Hot Stars as Targets for Intensity Interferometry

Hannes Jensen, Lund Observatory

Intensity interferometry is particularly sensitive to hot objects
(as opposed to phase interferometry).  Being insensitive to atmospheric
turbulence, observations in the violet or blue pose no problem (as opposed
to phase interferometry), enabling efficient study of hot stars over very
long baselines.

Star catalogs were examined to extract a subset of the hottest and
visually brightest stars. In the Bright Star Catalogue there are 2639 stars
with B-V < 0.1 (equivalent to an effective temperature of around 8600 K).
Of these stars, a subset of 34 bright and hot (V < 2 and Teff > 9000K) or
very hot (Teff > 25000 K) stars was selected for closer examination. Typical
angular sizes for the stellar disks range between 1 and 5 mas.

The literature was examined for previous studies of such bright and
hot stars, and a subset of especially interesting stars selected as
potential candidates for observation.  Objects with overall angular sizes on the
order of 10 mas have been studied with phase interferometers in the (infra)red,
revealing tantalizing and not yet resolved structures.  Examples include the
complex and unstable star Eta Carinae, and the rapidly rotating Be-star
Achernar.  Such objects would be suitable targets also for II over moderate
baselines (50-100 m).

Objects with expected angular sizes in the range 0.1-5 mas are as
yet not resolvable with current phase interferometers, but will become
realistic targets for intensity interferometry at shorter optical
wavelengths, with telescopes spaced by (several) hundreds of meters (LBII,
Long Baseline Intensity Interferometry).

A selection of about twenty objects for LBII includes several rapidly 
rotating stars (Vsini > 150 km/s),which can be expected to be flattened 
akin to the previously known cases of Altair and Achernar.  A number  of 
emission-line stars (Be, Wolf-Rayet) apparently are surrounded by 
circumstellar disks, whose  sizes and shapes (as measured in emission 
lines) can be expected to differ from the sizes and shapes of the stellar 
disks (as seen in the spectral continuum).  Some very hot stars may have 
clumped structures in their outermost atmospheres or stellar winds, etc., 
distinguishable in different emission lines.   Since the S/N ratio for II 
is independent of spectral bandpass, measurements in individual spectral 
lines should give data of similar quality as those for the continuum, making 
observations of such features realistic.