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HD 195019 Delphini

(The Planet Project)

HD 195019 is a double star, which makes it unusual for a planethost. The planet orbits the brighter primary star, the faintercompanion at least 150 AU away.

THE PLANET

The upper circle shows the location of the class G subgiant-dwarfstar HD 195019, found in the constellationDelphinus. The planet, whose mass is at least 3.43times that of Jupiter, orbits with a very short period of only 18.3days at an average distance of just 0.14 Astronomical Units (21million kilometers, 13 million miles, 36 percent the distance ofMercury from the Sun), the proximity to its star making it a "hotJupiter." Consistently, the orbit is close to circular, theeccentricity only about 5 percent.

THE STAR

HD 195019 is a seventh magnitude (6.91) star inDelphinus that is below naked eye vision (though easilyaccessible with binoculars) and classed as a G3 subgiant-dwarf(implying that it is close to giving up core hydrogen fusion). Much too faint to have aproperorGreek letter name, or even Flamsteed number, it is knownbest by its number in theHenry Draper (HD)Catalogue. From a distance of 122 light years, the starshines with a luminosity 2.0 times that of theSun with a surface temperature of 5840 Kelvin(only 60 degrees warmer than the Sun), from which we derive a massof 1.06 solar and a radius of 1.4 solar (after adjusting for thelight from the faint companion). The relatively high luminosityfor a near-solar-mass star shows that HD 195019 is older then theSun, is nearing the end of its hydrogen-fusing life, and may indeedbe close to true subgiant status. Typical of stars with planets,the iron abundance higher than normal, 20 percent greater thansolar. HD 195019 seems to be a truebinary. Four seconds of arcaway (3.5" as of 1988) is an 11th magnitude (10.6) class K3companion. While little orbital motion is seen, the two are movingtogether, implying real association. The two are at least 150 AUapart and orbit with a period of at least 1500 years. From HD195019's planet, the companion would shine with the brightness ofthree full Moons. A bit over a minute of arc away lie two otherstars of tenth magnitude, one of which is clearly just a line-of-sight coincidence. The status of the other one is not known.HD 195019 is "high velocity star" that speeds along at 112kilometers per second relative to the Sun, some 7 times higher thanaverage, suggesting that it is a visitor from a different part ofthe Galaxy.

HR 7907 Delphini, just below the centerof the picture, also has an orbiting planet.

Written byJim Kaler. Return toThe Planet Project or go toSTARS.

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