William O'Brien Observatory

The William O'Brien Observatory in Marine on St. Croix, Minnesota.
Photo credit
Cass0114 under CC BY-SA 3.0

The O'Brien Observatory (OBO) was built in 1967 in Marine on St. Croix. It was one of the earliest infrared observatories, built under University of Minnesota astronomer Edward Ney. Whereas other early infrared observatories had to be placed high on mountains to avoid the interference of water vapor, the O'Brien Observatory took advantage of the low dew point of cold Minnesota winters to achieve the same conditions. The namesake of the facility is William O'Brien, a lumber magnate, whose descendant Thomond O'Brien donated the land for the observatory. OBO houses a 30-inch, f/10, Cassagrain reflector telescope which can observe at both optical and infrared wavelengths.

Early Observations and Discoveries at OBO

  • The very first paper from OBO was published by Ed Ney and Wayne Stein, who used a 4 arcminute beam to measure the integrated synchrotron flux from the Crab Nebula at λ= 5800 Å 2.2 μm, and 3.5 μm. It was a continuation of their studies of low surface brightness sources like the Zodiacal Light, but marked a new extension of their interests to more traditional stellar astrophysical topics.
  • Nick Woolf’s involvement led to the classic paper reporting the discovery of thermal infrared emission from circumstellar silicate grains in M stars and carbon grains in C stars.
  • After learning that Stein had detected 3.5 μm emission from the Orion nebula using a 4 arcminute beam, Ney and David Allen discovered the optically thin Trapezium Nebula in Orion.
  • Stein and Fred Gillett used a new UCSD CVFW on the KPNO 36-inch telescope to show that the Trapezium dust had the same 10 μm emission feature seen in the M-supergiant μ Cephei spectrometer.
  • Shortly thereafter, Ray Maas, Ed Ney, and Nick Woolf discovered that a similar 10 μm feature appeared in the spectrum of Comet Bennett 1969i2. Thus, within two years of completing OBO, the UM/UCSD group had established that small carbon and silicate grains, the building blocks of the planets, were ubiquitous in circumstellar winds, regions of star formation, and the debris left over from planet building in the primitive Solar System.
  • In the meantime, by spring of 1969, Bob Gehrz had made a 3σ detection at 10μm on the RV Tauri star AC Her, suggesting that it had the largest infrared excess with respect to the continuum yet detected in a star to that time. Gehrz and Woolf followed up this tantalizing result using a UM bolometer on the 50-inch telescope at Kitt Peak National Observatory, and showed that RV Tauri stars, as a class, had very large excess infrared radiation due to circumstellar dust emission.
  • This discovery provided a PhD thesis for Gehrz. It is now known that the RV Tauri stars occupy an important spot in the HR Diagram and probably are objects in transition between the Post AGB Phase and planetary nebulae.
  • David Allen’s pioneering imaging studies of the lunar surface showed that there were thermal anomalies during eclipses and phasing that could be explained by the fact that large rocks connected deeply to the subsurface layers cooled more slowly than the loosely packed overlying regolith.
  • Murdock and Ney, comparing Allen’s lunar data with photometry of Mercury during its phase cycle, predicted that Mercury’s surface would look like the moon’s long before NASA sent back the first images of the Mercurian surface.
  • Gehrz, Ney, and Strecker discovered that luminous red supergiants as a class (the IC Variable stars) had extensive circumstellar dust envelopes rich in silicates.
  • Gehrz and Woolf subsequently used extensive OBO 4-color 3.6-11.4 μm infrared photometry on many classes of stars to show that it is plausible that mass loss winds can be driven by radiation pressure on circumstellar grains that carry away the gas as well by momentum coupling.