N. M. Ashok

Insights into the evolution of symbiotic recurrent novae from radio synchrotron emission: V745 Scorpii and RS Ophiuchi

We present observations at 610 and 235 MHz using the Giant Metrewave Radio Telescope (GMRT) of the recurrent nova V745 Scorpii which recorded its last outburst on 2014 February 6. This is the second symbiotic recurrent nova whose light curve at low frequencies has been followed in detail, the first being RS Ophiuchi in 2006. We fitted the 610 MHz light curve by a model of synchrotron emission from an expanding shell being modified by radiative transfer effects due to local absorbing gas consisting of a uniformly distributed and a clumpy component. Using our model parameters, we find that the emission at 235 MHz peaked around day 35 which is consistent with our GMRT observations. The two main results of our study are (1) The radio emission at a given frequency is visible sooner after the outburst in successive outbursts of both V745 Scorpii and RS Ophiuchi. The earlier detection of radio emission is interpreted to be caused by decreasing foreground densities. (2) The clumpy material, if exists, is close to the white dwarf and can be interpreted as being due to the material from the hot accretion disc. The uniform density gas is widespread and attributed to the winds blown by the white dwarf. We present implications of these results on the evolution of both novae. Such studies along with theoretical understanding have the potential of resolving several outstanding issues such as why all recurrent novae are not detectable in synchrotron radio and whether recurrent novae are progenitor systems of Type 1a supernova.

Near-infrared studies of V5558 Sgr: an unusually slow nova with multiple outbursts

We present near-infrared (1-2.5 µm) JHK photo-spectroscopic results of the unusually slow nova V5558 Sgr (2007). V5558 Sgr showed a slow climb to maximum that lasted for about 60 days and then underwent at least five strong secondary outbursts. We have analyzed the optical light curve to derive large t2 and t3 values of 281 ± 3 and 473 ± 3 days respectively. An alternate approach is adopted to derive a distance estimate of 1.55 ± 0.25 kpc as conventional MMRD relation may not be applicable for a slow nova. In the pre-maxima stage the spectra showed narrow (FWHM � 400 550 km s −1 ) and strong emission lines of Paschen and Brackett series with prominent P-Cygni components. In the later phase the spectra show significant changes with the development of strong and broad (� 1000 km s −1 ) emission lines of HI, HeI, OI, and NI and some uncommon Fe II emission lines. No evidence of dust formation is seen. V5558 Sgr has been shown to be a rare hybrid nova showing a transition from He/N to Fe II type from optical spectra. However the near-infrared data do not show such a transition and we discuss this anomalous behavior. A recombination analysis of the Brackett lines allows us to constrain the electron density and emission measure during the early optically thick phase and to estimate the mass of the ejecta to be (6.0 ± 1.5) × 10 −4 M⊙, assuming a filling factor of unity, from later observations.

Infrared Observations of Comet Hale–Bopp (C/1995 O1) from Gurushikhar Observatory

During the recent apparition of comet Hale—Bopp (1995 O1) near infrared photometric observations were carried out in the J, H, K filter bands and also in the 3.0—3.4 μm region at the 1.2 m telescope at Gurushikkar, India. The effective temperature of the comet was substantially higher than the equilibrium blackbody temperature. A mean superheat value of 1.83 was derived in the post-perihelion phase which implies that a large fraction of the grain population are made up of small and hot grains with radii <0.5 μm. High albedo values of ∼0.4 were also derived in the scattering angle range 135° to 160° which could explain the unusual brightness of comet Hale—Bopp.

Phase-dependent Photometric and Spectroscopic Characterization of the MASTER-Net Optical Transient J212444.87+321738.3: An Oxygen-rich Mira

We describe the time-dependent properties of a new spectroscopically confirmed Mira variable, which was discovered in 2013 as MASTER-Net Optical Transient (OT) J212444.87+321738.3 towards the Cygnus constellation. We have performed long-term optical/near-infrared (NIR) photometric and spectroscopic observations to characterize the object. From the optical/NIR light curves, we estimate a variability period of 465

Early formation of carbon monoxide in the Centaurus A supernova SN 2016adj

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Near-infrared and optical studies of the highly obscured nova V1831 Aquilae (Nova Aquilae 2015)

30 days. The wavelength-dependent amplitudes of the observed light-curves range from

Near-infrared studies of V2944 Ophiuchi (Nova Ophiuchi 2015)

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A near-infrared stellar spectral library: II. K-band spectra

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IR observations of Am stars

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A two channel high speed photometer for lunar occultation studies in the near - IR

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