Posted on by Mike Simonsen

Do Puny White Dwarfs Make Wimpy Supernovae?

The binary star system J0923+3028 consists of two white dwarfs: a visible star 23 percent as massive as our Sun and about four times the diameter of Earth, and an unseen companion 44 percent of the Sun's mass and about one Earth-diameter in size. The stars will spiral in toward each other and merge in about 100 million years. (Credit: Clayton Ellis (CfA))

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Based on results from a radial velocity survey, Warren Brown, (Smithsonian Astrophysical Observatory) and his team have placed a few more pieces into the supernova puzzle.

Supernovae come in many flavors. There are Type Ia, the “standard candles” everyone has heard of; and there are Type Ib and Ic, which also involve binary systems. We also have Type II supernovae that are believed to be the core collapse of single, super-massive stars. There are also super-luminous supernovae, which may be the explosive conversion of a neutron star into a quark star, and finally the weak-kneed cousins of the bunch, the under-performing underluminous supernovae.

Underluminous supernovae are a rare type of supernova explosion 10–100 times less luminous than a normal SN Type Ia and eject only 20% as much matter. Brown and his team have been investigating the connection between underluminous supernovae and merging pairs of white dwarfs.

In the 1980s, on the basis of our theoretical understanding of stellar and binary evolution it was predicted that many close double white dwarfs would exist. However, it was not until 1988 that the first one was actually discovered.

The way to find close double white dwarfs is to take high resolution spectra of the H-alpha absorption line of a white dwarf at several different times and look for variation that is caused by the orbital motion of the white dwarf around an unseen (dimmer) companion. The first systematic searches were not very unsuccessful. Only one system was found. Then, during the 1990s, Tom Marsh and collaborators concentrated their search on low-mass white dwarfs, which, based on current theories, could _only_ be formed in a binary system. In this way a dozen more systems were found.

Extremely low mass (ELM) white dwarfs (WDs) with less than 0.3 solar masses are the remnants of stars that never ignited helium in their cores. The Universe is not old enough to have produce ELM WDs by single star evolution. Therefore, ELM WDs must undergo significant mass loss sometime in their evolution. Producing WDs with 0.2 solar masses most likely requires compact binary systems.

“These white dwarfs have gone through a dramatic weight loss program,” said Carlos Allende Prieto, an astronomer at the Instituto de Astrofisica de Canarias in Spain and a co-author of the study. “These stars are in such close orbits that tidal forces, like those swaying the oceans on Earth, led to huge mass losses.”

Observational data for ELM WDs is pretty hard to come by because of their rarity. For example, of the 9316 WDs identified in the Sloan Digital Sky Survey, less than 0.2% have masses below 0.3 solar.

Half of the pairs discovered by Brown and collaborators are merging and might explode as supernovae in 100 million years or more.

“We have tripled the number of known, merging white-dwarf systems,” said Smithsonian astronomer and co-author Mukremin Kilic. “Now, we can begin to understand how these systems form and what they may become in the near future.” Unlike normal white dwarfs made of carbon and oxygen, these are made almost entirely of helium.

“The rate at which our white dwarfs are merging is the same as the rate of under-luminous supernovae – about one every 2,000 years,” explained Brown. “While we can’t know for sure whether our merging white dwarfs will explode as under-luminous supernovae, the fact that the rates are the same is highly suggestive.”

At least 25% of these ELM WDs belong to the old thick disk and halo components of the Milky Way. This helps astronomers know where to look for underluminous SNe and where they are unlikely to find them, if the models are correct. If merging ELM WD systems are the progenitors of underluminous SNe, the next generation of surveys such as the Palomar Transient Factory, Pan-STARRS, Skymapper, and the Large Synoptic Survey Telescope should find them amongst the older populations of stars in both elliptical and spiral galaxies.

The papers announcing their find are available online at: http://arxiv.org/abs/1011.3047 and http://arxiv.org/abs/1011.3050.

Posted on by Jon Voisey

Catching Planets in the Womb

Young stars have a disk of gas and dust around them called a protoplanetary disk. Credit: NASA/JPL-Caltech

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Awhile ago I wrote on the difficulty of finding young planets. There, I mentioned one team announcing the potential discovery of a planet a mere 1-5 million years old. But what are astronomers to do if they want to find even younger planets?

The chief difficulty in this instance is that such planets would still be hidden in the circumstellar disks from which they formed, hiding them from direct observation. Additionally, depending on how far along the process had advanced, they may not yet have accreted sufficient mass to show up in radial velocity surveys, if such surveys could even been conducted with interference from the disc.

One way astronomers have proposed to detect forming planets is to observe their effects on the disc itself. This could come in a number of ways. One would be for the planet to carve out grooves in the disc, clearing its orbit as it sweeps up matter. Another possibility is to look for the “shadows” caused by the local overdensity an accreting planet would cause.

But recently, another new method caught my eye. In this one, proposed by astronomers at the Crimean National Observatory in the Ukraine, astronomers could potentially look for again turns to the characteristics of the parent star. Earlier, astronomers had made a link between the properties of the disc around classes of protostars (such as T Tauri and Herbig Ae stars) and the variable luminosity of the star itself.

The authors suggest that, “[t]wo different mechanisms can be involved in interpretation of these results: 1) circumstellar extinction and 2) accretion.” In either scenario, a body present in the disc itself concentrating the material would be necessary to explain these results. In the first case, a protoplanet would draw a swarm of material around it again creating a local overdensity in the disc which would be dragged around with the planet, creating a dimming of the star as it passed near the line of sight. In the second, the planet would draw out tidal structures in the disc in much the same way tidal interactions can draw out spiral structure in galaxies. As these veins of matter fall onto the star, it feeds the star, temporarily causing an outburst and increasing the brightness.

The team conducted an analysis of periodicity in several protostellar systems and found several instances in which the periods were similar to those of planetary systems discovered around mature stars. Around one star, V866 Sco, they discovered, “two distinct periods in light variations, 6.78 and 24.78 days, that persist over several years.” They note that the shorter period is likely “due to axial rotation of the star” but could not offer an explanation for the longer period which leaves it open to the possibility of being a forming planet and they suggest that spectral observations may be possible. Other systems the team analyzed had periods ranging from 25 – 120 days also hinting at the possibility for young planetary systems.

The advantage to this method is that finding candidate systems can be done relatively easily using photometric systems which can survey great numbers of stars at once whereas radial velocity measurements generally require dedicated observations on a single object. This would allow astronomers to discriminate against candidates unlikely to harbor forming planets. Ultimately, finding young systems with forming planets will help astronomers understand how these systems form and evolve and why our own system is so different than many others found thus far.

Posted on by Nancy Atkinson

A Cosmologist’s Wish List: Four Most-Wanted Discoveries

Ancient woodcarving of where heaven and earth meet. Credit: Heikenwaelder Hugo at Wikimedia Commons.

Cosmology is a fairly young science, one which attempts to reconstruct the history of our Universe from billions of years ago. Looking back so far in time is extremely difficult, and adding to the complexity is that many of the pillars upon which the theories of cosmology rest have only been conceived within the last 20 years or so. That hasn’t given scientists and theorists much time to fully flesh out and comprehend the situation, and cosmologist Michael Turner says either some important new physics will have to be discovered or we’re going to find a fatal flaw in our prevailing view of the Universe.

So, what will it take to push cosmology over the edge, where it goes fully from theory to science, and we have at least a grasp of cosmological understanding? I had the chance to ask that question to Turner at last week’s National Association of Science Writers conference. Turner, who coined the term “dark energy,” is the Director of the Kavli Institute for Cosmological Physics at the University of Chicago. Here are his top four wishes for discoveries in cosmology:

Wish # 1: Figure out the nature of dark matter.

“I think we’re very close to solving this dark matter problem and I think its going to be stunning when it sinks in to everyone that most of the stuff in the Universe is made of something other than what we are,” Turner said.

Dark matter holds universe together, according to cosmologists. But since it does not emit electromagnetic radiation and we can’t see it, how do we know it is there? “It is needed to hold galaxies together, it is needed to hold clusters together, it is that simple,” Turner said. “There is not enough gravity in all the stars put together to hold clusters together.”

Turner has likened dark matter to an outdoor tree decorated with Christmas lights. From far away, all that can be seen are the lights, but it is the unseen tree that holds the lights where they are and gives them their shape. More poetically Turner said, “The universe is a web of dark matter that is decorated by stars.”

Turner made a bold prediction: “The 2010 is the decade of dark matter – we are going to finish this thing off.”

Dark matter in the Bullet Cluster. Otherwise invisible to telescopic views, the dark matter was mapped by observations of gravitational lensing of background galaxies. Credit: X-ray: NASA/CXC/CfA/ M.Markevitch et al.; Lensing Map: NASA/STScI; ESO WFI; Magellan/U.Arizona/ D.Clowe et al. Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.;

Wish # 2. Figure out the nature of dark energy.

“Dark energy may be most profound problem in cosmology today, and I’ve been wandering around for 10 years saying this,” Turner said. “If dark matter holds the Universe together, dark energy controls its destiny.”

Dark energy likely makes up 66% of the cosmos, and it’s existence has only been theorized since 1998 when astronomers realized that contrary to the prevailing notion that the expansion of the universe should be slowing down, it is actually moving faster as time goes on.

What is the current theoretical understanding of dark energy? “We don’t have a clue,” said Turner. “But let me go out here on a limb with dark energy, and say we may find it is not vacuum energy. Vacuum energy is mathematical equivalent to Einstein’s cosmological constant, and I hope we’ll figure out it is something weirder than the energy of nothing. That doesn’t solve the problem, but it would be a gift to my younger colleagues, because science is all about big questions and they need clues and something big they can sink their teeth into.”

Yes, dark energy is a big problem, but for theorists it’s a big opportunity. However, Turner has some doubts. “Dark energy is one of the big questions that will occupy the next decade, and I don’t know if we’ll be able to solve it,” he said.

Michael Turner at the 2010 National Association of Science Writers conference at Yale University. Image: Nancy Atkinson

Wish # 3: Confirming inflation with the discovery of B-Mode polarization.

Our current best theory about the earliest moments of the universe is called inflation, where during a tiny fraction of a second after the Big Bang, the Universe appears to have expanded exponentially. In particular, high precision measurements of the so-called B-modes (evidence of gravity waves) of the polarization of the cosmic microwave background radiation would be evidence of the gravitational radiation produced by inflation, and they will also show whether the energy scale of inflation predicted by the simplest models is correct.

“That is the smoking gun for inflation.” said Turner. “It explains where all the structure came from – that quantum mechanical fluctuations at the subatomic scale were blown up by this enormous expansion. That is an amazing idea, and in one equation we could figure out exactly when inflation took place. You’ll notice in all our talk of inflation no one ever tells you when it took place, because we don’t know. But those B-modes would tell us.”

Wish #4. Make the mulitiverse go away.

If there was inflation, that means there is also very likely a multitude of Universes out there.

Turner called the concept of the multiverse the 800 lb gorilla in the room.

“The dilemma is, we have evidence that inflation took place and the equations of inflation say that if it took place once, it took place twice and it’s sort of like the mouse and the cookie – if it took place twice it could have taken place an infinite number of times,” he said.

The multiverse hypothesizes multiple universes or parallel universes comprise everthing that is, not just our one “local” universe. “If there is a mulitverse structure, and if you marry this with string theory you end up with a picture of a Universe where there might be different local laws of physics and the different sub-universes might be incredibly different from each other – differences in space and time, some don’t have stable particles, many don’t have life, and so on. This is an incredibly bold idea and may even be the most important idea since Copernicus.”

But, Turner asked, how do you test it? “And if you can’t test it, therefore you can’t call it science,” he said. “So I call it the mulitiverse headache – you have this incredibly important idea, but is it science?”

So what do you end up with? An elephant, Turner declared, as in the story of the blind men describing an elephant.

“That’s where we are in cosmology,” he said. “We are the blind cosmologists feeling the Universe and each piece of data describes something. There are still big questions to be answered, and what we’re doing in cosmology is trying to put it all together, and we might actually, in the next 10-15 years put it all together. That is absolutely amazing; the universe is very big and our abilities are very primitive. But look what we’ve done so far.”

Slide from Turner's presentation showing the Universe as an elephant. Image: Nancy Atkinson
Posted on by Nancy Atkinson

New Geography Trivia Challenge From Space

The first image from NASA's new geography trivia contest.

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You can now test your knowledge of the world’s geography in a new trivia game on Twitter sponsored by NASA and the astronauts on board the International Space Station. It is kind of like our own “Where In the Universe Challenge” but strictly of images from Earth, and in this contest, there are even actual prizes. Astronaut Scott Kelly started the game this week, which is Geography Awareness Week. His vantage point is perfect for hosting the game, as where else can you get a better view of the various geographical features on our planet than from the International Space Station?

First of all, you have to be on Twitter, and follow Kelly: @StationCDRKelly. He’ll post a link to an image he took, and the first person to correctly identify the place depicted in his photos will win an autographed copy of the picture.

“Expanding our geography knowledge is essential to our economic well-being, our relationships with other nations and the environment,” Kelly said. “It helps us make sense of our world and allows us to make connections between people and places. Space exploration is a global endeavor, and the International Space Station is the result of these connections.”

The new trivia game is a way to engage the public in the activities of the ISS, and the pictures that Kelly, and other astronauts take from the station aren’t all just fun and games. “From the cupola, which is much like a bay window in a house, we are able to take pictures for many scientific reasons, but also to share with the public what we are learning about the planet on which we live,” Kelly said.

See this link for a complete rules of the Geography Trivia from Space contest (pdf).

Kelly launched to the space station along with two Russian cosmonauts, Alexander Kaleri and Oleg Skripochka on Oct. 8. He is set to return to Earth March 16, 2011. The space station and its six crew members orbit the Earth more than a dozen times each day, traveling more than 320 km (200 miles) above Earth at 28,000 kph (17,500 mph).

For more info about the ISS, see this NASA webpage.

Posted on by Jon Voisey

Dissolving Star Systems Create Mess in Orion

For young stars, stellar outflows are the rule. T Tauri stars and other young stars eject matter in generally collimated jets. However, a region in Orion’s giant molecular cloud known as the Becklin-Neugebauer/Kleinmann-Low (BN/KL) region, appears to have a clumpy, scattered set of outflows with “finger-like” projections in numerous directions. A new study, led by Luis Zapata at the National Autonomous University of Mexico, explores this odd region.

To conduct their study, the team used the Submillimeter Array to trace the motion of carbon monoxide gas in the area. Flying away from this region are three massive and young stars. Tracing their paths back, astronomers had previously determined that these stars likely had a common origin as members of a multiple system that for some reason, broke apart an estimated 500 years ago. Likely related to this, the new study discovered several new fingers of gas moving away as well with velocities that implied they came from the same point of origin near the same time. But what could send stars and gas hurtling outwards?

Nearby, the team also discovered a “hot core” of material as well as a “bubble” of empty space near the point of origin of the event. To explain the combination of these three events, the team proposes that an close interaction between the three stars (or perhaps more) occurred. At that time, the interaction tore apart any potential binary system throwing the stars outwards.

Since the stars are young and still embedded in a nebula, the team suggests it was likely they also contained circumstellar disks that had not yet formed planets. During the interaction, the outer portions which would be least strongly bound, were thrown outwards, creating the finger-like projections. Material that was bound more tightly but just enough to be torn off, “would find itself with an excess of kinetic energy, and will start to expand” creating the apparent bubble. If that bubble, expanding supersonically for the local medium, encountered a region that was overly dense, it would collide, heating the region and potentially forming the hot core.

This new discovery presents a potential first for the discovery of one or more destroyed circumstellar disks. Such findings could help impose new constraints on how planetary systems form since most stars form in open clusters and associations in which such interactions may be commonplace. Yet, the very fact that such destroyed systems have never been found until now imply that interactions sufficiently close to cause such disruption are rare. Regardless, such things will help astronomers form a better picture of the formation of planets.

Posted on by Nancy Atkinson

Mini-Asteroid Flying By Earth Tonight

A small asteroid will make a fairly close flyby of Earth tonight, November 16, 2010 at 10:44 p.m. EST (0344 GMT), but it is not a threat to hit the planet. Plus, at 3 meters wide, (10 ft) the asteroid, named 2010 WA, would break apart if it hit Earth’s atmosphere. Still, finding and tracking the small asteroid is “good practice in detection,” wrote NASA’s @AsteroidWatch Twitter feed.

2010 WA will pass about 1/10 lunar distance, or about 38,000 kilometers (24,000 miles) away, and have a magnitude of about 14.5, so it won’t be visible “without a good sized telescope” said @AsteroidWatch. But if you do have such a telescope, the asteroid could be seen over the middle to east coast of US.

For references, geostationary satellites are in orbit about 36,000 km (22,350 miles) up, while the International Space Station is about 350 km (220 miles) above Earth.

See more detailed information about 2010 WA on the Minor Planet Center Website

NASA is constantly on the lookout for asteroids, or Near Earth Objects (NEO), and has a mandate by Congress for the “Spaceguard” survey to find all asteroids around 40 meters and larger by 2020.

NASA says several teams of astronomers worldwide are surveying the sky to find NEOs. One of the most most productive NEO surveys is the LINEAR search program of the MIT Lincoln Lab, carried out in New Mexico with US Air Force and NASA support. The LINEAR team, which operates two search telescopes with one-meter aperture. Recently, the Catalina Sky Survey in Tucson, Arizona has been extremely productive, as well. Other active survey groups include the NEAT search program in Hawaii, carried out jointly by the NASA Jet Propulsion Lab and the US Air Force; the Spacewatch survey at the University of Arizona, and the LONEOS survey at Lowell Observatory in Flagstaff Arizona. Other astronomers — many of them amateurs — follow up the discoveries with supporting observations.

Graph of NEO discoveries. Credit: NASA

To see more details on NEO discoveries, see this page on the NASA’s NEO website.

Posted on by Nancy Atkinson

Cosmologist Allan Sandage Dies

Allan Sandage. Credit: Carnegie Institution for Science.

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Cosmologist Allan R. Sandage, who helped define the fields of observational cosmology and extragalactic astronomy, died November 13, 2010, at his home in San Gabriel, California, of pancreatic cancer. He was Edwin Hubble’s former observing assistant and one of the most prominent astronomers of the last century. Sandage was 84. Below is his biography from the Carnegie Institution for Science:

Allan Sandage became a Carnegie staff member in 1952 after serving as the observing assistant in observational cosmology to Edwin Hubble on both Mount Wilson and Palomar from 1950 to 1953, and Walter Baade’s PhD student in stellar evolution starting in 1949. Upon the death of Hubble in 1953, Sandage became responsible for developing the cosmology program using the 60- and 100-inch telescopes on Mount Wilson and with the newly commissioned Palomar 200-inch reflector. The programs centered on the recalibration of Hubble’s extragalactic distance scale and combining discoveries in stellar evolution with observational cosmology. Much of his research in the past 50 years has been directed toward these goals.

Early discoveries at Palomar showed that Hubble’s distances to galaxies were progressively incorrect, starting with Baade’s finding in 1950 that Hubble’s measured distance to the Andromeda Nebula, M31, was too small by a factor of about two. Sandage, first alone and later with G.A. Tammann professor of astronomy at the University of Basel, have carried the corrections progressively outward. This work indicates that by the time we reach the nearest cluster of galaxies in Virgo, the correction to Hubble’s scale is close to a factor of 10. Since 1988, Sandage and Tammann have led a consortium using the Hubble Space Telescope to determine distances to parent galaxies that have produced type Ia supernovae, shown earlier to be one of the best standard candles in luminosity known. From the results of the calibrations, Sandage, Tammann, and Abijit Saha of the Kitt Peak National Optical Observatory have determined at this writing (2005) the value of the Hubble constant to be 60 km s -1 Mpc -1.

Sandage’s other early research in observational stellar evolution led to a method developed in 1952 with Martin Schwarzschild of age-dating the stars from the luminosity turn-off from the main sequence of evolving stars in the Hertzsprung-Russell diagram. This method, improved over the years from theoretical calculations of stellar structure by many astronomers, remains the principal method of age dating. Sandage recently returned to problems related to the absolute magnitudes of RR Lyrae variable stars in globular clusters, important to the age dating of these most ancient of objects in the Galaxy.

Source: Carnegie Institution for Science

Posted on by Nancy Atkinson

Cool Chang’E 2 Videos

Emily Lakdawalla at the Planetary Society blog unearthed some really cool videos taken by the Chinese Chang’E 2 spacecraft at the Moon. The five engineering videos include Chang’E 2’s solar panel deployment, orbit insertion burn, the first and second orbital trim maneuvers, and low lunar orbit. They are all especially unique in that the video not only includes images from the Moon’s surface, but also the spacecraft itself can be seen, providing a perspective that is not often seen. The video above is of Chang’E 2’s second orbit trim maneuver. Check out Emily’s post to see all five, plus she provides great insights into the video clips, as well.

Posted on by Tammy Plotner

The Lion Tamer – Leonid Meteor Shower 2010

Are you ready to walk into the lion’s cage? Then break out your favorite skywatching gear because the 2010 Leonid meteor shower is underway…

In the pre-dawn hours on the mornings of November 17 and November 18, the offspring of Comet Temple/Tuttle will be flashing through our atmosphere and just taunting you to test your meteor watching skills against bright skies. Although the phat Moon will greatly interfere with fainter meteor trails, don’t let that stop you from enjoying your monring coffee with the sparkling “cubs” that will be shooting out from the constellation of Leo.

Where? For all observers the constellation of Leo is along the ecliptic plane and will be near its peak height during best viewing times. When? Because of the Moon, just a couple of hours before local dawn is the best time to watch. Why? Read on!

Although it has been a couple of years since Temple/Tuttle was at perihelion, don’t forget that meteor showers are wonderfully unpredictable and the Leonids are sure to please with fall rate of around 20 (average) per hour. Who knows what surprises it may bring! Each time the comet swings around our Sun it loses some of its material in the debris trail. Of course, we all know that is the source of a meteor shower, but what we don’t know is just how much debris was shed and where it may lay.

As our Earth passes through the dusty matter, it may encounter a place where the comet let loose with a large amount of its payload – or it may pass through an area where the “comet stuff” is thin. We might even pass through an area which produces an exciting “meteor storm” like the Leonids produced in 1883! For those in the know, the Leonid meteor shower also made a rather incredible appearance in 1866 and 1867 – dumping up to 1000 (not a typo, folks) shooting stars recorded even with a Moon present! It erupted again in 1966 and in 1998 and produced 3000 (yep. 3000!) video recorded meteors during the years of 2001 and 2002. But remember, human eyes may only be able to detect just a few…

And I ain’t lion!

Photo Courtesy of Stardate.org, Texas University

Posted on by Nancy Atkinson

Confirmed: Hayabusa Nabbed Asteroid Particles

An electron micrograph image of the edge of a special Teflon spatula that scraped the interior surfaces of Hayabusa's sample return capsule. Credit: JAXA

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The Japan Aerospace Exploration Agency (JAXA) has confirmed that the tiny particles inside the Hayabusa spacecraft’s sample return container are in fact from the asteroid Itokawa. Scientists examined the particles to determine if the probe successfully captured and brought back anything from the asteroid, and in a press release said “about 1,500 grains were identified as rocky particles, and most were determined to be of extraterrestrial origin, and definitely from Asteroid Itokawa.”

These are the first samples from an asteroid ever returned to Earth; the only other extraterrestrial samples brought back to Earth came from the Apollo missions to the Moon. See correction, below.

Previously, JAXA said that although particles were inside the container, it wasn’t clear if they were from the asteroid or if they could be of terrestrial origin (dust from Earth that could have been inside the container).

The particles samples were collected from the chamber by a specially shaped Teflon spatula and examined with a scanning electron microscope. There were two chambers inside the container, and from the press release (in Japanese) it appears all the particles were found in one chamber, Chamber A.

Most of the particles are extremely small, about 10 microns in size and require special handling and equipment. Unfortunately they aren’t the “peanut-sized” chunks of rock that the mission originally hoped to capture. This will make analyzing the particles difficult, but not impossible.

Hayabusa's sample return cannister and parachute on the ground in the Australian outback. Credit: JAXA

During the seven-year round trip journey, Hayabusa arrived at Itokawa in November, 2005. The mechanism that was intended to capture the samples apparently failed, but scientists were hopeful that at least some dust had made its way into the return canister. After a circuitous and troubled-filled return trip home, the sample return capsule was ejected and landed in Australia in June of this year.

Here are the other successful sample return missions:
Apollo Moon missions (1969-1972)
Soviet Union’s Luna 16 (1970) returned 101 grams of lunar soil
Luna 20 (1974) returned 30 grams
Luna 24 (1976) returned 170.1 grams.
The Orbital Debris Collection (ODC) experiment, deployed on the Mir space station for 18 months during 1996–1997, used aerogel to capture interplanetary dust particles in orbit.
Genesis (2001-2004) captured and returned molecules collected from the solar wind. It crashed in the Utah desert, but samples were able to be retreived.
Stardust (1999-2006) collected particles from the tail of a comet, as well as a few interstellar dust grains.

Source: JAXA