950 research outputs found
Dynamics of nonequilibrium quasiparticles in a double superconducting tunnel junction detector
We study a class of superconductive radiation detectors in which the
absorption of energy occurs in a long superconductive strip while the redout
stage is provided by superconductive tunnel junctions positioned at the two
ends of the strip. Such a device is capable both of imaging and energy
resolution. In the established current scheme, well studied from the
theoretical and experimental point of view, a fundamental ingredient is
considered the presence of traps, or regions adjacent to the junctions made of
a superconducting material of lower gap. We reconsider the problem by
investigating the dynamics of the radiation induced excess quasiparticles in a
simpler device, i.e. one without traps. The nonequilibrium excess
quasiparticles can be seen to obey a diffusion equation whose coefficients are
discontinuous functions of the position. Based on the analytical solution to
this equation, we follow the dynamics of the quasiparticles in the device,
predict the signal formation of the detector and discuss the potentiality
offered by this configuration.Comment: 16 pages, 5 figures Submitted to Superconducting Science and
Technolog
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
Two distinct ancient components in the Sculptor Dwarf Spheroidal Galaxy: First Results from DART
We have found evidence for the presence of two distinct ancient stellar
components (both geq 10 Gyr old) in the Sculptor dwarf spheroidal galaxy. We
used the ESO Wide Field Imager (WFI) in conjunction with the VLT/FLAMES
spectrograph to study the properties of the resolved stellar population of
Sculptor out to and beyond the tidal radius. We find that two components are
discernible in the spatial distribution of Horizontal Branch stars in our
imaging, and in the [Fe/H] and v_hel distributions for our large sample of
spectroscopic measurements. They can be generally described as a ``metal-poor''
component ([Fe/H] -1.7). The
metal-poor stars are more spatially extended than the metal-rich stars, and
they also appear to be kinematically distinct. These results provide an
important insight into the formation processes of small systems in the early
universe and the conditions found there. Even this simplest of galaxies appears
to have had a surprisingly complex early evolution.Comment: accepted for publication in ApJL 12 pages, 4 figure
Extragalactic Stellar Astronomy with the Brightest Stars in the Universe
A supergiants are objects in transition from the blue to the red (and vice
versa) in the uppermost HRD. They are the intrinsically brightest "normal"
stars at visual light with absolute visual magnitudes up to -9. They are ideal
to study young stellar populations in galaxies beyond the Local Group to
determine chemical composition and evolution, interstellar extinction,
reddening laws and distances. We discuss most recent results on the
quantitative spectral analysis of such objects in galaxies beyond the Local
Group based on medium and low resolution spectra obtained with the ESO VLT and
Keck. We describe the analysis method including the determination of
metallicity and metallicity gradients. A new method to measure accurate
extragalactic distances based on the stellar gravities and effective
temperatures is presented, the flux weighted gravity - luminosity relationship
(FGLR). The FGLR is a purely spectroscopic method, which overcomes the
untertainties introduced by interstellar extinction and variations of
metallicity, which plague all photometric stellar distance determination
methods. We discuss the perspectives of future work using the giant
ground-based telescopes of the next generation such as the TMT, the GMT and the
E-ELT.Comment: Invited review; to appear in "Massive Stars as Cosmic Engines", IAU
Symp. 250, ed. F. Bresolin, P. A. Crowther, and J. Puls (Cambridge University
Press
The Spatial Homogeneity of Nebular and Stellar Oxygen Abundances in the Local Group Dwarf Irregular Galaxy NGC 6822
To test the existence of a possible radial gradient in oxygen abundances
within the Local Group dwarf irregular galaxy NGC 6822, we have obtained
optical spectra of 19 nebulae with the EFOSC2 spectrograph on the 3.6-m
telescope at ESO La Silla. The extent of the measured nebulae spans
galactocentric radii in the range between 0.05 kpc and 2 kpc (over four
exponential scale lengths). In five H II regions (Hubble I, Hubble V, Kalpha,
Kbeta, KD28e), the temperature-sensitive [O III] 4363 emission line was
detected, and direct oxygen abundances were derived. Oxygen abundances for the
remaining H II regions were derived using bright-line methods. The oxygen
abundances for three A-type supergiant stars are slightly higher than nebular
values at comparable radii. Linear least-square fits to various subsets of
abundance data were obtained. When all of the measured nebulae are included, no
clear signature is found for an abundance gradient. A fit to only newly
observed H II regions with [O III] 4363 detections yields an oxygen abundance
gradient of -0.14 +/- 0.07 dex/kpc. The gradient becomes slightly more
significant (-0.16 +/- 0.05 dex/kpc) when three additional H II regions with [O
III] 4363 measurements from the literature are added. Assuming no abundance
gradient, we derive a mean nebular oxygen abundance 12+log(O/H) = 8.11 +/- 0.10
from [O III] 4363 detections in the five H II regions from our present data;
this mean value corresponds to [O/H] = -0.55.Comment: Accepted, Ap.J.; 25 pages (AASTeX 5.2; emulateapj) with 14 figures.
Full paper with color figures may be retrieved from
http://www.astro.umn.edu/~hlee
First Stellar Abundances in the Dwarf Irregular Galaxy Sextans A
We present the abundance analyses of three isolated A-type supergiant stars
in the dwarf irregular galaxy Sextans A from high-resolution spectra the UVES
spectrograph at the VLT. Detailed model atmosphere analyses have been used to
determine the stellar atmospheric parameters and the elemental abundances of
the stars. The mean iron group abundance was determined from these three stars
to be [(FeII,CrII)/H]=-0.99+/-0.04+/-0.06. This is the first determination of
the present-day iron group abundances in Sextans A. These three stars now
represent the most metal-poor massive stars for which detailed abundance
analyses have been carried out. The mean stellar alpha element abundance was
determined from the alpha element magnesium as
[alpha(MgI)/H]=-1.09+/-0.02+/-0.19. This is in excellent agreement with the
nebular alpha element abundances as determined from oxygen in the H II regions.
These results are consistent from star-to-star with no significant spatial
variations over a length of 0.8 kpc in Sextans A. This supports the nebular
abundance studies of dwarf irregular galaxies, where homogeneous oxygen
abundances are found throughout, and argues against in situ enrichment. The
alpha/Fe abundance ratio is [alpha(MgI)/FeII,CrII]=-0.11+/-0.02+/-0.10, which
is consistent with the solar ratio. This is consistent with the results from
A-supergiant analyses in other Local Group dwarf irregular galaxies but in
stark contrast with the high [alpha/Fe] results from metal-poor stars in the
Galaxy, and is most clearly seen from these three stars in Sextans A because of
their lower metallicities. The low [alpha/Fe] ratios are consistent with the
slow chemical evolution expected for dwarf galaxies from analyses of their
stellar populations.Comment: 40 pages, 8 figures, accepted for publication in A
Learning through social spaces: migrant women and lifelong learning in post-colonial London
This article shows how migrant women engage in learning through social spaces. It argues that such spaces are little recognised, and that there are multiple ways in which migrant women construct and negotiate their informal learning through socialising with other women in different informal modes. Additionally, the article shows how learning is shaped by the socio-political, geographical and multicultural context of living in London, outlining ways in which gendered and racialised identities shape, construct and constrain participation in lifelong learning. The article shows that one way in which migrant women resist (post)colonial constructions of difference is by engaging in informal and non-formal lifelong learning, arguing that the benefits are (at least) two-fold. The women develop skills (including language skills) but also use their informal learning to develop what is referred to in this article as 'relational capital'. The article concludes that informal lifelong learning developed through social spaces can enhance a sense of belonging for migrant women
A-type Supergiant Abundances in the SMC: Probes of Evolution
New abundances of N, O, Na, Mg, Si, Ca, Sc, Ti, Cr, Fe, Sr, Zr, and Ba are
presented for 10 A-type supergiants in the SMC, plus upper limits for C. In
interpreting the CNO results for constraints on stellar evolution theories,
careful attention has been paid to the comparison abundances, i.e., the present
day abundances of SMC nebulae and B-dwarf stars. These new results are also
compared to published results from F-K supergiant analyses, and found to be in
good agreement when both sets of data are carefully examined as differential
(SMC minus Galactic standard) abundances.
With the exception of nitrogen, very small star-to-star abundance variations
are found for all elements in this analysis. The N variations are not predicted
by standard stellar evolution models. Instead, the results support the new
predictions reported from rotating stellar models, where the range in nitrogen
is the result of partial mixing of CN-cycled gas from the stellar interior due
to main-sequence rotation at different rates (c.f., Langer & Heger 1998). The
overall overabundance of nitrogen in the sampled stars also implies these stars
have undergone the first dredge-up in addition to having been mixed while on
the main-sequence.
The alpha-elements (O, Mg, Si, Ca, Ti) have similar underabundances to Fe,
which is not the same as seen in metal-poor stars in the solar neighborhood of
the Galaxy. In addition, certain light s-process elements (Zr, Ba) are slightly
more underabundant than Fe, which is predicted by the bursting chemical
evolution model presented by Pagel & Tautvaisiene (1998) for the SMC.Comment: Accepted for publication in the Astrophysical Journal, Manuscript
#39295. Accepted January 4, 199
Early Results from APOKASC
Asteroseismology and spectroscopy provide complementary constraints on the
fundamental and chemical properties of stars. I describe the first results from
APOKASC, a collaboration between the Kepler asteroseismic science consortium
(KASC) and the SDSS-III APOGEE survey. These include (1) the first test of
asteroseismic scaling relationships in the metal-poor regime using halo and
thick disk stars identified in the APOKASC sample; and (2) the calibration of
spectroscopic parameters using precise asteroseismic measurements of surface
gravity. I also highlight future research avenues that are made possible by
this unique sample of thousands of well-characterized red giant stars.Comment: Proc. of the workshop "Asteroseismology of stellar populations in the
Milky Way" (Sesto, 22-26 July 2013), Astrophysics and Space Science
Proceedings, (eds. A. Miglio, L. Girardi, P. Eggenberger, J. Montalban
A Comparison of Elemental Abundance Ratios in Globular Clusters, Field Stars, and Dwarf Spheroidal Galaxies
We have compiled a sample of globular clusters with high quality stellar
abundances from the literature to compare to the chemistries of stars in the
Galaxy and those in dwarf spheroidal galaxies. Of the 45 globular clusters
examined, 29 also have kinematic information. Most of the globular clusters
belong to the Galactic halo, however a signficant number have disk kinematics
or belong to the bulge. Focusing on the [alpha/Fe] and light r-process element
ratios, we find that most globular cluster stars mimic those of the field stars
of similar metallicities, and neither clearly resembles the presently available
stellar abundances in the dwarf galaxies (including the globular clusters in
the Large Magellanic Cloud). The exceptions to these general elemental ratio
comparisons are already known in the literature, e.g., omega Centauri, Palomar
12, and Terzan 7 associated with the Sagittarius remnant, and Ruprecht 106
which has a high radial velocity and low [alpha/Fe] ratio. A few other globular
clusters show more marginal peculiarities. The most notable one being the halo
cluster M68 which has a high Galactocentric rotational velocity, a slightly
younger age, and a unique [Si/Ti] ratio. The [Si/Ti] ratios decrease with
increasing [Fe/H] at intermediate metallicities, which is consistent with very
massive stars playing a larger role in the early chemical evolution of the
Galaxy. The chemical similarities between globular clusters and field stars
with [Fe/H]<-1.0 suggests a shared chemical history in a well mixed early
Galaxy. The differences to the published chemistries of stars in the dwarf
spheroidal galaxies suggests that neither the globular clusters, halo stars,
nor thick disk stars had their origins in small isolated systems like the
present-day Milky Way dwarf satellites.Comment: 24 pages, including 10 figures (8 are in color) and 4 tables;
Accepted for publication in the Astronomical Journa
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