MPIfR Molecular jet simulation with a large precession angle and its application to emission from NGC 7538 IRS1

Recent high resolution bispectrum speckle interferometric observations in the near-infrared have revealed that the area surrounding NGC 7538 IRS1 is associated with a fan-like region that contains several stars and regions of diffuse emission. An interpretation of the speckle, CO, radio continuum, and methanol maser data, which shows the current position angle of the protostar’s accretion disk on the sky, is of a jet with a large precession angle. Here, we present a molecular jet simulation with a similarly wide precession angle. We analyze the simulation for properties associated with the flow and the calculated emission. We can reproduce the “older” average position angle for the CO emission, compared with a more recent value from molecular hydrogen (i.e., shocked gas) emission (which we use as a proxy for the K’-band emission). Thus, the model is consistent with the interpretation that a precessing jet is responsible for much of the molecular emission (covering different emission lines) in the IRS1 region. The Source and A Model IRS1 is a massive (30 M ) protostar which is associated with an ultracompact (UC) H ii region and a CO outflow. Remarkably, at the position of IRS1, a group of linear-aligned methanol masers has been detected, which most likely trace a Keplerian-rotating circumstellar disk (Pestalozzi et al. 2004). Examination of the data reveals a misalignment between the outflow direction expected from the orientation of the methanol maser disk and the other outflow tracers, which can be interpreted in the context of a disk precession model. A simple analytic precessional model for the S-shaped morphology in the K’-band images yields a best fit with precession period of 280 years and a precession half-angle of 40◦ (see Fig. 2). These values put strict constraints on possible precession mechanisms. After considering several mechanisms, we identify tidal interaction with a companion as most plausible. The short precession period implies a non-coplanar orbit which is causing the circumbinary disk to precess and maybe to warp (Larwood et al. 1997). A more complete analysis, including additional observational data, will appear in a manuscript (Kraus et al., in preparation). Figure 1: Speckle image reconstructed from MMT data with a resolution of 334 mas. The inset in the upper left corner shows a diffractionlimited image (73 mas, SAO-data) of IRS2, around which we discovered a companion (separation 195 mas). In the lower left, IRS1 is shown, emphasizing the elongation of the IRS1 Airy disk overplotted with the 15-GHz radio continuum and the position of the OH (circles) and methanol (crosses) masers (from Hutawarakorn & Cohen 2003). In the lower right, we show the integrated brightness of the methanol masers in order to stress the misalignment of the suspected maser disk with the outflow direction. Figure 2: MMT-Speckle image overplotted with the trajectory of ejecta from a precessing outflow projected onto the plane of the sky (thick blue line) in a simple analytic model. The precession period is 280 years and the half-opening precession angle is 40◦. The thin blue lines show the full extent of the projected trajectory when both the average position angle and the viewing angle (in the plane of the sky) for the precession axis is modified by ±5◦. The red contours show the 15-GHz radio continuum map by Hutawarakorn & Cohen (2003). Observations Initial observations were performed on 2002-09-24 using the 6m Special Astrophysical Observatory (SAO) telescope located on Mt. Pastukhov in Russia. Additional data was gathered on 2004-12-20 with the 6.5m Multiple Mirror Telescope (MMT) on Mt. Hopkins, Arizona. Images were reconstructed from the obtained speckle interferograms using the bispectrum speckle interferometry method (Weigelt 1977; Lohman et al. 1983). Detected NIR IRS1 Outflow Structure Extending from IRS1, we detect a diffuse fan-shaped structure with an ∼90◦ opening angle, which we interpret as scattered light from an outflow cavity excavated by strong outflow activity from IRS1. Contributions from the H2 1-0 S(1) (2.112 μm) line are also likely. Eighteen fainter stars and several blobs are embedded within this fan-shaped structure. The arrangement of the diffuse blobs suggests an S-shaped morphology. NGC 7538 IRS1 Maser Feature A The massive protostar IRS1, located at a distance of ∼ 2.8 kpc, is associated with an UC H ii region and is also the suspected driving source of a bipolar CO outflow. In 1998, Minier et al. reported the discovery of linear-aligned methanol masers at the position of IRS1 and interpreted them as an edge-on circumstellar disk. The accuracy of the alignment and the stunning precision with which the position-velocity diagram of these masers could be modelled makes the maser feature NGC 7538 IRS1-A one of the most intriguing candidates for Keplerian-rotating protostellar disks (see Pestalozzi et al. 2004). Therefore, for the following interpretation we assume that maser feature A resembles a circumstellar disk, mentioning that an alternative scenario was presented by De Buizer and Minier (2005) which suggests that the methanol masers might trace an outflow cavity. Indications for Disk and Jet Precession It is most remarkable that the maser disk is not perpendicular to the largescale outflow tracers (CO, H2, fan-shaped outflow cavity) but misaligned by ∼ 60◦. On scales 1′′, a bending (∼ 25◦) of the UC H ii region can be conceived (Campbell 1984), which seems to continue in the S-shaped morphology in our K’-band images. Based on these indications for a systematic change in the outflow direction, we suggest jet precession as one possible explanation. a) unconvolved b) convolved Figure 3: Integrated molecular emission line maps from the simulation for φ = 0◦. The images on the right are convolved with a Gaussian beam with σ = 6 zones.