  We report on the Fermi-LAT detection (with ~5.7 sigma significance) as well
as the multi-wavelength analysis of the 2017 nova eruption V549 Vel. Unlike the
recent shock-powered novae ASASSN-16ma and V906 Car, the optical and gamma-ray
light curves of V549 Vel show no correlation, likely implying relatively weak
shocks in the eruption. Gaia detected a candidate progenitor of V549 Vel and
found a parallax measurement of 1.91+/-0.39 mas, equivalent to a mode distance
of d=560 pc (90% credible interval of 380-1050 pc). The progenitor was also
observed by the 2MASS and WISE surveys. When adopting the Gaia distance, the
spectral energy distribution of the progenitor is close to that of a G-type
star. Swift/XRT detected the supersoft X-ray emission of the nova (kT=30-40
keV) since day 236, and the inferred blackbody size is comparable to that of
other novae assuming d=560 pc (i.e., R_bb~5*10^8 cm). However, there is also an
unknown astrometric excess noise of 3.2 mas found in the Gaia data, and the
inferred distance becomes controversial. If the Gaia distance is accurate, the
gamma-ray luminosity of V549 Vel will be as low as L_gamma~4*10^33 erg/s,
making it the least luminous gamma-ray nova known so far. This may imply that
the shock properties responsible for the gamma-ray emission in V549 Vel are
different from those of the more luminous events. If the nova is located
farther away, it is likely a symbiotic system with a giant companion as the
observed progenitor.
