1,508 research outputs found
Astrometric Positions and Proper Motions of 19 Radio Stars
We have used the Very Large Array, linked with the Pie Town Very Long
Baseline Array antenna, to determine astrometric positions of 19 radio stars in
the International Celestial Reference Frame (ICRF). The positions of these
stars were directly linked to the positions of distant quasars through phase
referencing observations. The positions of the ICRF quasars are known to 0.25
mas, thus providing an absolute reference at the angular resolution of our
radio observations. Average values for the errors in our derived positions for
all sources were 13 mas and 16 mas in R.A. and declination respectively, with
accuracies approaching 1-2 mas for some of the stars observed. Differences
between the ICRF positions of the 38 quasars, and those measured from our
observations showed no systematic offsets, with mean values of -0.3 mas in R.A.
and -1.0 mas in declination. Standard deviations of the quasar position
differences of 17 mas and 11 mas in R.A. and declination respectively, are
consistent with the mean position errors determined for the stars. Our measured
positions were combined with previous Very Large Array measurements taken from
1978-1995 to determine the proper motions of 15 of the stars in our list. With
mean errors of approximately 1.6 mas/yr, the accuracies of our proper motions
approach those derived from Hipparcos, and for a few of the stars in our
program, are better than the Hipparcos values. Comparing the positions of our
radio stars with the Hipparcos catalog, we find that at the epoch of our
observations, the two frames are aligned to within formal errors of
approximately 3 mas. This result confirms that the Hipparcos frame is inertial
at the expected level.Comment: 20 pages, 9 figures Accepted by the Astronomical Journal, 2003 March
1
First results from a VLBA proper motion survey of H2O masers in low-mass YSOs: the Serpens core and RNO15-FIR
This article reports first results of a long-term observational program aimed
to study the earliest evolution of jet/disk systems in low-mass YSOs by means
of VLBI observations of the 22.2 GHz water masers. We report here data for the
cluster of low-mass YSOs in the Serpens molecular core and for the single
object RNO~15-FIR. Towards Serpens SMM1, the most luminous sub-mm source of the
Serpens cluster, the water maser emission comes from two small (< 5 AU in size)
clusters of features separated by ~25 AU, having line of sight velocities
strongly red-shifted (by more than 10 km/s) with respect to the LSR velocity of
the molecular cloud. The two maser clusters are oriented on the sky along a
direction that is approximately perpendicular to the axis of the radio
continuum jet observed with the VLA towards SMM1. The spatial and velocity
distribution of the maser features lead us to favor the interpretation that the
maser emission is excited by interaction of the receding lobe of the jet with
dense gas in the accretion disk surrounding the YSO in SMM1. Towards
RNO~15-FIR, the few detected maser features have both positions and (absolute)
velocities aligned along a direction that is parallel to the axis of the
molecular outflow observed on much larger angular scales. In this case the
maser emission likely emerges from dense, shocked molecular clumps displaced
along the axis of the jet emerging from the YSO. The protostar in Serpens SMM1
is more massive than the one in RNO~15-FIR. We discuss the case where a high
mass ejection rate can generate jets sufficiently powerful to sweep away from
their course the densest portions of circumstellar gas. In this case, the
excitation conditions for water masers might preferably occur at the interface
between the jet and the accretion disk, rather than along the jet axis.Comment: 18 pages (postscript format); 9 figures; to be published into
Astronomy & Astrophysics, Main Journa
Response of East Asian climate to Dansgaard/Oeschger and Heinrich events in a coupled model of intermediate complexity
[1] Terrestrial records of loess-paleosol sequences from across the Asian interior (Chinese Loess Plateau) have been used to reconstruct climatic conditions through the Quaternary and they correlate, in general, with oxygen isotope records from deep sea cores (e. g., the grain-size maxima from Chinese loess with ages that match well those of the last six Heinrich events evidenced in the North Atlantic marine sediments during the last glacial period). Possible reasons for this teleconnection of the similar climate signal of the North Atlantic and China are investigated by using an Earth system model of intermediate complexity (CLIMBER-2) for the typical period of last glacial age (during 60 20 kyr BP). By using the CLIMBER- 2 model, we have studied the response of East Asian climate during the typical glacial age (60-20 kyr BP) to Dansgaard/Oeschger (D/O) and Heinrich events. TotriggerD/O and Heinrich events in the model, transient forcings in addition to changes in insolation caused by variations in the Earth orbit are prescribed in the modeling experiment. These additional forcings include changes in inland-ice volume over North America, in freshwater flux into the northern North Atlantic. The modeling results show that the variations of the annual-mean near-surface air temperature over Eurasia closely follow climate changes in North Atlantic. The stronger intensity of westerly wind in the midlatitude of northern hemisphere versus the weaker Asian summer monsoon as well as the slightly weaker Asian winter monsoon (north easterly flow near surface) corresponds well with the (prescribed) Heinrich events during 60-20 kyr BP. This suggests that the climate signals found in Chinese loess (grain-size maxima with ages that match those of the last six Heinrich events) during the last glaciation are likely related to the relatively stronger westerly winds over Eurasia in boreal winter and a relatively weaker Asian summer monsoon that intensified the aridity of northern China which lead to expansion of the northern deserts during the Heinrich events
How does the explicit treatment of convection alter the precipitation–soil hydrology interaction in the mid-Holocene African humid period?
Global climate models with coarse horizontal resolution are largely unable to reproduce the monsoonal precipitation pattern over North Africa during the mid-Holocene. Here we present the first regional, storm-resolving simulations with an idealized but reasonable mid-Holocene vegetation cover. In these simulations, the West African monsoon expands farther north by about 4–5∘, and the precipitation gradient between the Guinea coast and the Sahara decreases compared to simulations with a barren Sahara as it is today. The northward shift of monsoonal precipitation is caused by land surface–atmosphere interaction, i.e., the coupling of soil moisture and precipitation, as well as interactions of the land surface with the large-scale monsoon circulation (e.g., the African easterly jet).
The response of the monsoon circulation to an increased vegetation cover is qualitatively similar but more pronounced in parameterized convection simulations. We attribute the differences in monsoonal precipitation to differences in soil moisture that are strongly controlled by runoff and precipitation characteristics. If precipitation is intense and falls over a spatially small region, as in our storm-resolving simulations, about 35 % of all precipitation water goes into runoff instead of filling soil moisture storage. In contrast, in light and spatially more homogeneous precipitation, as produced in our parameterized convection simulations, only some 20 % leaves the grid cell as runoff. Therefore, much more water is available to maintain high soil moisture content. We confirm the significant role of soil moisture and runoff by performing simulations with the same constant soil moisture field in both storm-resolving and parameterized convection simulations. These constant soil moisture simulations cancel the effect of lower soil moisture on the land–atmosphere feedback cycle in our storm-resolving simulations. We show that precipitation strongly increases in the storm-resolving simulations, especially in moisture-controlled regions, such as the northern Sahel and Sahara, and reaches equally high values as in parameterized convection simulations. Our study highlights how the type of rainfall (e.g., local and intense or widespread and light) impacts soil moisture and thus land–atmosphere feedbacks. This is contrary to many studies that focus mainly on the amount of rainfall and how it modifies land–atmosphere feedbacks. Moreover, this study suggests that comprehensive land-surface schemes, which appropriately respond to varying precipitation characteristics, are needed for studying land surface–atmosphere interaction.</p
Impacts of snow and glaciers over Tibetan Plateau on Holocene climate change: Sensitivity experiments with a coupled model of intermediate complexity
An Earth system model of intermediate complexity has been used to investigate the sensitivity of simulated global climate to gradually increased snow and glacier cover over the Tibetan Plateau for the last 9000 years (9 kyr). The simulations show that in the mid-Holocene at about 6 kyr before present (BP) the imposed ice sheets over the Tibetan Plateau induces summer precipitation decreases strongly in North Africa and South Asia, and increases in Southeast Asia. The response of vegetation cover to the imposed ice sheets over the Tibetan Plateau is not synchronous in South Asia and in North Africa, showing an earlier and, hence, a more rapid decrease in vegetation cover in North Africa from 9 to 6 kyr BP while it has almost no influence on that in south Asia until 5 kyr BP. The simulation results suggest that the snow and glacier environment over the Tibetan Plateau is an important factor for Holocene climate variability in North Africa, South Asia and Southeast Asia
Analysis of the loop length distribution for the negative weight percolation problem in dimensions d=2 through 6
We consider the negative weight percolation (NWP) problem on hypercubic
lattice graphs with fully periodic boundary conditions in all relevant
dimensions from d=2 to the upper critical dimension d=6. The problem exhibits
edge weights drawn from disorder distributions that allow for weights of either
sign. We are interested in in the full ensemble of loops with negative weight,
i.e. non-trivial (system spanning) loops as well as topologically trivial
("small") loops. The NWP phenomenon refers to the disorder driven proliferation
of system spanning loops of total negative weight. While previous studies where
focused on the latter loops, we here put under scrutiny the ensemble of small
loops. Our aim is to characterize -using this extensive and exhaustive
numerical study- the loop length distribution of the small loops right at and
below the critical point of the hypercubic setups by means of two independent
critical exponents. These can further be related to the results of previous
finite-size scaling analyses carried out for the system spanning loops. For the
numerical simulations we employed a mapping of the NWP model to a combinatorial
optimization problem that can be solved exactly by using sophisticated matching
algorithms. This allowed us to study here numerically exact very large systems
with high statistics.Comment: 7 pages, 4 figures, 2 tables, paper summary available at
http://www.papercore.org/Kajantie2000. arXiv admin note: substantial text
overlap with arXiv:1003.1591, arXiv:1005.5637, arXiv:1107.174
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