Image of the Week

Gaia observes cosmic clock inside a heavenly jewel

Figure 1. Planetary nebula Heckathorn-Fesen-Gull 1 observed from Earth. Gaia observed the blue star V664 Cas at its center over a hundred times in 2014-2017. This close binary shows very regular brightness changes (top left inset panel) due to a bright surface spot. The bottom right panel shows bright lines of hydrogen gas in the Gaia spectrum, classifying it as an ´emission´ line star. Image credits: Peter Goodhew - Inset graphics: ESA/Gaia/DPAC

Since its launch in 2013 Gaia has repeatedly observed the central stars of Planetary Nebulae (abbreviated by PNe). The name ´Planetary Nebula’ is not related to planets, but arose in the 18th century because of the visual similarity between some round nebulae visible in the sky and the planets Uranus and Neptune as observed in small optical telescopes. The name has stuck, although modern telescopes make it obvious that they are not planets at all.

PNe are a type of emission nebula ejected during the final stages in the life of a star like our Sun. All single stars with masses between one and eight times the solar mass, will eventually develop a PN. They often reveal magnificent colourful displays of varied morphologies with intricate filaments and knots surrounding the parent star’s remnant. This schematic view of the origin of PNe is somehow complicated by the fact that many stars are born and live in binary systems, where the proximity of 2 stars can allow mass transfer between them, sometimes creating a common envelope surrounding the 2 stars. This makes more uncertain the relation between the existence of a PN and the mass of its progenitors, and specific observations are needed to understand the system.

The old PN Heckathorn-Fesen-Gull 1 (also HFG1), shown in Figure 1, seems to be one of such cases. This PN moves through the interstellar medium at a very high speed, creating a shock wave in the surrounding hot gas, seen as a bright blue arc to the lower left, just like a boat ploughing through water. Behind this PN is a long cometary-like wake of material, about one third of a degree long in the sky, shining in red light.

The blue star at the center of HFG1 is the known binary V664 Cassiopeiae (V664 Cas), observed by Gaia over one hundred times so far (and counting). The red drawn curves in the top left inset panel show the very periodic brightness changes (in Gaia's G, BP and RP bands) due to the relative motion of a close pair of stars separated by merely a few million kilometers. A small but very hot dwarf star irradiates part of a larger and cooler star (like our Sun or a red giant) heating a bright area of its surface. Gaia observes how the hot spot´s projected size and apparent brightness varies while both stars orbit each other. The new observations offered in Gaia´s Data Release 3 were published online in June 2022 and provide a very accurate orbital period of 13 hours, 57 minutes and 35 seconds. Truly a hefty clockwork in our Galaxy.

Within the Gaia Data Analysis and Processing Consortium (DPAC) a wide variety of objects showing so-called bright ´emission´ lines in their spectra are classified. This classification is part of the processing done by Gaia's Coordination Unit 8, the unit responsible for the processing of Astrophysical Parameters. In the case of V664 Cas, Gaia observed the bright lines of ionized hydrogen, marked with Halpha and Hbeta in the bottom right inset panel. Astronomers use these and other emission lines from other elements for measuring the chemical composition, temperature and density of the nebular gas, or in regions where they can also form closer to the central binary. The gas temperature of PNe can reach ten thousand degrees Celsius, whereas their central stars are among the hottest known stars in the Universe with temperatures ranging from twenty-five thousand to two hundred thousand degrees Celsius

Gaia has not only observed the precise period and the emission lines of V664 Cas, it also measured its parallax, allowing to place the star at 707 ±7 parsec (2,300 lightyears) from us. A measure of the distance allows to measure the true velocity of the star, and this is crucial to develop reliable theoretical models of the formation history of HFG1's shock region and the complex dynamics of its trailing outflow. The true size of the extended outer shell of shocked gas is important to explain the rare morphology of HFG1, resembling a giant jellyfish swimming faster than the speed of sound through the thin gas that exists in the interstellar space.

The red gas trail in Figure 1 of at least 20 arc minutes long in the sky is left behind by V664 Cas and estimated to be about ten thousand years old. HFG1 is therefore already old and will gradually dissolve in space, while its central binary star will cool and fade for billions of years. Our Sun is expected to experience the same process as HFG1, but fortunately not for another five billion years.

Gaia will continue observing HFG1 and other emission line stars in the next years. The combination of Gaia’s astrometric, photometric and spectroscopic measurements will provide new and fundamental insights in the physics of these enigmatic objects.

Credits: ESA/Gaia/DPAC, A. Lobel, R. Sordo, O. Creevey, P. Garcia-Lario, F. Thevenin, T. Roegiers on behalf of Gaia DPAC coordination unit 8.

[Published: 31/10/2023]