Observation and characterization of gravitationally strong and weak tied binary and multiple systems

This work consists of three main Parts, and as the title of this thesis suggests, it includes studies regarding both weakly and strongly bound objects of various masses, at various separations. In the first Part, we study the contribution of SOXS (Son Of X-Shooter) to studying the ElectroMagnetic (EM) counterparts of various sources of LIGO/VIRGO/KAGRA Gravitational Wave (GW) detectors. To this aim, we investigate the theoretical models of EM counterparts associated with Binary Black Holes (BBHs), Binary Neutron Stars (BNSs), and Black Hole Neutron Star (BHNS) pairs. We understood that SOXS is capable of detecting EM counterparts of BBHs up to a distance of 300 Mpc. For BNSs, this distance decreases to 200 Mpc, and for BHNSs the limiting distance is 150 Mpc. Considering the merger rates for each category of these sources, SOXS is expected to be capable of detecting the EM counterparts of LIGO/VIRGO/KAGRA sources under favorable conditions. In the second Part, we analyze the stellar spectra of four wide companion candidates of four different stellar systems. Discovering wide companions of stellar systems allows us to constrain these systems’ dynamical environment and age. Four probable wide companions of four different stellar systems have been studied in this work. The candidates were selected mainly based on their similar kinematic properties to the central star using Gaia DR2. The central stars are V4046 Sgr, HIP 74865, HIP 65426, and HIP 73145, and their probable wide companions are 2MASS J18152222-3249329, 2MASS J15174874-3028484, 2MASS J13242119-5129503, and 2MASS J14571503-3543505 respectively. We also performed an analysis on the HT Lup system located in the Lupus I cloud that was not a part of the proposal submitted to VLT/X-Shooter. Among the four objects analyzed in detail in this work, V4046 Sgr is a member of βPictoris Moving Group while the rest of the stellar systems are acknowledged as members of the Scorpius-Centaurus association. The selected stellar systems are particularly interesting since all of them are already acknowledged to possess a low-mass companion and/or a spatially resolved disk. Identifying wider companions of these stars can make them benchmarks for understanding various triple systems’ formation channels, and help us to determine the orbits of their possibly undiscovered inner/wider companions in case of higher multiplicity. By analyzing the X-Shooter spectra of the wide companion candidates of these stars, we obtained their stellar parameters and determined their ages. We also investigate the candidate wide companions of the HT Lup triple system located in Lupus I cloud as a complementary study. This last case is not included in Majidi et al. (2020), and does not belong to the same proposal as other targets studied in this paper. We found that 2MASS J15174874-3028484 (0.11 M, 7.4±0.5 Myr), an already recognized Pre-Main Sequence (PMS) member of Scorpius-Centaurus association, is a highly probable wide companion of HIP 74865. 2MASS J13242119-5129503 (0.04 M, 16±2.2 Myr) is ruled out as a plausible wide companion of HIP 65426, but confirmed to be a new sub-stellar member of the Scorpius-Centaurus association. 2MASS J14571503-3543505 (0.02 M, 17.75±4.15 Myr) is a probable sub-stellar member of the same association, but we can not confirm if it is gravitationally bound to HIP 73145. 2MASS J18152222-3249329 (0.3 M, older than 150 Myr) is determined to be a mildly active Main Sequence (MS) star, much older than members of β-Pictoris Moving Group, and unbound to V4046 Sgr despite their similar kinematic features. PMS wide companions such as 2MASS J15174874-3028484 might have formed through cascade fragmentation of their natal molecular core, hinting at high multiplicity in shorter separations which can be confirmed with future observations. In the third Part, we study bonafide and candidate members of Lupus I dark cloud in detail. We pursue two aims with this study: i) investigating the chance projection of a wide companion candidate of GQ Lup system which its analysis is partly included in this work, ii) finding new, PMS members of Lupus I based on their similar kinematic properties to Lupus I bonafide members - thanks to Gaia DR2. For the first item, we ruled out the chance projection of GQ Lup’s wide companion candidate. Regarding the second item, we submitted a proposal to ESO/VLT for observing 41 candidate members of Lupus I region by X-Shooter which is accepted. For the future works based on the results presented in this work, we would like to: 1. Address a more detailed characterization of the EM counterparts of GW sources detectable by SOXS, such as investigating the possibility of their spectroscopy or extending their photometric exposure times. 2. Analyze the potential new members of Lupus I cloud once the data acquisition is resumed by ESO.