| Description |
scientific abstract observations of population i wolf-rayet stars with the iso-sws in the grating mode are proposed in order to determine wind chemical abundances, wind velocities, free-free radiation by winds, mass loss rates, and, of late wc types, continuous and episodic dust formation rates and dust abundances. the selected priority-1 targets are the ir-brightest dust-free objects among the known galactic wr stars, justifying maximum use of the sws grating-mode capacities. this proposal has been coordinated with lws and phot. abundances: current wr star abundance ratio determinations, based on model atmospheres and mainly for he, n and c, are severely model dependent. determinations of the abundances of nucleosynthesis products o, ne, mg, s and ar, which have forbidden transitions in the sws wavelength range, will provide a direct test of evolutionary studies and of interior model abundance predictions by, e.g., maeder and by langer. presently the (ground-based and iras) observed ne abundance in the wc star gamma velorum (wr11) is in conflict with interior models. iso-sws observations of wr stars will be crucial to solve this problem. winds and mass loss: different observational methods yield different terminal wind velocity values per star, due to the stratification of these velocities. measurements in the constant velocity regime are essential. ir forbidden lines, formed in the constant wind velocity regime, have to date been measured for one wr star only: wr11. sws will change that. mass loss rates: current mass loss rate determinations for wr stars, based on ground-based ir and radio free-free radiation measurents (for density) and uv p-cygni profiles (for velocity), range from 5x10^-6 to 10^-4 m_sun/yr, with accuracies of at best a factor of 2. yet, even the lower estimates of wr mass loss rates indicate a wind momentum problem: the average wr star radiation field is not strong enough to drive the observed mass loss rates by radiation pressure alone. the assessment of more accurate mass loss rates with sws data will allow a better choice between possible accelleration mechanisms adding to the driving of the observed mass loss rates. energy distributions: discrepancies exists for many wr stars between the ir and radio spectral indices. where an ideal wind should display a spectral index of 0.6 for radiation originating in the constant velocity regime, wr stars show consistently larger values, at least between 2mu and 6cm. this may be due to their velocity fields and to recombination in the outer layers, and/or to the presence of non-thermal radiation sources in the winds. sws spectra will be allow the separation of emission lines and continuum in order to assess the real ir continuum. dust formation and episodic dust formation: the cause of dust formation in the hot winds of late wc-type stars, i.e., in a very hostile environment for dust formation, is not yet known. sws spectra will reveal dust energy distributions and dust features with unprecedented accuracy. a growing number of long-period eccentric wc+o binaries shows episodic dust formation. sws observations of the objects wr48a, wr98a, wr137 and wr140 will giving clues to the chemistry of very fresh dust. observation summary we plan to obtain full sws scans with aot sws06 for four wr stars and full sws scans with aot sws01 for ten wr stars. instrument: iso-sws grating mode aot sws06: grating scan aot sws01: low resolution full grating scan total tdt: 13.9 hours in case of orion hole total tdt: 13.7 hours in case of galactic center hole coordination: 9 objects with lws, 7 objects with phot priority 1: total tdt = 5.9 hr (orion hole), or 5.9 hr (galactic center hole) priority 2: total tdt = 4.3 hr (orion hole), or 5.7 hr (galactic center hole) priority 3: total tdt = 3.7 hr (orion hole), or 2.1 hr (galactic center hole) |
