A wide angle view of the Sagittarius dwarf spheroidal galaxy

Rathour

2021

A&A

Context. The origin of the enhanced abundances of both s- and r-process elements observed in a subclass of carbon-enhanced metal-poor (CEMP) stars, denoted CEMP-r/s stars, still remains poorly understood. The i-process nucleosynthesis has been suggested as one of the most promising mechanisms for the origin of these stars. Aims. Our aim is to better understand the chemical signatures and formation mechanism(s) of five previously claimed potential CH star candidates HE 0017+0055, HE 2144−1832, HE 2339−0837, HD 145777, and CD−27 14351 through a detailed systematic follow-up spectroscopic study based on high-resolution spectra. Methods. The stellar atmospheric parameters, the effective temperature Teff, the microturbulent velocity ζ, the surface gravity log g, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. Elemental abundances of C, N, α-elements, iron-peak elements, and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In the context of the double enhancement observed in four of the programme stars, we have critically examined whether the literature i-process model yields ([X/Fe]) of heavy elements can explain the observed abundance distribution. Results. The estimated metallicity [Fe/H] of the programme stars ranges from −1.63 to −2.74. All five stars show enhanced abundance for Ba, and four of them exhibit enhanced abundance for Eu. Based on our analysis, HE 0017+0055, HE 2144−1832, and HE 2339−0837 are found to be CEMP-r/s stars, whereas HD 145777 and CD−27 14351 show characteristic properties of CEMP-s stars. From a detailed analysis of different classifiers of CEMP stars, we have identified the one which best describes the CEMP-s and CEMP-r/s stars. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe] ≥ 1.0. However, CEMP-r/s stars satisfy [Eu/Fe] ≥ 1.0, 0.0 ≤ [Ba/Eu] ≤ 1.0, and/or 0.0 ≤ [La/Eu] ≤ 0.7. CEMP-s stars normally show [Eu/Fe] < 1.0 with [Ba/Eu] > 0.0 and/or [La/Eu] > 0.5. If [Eu/Fe] ≥ 1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu] > 1.0 and/or [La/Eu] > 0.7. Using a large sample of similar stars from the literature we have examined whether the ratio of heavy-s to light-s process elements [hs/ls] alone can be used as a classifier, and if there are any limiting values for [hs/ls] that can be used to distinguish between CEMP-s and CEMP-r/s stars. Even though they peak at different values of [hs/ls], CEMP-s and CEMP-r/s stars show an overlap in the range 0.0 < [hs/ls] < 1.5, and hence this ratio cannot be used to distinguish between CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] as well, in the range −1.6 < [Sr/Ba] < −0.5, and hence this ratio also cannot be used to separate the two subclasses.

Section: Agb and Accretion-induced Collapse (Aic) Pollution Of A Star...mentioning

confidence: 99%

Spectroscopic study of CEMP-(s & r/s) stars

Rathour

2021

A&A

“…Gull et al (2018) claimed that this object was the first bona fide CEMP-r+s star as its abundance pattern could be satisfactorily fitted only with AGB mass-transfer model taking into account initial r-process enhancement. Sbordone et al (2020) reported the abundance analysis of object GIU J190734.24-315102.1 located in the Sagittarius (Sgr) dwarf spheroidal (dSph) galaxy, and claimed that the abundance pattern could be best fit only with a model considering AGB-pollution in a binary system pre-enriched with a neutron star-neutron star merger event. Sbordone et al (2020) classify this object as the first CEMP-r/s star found in the Sgr dSph, but due to its formation scenario this object may be referred to as a bona fide CEMP-r+s star.…”

Section: Formation Scenarios Of Cemp-(s and R/s) Stars And Their Like...mentioning

confidence: 99%

Spectroscopic study of CEMP-(s & r/s) stars- Revisiting classification criteria and formation scenarios, highlighting i-process nucleosynthesis

Goswami,

Rathour,

Goswami

2021

Preprint

Context. The origin of the enhanced abundances of both s-and r-process elements observed in a subclass of carbon-enhanced metalpoor (CEMP) stars, denoted CEMP-r/s stars, still remains poorly understood. The i-process nucleosynthesis has been suggested as one of the most promising mechanisms for the origin of these stars. Aims. Our aim is to better understand the chemical signatures and formation mechanism(s) of five previously claimed potential CH star candidates HE 0017+0055, HE 2144−1832, HE 2339−0837, HD 145777, and CD−27 14351 through a detailed systematic follow-up spectroscopic study based on high-resolution spectra. Methods. The stellar atmospheric parameters, the effective temperature T e f f , the microturbulent velocity ζ, the surface gravity log g, and the metallicity [Fe/H] are derived from local thermodynamic equilibrium analyses using model atmospheres. Elemental abundances of C, N, α-elements, iron-peak elements, and several neutron-capture elements are estimated using the equivalent width measurement technique as well as spectrum synthesis calculations in some cases. In the context of the double enhancement observed in four of the programme stars, we have critically examined whether the literature i-process model yields ([X/Fe]) of heavy elements can explain the observed abundance distribution. Results. The estimated metallicity [Fe/H] of the programme stars ranges from −1.63 to −2.74. All five stars show enhanced abundance for Ba, and four of them exhibit enhanced abundance for Eu. Based on our analysis, HE 0017+0055, HE 2144−1832, and HE 2339−0837 are found to be CEMP-r/s stars, whereas HD 145777 and CD−27 14351 show characteristic properties of CEMP-s stars. From a detailed analysis of different classifiers of CEMP stars, we have identified the one which best describes the CEMP-s and CEMP-r/s stars. We found that for both CEMP-s and CEMP-r/s stars, [Ba/Eu] and [La/Eu] exhibit positive values and [Ba/Fe] ≥ 1.0. However, CEMP-r/s stars satisfy [Eu/Fe] ≥ 1.0, 0.0 ≤ [Ba/Eu] ≤ 1.0, and/or 0.0 ≤ [La/Eu] ≤ 0.7. CEMP-s stars normally show [Eu/Fe] < 1.0 with [Ba/Eu] > 0.0 and/or [La/Eu] > 0.5. If [Eu/Fe] ≥ 1.0, then the condition on [Ba/Eu] and/or [La/Eu] for a star to be a CEMP-s star is [Ba/Eu] > 1.0 and/or [La/Eu] > 0.7. Using a large sample of similar stars from the literature we have examined whether the ratio of heavy-s to light-s process elements [hs/ls] alone can be used as a classifier, and if there are any limiting values for [hs/ls] that can be used to distinguish between CEMP-s and CEMP-r/s stars. Even though they peak at different values of [hs/ls], CEMP-s and CEMP-r/s stars show an overlap in the range 0.0 < [hs/ls] < 1.5, and hence this ratio cannot be used to distinguish between CEMP-s and CEMP-r/s stars. We have noticed a similar overlap in the case of [Sr/Ba] as well, in the range −1.6 < [Sr/Ba] < −0.5, and hence this ratio also cannot be used to separate the two subclasses.

Evidence for multiple nucleosynthetic processes from carbon-enhanced metal-poor stars in the Carina dwarf spheroidal galaxy

Hansen¹,

Simon²,

Li³

et al. 2023

A&A

Context. Carbon-enhanced metal-poor (CEMP) stars ([C/Fe] > 0.7) are known to exist in large numbers at low metallicity in the Milky Way halo and are important tracers of early Galactic chemical evolution. However, very few stars of this kind have been identified in the classical dwarf spheroidal (dSph) galaxies, and detailed abundances, including neutron-capture element abundances, have only been reported for 13 stars. Aims. We aim to derive detailed abundances of six CEMP stars identified in the Carina dSph and compare the abundances to CEMP stars in other dSph galaxies and the Milky Way halo. This is the largest sample of CEMP stars in a dSph galaxy analysed to date. Methods. One-dimensional local thermodynamic equilibrium (LTE) elemental abundances are derived via equivalent width and spectral synthesis using high-resolution spectra of the six stars obtained with the MIKE spectrograph at Las Campanas Observatory. Results. We derived abundances or upper limits for up to 27 elements from C to Os in the six stars. Our analysis reveals one of the stars to be a CEMP-no star with very low neutron-capture element abundances. In contrast, the other five stars all show enhancements in neutron-capture elements in addition to their carbon enhancement, classifying them as CEMP-s and -r/s stars. The six stars have similar α and iron-peak element abundances to other stars in Carina, except for the CEMP-no star, which shows enhancement in Na, Mg, and Si. We explored the absolute carbon abundances (A(C)) of CEMP stars in dSph galaxies and find similar behaviour to that seen for Milky Way halo CEMP stars, but highlight that CEMP-r/s stars primarily have very high A(C) values. We also compared the neutron-capture element abundances of the CEMP-r/s stars in our sample to recent i-process yields, which provide a good match to the derived abundances.