369 research outputs found
An explanation for the curious mass loss history of massive stars: from OB stars, through Luminous Blue Variables to Wolf-Rayet stars
The stellar winds of massive stars show large changes in mass-loss rates and
terminal velocities during their evolution from O-star through the Luminous
Blue Variable phase to the Wolf-Rayet phase. The luminosity remains
approximately unchanged during these phases. These large changes in wind
properties are explained in the context of the radiation driven wind theory, of
which we consider four different models. They are due to the evolutionary
changes in radius, gravity and surface composition and to the change from
optically thin (in continuum) line driven winds to optically thick radiation
driven winds.Comment: Accepted for publication in Astronomy and Astrophysics (Letter to the
Editor
Synthesis, densification, and cation inversion in high entropy (Co,Cu,Mg,Ni,Zn)Al2O4 spinel
The synthesis, densification behavior, and crystallographic site occupancy were investigated for four different spinel-based ceramics, including a high-entropy spinel (Co0.2Cu0.2Mg0.2Ni0.2 Zn0.2)Al2O4. Each composition was reacted to form a single phase, but analysis of X-ray diffraction patterns revealed differences in cation site occupancy with the high-entropy spinel being nearly fully normal. Densification behavior was investigated and showed that fully dense ceramics could be produced by hot pressing at temperatures as low as 1375°C for all compositions. Vickersâ hardness values were at least 10 GPa for all compositions. The cations present in the high-entropy spinel appear to have a stabilizing effect that led to nearly normal site occupancy compared to full cation inversion behavior of nickel aluminate spinel. This is the first report that compares cation site occupancy of a high-entropy spinel to conventional spinel ceramics
Effects of rapid prey evolution on predator-prey cycles
We study the qualitative properties of population cycles in a predator-prey
system where genetic variability allows contemporary rapid evolution of the
prey. Previous numerical studies have found that prey evolution in response to
changing predation risk can have major quantitative and qualitative effects on
predator-prey cycles, including: (i) large increases in cycle period, (ii)
changes in phase relations (so that predator and prey are cycling exactly out
of phase, rather than the classical quarter-period phase lag), and (iii)
"cryptic" cycles in which total prey density remains nearly constant while
predator density and prey traits cycle. Here we focus on a chemostat model
motivated by our experimental system [Fussmann et al. 2000,Yoshida et al. 2003]
with algae (prey) and rotifers (predators), in which the prey exhibit rapid
evolution in their level of defense against predation. We show that the effects
of rapid prey evolution are robust and general, and furthermore that they occur
in a specific but biologically relevant region of parameter space: when traits
that greatly reduce predation risk are relatively cheap (in terms of reductions
in other fitness components), when there is coexistence between the two prey
types and the predator, and when the interaction between predators and
undefended prey alone would produce cycles. Because defense has been shown to
be inexpensive, even cost-free, in a number of systems [Andersson and Levin
1999, Gagneux et al. 2006,Yoshida et al. 2004], our discoveries may well be
reproduced in other model systems, and in nature. Finally, some of our key
results are extended to a general model in which functional forms for the
predation rate and prey birth rate are not specified.Comment: 35 pages, 8 figure
The Biogeography of Putative Microbial Antibiotic Production
Understanding patterns in the distribution and abundance of functional traits across a landscape is of fundamental importance to ecology. Mapping these distributions is particularly challenging for species-rich groups with sparse trait measurement coverage, such as flowering plants, insects, and microorganisms. Here, we use likelihood-based character reconstruction to infer and analyze the spatial distribution of unmeasured traits. We apply this framework to a microbial dataset comprised of 11,732 ketosynthase alpha gene sequences extracted from 144 soil samples from three continents to document the spatial distribution of putative microbial polyketide antibiotic production. Antibiotic production is a key competitive strategy for soil microbial survival and performance. Additionally, novel antibiotic discovery is highly relevant to human health, making natural antibiotic production by soil microorganisms a major target for bioprospecting. Our comparison of trait-based biogeographical patterns to patterns based on taxonomy and phylogeny is relevant to our basic understanding of microbial biogeography as well as the pressing need for new antibiotics
S Ori J053825.4-024241: A Classical T Tauri-like object at the substellar boundary
We present a spectrophotometric analysis of S Ori J053825.4-024241, a
candidate member close to the substellar boundary of the young (1-8 Myr),
nearby (~360 pc) sigma Orionis star cluster. Our optical and near-infrared
photometry and low-resolution spectroscopy indicate that S Ori J053825.4-024241
is a likely cluster member with a mass estimated from evolutionary models at
0.06+0.07-0.02 Msol, which makes the object a probable brown dwarf. The radial
velocity of S Ori J053825.4-024241 is similar to the cluster systemic velocity.
This target, which we have classified as an M 6.0+-1.0 low-gravity object,
shows excessemission in the near-infrared and anomalously strong photometric
variability for its type (from the blue to the J band), suggesting the presence
of a surrounding disc. The optical spectroscopic observations show a continuum
excess at short wavelengths and a persistent and resolved Halpha emission
(pseudo-equivalent width of ~-250 AA) in addition to the presence of other
forbidden and permitted emission lines, which we interpret as indicating
accretion from the disc and possibly mass loss. We conclude that despite the
low mass of S Ori J053825.4-024241, this object exhibits some of the properties
typical of active classical T Tauri stars.Comment: 12 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysics, section 5. Galactic structure, stellar clusters and
populations. The official date of acceptance is 24/08/2005. Acknowledgements
of the use of telescopes, instruments, catalogues and software are also give
MN112: a new Galactic candidate Luminous Blue Variable
We report the discovery of a new Galactic candidate Luminous Blue Variable
(cLBV) via detection of an infrared circular nebula and follow-up spectroscopy
of its central star. The nebula, MN112, is one of many dozens of circular
nebulae detected at m in the {\it Spitzer Space Telescope} archival
data, whose morphology is similar to that of nebulae associated with known
(c)LBVs and related evolved massive stars. Specifically, the core-halo
morphology of MN112 bears a striking resemblance to the circumstellar nebula
associated with the Galactic cLBV GAL 079.29+00.46, which suggests that both
nebulae might have a similar origin and that the central star of MN112 is a
LBV. The spectroscopy of the central star showed that its spectrum is almost
identical to that of the bona fide LBV P Cygni, which also supports the LBV
classification of the object. To further constrain the nature of MN112, we
searched for signatures of possible high-amplitude (\ga 1 mag) photometric
variability of the central star using archival and newly obtained photometric
data covering a 45 year period. We found that the B magnitude of the star was
constant ( 17.10.3 mag) over this period, while in the I band the
star brightened by mag during the last 17 years. Although the
non-detection of large photometric variability leads us to use the prefix
`candidate' in the classification of MN112, we remind that the long-term
photometric stability is not unusual for genuine LBVs and that the brightness
of P Cygni remains relatively stable during the last three centuries.Comment: 6 pages, 3 figures, accepted to MNRA
Robust estimation of microbial diversity in theory and in practice
Quantifying diversity is of central importance for the study of structure,
function and evolution of microbial communities. The estimation of microbial
diversity has received renewed attention with the advent of large-scale
metagenomic studies. Here, we consider what the diversity observed in a sample
tells us about the diversity of the community being sampled. First, we argue
that one cannot reliably estimate the absolute and relative number of microbial
species present in a community without making unsupported assumptions about
species abundance distributions. The reason for this is that sample data do not
contain information about the number of rare species in the tail of species
abundance distributions. We illustrate the difficulty in comparing species
richness estimates by applying Chao's estimator of species richness to a set of
in silico communities: they are ranked incorrectly in the presence of large
numbers of rare species. Next, we extend our analysis to a general family of
diversity metrics ("Hill diversities"), and construct lower and upper estimates
of diversity values consistent with the sample data. The theory generalizes
Chao's estimator, which we retrieve as the lower estimate of species richness.
We show that Shannon and Simpson diversity can be robustly estimated for the in
silico communities. We analyze nine metagenomic data sets from a wide range of
environments, and show that our findings are relevant for empirically-sampled
communities. Hence, we recommend the use of Shannon and Simpson diversity
rather than species richness in efforts to quantify and compare microbial
diversity.Comment: To be published in The ISME Journal. Main text: 16 pages, 5 figures.
Supplement: 16 pages, 4 figure
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