Browsing by Author "Youakim, Kris"

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    Pristine dwarf galaxy survey - I. A detailed photometric and spectroscopic study of the very metal-poor Draco II satellite
    (2018) Longeard, Nicolas; Martin, Nicolas; Starkenburg, Else; Ibata, Rodrigo A.; Collins, Michelle L. M.; Geha, Marla; Laevens, Benjamin P. M.; Rich, R. Michael; Aguado, David S.; Arentsen, Anke; Carlberg, Raymond G.; Cote, Patrick; Hill, Vanessa; Jablonka, Pascale; Gonzalez Hernandez, Jonay I.; Navarro, Julio F.; Sanchez-Janssen, Ruben; Tolstoy, Eline; Venn, Kim A.; Youakim, Kris
    We present a detailed study of the faint Milky Way satellite Draco II (Dra II) from deep CFHT/MegaCam broad-band g and i photometry and narrow-band metallicity-sensitive CaliK observations, along with follow-up Keck II/DEIMOS multi-object spectroscopy. Forward modelling of the deep photometry allows us to refine the structural and photometric properties of Dra II: the distribution of stars in colour-magnitude space implies Dra II is old (13.5 +/- 0.5 Gyr), very metal-poor, very faint (L-v = 180(-72)(+124) L-circle dot), and at a distance d = 21.5 +/- 0.4 kpc. The narrow-band, metallicity-sensitive Cal-IK Pristine photometry confirms this very low metallicity ([Fe/H] = -2.7 +/- 0.1 dex). Even though our study benefits from a doubling of the spectroscopic sample size compared to previous investigations, the velocity dispersion of the system is still only marginally resolved (sigma(vr) < 5.9 km s(-1) at the 95 per cent confidence level) and confirms that Dra II is a dynamically cold stellar system with a large recessional velocity (< v(r)> = -342.5(-1.2)(+1.1)km s(-)1). We further show that the spectroscopically confirmed members of Dra II have a mean proper motion of (mu(alpha)*, mu(delta)) = (1.26 +/- 0.27, 0.94 +/- 0.28) mas/yr in the Gaia DR2 data, which translates to an orbit with a pericentre and an apocentre of 21.3(-1.0)(+0.7) and 153.8(-34.7)(+56.7) kpc, respectively. Taken altogether, these properties favour the scenario of Dra II being a potentially disrupting dwarf galaxy. The low-significance extra-tidal features we map around the satellite tentatively support this scenario.
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    The Photometric Metallicity and Carbon Distributions of the Milky Way's Halo and Solar Neighborhood from S-PLUS Observations of SDSS Stripe 82
    (2021) Whitten, Devin D.; Placco, Vinicius M.; Beers, Timothy C.; An, Deokkeun; Lee, Young Sun; Almeida-Fernandes, Felipe; Herpich, Fabio R.; Daflon, Simone; Barbosa, Carlos E.; Perottoni, Helio D.; Rossi, Silvia; Tissera, Patricia B.; Yoon, Jinmi; Youakim, Kris; Schoenell, William; Ribeiro, Tiago; Kanaan, Antonio
    We report photometric estimates of effective temperature, T (eff), metallicity, [Fe/H], carbonicity, [C/Fe], and absolute carbon abundances, A(C), for over 700,000 stars from the Southern Photometric Local Universe Survey (S-PLUS) Data Release 2, covering a substantial fraction of the equatorial Sloan Digital Sky Survey Stripe 82. We present an analysis for two stellar populations: (1) halo main-sequence turnoff stars and (2) K-dwarf stars of mass 0.58 < M/M (circle dot) < 0.75 in the Solar Neighborhood. Application of the Stellar Photometric Index Network Explorer (SPHINX) to the mixed-bandwidth (narrow- plus wide-band) filter photometry from S-PLUS produces robust estimates of the metallicities and carbon abundances in stellar atmospheres over a wide range of temperatures, 4250 < T (eff)(K) < 7000. The use of multiple narrow-band S-PLUS filters enables SPHINX to achieve substantially lower levels of "catastrophic failures" (i.e., large offsets in metallicity estimates relative to spectroscopic determinations) than previous efforts using a single metallicity-sensitive narrow-band filter. We constrain the exponential slope of the Milky Way's K-dwarf halo metallicity distribution function (MDF), lambda (10,[Fe/H]) = 0.85 +/- 0.21, over the metallicity range -2.5 < [Fe/H] < -1.0; the MDF of our local-volume K-dwarf sample is well-represented by a gamma distribution with parameters alpha = 2.8 and beta = 4.2. S-PLUS photometry obtains absolute carbon abundances with a precision of similar to 0.35 dex for stars with T (eff) < 6500 K. We identify 364 candidate carbon-enhanced metal-poor stars, obtain assignments of these stars into the Yoon-Beers morphological groups in the A(C)-[Fe/H] space, and we derive the CEMP frequencies.
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    The R-process alliance: spectroscopic follow-up of low-metallicity star candidates from the best & brightest survey
    (2019) Placco, Vinicius M.; Santucci, Rafael M.; Beers, Timothy C.; Chaname, Julio; Sepulveda, Maria Paz; Coronado, Johanna; Rossi, Silvia; Sun Lee, Young; Starkenburg, Else; Youakim, Kris