Astrochemistry in high-mass star-forming regions

The cradles of massive stars are among the most chemically rich sources in the Galaxy. These high-mass star-forming regions usually harbour cores and young stellar objects in different evolutionary stages, from embedded infrared dark clouds (IRDCs), associated with prestellar cores or with very young stellar objects, to hot molecular cores (HMCs), hosting protostellar objects, to the more evolved (ultra-compact) HII regions (UC HIIs), developed by a young star ionizing the surroundings. The typical masses of these star-forming cores (few hundreds of solar masses) make them the most important reservoir of complex organic molecules (COMs), including key species for prebiotic processes. This rich chemistry is thought to be the result of the evaporation of dust grain mantles by the strong radiation of the deeply embedded early-type star(s). Our own Sun may have been born in a high-mass star-forming region, so our Earth may have inherited the primordial chemical composition of its parental hot core region, as suggested by recent studies of oxygen, sulfur and phosphorus chemistry in comets (see Fig. 1).

The Arcetri group has been studying the chemistry of high-mass star-forming regions for many years. This effort has led them to carry out important discoveries in the field, in particular associated with chemical evolution, the emission of prebiotic molecules, and fractionation of species.

Figure 1: Observations using the ALMA interferometer and the ROSINA instrument onboard the ESA Rosetta spacecraft have allowed us to follow the interstellar thread of phosphorus from star-forming regions, where the phosphorus-bearing molecules are created, all the way to Earth, where they played a crucial role in starting life. Credit: Rivilla et al. 2020 / ALMA (ESO/NAOJ/NRAO); ESO/L. Calçada; ESA/Rosetta/NAVCAM; M. Weigand, www.SkyTrip.de

Chemical evolution

Growing evidence shows that most stars in the Milky Way, including our Sun, are born in high-mass star-forming regions, but because of both observational and theoretical challenges, our comprehension of their chemical evolution is less clear than that of their low-mass counterparts. Our group is very active in this field, producing a growing amount of observational works aimed at the investigation of the chemical link from the very early cold phases (IRDCs) to the warmer, most evolved stages (HMCs and UC HIIs, see Figs. 2 and 3). Such works have important implications not only for our understanding of the (still mysterious) formation process of high-mass stars, but also for the chemistry that the primordial Solar System might have inherited from its birth environment.

Prebiotic chemistry

There are multiple and independent evidence that indicate that our Sun did not form in an isolated core but inside a stellar cluster, including massive stars too. For this reason, to understand the chemical heritage received by our Solar System, and in particular by our young Earth, we must study the chemical complexity of (massive) star-forming regions that resemble the birthplace of our Sun. The cradles of massive stars are indeed among the most chemically rich sources in the Galaxy, containing a plethora of molecules that could play important roles in the first steps of Life, like molecules with phosphorus (Fig. 1), one of the key ingredient for living organisms, or precursors of complex sugars and amino acids like glycolaldehyde and formamide, respectively.

Figure 2: Comparison of the chemical richness in the spectra of the IRDC G034C obtained with the IRAM 30m telescope (upper panel) and the HMC G31.41+0.31 obtained with the interferometer ALMA (lower panel): while the first one shows few lines of simple molecules (e.g. HCO⁺, HNC and HC₃N) in G31.41+0.31 we have the emission of more than 40 molecules including COMs (e.g. isomers of C₂H₄O₂). Credits: Fontani et al. (in prep.); Mininni et al. (in prep.)

Isotopic fractionation

Each chemical element can exist in the form of atoms with nuclei having the same number of protons but different neutron number, called isotopes. The relative abundance of stable isotopes of the same element is regulated locally in the Galaxy by nucleosynthesis processes, but in molecular species can vary also because of chemical reactions that lead the isotopes of an element to distribute differently into molecules. This effect is called isotopic fractionation. Because the rates of these reactions are strongly influenced by the physical properties of the cloud, in particular by the temperature, fractionation can change the isotopic ratio of some elements in molecules dramatically. For example, the elemental D/H ratio is about 10^-5, but can increase up to about 1 in molecules in star-forming cores. Our group is one of the most active in the world in the study of fractionation in high-mass star-forming regions for different purposes, from its use as indicator of physical and chemical evolution, to the comparison with galactic nucleosynthesis model predictions (see Fig. 3).

Figure 3: trend of the mean D/H ratio (left panels) computed from the molecules N₂H⁺, HNC, NH₃ and CH₃OH (from top to bottom) for high-mass star-forming cores divided in different evolutionary groups (x-axis), and ¹⁴N/¹⁵N ratios (right panels) for HCN (top) and HNC (bottom) as a function of their Galactocentric distance. Credit: Fontani et al. 2015; Colzi et al. 2018.