3,297 research outputs found
Stellar haloes in Milky-Way mass galaxies: From the inner to the outer haloes
We present a comprehensive study of the chemical properties of the stellar
haloes of Milky-Way mass galaxies, analysing the transition between the inner
to the outer haloes. We find the transition radius between the relative
dominance of the inner-halo and outer-halo stellar populations to be ~15-20 kpc
for most of our haloes, similar to that inferred for the Milky Way from recent
observations. While the number density of stars in the simulated inner-halo
populations decreases rapidly with distance, the outer-halo populations
contribute about 20-40 per cent in the fiducial solar neighborhood, in
particular at the lowest metallicities. We have determined [Fe/H] profiles for
our simulated haloes; they exhibit flat or mild gradients, in the range
[-0.002, -0.01 ] dex/kpc. The metallicity distribution functions exhibit
different features, reflecting the different assembly history of the individual
stellar haloes. We find that stellar haloes formed with larger contributions
from massive subgalactic systems have steeper metallicity gradients. Very
metal-poor stars are mainly contributed to the halo systems by lower-mass
satellites. There is a clear trend among the predicted metallicity distribution
functions that a higher fraction of low-metallicity stars are found with
increasing radius. These properties are consistent with the range of behaviours
observed for stellar haloes of nearby galaxies.Comment: 11 pages, 6 figures. Accepted MNRAS. Revised version after referee's
comment
A Search for Stars of Very Low Metal Abundance. V. Photoelectric UBV Photometry of Metal-Weak Candidates from the Northern HK Survey
We report photoelectric UBV data for 268 metal-poor candidates chosen from
the northern HK objective-prism/interference-filter survey of Beers and
colleagues. Over 40 % of the stars have been observed on more than one night,
and most have at least several sets of photometric measurements. Reddening
estimates, preliminary spectroscopic measurements of abundance, and type
classifications are reported.Comment: To Appear in the Astronomical Journal, October 200
On the origin of HE0107-5240, the most iron deficient star presently known
We show that the "puzzling" chemical composition observed in the extremely
metal poor star HE0107-5240 may be naturally explained by the concurrent
pollution of at least two supernovae. In the simplest possible model a
supernova of quite low mass (~15 Msun), underwent a "normal" explosion and
ejected ~0.06 Msun of 56Ni while a second one was massive enough (~35 Msun) to
experience a strong fall back that locked in a compact remnant all the
carbon-oxygen core. In a more general scenario, the pristine gas clouds were
polluted by one or more supernovae of relatively low mass (less than ~25 Msun).
The successive explosion of a quite massive star experiencing an extended fall
back would have largely raised the abundances of the light elements in its
close neighborhood.Comment: 10 pages; 3 figures; accepted for publication in the The
Astrophysical Journal Letter
Investigation of Smart Responses of Human Serum Albumin in Fever Condition: An In Vitro Approach
To move real objects, our hand needs to get in direct physical contact with the object. However, this is not necessarily the case when interacting with virtual objects, for example when displacing objects on tablets by swipe movements. Here, we performed two experiments to study the behavioral strategies of these movements, examining how visual information about the virtual object is mapped into a swipe that moves the object into a goal location. In the first experiment, we investigated how swiping behavior depends on whether objects were located within or outside the swiping workspace. Results show that participants do not start the swipe movement by placing their finger on the virtual object, as they do when reaching to real objects, but rather keep a systematic distance between the object location and the initial swipe location. This mismatch, which was experimentally imposed by placing the object outside the workspace, also occurred when the object was within the workspace. In the second experiment, we investigated which factors determine this mismatch by systematically manipulating the initial hand location, the location of the object and the location of the goal. Dimensionality reduction of the data showed that three factors are taken into account when participants choose the initial swipe location: the expected total movement distance, the distance between their finger on the screen and the object, and a preference not to cover the object. The weight given to each factor differed among individuals. These results delineate, for the first time, the flexibility of visuomotor associations in the virtual world
Chemical abundances of the metal-poor horizontal-branch stars CS 22186-005 and CS 30344-033
We report on a chemical-abundance analysis of two very metal-poor
horizontal-branch stars in the Milky Way halo: CS 22186-005 ([Fe/H]=-2.70) and
CS 30344-033 ([Fe/H]=-2.90). The analysis is based on high-resolution spectra
obtained at ESO, with the spectrographs HARPS at the 3.6 m telescope, and UVES
at the VLT. We adopted one-dimensional, plane-parallel model atmospheres
assuming local thermodynamic equilibrium. We derived elemental abundances for
13 elements for CS 22186-005 and 14 elements for CS 30344-033. This study is
the first abundance analysis of CS 30344-033. CS 22186-005 has been analyzed
previously, but we report here the first measurement of nickel (Ni; Z = 28) for
this star, based on twenty-two NiI lines ([Ni/Fe]=-0.210.02); the
measurement is significantly below the mean found for most metal-poor stars.
Differences of up to 0.5 dex in [Ni/Fe] ratios were determined by different
authors for the same type of stars in the literature, which means that it is
not yet possible to conclude that there is a real intrinsic scatter in the
[Ni/Fe] ratios. For the other elements for which we obtained estimates, the
abundance patterns in these two stars match the Galactic trends defined by
giant and turnoff stars well. This confirms the value of horizontal-branch
stars as tracers of the chemical properties of stellar populations in the
Galaxy. Our radial velocities measurements for CS 22186-005 differ from
previously published measurements by more than the expected statistical errors.
More measurements of the radial velocity of this star are encouraged to confirm
or refute its radial velocity variability
Extremely Metal-Poor Stars. VII. The Most Metal-Poor Dwarf, CS 22876-032
We report high-resolution, high-signal-to-noise, observations of the
extremely metal-poor double-lined spectroscopic binary CS 22876-032. The system
has a long period : P = 424.7 0.6 days. It comprises two main sequence
stars having effective temperatures 6300 K and 5600 K, with a ratio of
secondary to primary mass of 0.89 0.04. The metallicity of the system is
[Fe/H] = -3.71 0.11 0.12 (random and systematic errors) -- somewhat
higher than previous estimates. We find [Mg/Fe] = 0.50, typical of values of
less extreme halo material. [Si/Fe], [Ca/Fe], and [Ti/Fe], however, all have
significantly lower values, ~ 0.0-0.1, suggesting that the heavier elements
might have been underproduced relative to Mg in the material from which this
object formed. In the context of the hypothesis that the abundance patterns of
extremely metal-poor stars are driven by individual enrichment events and the
models of Woosley and Weaver (1995), the data for CS 22876-032 are consistent
with its having been enriched by a zero-metallicity supernova of mass 30
M. As the most metal-poor near-main-sequence-turnoff star currently
known, the primary of the system has the potential to strongly constrain the
primordial lithium abundance. We find A(Li) (= log(N(Li)/N(H)) + 12.00) = 2.03
0.07, which is consistent with the finding of Ryan et al. (1999) that for
stars of extremely low metallicity A(Li) is a function of [Fe/H].Comment: 27 pages, 9 figures, accepted for publication in The Astrophysical
Journal, Sept. 1, 2000 issu
Chronography of the Milky Way's Halo System with Field Blue Horizontal-Branch Stars
In a pioneering effort, Preston et al. reported that the colors of blue
horizontal-branch (BHB) stars in the halo of the Galaxy shift with distance,
from regions near the Galactic center to about 12 kpc away, and interpreted
this as a correlated variation in the ages of halo stars, from older to
younger, spanning a range of a few Gyrs. We have applied this approach to a
sample of some 4700 spectroscopically confirmed BHB stars selected from the
Sloan Digital Sky Survey to produce the first "chronographic map" of the halo
of the Galaxy. We demonstrate that the mean de-reddened gr color,
, increases outward in the Galaxy from 0.22 to 0.08 (over a
color window spanning [0.3:0.0]) from regions close to the Galactic center
to ~40 kpc, independent of the metallicity of the stars. Models of the expected
shift in the color of the field BHB stars based on modern stellar evolutionary
codes confirm that this color gradient can be associated with an age difference
of roughly 2-2.5 Gyrs, with the oldest stars concentrated in the central ~15
kpc of the Galaxy. Within this central region, the age difference spans a mean
color range of about 0.05 mag (~0.8 Gyrs). Furthermore, we show that
chronographic maps can be used to identify individual substructures, such as
the Sagittarius Stream, and overdensities in the direction of Virgo and
Monoceros, based on the observed contrast in their mean BHB colors with respect
to the foreground/background field population.Comment: 6 pages, 4 figures, ApJ letter
Lead abundance in the uranium star CS 31082-001
In a previous paper we were able to measure the abundance of uranium and
thorium in the very-metal poor halo giant BPS CS 31082-001, but only obtained
an upper limit for the abundance of lead (Pb). We have got from ESO 17 hours of
additional exposure on this star in order to secure a detection of the minimum
amount of lead expected to be present in CS 31082-001, the amount arising from
the decay of the original content of Th and U in the star. We report here this
successful detection. We find an LTE abundance log(Pb/H)+12=-0.55 \pm 0.15 dex,
one dex below the upper limits given by other authors for the similar stars CS
22892-052 and BD +17d3248, also enhanced in r-process elements. From the
observed present abundances of Th and U in the star, the expected amount of Pb
produced by the decay of 232Th, and 238U alone, over 12-15 Gyr is -0.73\pm 0.17
dex. The decay of 235U is more difficult to estimate, but is probably slightly
below the contribution of 238U, making the contribution of the 3 actinides only
slightly below, or even equal to, the measured abundance. The contribution from
the decay of 234U has was not included, for lack of published data. In this
sense our determination is a lower limit to the contribution of actinides to
lead production. We comment this result, and we note that if a NLTE analysis,
not yet possible, doubles our observed abundance, the decay of the 3 actinides
will still represent 50 per cent of the total lead, a proportion higher than
the values considered so far in the literature.Comment: 4 pages, LateX, A&A Letters Accepte
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