15 research outputs found
On the formation and evolution of magnetic chemically peculiar stars in the solar neighborhood
In order to put strict observational constraints on the evolutionary status
of the magnetic chemically peculiar stars (CP2) of the upper main sequence, we
have investigated a well established sample of galactic field CP2 objects
within a radius of 200pc from the Sun in the (X,Y) plane. In total, 182 stars
with accurate parallax measurements from the Hipparcos satellite were divided
into Si, SiCr and SrCrEu subgroups based on classification resolution data from
the literature. Primarily, it was investigated if the CP2 phenomenon occurs at
very early stages of the stellar evolution, significantly before these stars
reach 30% of their life-time on the main sequence. This result is especially
important for theories dealing with stellar dynamos, angular momentum loss
during the pre- as well as main sequence and stellar evolutionary codes for CP2
stars. For the calibration of the chosen sample, the well-developed framework
of the Geneva 7-color and Stromgren uvbybeta photometric system was used. We
are able to show that the CP2 phenomenon occurs continuously at the zero age
main sequence for masses between 1.5 and 4.5M(sun}. The magnetic field
strengths do not vary significantly during the evolution towards the terminal
age main sequence. Only the effective temperature and magnetic field strength
seem to determine the kind of peculiarity for those stars. We found several
effects during the evolution of CP2 stars at the main sequence, i.e. there are
two "critical" temperatures where severe changes take place. There is a
transition between Si, SiCr and SrCrEu stars at 10000K whereas a significant
decrease to almost zero of evolved SrCrEu objects with masses below 2.25M(sun)
at 8000K occurs.Comment: 7 pages, 4 figures, accepted by A&
Theoretical isochrones for the Delta a photometric system
We have calculated theoretical isochrones for the photometric Delta a system
to derive astrophysical parameters such as the age, reddening and distance
modulus for open clusters. The Delta a system samples the flux depression at
520 nm which is highly efficient to detect chemically peculiar (CP) objects of
the upper main sequence. The evolutionary status of CP stars is still a matter
of debate and very important to test, for example, the dynamo and diffusion
theories. In fact, the dynamo or fossil origin of the magnetic fields present
in this kind of stars it still not clear. Using the stellar evolutionary models
by Claret (1995), a grid of isochrones with different initial chemical
compositions for the Delta a system was generated. The published data of 23
open clusters were used to fit these isochrones with astrophysical parameters
(age, reddening and distance modulus) from the literature. As an additional
test, isochrones with the same parameters for Johnson UBV data of these open
clusters were also considered. The fits show a good agreement between the
observations and the theoretical grid. We find that the accuracy of fitting
isochrones to Delta a data without the knowledge of the cluster parameters is
between 5 and 15%.Comment: 5 pages, 4 figures, accepted by A&
A statistical method to determine open cluster metallicities
The study of open cluster metallicities helps to understand the local stellar
formation and evolution throughout the Milky Way. Its metallicity gradient is
an important tracer for the Galactic formation in a global sense. Because open
clusters can be treated in a statistical way, the error of the cluster mean is
minimized. Our final goal is a semi-automatic statistical robust method to
estimate the metallicity of a statistically significant number of open clusters
based on Johnson BV data of their members, an algorithm that can easily be
extended to other photometric systems for a systematic investigation. This
method incorporates evolutionary grids for different metallicities and a
calibration of the effective temperature and luminosity. With cluster
parameters (age, reddening and distance) it is possible to estimate the
metallicity from a statistical point of view. The iterative process includes an
intrinsic consistency check of the starting input parameters and allows us to
modify them. We extensively tested the method with published data for the
Hyades and selected sixteen open clusters within 1000pc around the Sun with
available and reliable Johnson BV measurements. In addition, Berkeley 29, with
a distance of about 15kpc was chosen. For several targets we are able to
compare our result with published ones which yielded a very good coincidence
(including Berkeley 29).Comment: 14 pages, 6 figures, accepted for publication in Astronomy &
Astrophysic
CCD photometric search for peculiar stars in open clusters. VI. NGC 1502, NGC 3105, Stock 16, NGC 6268, NGC 7235 and NGC 7510
In a sample of six young open clusters we investigated 1753 objects using the
narrow band, three filter Delta a photometric system resulting in the detection
of eleven bona-fide magnetic chemically peculiar (CP) stars and five Be or
metal-weak stars. The results for the distant cluster NGC 3105 is most
important because of the still unknown influence of the global metallicity
gradient of the Milky Way. These findings confirm that CP stars are present in
open clusters of very young ages (log t > 6.90) at galactocentric distances up
to 11.4 kpc. For all programme clusters the age, reddening, and distance
modulus were derived using the corresponding isochrones. Some additional
variable stars within Stock 16 could be identified by comparing different
photometric studies.Comment: 7 pages, 2 figures, accepted by A&
On the evolutionary status of chemically peculiar stars of the upper main sequence
We present further evidence that the magnetic chemically peculiar stars (CP2)
of the upper main sequence already occur at very early stages of the stellar
evolution, significantly before they reach 30% of their life-time on the main
sequence. This result is especially important for models dealing with dynamo
theories, angular momentum loss during the pre- as well as main sequence and
evolutionary calculations for CP2 stars. Results from the literature either
derived for objects in the Hyades and the UMa cluster or from the Hipparcos
mission contradict each other. A way out of this dilemma is to investigate
young open clusters with known ages and accurate distances (error < 10%),
including CP2 members. Up to now, four open clusters fulfill these
requirements: IC 2391, IC 2602, NGC 2451A and NGC 2516. In total, 13 CP2 stars
can be found within these clusters. We have used the measurements and
calibrations of the Geneva 7-color photometric system to derive effective
temperatures and luminosities. Taking into account the overall metallicity of
the individual clusters, isochrones and evolutionary tracks were used to
estimate ages and masses for the individual objects. The derived ages (between
10 and 140 Myr) are well in line with those of the corresponding clusters and
further strengthen the membership of the investigated CP2 stars.Comment: 7 pages, 1 figure, accepted by A&
A photometric long-term study of CP stars in open clusters
Photometric variability of chemically peculiar (CP) stars of the upper main
sequence is closely connected to their local stellar magnetic field and their
rotational period. Long term investigations, as presented here, help us to
identify possible stellar cycles (as in the Sun). Furthermore, these data
provide a basis for detailed surface mapping techniques. Photoelectric
Stroemgren uvby time series for 27 CP stars within the boundaries of open
clusters are presented. In addition, Hipparcos photometric data (from 1989 to
1993) are used for our analysis. Our observations cover a time period of about
six years (1986 to 1992) with typically fifteen measurements for each objects.
These observations help us to determine the rotational periods of these
objects. A standard reduction procedure was applied to the data. When possible,
we merged our data sets with already published ones to obtain a more
significant result. A detailed time series analysis was performed, involving
five different methods to minimize spurious detections. We established, for the
first time, variability for fourteen CP stars. For additional two stars, a
merging of already published data sets, resulted in more precise periods,
whereas for six objects, the published periods could be confirmed. Last, but
not least, no significant variations were found for five stars. Apart from six
stars, all targets seem to be members of their host open clusters.Comment: 10 pages, 3 figures, accepted by Astronomy and Astrophysic
Dietary nitrate supplementation does not alter exercise efficiency at high altitude â further results from the Xtreme Alps study
Introduction: Nitrate supplementation in the form of beetroot juice (BRJ) ingestion has been shown to improve exercise tolerance during acute hypoxia, but its effect on exercise physiology remains unstudied during sustained terrestrial high altitude exposure. We hypothesized that performing exercise at high altitude would lower circulating nitrate and nitrite levels and that BRJ ingestion would reverse this phenomenon while concomitantly improving key determinants of aerobic exercise performance.
Methods: Twenty seven healthy volunteers (21 male) underwent a series of exercise tests at sea level (SL, London, 75 m) and again after 5-8 days at high altitude (HA, Capanna Regina Margherita or "Margherita Hut," 4,559 m). Using a double-blind protocol, participants were randomized to consume a beetroot/fruit juice beverage (three doses per day) with high levels of nitrate (âŒ0.18 mmol/kg/day) or a nitrate-depleted placebo (âŒ11.5 ÎŒmoles/kg/day) control drink, from 3 days prior to the exercise trials until completion. Submaximal constant work rate cycle tests were performed to determine exercise efficiency and a maximal incremental ramp exercise test was undertaken to measure aerobic capacity, using breath-by-breath pulmonary gas exchange measurements throughout. Concentrations of nitrate, nitrite and nitrosation products were quantified in plasma samples collected at 5 timepoints during the constant work rate tests. Linear mixed modeling was used to analyze data.
Results: At both SL and HA, plasma nitrate concentrations were elevated in the nitrate supplementation group compared to placebo (P < 0.001) but did not change throughout increasing exercise work rate. Delta exercise efficiency was not altered by altitude exposure (P = 0.072) or nitrate supplementation (P = 0.836). VÌO2peak decreased by 24% at high altitude (P < 0.001) and was lower in the nitrate-supplemented group at both sea level and high altitude compared to placebo (P = 0.041). Dietary nitrate supplementation did not alter other peak exercise variables or oxygen consumption at anaerobic threshold. Circulating nitrite and S-nitrosothiol levels unexpectedly rose in a few individuals right after cessation of exercise at high altitude.
Conclusion: Whilst regularly consumed during an 8 days expedition to terrestrial high altitude, nitrate supplementation did not alter exercise efficiency and other exercise physiological variables, except decreasing VÌO2peak. These results and those of others question the practical utility of BRJ consumption during prolonged altitude exposure