Hubble's Variable Nebula
Glenlea Astronomical Observatory, University
A brief summary of current knowledge
From previous studies
From this study
R Mon is a massive Herbig Ae/Be star surrounded by an optically thick accretion
disk. It varies by more than 4 magnitudes in brightness.
Strong polar winds collimate at P.A. 350 degrees to clear a narrow "throat"
through the accetion disk. This throat is about 75 AU wide (0.1") and 45
AU north of R Mon and is the source of most of R Mon's light in the visible
Infalling envelope material is collimated into long filaments which are
carried northward by this wind.
This ejected material has formed the reflection nebula NGC 2261.
The nebula is thought to be a thin (<800 AU) walled parabolic shell.
Its major axis is aligned along P.A. 350 degrees and inclined 20 degrees
toward us. R Mon is at its base.
Visible and near infrared light escaping through the "throat" illuminates
the walls of the nebula. The walls are transparent at wavelengths greater
than 4 microns.
Rotating steamers of dust, formed 1-2 AU from R Mon, cast shadows onto
the walls of the nebula as they rise through the "throat".
- The proper motion velocities of some shadows exceed the speed of light.
The bright part of the nebula (all that can be easily seen in most of the
images at this site) is approximately 0.2 light-years wide (east-west) and
0.4 light-years long (north-south).
The nebula continues to change in brightness over a period of days and
weeks. This includes
- movement of well defined dark areas or "shadows" across the face
of the nebula
- large, less defined regions of the nebula that change in brightness
over several weeks
- a region several arcseconds north of R Mon that changes in brightness
and structure frequently
From image subtraction, it was found that the shadows move away from and/or
at right-angles to R Mon in a counterclockwise direction. This is the same
direction as seen in the Lampland images, however, the shadow transit seen
in this study had much greater northward velocity than the shadow transits
seen in the Lampland series.
Large parts of the nebula did not change during this study, especially
the fainter northern parts. In particular, images taken 50+ years ago show
many of the same features seen today.
Shadow transit structure suggests that the shape of the reflection nebula
is more complex than a simple parabolic shell.
From image subtraction, highly structured changes in brightness were detected
over large, irregularly illuminated areas. This suggests that the source
of the shadows is also highly structured.
The following links provide an overview of changes observed in the nebula.
Individual events are discussed below.
Overview of changes in the nebula
Analysis of changes
Effective wavelength of the GAO images
Shadow transit event
Analysis of the torus-like feature north of R Mon
Brightness changes immediately north of R Mon
Parts of the nebula that did not change
Analysis of historical images and changes
Persistant and reappearing structures in the nebula
Differences between current changes and historical changes
Required nebula structure based on observed shadow movement
Last modified April 18, 1999