2014ApJ...785..146H

Sakurai's object (V4334 Sgr), the final flash object discovered in the mid-1990s, underwent rapid cooling during the first decade of the 21st century becoming as faint as K ∼ 25. This stage of evolution has ceased. Between observations in 2010 September and 2013 April V4334 Sgr brightened >2 mag to K = 14.2 and the effective temperature increased to ∼590 K. AO images show a central source and two extended globules defining a 13° position angle. The globules span a spatial extent of ∼0.''3 in 2013. This spatial extent is consistent with sizes derived from spectral energy distributions taken over the previous decade and a debris cloud expanding at 0.055 mas/d since late 1998. Near-simultaneous 0.85-2.5 µm spectra reveal helium lines attributed to a wind-interaction shock. The He I 1.0830 µm emission has a spectral width of ∼1000 km/s and a spatial extent of ∼1.''4. The helium shell is fragmented, spatially asymmetric, and five times larger than the dust debris cloud. [C I] and [N I] forbidden lines are present in the 1 µm region spectrum. The forbidden line spectrum is similar to that of proto-planetary nebulae. The [C I] 9850 Å line is spatially extended. The expansion velocity and change of angular size limit the distance to 2.1-3.7 kpc.