Context. Hot exozodiacal dust is thought to be responsible for excess near-infrared (NIR) emission emanating from the innermost parts of some debris disks. The origin of this dust, however, is... Show moreContext. Hot exozodiacal dust is thought to be responsible for excess near-infrared (NIR) emission emanating from the innermost parts of some debris disks. The origin of this dust, however, is still a matter of debate.Aims: We test whether hot exozodiacal dust can be supplied from an exterior parent belt by Poynting-Robertson (P-R) drag, paying special attention to the pile-up of dust that occurs owing to the interplay of P-R drag and dust sublimation. Specifically, we investigate whether pile-ups still occur when collisions are taken into account, and if they can explain the observed NIR excess.Methods: We computed the steady-state distribution of dust in the inner disk by solving the continuity equation. First, we derived an analytical solution under a number of simplifying assumptions. Second, we developed a numerical debris disk model that for the first time treats the complex interaction of collisions, P-R drag, and sublimation in a self-consistent way. From the resulting dust distributions, we generated thermal emission spectra and compare these to observed excess NIR fluxes.Results: We confirm that P-R drag always supplies a small amount of dust to the sublimation zone, but find that a fully consistent treatment yields a maximum amount of dust that is about 7 times lower than that given by analytical estimates. The NIR excess due to this material is much less (≲10-3for A-type stars with parent belts at ≳1 AU) than the values derived from interferometric observations (~10-2). Pile-up of dust still occurs when collisions are considered, but its effect on the NIR flux is insignificant. Finally, the cross-section in the innermost regions is clearly dominated by barely bound grains. Show less
Context. Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened... Show moreContext. Broadband spectral surveys of protostars offer a rich view of the physical, chemical and dynamical structure and evolution of star-forming regions. The Herschel Space Observatory opened up the terahertz regime to such surveys, giving access to the fundamental transitions of many hydrides and to the high-energy transitions of many other species. Aims: A comparative analysis of the chemical inventories and physical processes and properties of protostars of various masses and evolutionary states is the goal of the Herschel CHEmical Surveys of Star forming regions (CHESS) key program. This paper focusses on the intermediate-mass protostar, OMC-2 FIR 4. Methods: We obtained a spectrum of OMC-2 FIR 4 in the 480 to 1902 GHz range with the HIFI spectrometer onboard Herschel and carried out the reduction, line identification, and a broad analysis of the line profile components, excitation, and cooling. Results: We detect 719 spectral lines from 40 species and isotopologs. The line flux is dominated by CO, H$_{2}$O, and CH$_{3}$OH. The line profiles are complex and vary with species and upper level energy, but clearly contain signatures from quiescent gas, a broad component likely due to an outflow, and a foreground cloud. Conclusions: We find abundant evidence for warm, dense gas, as well as for an outflow in the field of view. Line flux represents 2% of the 7 L$_{⊙}$ luminosity detected with HIFI in the 480 to 1250 GHz range. Of the total line flux, 60% is from CO, 13% from H$_{2}$O and 9% from CH$_{3}$OH. A comparison with similar HIFI spectra of other sources is set to provide much new insight into star formation regions, a case in point being a difference of two orders of magnitude in the relative contribution of sulphur oxides to the line cooling of Orion KL and OMC-2 FIR 4. Appendix A is available in electronic form at http://www.aanda.orgShow less
