by After a thermonuclear explosion in a binary star system about 3,400 light years away has been witnessed by the Hubble Space Telescope.
HM Sagittae, or HM Sge for short, is the so-called symbiotic system, in which a white ox feeding off a companion red giant star. The stolen material forms an accretion disk that surrounds the white dwarf. If too much material falls from the disk onto the white dwarf at the same time, the pressure and temperature become so great that a thermonuclear explosion detonates 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 brighten the system in what is called a “nova.”
Between April and September 1975, HM Sge nova passed in the constellation of Sagitta, the Arrow. He brightened in the the night sky by six magnitudes from magnitude +17 (visible only to telescopes with apertures greater than about 305mm/12 inches) to magnitude +10.5, at which point it was easier to see with telescopes with larger apertures less equal to about 102mm/4 inches, which allowed amateur astronomers to keep track of it. This brightness represents an increase in luminosity of 250 hours.
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Since its inception, HM Sge has not been following the rules. Most novas simmer down after a few days; HM Sge remained at its peak brightness for years, until the mid-1980s, before it began to slowly deteriorate, with more significant attenuation events. Even now, it’s only gone to about size +12.
“In 1975 HM Sge went from being a nondescript star to something that all the astronomers in the field were looking at, and at some point the flurry of activity slowed,” said Ravi Sankrit of the Space Telescope Science Institute (STScI) in a statement statement.
“Symbiotic stars like HM Sge are rare among us galaxyand seeing a Nova-like explosion is still rare,” Steven Goldman, also of STScI, said in the statement.
Observations over the years with many telescopes try to get to the bottom of what is happening in HM Sge. Now, Goldman and Sankrit and their team have reached new results, based on Hubble Space Telescope 2021 observations and data collected with NASA‘it is now obsolete SOFIA (Stratospheric Observatory for Infrared Astronomy), which had an infrared telescope on the back of a Boeing 747 aircraft, in 2021 and 2022.
The onset of the system’s slowdown in 1985 has so far been attributed, at least in part, to the behavior of the red star. It is called Mira variable (after the class prototype, Mira — omicron Ceti — in the constellation Cetus, the Great Whale) and has periodic pulses approximately every 534 days. The easing of the system began in the mid-1980s for one of two things. It could be that there was a mass event larger than normal loss from the red giant connected to its rags, which created a dust discharge that blocks some of the light, or it could be the result of the 90. -year, The non-circular orbits of the white dwarf and red giant around each other take 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 as the average distance between World and our sun, 149.6 million kilometers (93 million miles). For comparison, Neptune It is 30 AU from the sun.
Hubble’s observations also showed a strong emission line from ionized magnesium. This emission line was not present in the spectrum of HM Sge dating back to 1990, when the temperature of the white dwarf was 200,000 degrees Celsius (about 400,000 degrees Fahrenheit). For strongly ionized magnesium to exist in great abundance, the white dwarf’s temperature must have risen to 250,000 degrees Celsius (about 450,000 degrees Fahrenheit) at that time. As such, it is one of the hottest white dwarfs known, despite the fact that the system is collapsing as a whole. What is causing this rise in temperature is currently a mystery.
In addition, SOFIA was able to detect emission lines from water vapor in the disc for the first time in a symbiotic binary, and use its signal as a proxy to measure the properties of the accretion disc. The water molecules appear to be moving at 29 kilometers (18 miles) per second, which is attributed to their speed flowing around the edge of the disk.
However, most of the emission lines in the HM Sge spectrum are weakening compared to 1990, indicating that the system is changing and evolving slowly, perhaps as the red giant and white dwarf apart.
Goldman and Sankrit’s team conclude that the HM Sge system settled down to a “new normal” fairly quickly after the 1975 nova explosion with only a slow decrease in brightness on average over the years (there are some ups and downs in brightness, i. optical and infrared and not always the same time, again attributed to the behavior of the red giant). The overall fading could continue at its slow pace for many more years, until the white dwarf and red giant come close again in their orbits, increasing the amount of material flowing between them and sparking another new.
Finally, the white dwarf is a preview of what’s in store for the red companion. Both were sun-like once stars in a binary system, one star is slightly larger than the other. The massive star used up its nuclear fuel faster, and grew into a red giant that threw away its diffuse outer shell to reveal its bare, inert core — the white dwarf. The other star evolved a bit more slowly, but is now following the same path as its sibling, transforming first into a red giant and then a white dwarf after a million years or so.
The gravitational disturbance resulting from the transformation of the red giant could pull the two white dwarfs close to each other. One day, if they collide, they will explode as a Type Ia supernovabut that will not happen for hundreds of millions, or perhaps even billions, of years.
The results from Hubble and SOFIA were published in The Astrophysical Journal.
