4,659 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&
Space program: Space debris a potential threat to Space Station and shuttle
Experts estimate that more than 3.5 million man-made objects are orbiting the earth. These objects - space debris - include whole and fragmentary parts of rocket bodies and other discarded equipment from space missions. About 24,500 of these objects are 1 centimeter across or larger. A 1-centimeter man-made object travels in orbit at roughly 22,000 miles per hour. If it hit a spacecraft, it would do about the same damage as would a 400-pound safe traveling at 60 miles per hour. The Government Accounting Office (GAO) reviews NASA's plans for protecting the space station from debris, the extent and precision of current NASA and Defense Department (DOD) debris-tracking capabilities, and the extent to which debris has already affected shuttle operations. GAO recommends that the space debris model be updated, and that the findings be incorporated into the plans for protecting the space station from such debris. GAO further recommends that the increased risk from debris to the space shuttle operations be analyzed
The central spheroids of Milky Way mass-sized galaxies
Indexación: Scopus.PBT, DM and AM acknowledge partial support from the Nucleo UNAB 2015 DI-677-15/N of Universidad Andres Bello. PBT acknowledges partial support from Fondecyt Regular 1150334 and the Southern Astrophysics Network (SAN) collaboration funded by Conicyt, and PICT 2011-0959 and PIP 2012-0396 (Mincyt, Argentina). DM and MZ are supported by the BASAL Center for Astrophysics and Associated Technologies (CATA) through grant PFB-06, and the Ministry for the Economy, Development, and Tourism, Programa Iniciativa Cientifica Milenio through grant IC120009, awarded to the Millennium Institute of Astrophysics (MAS), and by FONDECYT Regular grant No. 1130196. DC and TCB acknowledge partial support for this work from grant PHY 14-30152; Physics Frontier Center/JINA Center for the Evolution of the Elements (JINA-CEE), awarded by the US National Science Foundation. REGM acknowledges support from Ci?ncia sem Fronteiras (CNPq, Brazil).We study the properties of the central spheroids located within 10 kpc of the centre of mass of MilkyWay mass-sized galaxies simulated in a cosmological context. The simulated central regions are dominated by stars older than 10 Gyr, mostly formed in situ, with a contribution of ~30 per cent from accreted stars. These stars formed in well-defined starbursts, although accreted stars exhibit sharper and earlier ones. The fraction of accreted stars increases with galactocentric distance, so that at a radius of~8-10 kpc, a fraction of~40 per cent, on average, is detected. Accreted stars are slightly younger, lower metallicity, and more α-enhanced than in situ stars. A significant fraction of old stars in the central regions come from a few (2-3) massive satellites (~1010M⊙). The bulge components receive larger contributions of accreted stars formed in dwarfs smaller than ~109.5M⊙. The difference between the distributions of ages and metallicities of old stars is thus linked to the accretion histories - those central regions with a larger fraction of accreted stars are those with contributions from more massive satellites. The kinematical properties of in situ and accreted stars are consistent with the latter being supported by their velocity dispersions, while the former exhibit clear signatures of rotational support. Our simulations demonstrate a range of characteristics, with some systems exhibiting a co-existing bar and spheroid in their central regions, resembling in some respect the central region of the Milky Way. © 2016 The Authors.https://academic.oup.com/mnras/article/473/2/1656/422260
The role of binaries in the enrichment of the early Galactic halo. I. r-process-enhanced metal-poor stars
The detailed chemical composition of most metal-poor halo stars has been
found to be highly uniform, but a minority of stars exhibit dramatic
enhancements in their abundances of heavy neutron-capture elements and/or of
carbon. The key question for Galactic chemical evolution models is whether
these peculiarities reflect the composition of the natal clouds, or if they are
due to later mass transfer of processed material from a binary companion. If
the former case applies, the observed excess of certain elements was implanted
within selected clouds in the early ISM from a production site at interstellar
distances. Our aim is to determine the frequency and orbital properties of
binaries among these chemically peculiar stars. This information provides the
basis for deciding whether mass transfer from a binary companion is necessary
and sufficient to explain their unusual compositions. This paper discusses our
study of a sample of 17 moderately (r-I) and highly (r-II) r-process-element
enhanced VMP and EMP stars. High-resolution, low signal-to-noise spectra of the
stars were obtained at roughly monthly intervals over 8 years with the FIES
spectrograph at the Nordic Optical Telescope. From these spectra, radial
velocities with an accuracy of ~100 m/s were determined by cross-correlation
against an optimized template. 14 of the programme stars exhibit no significant
RV variation over this period, while 3 are binaries with orbits of typical
eccentricity for their periods, resulting in a normal binary frequency of
~18+-6% for the sample. Our results confirm our preliminary conclusion from
2011, based on partial data, that the chemical peculiarity of the r-I and r-II
stars is not caused by any putative binary companions. Instead, it was
imprinted on the natal molecular clouds of these stars by an external, distant
source. Models of the ISM in early galaxies should account for such mechanisms.Comment: 14 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
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