Claudio Bastos Pereira

We present detailed chemical compositions of four stars on the first-ascent red giant branch that are classified as chemically peculiar, but lack comprehensive analyses at high spectral resolution. For BD+03^○2688, HE 0457-1805, HE 1255-2324, and HE 2207-1746, we derived metallicities [Fe/H] =−1.21, −0.19, −0.31, and −0.55, respectively, indicating a range in Galactic population membership. In addition to atmospheric parameters, we extracted elemental abundances for 28 elements, including the evolutionary-sensitive CNO group and ¹²C/¹³C ratios. Novel results are also presented for the heavy elements tungsten and thallium. All four stars have very large enhancements of neutron-capture elements, with high [La/Eu] ratios indicating enrichments from the slow neutron capture (s-process). To interpret these abundances, all indicative of [s/Fe] >1.0, we compared our results with data from literature, as well as with predictions from the Monash and fruitys-process nucleosynthesis models. BD+03^○2688, HE 1255-2324, and HE 2207-1746 show C/O >1, while HE 0457-1805 has C/O r/s star; the single object reported in literature so far with similar characteristics is the barium star HD 100503 ([Fe/H] =−0.72). A systematic monitoring is needed to confirm the binary nature of BD+03^○2688 and HE 2207-1746, which are probably CH stars.

We report the discovery of a new emission-line object, named SPH 4-South = (GAIA EDR3 5616553300192230272), toward the dark cloud LDN 1667. This object came to our attention after inspecting public images that show a faint diffuse nebula a few arcseconds south of SPH 4, an emission-line object previously classified as a T Tauri star. We present high-resolution spectra and analyzed JHK photometry of SPH 4 and SPH 4-South and new narrowband and archival broadband images of these objects. A comparison of the spectra of SPH 4 and SPH 4-South with high-resolution ones of DG Cir and R Mon strongly suggests that SPH 4 and SPH 4-South are Herbig Ae/Be stars. The classification of SPH 4-South is further supported by using a k-NN algorithm to its position in an H−K versus J−H color–color diagram. Both stars are detected in the four WISE bands and the WISE colors allow us to classify SPH 4 as a Class I and SPH 4-South as a Class II source. We also show that the faint nebula is most probably associated with SPH 4-South. Using published results on LDN 1667 and the Gaia Early Data Release 3, we conclude that SPH 4 is a member of LDN 1667. The case of SPH 4-South is not clear because the determination of its distance and proper motion could be affected by the nebulosity around the star, although membership of SPH 4-South to LDN 1667 cannot be ruled out.

New elemental abundances for the neutron-capture elements Sr, Nb, Mo, Ru, La, Sm, and Eu are presented for a large sample of 180 barium (Ba) giant stars, a class of chemically peculiar objects that exhibit in their spectra enhancements of the elements created by the s-process, as a consequence of mass transfer between the components of a binary system. The content of heavy elements in these stars, in fact, points to nucleosynthesis mechanisms that took place within a former asymptotic giant branch (AGB) companion, now an invisible white dwarf. From high-resolution (⁠|$R=48\, 000$|⁠) spectra in the optical, we derived the abundances either by equivalent width measurements or synthetic spectra computations, and compared them with available data for field giant and dwarf stars in the same range of metallicity. A re-determination of La abundances resulted in [La/Fe] ratios up to 1.2 dex lower than values previously reported in literature. The programme Ba stars show overabundance of neutron-capture elements, except for Eu, for which the observational data set behave similarly to field stars. Comparison to model predictions are satisfactory for second-to-first s-process peak ratios (e.g. [La/Sr]) and the ratios of the predominantly r-process element Eu to La. However, the observed [Nb, Mo, Ru/Sr] and [Ce, Nd, Sm/La] ratios show median values higher or at the upper limits of the ranges of the model predictions. This unexplained feature calls for new neutron capture models to be investigated.

We present a new abundance analysis of HD 55496, previously known as a metal-poor barium star. We found that HD 55496 has a metallicity [Fe/H] = −1.55 and is s-process enriched. We find that HD 55496 presents four chemical peculiarities: (i) a Na–O abundance anticorrelation; (ii) it is aluminium rich; (iii) it is carbon poor for an s-process enriched star, and (iv) the heavy second s-process peak elements, such as Ba, La, Ce, and Nd, present smaller abundances than the light s-process elements, such as Sr, Y, and Zr, which is not usually observed among the chemically peculiar binary stars at this metallicity. The heavy-element abundance pattern suggests that the main source of the neutrons is the 22Ne(α,n)25Mg reaction. Taking all these abundance evidences together into consideration strongly suggests that HD 55496 is a ‘second-generation of globular cluster stars’ formed from gas already strongly enriched in s-process elements and now is a field halo object. Our dynamical analysis, however, indicates that the past encounter probabilities with the known globular clusters are very low ($\le \!6{{\ \rm per\ cent}}$). This evidence, together with the retrograde motion, points to a halo intruder possibly originated from the tidal disruption of a dwarf galaxy.

Sodium-rich stars are often found in globular clusters giants. However, some sodium-rich stars have been found among field metal-poor stars. These stars are considered as evaporated from globular clusters. Identified such kind of stars among the field stars in the Galaxy may provide insights of which mechanism was responsible for the ejection from a globular cluster and may reveal some chemical peculiarity. Therefore, we started a search, using high-resolution spectroscopy, among metal-poor stars from several sources of the literature to find a sodium-rich star. Here we present the results for the temperature, gravity, metallicity, and sodium abundances for the stars of our sample. For many of them we determined the temperature, gravity, metallicity, and sodium abundances for the first time. As a result of our search we found one star, CD-23°16310, which has a [Na/Fe] ratio of +1.09. We also show that CD-23°16310 is not a carbon-enhanced metal-poor (CEMP) star since carbon is not enriched but is nitrogen-rich. We did not detect any variation of the radial velocity that would support the hypothesis of mass transfer. Thus, the high sodium and nitrogen abundance could be due to a strong internal mixing process, suggesting that CD-23°16310 is an early asymptotic giant branch star.

In this work we analysed, using high-resolution spectroscopy, a sample of 12 single and 4 spectroscopic binary stars of the open cluster NGC 2447. For the single stars, we obtained atmospheric parameters and chemical abundances of Li, C, N, O, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, Nd, Eu. Rotational velocities were obtained for all the stars. The abundances of the light elements and Eu and the rotational velocities were derived using spectral synthesis technique. We obtained a mean metallicity of [Fe/H] = −0.17 ± 0.05. We found that the abundances of all elements are similar to field giants and/or giants of open clusters, even for the s-process elements, which are enhanced as in other young open clusters. We show that the spectroscopic binaries NGC 2447-26, 38, and 42 are yellow-straggler stars, of which the primary is a giant star and the secondary a main-sequence A-type star.

Single stars in open clusters with known distances are important targets in constraining the nucleosynthesis process since their ages and luminosities are also known. In this work, we analyze a sample of 29 single red giants of the open clusters NGC 2360, NGC 3680, and NGC 5822 using high-resolution spectroscopy. We obtained atmospheric parameters, abundances of the elements C, N, O, Na, Mg, Al, Ca, Si, Ti, Ni, Cr, Y, Zr, La, Ce, and Nd, as well as radial and rotational velocities. We employed the local thermodynamic equilibrium atmospheric models of Kurucz and the spectral analysis code moog. Rotational velocities and light-element abundances were derived using spectral synthesis. Based on our analysis of the single red giants in these three open clusters, we could compare, for the first time, their abundance pattern with that of the binary stars of the same clusters previously studied. Our results show that the abundances of both single and binary stars of the open clusters NGC 2360, NGC 3680, and NGC 5822 do not have significant differences. For the elements created by the s-process, we observed that the open clusters NGC 2360, NGC 3680, and NGC 5822 also follow the trend already raised in the literature that young clusters have higher s-process element abundances than older clusters. Finally, we observed that the three clusters of our sample exhibit a trend in the [Y/Mg]-age relation, which may indicate the ability of the [Y/Mg] ratio to be used as a clock for the giants.

Carbon enhanced metal-poor (CEMP) stars are a particular class of low-metalicity halo stars whose chemical analysis may provide important contrains to the chemistry evolution of the Galaxy and to the models of mass-transfer and evolution of components in binary systems. Here, we present a detailed analysis of the CEMP star CD −50°776, using high resolution optical spectroscopy. We found that CD −50°776 has a metalicity [Fe/H] = −2.31 and a carbon abundance [C/Fe] = +1.21. Analysing the s-process elements and the europium abundances, we show that this star is actually a CEMP-s star, based on the criteria set in the literature to classify these chemically peculiar objects. We also show that CD −50°776 is a lead star, since it has a ratio [Pb/Ce] = +0.97. In addition, we show that CD −50°776 develops radial velocity variations that may be attributed to the orbital motion in a binary system. The abundance pattern of CD −50°776 is discussed and compared to other CEMP-s stars already reported in the literature to show that this star is a quite exceptional object among the CEMP stars, particularly due to its low nitrogen abundance. Explaining this pattern may require to improve the nucleosynthesis models, and the evolutionary models of mass transfer and binary interaction.

Carbon enhanced metal poor (CEMP) stars are a particular class of low metalicity halo stars whose chemical analysis may provide important constrains to the chemistry evolution of the Galaxy and to the models of mass transfer and evolution of components in binary systems. Here, we present a detailed analysis of the CEMP star CD-50$^\circ$776, using high resolution optical spectroscopy. We found that CD-50$^\circ$776 has a metalicity [Fe/H]=-2.31 and a carbon abundance [C/Fe]=+1.21. Analyzing the s-process elements and the europium abundances, we show that this star is actually a CEMP-s star, based on the criteria set in the literature to classify these chemically peculiar objects. We also show that CD-50$^\circ$776 is a lead star, since it has a ratio [Pb/Ce]=+0.97. In addition, we show that CD-50$^\circ$776 develops radial velocity variations that may be attributed to the orbital motion in a binary system. The abundance pattern of CD-50$^\circ$776 is discussed and compared to other CEMP-s stars already reported in the literature to show that this star is a quite exceptional object among the CEMP stars, particularly due to its low nitrogen abundance. Explaining this pattern may require to improve the nucleosynthesis models, and the evolutionary models of mass transfer and binary interaction.