Inclined Orbits and Their Causes

The abundance of giant planets among the more than 500 exoplanets thus far identified is largely the result of our detection methods — we can find larger planets far more readily than smaller ones. But even as we bring our detections down to ever more Earth-sized worlds, we can go to work on the questions that giant planets close to their star raise. Current thinking is that planets like these must have formed far from their host stars and migrated to their current locations. Still to be determined are the mechanisms at work to make migration happen.

Intriguing new evidence is coming in from the National Astronomical Observatory of Japan, which has been working with data from the Subaru Telescope to study the orbital characteristics of two exoplanets, HAT-P-11 b and XO-4 b. The former, about 130 light years from Earth in the constellation Cygnus, shows a mass of 0.081 that of Jupiter, making it a Neptune-sized world in an eccentric 4.89-day orbit. The latter is a Jupiter-class planet of about 1.3 Jupiter masses, with a circular orbit of 4.13 days. What’s intriguing about both is that their orbits are highly tilted.

We’re talking about the angle between the axis of a star’s rotation and the planetary orbital axis, and herein lies a tale. To study such angles, we rely on the Rossiter-McLaughlin effect, which relates to radial velocity irregularities of an exoplanet as observed during a planetary transit. Have a look at the diagram below and marvel as I do at the amount of information we’re able to tease out of these signals, all of which involve planets that have never been directly imaged.

Image: Because of stellar rotation called “spin”, the stellar surface or “disk” has two parts: the approaching side (blue) and the receding side (red). During a planetary passage or “transit”, the observed radial velocity (RV) or speed of the star exhibits an apparent irregularity because of the stellar spin. When the transiting planet blocks the approaching side of the disk (blue), the star appears to be receding, and the RV shows an apparent red-shift. When the transiting planet conceals the receding side of the stellar disk, the star appears to be approaching, and the RV exhibits an apparent blue-shift. These anomalous RV shifts occur along the trajectory of the planet relative to the stellar disk. The diagram shows two different trajectories. The left panels indicate alignment between the stellar spin axis and the planetary orbital axis while the right panels show misalignment of the two axes by 50 degrees. Therefore, precise measurements of RVs during a planetary transit enable an estimation of the angle between the two axes. Credit: NAOJ.

As you can see in the second image (below), HAT-P-11 b’s orbit is highly inclined compared to the star’s rotational axis. The Rossiter-McLaughlin effect has been investigated for approximately 35 exoplanetary systems and seems to be a reliable indicator of orbital inclination. The recent NAOJ findings take Rossiter-McLaughlin to a new level by detecting the effect using one of the smaller exoplanets known. It’s challenging work given that the signal of the RM effect is proportional to the size of the planet. In HAT-P-11 b’s case, that makes for a faint signal indeed.

But back to the planetary formation model. As we learn of more systems with inclined planetary orbits, the evidence is pointing to planet-planet models of migration. One such scenario is called Kozai migration. Here the gravitational interactions between an inner giant planet and another massive object in the system can alter the planet’s orbit and push it closer to its star. The Kozai mechanism, first developed by Japanese astronomer Yoshihide Kozai in relation to asteroid orbits, can produced the kind of inclined orbits we see in these results, as can models based on giant planets forcing mutual scattering as they are formed within the protoplanetary disk.

Image: Illustration of the HAT-P-11 System Based on Observations from Subaru Telescope. The planet orbits the star in a highly inclined orbit. Credit: NAOJ.

What doesn’t seem to work as well, given the number of systems with inclined planetary orbits, is a model of disk-to-planet interaction as the cause of migration. Here we can create a scenario where the planet falls toward the central star, but these models predict that the spin axis of the star and the orbital axis of the planet will be in substantial alignment. The Rossiter-McLaughlin effect can’t tell us whether planet-planet scattering or Kozai migration is at work in a given system, but by developing a large sample of RM data, we should eventually be able to perform statistical analyses to draw conclusions about which model is the most plausible.

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A Renewed Concern: Flares and Astrobiology

Before the recent American Astronomical Society meeting in Seattle gets too far behind us, I want to be sure to include an interesting story on red dwarfs in the coverage here. The story involves an extrasolar planet survey called SWEEPS — Sagittarius Window Eclipsing Extrasolar Planet Search, which used the Hubble Space Telescope to monitor 215,000 stars in the so-called Sagittarius Window (also called Baade’s Window, after Walter Baade, who discovered it with the 18” Schmidt camera on Mt. Palomar). The ‘window’ offers a view of the Milky Way’s central bulge stars, which are otherwise blocked by dark clouds of galactic dust.

M-dwarfs are by far the most common type of star in the Milky Way, and therefore have major implications for the search for extraterrestrial life. We now know from SWEEPS data that these small stars are given to stellar flares that can have major effects on a planetary atmosphere. Flares have often been mentioned as a serious problem for the development of life on M-dwarf planets, but the new data tell us they may be more dangerous than we had thought, occurring on a regular and frequent basis. This BBC story quotes planet hunter Geoff Marcy:

“Such powerful flares bode ill for any possible biology, life, on any planet that happens to be close to that flaring star. It’s extraordinary to think that the most numerous stars, the smallest ones in our galaxy, pose this threat to life.”

How Red Dwarf Flares Happen

The threat, vividly portrayed in the results presented by Adam Kowalski (University of Washington) at the conference, involves an eruption of hot plasma that happens when magnetic field lines in a stellar atmosphere reconnect and release an amount of energy that can surpass that of 100 million atomic bombs. From the perspective of life on a planet orbiting an M-dwarf, the planetary surface is blasted with ultraviolet light and a bath of X-rays, along with the charged particles of the stellar wind. The SWEEPS study, with observations over a seven-day period, found 100 stellar flares in this largest continuous monitoring of red dwarfs ever undertaken.

You wouldn’t think small M-dwarfs would pack an impressive punch, but it turns out they have a deep convection zone where cells of hot gas can bubble to the surface in a process Rachel Osten (Space Telescope Science Institute) likens to ‘boiling oatmeal.’ It’s within this zone that the magnetic field is generated that produces the flare, a magnetic field stronger than our Sun’s. I learned from reading papers related to this topic that while sunspots cover less than one percent of the Sun’s surface, the star spots that cover a red dwarf can occupy fully half their surface. And it’s not just young, active stars that pose the threat, according to Osten:

“We know that hyperactive young stars produce flares, but this study shows that even in fairly old stars that are several billion years old, flares are a fact of life. Life could be rough for any planets orbiting close enough to these flaring stars. Their heated atmospheres could puff up and might get stripped away.”

Most flares last for only a few minutes, but some have been observed to persist for up to eight hours. Older stars do seem to flare less frequently than younger ones, but this survey, taken from data originally compiled in 2006 as part of an exoplanet hunt, tells us that flares continue to be an issue for M-dwarfs that have moved past their youth. Some of the surveyed stars grew as much as 10 percent brighter in a short period of time, making their flares much brighter than those from our Sun, and a few of the stars surveyed produced more than one flare.

Waiting for Stellar Maturity

I haven’t found the paper on this work, but related papers using other surveys include Hilton et al., “The Galactic M Dwarf Flare Rate,” from the Proceedings of the Cool Stars 16 Workshop (preprint) and Hilton et al., “M Dwarf Flares from Time-Resolved SDSS Spectra,” accepted for publication in The Astrophysical Journal (preprint). The latter gets into the age issue re flares. From the abstract: “We find that the flare duty cycle is larger in the population near the Galactic plane and that the flare stars are more spatially restricted than the magnetically active but non-flaring stars. This suggests that flare frequency may be related to stellar age (younger stars are more likely to flare) and that the flare stars are younger than the mean active population.”

With red dwarfs comprising 75 percent and perhaps more of the stars in our galaxy, the question of life around them may come down to how long it takes a flare star to attain a more sedate existence, with flare activity slowing to less threatening levels. On that score, we have much to learn. Because their lifetimes are far longer than the current age of the universe, we have no senescent red dwarfs to study.

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Mapping Dark Matter in Ellipticals

Next week I’ll be reviewing Richard Panek’s The 4 Percent Universe (Houghton Mifflin Harcourt, 2011), a penetrating look at our investigations of dark matter and dark energy. But plenty of information has also come out of the American Astronomical Society’s 217th meeting, which ended yesterday. We looked at interesting gravitational lensing results in the previous post, pondering how they affected our census of high-redshift galaxies, but equally intriguing is a study of 14 massive galaxies that helps us map out the distribution of dark matter within them.

The work was led by David Pooley (Eureka Scientific), focusing on galaxies with strong gravitational lensing characteristics. The 14 galaxies average about 6 billion light years away, and they appear almost directly in front of even more distant galaxies that each include a supermassive black hole at galactic center with associated quasar. You know from our ongoing discussions of the FOCAL mission what to expect — light from the quasars is gravitationally lensed by the intervening galaxies to produce multiple images of the quasar.

Specifically, four images of the quasar appear as a result of light being ‘bent’ around the massive foreground object. Or perhaps the better way to put it is that the light is simply following the curvature of spacetime caused by the foreground galaxy. It’s this effect that makes light appear to bend. In any case, Pooley’s team went to work on the lensing images, using the Chandra X-ray Observatory to study them. Einstein’s general theory of relativity makes clear what the images should have looked like, but Chandra revealed some telling differences.

Image: Model prediction of what the four images of the background quasar RXJ 1131-1231 should look like, as lensed by an intervening galaxy (left). Chandra X-ray observations show a strong anomaly in the middle of the three images on the left side of the panel (right). Credit: D. Pooley (Eureka Scientific).

What’s going on? The aggregate gravitational field from all the matter in the foreground galaxy produces the gravitational lensing effect, creating the four distinct images of the distant quasar under study. And as the light passes through the lensing galaxy, it is also affected by the individual gravitational fields of the stars within the galaxy, what Pooley calls ‘lensing on top of lensing.’ How much the light is thus affected depends on both the number of stars and the amount of dark matter in the regions of the galaxy through which the quasar’s light passes.

If you simulate the effect with a galaxy made entirely of stars and completely devoid of dark matter, you do not get the Chandra results. Run the same simulation with a galaxy made entirely of dark matter and the results again diverge. Pooley’s team found that to match what Chandra sees, the galaxies must consist of 85 to 95 percent dark matter in the region through which the background light from the quasars passes. Interestingly, these regions are between 15,000 and 25,000 light years from the centers of the lensing galaxies.

Dark matter studies that work with cluster lensing, such as the famous Bullet Cluster analysis, focus on dark matter that is largely outside the individual galaxies in the cluster (we’ll be talking about the Bullet Cluster results again next week when I discuss Panek’s book). The new results look within elliptical galaxies to make their measurement. The more gravitationally lensed quasars we discover and observe with Chandra, the more these results will be refined, because working in the X-ray region of the spectrum gives far more precise results than optical studies. And because upcoming large-area surveys will doubtless increase our harvest of useful lensing phenomena, we can expect still more detailed maps of the dark matter within galaxies.

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A Deep-Sky Look at Lensing

As we continue to investigate the parameters of the proposed FOCAL mission to the Sun’s gravitational lens, it’s worth recalling how the idea of lensing has taken hold in recent decades. Einstein noted the possibilities of such lensing as far back as 1936, but it wasn’t until 1964 that Sydney Liebes (Stanford University) worked out the mathematical theory, explaining how a galaxy between the Earth and an extremely distant object like a quasar could focus the latter’s light in ways that should be detectable by astronomers. And it wasn’t until 1979 that Von Eshleman (also at Stanford) applied the notion to using the Sun as a focusing object.

It was Eshleman who suggested sending a spacecraft to the Sun’s gravitational focus at 550 AU for the first time, where magnifications, especially at microwave frequencies like the hydrogen line at 1420 MHz, are potentially enormous. This was a year after the first ‘twin quasar’ image caused by the gravitational field of a galaxy was identified by British astronomer Dennis Walsh. Frank Drake went on to champion the uses of the Sun’s gravitational lens in a paper presented to a 1987 conference, but since then it has been Claudio Maccone who has led work on a FOCAL mission, including a formal proposal to the European Space Agency in 1993.

Lensing and Galactic Surveys

Although ESA did not have funds for FOCAL, Maccone continues to write actively about the mission in papers and books. He’ll also be fascinated to see how the subject is being discussed at the American Astronomical Society’s 217th annual meeting, which concludes today. A team led by Stuart Wyithe (University of Melbourne) has made the case in a presentation and related paper in Nature that as many as 20 percent of the most distant galaxies we can detect appear brighter than they actually are, meaning that lensing has gone from a curious effect to a significant factor in evaluating galaxy surveys to make sure they are accurate.

Rogier Windhorst (Arizona State) is a co-author of the paper that sums up this work. Windhorst uses the analogy of looking through a glass Coke bottle at a distant light and noticing how the image is distorted as it passes through the bottle. The Wyithe team now believes that gravitational lensing distorted the measurements of the flux and number density of the most distant galaxies seen in recent near-infrared surveys with the Hubble Space Telescope.

The notion makes sense when you realize that the farther and older the object under study, the more likely there will be something massive in the foreground to distort its image — after all, there is more foreground universe to look through the farther out we look. But this is the first work I know that suggests that gravitational lensing dominates the observed properties of the earliest galaxies. These are objects that, as we observe them, are between 650 million and 480 million years old, seen with Hubble at redshifts of z > 8-10 respectively. Foreground galaxies from a later era, when the universe was 3-6 billion years old, will gravitationally distort these early objects.

Image: This diagram illustrates how gravitational lensing by foreground galaxies will influence the appearance of far more distant background galaxies. This means that as many as 20 percent of the most distant galaxies currently detected will appear brighter because their light is being amplified by the effects of foreground intense gravitational fields. The plane at far left contains background high-redshift galaxies. The middle plane contains foreground galaxies; their gravity amplifies the brightness of the background galaxies. The right plane shows how the field would look from Earth with the effects of gravitational lensing added. Distant galaxies that might otherwise be invisible appear due to lensing effects. Credit: NASA, ESA, and A. Feild (STScI).

Windhorst doesn’t doubt that the distortions play a major role in our observations:

“We show that gravitational lensing by foreground galaxies will lead to a higher number of galaxies to be counted at redshifts z>8-10. This number may be boosted significantly, by as much as an order of magnitude. If there existed only three galaxies above the detection threshold at redshifts z>10 in the Hubble field-of-view without the presence of lensing, the bias from gravitational lensing may make as many as 10-30 of them visible in the Hubble images. In this sense, the very distant universe is like a house of mirrors that you visit at the State Fair — there may be fewer direct lines-of-sight to a very distant object, and their images may reach us more often via a gravitationally-bent path. What you see is not what you’ve got!”

On the other hand, without lensing we would not be able to study many of these objects at all, as Haojing Yan (Ohio State) notes:

“On one hand, lensing is good for us in that it enables us to detect galaxies that would otherwise be invisible; but on the other hand, we will need to correct our surveys to obtain accurate tallies… We predict that many galaxies in the most remote universe will only ever be visible to us because they are magnified in this way.”

Consequences for Future Work

This work began with images from the Hubble Ultra Deep Field survey, as Yan and colleagues sought to understand why so many of the distant galaxies in the survey seemed to be located near the line of sight to galaxies in the foreground. Statistical analysis has determined that strong gravitational lensing is the most likely explanation, and Yan’s current estimate is that as many as 20 percent of the most distant galaxies currently detected appear brighter than they actually are. But he notes that the 20 percent figure is a tentative one, adding:

“We want to make it clear that the size of the effect depends on a number of uncertain factors. If, for example, very distant galaxies are much fainter than their nearby counterparts but much more numerous, the majority of such distant galaxies that we will detect in the foreseeable future could be lensed ones.”

All of this means that future surveys will have to incorporate a gravitational lensing bias in high-redshift galaxy samples. Fortunately, the James Webb Space Telescope is on the way. With its high resolution and sensitivity at longer wavelengths, it should be able to separate out the lensing effect, untangling the images to allow further study, a task that is beyond the Hubble Wide Field Camera 3. And because surveys of the first galaxies are a major part of JWST’s mission, we have a classic case of the right instrument emerging at the right time.

We’re also going to need some interesting software upgrades. Windhorst notes the necessity of developing “…a next generation of object finding algorithms, since the current software is simply not designed to find these rare background objects behind such dense foregrounds. It’s like finding a few ‘nano-needles’ in the mother-of-all-haystacks.” Such work has obvious implications for any future space mission to exploit the Sun’s gravitational lens.

The paper is Wyithe et al., “A distortion of very-high-redshift galaxy number counts by gravitational lensing,” Nature 469 (13 January 2011), pp. 181-184 (abstract).

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Planck Looks at the Interstellar Medium

Yesterday’s news conference on the Planck mission, held at the Millimeter and Submillimeter Sky in the Planck Mission Era conference in Paris, was so absorbing that I abandoned previous plans and stayed glued to the monitor most of the afternoon, replaying particular points from the various presenters (although keeping an eye on AAS happenings via Twitter as well). The video is available here, and it’s well worth a look given Planck’s interesting results so far, and the rich study of the Cosmic Microwave Background that will eventually flow from its data.

The European Space Agency has been offering broad coverage of the Planck findings, but before you check these out, bear in mind that the primary mission of the spacecraft is to measure the fluctuations in the CMB that both COBE and, to a higher level of detail, WMAP observed. It was fun to watch the sparring between the assembled Planck team and journalists at the conference when the question of data release came up. We won’t have the inside information on the CMB for another two years or so, the scientists arguing that such detailed work takes time (they’re right) and the journalists clearly looking for a bit of controversy.

Image: An artist’s impression depicts Planck against a background image of the Large-scale structure in the Milky Way. Credit and copyright: ESA.

Right now, though, Planck is engaged in a vital first step, which is to filter out the foreground emissions of cosmic structure located between us and the CMB. That means everything from galactic clusters to individual galaxies, gas and dust on large and small scales. And you can see where this leads — to filter out these factors, we have to study them with great precision, which is good science in itself and leads to yesterday’s discussion of the catalog of individual compact sources that Planck has made possible, along with maps of the galactic diffuse emission.

Anomalous Microwave Emission

But I want to focus on three aspects of the numerous Planck results announced yesterday, particularly its findings on the interstellar medium. If radiation from the ISM is a nuisance for cosmologists hoping to study the CMB, it’s a treasure trove of information for those specializing in the gas and dust in our own galaxy. And while dust in the ISM is known to shine at far infrared and submillimeter wavelengths, what we didn’t expect was a significant emission in the microwave band. First detected in the 1990s, this ‘Anomalous Microwave Emission’ (AME) has puzzled astrophysicists ever since, but Planck is now offering up a solution.

Clive Dickinson (University of Manchester) led one analysis of Planck’s maps re the AME:

“We are now becoming rather confident that the emission is due to nano-scale spinning grains of dust, which rotate up to ten thousand million times per second. These are the smallest dust grains known, comprising only 10 to 50 atoms; spun up by collisions with atoms or photons, they emit radiation at frequencies between 10 and 60 GHz.”

Dickinson’s work focused on two star-forming regions in the Milky Way, the Perseus and Rho Ophiuchus molecular clouds, pointing to nano-scale spinning dust grains as being a major factor, and perhaps the only factor, in creating the Anomalous Microwave Emission. A second study, led by Jean-Philippe Bernard (Institut de Recherche en Astrophysique et Planétologie, Toulouse), looked at the AME as manifested in the Small Magellanic Cloud. Bernard’s team confirms the idea that spinning dust grains play a major role in the excess radiation.

Image: This three-colour composite image of the Perseus molecular cloud (displayed on the right) is based on the three individual maps (shown on the left) of the complex, taken at 0.4 GHz from Haslam et al. (1982) and at 30 and 857 GHz by Planck, respectively. The colour composite highlights the correlation between the anomalous microwave emission, most likely due to nano-sized spinning dust grains and observed at 30 GHz (shown here in red) and the thermal dust emission, observed at 857 GHz (shown here in green). The complex structure of knots and filaments visible in this cloud of gas and dust represents striking evidence for on-going processes of star formation. Credit and copyright: ESA/Planck Collaboration.

Dark Gas in the Interstellar Medium

But Planck isn’t through with the interstellar medium yet. Its results also help astronomers measure an excess of dust emission that is sometimes called ‘dark gas,’ one which Bernard, speaking at yesterday’s news conference, was quick to point out has no relation whatsoever to ‘dark matter.’ Instead, this ‘dark gas’ component seems to be molecular gas which has been hard to quantify because it contains too little carbon monoxide (CO). The latter molecule is used by astronomers to measure the amount of molecular hydrogen in the interstellar medium.

Bernard adds: “”We believe that this ‘dark gas’ may be associated with the periphery of dense molecular clouds, where energetic ultraviolet photons destroy the CO molecules but leave the H2 undisturbed.” All of which points to Planck’s sensitivity to the temperature and density of gas in the ISM, helping us study components that have been the subject of theory but, until now, not of observation. The ISM studies help us extend our knowledge of galactic evolution and, for those of us with a long-term perspective, help clarify the medium through which future spacecraft will have to pass as we push outward into the galaxy.

A Catalog of Cold Cores

Planck is also finding a new set of targets for other telescopes to observe, among them cold and dense clumps of matter that emit most of their radiation in the sub-millimeter region of the spectrum. The complete Cold Core Catalogue of Planck Objects (with the delightful acronym C3PO) contains over 10,000 objects, with the most reliable of these, some 915 objects, being compiled in the Early Cold Core Catalogue. We’re talking about some of the coldest places in the universe, the dark birthplaces of stars whose formation is in its earliest stages.

Some of these cores have temperatures as low as 7 Kelvin (to see them, Planck’s detectors were chilled to 0.1 Kelvin), and as a by-product of their investigation, we wind up with a map of the coldest dust distribution throughout the Milky Way. Ludovic Montier (Institut de Recherche en Astrophysique et Planétologie, Toulouse) led the team that compiled and analyzed the C3PO sample:

“Instead of the compact cores that we expected to find, we have detected mainly objects which are rather elongated and have very low temperatures, between 7 and 16 Kelvin. These clumps are not isolated but appear to be all linked to one other, forming huge filamentary structures.”

Image: This map shows the density of cold cores in the Milky Way as detected by Planck during its first all-sky survey through its three highest frequency channels. Cold and dense clumps of dust, the coldest agglomerations of matter found within molecular clouds, are extremely important in order to understand the earliest phases of the formation of stars. Credit and copyright: ESA/Planck Collaboration.

The cold clumps tend to cluster along the Milky Way’s plane. Here’s Mika Juvela (University of Helsinki), who co-led the investigation with Montier:

“The majority of the cold material detected by Planck is either organised in filaments or located at the illuminated edges of pillars of dense gas. In addition, by combining the data with other observations we found that these cold clumps are aligned with hydrogen gas shells and other galactic regions of active star formation, supporting the scenario that the formation of stars might be triggered by earlier stellar populations.”

While Planck lacks the resolution to look deeply into the cores inside these cold clumps of matter, its work now feeds into future infrared observations by the Herschel instrument. Early Herschel work into a small number of the Planck detections has already been completed, but the Early Cold Core Catalogue offers hundreds of objects for further investigation. Understanding how cold dust factors into star formation helps us connect what we see in the Milky Way with broader galactic evolution.

That last point is significant because we’re talking about cold dust that has previously gone undetected in the study of local galaxies. Now we know that galaxies radiate more energy at far infrared and sub-millimeter wavelengths than we had assumed, and that changes how we draw conclusions about our observations of far more distant galaxies. Extremely cold dust is a clue to the history of star formation, giving us plenty to do as we wait for the Planck team to remove the veil from Planck’s whole-sky view and tell us what it has learned about the CMB.

The Planck Early Release Compact Source Catalogue, containing the Early Cold Core Catalogue, is available here.

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