377 research outputs found
Fine Scale Temperature Fluctuations in the the Orion Nebula and the t^2 Problem
We present a high spatial resolution map of the columnar electron temperature
(Tc) of a region to the south west of the Trapezium in the Orion Nebula. This
map was derived from Hubble Space Telescope images that isolated the primary
lines of HI for determination of the local extinction and of the OIII lines for
determination of Tc. Although there is no statistically significant variation
of Tc with distance from the dominant ionizing star theta1-Ori-C, we find small
scale variations in the plane of the sky down to a few arcseconds that are
compatible with the variations inferred from comparing the value of Te derived
from forbidden and recombination lines, commonly known as the t^2 problem. We
present other evidence for fine scale variations in conditions in the nebula,
these being variations in the surface brightness of the the nebula,
fluctuations in radial velocities, and ionization changes. From our Tc map and
other considerations we estimate that t^2=0.028 +-0.006 for the Orion nebula.
Shadowed regions behind clumps close to the ionization front can make a
significant contribution to the observed temperature fluctuations, but they
cannot account for the t^2 values inferred from several methods of temperature
determination. It is shown that an anomalous broadening of nebular emission
lines appears to have the same sense of correlation as the temperature
anomalies, although a causal link is not obvious.Comment: 53 pages, 13 images, many of the images have been downgraded to be
able to fit within the astro-ph file size limit
The qWR star HD 45166. II. Fundamental stellar parameters and evidence of a latitude-dependent wind
The enigmatic object HD 45166 is a qWR star in a binary system with an
orbital period of 1.596 day, and presents a rich emission-line spectrum in
addition to absorption lines from the companion star (B7 V). As the system
inclination is very small (i=0.77 +- 0.09 deg), HD 45166 is an ideal laboratory
for wind-structure studies. The goal of the present paper is to determine the
fundamental stellar and wind parameters of the qWR star. A radiative transfer
model for the wind and photosphere of the qWR star was calculated using the
non-LTE code CMFGEN. The wind asymmetry was also analyzed using a
recently-developed version of CMFGEN to compute the emerging spectrum in
two-dimensional geometry. The temporal-variance spectrum (TVS) was calculated
for studying the line-profile variations. Abundances, stellar and wind
parameters of the qWR star were obtained. The qWR star has an effective
temperature of Teff=50000 +- 2000 K, a luminosity of log(L/Lsun)=3.75 +- 0.08,
and a corresponding photospheric radius of Rphot=1.00 Rsun. The star is
helium-rich (N(H)/N(He) = 2.0), while the CNO abundances are anomalous when
compared either to solar values, to planetary nebulae, or to WR stars. The
mass-loss rate is Mdot = 2.2 . 10^{-7} Msun/yr, and the wind terminal velocity
is vinf=425 km/s. The comparison between the observed line profiles and models
computed under different latitude-dependent wind densities strongly suggests
the presence of an oblate wind density enhancement, with a density contrast of
at least 8:1 from equator to pole. If a high velocity polar wind is present
(~1200 km/s), the minimum density contrast is reduced to 4:1. The wind
parameters determined are unusual when compared to O-type stars or to typical
WR stars. (abridged)Comment: 16 pages, 13 figures, accepted for publication in A&
The tails in the Helix Nebula NGC 7293
We have examined a stream-source model for the production of the cometary
tails observed in the Helix Nebula NGC 7293 in which a transonic or moderately
supersonic stream of ionized gas overruns a source of ionized gas. Hydrodynamic
calculations reveal velocity structures which are in good agreement with the
observational data on tail velocities and are consistent with observations of
the nebular structure. The results also are indicative of a stellar atmosphere
origin for the cometary globules. Tail remnants persist for timescales long
enough for their identification with faint striations visible in the nebula gas
to be plausible.Comment: 7 pages, 6 figures, accepted for publication in A&
Age-Associated Metabolic and Morphologic Changes in Mitochondria of Individual Mouse and Hamster Oocytes
Background: In human oocytes, as in other mammalian ova, there is a significant variation in the pregnancy potential, with approximately 20% of oocyte-sperm meetings resulting in pregnancies. This frequency of successful fertilization decreases as the oocytes age. This low proportion of fruitful couplings appears to be influenced by changes in mitochondrial structure and function. In this study, we have examined mitochondrial biogenesis in both hamster (Mesocricetus auratus) and mouse (Mus musculus) ova as models for understanding the effects of aging on mitochondrial structure and energy production within the mammalian oocyte.
Methodology/Principal Findings: Individual metaphase II oocytes from a total of 25 young and old mice and hamsters were collected from ovarian follicles after hormone stimulation and prepared for biochemical or structural analysis. Adenosine triphosphate levels and mitochondrial DNA number were determined within individual oocytes from young and old animals. In aged hamsters, oocyte adenosine triphosphate levels and mitochondrial DNA molecules were reduced 35.4% and 51.8%, respectively. Reductions of 38.4% and 44% in adenosine triphosphate and mitochondrial genomes, respectively, were also seen in aged mouse oocytes. Transmission electron microscopic (TEM) analysis showed that aged rodent oocytes had significant alterations in mitochondrial and cytoplasmic lamellae structure.
Conclusions/Significance: In both mice and hamsters, decreased adenosine triphosphate in aged oocytes is correlated with a similar decrease in mtDNA molecules and number of mitochondria. Mitochondria in mice and hamsters undergo significant morphological change with aging including mitochondrial vacuolization, cristae alterations, and changes in cytoplasmic lamellae
The evolutionary status of the semiregular variable QYSge
Repeated spectroscopic observations made with the 6m telescope of yielded new
data on the radial-velocity variability of the anomalous yellow supergiant
QYSge. The strongest and most peculiar feature in its spectrum is the complex
profile of NaI D lines, which contains a narrow and a very wide emission
components. The wide emission component can be seen to extend from -170 to +120
km/s, and at its central part it is cut by an absorption feature, which, in
turn, is split into two subcomponents by a narrow (16km/s at r=2.5) emission
peak. An analysis of all the Vr values leads us to adopt for the star a
systemic velocity of Vr=-21.1 km/s, which corresponds to the position of the
narrow emission component of NaI. The locations of emission-line features of
NaI D lines are invariable, which point to their formation in regions that are
external to the supergiant's photosphere. Differential line shifts of about
10km/s are revealed. The absorption lines in the spectrum of QYSge have a
substantial width of FWHM~45 km/s. The method of model atmospheres is used to
determine the following parameters: Teff=6250K, lg g=2.0, and microturbulence
Vt=4.5km/s. The metallicity of the star is found to be somewhat higher than the
solar one with an average overabundance of iron-peak elements of [Met/H]=+0.20.
The star is found to be slightly overabundant in carbon and nitrogen,
[C/Fe]=+0.25, [N/Fe]=+0.27. The alpha-process elements Mg, Si, and Ca are
slightly overabundant [alpha/H]=+0.12. The strong sodium excess, [Na/Fe]=+0.75,
is likely to be due to the dredge-up of the matter processed in the NeNa cycle.
Heavy elements of the s-process are underabundant relative to the Sun. On the
whole, the observed properties of QYSge do not give grounds for including this
star into the group of RCrB or RVTau-type type objects.Comment: 29 pages, 8 figures, 4 tables; accepted by Astrophys. Bulleti
Transmission of mitochondrial DNA following assisted reproduction and nuclear transfer
Review of the articleMitochondria are the organelles responsible for producing the majority of a cell's ATP and also play an essential role in gamete maturation and embryo development. ATP production within the mitochondria is dependent on proteins encoded by both the nuclear and the mitochondrial genomes, therefore co-ordination between the two genomes is vital for cell survival. To assist with this co-ordination, cells normally contain only one type of mitochondrial DNA (mtDNA) termed homoplasmy. Occasionally, however, two or more types of mtDNA are present termed heteroplasmy. This can result from a combination of mutant and wild-type mtDNA molecules or from a combination of wild-type mtDNA variants. As heteroplasmy can result in mitochondrial disease, various mechanisms exist in the natural fertilization process to ensure the maternal-only transmission of mtDNA and the maintenance of homoplasmy in future generations. However, there is now an increasing use of invasive oocyte reconstruction protocols, which tend to bypass mechanisms for the maintenance of homoplasmy, potentially resulting in the transmission of either form of mtDNA heteroplasmy. Indeed, heteroplasmy caused by combinations of wild-type variants has been reported following cytoplasmic transfer (CT) in the human and following nuclear transfer (NT) in various animal species. Other techniques, such as germinal vesicle transfer and pronuclei transfer, have been proposed as methods of preventing transmission of mitochondrial diseases to future generations. However, resulting embryos and offspring may contain mtDNA heteroplasmy, which itself could result in mitochondrial disease. It is therefore essential that uniparental transmission of mtDNA is ensured before these techniques are used therapeutically
Academic Performance and Behavioral Patterns
Identifying the factors that influence academic performance is an essential
part of educational research. Previous studies have documented the importance
of personality traits, class attendance, and social network structure. Because
most of these analyses were based on a single behavioral aspect and/or small
sample sizes, there is currently no quantification of the interplay of these
factors. Here, we study the academic performance among a cohort of 538
undergraduate students forming a single, densely connected social network. Our
work is based on data collected using smartphones, which the students used as
their primary phones for two years. The availability of multi-channel data from
a single population allows us to directly compare the explanatory power of
individual and social characteristics. We find that the most informative
indicators of performance are based on social ties and that network indicators
result in better model performance than individual characteristics (including
both personality and class attendance). We confirm earlier findings that class
attendance is the most important predictor among individual characteristics.
Finally, our results suggest the presence of strong homophily and/or peer
effects among university students
- …