Prospects for Pre-supernova Neutrino Observation in Future Large Liquid-scintillator Detectors

Before massive stars heavier than $(8 \cdots 10)$ solar masses evolve to the phase of a gravitational core collapse, they will emit a huge number of MeV-energy neutrinos that are mainly produced in the thermal processes and nuclear weak interactions. The detection of such pre-supernova (pre-SN) neutrinos could provide an important and independent early warning for the optical observations of core-collapse SNe. In this paper, we investigate the prospects of future large liquid-scintillator detectors for the observation of pre-SN neutrinos in both $\bar{\nu}^{}_e + p \to e^+ + n$ and $\nu (\bar{\nu}) + e^- \to \nu (\bar{\nu}) + e^-$ reaction channels, where $\nu$ ($\bar{\nu}$) denotes neutrinos (antineutrinos) of all three flavors. We propose a quantitative assessment of the capability in terms of three working criteria, namely, how far the SN distance can be covered, how long the early warning before the core collapse can be sent out, and how well the direction pointing to the SN can be determined. The dependence of the final results on the different models of progenitor stars, neutrino flavor conversions and the relevant backgrounds is also discussed.