An international team of astronomers led by members of the Laboratory for Space Research (LSR) and the Department of Physics at the University of Hong Kong (HKU) has discovered a rare celestial gem, called a planetary nebula (PN) in the interior of a 500 million year old galactic open cluster (OC) called M37 (also known as NGC2099). This is a very rare find of great astrophysical value. Their results have just been published in the open access article Astrophysical Journal Letters.
PNs are the ejected bright envelopes of dying stars that glow with a rich spectrum of emission lines and therefore display their distinct colors and shapes that make them photogenic magnets for public interest. It is no coincidence that one of the first images from the James Webb Space Telescope (the largest optical telescope in space) to be published was a PN.
The PN, with the rather clumsy name “IPHASX J055226.2+323724”, is only the third example of an association between a PN and an OC of the ~4,000 known PNe in our galaxy. It also appears to be the oldest PN ever found. The small team led by Professor Quentin PARKER, director of the HKU LSR, determined some interesting properties of their discovery: The authors found that PR has a “kinematic age” of 70,000 years. This estimate is based on the rate of expansion of the nebula, as determined from the PN emission lines, and assuming that this rate has remained the same since the beginning, and is the time that has passed since the nebula first was ejected from the host. , a dying star. This compares with typical PN ages of 5,000 to 25,000 years. It is indeed a grand old lady in terms of PN, but of course just a “blink” in terms of the lifetime of the original star itself, which spans hundreds of millions of years.
Because this “grand old lady” lives in a star cluster, this environment allows the team to determine powerful additional parameters that are not possible for the general galactic PN population. These include the estimate of the mass of the PN progenitor star as it extinguished the main sequence star, as derived from the observed properties of the thousands of stars in the cluster when plotted in a so-called colour-magnitude diagram. The team can also estimate the remnant mass of the central star that ejected PNe via theoretical isochrones and observe the properties of the hot, blue central star. As a result, they calculated the mass of the star that ejected the gaseous PN shell at birth and the amount of mass left in its shrinking residual hot core (which is already a so-called “white dwarf” star) . The new “Gaia” data for the central hot blue star PN also provide a good estimate of the distance to determine the true size of the PN at this extreme age of 3.2 per cent. (parsec, an astronomical unit of measurement for interstellar space with 1 percent corresponding to 3.26 light years) in diameter – not surprisingly perhaps also at the extreme end of known physical PN sizes.
Former HKU PhD student Dr. Vasiliki FRAGKOU, the study’s first author, said: “I am so excited to be able to work on these fascinating rare cases of OC-PN associations as they continue to yield results. important scientists, like the three cases we found of butterflies (bipolar) PNs in terms of shape, they are all very weak and highly developed, and all have type I chemistry based on their emission lines, and of course all have intermediate to high progenitors.
Corresponding author Professor Quentin Parker said: “This is only the third example of a PN found in a galactic open star cluster, and my group has found all three confirmed examples. They are incredibly rare, but also very important because these beautiful objects give us possibility to independently determine points on the so-called Initial to Final Mass Relationship (IFMR) for stars – an important astrophysical relationship – independently of the traditional method of using white dwarfs in clusters. Curiously, all our points fall just below the currently established empirical IFMR trend, but adds to the “bent” in this ratio recently found in the 2-3 solar mass range of the original progenitor mass by Marigo et al in the journal Nature Astronomy Our OC-PN points happen to be in the sparsely populated regions by IFMR, which makes them even more valuable. »
Co-author Prof. Albert ZILJSTRA, Hung Hing Ying Distinguished Visiting Professor in Science and Technology at HKU LSR, University of Manchester commented on the previously much shorter PN visibility lifetimes in the general galaxy. “This new result implies that the location of a PN in an OC provides a suitable environment for the PNe to expand and dissipate without interference from the surrounding ISM (which is typically much weaker in an OC) and not as the would be in The Galaxy.”
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