The following scientific questions underpin my research activity:

How do massive stars form ?

How stars with ten Solar masses and more (hereafter, massive stars) form is still an open question within our paradigm of Star Formation. This lack of knowledge is due to the relatively small number of massive stars which form in the Galaxy that, for this reason, are statistically found far away from the Sun, at distances greater than three thousand light-years typically. As an example, we can roughly estimate that only a few hundreds of massive stars have been forming in the last million years within a radius of about a dozen thousand light-years from the Sun; this huge distance approximatively corresponds to half that between the Sun and the Galactic center. However, despite massive stars are "rare",  the history and evolution of our Milky Way, as well as that of other galaxies, cannot be understood without taking into account the life-cycle of massive stars, which are often said to regulate the life of the Galaxy itself.

For instance, at an early stage of stellar evolution, massive young stellar objects (YSOs) are deeply embedded in hot (︎100 K) and dense (︎a million particles per cubic centimeter) molecular cores of dust and gas (with visual extinction >100 mag), that are located inside much larger condensations of thousands times the mass of our Sun (named "clumps" and progenitors of massive clusters). When the proto-stellar environment heats up due to the increasing YSO luminosity, these dust-rich cocoons release large amounts of complex molecules in the Interstellar Medium (ISM), such as large organic carbon chains, by sublimation of the icy mantles of dust grains. The latter are believed to act as main catalyzers for molecular reactions in space, leading to complex chemistry. At the same time, once a massive protostar starts forming, it further injects large amounts of mechanical energy into the ISM by powerful outflow phenomena (namely, events of mass ejection coming from the inner circum-stellar regions, ≤ 100 au), which are a main outcome of mass accretion onto the protostar. In turn, this outflow activity sweeps up the proto-stellar envelope, scatters complex molecules through the ISM, and provide a major source of turbulent energy for the cluster gas. Besides the enormous (UV) energy that massive stars radiate away during their relatively short life, they burn out in a violent supernova event at last, injecting heavier elements (e.g., C, N, O, Si, S, Fe) into the ISM and triggering recursively star formation via shocks associated with the supernova explosions. Eventually, this massive star's cycle makes a full circle.  

In this context, we are still facing fundamental questions such as: (Q1) the accretion mechanism around massive YSOs — whether the mass is conveyed to the protostar through a stable accretion disk and/or via episodic accretion flows, and how the feedback between accretion processes and the ISM influences the final stellar mass; (Q2) the origin of the mass reservoir for the protostar — what maintains a large reservoir of diffused gas in virial equilibrium to feed the nascent massive star, and whether the protostar can gather mass only from its own cocoon (i.e., scales ≤ 0.1 pc, or 20000 au) or the mass reservoir is supplied from the whole clump (i.e., scales > 0.1 pc). 

To answer these questions, I am engaged in high angular-resolution observations (<1 arcsec) in the radio and sub-millimeter windows (see figure below), to resolve the physical processes happening near young stars and eventually link this information to what is seen further away, and finally put the puzzle together.

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                                                          Further related questions:

                                                                                                                                   How can we quantify the influence of outflows onto the ISM?

                                                                                                                                         How can maser emission help us understanding star formation?

How does our Milky Way look like ?

An image of the Milky Way, taken by an observer located in another galaxy, would probably reveal a spiral structure dotted with many bright HII regions: our current best "educated guess" is that the Milky Way is a barred Sb to Sc galaxy. However, since we are inside the Milky Way, the task of properly characterizing its spatial morphology has proven to be very difficult. Longitude-velocity plots of the most abundant atomic/molecular tracers of diffused gas, the 21 cm spin-flip line of HI and the first rotational CO transition at about 2.5 mm, clearly demonstrate the existence of coherent, large-scale, kinematic structures, which are interpreted in terms of spiral arms of the Milky Way. However, after decades of study, there is still little agreement on, for instance, how many spiral arms the Milky Way has, or how they look like. The primary reason for such a shortcoming is the lack of accurate distance measurements throughout the Galaxy, which makes the task of turning longitude-velocity data into a true plane-view of the Milky Way very uncertain. 

Sites of star formation which give birth to massive stars trace rich gas condensations along spiral arm segments of the Galaxy and are therefore the most suitable targets to pinpoint its spiral structure. Both their distances from the Sun and secular motions around the Galaxy can be measured by targeting the bright maser emission which arises in the ISM near to zero age main sequence (ZAMS) stars with B and O spectral types (namely, "massive"). Interstellar masers, whose acronym stands for Microwave Amplification by Stimulation Emission of Radiation, are compact cloudlets of gas with size of the order of an astronomical unit (the distance between Earth and the Sun) where microwave emission is amplified and beamed like a common laser pointer, and they act like galactic beacons guiding an observer from Earth (more on masers). The Milky Way is dotted with many maser cloudlets that, when observed at the highest resolution of the Very Long Baseline Interferometry (VLBI) technic, during a full Earth revolution around the Sun, allow us to measure trigonometric parallaxes with accuracies better than 10 micro-arcseconds (μas). That is to say: we can explore our own Galaxy observing objects with distances more than 10 kpc away from the Sun, or more than 33000 light-years,  and still locating them with an accuracy better than 10% (see right figure below).

In this context, I am member of a large project, called "The Bar and Spiral Structure Legacy (BeSSeL) Survey", which aims to reveal the spatial morphology and kinematics of the Milky Way (see left figure below) by observing distant masers through the most accurate trigonometric parallax measurements (aka, the gold standard for cosmic distances). These measurements are conducted with the finest astrometric capabilities of the Very Long Baseline Array (VLBA) facility in the United States.

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