Context
. Clumping in the radiation-driven winds of hot, massive stars severly affects the derivation of synthetic observables across the electromagnetic spectrum.
Aims
. We implement a formalism for ...treating wind clumping – focusing in particular on the light-leakage effects associated with a medium that is porous in physical and velocity space – into the global (photosphere + wind) NLTE model atmosphere and spectrum synthesis code
FASTWIND
.
Methods
. The basic method presented here assumes a stochastic, two-component wind consisting of a mixture of optically thick and thin clumps embedded in a rarefied inter-clump medium. We have accounted fully for the reductions in opacity associated with porosity in physical and velocity-space (the latter due to Doppler shifts in an accelerating medium), as well as for the well-known effect that opacities depending on ⟨
ρ
2
⟩ are higher in clumpy winds than in smooth ones of equal mass-loss rate. By formulating our method in terms of suitable mean and effective opacities for the clumpy wind, we are able to compute atmospheric models with the same speed (∼15 min on a modern laptop or desktop) as in previous generations of
FASTWIND
.
Results
. After verifying important analytic limits (smooth, optically thin, completely optically thick), we present some first, generic results of the new models. These include:
i)
Confirming earlier results that velocity-space porosity is critical for analysis of UV wind resonance lines in O-stars;
ii)
for the optical H
α
line, we show that optically thick clumping effects are small for O-stars, but potentially very important for late B and A-supergiants;
iii)
in agreement with previous work, we show that spatial porosity is a marginal effect for absorption of high-energy X-rays in O-stars, as long as the mean-free path between clumps are kept at realistic values ≲
R
*
;
iv)
whereas radio absorption in O-stars shows strong spatial porosity effects in near photospheric layers, it is negligible at their typical radio-photosphere radii ∼100
R
*
;
v)
regarding the wind ionization balance, a general trend is that increased rates of recombination in simulations with optically thin clumps lead to overall lower degrees of ionization than in corresponding smooth models, but that this effect now is counteracted by the increased levels of light-leakage associated with porosity in physical and velocity space (i.e., by an increase of ionization rates). We conclude by discussing future work and some planned applications for this new generation of
FASTWIND
models.
We present new or improved methods for calculating NLTE, line-blanketed model atmospheres for hot stars with winds (spectral types A to O), with particular emphasis on fast performance. These methods ...have been implemented into a previous, more simple version of the model atmosphere code FASTWIND and allow us to spectroscopically analyze large samples of massive stars in a reasonable time-scale, using state-of-the-art physics. Although this updated version of the code has already been used in a number of recent investigations, the corresponding methods have not been explained in detail so far, and no rigorous comparison with results from alternative codes has been performed. This paper intends to address both topics. In particular, we describe our (partly approximate) approach to solve the equations of statistical equilibrium for those elements that are primarily responsible for line-blocking and blanketing, as well as an approximate treatment of the line-blocking itself, which is based on a simple statistical approach using suitable means of line opacities and emissivities. Both methods are validated by specific tests. Furthermore, we comment on our implementation of a consistent temperature structure. In the second part, we concentrate on a detailed comparison with results from two codes used in alternative spectroscopical investigations, namely CMFGEN and WM-Basic. All three codes predict almost identical temperature structures and fluxes for lambda > 400 Ae, whereas at lower wavelengths a number of discrepancies are found. Particularly in the HeII continua, where fluxes and corresponding numbers of ionizing photons react extremely sensitively to subtle differences in the models, we consider any uncritical use of these quantities (e.g., in the context of nebula diagnostics) as unreliable. Optical H/He lines as synthesized by FASTWIND are compared with results from CMFGEN, obtaining a remarkable coincidence, except for the HeI singlets in the temperature range between 36 000 to 41 000 K for dwarfs and between 31 000 to 35 000 K for supergiants, where CMFGEN predicts much weaker lines. Consequences of these discrepancies are discussed. Finally, suggestions are presented as to adequately parameterize model-grids for hot stars with winds, with only one additional parameter compared to standard grids from plane-parallel, hydrostatic models.
Electronic coupling between Wannier and Frenkel excitons in an inorganic/organic semiconductor hybrid structure is experimentally observed. Time-resolved photoluminescence and excitation spectroscopy ...directly demonstrate that electronic excitation energy can be transferred with an efficiency of up to 50% from an inorganic ZnO quantum well to an organic 2,2-p-phenylenebis-(5-phenyloxazol), alpha-sexithiophene overlayer. The coupling is mediated via dipole-dipole-interaction analog to the Förster transfer in donor-acceptor systems.
We have studied the optical spectra of a sample of 31 O-and early B-type stars in the Small Magellanic Cloud, 21 of which are associated with the young massive cluster NGC 346. Stellar parameters are ...determined using an automated fitting method (Mokiem et al. 2005, A&A, 441, 711), which combines the stellar atmosphere code FASTWIND (Puls et al. 2005, A&A, 435, 669) with the genetic algorithm based optimisation routine PIKAIA (Charbonneau 1995, ApJS, 101, 309). Comparison with predictions of stellar evolution that account for stellar rotation does not result in a unique age, though most stars are best represented by an age of 1-3 Myr. The automated method allows for a detailed determination of the projected rotational velocities. The present day v_{\rm r}\sin i distribution of the 21 dwarf stars in our sample is consistent with an underlying rotational velocity ( v sub( )r distribution that can be characterised by a mean velocity of about 160-190 km s super(-1) and an effective half width of 100-150 km s super(-1). The v sub( )rdistribution must include a small percentage of slowly rotating stars. If predictions of the time evolution of the equatorial velocity for massive stars within the environment of the SMC are correct (Maeder & Meynet 2001, A&A, 373, 555), the young age of the cluster implies that this underlying distribution is representative for the initial rotational velocity distribution. The location in the Hertzsprung-Russell diagram of the stars showing helium enrichment is in qualitative agreement with evolutionary tracks accounting for rotation, but not for those ignoring v sub( )r The mass loss rates of the SMC objects having luminosities of \log \mbox{$L_{\star}$ }/\mbox{$L_{\odot}$ } \gtrsim 5.4 are in excellent agreement with predictions by Vink et al. (2001, A&A, 369, 574). However, for lower luminosity stars the winds are too weak to determine \dot{M} accurately from the optical spectrum. Three targets were classifiedas Vz stars, two of which are located close to the theoretical zero-age main sequence. Three lower luminosity targets that were not classified as Vz stars are also found to lie near the ZAMS. We argue that this is related to a temperature effect inhibiting cooler from displaying the spectral features required for the Vz luminosity class.
ABSTRACT
2MASS J20395358+4222505 is an obscured early B supergiant near the massive OB star association Cygnus OB2. Despite its bright infrared magnitude (Ks = 5.82) it has remained largely ignored ...because of its dim optical magnitude (B = 16.63, V = 13.68). In a previous paper, we classified it as a highly reddened, potentially extremely luminous, early B-type supergiant. We obtained its spectrum in the U, B and R spectral bands during commissioning observations with the instrument MEGARA at the Gran Telescopio CANARIAS. It displays a particularly strong Hα emission for its spectral type, B1 Ia. The star seems to be in an intermediate phase between supergiant and hypergiant, a group that it will probably join in the near (astronomical) future. We observe a radial velocity difference between individual observations and determine the stellar parameters, obtaining Teff = 24 000 K and log gc = 2.88 ± 0.15. The rotational velocity found is large for a B supergiant, $v$ sin i = 110 ± 25 $\rm km\, s^{-1}$. The abundance pattern is consistent with solar, with a mild C underabundance (based on a single line). Assuming that J20395358+4222505 is at the distance of Cyg OB2, we derive the radius from infrared photometry, finding R = 41.2 ± 4.0 R⊙, log(L/L⊙) = 5.71 ± 0.04 and a spectroscopic mass of 46.5 ± 15.0 M⊙. The clumped mass-loss rate (clumping factor 10) is very high for the spectral type, $\dot{M}$ = 2.4 × 10−6 M⊙ a−1. The high rotational velocity and mass-loss rate place the star at the hot side of the bi-stability jump. Together with the nearly solar CNO abundance pattern, they may also point to evolution in a binary system, J20395358+4222505 being the initial secondary.
We report on our efforts to cultivate the ternary compound ZnCdO as a semiconductor laser material. Molecular beam epitaxy far from thermal equilibrium allows us to overcome the standard solubility ...limit and to fabricate alloys with band gaps ranging from 3.4 down to 2.1 eV. Optimized structures containing well‐defined quantum wells as active zones are capable of low‐threshold lasing under optical pumping up to room temperature. The longest lasing wavelength achieved so far is 510 nm.
We report on our efforts to cultivate the ternary compound ZnCdO as a semiconductor laser material. Molecular beam epitaxy far from thermal equilibrium allows us to overcome the standard solubility limit and to fabricate alloys with band gaps ranging from 3.4 down to 2.1 eV. Optimized structures containing well‐defined quantum wells as active zones are capable of low‐threshold lasing under optical pumping up to room temperature. The longest lasing wavelength achieved so far is 510 nm.
The rotation rates of massive stars Sundqvist, J O; Simon-Diaz, S; Puls, J ...
Astronomy and astrophysics (Berlin),
11/2013, Volume:
559
Journal Article
Peer reviewed
Open access
Context. Rotation plays a key role in the life cycles of stars with masses above ~8 M sub(middot in circle). Hence, accurate knowledge of the rotation rates of such massive stars is critical for ...understanding their properties and for constraining models of their evolution. Aims. This paper investigates the reliability of current methods used to derive projected rotation speeds v sin i from line-broadening signatures in the photospheric spectra of massive stars, focusing on stars that are not rapidly rotating. Methods. We use slowly rotating magnetic O- stars with well-determined rotation periods to test the Fourier transform (FT) and goodness-of-fit (GOF) methods typically used to infer projected rotation rates of massive stars. Results. For our two magnetic test stars with measured rotation periods longer than one year, i.e., with v sin i <, ~ 1 kms super(-1), we derive v sin i approx = 40-50 kms super(-1)from both the FT and GOF methods. These severe overestimates are most likely caused by an insufficient treatment of the competing broadening mechanisms referred to as microturbulence and macroturbulence. Conclusions. These findings warn us not to rely uncritically on results from current standard techniques to derive projected rotation speeds of massive stars in the presence of significant additional line broadening, at least when v sin i <, ~ 50kms super(-1). This may, for example, be crucial for i) determining the statistical distribution of observed rotation rates of massive stars; ii) interpreting the evolutionary status and spin-down histories of rotationally braked B-supergiants; and iii) explaining the deficiency of observed O-stars with spectroscopically inferred v sin i approx = 0kms super(-1). Further investigations of potential shortcomings of the above techniques are presently under way.
The Gram-negative bacterium Helicobacter pylori is a causative agent of gastritis and peptic ulcer disease in humans. Strains producing the CagA antigen (cagA+) induce strong gastric inflammation and ...are strongly associated with gastric adenocarcinoma and MALT lymphoma. We show here that such strains translocate the bacterial protein CagA into gastric epithelial cells by a type IV secretion system, encoded by the cag pathogenicity island. CagA is tyrosine-phosphorylated and induces changes in the tyrosine phosphorylation state of distinct cellular proteins. Modulation of host cells by bacterial protein translocation adds a new dimension to the chronic Helicobacter infection with yet unknown consequences.
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