Star formation and evolution in the galaxy

Stars form in associations, from the fragmentation and contraction of large molecular clouds. During the first few million years after their formation young stars are surrounded by circumstellar disks of gas and dust, remnants of the gravitational collapse of their parent cloud. They also posses intense magnetic fields. Consequence of disks and magnetic fields are a variety of complex and dynamic phenomena, including: mass accretion from the circumstellar disk, outflows, both collimated (jets) and uncollimated (winds), intense high energy emission, and the formation of planetary systems.

Credit: Caramazza et al. 2008

Credit: Caramazza et al. 2008


Researchers at the OAPa study the formation and subsequent evolution of these complex systems and, also, the evolution of the stellar clusters that are in some cases the long-lasting remnants of the stars-forming associations, i.e. when these latter are gravitationally bound. Clusters and associations are also useful to follow-up the evolution of stars throughout their life, as well as the evolution of the Galaxy as a whole. Their members, in fact, having formed at the same time from the same cloud, share the same age and chemical composition.


The fragmentation and gravitational contraction of molecular clouds, and in particular the triggering of the process, its regulation and timescales, are not well understood. Useful constraints can be obtained from observations of star forming regions of different characteristics (e.g., mass, size, content of massive stars) and/or in different conditions and galactic environments (e.g. density, metallicity, radiation field). The outcome of star formation at different stages can be observed and compared with that in other regions, so to understand the effect of each physical quantity on star formation. The recent star formation history in the Solar neighborhood may be reconstructed comparing the young stellar population with theoretical galactic models. Identifying this young population is not straightforward, however, since the photometric and spectroscopic properties of young stars do not differ significantly from those of their older counterparts. Proxies of youth can be employed, however: X-ray observations are useful because stars are luminous in X-rays when young and become fainter as they age; in a similar fashion, stellar rotation periods decreases with stellar age during the main-sequence life of a star.


Following the cloud contraction phase and the main mass-building phase, during which protostars remain embedded in their accreting envelopes, young stellar objects remain surrounded for a few million years by optically thick and geometrically thin accretion disks. This phase, which is also important for the formation of planetary systems, is complex and not fully understood. Circumstellar accretion is critically mediated by the stellar magnetic field. This latter truncates the mildly ionized disk, and, from the truncation radius, channels the infalling material to the central star along its flux tubes. At the base of these accretion stream, the accreting material impacts the dense stellar surface at high velocity, thus creating a strong shock in which the gas is heated up to a few million degrees, emitting in the X-ray band. The whole surrounding region, also called the hot spot, is consequently heated to temperatures above that of the surrounding photosphere and emits conspicuously also in the UV and optical bands. Studying the accretion process in all the relevant bands, from X-rays to the optical, is important to understand the physics of pre-main-sequence low-mass stars and their disks. Circumstellar accretion, in fact, regulates the exchange of mass and angular momentum between the star and the disk. Moreover, because of the resulting high-energy radiation, accretion most likely influences the physics and the chemistry of the circumstellar disk, including its lifetime. A further contribution to the high energy emission comes from plasma shocked in collimated jets, a phenomenon related to accretion and especially relevant in the earlier phases of proto-stellar evolution. Maybe most important in this context, is the intense X-ray emission from very active magnetic coronae in which the magnetic field plays a fundamental role in confining and heating the plasma to tens of million of degrees. Although the observed phenomenology is qualitatively similar to that of the Solar corona, the stellar coronae of young stars are in many ways much more extreme (e.g. in terms of plasma temperatures, emission measures, variability). The physical origin of these coronae and their interplay with accretion and disks, of which there are strong indications, are not well understood.


X-rays from coronae, accretion shocks, and jets ionize and heat the disk. The ionization affects the disk viscosity, which, in turn, determines the accretion rate. The heating of the outer gas layers by the high energy radiation determines the disk evaporation which is now believed to be the main mechanism responsible for the eventual dispersal of disks. A joint study of high-energy phenomena and disk/accretion properties of young stars is, therefore, fundamental to understanding star and planet formation. A related research topic is the study of the environmental influences on the evolution of circumstellar disks and planet formation. Indeed, in addition to the above internal agents (i.e. from the central object), circumstellar disks located in regions of high stellar density can also be affected by gravitational interactions with other stars, and by intense optical/UV/X-ray irradiation by nearby massive stars.


OAPa scientists have been actively engaged in the investigation of all the science issues described above, through both observations and modeling efforts. Because of the complex and dynamic nature of the involved phenomena, a variety of observational techniques are required to try to derive a coherent physical picture. X-ray imaging and optical/infrared photometry and spectroscopy are routinely used to selected young cluster members and to study their physical properties. Particularly useful to elucidate the interplay between the various physical components are multi-wavelength and time-resolved observations, both photometric and spectroscopic. Researchers at OPAa participate to or lead several large international observational projects using the most powerful space- and ground-based instruments (e.g. Chandra and XMM-Newton in the X-ray band, Spitzer in the mIR, VLT@ESO in the optical). Researchers at OAPa are also active in modeling the relevant physical processes. They have, for example, developed and applied detailed magnetohydrodynamic (MHD) and hydrodynamic models describing the interaction of the circumstellar disk with the central star (through mass accretion) as well as the evolution of protostellar jets.

Research themes include:

Star formation

Star-disk interaction and related diagnostics

Star-planet interactions

Accretion phenomena

Protostellar jets physics and modeling

Research programs include:

  • CSI – NGC 2264 project: A simultaneous 3-satellites (Spitzer – CoRoT – Chandra) survey of NGC 2264 region plus a number of ground-based observations
  • DROXO (Deep Rho Ophiuchi X-ray Observation), an XMM-Newton LP
  • Cyg-OB2 1Ms Chandra Survey
  • Xshooter GTO Program
  • Participation to PLATO – Planetary Transits and Oscillation satellite
  • Participation to HARPS-N – The High Accuracy Radial velocity Planet Searcher/North
  • Participation to EChO – Exoplanets Characterization Observatory
  • Participation to ATHENA (Advanced Telescope for High Energy Astrophysics)
  • GES
  • EXTraS
  • JEDI
  • GAPS

Involved people:


Other research activities