- ID:
- ivo://CDS.VizieR/J/ApJ/821/37
- Title:
- Abundances of metal-poor star HD 94028
- Short Name:
- J/ApJ/821/37
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed analysis of the composition and nucleosynthetic origins of the heavy elements in the metal-poor ([Fe/H]=-1.62+/-0.09) star HD 94028. Previous studies revealed that this star is mildly enhanced in elements produced by the slow neutron-capture process (s process; e.g., [Pb/Fe]=+0.79+/-0.32) and rapid neutron-capture process (r process; e.g., [Eu/Fe]=+0.22+/-0.12), including unusually large molybdenum ([Mo/Fe]= +0.97+/-0.16) and ruthenium ([Ru/Fe]=+0.69+/-0.17) enhancements. However, this star is not enhanced in carbon ([C/Fe]=-0.06+/-0.19). We analyze an archival near-ultraviolet spectrum of HD 94028, collected using the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope, and other archival optical spectra collected from ground-based telescopes. We report abundances or upper limits derived from 64 species of 56 elements. We compare these observations with s-process yields from low-metallicity AGB evolution and nucleosynthesis models. No combination of s- and r-process patterns can adequately reproduce the observed abundances, including the super-solar [As/Ge] ratio (+0.99+/-0.23) and the enhanced [Mo/Fe] and [Ru/Fe] ratios. We can fit these features when including an additional contribution from the intermediate neutron-capture process (i process), which perhaps operated through the ingestion of H in He-burning convective regions in massive stars, super-AGB stars, or low-mass AGB stars. Currently, only the i process appears capable of consistently producing the super-solar [As/Ge] ratios and ratios among neighboring heavy elements found in HD 94028. Other metal-poor stars also show enhanced [As/Ge] ratios, hinting that operation of the i process may have been common in the early Galaxy.
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- ID:
- ivo://CDS.VizieR/J/A+A/645/A61
- Title:
- Abundances of 25 metal-poor stars
- Short Name:
- J/A+A/645/A61
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Among carbon-enhanced metal-poor (CEMP) stars, some are found to be enriched in slow-neutron capture (s-process) elements (and are then tagged CEMP-s), some have overabundances in rapid-neutron capture (r-process) elements (tagged CEMP-r), and some are characterized by both s- and r-process enrichments (tagged CEMP-rs). The current distinction between CEMP-s and CEMP-rs is based on their [Ba/Fe] and [Eu/Fe] ratios, since barium and europium are predominantly produced by the s- and the r-process, respectively. The origin of the abundance differences between CEMP-s and CEMP-rs stars is presently unknown. It has been claimed that the i-process, whose site still remains to be identified, could better reproduce CEMP-rs abundances than the s-process. We propose a more robust classification method for CEMP-s and CEMP-rs stars using additional heavy elements other than Ba and Eu. Once a secure classification is available, it should then be possible to assess whether the i-process or a variant of the s-process better fits the peculiar abundance patterns of CEMP-rs stars. We analyse high-resolution spectra of 24 CEMP stars and one r-process enriched star without carbon-enrichment, observed mainly with the high-resolution HERMES spectrograph mounted on the Mercator telescope (La Palma) and also with the UVES spectrograph on VLT (ESO Chile) and HIRES spectrograph on KECK (Hawaii). Stellar parameters and abundances are derived using MARCS model atmospheres. Elemental abundances are computed through spectral synthesis using the TURBOSPECTRUM radiative transfer code. Stars are re-classified as CEMP-s or -rs according to a new classification scheme using eight heavy element abundances. Within our sample of 25 objects, the literature classification is globally confirmed, except for HE 1429-0551 and HE 2144-1832, previously classified as CEMP-rs and now as CEMP-s stars. The abundance profiles of CEMP-s and CEMP-rs stars are compared in detail, and no clear separation is found between the two groups; it seems instead that there is an abundance continuum between the two stellar classes. There is an even larger binarity rate among CEMP-rs stars than among CEMP-s stars, indicating that CEMP-rs stars are extrinsic stars as well. The second peak s-process elements (Ba, La, Ce) are slightly enhanced in CEMP-rs stars with respect to first-peak s-process elements (Sr, Y, Zr), when compared to CEMP-s stars. Models of radiative s-process nucleosynthesis during the interpulse phases reproduce well the abundance profiles of CEMP-s stars, whereas those of CEMP-rs stars are explained well by low-metallicity 1M_{sun}_ models experiencing proton ingestion. The global fitting of our i-process models to CEMP-rs stars is as good as the one of our s-process models to CEMP-s stars. Stellar evolutionary tracks of an enhanced carbon composition (consistent with our abundance determinations) are necessary to explain the position of CEMP-s and CEMP-rs stars in the Hertzsprung-Russell (HR) diagram using Gaia DR2 parallaxes; they are found to lie mostly on the red giant branch (RGB). CEMP-rs stars present most of the characteristics of extrinsic stars such as CEMP-s, CH, barium, and extrinsic S stars; they can be explained as being polluted by a low-mass, low-metallicity thermally-pulsing asymptotic giant branch (TP-AGB) companion experiencing i-process nucleosynthesis after proton ingestion during its first convective thermal pulses. As such, they could be renamed CEMP-sr stars, since they represent a particular manifestation of the s-process at low-metallicities. For these objects a call for an exotic i-process site may not necessarily be required anymore.
- ID:
- ivo://CDS.VizieR/J/ApJ/852/99
- Title:
- Abundances of 3 metal-poor stars in Horologium I
- Short Name:
- J/ApJ/852/99
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present chemical abundance measurements of three stars in the ultrafaint dwarf galaxy Horologium I, a Milky Way satellite discovered by the Dark Energy Survey. Using high-resolution spectroscopic observations, we measure the metallicity of the three stars, as well as abundance ratios of several {alpha}-elements, iron-peak elements, and neutron-capture elements. The abundance pattern is relatively consistent among all three stars, which have a low average metallicity of [Fe/H]~-2.6 and are not {alpha}-enhanced ([{alpha}/Fe]~0.0). This result is unexpected when compared to other low-metallicity stars in the Galactic halo and other ultrafaint dwarfs and suggests the possibility of a different mechanism for the enrichment of Hor I compared to other satellites. We discuss possible scenarios that could lead to this observed nucleosynthetic signature, including extended star formation, enrichment by a Population III supernova, and or an association with the Large Magellanic Cloud.
- ID:
- ivo://CDS.VizieR/J/ApJ/856/142
- Title:
- Abundances of metal-poor stars in Sculptor
- Short Name:
- J/ApJ/856/142
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The study of the chemical abundances of metal-poor stars in dwarf galaxies provides a venue to constrain paradigms of chemical enrichment and galaxy formation. Here we present metallicity and carbon abundance measurements of 100 stars in Sculptor from medium-resolution (R~2000) spectra taken with the Magellan/Michigan Fiber System mounted on the Magellan-Clay 6.5m telescope at Las Campanas Observatory. We identify 24 extremely metal-poor star candidates ([Fe/H]{<}-3.0) and 21 carbon-enhanced metal-poor (CEMP) star candidates. Eight carbon-enhanced stars are classified with at least 2{sigma} confidence, and five are confirmed as such with follow-up R~6000 observations using the Magellan Echellette Spectrograph on the Magellan-Baade 6.5m telescope. We measure a CEMP fraction of 36% for stars below [Fe/H]=-3.0, indicating that the prevalence of carbon-enhanced stars in Sculptor is similar to that of the halo (~43%) after excluding likely CEMP-s and CEMP-r/s stars from our sample. However, we do not detect that any CEMP stars are strongly enhanced in carbon ([C/Fe]>1.0). The existence of a large number of CEMP stars both in the halo and in Sculptor suggests that some halo CEMP stars may have originated from accreted early analogs of dwarf galaxies.
- ID:
- ivo://CDS.VizieR/J/ApJ/824/73
- Title:
- Abundances of NGC 6362 member stars
- Short Name:
- J/ApJ/824/73
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present the first measure of Fe and Na abundances in NGC 6362, a low-mass globular cluster (GC) where first- and second-generation stars are fully spatially mixed. A total of 160 member stars (along the red giant branch (RGB) and the red horizontal branch (RHB)) were observed with the multi-object spectrograph FLAMES at the Very Large Telescope. We find that the cluster has an iron abundance of [Fe/H]=-1.09+/-0.01dex, without evidence of intrinsic dispersion. On the other hand, the [Na/Fe] distribution turns out to be intrinsically broad and bimodal. The Na-poor and Na-rich stars populate, respectively, the bluest and the reddest RGBs detected in the color-magnitude diagrams including the U filter. The RGB is composed of a mixture of first- and second-generation stars in a similar proportion, while almost all the RHB stars belong to the first cluster generation. To date, NGC 6362 is the least massive GC where both the photometric and spectroscopic signatures of multiple populations have been detected.
- ID:
- ivo://CDS.VizieR/J/A+A/490/625
- Title:
- Abundances of NGC 6121 red giants
- Short Name:
- J/A+A/490/625
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present abundance analysis based on high resolution spectra of 105 isolated red giant branch (RGB) stars in the Galactic Globular Cluster NGC 6121 (M4). The data have been collected with FLAMES+UVES, at the ESO/VLT@UT2 telescope. Spectroscopic data were coupled with high precision wide-field UBVI_C photometry from WFI@2.2m telescope and infrared JHK photometry from 2MASS. We derived an average [Fe/H]=-1.07+/-0.01, and an {alpha} enhancement of [{alpha}/Fe]=+0.39+/-0.05dex (internal errors). We confirm the presence of an extended Na-O anticorrelation, and find two distinct groups of stars with significantly different Na and O content. We find no evidence of a Mg-Al anticorrelation. By coupling our results with previous studies on the CN band strength, we find that the CN strong stars have higher Na and Al content and are more O depleted than the CN weak ones. The two groups of Na-rich, CN-strong and Na-poor, CN-weak stars populate two different regions along the RGB. In the U vs. U-B color magnitude diagram the RGB spread is present from the base of the RGB to the RGB-tip. Apparently, both spectroscopic and photometric results imply the presence of two stellar populations in M4. We briefly discuss the possible origin of these populations.
- ID:
- ivo://CDS.VizieR/J/A+A/279/567
- Title:
- Abundances of non-type I PNe in LMC
- Short Name:
- J/A+A/279/567
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Spectroscopic observations, plasma diagnostics and chemical composition of 15 non-type I planetary nebulae (PNe) in the Large Magellanic Cloud (LMC) are reported. Abundances of He, O, N, S, and Ar are determined and compared with recently obtained data for nebulae both in the Magellanic Clouds and in the Galaxy.
- ID:
- ivo://CDS.VizieR/J/MNRAS/496/2422
- Title:
- Abundances of 42 Pisces-Eridanus stream stars
- Short Name:
- J/MNRAS/496/2422
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Recently, a new cylindrical-shaped stream of stars, up to 700pc long, was discovered hiding in the Galactic disk using kinematic data enabled by the Gaia mission. This curious stream of stars, dubbed the Pisces-Eridanus stream, was initially thought to be as old as 1Gyr, yet its stars shared a rotation period distribution consistent with the 120-Myr-old Pleiades cluster. In this work, we explore the detailed chemical nature of this stellar stream. We carried out high-resolution spectroscopic follow-up of 42 Pisces-Eridanus stream stars using McDonald Observatory, and combined these data with information for 40 members observed with the low-resolution LAMOST spectroscopic survey. Together, these data enabled us to measure the abundance distribution of light/odd-Z (Li, C, Na, Al, Sc, V), {alpha} (Mg, Si, Ca, Ti), Fe-peak (Cr, Mn, Fe, Co, Ni, Zn), and neutron capture (Sr, Y, Zr, Ba, La, Nd, Eu) elements across the Pisces-Eridanus stream. We find that the stream is (1) near solar metallicity with [Fe/H] = -0.03 dex and (2) has a metallicity spread of 0.07 dex (or 0.04 dex when removing outliers). We also find that (3) the abundance of Li indicates that Pisces-Eridanus is ~120Myr old, consistent with the gyrochronology result. We find that (4) the stream has a [X/Fe] abundance spreads of 0.06<{sigma}_[X/Fe]_<0.20dex in most elements, and (5) no significant abundance gradients across its major axis except a potentially weak gradient in [Si/Fe]. These results together show that the Pisces-Eridanus stream is a uniquely close, young, chemically interesting laboratory for testing our understanding of star and planet formation.
- ID:
- ivo://CDS.VizieR/J/A+A/552/A12
- Title:
- Abundances of PNe in NGC 300
- Short Name:
- J/A+A/552/A12
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We have obtained deep spectroscopy of 26 planetary nebulae (PNe) and 9 compact HII regions in the nearby spiral galaxy NGC 300, and analyzed them together with the giant HII regions observed by Bresolin et al. (2009ApJ...700..309B). We have determined the physical properties of all those objects and their content in He, N, O, Ne, S and Ar in a consistent way. We find that compact HII regions have abundances similar to those of giant HII regions, while PNe have systematically larger N/O ratios and similar Ne/O and Ar/O ratios. We demonstrate that this nitrogen enhancement in PNe cannot be due to second dredge-up in the progenitor stars, since their initial masses are around 2-2.5M_{sun}_. An extra mixing process is required, perhaps be driven by rotation. Concerning the radial abundance distribution, PNe behave differently from HII regions: In the central parts, they show an average O/H smaller by 0.15dex. Their abundance dispersion at any galactocentric radius is significantly larger than for HII regions and many of them have O/H values higher than HII regions at the same galactocentric distance, suggesting that oxygen production in the PN progenitors in NGC 300 is common. PN abundance gradients in O/H, Ne/H and Ar/He are significantly shallower than those of HII regions. We argue that this indicates a steepening of the metallicity gradient in NGC 300 during the last Gr, rather than the effect of radial stellar motions.
- ID:
- ivo://CDS.VizieR/J/ApJ/872/137
- Title:
- Abundances of red clump & RGB stars with APOGEE
- Short Name:
- J/ApJ/872/137
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Internal mixing on the giant branch is an important process which affects the evolution of stars and the chemical evolution of the galaxy. While several mechanisms have been proposed to explain this mixing, better empirical constraints are necessary. Here, we use [C/N] abundances in 26097 evolved stars from the SDSS-IV/APOGEE-2 DR14 to trace mixing and extra mixing in old field giants with -1.7<[Fe/H]<0.1. We show that the APOGEE [C/N] ratios before any dredge-up occurs are metallicity dependent, but that the change in [C/N] at the first dredge-up is metallicity independent for stars above [Fe/H]~-1. We identify the position of the red giant branch (RGB) bump as a function of metallicity, note that a metallicity-dependent extra mixing episode takes place for low-metallicity stars ([Fe/H]{<}-0.4) 0.14dex in logg above the bump, and confirm that this extra mixing is stronger at low metallicity, reaching {Delta}[C/N]=0.58dex at [Fe/H]=-1.4. We show evidence for further extra mixing on the upper giant branch, well above the bump, among the stars with [Fe/H]{<}-1.0. This upper giant branch mixing is stronger in the more metal-poor stars, reaching 0.38 dex in [C/N] for each 1.0dex in logg. The APOGEE [C/N] ratios for red clump (RC) stars are significantly higher than for stars at the tip of the RGB, suggesting additional mixing processes occur during the helium flash or that unknown abundance zero points for C and N may exist among the RC sample. Finally, because of extra mixing, we note that current empirical calibrations between [C/N] ratios and ages cannot be naively extrapolated for use in low-metallicity stars specifically for those above the bump in the luminosity function.