Research Interests:
I study astrochemistry, the unified frontier of chemistry and astronomy. Chemistry—the science of atoms, molecules, and matter—and Astronomy —the science of stars, galaxies, and the Universe ‒ are related in two fundamental ways. First, the nuclei of C, N, O, and other heavy atoms, which make chemistry so rich, are produced in the cores of stars, the only place with sufficiently high temperature and density for nuclear fusion. The production of elements, their relative abundances, and their transport in galaxies are the results of the evolution of stars and explained by nuclear astrophysics. Second, stars are born from the gravitational condensation of molecular clouds, and molecules are indispensable as the coolant of the adiabatic process. Stars wouldn’t form without molecules, and molecules wouldn’t form without stars—it’s like the chicken and the egg.
Probing the Central Molecular Zone of the Galactic Center with H3+
The infrared spectrum of H3+, initially discovered in dense molecular clouds (Geballe & Oka, 1996 Nature 384, 334) has recently emerged as a powerful probe to study the Central Molecular Zone (CMZ), a region of a radius ~ 150 pc at the Galactic center (GC). The GC with the supermassive black hole at the center is astrophysically the most active area in the Galaxy. H3+ is very abundant in the CMZ; the region, which is a mere ~ 10-5 in volume, contains most of H3+ in the Galaxy.
We observe the 3.7 μm infrared spectrum of H3+ using spectrometers mounted on large (8 m) reflecting telescopes on high mountains in the Chilean Andes and on Mauna Kea Hawaii. The extremely simple chemistry and the quantum mechanics of H3+ allow us to reliably determine properties of the environment of the CMZ such as temperature, density, and ionization rate. Our investigations are radically changing the basic concept of the GC.
H3+, like H3O+, is an acid – a proton donor. It is a stronger acid than H3O+ because of the much lower proton affinity of H2 (3.39 eV) than H2O (7.22 eV). H3+ is the most important molecule in astrochemistry because it donates a proton to neutral atoms and molecules and initiates chains of reactions. Without H3+, molecules are not produced, stars do not form, and we are not here.
Solving the Mystery of the Diffuse Interstellar Bands
The Diffuse Interstellar Bands (DIBs) are a set of > 500 absorption bands mainly in the visible first observed more than 100 years ago. The reproducibility and intricate profiles of some DIBs indicate they are caused by molecules. The identity of these molecules had been a mystery until 2015, when five DIBs were shown to be caused by C60+.
In 2011 a DIBs team led by Don York of the University of Chicago made a spectacular discovery that some DIBs toward the star Herschel 36 have drastically different profiles from DIBs toward hundreds of other stars. Using this result as a Rosetta stone, I developed a
theoretical calculation and concluded that the carriers of those DIBs are polar carbon chain molecules with ~ 6 carbon atoms. I ascribe the enormous difference between the spectral profiles toward Herschel 36 and all other stars to radiative pumping dueto high far infrared flux from a nearby star. I work with undergraduate students to apply my theory for explaining DIBs with intricate structures and emissions from the Red Rectangle nebula.