Multi-wavelength spectroscopy can be used to constrain the dust and gas properties in debris disks. Circumstellar dust absorbs and scatters incident stellar light. The scattered light is sometimes resolved spatially at visual and near-infrared wavelengths using high contrast imaging techniques that suppress light from the central star. The thermal emission is inferred from infrared through submillimeter excess emission that may be 1-2 orders of magnitude brighter than the stellar photosphere alone. If the disk is not spatially resolved, then the radial distribution of the dust can be inferred from Spectral Energy Distribution (SED) modeling. If the grains are sufficiently small and warm, then their composition can be determined from mid-infrared spectroscopy. Otherwise, their composition may be determined from reflectance and/or far-infrared spectroscopy. Atomic and molecular gas absorb and resonantly scatter stellar light. Since the gas is believed to be secondary, detailed analysis analysis of the gas distribution, kinematics, and composition may also shed light on the dust composition and processing history.
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Deep Dive into A Short Guide to Debris Disk Spectroscopy.
Multi-wavelength spectroscopy can be used to constrain the dust and gas properties in debris disks. Circumstellar dust absorbs and scatters incident stellar light. The scattered light is sometimes resolved spatially at visual and near-infrared wavelengths using high contrast imaging techniques that suppress light from the central star. The thermal emission is inferred from infrared through submillimeter excess emission that may be 1-2 orders of magnitude brighter than the stellar photosphere alone. If the disk is not spatially resolved, then the radial distribution of the dust can be inferred from Spectral Energy Distribution (SED) modeling. If the grains are sufficiently small and warm, then their composition can be determined from mid-infrared spectroscopy. Otherwise, their composition may be determined from reflectance and/or far-infrared spectroscopy. Atomic and molecular gas absorb and resonantly scatter stellar light. Since the gas is believed to be secondary, detailed analysis a
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Debris disks are dusty disks around main sequence stars that are distinguished from proto-planetary disks by their small gas:dust ratios. In the absence of bulk gas, the dynamics of the planets, small bodies, and dust grains can be calculated based on stellar (e.g. stellar luminosity and mass loss rate) and planetary (e.g. mass, orbital inclination, eccentricity, and semi-major axis) properties alone. Without gas to retard the loss of particles against radiation pressure or corpuscular stellar wind and Poynting-Robertson drag, circumstellar grains typically possess lifetimes of <10,000 years, significantly shorter than the age of the central star, implying that the grains are replenished from a reservoir. In these systems, unseen planets are presumed to perturb minor bodies such as asteroids or comets into crossing orbits, generating small dust grains that are detected via remote sensing.
At the current time, radial velocity and transiting planet searches have discovered 291 companions to 251 solar-type stars with minimum masses <25 M Jup (see http://exoplanets.eu
), 41 of which transit their host stars. Statistical studies of their properties are expected to provide important constraints on planet formation models. For example, the observation that planets are found more frequently around metal-rich stars suggests that they typically form via core-accretion rather than gravitational instability [13]. Debris disk studies can provide complementary constraints to planet formation and evolution models such as the bulk composition of exo-planets, the frequency and location of dust producing events generated during terrestrial planet formation and giant planet migration, and the final architecture of planetary systems.
For example, the discovery of large masses of fine SiO 2 grains and Si gas around the β Pic moving group member HR 7012 (with an age of ∼12 Myr) indicates that a recent massive collision has occurred in the inner planetary system, perhaps analogous to the large impact that may have stripped Mercury’s mantle [21].
The most extensive spectroscopic observations of dust debris around main sequence stars have been carried out at mid-infrared wavelengths (5.5 -35 µm) using the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The majority of objects studied to date do not possess PAH or silicate emission features at 10 and/or 20 µm, suggesting that the dust grains are cold (T gr ≤ 110 K) and/or large (2πa » λ ) [6]. Despite the lack of spectral features, the shape of the SED can be used to infer the spatial distribution of the dust in the absence of resolved images. However, such studies only provide approximate guidelines for the location of the dust because SED fitting is degenerate. Dust distances predicted from SED modeling, assuming single temperature black bodies, may be as much as a factor of two smaller than resolved disk sizes.
The SEDs of the majority of debris disks are well fit using single temperature black bodies suggesting that the dust in these systems is located in rings [6]. As many as onethird of debris disk spectra may be better fit using multiple temperature components indicating either the presence of multiple components or a continuous disk with radii that extend several 10’s of AU [14]. The presence of central clearings may suggest that planets have already formed and are sculpting the disks [11]; however, other processes that do not require the presence of planets may also produce the inferred central clearings such as (1) radiation pressure if the disk is collisionally dominated and the grains rapidly shatter to sizes below the blow-out size, (2) sublimation if the grains are icy, and (3) grain sorting via gas drag if the disk has a dust:gas ratio of 0.1-10 [28]. Estimates for the grain lifetimes of IRAS-discovered debris disks (that are better fit using single temperature black bodies) suggest that these systems are collision dominated and that their central clearings are generated by radiation pressure [6].
The dust in our solar system is more tenuous and therefore spirals in under corpuscular solar wind and Poynting-Robertson (CPR) drag before it collides with other dust. If dust in debris disks, produced in collisions between parent bodies, spirals in under CPR drag, then it is expected to generate a disk with a uniform surface density and a flux,
[15] independent of the mass opacity wavelength dependence (i.e. for any κ ν ∝ ν p ).
Collisional cascade studies suggest that the boundary between the collisionally-and PRdrag dominated regimes around main sequence A-tye stars should occur at fractional luminosities, L IR /L * ∼ 10 -4 [19]; however, all of the IRAS-discovered debris disks appear to be collisionally-dominated even though some objects possess L IR /L * as small as 10 -6 . L IR /L * may not be the best metric to determine whether disks are collisionallyor CPR dominated because this quantity is proportional to dust mass rather than dust density. Rece
