Contents of: VI/111/./abstract/KVDHUCHT_WRSTARS.abs

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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)



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