This week in astronomy news, astronomers found planet-eating white dwarfs, a cluster of black holes, and a “farfarout” object that’s now the most distant known in the solar system.
Planet-eating White Dwarfs
White dwarfs are surprisingly dirty. Heavier elements sink due to the strong gravity of these collapsed cores of low-mass stars, "cleaning" their atmospheres of all elements but hydrogen and helium. But heavier elements nevertheless contaminate what should be pristine atmospheres, suggesting that white dwarfs are constantly feeding on debris from gravitationally ravaged systems.
Now, using spectroscopy from the Sloan Digital Sky Survey, Mark Hollands (University of Warwick, UK) and colleagues have shown that white dwarfs aren't picky eaters — even the remains of rocky bodies' crusts are found in white dwarf atmospheres.
The spectroscopy revealed the element lithium in the atmosphere of four ancient (5 to 10 billion years old) white dwarfs. One also has potassium. The abundance of these alkali metals compared to other elements, such as sodium and calcium, show chemical make-ups similar to Earth's crust. However, the amounts are small, suggesting what they're seeing are pieces of torn-off crust rather than a planet consumed whole.
Black Holes Cluster Among Stars
The globular cluster named NGC 6397 (above) is only about 70 light-years across, yet it packs in thousands of stars. It's a prime location for an intermediate-mass black hole (IMBH), one that has at least hundreds and maybe hundreds of thousands of times the Sun's mass but is still far less weighty than the supermassive black holes at the cores of galaxies. Gravitational-wave detections of surprisingly hefty (but largely still stellar-mass) black holes have lent credence to the existence of IMBHs.
Eduardo Vitral and Gary Mamon (both at the Institute of Astrophysics of Paris) sought to detect an IMBH hiding in the cluster's core by analyzing the motions of NGC 6397's stars using data from the Hubble Space Telescope and the Gaia satellite. Both space observatories provide the detailed measurements over years to help astronomers measure the stars' proper motions across the sky.
“We found very strong evidence for invisible mass in the dense central regions of the cluster, but we were surprised to find that this extra mass is not point-like but extended to a few percent of the size of the cluster,” Vitral explains. While some of this "dark mass" might include white dwarfs and neutron stars, the astronomers calculate that most of it is stellar-mass black holes.
"Farfarout" Is New Most Distant Solar System Object
Astronomers have found the most distant object in the solar system, a 400-kilometer rock. The team — Chad Trujillo (Northern Arizona University), Scott Sheppard (Carnegie Institution for Science), and David Tholen (University of Hawai‘i) — actually found the object in January 2018 using observations from the Subaru Telescope. But they needed additional observations from the Gemini North telescope in Hawai‘i and the Magellan telescopes in Chile to confirm its orbit and thus its distance.
The same astronomers had dubbed the previous record-holder, which they also discovered, “Farout.” So naturally, they bestowed this one with the moniker “Farfarout.” More officially, it's provisionally designated 2018 AG37 by the Minor Planet Center. And it really is far, far out, currently at 132 astronomical units from the Sun, nearly four times farther away than Pluto. (Farout, now known as 2018 VG18, is 124 a.u. from the Sun.) Its orbit is elongated, though, so while it travels as far out as 175 a.u., it also comes in as close as 27 a.u., taking it inside Neptune's orbit.
“Farfarout was likely thrown into the outer solar system by getting too close to Neptune in the distant past,” Trujillo says. “Farfarout will likely interact with Neptune again in the future since their orbits still intersect.”
Read more about the discovery in the press releases from the NSF's NOIRLab and the National Astronomical Observatory of Japan's Subaru Telescope (Japanese).