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Register a new userSTIS Imaging of the HR 4796A Circumstellar Debris Ring
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Added by
Alycia J. Weinberger
Publication date
2008
fields
Physics
and research's language is
English
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We have obtained high spatial resolution imaging observations of the HR 4796A circumstellar debris dust ring using the broad optical response of the Hubble Space Telescope Imaging Spectrograph in coronagraphic mode. We use our visual wavelength observations to improve upon the earlier measured geometrical parameters of the ring-like disk. Two significant flux density asymmetries are noted: (1) preferential forward scattering by the disk grains and (2) an azimuthal surface brightness anisotropy about the morphological minor axis of the disk with corresponding differential ansal brightness. We find the debris ring offset from the location of the star by ~1.4 AU, a shift insufficient to explain the differing brightnesses of the NE and SW ansae simply by the 1/$r^2$ dimmunition of starlight. The STIS data also better quantify the radial confinement of the starlight-scattering circumstellar debris, to a characteristic region <14 AU in photometric half-width, with a significantly steeper inner truncation than outward falloff in radial surface brightness. The inferred spatial distribution of the disk grains is consistent with the possibility of one or more unseen co-orbital planetary-mass perturbers, and the colors of the disk grains are consistent with a collisionally evolved population of debris, possibly including ices reddened by radiation exposure to the central star.
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The young A0V star HR 4796A is host to a bright and narrow ring of dust, thought to originate in collisions between planetesimals within a belt analogous to the Solar Systems Edgeworth-Kuiper belt. Here we present high spatial resolution 880$mu$m continuum images from the Atacama Large Millimeter Array. The 80au radius dust ring is resolved radially with a characteristic width of 10au, consistent with the narrow profile seen in scattered light. Our modelling consistently finds that the disk is also vertically resolved with a similar extent. However, this extent is less than the beam size, and a disk that is dynamically very cold (i.e. vertically thin) provides a better theoretical explanation for the narrow scattered light profile, so we remain cautious about this conclusion. We do not detect $^{12}$CO J=3-2 emission, concluding that unless the disk is dynamically cold the CO+CO$_2$ ice content of the planetesimals is of order a few percent or less. We consider the range of semi-major axes and masses of an interior planet supposed to cause the rings eccentricity, finding that such a planet should be more massive than Neptune and orbit beyond 40au. Independent of our ALMA observations, we note a conflict between mid-IR pericenter-glow and scattered light imaging interpretations, concluding that models where the spatial dust density and grain size vary around the ring should be explored.
We have obtained Gemini Planet Imager (GPI) J-, H-, K1-, and K2-Spec observations of the iconic debris ring around the young, main-sequence star HR 4796A. We applied several point-spread function (PSF) subtraction techniques to the observations (Mask-and-Interpolate, RDI-NMF, RDI-KLIP, and ADI-KLIP) to measure the geometric parameters and the scattering phase function for the disk. To understand the systematic errors associated with PSF subtraction, we also forward-modeled the observations using a Markov Chain Monte Carlo framework and a simple model for the disk. We found that measurements of the disk geometric parameters were robust, with all of our analyses yielding consistent results; however, measurements of the scattering phase function were challenging to reconstruct from PSF-subtracted images, despite extensive testing. As a result, we estimated the scattering phase function using disk modeling. We searched for a dependence of the scattering phase function with respect to the GPI filters but found none. We compared the H-band scattering phase function with that measured by Hubble Space Telescope STIS at visual wavelengths and discovered a blue color at small scattering angles and a red color at large scattering angles, consistent with predictions and laboratory measurements of large grains. Finally, we successfully modeled the SPHERE H2 HR 4796A scattered phase function using a distribution of hollow spheres composed of silicates, carbon, and metallic iron.
We combine HST/NICMOS imaging photometry of the HR 4796A disk at previously unobserved wavelengths between 1.71-2.22micron with reprocessed archival observations to produce a measure of the dusts scattering efficiency as a function of wavelength. The spectrum of the dust, synthesized from the seven photometric measures, is characterized by a steep red slope increasing from 0.5 micron to 1.6 micron followed by a flattening of the spectrum at wavelengths $>$ 1.6 micron. We fit the spectrum with a model population of dust grains made of tholins, materials comprised of complex organic materials seen throughout the outer parts of our Solar System. The presence of organic material around a star that may be in the later stages of giant planet formation implies that the basic building blocks for life may be common in planetary systems.
The scattering properties of the dust originating from debris discs are still poorly known. The analysis of scattered light is however a powerful remote-sensing tool to understand the physical properties of dust particles orbiting other stars. Scattered light is indeed widely used to characterise the properties of cometary dust in the solar system. We aim to measure the morphology and scattering properties of the dust from the debris ring around HR4796A in polarised optical light. We obtained high-contrast polarimetric images of HR4796A in the wavelength range 600-900nm with the SPHERE / ZIMPOL instrument on the Very Large Telescope. We measured for the first time the polarised phase function of the dust in a debris system over a wide range of scattering angles in the optical. We confirm that it is incompatible with dust particles being compact spheres under the assumption of the Mie theory, and propose alternative scenarios compatible with the observations, such as particles with irregular surface roughness or aggregate particles.
We present the first results from the polarimetry mode of the Gemini Planet Imager (GPI), which uses a new integral field polarimetry architecture to provide high contrast linear polarimetry with minimal systematic biases between the orthogonal polarizations. We describe the design, data reduction methods, and performance of polarimetry with GPI. Point spread function subtraction via differential polarimetry suppresses unpolarized starlight by a factor of over 100, and provides sensitivity to circumstellar dust reaching the photon noise limit for these observations. In the case of the circumstellar disk around HR 4796A, GPIs advanced adaptive optics system reveals the disk clearly even prior to PSF subtraction. In polarized light, the disk is seen all the way in to its semi-minor axis for the first time. The disk exhibits surprisingly strong asymmetry in polarized intensity, with the west side >9 times brighter than the east side despite the fact that the east side is slightly brighter in total intensity. Based on a synthesis of the total and polarized intensities, we now believe that the west side is closer to us, contrary to most prior interpretations. Forward scattering by relatively large silicate dust particles leads to the strong polarized intensity on the west side, and the ring must be slightly optically thick in order to explain the lower brightness in total intensity there. These findings suggest that the ring is geometrically narrow and dynamically cold, perhaps shepherded by larger bodies in the same manner as Saturns F ring.
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