Webb Reveals Shells of Dust Surrounding Brilliant Binary Star System

The latest image from NASA‘s James Webb Space Telescope is a new perspective on the binary star Wolf-Rayet 140, revealing details and structure in a new light. Astronomer Ryan Lau of NSF’s NOIRLab, principal investigator of the Webb Early Release Science program that observed the star, shares his thoughts on the observations.

“On the night that my team’s Early Release Science observations of the dust-forming massive binary star Wolf-Rayet (WR) 140 were taken, I was puzzled by what I saw in the preview images from the Mid-Infrared Instrument (MIRI). There seemed to be a strange-looking diffraction pattern, and I worried that it was a visual effect created by the stars’ extreme brightness. However, as soon as I downloaded the final data I realized that I was not looking at a diffraction pattern, but instead rings of dust surrounding WR 140 – at least 17 of them.

A bright white point of light is surrounded by ten to fifteen regularly spaced, hazy rings at its bottom, right, and upper right. The central point, where the stars are located, has a rough hexagon shape. The innermost ring is highlighted blueish white and is much brighter to the right. The outer rings fade from view to the upper left, with only a few close rings visible there. The central light seems to highlight the misshapen rings like a spotlight, with rays coming out diagonally from the upper left to lower right. One ray illuminates even more rings as it travels to the upper right.
Shells of cosmic dust created by the interaction of binary stars appear like tree rings around Wolf-Rayet 140. The remarkable regularity of the shells’ spacing indicates that they form like clockwork during the stars’ eight-year orbit cycle, when the two members of the binary make their closest approach to one another. In this image, blue, green, and red were assigned to Webb’s Mid-Infrared Instrument (MIRI) data at 7.7, 15, and 21 microns (F770W, F1500W, and F2100W filters, respectively). Credit: NASA, ESA, CSA, STScI, JPL-Caltech. Download/View the full-resolution version and supporting visuals from the Space Telescope Science Institute.

“I was amazed. Although they resemble rings in the image, the true 3D geometry of those semi-circular features is better described as a shell. The shells of dust are formed each time the stars reach a point in their orbit where they are closest to each other and their stellar winds interact. The even spacing between the shells indicates that dust formation events are occurring like clockwork, once in each eight-year orbit. In this case, the 17 shells can be counted like tree rings, showing more than 130 years of dust formation. Our confidence in this interpretation of the image was strengthened by comparing our findings to the geometric dust models by Yinuo Han, a doctoral student at the University of Cambridge, which showed a near-perfect match to our observations.

“One of the biggest surprises was how many shells the telescope was able to detect. The shells furthest from the binary star have traversed over 70,000 times the distance from Earth to the Sun, at speeds of around 6 million miles per hour, through the harsh environment around a WR star—some of the hottest and most luminous stars known. The survival of these distant shells shows that the dust formed by WR binaries like WR 140 will likely survive to enrich the surrounding interstellar environment. However, it wasn’t enough for us to see these dusty shells. We wanted to know their spectroscopic signature and chemical composition. What will they add to the interstellar medium?

“With the Medium-Resolution Spectroscopy (MRS) mode on MIRI, we obtained the first spatially resolved mid-infrared spectra of a dust-forming WR binary in our observation of WR 140, and were able to directly probe the chemical signatures of its dust shells. Broad and prominent features in the spectral lines at 6.4 and 7.7 microns told us that the dust was composed of compounds consistent with Polycyclic Aromatic Hydrocarbons (PAHs). This carbonaceous material plays an important role in the interstellar medium and the formation of stars and planets, but its origin is a long-standing mystery. With the combined results of JWST’s MRS spectra and MIRI imaging, we now have evidence that WR binaries can be an important source of carbon-rich compounds that enrich the interstellar environment of our galaxy, and likely galaxies beyond our own.”

About the author:
Ryan Lau is an Assistant Astronomer at the National Science Foundation’s NOIRLab. His team’s observations of WR 140 are the results of the Director’s Discretionary-Early Release Science program 1349. Learn more about the findings here.

Editor’s Note: This post highlights data from a paper appearing today in Nature Astronomy.