17,614 research outputs found
Spitzer IRS Spectra of Luminous 8 micron Sources in the Large Magellanic Cloud: Testing color-based classifications
We present archival Spitzer IRS spectra of 19 luminous 8 micron selected
sources in the Large Magellanic Cloud (LMC). The object classes derived from
these spectra and from an additional 24 spectra in the literature are compared
with classifications based on 2MASS/MSX (J, H, K, and 8 micron) colors in order
to test the "JHK8" classification scheme (Kastner et al. 2008). The IRS spectra
confirm the classifications of 22 of the 31 sources that can be classified
under the JHK8 system. The spectroscopic classification of 12 objects that were
unclassifiable in the JHK8 scheme allow us to characterize regions of the
color-color diagrams that previously lacked spectroscopic verification,
enabling refinements to the JHK8 classification system. The results of these
new classifications are consistent with previous results concerning the
identification of the most infrared-luminous objects in the LMC. In particular,
while the IRS spectra reveal several new examples of asymptotic giant branch
(AGB) stars with O-rich envelopes, such objects are still far outnumbered by
carbon stars (C-rich AGB stars). We show that Spitzer IRAC/MIPS color-color
diagrams provide improved discrimination between red supergiants and
oxygen-rich and carbon-rich asymptotic giant branch stars relative to those
based on 2MASS/MSX colors. These diagrams will enable the most luminous IR
sources in Local Group galaxies to be classified with high confidence based on
their Spitzer colors. Such characterizations of stellar populations will
continue to be possible during Spitzer's warm mission, through the use of IRAC
[3.6]-[4.5] and 2MASS colors.Comment: 31 pages, 10 figures, to be published in A
Radio-Excess IRAS Galaxies: IV. Optical Spectroscopy
This is the fourth in our series of papers investigating radio-excess
galaxies, which have radio emission associated with an active nucleus but which
do not fit into the traditional categories of either radio-loud or radio-quiet
active galaxies. In this paper, we present optical spectra of our sample of
FIR-luminous radio-excess galaxies. Optical emission line diagnostics are used
to determine the dominant source of the ionizing radiation. We find that radio
excess is an excellent indicator of the presence of an active nucleus: the
radio-excess sample contains a much higher fraction of AGN than samples
selected on FIR luminosity alone, or using other criteria such as warm FIR
colors. Several objects have ambiguous classifications and are likely to be
composite objects with mixed excitation. The type of optical spectrum appears
to be associated with the radio-loudness: radio-loud objects may be more `pure'
AGN than radio-intermediate objects. We find strong evidence for interaction
between the radio plasma and the surrounding gas. The jet energy fluxes of the
radio-excess objects, inferred from the [O III] luminosities, are lower than in
powerful radio sources, consistent with our previous results. We conclude that
the jets of radio-intermediate sources are intrinsically weaker than those in
sources with more powerful radio emission. A significant fraction of the sample
spectra show post-starburst stellar continuum, with A-star absorption lines,
consistent with the large fraction of merging or disturbed host galaxies in the
sample. The ages of the radio sources are significantly less than those of A
stars indicating that, if the radio sources are associated with merging
activity, there is a delay between the interaction and the initiation of the
radio activity. (Abridged.)Comment: Accepted for publication in AJ; version with high resolution figures
available from http://www.cis.rit.edu/~clbsps/papers/paper4.pd
What makes Cryptococcus neoformans a pathogen?
Life-threatening infections caused by the encapsulated fungal pathogen Cryptococcus neoformans have been increasing steadily over the past 10 years because of the onset of AIDS and the expanded use of immunosuppressive drugs. Intricate host-organism interactions make the full understanding of pathogenicity and virulence of C. neoformans difficult. We discuss the current knowledge of the characteristics C. neoformans must possess to enter the host and establish progressive disease: basic growth requirements and virulence factors, such as the polysaccharide capsule; shed products of the organism; melanin production; mannitol secretion; superoxide dismutase; proteases; and phospholipases
Analyses of Dental Pulp in Restored Teeth
Restored teeth were extracted from test animals at four time intervals (1 hour, 1 day, 1 week, and 3 months) following amalgam insertion. Extracted teeth were frozen in liquid nitrogen, cryo-fractured so as to expose the pulps and then freeze-dried. Pulps were analyzed for mercury content by energy dispersive spectrometry (EDS) and atomic absorption spectrophotometry (AAS). Mercury levels appeared below the detection limits of EDS but could be detected by AAS which showed the highest readings seven days after amalgam insertion
EPR identification of defects responsible for thermoluminescence in Cu-doped lithium tetraborate (Li2B4O7) crystals
Electron paramagnetic resonance (EPR) is used to identify the electron and hole traps responsible for thermoluminescence (TL) peaks occurring near 100 and 200 â—¦C in copper-doped lithium tetraborate (Li2B4O7) crystals. As-grown crystals have Cu+ and Cu2+ ions substituting for lithium and have Cu+ ions at interstitial sites. All of the substitutional Cu2+ ions in the as-grown crystals have an adjacent lithium vacancy and give rise to a distinct EPR spectrum. Exposure to ionizing radiation at room temperature produces a second and different Cu2+ EPR spectrum when a hole is trapped by substitutional Cu+ ions that have no nearby defects. These two Cu2+ trapped-hole centers are referred to as Cu2+-VLi and Cu2+active, respectively. Also during the irradiation, two trapped-electron centers in the form of interstitial Cu0 atoms are produced when interstitial Cu+ ions trap electrons. They are observed with EPR and are labeled Cu0A and Cu0B. When an irradiated crystal is warmed from 25 to 150 â—¦C, the Cu2+active centers have a partial decay step that correlates with the TL peak near 100 â—¦C. The concentrations of Cu0A and Cu0B centers, however, increase as the crystal is heated through this range. As the crystal is futher warmed between 150 and 250 â—¦C, the EPR signals from the Cu2+active hole centers and Cu0A and Cu0B electron centers decay simultaneously. This decay step correlates with the intense TL peak near 200 â—¦C
WFPC2 LRF Imaging of Emission Line Nebulae in 3CR Radio Galaxies
We present HST/WFPC2 Linear Ramp Filter images of high surface brightness
emission lines (either [OII], [OIII], or H-alpha+[NII]) in 80 3CR radio
sources. We overlay the emission line images on high resolution VLA radio
images (eight of which are new reductions of archival data) in order to examine
the spatial relationship between the optical and radio emission. We confirm
that the radio and optical emission line structures are consistent with weak
alignment at low redshift (z < 0.6) except in the Compact Steep Spectrum (CSS)
radio galaxies where both the radio source and the emission line nebulae are on
galactic scales and strong alignment is seen at all redshifts. There are weak
trends for the aligned emission line nebulae to be more luminous, and for the
emission line nebula size to increase with redshift and/or radio power. The
combination of these results suggests that there is a limited but real capacity
for the radio source to influence the properties of the emission line nebulae
at these low redshifts (z < 0.6). Our results are consistent with previous
suggestions that both mechanical and radiant energy are responsible for
generating alignment between the radio source and emission line gas.Comment: 80 pages, 54 figures. Accepted for publication in ApJ
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