166 research outputs found
Chemical abundance ratios of galactic globular clusters from modelling integrated light spectroscopy
We use our new, flux-calibrated stellar population model of absorption-line
indices to derive ages, metallicities, and various element abundance ratios
from integrated light spectroscopy of galactic globular clusters. The ages
agree well with the literature and are all consistent with the age of the
universe. There is a considerable scatter, though, and we obtain systematically
larger ages than CMD determinations mostly for metal-rich globular clusters.
The metallicities agree well with literature values on the Zinn & West scale,
if we adopt iron abundance [Fe/H] for those clusters whose ages agree with the
CMD ages. It turns out that the derivation of individual element abundance
ratios is not reliable at [Fe/H]<-1 dex, while the [alpha/Fe] ratio is robust
at all metallicities. We find general enhancement of light and alpha elements,
as expected, with significant variations for some elements. The elements O and
Mg follow the same general enhancement with almost identical distributions of
[O/Fe] and [Mg/Fe]. We obtain slightly lower [C/Fe] and very high [N/Fe]
ratios, instead. This chemical anomaly, commonly attributed to self-enrichment,
is well known in globular clusters from individual stellar spectroscopy. It is
the first time that this pattern is obtained also from the integrated light.
The alpha elements follow a pattern such that the heavier elements Ca and Ti
are less enhanced. More specifically, the [Ca/Fe] and [Ti/Fe] ratios are lower
than [O/Fe] and [Mg/Fe] by about 0.2 dex. This trend is also seen in recent
determinations of element abundances in globular cluster and field stars of the
Milky Way. This suggests that Type Ia supernovae contribute significantly to
the enrichment of the heavier alpha elements as predicted by nucleosynthesis
calculations and galactic chemical evolution models.Comment: MNRAS, re-submitted including referee's comments (minor revision),
update on CMD globular cluster age
Measuring and improving Agile Processes in a small-size software development company
Context: Agile software development has become commonplace in software development companies due to the numerous benefits it provides. However, conducting Agile projects is demanding in Small and Medium Enterprises (SMEs), because projects start and end quickly, but still have to fulfil customers' quality requirements. Objective: This paper aims at reporting a practical experience on the use of metrics related to the software development process as a means supporting SMEs in the development of software following an Agile methodology. Method: We followed Action-Research principles in a Polish small-size software development company. We developed and executed a study protocol suited to the needs of the company, using a pilot case. Results: A catalogue of Agile development process metrics practically validated in the context of a small-size software development company, adopted by the company in their Agile projects. Conclusions: Practitioners may adopt these metrics in their Agile projects, especially if working in an SME, and customise them to their own needs and tools. Academics may use the findings as a baseline for new research work, including new empirical studies.The authors would like to thank all the members of the QRapids H2020 project consortium.Peer ReviewedPostprint (published version
The Age of Ellipticals and the Color-Magnitude Relation
Using new narrowband color observations of early-type galaxies in clusters,
we reconstruct the color-magnitude relation (CMR) with a higher degree of
accuracy than previous work. We then use the spectroscopically determined ages
and metallicities from three samples (Trager et al 2008, Thomas et al 2005,
Gallazzi et al 2006), combined with multi-metallicity SED models, to compare
predicted colors for galaxies with young ages (less than 8 Gyr) with the known
CMR. We find that the CMR cannot by reproduced by the spectroscopically
determined ages and metallicities in any of the samples despite the high
internal accuracies to the spectroscopic indices. In contrast, using only the
index to determine [Fe/H], and assuming a mean age of 12 Gyr for a
galaxy's stellar population, we derive colors that exactly match not only the
color zeropoint of the CMR but also its slope. We consider the source of young
age estimates, the H index, and examine the conflict between red
continuum colors and large H values in galaxy spectra. We conclude that
our current understanding of stellar populations is insufficient to correctly
interpret H values and that the sum of our galaxy observations supports
an old and monolithic scenario of galaxy formation. This result has a
devastating impact on every study that has used the H index to calculate
galaxy age, as the use of the H versus MgFe diagram will result in
incorrectly deduced young ages.Comment: 29 pages, 9 figures. This version deviates from the version accepted
by Ap
How do practitioners perceive the relevance of requirements engineering research? An ongoing study
The relevance of Requirements Engineering (RE) research to practitioners is a prerequisite for problem-driven research in the area and key for a long-term dissemination of research results to everyday practice. To understand better how industry practitioners perceive the practical relevance of RE research, we have initiated the RE-Pract project, an international collaboration conducting an empirical study. This project opts for a replication of previous work done in two different domains and relies on survey research. To this end, we have designed a survey to be sent to several hundred industry practitioners at various companies around the world and ask them to rate their perceived practical relevance of the research described in a sample of 418 RE papers published between 2010 and 2015 at the RE, ICSE, FSE, ESEC/FSE, ESEM and REFSQ conferences. In this paper, we summarize our research protocol and present the current status of our study and the planned future steps.Peer ReviewedPostprint (author's final draft
Stellar population models at high spectral resolution
We present new, high-to-intermediate spectral resolution stellar population
models, based on four popular libraries of empirical stellar spectra, namely
Pickles, ELODIE, STELIB and MILES. These new models are the same as our
previous models, but with higher resolution and based on empirical stellar
spectra, while keeping other ingredients the same including the stellar
energetics, the atmospheric parameters and the treatment of the
Thermally-Pulsating Asymptotic Giant Branch and the Horizontal Branch
morphology. We further compute very high resolution (R=20,000) models based on
the theoretical stellar library MARCS which extends to the near-infrared. We
therefore provide merged high resolution stellar population models, extending
from ~1000 AA to 25,000 AA. We compare how these libraries perform in stellar
population models and highlight spectral regions where discrepancies are found.
We confirm our previous findings that the flux around the V-band is lower (in a
normalised sense) in models based on empirical libraries than in those based on
the BaSeL-Kurucz library, which results in a bluer B-V colour. Most noticeably
the theoretical library MARCS gives results fully consistent with the empirical
libraries. This same effect is also found in other models using MILES, namely
Vazdekis et al. and Conroy & Gunn, even though the latter authors reach the
opposite conclusion. The bluer predicted B-V colour (by 0.05 magnitudes in our
models) is in better agreement with both the colours of Luminous Red Galaxies
and globular cluster data. We test the models on their ability to reproduce,
through full spectral fitting, the ages and metallicities of galactic globular
clusters as derived from CMD fitting and find overall good agreement.
{Abridged}Comment: 30 pages, 36 figures, Monthly Notices of the Royal Astronomical
Society in pres
Globular cluster formation within the Aquarius simulation
The Aquarius project is a very high-resolution simulation capable of resolving the full mass range of potential globular cluster (GC) formation sites. With a particle mass mp= 1.4 × 104 M¿, Aquarius yields more than 100 million particles within the virial radius of the central halo which has a mass of 1.8 × 1012 M¿, similar to that of the Milky Way. With this particle mass, dark matter concentrations (haloes) that give rise to GCs via our formation criteria contain a minimum of ~2000 particles. Here, we use this simulation to test a model of metal-poor GC formation based on collapse physics. In our model, GCs form when the virial temperatures of haloes first exceed 104 K as this is when electronic transitions allow the gas to cool efficiently. We calculate the ionizing flux from the stars in these first clusters and stop the formation of new clusters when all the baryonic gas of the Galaxy is ionized. This is achieved by adopting reasonable values for the star formation efficiencies and escape fraction of ionizing photons which result in similar numbers and masses of clusters to those found in the Milky Way. The model is successful in that it predicts ages (peak age ~13.3 Gyr) and a spatial distribution of metal-poor GCs which are consistent with the observed populations in the Milky Way. The model also predicts that less than 5 per cent of GCs within a radius of 100 kpc have a surviving dark matter halo, but the more distant clusters are all found in dark matter concentrations. We then test a scenario of metal-rich cluster formation by examining mergers that trigger star formation within central gas discs. This results in younger (~7¿13.3 Gyr), more centrally located clusters (40 metal-rich GCs within 18 kpc from the centre of the host) which are consistent with the Galactic metal-rich population. We test an alternate model in which metal-rich GCs form in dwarf galaxies that become stripped as they merge with the main halo. This process is inconsistent with observed metal-rich globulars in the Milky Way because it predicts spatial distributions that are far too extended
The Blue Hook Populations of Massive Globular Clusters
We present new HST ultraviolet color-magnitude diagrams of 5 massive Galactic
globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These
observations were obtained to investigate the "blue hook" phenomenon previously
observed in UV images of the globular clusters omega Cen and NGC 2808. Blue
hook stars are a class of hot (approximately 35,000 K) subluminous horizontal
branch stars that occupy a region of the HR diagram that is unexplained by
canonical stellar evolution theory. By coupling new stellar evolution models to
appropriate non-LTE synthetic spectra, we investigate various theoretical
explanations for these stars. Specifically, we compare our photometry to
canonical models at standard cluster abundances, canonical models with enhanced
helium (consistent with cluster self-enrichment at early times), and
flash-mixed models formed via a late helium-core flash on the white dwarf
cooling curve. We find that flash-mixed models are required to explain the
faint luminosity of the blue hook stars, although neither the canonical models
nor the flash-mixed models can explain the range of color observed in such
stars, especially those in the most metal-rich clusters. Aside from the
variation in the color range, no clear trends emerge in the morphology of the
blue hook population with respect to metallicity.Comment: Accepted for publication in The Astrophysical Journal. Latex, 14
pages, 1 B&W and 6 color figure
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