652 research outputs found
FUSE Detection of Galactic OVI Emission in the Halo above the Perseus Arm
Background observations obtained with the Far Ultraviolet Spectroscopic
Explorer (FUSE) toward l=95.4, b=36.1 show OVI 1032,1038 in emission. This
sight line probes a region of stronger-than-average soft X-ray emission in the
direction of high-velocity cloud Complex C above a part of the disk where
Halpha filaments rise into the halo. The OVI intensities, 1600+/-300
ph/s/cm^2/sr (1032A) and 800+/-300 ph/s/cm^2/sr (1038A), are the lowest
detected in emission in the Milky Way to date. A second sight line nearby
(l=99.3, b=43.3) also shows OVI 1032 emission, but with too low a
signal-to-noise ratio to obtain reliable measurements. The measured
intensities, velocities, and FWHMs of the OVI doublet and the CII* line at
1037A are consistent with a model in which the observed emission is produced in
the Galactic halo by hot gas ejected by supernovae in the Perseus arm. An
association of the observed gas with Complex C appears unlikely.Comment: accepted for publication in ApJL, 11 pages including 3 figure
First optical images of circumstellar dust surrounding the debris disk candidate HD 32297
Near-infrared imaging with the Hubble Space Telescope recently revealed a
circumstellar dust disk around the A star HD 32297. Dust scattered light is
detected as far as 400 AU radius and the linear morphology is consistent with a
disk ~10 degrees away from an edge-on orientation. Here we present the first
optical images that show the dust scattered light morphology from 560 to 1680
AU radius. The position angle of the putative disk midplane diverges by 31
degrees and the color of dust scattering is most likely blue. We associate HD
32297 with a wall of interstellar gas and the enigmatic region south of the
Taurus molecular cloud. We propose that the extreme asymmetries and blue disk
color originate from a collision with a clump of interstellar material as HD
32297 moves southward, and discuss evidence consistent with an age of 30 Myr or
younger.Comment: 5 pages; Accepted for publication in ApJ Letter
Surprisingly Little O VI Emission Arises in the Local Bubble
This paper reports the first study of the O VI resonance line emission (1032,
1038 Angstroms) originating in the Local Bubble (or Local Hot Bubble)
surrounding the solar neighborhood. In spite of the fact that O VI absorption
within the Local Bubble has been observed, no resonance line emission was
detected during our 230 ksec Far Ultraviolet Spectroscopic Explorer observation
toward a ``shadowing'' filament in the southern Galactic hemisphere. As a
result, tight 2 sigma upper limits are set on the intensities in the 1032 and
1038 Angstrom emission lines: 500 and 530 photons cm^{-2} s^{-1} sr^{-1},
respectively. These values place strict constraints on models and simulations.
They suggest that the O VI-bearing plasma and the X-ray emissive plasma reside
in distinct regions of the Local Bubble and are not mixed in a single plasma,
whether in equilibrium with T ~ 10^6 K or highly overionized with T ~ 4 to 6 x
10^4 K. If the line of sight intersects multiple cool clouds within the Local
Bubble, then the results also suggest that hot/cool transition zones differ
from those in current simulations. With these intensity upper limits, we
establish limits on the electron density, thermal pressure, pathlength, and
cooling timescale of the O VI-bearing plasma in the Local Bubble. Furthermore,
the intensity of O VI resonance line doublet photons originating in the
Galactic thick disk and halo is determined (3500 to 4300 photons cm^{-2} s^{-1}
sr^{-1}), and the electron density, thermal pressure, pathlength, and cooling
timescale of its O VI-bearing plasma are calculated. The pressure in the
Galactic halo's O VI-bearing plasma (3100 to 3800 K cm^{-3}) agrees with model
predictions for the total pressure in the thick disk/lower halo. We also report
the results of searches for other emission lines.Comment: accepted by ApJ, scheduled for May 2003, replacement astro-ph
submission corrects typos and grammatical errors in original versio
Evidence for deuterium astration in the planetary nebula Sh2-216?
We present FUSE observations of the line of sight to WD0439+466 (LS V +46
21), the central star of the old planetary nebula Sh2-216. The FUSE data shows
absorption by many interstellar and stellar lines, in particular D I, H2 (J = 0
- 9), HD (J = 0 - 1), and CO. Many other stellar and ISM lines are detected in
the STIS E140M HST spectra of this sightline, which we use to determine N(HI).
We derive, for the neutral gas, D/H=(0.76 +0.12 -0.11)E-5, O/H = (0.89 +0.15
-0.11)E-4 and N/H = (3.24 +0.61-0.55)E-5. We argue that most of the gas along
this sightline is associated with the planetary nebula. The low D/H ratio is
likely the result of this gas being processed through the star (astrated) but
not mixed with the ISM. This would be the first time that the D/H ratio has
been measured in predominantly astrated gas. The O/H and N/H ratios derived
here are lower than typical values measured in other planetary nebulae likely
due to unaccounted for ionization corrections.Comment: Accepted for publication is ApJ
Ultrafast Thermal Modification of Strong Coupling in an Organic Microcavity
There is growing interest in using strongly coupled organic microcavities to
tune molecular dynamics, including the electronic and vibrational properties of
molecules. However, very little attention has been paid to the utility of
cavity polaritons as sensors for out-of-equilibrium phenomena, including
thermal excitations. Here, we demonstrate that non-resonant infrared excitation
of an organic microcavity system induces a transient response in the visible
spectral range near the cavity polariton resonances. We show how these optical
response can be understood in terms of ultrafast heating of electrons in the
metal cavity mirror, which modifies the effective refractive index and
subsequently the strong coupling conditions. The temporal dynamics of the
microcavity are strictly determined by carriers in the metal, including the
cooling of electrons via electron-phonon coupling and excitation of propagating
coherent acoustic modes in the lattice. We rule out multiphoton excitation
processes and verify that no real polariton population exists despite their
strong transient features. These results suggest the promise of cavity
polaritons as sensitive probes of non-equilibrium phenomena
The Millennium Arecibo 21-CM Absorption Line Survey. II. Properties of the Warm and Cold Neutral Media
We use the Gaussian-fit results of Paper I to investigate the properties of
interstellar HI in the Solar neighborhood. The Warm and Cold Neutral Media (WNM
and CNM) are physically distinct components. The CNM spin temperature histogram
peaks at about 40 K. About 60% of all HI is WNM. At z=0, we derive a volume
filling fraction of about 0.50 for the WNM; this value is very rough. The
upper-limit WNM temperatures determined from line width range upward from about
500 K; a minimum of about 48% of the WNM lies in the thermally unstable region
500 to 5000 K. The WNM is a prominent constituent of the interstellar medium
and its properties depend on many factors, requiring global models that include
all relevant energy sources, of which there are many. We use Principal
Components Analysis, together with a form of least squares fitting that
accounts for errors in both the independent and dependent parameters, to
discuss the relationships among the four CNM Gaussian parameters. The spin
temperature T_s and column density N(HI) are, approximately, the two most
important eigenvectors; as such, they are sufficient, convenient, and
physically meaningful primary parameters for describing CNM clouds. The Mach
number of internal macroscopic motions for CNM clouds is typically 2.5, but
there are wide variations. We discuss the historical tau-T_s relationship in
some detail and show that it has little physical meaning. We discuss CNM
morphology using the CNM pressure known from UV stellar absorption lines.
Knowing the pressure allows us to show that CNM structures cannot be isotropic
but instead are sheetlike, with length-to-thickness aspect ratios ranging up to
about 280. We present large-scale maps of two regions where CNM lies in very
large ``blobby sheets''.Comment: Revised submission to Ap.J. Changes include: (1) correction of
turbulent Mach number in equation 16 and figure 12; the new typical value is
1.3 versus the old, incorrect value 2.5. (2) smaller typeface for the
astro-ph version to conserve paper. 60 pages, 16 figure
The Deuterium, Oxygen, and Nitrogen Abundance Toward LSE 44
We present measurements of the column densities of interstellar DI, OI, NI,
and H2 made with FUSE, and of HI made with IUE toward the sdO star LSE 44, at a
distance of 554+/-66 pc. This target is among the seven most distant Galactic
sight lines for which these abundance ratios have been measured. The column
densities were estimated by profile fitting and curve of growth analyses. We
find D/H = (2.24 +1.39 -1.32)E-5, D/O = (1.99 +1.30 -0.67)E-2, D/N = (2.75
+1.19 -0.89)E-1, and O/H = (1.13 +0.96 -0.71)E-3 (2 sigma). Of the most distant
Galactic sight lines for which the deuterium abundance has been measured LSE 44
is one of the few with D/H higher than the Local Bubble value, but D/O toward
all these targets is below the Local Bubble value and more uniform than the D/H
distribution. (Abstract abridged.)Comment: 20 pages, including 9 figures. Accepted for publication in Ap
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