We have studied the stability of low-degree g modes in uniformly rotating B-type stars, taking into account the effects of the Coriolis force and the rotational deformation. From an analysis treating rotation frequency as a small parameter, it is found that slow rotation tends to destabilize high-radial-order retrograde g modes, although the effect is very small or absent for relatively low-order modes. Calculating eigenfrequencies at selected rotation rates, we find, on the other hand, that rapid rotation tends to stabilize retrograde g modes. The stabilizing effect appears stronger for less-massive B-type stars having low effective temperatures. If we change the rotation rate continuously, the frequency of a g mode belonging to ( , m) crosses frequencies of other g modes belonging to ( , m). If the parity of the two encountering modes is the same, then they interact with each other and the stability (i.e. imaginary part of eigenfrequency) of each mode is modified. Using an asymptotic method, we discuss the property of such mode crossings and couplings. For rapidly rotating stars, mode couplings are important for the stability of low-degree g modes. In particular, we find that the stabilization of retrograde g modes in rapidly rotating stars is due to many strong mode couplings, while prograde sectoral modes are exceptionally immune to the damping effects from the mode couplings.

We study the effect of density distribution evolution on the global one-armed oscillation modes in low viscosity disks around isolated and binary Be stars. Observations show that some Be stars exhibit evidence of formation and dissipation of the equatorial disk. In this paper, we first calculate the density evolution in disks around isolated Be stars. To model the formation stage of the disk, we inject mass at a radius just outside the star at a constant rate for 30 − 50 years. As the disk develops, the density distribution approaches the form of the steady disk solution. Then, we turn off the mass injection to model the disk dissipation stage. The innermost part of the disk starts accretion, and a gap forms between the star and the disk. Next, we calculate the one-armed modes at several epochs. We neglect the effect of viscosity because the time-scale of oscillations is much shorter than the disk evolution time-scale for low viscosity. In the disk formation stage, the eigenfrequency increases with time toward the value for the steady state disk. On the other hand, one-armed eigenmodes in dissipating Be disks have significantly higher eigenfrequencies and narrower propagation regions. Observationally, such a change of mode characteristics can be taken as an evidence for gap opening around the star. In binary Be stars, the characteristics of the disk evolution and the eigenmodes are qualitatively the same as in isolated Be stars, but quantitatively they have shorter evolution time-scales and higher eigenfrequencies, which is in agreement with the observed trend.

Two star clusters can be located near each other as a result of either simultaneous birth or system encounters, and thus are called a binary star cluster system. We analyzed three candidates of binary cluster: ASCC 16-ASCC 21, NGC 6716-Collinder 394, and NGC 2547-Pozzo 1 based on Gaia DR2. Each pair have physical separations of 13 pc, 13 pc, and 42 pc. In order to constrain the binarity of the candidates, we investigated their morphology, age estimates, photometric mass, and kinematics.

Global history of star or cluster formation in the Large Magellanic Cloud (LMC) has been the center of interest in several studies as it is thought to be influenced by tidal interaction with the Small Magellanic Cloud and even the Milky Way. This study focus on the formation history of the LMC in relation with the context of binary star clusters population, the apparent binary fraction (e.g., percentage of cluster pairs) in different epoch were calculated and analyzed. From the established distributions, it can be deduced that the binary clusters tend to be young (∽ 100 Myr) while their locations coincide with the locations of star forming complexes. There is an indication that the binary fraction increases as the rise of star formation rate in the last millions years. In the LMC, the increase of binary fraction at age ∽ 100 Myr can be associated to the last episode of close encounter with the Small Magellanic Cloud at ∽ 150 Myr ago. This observational evidence supports the theory of binary cluster formation through the fission of molecular cloud where the encounter between galaxies enhanced the clouds velocity dispersion which in turn increased the probability of cloud-cloud collisions that produce binary clusters.

Spectroscopic observations of standard stars (spectrophotometric) have been carried out at Bosscha Observatory ITB (latitude= 6°49′28″ S, longitude= 107°36′56″ E, altitude=1310 m) using the Celestron C-11 reflector (D=11 inches, F/10.0), equipped with NEO R-1000 spectrograph (Resolution of 1000), and ST-8XME CCD camera to deduce total atmospheric extinction curve. Spectrograms were reduced with the long-slit task in the Image Reduction and Analysis Facility (IRAF). The total extinction curve obtained from observation is decomposed into three main components, i.e. Rayleigh scattering, Ozone and water vapor absorptions, and extinction by aerosol. The extinction profile by Aerosol serves as an important indication on size and distribution of particulates as the main constituent of atmospheric pollutant. This pollutant can be resulted from natural process or anthropogenic activities. The behavior of atmospheric extinction over Bosscha Observatory based on long-term astronomical photometric database (1982-1993) was previously studied by one of us in 1993. In this study we analyze new data and study the dynamics of atmosphere by comparing our recent result with that from long-term photometric observations to indicate variation of degree of atmospheric turbidity, particulate size and its distribution over the atmosphere. We recommend that long-term regular based spectrophotometry at an Observatory is imperative as an effective means to gain our insight on atmospheric dynamic.