170 research outputs found
Rapid onset of neuronal death induced by blockade of either axoplasmic transport or action potentials in afferent fibers during brain development.
We have investigated how neurons in the optic tecta of embryonic day 16 chick embryos depend for survival on their afferents from the retina. To distinguish between activity-mediated effects and other, "trophic," ones, we compared the effects on the tectal neurons of blocking intraocular axoplasmic transport (with colchicine) or action potentials (by means of TTX). Both interventions rapidly induced the appearance of dying (pyknotic) neurons in the tectum, with major increases in their number occurring within 13 hr post-colchicine and within 9 hr post-TTX. Following both drugs, the dying neurons were morphologically similar, and in both cases the cell death depended on protein synthesis. However, the effects of colchicine and of TTX could be dissociated, since the most superficial tectal neurons became pyknotic only in response to colchicine, and, with a sufficiently short survival time (9 hr), the deep cells of the stratum griseum centrale became pyknotic only in response to TTX. We hence argue that the survival of the tectal neurons depends on their ongoing maintenance by substances released from retinotectal axon terminals, the release being activity dependent in the case of the deep neurons but independent of activity in the case of the superficial ones
Rubidium in the Interstellar Medium
We present observations of interstellar rubidium toward o Per, zeta Per, AE
Aur, HD 147889, chi Oph, zeta Oph, and 20 Aql. Theory suggests that stable 85Rb
and long-lived 87Rb are produced predominantly by high-mass stars, through a
combination of the weak s- and r-processes. The 85Rb/87Rb ratio was determined
from measurements of the Rb I line at 7800 angstroms and was compared to the
solar system meteoritic ratio of 2.59. Within 1-sigma uncertainties all
directions except HD 147889 have Rb isotope ratios consistent with the solar
system value. The ratio toward HD 147889 is much lower than the meteoritic
value and similar to that toward rho Oph A (Federman et al. 2004); both lines
of sight probe the Rho Ophiuchus Molecular Cloud. The earlier result was
attributed to a deficit of r-processed 85Rb. Our larger sample suggests instead
that 87Rb is enhanced in these two lines of sight. When the total elemental
abundance of Rb is compared to the K elemental abundance, the interstellar Rb/K
ratio is significantly lower than the meteoritic ratio for all the sight lines
in this study. Available interstellar samples for other s- and r- process
elements are used to help interpret these results.Comment: 24 pages, 3 figures. Accepted for publication in Ap
NGC 2579 and the carbon and oxygen abundance gradients beyond the solar circle
We present deep echelle spectrophotometry of the Galactic HII region NGC
2579. The data have been taken with the Very Large Telescope Ultraviolet-Visual
Echelle Spectrograph in the 3550--10400 \AA\ range. This object, which has been
largely neglected, shows however a rather high surface brightness, a high
ionization degree and is located at a galactocentric distance of 12.4 0.7
kpc. Therefore, NGC 2579 is an excellent probe for studying the behaviour of
the gas phase radial abundance gradients in the outer disc of the Milky Way. We
derive the physical conditions of the nebula using several emission
line-intensity ratios as well as the abundances of several ionic species from
the intensity of collisionally excited lines. We also determine the ionic
abundances of C, O and O -- and therefore the total O
abundance -- from faint pure recombination lines. The results for NGC 2579
permit to extend our previous determinations of the C, O and C/O gas phase
radial gradients of the inner Galactic disc (Esteban etal. 2005) to larger
galactocentric distances. We find that the chemical composition of NGC 2579 is
consistent with flatten gradients at its galactocentric distance. In addition,
we have built a tailored chemical evolution model that reproduces the observed
radial abundance gradients of O, C and N and other observational constraints.
We find that a levelling out of the star formation efficiency about and beyond
the isophotal radius can explain the flattening of chemical gradients observed
in the outer Galactic disc.Comment: 10 pages, 5 figures. Accepted for publication in Monthly Notices of
the Royal Astronomical Societ
Recommended from our members
Detection of [O I] 63 <i>μ</i>m in absorption toward Sgr B2
A high signal-to-noise 52-90 μm spectrum is presented for the central part of the Sagittarius B2 complex. The data were obtained with the Long Wavelength Spectrometer on board the Infrared Space Observatory (ISO). The [O I] 63 μm line is detected in absorption even at the grating spectral resolution of 0.29 μm. A lower limit for the column density of atomic oxygen of the order of 1019 cm-2 is derived. This implies that more than 40% of the interstellar oxygen must be in atomic form along the line of sight toward the Sgr B2 molecular cloud
Heavy elements in Galactic and Magellanic Cloud HII regions: recombination-line versus forbidden-line abundances
We have obtained deep optical, long-slit spectrophotometry of the Galactic
HII regions M 17, NGC 3576 and of the Magellanic Cloud HII regions 30 Doradus,
LMC N11B and SMC N66, recording the optical recombination lines (ORLs) of CII,
NII and OII. Temperature-insensitive ORL C2+/O2+ and N2+/O2 ratios are obtained
for all nebulae except SMC N66. The ORL C2+/O2+ ratios show remarkable
agreement within each galactic system, while also being in agreement with the
corresponding CEL ratios. For all five nebulae, the O2+/H+ abundance derived
from multiple OII ORLs is found to be higher than the corresponding value
derived from the strong [OIII] 4959, 5007A CELs, by factors of 1.8--2.7 for
four of the nebulae. The LMC N11B nebula exhibits a more extreme discrepancy
factor for the O2+ ion, ~5. Thus these HII regions exhibit ORL/CEL abundance
discrepancy factors that are similar to those previously encountered amongst
planetary nebulae.
Our optical CEL O2+/H+ abundances agree to within 20-30 per cent with
published O2+/H+ abundances that were obtained from observations of infrared
fine-structure lines. Since the low excitation energies of the latter make them
insensitive to variations about typical nebular temperatures, fluctuations in
temperature are ruled out as the cause of the observed ORL/CEL O2+ abundance
discrepancies. We present evidence that the observed OII ORLs from these HII
regions originate from gas of very similar density (<3500 cm-3) to that
emitting the observed heavy-element optical and infrared CELs, ruling out
models that employ high-density ionized inclusions in order to explain the
abundance discrepancy. We consider a scenario whereby much of the heavy-element
ORL emission originates from cold (<=500 K) metal-rich ionized regions.Comment: 24 pages; 9 figures; accepted by Monthly Notices of the Royal
Astronomical Societ
Misadventure during laparoscopic sleeve gastrectomy: why it happened? how to prevent and recover from it?
Slit Observations and Empirical Calculations for HII Regions
When analysing HII regions, a possible source of systematic error on
empirically derived physical quantities is the limited size of the slit used
for the observations. A grid of photoionization models was built through the
Aangaba code varying the ionizing radiation spectrum emitted by a stellar
cluster, as well as the gas abundance. The calculated line surface brightness
was then used to simulate slit observations and to derive empirical parameters
using the usual methods described in the literature. Depending on the fraction
of the object covered by the slit, the parameters can be different from those
obtained from observations of the whole object, an effect that is mainly
dependent on the age of the ionizing stellar cluster. The low-ionization
forbidden lines are more sensitive to the size of the area covered by the slit
than the high-ionization forbidden lines or recombination lines. Regarding the
temperature indicator T[OIII], the slit effects are small since this
temperature is derived from [OIII] lines. On the other hand, for the abundance
indicator R23, which depends also on the [OII] line, the slit effect is
slightly higher. Therefore, the systematic error due to slit observations on
the O abundance is low, being usually less than 10%, except for HII regions
powered by stellar clusters with a relative low number of ionizing photons
between 13.6 and 54.4 eV, which create a smaller O++ emitting volume. In this
case, the systematic error on the empirical O abundance deduced from slit
observations is more than 10% when the covered area is less than 50%.Comment: To be published in MNRAS, accepted in 09/09/2005, 17 pages and 6
figure
Unveiling the structure of the planetary nebula M 2-48: Kinematics and physical conditions
The kinematics and physical conditions of the bipolar planetary nebula M 2-48
are analysed from high and low dispersion long-slit spectra. Previous CCD
narrow-band optical observations have suggested that this nebula is mainly
formed by a pair of symmetric bow-shocks, an off-center semi-circular shell,
and an internal bipolar structure. The bipolar outflow has a complex structure,
characterised by a series of shocked regions located between the bright core
and the polar tips. There is an apparent kinematic discontinuity between the
bright bipolar core and the outer regions. The fragmented ring around the
bright bipolar region presents a low expansion velocity and could be associated
to ejection in the AGB-PN transition phase, although its nature remains
unclear. The chemical abundances of the central region are derived, showing
that M 2-48 is a Type I planetary nebula (PN)
Early evolution of the extraordinary Nova Del 2013 (V339 Del)
We determine the temporal evolution of the luminosity L(WD), radius R(WD) and
effective temperature Teff of the white dwarf (WD) pseudophotosphere of V339
Del from its discovery to around day 40. Another main objective was studying
the ionization structure of the ejecta. These aims were achieved by modelling
the optical/near-IR spectral energy distribution (SED) using low-resolution
spectroscopy (3500 - 9200 A), UBVRcIc and JHKLM photometry. During the fireball
stage (Aug. 14.8 - 19.9, 2013), Teff was in the range of 6000 - 12000 K, R(WD)
was expanding non-uniformly in time from around 66 to around 300 (d/3 kpc)
R(Sun), and L(WD) was super-Eddington, but not constant. After the fireball
stage, a large emission measure of 1.0-2.0E+62 (d/3 kpc)**2 cm**(-3)
constrained the lower limit of L(WD) to be well above the super-Eddington
value. The evolution of the H-alpha line and mainly the transient emergence of
the Raman-scattered O VI 1032 A line suggested a biconical ionization structure
of the ejecta with a disk-like H I region persisting around the WD until its
total ionization, around day 40. It is evident that the nova was not evolving
according to the current theoretical prediction. The unusual non-spherically
symmetric ejecta of nova V339 Del and its extreme physical conditions and
evolution during and after the fireball stage represent interesting new
challenges for the theoretical modelling of the nova phenomenon.Comment: 14 pages, 9 figures, 3 tables, accepted for Astronomy and
Astrophysic
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