Hubble Captures Strobe Flashes From A Young Star

Two of NASA’s Great Observatories, the Spitzer and Hubble space telescopes, have teamed up to uncover a mysterious infant star that behaves like a police strobe light.

Every 25.34 days, the object, designated LRLL 54361, unleashes a burst of light. Though a similar phenomenon has been seen in two other young stellar objects, this is the most powerful such beacon seen to date.

 

This infrared image from the NASA/ESA Hubble Space Telescope shows an image of protostellar object LRLL 54361 and its rich cosmic neighbourhood, a region called IC 348. The protostar, which is the bright object with fan-like beams of light coming from it, located towards the right of the image, is letting off flashes of light every 25.3 days.

Credit: NASA, ESA, and J. Muzerolle (STScI)

The heart of the fireworks is hidden behind a dense disk and envelope of dust. Astronomers propose that the light flashes are caused by periodic interactions between two newly formed stars that are gravitationally bound to each other. The newly discovered object offers insights into the early stages of star formation when a lot of gas and dust is being rapidly accreted to form a new binary star.

 

This time-lapse movie from the NASA/ESA Hubble Space Telescope shows a pulse of light emanating from the protostellar object LRLL 54361. Most if not all of this light results from scattering off circumstellar dust in the protostellar envelope.

Credit: NASA, ESA, and J. Muzerolle (STScI)

Astronomers propose that the flashes are due to material suddenly being dumped onto the growing stars, known as protostars, unleashing a blast of radiation each time the stars get close to each other in their orbit. This is the so-called pulsed-accretion model. The phenomenon has been seen in later stages of star birth but never in such a young system, nor with such intensity and regularity.

 

“This protostar has large brightness variations with a precise period that is very difficult to explain,” said James Muzerolle of the Space Telescope Science Institute in Baltimore, Md. His paper appeared in the January 17 issue of the science journal Nature.

LRLL 54361 was discovered by the Spitzer Space Telescope as a variable object inside the star-forming region IC 348, located 950 light-years away. The infrared spectrum as measured by Spitzer has the typical signature of a protostar. These stars are estimated to be no more than a few hundred thousand years old, based on statistical analysis.

The Spitzer infrared data, taken repeatedly over a period of seven years, showed unusual outbursts in the brightness of the star. Surprisingly, the outbursts recurred every 25.34 days, a very rare phenomenon. Further analysis of the data led the authors to propose the pulsed-accretion model.

  

This sequence of images from the NASA/ESA Hubble Space Telescope shows a pulse of light emanating from the protostellar object LRLL 54361. Most if not all of this light results from scattering off circumstellar dust in the protostellar envelope.

An apparent edge-on disk visible at the centre of the object, and three separate structures are interpreted as outflow cavities. The extent and shape of the scattered light changes substantially over a 25.3-day period.

This is caused by the propagation of the light pulse through the nebula. Astronomers propose that the flashes are due to material in a circumstellar disk suddenly being dumped onto a binary pair of forming stars. This unleashes a blast of radiation each time the stars get close to each other in their orbit.

These near-infrared light images are from Hubble’s Wide Field Camera 3.

Credit: NASA, ESA, and J. Muzerolle (STScI)

The Hubble Space Telescope was used to confirm the Spitzer observations and revealed the detailed structure around the protostar. Hubble resolved two cavities that are traced by light scattered off their edges above and below a dusty disk. The cavities were likely blown out of the surrounding natal envelope of dust and gas by an outflow launched near the central stars. The envelope and disk prevent the suspected binary star pair from being directly observed. By capturing multiple images over the course of one pulse event, the Hubble observations uncovered a spectacular movement of light away from the center of the system, an optical illusion known as a light echo.

Muzerolle and his team hypothesized that a pair of stars lie in the center of the dust cloud, moving about each other in a very eccentric orbit. As the stars approach each other, dust and gas are dragged from the inner edge of a surrounding disk. The material ultimately crashes onto one or both stars, which triggers a flash of light that then illuminates the circumstellar dust. The system is rare because close binaries account for only a few percent of our galaxy’s stellar population. And this is likely a brief, transitory phase in the birth of a star system. “Assuming the pulsed accretion hypothesis is correct, the disk is circumbinary,” which means it encircles both stars. “There may be small circumstellar disks around each star, though that is perhaps less likely given the strong pulse signature,” said Muzerolle.

Muzerolle’s team plans to continue monitoring LRLL 54361 using other facilities including the Herschel Space Telescope, and hopes to eventually obtain more direct measurements of the binary star and its orbit. “This system continues to surprise us, and we can’t wait to see what happens next!”

 

Contacts and sources:
Ray Villard
Space Telescope Science Institute, Baltimore, Md.

James Muzerolle
Space Telescope Science Institute, Baltimore, Md.