757 research outputs found
Making the best of mixed-field orientation of polar molecules: A recipe for achieving adiabatic dynamics in an electrostatic field combined with laser pulses
We have experimentally and theoretically investigated the mixed-field
orientation of rotational-state-selected OCS molecules and we achieve strong
degrees of alignment and orientation. The applied moderately intense nanosecond
laser pulses are long enough to adiabatically align molecules. However, in
combination with a weak dc electric field, the same laser pulses result in
nonadiabatic dynamics in the mixed-field orientation. These observations are
fully explained by calculations employing, both, adiabatic and non-adiabatic
time-dependent models.Comment: 5 pages, 4 figure
Strongly driven quantum pendulum of the OCS molecule
We demonstrate and analyze a strongly driven quantum pendulum in the angular
motion of stateselected and laser aligned OCS molecules. Raman-couplings during
the rising edge of a 50-picosecond laser pulse create a wave packet of pendular
states, which propagates in the confining potential formed by the
polarizability interaction between the molecule and the laser field. This
wave-packet dynamics manifests itself as pronounced oscillations in the degree
of alignment with a laser-intensity dependent period.Comment: 6 pages, 4 figure
A Spitzer Study of the Mass Loss Histories of Three Bipolar Pre-Planetary Nebulae
We present the results of far-infrared imaging of extended regions around
three bipolar pre-planetary nebulae, AFGL 2688, OH 231.8+4.2, and IRAS
163423814, at 70 and 160 m with the MIPS instrument on the Spitzer
Space Telescope. After a careful subtraction of the point spread function of
the central star from these images, we place constraints on the existence of
extended shells and thus on the mass outflow rates as a function of radial
distance from these stars. We find no apparent extended emission in AFGL 2688
and OH 231.8+4.2 beyond 100 arcseconds from the central source. In the case of
AFGL 2688, this result is inconsistent with a previous report of two extended
dust shells made on the basis of ISO observations. We derive an upper limit of
M yr and M
yr for the dust mass loss rate of AFGL 2688 and OH 231.8, respectively,
at 200 arcseconds from each source. In contrast to these two sources, IRAS
163423814 does show extended emission at both wavelengths, which can be
interpreted as a very large dust shell with a radius of 400 arcseconds
and a thickness of 100 arcseconds, corresponding to 4 pc and 1 pc,
respectively, at a distance of 2 kpc. However, this enhanced emission may also
be galactic cirrus; better azimuthal coverage is necessary for confirmation of
a shell. If the extended emission is a shell, it can be modeled as enhanced
mass outflow at a dust mass outflow rate of M
yr superimposed on a steady outflow with a dust mass outflow rate of
M yr. It is likely that this shell has swept
up a substantial mass of interstellar gas during its expansion, so these
estimates are upper limits to the stellar mass loss rate.Comment: 31 pages, 12 figures, accepted to A
The Kinematics of HH 34 from HST Images with a Nine-year Time Baseline
We study archival HST [S II] 6716+30 and Hα images of the HH 34 outflow, taken in 1998.71 and in 2007.83. The ~9 yr time baseline and the high angular resolution of these observations allow us to carry out a detailed proper-motion study. We determine the proper motions of the substructure of the HH 34S bow shock (from the [S II] and Hα frames) and of the aligned knots within ~30'' from the outflow source (only from the [S II] frames). We find that the present-day motions of the knots along the HH 34 jet are approximately ballistic, and that these motions directly imply the formation of a major mass concentration in ~900 yr, at a position similar to the one of the present-day HH 34S bow shock. In other words, we find that the knots along the HH 34 jet will merge to form a more massive structure, possibly resembling HH 34S
HST Scattered Light Imaging and Modeling of the Edge-on Protoplanetary Disk ESO-H 569
We present new HST ACS observations and detailed models for a recently
discovered edge-on protoplanetary disk around ESO H 569 (a low-mass T
Tauri star in the Cha I star forming region). Using radiative transfer models
we probe the distribution of the grains and overall shape of the disk
(inclination, scale height, dust mass, flaring exponent and surface/volume
density exponent) by model fitting to multiwavelength (F606W and F814W) HST
observations together with a literature compiled spectral energy distribution.
A new tool set was developed for finding optimal fits of MCFOST radiative
transfer models using the MCMC code emcee to efficiently explore the high
dimensional parameter space. It is able to self-consistently and simultaneously
fit a wide variety of observables in order to place constraints on the physical
properties of a given disk, while also rigorously assessing the uncertainties
in those derived properties. We confirm that ESO H 569 is an optically
thick nearly edge-on protoplanetary disk. The shape of the disk is well
described by a flared disk model with an exponentially tapered outer edge,
consistent with models previously advocated on theoretical grounds and
supported by millimeter interferometry. The scattered light images and spectral
energy distribution are best fit by an unusually high total disk mass (gas+dust
assuming a ratio of 100:1) with a disk-to-star mass ratio of 0.16.Comment: Accepted for publication in Ap
Laboratory Determination of the Infrared Band Strengths of Pyrene Frozen in Water Ice: Implications for the Composition of Interstellar Ices
Broad infrared emission features (e.g., at 3.3, 6.2, 7.7, 8.6, and 11.3
microns) from the gas phase interstellar medium have long been attributed to
polycyclic aromatic hydrocarbons (PAHs). A significant portion (10%-20%) of the
Milky Way's carbon reservoir is locked in PAH molecules, which makes their
characterization integral to our understanding of astrochemistry. In molecular
clouds and the dense envelopes and disks of young stellar objects (YSOs), PAHs
are expected to be frozen in the icy mantles of dust grains where they should
reveal themselves through infrared absorption. To facilitate the search for
frozen interstellar PAHs, laboratory experiments were conducted to determine
the positions and strengths of the bands of pyrene mixed with H2O and D2O ices.
The D2O mixtures are used to measure pyrene bands that are masked by the strong
bands of H2O, leading to the first laboratory determination of the band
strength for the CH stretching mode of pyrene in water ice near 3.25 microns.
Our infrared band strengths were normalized to experimentally determined
ultraviolet band strengths, and we find that they are generally ~50% larger
than those reported by Bouwman et al. based on theoretical strengths. These
improved band strengths were used to reexamine YSO spectra published by Boogert
et al. to estimate the contribution of frozen PAHs to absorption in the 5-8
micron spectral region, taking into account the strength of the 3.25 micron CH
stretching mode. It is found that frozen neutral PAHs contain 5%-9% of the
cosmic carbon budget, and account for 2%-9% of the unidentified absorption in
the 5-8 micron region.Comment: Accepted for publication in ApJ on 14 Feb 201
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