Near Infrared ( NIR ) and optical photometry and spectroscopy are presented for the nova V1831 Aquilae , covering the early decline and dust forming phases during the first \sim 90 days after its discovery .
The nova is highly reddened due to interstellar extinction .
Based solely on the nature of NIR spectrum we are able to classify the nova to be of the Fe II class .
The distance and extinction to the nova are estimated to be 6.1 \pm 0.5 kpc and A _ { v } \sim 9.02 respectively .
Lower limits of the electron density , emission measure and ionized ejecta mass are made from a Case B analysis of the NIR Brackett lines while the neutral gas mass is estimated from the optical [ OI ] lines .
We discuss the cause for a rapid strengthening of the He I 1.0830 \mu m line during the early stages .
V1831 Aql formed a modest amount of dust fairly early ( \sim 19.2 days after discovery ) ; the dust shell is not seen to be optically thick .
Estimates are made of the dust temperature , dust mass and grain size .
Dust formation commences around day 19.2 at a condensation temperature of 1461 \pm 15 K , suggestive of a carbon composition , following which the temperature is seen to gradually decrease to 950K .
The dust mass shows a rapid initial increase which we interpret as being due to an increase in the number of grains , followed by a period of constancy suggesting the absence of grain destruction processes during this latter time .
A discussion is made of the evolution of these parameters , including certain peculiarities seen in the grain radius evolution .