114 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
Rocket and FUSE Observations of IC 405: Differential Extinction and Fluorescent Molecular Hydrogen
We present far-ultraviolet spectroscopy of the emission/reflection nebula IC
405 obtained by a rocket-borne long-slit spectrograph and the Far Ultraviolet
Spectroscopic Explorer. Both data sets show a rise in the ratio of the nebular
surface brightness to stellar flux (S/F_*) of approximately two orders of
magnitude towards the blue end of the far-UV bandpass. Scattering models using
simple dust geometries fail to reproduce the observed S/F_* for realistic grain
properties. The high spectral resolution of the FUSE data reveals a rich
fluorescent molecular hydrogen spectrum ~1000" north of the star that is
clearly distinguished from the steady blue continuum. The S/F_* remains roughly
constant at all nebular pointings, showing that fluorescent molecular hydrogen
is not the dominant cause for the blue rise. We discuss three possible
mechanisms for the ``Blue Dust'': differential extinction of the dominant star
(HD 34078), unusual dust grain properties, and emission from nebular dust. We
conclude that uncertainties in the nebular geometry and the degree of dust
clumping are most likely responsible for the blue rise. As an interesting
consequence of this result, we consider how IC 405 would appear in a spatially
unresolved observation. If IC 405 was observed with a spatial resolution of
less than 0.4 pc, for example, an observer would infer a far-UV flux that was
2.5 times the true value, giving the appearance of a stellar continuum that was
less extinguished than radiation from the surrounding nebula, an effect that is
reminiscent of the observed ultraviolet properties of starburst galaxies.Comment: 8 pages, 9 figures, using emulateapj, ApJ - accepte
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
Fluorescent Molecular Hydrogen Emission in IC 63: FUSE, HUT, and Rocket Observations
We present far-ultraviolet observations of IC 63, an emission/reflection
nebula illuminated by the B0.5IV star gamma Cassiopeia, located 1.3 pc from the
nebula. Molecular hydrogen fluorescence was detected first in IC 63 by IUE and
later at shorter wavelengths by ORFEUS. Here we present Far Ultraviolet
Spectroscopic Explorer (FUSE) observations towards three locations in the
nebula, complemented by Hopkins Ultraviolet Telescope (HUT) data on the central
nebular position. In addition, we present a sounding rocket calibration of a
FUSE spectrum of gamma Cas. Molecular hydrogen fluorescence is detected in all
three FUSE pointings. The intensity of this emission as well as the
contributions from other species are seen to vary with position. The absolute
flux calibration of the sounding rocket data allows us to reliably predict the
radiation field incident on IC 63. We use these data to test models of the
fluorescent process. Our modeling resolves the perceived discrepancy between
the existing ultraviolet observations and achieves a satisfactory agreement
with the H_2 rotational structure observed with FUSE.Comment: 7 pages, 6 figures, using emulateapj. Accepted by Ap
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
Central Stars of Planetary Nebulae in the Large Magellanic Cloud: A Far-UV Spectroscopic Analysis
We observed seven central stars of planetary nebulae (CSPN) in the Large
Magellanic Cloud (LMC) with the Far Ultraviolet Spectroscopic Explorer (FUSE),
and performed a model-based analysis of these spectra in conjunction with
Hubble Space Telescope (HST) spectra in the UV and optical range to determine
the stellar and nebular parameters. Most of the objects show wind features, and
they have effective temperatures ranging from 38 to 60 kK with mass-loss rates
of ~= 5x10^-8 Msun/yr. Five of the objects have typical LMC abundances. One
object (SMP LMC 61) is a [WC4] star, and we fit its spectra with He/C/O-rich
abundances typical of the [WC] class, and find its atmosphere to be
iron-deficient. Most objects have very hot (T ~> 2000 K) molecular hydrogen in
their nebulae, which may indicate a shocked environment. One of these (SMP LMC
62) also displays OVI 1032-38 nebular emission lines, rarely observed in PN.Comment: 53 pages, 15 figures (11 color). Accepted for publication in Ap
Letter Binding and Invariant Recognition of Masked Words. Behavioral and Neuroimaging Evidence
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