Kinematics of Highly r-process-enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites

Roederer, Ian U.; Hattori, Kohei; Valluri, Monica

doi:10.3847/1538-3881/aadd9c

Cited by 97 publications

(193 citation statements)

References 133 publications

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“…After correcting for the difference in metallicity (0.13 dex), we find a remarkable match between the two stars in the considered range (reduced χ 2 of 0.49). Similarities between the two stars have already been reported in the literature from a kinematical point of view (Roederer et al 2018a) and based on their metallicity (Barklem et al 2005;Roederer et al 2018b). We emphasize, however, that the similarities do not extend to the neutron-capture regime, since HD 222925 is an r-II and HD 20 an r-I star with possible sprocess contamination, as outlined in the following section.…”

Section: Abundances Up To Zn (11 ≤ Z ≤ 30)supporting

confidence: 81%

“…Based on the full 6D phase-space information from the second data release (DR2) of the Gaia mission (Gaia Collaboration et al 2018), Roederer et al (2018a) concluded that HD 20 may be chemodynamically associated with two other metal-poor halo stars with observed r-process excess. Based on its kinematicscharacterized by a highly eccentric orbit (e = 0.975 +0.002 −0.004 ) and a close pericentric passage (r peri = 0.19 +0.04 −0.02 kpc) -and its low metallicity, the authors speculate that HD 20 and its associates may have been accreted from a disrupted satellite.…”

Section: Introductionmentioning

confidence: 99%

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A high-precision abundance analysis of the nuclear benchmark star HD 20

Hanke

Hansen

Ludwig

et al. 2020

A&A

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Metal-poor stars with available detailed information about their chemical inventory pose powerful empirical benchmarks for nuclear astrophysics. Here we present our spectroscopic chemical abundance investigation of the metal-poor ([Fe/H] = −1.60 ± 0.03 dex), r-process-enriched ([Eu/Fe] = 0.73 ± 0.10 dex) halo star HD 20 using novel and archival high-resolution data at outstanding signalto-noise ratios (up to ∼ 1000 Å −1 ). By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with the spectroscopic analysis of iron lines under non-local thermodynamic equilibrium conditions, we derive a set of highly accurate and precise stellar parameters. These allow us to delineate a reliable chemical pattern that is comprised of solid detections of 48 elements, including 28 neutron-capture elements. Hence, we establish HD 20 among the few benchmark stars that have almost complete patterns and possess low systematic dependencies on the stellar parameters. Our light-element (Z ≤ 30) abundances are representative of other, similarly metal-poor stars in the Galactic halo with contributions from core-collapse supernovae of type II. In the realm of the neutron-capture elements, our comparison to the scaled solar r-pattern shows that the lighter neutron-capture elements (Z 60) are poorly matched. In particular, we find imprints of the weak r-process acting at low metallicities. Nonetheless, by comparing our detailed abundances to the observed metal-poor star BD +17 3248, we find a persistent residual pattern involving mainly the elements Sr, Y, Zr, Ba, and La. These are indicative of enrichment contributions from the s-process and we show that mixing with material from predicted yields of massive, rotating AGB stars at low metallicity considerably improves the fit. Based on a solar ratio of heavy-to light-s elements -at odds with model predictions for the i-process -and a missing clear residual pattern with respect to other stars with claimed contributions from this process, we refute (strong) contributions from such astrophysical sites providing intermediate neutron densities. Finally, nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of 11.0 ± 3.8 Gyr.

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Section: Abundances Up To Zn (11 ≤ Z ≤ 30)supporting

confidence: 81%

Section: Introductionmentioning

confidence: 99%

A high-precision abundance analysis of the nuclear benchmark star HD 20

Hanke

Hansen

Ludwig

et al. 2020

A&A

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“…Surviving UFD galaxies have average densities 1 M pc −3 within their core radii (Simon & Geha 2007), so any UFD-like systems on orbits like that of the Sylgr stream would have disrupted long ago (cf. Roederer et al 2018a). We conclude that the progenitor must have been substantially more dense than the surviving population of UFD galaxies.…”

Section: Light Elements Whose Abundances Vary In Gcsmentioning

confidence: 76%

High-resolution Optical Spectroscopy of Stars in the Sylgr Stellar Stream*

Roederer

Gnedin

2019

ApJ

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We observe two metal-poor main sequence stars that are members of the recently-discovered Sylgr stellar stream. We present radial velocities, stellar parameters, and abundances for 13 elements derived from high-resolution optical spectra collected using the Magellan Inamori Kyocera Echelle spectrograph. The two stars have identical compositions (within 0.13 dex or 1.2σ) among all elements detected. Both stars are very metal poor ([Fe/H] = −2.92 ± 0.06). Neither star is highly enhanced in C ([C/Fe] < +1.0). Both stars are enhanced in the α elements Mg, Si, and Ca ([α/Fe] = +0.32 ± 0.06), and ratios among Na, Al, and all Fe-group elements are typical for other stars in the halo and ultrafaint and dwarf spheroidal galaxies at this metallicity. Sr is mildly enhanced ([Sr/Fe] = +0.22 ± 0.11), but Ba is not enhanced ([Ba/Fe] < −0.4), indicating that these stars do not contain high levels of neutron-capture elements. The Li abundances match those found in metal-poor unevolved field stars and globular clusters (log (Li) = 2.05 ± 0.07), which implies that environment is not a dominant factor in determining the Li content of metal-poor stars. The chemical compositions of these two stars cannot distinguish whether the progenitor of the Sylgr stream was a dwarf galaxy or a globular cluster. If the progenitor was a dwarf galaxy, the stream may originate from a dense region such as a nuclear star cluster. If the progenitor was a globular cluster, it would be the most metal-poor globular cluster known.

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“…Chemical abundances also show variations between old metal-poor stars in different environments such as dwarf galaxies, suggesting that the first stages of enrichment were not uniform. Stars in the nearby dwarf galaxies typically have lower abundances of αand odd-Z elements, attributed to their slower star formation histories and/or fewer number of high mass stars overall (Venn et al 2004;Tolstoy et al 2009;Nissen & Schuster 2010;McWilliam et al 2013;Frebel & Norris 2015;Hayes et al 2018), while significant variations in heavy r-process elements in some dwarf galaxies, and globular clusters, are discussed in terms of contributions from individual compact binary merger events, like GW170817 (e.g., Roederer 2011;Roederer et al 2018a;Ji et al 2016Ji et al , 2019. In addition, about a third of the [Fe/H] < −2.5 stars 1 in the Galactic halo show very high enhancements in carbon (the carbon-enhanced metal-poor stars, "CEMP"; Yong et al 2013;Aguado et al 2019a also see Kielty et al 2017;Mardini et al 2019), discussed as a signature of the earliest chemical enrichment in the Universe.…”

Section: Introductionmentioning

confidence: 99%

The Pristine survey – IX. CFHT ESPaDOnS spectroscopic analysis of 115 bright metal-poor candidate stars

Venn

Kielty

Sestito

et al. 2019

Monthly Notices of the Royal Astronomical Society

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A chemo-dynamical analysis of 115 metal-poor candidate stars selected from the narrow-band Pristine photometric survey is presented based on CFHT highresolution ESPaDOnS spectroscopy. We have discover 28 new bright (V < 15) stars with [Fe/H]< −2.5 and 5 with [Fe/H]< −3.0 for success rates of 40% (28/70) and 19% (5/27), respectively. A detailed model atmospheres analysis is carried out for the 28 new metal-poor stars. Stellar parameters were determined from SDSS photometric colours, Gaia DR2 parallaxes, MESA/MIST stellar isochrones, and the initial Pristine survey metallicities, following a Bayesian inference method. Chemical abundances are determined for 10 elements (Na, Mg, Ca, Sc, Ti, Cr, Fe, Ni, Y, Ba). Most stars show chemical abundance patterns that are similar to the normal metal-poor stars in the Galactic halo; however, we also report the discoveries of a new r-process rich star, a new CEMP-s candidate with [Y/Ba]>0, and a metal-poor star with very low [Mg/Fe]. The kinematics and orbits for all of the highly probable metal-poor candidates are determined by combining our precision radial velocities with Gaia DR2 proper motions. Some stars show unusual kinematics for their chemistries, including planar orbits, unbound orbits, and highly elliptical orbits that plunge deeply into the Galactic bulge (R peri < 0.5 kpc); also, eight stars have orbital energies and actions consistent with the Gaia-Enceladus accretion event. This paper contributes to our understanding of the complex chemo-dynamics of the metal-poor Galaxy, and increases the number of known bright metal-poor stars available for detailed nucleosynthetic studies.

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Kinematics of Highly r-process-enhanced Field Stars: Evidence for an Accretion Origin and Detection of Several Groups from Disrupted Satellites

Cited by 97 publications

References 133 publications

A high-precision abundance analysis of the nuclear benchmark star HD 20

A high-precision abundance analysis of the nuclear benchmark star HD 20

High-resolution Optical Spectroscopy of Stars in the Sylgr Stellar Stream*

The Pristine survey – IX. CFHT ESPaDOnS spectroscopic analysis of 115 bright metal-poor candidate stars

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