We are a group of European scientists interested in the star formation properties of young clusters in the Local Group, mostly the Galaxy and Magellanic Clouds. This site provides an overview of our research with a selection of our papers.

Our research addresses broadly the topics of star formation and stellar populations in the Local Group, mainly in the Magellanic Clouds and in starburst clusters in the Milky Way. The programme hinges on photometric observations with the Hubble Space Telescope. In recent years we have worked mostly on the initial stages of star formation, the pre-main sequence (PMS) phase, to understand the role played by the environment on the formation of low-mass stars, which represent the bulk of the stellar mass in the universe and which allow us to trace how star formation has proceeded over extended periods of time in the past (several 10 Myr). We have developed a novel and very robust method to securely identify these objects in a variety of Galactic and extragalactic environments with very different physical properties and to study their temporal evolution. At the same time, we have undertaken an innovative and systematic study of the properties of extinction due to dust grains in the same environments, also using HST observations, to probe the effects of star formation on the interstellar medium (ISM).

The two areas of research are intimately connected because the properties of PMS objects cannot be completely understood until the effects of the ISM, i.e. the extinction, are taken into account in their photometry. Furthermore, they are both based on the comparison of the same type of objects between our Galaxy and the Magellanic Clouds. With a metallicity between 1/10 and 1/3 of the solar value, the Magellanic Clouds allow us to probe the prevailing conditions at redshifts z ~ 2, when star formation was at its peak in the Universe. Understanding how star formation proceeds in these environments is therefore crucial also in a cosmological context.

Star formation

We have undertaken a systematic study of PMS stars spanning a wide range of masses (0.5 – 4 M), metallicities (0.1 – 1 Z) and ages (0.5 – 30 Myr). We have used the HST to identify and characterise a large sample of PMS objects in several star-forming regions in the Magellanic Clouds, namely 30 Dor, the SN1987A field and NGC 1850 in the LMC and NGC 346 and NGC 602 in the SMC, and have compared them to PMS stars in similar regions in the Milky Way, such as NGC 3603 and Trumpler 14. Thanks to a novel method that we have developed to combine broad-band (V, I) photometry with narrow-band Hα imaging, we have determined the physical parameters (temperature, luminosity, age, mass and mass accretion rate) of more than 3000 bona-fide PMS stars still undergoing active mass accretion. This is presently the largest and most homogeneous sample of PMS objects with known physical properties and it includes not only very young objects, but also PMS stars older than 10 – 20 Myr that are approaching the main sequence (MS). We find that the mass accretion rate scales roughly with the square root of the age, with the mass of the star to the power of 1.5, and with the inverse of the cube root of the metallicity. The mass accretion for stars of the same mass and age is thus systematically higher in the Magellanic Clouds than in the Milky Way. These results are bound to have important implications for, and constraints on our understanding of the star formation process.

A short four-page summary in PDF format is available here.


We have initiated a systematic study of the extinction law in the Magellanic Clouds, starting from two regions located at the centre and in the periphery of the 30 Doradus nebula. Interstellar extinction is traditionally derived through spectroscopy of massive early-type stars, which are necessarily few and short-lived and only allow one to probe the most active star forming environments. Instead, using observations at optical and near-infrared wavelengths obtained with the WFPC2, ACS and WFC3 cameras on board the HST, we have developed a new method to determine the extinction law from the much more numerous and ubiquitous stars in the red giant clump of intermediate age populations. When the levels of extinction are high and uneven, like in the regions of interest for this study, these objects are spread across the CMD. Since they share very similar physical properties and are all at the same distance, they are located on a narrow strip along the reddening vector and allow us to derive the absolute extinction in a straightforward and reliable way. Thus we have measured the extinction law in the range 0.3 – 1.6 µm and the absolute extinction towards hundreds of objects in each field, or two orders of magnitude more than allowed by spectroscopy of early-type stars. At optical wavelengths, the extinction curve that we find in 30 Dor is almost parallel to that of the diffuse Galactic ISM, but the value of RV ∼ 5 that we measure indicates that there is a grey component due to a larger fraction of large grains. At wavelengths longer than ∼ 1 µm, the contribution of the grey component tapers off as λ–1.5, like in the Milky Way, suggesting that the nature of the grains is otherwise similar to those in our Galaxy. This extinction law leaves no doubts that in these regions large grains are more important than in the diffuse Galactic ISM, suggesting that in star-forming regions either grains are generally larger, or large grains are more frequent, or both. The most straightforward explanation of our findings is the addition of "fresh" large grains to a Galactic mix by supernova explosions, as recently revealed by Herschel and ALMA observations of SN1987A.

A short four-page summary in PDF format is available here.