Spectroscopic and photometric monitoring of a poorly known high-luminous OH/IR star: IRAS 18278+0931

Abstract

We present the time-dependent properties of a poorly known OH/IR star − IRAS 18278+0931 (hereafter, IRAS 18+09) towards the Ophiuchus constellation. We have carried out long-term optical/nearinfrared (NIR) photometric and spectroscopic observations to study the object. From optical R-and I-band light curves, the period of IRAS 18+09 is estimated to be 575 ± 30 days and the variability amplitudes ranges from ∆R ∼ 4.0 mag to ∆I ∼ 3.5 mag. From the standard Period-Luminosity (PL) relations, the distance (D) to the object, 4.0±1.3 kpc, is estimated. Applying this distance in the radiative transfer model, the spectral energy distribution (SED) are constructed from multi-wavelength photometric and IRAS-LRS spectral data which provides the luminosity, optical depth, and gas massloss rate (MLR) of the object to be 9600 ± 500 L , 9.1 ± 0.6 at 0.55 µm and 1.0×10 −6 M yr −1 , respectively. The current mass of the object infers in the range 1.0−1.5M assuming solar metallicity. Notably, the temporal variation of atomic and molecular features (e.g., TiO, Na I, Ca I, CO, H 2 O) over the pulsation cycle of the OH/IR star illustrates the sensitivity of the spectral features to the dynamical atmosphere as observed in pulsating AGB stars.