Recent stellar evolution models show consistently that very massive
metal-free stars evolve into red supergiants shortly before they explode. We
argue that the envelopes of these stars, which will form pair-instability
supernovae, become pulsationally unstable and that this will lead to extreme
mass-loss rates despite the tiny metal content of the envelopes. We investigate
the pulsational properties of such models and derive pulsationally induced
mass-loss rates, which take the damping effects of the mass loss on the
pulsations selfconsistently into account. We find that the pulsations may
induce mass-loss rates of ~ 1e-4 - 1e-2 Msun/yr shortly before the explosions,
which may create a dense circumstellar medium. Our results show that very
massive stars with dense circumstellar media may stem from a wider initial mass
range than pulsational-pair instability supernovae. The extreme mass loss will
cease when so much of the hydrogen-rich envelope is lost that the star becomes
more compact and stops pulsating. The helium core of these stars therefore
remains unaffected, and their fate as pair-instability supernovae remains
unaltered. The existence of dense circumstellar media around metal-free
pair-instability supernovae can make them brighter and bluer, and they may be
easier to detect at high redshifts than previously expected. We argue that the
mass-loss enhancement in pair-instability supernova progenitors can naturally
explain some observational properties of superluminous supernovae: the
energetic explosions of stars within hydrogen-rich dense circumstellar media
with little 56Ni production and the lack of a hydrogen-rich envelope in
pair-instability supernova candidates with large 56Ni production.Comment: 11 pages, 10 figures, 1 table, accepted by Astronomy & Astrophysics,
proofed in v
