Our Sun is surrounded by the Oort Cloud (in radius 0.5 pc) which can be perturbed by various external factors. One of those is the stellar close encounter with our Sun. This kind of perturbation can induce the cometary showers in our Solar System. In this work, we attempt to make numerical simulations to trace the orbit of stars which close encounter with our Sun in the cases of Milky Way’s axisymmetric only and with non-axisymmetric potentials. We have 306 selected solar neighborhood stars from GAIA DR2, LAMOST DR4, and RAVE DR5 which have highly precise kinematics. In this work, we find a few stars that have counter parameter (dm ) less than or equal to 2 pc in both of past and future close encounters with the Sun. We also find a few stars (ID 283, 290, 297, 298) even with dm ≤ 0.5 pc within their errors, for past close encounters at time tm ≥ 0.5 Myr ago. These stars should have perturbed the Oort Cloud’s stability long time ago. Furthermore, we find a few stars (ID 293, 299, 300) with dm ≤ 1 pc within their errors, at tm > 0.6 Myr for future close encounters. Besides that,cadding non-axisymmetric component of Milky Way does not change the results. This suggests that the non-axisymmetric component of Milky Way potential has small effect in perturbing the orbital motion of stars for short timescale. That’s why the values of dm are relatively similar within their errors, for both cases of the Milky Way potential.

Recent observations of young embedded clumpy clusters and statistical identifications of binary star clusters have provided new insights into the formation process and subsequent dynamical evolution of star clusters. The early dynamical evolution of clumpy stellar structures provides the conditions for the origin of binary star clusters. Here, we carry out N-body simulations in order to investigate the formation of binary star clusters in the Milky Way and in the Large Magellanic Cloud (LMC). We find that binary star clusters can form from stellar aggregates with a variety of initial conditions. For a given initial virial ratio, a higher degree of initial substructure results in a higher fraction of binary star clusters. The number of binary star clusters decreases over time due to merging or dissolution of the binary system. Typically, $\sim 45{{\ \rm per\ cent}}$ of the aggregates evolve into binary/multiple clusters within t = 20 Myr in the Milky Way environment, while merely $\sim 30{{\ \rm per\ cent}}$ survives beyond t = 50 Myr, with separations ≲ 50 pc. On the other hand, in the LMC, $\sim 90{{\ \rm per\ cent}}$ of the binary/multiple clusters survive beyond t = 20 Myr and the fraction decreases to $\sim 80{{\ \rm per\ cent}}$ at t = 50 Myr, with separations ≲ 35 pc. Multiple clusters are also rapidly formed for highly-substructured and expanding clusters. The additional components tend to detach and the remaining binary star cluster merges. The merging process can produce fast rotating star clusters with mostly flat rotation curves that speed up in the outskirts.

Observations showed that there are star clusters in paired condition, called as binary star cluster. Some statistical studies attempted to identify the existence of binary and multiple clusters in our Galaxy and Magellanic Clouds. MIStiX survey also found that most of embedded younger star clusters (a few Myr) have clumpy structures, indicating star distribution in young clusters cannot be simply described as spherical shape. This structure can be the key-factor of the formation of primordial binary cluster in our Galaxy. In this work, we simulate the formation of binary star cluster from fractal distribution in isolated condition and under Galactic tidal field for 50 Myr. This aims to investigate the most probable condition to let the highly formation of binary star cluster in our Galaxy and the role of Galactic tidal field in that formation. We find that the clumpier structure of star cluster, the number of binary/multiple cluster will increase, and the same condition also occurs for the warmer condition or higher αvir (the kinetic to potential energy ratio). But this number will decrease at larger time due to the merger processes between the star clusters. By these arguments, we conclude that the most probable condition of star cluster to let the highly formation of binary star cluster is the one with highly sub structured and warm condition. For such condition of star cluster, the fraction of binary/multiple in our Galaxy is 0.87 ± 0.06 at time 20 Myr and decreases to be 0.63 ± 0.09 at time 50 Myr, with reduction 0.24 ± 0.08. As comparison, this reduction is only 0.07 ± 0.05 for the same condition in isolated. The higher deflation shows that the Galactic tidal field has important role in the formation of binary star cluster, as well as the initial mass of star cluster. And we suggest the discovery of multiple clusters in this work is interesting to learn more deeply in the future work.

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.

Sebagian besar bintang-bintang di galaksi terbentuk di dalam gugus bintang, baik gugus terbuka maupun gugus bola. Begitu juga dengan Matahari, yang diperkirakan terbentuk pada 4,6 milyar tahun lalu di dalam sebuah gugus terbuka (sun's birth cluster). Bintang-bintang yang terbentuk bersamaan dengan Matahari di dalam sun's birth cluster, disebut sebagai solar siblings. Argumen ini didukung oleh penemuan radioisotop berumur pendek dan objek-objek Edgeworth-Kuiper yang memiliki inklinasi dan eksentrisitas tinggi di Tata Surya. Pekerjaan ini bertujuan untuk mempelajari evolusi dinamik sun's birth cluster selama 4,6 milyar tahun dan mencari keberadaan solar siblings di Bimasakti dalam radius 1 kpc dari Matahari. Simulasi N-benda dilakukan untuk memahami evolusi dinamik sun's birth cluster dan sebaran solar siblings saat ini dalam pengaruh potensial gravitasi Bimasakti. Sebanyak 2 juta bintang dari katalog GAIA DR2 digunakan untuk mencari solar siblings dalam radius 1 kpc dari Matahari. Hasil dari pekerjaan ini menunjukkan bahwa sun's birth cluster mengalami kehancuran pada usia kurang dari 1 milyar tahun setelah pembentukannya, akibat adanya gangguan pasang surut Bimasakti yang menyebabkan solar siblings terlepas dari sun's birth cluster dan menyebar di sekitar orbit Matahari saat ini. Semakin besar jumlah bintang sun's birth cluster yang dimodelkan, semakin banyak solar siblings yang dapat ditemukan di sekitar Matahari. Analisis kinematika dan fotometri solar siblings menunjukkan setidaknya terdapat 14 hingga 19 kandidat solar siblings pada katalog GAIA DR2 dalam radius 1 kpc dari Matahari.

α Tau is an interesting Red Giant Branch (RGB) star with spectral type K5, yet not well studied. Helium core contraction and hydrogen burning in the shell are occurring in this star. Stars like α Tau are interested to be investigated in order to learn and understand how their evolution processes at the next stage, i.e. Asymptotic Giant Branch (AGB) stage. At AGB stage, a star will suffer stronger oscillations with larger radius than RGB stage. This stage is important because it will be the key-factor on most recent observational data, and Modules for Experiments in Stellar Astrophysics (MESA) and SPECTRUM packages to compute accurate physical parameters and atmospheric models of α Tau at AGB stage. Our computational results show that α Tau will be an AGB star with M = (1.52 ± 0.07) M⊙, Teff = (4545 ± 18) K, and log L = (1.98 ± 0.07) L⊙ at age (2.11 ± 0.27) Gyr. Its radius at this stage is determined to be log R = (1.20 ± 0.92) R⊙. Mass loss at the initial of this stage is still small, i.e. (2.07 ± 0.09) × 10–10 M⊙/yr and it will increase during this stage. The core at this stage contains carbon and oxygen with C/O ∼ 0.26 which shows that α Tau will be type M-AGB. Atmospheric model of α Tau at AGB stage shows that this star will have higher effective temperature and gas pressure than at RGB stage. These conditions will cause the increasing of electron density and Rosseland absorption coefficient in its atmosphere. The hotter atmosphere of AGB stage causes its peak of continuum shifts toward smaller wavelength and yields in three times higher intensity than at RGB stage.

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