Technical paper here:
https://arxiv.org/pdf/1608.02945.pdf
I was wrong about A stars being outside the instability strip. This paper is specifically about main sequence stars that are inside the instability strip, to whit Delta Scuti variables. These are class A and F stars almost on the main sequence, as shown in the diagram below.
Radial velocity observations of cool sub-giants, most
notably via the ‘Retired A stars’ project, suggest panets around
A stars in the classical instability strip, where
delta Scuti (δ Sct) pulsators are common. The apparent
planet deficit around these main-sequence stars can
be explained as an observational selection effect, caused
by problems in the application of the most successful
planet-hunting methods to these types of stars.
The transit method has difficulty because of the pulsational
luminosity variations, which amount to several mmag,
and because planets occupy wider orbits around A stars, resulting in a
lower transit probability.
The radial velocity method,
on the other hand, is particularly hindered by the nature
of A-type spectra. A stars are typically fast rotators,
with the mean of the equatorial rotational velocity
distribution exceeding 100 km/s. Their high effective temperatures
lead to fewer, shallower absorption lines, and these
lines can be distorted by pulsation. Therefore the wavelengths
of their spectral lines are not a precise standard
of measure.
Fortunately, the same pulsations can themselves be used
as precise clocks for the detection of orbital motion. The pulsations
of δ Sct stars are particularly well suited to this task.
