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Steady Twin-Jets Orientation: Implications For Their Formation Mechanism

We compare the structures of the jets of the pre-planetary nebulae (PN) CRL618 and the young stellar object (YSO) NGC 1333 IRAS4A2 and propose that in both cases the jets are launched near periastron passages of a highly eccentric binary system. The pre-PN CRL618 has two `twin-jets' on each side, where by `twin-jets' we refer to a structure where one side is composed of two very close and narrow jets that were launched at the same time. We analyze the position-velocity diagram of NGC 1333 IRAS4A2, and find that it also has the twin-jets structure. In both systems the orientation of the two twin-jets does not change with time. By comparing these two seemingly different objects, we speculate that the constant relative direction of the two twin-jets is fixed by the direction of a highly eccentric orbit of a binary star. For example, a double-arm spiral structure in the accretion disk induced by the companion might lead to the launching of the twin-jets. We predict the presence of a low-mass stelar companion in CRL618 that accretes mass and launches the jets, and a substellar (a planet of a brown dwarf) companion to the YSO NGC 1333 IRAS4A2 that perturbed the accretion disk. In both cases the orbit has a high eccentricity.