Astrochemistry

Scientific illustration of spectral lines — Spectra of two hot molecular cores embedded in the high-mass star forming region Sagittarius B2 obtained with the ALMA interferometer. Many lines of various complex organic molecules are detected. The red spectra show the models used to identify these molecules.

© Arnaud Belloche, MPIfR

Spectra of two hot molecular cores embedded in the high-mass star forming region Sagittarius B2 obtained with the ALMA interferometer. Many lines of various complex organic molecules are detected. The red spectra show the models used to identify these molecules.

© Arnaud Belloche, MPIfR

Molecules allow us to probe the physical properties of the interstellar medium, such as its temperature, density, and kinematics, or the radiation field and cosmic rays that impinge on it. They also reveal the chemical composition of astronomical environments, in particular star forming regions. This chemical composition is a powerful tool to probe the history of these regions and reveal their evolutionary stage. Studying this chemical composition across the phases of star formation tells us about the inheritance or reprocessing of the outcome of interstellar chemistry from the earliest stages (molecular cloud cores prior to the formation of stars) to the latest ones (circumstellar disks where planets form and life may appear).

Our group focuses in particular on the protostellar phase when complex organic molecules, that is carbon-bearing molecules consisting of at least six atoms, desorb from the surface of icy dust grains and reveal, through their rotational emission, so called hot cores and hot corinos. We employ mm/submm single-dish telescopes such as APEX or the IRAM 30m telescope and interferometers such as ALMA or NOEMA to perform imaging spectral line surveys of these chemically rich young stellar objects, both in the inner and outer Galaxy. We investigate in particular the impact of metallicity on the chemical composition of these star forming regions.

Links to projects:

The ALCHEMI survey

Mapping of molecular lines — Velocity-integrated maps of several lines of HCO⁺, HOC⁺ and H¹³CO⁺ in NGC253 obtained with the ALCHEMI project (Nanase et al. 2021). The HOC⁺ emission is found to be a good tracer of super bubbles, created by either supernovae or expanding HII regions. Its abundance with respect to HCO⁺ is boosted in regions dominated by cosmic-rays and/or UV photons, sources of starburst energy feedback.

© H. Nanase 2021

Velocity-integrated maps of several lines of HCO⁺, HOC⁺ and H¹³CO⁺ in NGC253 obtained with the ALCHEMI project (Nanase et al. 2021). The HOC⁺ emission is found to be a good tracer of super bubbles, created by either supernovae or expanding HII regions. Its abundance with respect to HCO⁺ is boosted in regions dominated by cosmic-rays and/or UV photons, sources of starburst energy feedback.

© H. Nanase 2021

The large program ALCHEMI used ALMA to image the Central Molecular Zone (CMZ) of the iconic starburst galaxy NGC253 in a nearly contiguous 289 GHz frequency range between 84 and 373 GHz, with a 1.6" angular resolution (28 pc) and a sensitivity of 30-50 mK.

Results already published from this unprecedented high-resolution and high-sensitivity extragalactic project are the first extragalactic detections of some Complex Organic Molecules (COMs), such as C₂H₅OH, HOCN, and HC₃HO, newly detected vibrationally excited lines of HCN, HNC and HC₃N originating in the hot cores of star-forming Giant Molecular Clouds (GMCs) (Martin et al. 2021), the starburst energy feedback measured by the HCO⁺/HOC⁺ ratio (Harada et al. 2021), and the distribution and origin of C2H, a tracer of photon dissociation regions (PDRs) and high cosmic-ray ionisation rates (Holdship et al. 2021).

Many other studies are currently being developed as part of the ALCHEMI project, which will virtually cover all physical and chemical aspects of this galaxy, as well as its comparison to the Milky Way properties. ALCHEMI also offers a unique template for comparing it to star-forming galaxies in the early Universe.