Discovery of a Symmetrical Highly Collimated Bipolar Jet in Hen 2-90

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Abstract
Using the Hubble Space Telescope, we have obtained Hα imaging of the object Hen 2-90, which has long been classified as a planetary nebula (PN). We find that the morphology of Hen 2-90 does not look like that of any known PN, but resembles that of a classical young stellar object (YSO)—a bipolar nebula bisected by a flaring disklike structure and a highly collimated bipolar jet perpendicular to the disk. The linear jet shows at least six pairs of emission knots located symmetrically on either side of the nebular center. Taking a kinematic distance of 2.5 kpc, we find that the gas density in the knots decreases steadily from about 104 cm-3 in the knots closest to the center to 1.1 × 103 cm-3 in the more distant knots, and their masses lie in the range of (0.7-3.6) × 10-6 M☉. The jet opening angle is about 4°, from which we estimate its speed to be ~150 km s-1. Hen 2-90's near- and mid-infrared fluxes imply the presence of a massive dusty nebula containing "warm" (183 K) and "hot" (513 K) dust in components with masses 5 × 10-3 and 2.4 × 10-5 M☉, respectively (assuming a gas-to-dust ratio of 100); the source luminosity is 5280 L☉. Millimeter-wave line observations show no molecular gas directly associated with the source and the absence of star-forming activity, indicating that Hen 2-90 is probably not a YSO. The most likely hypothesis for explaining Hen 2-90 requires a binary with a cool giant and a compact companion with an accretion disk.

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