3,964 research outputs found
A state-of-the-art analysis of the dwarf irregular galaxy NGC 6822
We present a detailed photometric study of the dwarf irregular galaxy NGC
6822 aimed at investigating the properties of its stellar populations and, in
particular, the presence of stellar radial gradients. Our goal is to analyse
the stellar populations in six fields, which cover the whole bar of this dwarf
galaxy. We derived the quantitative star formation history (SFH) of the six
fields using the IAC method, involving IAC-pop/MinnIAC codes. The solutions we
derived show an enhanced star formation rate (SFR) in Fields 1 and 3 during the
past 500 Myr. The SFRs of the other fields are almost extinguished at very
recent epochs and. We study the radial gradients of the SFR and consider the
total mass converted into stars in two time intervals (between 0 and 0.5 Gyr
ago and between 0.5 and 13.5 Gyr ago). We find that the scale lengths of the
young and intermediate-to-old populations are perfectly compatible, with the
exception of the young populations in Fields 1 and 3. The recent SF in these
two fields is greater than in the other ones. This might be an indication that
in these two fields we are sampling incipient spiral arms. Further evidence and
new observations are required to prove this hypothesis. In addition, we derived
the age-metallicity relations. As expected, the metallicity increases with time
for all of the fields. We do not observe any radial gradient in the
metallicity.Comment: 9 pages, 11 figures, Accepted to A&
A theoretical study on trehalose + water mixtures for dry preservation purposes
The properties of trehalose + water mixtures are studied as a function of mixture composition and temperature using molecular dynamics simulations. As trehalose disaccharide has been proposed for dry preservation purposes, the objective of this work is to analyse the nanoscopic properties of the considered mixtures, in terms of aggregation, clustering, interactions energies, and local dynamics, and their relationships with hydrogen bonding. The reported results allow a detailed characterization of hydrogen bonding and its evolution with mixture composition and thus inferring the effects of trehalose on water structuring providing results to justify the mechanisms of trehalose acting as preservation agent
Innovation through R&D activities in the European context: Antecedents and consequences
Despite the fact that the determinants and the consequences of R&D activities have been extensively studied in previous research, further efforts to integrate disparate streams of literature might bring new insights into innovation decision-making by firms. In particular, this article studies the simultaneous effects that a set of factors (at both company and environmental levels of analysis) have on R&D activity, which explain firm growth. A two-stage probit least squares (2SPLS) estimation is applied to data from the EU-EFIGE/Bruegel-UniCredit dataset for seven European countries for the years 2007-2009. The main findings show that not all the R&D determinants lead to firm growth. In particular, R&D activities are affected by the employment of a significant number of foreign executives, a higher percentage of employees with fixed-term contracts, appropriate labour regulations and access to employees who have received external training, all of which are positively related to firm growth. Based on these results, policy and practical implications to improve firms’ performances are discussed
Spatially resolved LMC star formation history: I. Outside in evolution of the outer LMC disk
We study the evolution of three fields in the outer LMC disk Rgc=3.5-6.2 Kpc.
Their star formation history indicates a stellar populations gradient such that
younger stellar populations are more centrally concentrated. We identify two
main star forming epochs, separated by a period of lower activity between ~7
and ~4 Gyr ago. Their relative importance varies from a similar amount of stars
formed in the two epochs in the innermost field, to only 40% of the stars
formed in the more recent epoch in the outermost field. The young star forming
epoch continues to the present time in the innermost field, but lasted only
till ~0.8 and 1.3 Gyr ago at Rgc=5.5 degrees and 7.1 degrees, respectively.
This gradient is correlated with the measured HI column density and implies an
outside-in quenching of the star formation, possibly related to a variation of
the size of the HI disk. This could either result from gas depletion due to
star formation or ram-pressure stripping, or from to the compression of the gas
disk as ram-pressure from the Milky Way halo acted on the LMC interstellar
medium. The latter two situations may have occurred when the LMC first
approached the Milky Way.Comment: 15 pages, 13 figures, 4 tables. MNRAS, in pres
Multiple stellar populations in Magellanic Cloud clusters. III. The first evidence of an extended main sequence turn-off in a young cluster: NGC1856
Recent studies have shown that the extended main-sequence turn off (eMSTO) is
a common feature of intermediate-age star clusters in the Magellanic Clouds
(MCs). The most simple explanation is that these stellar systems harbor
multiple generations of stars with an age difference of a few hundred Myrs.
However, while an eMSTO has been detected in a large number of clusters with
ages between ~1-2 Gyrs, several studies of young clusters in both MCs and in
nearby galaxies do not find any evidence for a prolonged star-formation
history, i.e. for multiple stellar generations. These results have suggested
alternative interpretation of the eMSTOs observed in intermediate-age star
clusters. The eMSTO could be due to stellar rotation mimicking an age spread or
to interacting binaries. In these scenarios, intermediate-age MC clusters would
be simple stellar populations, in close analogy with younger clusters.
Here we provide the first evidence for an eMSTO in a young stellar cluster.
We exploit multi-band Hubble Space Telescope photometry to study the ~300-Myr
old star cluster NGC1856 in the Large Magellanic Cloud and detected a broadened
MSTO that is consistent with a prolonged star-formation which had a duration of
about 150 Myrs. Below the turn-off, the MS of NGC1856 is split into a red and
blue component, hosting 33+/-5% and 67+/-5% of the total number of MS stars,
respectively. We discuss these findings in the context of
multiple-stellar-generation, stellar-rotation, and interacting-binary
hypotheses.Comment: 14 pages, 10 figures, accepted for publication in MNRA
A WFC3/HST view of the three stellar populations in the Globular Cluster NGC6752
Multi-band Hubble Space Telescope photometry reveals that the main sequence,
sub-giant, and the red giant branch of the globular cluster NGC6752 splits into
three main components in close analogy with the three distinct segments along
its horizontal branch stars. These triple sequences are consistent with three
stellar groups: a stellar population with a chemical composition similar to
field halo stars (population a), a population (c) with enhanced sodium and
nitrogen, depleted carbon and oxygen and enhanced helium abundance (Delta Y
~0.03), and a population (b) with an intermediate (between population a and c)
chemical composition and slightly helium enhanced (Delta Y ~0.01). These
components contain ~25% (population a), ~45% (population b), and ~30%
(population c) of the stars. No radial gradient for the relative numbers of the
three populations has been identified out to about 2.5 half mass radii.Comment: 42 pages, 24 figures, accepted for publication in Ap
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