Spectral Variations, Quiet Chromosphere and Flaring of Transition Region of AR Lac in UV and Gaia DR3 in the Field Open Cluster NGC 7209

Abstract

We present Gaia DR3 data and estimated the cluster radius, color-magnitude diagram and the membership probability of the open cluster NGC 7209 using the machine learning algorithm pyUPMASK. We evaluated the membership probability of the RS CVn eclipsing binary system AR Lac. We reported International Ultraviolet Explorer (IUE) observations during the period (1979–1994) of the RS CVn eclipsing binary system AR Lac to describe the spectral behavior and the physical conditions in its atmosphere. The observations reveal indications of flare activity in the transition region. Besides the flaring activity, the emission lines show a range of variations between high, intermediate, and low flux values. The relation between line fluxes and orbital phases is found. The flaring activity is found in 1979, 1990, and 1994. The reddening of AR Lac was estimated from the 2200 Å absorption feature to be $E\left(B-V\right)=0$. The average mass loss rate is found to be $\approx 3.2\times {10}^{-9}{M}_{\odot }{\text{year}}^{-1}$, the average temperature of the emitting region is determined by using Planck's equation to be $\approx 9.2\times {10}^{4}K$, the energy of flare to be $\approx 3.9\times {10}^{38}\mathrm{erg}$. We attributed the spectral variations to a cyclic behavior of the underlying magnetic field and the flaring activity to a three-component model.