GMTIFS science: the physics of star formation

The manner in which stars and planets form is not well understood. Stars form from gas clouds that collapse gravitationally. They probably experience a phase of spherical accretion. However, the core takes on an axially symmetric structure early in this process due to angular momentum conservation as the cloud collapses. Mass accretion onto the central star is then regulated by angular momentum loss. Low-mass stars (M < 2 MSun) may accumulate the bulk of their mass in this disk-accretion phase. The relative importance of disk accretion in the formation of more massive stars is not known.

Much of our knowledge of star and planet formation is inferred because we have not been able to spatially resolve the regions where these processes are occurring, even in the closest young stars. With GMTIFS working at the diffraction-limit of GMT, we will achieve angular resolutions of ~ 25 mas, comparable to the orbital radius of Jupiter (~ 35 mas) at the distance of ~ 150 pc of the nearest regions of significant low-mass star formation. The GMTIFS IFS will spatially resolve structures in the bipolar outflows from young stars and detect molecular emission from the surfaces of their circumstellar disks. ALMA will soon begin probing the molecular components of circumstellar disk interiors on even smaller scales. Consequently, powerful synergies will exist between GMTIFS and ALMA presenting different views of nearby star formation.

Changes in the structures of the "micro-jets" emanating from actively-accreting low-mass stars are already observed on timescales of ~ 1 yr with adaptive-optics-corrected integral-field spectrographs on 8-10 m telescopes. With GMTIFS, we expect to see changes in the jet structure of southern T Tauri stars on timescales of ~ 1-2 months. This potentially creates a new field of "jet astrometry" that has only been marginally possible to date. The southern star-forming regions of Chamaeleon, ρ Ophiuchus, Lupus, and R Corona Australis are ripe with actively-accreting T Tauri stars suitable for study.

A key question to be addressed with the higher angular resolution and higher spectral resolution of the GMTIFS IFS operating at the diffraction limit is the extent to which the outflows from young stars show axial rotation. Unambiguous detection of axial rotation would provide clear evidence that their jets are driven by magneto-centrifugal forces at the disk surface, rather than by other magnetospheric effects originating much closer the the star. A simulated observation of a typical T Tauri star with the high-resolution hHgrating and the 6 mas spaxel-1 IFS scale is shown below. A 30 mas occulting disk is used to attenuate the light of the central star so that the observation is sensitive to [Fe II] line emission arising in the extended jet. The jet is assumed to have an intrinsic width of 3 AU, an outflow velocity of 100 km s-1, and axial solid-body rotation is imposed on the jet with a velocity amplitude of 30 km s-1 at its outer edge.

Emission-line fits to the resulting data cube recover the imposed axial rotation of both the approaching and receding jets with a velocity amplitude that is approximately half of the maximum imposed values, as is expected.

Figure 1 - Line-of-sight velocities (top) and central line intensities (bottom) recovered from a T Tauri star jet simulation using the GMTIFS IFS plus occulting disk. The approaching jet is shown at left and the receding jet is shown at right. The black cross indicates the location of the central star in each panel. The color scale is shown to the right of each panel.