An r-process Enhanced Star in the Dwarf Galaxy Tucana III*

Hansen, Terese T.; Simon, Joshua D.; Marshall, J. L.; Li, T. S.; Carollo, D.; DePoy, D. L.; Nagasawa, D. Q.; Bernstein, R. A.; Drlica-Wagner, A.; Abdalla, F. B.; Allam, S.; Annis, J.; Bechtol, K.; Benoit-Lévy, A.; Brooks, D.; Buckley-Geer, E.; Rosell, A. Carnero; Kind, M. Carrasco; Carretero, J.; Cunha, C. E.; Costa, L. N. da; Desai, S.; Eifler, T. F.; Neto, A. Fausti; Flaugher, B.; Frieman, Joshua A.; García-Bellido, J.; Gaztañaga, E.; Gerdes, D. W.; Gruen, D.; Gruendl, R. A.; Gschwend, J.; Gutiérrez, G.; James, D. J.; Krause, E.; Kuêhn, K.; Kuropatkin, N.; Lahav, O.; Miquel, R.; Plazas, A. A.; Romer, A. K.; Sánchez, E.; Santiago, B.; Scarpine, V.; Smith, R. C.; Soares-Santos, M.; Sobreira, F.; Suchyta, E.; Swanson, M. E. C.; Tarlè, G.; Walker, A. R.

doi:10.3847/1538-4357/aa634a

Cited by 123 publications

(91 citation statements)

References 82 publications

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“…Incorporating this ejecta into ∼ 10 3 to 10 4 M of stars formed after the merger would produce the observed [Eu/H] ratios, assuming a star formation efficiency of ∼ 1% and no loss of r-process metals. This mass is comparable to that of the lowest-mass dwarf galaxies known today, including Reticulum II (Ji et al 2016c) and Tucana III (Hansen et al 2017), which are known to contain r-process-enhanced stars. There is considerable scatter among the [Eu/H] ratios of individual stars in these systems (−1.6 ≤ [Eu/H] ≤ −0.3), and the average [Eu/H] ratio of stars in a system may provide a more meaningful measure than that from any one star.…”

Section: The Environment Of Hd 222925mentioning

confidence: 66%

The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme R-process Enhancement of [Eu/H] = −0.14*

Roederer¹,

Sakari²,

Placco³

et al. 2018

ApJ

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113

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We present a detailed abundance analysis of the bright (V = 9.02), metal-poor ([Fe/H] = −1.47 ± 0.08) field red horizontal-branch star HD 222925, which was observed as part of an ongoing survey by the R-Process Alliance. We calculate stellar parameters and derive abundances for 46 elements based on 901 lines examined in a high-resolution optical spectrum obtained using the Magellan Inamori Kyocera Echelle spectrograph. We detect 28 elements with 38 ≤ Z ≤ 90; their abundance pattern is a close match to the Solar r -process component. The distinguishing characteristic of HD 222925 is an extreme enhancement of r -process elements ([Eu/Fe] = +1.33 ± 0.08, [Ba/Eu] = −0.78 ± 0.10) in a moderately metal-poor star, so the abundance of r -process elements is the highest ([Eu/H] = −0.14 ± 0.09) in any known r -process-enhanced star. The abundance ratios among lighter (Z ≤ 30) elements are typical for metal-poor stars, indicating that production of these elements was dominated by normal Type II supernovae, with no discernible contributions from Type Ia supernovae or asymptotic giant branch stars. The chemical and kinematic properties of HD 222925 suggest it formed in a low-mass dwarf galaxy, which was enriched by a high-yield r -process event before being disrupted by interaction with the Milky Way.

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Section: The Environment Of Hd 222925mentioning

confidence: 66%

The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme R-process Enhancement of [Eu/H] = −0.14*

Roederer¹,

Sakari²,

Placco³

et al. 2018

ApJ

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113

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“…This mechanism could enhance halo stars in r-process elements already at early times, when the metallicity of the halo stars is low and many building blocks of the stellar halo still have shallow potential wells from which NS binaries easily escape after receiving a standard kick velocity. This scenario is supported by the recent observation of a only mildly r-process enhanced star in the ultra-faint dwarf galaxy Tucana III (Hansen et al 2017), contrary to the highly r-process enhanced stars observed in Ret II.…”

Section: Summary and Discussionmentioning

confidence: 58%

Semi-analytic modelling of the europium production by neutron star mergers in the halo of the Milky Way

Oirschot

Nelemans

Pols

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Neutron star mergers (NSM) are likely to be the main production sites for the rapid (r-) neutron capture process elements. We study the r-process enrichment of the stellar halo of the Milky Way through NSM, by tracing the typical r-process element Eu in the Munich-Groningen semi-analytic galaxy formation model, applied to three high resolution Aquarius dark matter simulations. In particular, we investigate the effect of the kick velocities that neutron star binaries receive upon their formation, in the building block galaxies (BBs) that partly formed the stellar halo by merging with our Galaxy. When this kick is large enough to overcome the escape velocity of the BB, the NSM takes place outside the BB with the consequence that there is no rprocess enrichment. We find that a standard distribution of NS kick velocities decreases [Eu/Mg] abundances of halo stars by ∼ 0.5 dex compared to models where NS do not receive a kick. With low NS kick velocities, our simulations match observed [Eu/Mg] abundances of halo stars reasonably well, for stars with metallicities [Mg/H] −1.5. Only in Aquarius halo B-2 also the lower metallicity stars have [Eu/Mg] values similar to observations. We conclude that our assumption of instantaneous mixing is most likely inaccurate for modelling the r-process enrichment of the Galactic halo, or an additional production site for r-process elements is necessary to explain the presence of low-metallicity halo stars with high Eu abundances.

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“…It does not present significant s-process or r-process enrichment like the stars in Ret II (Ji et al 2016c) or Tuc III (Hansen et al 2017). The upper limit of Eu found in DES J025535-540643 ([Eu/Fe] < +1.02) excludes it from being an r-II star (defined as [Eu/Fe]>+1.0) but does not exclude the possibility that it is an r-I star (defined as [Eu/Fe]>+0.3), where these definitions are taken from Beers & Christlieb (2005).…”

Section: Resultsmentioning

confidence: 99%

“…Four DES-discovered ultrafaint dwarfs have been chemically studied in detail to date: Ret II (Ji et al 2016a;Roederer et al 2016), Tuc II (Ji et al 2016b), Tuc III (Hansen et al 2017), and now Hor I. In each case (with the possible exception of Tuc II), the brightest confirmed member stars show an unexpected and peculiar abundance pattern.…”

Section: Discussionmentioning

confidence: 99%

“…The chemical diversity of stars in Tuc II is somewhat unlike that observed in previously studied ultrafaint dwarfs and could be explained by a range of phenomena, not all of which follow the standard nucleosynthetic processes (Ji et al 2016b). Hansen et al (2017) report the discovery of a moderately r-process-enhanced (r-I) star in Tuc III, a rare chemical signature when compared to the bulk of field stars in the Milky Way halo, though not as rare as r-II stars. The diverse abundance patterns observed in these galaxies and the range of unusual phenomena invoked to explain them suggest that star formation in the early universe must have been a stochastic process that was highly variable on the mass scales of ultrafaint dwarf galaxies.…”

Section: Introductionmentioning

confidence: 99%

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Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I*

Nagasawa

Marshall

et al. 2018

ApJ

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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 α-elements, iron-peak elements, and neutron-capture elements. The abundance pattern is relatively consistent among all three stars, which have a 1 low average metallicity of [Fe/H]∼−2.6 and are not α-enhanced ([α/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.

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An r-process Enhanced Star in the Dwarf Galaxy Tucana III*

Cited by 123 publications

References 82 publications

The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme R-process Enhancement of [Eu/H] = −0.14*

The R-Process Alliance: A Comprehensive Abundance Analysis of HD 222925, a Metal-poor Star with an Extreme R-process Enhancement of [Eu/H] = −0.14*

Semi-analytic modelling of the europium production by neutron star mergers in the halo of the Milky Way

Chemical Abundance Analysis of Three α-poor, Metal-poor Stars in the Ultrafaint Dwarf Galaxy Horologium I*

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