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Arcetri Star-formation Group Research Topics

Research topics

Our group is involved in theoretical and observational research in several topics of star formation. A list of recent publications is available. In the following, we give a brief guide to our research interests.

Theory and Observation

Molecular clouds:

Low-mass SFR, High-mass SFR:

The Arcetri group is carrying a coordinated effort to determine the kinematics and physical parameters of clouds where high-mass stars form. Massive stars form in association with solar mass objects from gas clouds of mass anywhere between one hundred and ten thousand times that of the sun. Most stars form in such environments and understanding the formation process of massive stars has an impact on the history of the evolution of our Galaxy and of galaxies in general. The Arcetri group carries out long wavelength studies (radio to infrared) which are not affected by dust obscuration. In particular, the rotational transitions of interstellar molecules at millimeter wavelengths offer one of the best diagnostics to probe the physical and chemical conditions at the highest spatial resolution available with current generation interferometers.

Star formation theory:

The star formation process has been modeled starting from the initial conditions within magnetized dense molecular cloud cores, through the dynamical phase of protostellar accretion and onto the hydrostatic contraction of pre-main-sequence stars. Magnetic fields have been recognized to play a fundamental role in the equilibrium and dynamical evolution of molecular clouds. Hydrodynamical models have been developed that incorporate both the effects of large scale magnetic fields and the contribution of MHD waves. The formation of protostars has been followed from low- to intermediate-mass objects. The initial conditions set by the accretion process have been used to construct self-consistent pre-main-sequence evolutionary tracks. The role and properties of circumstellar disks around young stars has been analyzed by means of theoretical models.

Young stars:

Once a young star has emerged from its natal cocoon and become visible to optical telescopes, it can be studied using the classical techniques of stellar evolution. From the observed surface (photospheric) temperature and luminosity, one can infer the stars age and mass. This in turn allows a look into past history since once can then determine the rate at which stars have been forming in a given molecular cloud and the distribution of their masses. These ideas can be tested e.g. by observing binaries or clusters of stars which are expected to have the same age. Critical for the above is to be able to select young stars (less than a hundred million years old) from the much more numerous stars like our sun which have settled down to burn hydrogen. For this reason, we are interested in studying age indicators for young stars. One such is X-ray activity. Another is the presence of Lithium in the stars atmosphere since Lithium gets burnt and is not present in a star after a hundred million years or so.

Disks around young stars:

Inferring planet formation

Young newly formed stars are surrounded by rotating disks of gas and dust particles. These gradually disappear on a timescale of ten million years as the young star contracts towards the state in which it will burn hydrogen in its nucleus. It is natural to link the disappearance of these disks with the formation of a planetary system but at the moment, the theory of this process is extremely sketchy and biased by the example we know best : our own solar system. Nevertheless, we can observe the emission of the dust grains surrounding the young star at wavelengths from the infrared to the radio. From such observations, we can build up an idea of the properties of such disks as a function of age. The Arcetri group is heavily involved in such studies.

Jets from young stars:

Young stars power supersonic 'Herbig-Haro' (HH) jets, constituted of atomic (partially ionized) gas. These beautiful outflows seen in optical emission lines are believed to be a crucial element in the star formation process, as they can remove the excess angular momentum from the system, and disperse the residual infalling envelope. According to the current theoretical scenario, magnetic and centrifugal forces act together to launch these jets along magnetic field lines in a 'bead-on-a-wire' sling mechanism. The 'knotty' structure within the flow is believed to arise from variabilty in the ejection properties, probably due to magnetic instabilities. This fascinating picture, however, is hardly tested observationally, because of the small scales involved (from hundreths to a few tens of astronomical units). Within this framework, the Arcetri group is actively working on high angular resolution observational programs, both from space and ground.

Chemistry and evolution of the gas:

In our galaxy

In the early universe

Roughly half of the interstellar gas in the Milky Way is in molecular form. This gas is of importance because it is within the dense molecular gas that new stars form. The Arcetri group has been carrying out a series of observational and theoretical studies on topics such as the ionization degree in molecular clouds (which plays a vital role in determining the coupling between the magnetic field and the gas), the role of interstellar shocks (which can cause the ejection of ice mantles condensed on grain surfaces), and the interaction between the radiation fields of young stars and their environments (especially important for high-mass stars). In a broader context, the chemical properties of the primordial gas, both atomic and molecular, after the recombination epoch and the implications for the conditions driving the formation of the first structures in the universe have been fully explored by the Arcetri group.

Gravitational collapse:

Search for the infall signature

The identification of a molecular cloud undergoing gravitational collapse is still lacking and researchers in Arcetri have been deeply involved in this exciting search. These studies require the use of millimeter and submilliter arrays to obtain the highest spatial and spectral resolution to resolve the regions where collapse is occurring. The main contribution comes from the identification and characterization of the best candidates in nearby low-mass star forming regions.

Maser variability:

Our project consists in observing and cataloguing masers in different physical environments, including star forming regions in the Galaxy and late-type stars. Over the last 11 years we have been observing with the Medicina 32-m radio telescope, with about 100 runs to date beginning in March 1987. At present the catalog contains 30177 entries. We started in 1995 a maser time-variability program with 52 sources that are monitored several times per year.

High angular resolution observations:

The contribution of the Arcetri group in this field is based on observations by means of techniques such as lunar occultations and speckle interferometry. Results have been obtained for a large range of astrophysical objects, from young stars in the pre-main-sequence phase, to main sequence stars, to evolved objects such as red giants and AGB stars. In the field of star formation, studies of young stars address the fundamental issue of the binary frequency of stars at birth and have already provided significant results in regions such Taurus-Auriga and Orion. These studies will greatly benefit from the use of large interferometric facilities, such as LBT and VLTI, in which Arcetri scientists are making important contributions.

Instrumentation and Observations