Author: Anne Minard

The red supergiant star Betelgeuse is undoubtedly enormous. But it’s shrinking, and astronomers aren’t sure why.

Researchers at the University of California at Berkeley have been monitoring the star by aiming the Infrared Spatial Interferometer, atop Mt. Wilson in Southern California, toward the star’s home in the constellation Orion. Since 1993, the Betelgeuse star (pictured in a NASA image at left) has shrunk in diameter by more than 15 percent.

Betelgeuse is so big that in our solar system it would reach to the orbit of Jupiter. Its radius is about five astronomical units, or five times the radius of Earth’s orbit. Its measured shrinkage means the star’s radius has shrunk by a distance equal to the orbit of Venus.

“To see this change is very striking,” said Charles Townes, a UC Berkeley professor emeritus of physics. “We will be watching it carefully over the next few years to see if it will keep contracting or will go back up in size.”

Townes and his colleague, Edward Wishnow, a research physicist at UC Berkeley, presented their findings at a press conference on Tuesday during the Pasadena meeting of the American Astronomical Society. The results also appeared June 1 in The Astrophysical Journal Letters.

Despite Betelgeuse’s diminished size, Wishnow pointed out that its visible brightness, or magnitude, which is monitored regularly by members of the American Association of Variable Star Observers, has shown no significant dimming over the past 15 years.

The ISI has been focusing on Betelgeuse for more than 15 years in an attempt to learn more about these giant massive stars and to discern features on the star’s surface, Wishnow said. He speculated that giant convection cells on the star’s surface might affect the measurements. Like convection granules on the Sun, the cells are so large that they bulge out from the surface. Townes and a former graduate student observed a bright spot on the surface of Betelgeuse in recent years, although at the moment, the star appears spherically symmetrical.

“But we do not know why the star is shrinking,” Wishnow said. “Considering all that we know about galaxies and the distant universe, there are still lots of things we don’t know about stars, including what happens as red giants near the ends of their lives.”

Betelgeuse was the first star ever to have its size measured, and even today is one of only a handful of stars that appears through the Hubble Space Telescope as a disk rather than a point of light. In 1921, Francis G. Pease and Albert Michelson used optical interferometry to estimate its diameter was equivalent to the orbit of Mars. Last year, new measurements of the distance to Betelgeuse raised it from 430 light-years to 640, which increased the star’s diameter from about 3.7 to about 5.5 AU.

“Since the 1921 measurement, its size has been re-measured by many different interferometer systems over a range of wavelengths where the diameter measured varies by about 30 percent,” Wishnow said. “At a given wavelength, however, the star has not varied in size much beyond the measurement uncertainties.”

The measurements cannot be compared anyway, because the star’s size depends on the wavelength of light used to measure it, Townes said. This is because the tenuous gas in the outer regions of the star emits light as well as absorbs it, which makes it difficult to determine the edge of the star.

The Infrared Spatial Interferometer, which Townes and his colleagues first built in the early 1990s, sidesteps these confounding emission and absorption lines by observing in the mid-infrared with a narrow bandwidth that can be tuned between spectral lines. The technique of stellar interferometry is highlighted in the June 2009 issue of Physics Today magazine.

Townes, who turns 94 in July, plans to continue monitoring Betelgeuse in hopes of finding a pattern in the changing diameter, and to improve the ISI’s capabilities by adding a spectrometer to the interferometer.

“Whenever you look at things with more precision, you are going to find some surprises,” he said, “and uncover very fundamental and important new things.”

Sources: AAS and UC Berkeley. The paper is available here.

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A now-you-see-it, now-you don’t accretion disk (white and blue in the artist’s rendering at left) has tipped astronomers to the birth of a superfast, “millisecond” pulsar that was happening right before their eyes — er, their radio telescopes.

The new finding confirms the long-suspected evolutionary connection between a neutron star and a millisecond pulsar: they are two life stages of the same object.

Anne Archibald, of McGill University in Montreal, Canada and her colleagues announced their discovery in the May 21 online issue of the journal Science.

Pulsars are superdense neutron stars, the remnants left after massive stars have exploded as supernovae. Their powerful magnetic fields generate lighthouse-like beams of light and radio waves that sweep around as the star rotates, and are detectable as pulses on Earth.

Some, dubbed millisecond pulsars, rotate hundreds of times a second. Astronomers believe the fast rotation is caused by a companion star dumping material onto the neutron star and spinning it up.

The material from the companion would form a flat, spinning disk around the neutron star, and during this period, the radio waves characteristic of a pulsar would not be seen coming from the system. As the amount of matter falling onto the neutron star decreased and stopped, the radio waves could emerge, and the object would be recognized as a pulsar.

This sequence of events is apparently what happened with a binary-star system some 4000 light-years from Earth, in the constellation of Sextans just south of Leo. The millisecond pulsar in this system, called J1023, was discovered by the National Science Foundation’s Robert C. Byrd Green Bank Telescope (GBT) in West Virginia in 2007 in a survey led by astronomers at West Virginia University and the National Radio Astronomy Observatory.

The astronomers then found that the object had been detected by National Science Foundation’s Very Large Array radio telescope in New Mexico, during a large sky survey in 1998, and had been observed in visible light by the Sloan Digital Sky Survey in 1999, revealing a Sun-like star.

When observed again in 2000, the object had changed dramatically, showing evidence for a rotating disk of material, called an accretion disk, surrounding the neutron star. By May of 2002, the evidence for this disk had disappeared.

“This strange behavior puzzled astronomers, and there were several different theories for what the object could be,” said Ingrid Stairs of the University of British Columbia.

The 2007 GBT observations showed that the object is a millisecond pulsar, spinning 592 times per second.

“No other millisecond pulsar has ever shown evidence for an accretion disk,” Archibald said. “We know that another type of binary-star system, called a low-mass X-ray binary (LMXB), also contains a fast-spinning neutron star and an accretion disk, but these don’t emit radio waves. We’ve thought that LMXBs probably are in the process of getting spun up, and will later emit radio waves as a pulsar. This object appears to be the ‘missing link’ connecting the two types of
systems.”

The scientists have studied J1023 in detail with the GBT, with the Westerbork radio telescope in the Netherlands, with the Arecibo radio telescope in Puerto Rico, and with the Parkes radio telescope in Australia. Their results indicate that the neutron star’s companion has less than half the Sun’s mass, and orbits the neutron star once every four hours and 45 minutes.

Image caption: Material from distended “normal” star. right, streams onto accretion disk (white and blue) surrounding neutron star, left. Credit: Bill Saxton, NRAO/AUI/NSF

Source: National Radio Astronomy Observatory. Animations are here and here. Warning: that last one may cause dizziness.

Rocky planets like Earth are all believed to have begun as dust circling newly born stars, and clues about the origin of such dust comes to us in today’s meteorites and comets, as well as observations of circumstellar disks around young stars.

But mystery has shrouded the details of the evolution of dust and how it eventually comes to form larger objects. Now, two papers in the journal Nature are proposing a new mechanism to explain it.

The new mechanism hinges on heat-shocked crystalline dust grains, which somehow migrated from where they were created — presumably close to the Sun — to the outer Solar System.  By implication, the same process should occur around other young stars. 

A trio of past hypotheses had been proposed to explain the migration, but none of them quite fit. They included, according to physicist Dejan Vinkovic of the University of Split in Croatia, turbulent mixing, ballistic launching of particles in a dense wind created by interaction of the accretion disk with the young star’s magnetic field (called the X-wind model), and mixing mediated by transient spiral arms in marginally gravitationally unstable disks. Vinkovic is lead author on one of the Nature papers.

“The turbulent mixing requires a source of efficient turbulent viscosity and the magnetorotational instability is invoked as the most promising candidate, but large stretches of the disk are considered not sufficiently ionized to keep this instability active,” he wrote. “The X-wind model relies on the theoretical notion of magnetic field configurations in the immediate vicinity of pre-main-sequence stars and high hopes are put on future observations to resolve this predicament.”

And finally, “The spiral arms model is in the domain of discussions on whether the underlying numerics, physical approximations and assumptions about the initial conditions are realistic enough to make results plausible.”

In the other paper, Peter Abraham of the Hungarian Academy of Sciences and his colleagues find the signature of crystalline dust after a young star flared, whereas archival data showed no sign of it before the flare.

The Vinkovic paper investigates the mixing of large crystalline dust particles in the protoplanetary nebula around the young Sun.

The force produced by the light shining on an object is a well known phenomenon called radiation pressure. We do not feel it in daily lives because we are too massive for this effect to be noticeable. For very small particles, on the other hand, this force can be larger even than the gravity that keeps particles in the orbit around the star. Investigations have been focused so far only to the radiation pressure due to the starlight. The results showed that individual grains would not travel far and would be pushed deeper into the disk. 

Vinkovic reports that infrared radiation arising from the dusty disk can loft grains bigger than one micrometer out of the inner disk, where they are pushed outwards by stellar radiation pressure while gliding above the disk. Grains re-enter the disk at radii where it is too cold to produce sufficient infrared radiation pressure support for a given grain size and solid density.

However, Vinkovic points out that it is not only the star, but also the disk that shines. When studying effects on protoplanetary dust grains larger than one micrometer, which is comparable to the particle size of cigarette smoke, Vinkovic has discovered that the intense infrared light from the hottest regions of the protoplanetary disk is capable of pushing such dust out of the disk. Infrared radiation is what we can feel as “heat” on our skin. Combination of radiation pressure from the star and the disk creates a net force that enables dust grains to surf along the disk surface from inner to outer regions of the disk. 

The temperatures in this hot region reach around 1500 degrees Kelvin (2200 degrees Fahrenheit), enough to vaporize solid dust particles or to alter their physical and chemical structure. The mechanism that Vinkovic describes in his paper would transfer such altered dust particles to colder disk regions away from the star. This can explain why comets contain a puzzling combination of ices and particles altered at high temperatures. Astronomers have been perplexed by this mixture, since comets form in cold disk regions out of frozen substances like water, carbon dioxide or methane. Rocky dust particles that end up mixed with ices are therefore expected to never experience high temperatures.   

In an editorial accompanying the studies, University of Missouri astrophysicist Aigen Li wrote that the origin of crystalline silicates in comets “has been a matter of debate since their first detection 20 years ago.”

While Li touts promise in the new theory, “It would be interesting to see whether other mechanisms such as turbulent mixing and the ‘X-wind’ model would effectively carry submicrometre grains, which are efficient mid-IR emitters, outwards and incorporate them into comets,” he wrote. “It is also possible that some — but not all — crystalline silicates are made in situ in cometary comae.”

Source: Vinkovic’s press release. Watch a short animation showing how the newly proposed mechanism of dust movement works.