The research of MRH focuses on molecular line and dust-continuum observations at (sub) millimeter wavelengths to study the physics of protostellar cloud cores and disks around young stars.
Combining submillimeter continuum imaging from the SCUBA array on JCMT with molecular line profile analysis of a sample of embedded protostars, Hogerheijde & Sandell (2000) investigated the velocity fields in these collapsing cores. They concluded that current theoretical descriptions suffice to describe these objects on 1000 AU scales. On smaller scales, embedded disks and the activity of outflows is apparent, as traced by interferometric observations of various molecules like HCO+, N2H+, and HCN (e.g., Joergensen et al. 2004a, 2004b).
One object, L1489 IRS, turns out to be very interesting. Using OVRO and BIMA, interferometer observations in lines of HCO+, HCN, 13CO, and C18O showed this object to be surrounded by an unusually large (2000 AU radius) disk (Hogerheijde 2001). In addition to Keplerian rotation, the line profiles clearly indicated inward motions. Hogerheijde (2001) suggested that this disk is in the earliest evolution stages and is still settling to its rotationally supported size. Using the NIRSPEC near-infrared instrument on the Keck 10-meter telescope, Boogert, Hogerheijde & Blake (2002) showed that the inward motions around L1489 IRS continue to within 0.1 AU from the star. This is puzzling in the context of accepted viscous accretion disk models.
To analyze the molecular line data, Hogerheijde & van der Tak (2000) developed an advanced code to simulate radiative transfer and molecular excitation in arbitrary axisymmetric geometries. Coupled with techniques to synthesize observations, this code lies at the heart of much of MRH's work. It has been applied to the modeling the physical conditions in protoplanetary disks in collaboration with specialists in disk chemistry (e.g., van Zadelhoff et al. 2003).
MRH also investigates protoplanetary disks from another viewpoint, that of the volatile content of comets. This is believed to reflect the `frozen' composition of the disk early in the history of our Solar System. Millimeter interferometric observations of passing comets involving MRH were reported in Blake et al. (1998) and Hogerheijde et al. (2004).