1,189 research outputs found
SMA Imaging of the Maser Emission from the H30 Radio Recombination Line in MWC349A
We used the Submillimeter Array to map the angular distribution of the
H30 recombination line (231.9 GHz) in the circumstellar region of the
peculiar star MWC349A. The resolution was 1\farcs2, but because of high
signal-to-noise ratio we measured the positions of all maser components to
accuracies better than 0\farcs01, at a velocity resolution of . The
two strongest maser components (called high velocity components) at velocities
near -14 and are separated by 0\farcs048 \pm 0\farcs001 (60 AU)
along a position angle of 102 \pm 1\arcdeg. The distribution of maser
emission at velocities between and beyond these two strongest components were
also provided. The continuum emission lies at the center of the maser
distribution to within 10 mas. The masers appear to trace a nearly edge-on
rotating disk structure, reminiscent of the water masers in Keplerian rotation
in the nuclear accretion disk of the galaxy NGC4258. However, the maser
components in MWC349A do not follow a simple Keplerian kinematic prescription
with , but have a larger power law index. We explore the
possibility that the high velocity masers trace spiral density or shock waves.
We also emphasize caution in the interpretation of relative centroid maser
positions where the maser is not clearly resolved in position or velocity, and
we present simulations that illustrate the range of applicability of the
centroiding method.Comment: 23 pages with 9 figures (two of these figures are vertically aligned
as Figure 4) submitted to the Astrophysical Journa
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
A new intermediate mass protostar in the Cepheus A HW2 region
We present the discovery of the first molecular hot core associated with an
intermediate mass protostar in the CepA HW2 region. The hot condensation was
detected from single dish and interferometric observations of several high
excitation rotational lines (from 100 to 880K above the ground state) of SO2 in
the ground vibrational state and of HC3N in the vibrationally excited states
v7=1 and v7=2. The kinetic temperature derived from both molecules is 160K. The
high-angular resolution observations (1.25'' x 0.99'') of the SO2
J=28(7,21)-29(6,24) line (488K above the ground state) show that the hot gas is
concentrated in a compact condensation with a size of 0.6''(430AU), located
0.4'' (300AU) east from the radio-jet HW2. The total SO2 column density in the
hot condensation is 10E18cm-2, with a H2 column density ranging from 10E23 to 6
x 10E24cm-2. The H2 density and the SO2 fractional abundance must be larger
than 10E7cm-3 and 2 x 10E-7 respectively. The most likely alternatives for the
nature of the hot and very dense condensation are discussed. From the large
column densities of hot gas, the detection of the HC3N vibrationally excited
lines and the large SO2 abundance, we favor the interpretation of a hot core
heated by an intermediate mass protostar of 10E3 Lo. This indicates that the
CepA HW2 region contains a cluster of very young stars
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
Evaluating two soil carbon models within the global land surface model JSBACH using surface and spaceborne observations of atmospheric CO<sub>2</sub>
The trajectories of soil carbon (C) in the changing climate are of utmost importance, as soil carbon is a substantial carbon storage with a large potential to impact the atmospheric carbon dioxide (CO2) burden. Atmospheric CO2 observations integrate all processes affecting C exchange between the surface and the atmosphere. Therefore they provide a benchmark for carbon cycle models. We evaluated two distinct soil carbon models (CBALANCE and YASSO) that were implemented to a global land surface model (JSBACH) against atmospheric CO2 observations. We transported the biospheric carbon fluxes obtained by JSBACH using the atmospheric transport model TM5 to obtain atmospheric CO2. We then compared these results with surface observations from Global Atmosphere Watch (GAW) stations as well as with column XCO2 retrievals from the GOSAT satellite. The seasonal cycles of atmospheric CO2 estimated by the two different soil models differed. The estimates from the CBALANCE soil model were more in line with the surface observations at low latitudes (0 N–45 N) with only 1 % bias in the seasonal cycle amplitude (SCA), whereas YASSO was underestimating the SCA in this region by 32 %. YASSO gave more realistic seasonal cycle amplitudes of CO2 at northern boreal sites (north of 45 N) with underestimation of 15 % compared to 30 % overestimation by CBALANCE. Generally, the estimates from CBALANCE were more successful in capturing the seasonal patterns and seasonal cycle amplitudes of atmospheric CO2 even though it overestimated soil carbon stocks by 225 % (compared to underestimation of 36 % by YASSO) and its predictions of the global distribution of soil carbon stocks was unrealistic. The reasons for these differences in the results are related to the different environmental drivers and their functional dependencies of these two soil carbon models. In the tropical region the YASSO model showed earlier increase in season of the heterotophic respiration since it is driven by precipitation instead of soil moisture as CBALANCE. In the temperate and boreal region the role of temperature is more dominant. There the heterotophic respiration from the YASSO model had larger annual variability, driven by air temperature, compared to the CBALANCE which is driven by soil temperature. The results underline the importance of using sub-yearly data in the development of soil carbon models when they are used in shorter than annual time scales
Discovery of Interstellar Propylene (CH_2CHCH_3): Missing Links in Interstellar Gas-Phase Chemistry
We report the discovery of propylene (also called propene, CH_2CHCH_3) with
the IRAM 30-m radio telescope toward the dark cloud TMC-1. Propylene is the
most saturated hydrocarbon ever detected in space through radio astronomical
techniques. In spite of its weak dipole moment, 6 doublets (A and E species)
plus another line from the A species have been observed with main beam
temperatures above 20 mK. The derived total column density of propylene is 4
10^13 cm^-2, which corresponds to an abundance relative to H_2 of 4 10^-9,
i.e., comparable to that of other well known and abundant hydrocarbons in this
cloud, such as c-C_3H_2. Although this isomer of C_3H_6 could play an important
role in interstellar chemistry, it has been ignored by previous chemical models
of dark clouds as there seems to be no obvious formation pathway in gas phase.
The discovery of this species in a dark cloud indicates that a thorough
analysis of the completeness of gas phase chemistry has to be done.Comment: 13 pages, 2 figures, accepted for publication in ApJ
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