80 research outputs found
Design principles of a rotating medium speed mechanism
Design principles of a medium speed mechanism (MSM) are presented, including discussion on the relative merits of beryllium and aluminium as structural materials. Rotating at a speed of 60 rpm, the application envisaged for the MSM was as a despin bearing for the despun platform or despun antenna of a spin stabilized satellite. The MSM was built and tested to qualification level and is currently undergoing real time life testing
High resolution spectroscopy of the 11.3 micron emission band
High resolution spectra of the 11.3 micron emission band in M82 and NGC 7027 were obtained using the University of Texas IR echelle spectrometer on the IRTF in April 1988. The spectral resolution was 0.004 micron, with coverage from 11.0 to 11.6 microns. Spectra were measured at ten positions along a 10 min. long slit. Analysis of the data is still in progress, but initial results show no clear evidence of narrow structure within the feature. The analysis will involve comparison of the observed spectra to laboratory and predicted spectra of Polycylic Aromatic Hydrocarbons (PAHs) and Quenched Carbonaceous Composite (QCCs) to determine which may be responsible for the emission. The spectra will be examined with a goal of determining whether the emission is caused by molecular or solid state material. The data are also examined for evidence of variations in the shape and strength of the 11.3 micron feature with position on the sky. In NGC 7027 the 10 min. long slit went across the edge of the ionized nebulae, allowing comparison of emission from both ionized and neutral regions
Detailed investigations of PMTs in optical sensors for neutrino telescopes such as IceCube Upgrade
Photomultiplier tubes (PMTs) are a central component of neutrino telescopes
such as IceCube and KM3NeT, and an accurate understanding and measurement of
their properties is indispensable for improvements of these experiments. In
this contribution we focus on a detailed investigation of the photocathode and
the dynode system and their influence on the performance of the PMT. Three
methods are used for the investigation. Ellipsometry measurements of the
photocathode analyze its optical properties in terms of absorption probability
and refractive index. Scans of the photocathode in single photon illumination
probe performance differences along the photocathode surface. Systematic
deviations in the resulting amplifications are compared to electric field and
electron tracing simulations through the dynode system to understand the
measured values. The goal is an extensive understanding of efficiency,
amplification, and timing as functions of wavelength and impact point as well
as angle.Comment: Presented at the 38th International Cosmic Ray Conference (ICRC2023).
See arXiv:2307.13047 for all IceCube contribution
A Parsec-Scale Study of the 5/15 GHz Spectral Indices of the Compact Radio Sources in M82
Observations of the starburst galaxy, M82, have been made with the VLA in its
A-configuration at 15 GHz and MERLIN at 5 GHz enabling a spectral analysis of
the compact radio structure on a scale of < 0.1'' (1.6 pc). Crucial to these
observations was the inclusion of the Pie Town VLBA antenna, which increased
the resolution of the VLA observations by a factor of ~2. A number of the
weaker sources are shown to have thermal spectra and are identified as HII
regions with emission measures ~10^7 cm^-6 pc. Some of the sources appear to be
optically thick at 5 GHz implying even higher emission measures of ~10^8 cm^-6
pc. The number of compact radio sources in M82 whose origin has been determined
is now 46, of which 30 are supernova related and the remaining 16 are HII
regions. An additional 15 sources are noted, but have yet to be identified,
meaning that the total number of compact sources in M82 is at least 61. Also,
it is shown that the distribution of HII regions is correlated with the
large-scale ionised gas distribution, but is different from the distribution of
supernova remnants. In addition, the brightest HII region at (B1950) 09h 51m
42.21s +69 54' 59.2'' shows a spectral index gradient across its resolved
structure which we attribute to the source becoming optically thick towards its
centre.Comment: Accepted for publication in MNRAS. 15 pages, 9 figure
The Mid-Infrared [SIV]/[NeII] versus [NeIII]/[NeII] Correlation
The mid-infrared ratio [NeIII]15.6mum/[NeII]12.8mum is a strong diagnostic of
the ionization state of emission line objects, due to its use of only strong
neon emission lines only weakly affected by extinction. However this ratio is
not available to ground-based telescopes as only a few spectroscopic windows
are available in the MIR. To deal with this problem we aimed to verify if there
exists a conversion law between ground-accessible, strong MIR line ratio
[SIV]/[NeII] and the diagnostic [NeIII]/[NeII] ratio that can serve as a
reference for future ground-based observations. We collated the [SIV]10.5mum,
[NeII]12.8mum, [NeIII]15.6\mum and [SIII]18.7mum emission line fluxes from a
wide range of sources in the rich Spitzer and ISO archives, and compared the
[NeIII]/[NeII], [SIV]/[SIII], and [SIV]/[NeII] ratios. We find a strong
correlation between the [SIV]/[NeII] and [\neiii]/[\neii] ratio, with a linear
fit of log([NeIII]/[NeII]) = 0.81log([SIV]/[NeII])+0.36, accurate to a factor
of ~2 over four orders of magnitude in the line ratios. This demonstrates
clearly the ability of ground-based infrared spectrographs to do ionization
studies of nebulae.Comment: 5 pages, Accepted for MNRAS letter
High-velocity-resolution observations of OH main line lasers in the M82 starburst
Using the VLA, a series of high velocity resolution observations have been
made of the M82 starburst at 1.6 GHz. These observations follow up on previous
studies of the main line OH maser emission in the central kiloparsec of this
starburst region, but with far greater velocity resolution, showing significant
velocity structure in some of the maser spots for the first time. A total of
thirteen masers were detected, including all but one of the previously known
sources. While some of these masers are still unresolved in velocity, these new
results clearly show velocity structure in spectra from several of the maser
regions. Position-velocity plots show good agreement with the distribution of
H{\sc i} including interesting velocity structure on the blueward feature in
the west of the starburst which traces the velocity distribution seen in the
ionised gas.Comment: MNRAS in press. 15 pages, 9 figure
Rayleigh Scattering in Rare Gas Liquids
The Rayleigh scattering length has been calculated for rare-gas liquids in
the ultraviolet for the frequencies at which they luminesce. The calculations
are based on the measured dielectric constants in the gas phase, except in the
case of xenon for which measurements are available in the liquid. The
scattering length mayplace constraints on the design of some large-scale
detectors, using uv luminescence, being proposed to observe solar neutrinos and
dark matter. Rayleigh scattering in mixtures of rare-gas mixtures is also
discussed.Comment: 8 pages, 4 tables; This version corrects erratum in table and has
expanded discussion in Section II. Accepred for publication in NIM
Index of refraction, Rayleigh scattering length, and Sellmeier coefficients in solid and liquid argon and xenon
Large liquid argon detectors have become widely used in low rate experiments,
including dark matter and neutrino research. However, the optical properties of
liquid argon are not well understood at the large scales relevant for current
and near-future detectors.The index of refraction of liquid argon at the scin-
tillation wavelength has not been measured, and current Rayleigh scattering
length calculations disagree with measurements. Furthermore, the Rayleigh
scattering length and index of refraction of solid argon and solid xenon at
their scintillation wavelengths have not been previously measured or
calculated. We introduce a new calculation using existing data in liquid and
solid argon and xenon to extrapolate the optical properties at the
scintillation wavelengths using the Sellmeier dispersion relationship.Comment: 11 pages, 4 figure
The Nuclear Starburst in NGC 253
We have obtained long-slit spectra of NGC 253 in the J, H, K, and N bands,
broadband images in the J, H, and Ks bands, narrowband images centered at the
wavelengths of BrGamma and H2(1,0)S(1), and imaging spectroscopy centered on
[NeII](12.8um). We use these data and data from the literature in a
comprehensive re-assessment of the starburst in this galaxy. We derive the
supernova rate from the strength of the infrared [FeII] lines. We find that
most of the H2 infrared luminosity is excited by fluorescence in low density
gas. We derive a strong upper limit of ~37,000K for the stars exciting the
emission lines. We use velocity-resolved infrared spectra to determine the mass
in the starburst region. Most of this mass appears to be locked up in the old,
pre-existing stellar population. Using these constraints and others to build an
evolutionary synthesis model, we find that the IMF originally derived to fit
the starburst in M 82 (similar to a Salpeter IMF) also accounts for the
properties of NGC 253. The models indicate that rapid massive star formation
has been ongoing for 20-30 million years in NGC 253---that is, it is in a late
phase of its starburst. We model the optical emission line spectrum expected
from a late phase starburst and demonstrate that it reproduces the observed
HII/weak-[OI] LINER characteristics.Comment: 48 pages, 14 figures, uses AASTeX macros, to appear in Ap
ISOCAM view of the starburst galaxies M82, NGC253, and NGC1808
We present results of mid-infrared 5.0-16.5 micron spectrophotometric imaging
of the starburst galaxies M82, NGC253, and NGC1808 from the ISOCAM instrument
on board the Infrared Space Observatory. The mid-infrared spectra of the three
galaxies are very similar in terms of features present. The > 11 micron
continuum attributed to very small dust grains (VSGs) exhibits a large spread
in intensity relative to the short-wavelength emission. We find that the 15
micron dust continuum flux density correlates well with the fine-structure
[ArII] 6.99 micron line flux and thus provides a good quantitative indicator of
the level of star formation activity. By contrast, the 5-11 micron region
dominated by emission from polycyclic aromatic hydrocarbons (PAHs) has a nearly
invariant shape. Variations in the relative intensities of the PAH features are
nevertheless observed, at the 20%-100% level. We illustrate extinction effects
on the shape of the mid-infrared spectrum of obscured starbursts, emphasizing
the differences depending on the applicable extinction law and the consequences
for the interpretation of PAH ratios and extinction estimates. The relative
spatial distributions of the PAH, VSG, and [ArII] 6.99 micron emission between
the three galaxies exhibit remarkable differences. The < 1 kpc size of the
mid-infrared source is much smaller than the optical extent of our sample
galaxies and 70%-100% of the IRAS 12 micron flux is recovered within the ISOCAM
< 1.5 arcmin squared field of view, indicating that the nuclear starburst
dominates the total mid-infrared emission while diffuse light from quiescent
disk star formation contributes little.Comment: 25 pages, 12 figures, accepted for publication in Astronomy and
Astrophysics; Figs. 3, 4, 5, 6, 7, 9, 10, 12 appear after Sect.
- …