186 research outputs found
RESIK observations of He-like Ar X-ray line emission in solar flares
The Ar XVII X-ray line group principally due to transitions 1s2 - 1s2l (l=s,
p) near 4 Anstroms was observed in numerous flares by the RESIK bent crystal
spectrometer aboard CORONAS-F between 2001 and 2003. The three line features
include the Ar XVII w (resonance line), a blend of x and y (intercombination
lines), and z (forbidden line), all of which are blended with Ar XVI
dielectronic satellites. The ratio G, equal to [I(x+y) + I(z)]/I(w), varies
with electron temperature Te mostly because of unresolved dielectronic
satellites. With temperatures estimated from GOES X-ray emission, the observed
G ratios agree fairly well with those calculated from CHIANTI and other data.
With a two-component emission measure, better agreement is achieved. Some S XV
and S XVI lines blend with the Ar lines, the effect of which occurs at
temperatures greater than 8MK, allowing the S/Ar abundance ratio to be
determined. This is found to agree with coronal values. A nonthermal
contribution is indicated for some spectra in the repeating-pulse flare of 2003
February 6.Comment: Latex file and 3 ps files. Astrophysical Journal Letters (accepted,
June 2008
Highly Ionized Potassium Lines in Solar X-ray Spectra and the Abundance of Potassium
The abundance of potassium is derived from X-ray lines observed during flares
by the RESIK instrument on the solar mission CORONAS-F between 3.53 A and 3.57
A. The lines include those emitted by He-like K and Li-like K dielectronic
satellites, which have been synthesized using the CHIANTI atomic code and newly
calculated atomic data. There is good agreement of observed and synthesized
spectra, and the theoretical behavior of the spectra with varying temperature
estimated from the ratio of the two GOES channels is correctly predicted. The
observed fluxes of the He-like K resonance line per unit emission measure gives
log A(K) = 5.86 (on a scale log A(H) = 12), with a total range of a factor 2.9.
This is higher than photospheric abundance estimates by a factor 5.5, a
slightly greater enhancement than for other elements with first ionization
potential (FIP) less than about 10 eV. There is, then, the possibility that
enrichment of low-FIP elements in coronal plasmas depends weakly on the value
of the FIP which for K is extremely low (4.34 eV). Our work also suggests that
fractionation of elements to form the FIP effect occurs in the low chromosphere
rather than higher up, as in some models.Comment: 14 pages, 3 figure
Plasticity and regeneration of gonads in the annelid Pristina leidyi
International audienceGonads are specialized gamete-producing structures that, despite their functional importance, are generated by diverse mechanisms across groups of animals and can be among the most plastic organs of the body. Annelids, the segmented worms, are a group in which gonads have been documented to be plastic and to be able to regenerate, but little is known about what factors influence gonad development or how these structures regenerate. In this study, we aimed to identify factors that influence the presence and size of gonads and to investigate gonad regeneration in the small asexually reproducing annelid, Pristina leidyi.We found that gonad presence and size in asexual adult P. leidyi are highly variable across individuals and identified several factors that influence these structures. An extrinsic factor, food availability, and two intrinsic factors, individual age and parental age, strongly influence the presence and size of gonads in P. leidyi. We also found that following head amputation in this species, gonads can develop by morphallactic regeneration in previously non-gonadal segments. We also identified a sexually mature individual from our laboratory culture that demonstrates that, although our laboratory strain reproduces only asexually, it retains the potential to become fully sexual.Our findings demonstrate that gonads in P. leidyi display high phenotypic plasticity and flexibility with respect to their presence, their size, and the segments in which they can form. Considering our findings along with relevant data from other species, we find that, as a group, clitellate annelids can form gonads in at least four different contexts: post-starvation refeeding, fission, morphallactic regeneration, and epimorphic regeneration. This group is thus particularly useful for investigating the mechanisms involved in gonad formation and the evolution of post-embryonic phenotypic plasticity
The Solar Flare Chlorine Abundance from RESIK X-ray Spectra
The abundance of chlorine is determined from X-ray spectra obtained with the
RESIK instrument on {\em CORONAS-F} during solar flares between 2002 and 2003.
Using weak lines of He-like Cl, \ion{Cl}{16}, between 4.44 and 4.50 \AA, and
with temperatures and emission measures from {\em GOES} on an isothermal
assumption, we obtained on a scale . The uncertainty reflects an approximately factor 2 scatter in measured
line fluxes. Nevertheless our value represents what is probably the best solar
determination yet obtained. It is higher by factors of 1.8 and 2.7 than Cl
abundance estimates from an infrared sunspot spectrum and nearby \ion{H}{2}
regions. The constancy of the RESIK abundance values over a large range of
flares ({\em GOES} class from below C1 to X1) argues for any fractionation that
may be present in the low solar atmosphere to be independent of the degree of
solar activity.Comment: 13 pages, 3 colour figures. To be published, Astrophysical Journa
Thin deformable mirrors for a reconfigurable space telescope
As part of a small satellite technology demonstration that will utilize autonomous assembly, reconfiguration, and docking technology to form the primary mirror for the mission’s telescope payload, the mirror segments are required to modify and control their shape, in order to allow for imaging in different configurations. This paper focuses on the development of 10 cm diameter active lightweight mirrors. The current mirror design, control scheme, and fabrication methods are described, as well as experimental results on initial samples. The data demonstrates that the mirrors are capable of at least 100 microns of displacement during operation, and that fabrication on polished molds can result in high quality reflective surfaces
Interelectronic-interaction effects on the two-photon decay rates of heavy He-like ions
Based on a rigorous QED approach a theoretical analysis is performed for the
two-photon transitions in heavy He-like ions. Special attention is paid to the
interelectronic-interaction corrections to the decay rates that are taken into
account within the two-time Green-function method. Detailed calculations are
carried out for the two-photon transitions 2^1S_0 -> 1^1S_0 and 2^3S_1 ->
1^1S_0 in He-like ions within the range of nuclear numbers Z=28-92. The total
decay rates together with the spectral distributions are given. The obtained
results are compared with experimental values and previous calculations.Comment: 17 pages, 7 figure
Excitation of the 3.071mm Hyperfine Line in Li-Like 57-Fe in Astrophysical Plasmas
As noted first by Sunyaev & Churazov (1984), the 3.071 mm hyperfine line from
might be observable in astrophysical plasmas. We assess the
atomic processes which might contribute to the excitation of this line. We
determine the intensity of the hyperfine line from an isothermal, coronal
plasma in collisional ionization equilibrium and for a coronal plasma cooling
isobarically due to its own radiation. Comparisons of the hyperfine line to
other lines emitted by the same ion, Fe, are shown to be useful for
deriving the isotopic fraction of Fe. We calculate the ratios of the
hyperfine line to the 2s--2p EUV lines at 192 \AA and 255 \AA, and the 2s--3p
X-ray doublet at 10.6 \AA.Comment: 28 pages text+figures, Accepted to ApJ in Jan 98, also at
http://www.astro.virginia.edu/~nld2n/research.htm
DNA Barcoding Reveals Cryptic Diversity in Lumbricus terrestris L., 1758 (Clitellata): Resurrection of L. herculeus (Savigny, 1826)
The widely studied and invasive earthworm, Lumbricus terrestris L., 1758 has been the subject of nomenclatural debate for many years. However these disputes were not based on suspicions of heterogeneity, but rather on the descriptions and nomenclatural acts associated with the species name. Large numbers of DNA barcode sequences of the cytochrome oxidase I obtained for nominal L. terrestris and six congeneric species reveal that there are two distinct lineages within nominal L. terrestris. One of those lineages contains the Swedish population from which the name-bearing specimen of L. terrestris was obtained. The other contains the population from which the syntype series of Enterion herculeum Savigny, 1826 was collected. In both cases modern and old representatives yielded barcode sequences allowing us to clearly establish that these are two distinct species, as different from one another as any other pair of congeners in our data set. The two are morphologically indistinguishable, except by overlapping size-related characters. We have designated a new neotype for L. terrestris. The newly designated neotype and a syntype of L. herculeus yielded DNA adequate for sequencing part of the cytochrome oxidase I gene (COI). The sequence data make possible the objective determination of the identities of earthworms morphologically identical to L. terrestris and L. herculeus, regardless of body size and segment number. Past work on nominal L. terrestris could have been on either or both species, although L. herculeus has yet to be found outside of Europe
What Role Do Annelid Neoblasts Play? A Comparison of the Regeneration Patterns in a Neoblast-Bearing and a Neoblast-Lacking Enchytraeid Oligochaete
The term ‘neoblast’ was originally coined for a particular type of cell that had been observed during annelid regeneration, but is now used to describe the pluripotent/totipotent stem cells that are indispensable for planarian regeneration. Despite having the same name, however, planarian and annelid neoblasts are morphologically and functionally distinct, and many annelid species that lack neoblasts can nonetheless substantially regenerate. To further elucidate the functions of the annelid neoblasts, a comparison was made between the regeneration patterns of two enchytraeid oligochaetes, Enchytraeus japonensis and Enchytraeus buchholzi, which possess and lack neoblasts, respectively. In E. japonensis, which can reproduce asexually by fragmentation and subsequent regeneration, neoblasts are present in all segments except for the eight anterior-most segments including the seven head-specific segments, and all body fragments containing neoblasts can regenerate a complete head and a complete tail, irrespective of the region of the body from which they were originally derived. In E. japonensis, therefore, no antero-posterior gradient of regeneration ability exists in the trunk region. However, when amputation was carried out within the head region, where neoblasts are absent, the number of regenerated segments was found to be dependent on the level of amputation along the body axis. In E. buchholzi, which reproduces only sexually and lacks neoblasts in all segments, complete heads were never regenerated and incomplete (hypomeric) heads could be regenerated only from the anterior region of the body. Such an antero-posterior gradient of regeneration ability was observed for both the anterior and posterior regeneration in the whole body of E. buchholzi. These results indicate that the presence of neoblasts correlates with the absence of an antero-posterior gradient of regeneration ability along the body axis, and suggest that the annelid neoblasts are more essential for efficient asexual reproduction than for the regeneration of missing body parts
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