4,366 research outputs found
Operation of a Stark decelerator with optimum acceptance
With a Stark decelerator, beams of neutral polar molecules can be
accelerated, guided at a constant velocity, or decelerated. The effectiveness
of this process is determined by the 6D volume in phase space from which
molecules are accepted by the Stark decelerator. Couplings between the
longitudinal and transverse motion of the molecules in the decelerator can
reduce this acceptance. These couplings are nearly absent when the decelerator
operates such that only every third electric field stage is used for
deceleration, while extra transverse focusing is provided by the intermediate
stages. For many applications, the acceptance of a Stark decelerator in this
so-called mode significantly exceeds that of a decelerator in the
conventionally used () mode. This has been experimentally verified by
passing a beam of OH radicals through a 2.6 meter long Stark decelerator. The
experiments are in quantitative agreement with the results of trajectory
calculations, and can qualitatively be explained with a simple model for the 6D
acceptance. These results imply that the 6D acceptance of a Stark decelerator
in the mode of operation approaches the optimum value, i.e. the value
that is obtained when any couplings are neglected.Comment: 13 pages, 11 figure
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
Loading Stark-decelerated molecules into electrostatic quadrupole traps
Beams of neutral polar molecules in a low-field seeking quantum state can be
slowed down using a Stark decelerator, and can subsequently be loaded and
confined in electrostatic quadrupole traps. The efficiency of the trap loading
process is determined by the ability to couple the decelerated packet of
molecules into the trap without loss of molecules and without heating. We
discuss the inherent difficulties to obtain ideal trap loading, and describe
and compare different trap loading strategies. A new "split-endcap" quadrupole
trap design is presented that enables improved trap loading efficiencies. This
is experimentally verified by comparing the trapping of OH radicals using the
conventional and the new quadrupole trap designs
HI Observations of the Asymptotic Giant Branch Star X Herculis: Discovery of an Extended Circumstellar Wake Superposed on a Compact High-Velocity Cloud
We report HI 21-cm line observations of the AGB star X Her obtained with the
Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI
emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope
of the star. The HI distribution exhibits a head-tail morphology, similar to
those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24
pc) in the plane of the sky, along the direction of the star's space motion. We
also detect a velocity gradient of ~6.5 km/s across the envelope, consistent
with the HI tracing a turbulent wake that arises from the motion of a
mass-losing star through the ISM. GBT mapping of a 2x2deg region around X Her
reveals that the star lies (in projection) near the periphery of a much larger
HI cloud that also exhibits signatures of ISM interaction. The properties of
the cloud are consistent with those of compact high-velocity clouds. Using CO
observations, we have placed an upper limit on its molecular gas content of
N_H2<1.3e20 cm^-2. Although the distance to the cloud is poorly constrained,
the probability of a chance coincidence in position, velocity, and apparent
position angle of space motion between X Her and the cloud is extremely small,
suggesting a possible physical association. However, the large HI mass of the
cloud (~>2.4~M_sun) and the blueshift of its mean velocity relative to X Her
are inconsistent with an origin tied directly to stellar ejection. (abridged)Comment: Accepted to AJ; 47 pages, 15 figures; version with full resolution
figures available at
http://www.haystack.mit.edu/hay/staff/lmatthew/matthews_XHer.pd
Deceleration and electrostatic trapping of OH radicals
A pulsed beam of ground state OH radicals is slowed down using a Stark
decelerator and is subsequently loaded into an electrostatic trap.
Characterization of the molecular beam production, deceleration and trap
loading process is performed via laser induced fluorescence detection inside
the quadrupole trap. Depending on details of the trap loading sequence,
typically OH () radicals are trapped at a density
of around cm and at temperatures in the 50-500 mK range. The 1/e
trap lifetime is around 1.0 second.Comment: 4 pages, 3 figure
Direct measurement of the radiative lifetime of vibrationally excited OH radicals
Neutral molecules, isolated in the gas-phase, can be prepared in a long-lived
excited state and stored in a trap. The long observation time afforded by the
trap can then be exploited to measure the radiative lifetime of this state by
monitoring the temporal decay of the population in the trap. This method is
demonstrated here and used to benchmark the Einstein -coefficients in the
Meinel system of OH. A pulsed beam of vibrationally excited OH radicals is
Stark decelerated and loaded into an electrostatic quadrupole trap. The
radiative lifetime of the upper -doublet component of the level is determined as ms, in good
agreement with the calculated value of ms.Comment: 4 pages, 3 figures, submitted to Phys. Rev. Let
Multiple packets of neutral molecules revolving for over a mile
The level of control that one has over neutral molecules in beams dictates
their possible applications. Here we experimentally demonstrate that
state-selected, neutral molecules can be kept together in a few mm long packet
for a distance of over one mile. This is accomplished in a circular arrangement
of 40 straight electrostatic hexapoles through which the molecules propagate
over 1000 times. Up to 19 packets of molecules have simultaneously been stored
in this ring structure. This brings the realization of a molecular low-energy
collider within reach
- …