The aftermath of a thermonuclear explosion in a double star system about 3,400 light-years away has been observed by the Hubble Space Telescope.
HM Sagittae, or simply HM Sge, is a so-called symbiotic system, in which a white dwarf feeds on a companion red giant star. The stolen material forms an accretion disk that swirls 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 detonates on the white dwarf's surface.
Although this explosion is not enough to destroy the white dwarf in a supernova, it does release enough energy to make the system brighten in what is called a 'nova'.
Between April and September 1975, HM Sge went nova in the constellation Sagitta, the Arrow. It cleared up in the night sky with six magnitudes from magnitude +17 (visible only to telescopes with apertures larger than about 305 mm/12 in) to magnitude +10.5, at which point it became more easily visible to telescopes with smaller apertures equal to about 102 mm/4 in. , allowing amateur astronomers to keep an eye on it. This clarification corresponds to an increase of Brightness of 250 times.
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Since the nova, HM Sge has not followed the rules. Most novas simmer after a few days; HM Sge remained at its maximum brightness for many years, until the mid-1980s, before the brightness slowly began to fade, interrupted by more noticeable dimming events. Even now it has only faded to about magnitude +12.
"In 1975, HM Sge went from an unremarkable star to something that all astronomers in the field were watching, and at some point the wave of activity subsided," says Ravi Sankrit of the Space Telescope Science Institute (STScI). rack.
'Symbiotic stars like HM Sge are rare in our world universe and witnessing a nova-like explosion is even rarer," Steven Goldman, also of STScI, added in the statement. "This unique event is a treasure for astrophysicists for decades."
Observations over the years with a large number of telescopes have tried to get to the bottom of what is happening in HM Sge. Now Goldman and Sankrit and their team have achieved new results, based on observations from the 2021 Hubble Space Telescope and data collected with NASA'is now defunct SOFIA (Stratospheric Observatory for Infrared Astronomy), with an infrared telescope in the back of one Boeing 747 aircraft, in 2021 and 2022.
The onset of dimming related to the system in 1985 to date is at least partially attributed to the behavior of the red giant star. It's what's called a Mira variable (after the prototype of the class Mira - omicron Ceti - in the constellation Cetus, the Whale) and undergoes periodic pulsations approximately every 534 days. The beginning of the system's dimming in the mid-1980s has been attributed to one of two things. It could either have been caused by a larger-than-normal mass loss from the red giant, coupled with its pulsations, which would have caused an outpouring of dust that blocks some of the light, or it could be the result of the 90-year , non-circular orbit of the white dwarf and red giant around each other, pushing them further apart, reducing the amount of material flowing between the two. Currently the separation between the two components of the system is approximately 40 astronomical units (AU), where 1 AU is defined by the average distance between Soil And our sun, 149.6 million kilometers (93 million miles). For comparison, Neptune is located 30 AU from the Sun.
Hubble's observations also revealed a strong emission line from ionized magnesium. This emission line was not present in spectra of HM Sge dating back to 1990, when the white dwarf's temperature was 200,000 degrees Celsius (about 400,000 degrees Fahrenheit). For highly ionized magnesium to exist in large quantities, the white dwarf's temperature must have risen to 250,000 degrees Celsius (about 450,000 degrees Fahrenheit) during that time. This makes it one of the hottest white dwarfs known, despite the system's overall brightness decreasing. What is causing this temperature increase is currently a mystery.
Furthermore, for the first time, SOFIA was able to detect emission lines of water vapor in the disk in a symbiotic binary, and use its signal as a proxy for measuring the properties of the accretion disk. It appears that the water molecules are moving at a speed of 29 kilometers per second, which is attributed to their speed flowing along the edge of the disk.
However, most of the emission lines in HM Sge's spectrum have become fainter compared to 1990, showing that the system is slowly changing and evolving, possibly as the red giant and white dwarf move apart.
The Goldman and Sankrit team conclude that the HM Sge system settled into a "new normal" quite quickly after the 1975 nova explosion, with on average only a slow decline in brightness over the years (there has been some increase and decrease in brightness, both in optical and infrared and not always the same time, again attributed to the behavior of the red giant). The overall fading may continue at its slow pace for many years until the white dwarf and red giant come close to each other in their orbits again, increasing the amount of material flowing between them and creating a new nova.
Finally, the white dwarf is a foretaste of what fate has in store for the red giant. Both were once sunny stars in a binary system, one star is slightly more massive than the other. The more massive star consumed its nuclear fuel more quickly and evolved into a red giant that eventually shed its diffuse outer envelope and exposed its exposed, inert core: the white dwarf. The other star evolved a little slower, but is now following the same path as its sibling, first evolving into a red giant and then after about a million years into a white dwarf.
The gravity that will cause the red giant's transformation could bring the two white dwarfs closer together. If they collide one day, they will explode like a Type Ia supernova but that won't happen for hundreds of millions, or perhaps even billions, of years.
The Hubble and SOFIA findings were published in The Astrophysical Journal.
