HD 120678 shows evidence of a tilting heartbeat disk

The classical Be star HD 120678 exhibits unique brightness variations caused by a disk of gas being gravitationally tilted by a companion star. This process, known as a heartbeat disk, creates significant photometric changes that differ from the typical behavior of other Be stars. The findings suggest that the disk orientation changes periodically, causing the star to appear differently from our perspective over time.

Star system: HD 120678 | Distance from the Sun: 5829 light years

Classical Be stars are massive, hot stars that rotate rapidly and eject gas into a disk around their equators. This circumstellar gas produces characteristic emission lines in the star's spectrum and causes various changes in brightness over time as the amount and distribution of the gas shift. The star HD 120678 has shown unusual behavior, including a notable photometric event between 2005 and 2008 where its brightness shifted significantly, reaching a peak-to-valley amplitude of approximately 0.9 magnitudes.

Observers have linked this variability to the presence of a heartbeat disk, a phenomenon where the gravitational pull of an additional companion star causes the disk of gas to tilt. During specific phases, the star displays shell-type spectral profiles, which indicate that the disk is oriented edge-on relative to our line of sight. Similar patterns of shifting between standard Be star phases and shell-star phases have been observed in other stars like gamma Cas, suggesting a rotation of the disk plane.

While heartbeat stars typically show minor variations in brightness, heartbeat disks in Be stars can produce amplitudes ten times larger. This mechanism provides a new explanation for the complex photometric patterns observed in classical Be stars. By analyzing the light curve data, the study clarifies that the observed variations in HD 120678 are consistent with the geometric changes of a disk influenced by an external gravitational source.

Key findings
HD 120678 experiences significant brightness variations driven by a heartbeat disk.
The disk of gas around the star is gravitationally tilted by the influence of a companion star.
Spectral profiles indicate the disk shifts between different orientations, sometimes appearing edge-on from Earth.
This heartbeat disk mechanism explains brightness fluctuations that are ten times larger than those seen in standard heartbeat stars.

Scientific study

HD 120678: a classical Be star with a tilting heartbeat disk (external link)