  Presolar stardust grains found in primitive meteorites are believed to retain
the isotopic composition of stellar outflows at the time of grain condensation.
Therefore, laboratory measurements of their isotopic ratios represent sensitive
probes for investigating open questions related to stellar evolution, stellar
explosions, nucleosynthesis, mixing mechanisms, dust formation, and galactic
chemical evolution. For a few selected presolar grains, classical novae have
been discussed as a potential source. For SiC, silicate, and graphite presolar
grains, the association is based on the observation of small
$N(^{12}$C)/$N(^{13}$C) and $N(^{14}$N)/$N(^{15}$N) number abundance ratios
compared to solar values, and abundance excesses in $^{30}$Si relative to
$^{29}$Si, as previously predicted by models of classical novae. We report on a
direct measurement of the $^{29}$Si(p,$\gamma$)$^{30}$P reaction, which
strongly impacts simulated $\delta ^{29}$Si values from classical novae. Our
new experimental $^{29}$Si(p,$\gamma$)$^{30}$P thermonuclear reaction rate
differs from previous results by up to 50\% in the classical nova temperature
range ($T$ $=$ $100$ $-$ $400$~MK), while the rate uncertainty is reduced by up
to a factor of $3$. Using our new reaction rate in Monte Carlo reaction network
and hydrodynamic simulations of classical novae, we estimate $\delta ^{29}$Si
values with much reduced uncertainties. Our results establish $\delta ^{29}$Si
values measured in presolar grains as a sensitive probe for assessing their
classical nova paternity. We also demonstrate that $\delta ^{30}$Si values from
nova simulations are presently not a useful diagnostic tool unless the large
uncertainty of the $^{30}$P(p,$\gamma$)$^{31}$S reaction rate can be
significantly reduced.
