William O'Brien Observatory
The O'Brien Observatory (OBO) was built in 1967 in Marine on St. Croix. It is one of the first infrared observatories, built under University of Minnesota astronomer Edward Ney. Whereas other infrared observatories have to be placed high on mountains to avoid the interference of water vapor, the O'Brien Observatory takes 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.
Any instrumentation used at the O'Brien and MLOF observatories must be user-supplied. The interface is a 16-inch diameter mounting plate with three 1/2-inch NFT bolt holes spaced at 120 degrees on a 13 5/8-inch diameter circle. A liquid helium cooled GaGe bolometer can be made available through collaboration with the MIfA infrared (IR) Group. Operations at these observatories are conducted by the MIfA IR Group.
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.
- Ney and Stein, 1968. Observations of the Crab Nebula at λ= 5800 Å 2.2 μ, and 3.5 μ with a 4-MINUTE Beam.
- 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.
- Woolf and Ney, 1969. Circumstellar Infrared Emission from Cool 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.
- Ney and Allen, 1969. The Infrared Sources in the Trapezium Region of M42.
- 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.
- Gillett and Stein, 1969. Detection of the 12.8-micron Ne⁺ Emission Line from the Planetary Nebula IC 418.
- 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.
- Maas, Ney, and Woolf, 1970. The 10-Micron Emission Peak of Comet Bennett 1969i.
- 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.
- Gehrz and Woolf, 1970. RV Tauri Stars: A New Class of Infrared Object.
- 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.
- Gehrz, 1971. Infrared Radiation from RV Tauri Stars.
- 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.
- Allen, 1971. Infrared Studies of the Lunar Terrain: The Background Moon.
- Allen, 1971. Infrared Studies of the Lunar Terrain: Thermal Anomalies.
- 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.
- Murdock and Ney 1970. Mercury: The Dark-Side Temperature.
- 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, Ney, and Strecker, 1970. Observations of Anomalous Radiation at Long Wavelengths from Ic Class Variables.
- 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.
- Gehrz and Woolf, 1971. Mass Loss from M Stars.