Of the trillions of stars scattered throughout the universe, one of the most common are the white dwarfs, which are the dormant, burned out, and leftover cores of low/medium mass stars. For decades, scientists have only measured the masses of white dwarfs within binary star systems. While these measurements provide insight into the true masses of white dwarfs, the measurements typically feature high amounts of uncertainty.
With help from the joint NASA/European Space Agency (ESA) Hubble Space Telescope, a team of researchers directly measured the mass of an isolated white dwarf outside of a binary star system. Found to be approximately 56% of the Sun’s mass, the team’s results agree with previous white dwarf mass predictions and provide insight into the evolutionary processes of dead stars.
To measure the mass of the white dwarf, named LAWD 37, the team utilized gravitational microlensing — a natural phenomenon in which the gravity from a cosmic object in front of another warps the light from the background object. In the case of LAWD 37’s observations, light from a star behind the white dwarf was slightly warped as the white dwarf passed in front of the star, seemingly moving the background star to a different location in the sky.
Precisely measuring the amount of light deflected by LAWD 37’s gravity allowed the team to determine the mass of the white dwarf. Interestingly, using gravitational microlensing to measure the mass of a white dwarf is not new, and was first used in 2017 by Kailash Sahu of the Space Telescope Science Institute in Maryland to measure the mass of Stein 2051 b. However, much like the observations before Sahu’s, the white dwarf was a member of a binary star system.
“Our latest observation provides a new benchmark because LAWD 37 is all by itself,” said Sahu, who served as the principal Hubble investigator during the LAWD 37 observations.
But what about LAWD 37 made it the perfect candidate for the team to study?
First, LAWD 37 is a single, isolated white dwarf that is not part of a binary star system. This means that the gravity measurements the team collected would not be warped by the gravity of a second star within a binary, giving them clear and precise measurements for LAWD 37’s mass.
Second, LAWD 37 is very close to Earth — only 15 light-years away from our solar system in the constellation Musca. Using multiple observations from ESA’s Gaia space observatory, the team was able to predict when LAWD 37 would pass in front of a background star. Gaia takes extraordinarily precise measurements of the positions of nearly two billion stars in the universe, and the motion of a specific star can be determined over multiple observations by Gaia.
“Because this white dwarf is relatively close to us, we’ve got lots of data on it — we’ve got information about its spectrum of light, but the missing piece of the puzzle has been a measurement of its mass,” said lead author Peter McGill of the University of California, Santa Cruz.
The Gaia data showed McGill et al. that LAWD 37 would pass in front of the background star in November 2019. With this information, the team then enlisted the help of Hubble, which was used to precisely measure the (apparent) positional change of the background star over several years.
“These events are rare, and the effects are tiny. For instance, the size of our measured offset is like measuring the length of a car on the Moon as seen from Earth,” McGill said.
What’s more, extracting the light from the faint background star proved challenging for the team, as the glare from LAWD 37 is approximately 400 times brighter than the observed light of the background star. Hubble is the only space telescope that can perform high-contrast observations in visible light, making it the perfect tool for the team.
After Hubble had collected all the needed data, McGill et al. were able to precisely calculate the mass of LAWD 37, which came to be around 56% of the mass of our Sun. The mass measurements will allow the team to test the mass-radius relationship for white dwarfs, which will then allow them to test the theory of degenerate matter — which states that within stars and other extreme cosmic objects, gas is so compressed under the object’s gravity that it begins to behave like solid matter.
“The precision of LAWD 37’s mass measurement allows us to test the mass-radius relationship for white dwarfs. This means testing the theory of degenerate matter under the extreme conditions inside this dead star,” said McGill.
McGill et al.’s mass calculation method is expected to open the door for predicting future white dwarf occultation events using Gaia. Furthermore, NASA’s James Webb Space Telescope is capable of observing these occultation events like Hubble. With Webb’s extreme sensitivity to infrared light, the white dwarf in Webb’s observations will appear dimmer than the background star — perfect for measuring the change in the apparent position of the background star.
With Hubble, astronomers measured the mass of a single, isolated white dwarf – which is the surviving core of a burned-out star.
The white dwarf, called LAWD 37, is 56% the mass of our Sun: https://t.co/wIg48n2jPY
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— Hubble (@NASAHubble) February 2, 2023
“Gaia has really changed the game — it’s exciting to be able to use Gaia data to predict when events will happen, and then observe them happening. We want to continue measuring the gravitational microlensing effect and obtain mass measurements for many more types of stars,” McGill said.
Webb is already being used to observe a white dwarf occultation event. With help from Gaia, Kailash Sahu is currently using Webb to observe LAWD 66, which is currently in the process of occulting the background star Sahu will use to measure the white dwarf’s mass. The first observation of LAWD 66 was performed in 2022, and Webb is expected to continue observing the white dwarf through 2024.
(Lead image: White dwarf LAWD 37, as imaged by Hubble. Credit: NASA/ESA/Peter McGill (UC Santa Cruz, IoA)/Kailash Sahu (STScI)/Joseph DePasquale (STScI))