2,909 research outputs found
Structural concepts and experimental considerations for a versatile high-speed research airplane
Future aircraft may be hydrogen fueled and fly at hypersonic speeds. The resulting environments will require new structural concepts to satisfy performance goals. Large representative structures will have to be flight tested prior to commitment to a costly vehicle fleet. To perform flight tests, a versatile, economical, high-speed research airplane is defined. Results of this study including experimental considerations for a hypersonic research airplane are reported
Sexually Transmitted Disease and Male Infertility: A Systematic Review
Context Theoretically, sexually transmitted diseases (STDs) have the potential to disrupt male fertility; however, the topic remains controversial. Objective To describe the possible association between STDs and male infertility and to explore possible pathophysiologic mechanisms. Evidence acquisition We performed a systematic literature review in accordance with the PRISMA guidelines. PubMed, Embase, and the Cochrane Library were searched for articles published before January 1, 2016, using the MeSH terms for a variety of STDs and infertility. The search was restricted to human studies performed in men and published in English. Studies were included if they contained original data on a possible association or a cause-and-effect relationship between STD and male infertility. Studies were considered only if they included an appropriate control group and/or comprehensive laboratory data. Due to heterogeneity in the literature, a qualitative analysis was performed. Evidence synthesis Relevant studies on Chlamydia trachomatis, genital mycoplasmas, Neisseria gonorrhoeae, Trichomonas vaginalis, and viral infections were identified. For all pathogens, the studies were contradictory and generally of limited quality. In studies confirming an association, there was a tendency for authors to perform multiple analyses without appropriate corrections and to subsequently focus solely on outcomes that seemed to suggest a positive association; however, the body of literature that does not confirm an association between STDs and male infertility is also of inadequate quality. The data regarding possible pathophysiologic mechanisms are inconclusive. Conclusions There may be an association between STDs and male infertility of unknown genesis and possibly with different pathogenic mechanisms for different pathogens. Alternatively, some STDs may cause male infertility, whereas others may not; however, there is hardly a strong correlation. High-quality studies of the subject are needed. Patient summary Sexually transmitted diseases may cause male infertility through unknown mechanisms; however, from the available research, we cannot be sure that there is an association, and more studies are needed
Manned Earth Observatory
The Manned Earth Observatory (MEO) study being conducted by TRW under the management of NASA/MSFC will establish the conceptual design of and the mission requirements for an Earth Observation Laboratory that will be flown on Shuttle missions beginning in 1980. MEO offers a variety of unique inroads to improving our understanding of the marine environment. The Shuttle-MEO is a valuable addition to a multi-level multi-disciplinary remote sensing program. The unique attributes of MEO are its experimental flexibility due to man-instrument interaction, its complimentary orbit (intermediate between nonorbital and high-orbital platforms), its high weight and volume capacity and short duration missions
The chemical evolution of galaxies within the IGIMF theory: the [alpha/Fe] ratios and downsizing
The chemical evolution of galaxies is investigated within the framework of
the star formation rate (SFR) dependent integrated galactic initial mass
function (IGIMF). We study how the global chemical evolution of a galaxy and in
particular how [alpha/Fe] abundance ratios are affected by the predicted
steepening of the IGIMF with decreasing SFR. We use analytical and
semi-analytical calculations to evaluate the mass-weighted and
luminosity-weighted [alpha/Fe] ratios in early-type galaxies of different
masses. The models with the variable IGIMF produce a [alpha/Fe] vs. velocity
dispersion relation which has the same slope as the observations of massive
galaxies, irrespective of the model parameters, provided that the star
formation duration inversely correlates with the mass of the galaxy
(downsizing). These models also produce steeper [alpha/Fe] vs. sigma relations
in low-mass early-type galaxies and this trend is consistent with the
observations. Constant IMF models are able to reproduce the [alpha/Fe] ratios
in large elliptical galaxies as well, but they do not predict this change of
slope for small galaxies. In order to obtain the best fit between our results
and the observations, the downsizing effect (i.e. the shorter duration of the
star formation in larger galaxies) must be milder than previously thought.Comment: 13 pages, 18 figures, accepted for publication in Astronomy and
Astrophysic
Effects of the integrated galactic IMF on the chemical evolution of the solar neighbourhood
The initial mass function determines the fraction of stars of different
intial mass born per stellar generation. In this paper, we test the effects of
the integrated galactic initial mass function (IGIMF) on the chemical evolution
of the solar neighbourhood. The IGIMF (Weidner & Kroupa 2005) is computed from
the combination of the stellar intial mass function (IMF), i.e. the mass
function of single star clusters, and the embedded cluster mass function, i.e.
a power law with index beta. By taking into account also the fact that the
maximum achievable stellar mass is a function of the total mass of the cluster,
the IGIMF becomes a time-varying IMF which depends on the star formation rate.
We applied this formalism to a chemical evolution model for the solar
neighbourhood and compared the results obtained by assuming three possible
values for beta with the results obtained by means of a standard, well-tested,
constant IMF. In general, a lower absolute value of beta implies a flatter
IGIMF, hence a larger number of massive stars and larger metal ejection rates.
This translates into higher type Ia and II supernova rates, higher mass
ejection rates from massive stars and a larger amount of gas available for star
formation, coupled with lower present-day stellar mass densities. (abridged) We
also discuss the importance of the present day stellar mass function (PDMF) in
providing a way to disentangle among various assumptions for beta. Our results
indicate that the model adopting the IGIMF computed with beta ~2 should be
considered the best since it allows us to reproduce the observed PDMF and to
account for most of the chemical evolution constraints considered in this work.Comment: 22 pages, 19 figure
Testing the universal stellar IMF on the metallicity distribution in the bulges of the Milky Way and M31
We test whether the universal initial mass function (UIMF) or the integrated
galaxial IMF (IGIMF) can be employed to explain the metallicity distribution
(MD) of giants in the Galactic bulge. We make use of a single-zone chemical
evolution model developed for the Milky Way bulge in the context of an
inside-out model for the formation of the Galaxy. We checked whether it is
possible to constrain the yields above 80 M_{\sun} by forcing the UIMF and
required that the resulting MD matches the observed ones. We also extended the
analysis to the bulge of M31 to investigate a possible variation of the IMF
among galactic bulges. Several parameters that have an impact on stellar
evolution (star-formation efficiency, gas infall timescale) are varied. We show
that it is not possible to satisfactorily reproduce the observed metallicity
distribution in the two galactic bulges unless assuming a flatter IMF () than the universal one. We conlude that it is necessary to assume a
variation in the IMF among the various environments.Comment: 9 pages, 4 figures, accepted for publication in A&
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