1,700 research outputs found
Analysis of IUE observations of hydrogen in comets
The large body of hydrogen Lyman-alpha observations of cometary comae obtained with the International Ultraviolet Explorer satellite has gone generally unanalyzed because of two main modeling complications. First, the inner comae of many bright (gas productive) comets are often optically thick to solar Lyman-alpha radiation. Second, even in the case of a small comet (low gas production) the large IUE aperture is quite small as compared with the immense size of the hydrogen coma, so an accurate model which properly accounts for the spatial distribution of the coma is required to invert the inferred brightnesses to column densities and finally to H atom production rates. Our Monte Carlo particle trajectory model (MPTM), which for the first time provides the realistic full phase space distribution of H atoms throughout the coma was used as the basis for the analysis of IUE observations of the inner coma. The MCPTM includes the effects of the vectorial ejection of the H atoms upon dissociation of their parent species (H2O and OH) and of their partial collisional thermalization. Both of these effects are crucial to characterize the velocity distribution of the H atoms. A new spherical radiative transfer calculation based on our MCPTM was developed to analyze IUE observations of optically thick H comae. The models were applied to observations of comets P/Giacobini-Zinner and P/Halley
Re-Examination of Globally Flat Space-Time
In the following, we offer a novel approach to modeling the observed effects
currently attributed to the theoretical concepts of `dark energy', `dark
matter', and `dark flow'. Instead of assuming the existence of these
theoretical concepts, we take an alternative route and choose to redefine what
we consider to be inertial motion as well as what constitutes an inertial frame
of reference in flat space-time. We adopt none of the features of our current
cosmological models except for the requirement that special and general
relativity be local approximations within our revised definition of inertial
systems. Implicit in our ideas is the assumption that at "large enough" scales
one can treat objects within these inertial systems as point-particles having
an insignificant effect on the curvature of space-time. We then proceed under
the assumption that time and space are fundamentally intertwined such that
time- and spatial-translational invariance are not inherent symmetries of flat
space-time (i.e. observable clock rates depend upon both relative velocity and
spatial position within these inertial systems) and take the geodesics of this
theory in the radial Rindler chart as the proper characterization of inertial
motion. With this commitment, we are able to model solely with inertial motion
the observed effects expected to be the result of `dark energy', `dark matter',
and `dark flow'. In addition, we examine the potential observable implications
of our theory in a gravitational system located within a confined region of an
inertial reference frame, subsequently interpreting the Pioneer anomaly as
support for our redefinition of inertial motion. As well, we extend our
analysis into quantum mechanics by quantizing for a real scalar field and find
a possible explanation for the asymmetry between matter and antimatter within
the framework of these redefined inertial systems.Comment: 55 pages, 3 figures, 2 tables, 6 appendices; added two unpublished
sections with supplementary material (see Comments section of PLoS ONE
publication link). None of the physical content of the original article has
change
Deep space experiment to measure
Responding to calls from the National Science Foundation (NSF) for new
proposals to measure the gravitational constant , we offer an interesting
experiment in deep space employing the classic gravity train mechanism. Our
setup requires three bodies: a larger layered solid sphere with a cylindrical
hole through its center, a much smaller retroreflector which will undergo
harmonic motion within the hole and a host spacecraft with laser ranging
capabilities to measure round trip light-times to the retroreflector but
ultimately separated a significant distance away from the sphere-retroreflector
apparatus. Measurements of the period of oscillation of the retroreflector in
terms of host spacecraft clock time using existing technology could give
determinations of nearly three orders of magnitude more accurate than
current measurements here on Earth. However, significant engineering advances
in the release mechanism of the apparatus from the host spacecraft will likely
be necessary. Issues with regard to the stability of the system are briefly
addressed.Comment: 13 pp, 3 figs, accepted CQ
GLUCOCORTICOIDS AND CELLULAR IMMUNITY IN VITRO : FACILITATION OF THE SENSITIZATION PHASE AND INHIBITION OF THE EFFECTOR PHASE OF A LYMPHOCYTE ANTI-FIBROBLAST REACTION
We studied the influence of glucocorticoids on the sensitization phase as well as on the cytolytic effector phase of an in vitro lymphocyte-mediated immune reaction. Lymphocytes obtained from the spleens or lymph nodes of unimmunized inbred rats were sensitized against foreign rat or mouse embryonic fibroblasts in cell culture. The capacity of the sensitized lymphocytes to produce a cytolytic effect was tested by transferring them to target fibroblast cultures. Injury to target fibroblasts was measured by release of radioactive 51Cr from previously labeled fibroblasts or by direct count of viable fibroblasts after incubation with sensitized lymphocytes. Various concentrations of water-soluble hydrocortisone or prednisolone were added to cell cultures during the 5 day sensitization phase and/or during the subsequent cytolytic effector phase and the influence of these hormones on the number and cytolytic capacity of the lymphocytes was measured. During the sensitization phase, the presence of glucocorticoid hormones, at concentrations of about 1 µg/ml, led to a profound decrease in the total number of recoverable lymphocytes. However, the per cent of large transformed lymphocytes was much greater in these treated cultures. The antigen-specific cytolytic capacity per cell of the glucocorticoid-treated lymphocytes, after the hormone was removed, was several times greater than that of lymphocytes sensitized in the absence of added hormones. Glucocorticoids influenced the effector phase of the reaction by inhibiting lymphocyte-mediated injury to target fibroblasts. The hormones, at concentrations of about 1 µg/ml, inhibited the cytolytic effect by about 50% without reducing the viability of the sensitized lymphocytes. Dose-dependent toxicity to lymphocytes and increasing inhibition of cytolytic effect appeared at higher concentrations of hormones. Thus, hydrocortisone and prednisolone, at concentrations of about 1 µg/ml, did not suppress the induction of sensitization, a process which they seem to facilitate in vitro. However, similar concentrations of these hormones appear to inhibit the cytolytic effector mechanism of sensitized lymphocytes. These findings may be relevant to the use of glucocorticoids as immunosuppressive agents in vivo
REJECTION OF TUMOR ALLOGRAFTS BY MOUSE SPLEEN CELLS SENSITIZED IN VITRO
This paper reports a model system of cellular immunity in which allosensitization of mouse spleen cells is induced in vitro. Allosensitization was achieved by culturing spleen cells upon monolayers of allogeneic fibroblasts. The ability of the spleen cells to inhibit the growth of tumor allografts in vivo served as a functional assay of sensitization. We found that unsensitized spleen cells or spleen cells sensitized against unrelated fibroblast antigens had no inhibitory effect on the growth of allogeneic fibrosarcoma cells when they were injected together into irradiated recipients. In contrast, spleen cells which were specifically allosensitized in vitro were found to be highly effective in inhibiting the growth of an equal number of allogeneic tumor cells. Several times more spleen cells from mice sensitized in vivo were required to produce a similar immune effect. This confirms the findings of previous studies which indicate that sensitization in cell culture can promote the selection of specifically sensitized lymphocytes. Preincubating sensitizing fibroblasts with allo-antisera blocked the allosensitization of spleen cells. This suggests that antibodies binding to fibroblasts may inhibit the induction of sensitization by competing with lymphocytes for antigenic sites. Mouse spleen cells which were able to recognize and reject tumor allografts in vivo were unable to cause lysis of target fibroblasts in vitro. Such fibroblasts, however, were susceptible to lysis by rat lymphoid cells sensitized by a similar in vitro method. These findings indicate that the conditions required for lymphocyte-mediated lysis of target cells may not be directly related to the processes of antigen recognition and allograft rejection in vivo
AUTOSENSITIZATION IN VITRO
Autosensitization of rat or mouse lymphoid cells against syngeneic fibroblast antigens was induced in cell culture. Rat lymphoid cells autosensitized by this method were able to produce immunospecific lysis of syngeneic target fibroblasts in vitro or GvH reactions in newborn rats. Autosensitized mouse spleen cells mediated similar GvH reactions when injected into newborn mice. The nature of the system used to induce immunity in vitro appears to argue against the possibility that lymphocytes capable of reacting against self-antigens could arise by mutation in cell culture. Hence, it is likely that cells potentially reactive against self-antigens preexisted in the lymphoid cell donors. The ability of autosensitized cells to mediate immune reactions in vivo suggests that the immunogenic self-antigens present on sensitizing fibroblasts also were accessible in the intact animals. Loss of natural self-tolerance in vitro, therefore, can be explained most simply by the existence of lymphocytes which are reversibly tolerant to self. Hence, ontogenic elimination of potentially self-reactive cells may not be the only basis for natural tolerance. Regulatory mechanisms, such as antigen excess, may have to function in vivo to prevent differentiation of self-tolerant lymphocytes. These regulatory mechanisms appear to be annulled in the cell-culture system. The present system thus may offer a new approach to studies of tolerance and regulation of cellular immunity
Analysis of IUE Observations of Hydrogen in Comets
The 15-years worth of hydrogen Lyman-alpha observations of cometary comae obtained with the International Ultraviolet Explorer (IUE) satellite had gone generally unanalyzed because of two main modeling complications. First, the inner comae of many bright (gas productive) comets are often optically thick to solar Lyman-alpha radiation. Second, even in the case of a small comet (low gas production) the large IUE aperture is quite small as compared with the immense size of the hydrogen coma, so an accurate model which properly accounts for the spatial distribution of the coma is required to invert the infrared brightnesses to column densities and finally to H atom production rates. Our Monte Carlo particle trajectory model (MCPTM), which for the first time provides the realistic full phase space distribution of H atoms throughout the coma has been used as the basis for the analysis of IUE observations of the inner coma. The MCPTM includes the effects of the vectorial ejection of the H atoms upon dissociation of their parent species (H2O and OH) and of their partial collisional thermalization. Both of these effects are crucial to characterize the velocity distribution of the H atoms. This combination of the MCPTM and spherical radiative transfer code had already been shown to be successful in understanding the moderately optically thick coma of comet P/Giacobini-Zinner and the coma of comet Halley that varied from being slightly to very optically thick. Both of these comets were observed during solar minimum conditions. Solar activity affects both the photochemistry of water and the solar Lyman-alpha radiation flux. The overall plan of this program here was to concentrate on comets observed by IUE at other time during the solar cycle, most importantly during the two solar maxima of 1980 and 1990. Described herein are the work performed and the results obtained
Hubble Space Telescope Observations of Comet 9P/Tempel 1 during the Deep Impact Encounter
We report on the Hubble Space Telescope program to observe periodic comet
9P/Tempel 1 in conjunction with NASA's Deep Impact mission. Our objectives were
to study the generation and evolution of the coma resulting from the impact and
to obtain wide-band images of the visual outburst generated by the impact. Two
observing campaigns utilizing a total of 17 HST orbits were carried out: the
first occurred on 2005 June 13-14 and fortuitously recorded the appearance of a
new, short-lived fan in the sunward direction on June 14. The principal
campaign began two days before impact and was followed by contiguous orbits
through impact plus several hours and then snapshots one, seven, and twelve
days later. All of the observations were made using the Advanced Camera for
Surveys (ACS). For imaging, the ACS High Resolution Channel (HRC) provides a
spatial resolution of 36 km (16 km/pixel) at the comet at the time of impact.
Baseline images of the comet, made prior to impact, photometrically resolved
the comet's nucleus. The derived diameter, 6.1 km, is in excellent agreement
with the 6.0 +/- 0.2 km diameter derived from the spacecraft imagers. Following
the impact, the HRC images illustrate the temporal and spatial evolution of the
ejecta cloud and allow for a determination of its expansion velocity
distribution. One day after impact the ejecta cloud had passed out of the
field-of-view of the HRC.Comment: 15 pages, 14 postscript figures. Accepted for publication in Icarus
special issue on Deep Impac
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
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