Research
My main area of interest is observational astrochemistry, in particular in connection with galactic star formation. Since stars forms in dense clouds composed by molecules, molecular transitions represent the ideal diagnostic tool to probe the different regimes of density and temperature and to infer the physical and chemical structure of star-forming regions. I am hence an experienced user of most of the current facilities operating at radio/microwave/sub-millimeter wavelengths, such as the IRAM 30m telescope, the Green Bank telescope, the Atacama Large Millimeter and submillemeter Array, and the Very Large Array.
High-mass star formation: the ALMA-UNIC Large Program
High-mass stars dominate the dynamics of the interstellar medium at a Galactic level. They were likely present in the cluster from which the Sun was born. Studying their formation is hence crucial to provide a quantitative model also of Solar-like stellar systems. However, the initial stages of high-mass-star formation are challenging to observe, and hence crucial constrains to our current theoretical models are missing. In order to fill this gap between observations and theory, I am the PI of the ALMA Large Program UNIC: Unveiling the Initial Conditions of high-mass star formation (ID: 2023.1.00360.L, co-PIs: S. Bovino, P. Sanhueza, R. Friesen, V. H. R. Chen). The project was approved for Cycle 10 and awarded more than 80 hours of Main Array time and 900 hours on the ACA, and it will observe ten very young massive clumps extracted from the ATLASGAL-top1000 sample. We will use four setups in three bands (3, 6, and 7), targeting most of the relevant cold-gas tracers. N2H+ (1-0) observations will be used to trace the large, clump-scale gas kinematics at ~8000AU of resolution. By applying a dendrogram approach to the H2D+ data at higher resolution, we will characterise the populations of prestellar cores embedded in the clumps. Finally, the two setups in Band 6 will allow to infer the HCO+ deuteration level and the CO depletion factor, which will be used (together with H2D+) produce the first large sample of spatially-resolved maps of ionisation rates.
Kinematics of magnetised star-forming regions
Dense cores are embedded in filamentary structures, from which they form and to which they are kinematically connected. This connection has profound impacts on the dynamic evolution of star-forming regions. Recently, I observed a large-scale map of HCO+ (1-0) towards the well known low-mass core L1544. This abundant molecule traces preferentially the more diffuse envelope surrounding the core, making it a perfect tracer of the kinematics of the source along the line-of-sight. I combined the non-LTE radiative transfer analysis at the core’s centre, with a modelling of the whole map, studying the spatial variations of the infall velocity. For the first time, I reported extended inward motions in the whole envelope around the core. Furthermore, this envelope must be rather diffuse (tens of 10cm-3 at most) and extended (~1 pc).
Furthermore, the interstellar medium is a magnetised environment, and magnetic fields are known to play a key role in its evolution. By means of observations of the polarised dust emission, we can probe the magnetic field morphology, and how it is linked to the gas kinematics and dynamics. This is the focus of the Ph.D .thesis of Mrs. Farideh S. Tabatabaei, one of the students in my group. She is combining the analysis of molecular tracers and polarimetric data to investigate the interplay between magnetic fields and gas flows, especially in low-mass star-forming regions.
The ionisation properties of star-forming gas
Cosmic rays (CRs) are energetic, ionised particles found ubiquitously in the ISM. In the densest gas phases, where prestellar sources are found, the interstellar ultraviolet flux is completely absorbed, and CRs represent the only ionising agent. By producing H3+ after ionising H2 molecules, CRs determine the ionisation fraction of the dense matter, which in turn controls important dynamic processes such as the ambipolar diffusion. Furthermore, H3+ is a pivotal species for the chemical evolution of star-forming regions, because it drives the rich ion chemistry. Hence, CRs deeply impact the chemistry and physics of the dense ISM, affecting its chemical composition and determining its subsequent evolution.
The CR ionisation rate, however, is an elusive quantity to be measured, particularly at high-densities. We have exploited two main methods to constrain this parameter. In Redaelli et al. (2021b), I analysed a large set of high-sensitivity observations comprising several rotational transitions of N2H+, N2D+, HCO+, and DCO+ in the pre-stellar core L1544, coupling the known physical model of the source to the gas-grain chemical code pyrate (Sipilä et al. 2015), to perform the first study of the spatial variation of the cosmic-ray ionisation rate at high densities. We showed that the observations are in agreement with he theoretical models developed from the Voyager mission data (Padovani et al. 2009, 2018).
This approach is effective but time-consuming, which limits its application to large surveys. This is why we have been focusing on finding an analytical expression that is sufficiently reliable to estimate ionisation rates and to compare them with theoretical predictions. Bovino et al. (2020) suggested an equation based on observations of H2D+. I currently tested it in a controlled environments, starting from 3D simulations of a dense cores and post-processing them with the radiative transfer code polaris. The results are encouraging, as the equation is usually correct within a factor of a few. I was involved in the work that using this approach on ALMA data, obtained the first spatially-resolved map of the CR ionisation rate (Sabatini et al. 2023). In the future, this method will allow to make a leap in statistics of measurements of this key parameter for the evolution of star-forming regions.
