Near Infrared photometric and spectroscopic observations of the bright optical transient J212444.87+321738.3

ATel #4931; Soumen Mondal (1), Ramkrishna Das (1), N. M. Ashok (2), D. P. K. Banerjee (2), Somnath Dutta (1), Supriyo Ghosh (1), Anindita Mondal (1) and Arpita Nandi (1). [1]: S N Bose National Centre for Basic Sciences, Kolkata, India [2]: Physical Research Laboratory, Ahmedabad, India
on 1 Apr 2013; 14:51 UT
Credential Certification: Dipankar P.K. Banerjee (orion@prl.res.in)

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We report near infrared JHK-band photometry and spectroscopic observations of the recently reported bright optical transient J212444.87+321738.3 using the Near-IR Imager cum spectrograph (NICMOS-3) installed on the Mount Abu 1.2-m telescope of the Physical Research Laboratory, India following the outburst announcement by Tiurina et al. in ATel #4888.

The photometric observations were carried out on 2013 March 21.020 UT and 23.010 UT yielding magnitudes of J = 5.85 +/- 0.06, H = 4.47 +/- 0.06, K = 3.77 +/- 0.05 ; and J= 5.64 +/- 0.04, H= 4.48 +/- 0.04 , K = 3.77 +/- 0.03 respectively. These present values are significantly brighter than the 2MASS JHKs measurements made on 1998 June 18 of J = 7.82+/-0.03, H = 5.97+/-0.02 and K = 4.82+/-0.02. The large (J-K) colour index of about 2.0 suggests that the source might be a late Mira or semi-regular variable having a large amplitude of variability. The amplitude of variability in comparison with 2MASS measurements shows wavelength-dependence with the amplitude being less in the near-infrared than in the optical (ATel #4888). This may be suggestive of a Mira type behavior (Smith et al., 2002, AJ, 123, 948 ) but the exact nature of the source can be confirmed only from further photometric monitoring.

The near-IR spectra are dominated by strong first overtone bands of 12CO in the K band at 2.2935 micron and beyond. 13CO first overtone bands are also seen. The second overtone 12CO bandheads are also very prominent in the H Band (for e.g 3-0, 4-1, 5-2, 6-3, 7-4, 8-5 at 1.5582, 1.5780, 1.5982, 1.6187, 1.6397, 1.6610 micron respectively). Each of the J, H and K spectra turn downwards at the ends of the band indicating absorption due to water in the source's atmosphere. The spectra are typical of a cool M type star. More observations are needed to be certain if the source is O or C rich. But certain indications (Lancon and Wood, 2000, A &A Suppl. Ser., 146, 217) suggest it may be O rich such as the absence of the C2 bandhead at 1.77 micron and the likely presence of Si I absorption at 1.59 microns which is generally seen in O-rich sources. OH bands are also seen in the H band which are generally absent in carbon star spectra, as expected if all oxygen is locked up in CO.