406 research outputs found
Spectroscopic analyses of the blue hook stars in NGC 2808: A more stringent test of the late hot flasher scenario
No full textRecent UV observations of the globular cluster NGC 2808
(Brown et al. [CITE]) show a significant population of hot
stars fainter than the zero-age horizontal branch (“blue hook”
stars), which cannot be explained by canonical stellar evolution.
Their results suggest that stars which experience unusually large mass
loss on the red giant branch and which subsequently undergo the helium
core flash while descending the white dwarf cooling curve could
populate this region. Theory predicts that these “late hot
flashers” should show higher temperatures than the hottest canonical
horizontal branch stars and should have helium- and carbon-rich
atmospheres. As a test of this late hot flasher scenario, we have
obtained and analysed medium resolution spectra of a sample of blue
hook stars in NGC 2808 to derive their atmospheric parameters. Using
the same procedures, we have also re-analyzed our earlier spectra of
the blue hook stars in ω Cen (Moehler et al. [CITE]) for
comparison with the present results for NGC 2808. The blue hook stars
in these two clusters are both hotter ( K) and more
helium-rich than canonical extreme horizontal branch stars in
agreement with the late hot flasher scenario. Moreover, we find
indications for carbon enhancement in the three most helium-enriched
stars in NGC 2808. However, the blue hook stars still show some
hydrogen in their atmospheres, perhaps indicating that some residual
hydrogen survives a late hot flash and then later diffuses to the
surface during the horizontal branch phase. We note that the presence
of blue hook stars apparently depends mostly on the total mass of the
globular cluster and not so much on its horizontal branch morphology.
Knowledge sharing strategies for Project Knowledge Management in the automotive sector
No full textProject Knowledge Management is regarded as a field of increasing importance for both researchers and organisations. This led Volvo Technology, the innovation business unit within a Swedish automotive multinational corporation, to explore through a qualitative case study how Project Knowledge Management could be improved to support knowledge sharing between projects within the organisation. The current situation of Project Knowledge Management is described through a developed theoretical framework and with input from thirteen semi-structured interviews conducted and analysed in an iterative fashion. The description shows that the contributor employs the codification strategy to share knowledge with other projects while the receiver adapts a personalisation strategy to retrieve knowledge from other projects. This description was analysed and compared with current research through brainstorming. Since there are current initiatives within the organisation to improve the codification strategy, this research focuses on improving the personalisation strategy. The recommendation was to promote Communities of Practice. Six semi-structured interviews were conducted to evaluate the relevance of the recommendations. The recommendation was found to be relevant for improving Project Knowledge Management within Volvo Technology
Spectral types and masses of white dwarfs in globular clusters
No full textWhite dwarfs in globular clusters offer additional possibilities to determine distances and ages of globular clusters, provided their spectral types and masses are known. We therefore started a project to obtain spectra of white dwarfs in the globular clusters NGC 6397 and NGC 6752. All observed white dwarfs show hydrogen-rich spectra and are therefore classified as DA. Analysing the multi-colour photometry of the white dwarfs in NGC 6752 yields an average gravity of log g = 7.84 and 0.53 M⊙ as the most probable average mass for globular cluster white dwarfs. Using this average gravity we try to determine independent temperatures by fitting the white dwarf spectra. While the stellar parameters determined from spectroscopy and photometry usually agree within the mutual error bars, the low resolution and S/N of the spectra prevent us from setting constraints stronger than those derived from the photometry alone. For the same reasons the white dwarf spectra obtained for NGC 6397 unfortunately do not provide an independent distance estimate of sufficient accuracy to distinguish between the long and short distance scale for globular clusters
New Results from the Supernova Ia Progenitor Survey
No full textContains fulltext :
32777.pdf (Publisher’s version ) (Open Access)We report on a systematic radial velocity survey for double degenerate (DD) binaries as potential progenitors of type Ia supernovae: SPY (ESO Supernovae Ia Progenitor surveY). More than 1000 white dwarfs and pre-white dwarfs were observed with the VLT. Our aim is to perform a statistically significant test of the DD scenario. SPY detected more than 100 new binary white dwarfs, dramatically increasing the number of known DDs. System parameters are determined from ongoing follow-up observations. Our sample includes systems with masses close to the Chandrasekhar limit and a probable SN Ia progenitor candidate
Hot horizontal branch stars in NGC 288 – effects of diffusion and stratification on their atmospheric parameters
No full textWe found that the script to determine the masses of the stars contains two errors. This script and a related one have been used to determine masses of globular cluster stars and distances to field stars in 12 papers published between 1990 and 2014. While the numerical values need to be revised none of the conclusions are affected. We provide the updated numerical values and figures for all 12 publications here. In addition we describe the effects on those refereed publications that used the distances to the field stars
The Hottest Horizontal-Branch Stars in Omega Centauri: Late Hot Flasher vs. Helium Enrichment
No full textUV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario here stars experience the helium flash while on the white dwarf cooling curve or by the helium-rich sub-population recently postulated to exist in some clusters. Spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C, N enrichment could not be verified from existing data. We want to determine effective temperatures, surface gravities and abundances of He, C, N in blue hook and canonical extreme horizontal branch (EHB) star candidates. Moderately high resolution spectra of stars at the hot end of the blue horizontal branch in the globular cluster omega Cen were analysed for atmospheric parameters (T(sub eff), log g) and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000 K to 50,000 K we find that 37% of our stars are helium-poor (log nHe/nH less than -2), 49% have solar helium abundance within a factor of 3 (-1.5 less than or equal to log nHe/nH less than or equal to -0.5) and 14% are helium rich (log nHe/nH greater than -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 30% of the hottest HB stars in omega Centauri show helium abundances well above the predictions from the helium enrichment scenario (Y = 0.42 corresponding to log nHe/nH approximately equal to -0.74). In addition the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence
Construction skills development in the UK : transitioning between the formal and informal
No full textResearch reported here is part of a wider study that seeks to examine the practices involved in encouraging and enabling employers to engage with the skills development agenda. A series of exploratory interviews and ethnographic observations reveal potential disconnections between skills policies at the governmental level and what actually happens in employer practices regarding skills development. On the one hand, the formal education and training system focuses on such targets as the attainment of narrowly-defined occupational standards, levels of competence, and quantitative performance measures like completion rates. On the other hand, the socialised concept of skills development takes place informally at the workplace through on-the-job training and mentoring relationships between senior and junior employees. Both the formal and informal systems appear to co-exist alongside each other, although tensions are mounting in terms of confidence that employers and the wider industry place on the efficacy of the formal system
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