Overview of Molecular Astrophysics and Star Formation

All stars, as far as we know, are born from the gravitational collapse of the core of a molecular cloud.

Molecular clouds are regions of relatively dense interstellar gas and dust that can shield their contents against the destructive ambient ultraviolet (UV) radiation field. In such a cold, protected environment the predominant form of matter, atomic hydrogen, preferentially associates into molecular hydrogen, or H₂. However, owing to its simplifying symmetry, cold H₂ has no emission line spectrum and therefore remains essentially invisible. However other atomic species, although far less abundant, also associate into common molecules like CO₂, H₂O, HCN, and so forth, and for the most part glow profusely at microwave radio frequencies and are used as "tracers", or "placeholders" for the otherwise invisible H₂ molecule. In fact, over 118 molecules have been detected in dense molecular clouds, some as complicated as the amino acid glycine. This is a stunningly interesting point in regards to the development of life. Consider: if amino acids can form in "empty" space, and amino acids are the building blocks of proteins, and proteins are the building blocks of DNA, and DNA is the building block of life as we know it, is it so far-fetched to imagine the possibility of life elsewhere? Or, stated a different way, it appears that the most fundamental physical processes that serve as necessary conditions for the formation of life on Earth appear to happen elsewhere, and maybe everywhere.

A millimeter-wavelength spectrum of the core of the Orion giant molecular cloud, made at the Owens Valley Radio Observatory. The spectrum covers a small interval in the atmospheric window at 1.3 mm. Over 800 spectral features are seen! This type of data is needed to understand the chemistry of cold and dense interstellar regions.

Much data has been gathered on molecular clouds in relation to their ability to form stars, mostly via detection of the molecular emission lines at (sub)millimeter wavelengths. In these clouds, molecules radiate like little microwave radio transmitters as they spontaneously change rotational energy levels. For example, the carbon monoxide molecule (CO) is the most abundant molecule after H₂. The first rotational excited state lies only 5 degrees Kelvin (using temperature as a synonym for energy) above the ground state, and therefore is readily excited by the ambient cosmic microwave background radiation or collisions with neighboring molecules (usually H₂, since it's 10⁴ times more abundant than even CO). When the CO molecule drops back to the ground state, it gives off a photon of light, in an effort to conserve energy. Because the difference in energy levels is so small, the photon emitted carries away a small amount of energy. For this particular transition in CO, the wavelength of the photon emitted is around 2.6 millimeters, or 115 GHz, in the microwave (radio) portion of the spectrum. This is 1000 times higher in frequency (energy) than what you receive with your FM radio.

Molecules can not only change electronic energy levels like atoms, but also can vibrate and rotate. Each of these new degrees of freedom complicates the spectrum substantially. A spectrum of a nearby star-forming cloud like the Orion Nebula can have thousands of detectable molecular spectral lines even within a narrow range of observed frequency! This is not a nightmare best forgotten, but rather provides a unique diagnostic tool. Each transition of each molecule probes diffeent physical conditions within the cloud -- each spectral line tells a different story, a different perspective. Putting the chorus together remains our best hope for disentangling the complicated physical and chemical structure of molecular clouds, and understanding the initial conditions, or "seeds" of star formation.

In spite of over 30 years of observing molecular clouds using CO as a tracer for the otherwise invisible H₂ molecule, there are still many fundamental questions which remain unanswered:

• What is the physical and chemical structure of a molecular cloud? i.e. what do they really look like?
• How are molecular clouds formed? Where do they come from? What are their evolutionary stages? How do they "die"? How long do they live?
• What is the actual H₂ content of molecular clouds?
• What are the initial conditions for forming stars? There are a number of dense cloud cores in the Taurus molecular cloud complex, about 140 parsecs away. About 3% of the cores are forming stars. Why are they forming stars and the other 97% aren't?
• What is the evolution of a star-forming cloud, physically and chemically? How can we determine the evolutionary state of a young protostar or collapsing cloud? How does the collapse and fragmentation process proceed, and what dictates the mass of the emergent star?
• What is the efficiency of star formation? That is, what fraction of molecular mass is converted into stars? What is the mass spectrum of stars formed from a giant molecular cloud?
• How do newly formed stars alter the cloud from which they formed? The standard picture of star formation considers a single low-mass star forming in isolation. But most stars are formed in clusters. How does clustered star formation differ from the isolated case?
• Can clustered star formation be "triggered" by a nearby violent event, like a supernova explosion or passing of an interstellar shock wave?
• How do we tie the evolution of a molecular cloud to the evolution of a galaxy as a whole?
• How do molecular clouds and star-forming regions evolve at large lookback times, at high redshifts, when galaxies were just forming? Are they analogous to local molecular clouds, or are other processes relevant?