871 research outputs found
Resolving the electron temperature discrepancies in HII Regions and Planetary Nebulae: kappa-distributed electrons
The measurement of electron temperatures and metallicities in H ii regions
and Planetary Nebulae (PNe) has-for several decades-presented a problem:
results obtained using different techniques disagree. What it worse, they
disagree consistently. There have been numerous attempts to explain these
discrepancies, but none has provided a satisfactory solution to the problem. In
this paper, we explore the possibility that electrons in H ii regions and PNe
depart from a Maxwell-Boltzmann equilibrium energy distribution. We adopt a
"kappa-distribution" for the electron energies. Such distributions are widely
found in Solar System plasmas, where they can be directly measured. This simple
assumption is able to explain the temperature and metallicity discrepancies in
H ii regions and PNe arising from the different measurement techniques. We find
that the energy distribution does not need to depart dramatically from an
equilibrium distribution. From an examination of data from Hii regions and PNe
it appears that kappa ~ 10 is sufficient to encompass nearly all objects. We
argue that the kappa-distribution offers an important new insight into the
physics of gaseous nebulae, both in the Milky Way and elsewhere, and one that
promises significantly more accurate estimates of temperature and metallicity
in these regions.Comment: 16 pages, 11 figures, 2 tables, published in Ap
Metal-poor dwarf galaxies in the SIGRID galaxy sample. I. HII region observations and chemical abundances
In this paper we present the results of observations of seventeen HII regions
in thirteen galaxies from the SIGRID sample of isolated gas rich irregular
dwarf galaxies. The spectra of all but one of the galaxies exhibit the auroral
[OIII] 4363A line, from which we calculate the electron temperature, Te, and
gas-phase oxygen abundance. Five of the objects are blue compact dwarf (BCD)
galaxies, of which four have not previously been analysed spectroscopically. We
include one unusual galaxy which exhibits no evidence of the [NII]
{\lambda}{\lambda} 6548,6584A lines, suggesting a particularly low metallicity
(< Zsolar/30). We compare the electron temperature based abundances with those
derived using eight of the new strong line diagnostics presented by Dopita et
al. (2013). Using a method derived from first principles for calculating total
oxygen abundance, we show that the discrepancy between the Te-based and strong
line gas-phase abundances have now been reduced to within ~0.07 dex. The
chemical abundances are consistent with what is expected from the
luminosity-metallicity relation. We derive estimates of the electron densities
and find them to be between ~5 and ~100 cm-3. We find no evidence for a
nitrogen plateau for objects in this sample with metallicities 0.5 > Zsolar >
0.15.Comment: 46 pages, 15 figures, 6 tables, 1 appendix. Accepted for publication
in the Astrophysical Journa
A mitotic SKAP isoform regulates spindle positioning at astral microtubule plus ends
The Astrin/SKAP complex plays important roles in mitotic chromosome alignment and centrosome integrity, but previous work found conflicting results for SKAP function. Here, we demonstrate that SKAP is expressed as two distinct isoforms in mammals: a longer, testis-specific isoform that was used for the previous studies in mitotic cells and a novel, shorter
mitotic isoform. Unlike the long isoform, short SKAP rescues SKAP depletion in mitosis and displays robust microtubule plus-end tracking, including localization to astral microtubules. Eliminating SKAP microtubule binding results in severe chromosome segregation defects. In contrast, SKAP mutants specifically defective for plus-end tracking facilitate proper
chromosome segregation but display spindle positioning defects. Cells lacking SKAP plus-end tracking have reduced Clasp1 localization at microtubule plus ends and display increased lateral microtubule contacts with the cell cortex, which we propose results in unbalanced dynein-dependent cortical pulling forces. Our work reveals an unappreciated role for the Astrin/SKAP complex as an astral microtubule mediator of mitotic spindle positioning.Leukemia & Lymphoma Society of America (Scholar Award)National Institute of General Medical Sciences (U.S.) (GM088313)American Cancer Society (121776
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