[Spitzer News] Scientists Hold Séance for Supernova

spitzer-news at lists.ipac.caltech.edu spitzer-news at lists.ipac.caltech.edu
Thu May 29 12:57:11 PDT 2008

In this issue:

1) Scientists Hold Séance for Supernova
2) Strange Ring Found Circling Dead Star
3) Spitzer Finds Cosmic Neon's Sweet Spot
4) New Software Brings the Universe to Your Computer
5) The Second Stellar Baby Boom



Astronomers have unearthed secrets from the grave of a star that  
blasted apart in a supernova explosion long ago. By decoding ghostly  
echoes of light traveling away from the remains of a supernova called  
Cassiopeia A, the scientists have pieced together what the star  
looked like in life, and ultimately how it met its demise.

The discovery, made using primarily NASA's Spitzer Space Telescope  
and Japan's Subaru telescope on Mauna Kea in Hawaii, represents the  
first time astronomers have been able to resurrect the life history  
of a supernova remnant in our own galaxy.

"Cassiopeia A lies in our cosmic backyard and offers the sharpest  
view of what is left hundreds of years after a supernova explosion,"  
said Oliver Krause of the Max Planck Institute for Astronomy in  
Germany, lead author of a paper about the discovery appearing in this  
week's Science. "The echoes of light we found around Cassiopeia A  
provide us with a time machine to go back and see its past."

Cassiopeia A is one of the most explored objects in our sky and the  
subject of more than 1,000 scientific papers. It is the burnt-out  
corpse of a massive star that ended its life in a fiery supernova  
about 11,300 years ago. In fact, until recently, it was the youngest  
supernova remnant in our Milky Way galaxy (the new record holder, G1.9 
+0.3, was recently discovered using NASA's Chandra X-ray Observatory  
and other ground-based telescopes). Because Cassiopeia A is 11,000  
light-years from Earth, the light from its explosion would have  
reached Earth, sweeping right past it, about 300 years ago.

Astronomers had thought this supernova light was never to be seen  
again, until 2005, when Krause and his colleagues discovered hints of  
it still bouncing around clouds surrounding the remnant (http:// 
www.spitzer.caltech.edu/Media/releases/ssc2005-14/index.shtml). Using  
Spitzer's infrared eyes, they found so-called infrared echoes, which  
occur when a flash of light from the supernova blasts through clouds,  
heating them up and causing them to glow in infrared. As the light  
rolls outward, the infrared echoes continue to flare up and travel  
away from the star.

In the new study, the astronomers used Cassiopeia A's infrared echoes  
to hone in on faint visible-light echoes with Subaru and other ground- 
based telescopes. Visible-light echoes, known simply as light echoes,  
occur when visible light from the supernova scatters off dust. Unlike  
infrared echoes, they are direct signals from the graves of exploded  
stars, bearing all the information about the nature of the original  

Next, the astronomers had to act quickly because these echoes can  
fade within weeks. They used Subaru's spectrometer instrument to  
break the light apart and reveal signatures of atoms present when  
Cassiopeia A exploded. The resulting spectrum of light revealed  
hydrogen and helium -- telltale signs that Cassiopeia A was once a  
huge red supergiant star whose core collapsed in a rare supernova  
referred to as Type IIb. Previously, scientists did not know the  
supernova class to which Cassiopeia A belonged.

"This is an exciting result," said Alex Filippenko of the University  
of California, Berkeley, a supernova expert not affiliated with the  
study. "Cassiopeia A has been studied extensively with many  
telescopes over a wide range of wavelengths. It is gratifying that we  
finally know what kind of star exploded so long ago."

The findings also offer insight into another mystery shrouding  
Cassiopeia A. When Cassiopeia A's original star erupted, the event  
should have been widely witnessed on Earth as a bright star lighting  
up the sky. The most likely possible sighting is by the Astronomer  
Royal John Flamsteed in 1680, but he made just one observation of a  
dim star. The fact that almost no one saw the event is a classic  
problem in supernova lore.

Now that astronomers have learned how Cassiopeia A was forged, they  
think they might know why its death went unnoticed. "Type IIb  
supernovas fade quickly," said co-author George Rieke of the  
University of Arizona in Tucson. "This, plus a few cloudy nights,  
might explain the historical enigma around Cassiopeia A."

Recently, astronomers using Chandra, ESA's XMM-Newton Observatory and  
the Gemini Observatory in Chile, were able to use light echoes to  
identify the origins of a supernova outside our galaxy. That study,  
together with the new one, demonstrates the power of light echoes for  
conjuring up the "ghosts" of long-dead stars.

Other co-authors include Stephan Birkmann and Miwa Goto of the Max  
Planck Institute for Astronomy; Tomonori Usuda and Takashi Hattori of  
the National Astronomical Observatory of Japan in Hawaii; and Karl  
Misselt of the University of Arizona. NASA's Jet Propulsion  
Laboratory, Pasadena, Calif., manages the Spitzer Space Telescope  
mission for NASA's Science Mission Directorate, Washington. Science  
operations are conducted at the California Institute of Technology,  
also in Pasadena. For more information about Subaru, operated by the  
National Astronomical Observatory of Japan, visit http:// 
subarutelescope.org .




NASA's Spitzer Space Telescope has found a bizarre ring of material  
around the magnetic remains of a star that blasted to smithereens.

The stellar corpse, called SGR 1900+14, belongs to a class of objects  
known as magnetars. These are the cores of massive stars that blew up  
in supernova explosions, but unlike other dead stars, they slowly  
pulsate with X-rays and have tremendously strong magnetic fields.

"The universe is a big place and weird things can happen," said  
Stefanie Wachter of NASA's Spitzer Science Center at the California  
Institute of Technology, Pasadena, who found the ring  
serendipitously. "I was flipping through archived Spitzer data of the  
object, and that's when I noticed it was surrounded by a ring we'd  
never seen before." Wachter is lead author of a paper about the  
findings in this week's Nature.

Wachter and her colleagues think that the ring, which is unlike  
anything ever seen before, formed in 1998 when the magnetar erupted  
in a giant flare. They believe the crusty surface of the magnetar  
cracked, sending out a flare, or blast of energy, that excavated a  
nearby cloud of dust, leaving an outer, dusty ring. This ring is  
oblong, with dimensions of about seven by three light-years. It  
appears to be flat, or two-dimensional, but the scientists said they  
can't rule out the possibility of a three-dimensional shell.

"It's as if the magnetar became a huge flaming torch and obliterated  
the dust around it, creating a massive cavity," said Chryssa  
Kouveliotou, senior astrophysicist at NASA's Marshall Space Flight  
Center, Huntsville, Ala., and a co-author of the paper. "Then the  
stars nearby lit up a ring of fire around the dead star, marking it  
for eternity."

The discovery could help scientists figure out if a star's mass  
influences whether it becomes a magnetar when it dies. Though  
scientists know that stars above a certain mass will "go supernova,"  
they do not know if mass plays a role in determining whether the star  
becomes a magnetar or a run-of-the-mill dead star. According to the  
science team, the ring demonstrates that SGR 1900+14 belongs to a  
nearby cluster of young, massive stars. By studying the masses of  
these nearby stars, the scientists might learn the approximate mass  
of the original star that exploded and became SGR 1900+14.

"The ring has to be lit up by something, otherwise Spitzer wouldn't  
have seen it," said Enrico Ramirez-Ruiz of the University of  
California, Santa Cruz. "The nearby massive stars are most likely  
what's heating the dust and lighting it up, and this means that the  
magnetar, which lies at the exact center of the ring, is associated  
with the massive star-forming region."

Rings and spheres are common in the universe. Young, hot stars blow  
bubbles in space, carving out dust into spherical shapes. When stars  
die in supernova explosions, their remains are blasted into space,  
forming short-lived beautiful orbs called supernova remnants. Rings  
can also form around exploded stars whose expanding shells of debris  
ram into pre-existing dust rings, causing the dust to glow, as is the  
case with the supernova remnant called 1987A.

But the ring around the magnetar SGR 1900+14 fits into none of these  
categories. For one thing, supernova remnants and the ring around  
1987A cry out with X-rays and radio waves. The ring around SGR 1900 
+14 only glows at specific infrared wavelengths that Spitzer can see.

At first, the astronomers thought the ring must be what's called an  
infrared echo. These occur when an object sends out a blast wave that  
travels outward, heating up dust and causing it to glow with infrared  
light. But when they went back to observe SGR 1900+14 later, the ring  
didn't move outward as it should have if it were an infrared echo.

A closer analysis of the pictures later revealed that the ring is  
most likely a carved-out cavity in a dust cloud -- a phenomenon that  
must be somewhat rare in the universe since it had not been seen  
before. The scientists plan to look for more of these rings.

"This magnetar is still alive in many ways," said Ramirez-Ruiz. "It  
is interacting with its environment, making a big impact on the young  
star-forming region where it was born."

Other paper authors include V. Dwarkadas of the University of  
Chicago, Ill.; J. Granot of the University of Hertfordshire, England;  
S.K. Patel of the Optical Sciences Corporation, Huntsville, Ala.; and  
D. Figer of the Rochester Institute of Technology, N.Y. NASA's Jet  
Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission  
for NASA's Science Mission Directorate, Washington. Science  
operations are conducted at the Spitzer Science Center. Caltech  
manages JPL for NASA. Spitzer's infrared array camera, which made the  
observations, was built by NASA's Goddard Space Flight Center,  
Greenbelt, Md. Its principal investigator is Giovanni Fazio of the  
Harvard-Smithsonian Center for Astrophysics.




On Earth, neon is known for being flashy. Any Las Vegas tourist knows  
that signs sporting this noble gas are hard to miss, but in space  
this is not always true. Neon is the fifth most abundant element in  
the cosmos, but until recently, astronomers couldn't seem to get a  
precise measurement of it in the Universe.

Now, new research shows that NASA's Spitzer Space Telescope has a  
"sweet spot" for detecting neon in star-forming regions.

"By the good fortune of excellent instrument planning, Spitzer's  
infrared spectrometer instrument can measure all the dominant forms  
of neon in star-forming regions, at the exact same time and area,"  
says Dr. Robert Rubin, of the NASA Ames Research Center in Moffett  
Field, Calif.

Rubin and his collaborators used Spitzer's spectrometer to measure  
neon and sulfur abundances in 25 star-forming regions across the  
nearby spiral galaxy M33. For the first time, they found that the  
ratio of neon to sulfur in all of these areas is relatively constant  
at about 16. They note that this observational result is not  
surprising because it is consistent with current models for how these  
chemical elements are created in the cosmos.

"Both neon and sulfur are produced in nuclear reactions deep inside  
the core of the most massive stars following the stages where carbon  
and oxygen are synthesized by nuclear processes in these stars.  
Theory predicts that neon and sulfur yields depend very little on how  
enriched in the 'heavy elements' -- all chemical elements except  
hydrogen and helium -- that the star begins its life with. Thus, it  
is expected that the neon to sulfur ratio remains relatively constant  
throughout a galaxy," says Rubin.

Team members believe that this research may eventually provide  
valuable insights into the amount of neon in our Sun, which is  
currently a controversial topic among astronomers.

Because cosmic neon can take on a variety of "forms" in a given  
region of space, astronomers must account for all of its  
possibilities to get an accurate measurement of it. The neon atom  
contains 10 electrons, and an energetic environment can rip one or  
multiple electrons off. This altered atom is called ionized neon.

According to Rubin, neon atoms in star-forming regions exist  
primarily in two states -- singly and doubly ionized neon, meaning  
that it is missing one and two electrons, respectively. While prior  
infrared missions could only detect one form of neon at a time in any  
given area, Spitzer can see all the dominant forms at the same exact  
time. This makes Spitzer's measurements more precise. The telescope's  
unique sensitivity also allows it to measure this ratio for very  
distant objects.

"In addition to neon, Spitzer's infrared spectrometer can do nearly  
as well to measure the sulfur abundance for star-forming regions at  
the same time and location... Spitzer is a lean, mean neon and sulfur  
abundance machine," says Rubin. Sulfur is the eighth most abundant  
element in the cosmos.

By taking accurate measurements of both neon and sulfur in a star- 
forming region, astronomers can then create an abundance ratio, which  
helps them keep everything in perspective.

"The rationale behind ratios is to take away the variability due to  
other effects such as distance. If we looked at nearby star-forming  
regions everything would look brighter, not because there is more  
neon and sulfur, but because it is closer. By putting neon in a ratio  
with sulfur, we normalize this measurement and put everything on the  
same level," says Rubin.

Team members chose to compare neon to sulfur because Spitzer was able  
to measure all the dominant forms of these elements at the same time,  
in the same area.

Rubin and his collaborators will continue their research with Spitzer  
in the next few months by observing nearly two dozen star-forming  
regions in the nearby galaxy NGC 6822. A paper on the M33 research  
findings will be in a forthcoming issue of the Monthly Notices of the  
Royal Astronomical Society.

Other authors on the paper include Janet Simpson, Sean Colgan, Ian  
McNabb, Edwin Erickson, Michael Haas, and Robert Citron, of the NASA  
Ames Research Center. Reginald Dufour and Gregory Brunner, of Rice  
University. Adalbert Pauldrach, of the University of Munich.

A copy of the paper is posted at: http://arxiv.org/abs/0804.0828 .




The incredible images from NASA's "Great Observatories" and many  
other NASA space- and ground-based telescopes are now available to  
the public in an educational and innovative manner through the  
release of the free WorldWide Telescope software from Microsoft.

Views of the cosmos from such observatories as NASA's Hubble Space  
Telescope, Spitzer Space Telescope, and Chandra X-ray Observatory can  
all be accessed through the same intuitive interface of exploring the  
night sky. Several all-sky surveys are also available through the  
WorldWide Telescope, including the Two Micron All-Sky Survey and the  
Infrared Astronomical Satellite survey. The rich multimedia software  
enables browsing through the visible, infrared, x-ray and other views  
of the universe, allowing for direct comparison of multi-wavelength  
observations that reveal surprising contrasts.

Other innovative features include guided tours created by scientists  
and educators. These tours guide users through various aspects of  
astronomy with narration, music, text and graphics. Members of the  
public, including children, will also be able to make their own tours  
to share with others.

The Two Micron All-Sky Survey is a collaborative effort between the  
University of Massachusetts, Amherst, and the Infrared Processing and  
Analysis Center in Pasadena, Calif., operated by NASA's Jet  
Propulsion Laboratory and the California Institute of Technology,  
both in Pasadena.

The Infrared Astronomical Satellite is a joint project between NASA,  
the Netherlands and the United Kingdom. Its data are archived at the  
Infrared Processing and Analysis Center.

JPL manages the Spitzer Space Telescope mission for NASA's Science  
Mission Directorate, Washington. Science operations are conducted at  
the Spitzer Science Center at Caltech, which manages JPL for NASA.

The WorldWide Telescope is available as of May 13, 2008, at http:// 
www.worldwidetelescope.org .




When it comes to giving birth, galaxies don't seem to have a "ticking  
biological clock." In fact, observations from NASA's Spitzer Space  
Telescope show that old galaxies were the biggest producers of new  
stars when our universe was half of its current age of 13.6 billion  

"The idea that galaxies might form their stars in different  
generations at different times is an old one... What our work proves  
is that this is the 'typical' behavior of the most luminous infrared  
galaxies between five and eight billion years ago," says Dr. Karina  
Caputi, of the Institute of Astronomy ETH Hoenggerberg, in Zurich,  

Infrared galaxies are extremely dusty, and most are furiously forming  
new stars. Astronomers suspect that the source of the galaxy's  
infrared glow comes from the warm dust around newborn stars. Using  
Spitzer data, Caputi and her colleagues identified approximately 600  
of the brightest infrared galaxies within eight billion light-years  
of Earth.

"The Spitzer data allowed us to estimate how luminous these galaxies  
were at infrared wavelengths and, how many stars were forming per  
unit time. The most luminous infrared galaxies were forming stars at  
a rate equivalent to a few tens to several hundreds of Suns per  
year," says Caputi.

Once team members identified the galaxies, they used data from the  
ground-based observations from the European Southern Observatory's  
Very Large Telescope (VLT), in Chile, to learn about the stellar  
population of these infrared galaxies.

"For most of the galaxies in our study, the VLT data revealed an  
older population of stars mingling with newborn stars. This indicates  
that the galaxies are 'old,' and undergoing a new 'burst' of star  
formation," says Caputi.

Team members suspect that the older stellar population was  
responsible for filling the infrared galaxies with dust. This dust  
eventually absorbed ultraviolet light from the new generation of  
stars, and re-emitted the absorbed energy in infrared, giving the  
galaxies their infrared shine.

In addition, the scientists found a 10 to 100 million-year lag  
between when the starbursts began, and when the galaxy got its  
brilliant infrared glow.

"We suspect that it must take that long for the dust to absorb the  
ultraviolet-light that is emitted by young stars, and re-emit it in  
the infrared," says Caputi, who notes that this research will help  
astronomers better understand how galaxies develop over time.

A paper on the topic was published in the June 2008 issue of  
Astrophysical Journal. It incorporated data from the Spitzer's S- 
COSMOS Legacy Project, led by Dr. David Sanders, of the University of  
Hawaii, Honolulu. And, VLT data collected by Dr. Simon Lilly, of the  
Institute of Astronomy ETH Hoenggerberg, in Zurich, Switzerland.



More information about the Spitzer-news mailing list