Universe with Metal-Poor Stars
The early chemical evolution of the Galaxy and the Universe is vital to our
understanding of a host of astrophysical phenomena. Since the most metal-poor
Galactic stars (with metallicities down to [Fe/H]\sim-5.5) are relics from the
high-redshift Universe, they probe the chemical and dynamical conditions of the
Milky Way and the origin and evolution of the elements through nucleosynthesis.
They also provide constraints on the nature of the first stars, their
associated supernovae and initial mass function, and early star and galaxy
formation. The Milky Way's dwarf satellites contain a large fraction (~30%) of
the known most metal-poor stars that have chemical abundances that closely
resemble those of equivalent halo stars. This suggests that chemical evolution
may be universal, at least at early times, and that it is driven by massive,
energetic SNe. Some of these surviving, ultra-faint systems may show the
signature of just one such PopIII star; they may even be surviving first
galaxies. Early analogs of the surviving dwarfs may thus have played an
important role in the assembly of the old Galactic halo whose formation can now
be studied with stellar chemistry. Following the cosmic evolution of small
halos in simulations of structure formation enables tracing the cosmological
origin of the most metal-poor stars in the halo and dwarf galaxies. Together
with future observations and additional modeling, many of these issues,
including the reionization history of the Milky Way, may be constrained this
way. The chapter concludes with an outlook about upcoming observational
challenges and ways forward is to use metal-poor stars to constrain theoretical
studies.Comment: 34 pages, 11 figures. Book chapter to appear in "The First Galaxies -
Theoretical Predictions and Observational Clues", 2012 by Springer, eds. V.
Bromm, B. Mobasher, T. Wiklin
The Nucleosynthetic Imprint of 15-40 Solar Mass Primordial Supernovae on Metal-Poor Stars
The inclusion of rotationally-induced mixing in stellar evolution can alter
the structure and composition of presupernova stars. We survey the effects of
progenitor rotation on nucleosynthetic yields in Population III and II
supernovae using the new adaptive mesh refinement (AMR) code CASTRO. We examine
spherical explosions in 15, 25 and 40 solar mass stars at Z = 0 and 10^-4 solar
metallicity with three explosion energies and two rotation rates. Rotation in
the Z = 0 models resulted in primary nitrogen production and a stronger
hydrogen burning shell which led all models to die as red supergiants. On the
other hand, the Z=10^-4 solar metallicity models that included rotation ended
their lives as compact blue stars. Because of their extended structure, the
hydrodynamics favors more mixing and less fallback in the metal free stars than
the Z = 10^-4 models. As expected, higher energy explosions produce more
enrichment and less fallback than do lower energy explosions, and less massive
stars produce more enrichment and leave behind smaller remnants than do more
massive stars. We compare our nucleosynthetic yields to the chemical abundances
in the three most iron-poor stars yet found and reproduce the abundance pattern
of one, HE 0557-4840, with a zero metallicity 15 solar mass, 2.4 x 10^51 erg
supernova. A Salpeter IMF averaged integration of our yields for Z=0 models
with explosion energies of 2.4x10^51 ergs or less is in good agreement with the
abundances observed in larger samples of extremely metal-poor stars, provided
15 solar mass stars are included. Since the abundance patterns of extremely
metal-poor stars likely arise from a representative sample of progenitors, our
yields suggest that low-mass supernovae contributed the bulk of the metals to
the early universe.Comment: 16 pages, 11 figures; submitted to Ap
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