The consequences of a thermonuclear explosion in a binary star system about 3,400 light years away has been witnessed by Hubble Space Telescope.
HM Sagittae, or HM Sge for short, is what is known as a symbiotic system, in which a white dwarf is eating with a friend red giant star The stolen material forms an accretion disk orbiting the white dwarf. If too much material falls from the disc onto the white dwarf at once, the pressure and temperature become so great that a thermonuclear explosion erupts on the surface of the white dwarf.
Although this explosion is not enough to destroy the white dwarf into a supernova, it releases enough energy to cause the system to glow in what is called a “nova.”
Between April and September 1975, HM Sge went nova in the constellation Sagittarius, Sagittarius. It was lit in the night sky by six magnitudes from magnitude +17 (visible only to telescopes with apertures larger than about 305 mm/12 inches) to magnitude +10.5, at which point it became easier for telescopes with smaller apertures to equal to about 102 mm/4 inches, allowing amateur astronomers to keep track of it. This illumination equates to an increase in LIGHTENING of 250 times.
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Since nova went, HM Sge has not followed the rules. Most novas boil after a few days; HM Sge remained at the height of its brightness for years, until the mid-1980s, before it began to slowly fade, accompanied by more visible dimming events. Even now, it has only faded to about +12.
“In 1975 HM Sge went from a nondescript star to something every astronomer in the field was looking at, and in a moment the flurry of activity slowed,” said Ravi Sankrit of the Space Telescope Science Institute (STScI) in a. STATEMENT.
“Symbiotic stars like HM Sge are rare in our country galaxyand witnessing a nova-like explosion is even rarer,” added Steven Goldman, also of STScI, in the statement. “This unique event is a treasure trove for astrophysicists that spans decades.”
Observations over the years with a multitude of telescopes have tried to get to the bottom of what is going on in HM Sge. Now, Goldman and Sankrit have achieved new results with their team, based on Hubble Space Telescope 2021 observations and data collected with NASAnow it is powerless Sofia (Stratospheric Observatory for Infrared Astronomy), which featured an infrared telescope on the back of a Boeing 747 aircraft, in 2021 and 2022.
The onset of dimming relative to the system in 1985 has until now been attributed at least in part to the behavior of the red giant star. It is the one called Mira variable (after the class prototype, Mira – omicron Ceti – in the constellation Cetus, the Whale) and endures periodic pulsations approximately every 534 days. The beginning of the system’s decline in the mid-1980s has been attributed to one of two things. It could either have been driven by a larger-than-usual mass-loss event from the red giant associated with its pulsations, which would have created a dust plume that blocks some of the light, or it could be the result of years 90 -year, non-circular orbit of the white dwarf and red giant around each other pushing them apart, reducing the amount of material flowing between the two. Currently, the separation between the two system components is approximately 40 astronomical unit (AU), where 1 AU is defined as the average distance between earth AND our sun, 149.6 million kilometers (93 million miles). By comparison, Neptune it is 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 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. This makes it one of the hottest white dwarfs known, despite the system fading in brightness overall. What is causing this rise in temperature is currently a mystery.
Additionally, SOFIA was able to detect emission lines from water vapor in the disk for the first time in a symbiotic binary, and use its signal as a proxy for measuring the properties of the accretion disk. 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 spectrum of HM Sge are weakening compared to 1990, indicating that the system is changing and evolving slowly, perhaps as the red giant and white dwarf move away.
Goldman and Sankrit’s team concluded that the HM Sge system settled into a “new normal” fairly quickly after the 1975 nova with only a slow decline in brightness on average over the years (there have been some ups and downs in brightness , both in optical and infrared and not always the same time, again attributed to red giant behavior). The overall dimming may continue at its slow pace for many more years, until the white dwarf and red giant come closer together in their orbits, increasing the amount of material flowing between them and sparking another nova.
Finally, the white dwarf is a preview of what fate has in store for the red giant companion. Both were once like the sun STARS in a binary system, one star slightly more massive than the other. The more massive star used up 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 bit more slowly, but is now following the same path as its sibling, first becoming a red giant and then a white dwarf after a million or so years.
The gravitational upheaval that the transformation of the red giant will cause may pull the two white dwarfs close to each other. One day, if they collide, they will explode as a type Ia supernovabut that won’t happen for hundreds of millions, or maybe even billions of years.
Findings from Hubble and SOFIA were published in The Astrophysical Journal.