  Recent observations have shown that [P/Fe] in the Galactic stars decreases
with increasing [Fe/H] for [Fe/H] > -1 whereas it is almost subsolar for
[Fe/H]< -2. These [P/Fe] trends with [Fe/H] have not been well reproduced by
previous theoretical models incorporating phosphorus (P) enrichment only by
core collapse supernoave. We here show, for the first time, that the trends can
be naturally explained by our new models incorporating P enrichment by
oxygen-neon (ONe) novae which occur at the surface of massive white dwarfs
whose masses are larger than 1.25 M_sun with a metallicity-dependence rate. We
also show that the observations can be better reproduced by the models by
assuming that (i) the total mass of gaseous ejecta per ONe nov (M_ej) is as
high as 4 * 10^{-5} M_sun and (ii) the number of such novae per unit mass
(N_ONe) is as large as 0.01 at [Fe/H]~-3. The assumed M_ej is consistent with
observations, and the high N_ONe is expected from recent theoretical models for
ONe nova fractions. We predict that (i) [P/Fe] increases with increasing [Fe/H]
for -2 < [Fe/H] < -1 and (ii) [P/Fe] and [Cl/Fe] trends with [Fe/H] are very
similar with each other due to very large yields of P and Cl from ONe nova. It
is thus worthwhile for future observations to assess the validity of the
proposed P enrichment by ONe novae by confirming or ruling out these two
predictions.
