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Register a new userThe role of faint population III supernovae in forming CEMP stars in ultra-faint dwarf galaxies
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CEMP-no stars, a subset of carbon enhanced metal poor (CEMP) stars ($rm [C/Fe]geq0.7$ and $rm [Fe/H]lesssim-1$) have been discovered in ultra-faint dwarf (UFD) galaxies, with $M_{rm vir} sim 10^8$ Msun and $M_{ast}sim10^3-10^4$ Msun at $z=0$, as well as in the halo of the Milky Way (MW). These CEMP-no stars are local fossils that may reflect the properties of the first (Pop~III) and second (Pop~II) generation of stars. However, cosmological simulations have struggled to reproduce the observed level of carbon enhancement of the known CEMP-no stars. Here we present new cosmological hydrodynamic zoom-in simulations of isolated UFDs that achieve a gas mass resolution of $m_{rm gas}sim60$ Msun. We include enrichment from Pop~III faint supernovae (SNe), with $ E_{rm SN}=0.6times10^{51}$ erg, to understand the origin of CEMP-no stars. We confirm that Pop~III and Pop~II stars are mainly responsible for the formation of CEMP and C-normal stars respectively. New to this study, we find that a majority of CEMP-no stars in the observed UFDs and the MW halo can be explained by Pop~III SNe with normal explosion energy ($ E_{rm SN}=1.2times10^{51}$~erg) and Pop~II enrichment, but faint SNe might also be needed to produce CEMP-no stars with $rm [C/Fe]gtrsim2$, corresponding to the absolute carbon abundance of $rm A(C)gtrsim6.0$. Furthermore, we find that while we create CEMP-no stars with high carbon ratio $rm [C/Fe]approx3-4$, by adopting faint SNe, it is still challenging to reproduce CEMP-no stars with extreme level of carbon abundance of $rm A(C)approx 7.0-7.5$, observed both in the MW halo and UFDs.
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Highly r-process enhanced metal-poor stars (MP r-II, $rm [Eu/Fe]>1$ and $rm [Fe/H]lesssim-1.5$) have been observed in ultra-faint dwarf (UFD) galaxy, specifically in Reticulum~II (Ret~II). The fact that only a few UFDs contain such stars implies that the r-process site may reflect very rare, but individually prolific events, such as neutron star mergers (NSMs). Considering the relatively short star formation history (SFH) of UFDs, it is puzzling how they could experience such a rare phenomenon. In this work, we show the results of cosmological hydrodynamic zoom-in simulations of isolated UFDs ($M_{vir}approx10^7-10^8$ solar mass and $M_{ast}approx10^3-10^4$ solar mass at $z=0$) to explain the formation of MP r-II stars in UFDs. We employ a simple toy model for NSM events, adopting parameters consistent with observations, such as the NSM rate (1 per $M_{ast}approx10^5$ solar mass) and europium (Eu) mass ($M_{Eu}approx10^{-5}$ solar mass). We identify only one simulated galaxy ($ M_{vir}approx4.6times10^7$, $M_{ast}approx 3.4times 10^3$ solar mass at $z=0$) with abundances similar to Ret~II in a simulation volume that hosts $sim30$ UFD analogs, indicating that such abundances are possible but rare. By exploring a range of key parameters, we demonstrate that the most important factor in determining the formation of MP r-II stars in UFDs is how quickly subsequent stars can be formed out of r-process enriched gas. We find that it takes between 10 to 100~Myr to form the first and second burst of MP r-II stars. Over this period, Eu-polluted gas maintains the required high abundance ratios of $rm [Eu/Fe]>1$.
We search for RR Lyrae stars in 27 nearby ($<100$ kpc) ultra-faint dwarf satellite galaxies using the Gaia DR2 catalog of RR Lyrae stars. Based on proper motions, magnitudes and location on the sky, we associate 47 Gaia RR Lyrae stars to 14 different satellites. Distances based on RR Lyrae stars are provided for those galaxies. We have identified RR Lyrae stars for the first time in the Tucana II dwarf galaxy, and find additional members in Ursa Major II, Coma Berenices, Hydrus I, Bootes I and Bootes III. In addition we have identified candidate extra-tidal RR Lyrae stars in six galaxies which suggest they may be undergoing tidal disruption. We found 10 galaxies have no RR Lyrae stars neither in Gaia nor in the literature. However, given the known completeness of Gaia DR2 we cannot conclude these galaxies indeed lack variable stars of this type.
We investigate the star formation history and chemical evolution of isolated analogues of Local Group (LG) ultra faint dwarf galaxies (UFDs; stellar mass range of 10^2 solar mass < M_star <10^5 solar mass) and gas rich, low mass dwarfs (Leo P analogs; stellar mass range of 10^5 solar mass < M_star <10^6 solar mass). We perform a suite of cosmological hydrodynamic zoom-in simulations to follow their evolution from the era of the first generation of stars down to z=0. We confirm that reionization, combined with supernova (SN) feedback, is primarily responsible for the truncated star formation in UFDs. Specifically, haloes with a virial mass of M_vir < 2 x 10^9 solar mass form> 90% of stars prior to reionization. Our work further demonstrates the importance of Population~III (Pop~III) stars, with their intrinsically high $rm [C/Fe]$ yields, and the associated external metal-enrichment, in producing low-metallicity stars ($rm [Fe/H]lesssim-4$) and carbon-enhanced metal-poor (CEMP) stars. We find that UFDs are composite systems, assembled from multiple progenitor haloes, some of which hosted only Population~II (Pop~II) stars formed in environments externally enriched by SNe in neighboring haloes, naturally producing, extremely low-metallicity Pop~II stars. We illustrate how the simulated chemical enrichment may be used to constrain the star formation histories (SFHs) of true observed UFDs. We find that Leo P analogs can form in haloes with M_vir ~ 4 x 10^9 solar mass (z=0). Such systems are less affected by reionization and continue to form stars until z=0, causing higher metallicity tails. Finally, we predict the existence of extremely low-metallicity stars in LG UFD galaxies that preserve the pure chemical signatures of Pop~III nucleosynthesis.
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We develop a technique to investigate the possibility that some of the recently discovered ultra-faint dwarf satellites of the Milky Way might be cusp caustics rather than gravitationally self-bound systems. Such cusps can form when a stream of stars folds, creating a region where the projected 2-D surface density is enhanced. In this work, we construct a Poisson maximum likelihood test to compare the cusp and exponential models of any substructure on an equal footing. We apply the test to the Hercules dwarf (d ~ 113 kpc, M_V ~ -6.2, e ~ 0.67). The flattened exponential model is strongly favored over the cusp model in the case of Hercules, ruling out at high confidence that Hercules is a cusp catastrophe. This test can be applied to any of the Milky Way dwarfs, and more generally to the entire stellar halo population, to search for the cusp catastrophes that might be expected in an accreted stellar halo.
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