Takeshi Oka
University of Chicago
The Ubiquitous H3+, from Giant Planets to Star Forming Regions
Although it took many years from the discovery of the laboratory
spectrum1 of H3+ and its first search in
interstellar space2 to its detection,3 subsequent
observations have shown surprising ubiquity of this molecular ion. H3+
has been detected not only in dense clouds where its presence had been predicted
by model calculations, but also in diffuse clouds4 where its
abundance was not anticipated. The recent advent of large diameter telescopes
equipped with high resolution-sensitivity spectrometer with wide wavelength
coverage is causing an avalanche of H3+ observation in a
wide variety of astronomical objects.
Because of cosmic ray ionization of H2, followed by the
efficient reaction H2+ + H2 H3+ + H,
H3+ is produced wherever H2 abounds, and acts
as the universal proton donor (acid) initiating myriads of chain chemical
reactions. It was first observed towards infrared stars that are deeply
embedded in dense molecular clouds with high extinction (Av ~ 100)
but later towards brighter stars including bright constellation stars with low
extinction (Av ~ 1). The recent detection toward
z Persei is particularly noteworthy.5
The strongest spectrum of H3+ is observed
toward bright infrared sources near the Galactic center. The observations
samples the local core clouds as well as several clouds in intervening spiral
arms and produce a rich spectrum6. Some high temperature clouds show
presence of H3+ in a high energy metastable rotational
level. An emission spectrum from a gas giant protoplanet7 and an
absorption spectrum in an obscured active galactic nucleus8 have also
been reported.
I shall discuss the observed spectra and related physics of H3+.
1
Observation of the Infrared Spectrum of H3+, T.
Oka, Phys. Rev. Lett. 45, 531 (1980)
2 A Search for Interstellar H3+, T. Oka,
Phil. Trans. R. Soc. Lond. A303, 543 (1981)
3 Detection of H3+in Interstellar Space,
T. R. Geballe and T. Oka, Nature 384, 334 (1996)
4 Observation of H3+in the Diffuse Interstellar
Medium, B. J. McCall et al. ApJ 567, 391 (2002)
5 An enhanced Cosmic-Ray Flux towards
z Persei inferred from a Laboratory Study of the H3+-e-
Recombination Rate, B. J. McCall et al. Nature 422, 500 (2003)
6 Aborption Line Survey of H3+ toward the
Galactic Center Sources I. GCS 3-2 and GC IRS3, M. G M.
Goto, et al. PASJ 54, 951 (2002)
7 CO and H3+ in the Protoplanetary Disk around
the Star HD 141569, S. D. Brittain and T. W. Rettig, Nature 418,
57 (2002)
8 Detection of H3+ in the Interstellar Medium of
IRAS 08572+3915, T. R. Geballe, Astro-ph/0109166 (2001)