In November 2019, the nearby single, isolated white dwarf LAWD 37 aligned closely with a distant background source and caused a so-called microlensing event. Leveraging astrometry from ESA’s Gaia mission and follow-up data from the NASA/ESA Hubble Space Telescope, astronomer measured the astrometric deflection of the background source and obtain a mass for LAWD 37.
This image shows how microlensing was used to measure the mass of the white dwarf LAWD 37. The inset box plots how the dwarf passed in front of a background star in 2019. The wavy blue line traces the dwarf’s apparent motion across the sky as seen from Earth. Image credit: NASA / ESA / Peter McGill, UC Santa Cruz, IoA / Kailash Sahu, STScI / Joseph DePasquale, STScI.
LAWD 37 is located approximately 15 light-years away in the constellation of Musca.
Also known as WD 1142-645, this white dwarf has been extensively studied.
“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 Dr. Peter McGill, an astronomer at the University of California, Santa Cruz.
In his general theory of relativity, Albert Einstein predicted that when a massive compact object passes in front of a distant star, the light from the star would bend around the foreground object due to its gravitational field. This effect is known as gravitational microlensing.
In 1919, British astronomers Arthur Eddington and Frank Dyson first detected this effect during a solar eclipse, in what was the first popular confirmation of general relativity.
In 2017, astronomers detected this gravitational microlensing effect for another nearby white dwarf in a binary system, Stein 2051b, which marked the first detection of this effect for a star other than our Sun.
Now, Dr. McGill and colleagues have detected the effect for LAWD 37, giving the first direct mass measurement for a single white dwarf.
Using data from Gaia, they were able to predict the movement of the star and identify the point where it would align close enough to a background star to detect the lensing signal.
They then pointed Hubble in the right place at the right time to observe this phenomenon, which happened in November 2019, 100 years after the famous Eddington/Dyson experiment.
Since the light from the background star was so faint, the main challenge for astronomers was extracting the lensing signal from the noise.
“These events are rare, and the effects are tiny,” Dr. McGill said.
“For instance, the size of our measured effect is like measuring the length of a car on the Moon as seen from Earth, and is 625 times smaller than the effect measured at the 1919 solar eclipse.”
Once they had extracted the lensing signal, the researchers were able to measure the size of the astrometric deflection of the background source, which scales with the mass of the white dwarf, and obtain a gravitational mass for LAWD 37 that is 56% the mass of our Sun.
This agrees with earlier theoretical predictions of LAWD 37’s mass, and corroborates current theories of how white dwarfs evolve.
“The precision of LAWD 37’s mass measurement allows us to test the mass-radius relationship for white dwarfs,” Dr. McGill said.
“This means testing the properties of matter under the extreme conditions inside this star.”
The team’s results open the door for future event predictions with Gaia data that can be detected with space-based observatories such as Webb, the successor to Hubble.
“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,” Dr. McGill said.
“We want to continue measuring the gravitational microlensing effect and obtain mass measurements for many more types of stars.”
A paper on the findings was published in the Monthly Notices of the Royal Astronomical Society.
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Peter McGill et al. 2023. First semi-empirical test of the white dwarf mass-radius relationship using a single white dwarf via astrometric microlensing. MNRAS 520 (1): 259-280; doi: 10.1093/mnras/stac3532
