Dust grain growth in the interstellar medium of 5<z<6.5 quasars
  We investigate whether stellar dust sources i.e. asymptotic giant branch
(AGB) stars and supernovae (SNe) can account for dust detected in 5<z<6.5
quasars (QSOs). We calculate the required dust yields per AGB star and per SN
using the dust masses of QSOs inferred from their millimeter emission and
stellar masses approximated as the difference between the dynamical and the H_2
gas masses of these objects. We find that AGB stars are not efficient enough to
form dust in the majority of the z>5 QSOs, whereas SNe may be able to account
for dust in some QSOs. However, they require very high dust yields even for a
top-heavy initial mass function. This suggests additional non-stellar dust
formation mechanism e.g. significant dust grain growth in the interstellar
medium of at least three out of nine z>5 QSOs. SNe (but not AGB stars) may
deliver enough heavy elements to fuel this growth.
