Two Views of a Stellar System in the Making

A flattened envelope of gas and dust surrounding the young protostar L1157 gives us some idea of what our Solar System may have looked like as it began to form. The object is only a few thousand years old, the central star hidden, with its envelope detectable in silhouette as a black bar. The view from the Spitzer Space Telescope (below) shows how infrared can look within the dust to see structure. While the telescope cannot penetrate the envelope (itself hard to see in this image), enormous jets whose hottest points appear in white are clearly defined.

Spitzer view of L1157

These jets are interesting. They’re being emitted from the protostar’s two magnetic poles, and are approximately one and one half light years from end to end. The envelope of material is too thick for Spitzer to penetrate and appears in black, its thickest part visible as a black line crossing the jets. The envelope is roughly centered on the polar jets and perpendicular to them, showing up more clearly in the grayscale image below, which is drawn from the paper soon to be published on this work.

Image (above): A rare, infrared view of a developing star and its flaring jets taken by NASA’s Spitzer Space Telescope shows us what our own solar system might have looked like billions of years ago. In visible light, this star and its surrounding regions are completely hidden in darkness. The color white shows the hottest parts of the jets, with temperatures around 100 degrees Celsius. Most of the material in the jets, seen in orange, is roughly zero degrees on the Celsius and Fahrenheit scales. The reddish haze all around the picture is dust. Credit: NASA/JPL-Caltech/UIUC.

View of L1157 envelope

The collapsing cloud that will give rise to this system rotates faster as it coalesces, the growing magnetic field ejecting some of the gas and dust along the magnetic axis. Leslie Looney (UIUC) notes the significance of the jets: “If material was not shed in this fashion, the protostar’s spin would speed up so fast it would break apart.”

Image: This grayscale view of L1157 makes the presence (and size) of the surrounding envelope more apparent. Credit: Leslie Looney/UIUC.

Some 800 light years from Earth in the constellation Cepheus, L1157 appears black in visible light but infrared reveals the structures that should one day produce planets around this star. But keep the scale in mind. Although L1157 will one day be a star of about solar mass, the envelope of gas and dust around it is huge, big enough to hold tens of thousands of solar systems like ours. What will eventually become a planet-forming disk would be in the first photograph smaller than a pixel.

The paper is Looney, Tobin et al., “A Flattened Protostellar Envelope in Absorption around L1157,” accepted for publication in Astrophysical Journal Letters and available online.

Voyager 2 Closes on Termination Shock

When I use the term ‘interstellar mission,’ people assume I’m talking about a far future crewed mission to a star like Alpha Centauri or Epsilon Eridani. But the two Voyager spacecraft are on an interstellar mission of a sort, meaning they’re eventually going to leave the Solar System entirely and head into true interstellar space. Because the Voyagers’ power looks sound enough to keep sending data for another decade or more, we should thus get an interesting look at how our solar neighborhood differs from the medium that Sol and all the other stars in the Orion Arm swim in.

Voyagers leave the Solar System

Image: Voyager 1 and Voyager 2 leaving the solar system. Image Credit: NASA/Walt Feimer.

The termination shock is that place where the solar wind — charged particles flowing outward from the Sun — slows below the speed of sound. It should be a tricky and mutable place, there being no fixed boundary out there some eight billion or so miles from our star. Instead, the termination shock should vary depending on solar activity and other factors we may learn more about as we study the plasma, gas and dust outside. Thus Voyager gets the probable chance to pass through the shock more than once, as Haruichi Washimi (UC Riverside) notes:

“After it crosses this boundary, Voyager 2 will be in the outer heliosphere beyond which lies the interstellar medium and galactic space. Our simulations also show that the spacecraft will cross the termination shock again in the middle of 2008. This will happen because of the back and forth movement of the termination-shock boundary. This means Voyager 2 will experience multiple crossings of the termination shock. These crossings will come to an end after the spacecraft escapes into galactic space.”

Washimi’s simulations say that Voyager will cross the termination shock almost any time now, perhaps as late as early next year. He and his team are making their predictions based on geomagnetic disturbances caused by what’s happening on the Sun, their data drawing on what Voyager 2 has already passed along. Voyager 1 passed the termination shock in December of 2004. Up next for both craft: The heliopause, where the solar wind comes to a halt and interstellar space really begins.

The work of Washimi and team is scheduled to appear in the December 1 issue of The Astrophysical Journal.

Messier 74 In All Its Glory

Messier 74

Image (click to enlarge): Hubble has sent back an early Christmas card with this new NASA/ESA Hubble Space Telescope image of the nearby spiral galaxy Messier 74. It is an enchanting reminder of the impending season. Resembling glittering baubles on a holiday wreath, bright knots of glowing gas light up the spiral arms; regions of new star birth shining in pink. Credit: NASA, ESA and the Hubble Heritage (STScI/AURA)-ESA/Hubble Collaboration.

Simply too beautiful not to post immediately on an otherwise quiet day.

On Planets in the Galactic Bulge

One thing we’d like to know about exoplanets is where they are likely to be found. We’ve located more than 250 of them, but most are confined within about 650 light years. That’s very much in the local neighborhood by galactic standards — our methods have led us to nearby, bright stars. We do have a small number of planets detected through microlensing, some as far away as 6000 parsecs (about 19,500 light years), but our radial velocity detections, which form the great bulk of the current catalog, tend to be confined to relatively close higher mass stars.

Other similarities? The exoplanet host stars we know about are generally metal rich. And because they’re nearby, they’re located in the galactic disk. This leaves us with some key questions, among them whether planets are equally abundant elsewhere in the galaxy. Other issues:

  • Do planets occur with the same frequency around lower mass stars?
  • Does the presence of heavy elements favor particular parts of the galaxy for planet formation?
  • How do ‘hot Jupiters’ fit into the galactic map of planet formation?

SWEEPS field

The SWEEPS Project (Sagittarius Window Eclipsing Extrasolar Planet Search) sets out to study these questions. Using the Hubble Space Telescope and the Wide Field Camera of the Advanced Camera for Surveys, this effort has looked at approximately 180,000 F, G, K and M dwarfs in a dense stellar field in the galactic bulge. The goal is to find transits of Jupiter-size planets in an area over 27,000 light years from Earth. SWEEPS monitored this field over a seven day period in 2004. Remarkably, Hubble can work with an M dwarf having an apparent visual magnitude of 25.5 at this range and still detect planetary transits.

Image (click to enlarge): Part of the stellar field observed in the SWEEPS survey. The green circles indicate the position of 11 of the 16 host stars that have been found. Copyright : NASA/HST, ESA, K. Sahu (STScI) and the SWEEPS Science Team.

The results: SWEEPS found sixteen candidate transiting exoplanets. From the paper:

After correcting for geometric transit probability and our detection efficiency, our detections suggest that the frequency of planets in the SWEEPS ?eld is similar to that in the local neighborhood.

The frequency of planets around low-mass stars is also similar to the frequency of planets around higher-mass stars, but given the small number statistics, the uncertainty is large which can easily be a factor of 2 or 3.

Interestingly, the project identified a new class of ultra-short period planets with orbital periods shorter than one day. The host stars for this category are all low mass, suggesting “…that planets orbiting very close to more massive stars might be evaporatively destroyed, or that planets can migrate to close-in orbits and survive there only around such old and low-mass stars.” A final SWEEPS finding: Planets seem to occur more frequently with higher metallicity even in the galactic bulge.

The paper is Sahu et al., “Planets in the Galactic Bulge: Results from the SWEEPS Project,” scheduled to appear in Extreme Solar Systems, eds. D. Fischer, F. Rasio, S. Thorsett, A. Wolszczan (ASP Conf. Series). The paper is available online.

Interstellar Sails and Their Precursors

Lou Friedman’s work on solar sails dates back to his days at the Jet Propulsion Laboratory where, in the 1970s, his team began work on a rendezvous mission with Halley’s Comet. It was a mission that never flew, but you can read about its planning stages in Friedman’s book Starsailing: Solar Sails and Interstellar Travel (Wiley, 1988). That title is, as far as I know, the first book-length study of this technology, though it has since been joined by Colin McInnes’ key text Solar Sailing: Technology, Dynamics and Mission Applications (Springer/Praxis, 1999).

Now executive director of The Planetary Society, Friedman’s interest in solar sails led to his work on the Society’s Cosmos I mission, unfortunately lost during the launch attempt in 2005. His interest in interstellar issues remains keen as well, as evidenced by an article he recently wrote for Professional Pilot magazine. “Making Light Work” runs through solar sail basics for an audience that may seem surprising, but I can tell you from my own flying days that as we used to wait in the pilot’s lounge for students to arrive, we would often kick around outlandish concepts like deep space missions (and there were always a few dog-eared copies of Professional Pilot scattered around the room, out of date and thoroughly read).

Friedman speculates about sails kilometers wide in the area of 0.1 microns in thickness, ultralight films that would, when the photons from sunlight lost their punch, take advantage of huge laser installations that could be focused for interstellar distances. Now we’re into Robert Forward territory and also in range of feasible interstellar missions. For as Friedman notes, solar sails are the only technology we currently have that could complete such missions in a single human lifetime:

What is exciting is that we know the way forward. We don’t have to invent some new physics (like matter/antimatter engines) and we don’t have to conjure up new technologies from science fiction (such as interstellar ramjets scooping up and using interstellar hydrogen molecules). Rather, it’s all a matter of engineering—make the light sail materials thinner, the spacecraft lighter and the lasers more powerful.

Of course, the demands are still huge, power on the order of 100 gigawatts, which means power stations located in space, assembled in the inner solar system where solar radiation is much higher than here on Earth (presumably sails would be involved in ferrying the needed materials). And then there’s the problem of sail construction, conceivably handled by making the sail out of plastics whose evaporation would leave only the needed molecules to reflect sunlight and laser photons. Imagine a square kilometer sail weighing just a few kilograms, its electronics sprayed onto the sail rather than flying as a separate payload.

Solar sail technology is no idle dream. After extensive study at Marshall Space Flight Center, NASA’s basic sail design has reached the point where space testing is the logical next step even as research continues in European venues like Germany’s DLR. When we begin a serious push into solar sail technologies, we’ll need to test these designs in near-Earth orbit, and then move out into the Solar System. A logical mission for early sails will be, as Friedman notes, a replacement for the Advanced Composition Explorer (ACE), a mission nearing the end of its lifetime.

ACE operates at a libration point where the gravitational forces of Sun and Earth balance, some 1.5 million kilometers from Earth. A sail mission that could monitor solar weather (and warn us of solar storms) could offer a new kind of station-keeping, one that uses the momentum imparted by photons to stay in position closer to the Sun without the need of remaining at the libration point. Such a position would, among other things, allow greater early warning of potential ionospheric disruptions.

The range of sail missions available in coming decades will be huge, but if we keep at it, we may get to the point where building the kind of laser we’ll need for an interstellar mission becomes possible. Solar sailing is the kind of next-step technology that moves us from one-shot mission spectaculars like Apollo into the realm of a stable and long-term human presence expanding into the Solar System. For the short term, we need to keep doing what Friedman and sail advocate Gregory Matloff are doing, explaining and arguing for the needed steps to get sails into nearby space where their value for more complex missions will be obvious.