John Barentine and Gilbert Esquerdo (University of Arizona undergraduate students), were granted time on the KPNO Coudé Feed Telescope to measure the velocity of Barnard's Merope Nebula. Prior observations had suggested that the nebula is virtually at rest relative to Merope, implying that it was formed as part of the cluster. The students' goal was to obtain new velocity measurements to determine if the Merope Nebula is associated with the Pleiades. Their results suggest instead that it may be associated with the passing molecular cloud that is responsible for the Pleiades reflection nebula.
The Pleiades is a spectacular star cluster. The wispy reflection nebula surrounding the cluster is made bright by the light of the luminous, young, cluster B stars well known to naked eye observers. But the nebula, which so enhances the appearance of the Pleiades is just a chance encounter between the stars and a passing molecular cloud, perhaps a fragment of the nearby Taurus-Aurigae nebula lying east and south of the star cluster.
In 1891, E. E. Barnard visually identified a small knot of condensation within the Pleiades nebulosity, just 36" south-southeast of the bright star Merope. The knot is some 15× brighter than the brightest areas of the reflection nebula, and has a diameter of just a few arc seconds, with two arms, about 10'' long, directed away from Merope (see the Figure). The knot is known as "Barnard's Merope Nebula" and was later cataloged as IC 349.
Observation of IC 349 is difficult because of Merope's brightness. The nebula was not even visible on the TV guiding camera. Nevertheless, Barentine and Esquerda obtained 5 one-hour integrations of the nebula, as well as a high signal-to-noise spectrum of Merope itself. A 15" slit was used to allow subtraction of scattered light from the star. They also obtained a second spectrum at a similar distance from Merope, but at a position angle 180º away from the Merope Nebula, to investigate the contribution of instrumentally scattered light. The radial velocity of the nebula was determined by cross-correlation, using the spectrum of Merope as a template.
The new velocity measurement suggests that the Merope Nebula is not associated with the Pleiades cluster. Barentine and Esquerda were also able to calculate the space motion of the Nebula, using proper motions recently measured by B. Jones (Lick Observatory). The Merope Nebula is not only kinematically distinct from the Pleiades cluster, but also from T Tauri stars in the nearby Taurus-Aurigae clouds.
Using L' observations obtained with the Diffraction Limited Infrared Imager on the Mayall 4-m Telescope, the students were also able to set an upper limit to the luminosity of any object embedded in the nucleus of the Merope Nebula. The absence of infrared flux and also of emission lines in the optical spectrum suggests that the Merope Nebula is not hiding a protostar. The nebula may just be a large clump of dust, and its morphology may result from radiation pressure from nearby Merope.
Barnard described the Merope Nebula as "one of the most singular objects in the heavens." It has certainly proved to be fertile ground for two young astronomers at the beginning of their careers. A paper by Barentine and Esquerda will soon appear in the Astronomical Journal.