SETI S&T's News Bulletin for June 4, 1999


Larry Klaes (lklaes@bbn.com)
Mon, 07 Jun 1999 16:02:02 -0400


>From: SkyGlance@aol.com >Date: Sat, 5 Jun 1999 14:56:09 EDT >Subject: S&T's News Bulletin for June 4, 1999 >X-Mailer: AOL 4.0 for Windows 95 sub 15 >To: undisclosed-recipients:; > >=========================================================================== > SKY & TELESCOPE'S NEWS BULLETIN - JUNE 4, 1999 >=========================================================================== > For images and Web links for these and additional news items, visit > http://www.skypub.com/news/news.html >=========================================================================== > >LUNAR PROSPECTOR TO MAKE A SPLASH > >If David B. Goldstein has his way, NASA's Lunar Prospector spacecraft will >finish its life as a cosmic projectile by intentionally crashing onto the >Moon's south pole. Goldstein hopes that his team's proposed July 31st >impact into an unnamed, 50-kilometer-wide crater will eject a plume of >debris containing water from the ice that is suspected to lie in the >perpetual polar shadows. The Hubble Space Telescope and McDonald >Observatory will be watching, looking for the telltale spectral signatures >of water or its molecular byproduct OH (hydroxyl). But seeing clear >evidence of polar ice will be a long shot. As Goldstein and his team note >in the June 15th issue of *Geophysical Research Letters,* "This is, >admittedly, a high-risk experiment for which only a positive result has >definite meaning." > >Here's the plan: Ground controllers will use the remaining onboard fuel >first to raise Lunar Prospector's orbital high point from 30 km to 200 km, >then to reduce its velocity and force a collision near the Moon's south >pole at 9:51 Universal Time on July 31st. The approach angle will be no >more than about 7 deg. above horizontal, meaning the 160-kilogram craft >will barely clear the crater rim before slamming into its shadowed floor at >1.3 km per second. Prior measurements suggest that the near-surface >regolith there contains up to 2 percent frozen water, and Prospector's >impact energy could in theory heat as much as 18 kg of the ice to 127 deg. >Celsius (260 deg. Fahrenheit). Earth-based observers hope to record the >cloud of vapor as it expands beyond the lunar limb. (The Moon will be three >days past full.) Or they might detect tenuous wisps of OH for several hours >thereafter, once the impact debris settles back onto the surface. > >An aerospace engineer at the University of Texas at Austin, Goldstein notes >that his plan is supported by the Lunar Prospector team but awaits final >approval by NASA administrator Daniel Goldin. Notably, this flashy finale >is being considered only because funding for the project is being >terminated. Investigator William C. Feldman (Los Alamos National >Laboratory) says the spacecraft continues to operate well and could >insteadcontinue returning scientific data from its present orbit. > >IN CHICAGO, IT'S ALL THINGS ASTRONOMICAL > >Astronomers from around the world gathered in Chicago this past week for >the 194th meeting of the American Astronomical Society (AAS). More than >1,600 researchers in disciplines ranging from solar physics to cosmology >shared information on a wide variety of astronomical topics. The meeting, >hosted by the University of Chicago, Northwestern University, and the Adler >Planetarium, also marked the 100th anniversary of the founding of the AAS. >Sky & Telescope's Carolyn Collins Petersen and Joshua Roth attended the >meeting and filed daily reports. Visit http://www.skypub.com/ for >additional stories. > >PLANETARY NEBULAE: THE EXTRAORDINARY END OF ORDINARY STARS > >Two scientists using Hubble Space Telescope's Wide Field and Planetary >Camera 2 instrument continue to probe the structures of dying stars called >planetary nebulae. Raghavendra Sahai and John Trauger (Jet Propulsion >Laboratory) presented several images as part of their ongoing survey of >these beautiful and ghostly looking death stars. > >The false-color images, taken in the red-light emission of hydrogen atoms, >show an amazing amount of structure. The complexity of the images have long >presented a challenge to astronomers trying to explain why they look the >way they do. "Planetary nebulae evolve from round gas and dusty clouds >surrounding red giant stars," said Sahai. "We don't have a consensus yet on >the exact mechanism responsible for the structure, our images strongly >suggest that fast collimated outflow patterns that change direction could >be shaping these nebulae." > >In four examples presented at the meeting, Sahai and Trauger have >determined that the intricate shells of gas were ejected several thousand >years ago from the multi-billion-year-old, 30- to 40-thousand-degree-hot >central stars. The gas is rushing out from the stars are speeds between >20,000 and 60,000 kilometers per hour. Based on their work, the two >researchers theorize that the slowly expanding cloud of gas surrounding >each dying star starts out round. Then, jetlike outflows carve away at the >inner part of the shell. Hot stellar winds continually push on the shell, >eventually producing the delicate shapes we see in planetary nebulae. > >It's also possible that a binary companion to each dying star also stirs up >the picture, further complicating the shape of each nebula. Trauger and >Sahai will continue their survey of planetary nebulae in an effort to >solidify their theory. > >STARQUAKES IN NEUTRON STARS > >At the other end of the mass spectrum, supergiant stars, which end their >days as neutron stars and pulsars (as reported in the July 1999 issue of >SKY & TELESCOPE), could be exhibiting some unexpectedly familiar behaviors. >A University of Chicago scientist and her collaborators are theorizing that >such Earthlike geological phenomena as faulting, quakes and mountain >building may be part of the death dance that neutron stars perform as they >move into the last part of their lives. > >"Starquakes are different from earthquakes," said graduate student Lucia M. >Franco. "In some ways however, they're similar. You're still moving matter >and you're still forming mountains." In support of this seemingly >outrageous vision of neutron stars, Franco presented a new theory at the >AAS meeting. Her work, motivated by more than three decades of neutron-star >data, appears to explain the mysterious rotational slowdowns often seen in >these immense stellar graveyards. > >Neutron stars contain approximately 1.5 times the mass of the Sun in a >sphere about 16 kilometers (10 miles) in diameter. They are formed after a >supermassive star goes supernova, and the leftover is an ultracompressed >core of nothing but neutrons spinning very fast. > >The earthquake theory comes in when a neutron star must shift its rotation >to keep up with misalignments in its strong magnetic field. It starts to >"spin down," and stresses start to build up on the star's crust. Its shape >is warped. In response the brittle crust breaks at the equator. "Little >mountains form, throwing the distribution of matter at the surface out of >balance, causing a wobble," Franco said. > >This theory fits well with changes in rotational spin-down rates observed >in several "glitching" pulsars for which good data are available. Now >Franco is putting out a call for more and better data to see if her theory >will play out. "It will be interesting to see if changes in spin-down rates >of other neutron stars match what this theory predicts," she said. > >OTHER STEPS TOWARD SOLAR SYSTEMS? > >On June 2nd, astronomers made cases for planetary constituents around two >star systems in the Milky Way. In one study, Carol Grady (Space Telescope >Science Institute) and her colleagues used the Hubble Space Telescope's >imaging spectrometer to take pictures of material around AB Aurigae, a >4-million-year-old, type-A star some 470 light-years distant. Grady used >the spectrometer's coronagraph to block light from AB Aur itself; this >revealed a disklike structure, 1,300 astronomical units (a.u.) in radius, >that reflects light from the star. Striations in the reflective disk >suggest to Grady that material may already be clumping, perhaps presaging a >Kuiper Belt of cometlike bodies or a bona fide planetary system that will >come into being some millions of years hence. > >A second team has inferred the presence of a zodiacal dust band around HD >98800B, a spectroscopic binary star. The narrow belt of material, 4.5 a.u. >in radius but less than 1 a.u. thick, "is remarkably similar to the >zodiacal dust bands sustained by the asteroid families in our solar >system," according to Dean Hines and Glenn Schneider (University of >Arizona). The 165 deg. Kelvin band of dark, 200-micron-wide dust grains was >not seen in the team's near-infrared images of HD 98800B, taken with >Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS); its >presence was inferred from ground-based infrared and submillimeter >observations and from the Infrared Astronomical Satellite's (IRAS) >venerable database. However, Hubble's failure to detect the dust was >essential to proving its frigid 156 deg. K temperature and its very narrow >range of distances from the binary. With an estimated total mass just under >Earth's, "the debris may have resulted from a failed terrestrial planet >like that which created the asteroid belt between Mars and Jupiter," said >Hines. Shining at 9th magnitude in the constellation Crater, the HD 98800 >system has an estimated age of 5 million years. > >ETA CARINAE IS ACTING UP > >Life for the people who study Eta Carinae got a whole lot more interesting >a few short weeks ago when astronomer Kris Davidson (University of >Minnesota) and his colleagues Theodore Gull (NASA/Goddard Space Flight >Center) and Minnesota's Roberta Humphreys noticed that this massive star >had gotten a whole lot brighter very fast. They alerted other observers, >including a network of amateur astronomers in the Southern Hemisphere. The >results of their observations were presented at the AAS meeting in a series >of papers outlining the visual and spectroscopic changes that are >transforming the face of Eta Carina for the second time in 130 years. > >Eta Carinae is referred to as a luminous blue variable star. It dwarfs the >Sun in every way: it's 100 times more massive, it radiates five million >times more power, and its stellar winds blow out from 100 to 1,000 times >stronger than stars 30 or 40 times more massive than our star. It is >expected to end its days as a supernova, or as some researchers now >suspect, as a hypernova. Yet, Eta Carinae's suffocating double-lobed cloud >have kept astronomers from seeing everything that happens to the star. >Interestingly enough, that cloud is a relatively new phenomenon. In the >1840s, this supermassive giant star flared brightly and, over a 20-year >period, blew off enough material to create three Suns. We see this eruption >today as the nebula called the "Homunculus." > >In order to understand why Eta Carinae seemed to be brightening again this >year, Davidson and his colleagues turned the Hubble Space Telescope's Space >Telescope Imaging Spectrograph to look at the star and the inner 0.5 >arcsecond of the Homunculus where it is embedded. What they found is so >strange that they're still trying to explain what's happening. It appears >that a smaller double-lobed structure is growing within the larger pair of >lobes. It is this inner set of cones that has tripled in brightness since >April. According to Theodore Gull, the tremendous heat inside in these >cones may be responsible for the brightness. "Some of the brightness we're >seeing could be the result of ionization within the nebula," he said. >"However, it's also possible that Eta Carinae is about to erupt again." > >Astronomers are also trying to determine what spectra of the star actually >show. What is so astounding is that Davidson and others who study luminous >blue variables thought they had a good handle on how these stars work. "We >basically understand how all naked-eye stars work," he said. "All except >Eta Carinae. And now, we're stumped. This brightness increase was a >complete surprise because we weren't expecting any major outburst for a few >decades yet." The researchers were expecting a different kind of eruption >and brightening from Eta Carinae. They thought the spectrum would shift >from very hot emissions to lower-temperature ones. The data released on >Wednesday aren't playing along, leading Davidson and his colleagues to >re-think their understanding of Eta Carinae and the unstable, violent and >massive class of stars to which it belongs. > >FOCUS ON THE TRIFID > >Astronomer J. Jeff Hester and his colleagues at Arizona State University, >Jet Propulsion Laboratory, and Space Telescope Science Institute used HST's >WFPC2 to look at regions of star birth in the well-known Trifid Nebula >(M20). Their image shows a region of ongoing star formation that is being >torn apart by radiation from the star at the center of the nebula, which >lies about 9,000 light-years away from Earth. Like their previous work with >M16 (nicknamed "The Pillars of Creation"), the team found evaporating >gaseous globules (EGGs) in the Trifid. > >One of these globules sports a jet of material three-quarters of a >light-year long, moving out at speeds up to 370 kilometers per second >(about 800,000 miles per hour). Hester described this jet as a "ticker >tape" that tells the story of the stellar object hidden from view in the >EGG. The jet turned on about 600 years ago, and is being powered by the >star formation. It is also being lit by the star at the center of the >Trifid. Within the next 10,000 years or so, the advancing ionization front >from the nebula's main star will overrun the forming star. Depending on the >strength of the front, the newborn star's formation could stop dead in its >tracks. This may have already happened to another star in the EGG. A tiny >microjet pointing out from one of the "fingers" in the EGG may be the last >gasp of a star that was cut off from its supply of nurturing gas and dust >by the ionizing radiation more than 100,000 years ago. > >The vast majority of stars like our Sun form in regions like these, in the >neighborhood of massive powerful stars. It's a matter of luck that our Sun >survived this "survival of the fittest" regime. Ongoing HST observations of >the Trifid provide a snapshot of the ecology from which stars form. > >X-RAYS FROM HOT YOUNG STARS > >Newborn stars in another cluster were the subject of a series of >observations made by the ROSAT spacecraft before it was turned off earlier >this year. Scientists at Pennsylvania State University and the University >of New South Wales were looking for pre-main-sequence stars (stars that >have not yet begun to burn hydrogen, but that are very hot and bright). >They found a previously unknown open cluster of stars surrounding the star >Eta Chamaeleontis, located about 315 light-years from Earth in the southern >hemisphere of Chameleon. Very few young stars are as close to Earth as >these, and their discovery took researchers by surprise. They may be the >only infant stars that can be seen from Earth with just binoculars or a >small telescope. > >Oddly enough, these stellar newborns appear isolated in space, far from >their gaseous nursery. According to principal investigator Eric Feigelson, >it is very hard to find these pre-main-sequence stars apart from their >clouds without an X-ray telescope. Feigelson, who discovered in 1981 that >such stars have dramatic and distinctive X-ray signatures, worked with >graduate student Eric Mamajek to locate these young stellar objects by >their X-ray emission. "The intensity of X-ray signals emitted by these >stars is elevated far above normal main sequence stars," Feigelson said. >"They have magnetic flares, which are similar to the Sun's flares, but >thousands of times more powerful." > >The cluster is the first to be discovered with X-rays and the nearest to >Earth discovered this century. The team speculates that the Eta >Chamaeleontis stars were born in the Scorpius-Centaurus OB star >association, but they now lie dozens of light-years away from their stellar >nursery. "We suspect that this cluster was once inside the Sco-Cen >association," said Mamajek, "but its surrounding gas cloud was stripped >away by massive explosions of the supergiant Sco-Cen stars," said Mamajek. > >Because this open cluster is so recently formed, the researchers are also >searching through its members for small, low-mass objects called brown >dwarfs. It's possible that the cluster could harbor some of these >mysterious objects. If it does, the group hopes to learn more about the >birth of these objects that some astronomers consider the leftovers of star >birth. > >BROWN DWARFS: TWO'S COMPANY, SEVEN'S A CROWD > >Half a decade ago, brown dwarfs were mere conjecture. The term was coined >to describe objects that formed like stars but don't have enough mass to >sustain nuclear fusion in their cores. Theory suggests that gas balls with >less than 8 percent of our Sun's mass, or 80 Jupiters, would be mere brown >dwarfs. (It remains contentious just what differentiates brown dwarfs from >giant planets on the other end of the mass range; brown dwarfs all have >more or less the size of Jupiter, regardless of their mass.) In the last >few years, some compelling brown-dwarf candidates have surfaced in surveys >of young star clusters like the Pleiades. But skeptics awaited a >not-quite-star with the spectral signature of methane (CH4), a compound >that cannot survive at temperatures much above 900 deg. Celsius. Methane is >conspicuous in the near-infrared spectrum of Jupiter's atmosphere, and its >presence would be proof of a failed nuclear furnace. Their wait ended in >late 1995 with the discovery of a methane brown dwarf in orbit about a >nearby star called Gliese 229. > >Yet for nearly four years Gl229B, as the substellar companion is known, >stood alone. But no longer. Two ambitious sky surveys now underway -- the >Sloan Digital Sky Survey and the 2 Micron All Sky Survey -- have turned up >a half dozen more methane dwarfs between them. The objects were suspected >to be brown dwarfs on the basis of their unusual colors; near-infrared >spectra (some of which were taken only days before the AAS meeting) then >confirmed the spectral signature of methane. These objects "really fill in >the gap" between planets and stars, said Sloan coinvestigator Xiaohui Fan >(Princeton University), and according to his colleague David Golimowski >(Johns Hopkins University), they may be as common in our galaxy as bona >fide stars. > >Establishing the overall abundance of these very dim dwarfs goes beyond >mere novelty; it's crucial to our understanding of star formation and the >composition of the Milky Way's still-mysterious dark matter. > >COMET LEE AT 7th MAGNITUDE > >Comet Lee (C/1999 H1) moves through Cancer this coming week. It is >currently about 7th magnitude and has a coma about 8 arcminutes in >diameter. While still visible to those in the Southern Hemisphere above the >northwestern horizon after twilight, Comet Lee is dropping steadily. >Observers in midnorthern latitudes will find it 15 to 25 degrees above the >western horizon at the end of twilight. Here are positions (2000.0 >coordinates) at 0 hours Universal Time for the coming week: > > Date R.A. Dec. > >June 5 8h 30.1m +7.7 deg. >June 7 8h 28.3m +9.5 deg. >June 9 8h 26.5m +11.1 deg. >June 11 8h 24.8m +12.6 deg. > > >THIS WEEK'S "SKY AT A GLANCE" > > Some daily events in the changing sky, from the editors of SKY & >TELESCOPE. > >JUNE 6 -- SUNDAY > > * Look southwest during evening; you can't miss orange Mars and >blue-white Spica shining high, less that 2 degrees apart. The bright >yellow-orange star high above them is Arcturus. Well to their lower right, >look for the four-star quadrilateral of the constellation Corvus, the Crow. > >JUNE 7 -- MONDAY > > * Last-quarter Moon (exact at 12:20 a.m. Eastern Daylight Time). > >JUNE 8 -- TUESDAY > > * The red long-period variable star R Corvi should be at maximum light >(7th or 8th magnitude) around this date. > > * The waning Moon shines to the right of Jupiter in the east during early >dawn Wednesday morning. > >JUNE 9 -- WEDNESDAY > > * The Moon shines below Jupiter at dawn Thursday morning. > >JUNE 10 -- THURSDAY > > * Venus is at greatest elongation, 45 degrees east of the Sun. > >JUNE 11 -- FRIDAY > > * Look rather low in the south-southeast as evening grows late to spot >the orange-red star Antares. Fainter white stars of the constellation >Scorpius are scattered around it. > >JUNE 12 -- SATURDAY > > * Venus is just 1 degree north of the Beehive star cluster, M44 in >Cancer. As twilight ends, take a look with binoculars! > > > ============================ > THIS WEEK'S PLANET ROUNDUP > ============================ > >MERCURY is hidden in the glare of sunset. > >VENUS is the brilliant "Evening Star" (magnitude -4.3) shining in the >west-northwest during and after dusk. Pollux and Castor are to its lower >right. > >MARS (magnitude -0.9) blazes bright orange in the southwest during evening. >The fainter star just below it is blue-white Spica; this week the two >appear as close as they'll get. In a telescope, Mars is about 13.5 >arcseconds in apparent diameter -- pretty small, but the biggest we'll see >it for another two years. Mars will shrink and fade in coming weeks. > >JUPITER is low in the east at dawn. > >SATURN is emerging from the glow of sunrise well to Jupiter's lower left. >Look about 60 minutes before sunrise. > >URANUS and NEPTUNE, dim at magnitudes 6 and 8, respectively, are in the >southeast during early morning hours. See the finder chart in the May Sky & >Telescope, page 108, or at >http://www.skypub.com/sights/moonplanets/urnepplu.html. > >PLUTO, extremely dim at magnitude 14, is in Ophiuchus in the south around >midnight. See the finder chart in the March Sky & Telescope, page 103, or >at http://www.skypub.com/sights/moonplanets/plutochart99.html. > >(All descriptions that relate to the horizon or zenith are written for the >world's midnorthern latitudes. Descriptions that also depend on longitude >are for North America. Eastern Daylight Time, EDT, equals Universal Time >minus 4 hours.) > >More details, sky maps, and news of other celestial events appear each >month in SKY & TELESCOPE, the essential magazine of astronomy. See our >enormous Web site at www.skypub.com. Clear skies! > >SKY & TELESCOPE, P.O. Box 9111, Belmont, MA 02478 * 617-864-7360 > >=========================================================================== >Copyright 1999 Sky Publishing Corporation. S&T's Weekly News Bulletin and >Sky at a Glance stargazing calendar are provided as a service to the >astronomical community by the editors of SKY & TELESCOPE magazine. >Widespread electronic distribution is encouraged as long as these >paragraphs are included. But the text of the bulletin and calendar may not >be published in any other form without permission from Sky Publishing >(contact permissions@skypub.com or phone 617-864-7360). Updates of >astronomical news, including active links to related Internet resources, >are available via SKY & TELESCOPE's site on the World Wide Web at >http://www.skypub.com/. > >In response to numerous requests, and in cooperation with the Astronomical >League (http://www.mcs.net/~bstevens/al/) and the American Association of >Amateur Astronomers (http://www.corvus.com/), S&T's Weekly News Bulletin >and Sky at a Glance are available via electronic mailing list too. For a >free subscription, send e-mail to skyline@corvus.com and put the word >"join" on the first line of the body of the message. To unsubscribe, send >e-mail to skyline@corvus.com and put the word "unjoin" on the first line of >the body of the message. >--------------------------------------------------------------------------- >SKY & TELESCOPE, the Essential Magazine of Astronomy, is read by more than >200,000 enthusiasts each month. It is available on newsstands worldwide. >For subscription information, or for a free copy of our catalog of fine >astronomy books and products, please contact Sky Publishing Corp., 49 Bay >State Rd., Cambridge, MA 02138-1200, U.S.A. Phone: 800-253-0245 (U.S. and >Canada); 617-864-7360 (International). Fax: 617-864-6117. E-mail: >>custserv@skypub.com. WWW: http://www.skypub.com/. Clear skies! >=========================================================================== >



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