556 research outputs found
Non-thermal radio emission from O-type stars. V. 9 Sgr
The colliding winds in a massive binary system generate synchrotron emission
due to a fraction of electrons that have been accelerated to relativistic
speeds around the shocks in the colliding-wind region. We studied the radio
light curve of 9 Sgr = HD 164794, a massive O-type binary with a 9.1-yr period.
We investigated whether the radio emission varies consistently with orbital
phase and we determined some parameters of the colliding-wind region. We
reduced a large set of archive data from the Very Large Array (VLA) to
determine the radio light curve of 9 Sgr at 2, 3.6, 6 and 20 cm. We also
constructed a simple model that solves the radiative transfer in the
colliding-wind region and both stellar winds. The 2-cm radio flux shows clear
phase-locked variability with the orbit. The behaviour at other wavelengths is
less clear, mainly due to a lack of observations centred on 9 Sgr around
periastron passage. The high fluxes and nearly flat spectral shape of the radio
emission show that synchrotron radiation dominates the radio light curve at all
orbital phases. The model provides a good fit to the 2-cm observations,
allowing us to estimate that the brightness temperature of the synchrotron
radiation emitted in the colliding-wind region at 2 cm is at least 4 x 10^8 K.
The simple model used here already allows us to derive important information
about the colliding-wind region. We propose that 9 Sgr is a good candidate for
more detailed modelling, as the colliding-wind region remains adiabatic during
the whole orbit thus simplifying the hydrodynamics.Comment: 10 pages, 3 figures, accepted for publication in A&
Massive non-thermal radio emitters: new data and their modelling
During recent years some non-thermal radio emitting OB stars have been
discovered to be binary, or multiple systems. The non-thermal emission is due
to synchrotron radiation that is emitted by electrons accelerated up to high
energies. The electron acceleration occurs at the strong shocks created by the
collision of radiatively-driven winds. Here we summarize the available radio
data and more recent observations for the binary Cyg OB2 No. 9. We also show a
new emission model which is being developed to compare the theoretical total
radio flux and the spectral index with the observed radio light curves. This
comparison will be useful in order to solve fundamental questions, such as the
determination of the stellar mass loss rates, which are perturbed by clumping.Comment: 3 pages, 1 figure, poster at Four Decades of Research on Massive
Stars-A Scientific Meeting in Honour of Anthony F.J.Moffa
ALMA observations of the supergiant B[e] star Wd1-9
Mass-loss in massive stars plays a critical role in their evolution, although the precise mechanism(s) responsible – radiatively driven winds, impulsive ejection and/or binary interaction – remain uncertain. In this Letter, we present Atacama Large Millimetre/Submillimeter Array line and continuum observations of the supergiant B[e] star Wd1-9, a massive post-main-sequence object located within the starburst cluster Westerlund 1 (Wd1). We find it to be one of the brightest stellar point sources in the sky at millimetre wavelengths, with (serendipitously identified) emission in the H41α radio recombination line. We attribute these properties to a low velocity (∼100 km s-1 ) ionized wind, with an extreme mass-loss rate ≳6.4 × 105(d/5 kpc)1.5 Mȯyr-1. External to this is an extended aspherical ejection nebula indicative of a prior phase of significant mass-loss. Taken together, the millimetre properties of Wd1-9 show a remarkable similarity to those of the highly luminous stellar source MWC349A. We conclude that these objects are interacting binaries evolving away from the main sequence and undergoing rapid case-A mass transfer. As such they – and by extension the wider class of supergiant B[e] stars – may provide a unique window into the physics of a process that shapes the life-cycle of ∼70 per cent of massive stars found in binary systems
A multi-method approach to radial-velocity measurement for single-object spectra
The derivation of radial velocities from large numbers of spectra that
typically result from survey work, requires automation. However, except for the
classical cases of slowly rotating late-type spectra, existing methods of
measuring Doppler shifts require fine-tuning to avoid a loss of accuracy due to
the idiosyncrasies of individual spectra. The radial velocity spectrometer
(RVS) on the Gaia mission, which will start operating very soon, prompted a new
attempt at creating a measurement pipeline to handle a wide variety of spectral
types.
The present paper describes the theoretical background on which this software
is based. However, apart from the assumption that only synthetic templates are
used, we do not rely on any of the characteristics of this instrument, so our
results should be relevant for most telescope-detector combinations.
We propose an approach based on the simultaneous use of several alternative
measurement methods, each having its own merits and drawbacks, and conveying
the spectral information in a different way, leading to different values for
the measurement. A comparison or a combination of the various results either
leads to a "best estimate" or indicates to the user that the observed spectrum
is problematic and should be analysed manually.
We selected three methods and analysed the relationships and differences
between them from a unified point of view; with each method an appropriate
estimator for the individual random error is chosen. We also develop a
procedure for tackling the problem of template mismatch in a systematic way.
Furthermore, we propose several tests for studying and comparing the
performance of the various methods as a function of the atmospheric parameters
of the observed objects. Finally, we describe a procedure for obtaining a
knowledge-based combination of the various Doppler-shift measurements.Comment: 16 pages, 4 figure
The 2.35 year itch of Cyg OB2 #9. II. Radio monitoring
Cyg OB2 #9 is one of a small set of non-thermal radio emitting massive O-star
binaries. The non-thermal radiation is due to synchrotron emission in the
colliding-wind region. Cyg OB2 #9 was only recently discovered to be a binary
system and a multi-wavelength campaign was organized to study its 2011
periastron passage. We report here on the results of the radio observations
obtained in this monitoring campaign. We used the Expanded Very Large Array
(EVLA) radio interferometer to obtain 6 and 20 cm continuum fluxes. The
observed radio light curve shows a steep drop in flux sometime before
periastron. The fluxes drop to a level that is comparable to the expected
free-free emission from the stellar winds, suggesting that the non-thermal
emitting region is completely hidden at that time. After periastron passage,
the fluxes slowly increase. We introduce a simple model to solve the radiative
transfer in the stellar winds and the colliding-wind region, and thus determine
the expected behaviour of the radio light curve. From the asymmetry of the
light curve, we show that the primary has the stronger wind. This is somewhat
unexpected if we use the astrophysical parameters based on theoretical
calibrations. But it becomes entirely feasible if we take into account that a
given spectral type - luminosity class combination covers a range of
astrophysical parameters. The colliding-wind region also contributes to the
free-free emission, which can help to explain the high values of the spectral
index seen after periastron passage. Combining our data with older Very Large
Array (VLA) data allows us to derive a period P = 860.0 +- 3.7 days for this
system. With this period, we update the orbital parameters that were derived in
the first paper of this series.Comment: 10 pages, 4 figures, accepted for publication in A&
Banana Xanthomonas wilt in Ethiopia: Occurrence and insect vector transmission
Bacterial wilt caused by Xanthomonas vasicola pv. musacearum (Xvm) is an important disease of enset and banana in south and south-western Ethiopia where, the diversity of the insect fauna on banana inflorescences was unknown and the role of insects as vectors of the disease had not been studied. The objectives of this study wereto assess the occurrence of bacterial wilt and male bud infection, the diversity of insect families in banana plantations and the presence of the bacteria on insects collected from diseased inflorescences in south and southwestern Ethiopia. Surveys were carried out and insects were collected from three different zones in 2005. The diversity and richness of the insect families was assessed across sites and genotypes and comparisons were made using the Shannon Diversity Index and the Jack knife estimator, respectively. Correlations were made betweenthe abundance and incidence of insects with the incidence of male bud infection on ‘Pisang Awak’ plants. A wide range of insect families were recorded and they varied according to banana genotype and altitude. The Drosophilidae and Apinae families were most frequently recorded across sites and genotypes. The ‘Wendo’ variety (AAACavendish group) had the highest diversity and richness of insect families within and across sites. In contrast to the Kembata Tembaro and Bench Maji zones, severe and widespread male bud infection of banana was found in Kaffa, where there was a high diversity of insects on the ‘Pisang Awak’ and ‘Abesha muz’ plants. The incidenceof male bud infection on ‘Pisang Awak’ plants was highly correlated with the incidence of insects (R2 = 0.964). The incidence of male bud infection however depends on the floral morphology and altitude. Artificial inoculation with Xvm ooze on fresh male bract and flower scars resulted in infections on ‘Pisang Awak’ and ‘Abesha muz’ plants, but the ‘Dwarf Cavendish’ plants with persistent bracts and flowers remained healthy. Few male bud infections were observed at altitudes above 1,700 masl. Xvm was isolated from Apinae, Lonchaeidae, Muscidae,Tephritidae and Vespidae insect families. Lonchaeidae (Silba spp.) were frequently observed on banana bract and flower scars and could thus be an important insect vector of Xvm in Ethiopia
Non-thermal radio emission from O-type stars. IV. Cyg OB2 No. 8A
We study the non-thermal radio emission of the binary Cyg OB2 No. 8A, to see
if it is variable and if that variability is locked to the orbital phase. We
investigate if the synchrotron emission generated in the colliding-wind region
of this binary can explain the observations and we verify that our proposed
model is compatible with the X-ray data. We use both new and archive radio data
from the Very Large Array (VLA) to construct a light curve as a function of
orbital phase. We also present new X-ray data that allow us to improve the
X-ray light curve. We develop a numerical model for the colliding-wind region
and the synchrotron emission it generates. The model also includes free-free
absorption and emission due to the stellar winds of both stars. In this way we
construct artificial radio light curves and compare them with the observed one.
The observed radio fluxes show phase-locked variability. Our model can explain
this variability because the synchrotron emitting region is not completely
hidden by the free-free absorption. In order to obtain a better agreement for
the phases of minimum and maximum flux we need to use stellar wind parameters
for the binary components which are somewhat different from typical values for
single stars. We verify that the change in stellar parameters does not
influence the interpretation of the X-ray light curve. Our model has trouble
explaining the observed radio spectral index. This could indicate the presence
of clumping or porosity in the stellar wind, which - through its influence on
both the Razin effect and the free-free absorption - can considerably influence
the spectral index. Non-thermal radio emitters could therefore open a valuable
pathway to investigate the difficult issue of clumping in stellar winds.Comment: 19 pages, 10 figures, accepted by A&
Variable millimetre radiation from the colliding-wind binary Cygnus OB2 #8A
Context. Massive binaries have stellar winds that collide. In the colliding-wind region, various physically interesting processes occur, leading to enhanced X-ray emission, non-thermal radio emission, as well as non-thermal X-rays and gamma-rays. Non-thermal radio emission (due to synchrotron radiation) has so far been observed at centimetre wavelengths. At millimetre wavelengths, the stellar winds and the colliding-wind region emit more thermal free-free radiation, and it is expected that any non-thermal contribution will be difficult or impossible to detect. Aims. We aim to determine if the material in the colliding-wind region contributes substantially to the observed millimetre fluxes of a colliding-wind binary. We also try to distinguish the synchrotron emission from the free-free emission. Methods. We monitored the massive binary Cyg OB2 #8A at 3 mm with the NOrthern Extended Millimeter Array (NOEMA) interferometer of the Institut de Radioastronomie Millimétrique (IRAM). The data were collected in 14 separate observing runs (in 2014 and 2016), and provide good coverage of the orbital period. Results. The observed millimetre fluxes range between 1.1 and 2.3 mJy, and show phase-locked variability, clearly indicating that a large part of the emission is due to the colliding-wind region. A simple synchrotron model gives fluxes with the correct order of magnitude, but with a maximum that is phase-shifted with respect to the observations. Qualitatively this phase shift can be explained by our neglect of orbital motion on the shape of the colliding-wind region. A model using only free-free emission results in only a slightly worse explanation of the observations. Additionally, on the map of our observations we also detect the O6.5 III star Cyg OB2 #8B, for which we determine a 3 mm flux of 0.21 ± 0.033 mJy. Conclusions. The question of whether synchrotron radiation or free-free emission dominates the millimetre fluxes of Cyg OB2 #8A remains open. More detailed modelling of this system, based on solving the hydrodynamical equations, is required to give a definite answer
Musa germplasm diversity status across a wide range of agro-ecological zones in Rwanda
This study assessed the on-farm Musa germplasm diversity across different agro-ecologies of Rwanda and the socio-economic utilization options and selection practices that create/maintain this diversity on-farm. Methodology and results: A Musa germplasm diagnostic survey was carried out in 2007 in five Rwandan districts on a transect from Lake Kivu (West) to Kirehe district (East) bordering Tanzania. Across all sites, 118 farms, each having at least 50 mats were sampled for determining Musa diversity. Forty three Musa cultivars were recorded across the five districts. Higher diversity was observed in the east declining westwards to the Lake Kivu region as reflected by the number of cultivars and their relative abundance. Nearly half of the recorded cultivars had a low diversity index (Gini-Simpson 1-D < 0.2) and therefore prone to genetic erosion. Cooking cultivars only dominate in the district of Kirehe, while beer cultivars dominate the banana production landscape in the other districts. Taste/flavor, bunch size and market demand were the most important criteria for banana cultivar selection and thus greatly influenced cultivar conservation and distribution on-farm. Diseases such as Fusarium wilt and Xanthomonas wilt greatly contributed to genetic erosion. Conclusion and application of results: Musa cultivar diversity in Rwanda is under threat. Ex-situ conservation of the menaced cultivars is of crucial importance. Beer cultivars dominated the landscape. Cultivar diversity on-farm was influenced by the prevailing altitudes; taste/flavor, bunch size, and market demand of the cultivars; and their susceptibility to diseases especially Fusarium and Xanthomonas wilt. Banana breeding or adaptation strategies therefore should take into account the farmer preferred traits. In addition, strategies for managing these diseases are critical in the prevention of the genetic erosion of the affected cultivars
Banana and plantain (Musa spp.) cultivar preference, local processing techniques and consumption patterns in eastern Democratic Republic of Congo
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