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ABSTRACT We study the production of barium (Ba) and strontium (Sr) in ultrafaint dwarf (UFDs) galaxies. Both r- and s- processes produce these elements, and one can infer the contribution of the r-process from the characteristic r-process abundance pattern, whereas the s-process contribution remains largely unknown. We show that the current s-process yield from asymptotic giant branch (AGB) stars is not sufficient to explain the Ba and Sr abundances observed in UFDs. Production of these elements would need to be efficient from the beginning of star formation in the galaxies. The discrepancy of nearly or more than 1 dex is not reconciled even if we consider s-process in super-AGB stars. We consider a possible resolution by assuming rotating massive stars (RMSs) and electron-capture supernovae (ECSNe) as additional contributors. We find that the RMSs could be the origin of Ba in UFDs if ∼10 per cent of massive stars are rotating at 300 km s−1. As for ECSNe, we argue that their fraction is less than 2 per cent of core-collapse supernova. It narrows the progenitor mass-range to ${\lesssim}0.1\, \mathrm{M}_\odot$ at −3 ≲ [Fe/H] ≲ −2. We also explore another resolution by modifying the stellar initial mass function (IMF) in UFDs and findmore »a top-light IMF model that reproduces the observed level of Ba-enrichment. Future observations that determine or tightly constrain the europium and nitrogen abundances are crucial to identify the origin of Ba and Sr in UFDs.« less

We present an overview of a deep transient survey of the COSMOS field with the Subaru Hyper Suprime-Cam (HSC). The survey was performed for the 1.77 deg2 ultra-deep layer and 5.78 deg2 deep layer in the Subaru Strategic Program over six- and four-month periods from 2016 to 2017, respectively. The ultra-deep layer reaches a median depth per epoch of 26.4, 26.3, 26.0, 25.6, and 24.6 mag in g, r, i, z, and y bands, respectively; the deep layer is ∼0.6 mag shallower. In total, 1824 supernova candidates were identified. Based on light-curve fitting and derived light-curve shape parameter, we classified 433 objects as Type Ia supernovae (SNe); among these candidates, 129 objects have spectroscopic or COSMOS2015 photometric redshifts and 58 objects are located at z > 1. Our unique data set doubles the number of Type Ia SNe at z > 1 and enables various time-domain analyses of Type II SNe, high-redshift superluminous SNe, variable stars, and active galactic nuclei.