By José Pablo Fonfría, Ed Montiel & Joan Schmelz
The ability of atoms to combine and form molecules is perhaps the most fundamental process leading to the evolution of life on Earth. This “molecular lifecycle” starts in space where giant molecular clouds gave birth to the first stars. As these stars approach the end of their lives, they swell to hundreds of times their initial size and shed their outer envelopes. As this circumstellar material cools, it creates an environment favorable to the formation of molecules. Mixing of these enriched envelopes with the interstellar medium enables the next generation of stars to form with the necessary ingredients for planets and possibly life.
The rotational spectra of most molecules are detectable with (sub)millimeter telescopes (e.g., SOFIA, IRAM 30m telescope, GBT, APEX) and interferometers (e.g., ALMA, SMA, NOEMA). These signals enable astronomers to determine abundances, temperature, and density, fundamental parameters required to model the environment of the emitting gas. However, symmetric molecules do not show rotational spectra and can, for the most part, only be observed in the infrared range. Examples include H 2 , which accounts for most of the molecular mass in interstellar space, carbon dioxide, methane, and acetylene, all of which play important roles in astro-chemistry. Hence, observing their spectra helps us understand the evolution of stars, the Universe, and life itself.
The sensitive, powerful instruments of JWST are already unveiling previously hidden astrophysical mysteries, but the science might be limited by the relatively low spectral resolution. This is a significant impediment in infrared astronomy because many molecules vibrate in a similar way so their spectral bands overlap. Consequently, molecules with the weakest bands, which are usually from the least abundant molecules, will be overlooked in low-spectral resolution observations. In this respect, the EXES instrument on SOFIA is a valued partner for JWST. Its high-spectral resolution spectroscopy can separate the lines of the vibrational bands and reveal the weak signals of low-abundance molecules that would otherwise be hidden among a forest of stronger lines from other species.
The EXES Legacy program was designed to create a library of these weaker lines so they could be incorporated into the JWST spectral fitting software, substantially enhancing the scientific value of the JWST molecular observations. Initially more ambitious, the program was tailored (due to the end of SOFIA mission) to include the three most promising sources: the hot cores, NGC 7538 IRS1 and AFGL 2136, as well as the asymptotic giant branch star IRC+10216. The observations included most of the mid-infrared range (from 5 to 28 µm) with a spectral resolution better than 6 km/s. This high spectral resolution can not only isolate molecular lines but also resolve their profiles making it possible to analyze in detail the gas kinematics in the layers of the emitting molecules.
Analysis of the IRC+10216 observations is already underway. IRC+10216 is a carbon-rich, evolved star located 120 pc from Earth with a dense circumstellar envelope composed of dust grains and molecular gas. To date, there are 298 different molecular species detected in space, and 95 of them are seen towards IRC +10216. Of particular importance are the spectral bands around 6.3 µm and 15.0 µm, which are unobservable by ground-based telescopes due to the presence of atmospheric water and carbon dioxide, respectively. Likewise, the short- and long-wavelength edges of the covered range were barely explored in the past and could reveal previously undetected molecules.
A detailed analysis of the thousands of molecular lines in the observed spectral range is on-going. Many lines, primarily beyond 7 µm, show P-Cygni profiles (features with emission and absorption components) that belong to low-excitation, vibrational bands of the so-called parent molecules that form at the stellar photosphere or very close to it. The rest of the lines are seen only in absorption and are either produced by molecules that form in the intermediate and outer layers of the envelope, or are lines of highly excited vibrational bands.
The observations of the EXES Legacy program will help improve our understanding of the spectroscopically rich mid-infrared region. In addition, the identification and analysis of these lines is crucial to refine the circumstellar chemistry models that will help interpret JWST data and increase our understanding of how matter behaves in the astrophysical environments where molecules form.