Light Echoes Map Planet-forming Disk

Light’s finite speed helped astronomers pinpoint the location of the “inner wall” of the disk of dust and gas that’s feeding a fast-growing baby star.

This illustration shows a star surrounded by a protoplanetary disk. Gas funnels from the disk onto the stellar poles, feeding the newborn star.
NASA / JPL-Caltech

Flick a switch and the room lights up. We make this trivial motion several times a day, and every time the room appears to brighten instantaneously. But the universe doesn’t actually bend to our will  as easily as we think — light takes time to travel. For a typical living room, light takes several nanoseconds to travel from the center to the surrounding walls.

It’s a reality too fast for our eyes to see, but in space, where events transpire on a somewhat grander scale than your living room, enough time passes while light is traveling that “light echoes” (or, if you prefer the technical term, photo-reverberations) are measurable.

Now a team of astronomers has used this phenomenon to take an unprecedented measure of the inner edge of a disk of dust and gas that's feeding a newborn star.

From Black Holes to Young Stars

For years, astronomers have used light echoes to measure the mass of supermassive black holes enthroned in galaxies’ centers. First they monitor gas that’s feeding a black hole, waiting for a momentary brightening — the flick of the light switch. Then they wait some more (days or often weeks) before gas circling far from the black hole lights up in response — when the central light hits the metaphorical wall.

That delay tells astronomers just how far away the gas is from the black hole. Combined with a measure of the gas’s speed, astronomers can then calculate the central mass that keeps the gas circling.

Now for the first time Huan Meng (Caltech and University of Arizona) and a team of astronomers have applied light echoes to measure an entirely different type of celestial target: protoplanetary disks. These thick disks of gas and dust feed newborn stars, before the growing star radiates enough photons to push the shroud away.

Such disks, especially those around stellar young’uns that will grow up to be like our Sun or smaller, are too tiny to image directly, even with the biggest adaptive-optics-equipped telescopes we’ve got. And near-infrared interferometry, which can sometimes tease out structures too small to see directly, hasn’t produced consistent results. So Meng and his colleagues resorted to light echoes.

The key to this type of measurement is variability — steady starlight won’t create an echo. But like toddlers who have just discovered the joy of light switches, young stars tend to flicker. So Meng’s team picked a part of the star-forming region known as Rho Ophiuchus to image simultaneously with a cadre of ground- and space-based telescopes: the Spitzer Space Telescope, the 4-meter Mayall telescope at Kitt Peak, the 4-meter SOAR telescope and 1.3-meter SMARTS telescope at the Cerro Tololo Inter-American Observatory in Chile, and the 1.5-meter National Astronomical Observatory scope at Sierra San Pedro Mártir in Mexico.

Out of the 27 stars observed in this region, the team caught only one that flickered in just the right way: YLW 16B. But one was enough.

Light Echoes from a Protoplanetary Disk

During the observing run, the star’s brightness in two near-infrared bands dipped and then brightened again. The team attributes this change to random variations in the gas flowing from the protoplanetary disk onto the star’s poles.

Then, at longer infrared wavelengths, the brightness changed in the exact same way — but it lagged behind by 75 seconds. This echo, the authors argue in an upcoming Astrophysical Journal, comes from the inner edge of the disk’s far wall, where dust particles heat and cool (brighten and dim) in response to the changing starlight.

This diagram illustrates the concept of light echoes. When a star varies in brightness, the inner wall of the disk lit by the star will vary too, but with a time delay. This time delay, or light echo, lets astronomers measure the distance between the star and the disk's inner edge.
NASA / JPL-Caltech

The time delay corresponds to a gap of about 0.08 astronomical unit (that is, 0.08 times the distance between Earth and the Sun) between the growing star and the inner edge of the disk that feeds it. The result is exactly what’s expected for a dusty disk: inside about 0.1 a.u., the dust sublimates, turning from solid into gas under the star’s harsh glare. And, importantly, the gap is too big for the star’s magnetic field to play a role in cutting off the inner disk.

Following this proof-of-concept, astronomers are sure to deploy this technique to “see” the protoplanetary disks surrounding other young stars. Soon we’ll have a much better picture of how these stars feed, grow, and eventually host their own planets.