  The general theory of relativity predicts the relativistic effect in the
orbital motions of S-stars which are orbiting around our Milky-way galactic
center. The post-Newtonian or higher-order approximated Schwarzschild black
hole models have been used by GRAVITY and UCLA galactic center groups to
carefully investigate the S2 star's periastron precession. In this paper, we
investigate the scalar field effect on the orbital dynamics of S2 star. Hence,
we consider a spacetime, namely Janis-Newman-Winicour (JNW) spacetime which is
seeded by a minimally coupled, mass-less scalar field. The novel feature of
this spacetime is that one can retain the Schwarzschild spacetime from JNW
spacetime considering zero scalar charge. We constrain the scalar charge of JNW
spacetime by best fitting the astrometric data of S2 star using the
Monte-Carlo-Markov-Chain (MCMC) technique assuming the charge to be positive.
Our best-fitted result implies that similar to the Schwarzschild black hole
spacetime, the JNW naked singularity spacetime with an appropriate scalar
charge also offers a satisfactory fitting to the observed data for S2 star.
Therefore, the JNW naked singularity could be a contender for explaining the
nature of Sgr A* through the orbital motions of the S2 star.
