Planetary nebulae are short-lived “remnants” of Sun-like stars. Most of these “star ghosts” only last – at most – about 25,000 years. Usually, debris clouds are so widely dispersed that they vanish fairly quickly. However, there is one that lasted at least 70,000 years. This makes her the “Great Lady” of the planetary nebulae.
A team of astronomers led by members of the Space Research Laboratory (LSR) and the Department of Physics at the University of Hong Kong have discovered this rare celestial gem in the open star cluster M37. It orbits our galaxy in the same galactic arm as the sun and contains about 1,500 solar masses. The object called IPHASX J055226.2 + 323724 is the third known planetary nebula associated with an open cluster in our galaxy. So, how do astronomers know it’s so old?
Understanding This Elderly Planetary Nebula
A planetary nebula is a hot white dwarf star surrounded by a crust of material that has been ejected with age. In some planetary nebulae, the shell is nearly circular, while in others, it can be bipolar. The radiation from the star heats up the nebula, causing it to glow. It may seem difficult to determine the age of the expanding cortex. However, there are ways. The team that discovered this object, led by Quentin Parker of the University of Hong Kong, found that it has a “kinetic age” of 70,000 years. This is an estimate, but a good one, based on how fast the nebula is expanding. The clues lie in the light-emission spectrum of hot, glowing gas in the expanding envelope around the dying star. Those are the “emission lines”.
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The team also assumed that the speed of expansion had remained the same from the start. Combining all of that, you get the time elapsed since the star first died and pulled out its outer layers. In this case, it goes back 70 thousand years. By comparison, most “typical” planetary nebulae only last about 5,000 to 25,000 years. This is a relatively fast time, compared to the age of the star, which would have been about hundreds of millions or billions of years.
Additionally, the fact that the IPHASX J055226.2 + 323724 is still visible despite its age and expanded size, makes it doubly rare. His location states that he “lives” in a relatively “safe” environment. This house in an open block allows the expanding cloud of gas and dust to vanish. If the star is in the interstellar medium, the crust can be disrupted by other effects.
Tech Talk About IPHASX J055226.2 + 323724
The environment of this planetary nebula within a star cluster gave the science team further insight into the original star that created the nebula. Right now, it’s a white dwarf, the aging superstar after it shrinks in on itself and sheds its outer layers. It is an extremely hot object that will take billions of years to cool down. When it was still a sun-like star, it was about 2-3 solar masses. The team was able to estimate the mass it contained when it first expelled the material that became the crust and know its mass now. Using the Gaia distance data, they also discovered that the expanding shell of matter is now about 3.2 parsecs. (For comparison, the distance between the Sun and its nearest star, Proxima Centauri, is 1.3 parsecs.) This places the crust at the very large end of known planetary nebula sizes.
Dr Vasiliki Frajko, first author of the paper describing work on this nebula, described the Open Cluster Nebula Planetary Environment (OC-PN) as “I am very excited to be able to work on these wonderful rare instances of OC-PN correlations as they continue to show scientific results.” Important, like the three states we found are butterfly (bipolar) PN shaped, all very faint, highly developed, all have type I chemistry according to their emission lines, and of course, all have medium to high ancestral masses.”
It’s rare but useful
As this is only the third known example of a planetary nebula in an open star cluster in our galaxy, it provides intriguing clues to the formation of the cluster where IPHASX J055226.2 + 323724 lives. Corresponding author Professor Quentin Parker noted that his group found all three confirmed examples. “It’s incredibly rare, but it’s also very important,” he said, “that these beautiful things allow us to independently determine points on the so-called initial-to-final mass relationship (IFMR) of stars.”
The initial mass function describes the distribution of stellar masses that form in a single star formation event of a given size in space. They cover all stellar bodies, from low-mass brown dwarfs to the most massive stars ever formed. It is important in clusters as a way to understand the range of stars they contain, and, in addition to the presence of IPHASX J055226.2 + 323724, gives insight into the types of stars in M37.
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Oldest visible planetary nebula discovered
The planetary nebula in the open cluster M37 of 500 Myr