V1794 Cygni

Lore & Background

Spectrographs of HD 199178 taken in 1926 showed unusually diffuse lines for a star of its type. In 1981, a broad, variable emission of Hydrogen alpha was noted, and the star showed photometric variability with a period of a few days. It showed extreme rotational broadening of the absorption lines but no variation in radial velocity. The existence of a class of single, cool, rapidly rotating giant stars such as FK Com and HD 199178 presented a puzzle for astronomers. The star showed a variable polarization with a period of four days, suggesting chromospheric activity with an asymmetric distribution of star spots and faculae, modulated by rotation. Photometric measurements established a rotation period of 3.337 days by 1983.

V1794 Cygni was classified as a Herbig Ae/Be star in 1999 in a catalog of UX Orionis stars. However, this proved incorrect because of the star's fast rotation compared to other giants and it is now classified as an FK Comae Berenices variable. The stellar classification of V1794 Cygni is G7III-IVne, matching an evolved subgiant/giant star with emission lines (e) plus 'nebulous' lines (n) due to rapid rotation. It is currently crossing the Hertzsprung gap, which indicates it has ceased core hydrogen burning.

This star has 165% of the mass and 6.1 times the radius of the Sun, but, with an age of around 2 billion years is younger than the latter. It is spinning with a projected rotational velocity of 84 km/s. V1794 Cygni is radiating 13 times the luminosity of the Sun from its enlarged photosphere at an effective temperature of 5,180 K, which gives it a yellow hue. For a giant star it has an exceptionally high X-ray emission, which suggests there is a high surface density of active regions.

Reader's Guide

V1794 Cygni is significant as a member of the FK Comae Berenices class of single, cool, rapidly rotating giant stars, which initially puzzled astronomers due to the lack of a binary companion to explain the high rotation. Its history includes an initial misclassification as a Herbig Ae/Be star in 1999, later corrected based on its fast rotation relative to other giants. The star's extreme rotational broadening and lack of radial velocity variation, combined with variable polarization and photometric periodicity, point to chromospheric activity modulated by rotation, with star spots and faculae. Its exceptionally high X-ray emission for a giant star indicates a high surface density of active regions. The star's current crossing of the Hertzsprung gap confirms it has ceased core hydrogen burning, placing it in a transitional evolutionary phase. The star's properties—mass 165% solar, radius 6.1 solar, luminosity 13 solar, and effective temperature 5,180 K—provide a detailed portrait of an evolved star in rapid rotation.

Did You Know?

• V1794 Cygni has a rotation period of 3.337 days, established by photometric measurements by 1983.
• It was misclassified as a Herbig Ae/Be star in 1999 before being reclassified as an FK Comae Berenices variable.
• The star shows variable polarization with a period of four days, suggesting chromospheric activity with asymmetric star spots and faculae.
• For a giant star, V1794 Cygni has exceptionally high X-ray emission, indicating a high surface density of active regions.