4,175 research outputs found
Molecular Hydrogen in Orion as Observed by the Far Ultraviolet Spectroscopic Explorer
Diffuse far-ultraviolet stellar emission scattered by dust grains has been
observed in a region near the Orion Nebula. In addition to the scattered
stellar continuum, emission and absorption features produced by molecular
hydrogen have been identified. In this Letter, we present an analysis of this
absorption and fluorescent emission from molecular hydrogen in Orion. We model
the spectra obtained with the Far Ultraviolet Spectroscopic Explorer using
optical depth templates and a fluorescent emission code. These results are
surprising because previous studies have found little ultraviolet absorption
from H_2 in this region, and the emission is coming from a seemingly empty part
of the nebula. We find that the emission fills in the observed absorption lines
where the two overlap. These data support the claim that fluorescent excitation
by ultraviolet photons is the primary mechanism producing the near-infrared
emission spectrum observed in the outer regions of the Orion Nebula.Comment: 4 pages, 2 figures, uses emulate apj. ApJL - accepte
Far-Ultraviolet Dust Albedo Measurements in the Upper Scorpius Cloud Using the SPINR Sounding Rocket Experiment
The Spectrograph for Photometric Imaging with Numeric Reconstruction (SPINR)
sounding rocket experiment was launched on 2000 August 4 to record
far-ultraviolet (912-1450 A) spectral and spatial information for the giant
reflection nebula in the Upper Scorpius region. The data were divided into
three arbitrary bandpasses (912-1029 A, 1030-1200 A, and 1235-1450 A) for which
stellar and nebular flux levels were derived. These flux measurements were used
to constrain a radiative transfer model and to determine the dust albedo for
the Upper Scorpius region. The resulting albedos were 0.28+/-0.07 for the
912-1029 A bandpass, 0.33+/-0.07 for the 1030-1200 A bandpass, and 0.77+/-0.13
for the 1235-1450 A bandpass
Spitzer Mapping of PAHs and H2 in Photodissociation Regions
The mid-infrared (MIR) spectra of dense photodissociation regions (PDRs) are
typically dominated by emission from polycyclic aromatic hydrocarbons (PAHs)
and the lowest pure rotational states of molecular hydrogen (H2); two species
which are probes of the physical properties of gas and dust in intense UV
radiation fields. We utilize the high angular resolution of the Infrared
Spectrograph on the Spitzer Space Telescope to construct spectral maps of the
PAH and H2 features for three of the best studied PDRs in the galaxy, NGC 7023,
NGC 2023 and IC 63. We present spatially resolved maps of the physical
properties, including the H2 ortho-to-para ratio, temperature, and G_o/n_H. We
also present evidence for PAH dehydrogenation, which may support theories of H2
formation on PAH surfaces, and a detection of preferential self-shielding of
ortho-H2. All PDRs studied exhibit average temperatures of ~500 - 800K, warm H2
column densities of ~10^20 cm^-2, G_o/n_H ~ 0.1 - 0.8, and ortho-to-para ratios
of ~ 1.8. We find that while the average of each of these properties is
consistent with previous single value measurements of these PDRs, when
available, the addition of spatial resolution yields a diversity of values with
gas temperatures as high as 1500 K, column densities spanning ~ 2 orders of
magnitude, and extreme ortho-to-para ratios of 3.Comment: 14 figure
Revisiting the mechanism of reversed thermoremanent magnetization based on observations from synthetic ferrian ilmenite (y = 0.7)
International audienceThis study investigates the magnetic behavior of three well-characterized synthetic single-phase ferrian ilmenite (y = 0.7) specimens over the temperature range between 10 K and 573 K. Careful experiments measuring induced and remanent magnetizations in variable temperatures, applied magnetic fields, and pretreatment conditions are conducted in order to elucidate the mechanism leading to reversed thermoremanent magnetization (RTRM). Magnetic ordering temperatures of the cation ordered domains, in all three samples, are estimated at 380 K, suggesting that their Curie temperatures (T C) are independent of the sample's thermal history. This is not the case for cation disordered boundaries resulting from quenching from high temperatures. These cation disordered domains have estimated magnetic ordering temperatures of 418 K (Q1300), 410 K (Q1050), and 425 K (Q900). The data unambiguously support a less than perfect ferrimagnetic–antiferromagnetic exchange interaction as the fundamental source of RTRM. Furthermore, the magnetic field strength of the ''effective'' exchange anisotropies in such polycrystalline samples are estimated at 12 mT (Q1050), and 0 mT (Q900). However, from the results presented herein we conclude that favorable conditions for the acquisition of RTRM are dependent not only on the strength of the exchange anisotropy but also on the crucial role played by the size of the cation ordered domains. INDEX TERMS: 1519 Geomagnetism and Paleomagnetism: Magnetic mineralogy and petrology; 1540 Geomagnetism and Paleomagnetism: Rock and mineral magnetism; 1714 History of Geophysics: Geomagnetism and paleomagnetism; KEYWORDS: ferrian ilmenite, reversed thermoremanent magnetization, exchange anisotropy Citation: Lagroix, F., S. K. Banerjee, and B. M. Moskowitz (2004), Revisiting the mechanism of reversed thermoremanent magnetization based on observations from synthetic ferrian ilmenite (y = 0.7)
Atmospheric nitrogen oxides (NO and NO2) at Dome C, East Antarctica, during the OPALE campaign
Mixing ratios of the atmospheric nitrogen oxides NO and NO2 were measured as part of the OPALE (Oxidant Production in Antarctic Lands & Export) campaign at Dome C, East Antarctica (75.1 degrees S, 123.3 degrees E, 3233 m), during December 2011 to January 2012. Profiles of NOx mixing ratios of the lower 100m of the atmosphere confirm that, in contrast to the South Pole, air chemistry at Dome C is strongly influenced by large diurnal cycles in solar irradiance and a sudden collapse of the atmospheric boundary layer in the early evening. Depth profiles of mixing ratios in firn air suggest that the upper snowpack at Dome C holds a significant reservoir of photolytically produced NO2 and is a sink of gas-phase ozone (O-3). First-time observations of bromine oxide (BrO) at Dome C show that mixing ratios of BrO near the ground are low, certainly less than 5 pptv, with higher levels in the free troposphere. Assuming steady state, observed mixing ratios of BrO and RO2 radicals are too low to explain the large NO2 : NO ratios found in ambient air, possibly indicating the existence of an unknown process contributing to the atmospheric chemistry of reactive nitrogen above the Antarctic Plateau. During 2011-2012, NOx mixing ratios and flux were larger than in 2009-2010, consistent with also larger surface O-3 mixing ratios resulting from increased net O-3 production. Large NOx mixing ratios at Dome C arise from a combination of continuous sunlight, shallow mixing height and significant NOx emissions by surface snow (F-NOx). During 23 December 2011-12 January 2012, median F-NOx was twice that during the same period in 20092010 due to significantly larger atmospheric turbulence and a slightly stronger snowpack source. A tripling of F-NOx in December 2011 was largely due to changes in snowpack source strength caused primarily by changes in NO3- concentrations in the snow skin layer, and only to a secondary order by decrease of total column O-3 and associated increase in NO3- photolysis rates. A source of uncertainty in model estimates of F-NOx is the quantum yield of NO3- photolysis in natural snow, which may change over time as the snow ages
Multiwavelength Observations of Swift J1753.5-0127
We present contemporaneous X-ray, ultraviolet, optical and near-infrared
observations of the black hole binary system, Swift J1753.5-0127, acquired in
2012 October. The UV observations, obtained with the Cosmic Origins
Spectrograph on the Hubble Space Telescope, are the first UV spectra of this
system. The dereddened UV spectrum is characterized by a smooth, blue continuum
and broad emission lines of CIV and HeII. The system was stable in the UV to
<10% during our observations. We estimated the interstellar reddening by
fitting the 2175 A absorption feature and fit the interstellar absorption
profile of Ly to directly measure the neutral hydrogen column density
along the line of sight. By comparing the UV continuum flux to steady-state
thin accretion disk models, we determined upper limits on the distance to the
system as a function of black hole mass. The continuum is well fit with disk
models dominated by viscous heating rather than irradiation. The broadband
spectral energy distribution shows the system has declined at all wavelengths
since previous broadband observations in 2005 and 2007. If we assume that the
UV emission is dominated by the accretion disk the inner radius of the disk
must be truncated at radii above the ISCO to be consistent with the X-ray flux,
requiring significant mass loss from outflows and/or energy loss via advection
into the black hole to maintain energy balance.Comment: To appear in the Ap
Philosophy and Purposes of Distance Education
In considering how best to tackle this topic, I ultimately decided that in Nebraska the best approach is usually the most straight forward one. Given that much of the conversation about distance education is filled with myth and marketing hype, I decided what we could best contribute to the conversation would be the facts. While some of the other authors may have a better in-depth grasp of distance education opportunities and realities in Nebraska, Erik and I come as native Nebraskans with a good understanding of the history, economy, communities and cultures in our home state. My early faculty career here at UNL included producing thousands of radio and television programs in home economics and agricultural areas including a weekly educational television program in consumer affairs. I spent approximately 20 years of my career outside Nebraska in the U.S. Department of Agriculture in international training and development positions and as head of Communication, Information and Technology for the USDA Extension Service. Six of those 20 years were spent in Tanzania, East Africa, establishing a large human resource development program. My academic degrees are in journalism, nutrition and adult and continuing education - all from UNL. My current position as president of the ADEC distance education consortium which includes approximately 55 state university and land grant college members provides a unique perspective on the field of distance education locally, nationally and internationally. Erik France, who is assisting me in this work, is a journalism and business graduate from UNL and is currently doing his masters degree in the Department of Agricultural Leadership, Education and Communication. Erik conducted a series of Nebraska interviews related to the issues we were asked to cover. I also think he provides a youthful, up and coming perspective that is important for us to hear. We also consider this paper to be a draft, a document to stimulate those of you attending this conference and others who may want to contribute ideas following this conference on our ADEC website. We posted the paper to the website - http://www.adec.edu and would welcome additional insights that will assist us in completing the document by November 15. We have chosen to organize the document around five propositions - for each of these, I will provide rationale and discussion material, Erik will then provide perspectives from his interviews that relate and then we will ask for reactions from those of you in this room
X-ray to NIR emission from AA Tauri during the dim state - Occultation of the inner disk and gas-to-dust ratio of the absorber
AA Tau is a well-studied, nearby classical T Tauri star, which is viewed
almost edge-on. A warp in its inner disk periodically eclipses the central
star, causing a clear modulation of its optical light curve. The system
underwent a major dimming event beginning in 2011 caused by an extra absorber,
which is most likely associated with additional disk material in the line of
sight toward the central source. We present new XMM-Newton X-ray, Hubble Space
Telescope FUV, and ground based optical and near-infrared data of the system
obtained in 2013 during the long-lasting dim phase. The line width decrease of
the fluorescent H disk emission shows that the extra absorber is located at
au. Comparison of X-ray absorption () with dust extinction (),
as derived from measurements obtained one inner disk orbit (eight days) after
the X-ray measurement, indicates that the gas-to-dust ratio as probed by the
to ratio of the extra absorber is compatible with the ISM ratio.
Combining both results suggests that the extra absorber, i.e., material at
au, has no significant gas excess in contrast to the elevated
gas-to-dust ratio previously derived for material in the inner region
(au).Comment: 16 pages, 12 figures, accepted by A&
The UV Perspective of Low-Mass Star Formation
The formation of low-mass stars in molecular clouds involves accretion disks
and jets, which are of broad astrophysical interest. Accreting stars represent
the closest examples of these phenomena. Star and planet formation are also
intimately connected, setting the starting point for planetary systems like our
own. The ultraviolet (UV) spectral range is particularly suited to study star
formation, because virtually all relevant processes radiate at temperatures
associated with UV emission processes or have strong observational signatures
in the UV. In this review, we describe how UV observations provide unique
diagnostics for the accretion process, the physical properties of the
protoplanetary disk, and jets and outflows.Comment: 26 pages, 12 figures. Published in Galaxies special issue: "Star
Formation in the UV", ed. Jorick Vin
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