Charles T. Troy
YORKTOWN HEIGHTS, N.Y. -- Two IBM scientists, calling on their years of expertise in lasers, believe they have solved the missing starlight mystery -- a.k.a. the diffuse interstellar bands, or DIBs.
Astronomers have long known that light from many young stars exhibited less intensity than expected in portions of the spectrum and have attributed this to some mysterious interstellar material.
For years researchers tried, without luck, to match the missing DIBs' spectra with materials that were, or could be, in outer space. Now the scientist-sleuths Peter Sorokin and James Glownia blame the spectra theft on hydrogen.
DIBs, they say, are caused by dual optical effects: Rayleigh scattering and complex nonlinear interactions related to those that make lasers possible. The process allows hydrogen molecules in clouds near bright stars to absorb visible light from the stars at specific frequencies that exactly match portions of DIBs' spectra patterns.
Trapped in the clouds
Specifically, DIBs can occur when sheet-like hydrogen clouds lie within 30 light-years of very bright stars along a line of sight to Earth. While most light passes through the clouds, some vacuum ultraviolet light (VUV) near absorption wavelengths that correspond to molecular-hydrogen excited-state transitions are trapped in the cloud by Rayleigh scattering. The intensified nearly resonant VUV light combines with certain visible starlight wavelengths and excites the hydrogen molecules in simultaneous two-photon events.
In their detective work, the scientists began analyzing studies of light-absorbing properties of excited states of hydrogen. Eventually they found that in at least 70 instances, the color of the visible light needed to reach a quantum-mechanically permitted excited state of hydrogen matched one of the 130 known DIBs, including two of the strongest.
The researchers noted that for the DIB at 5797 Å the absorption is far greater than the VUV part of the assigned two-photon transition would indicate. "If our basic model is correct, there has to be some supercharging mechanism that would greatly enhance the VUV associated with the 5797-Å DIB," Sorokin said.
In the scientists' view, two other nonlinear processes, stimulated Raman scattering and four-wave parametric oscillation (FWPO) appear to provide such a mechanism.
In addition, FWPO offers an explanation for the unusual emission seen in the Red Rectangle, a star-driven emission nebula in the Milky Way consisting of two conical gas shells pointing in opposite directions. From Earth, it appears X-shaped, but in early overexposed photographs it appeared rectangular, hence the name. The rectangle is the only such system where DIBs are seen in emission, rather than absorption.