The progenitors of Type-IIb supernovae (SNe IIb) are believed to have lost their H-rich envelopes almost completely in the direct pre-SN phase. Recently the first ‘flash spectrum’ of an SN IIb ...(SN 2013cu) has been presented, taken early enough to study its immediate circumstellar medium (CSM). Similar to a previous study by Groh, we analyse the structure and chemical composition of the optically thick CSM using non-local thermodynamic equilibrium(non-LTE) model atmospheres. For the first time, we take light-travel time effects on the spectrum formation into account, which affect the shapes and strengths of the observable emission lines, as well as the inferred SN luminosity. Based on the new CSM parameters, we estimate a lower limit of ∼0.3 M⊙ for the CSM mass, which is a factor 10–100 higher than previous estimates. The spectral fit implies a CSM in the form of a homogeneous and spherically symmetric superwind whose mass-loss rate exceeds common expectations by up to two orders of magnitude. The derived chemical composition is in agreement with a progenitor that has just left, or is just about to leave the Red-Supergiant stage, confirming the standard picture for the origin of SNe IIb. Due to its extreme mass-loss, the SN progenitor will likely appear as extreme RSG, Luminous Blue Variable, or Yellow Hypergiant. The direct detection of a superwind, and the high inferred CSM mass suggest that stellar wind mass-loss may play an important role in the formation of SNe IIb.
It is thought that Type Ia supernovae (SNe Ia) are explosions of carbon-oxygen white dwarfs (CO WDs). Two main evolutionary channels are proposed for the WD to reach the critical density required for ...a thermonuclear explosion: the single degenerate (SD) scenario, in which a CO WD accretes from a non-degenerate companion, and the double degenerate (DD) scenario, in which two CO WDs merge. However, it remains difficult to reproduce the observed SN Ia rate with these two scenarios. With a binary population synthesis code we study the main evolutionary channels that lead to SNe Ia and we calculate the SN Ia rates and the associated delay-time distributions. We find that the DD channel is the dominant formation channel for the longest delay times. The SD channel with helium-rich donors is the dominant channel at the shortest delay times. Our standard model rate is a factor of five lower than the observed rate in galaxy clusters. We investigate the influence of ill-constrained aspects of single- and binary-star evolution and uncertain initial binary distributions on the rate of Type Ia SNe. These distributions, as well as uncertainties in both helium star evolution and common envelope evolution, have the greatest influence on our calculated rates. Inefficient common envelope evolution increases the relative number of SD explosions such that for αce = 0.2 they dominate the SN Ia rate. Our highest rate is a factor of three less than the galaxy-cluster SN Ia rate, but compatible with the rate determined in a field-galaxy dominated sample. If we assume unlimited accretion onto WDs, to maximize the number of SD explosions, our rate is compatible with the observed galaxy-cluster rate.
We present a 3D map of extinction in the northern Galactic plane derived using photometry from the INT/WFC Photometric Hα Survey of the northern Galactic plane. The map has fine angular ( ∼ 10 ...arcmin) and distance (100 pc) sampling allied to a significant depth (≳5 kpc). We construct the map using a method based on a hierarchical Bayesian model described in a previous article by Sale. In addition to mean extinction, we also measure differential extinction, which arises from the fractal nature of the interstellar medium, and show that it will be the dominant source of uncertainty in estimates of extinction to some arbitrary position. The method applied also furnishes us with photometric estimates of the distance, extinction, effective temperature, surface gravity, and mass for ∼38 million stars. Both the extinction map and the catalogue of stellar parameters are made publicly available via http://www.iphas.org/extinction.
The 30 Doradus star-forming region in the Large Magellanic Cloud is a nearby analog of large star-formation events in the distant universe. We determined the recent formation history and the initial ...mass function (IMF) of massive stars in 30 Doradus on the basis of spectroscopic observations of 247 stars more massive than 15 solar masses (Formula: see text). The main episode of massive star formation began about 8 million years (My) ago, and the star-formation rate seems to have declined in the last 1 My. The IMF is densely sampled up to 200 Formula: see text and contains 32 ± 12% more stars above 30 Formula: see text than predicted by a standard Salpeter IMF. In the mass range of 15 to 200 Formula: see text, the IMF power-law exponent is Formula: see text, shallower than the Salpeter value of 2.35.
Abstract
Classical Wolf-Rayet (WR) stars are at a crucial evolutionary stage for constraining the fates of massive stars. The feedback of these hot, hydrogen-depleted stars dominates their ...surrounding by tremendous injections of ionizing radiation and kinetic energy. The strength of a WR wind decides the eventual mass of its remnant, likely a massive black hole. However, despite their major influence and importance for gravitational wave detection statistics, WR winds are particularly poorly understood. In this paper, we introduce the first set of hydrodynamically consistent stellar atmosphere models for classical WR stars of both the carbon (C) and nitrogen (N) sequence, i.e. WC and WN stars, as a function of stellar luminosity-to-mass ratio (or Eddington Gamma), and metallicity. We demonstrate the inapplicability of the CAK wind theory for classical WR stars and confirm earlier findings that their winds are launched at the (hot) iron (Fe) opacity peak. For log Z/Z⊙ > −2, Fe is also the main accelerator throughout the wind. Contrasting previous claims of a sharp lower mass-loss limit for WR stars, we obtain a smooth transition to optically thin winds. Furthermore, we find a strong dependence of the mass-loss rates on Eddington Γ, both at solar and sub-solar metallicity. Increases in WC carbon and oxygen abundances turn out to slightly reduce the predicted mass-loss rates. Calculations at subsolar metallicities indicate that below the metallicity of the SMC, WR mass-loss rates decrease much faster than previously assumed, potentially allowing for high black hole masses even in the local universe.
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.
Context. The “mass discrepancy” in massive O stars represents a long-standing problem in stellar astrophysics with far-reaching implications for the chemical and dynamical feedback in galaxies. Aims. ...Our goal is to investigate this mass discrepancy by comparing state-of-the-art model masses with model-independent masses determined from eclipsing binaries. Methods. Using stellar evolution models and a recent calibration of stellar parameters for O-star spectral sub-classes, we present a convenient way to convert observed solar metallicity O star spectral types into model masses, which we subsequently compare to our dynamical mass compilation. We also derive similar conversions for Large and Small Magellanic Cloud metallicities. Results. We obtain a good agreement between model and dynamical masses, suggesting the long-standing problem of a systematic mass discrepancy problem may have been solved. We also provide error ranges for the model masses, as well as minimal and maximal age estimates for when the model stars are in a given spectral type box.
Interactions between proteins frequently involve recognition sequences based on multivalent binding events. Dimeric 14‐3‐3 adapter proteins are a prominent example and typically bind partner proteins ...in a phosphorylation‐dependent mono‐ or bivalent manner. Herein we describe the development of a cucurbit8uril (Q8)‐based supramolecular system, which in conjunction with the 14‐3‐3 protein dimer acts as a binary and bivalent protein assembly platform. We fused the phenylalanine–glycine–glycine (FGG) tripeptide motif to the N‐terminus of the 14‐3‐3‐binding epitope of the estrogen receptor α (ERα) for selective binding to Q8. Q8‐induced dimerization of the ERα epitope augmented its affinity towards 14‐3‐3 through a binary bivalent binding mode. The crystal structure of the Q8‐induced ternary complex revealed molecular insight into the multiple supramolecular interactions between the protein, the peptide, and Q8.
Binary bivalent binding: The combination of a bivalent cucurbit8uril (Q8) host–guest complex with the bivalent protein 14‐3‐3 generated a binary assembly platform. Supramolecular induced switching between mono‐ and bivalent protein modes was observed and the elucidation of the first Q8‐protein cocrystal structure reported.
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
.
The cosmic ray electron spectrum exhibits a break at a particle energy of ∼1 TeV and extends without any attenuation up to ∼20 TeV. Synchrotron and inverse Compton energy losses strongly constrain ...the time of emission of ∼20 TeV electrons to ≈2×104 yr and the distance of the potential source(s) to ≈100–500 pc, depending on the cosmic ray diffusion coefficient. This suggests that maybe one nearby discrete source may explain the observed spectrum of high energy electrons. Given the strong energy dependence (∝1/E) of the cooling time of TeV electrons, the spectral shape of the electron spectrum above the ∼1 TeV break strongly depends on the history of injection of these electrons from the source. In this paper we show that a local, continuous (on timescales of ∼105 yr) but fading electron accelerator, with a characteristic decay time of ∼104 yr, can naturally account for the entire spectrum of cosmic ray electrons in the TeV domain. Although the standard "nearby pulsar" scenario naturally meets this time condition, it is (almost) excluded by recent measurements of the positron fraction, which above ∼100 GeV saturates at a level well below 0.5 and drops above ∼400–500 GeV. The second potential source population, the supernova remnants, accelerate mostly electrons, rather than positrons. However, they hardly can provide an effective production of multi-TeV electrons via the standard diffusive shock acceleration scenario for ∼105 yr. A third possibility are stellar wind shocks, which however are likely to be continuous with nearly constant luminosity on timescales ≫10 kyr and probably cannot match the time requirement of our potential source. Therefore, we face a real challenge in the identification of the origin of the source of multi-TeV electrons. Thus, the link of this source with known particle accelerators would require a dramatic revision of the standard paradigms of acceleration and escape in such objects.
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