Playing shadow puppets with NASA's Hubble Space Telescope
IMAGE: THIS ARTIST'S CONCEPT IS BASED ON HUBBLE SPACE TELESCOPE IMAGES OF GAS-AND-DUST DISKS AROUND THE YOUNG STAR TW HYDRAE. HUBBLE SPACE TELESCOPE PHOTOS SHOW SHADOWS SWEEPING ACROSS THE DISKS ENCIRCLING THE SYSTEM. THE INTERPRETATION IS THESE SHADOWS ARE FROM SLIGHTLY INCLINED INNER DISKS THAT BLOCK STARLIGHT FROM REACHING THE OUTER DISK, AND THEREFORE ARE CASTING A SHADOW. THE DISKS ARE SLIGHTLY INCLINED TO EACH OTHER DUE TO THE GRAVITATIONAL PULL OF UNSEEN PLANETS WARPING THE DISK STRUCTURE. view more
CREDIT: ARTWORK: NASA, AURA/STSCI FOR ESA, LEAH HUSTAK (STSCI)
Scientists' Hubble Space Telescope observations of the young star, TW Hydrae may signal new planets under construction.
In 2017 astronomers reported discovering a shadow sweeping across the face of a vast pancake-shaped gas-and-dust disk surrounding the red dwarf star. The shadow isn't from a planet, but from an inner disk slightly inclined relative to the much larger outer disk – causing it to cast a shadow. One explanation is that an unseen planet's gravity is pulling dust and gas into the planet's inclined orbit.
Now, a second shadow – playing a game of peek-a-boo – has emerged in just a few years between observations stored in the Hubble's MAST archive. This could be from yet another disk nestled inside the system. The two disks are likely evidence of a pair of planets under construction.
George Mason University Physics and Astronomy Professor, Peter Plavchan, collaborated with the team making the observations. He studied ways a forming planet in the disk could potentially play a role in the formation of the disk structures and the shadows they case.
TW Hydrae is less than 10 million years old and resides about 200 light-years away. In its infancy, our solar system may have resembled the TW Hydrae system, some 4.6 billion years ago. Because the TW Hydrae system is tilted nearly face-on to our view from Earth, it is an optimum target for getting a bull's-eye-view of a planetary construction yard.
The second shadow was discovered in observations obtained June 6, 2021, as part of a multi-year program designed to track the shadows in circumstellar disks. John Debes of AURA/STScI for the European Space Agency at the Space Telescope Science Institute in Baltimore, Maryland, compared the TW Hydrae disk to Hubble observations made several years ago.
"We found out that the shadow had done something completely different," said Debes, who is principal investigator and lead author of the study published in The Astrophysical Journal. "When I first looked at the data, I thought something had gone wrong with the observation because it wasn't what I was expecting. I was flummoxed at first, and all my collaborators were like: what is going on? We really had to scratch our heads and it took us a while to actually figure out an explanation." Debes shared.
“We haven't found any direct evidence for a planet at this time, but can rule out planets more massive than Jupiter from precisely monitoring the position of the star as a function of time,” said Plavchan, who also serves as Director of the Mason Observatory.
The best solution the team identified is that there are two misaligned disks casting shadows. They were so close to each other in the earlier observation, they were missed. Over time, they've now separated and split into two shadows. "We've never really seen this before on a protoplanetary disk. It makes the system much more complex than we originally thought," Debes said.
The simplest explanation is that the misaligned disks are likely caused by the gravitational pull of two planets in slightly different orbital planes. Hubble is piecing together a holistic view of the architecture of the system.
The disks may be proxies for planets that are lapping each other as they whirl around the star. It's sort of like spinning two vinyl phonograph records at slightly different speeds. Sometimes labels will match up but then one gets ahead of the other.
"It does suggest that the two planets have to be fairly close to each other. If one was moving much faster than the other, this would have been noticed in earlier observations. It's like two race cars that are close to each other, but one slowly overtakes and laps the other," said Debes.
The suspected planets are located in a region roughly the distance of Jupiter from our Sun. And, the shadows complete one rotation around the star about every 15 years – the orbital period that would be expected at that distance from the star.
Also, these two inner disks are inclined about five to seven degrees relative to the plane of the outer disk. This is comparable to the range of orbital inclinations inside our solar system. "This is right in line with typical solar system style architecture," said Debes.
The outer disk that the shadows are falling on may extend as far as several times the radius of our solar system's Kuiper belt. This larger disk has a curious gap at twice Pluto's average distance from the Sun. This might be evidence for a third planet in the system.
Any inner planets would be difficult to detect because their light would be lost in the glare of the star. Also, dust in the system would dim their reflected light. ESA's Gaia space observatory may be able to measure a wobble in the star if Jupiter-mass planets are tugging on it, but this would take years given the long orbital periods.
The TW Hydrae data are from Hubble's Space Telescope Imaging Spectrograph. The James Webb Space Telescope's infrared vision may also be able to show the shadows in more detail.
The Hubble Space Telescope is a project of international cooperation between NASA and ESA. NASA's Goddard Space Flight Center in Greenbelt, Maryland, manages the telescope. The Space Telescope Science Institute (STScI) in Baltimore conducts Hubble science operations. STScI is operated for NASA by the Association of Universities for Research in Astronomy, in Washington, D.C.
JOURNAL
The Astrophysical Journal
METHOD OF RESEARCH
Observational study
SUBJECT OF RESEARCH
Not applicable
ARTICLE TITLE
The Surprising Evolution of the Shadow on the TW Hya Disk
ARTICLE PUBLICATION DATE
4-May-2023
Precision mass measurements of nuclei reveal neutron star properties
Peer-Reviewed PublicationIMAGE: THE DETECTOR SYSTEM OF NUCLEAR MASS SPECTROMETER BASED ON THE COOLER STORAGE RING (CSR) IN LANZHOU. view more
CREDIT: IMAGE BY IMP
Researchers at the Institute of Modern Physics (IMP) of the Chinese Academy of Sciences (CAS) and their collaborators recently measured the masses of several key nuclei with high-precision by employing a state-of-the-art storage-ring mass spectrometry technique. Using the new mass data, they investigated X-ray bursts on the surface of a neutron star, thus deepening the understanding of neutron star properties. The study was published in Nature Physics.
Neutron stars are considered to be the densest objects besides black holes. Type-I X-ray bursts, among the brightest stellar objects frequently observed in the sky by space-based telescopes, are violent thermonuclear explosions occurring on the surface of neutron stars.
Due to the strong gravity of the neutron star, hydrogen- and helium-rich matter from a companion star accretes on the surface of a neutron star for hours or days before igniting thermonuclear burning. The explosion lasts for 10 to 100 seconds, causing a bright X-ray burst. These frequent X-ray bursts offer an opportunity to study the properties of neutron stars.
The bursts are powered by a nuclear reaction sequence, known as the rapid proton capture nucleosynthesis process (rp-process), which involves hundreds of exotic neutron-deficient nuclides. Among them, the waiting-point nuclides, including germanium-64, play a decisive role.
“Germanium-64, like a crossroad on the path of nuclear reaction processes, is an important congested section encountered when the nuclear reaction proceeds to the medium mass region. The masses of the relevant nuclei are decisive in setting the reaction path and thereby the X-ray flux produced,” explained ZHOU Xu, first author of the paper and a Ph.D. student at IMP.
Therefore, precision mass measurements of the nuclei around germanium-64 are essential for understanding X-ray bursts and the properties of neutron stars. However, due to extremely low production yield, it has been very challenging to measure the masses of these short-lived nuclei. As a result, few breakthroughs have been seen for many years worldwide.
After more than ten years of effort, the researchers from the Storage Ring Nuclear Physics Group at IMP have developed a new ultrasensitive mass spectrometry technique at the Cooler Storage Ring (CSR) of the Heavy Ion Research Facility in Lanzhou (HIRFL). This technique, named as Bρ-defined Isochronous Mass Spectrometry (Bρ-IMS), is fast and efficient, thus particularly suitable for measuring short-lived nuclei with extremely low production yields.
"Our experiment is capable of precisely determining the mass of a single nuclide within a millisecond after its production, and it is essentially background free in the measured spectrum,” said Prof. WANG Meng from IMP.
The researchers precisely measured the masses of arsenic-64, arsenic-65, selenium-66, selenium-67 and germanium-63. The masses of arsenic-64 and selenium-66 were experimentally measured for the first time, and the mass precision was significantly improved for the others. With the newly measured masses, all nuclear reaction energies related to the waiting point nucleus germanium-64 have been experimentally determined for the first time or the precision of these measurements has been greatly improved compared to old values.
The researchers then used the new masses as inputs for X-ray burst model calculations. They found that the new data led to changes in the rp-process path. As a result, the X-ray burst light curve from the surface of the neutron star shows an increased peak luminosity and a prolonged tail duration.
By comparing model calculations with the observed X-ray bursts of GS 1826-24, the researchers found that the distance from Earth to the burster should be increased by 6.5%, and the neutron star surface gravitational redshift coefficient needs to be reduced by 4.8% to match astronomical observations. These results indicate that the density of the neutron star is lower than expected. In addition, the product abundances from the rp-process reveal that the temperature of the outer shell of the neutron star should be higher than generally believed after the X-ray burst.
"Through precise nuclear mass measurement, we obtained a more accurate X-ray burst light curve on the surface of the neutron star. By comparing it with astronomical observations, we set constraints on the relationship between the mass and radius of the neutron star from a new perspective,” said Prof. ZHANG Yuhu from IMP.
This work was conducted in collaboration with researchers from GSI Helmholtzzentrum für Schwerionenforschung, Max-Planck-Institut für Kernphysik, Ohio University, Advanced Energy Science and Technology Guangdong Laboratory, Beijing University, Lanzhou University, Beijing Normal University and East China University of Technology.
JOURNAL
Nature Physics
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