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[abridged] We present multi-epoch high-resolution optical spectroscopy,UV/radio/X-ray imaging, and archival Hubble and Spitzer observations of anintermediate luminosity optical transient recently discovered in the nearbygalaxy NGC300. We find that the transient (NGC300 OT2008-1) has a peak absolutemagnitude of M_bol~-11.8 mag, intermediate between novae and supernovae, andsimilar to the recent events M85 OT2006-1 and SN2008S. Our high-resolutionspectra, the first for this event, are dominated by intermediate velocity(~200-1000 km/s) hydrogen Balmer lines and CaII emission and absorption linesthat point to a complex circumstellar environment, reminiscent of the yellowhypergiant IRC+10420. In particular, we detect broad CaII H&K absorption withan asymmetric red wing extending to ~1000 km/s, indicative of gas infall onto amassive and relatively compact star (blue supergiant or Wolf-Rayet star); anextended red supergiant progenitor is unlikely. The origin of the inflowing gasmay be a previous ejection from the progenitor or the wind of a massive binarycompanion. The low luminosity, intermediate velocities, and overall similarityto a known eruptive star indicate that the event did not result in a completedisruption of the progenitor. We identify the progenitor in archival Spitzerobservations, with deep upper limits from Hubble data. The spectral energydistribution points to a dust-enshrouded star with a luminosity of about 6x10^4L_sun, indicative of a ~10-20 M_sun progenitor (or binary system). Thisconclusion is in good agreement with our interpretation of the outburst andcircumstellar properties. The lack of significant extinction in the transientspectrum indicates that the dust surrounding the progenitor was cleared by theoutburst.

An Intermediate Luminosity Transient in NGC300: The Eruption of a Dust-Enshrouded Massive Star