9 research outputs found
Massive stars at low metallicity: Evolution and surface abundances of O dwarfs in the SMC
We study the evolution, rotation, and surface abundances of O-type dwarfs in
the Small Magellanic Cloud. We analyzed the UV and optical spectra of
twenty-three objects and derived photospheric and wind properties. The observed
binary fraction of the sample is ~ 26%, which is compatible with more
systematic studies, if one considers that the actual binary fraction is
potentially larger owing to low-luminosity companions and that the sample
excluded obvious spectroscopic binaries. The location of the fastest rotators
in the H-R diagram indicates that these could be several Myr old. The offset in
the position of these fast rotators compared with the other stars confirms the
predictions of evolutionary models that fast-rotating stars tend to evolve more
vertically in the H-R diagram. Only one star of luminosity-class Vz, expected
to best characterize extreme youth, is located on the ZAMS, the other two stars
are more evolved. The distribution of nitrogen abundance of O and B stars
suggests that the mechanisms responsible for the chemical enrichment of slowly
rotating massive stars depends only weakly on the star's mass. We confirm that
the group of slowly rotating N-rich stars is not reproduced by the evolutionary
tracks. Our results call for stronger mixing in the models to explain the range
of observed N abundances. All stars have an N/C ratio as a function of stellar
luminosity that matches the predictions of the stellar evolution models well.
More massive stars have a higher N/C ratio than the less massive stars. Faster
rotators show on average a higher N/C ratio than slower rotators. The N/O
versus N/C ratios agree qualitatively well with those of stellar evolution
models. The only discrepant behavior is observed for the youngest two stars of
the sample, which both show very strong signs of mixing, which is unexpected
for their evolutionary status.Comment: Accepted for publication in A&A (43 pages, 60 figures
Clumping and X-Rays in cooler B supergiant stars
B supergiants (BSGs) are evolved stars with effective temperatures between 10
to 30 kK and are important to understand massive star evolution. Located on the
edge of the line-driven wind regime, the study of their atmospheres is helpful
to understand phenomena such as the bi-stability jump. Key UV features of their
spectra have so far not been reproduced by models for types later than B1.
Here, we aim to remedy this situation via spectral analysis that accounts for
wind clumping and X-rays. In addition, we investigate the evolutionary status
of our sample stars based on the obtained stellar parameters. We determined
parameters via quantitative spectroscopy using CMFGEN and PoWR codes. The
models were compared to UV and optical data of four BSGs: HD206165, HD198478,
HD53138, and HD164353. We also study the evolutionary status of our sample
using GENEC and MESA tracks. When including clumping and X-rays, we find good
agreements between synthetic and observed spectra for our sample stars. For the
first time, we reproduced key lines in the UV. For that, we require a
moderately clumped wind (f_infty > ~0.5). We also infer relative X-ray
luminosities of ~10^-7.5 to 10^-8 -- lower than the typical ratio of 10^-7.
Moreover, we find a possible mismatch between evolutionary and spectroscopic
masses, which could be related to the mass-discrepancy problem present in other
OB stars. Our results provide evidence that X-rays and clumping are needed to
describe the winds of cool BSGs. However, their winds seem less structured than
in earlier type stars. This aligns with observational X-rays and clumping
constraints as well as recent hydrodynamical simulations. The BSGs'
evolutionary status appears diverse: some objects are potentially post-red
supergiants or merger products. The wind parameters provide evidence for a
moderate mass-loss rate increase around the bi-stability jump. Abstract
abridgedComment: 27 pages, 22 figures, accepted for publication in A&
Genome of the Avirulent Human-Infective Trypanosome—Trypanosoma rangeli
Background: Trypanosoma rangeli is a hemoflagellate protozoan parasite infecting humans and other wild and domestic mammals across Central and South America. It does not cause human disease, but it can be mistaken for the etiologic agent of Chagas disease, Trypanosoma cruzi. We have sequenced the T. rangeli genome to provide new tools for elucidating the distinct and intriguing biology of this species and the key pathways related to interaction with its arthropod and mammalian hosts. Methodology/Principal Findings: The T. rangeli haploid genome is ,24 Mb in length, and is the smallest and least repetitive trypanosomatid genome sequenced thus far. This parasite genome has shorter subtelomeric sequences compared to those of T. cruzi and T. brucei; displays intraspecific karyotype variability and lacks minichromosomes. Of the predicted 7,613 protein coding sequences, functional annotations could be determined for 2,415, while 5,043 are hypothetical proteins, some with evidence of protein expression. 7,101 genes (93%) are shared with other trypanosomatids that infect humans. An ortholog of the dcl2 gene involved in the T. brucei RNAi pathway was found in T. rangeli, but the RNAi machinery is non-functional since the other genes in this pathway are pseudogenized. T. rangeli is highly susceptible to oxidative stress, a phenotype that may be explained by a smaller number of anti-oxidant defense enzymes and heatshock proteins. Conclusions/Significance: Phylogenetic comparison of nuclear and mitochondrial genes indicates that T. rangeli and T. cruzi are equidistant from T. brucei. In addition to revealing new aspects of trypanosome co-evolution within the vertebrate and invertebrate hosts, comparative genomic analysis with pathogenic trypanosomatids provides valuable new information that can be further explored with the aim of developing better diagnostic tools and/or therapeutic targets
Mid-infrared observations of O-type stars: spectral morphology
16 pages, 9 figures, accepted for publication in MNRASInternational audienceWe present mid-infrared observations for a sample of 16 O-type stars. The data were acquired with the NASA Spitzer Space Telescope, using the IRS instrument at moderate resolution (R 600), covering the range microns. Our sample includes early, mid and late O supergiants and dwarfs. We explore for the first time their mid-IR spectral morphology in a quantitative way. We use NLTE expanding atmosphere models to help with line identifications, analyze profile contributions and line-formation regions. The O supergiants present a rich emission line spectra. The most intense features are from hydrogen - , 7, and - which have non-negligible contributions of HeI or HeII lines, depending on the spectral type. The spectrum of early O supergiants is a composite of HI and HeII lines, HeI lines being absent. On the other hand, late O supergiants present features composed mainly by HI and HeI lines. All emission lines are formed throughout the stellar wind. We found that O dwarfs exhibit a featureless mid-IR spectrum. Two stars of our sample exhibit very similar mid-IR features, despite having a very different optical spectral classification. The analysis of O-type stars based on mid-IR spectra alone to infer spectral classes or to estimate physical parameters may thus be prone to substantial errors. Our results may therefore inform spectroscopic observations of massive stars located in heavily obscured regions and help establish an initial framework for observations of massive stars using the Mid-Infrared Instrument on the James Webb Space Telescope
CRISPR-based genomic tools for the manipulation of genetically intractable microorganisms
Genetic manipulation of microorganisms has been crucial in understanding their biology, yet for many microbial species, robust tools for comprehensive genetic analysis were lacking until the advent of CRISPR–Cas-based gene editing techniques. In this Progress article, we discuss advances in CRISPR-based techniques for the genetic analysis of genetically intractable microorganisms, with an emphasis on mycobacteria, fungi and parasites. We discuss how CRISPR-based analyses in these organisms have enabled the discovery of novel gene functions, the investigation of genetic interaction networks and the identification of virulence factors