Nova Sgr 2015 #2 very likely to form dust

ATel #7299; D. P. K. Banerjee, N. M. Ashok and M. Srivastava (Physical Research Laboratory, Ahmedabad, India)
on 27 Mar 2015; 03:33 UT
Credential Certification: Dipankar P.K. Banerjee (orion@prl.res.in)

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Near-IR spectra of Nova Sgr 2015 #2 are reported on 2014 March 26.02 UT with the Mount Abu 1.2 meter telescope and the Near-Infrared Imager/Spectrograph (NICS) at R ~ 1000 in the 0.85-2.4 micron region. Earlier spectra were reported in ATel #7265.

All lines are now clearly in emission and the P Cygni absorption components have weakened considerably or disappeared This is in stark contrast to the spectrum just a few days earlier on March 22.0 UT, when the nova was close to peak brightness. At that phase, the NIR lines were almost completely in absorption with very weak emission components indicating high mass loss around maximum brightness. Apart from the customary lines of HI, HeI, OI and NI seen in the spectrum of March 26.02UT, the striking change from earlier spectra is in the development of the neutral carbon lines. These CI lines, which differentiate the FeII from the He/N class of novae in the NIR (Banerjee and Ashok, 2012, BASI, 40, 243) are now extremely strong. in Nova Sgr no. 2. They overshadow even the strongest hydrogen lines in our region of study viz Paschen beta and Brackett gamma.

Clear detections are made of several low excitation lines of NaI and MgI in emission. These lines we have argued, forming from elements of low ionization potential (~5 to 7 eV) relative to that of H, O or N ( ~13 to 14 eV) are good indicators of imminent dust formation as they imply the presence of a cool zone (details in Das, Banerjee et al., 2008, MNRAS , 391, 1874 ). Dust has never failed to form in a nova when these lines were seen in its NIR spectrum and we thus expect Nova Sgr 2015 #2 to also form dust.