Properties and evolution of circumstellar disks and envelopes around young, low-mass stars in the Taurus star-forming region

This thesis presents an analysis of mid-infrared spectra of a representative sample of protostars (Class I objects) and T Tauri stars stars (Class II and III objects) in the Taurus star-forming region. The data were obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope, whose unprecedented sensitivity allows us to study the evolution of faint, low-mass stars in nearby star-forming regions. Since young stars are surrounded by disks and envelopes which emit a large fraction of their radiation in the infrared, this wavelength region is particularly useful to study them. Understanding how circumstellar envelopes and disks evolve in time is essential for unraveling the processes involved in the transformation of interstellar material to planetary bodies, which are believed to form in circumstellar disks. In protostars, which are surrounded by large envelopes, in addition to accretion disks, the mid-infrared spectrum reveals ice absorption features and a usually rising continuum; in most T Tauri stars, which have dissipated their envelopes, the IRS spectrum reveals dust continuum emission from the inner few AU of flared disks, and, in many cases, emission bands due to amorphous and crystalline silicates. The IRS spectra offer new constraints to models created to interpret the structure and composition of circumstellar disks and envelopes. We generated envelope models that reproduce the observed spectral energy distribution of a large fraction of the Class I objects in our sample. We determined a morphological sequence for the T Tauri stars in our sample based on their IRS spectra, and compared their continuum emission with a grid of accretion disk models that include dust grain growth and settling. Our main conclusions are as follows: (1) protostars exhibit a wide range of envelope characteristics, like the density and accretion disk size, and the inclination angle plays a major role in their appearance at all wavelengths; (2) T Tauri stars already show indications of dust settling and growth in their disks, which are first steps towards the formation of planets; (3) very low-mass young stars close to the hydrogen-burning mass limit can also be surrounded by accretion disks, like their more massive counterparts.