ABSTRACT
Evolved Wolf–Rayet stars form a key aspect of massive star evolution, and their strong outflows determine their final fates. In this study, we calculate grids of stellar models for a wide ...range of initial masses at five metallicities (ranging from solar down to just 2 per cent solar). We compare a recent hydrodynamically consistent wind prescription with two earlier frequently used wind recipes in stellar evolution and population synthesis modelling, and we present the ranges of maximum final masses at core He-exhaustion for each wind prescription and metallicity Z. Our model grids reveal qualitative differences in mass-loss behaviour of the wind prescriptions in terms of ‘convergence’. Using the prescription from Nugis & Lamers the maximum stellar black hole is found to converge to a value of 20–30 M⊙, independent of host metallicity; however, when utilizing the new physically motivated prescription from Sander & Vink there is no convergence to a maximum black hole mass value. The final mass is simply larger for larger initial He-star mass, which implies that the upper black hole limit for He-stars below the pair-instability gap is set by prior evolution with mass loss, or the pair instability itself. Quantitatively, we find the critical Z for pair-instability (ZPI) to be as high as 50 per cent Z⊙, corresponding to the host metallicity of the Large Magellanic Cloud. Moreover, while the Nugis & Lamers prescription would not predict any black holes above the approx 130 M⊙ pair-instability limit, with Sander & Vink winds included, we demonstrate a potential channel for very massive helium stars to form such massive black holes at ∼2 per cent Z⊙ or below.
ABSTRACT
Mass-loss rates and terminal wind velocities are key parameters that determine the kinetic wind energy and momenta of massive stars. Furthermore, accurate mass-loss rates determine the mass ...and rotational velocity evolution of mass stars, and their fates as neutron stars and black holes in function of metallicity (Z). Here, we update our Monte Carlo mass-loss Recipe with new dynamically consistent computations of the terminal wind velocity – as a function of Z. These predictions are particularly timely as the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) project will observe ultraviolet spectra with blue-shifted P Cygni lines of hundreds of massive stars in the low-Z Large and Small Magellanic Clouds (SMC), as well as sub-SMC metallicity hosts. Around 35 000 K, we uncover a weak-wind ‘dip’ and we present diagnostics to investigate its physics with ULLYSES and X-Shooter data. We discuss how the dip may provide important information on wind-driving physics, and how this is of key relevance towards finding a new gold-standard for OB star mass-loss rates. For B supergiants below the Fe iv to iii bi-stability jump, the terminal velocity is found to be independent of Z and M, while the mass-loss rate still varies as $\dot{M} \propto Z^{0.85}$. For O-type stars above the bi-stability jump we, find a terminal-velocity dependence of $\mbox{$v _{\infty }$}\propto Z^{0.19}$ and the Z-dependence of the mass-loss rate is found to be as shallow as $\dot{M} \propto Z^{0.42}$, implying that to reproduce the ‘heavy’ black holes from LIGO/Virgo, the ‘low Z’ requirement becomes even more stringent than was previously anticipated.
Context.
Classical Wolf-Rayet (WR) stars are massive, hydrogen-depleted, post main-sequence stars that exhibit emission-line dominated spectra. For a given metallicity
Z
, stars exceeding a certain ...initial mass
M
single
WR
(Z) can reach the WR phase through intrinsic mass-loss or eruptions (single-star channel). In principle, stars of lower masses can reach the WR phase via stripping through binary interactions (binary channel). Because winds become weaker at low
Z
, it is commonly assumed that the binary channel dominates the formation of WR stars in environments with low metallicity such as the Small and Large Magellanic Clouds (SMC, LMC). However, the reported WR binary fractions of 30−40% in the SMC (
Z
= 0.002) and LMC (
Z
= 0.006) are comparable to that of the Galaxy (
Z
= 0.014), and no evidence for the dominance of the binary channel at low
Z
could be identified observationally. Here, we explain this apparent contradiction by considering the minimum initial mass
M
spec
WR
(Z) needed for the stripped product to appear as a WR star.
Aims.
By constraining
M
spec
WR
(Z) and
M
single
WR
(Z), we estimate the importance of binaries in forming WR stars as a function of
Z
.
Methods.
We calibrated
M
spec
WR
using the lowest-luminosity WR stars in the Magellanic Clouds and the Galaxy. A range of
M
single
WR
values were explored using various evolution codes. We estimated the additional contribution of the binary channel by considering the interval
M
spec
WR
(Z),
M
single
WR
(Z), which characterizes the initial-mass range in which the binary channel can form additional WR stars.
Results.
The WR-phenomenon ceases below luminosities of log
L
≈ 4.9, 5.25, and 5.6
L
⊙
in the Galaxy, the LMC, and the SMC, respectively, which translates to minimum He-star masses of 7.5, 11, 17
M
⊙
and minimum initial masses of
M
spec
WR
= 18, 23, 37
M
⊙
. Stripped stars with lower initial masses in the respective galaxies would tend not to appear as WR stars. The minimum mass necessary for self-stripping,
M
single
WR
(Z), is strongly model-dependent, but it lies in the range 20−30, 30−60, and ≳40
M
⊙
for the Galaxy, LMC, and SMC, respectively. We find that that the additional contribution of the binary channel is a non-trivial and model-dependent function of
Z
that cannot be conclusively claimed to be monotonically increasing with decreasing
Z
.
Conclusions.
The WR spectral appearance arises from the presence of strong winds. Therefore, both
M
spec
WR
and
M
single
WR
increase with decreasing metallicity. Considering this, we show that one should not a-priori expect that binary interactions become increasingly important in forming WR stars at low
Z
, or that the WR binary fraction grows with decreasing
Z
.
Exosome mimetics: a novel class of drug delivery systems Kooijmans, Sander A A; Vader, Pieter; van Dommelen, Susan M ...
International Journal of Nanomedicine,
01/2012, Letnik:
7, Številka:
default
Journal Article, Book Review
Recenzirano
Odprti dostop
The identification of extracellular phospholipid vesicles as conveyors of cellular information has created excitement in the field of drug delivery. Biological therapeutics, including short ...interfering RNA and recombinant proteins, are prone to degradation, have limited ability to cross biological membranes, and may elicit immune responses. Therefore, delivery systems for such drugs are under intensive investigation. Exploiting extracellular vesicles as carriers for biological therapeutics is a promising strategy to overcome these issues and to achieve efficient delivery to the cytosol of target cells. Exosomes are a well studied class of extracellular vesicles known to carry proteins and nucleic acids, making them especially suitable for such strategies. However, the considerable complexity and the related high chance of off-target effects of these carriers are major barriers for translation to the clinic. Given that it is well possible that not all components of exosomes are required for their proper functioning, an alternative strategy would be to mimic these vesicles synthetically. By assembly of liposomes harboring only crucial components of natural exosomes, functional exosome mimetics may be created. The low complexity and use of well characterized components strongly increase the pharmaceutical acceptability of such systems. However, exosomal components that would be required for the assembly of functional exosome mimetics remain to be identified. This review provides insights into the composition and functional properties of exosomes, and focuses on components which could be used to enhance the drug delivery properties of exosome mimetics.
Atmospheric correction over inland and coastal waters is one of the major remaining challenges in aquatic remote sensing, often hindering the quantitative retrieval of biogeochemical variables and ...analysis of their spatial and temporal variability within aquatic environments. The Atmospheric Correction Intercomparison Exercise (ACIX-Aqua), a joint NASA – ESA activity, was initiated to enable a thorough evaluation of eight state-of-the-art atmospheric correction (AC) processors available for Landsat-8 and Sentinel-2 data processing. Over 1000 radiometric matchups from both freshwaters (rivers, lakes, reservoirs) and coastal waters were utilized to examine the quality of derived aquatic reflectances (ρ̂w). This dataset originated from two sources: Data gathered from the international scientific community (henceforth called Community Validation Database, CVD), which captured predominantly inland water observations, and the Ocean Color component of AERONET measurements (AERONET-OC), representing primarily coastal ocean environments. This volume of data permitted the evaluation of the AC processors individually (using all the matchups) and comparatively (across seven different Optical Water Types, OWTs) using common matchups. We found that the performance of the AC processors differed for CVD and AERONET-OC matchups, likely reflecting inherent variability in aquatic and atmospheric properties between the two datasets. For the former, the median errors in ρ̂w560 and ρ̂w664 were found to range from 20 to 30% for best-performing processors. Using the AERONET-OC matchups, our performance assessments showed that median errors within the 15–30% range in these spectral bands may be achieved. The largest uncertainties were associated with the blue bands (25 to 60%) for best-performing processors considering both CVD and AERONET-OC assessments. We further assessed uncertainty propagation to the downstream products such as near-surface concentration of chlorophyll-a (Chla) and Total Suspended Solids (TSS). Using satellite matchups from the CVD along with in situ Chla and TSS, we found that 20–30% uncertainties in ρ̂w490≤λ≤743nm yielded 25–70% uncertainties in derived Chla and TSS products for top-performing AC processors. We summarize our results using performance matrices guiding the satellite user community through the OWT-specific relative performance of AC processors. Our analysis stresses the need for better representation of aerosols, particularly absorbing ones, and improvements in corrections for sky- (or sun-) glint and adjacency effects, in order to achieve higher quality downstream products in freshwater and coastal ecosystems.
•Performances of eight state-of-the-art methods were examined.•Best-performers per optical water types were identified.•Best-performer's uncertainties across the visible bands are not uniformly <30%.•Performances are best in the 560 nm band and worst in the 443 nm band.•Impacts of uncertainties in atmospheric correction on Chla and TSS were assessed.
The study of massive stars in different metallicity environments is a central topic of current stellar research. The spectral analysis of massive stars requires adequate model atmospheres. The ...computation of such models is difficult and time-consuming. Therefore, spectral analyses are greatly facilitated if they can refer to existing grids of models. Here we provide grids of model atmospheres for OB-type stars at metallicities corresponding to the Small and Large Magellanic Clouds, as well as to solar metallicity. In total, the grids comprise 785 individual models. The models were calculated using the state-of-the-art Potsdam Wolf-Rayet (PoWR) model atmosphere code. The parameter domain of the grids was set up using stellar evolution tracks. For all these models, we provide normalized and flux-calibrated spectra, spectral energy distributions, feedback parameters such as ionizing photons, Zanstra temperatures, and photometric magnitudes. The atmospheric structures (the density and temperature stratification) are available as well. All these data are publicly accessible through the PoWR website.
Extracellular vesicles (EVs) are increasingly being recognized as candidate drug delivery systems due to their ability to functionally transfer biological cargo between cells. However, manipulation ...of targeting properties of EVs through engineering of the producer cells can be challenging and time-consuming. As a novel approach to confer tumor targeting properties to isolated EVs, we generated recombinant fusion proteins of nanobodies against the epidermal growth factor receptor (EGFR) fused to phosphatidylserine (PS)-binding domains of lactadherin (C1C2). C1C2-nanobody fusion proteins were expressed in HEK293 cells and isolated from culture medium with near-complete purity as determined by SDS-PAGE. Fusion proteins specifically bound PS and showed no affinity for other common EV membrane lipids. Furthermore, C1C2 fused to anti-EGFR nanobodies (EGa1-C1C2) bound EGFR with high affinity and competed with binding of its natural ligand EGF, as opposed to C1C2 fused to non-targeting control nanobodies (R2-C1C2). Both proteins readily self-associated onto membranes of EVs derived from erythrocytes and Neuro2A cells without affecting EV size and integrity. EV-bound R2-C1C2 did not influence EV-cell interactions, whereas EV-bound EGa1-C1C2 dose-dependently enhanced specific binding and uptake of EVs by EGFR-overexpressing tumor cells. In conclusion, we developed a novel strategy to efficiently and universally confer tumor targeting properties to PS-exposing EVs after their isolation, without affecting EV characteristics, circumventing the need to modify EV-secreting cells. This strategy may also be employed to decorate EVs with other moieties, including imaging probes or therapeutic proteins.
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