49 research outputs found
Rocket Observations of Far-Ultraviolet Dust Scattering in NGC 2023
The reflection nebula NGC 2023 was observed by a rocket-borne long-slit
imaging spectrograph in the 900 -- 1400 Angstrom bandpass on 2000 February 11.
A spectrum of the star, as well as that of the nebular scattered light, was
recorded. Through the use of a Monte Carlo modeling process, the scattering
properties of the dust were derived. The albedo is low, 0.2 -- 0.4, and
decreasing toward shorter wavelengths, while the phase function asymmetry
parameter is consistent with highly forward-scattering grains, g~0.85. The
decrease in albedo, while the optical depth increases to shorter wavelengths,
implies that the far-UV rise in the extinction curve is due to an increase in
absorption efficiency.Comment: 16 pages, 11 figures, accepted for publication in the Astrophysical
Journa
GALEX Observations of CS and OH Emission in Comet 9P/Tempel 1 During Deep Impact
GALEX observations of comet 9P/Tempel 1 using the near ultraviolet (NUV)
objective grism were made before, during and after the Deep Impact event that
occurred on 2005 July 4 at 05:52:03 UT when a 370 kg NASA spacecraft was
maneuvered into the path of the comet. The NUV channel provides usable spectral
information in a bandpass covering 2000 - 3400 A with a point source spectral
resolving power of approximately 100. The primary spectral features in this
range include solar continuum scattered from cometary dust and emissions from
OH and CS molecular bands centered near 3085 and 2575 A, respectively. In
particular, we report the only cometary CS emission detected during this event.
The observations allow the evolution of these spectral features to be tracked
over the period of the encounter. In general, the NUV emissions observed from
Tempel 1 are much fainter than those that have been observed by GALEX from
other comets. However, it is possible to derive production rates for the parent
molecules of the species detected by GALEX in Tempel 1 and to determine the
number of these molecules liberated by the impact. The derived quiescent
production rates are Q(H2O) = 6.4e27 molecules/s and Q(CS2) = 6.7e24
molecules/s, while the impact produced an additional 1.6e32 H2O molecules and
1.3e29 CS2 molecules, a similar ratio as in quiescent outgassing.Comment: 15 pages, 4 figures, accepted for publication in the Astrophysical
Journa
Direct Measurement of the Ratio of Carbon Monoxide to Molecular Hydrogen in the Diffuse Interstellar Medium
We have used archival far-ultraviolet spectra from observations made by
HST/STIS and FUSE to determine the column densities and rotational excitation
temperatures for CO and H2, respectively, along the lines of sight to 23
Galactic O and B stars. The sightlines have reddening values in the range
E(B-V)= 0.07-0.62, sampling the diffuse to translucent interstellar medium. We
find that the H2 column densities range from 5x10^18-8x10^20 cm^-2 and the CO
from upper limits around 2x10^12 cm^-2 to detections as high as 1.4x10^16
cm^-2. CO increases with increasing H2, roughly following a power law of factor
\~2. The CO/H2 column density ratio is thus not constant, and ranges from 10^-7
- 10^-5, with a mean value of 3x10^-6. The sample segregates into "diffuse" and
"translucent" regimes, the former having a molecular fraction less than ~0.25
and A_V/d<1 mag kpc^-1. The mean CO/H2 for these two regimes are 3.6x10^-7 and
9.3x10^-6, respectively, significantly lower than the canonical dark cloud
value of 10^-4. In six of the sightlines, 13CO is observed, and the isotopic
ratio we observe (~50-70) is consistent with, if perhaps a little below, the
average 12C/13C for the ISM at large. The average H2 rotational excitation
temperature is 74+/-24 K, in good agreement with previous studies, and the
average CO temperature is 4.1 K, with some sightlines as high as 6.4 K. The
higher excitation CO is observed with higher column densities, consistent with
the effects of photon trapping in clouds with densities in the 20-100 cm^-3
range. We discuss the implications for the structure of the diffuse/translucent
regimes of the interstellar medium and the estimation of molecular mass in
galaxies.Comment: emualateapj style, 6 figures, 3 tables, accepted on 21 Nov 2006 for
publication in The Astrophysical Journa
Calibration and flight performance of the long-slit imaging dual order spectrograph
We present a preliminary calibration and flight performance of the Long-Slit
Imaging Dual Order Spectrograph (LIDOS), a rocket-borne instrument with a large
dynamic range in the 900 - 1700A bandpass. The instrument observes UV-bright
objects with a CCD channel and fainter nebulosity with an MCP detector. The
image quality and the detector quantum efficiencies were determined using the
calibration and test equipment at the Johns Hopkins University, and further
monitored using an on-board electron-impact calibration lamp. We review results
from each of the three flights of the instrument.Comment: 12 pages. to appear in Proc. SPIE 701
Potential Variations in the Interstellar N I Abundance
We present Far Ultraviolet Spectroscopic Explorer (FUSE) and Space Telescope
Imaging Spectrograph observations of the weak interstellar N I doublet at 1160
Angstroms toward 17 high-density sight lines [N(Htot)>=10^21 cm^-2]. When
combined with published data, our results reveal variations in the fractional N
I abundance showing a systematic deficiency at large N(Htot). At the FUSE
resolution (~20 km s^-1), the effects of unresolved saturation cannot be
conclusively ruled out, although O I at 1356 Angstroms shows little evidence of
saturation. We investigated the possibility that the N I variability is due to
the formation of N_2 in our mostly dense regions. The 0-0 band of the c'_4
^1Sigma^+_u - X ^1Sigma^+_g transition of N_2 at 958 Angstroms should be easily
detected in our FUSE data; for 10 of the denser sight lines, N_2 is not
observed at a sensitivity level of a few times 10^14 cm^-2. The observed N I
variations are suggestive of an incomplete understanding of nitrogen chemistry.
Based on observations made with the NASA-CNES-CSA Far Ultraviolet
Spectroscopic Explorer, which is operated for NASA by the Johns Hopkins
University under NASA contract NAS 5-32985, and the NASA/ESA Hubble Space
Telescope, obtained from the Multimission Archive at the Space Telescope
Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under the NASA contract NAS 5-26555.Comment: 12 pages, 3 figures, accepted for publication in ApJ Letter
On the Correlation Between CO Absorption and Far-Ultraviolet Non-Linear Extinction Toward Galactic OB Stars
A sample of 59 sight lines to reddened Galactic OB stars was examined for
correlations of the strength of the CO Fourth Positive (A - X) absorption band
system with the ultraviolet interstellar extinction curve parameters. We used
archival high-dispersion NEWSIPS IUE spectra to measure the CO absorption for
comparison to parametric fits of the extinction curves from the literature. A
strong correlation with the non-linear far-UV curvature term was found with
greater absorption, normalized to E(B-V), being associated with more curvature.
A weaker trend with the linear extinction term was also found. Mechanisms for
enhancing CO in dust environments exhibiting high non-linear curvature are
discussed.Comment: 10 pages, including 6 figures. LaTeX2e (emulateapj5.sty). To appear
in ApJ, Sep 20, 200