1,350 research outputs found
Detection of thermal radio emission from a single coronal giant
We report the detection of thermal continuum radio emission from the K0 III
coronal giant Pollux ( Gem) with the Karl G. Jansky Very Large Array
(VLA). The star was detected at 21 and 9 GHz with flux density values of
and Jy, respectively. We also place a
upper limit of Jy for the flux density at 3
GHz. We find the stellar disk-averaged brightness temperatures to be
approximately 9500, 15000, and K, at 21, 9, and 3 GHz, respectively,
which are consistent with the values of the quiet Sun. The emission is most
likely dominated by optically thick thermal emission from an upper chromosphere
at 21 and 9 GHz. We discuss other possible additional sources of emission at
all frequencies and show that there may also be a small contribution from
gyroresonance emission above active regions, coronal free-free emission and
free-free emission from an optically thin stellar wind, particularly at the
lower frequencies. We constrain the maximum mass-loss rate from Pollux to be
less than yr (assuming a wind terminal
velocity of 215 km s), which is about an order of magnitude smaller than
previous constraints for coronal giants and is in agreement with existing
predictions for the mass-loss rate of Pollux. These are the first detections of
thermal radio emission from a single (i.e., non-binary) coronal giant and
demonstrate that low activity coronal giants like Pollux have atmospheres at
radio frequencies akin to the quiet Sun
For the good of the group? Exploring group-level evolutionary adaptations using multilevel selection theory.
In this paper, we present an evolutionary framework, multilevel selection theory (MLS), that is highly amenable to existing social psychological theory and empiricism. MLS provides an interpretation of natural selection that shows how group-beneficial traits can evolve, a prevalent implication of social psychological data. We outline the theory and provide a number of example topics, focusing on prosociality, policing behavior, gossip, brainstorming, distributed cognition, and social identity. We also show that individual differences can produce important group-level outcomes depending on differential aggregation of individual types and relate this to the evolutionary dynamics underlying group traits. Drawing on existing work, we show how social psychologists can integrate this framework into their research program and suggest future directions for research
Multi-wavelength Radio Continuum Emission Studies of Dust-free Red Giants
Multi-wavelength centimeter continuum observations of non-dusty,
non-pulsating K spectral-type red giants directly sample their chromospheres
and wind acceleration zones. Such stars are feeble emitters at these
wavelengths however, and previous observations have provided only a small
number of modest S/N measurements slowly accumulated over three decades. We
present multi-wavelength Karl G. Jansky Very Large Array thermal continuum
observations of the wind acceleration zones of two dust-free red giants,
Arcturus (Alpha Boo: K2 III) and Aldebaran (Alpha Tau: K5 III). Importantly,
most of our observations of each star were carried out over just a few days, so
that we obtained a snapshot of the different stellar atmospheric layers sampled
at different wavelengths, independent of any long-term variability. We report
the first detections at several wavelengths for each star including a detection
at 10 cm (3.0 GHz: S band) for both stars and a 20 cm (1.5 GHz: L band)
detection for Alpha Boo. This is the first time single luminosity class III red
giants have been detected at these continuum wavelengths. Our long-wavelength
data sample the outer layers of Alpha Boo's atmosphere where its wind velocity
is approaching its terminal value and the ionization balance is becoming
frozen-in. For Alpha Tau, however, our long-wavelength data are still sampling
its inner atmosphere, where the wind is still accelerating probably due to its
lower mass-loss rate. We compare our data with published semi-empirical models
based on ultraviolet data, and the marked deviations highlight the need for new
atmospheric models to be developed. Spectral indices are used to discuss the
possible properties of the stellar atmospheres, and we find evidence for a
rapidly cooling wind in the case of Alpha Boo. Finally, we develop a simple
analytical wind model for Alpha Boo based on our new long-wavelength flux
measurements
Discrete mode lasers for applications in access networks
Fast development of the modern telecommunication networks such as fiber-to-the-home or radio-over-fiber systems require an inexpensive yet reliable optical transmitter for electro-optic conversion. Such devices should be able to generate stable, single moded optical signals suitable for athermal operation. Discrete Mode Lasers (DMLs) are able to fulfill all the above-mentioned requirements with the added benefit of low sensitivity to optical feedback. DMLs are essentially Fabry-Perot lasers in which the refractive index is modified by introducing perturbations along very small sections of the laser cavity. These modifications result in a single mode laser output with a very narrow linewidth (order of 400 kHz). In this paper, we demonstrate how a DML can outperform the commonly used/commercially available DFB lasers in terms of linewidth, sensitivity to optical feedback and transmission performance in the presence of feedback
Temporal Evolution of the Size and Temperature of Betelgeuse's Extended Atmosphere
We use the Very Large Array (VLA) in the A configuration with the Pie Town
(PT) Very Long Baseline Array (VLBA) antenna to spatially resolve the extended
atmosphere of Betelgeuse over multiple epochs at 0.7, 1.3, 2.0, 3.5, and 6.1
cm. The extended atmosphere deviates from circular symmetry at all wavelengths
while at some epochs we find possible evidence for small pockets of gas
significantly cooler than the mean global temperature. We find no evidence for
the recently reported e-MERLIN radio hotspots in any of our multi-epoch VLA/PT
data, despite having sufficient spatial resolution and sensitivity at short
wavelengths, and conclude that these radio hotspots are most likely
interferometric artefacts. The mean gas temperature of the extended atmosphere
has a typical value of 3000 K at 2 and decreases to 1800 K at 6
, in broad agreement with the findings of the single epoch study
from Lim et al. (1998). The overall temperature profile of the extended
atmosphere between can be
described by a power law of the form ,
with temporal variability of a few 100 K evident at some epochs. Finally, we
present over 12 years of V band photometry, part of which overlaps our
multi-epoch radio data. We find a correlation between the fractional flux
density variability at V band with most radio wavelengths. This correlation is
likely due to shock waves induced by stellar pulsations, which heat the inner
atmosphere and ionize the more extended atmosphere through radiative means.
Stellar pulsations may play an important role in exciting Betelgeuse's extended
atmosphere
Self-pulsation at 480 GHz from a two-color discrete mode laser diode
A discrete mode Fabry-PĂ©rot laser is designed and fabricated to achieve two-color lasing. We demonstrate beating between the two laser modes and self-pulsation at 480 GHz
ALMA observations of TiO around VY Canis Majoris
Titanium dioxide, TiO, is a refractory species that could play a crucial
role in the dust-condensation sequence around oxygen-rich evolved stars. To
date, gas phase TiO has been detected only in the complex environment of
the red supergiant VY CMa. We aim to constrain the distribution and excitation
of TiO around VY CMa in order to clarify its role in dust formation. We
analyse spectra and channel maps for TiO extracted from ALMA science
verification data. We detect 15 transitions of TiO, and spatially resolve
the emission for the first time. The maps demonstrate a highly clumpy,
anisotropic outflow in which the TiO emission likely traces gas exposed to
the stellar radiation field. A roughly east-west oriented, accelerating
bipolar-like structure is found, of which the blue component runs into and
breaks up around a solid continuum component. A distinct tail to the south-west
is seen for some transitions, consistent with features seen in the optical and
near-infrared. We find that a significant fraction of TiO remains in the
gas phase outside the dust-formation zone and suggest that this species might
play only a minor role in the dust-condensation process around extreme
oxygen-rich evolved stars like VY CMa.Comment: Accepted for publication in Astronomy & Astrophysics, 25 pages, 20
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Inverse scattering approach to multiwavelength Fabry-PĂ©rot laser design
A class of multiwavelength Fabry-Pérot lasers is introduced where the spectrum is tailored through a patterning of the cavity effective index. The cavity geometry is obtained using an inverse scattering approach and can be designed such that the spacing of discrete Fabry-Pérot lasing modes is limited only by the bandwidth of the inverted gain medium. A specific two-color semiconductor laser with a mode spacing in the THz region is designed, and measurements are presented demonstrating the simultaneous oscillation of the two wavelengths. The nonperiodic effective index profile of the particular two-color device considered is shown to be related to a Moiré or superstructure grating
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