14 research outputs found
A Stromgren view of the multiple populations in globular clusters
We discuss a variety of photometric indices assembled from the uvby Stromgren
system. Our aim is to examine the pros and cons of the various indices to find
the most suitable one(s) to study the properties of multiple populations in
globular clusters (GCs) discovered by spectroscopy. We explore in particular
the capabilities of indices like m_1 and c_y at different metallicities. We
define a new index delta_4=(u-v)-(b-y) to separate first and second stellar
generations in GCs of any metal abundance, since it keeps the sensitivity to
multiple stellar populations over all the metallicity range and at the same
time minimizes the sensitivity to photometric errors. We detecte clear
differences in the red giant branches of the GCs examined, like skewness or
bi/multi-modality in color distribution. We connect the photometric information
with the spectroscopic results on O, Na abundances we obtained in our survey of
GCs. Finally, we compute the effects of different chemical composition on the
Stromgren filters and indices using synthetic spectra.Comment: Accepted for publication on Astronomy and Astrophysics. Figures 1,3,5
degraded in resolutio
Accretion of gas by globular cluster stars
Some recent observations of the abundances of s-process, r-process, and alpha
elements in metal-poor stars have led to a new scenario for their formation.
According to this scenario, these stars were born in a globular cluster and
accreted the s-process enriched gas expelled by cluster stars of higher-mass,
thereby modifying their surface abundances. Later on, these polluted stars
evaporated from the globular cluster to constitute an important fraction of the
current halo population. In addition, there are now many direct observations of
abundance anomalies not only in globular cluster giant stars but also in
subgiant and main-sequence stars. Accretion provides again a plausible
explanation for (at least some of) these peculiarities. Here we investigate
further the efficiency of the accretion scenario. We find that in concentrated
clusters with large escape velocities, accretion is very efficient and can
indeed lead to major modifications of the stellar surface abundances.Comment: 11 pages; accepted for publication in Astronomy and Astrophysic
Abundances of C, N, O in slightly evolved stars in the globular clusters NGC 6397, NGC 6752 and 47 Tuc
Abundances of C and N are derived from features due to the CH G-band and to
the UV CN band measured on high resolution (R ~ 40,000) UVES spectra of more
than 40 dwarfs and subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygen
abundances (or upper limits) are available for all stars in the sample.
Isotopic ratios 12C/13C were derived from the CH molecular band. This is the
first determination of this ratio in unevolved dwarf stars in globular
clusters. By enlarging the sample of subgiants in NGC 6397 studied in Gratton
et al. (2001), we uncovered, for the first time, large variations in both Na
and O also in this cluster. The origin of the chemical inhomogeneities must be
searched for outside the stars under scrutiny. Our data indicate that in
unevolved or slightly evolved stars in these clusters C abundances are low but
not zero, also in stars with large N-enhancements and O-depletions, and that
the isotopic ratios 12C/13C are low, but never reach the equilibrium value of
the CN-cycle. When coupled with the run of O and Na abundances, these findings
possibly require that, in addition to CNO burning and p-captures, some
triple-alpha process is also involved: previously evolved intermediate-mass AGB
stars are then the most likely polluters.Comment: 16 pages, 18 figures, accepted for publication on A&
Multiple populations in globular clusters. Lessons learned from the Milky Way globular clusters
Recent progress in studies of globular clusters has shown that they are not
simple stellar populations, being rather made of multiple generations. Evidence
stems both from photometry and spectroscopy. A new paradigm is then arising for
the formation of massive star clusters, which includes several episodes of star
formation. While this provides an explanation for several features of globular
clusters, including the second parameter problem, it also opens new
perspectives about the relation between globular clusters and the halo of our
Galaxy, and by extension of all populations with a high specific frequency of
globular clusters, such as, e.g., giant elliptical galaxies. We review progress
in this area, focusing on the most recent studies. Several points remain to be
properly understood, in particular those concerning the nature of the polluters
producing the abundance pattern in the clusters and the typical timescale, the
range of cluster masses where this phenomenon is active, and the relation
between globular clusters and other satellites of our Galaxy.Comment: In press (The Astronomy and Astrophysics Review
Heavy elements abundances in turn-off stars and early subgiants in NGC 6752
High resolution spectra (R > 40 000) for 9 main sequence turn-off stars and 9
subgiants in the globular cluster NGC 6752 were acquired with UVES on the
VLT-Kueyen (UT2). These data have been used to determine the abundances of some
heavy elements (strontium, yttrium, barium and europium). This paper presents
for the first time accurate results for heavy elements in this globular
cluster. We did not find any systematic effect between the abundances found in
turn-off stars, subgiants, and giants. We obtain the following mean abundances
for these elements in our sample (turn-off stars and subgiants): [Sr/Fe] = 0.06
+/- 0.16, [Y/Fe] = -0.01 +/- 0.12, [Ba/Fe] = 0.18 +/- 0.11, and [Eu/Fe] = 0.41
+/- 0.09. The dispersion in the abundance ratios of the different elements is
low and can be totally explained by uncertainties in their derivation. These
ratios are in agreement with the values found in field halo stars with the same
metallicity. We did not observe any correlation between the [n-capture/Fe]
ratios and the star-to-star variations of the O and Na abundances. The [Ba/Eu]
and [Sr/Ba] ratios show clearly that this globular cluster has also been
uniformly enriched by r- and s-process synthesis.Comment: 10 pages, 8 figures. Accepted for publication in A&
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Students using digital imagery as tools to scaffold an investigation of light pollution
Students must do science. Both the National Science Education Standards (NSES) and Texas Essential Knowledge and Skills (TEKS) clearly direct teachers to engage their students in scientific investigations. These investigations should be authentic, not contrived. Students should engage in interesting problems and produce meaningful results. Light pollution - a hazy by-product of artificial illumination that washes out star light - is a local and global phenomena that students can investigate using astronomy with sophisticated tools. This study describes how students approach "authentic science inquiry" while they investigate light pollution at their school. Two groups of students measured light pollution with different tools. The control group students used star maps of small areas of sky to record the stars they could see using their eyes alone. The faintest stars they saw are close to the sky's brightness. Treatment group students take digital images of sky and analyze them with freely available image analysis software to determine the sky's brightness. Both groups engage in an authentic problem, but the different tools they use affect their approach toward authentic science inquiry. Control group students show an unexpected score increase for Texas Assessment of Knowledge and Skills (TAKS) related posttest items. Both treatment and control students approach scientific inquiry; but, analysis of the treatment students' presentations and interviews show that they made subtle additional steps toward authentic inquiry. Students enjoyed using the tools, and thought that their new tool skills helped them organize and explore their data and understand their results.Science, Technology, Engineering, and Mathematics Educatio