265 research outputs found
The R136 star cluster hosts several stars whose individual masses greatly exceed the accepted 150 Msun stellar mass limit
Spectroscopic analyses of H-rich WN5-6 stars within the young star clusters
NGC 3603 and R136 are presented, using archival HST & VLT spectroscopy, & high
spatial resolution near-IR photometry. We derive high T* for the WN stars in
NGC 3603 (T*~42+/-2 kK) & R136 (T*~53+/-3 kK) plus clumping-corrected dM/dt ~
2-5x10^-5 Msun/yr which closely agree with theoretical predictions. These stars
make a disproportionate contribution to the global budget of their host
clusters. R136a1 alone supplies ~7% of N(LyC) of the entire 30 Dor region.
Comparisons with stellar models calculated for the main-sequence evolution of
85-500 Msun suggest ages of ~1.5 Myr & M_init in the range 105 - 170 Msun for 3
systems in NGC 3603, plus 165-320 Msun for 4 stars in R136. Our high stellar
masses are supported by dynamical mass determinations for the components of NGC
3603 A1. We consider the predicted L_X of the R136 stars if they were close,
colliding wind binaries. R136c is consistent with a colliding wind binary
system. However, short period, colliding wind systems are excluded for R136a WN
stars if mass ratios are of order unity. Widely separated systems would have
been expected to harden owing to early dynamical encounters with other massive
stars in such a dense environment. From simulated star clusters, whose
constituents are randomly sampled from the Kroupa IMF, both clusters are
consistent with a tentative upper mass limit of ~300 Msun. The Arches cluster
is either too old, exhibits a deficiency of very massive stars, or more likely
stellar masses have been underestimated - M_init for the most luminous stars in
the Arches cluster approach 200 Msun according to contemporary stellar &
photometric results. The potential for stars greatly exceeding 150 Msun within
metal-poor galaxies suggests that such pair-instability SNe could occur within
the local universe, as has been claimed for SN 2007bi (abridged).Comment: 20 pages, 14 figures, accepted for MNRAS. Version with higher
resolution figures is available from
http://pacrowther.staff.shef.ac.uk/R136.pdf See also
http://www.eso.org/public/news/eso1030/ from Wed 21 from noon (CEST
Ion beam irradiation of ABO4 compounds with the fergusonite, monazite, scheelite, and zircon structures
First lensing measurements of SZ-discovered clusters
We present the first lensing mass measurements of Sunyaev-Zel'dovich (SZ)
selected clusters. Using optical imaging from the Southern Cosmology Survey
(SCS), we present weak lensing masses for three clusters selected by their SZ
emission in the South Pole Telescope survey (SPT). We confirm that the SZ
selection procedure is successful in detecting mass concentrations. We also
study the weak lensing signals from 38 optically-selected clusters in ~8 square
degrees of the SCS survey. We fit Navarro, Frenk and White (NFW) profiles and
find that the SZ clusters have amongst the largest masses, as high as 5x10^14
Msun. Using the best fit masses for all the clusters, we analytically calculate
the expected SZ integrated Y parameter, which we find to be consistent with the
SPT observations.Comment: Minor changes to match accepted version, 5 pages, 3 figure
New insights into phase distribution, phase composition and disorder in Y2(Zr,Sn) 2O7 ceramics from NMR spectroscopy
A combination of 89Y and 119Sn NMR spectroscopy and DFT calculations are used to investigate phase evolution, local structure and disorder in Y2Zr2−xSnxO7 ceramics, where a phase change is predicted, from pyrochlore to defect fluorite, with increasing Zr content. The ability of NMR to effectively probe materials that exhibit positional and compositional disorder provides insight into the atomic-scale structure in both ordered and disordered phases and, by exploiting the quantitative nature of the technique, we are able to determine detailed information on the composition of the phase(s) present and the average coordination number (and next-nearest neighbour environment) of the cations. In contrast to previous studies, a more complex picture of the phase variation with composition emerges, with single-phase pyrochlore found only for the Sn end member, and a single defect fluorite phase only for x = 0 to 0.6. A broad two-phase region is observed, from x = 1.8 to 0.8, but the two phases present have very different composition, with a maximum of 13% Zr incorporated into the pyrochlore phase, whereas the composition of the defect fluorite phase varies throughout. Preferential ordering of the anion vacancies in the defect fluorite phase is observed, with Sn only ever found in a six-coordinate environment, while remaining vacancies are shown to be more likely to be associated with Zr than Y. Our findings are then discussed in the light of those from previous studies, many of which utilize diffraction-based approaches, where, in most cases, a single phase of fixed composition has been assumed for the refinement procedure. The significant and surprising differences encountered demonstrate the need for complementary approaches to be considered for a detailed and accurate picture of both the long- and short-range structure of a solid to be achieved
Weak lensing, dark matter and dark energy
Weak gravitational lensing is rapidly becoming one of the principal probes of
dark matter and dark energy in the universe. In this brief review we outline
how weak lensing helps determine the structure of dark matter halos, measure
the expansion rate of the universe, and distinguish between modified gravity
and dark energy explanations for the acceleration of the universe. We also
discuss requirements on the control of systematic errors so that the
systematics do not appreciably degrade the power of weak lensing as a
cosmological probe.Comment: Invited review article for the GRG special issue on gravitational
lensing (P. Jetzer, Y. Mellier and V. Perlick Eds.). V3: subsection on
three-point function and some references added. Matches the published versio
Precision cluster mass determination from weak lensing
Weak gravitational lensing has been used extensively in the past decade to
constrain the masses of galaxy clusters, and is the most promising
observational technique for providing the mass calibration necessary for
precision cosmology with clusters. There are several challenges in estimating
cluster masses, particularly (a) the sensitivity to astrophysical effects and
observational systematics that modify the signal relative to the theoretical
expectations, and (b) biases that can arise due to assumptions in the mass
estimation method, such as the assumed radial profile of the cluster. All of
these challenges are more problematic in the inner regions of the cluster,
suggesting that their influence would ideally be suppressed for the purpose of
mass estimation. However, at any given radius the differential surface density
measured by lensing is sensitive to all mass within that radius, and the
corrupted signal from the inner parts is spread out to all scales. We develop a
new statistic that is ideal for estimation of cluster masses because it
completely eliminates mass contributions below a chosen scale (which we suggest
should be about 20 per cent of the virial radius), and thus reduces sensitivity
to systematic and astrophysical effects. We use simulated and analytical
profiles to quantify systematic biases on the estimated masses for several
standard methods of mass estimation, finding that these can lead to significant
mass biases that range from ten to over fifty per cent. The mass uncertainties
when using our new statistic are reduced by up to a factor of ten relative to
the standard methods, while only moderately increasing the statistical errors.
This new method of mass estimation will enable a higher level of precision in
future science work with weak lensing mass estimates for galaxy clusters.Comment: 27 pages, 7 figures, submitted to MNRAS; v2 has expanded explanation
for clarity, no change in results or conclusion
The Mechanisms of Codon Reassignments in Mitochondrial Genetic Codes
Many cases of non-standard genetic codes are known in mitochondrial genomes.
We carry out analysis of phylogeny and codon usage of organisms for which the
complete mitochondrial genome is available, and we determine the most likely
mechanism for codon reassignment in each case. Reassignment events can be
classified according to the gain-loss framework. The gain represents the
appearance of a new tRNA for the reassigned codon or the change of an existing
tRNA such that it gains the ability to pair with the codon. The loss represents
the deletion of a tRNA or the change in a tRNA so that it no longer translates
the codon. One possible mechanism is Codon Disappearance, where the codon
disappears from the genome prior to the gain and loss events. In the
alternative mechanisms the codon does not disappear. In the Unassigned Codon
mechanism, the loss occurs first, whereas in the Ambiguous Intermediate
mechanism, the gain occurs first. Codon usage analysis gives clear evidence of
cases where the codon disappeared at the point of the reassignment and also
cases where it did not disappear. Codon disappearance is the probable
explanation for stop to sense reassignments and a small number of reassignments
of sense codons. However, the majority of sense to sense reassignments cannot
be explained by codon disappearance. In the latter cases, by analysis of the
presence or absence of tRNAs in the genome and of the changes in tRNA
sequences, it is sometimes possible to distinguish between the Unassigned Codon
and Ambiguous Intermediate mechanisms. We emphasize that not all reassignments
follow the same scenario and that it is necessary to consider the details of
each case carefully.Comment: 53 pages (45 pages, including 4 figures + 8 pages of supplementary
information). To appear in J.Mol.Evo
Discovery of a Be/X-ray pulsar binary and associated supernova remnant in the Wing of the SMC
We report on a new Be/X-ray pulsar binary located in the Wing of the Small
Magellanic Cloud (SMC). The strong pulsed X-ray source was discovered with the
Chandra and XMM-Newton X-ray observatories. The X-ray pulse period of 1062 s is
consistently determined from both Chandra and XMM-Newton observations,
revealing one of the slowest rotating X-ray pulsars known in the SMC. The
optical counterpart of the X-ray source is the emission-line star 2dFS 3831.
Its B0-0.5(III)e+ spectral type is determined from VLT-FLAMES and 2dF optical
spectroscopy, establishing the system as a Be/X-ray binary (Be-XRB). The hard
X-ray spectrum is well fitted by a power-law with additional thermal and
blackbody components, the latter reminiscent of persistent Be-XRBs. This system
is the first evidence of a recent supernova in the low density surroundings of
NGC 602. We detect a shell nebula around 2dFS 3831 in H-alpha and [O III]
images and conclude that it is most likely a supernova remnant. If it is linked
to the supernova explosion that created this new X-ray pulsar, its kinematic
age of (2-4)x10^4 yr provides a constraint on the age of the pulsar.Comment: 5 pages, 5 figures, accepted for publication in MNRAS Letter
From Nuevo León to the USA and Back Again: Transnational Students in Mexico
The movement of Mexicans to the United States is both longstanding and long studied and from that study we know that for many newcomers the attachment to the receiving community is fraught and tentative. The experience of immigrant children in U.S. schools is also relatively well studied and reveals challenges of intercultural communication as well as concurrent and contradictory features of welcome and unwelcome. What is less well known, in the study of migration generally and of transnational students in particular, is how students moving in a less common direction — from the U.S. to Mexico — experience that movement. Based on visits to 173 randomly selected classrooms in the state of Nuevo León Mexico, this study shares survey and interview data from 208 of the 242 students encountered who had previous experience attending school in the United States
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