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Cool stars can form molecules in their atmospheres, which make for interesting spectra. I am interested in this subject for two reasons. The brightest cool stars are known as red giants, and they serve as convenient calibrators, because there are so many of them and they are so bright. Calibration is not the most exciting topic, so I'll come back to it at the bottom of the page. What people find more interesting are the brown dwarfs, those stars with so little mass that they aren't true stars at all. They are sort of a cross between big Jupiters and tiny stars.
My interest in brown dwarfs began on an observing run in 1995 at the United Kingdom Infrared Telescope (UKIRT). The plan was to study something completely different, but my run was with Tom Geballe, then director of UKIRT, and he had just had a special request from Shri Kulkarni at Caltech. They had recently discovered Gliese 229B, had taken a preliminary infrared spectrum of it at Palomar, and wanted us to take a better spectrum of it at UKIRT before they announced their discovery. So we did.
The spectrum of a brown dwarf stands out because it is so cool, molecules like methane (CH4) can survive in its atmosphere. The Sun is too hot for any molecules, but lower mass stars can have some simple molecules. Not only did we verify the previous detection of methane, but we also found absorption from steam (H2O) as well.
The discovery of Gliese 229B was only the start. Since 1995, hundreds of brown dwarfs have been discovered. They even have their own spectral classes now. The warmer ones are known as L dwarfs, and the cooler ones are T dwarfs. Davy Kirkpatrick maintains an excellent archive of all known brown dwarfs. At last count (13 January, 2007), he had 490 L dwarfs and 101 T dwarfs listed.
My interest in brown dwarfs was renewed with the launch of the Spitzer Space Telescope in 2003. For the past five years, I have been working for the Infrared Spectrograph team at Cornell, and one of the science teams we formed has been focusing on brown dwarfs. Jeff Van Cleve christened us the Dim Suns team! In 2004, we published our first results. These were the first mid-infrared spectra ever of brown dwarfs and in them we discovered absorption from ammonia (NH3), another molecule too fragile to exist in normal stars.
Red giants are also cool stars, and they also have interesting molecules in their atmospheres. Red giants are bright and plentiful, and they have served as calibrators for infrared spectroscopy since the beginnings of the field in the late 1960s. Of course, if you are using a star as a standard, your calibration is only as good as your understanding of your standard. That makes it important to understand the spectra of red giants in as much detail as we can.
Geballe, T.R., Kulkarni, S.R., Matthews, K., Woodward, C.E., & Sloan, G.C. 1996, “The near-infrared spectrum of the recently discovered brown dwarf Gliese 229B,” ApJ Letters, 467, L101.
Roellig, T.L., et al. 2004, “Spitzer Infrared Spectrograph (IRS) Observations of M, L, and T dwarfs,” ApJ Supplement, 154, 418.
Cushing, M.C., et al. 2006, “A Spitzer Infrared Spectrograph (IRS) spectral sequence of M, L, and T dwarfs,” ApJ, 648, 614.
Mainzer, A.K., et al. 2007, “Moderate Resolution Spitzer Infrared Spectrograph (IRS) Observations of M, L, and T Dwarfs,” ApJ, 662, 1245.
Last modified 27 May, 2008. © Gregory C. Sloan.