Interstellar Matter and Star Formation
Dust and Ice
Only one percent of the mass of the interstellar medium is composed of dust, but this refractory material plays key roles in star, planet, and complex molecule formation. The dust grains absorb light, accrete gas phase atoms that form icy mantles, and they coagulate to larger grains, and eventually pebbles, and larger bodies. Adwin Boogert and collaborators study the dust and ices using infrared spectroscopy at the Keck, IRTF, Spitzer, and future James Webb Space Telescope. Toward dense clouds, they found an enhancement in the reddening at 2 micron, indicative of grain growth by coagulation. This is correlated with ice mantle growth, which makes the grains stickier. The icy mantles were found to have a rich composition quite similar to cometary ices, but the identification of several observed spectroscopic features is still debated. As a co-princiipal investigator of the approved program ‘Ice Age’ at the JWST telescope, Boogert is preparing for observations that will spectroscopically map the dust and ice properties in dense clouds and investigate the link with solar system ices.
Infrared spectrum of a highly reddened star tracing silicate and carbon dust with icy mantles in a dense molecular cloud (black lines). It was observed with the Keck II telescope on Maunakea (λ < 4μm) and the Spitzer Space Telescope (λ > 5μm). The red and green lines are models used to extract the dust and ice signatures. By observing many of such background stars we are able measure dust and ice mantle evolution from quiescent dense clouds to star formating regions.
Star Formation and Orphaned Protostars
Observations from optical to centimeter wavelengths have demonstrated that multiple systems of two or more bodies are the norm at all stellar evolutionary stages. Multiple systems are widely agreed to result from the collapse and fragmentation of cloud cores. Surveys of embedded protostars by Bo Reipurth have revealed a very high multiplicity frequency, and it has even been suggested that all stars could be born in multiple systems. Adaptive optics observations of the slightly older T Tauri stars show a lower binary frequency, a declining trend that continues to the old field population. Newly born multiple systems are dynamically unstable, leading to ejection of members, which then explains the decline in multiplicity with age. However, the details are not well understood, so several programs are underway at IfA to discover and analyze very young multiple systems by using advanced adaptive optics.
Bo Reipurth has been performing numerical N-body simulations revealing that such break-ups of triple systems most frequently occur during the protostellar stage, when the stellar embryos are embedded in their placental cloud cores and are still growing. When an embryo is ejected before it has grown to a mass of 0.08 solar masses, it will forever remain a brown dwarf.
In many cases, protostellar objects are ejected with insufficient momentum to climb out of the potential well of the cloud core and associated binary. These loosely bound companions can travel out of their dense cloud cores to distances of many thousands of AU before falling back and eventually being ejected into escapes as the cloud cores gradually disappear and the gravitational bonds weaken. Such orphaned protostars offer an intriguing glimpse of newborn stars that are normally hidden from view. A number of such orphans have been identified in nearby star-forming regions in the vicinity of deeply embedded protostars, for the first time allowing detailed studies of protostars at near-infrared and even at optical wavelength.
Mike Connelley uses near-IR spectroscopy of Class I embedded protostars to understand their formation. He uses medium resolution broad bandwidth spectra from SpeX to investigate spectroscopic variability on time scales from days to over a decade. We use high resolution spectra with iSHELL to measure the physical properties of these stars, including their gravity and magnetic field strength, to test pre-main sequence evolutionary models and determine if Class I protostars really are younger than the less embedded T Tauri stars.