2 research outputs found
The RR Lyrae Delay-Time Distribution: A Novel Perspective on Models of Old Stellar Populations
The delay-time distribution (DTD) is the occurrence rate of a class of
objects as a function of time after a hypothetical burst of star formation.
DTDs are mainly used as a statistical test of stellar evolution scenarios for
supernova progenitors, but they can be applied to many other classes of
astronomical objects. We calculate the first DTD for RR Lyrae variables using
29,810 RR Lyrae from the OGLE-IV survey and a map of the stellar-age
distribution (SAD) in the Large Magellanic Cloud (LMC). We find that of the OGLE-IV RR Lyrae are associated with delay-times older than 8 Gyr
(main-sequence progenitor masses less than 1 M), and consistent with
existing constraints on their ages, but surprisingly about of RR Lyrae
appear have delay times Gyr (main-sequence masses between
M at LMC metallicity). This intermediate-age signal also persists
outside the Bar-region where crowding is less of a concern, and we verified
that without this signal, the spatial distribution of the OGLE-IV RR Lyrae is
inconsistent with the SAD map of the LMC. Since an intermediate-age RR Lyrae
channel is in tension with the lack of RR Lyrae in intermediate-age clusters
(noting issues with small-number statistics), and the age-metallicity
constraints of LMC stars, our DTD result possibly indicates that systematic
uncertainties may still exist in SAD measurements of old-stellar populations,
perhaps stemming from the construction methodology or the stellar evolution
models used. We described tests to further investigate this issue.Comment: 21 pages, 11 figures. Accepted to Ap
Spectroscopic Analysis of Milky Way Outer Halo Satellites: Aquarius II and Boötes II
In this paper, we present a chemical and kinematic analysis of two ultrafaint dwarf galaxies (UFDs), Aquarius II (Aqu II) and Boötes II (Boo II), using Magellan/IMACS spectroscopy. We present the largest sample of member stars for Boo II (12), and the largest sample of red giant branch members with metallicity measurements for Aqu II (eight). In both UFDs, over 80% of targets selected based on Gaia proper motions turned out to be spectroscopic members. In order to maximize the accuracy of stellar kinematic measurements, we remove the identified binary stars and RR Lyrae variables. For Aqu II, we measure a systemic velocity of −65.3 ± 1.8 km s ^−1 and a metallicity of [Fe/H] = . When compared with previous measurements, these values display a ∼6 km s ^−1 difference in radial velocity and a decrease of 0.27 dex in metallicity. Similarly for Boo II, we measure a systemic velocity of km s ^−1 , more than 10 km s ^−1 different from the literature, a metallicity almost 1 dex smaller at [Fe/H] = , and a velocity dispersion 3 times smaller at km s ^−1 . Additionally, we derive systemic proper-motion parameters and model the orbits of both UFDs. Finally, we highlight the extremely dark-matter-dominated nature of Aqu II and compute the J-factor for both galaxies to aid searches of dark matter annihilation. Despite the small size and close proximity of Boo II, it is an intermediate target for the indirect detection of dark matter annihilation due to its low-velocity dispersion and corresponding low dark matter density