125 research outputs found
Vertical stratification of iron in atmospheres of blue horizontal-branch stars
The aim of this study is to search for observational evidence of vertical
iron stratification in the atmosphere of fourteen blue horizontal-branch (BHB)
stars. We have found from our numerical simulations that five BHB stars: B22,
B186 in the globular cluster NGC 288, WF2-820, WF2-2692 in M13 and B203 in M15
show clear signatures of the vertical stratification of iron whose abundance
increases toward the lower atmosphere. Two other BHB stars (B334 in M15 and
B176 in M92) also show possible iron stratification in their atmosphere. A
dependence of the slope of iron stratification on the effective temperature was
also discovered. It is found that the vertical stratification of iron is
strongest in BHB stars with Teff around 11,500K. The slope of iron abundance
decreases as Teff increases and becomes negligible for the BHB stars with Teff=
14,000K. These results support the hypothesis regarding the efficiency of
atomic diffusion in the stellar atmospheres of BHB stars with Teff > 11,500K.Comment: 6 pages, 2 figures, 3 table
Stratification of the elements in the atmospheres of blue horizontal-branch stars
Blue horizontal-branch (BHB) stars with approximately larger
than 11500 K show several observational anomalies. In globular clusters, they
exhibit low rotational velocities, abundance anomalies (as compared to cluster
abundances), photometric jumps and gaps and spectroscopic gravities lower than
predicted by canonical models. It is commonly believed that the low rotational
velocities of these stars permit atomic diffusion to be efficient in their
atmosphere thereby causing the observed anomalies. Recent detections of
vertical stratification of iron (and some other chemical elements) in several
BHB stars concur with this framework. In this paper, improved model atmospheres
that include the vertical stratification of the elements are applied to BHB
stars to verify if they can explain their observational anomalies. The results
from theoretical model atmospheres are consistent with the photometric jumps
and gaps observed for BHB stars in globular clusters. It is found that iron
stratification in the theoretical models and that obtained from observations
have similar tendancies. Our results also show that the spectroscopic gravities
obtained while using chemically homogeneous model atmospheres to fit
observations are underestimated. These results significantly strengthen the
belief that atomic diffusion is responsible for these BHB-star anomalies.Comment: 6 pages, 6 figure
Search for vertical stratification of metals in atmospheres of blue horizontal-branch stars
The observed abundance peculiarities of many chemical species relative to the
expected cluster metallicity in blue horizontal-branch (BHB) stars presumably
appear as a result of atomic diffusion in the photosphere. The slow rotation
(typically 10 km s) of BHB stars with effective temperatures
11,500 K supports this idea since the diffusion mechanism is
only effective in a stable stellar atmosphere. In this work we search for
observational evidence of vertical chemical stratification in the atmospheres
of six hot BHB stars: B84, B267 and B279 in M15 and WF2-2541, WF4-3085 and
WF4-3485 in M13. We undertake an abundance stratification analysis of the
stellar atmospheres of the aforementioned stars, based on acquired Keck HIRES
spectra. We have found from our numerical simulations that three stars (B267,
B279 and WF2-2541) show clear signatures of the vertical stratification of iron
whose abundance increases toward the lower atmosphere, while the other two
stars (B84 and WF4-3485) do not. For WF4-3085 the iron stratification results
are inconclusive. B267 also shows a signature of titanium stratification. Our
estimates for radial velocity, and overall iron, titanium and
phosphorus abundances agree with previously published data for these stars
after taking the measurement errors into account. The results support the
hypothesis regarding the efficiency of atomic diffusion in the stellar
atmospheres of BHB stars with 11,500 K.Comment: 8 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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