Properties of the host stars of exoplanets are derived from a combination of astrometric, photometric, and spectroscopic observations, interpreted primarily within the context of stellar evolutionary models.

Planets are now known to exist around a wide variety of stellar types: not only around main sequence stars like the Sun, but around M dwarfs (§2.4.4) and as lower mass companions to brown dwarfs (§7.5), around pulsating stars including hot subdwarfs (§4.2.2) and δ Scu variables (WASP–33), around giants (§2.4.5), and around objects in the terminal stages of evolution including pulsars (§4.1) and probably white dwarfs (§4.2.1). They are found in binary systems (§4.3), around stars of the thick disk (e.g. WASP–21), around stars of low-metallicity both from radial velocity surveys (§2.4.6) and from transit surveys (e.g. WASP–37), in open clusters (§2.4.6), and perhaps in the bulge (MOA–2008–BLG–310).

Knowledge from astrometry

Hipparcos distances and proper motions

The majority of stars monitored for radial velocity or photometric transit observations are bright and consequently relatively nearby (d < 50 – 70 pc). Distances and proper motions are therefore generally well determined from the Hipparcos satellite measurements. Operated between 1989–93, Hipparcos provided 1 mas accuracy in positions, parallaxes and annual proper motions for about 120,000 stars (Perryman, 1997; Perryman et al., 1997), subsequently improved through an enhanced satellite attitude solution (van Leeuwen, 2007). These precise distances (Figure 8.1) translate into improved determination of the host star properties.