On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are yet not understood. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star, precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. A helium-dominated stellar evolutionary sequence exists, that may be fed by different types of mergers or CE scenarios. It appears likely, that all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&

Planting stock cultivation in combined schools of forest nurseries

Various methods of initiation and cultivation agrotechnology of the planting stock were studied in the combined schools and combined school-sowing departments of forest nurseries. Differences in biometric indices of saplings and seedlings of different wood species aspects were detected. The comparative analysis of the planting stock growth indices was carried out at cultivation in an open ground and in school-sowing department of forest nursery

Influence of seeding rates seeds at exit standard seedlings pine in indoor growing

Investigations to determine the output of standard pine seedlings at different seeding rate of pine seeds in the greenhouse conditions. For the cultivation of seedlings it was used as a substrate of peat bogs transition milling work piece with a dose of application of mineral fertilizers, N70P150K 90. Pine seeds were sown in Egedal a scatter planter beds with a width of one meter seeding rates of 6.8; 11.0; 14.0 g/m 2. To estimate the output of seedlings that meet regulatory requirements, use trial-analysis based on a normal distribution. With the increase of pine seed sowing rate was observed decrease in the yield percent of standard seedlings and increase custom. It is not noted a significant change in percent yield seedlings height of 7 cm. With seeding rate is 6.8 g/m 2 output of standard seedlings was 94.5%, while and 14.0 g/m 2 – 88.4%. Significant changes were observed in the output of larger seedlings 12 cm. When seeding rate of 6.8 g/m 2 yield was 57.9%, with 11.0 g/m 2 – 46.0%, and at 14.0 g/m 2 – 27.2%. Therefore, the indoor planting pine material should be grown in a medium-density plantings with a seeding rate of 11–12 g per 1 m 2, which will be combined with economical consumption of seeds with an optimal output of standard planting material

Agricultural technology of growing deciduous seedlings for reforestation purposes

A large variety of soil conditions in the forest fundof the Republic of Belarus requires reforestation by various tree species, of which a significant roleis played by deciduous species. Growing seedlings of deciduous species requires strict observance of all agricultural activities (preparation of soil, eradication of weeds, pests control, crown care, application of mineral and organic fertilizers, etc.). The paper dwells upon the study of technology and the growth of deciduous seedlings grown in close-planting forest nurseries of forest enterprises of the Republic of Belarus

The rapid evolution of the exciting star of the Stingray Nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&

On the Evolution of O(He)-Type Stars

O(He) stars represent a small group of four very hot post-AGB stars whose atmospheres are composed of almost pure helium. Their evolution deviates from the hydrogen-deficient post-AGO evolutionary sequence of carbon-dominated stars like e.g. PG 1159 or Wolf- Rayet stars. While (very) late thermal pulse evolutionary models can explain the observed He/C/O abundances in these objects, they do not reproduce He-dominated surface abundances. Currently it seems most likely that the O(He) stars originate from a double helium white dwarf merger and so they could be the successors of the luminous helium-rich sdO-stars. An other possibility is that O(He)-stars could be successors of RCB or EHe stars

Frontline health workers’ experiences of providing care for people living with non-communicable diseases during the COVID-19 pandemic in Ghana: a qualitative study

Background: The COVID-19 pandemic has significantly impacted frontline health workers. However, a neglected dimension of this discourse was the extent to which the pandemic impacted frontline healthcare workers providing non-communicable diseases (NCDs) care. This study aims to understand the experiences of healthcare workers with no prior exposure to pandemics who provided care to people living with NCDs (PLWNCDs). Methods: A qualitative study design was employed, using a face-to-face in-depth interviews. Interviews were conducted in primary healthcare facilities in three administrative regions of Ghana, representing the Northern, Southern and Middle Belts. Only frontline health workers with roles in providing care for PLWNCDs were included. Purposive snowballing and convenience sampling methods were employed to select frontline health workers. An open-ended interview guide was used to facilitate data collection, and thematic content analysis was used to analyse the data. Results: A total of 47 frontline health workers were interviewed. Overall, these workers experienced diverse patient-driven and organisational challenges. Patient-level challenges included a decline in healthcare utilisation, non-adherence to treatment, a lack of continuity, fear and stigma. At the organisational levels, there was a lack of medical logistics, increased infection of workers and absenteeism, increased workload and burnout, limited motivational packages and inadequate guidelines and protocols. Workers coped and responded to the pandemic by postponing reviews and consultations, reducing inpatient and outpatient visits, changing their prescription practices, using teleconsultation and moving to long-shift systems. Conclusion: This study has brought to the fore the experiences that adversely affected frontline health workers and, in many ways, affected the care provided to PLWNCDs. Policymakers and health managers should take these experiences into account in plans to mitigate the impact of future pandemics

Using Semantic Technologies in Digital Libraries- A Roadmap to Quality Evaluation

Abstract. In digital libraries semantic techniques are often deployed to reduce the expensive manual overhead for indexing documents, maintaining metadata, or caching for future search. However, using such techniques may cause a decrease in a collection’s quality due to their statistical nature. Since data quality is a major concern in digital libraries, it is important to be able to measure the (loss of) quality of metadata automatically generated by semantic techniques. In this paper we present a user study based on a typical semantic technique use

Wide-Field InfraRed Survey Telescope (WFIRST) Final Report

In December 2010, NASA created a Science Definition Team (SDT) for WFIRST, the Wide Field Infra-Red Survey Telescope, recommended by the Astro 2010 Decadal Survey as the highest priority for a large space mission. The SDT was chartered to work with the WFIRST Project Office at GSFC and the Program Office at JPL to produce a Design Reference Mission (DRM) for WFIRST. Part of the original charge was to produce an interim design reference mission by mid-2011. That document was delivered to NASA and widely circulated within the astronomical community. In late 2011 the Astrophysics Division augmented its original charge, asking for two design reference missions. The first of these, DRM1, was to be a finalized version of the interim DRM, reducing overall mission costs where possible. The second of these, DRM2, was to identify and eliminate capabilities that overlapped with those of NASA's James Webb Space Telescope (henceforth JWST), ESA's Euclid mission, and the NSF's ground-based Large Synoptic Survey Telescope (henceforth LSST), and again to reduce overall mission cost, while staying faithful to NWNH. This report presents both DRM1 and DRM2.Comment: 102 pages, 57 figures, 17 table