Chapter 28:

Stellar Evolution: Images, spectra, and explanation of the life cycle of stars.
Gallery of Planetary Nebulae from NOAO
Terzian Planetary Nebula Web Resources

A White Dwarf Supernova Inside A Dense Circumstellar Disk

BERKELEY, CA By measuring polarized light from an unusual exploding star, an international team of astrophysicists and astronomers has worked out the first detailed picture of a Type Ia supernova and the distinctive star system in which it exploded.

Using the European Southern Observatory's Very Large Telescope in Chile, the researchers determined that supernova 2002ic exploded inside a flat, dense, clumpy disk of dust and gas, previously blown away from a companion star. Their work suggests that this and some other precursors of Type Ia supernovae resemble the objects known as protoplanetary nebulae, well known in our own Milky Way galaxy.

Lifan Wang of Lawrence Berkeley National Laboratory, Dietrich Baade of the European Southern Observatory (ESO), Peter H=F6flich and J. Craig Wheeler of the University of Texas at Austin, Koji Kawabata of the National Astronomical Observatory of Japan, and Ken'ichi Nomoto of the University of Tokyo report their findings in the 20 March 2004 issue of Astrophysical Journal Letters.

Casting supernovae to type

Type II and some other supernovae occur when the cores of very massive stars collapse and explode, leaving behind extremely dense neutron stars or even black holes. Type Ia supernovae, however, explode by a very different mechanism.

"A Type Ia supernova is a metallic fireball," explains Berkeley Lab's Wang, a pioneer in the field of supernova spectropolarimetry. "A Type Ia has no hydrogen or helium but lots of iron, plus radioactive nickel, cobalt, and titanium, a little silicon, and a bit of carbon and oxygen. So one of its progenitors must be an old star that has evolved to leave behind a carbon-oxygen white dwarf. But carbon and oxygen, as nuclear fuels, do not burn easily. How can a white dwarf explode?"

The most widely accepted Type Ia models assume that the white dwarf -- roughly the size of Earth but packing most of the mass of the sun -- accretes matter from an orbiting companion until it reaches 1.4 solar masses, known as the Chandrasekhar limit. The now superdense white dwarf ignites in a mighty thermonuclear explosion, leaving behind nothing but stardust.

Other schemes include the merger of two white dwarfs or even a lone white dwarf that re-accretes the matter shed by its younger self. Despite three decades of searching, however, until the discovery and subsequent spectropolarimetric studies of SN 2002ic, there was no firm evidence for any model.

In November of 2002, Michael Wood-Vasey and his colleagues in the Department of Energy's Nearby Supernova Factory based at Berkeley Lab reported the discovery of SN 2002ic, shortly after its explosion was detected almost a billion light-years away in an anonymous galaxy in the constellation Pisces.

In August of 2003, Mario Hamuy from the Carnegie Observatories and his colleagues reported that the source of the copious hydrogen-rich gas in SN 2002ic was most likely a so-called Asymptotic Giant Branch (AGB) star, a star in the final phases of its life, with three to eight times the mass of the sun -- just the sort of star that, after it has blown away its outer layers of hydrogen, helium, and dust, leaves behind a white dwarf.

Moreover, this seemingly self-contradictory supernova -- a Type Ia with hydrogen -- was in fact similar to other hydrogen-rich supernovae previously designated Type IIn. This in turn suggested that, while Type Ia supernovae are indeed remarkably similar, there may be wide differences among their progenitors.

Because Type Ia supernovae are so similar and so bright -- as bright or brighter than whole galaxies -- they have become the most important astronomical standard candles for measuring cosmic distances and the expansion of the universe. Early in 1998, after analyzing dozens of observations of distant Type Ia supernovae, members of the Department of Energy's Supernova Cosmology Project based at Berkeley Lab, along with their rivals in the High-Z Supernova Search Team based in Australia, announced the astonishing discovery that the expansion of the universe is accelerating.

Cosmologists subsequently determined that over two-thirds of the universe consists of a mysterious something dubbed "dark energy," which stretches space and drives the accelerating expansion. But learning more about dark energy will depend on careful study of many more distant Type Ia supernovae, including a better knowledge of what kind of star systems trigger them.

Picturing structure with spectropolarimetry

If the dust cloud or explosion is spherical and uniformly smooth, all orientations are equally represented and the net polarization is zero. But if the object is not spherical -- shaped like a disk or a cigar, for example -- more light will oscillate in some directions than in others.

Even for quite noticeable asymmetries, net polarization rarely exceeds one percent. Thus it was a challenge for the ESO spectropolarimetry instrument to measure faint SN 2002ic, even using the powerful Very Large Telescope. It took several hours of observation on four different nights to acquire the necessary high-quality polarimetry and spectroscopy data.

The team's observations came nearly a year after SN 2002ic was first detected. The supernova had grown much fainter, yet its prominent hydrogen emission line was six times brighter. With spectroscopy the astronomers confirmed the observation of Hamuy and his associates, that ejecta expanding outward from the explosion at high velocity had run into surrounding thick, hydrogen-rich matter.

Only the new polarimetric studies, however, could reveal that most of this matter was shaped as a thin disk. The polarization was likely due to the interaction of high-speed ejecta from the explosion with the dust particles and electrons in the slower-moving surrounding matter. Because of the way the hydrogen line had brightened long after the supernova was first observed, the astronomers deduced that the disk included dense clumps and had been in place well before the white dwarf exploded.

"These startling results suggest that the progenitor of SN 2002ic was remarkably similar to objects that are familiar to astronomers in our own Milky Way, namely protoplanetary nebulae," says Wang. Many of these nebulae are the remnants of the blown-away outer shells of Asymptotic Giant Branch stars. Such stars, if rotating rapidly, throw off thin, irregular disks.

A matter of timing

Thus it's more likely that a white dwarf companion in the SN 2002ic system was already busily collecting matter long before the nebula formed. Because the protoplanetary phase lasts only a few hundred years, and assuming a Type Ia supernova typically takes a million years to evolve, only about a thousandth of all Type Ia supernovae are expected to resemble SN 2002ic. Fewer still will exhibit its specific spectral and polarimetric features, although "it would be extremely interesting to search for other Type Ia supernovae with circumstellar matter," Wang says.

Nevertheless, says Dietrich Baade, principal investigator of the polarimetry project that used the VLT, "it's the assumption that all Type Ia supernovae are basically the same that permits the observations of SN 2002ic to be explained."

Binary systems with different orbital characteristics and different kinds of companions at different stages of stellar evolution can still give rise to similar explosions, through the accretion model. Notes Baade, "The seemingly peculiar case of SN 2002ic provides strong evidence that these objects are in fact very much alike, as the stunning similarity of their light curves suggests."

By showing the distribution of the gas and dust, spectropolarimetry has demonstrated why Type Ia supernovae are so much alike even though the masses, ages, evolutionary states, and orbits of their precursor systems may differ so widely.

Pulsating White Dwarfs and the Search for Exoplanets

Austin, Texas University of Texas at Austin astronomers have invented an inexpensive method to determine if other solar systems like our own exist.

Among the more than 100 stars now known to have planets, astronomers have found few systems similar to ours. It's unknown if this is because of technological limitations or if our system is truly a rare configuration. The McDonald Observatory astronomers=B9 novel search method uses a Depression-era telescope mated with today=B9s technology.

Astronomers Don Winget and Edward Nather, graduate students Fergal Mullally and Anjum Mukadem, and colleagues are looking for the "leftovers" of solar systems like ours. Their method searches for the pieces of such a solar system after its star has died, by exploiting a trait of ancient, burnt-out Suns called "white dwarfs."

University of Texas astronomers Bill Cochran and Ted von Hippel are also involved, along with S.O. Kepler of Brazil's Universidade Federal de Rio Grande dol Sul and Antonio Kanaan of Brazil's Universidade Federal de Santa Catarina.

Astronomers know that as Sun-like stars use up their nuclear fuel, their outer layers will expand, and the star will become a "red giant" star. When this happens to the Sun, in about five billion years, they expect it will swallow Mercury and Venus, perhaps not quite reaching Earth. Then the Sun will blow off its outer layers and will exist for a few thousand years as a beautiful, wispy planetary nebula. The Sun's leftover core will then be a white dwarf, a dense, dimming cinder about the size of Earth. And, most importantly, it likely will still be orbited by the outer planets of our solar system.

Once a Sun-like system reaches this state, WingeT's team may be able to find it. Their method is based on more than three decades of research on the variability (that is, changes in brightness) of white dwarfs. In the early 1980s, University of Texas astronomers discovered that some white dwarfs vary, or "pulsate," in regular bursts. More recently, Winget and colleagues discovered that about one-third of these pulsating white dwarfs (PWDs) are more reliable timekeepers than atomic clocks and most millisecond pulsars.

These pulsations are the key to detecting planets. Planets orbiting a stable PWD star will affect observations of its timekeeping, appearing to cause periodic variations in the patterns of pulses coming from the star. That=B9s because the planet orbiting the PWD drags the star around as it moves. The change in distance between the star and Earth with change the amount of time taken for the light from the pulsations to reach Earth. Because the pulses are very stable, astronomers can calculate the difference between the observed and expected arrival time of the pulses and deduce the presence and properties of the planet. (This method is similar to that used in the discoveries of the so-called "pulsar planets." The difference is, the pulsar companions are not thought to have formed with their stars, but only after those stars had exploded in supernovae.)

"This search will be sensitive to white dwarfs which were initially between one and four times as massive as the Sun, and should be able to detect planets within two to 20 AU from their parent star. This means we'll be probing inside the habitable zone for some stars," Winget said. (An AU, or astronomical unit, is the distance between Earth and the Sun.) "Basically, detecting Jupiter at Jupiter's distance with this technique is easy. It's duck soup," he said.

Easy, but not quick. Outer planets, orbiting their stars at large distances, can take more than a decade to complete one orbit. Therefore, it can take many years of observations to definitively detect a planet orbiting a white dwarf.

"You need to look for a long time for a full orbit," Winget said. "A half-orbit or a third of an orbit will tell us something's going on there. But for a planet at Jupiter's distance, a half-orbit is still six years." Winget added that for this method, "detecting Jupiter at Uranus' distance is easier, but takes even longer."

For the PWD planet search, Nather conceived a specialized new instrument for McDonald Observatory's 2.1-meter Otto Struve Telescope. He and Mukadam designed and built the instrument, called Argos, to measure the amount of light coming from target stars. Specifically, Argos is a "CCD photometer" -- a photon counter that uses a charge-coupled device to record images. Located at the prime focus of the Struve Telescope, Argos has no optics other than the telescope's 2.1-meter primary mirror. Copies of Argos are now being built at other observatories around the world.

Mullally continues the search for planets around white dwarfs with Argos on the Struve Telescope. He currently has 22 target stars, most of which were identified through the Sloan Digital Sky Survey.

When the team finds promising planet candidates with Argos, they will follow up using the 9.2-meter Hobby-Eberly Telescope (HET) at McDonald Observatory.

"If we find large planets orbiting at large distances, that's a good clue that there might be smaller planets closer in. In that case, what you do is pound away on that target with the largest telescope you have access to," Winget said. The HET will enable more precise timing of the PWD's pulses, and thus be able to pinpoint smaller planets.

This search will be able to study types of stars unable to be studied with the doppler spectroscopy method =8B the most successful planet search method to date =8B Winget said. Because of idiosyncrasies in the make-up of Sun-like stars, the doppler spectroscopy method is not very sensitive in looking for planets around stars twice as massive as the Sun. Roughly half of the stars in Winget=B9s study will be white dwarfs that were originally these types of stars. For this reason, the PWD study at McDonald can be instrumental in scouting and assessing targets and observing strategies for NASA space missions planned in the next two decades, specifically the Space Interferometry Mission, Terrestrial Planet Finder, and Kepler spacecraft.

A Weird and Beautiful Star Light Echo

see also the
ESA Press Release, March 26, 2003

In January 2002, a moderately dim star in the constellation Monoceros, the Unicorn, suddenly became 600 000 times more luminous than our Sun. This made it temporarily the brightest star in our Milky Way. The light from this eruption created a unique phenomenon known as a 'light echo' when it reflected off dust shells around the star.

More at: http://sci.esa.int/hubble/news/index.cfm?oid=32026

University of Arizona Press Release

The mysterious star has since faded back to obscurity. But between late April and late October, astronomers using NASA's Hubble Space Telescope observed the most spectacular "light echo" ever seen.

Light from the explosively brightened star lit up surrounding circumstellar dust like a flashbulb lights up fog. Astronomers got detailed, color CAT scan-like views of the three-dimensional structure of dust shells surrounding the star.

They report their results March 27, 2003, in the journal Nature.

Light echoing off circumstellar dust in our Milky Way galaxy was last seen in 1936, long before Hubble was available to study the tidal wave of light and reveal the netherworld of dusty black interstellar space.

"The light echo gives us a recording of the star's unusual eruption and allows us to follow a detective hunt to figure out how this star is evolving. Hubble's view is so sharp that we for the first time can do 'astronomical tomography' of the space around the star," says the lead observer, astronomer Howard Bond of the Space Telescope Science Institute in Baltimore.

Other authors on the report are the Zoltan G. Levay, Nino Panagia and William B. Sparks of the Space Telescope Science Institute; Arne Henden of the U.S. Naval Observatory; Sumner Starrfield of Arizona State University; R. Mark Wagner with the Large Binocular Telescope Observatory at the University of Arizona; Romano L.M. Corradi from the Isaac Newton Group of Telescopes in Canarias, Spain; and Ulisse Munari from the INAF-Osservatorio Astronomico de Padova in Asiago, Italy.

Web links:
V838 Mon movie - http://vela.as.arizona.edu/~rmw/v838mon.movie.gif
R. Mark Wagner - http://vela.as.arizona.edu/~rmw/
__________________________

The star presumably ejected the illuminated dust shells in previous outbursts. Some of the light from the latest outburst travels to the dust and then is reflected to Earth. Because of this indirect path, the light arrives at Earth long after light coming directly toward Earth from the star itself.

V838 Mon did not explosively expel its outer layers like a typical nova outburst, where a normal star dumps hydrogen onto a compact white-dwarf companion star. In that case, the piled-up hydrogen spontaneously explodes by nuclear fusion -- like a titanic hydrogen bomb. Instead, V838 Mon slowly grew to an enormous size, with its surface temperature dropping to temperatures not much hotter than a light bulb.

"We don't have any other objects that really compare to this event," UA astronomer R. Mark Wagner said. "It is unique both in its evolution and the presence of the spectacular light echo."

Wagner and ASU Regents' Professor Sumner Starrfield, a scientist with the University of Arizona Theoretical Astrophysics Program, have found by ground-based spectroscopy that V838 Mon is actually a binary system. Only the cooler of the two stars underwent the outburst.

At its peak brightness, the star was relatively blue in color, Wagner noted. But several months later, it was much redder. That's why the rings of the light echoes in the Hubble photos appear blue at the outer edge and red on the inner edge.

Eventually, astronomers will see the light echo close entirely, as light reflected from dust behind the star finally reaches Earth, Wagner predicts. The astronomers hope to use the light echo to more precisely determine how far away the star is, which is a key to understanding the physics of what happened, Wagner said.

"The distance is hard to estimate," he said. "But we need to pin down how far away this is to determine how much energy was involved in its outburst. Once we know how bright the outburst really was, we may discover what triggered it, amd what caused the slow ejection of the star's envelope."

The circular light-echo feature has now expanded to twice the angular size of Jupiter on the sky. Astronomers expect it to continue expanding as reflected light from farther out in the dust envelope finally arrives at Earth. Bond predicts that the echo will be observable for the rest of this decade.

MOVIE DESCRIPTION: HST images of V838 Mon in the movie online at http://vela.as.arizona.edu/~rmw/v838mon.movie.gif were taken May 20, Sept. 2, Oct. 28 and Dec. 17, 2002.

ELECTRONIC IMAGE FILES and additional information are available at:
http://hubblesite.org/news/2003/10

STScI PRESS RELEASE NO.: STScI-PR03-10

The mysterious star has long since faded back to obscurity, but observations by NASA's Hubble Space Telescope of a phenomenon called a "light echo" have uncovered remarkable new features. These details promise to provide astronomers with a CAT-scan-like probe of the three-dimensional structure of shells of dust surrounding an aging star. The results appear tomorrow in the journal Nature.

"Like some past celebrities, this star had its 15 minutes of fame," says Anne Kinney, director of NASA's Astronomy and Physics program, Headquarters, Washington. "But its legacy continues as it unveils an eerie light show in space. Thankfully, NASA's Hubble has a front row seat to this unique event in our galaxy."

Light from a stellar explosion echoing off circumstellar dust in our Milky Way galaxy was last seen in 1936, long before Hubble was available to study the tidal wave of light and reveal the netherworld of dusty black interstellar space.

"As light from the outburst continues to reflect off the dust surrounding the star, we view continuously changing cross-sections of the dust envelope. Hubble's view is so sharp that we can do an 'astronomical cat-scan' of the space around the star," says the lead observer, astronomer Howard Bond of the Space Telescope Science Institute in Baltimore.

Bond and his team used the Hubble images to determine that the petulant star, called V838 Monocerotis (V838 Mon) is about 20,000 light-years from Earth. The star put out enough energy in a brief flash to illuminate surrounding dust, like a spelunker taking a flash picture of the walls of an undiscovered cavern. The star presumably ejected the illuminated dust shells in previous outbursts. Light from the latest outburst travels to the dust and then is reflected to Earth. Because of this indirect path, the light arrives at Earth months after light coming directly toward Earth from the star itself.

The outburst of V838 Mon was somewhat similar to that of a nova, a more common stellar outburst. A typical nova is a normal star that dumps hydrogen onto a compact white-dwarf companion star. The hydrogen piles up until it spontaneously explodes by nuclear fusion -- like a titanic hydrogen bomb. This exposes a searing stellar core, which has a temperature of hundreds of thousands of degrees Fahrenheit.

By contrast, however, V838 Mon did not expel its outer layers. Instead, it grew enormously in size, with its surface temperature dropping to temperatures not much hotter than a light bulb. This behavior of ballooning to an immense size, but not losing its outer layers, is very unusual and completely unlike an ordinary nova explosion.

"We are having a hard time understanding this outburst, which has shown a behavior that is not predicted by present theories of nova outbursts," says Bond. "It may represent a rare combination of stellar properties that we have not seen before."

The star is so unique it may represent a transitory stage in a star's evolution that is rarely seen. The star has some similarities to highly unstable aging stars called eruptive variables, which suddenly and unpredictably increase in brightness.

The circular light-echo feature has now expanded to twice the angular size of Jupiter on the sky. Astronomers expect it to continue expanding as reflected light from farther out in the dust envelope finally arrives at Earth. Bond predicts that the echo will be observable for the rest of this decade.

The research team included investigators from the Space Telescope Institute in Baltimore; the Universities Space Research Association at the U.S. Naval Observatory in Flagstaff, Ariz.; the European Space Agency; Arizona State University; the Large Binocular Telescope Observatory at the University of Arizona at Tucson; the Isaac Newton Group of Telescopes in Spain's Canary Islands; and the INAF-Osservatorio Astronomico di Padova in Asiago, Italy.

Electronic image files and additional information are available at:
http://hubblesite.org/news/2003/10

Can You See a White Dwarf, Sirius B?

As of 2002, Sirius B is 5.5" southeast of Sirius, and will gradually increase its separation until it is 11.3" away toward the east-northeast in 2022. Sue French describes her observations and these details in Sky & Telescope for March 2002, p. 88. She saw Sirius B with her 105-mm (4-inch) refractor.

VLA Sees Star Turning Into Planetary Nebula

A team of astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope has caught an old star during the very brief period of its transformation into a planetary nebula, a shining bubble of glowing gas with a hot remnant star at its center.

"This is the first time that anyone has seen a star that is so clearly going through this transformation stage," said Yolanda Gomez, an astronomer at the Institute for Astronomy at the National Autonomous University in Mexico City, Mexico. "We believe this star began to enter its planetary-nebula phase only after 1984," she added. The researchers reported their findings in the November 15 edition of the scientific journal Nature.

At the end of their lives, stars like our Sun eject gas into space before starting to contract under their own gravity into white dwarf stars. The gravitational contraction heats up the star, making it pour out energetic ultraviolet light. The ultraviolet light tears apart molecules in the gas ejected earlier by the star and rips electrons from the atoms in the gas. This makes the gas glow, producing often-beautiful shining shells and other shapes.

Once the remnant star has heated up sufficiently to produce large amounts of ultraviolet light, molecules in the gas ejected earlier are destroyed rapidly. "We are seeing radio waves emitted by water molecules in this planetary nebula," said Gomez, who added, "The water molecules, we believe, are all destroyed within only 100 years of the beginning of this stage, so we are seeing this star during an extremely brief transition period of its life."

The astronomers used the VLA to observe a planetary nebula called K3-35, 16,000 light-years from Earth in the constellation Vulpecula (the small fox). This object has a doughnut-shaped ring of gas around its center and lobes of outflowing material, similar to structures seen in other planetary nebulae.

The researchers were surprised to find regions near the star in which water molecules are amplifying, or strengthening radio-wave emission at a frequency of 22 GigaHertz, in the same manner that a laser amplifies light waves. They found these regions, called masers, in the doughnut-shaped structure surrounding the central star, as well as at the end of much larger lobes of gas extending from the star. The doughnut-shaped ring has a radius of more than twice the distance from the Sun to Pluto. The masers at the ends of the lobes are more than 100 times more distant from the star.

By analyzing their VLA observations as well as earlier observations of the object by other astronomers, the research team concludes that K3-35 has only just begun its transformation into a planetary nebula.

"This is extremely exciting, because we now have a 'laboratory' for watching this process take place over the next few years," Luis Miranda of the Institute of Andalucia in Spain said. "We don't fully understand everything we see in this object, but know that we are going to learn much valuable information about this process by watching it develop," he added.

"We are very lucky to have caught this star during such a very brief but important period of its life," agreed Guillem Anglada of the Harvard- Smithsonian Institute for Astrophysics in Cambridge, MA; and Jose Torrelles of the Institute for Space Studies of Catalyunya in Barcelona, Spain, the other members of the team.

A VLA image of K3-35 is available on the NRAO Web page, at:
http://www.nrao.edu/pr/k335.html