The molecular environment of low-mass protostars

Details The molecular environment of low-mass protostars The molecular environment of low-mass protostars

Jun 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998phdt........39h&link_type=abstract

Ph.D. thesis, Leiden University, (1998)

Computer Science

12

Scientific paper

Although well understood qualitatively, many important details about the formation of low-mass stars remain ill determined. In this thesis we try to find some answers to questions like: what is the density, temperature, and velocity distribution in the envelopes around embedded young stellar objects (YSOs)? What is the role of rotation and magnetic fields? At what stage do circumstellar disks form, and what is their evolution? What is the influence of the object's mass, multiplicity, outflow, and star-forming environment? This thesis investigates (sub) millimeter molecular-line and dust-continuum observations of 13 YSOs: 9 class I objects in Taurus (d≅ 140 pc) and 4 more deeply embedded, and possibly more massive, class 0 sources in Serpens (d≅ 400 pc). Whereas the Taurus region is prototypical of the isolated mode of star formation, Serpens is forming a dense cluster of stars. Single-dish observations observations were obtained with the James Clerk Maxwell Telescope, the Caltech Submillimeter Observatory, and the IRAM 30m telescope of low- and mid-J lines of CO, HCO+, HCN, and isotopes. These data are complemented by aperture synthesis observations from the Owens Valley Millimeter Array of HCO+, HCN, 13CO, and C18O 1--0. The latter data also include continuum measurements at 3.4--1.3 mm. Together, our data set traces densities of 104 -- 107 cm-3, temperatures of 10--100 K, and scales of a few hundred to 10,000 AU, typical of YSO envelopes. An important step in the analysis is formed by Monte Carlo calculations of the radiative transfer and molecular excitation employing axisymmetric source models. The main conclusions of this thesis are: 1. Throughout the embedded class 0 and I phases single-dish molecular line observations trace the envelope density structure, especially in HCO+ 3--2 and 4--3, which exclusively sample dense envelope gas. The class 0 envelopes are sufficiently massive that interferometric observations of spatial resolved continuum emission provide additional, accurate constraints on the density distribution. 2. The inside-out collapse model formulated by Shu (1977) describes well the observed molecular lines on scales of a few thousand AU as sampled by single-dish instruments. The resolved continuum emission from class 0 envelopes indicates a radial power law for the density with slope -2.0+/- 0.5, consistent with the molecular line fits. Whereas the models invariably predict ``infall asymmetry'' for optically thick molecular lines, the observations only show these unambiguously in lines like HCO+ 3--2 and 4--3, which exclusively sample the envelopes. 3. On smaller scales of 700--1500 AU as sampled by the interferometer, the envelopes deviate from spherical symmetry, suggesting flattening and rotation. Collapse models including these effects reproduce the observed characteristics, although a more detailed description is warranted. 4. Two-thirds of the class I Taurus sources are surrounded by a circumstellar disk, as evidenced by the presence of unresolved (<3'') continuum emission. Their similar fluxes compared to class II sources suggest that there is little disk evolution from the embedded to the early T Tauri phases. The detection of disks around the more embedded class 0 objects is prohibited by their massive, centrally concentrated envelopes which dominate the unresolved flux. 5. Sub-arcsecond resolution observations of the young binary system T Tau reveal that only the optical star T Tau N is surrounded by a sizeable disk. Strong upper limits on any circumstellar material are obtained for the infrared star T Tau S. It is proposed that the embedded appearance of this source may be due to T Tau N's disk, if this obscures T Tau S as seen from Earth. 6. The observed correlation between envelope mass, disk continuum flux, and outflow strength supports the notion that outflows are driven by disk accretion, and suggests that the infall rate increases with envelope mass. In lines of HCO+ the interferometer traces the interaction of the outflow with the surrounding envelope, which can be successfully modeled as a mixing layer. For a postscript version of this thesis, see http://www.strw.leidenuniv.nl/~michiel/publications.html

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