Astro News Briefs: June 3–9

This week astronomers from around the world have gathered in Albuquerque, New Mexico, for the 200th meeting of the American Astronomical Society. Below are assorted news briefs highlighting some of the announced research findings. Come back for more updates throughout the week.

Image of Betelgeuse Reveals Bloated Atmosphere

June 6, 2002 | Seeing stars as anything other than points of light is almost impossible. One exception is the red supergiant star Betelgeuse, located 425 light-years away in Orion. It's so large that astronomers can directly image activity on its surface and in its atmosphere. Using the Hubble Space Telescope, J. Gethyn Timothy (Catholic University of America) recently observed Betelgeuse in the far ultraviolet (FUV) and found its atmosphere to be three times larger than the diameter of the star itself. Betelgeuse is thought to be about two billion kilometers across — so large that if it replaced the Sun in the center of our solar system it would extend past the orbit of Jupiter. Its atmosphere, as seen in FUV, would reach out beyond the orbit of Saturn. According to Timothy, who presented his findings Wednesday at the AAS meeting, the atmosphere of Betelgeuse is larger in extent and more structured than when it was previously observed in the near ultraviolet. In addition to the extended atmosphere, the FUV image revealed two clearly separated hot spots at or near the surface of the star. Similar spots have been observed before at other wavelengths.

Images associated with this story are available at:
http://www.cis.rit.edu/people/faculty/horch/aori/index.html


Another Prediction for November's Leonids

June 6, 2002 | Until recently, predicting the arrival of meteor storms was iffy at best. However, some scientists did make very accurate predictions of the strong Leonid showers in 1999, 2000, and 2001 (S&T: November 2001, page 109). One such prognosticator was Esko Lyytinen of Finland. His forecasts were based on an unusual cometary model created by Tom Van Flandern (Meta Research) that assumes that comets are enveloped by a cloud of particulate debris orbiting a relatively solid nucleus. Van Flandern explained his model at the AAS meeting in Albuquerque Wednesday. In it, every time the comet approaches perihelion, the debris slowly escapes via the comet's L1 and L2 Lagrangian points — regions where the pull of solar and cometary gravity balance to keep dust "hovering" between them. Van Flandern calculates the time, velocity, and direction of the escaping material and predicts the time and strength of any meteor stream that might intercept Earth. Based on this model, he believes that the next Leonid meteor shower will be a good one. He predicts two peaks on November 19, 2002: the first at 4:10 Universal Time (visible in Europe and Africa with a zenith hourly rate of 3,500) and the second at 10:46 UT (visible in eastern North America) with a rate of 2,600 meteors visible by a single observer per hour. Unfortunately light from the full Moon will obliterate many of the fainter meteors associated with the storm.


Tsunamis and Asteroids

June 6, 2002 | The 1998 movie Deep Impact depicts a fragment of a comet slamming into the Atlantic Ocean just off the east coast of North America (S&T: June 1998, page 26). The resulting tsunami destroys New York and other cities. But how realistic was the depiction of that giant tidal wave? As the AAS meeting today in Albuquerque, Michael Gittings (Science Applications International Corp.) described simulations of six different asteroid-impact scenarios that he and his colleagues have created. Their impactors were iron asteroids 250 meters, 500 meters, and one kilometer in diameter. The simulation started with the asteroid 30 km above the ocean headed straight down at 25 km per second. Gittings found that the waves generated by the impact were double the height predicted in previous simulations, but their speed was 25 percent slower. The one-kilometer iron asteroid produced a jet of water more than 12 miles high. The initial tsunami waves moved at 380 miles per hour and were more than half a mile high, abating to about two-thirds that height 40 miles from the point of impact. "The simulations are currently geared to exploring the results of asteroid impacts in water," said Gittings, "but the computer code can easily be adapted to other uses." At a meeting of the Tsunami Society in Hawaii in late May, Gittings used his simulations to predict the size of tsunamis generated by underwater landslides caused by earthquakes instead of asteroid impacts.


New Light on Dark Matter

JUNE 5, 2002 | An X-ray study of a small group of galaxies in Virgo has led to a better understanding of the amount of dark matter present in the universe. John Mulchaey (Carnegie Observatories) reported Tuesday at the AAS meeting in Albuquerque that the NGC 4325, a small cluster in the constellation Virgo with fewer than 10 galaxies, contains the same percentage of dark matter as do much larger galaxy clusters. Observations of the group by the European Space Agency's XMM-Newton X-ray telescope reveal that it is immersed in a huge cloud of hot (10 million° Kelvin) gas, 1.9 million light-years in diameter. Given its high temperature, the cloud should have dissipated long ago. Instead, it's maintained by a balance between the force of gravity and the pressure of the hot gas. Based on the temperature of the enveloping gas, Mulchaey calculates the total mass of the system to be 20 trillion times the mass of the Sun. "But the stars and gas contribute only two trillion solar masses of material, so 90 percent of the mass in the system doesn't emit light," says Mulchaey. This percentage of dark matter is consistent with that found in larger clusters with many more members. "This is the first study done for a small group of galaxies," notes Mulchaey, "and they provide a much better picture of what the universe looks like on average."

A combined X-ray/optical image of the NGC 4325 group can be found at:
http://www.carnegieinstitution.org/mulchaey.html


Faint Background X-ray Source Identified

JUNE 5, 2002 | When the Chandra and XMM-Newton X-ray observatories peered into an apparently empty section of space, they discovered a pervasive glow from extremely distant galaxies inhabiting the early universe. According to Richard Griffiths (Carnegie Mellon University), that pervasive X-ray glow comes from more than just massive black holes at the center of distant galaxies. As he explained during a presentation at the AAS conference on Tuesday, a statistical analysis of the latest deep-field observations finds several times as many X-ray sources as expected from active galactic nuclei. According to Griffiths, "a large number of the very faintest sources are from normal galaxies containing hundreds of binary X-ray sources." Such sources usually contain a star orbiting a compact object such as a neutron star or black hole. It's known that young spiral galaxies contain more X-ray binaries than do old ones, in part because of their higher rates of star formation. Griffiths speculates that these new results may provide insight into the rate of star formation in the early universe.

Initial findings appear in the Astrophysical Journal. An image associated with this story can be found at:
http://astro.phys.cmu.edu/~griffith/xmm.jpg


Elliptical Galaxies Hide Black Holes

June 5, 2002 | Because they're visually bland, elliptical galaxies don't always get the same attention as their more spectacular spiral compatriots. That could change now that the Chandra X-ray Observatory has found hundreds of X-ray binary stars within three ellipticals — NGC 4697, 4649, and 1553. The binary systems contain a normal star in orbit around either a neutron star or a black hole. "It's surprising to see binary X-ray stars in ellipticals since they're geriatric stellar communities," said Craig Sarazin (University of Virginia) during a presentation to the AAS on Tuesday. "This implies they had an interesting past." The presence of these X-ray binaries indicates that ellipticals once contained many bright, massive stars, in marked contract to the present population of lower-mass, faint old stars that now dominate these galaxies. But the Chandra observations also revealed that the binary systems are not randomly scattered throughout each galaxy — many appear to be located in densely populated globular clusters that surround each elliptical. This raises the possibility that an X-ray binary in a globular cluster might result when an individual black hole either captures a single star or pulls one away from its original companion. Sarazin cautioned that more study is needed to determine if a majority of elliptical galaxies also have X-ray binaries within their globular clusters.

The full text of the press release can be found here:
http://chandra.harvard.edu/photo/2002/1140/index.html


Veil Nebula Closer than Previously Thought

June 3, 2002 | A pair of images of the northeastern edge of the Veil Nebula (Cygnus Loop) supernova remnant, acquired by the Hubble Space Telescope in 1997 and 2001, has strengthened the idea that the loop is only half as far away as previously believed. In a poster paper presented today at the American Astronomical Society meeting in Albuquerque, New Mexico, Ravi Sankrit (Johns Hopkins University), William P. Blair (JHU), and John C. Raymond (Harvard-Smithsonian Center for Astrophysics) derive a proper motion for the filament of 0.08 arcsecond per year, consistent with results published in 1999. However, to get their 1999 result they had to degrade the HST image to match an image taken 44 years earlier by the Palomar Sky Survey (POSS). "Because we had to soften the HST image so it matched the POSS plate, we couldn't be sure that the changes we saw were due entirely to proper motion," says Sankrit. "It was possible that we were also recording brightness changes." The 2001 HST image not only shows that the entire shock front has moved outward, but that the substructure of the filament is unchanged over four years. Based on this proper motion, the Cygnus Loop is estimated to be 1,400 light-years away, using the nebula's known rate of expansion (found from the blue- and redshifts of the parts moving directly toward and away from us). This is significantly closer than the 2,500 light-year distance that has been accepted for more than 40 years.


Stellar Black Holes Getting Lighter

June 3, 2002 | Theory predicts that stellar black-hole masses should range from roughly 1.4 to 15 times the mass of the Sun. Trouble is, finding black holes at either end of the scale has proven difficult. By observing soft X-ray transients in binary systems (believed to consist of a black hole or neutron star and a late type dwarf star), Dawn M. Gelino, (UCSD Center for Astrophysics and Space Sciences) and Thomas E. Harrison (New Mexico State University) discovered two systems containing black holes at the lower end of the mass spectrum. "It's nice to finally have some confirmation that low-mass black holes actually exist," says Gelino who presented a poster paper at today's AAS meeting. One of the systems — Nova Velorum 1993 — contains a 5.25 solar-mass black hole separated from its companion by just 3.3 solar radii. The other — JO422+32 — seems to have a 4.25 solar-mass black hole orbiting only 2.5 solar radii from its companion. Gelino plans to observe other soft X-ray transient systems in hopes of finding more low-mass black holes.

The full text of the press release can be found at:
http://www.noao.edu/outreach/press/pr02/pr0205.html


O Stars Aren't So Hot After All

June 3, 2002 | Type O stars, the most massive, hottest stars known, are not as hot as we thought, according to a poster paper presented at today's AAS meeting. Sarah R. Heap (NASA Goddard Space Flight Center) and Thierry Lanz (University of Maryland) collected the spectra of a number of O-type stars in the Small Magellanic Cloud using the Hubble Space Telescope and the Far Ultraviolet Spectroscopic Explorer. They compared their results with theoretical stellar spectra developed by Lanz and Ivan Hubeny (National Optical Astronomy Observatory) and found that O stars appear to be upwards of 20 percent cooler than earlier estimates. If true, then their luminosities (mostly in the ultraviolet) and masses are subject to downward revision as well. A more accurate temperature estimate will also provide a better estimate of their ages.

The full text of the press release can be found at:
http://www.gsfc.nasa.gov/topstory/20020603startemp.html