Hubble's Variable Nebula I: ripples on a big screen

NGC 2261 is a reflection nebula illuminated by the young star R Monocerotis. Objects moving near the star occasionally cast shadows on the nebula, giving rise to its alternative name: Hubble’s Variable Nebula. For 7 yr since Spring 2017 robotic telescopes have been used to compile a roughly twice-weekly record of changes in the object. The results, over 1000 images at separate epochs, have been compiled into a movie. This shows that, as well as the large scale but infrequent variability for which it is famous, the nebula is continually traversed by low level ‘ripples’ of light and dark. These record changes in the light output from R Mon and analysis of their progress indicates that the reflecting material takes the form of a thin (< 3 × 10¹⁶ cm) screen whose shape resembles a half paraboloid, rooted at the star and bowed towards us. The brightness of the screen in Herschel far-infrared maps indicates a density n_H > 1.7 × 10⁵ cm⁻³ and CO observations show the material is moving towards us at a few km s⁻¹ relative to the rest cloud, consistent it with being a dense shell of material displaced by R Mon’s outflow. The results demonstrate the value of studying such objects in the time domain, and are a glimpse of what will be achieved by instruments like the Zwicky Transient Facility and Vera Rubin Observatory.