848 research outputs found
The First Simultaneous 3.5 and 1.3mm Polarimetric Survey of Active Galactic Nuclei in the Northern Sky
Short millimeter observations of radio-loud AGN offer the opportunity to
study the physics of their inner relativistic jets, from where the bulk
millimeter emission is radiated. Millimeter jets are significantly less
affected by Faraday rotation and depolarization than in radio. Also, the
millimeter emission is dominated by the innermost jet regions, that are
invisible in radio owing to synchrotron opacity. We present the first dual
frequency simultaneous 86GHz and 229GHz polarimetric survey of all four Stokes
parameters of a large sample of 211 radio loud active galactic nuclei, designed
to be flux limited at 1Jy at 86GHz. The observations were most of them made in
mid August 2010 using the XPOL polarimeter on the IRAM 30 m millimeter radio
telescope. Linear polarization detections above 3 sigma median level of ~1.0%
are reported for 183 sources at 86GHz, and for 23 sources at 229GHz, where the
median 3 sigma level is ~6.0%. We show a clear excess of the linear
polarization degree detected at 229GHz with regard to that at 86GHz by a factor
of ~1.6, thus implying a progressively better ordered magnetic field for blazar
jet regions located progressively upstream in the jet. We show that the linear
polarization angle, both at 86 and 229GHz, and the jet structural position
angle for both quasars and BL Lacs do not show a clear preference to align in
either parallel or perpendicular directions. Our variability study with regard
to the 86GHz data from our previous survey points out a large degree variation
of total flux and linear polarization in time scales of years by median factors
of ~1.5 in total flux, and ~1.7 in linear polarization degree -maximum
variations by factors up to 6.3, and ~5, respectively-, with 86% of sources
showing linear polarization angles evenly distributed with regard to our
previous measurements.Comment: Submitted for Publication in Astronomy & Astrophysics. 14 pages
(including 2 tables and 18 figures
CO and HI observations of an enigmatic cloud
An isolated HI cloud with peculiar properties has recently been discovered by
Dedes, Dedes, & Kalberla (2008, A&A, 491, L45) with the 300-m Arecibo
telescope, and subsequently imaged with the VLA. It has an angular size of ~6',
and the HI emission has a narrow line profile of width ~ 3 km/s.
We explore the possibility that this cloud could be associated with a
circumstellar envelope ejected by an evolved star.
Observations were made in the rotational lines of CO with the IRAM-30m
telescope, on three positions in the cloud, and a total-power mapping in the HI
line was obtained with the Nancay Radio Telescope.
CO was not detected and seems too underabundant in this cloud to be a
classical late-type star circumstellar envelope. On the other hand, the HI
emission is compatible with the detached-shell model that we developed for
representing the external environments of AGB stars.
We propose that this cloud could be a fossil circumstellar shell left over
from a system that is now in a post-planetary-nebula phase. Nevertheless, we
cannot rule out that it is a Galactic cloud or a member of the Local Group,
although the narrow line profile would be atypical in both cases.Comment: Accepted for publication in Astronomy and Astrophysic
Circumstellar HI and CO around the carbon stars V1942 Sgr and V CrB
Context. The majority of stars that leave the main sequence are undergoing
extensive mass loss, in particular during the asymptotic giant branch (AGB)
phase of evolution. Observations show that the rate at which this phenomenon
develops differs highly from source to source, so that the time-integrated mass
loss as a function of the initial conditions (mass, metallicity, etc.) and of
the stage of evolution is presently not well understood. Aims. We are
investigating the mass loss history of AGB stars by observing the molecular and
atomic emissions of their circumstellar envelopes. Methods. In this work we
have selected two stars that are on the thermally pulsing phase of the AGB
(TP-AGB) and for which high quality data in the CO rotation lines and in the
atomic hydrogen line at 21 cm could be obained. Results. V1942 Sgr, a carbon
star of the Irregular variability type, shows a complex CO line profile that
may originate from a long-lived wind at a rate of ~ 10^-7 Msol/yr, and from a
young (< 10^4 years) fast outflow at a rate of ~ 5 10^-7 Msol/yr. Intense HI
emission indicates a detached shell with 0.044 Msol of hydrogen. This shell
probably results from the slowing-down, by surrounding matter, of the same
long-lived wind observed in CO that has been active during ~ 6 10^5 years. On
the other hand, the carbon Mira V CrB is presently undergoing mass loss at a
rate of 2 10^-7 Msol/yr, but was not detected in HI. The wind is mostly
molecular, and was active for at most 3 10^4 years, with an integrated mass
loss of at most 6.5 10^-3 Msol. Conclusions. Although both sources are carbon
stars on the TP-AGB, they appear to develop mass loss under very different
conditions, and a high rate of mass loss may not imply a high integrated mass
loss.Comment: Accepted for publication in Astron. Astrophy
3C 286: a bright, compact, stable, and highly polarized calibrator for millimeter-wavelength observations
(Context.) A number of millimeter and submillimeter facilities with linear
polarization observing capabilities have started operating during last years.
These facilities, as well as other previous millimeter telescopes and
interferometers, require bright and stable linear polarization calibrators to
calibrate new instruments and to monitor their instrumental polarization. The
current limited number of adequate calibrators implies difficulties in the
acquisition of these calibration observations. (Aims.) Looking for additional
linear polarization calibrators in the millimeter spectral range, in mid-2006
we started monitoring 3C 286, a standard and highly stable polarization
calibrator for radio observations. (Methods.) Here we present the 3 and 1 mm
monitoring observations obtained between September 2006 and January 2012 with
the XPOL polarimeter on the IRAM 30 m Millimeter Telescope. (Results.) Our
observations show that 3C 286 is a bright source of constant total flux with 3
mm flux density S_3mm = (0.91 \pm 0.02) Jy. The 3mm linear polarization degree
(p_3mm =[13.5\pm0.3]%) and polarization angle (chi_3mm
=[37.3\pm0.8]deg.,expressed in the equatorial coordinate system) are also
constant during the time span of our observations. Although with poorer time
sampling and signal-to-noise ratio, our 1 mm observations of 3C 286 are also
reproduced by a constant source of 1 mm flux density (S_1mm = [0.30 \pm 0.03]
Jy), polarization fraction (p_1mm = [14.4 \pm 1.8] %), and polarization angle
(chi_1mm = [33.1 \pm 5.7]deg.). (Conclusions.) This, together with the
previously known compact structure of 3C 286 -extended by ~3.5" in the sky-
allow us to propose 3C 286 as a new calibrator for both single dish and
interferometric polarization observations at 3 mm, and possibly at shorter
wavelengths.Comment: Accepted for publication in A&A. 7 pages, 4 figures, 8 tables.
Updated data sets with regard to previous version. New discussion about multi
frequency properties of the source. Section 3.3, Figures 3 and 4, and Tables
7 and 8 are ne
The neuronal and molecular basis of quinine-dependent bitter taste signaling in Drosophila larvae.
The sensation of bitter substances can alert an animal that a specific type of food is harmful and should not be consumed. However, not all bitter compounds are equally toxic and some may even be beneficial in certain contexts. Thus, taste systems in general may have a broader range of functions than just in alerting the animal. In this study we investigate bitter sensing and processing in Drosophila larvae using quinine, a substance perceived by humans as bitter. We show that behavioral choice, feeding, survival, and associative olfactory learning are all directly affected by quinine. On the cellular level, we show that 12 gustatory sensory receptor neurons that express both GR66a and GR33a are required for quinine-dependent choice and feeding behavior. Interestingly, these neurons are not necessary for quinine-dependent survival or associative learning. On the molecular receptor gene level, the GR33a receptor, but not GR66a, is required for quinine-dependent choice behavior. A screen for gustatory sensory receptor neurons that trigger quinine-dependent choice behavior revealed that a single GR97a receptor gene expressing neuron located in the peripheral terminal sense organ is partially necessary and sufficient. For the first time, we show that the elementary chemosensory system of the Drosophila larva can serve as a simple model to understand the neuronal basis of taste information processing on the single cell level with respect to different behavioral outputs
Ionization structure in the winds of B[e] supergiants: I. Ionization equilibrium calculations in a H plus He wind
The non-spherically symmetric winds of B[e] supergiants are investigated. An
empirical density distribution is chosen that accounts for the density
concentrations and ratios derived from observations, and our model winds are
assumed to contain only hydrogen and helium. We first calculate the approximate
ionization radii for H and He and compare the results with the ionization
fractions calculated from the more accurate ionization balance equations. We
find that winds with a r^-2 density distribution turn out to reach a constant
ionization fraction as long as the wind density is low, i.e. in polar
direction. For the high density equatorial regions, however, we find that the
winds become neutral just above the stellar surface of the hot and massive B[e]
supergiants forming a disk-like neutral region. In such a disk molecules and
dust can form even very near the hot central star.Comment: 11 pages, 8 figures, accepted for publication in A&
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