Outflows from the high-mass protostars NGC 7538 IRS1/2 observed with bispectrum speckle interferometry. Signatures of flow precession

Abstract

Context. NGC 7538 IRS1 is a high-mass (30 M) protostar with a CO outflow, an associated ultracompact H  region, and a linear methanol maser structure, which might trace a Keplerian-rotating circumstellar disk. The directions of the various associated axes are misaligned with each other. Aims. We investigate the near-infrared morphology of the source to clarify the relations among the various axes. Methods. K -band bispectrum speckle interferometry was performed at two 6-meter-class telescopes – the BTA 6 m telescope and the 6.5 m MMT. Complementary IRAC images from the Spitzer Space Telescope Archive were used to relate the structures detected with the outflow at larger scales. Results. High-dynamic range images show fan-shaped outflow structure in which we detect 18 stars and several blobs of diffuse emission. We interpret the misalignment of various outflow axes in the context of a disk precession model, including numerical hydrodynamic simulations of the molecular emission. The precession period is ∼280 years and its half-opening angle is ∼40◦. A possible triggering mechanism is non-coplanar tidal interaction of an (undiscovered) close companion with the circumbinary protostellar disk. Our observations resolve the nearby massive protostar NGC 7538 IRS2 as a close binary with separation of 195 mas. We find indications for shock interaction between the outflow activities in IRS1 and IRS2. Finally, we find prominent sites of star formation at the interface between two bubble-like structures in NGC 7538, suggestive of a triggered star formation scenario. Conclusions. Indications of outflow precession have been discovered to date in a number of massive protostars, all with large precession angles (∼20–45◦). This might explain the difference between the outflow widths in low- and high-mass stars and add support to a common collimation mechanism.