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.
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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.
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Star formation theory:
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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.
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Young stars:
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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.
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Jets from young stars:
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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.
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Instrumentation and Observations
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Publications from our Group
This page was updated on December 3, 2007.
For
comments please contact C.M.
Walmsley