200 research outputs found
The He abundance in NGC 1850 A and B: are we observing the early stage of formation of multiple populations in a stellar cluster?
We present the result of a sample of B-stars in the Large Magellanic Cloud
young double stellar cluster NGC 1850 A and NGC 1850 B, observed with the
integral-field spectrograph at the Very Large Telescope, the Multi Unit
Spectroscopic Explorer. We compare the observed equivalent widths (EWs) of four
He lines (4922 , 5015 , 6678
, and 7065 ) with the ones
determined from synthetic spectra computed with different He mass fraction
(Y=0.25, 0.27, 0.30 and 0.35) with the code SYNSPEC, that takes into account
the non-LTE effect. From this comparison, we determined the He mass fraction of
the B stars, finding a not homogeneous distribution. The stars can be divided
in three groups, He-weak (Y 0.24) and the He-normal (0.24 Y
0.26) belonging to the MS of NGC 1850 A, and the He-rich stars
(0.33 Y 0.38) situated in the MS associated to NGC 1850
B. We have analyzed the stellar rotation as possible responsible of the
anomalous features of the He lines in the He-rich stars. We provide a simple
analysis of the differences between the observed EWs and the ones obtained from
the theoretical models with different rotation velocity (V = 0 and 250
Km/s). The resolution of the MUSE spectra do not allow to get a conclusive
result, however our analysis support the He-enhanced hypothesis.Comment: Accepted for publication by MNRAS, 10 pages, 8 figure
MUSE observations of NGC330 in the Small Magellanic Cloud. Helium abundance of bright main sequence stars
We present observations of the most bright main sequence stars in the Small
Magellanic Cloud stellar cluster NGC330 obtained with the integral field
spectrograph MUSE@VLT. The use of this valuable instrument allows us to study
both photometric and spectroscopic properties of stellar populations of this
young star cluster.
The photometric data provide us a precise color magnitude diagram, which
seems to support the presence of two stellar populations of ages of 18
Myr and 30 Myr assuming a metallicity of Z = 0.002. Thanks to the
spectroscopic data, we derive helium abundance of 10 main sequence stars within
the effective radius Reff= 20" of NGC330, thus leading to an estimation of
= 10.93 0.05 (1 ). The helium elemental abundances
of stars likely belonging to the two possible stellar populations, do not show
differences or dichotomy within the uncertainties. Thus, our results suggest
that the two stellar populations of NGC330, if they exist, share similar
original He abundances.
If we consider stellar rotation velocity in our analysis, a coeval (30 Myr)
stellar population, experiencing different values of rotation, cannot be
excluded. In this case, the mean helium abundance rot
obtained in our analysis is 11.00 0.05 dex. We also verified that
possible NLTE effects cannot be identified with our analysis because of the
spectral resolution and they are within our derived abundance He uncertainties.
Moreover, the analysis of the He abundance as a function of the distance from
the cluster center of the observed stars do not show any correlation
CCD Photometry of Galactic Globular Clusters. IV. The NGC 1851 RR Lyraes
The variable star population of the galactic globular cluster NGC 1851
(C0512-400) has been studied by CCD photometry, from observations made in the
B, V, and I bands during 1993-4. Light curves are presented for 29 variables,
seven of which are new discoveries. The behavior of the RR lyraes in the
period-temperature diagram appears normal when compared to clusters which
bracket the NGC 1851 metallicity. Reddening and metallicity are re-evaluated,
with no compelling evidence to change from accepted values. Photometry for
stars within an annulus with radii 80 and 260 arcsec agrees to better than 0.02
mag in all colors with extensive earlier photometry, to at least V = 18.5.
Instability strip boundary positions for several clusters shows a trend for the
red boundary to move to redder colors as the metallicity increases.Comment: 29 pages, 9 figures, accepted by A.
Hubble Space Telescope Observations of the Oldest Star Clusters in the LMC
We present V, V-I color-magnitude diagrams (CMDs) for three old star clusters
in the Large Magellanic Cloud (LMC): NGC 1466, NGC 2257 and Hodge 11. Our data
extend about 3 magnitudes below the main-sequence turnoff, allowing us to
determine accurate relative ages and the blue straggler frequencies. Based on a
differential comparison of the CMDs, any age difference between the three LMC
clusters is less than 1.5 Gyr. Comparing their CMDs to those of M 92 and M 3,
the LMC clusters, unless their published metallicities are significantly in
error, are the same age as the old Galactic globulars. The similar ages to
Galactic globulars are shown to be consistent with hierarchial clustering
models of galaxy formation. The blue straggler frequencies are also similar to
those of Galactic globular clusters. We derive a true distance modulus to the
LMC of (m-M)=18.46 +/- 0.09 (assuming (m-M)=14.61 for M 92) using these three
LMC clusters.Comment: 22 pages; to be published in Ap
Integrated parameters of star clusters: A comparison of theory and observations
(Abridged) This paper presents integrated magnitude and colours for synthetic
clusters. The integrated parameters have been obtained for the whole cluster
population as well as for the main-sequence (MS) population of star clusters.
We have also estimated observed integrated magnitudes and colours of MS
population of galactic open clusters, LMC and SMC star clusters. It is found
that the colour evolution of MS population of star clusters is not affected by
the stochastic fluctuations, however these fluctuations significantly affect
the colour evolution of the whole cluster population. The fluctuations are
maximum in colour in the age range 6.7 log (age) 7.5. Evolution
of integrated colours of MS population of the clusters in the Milky Way, LMC
and SMC, obtained in the present study are well explained by the present
synthetic cluster model. The observed integrated colours of MS
population of LMC star clusters having age 500 Myr seem to be
distributed around 0.004 model, whereas colours are found to be
more bluer than those predicted by the 0.004 model. vs
two-colour diagram for the MS population of the Milky Way star clusters shows a
fair agreement between the observations and present model, however the diagrams
for LMC and SMC clusters indicate that observed colours are relatively
bluer. Possible reasons for this anomaly have been discussed.Comment: 35 pages, 20 figs, accepted for publication in MNRA
The CHilean Automatic Supernova sEarch (CHASE)
The CHASE project started in 2007 with the aim of providing young southern
supernovae (SNe) to the Carnegie Supernova Project (CSP) and Millennium Center
for Supernova Studies (MCSS) follow-up programs. So far CHASE has discovered 33
SNe with an average of more than 2.5 SNe per month in 2008. In addition to the
search we are carrying out a follow-up program targeting bright SNe. Our fully
automated data reduction allows us to follow the evolution on the light curve
in real time, triggering further observations if something potentially
interesting is detectedComment: 4 pages, 2 figures, conference proceedin
Solar analogues in open clusters: The case of M67
Solar analogues are fundamental targets for a better understanding of our Sun
and our Solar System. Usually, this research is limited to field stars, which
offer several advantages and limitations. In this work, we present the results
of a research of solar twins performed for the first time in a open cluster,
namely M67. Our analysis allowed us to find five solar twins and also to derive
a solar colour of (B-V)0=0.649+/-0.016 and a cluster distance modulus of
9.63+/-0.08. This study encourages us to apply the same method to other open
clusters, and to do further investigations for planet search in the solar twins
we find.Comment: 4 pages, 2 figures; Proceedings of the conference "Probing Stellar
Populations out to the Distant Universe", Cefalu' (Italy), September 7 - 19,
2008. To be published in the AIP Conf. Proc. Serie
Abundances in intermediate-mass AGB stars undergoing third dredge-up and hot-bottom burning
High dispersion near-infrared spectra have been taken of seven
highly-evolved, variable, intermediate-mass (4-6 Msun) AGB stars in the LMC and
SMC in order to look for C, N and O variations that are expected to arise from
third dredge-up and hot-bottom burning. The pulsation of the objects has been
modelled, yielding stellar masses, and spectral synthesis calculations have
been performed in order to derive abundances from the observed spectra. For two
stars, abundances of C, N, O, Na, Al, Ti, Sc and Fe were derived and compared
with the abundances predicted by detailed AGB models. Both stars show very
large N enhancements and C deficiencies. These results provide the first
observational confirmation of the long-predicted production of primary nitrogen
by the combination of third dredge-up and hot-bottom burning in
intermediate-mass AGB stars. It was not possible to derive abundances for the
remaining five stars: three were too cool to model, while another two had
strong shocks in their atmospheres which caused strong emission to fill the
line cores and made abundance determination impossible. The latter occurrence
allows us to predict the pulsation phase interval during which observations
should be made if successful abundance analysis is to be possible.Comment: Accepted for publication in MNRA
Emergence of Anti-Cancer Drug Resistance: Exploring the Importance of the Microenvironmental Niche via a Spatial Model
Practically, all chemotherapeutic agents lead to drug resistance. Clinically,
it is a challenge to determine whether resistance arises prior to, or as a
result of, cancer therapy. Further, a number of different intracellular and
microenvironmental factors have been correlated with the emergence of drug
resistance. With the goal of better understanding drug resistance and its
connection with the tumor microenvironment, we have developed a hybrid
discrete-continuous mathematical model. In this model, cancer cells described
through a particle-spring approach respond to dynamically changing oxygen and
DNA damaging drug concentrations described through partial differential
equations. We thoroughly explored the behavior of our self-calibrated model
under the following common conditions: a fixed layout of the vasculature, an
identical initial configuration of cancer cells, the same mechanism of drug
action, and one mechanism of cellular response to the drug. We considered one
set of simulations in which drug resistance existed prior to the start of
treatment, and another set in which drug resistance is acquired in response to
treatment. This allows us to compare how both kinds of resistance influence the
spatial and temporal dynamics of the developing tumor, and its clonal
diversity. We show that both pre-existing and acquired resistance can give rise
to three biologically distinct parameter regimes: successful tumor eradication,
reduced effectiveness of drug during the course of treatment (resistance), and
complete treatment failure
The Carina Project: II. Stellar Populations
We present a new (V,B-V) Color-Magnitude Diagram (CMD) of the Carina dwarf
spheroidal (dSph) that extends from the tip of the Red Giant Branch (RGB) down
to about V=25 mag. Data were collected with the Wide Field Imager (WFI)
available at 2.2m ESO/MPI telescope and cover an area of about 0.3 square
degrees around the center of the galaxy. We confirm the occurrence of a
substantial amount of old stars with ages around 11 Gyr, together with an
intermediate-age population around 5 Gyr. Moreover, we also detected a new
well-defined blue plume of young Main-Sequence (MS) stars with an age at most
of the order of 1 Gyr. At odds with previous results available in the
literature we found thatstars along the RGB of the old and of the
intermediate-age stellar populationindicate a mean metallicity roughly equal to
Z=0.0004 and a small dispersion around this value. We find evidence for a
smooth spatial distribution of the "intermediate-age" stellar population, which
appears more centrally concentrated than the oldest one.Comment: 39 pages, 24 figures (PNG format). Accepted for publication on
Astronomical Journal. A version with all figures included is available at
http://www.mporzio.astro.it/~marco/carinaproject
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