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تسجيل مستخدم جديدOn the neutron-capture elements across the Galactic thin disk using Cepheids
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We present new accurate abundances for five neutron-capture (Y, La, Ce, Nd, Eu) elements in 73 classical Cepheids located across the Galactic thin disk. Individual abundances are based on high spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300) spectra collected with UVES at ESO VLT for the DIONYSOS project. Taking account for similar Cepheid abundances provided either by our group (111 stars) or available in the literature, we end up with a sample of 435 Cepheids covering a broad range in iron abundances (-1.6 < [Fe/H] < 0.6). We found, using homogeneous individual distances and abundance scales, well defined gradients for the above elements. However, the slope of the light s-process element (Y) is at least a factor of two steeper than the slopes of heavy s- (La, Ce, Nd) and r- (Eu) process elements. The s to r abundance ratio ([La/Eu]) of Cepheids shows a well defined anticorrelation with of both Eu and Fe. On the other hand, Galactic field stars attain an almost constant value and only when they approach solar iron abundance display a mild enhancement in La. The [Y/Eu] ratio shows a mild evidence of a correlation with Eu and, in particular, with iron abundance for field Galactic stars. We also investigated the s-process index - [hs/ls] - and we found a well defined anticorrelation, as expected, between [La/Y] and iron abundance. Moreover, we found a strong correlation between [La/Y] and [La/Fe] and, in particular, a clear separation between Galactic and Sagittarius red giants. Finally, the comparison between predictions for low-mass asymptotic giant branch stars and the observed [La/Y] ratio indicate a very good agreement over the entire metallicity range covered by Cepheids. However, the observed spread, at fixed iron content, is larger than predicted by current models.
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We present new homogeneous measurements of Na, Al and three alpha-elements (Mg, Si, Ca) for 75 Galactic Cepheids. The abundances are based on high spectral resolution (R ~ 38,000) and high signal-to-noise ratio (S/N ~ 50-300) spectra collected with U
VES at ESO VLT. The current measurements were complemented with Cepheid abundances either provided by our group (75) or available in the literature, for a total of 439 Galactic Cepheids. Special attention was given in providing a homogeneous abundance scale for these five elements plus iron (Genovali et al. 2013, 2014). In addition, accurate Galactocentric distances (RG) based on near-infrared photometry are also available for all the Cepheids in the sample (Genovali et al. 2014). They cover a large fraction of the Galactic thin disk (4.1 <= RG <= 18.4 kpc). We found that the above five elements display well defined linear radial gradients and modest standard deviations over the entire range of RG. Moreover, the [element/Fe] abundance ratios are constant across the entire thin disk; only the Ca radial distribution shows marginal evidence of a positive slope. These results indicate that the chemical enrichment history of iron and of the quoted five elements has been quite similar across the four quadrants of the Galactic thin disk. The [element/Fe] ratios are also constant over the entire period range. This empirical evidence indicates that the chemical enrichment of Galactic Cepheids has also been very homogenous during the range in age that they cover (~10-300 Myr). Once again, [Ca/Fe] vs. log(P) shows a (negative) gradient, being underabundant among youngest Cepheids. Finally, we also found that Cepheid abundances agree quite well with similar abundances for thin and thick disk dwarf stars and they follow the typical Mg-Al and Na-O correlations.
The enrichment history of heavy neutron-capture elements in the Milky Way disc provides fundamental information about the chemical evolution of our Galaxy and about the stellar sources that made those elements. In this work we give new observational
data for Sr, the element at the first neutron-shell closure beyond iron, N=50, based on the analysis of the high resolution spectra of 276 Galactic disc stars. The Sr abundance was derived by comparing the observed and synthetic spectra in the region of the SrI 4607 A line, making use of the LTE approximation. NLTE corrections lead to an increase of the abundance estimates obtained under LTE, but for these lines they are minor near solar metallicity. The average correction that we find is 0.151 dex. The star that is mostly affected is HD 6582, with a 0.244 dex correction. The behavior of the Sr abundance as a function of metallicity is discussed within a stellar nucleosynthesis context, in comparison with the abundance of the heavy neutron-capture elements Ba (Z=56) and Eu (Z=63). The comparison of the observational data with the current GCE models confirm that the s-process contributions from Asymptotic Giant Branch stars and from massive stars are the main sources of Sr in the Galactic disc and in the Sun, while different nucleosynthesis sources can explain the high [Sr/Ba] and [Sr/Eu] ratios observed in the early Galaxy.
We present homogeneous and accurate iron abundances for almost four dozen (47) of Galactic Cepheids using high-spectral resolution (R$sim$40,000) high signal-to-noise ratio (S/N $ge$ 100) optical spectra collected with UVES at VLT. A significant frac
tion of the sample (32) is located in the inner disk (RG $le$ 6.9 kpc) and for half of them we provide new iron abundances. Current findings indicate a steady increase in iron abundance when approaching the innermost regions of the thin disk. The metallicity is super-solar and ranges from 0.2 dex for RG $sim$ 6.5 kpc to 0.4 dex for RG $sim$ 5.5 kpc. Moreover, we do not find evidence of correlation between iron abundance and distance from the Galactic plane. We collected similar data available in the literature and ended up with a sample of 420 Cepheids. Current data suggest that the mean metallicity and the metallicity dispersion in the four quadrants of the Galactic disk attain similar values. The first-second quadrants show a more extended metal-poor tail, while the third-fourth quadrants show a more extended metal-rich tail, but the bulk of the sample is at solar iron abundance. Finally, we found a significant difference between the iron abundance of Cepheids located close to the edge of the inner disk ([Fe/H]$sim$0.4) and young stars located either along the Galactic bar or in the nuclear bulge ([Fe/H]$sim$0). Thus suggesting that the above regions have had different chemical enrichment histories. The same outcome applies to the metallicity gradient of the Galactic bulge, since mounting empirical evidence indicates that the mean metallicity increases when moving from the outer to the inner bulge regions.
Context: Galactic abundance gradients set strong constraints to chemo-dynamical evolutionary models of the Milky Way. Given the PL relations that provide accurate distances and the large number of spectral lines, Cepheids are excellent tracers of the
present-day abundance gradients. Aims: We want to measure the Galactic abundance gradient of several chemical elements. While the slope of the Cepheid iron gradient did not vary much from the very first studies, the gradients of the other elements are not that well constrained. In this paper we focus on the inner and outer regions of the Galactic thin disk. Methods: We use HR spectra (FEROS, ESPADONS, NARVAL) to measure the abundances of several light (Na, Al), alpha (Mg, Si, S, Ca), and heavy elements (Y, Zr, La, Ce, Nd, Eu) in a sample of 65 Milky Way Cepheids. Combining these results with accurate distances from period-Wesenheit relations in the NIR enables us to determine the abundance gradients in the Milky Way. Results: Our results are in good agreement with previous studies on either Cepheids or other tracers. In particular, we confirm an upward shift of approximatively 0.2 dex for the Mg abundances, as has recently been reported. We also confirm the existence of a gradient for all the heavy elements studied in the context of a LTE analysis. However, for Y, Nd, and especially La, we find lower abundances for Cepheids in the outer disk than reported in previous studies, leading to steeper gradients. This effect can be explained by the differences in the line lists used by different groups. Conclusions: Our data do not support a flattening of the gradients in the outer disk, in agreement with recent Cepheid studies and chemo-dynamical simulations. This is in contrast to the open cluster observations but remains compatible with a picture where the transition zone between the inner disk and the outer disk would move outward with time.
High resolution spectra obtained from the Subaru Telescope High Dispersion Spectrograph have been used to update the stellar atmospheric parameters and metallicity of the star HD 209621. We have derived a metallicity of [Fe/H] = -1.93 for this star,
and have found a large enhancement of carbon and of heavy elements, with respect to iron. Updates on the elemental abundances of four s-process elements (Y, Ce, Pr, Nd) along with the first estimates of abundances for a number of other heavy elements (Sr, Zr, Ba, La, Sm, Eu, Er, Pb) are reported. The stellar atmospheric parameters, the effective temperature, Teff, and the surface gravity, log g (4500 K, 2.0), are determined from LTE analysis using model atmospheres. Estimated [Ba/Eu] = +0.35, places the star in the group of CEMP-(r+s) stars; however, the s-elements abundance pattern seen in HD 209621 is characteristic of CH stars; notably, the 2nd-peak s-process elements are more enhanced than the first peak s-process elements. HD 209621 is also found to show a large enhancement of the 3rd-peak s-process element lead (Pb) with [Pb/Fe] = +1.88. The relative contributions of the two neutron-capture processes, r- and s- to the observed abundances are examined using a parametric model based analysis, that hints that the neutron-capture elements in HD 209621 primarily originate in s-process.
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