305 research outputs found
On a new theoretical calibration of the Stroemgren hk metallicity index: NGC6522 as a first test case
We present a new theoretical calibration of the Stroemgren metallicity index
hk by using alpha-enhanced evolutionary models transformed into the
observational plane by using atmosphere models with the same chemical mixture.
We apply the new Metallicity--Index--Color (MIC) relations to a sample of 85
field red giants (RGs) and find that the difference between photometric
estimates and spectroscopic measurements is on average smaller than 0.1 dex
with a dispersion of sigma = 0.19 dex. The outcome is the same if we apply the
MIC relations to a sample of eight RGs in the bulge globular cluster NGC6522,
but the standard deviation ranges from 0.26 (hk, v-y) to 0.49 (hk, u-y). The
difference is mainly caused by a difference in photometric accuracy. The new
MIC relations based on the (Ca-y) color provide metallicities systematically
more metal-rich than the spectroscopic ones. We found that the Ca-band is
affected by Ca abundance and possibly by chromospheric activity.Comment: Accepted for publication on The Astrophysical Journal Letter
Hiding its age: the case for a younger bulge
The determination of the age of the bulge has led to two contradictory
results. On the one side, the color-magnitude diagrams in different bulge
fields seem to indicate a uniformly old (10 Gyr) population. On the other
side, individual ages derived from dwarfs observed through microlensing events
seem to indicate a large spread, from 2 to 13 Gyr. Because the
bulge is now recognised as being mainly a boxy peanut-shaped bar, it is
suggested that disk stars are one of its main constituents, and therefore also
stars with ages significantly younger than 10 Gyr. Other arguments as well
point to the fact that the bulge cannot be exclusively old, and in particular
cannot be a burst population, as it is usually expected if the bulge was the
fossil remnant of a merger phase in the early Galaxy. In the present study, we
show that given the range of metallicities observed in the bulge, a uniformly
old population would be reflected into a significant spread in color at the
turn-off which is not observed. Inversely, we demonstrate that the correlation
between age and metallicity expected to hold for the inner disk would conspire
to form a color-magnitude diagram with a remarkably small spread in color, thus
mimicking the color-magnitude diagram of a uniformly old population. If stars
younger than 10 Gyr are part of the bulge, as must be the case if the bulge has
been mainly formed through dynamical instabilities in the disk, then a very
small spread at the turn-off is expected, as seen in the observations.Comment: 11 pages, 11 figures. Accepted for publication in A&
Detailed abundances of Red Giants in the Globular Cluster NGC~1851: C+N+O and the Origin of Multiple Populations
We present chemical abundance analysis of a sample of 15 red giant branch
(RGB) stars of the Globular Cluster NGC~1851 distributed along the two RGBs of
the (v, v-y) CMD. We determined abundances for C+N+O, Na, , iron-peak,
and s-elements. We found that the two RGB populations significantly differ in
their light (N,O,Na) and s-element content. On the other hand, they do not show
any significant difference in their and iron-peak element content.
More importantly, the two RGB populations do not show any significant
difference in their total C+N+O content. Our results do not support previous
hypotheses suggesting that the origin of the two RGBs and the two subgiant
branches of the cluster is related to a different content of either
(including Ca) or iron-peak elements, or C+N+O abundance, due to a second
generation polluted by SNeII.Comment: 9 pages, 4 figures, accepted for publication on ApJ Lette
The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment
UV 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 as
results of the late hot flasher scenario where stars experience the helium
flash while on the white dwarf cooling curve or by the progeny of the
helium-enriched sub-population recently postulated to exist in some clusters.
Moderately high resolution spectra of stars at the hot end of the blue HB in
omega Cen were analysed for atmospheric parameters and abundances using LTE and
Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find
that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar
helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14%
are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step
with helium enrichment, with a maximum carbon enrichment of 3% by mass. At
least 14% of the hottest HB stars in omega Cen show helium abundances well
above the highest predictions from the helium enrichment scenario (Y = 0.42
corresponding to log(n_He/n_H) ~ -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. (Abridged)Comment: 4 pages, 3 figures, uses aa.cls (enclosed), accepted as A&A Lette
Just how hot are the Centauri extreme horizontal branch pulsators?
Past studies based on optical spectroscopy suggest that the five Cen
pulsators form a rather homogeneous group of hydrogen-rich subdwarf O stars
with effective temperatures of around 50 000 K. This places the stars below the
red edge of the theoretical instability strip in the log Teff diagram,
where no pulsation modes are predicted to be excited. Our goal is to determine
whether this temperature discrepancy is real, or whether the stars' effective
temperatures were simply underestimated. We present a spectral analysis of two
rapidly pulsating extreme horizontal branch (EHB) stars found in Cen.
We obtained Hubble Space Telescope/COS UV spectra of two Cen
pulsators, V1 and V5, and used the ionisation equilibrium of UV metallic lines
to better constrain their effective temperatures. As a by-product we also
obtained FUV lightcurves of the two pulsators. Using the relative strength of
the N IV and N V lines as a temperature indicator yields Teff values close to
60 000 K, significantly hotter than the temperatures previously derived. From
the FUV light curves we were able to confirm the main pulsation periods known
from optical data. With the UV spectra indicating higher effective temperatures
than previously assumed, the sdO stars would now be found within the predicted
instability strip. Such higher temperatures also provide consistent
spectroscopic masses for both the cool and hot EHB stars of our previously
studied sample.Comment: 9 pages, accepted for publication in Astronomy and Astrophysic
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