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Register a new userApocenter glow in eccentric debris disks: implications for Fomalhaut and epsilon Eridani
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Authors
Margaret Pan
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Debris disks often take the form of eccentric rings with azimuthal asymmetries in surface brightness. Such disks are often described as showing pericenter glow, an enhancement of the disk brightness in regions nearest the central star. At long wavelengths, however, the disk apocenters should appear brighter than their pericenters: in the long wavelength limit, we find the apocenter/pericenter flux ratio scales as 1+e for disk eccentricity e. We produce new models of this apocenter glow to explore its causes and wavelength dependence and study its potential as a probe of dust grain properties. Based on our models, we argue that several far-infrared and (sub)millimeter images of the Fomalhaut and epsilon Eridani debris rings obtained with Herschel, JCMT, SHARC II, ALMA, and ATCA should be reinterpreted as suggestions or examples of apocenter glow. This reinterpretation yields new constraints on the disks dust grain properties and size distributions.
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The evidence for a rotation of the epsilon Eridani debris disc is examined. Data at 850 micron wavelength were previously obtained using the Submillimetre Common User Bolometer Array (SCUBA) over periods in 1997-1998 and 2000-2002. By chi-square fitting after shift and rotation operations, images from these two epochs were compared to recover proper motion and orbital motion of the disc. The same procedures were then performed on simulated images to estimate the accuracy of the results. Minima in the chi-square plots indicate a motion of the disc of approximately 0.6 per year in the direction of the stars proper motion. This underestimates the true value of 1 per year, implying that some of the structure in the disc region is not associated with epsilon Eridani, originating instead from background galaxies. From the chi-square fitting for orbital motion, a counterclockwise rotation rate of ~2.75 degrees per year is deduced. Comparisons with simulated data in which the disc is not rotating show that noise and background galaxies result in approximately Gaussian fluctuations with a standard deviation +/-1.5 degrees per year. Thus counterclockwise rotation of disc features is supported at approximately a 2-sigma level, after a 4-year time difference. This rate is faster than the Keplerian rate of 0.65 degrees per year for features at ~65 AU from the star, suggesting their motion is tracking a planet inside the dust ring. Future observations with SCUBA-2 can rule out no rotation of the epsilon Eridani dust clumps with ~4-sigma confidence. Assuming a rate of about 2.75 degrees per year, the rotation of the features after a 10-year period could be shown to be >1 degree per year at the 3-sigma level.
Vega and Fomalhaut, are similar in terms of mass, ages, and global debris disk properties; therefore, they are often referred as debris disk twins. We present Spitzer 10-35 um spectroscopic data centered at both stars, and identify warm, unresolved excess emission in the close vicinity of Vega for the first time. The properties of the warm excess in Vega are further characterized with ancillary photometry in the mid infrared and resolved images in the far-infrared and submillimeter wavelengths. The Vega warm excess shares many similar properties with the one found around Fomalhaut. The emission shortward of ~30 um from both warm components is well described as a blackbody emission of ~170 K. Interestingly, two other systems, eps Eri and HR 8799, also show such an unresolved warm dust using the same approach. These warm components may be analogous to the solar systems zodiacal dust cloud, but of far greater. The dust temperature and tentative detections in the submillimeter suggest the warm excess arises from dust associated with a planetesimal ring located near the water-frost line and presumably created by processes occurring at similar locations in other debris systems as well. We also review the properties of the 2 um hot excess around Vega and Fomalhaut, showing that the dust responsible for the hot excess is not spatially associated with the dust we detected in the warm belt. We suggest it may arise from hot nano grains trapped in the magnetic field of the star. Finally, the separation between the warm and cold belt is rather large with an orbital ratio >~10 in all four systems. In light of the current upper limits on the masses of planetary objects and the large gap, we discuss the possible implications for their underlying planetary architecture, and suggest that multiple, low-mass planets likely reside between the two belts in Vega and Fomalhaut.
Debris disks around young stars (analogues of the Kuiper Belt in our Solar System) show a variety of non-trivial structures attributed to planetary perturbations and used to constrain the properties of the planets. However, these analyses have largely ignored the fact that some debris disks are found to contain small quantities of gas, a component that all such disks should contain at some level. Several debris disks have been measured with a dust-to-gas ratio around unity at which the effect of hydrodynamics on the structure of the disk cannot be ignored. Here we report linear and nonlinear modelling that shows that dust-gas interactions can produce some of the key patterns attributed to planets. We find a robust clumping instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk. The conclusion that such disks might contain planets is not necessarily required to explain these systems.
Context: Since circumstellar dust in debris disks is short-lived, dust-replenishing requires the presence of a reservoir of planetesimals. These planetesimals in the parent belt of debris disks orbit their host star and continuously supply the disk with fine dust through their mutual collisions. Aims: We aim to understand effects of different collisional parameters on the observational appearance of eccentric debris disks. Methods: The collisional evolution of selected debris disk configurations was simulated with the numerical code ACE. Subsequently, selected observable quantities are simulated with our newly developed code DMS. The impact of the eccentricity, dynamical excitation, and the material strength is discussed with respect to the grain size distribution, the spectral energy distribution, and spatially resolved images of debris disk systems. Results: The most recognizable features in different collisional evolutions are as follows. First, both the increase of dynamical excitation in the eccentric belt of the debris disk system and the decrease of the material strength of dust particles result in a higher production rate of smaller particles. This reduces the surface brightness differences between the periastron and the apastron sides of the disks. For very low material strengths, the pericenter glow phenomenon is reduced and eventually even replaced by the opposite effect, the apocenter glow. Second, it is possible to constrain the level of collisional activity from the appearance of the disk, for example, the wavelength-dependent apocenter-to-pericenter flux ratio. Within the considered parameter space, the impact of the material strength on the appearance of the disk is stronger than that of dynamical excitation of the system. Finally, we find that the impact of the collisional parameters on the net spectral energy distribution is weak.
Fomalhaut plays an important role in the study of debris disks and small bodies in other planetary systems. The proximity and luminosity of the star make key features of its debris, like the water ice-line, accessible. Here we present ALMA cycle 1, 870 mu m (345 GHz) observations targeted at the inner part of the Fomalhaut system with a synthesized beam of 0.45x0.37 (~3 AU linear resolution at the distance of Fomalhaut) and a rms of 26 mu Jy/beam. The high angular resolution and sensitivity of the ALMA data enable us to place strong constraints on the nature of the warm excess revealed by Spitzer and Herschel observations. We detect a point source at the star position with a total flux consistent with thermal emission from the stellar photosphere. No structures that are brighter than 3sigma are detected in the central 15 AU x 15 AU region. Modeling the spectral energy distribution using parameters expected for a dust-producing planetesimal belt indicates a radial location in the range ~8-15 AU. This is consistent with the location where ice sublimates in Fomalhaut, i.e., an asteroid-belt analog. The 3sigma upper limit for such a belt is <1.3 mJy at 870 mu m. We also interpret the 2 and 8-13 mu m interferometric measurements to reveal the structure in the inner 10 AU region as dust naturally connected to this proposed asteroid belt by Poynting-Robertson drag, dust sublimation, and magnetically trapped nano grains.
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