14,818 research outputs found
Solar type II radio bursts associated with CME expansions as shown by EUV waves
We investigate the physical conditions of the sources of two metric Type-II
bursts associated with CME expansions with the aim of verifying the
relationship between the shocks and the CMEs, comparing the heights of the
radio sources and the heights of the EUV waves associated with the CMEs. The
heights of the EUV waves associated with the events were determined in relation
to the wave fronts. The heights of the shocks were estimated by applying two
different density models to the frequencies of the Type-II emissions and
compared with the heights of the EUV waves. For the 13 June 2010 event, with
band-splitting, the shock speed was estimated from the frequency drifts of the
upper and lower branches of the harmonic lane, taking into account the H/F
frequency ratio fH/fF = 2. Exponential fits on the intensity maxima of the
branches revealed to be more consistent with the morphology of the spectrum of
this event. For the 6 June 2012 event, with no band-splitting and with a clear
fundamental lane on the spectrum, the shock speed was estimated directly from
the frequency drift of the fundamental emission, determined by linear fit on
the intensity maxima of the lane. For each event, the most appropriate density
model was adopted to estimate the physical parameters of the radio source. The
13 June 2010 event presented a shock speed of 664-719 km/s, consistent with the
average speed of the EUV wave fronts of 609 km/s. The 6 June 2012 event was
related to a shock of speed of 211-461 km/s, also consistent with the average
speed of the EUV wave fronts of 418 km/s. For both events, the heights of the
EUV wave revealed to be compatible with the heights of the radio source,
assuming a radial propagation of the shock.Comment: Accepted for publication in Astronomy and Astrophysic
Recording from two neurons: second order stimulus reconstruction from spike trains and population coding
We study the reconstruction of visual stimuli from spike trains, recording
simultaneously from the two H1 neurons located in the lobula plate of the fly
Chrysomya megacephala. The fly views two types of stimuli, corresponding to
rotational and translational displacements. If the reconstructed stimulus is to
be represented by a Volterra series and correlations between spikes are to be
taken into account, first order expansions are insufficient and we have to go
to second order, at least. In this case higher order correlation functions have
to be manipulated, whose size may become prohibitively large. We therefore
develop a Gaussian-like representation for fourth order correlation functions,
which works exceedingly well in the case of the fly. The reconstructions using
this Gaussian-like representation are very similar to the reconstructions using
the experimental correlation functions. The overall contribution to rotational
stimulus reconstruction of the second order kernels - measured by a chi-squared
averaged over the whole experiment - is only about 8% of the first order
contribution. Yet if we introduce an instant-dependent chi-square to measure
the contribution of second order kernels at special events, we observe an up to
100% improvement. As may be expected, for translational stimuli the
reconstructions are rather poor. The Gaussian-like representation could be a
valuable aid in population coding with large number of neurons
Probing the ejecta of evolved massive stars in transition: A VLT/SINFONI K-band survey
Massive evolved stars in transition phases, such as Luminous Blue Variables
(LBVs), B[e] Supergiants (B[e]SGs), and Yellow Hypergiants (YHGs), are not well
understood, and yet crucial steps in determining accurate stellar and galactic
evolution models. The circumstellar environments of these stars reveal their
mass-loss history, identifying clues to both their individual evolutionary
status and the connection between objects of different phases. Here we present
a survey of 25 such evolved massive stars (16 B[e]SGs, 6 LBVs, 2 YHGs, and 1
Peculiar Oe star), observed in the K-band with the Spectrograph for INtegral
Field Observation in the Near-Infrared (SINFONI; R = 4500) on the ESO VLT UT4 8
m telescope. The sample can be split into two categories based on spectral
morphology: one group includes all of the B[e]SGs, the Peculiar Oe star, and
two of the LBVs, while the other includes the YHGs and the rest of the LBVs.
The difference in LBV spectral appearance is due to some objects being in a
quiescent phase and some objects being in an active or outburst phase. CO
emission features are found in 13 of our targets, with first time detections
for MWC 137, LHA 120-S 35, and LHA 115-S 65. From model fits to the CO band
heads, the emitting regions appear to be detached from the stellar surface.
Each star with ^12CO features also shows ^13CO emission, signaling an evolved
nature. Based on the level of ^13C enrichment, we conclude that many of the
B[e]SGs are likely in a pre-Red Supergiant phase of their evolution. There
appears to be a lower luminosity limit of log L/L_solar = 5.0 below which CO is
not detected. The lack of CO features in several high luminosity B[e]SGs and
variability in others suggests that they may in fact be LBV candidates,
strengthening the connection between these two very similar transition phases.Comment: 20 pages, 13 figures, 7 tables. Accepted for publication in A&
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