Polarimetry of stars and planetary systems
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Wilner, N. Qi, J. Williams, and M. Pyle SSC. Here the panel poses some key questions about disks that can be answered definitively in the coming decade, provided the requisite observational capabilities and theoretical developments are mustered. There are still many unknowns regarding the mass, structure, and evolution of disks in the pre-planetary state. How does disk mass evolve with time? How are mass and angular momentum transported to form stars and cause planets to migrate? To what extent is steady accretion punctuated by violent episodes? What are the thermal and chemical structures of disks?
Where are the molecules regarded as prebiotic—that is, where are the molecules formed of the simple sugars and amino acids from which living organisms derive? Are the most massive stars also assembled from disks and thus likely to harbor planets? Essential to answering these questions are improvements in angular and spectral resolution and imaging contrast, especially at wavelengths from the near-IR through the millimeter.
A major goal for the next decade is to improve significantly the spatial resolution in imaging such disks. One outcome of AU-resolution imaging of protoplanetary disks would be a clear definition of gaps and small-scale structures that will illuminate the dynamics of planet-disk interaction and early planetary migration. Of special significance would be the detection in disks of gravitational instabilities, such as spiral density waves, that would indicate self-gravitating disks.
Compression amplitudes in gravitational instabilities should exceed a factor of 10, making their contrast with respect to the disk very large—brighter than the unperturbed disk—in molecular lines that trace high-density gas Figure 4. Currently the role of self-gravity in disk evolution is a controversial topic, but if density waves are discovered, disk masses could be derived independently of uncertain gas-to-dust ratios and grain-size distributions, and the role of such instabilities in giant-planet formation could be assessed.
Detecting waves in disks with the normal range of accretion rates will. Left: Three-dimensional model at full resolution shows a Jovian-mass collapsing fragment in the outer ring. Right: The same scene convolved to the resolution of ALMA in its longest-baseline configuration, with 0. Narayanan, C. Kulesa, A. Boss, and C. Walker, Molecular line emission from gravitationally unstable protoplanetary disks, Astrophysical Journal , , reproduced by permission of the AAS.
Breakthroughs in the theory of accretion disks will require disk thermodynamics to be treated more realistically. For example, gravito-turbulent accretion is driven by net cooling, so the next generation of disk models should include in their energy budgets the feedback from stellar irradiation.
The panel recommends several key measurements and measurement capabilities that will advance the understanding of the structure and evolution of pre-planetary disks, as follows:. Sensitive millimeter-wave imaging with ALMA at few-AU resolution plus high-spectral-resolution observations by ALMA and Herschel in column-density tracers and density-sensitive molecular lines to provide the first maps of the opacity, density, thermal, and chemical structure of disks. In turn this will improve estimates of disk masses, enable measurements of the abundances of molecular.
Longer-wavelength imaging with the EVLA, and high-resolution, molecular-vibration spectroscopy with SOFIA, to provide important constraints on the chemical and radiation environments. These observations should also yield measurements of the disk ionization fraction, which is crucial for assessing the role of magnetic fields in disk transport and distinguishing actively accreting gas from dead zones. The latter may promote the survival of giant planets by acting as barriers to planet migration. Gap contrasts can be several times higher than for thermal emission from the midplane Figure 4.
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The near-IR will also be a fertile ground for identifying gravitationally induced density waves, which can appear at a contrast of about 0. High-contrast imaging with extreme adaptive-optical ExAO correction to see the disk against the glare of the star in the near-IR. Achieving mas resolution at submillimeter and near-IR wavelengths would also illuminate the role of disks in the formation of high-mass stars, by resolving distant, crowded young clusters AU at 10 kpc. Disks around young, high-mass stars could be identified by means of direct imaging, and the properties of these necessarily massive, short-lived, and externally ionized disks could be studied for the first time and could be used to discriminate among models for massive star formation.
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A further goal in the study of protoplanetary disks is to characterize their accretion history by means of wide-field long-term synoptic surveys in both the. In the lower panels the disk has a 4-AU-wide gap at 10 AU, created by a Earth-mass planet, showing a contrast ratio of about 0. At the distance of the Taurus star-formation region each image is 0. Jang-Condell, personal communication. This monitoring can be carried out with conventional resolution and sensitivity limiting K magnitude of 18 per epoch , but it would ideally include measures of stellar accretion rate, such as near-IR hydrogen recombination lines.
Such studies would clarify how important eruptive FU Ori-like events are in the buildup of stellar mass, and whether planet formation may be periodically disrupted. Ground-based near-IR interferometry also has a vital role in the study of protoplanetary disks, probing the inner 0. In the coming decade it will be important to extend the sensitivity of this technique so that more than just the few brightest systems can be observed. This is illustrated in Figure 4. At this young age, Jovian planets are relatively luminous.
Polarimetry of Stars and Planetary Systems by Ludmilla Kolokolova, Hardcover | Barnes & Noble®
A handful of targets may be detectable with JWST and extreme-AO near-IR images on 8- to m telescopes, but most of the known candidates will require the higher resolution of m telescopes, fitted with similar high-contrast instrumentation. Finding newborn giant planets will enable astronomers to test, for the first time, theories of how these planets nucleate from the disk and feed from it. Characterization of the atmospheres and neighbor-.
The protoplanets are assumed to be the same age as the host star and to lie just inside the outer edges of the inferred gaps. Also shown are the domains in which one could observe older Jovian planets and smaller terrestrial planets. Macintosh, M. Troy, R. Doyon, J. Graham, K.
Baker, B. Bauman, C. Marois, D. Palmer, D. Phillion, L. Poyneer, I. Crossfield, et al. Additional data from J. Brown, G. Blake, C. Dullemond, B. Augereau, A. Boogert, N. Evans II, V. Geers, F. Lahuis, J. Kessler-Silacci, K. Pontoppidan, and E. Kim, D. Watson, P. Manoj, E. Najita, W. Forrest, B. Sargent, C. Espaillat, N. Calvet, K. Luhman, M. McClure, J. Green, and S. Kim, personal communication; B. Macintosh, J. Graham, D. Palmer, R. Dunn, D. Gavel, J. Larkin, B.
Oppenheimer, L. Saddlemyer, A. Sivaramakrishnan, J. Wallace, B. Bauman, D. Erickson, C. Marois, L. Poyneer, and R. SPIE , Some infant giant planets may be detectable along with their cooler accretion streams, by ALMA, as illustrated in Figure 4. The distance to this hypothetical system is pc. Wolf and G. An important milestone will be the measurement of the masses of infant planets through stellar reflex motion. Magnetic activity and high rotation rates impose fundamental limits to radial-velocity precision on very young stars, which would only be sensitive to close-in or very massive planets.
Future space-based astrometric surveys could also further extend the census of infant planets in the AU region, where direct imaging is most challenging. Somewhat older giant planets Myr , including those producing gaps in nearby debris disks, also await direct detection by high-contrast imagers.
Planets at stellocentric distances of AU, where a handful of debris disks have been resolved and where high-contrast imagers operate best, could be the result of long-range migration, but also might provide long-sought examples of planet formation by gravitational instability. It is here, in the outermost reaches of disks, that fragmentation by gravitational instability during the protoplanetary phase is most viable, since cooling times are. Indeed a system of such planets has recently been reported, orbiting an A4V star with a debris disk, HR In the future, as higher angular resolution and high contrast imaging become available, the inner regions of these systems can also be searched for young planets.
In this age range, a Jovian planet luminosity is predicted to be. Planets in some nearby debris-disk systems are thus within the grasp of the next generation of high-contrast instruments on 8- to m telescopes see Figure 4. Major advances in the theoretical understanding of planet formation, migration, and interactions with their disks—both gaseous and debris—will be required to take advantage of the wealth of observational data that will be obtained in the coming decade. The required efforts span a wide range of problems, including investigations of turbulence driven by magnetic fields in partially ionized gas; coagulation of solids with settling; disk-driven migration; and fragmentation in irradiated, self-gravitating disks.
Many if not all of these problems will require major computational resources. The orbits of known extrasolar planets reflect a host of dynamical processes—some violent—including migration, planet-planet scattering, and forcing by stars. Similar dynamical evolution is currently underway in extrasolar debris disks, and the variety of structures observed to date in a handful of systems already hints at the diversity of their planetary systems.
Careful modeling of the observed disk morphology will enable the masses and orbits of sculpting bodies to be inferred. New theoretical approaches need development, since extrasolar debris disk densities are so large that interparticle collisions outweigh radiation drag, with consequences for disk structure that are only beginning to be explored.
Future models should track not only forcing by planets, but also particle fragmentation in collisional cascades and momentum exchanges between colliding particles. Multi-epoch observations will break modeling degeneracies by measuring proper motions of debris disk clumps—that is, speeds of wave patterns driven by plan-. Left: IRAM image at 1. Center: Model based on resonant trapping of planetesimals by migration of Neptune-mass planet.
Total flux density in the simulated image is about 42 mJy. Wilner, M. Holman, M. Kuchner, and P. Center: M. Right: Courtesy of R. Reid, L. Mundy, and A. Wooten, personal communication. Characterizing debris disks of different ages will enable the understanding of how planets and destructive collisions scour disks of their smallest planetesimals, leaving only large bodies. A more secure understanding of the dynamical history of our own solar system will come from an unbiased characterization, both kinematic and physical, of the Kuiper belt.
The current census of KBOs is pieced together from a patchwork of surveys, each with different areal coverage and limiting magnitude, giving an incomplete picture at best. Surface compositions will also provide important clues to the chemistry of planetesimal formation. KBOs show a broad range of colors, suggesting a wide. About half possess ultra-red matter that may be extremely pristine, preserved only in the deep freeze of the Kuiper belt. Key questions include whether there is a radial composition gradient across the belt and whether resonantly trapped KBOs are of similar composition or have migrated from different source regions.
To answer these questions requires the following:.
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Time-resolved photometry with similar precision can reveal the shapes and rotation states of the KBOs, two further constraints on their origins and collisional histories. The detection and characterization of these worlds will invigorate the efforts both to understand their origin in the grand context of star formation science question PSF 1 and protoplanetary disk evolution science question PSF 2 , and to compare their compositions, structures, and atmospheres with those of the planets of the solar system.
It is this great quest for detection and characterization that concerns the panel in the present section. In the subsequent section on science question PSF 4, the panel focuses on the particular question of life on these worlds, arguably one of the greatest intellectual endeavors ever undertaken by humanity. The panel then identifies a fast-track opportunity that may permit the study of habitable worlds within 5 years the PSF discovery area , albeit for stars very different from the Sun.
The dominant methods of discovery of planets orbiting other stars known as exoplanets are measurements of radial velocities wherein one infers the existence of a planet through its acceleration of the central star and photometric transits in which one measures the periodic dimming of the central star due to the intervening. Together these methods account for nearly all of the more than known planetary systems, in which more than exoplanets have been detected.
The nascent methods of microlensing in which the gravitational field of a planetary system causes the magnification of an unassociated background star and direct imaging wherein one spatially separates the light from the planet from the glare of the central star have each yielded a handful of detections.
In contrast, the past decade of discovery has revealed much shorter timescales for giant-planet formation see the subsection on giant-planet accretion and startling diversity in the architectures of mature planetary systems Figure 4. Although many stars indeed host Jupiter-mass planets at greater separations, the median eccentricity of the population is roughly 0.
Furthermore, many of these eccentric planets are members of multiplanet systems, and in some cases the mutual dynamical interactions of the member planets can be directly observed. There are several strong pieces of. Yet it is clear that the hot Jupiters could not have formed in place by core accretion. It also appears that gas and ice giants of our own solar system have moved significantly from their birth place. Such results have underlined the importance of studies of planetary migration and re-opened questions of the basic mechanisms of the formation of giant planets see the subsection on giant-planet accretion.
Nature has proved far more inventive in forming planets than scientific theories have in predicting their properties. Exoplanetary studies have progressed admirably during the past decade, but it still is not known whether solar systems like ours are commonplace or are cosmically rare. For a true understanding of the process of planet formation, the end states need to be understood: the planetary census must be completed. The fundamental starting point for characterizing planetary systems is the determination of orbital properties and masses; this is vital not only for understanding the dynamics and composition of these systems but also for understanding how planetary systems originate.
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In addition, a further step should be taken to develop the sensitivity to detect and characterize the bulk properties of rocky planets akin to the solar system terrestrial planets. Novel methods need to be encouraged for the detection of planets far from their stars, at distances where the radial velocity and transit methods are precluded due to the very long orbital periods and low transit probabilities.
Astrometry with Gaia will also contribute to the census of massive exoplanets in the coming decade. Fischer and J. Valenti, The planet-metallicity correlation, Astrophysical Journal , , reproduced by permission of the AAS. Left: The original demonstration by Zapolsky and Salpeter that Jupiter and Saturn are predominantly hydrogen and helium i. The two smallest exoplanets shown lie near Neptune and similarly have densities that require a bulk composition dominated by ice and rock.
Zapolsky and E. Salpeter, The mass-radius relation for cold spheres of low mass, Astrophysical Journal , , reproduced by permission of the AAS. Right: J. Fortney, University of California, Santa Cruz. The recently commissioned NASA Kepler mission, with a modest contribution from the CoRoT spacecraft, will offer humanity its first opportunity to study the population of terrestrial exoplanets within 1. This mission employs the transit method and hence will deliver estimates of the planetary radii and orbital periods. However, the scientific return will be fully realized only if mass estimates can be obtained for a significant number of these planets.
The de-. The challenges for radial velocities are that the targets are faint and the signal is small: An Earth-mass planet 1 AU from a Sun-like star produces a peak-to-peak radial-velocity difference of only 0. Even if the limiting precision were to remain fixed , the panel recommends the following:. A substantial expansion of the telescope time available to pursue radial-velocity work, since the mass determination for rocky bodies several times the mass of Earth or at smaller orbital separations is already within reach.
However, given the fold improvement in radial-velocity precision of the past 15 years, the panel finds that it is not unreasonable to aim to accomplish the following:. Develop advanced radial-velocity techniques capable of achieving 0. This will require the development of dedicated and highly specialized spectrographs and novel means for wavelength calibration. Beyond the vital follow-up of Kepler-detected worlds, this investment in radial-velocity observations will also drive the understanding of more massive worlds located at distances of 1.
There is not space here to mention the large number of ongoing radial-velocity projects that will continue to make important contributions to the planetary census; the panel necessarily groups all these efforts together in its endorsement of the expansion of ground-based radial-velocity measurements. If implemented on next-generation large-aperture telescopes needed to gather a sufficient number of photons , this could permit a survey of the closest Sun-like stars to find the nearest Earth-like planets orbiting within their stellar habitable zones.
These systems would be invaluable for subsequent efforts to search for atmospheric biomarkers science question PSF 4. The method of microlensing complements that of radial velocities and transits. It does not require that data be gathered over a full orbital cycle and thus can provide in relatively short order the detailed statistics on the masses and orbital separations of planets in the outer reaches of planetary systems. Microlensing is demanding of instrumental stability: a satellite borne survey instrument, for which the point spread function profile would be stable over times very long compared to the duration of a planetary microlensing event, would—compared with a ground-.
The results of a space-based microlensing survey would be particularly dramatic for several reasons. First, the sensitivity of Kepler to exoplanets declines sharply beyond 1. Second, microlensing will naturally survey a wider range of host stars and will include a large number of low-mass stars, as they are the most numerous type of star in the galaxy.
Kepler will survey few low-mass stars owing to their intrinsic dimness. Importantly, a space-based microlensing survey can determine the planetary masses and projected separations in physical units and can frequently detect the light emitted by the host star. This is in contrast to ground-based microlensing surveys, which must generally rely on HST imaging to accomplish this, and there is insufficient time available to follow up a significant number of detections. To obtain the fundamental data set to test the current picture of how planetary systems form, how their properties depend on the properties of the central star and, by inference, the conditions of the circumstellar disk, the panel recommends a combination of the following:.
A space-based microlensing survey, which will provide a statistical picture of planetary architectures, and. The Kepler findings on terrestrial planet frequency within 1. Expanded radial-velocity measurements on 4- to m-class telescopes to provide masses. More generally, these efforts will demonstrate how typical our own solar system is.
When both photometric and Doppler signals are measured for the same planet-hosting star, the planetary mass and radius are determined directly. These permit, in turn, an estimate of the density and, by inference, the bulk composition and the formation history. Fifty such transiting worlds are now known, and these objects have proven immensely valuable for constraining models of the physical structures of planets as a function of mass, composition, and external environment. Indeed, many show significant discrepancies from the theoretical prediction for a degenerate body with solar abundances see Figure 4.
In many cases the planets are larger than expected for their age, indicating that a mechanism to prevent cooling and contraction is in operation. A handful of systems are smaller than expected, indicating the presence of a large fraction of heavy elements, usu-. For nontransiting planets, the radial-velocity method provides only a lower bound on the mass, but.
The true masses could be determined with space-based astrometry. Since these would be the systems nearest to Earth, knowledge of the true masses for each of the planets in these systems would be extremely valuable. Progressing downward through the mass range corresponding to Neptune-like bodies, two transiting examples of which are known at the time of writing, the interpretation of radii becomes increasingly ambiguous, because there are three ingredients—gas, ice, and rock—and thus multiple ways to obtain the observed radius and mass.
Even lacking unique solutions, the observed diversity yields important constraints on the range of planetary-formation conditions. Deciding whether the atmosphere is small in this sense may prove to be very challenging, but for sufficiently low-mass bodies at modest orbital radii e. At the rather low level of accuracy attainable for determining bulk compositions, the present understanding of thermodynamics and pressure-density relationships for candidate materials is adequate for the task. The expected diversity of observations will thus be traceable to the diversity of conditions and environments of planet formation.
The tremendously exciting opportunity to make informed estimates of the compositions of perhaps hundreds of Earth-like planets detected by Kepler serves as a compelling motivation for increased radial-velocity precision and the expansion of available observatory time to undertake these measurements. It is essential that spectra be gathered to determine the chemistries, structures, and dynamics of exoplanetary atmospheres.
Nearly all of the available data on the atmospheres of exoplanets comes from one of two techniques that are possible only for transiting systems: 1 In transmission spectroscopy, one takes the ratio of a spectrum gathered when the planet is in front of the star with a spectrum of the unocculted star, interpreting any residual absorption features as arising in the atmosphere of the planet. The residual emission is that.
Together these studies have proven immensely valuable, permitting the detection of numerous atoms and molecules including Na, H, H 2 O, and CH 4 , as well as clouds, and the direct determination of atmospheric temperatures and studies of temperature inversions. These studies are rendered all the more penetrating by the fact that the masses and radii of the planets are accurately known. More recently, planetary emission has been studied as a function of planetary longitude Figure 4.
Upper: The light curve at mid-infrared wavelengths, showing the passage of the planet in front of phase 0 and then behind phase 0. Since the planetary emission is not observed at phase 0. Lower: The temperature map of HD b derived from the light curve. The substellar point corresponding to high noon on the planet is in the center of the diagram. The hottest point on the planet indicated with the red arrow is eastward of the substellar point, indicating the action of strong super-rotating winds on the planet.
Knutson, D. Charbonneau, L. Allen, J. Fortney, E. Agol, N. Cowan, A. Showman, C. Cooper, and S. Megeath, A map of the day-night contrast of the extrasolar planet HD b, Nature , These techniques hold great promise for the future, and indeed the panel foresees a fast-track opportunity to use these methods to search for atmospheric biomarkers such as molecular oxygen.
The panel encourages the following:. A detailed study of the optimal observing methods with JWST, and if these studies confirm their feasibility, the acquisition of very high signal-to-noise-ratio spectra of hot Jupiters and even-lower-mass planets that are predominantly ice and rock in composition. An extension of the warm phase of Spitzer would bridge the gap, permitting the continued study of exoplanet atmospheres until the launch of JWST. Abstract : Summarising the striking advances of the last two decades, this reliable introduction to modern astronomical polarimetry provides a comprehensive review of state-of-the-art techniques, models and research methods.
Focusing on optical and near-infrared wavelengths, each detailed, up-to-date chapter addresses a different facet of recent innovations, including new instrumentation, techniques and theories; new methods based on laboratory studies, enabling the modelling of polarimetric characteristics for a wide variety of astronomical objects; emerging fields of polarimetric exploration, including proto-planetary and debris discs, icy satellites, transneptunian objects, exoplanets, and the search for extraterrestrial life; and unique results produced by space telescopes, and polarimeters aboard exploratory spacecraft.
With contributions from an international team of accomplished researchers, this is an ideal resource for astronomers and researchers working in astrophysics, earth sciences, and remote sensing keen to learn more about this valuable diagnostic tool. The book is dedicated to the memory of renowned polarimetrist Tom Gehrels. Keywords : techniques and instrumentation Observational astronomy Astrophysics. Toggle navigation. Have you forgotten your login?
HAL-Inria Publications, software Ludmilla Kolokolova 1 AuthorId : Author. James Hough 2 AuthorId : Author. Hide details.