HR 4796A Research Articles

PRE-DISCOVERY 2007 IMAGE OF THE HR 8799 PLANETARY SYSTEM

We present a pre-discovery H-band image of the HR 8799 planetary system that reveals all three planets in August 2007. The data were obtained with the Keck adaptive optics system, using angular differential imaging and a coronagraph. We confirm the physical association of all three planets, including HR 8799d, which had only been detected in 2008 images taken two months apart, and whose association with HR 8799 was least secure until now. We confirm that the planets are 2-3 mag fainter than field brown dwarfs of comparable near-infrared colors. We note that similar under-luminosity is characteristic of young substellar objects at the L/T spectral type transition, and is likely due to enhanced dust content and non-equilibrium CO/CH_4 chemistry in their atmospheres. Finally, we place an upper limit of 18 mag per square arc second on the >120 AU H-band dust-scattered light from the HR 8799 debris disk. The upper limit on the integrated scattered light flux is 1e-4 times the photospheric level, 24 times fainter than the debris ring around HR 4796A.

SPECKLE SUPPRESSION THROUGH DUAL IMAGING POLARIMETRY, AND A GROUND-BASED IMAGE OF THE HR 4796A CIRCUMSTELLAR DISK

We demonstrate the versatility of a dual imaging polarimeter working in tandem with a Lyot coronagraph and adaptive optics to suppress the highly static speckle noise pattern—the greatest hindrance to ground-based direct imaging of planets and disks around nearby stars. Using a double difference technique with the polarimetric data, we quantify the level of speckle suppression, and hence improved sensitivity, by placing an ensemble of artificial faint companions into real data, with given total brightness and polarization. For highly polarized sources within 0. 5, we show that we achieve 3 to 4 mag greater sensitivity through polarimetric speckle suppression than simply using a coronagraph coupled to a high-order adaptive optics system. Using such a polarimeter with a classical Lyot coronagraph at the 3.63 m Advanced Electro-Optical System telescope, we have obtained a 6.5! detection in the H band of the 76 AU diameter circumstellar debris disk around the star HR 4796A. Our data represent the first definitive ground-based near-IR polarimetric image of the HR 4796A debris disk and clearly show the two outer ansae of the disk, evident in Hubble Space Telescope NICMOS/STIS imaging. Comparing our peak linearly polarized flux with the total intensity in the lobes as observed by NICMOS, we derive a lower limit to the fractional linear polarization of >29% caused by dust grains in the disk. In addition, we fit simple morphological models of optically thin disks to our data allowing us to constrain the dust disk scale height (2.5 +5.0 # 1.3 AU) and scattering asymmetry parameter (g =$ cos %= 0.20 +.07 # .10 ). These values are consistent with several lines of evidence suggesting that the HR 4796A disk is dominated by a micron-sized dust population, and are indeed typical of disks in transition between those surrounding the Herbig Ae stars to those associated with Vega-like stars.

STIS IMAGING OF THE HR 4796A CIRCUMSTELLAR DEBRIS RING

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.

Complex Organic Materials in the HR 4796A Disk?

The red spectral shape of the visible to near-infrared reflectance spectrum of the sharply edged ringlike disk around the young main-sequence star HR 4796A was recently interpreted as the presence of tholin-like complex organic materials which are seen in the atmosphere and surface of Titan and the surfaces of icy bodies in the solar system. However, we show in this Letter that porous grains composed of common cosmic dust species (amorphous silicate, amorphous carbon, and water ice) also closely reproduce the observed reflectance spectrum, suggesting that the presence of complex organic materials in the HR 4796A disk is still not definitive.

Outer edges of debris discs

Context. Rings or annulus-like features have been observed in most imaged debris discs. Outside the main ring, while some systems (e.g., β Pictoris and AU Mic) exhibit smooth surface brightness profiles (SB) that fall off roughly as ~r-3.5, others (e.g. HR 4796A and HD 139664) display large drops in luminosity at the ring's outer edge and steeper radial luminosity profiles.

Complex Organic Materials in the Circumstellar Disk of HR 4796A

We combine HST/NICMOS imaging photometry of the HR 4796A disk at previously unobserved wavelengths between 1.71-2.22\micron with reprocessed archival observations to produce a measure of the dust's 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.

Effects of photophoresis on the evolution of transitional circumstellar disks

Although known for almost a century, the photophoretic force has only recently been considered in astrophysical context for the first time. In our work, we have examined the effect of photophoresis, acting together with stellar gravity, radiation pressure, and gas drag, on the evolution of solids in transitional circumstellar disks. We have applied our calculations to four different systems: the disks of HR 4796A and HD 141569A, which are several Myr-old AB-type stars, and two hypothetical systems that correspond to the solar nebula after disk dispersal has progressed sufficiently for the disk to become optically thin. Our results suggest that solid objects migrate inward or outward, until they reach a certain size-dependent stability distance from the star. The larger the bodies, the closer to the star they tend to accumulate. Photophoresis increases the stability radii, moving objects to larger distances. What is more, photophoresis may cause formation of a belt of objects, but only in a certain range of sizes and only around low-luminosity stars. The effects of photophoresis are noticeable in the size range from several micrometers to several centimeters (for older transitional disks) or even several meters (for younger, more gaseous, ones). We argue that due to gas damping, rotation does not substantially inhibit photophoresis.

High‐Resolution Imaging of the Dust Disk around 49 Ceti

Sub-arcsecond scale Keck images of the young A1V star, 49 Ceti, resolve emission at lambda = 12.5 and 17.9 microns from a disk with long axis at PA 125 pm 10 degrees and inclination phi = 60 pm 15 degrees . At 17.9 microns, the emission is brighter and more extended toward the NW than the SE. Modeling of the mid-infrared images combined with flux densities from the literature indicate that the bulk of the mid-infrared emission comes from very small grains (a ~ 0.1 microns) confined between 30 and 60 AU from the star. This population of dust grains contributes negligibly to the significant excess observed in the spectral energy distribution. Most of the non- photospheric energy is radiated at longer wavelengths by an outer disk of larger grains (a ~ 15 microns), inner radius ~ 60 AU, and outer radius ~ 900 AU. Global properties of the 49 Cet disk show more affinity with the Beta Pic and HR 4796A disks than with other debris disks. This may be because they are all very young (t < 20 Myr), adding strength to the argument that they are transitional objects between Herbig Ae and Vega-like A stars with more tenuous circumstellar disks.

Formation of Narrow Dust Rings in Circumstellar Debris Disks

Narrow dust rings observedaround some young stars (e.g., HR 4796A) need to be confined. We present a possible explanation for the formation and confinement of such rings in optically thin circumstellar disks, without invoking shepherding planets. If an enhancement of dust grains (e.g., due to a catastrophic collision) occurs somewhere in the disk, photoelectric emission from the grains can heat the gas to temperatures well above that of the dust. The gas orbits with super (sub)-Keplerian speeds inward (outward) of the associated pressure maximum. This tends to concentrate the grains into a narrow region. The rise in dust density leads to further heating and a stronger concentration of grains. A narrow dust ring forms as a result of this instability.We show that this mechanism not only operates around early-type stars that have high UV fluxes, but also around stars with spectral types as late as K. This implies that this process is generic and may have occurred during the lifetime of each circumstellar disk. We examine the stringent upper limit on the H2 column density in the HR 4796A disk and find it to be compatible with the presence of a significant amount of hydrogen gas in the disk. We also compute the O i and C ii infrared line fluxes expected from variousdebrisdisksandshowthatthesewillbeeasilydetectablebytheupcomingHerschelmission.Herschelwillbe instrumental in detecting and characterizing gas in these disks. Subject headingg: circumstellar matter — hydrodynamics — infrared: stars — instabilities Online material: color figuresONLINE MATERIALCOLOR FIGURES

Dust Distribution in Gas Disks. II. Self‐induced Ring Formation through a Clumping Instability

Debris rings of dust are found around young luminous stars such as HR 4796A and HD 141569. Some of these entities have sharp edges and gaps, which have been interpreted as evidence for the presence of shepherding and embedded planets. Here we show that gaps and sharp edges in the debris disks of dust can also be spontaneously self-generated if they are embedded in optically thin regions of gaseous disks. This clumping instability arises in regions where an enhancement in the dust density leads to local gas temperature and pressure increases. Consequently, the relative motion between the gas and the dust is modified. The subsequent hydrodynamic drag on the dust particles leads to further enhancement of their concentration. We show that this process is linearly unstable and leads to the formation of ringlike structures within the estimated lifetime of such young objects. Once the gas is removed (e.g., by photoevaporation), the structures are frozen and will persist, even when the gas might not be observable anymore.

Photophoresis and the Pile‐up of Dust in Young Circumstellar Disks

A rapidly growing number of observations reveal ever more structure in young circumstellar disks that are presumed to be forming planetary systems. Prominent features observed are ring-shaped dust distributions with sharp inner edges around stars like the young, main-sequence star HR 4796A. Models aiming to explain the formation of these dust rings by grain migration incorporate radiation pressure of the central star as one shaping force in radial direction. However, the radiometric effect of photophoresis has been ignored, so far, in this context. This effect is based on a radiation-induced temperature gradient on the surface of a particle and the consequential nonuniform interaction with surrounding gas. The resulting force is able to effectively influence the motion of particles in gaseous environments, but so far photophoresis has been limited to applications in the field of aerosol science. Here we present calculations that underline the relevance of the photophoretic force for the dynamics of particles in gas-rich, optically thin circumstellar disks. Depending on the gas pressure, photophoresis can be stronger than radiation pressure, gas drag, and gravity by orders of magnitude. Then the motion of particles ranging in size from 1 μm to 10 m will be dominated by photophoresis. Since the photophoretic force is a function of the gas density, it provides an efficient mechanism for fast radial migration of particles to a definite distance from the star where the gas density reaches a value at which photophoresis is in equilibrium with all other forces at work. By this effect, material is swept out from the inner region of the disk and piled-up in a more or less confined belt around the star. Thus, the formation of ringlike structures of the dust distribution can most naturally be explained without any further assumptions. Since photophoretic pile-up also works for larger bodies, it might even trigger the formation of Kuiper belts.

Exploring Terrestrial Planet Formation in the TW Hydrae Association

Spitzer Space Telescope infrared measurements are presented for 24 members of the TW Hya association (TWA). High signal-to-noise ratio 24 μm photometry is presented for all these stars, including 20 stars that were not detected by IRAS. Among these 20 stars, only a single object, TWA 7, shows excess emission at 24 μm at the level of only 40% above the star's photosphere. TWA 7 also exhibits a strong 70 μm excess that is a factor of 40 brighter than the stellar photosphere at this wavelength. At 70 μm, an excess of similar magnitude is detected for TWA 13, although no 24 μm excess was detected for this binary. For the 18 stars that failed to show measurable IR excesses, the sensitivity of the current 70 μm observations does not rule out substantial cool excesses at levels 10-40 times above their stellar continua. Measurements of two T Tauri stars, TW Hya and Hen 6-300, confirm that their spectacular IR spectral energy distributions (SEDs) do not turn over even by 160 μm, consistent with the expectation for their active accretion disks. In contrast, the Spitzer data for the luminous planetary debris systems in the TWA, HD 98800B and HR 4796A, are consistent with single-temperature blackbody SEDs and agree with previous IR, submillimeter, and millimeter measurements. The major new result of this study is the dramatic bimodal distribution found for the association in the form of excess emission at a wavelength of 24 μm, indicating negligible amounts of warm (100 K) dust and debris around 20 of 24 stars in this group of very young stars. This bimodal distribution is especially striking given that the four stars in the association with strong IR excesses are 100 times brighter at 24 μm than their photospheres. Clearly, two terrestrial planetary systems, HD 98800B and HR 4796A, exist in some form. In addition, there are at least two active accreting objects, TW Hya and Hen 6-300, that may still be forming planetesimals. The remaining stars may possess significant amounts of cold dust, as in TWA 7 and 13, that have yet to be found.

The Differential Lifetimes of Protostellar Gas and Dust Disks

We construct a protostellar disk model that takes into account the combined effect of viscous evolution, photoevaporation, and the differential radial motion of dust grains and gas. For T Tauri disks, the lifetimes of dust disks that are mainly composed of millimeter-sized grains are always shorter than the gas disks' lifetimes and become similar only when the grains are fluffy (density 0.1 g cm-3). If grain growth during the classical T Tauri phase produces plenty of millimeter-sized grains, such grains completely accrete onto the star in 107 yr, before photoevaporation begins to drain the inner gas disk and the star evolves to the weak-line T Tauri phase. In the weak-line phase, only dust-poor gas disks remain at large radii (10 AU), without strong signs of gas accretion or of millimeter thermal emission from the dust. For Herbig Ae/Be stars, the strong photoevaporation clears the inner disks in 106 yr, before the dust grains in the outer disk migrate to the inner region. In this case, the grains left behind in the outer gas disk accumulate at the disk inner edge (at 10-100 AU from the star). The dust grains remain there even after the entire gas disk has been photoevaporated and form a gas-poor dust ring similar to that observed around HR 4796A. Hence, depending on the strength of the stellar ionizing flux, our model predicts opposite types of products around young stars. For low-mass stars with a low photoevaporation rate, dust-poor gas disks with an inner hole would form, whereas for high-mass stars with a high photoevaporation rate, gas-poor dust rings would form. This prediction should be examined by observations of gas and dust around weak-line T Tauri stars and evolved Herbig Ae/Be stars.

Radial Distribution of Dust Grains around HR 4796A

We present high-dynamic-range images of circumstellar dust around HR 4796A that were obtained with MIRLIN at the Keck II telescope at lambda = 7.9, 10.3, 12.5 and 24.5 um. We also present a new continuum measurement at 350 um obtained at the Caltech Submillimeter Observatory. Emission is resolved in Keck images at 12.5 and 24.5 um with PSF FWHM's of 0.37" and 0.55", respectively, and confirms the presence of an outer ring centered at 70 AU. Unresolved excess infrared emission is also detected at the stellar position and must originate well within 13 AU of the star. A model of dust emission fit to flux densities at 12.5, 20.8, and 24.5 um indicates dust grains are located 4(+3/-2) AU from the star with effective size, 28+/-6 um, and an associated temperature of 260+/-40 K. We simulate all extant data with a simple model of exozodiacal dust and an outer exo-Kuiper ring. A two-component outer ring is necessary to fit both Keck thermal infrared and HST scattered-light images. Bayesian parameter estimates yield a total cross-sectional area of 0.055 AU^2 for grains roughly 4 AU from the star and an outer-dust disk composed of a narrow large-grain ring embedded within a wider ring of smaller grains. The narrow ring is 14+/-1 AU wide with inner radius 66+/-1 AU and total cross-sectional area 245 AU^2. The outer ring is 80+/-15 AU wide with inner radius 45+/-5 AU and total cross-sectional area 90 AU^2. Dust grains in the narrow ring are about 10 times larger and have lower albedos than those in the wider ring. These properties are consistent with a picture in which radiation pressure dominates the dispersal of an exo-Kuiper belt.

Mid‐infrared Spectroscopy of Disks around Classical T Tauri Stars

We present the first Spitzer Infrared Spectrograph (IRS; The IRS was a collaborative venture between Cornell University and Ball Aerospace Corporation funded by NASA through the Jet Propulsion Laboratory and the Ames Research Center.) observations of the disks around classical T Tauri stars: spectra in the 5.2-30 micron range of six stars. The spectra are dominated by emission features from amorphous silicate dust, and a continuous component from 5 to 8 microns that in most cases comprises an excess above the photosphere throughout our spectral range. There is considerable variation in the silicate feature/continuum ratio, which implies variations of inclination, disk flaring, and stellar mass accretion rate. In most of our stars, structure in the silicate feature suggests the presence of a crystalline component. In one, CoKu Tau/4, no excess above the photosphere appears at wavelengths shortward of the silicate features, similar to 10 Myr old TW Hya, Hen 3-600, and HR 4796A. This indicates the optically thick inner disk is largely absent. The silicate emission features with peaks at 9.7 and 18 microns indicate small dust grains are present. The extremely low 10-20 micron color temperature of the dust excess, 135 K, indicates these grains are located more than 10 AU from the star. These features are suggestive of gravitational influence by planets or close stellar companions and grain growth in the region within 10 AU of the star, somewhat surprising for a star this young (1 Myr).

Collisional Cascades in Planetesimal Disks. II. Embedded Planets

We use a multiannulus planetesimal accretion code to investigate the growth of icy planets in the outer regions of a planetesimal disk. In a quiescent minimum mass solar nebula, icy planets grow to sizes of 1000--3000 km on a timescale t = 15-20 Myr (a/30 AU)^3 where a is the distance from the central star. Planets form faster in more massive nebulae. Newly-formed planets stir up leftover planetesimals along their orbits and produce a collisional cascade where icy planetesimals are slowly ground to dust. The dusty debris of planet formation has physical characteristics similar to those observed in beta Pic, HR 4796A, and other debris disks. We derive dust masses for small particles, 1 mm and smaller, and large particles, 1 mm and larger, as a function of the initial conditions in the planetesimal disk. The dust luminosities derived from these masses are similar to those observed in Vega, HR 4796A, and other debris disks. The calculations produce bright rings and dark gaps. Bright rings occur where 1000 km and larger planets have recently formed. Dark gaps are regions where planets have cleared out dust or shadows where planets have yet to form.

A Comparative Anatomy of Dusty Disks Imaged by NICMOS

Using the Near Infrared Camera and Multi-Object Spectrometer (NICMOS) on the Hubble Space Telescope we have conducted a coronagraphic imaging survey of 18 main sequence stars with large infrared excesses, searching for circumstellar dust (debris) in scattered light. Dusty disks with radial and hemispheric brightness anisotropies and complex morphologies, both possibly indicative of dynamical interactions with unseen planetary mass companions, were spatially resolved and imaged around three young (≲ 10Myr old) stars. From these observations we describe the debris systems around: a) HR 4796A (A0V), a 70 AU radius ring less than 14 AU wide with unequal ansal flux densities; b) HD 141569A (Herbig Ae/Be), a 400 AU radius disk with a 40 AU wide gap; and c) TW Hya (K7 T-Tauri), a pole-on circularly symmetric disk with a radial break in its surface density of scattering particles. Additionally, our non-detection of scattered light and high precision photometry of a fourth system of similar age, HD 98800 A/B, coupled with mid and thermal IR measurements, greatly constrain a likely model for the debris about the B component.

Mid-IR Spectroscopy of the Debris Disks in the TW Hydrae Association

Spectroscopic observation between 3-14 μm of 3 stars in the TW Hya Association were obtained using the Aerospace Corporation Broadband Array Spectrograph System (BASS) on the NASA IRTF telescope. The targets observed were TW Hya, HR 4796A, and HD 98800. Both of the late-type stars, TW Hya and HD 98800, exhibit a strong 10 μm silicate emission band. The strong emission indicates the presence of small dust grains close to the star, at angular distances not currently accessible to coronagraphic techniques. The spectral structure due to crystalline material that is present in some young early-type stars (HD 163296, HD 31648, etc.) is not apparent in these two objects. For HR 4796A, the dust emission is weak.

Ten Micron Observations of Nearby Young Stars

We present new 10 μm photometry of 21 nearby young stars obtained at the Palomar 5 m and at the Keck I 10 m telescopes as part of a program to search for dust in the habitable zone of young stars. Thirteen of the stars are in the F-K spectral type range (solar analogs), four have B or A spectral types, and four have spectral type M. We confirm existing IRAS 12 μm and ground-based 10 μm photometry for 10 of the stars and present new insight into this spectral regime for the rest. Excess emission at 10 μm is not found in any of the young solar analogs, except for a possible 2.4 σ detection in the G5 V star HD 88638. The G2 V star HD 107146, which does not display a 10 μm excess, is identified as a new Vega-like candidate, based on our 10 μm photospheric detection, combined with previously unidentified 60 and 100 μm IRAS excesses. Among the early-type stars, a 10 μm excess is detected only in HD 109573A (HR 4796A), confirming prior observations; among the M dwarfs, excesses are confirmed in AA Tau, CD -40°8434, and Hen 3-600A. A previously suggested N-band excess in the M3 dwarf CD -33°7795 is shown to be consistent with photospheric emission. We calculate infrared to stellar bolometric luminosity ratios for all stars exhibiting mid-infrared excesses and infer the total mass of orbiting dust in the cases of optically thin disks. For a derived median photometric precision of ±0.11 mag, we place an upper limit of M_(dust) ≈ 2 × 10^(-5) M_⊕ on the dust mass (assuming a dust temperature of 300 K) around solar analogs not seen in excess at 10 μm. Our calculations for the nearby K1 V star HD 17925 show that it may have the least massive debris disk known outside our solar system (M_(dust) ≳ 7 × 10^(-6) M_⊕). Our limited data confirm the expected tendency of decreasing fractional dust excess f_d = L_(IR)/L^* with increasing stellar age. However, we argue that estimates of f_d suffer from a degeneracy between the temperature and the amount of circumstellar dust M_(dust), and we propose a relation of M_(dust) as a function of age instead.

Are Giant Planets Forming around HR 4796A?

We have obtained FUSE and HST STIS spectra of HR 4796A, a nearby 8 Myr old main sequence star that possesses a dusty circumstellar disk whose inclination has been constrained from high resolution near-infrared observations to be ~17 deg from edge-on. We searched for circumstellar absorption in the ground states of C II at 1036.3 A, O I at 1039.2 A, Zn II at 2026.1 A, Lyman series H2, and CO (A-X) and failed to detect any of these species. We place upper limits on the column densities and infer upper limits on the gas masses assuming that the gas is in hydrostatic equilibrium, is well-mixed, and has a temperature, Tgas ~ 65 K. Our measurements suggest that this system possesses very little molecular gas. Therefore, we infer an upper limit for the gas:dust ratio (<4.0) assuming that the gas is atomic. We measure less gas in this system than is required to form the envelope of Jupiter.