The consequences of a thermonuclear explosion in a binary star system about 3,400 light years away was observed by the The Hubble Space Telescope.
HM Sagittae, or HM Sge for short, is what is called a symbiotic system, in which a white dwarf feeds on a mate red giant star. The stolen material forms an accretion disk swirling around the white dwarf. If too much material falls from the disk onto the white dwarf at once, the pressure and temperature become so high that a thermonuclear explosion explodes on the surface of the white dwarf.
Although this explosion is not enough to destroy the white dwarf in a supernova, it releases enough energy to cause the system to brighten in what is called a “nova.”
Between April and September 1975, HM Sge went nova in the constellation Sagitta, the Arrow. It lit up in the night sky of six magnitudes, from magnitude +17 (visible only to telescopes with apertures larger than about 305 mm/12 in) to magnitude +10.5, in which case it became more readily visible to telescopes with equal smaller apertures at approximately 102 mm/4 inches, allowing amateur astronomers to track it. This lightening is equivalent to an increase in brightness 250 times.
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Since it became Nova, HM Sge has not followed the rules. Most novas simmer after a few days; HM Sge remained at its peak brightness for years, until the mid-1980s, before it began to slowly fade, punctuated by more notable dimming events. Even now it has only diminished to a magnitude of around +12.
“By 1975, HM Sge went from being a nondescript star to something that every astronomer in the field was observing, and at some point the whirlwind of activity slowed,” said Ravi Sankrit of the Space Telescope Science Institute ( STScI) in a report. statement.
“Symbiotic stars like HM Sge are rare in our galaxyand witnessing a nova-like explosion is even rarer,” Steven Goldman, also of STScI, added in the release. “This unique event is a decades-long treasure trove for astrophysicists.”
Over the years, observations with a multitude of telescopes have attempted to shed light on what is happening at HM Sge. Today, Goldman and Sankrit obtained new results with their team, based on observations from the 2021 Hubble Space Telescope and data collected with NASAis now gone SOFIA (Stratospheric Observatory for Infrared Astronomy), which featured an infrared telescope on the back of a Boeing 747 aircraft, in 2021 and 2022.
The beginning of the system’s dimming in 1985 to the present has been attributed at least in part to the behavior of the red giant star. It’s called a Mira variable (based on the prototype of the class, Mira — omicron Ceti — in the constellation Cetus, the Whale) and undergoes periodic pulsations approximately every 534 days. The start of system attenuation in the mid-1980s has been attributed to one of two reasons. This could have been caused either by a larger than usual mass loss event of the red giant linked to its pulsations, which would have created a dust spill blocking some of the light, or by the result of the 1990s .-year, the non-circular orbit of the white dwarf and the red giant relative to each other pushes them further apart, thereby reducing the amount of material flowing between the two. Currently, the separation between the two system components is approximately 40 astronomical units (AU), where 1 AU is defined by the average distance between Earth And our sun, 149.6 million kilometers (93 million miles). In comparison, Neptune is 30 AU from the sun.
Hubble observations also revealed a strong emission line from ionized magnesium. This emission line was not present in HM Sge spectra from 1990, when the white dwarf’s temperature was 200,000 degrees Celsius (about 400,000 degrees Fahrenheit). For highly ionized magnesium to exist in great abundance, the temperature of the white dwarf must have increased by that time to 250,000 degrees Celsius (about 450,000 degrees Fahrenheit). This makes it one of the hottest white dwarfs known, despite the overall decrease in the system’s brightness. The cause of this temperature increase currently remains a mystery.
Additionally, SOFIA was able to detect for the first time the water vapor emission lines in the disk in a symbiotic binary and use its signal as an indicator to measure the properties of the accretion disk. The water molecules appear to be moving at a speed of 29 kilometers (18 miles) per second, which is attributed to their speed traveling around the edge of the disk.
However, most of the emission lines in the HM Sge spectrum are weakening compared to 1990, showing that the system is changing and evolving slowly, perhaps as the red giant and white dwarf move away.
Goldman and Sankrit’s team conclude that the HM Sge system settled into a “new normal” fairly quickly after the 1975 nova explosion, with only a slow decrease in brightness on average over the years ( there were ups and downs in brightness, both in optical and infrared and not always at the same time time, again attributed to the behavior of the red giant). The global disappearance could continue at its slow pace for many more years, until the white dwarf and the red giant approach each other again in their orbit, increasing the amount of matter flowing between them and triggering another nova.
Finally, the white dwarf is a glimpse of what fate has in store for the fellow red giant. Both once looked like the sun stars in a binary system, one star a little more massive than the other. The more massive star exhausted its nuclear fuel more quickly and evolved into a red giant that eventually shed its diffuse outer envelope to reveal its exposed inert core – the white dwarf. The other star evolved a little more slowly, but is now following the same path as its sister, evolving first into a red giant and then into a white dwarf after about a million years.
The gravitational upheaval caused by the transformation of the red giant could bring the two white dwarfs closer together. One day, if they collide, they will explode as Type Ia. supernovabut that won’t happen for hundreds of millions or even billions of years.
The Hubble and SOFIA findings were published in The Astrophysics Journal.