Young Stellar Objects in a Nutshell

Péter Ábrahám

Stars are formed by gravitational collapse of dense gas condensations in the interstellar space. Figure 1 is an image of the Elephant's Trunk Nebula (a globule in IC 1396) where protostars are visible as bright redish dots mostly along the southern rim of the globule.

A protostar collects its mass via accretion. Due to angular momentum constrains the infalling material cannot reach directly the stellar surface but flows through a flattened circumstellar structure. Such a disk is seen edge-on in the HH30 system (Figure 2) where the circumstellar material surrounds and hides the central source. The characteristic sizes of typical disks is several 100 AU.
Figure 2: Left panel: the young star in the HH30 system is hidden by a flattened edge-on circumstellar disk visible as a dark horizontal lane (from the Hubble Space Telescope). The green structures are powerful gaseous jets emitted by the source. Right panel: artist's impression of a HH30-like system.

Figure 1: Newborn stars in the Elephant's Trunk Nebula visible as bright redish dots mostly along the southern rim of the globule (from Spitzer Space Telescope).

Newborn stars evolve from an embrionic embedded state towards the main sequence. The amount of circumstellar material is continuously decreasing with time, starting from a mass comparable to the stellar mass and arriving to the mass range assumed for the early solar system. A schematic view of the pre-main sequence stellar evolution of a solar mass star is depicted in Figure 3. The timescale of pre-main sequence evolution is about 10 million years for solar mass stars and is somewhat shorter for intermediate mass stars.

The circumstellar disk/envelope is heated by the stellar radiation and/or the release of accretion energy, and is cooled by radiating away the energy mainly at infrared wavelengths. The shape of the infrared spectral energy distribution is closely linked to the temperature and density profiles of the circumstellar structure (see Figure 4, and also Figure 3).
Figure 4: The radial temperature distribution determines spectral shape (from M. Meyer).

Figure 3: Stages of pre-main sequence evolution.