Edge-on Debris Disk Research Articles

An inner warp discovered in the disk around HD 110058 using VLT/SPHERE and HST/STIS

Context. An edge-on debris disk was detected in 2015 around the young, nearby A0V star HD 110058. The disk showed features resembling those seen in the disk of β Pictoris that could indicate the presence of a perturbing planetary-mass companion in the system. Aims. We investigated new and archival scattered light images of the disk in order to characterise its morphology and spectrum. In particular, we analysed the disk’s warp to constrain the properties of possible planetary perturbers. Methods. Using data from two VLT/SPHERE observations taken with the Integral Field Spectrograph (IFS) and near InfraRed Dual-band Imager and Spectrograph (IRDIS), we obtained high-contrast images of the edge-on disk. Additionally, we used archival data from HST/STIS with a poorer inner-working angle but a higher sensitivity to detect the outer parts of the disk. We measured the morphology of the disk by analysing vertical profiles along the length of the disk to extract the centroid spine position and vertical height. We extracted the surface brightness and reflectance spectrum of the disk. Results. We detect the disk between 20 au (with SPHERE) and 150 au (with STIS), at a position angle of 159.6° ± 0.6°. Analysis of the spine shows an asymmetry between the two sides of the disk, with a 3.4° ± 0.9° warp between ~20au and 60 au. The disk is marginally vertically resolved in scattered light, with a vertical aspect ratio of 9.3 ± 0.7% at 45 au. The extracted reflectance spectrum is featureless, flat between 0.95 µm and 1.1 µm, and red from 1.1 µm to 1.65 µm. The outer parts of the disk are also asymmetric with a tilt between the two sides compatible with a disk made of forward-scattering particles and seen not perfectly edge-on, suggesting an inclination of <84°. Conclusions. The presence of an undetected planetary-mass companion on an inclined orbit with respect to the disk could explain the warp. The misalignment of the inner parts of the disk with respect to the outer disk suggests a warp that has not yet propagated to the outer parts of the disk, favouring the scenario of an inner perturber as the origin of the warp.

The β Pictoris system: Setting constraints on the planet and the disk structures at mid-IR wavelengths with NEAR

Context. β Pictoris is a young nearby system hosting a well-resolved edge-on debris disk, along with at least two exoplanets. It offers key opportunities for carrying out detailed studies of the evolution of young planetary systems and their shaping soon after the end of the planetary formation phase. Aims. We analyzed high-contrast coronagraphic images of this system, obtained in the mid-infrared, taking advantage of the NEAR experiment using the VLT/VISIR instrument, which provides access to adaptive optics, as well as phase coronagraphy. The goal of our analysis is to investigate both the detection of the planet β Pictoris b and of the disk features at mid-IR wavelengths. In addition, by combining several epochs of observation, we expect to constrain the position of the known clumps and improve our knowledge on the dynamics of the disk. Methods. We observed the β Pictoris system over two nights in December 2019 in the 10–12.5 µm coronagraphic filter. To evaluate the planet b flux contribution, we extracted the photometry at the expected position of the planet and compared it to the flux published in the literature. In addition, we used previous data from T-ReCS and VISIR in the mid-IR, updating the star's distance, to study the evolution of the position of the southwest clump that was initially observed in the planetary disk back in 2003. Results. While we did not detect the planet b, we were able to put constraints on the presence of circumplanetary material, ruling out the equivalent of a Saturn-like planetary ring around the planet. The disk presents several noticeable structures, including the known southwest clump. Using a 16-yr baseline, sampled with five epochs of observations, we were able to examine the evolution of the clump. We found that the clump orbits in a Keplerian motion with a semi-major axis of 56.1−0.3+0.4 au. In addition to the known clump, the images clearly show the presence of a second clump on the northeast side of the disk as well as possibly fainter and closer structures that are yet to be confirmed. Furthermore, we found correlations between the CO clumps detected with ALMA and the northeastern and southwestern clumps in the mid-IR images. Conclusions. If the circumplanetary material were located at the Roche radius, the maximum amount of dust determined from the flux upper limit around β Pictoris b would correspond to the mass of an asteroid of 5 km in diameter. Finally, the Keplerian motion of the southwestern clump is possibly indicative of a yet-to-be detected planet or signals the presence of a vortex.

Radial velocity survey for planets around young stars (RVSPY)

Aims. We aim to detect planetary companions to young stars with debris disks via the radial velocity method. Methods. We observed HD 114082 during April 2018–August 2022 as one of the targets of our RVSPY program (Radial Velocity Survey for Planets around Young stars). We use the FEROS spectrograph, mounted to the MPG/ESO 2.2 m telescope in Chile, to obtain high signal-to-noise spectra and time series of precise radial velocities (RVs). Additionally, we analyze archival HARPS spectra and TESS photometric data. We use the CERES, CERES++ and SERVAL pipelines to derive RVs and activity indicators and ExoStriker for the independent and combined analysis of the RVs and TESS photometry. Results. We report the discovery of a warm super-Jovian companion around HD 114082 based on a 109.8±0.4 day signal in the combined RV data from FEROS and HARPS, and on one transit event in the TESS photometry. The best-fit model indicates a 8.0±1.0 MJup companion with a radius of 1.00±0.03 RJup in an orbit with a semi-major axis of 0.51±0.01 au and an eccentricity of 0.4±0.04. The companions orbit is in agreement with the known near edge-on debris disk located at ∼28 au. HD 114082 b is possibly the youngest (15±6 Myr), and one of only three young (< 100 Myr) giant planetary companions for which both their mass and radius have been determined observationally. It is probably the first properly model-constraining giant planet that allows distinguishing between hot and cold-start models. It is significantly more compatible with the cold-start model.

A Multiwavelength Study of the Highly Asymmetrical Debris Disk around HD 111520

We observed the nearly edge-on debris disk system HD 111520 at the HJ and K1 near-infrared (NIR) bands using both the spectral and polarization modes of the Gemini Planet Imager. With these new observations, we have performed an empirical analysis in order to better understand the disk morphology and its highly asymmetrical nature. We find that the disk features a large brightness and radial asymmetry, most prominent at shorter wavelengths. We also find that the radial location of the peak polarized intensity differs on either side of the star by 11 au, suggesting that the disk may be eccentric, although, such an eccentricity does not fully explain the large brightness and radial asymmetry observed. Observations of the disk halo with the Hubble Space Telescope also show the disk to be warped at larger separations, with a bifurcation feature in the northwest, further suggesting that there may be a planet in this system creating an asymmetrical disk structure. Measuring the disk color shows that the brighter extension is bluer compared to the dimmer extension, suggesting that the two sides have different dust grain properties. This finding, along with the large brightness asymmetry, are consistent with the hypothesis that a giant impact occurred between two large bodies in the northern extension of the disk, although confirming this based on NIR observations alone is not feasible. Follow-up imaging with the Atacama Large Millimeter/submillimeter Array to resolve the asymmetry in the dust mass distribution is essential in order to confirm this scenario.

Polarization from Aligned Dust Grains in the β Pic Debris Disk

We present 870 μm Atacama Large Millimeter/submillimeter Array polarization observations of thermal dust emission from the iconic, edge-on debris disk β Pic. While the spatially resolved map does not exhibit detectable polarized dust emission, we detect polarization at the ∼3σ level when averaging the emission across the entire disk. The corresponding polarization fraction is P frac = 0.51% ± 0.19%. The polarization position angle χ is aligned with the minor axis of the disk, as expected from models of dust grains aligned via radiative alignment torques (RAT) with respect to a toroidal magnetic field (B-RAT) or with respect to the anisotropy in the radiation field (k-RAT). When averaging the polarized emission across the outer versus inner thirds of the disk, we find that the polarization arises primarily from the SW third. We perform synthetic observations assuming grain alignment via both k-RAT and B-RAT. Both models produce polarization fractions close to our observed value when the emission is averaged across the entire disk. When we average the models in the inner versus outer thirds of the disk, we find that k-RAT is the likely mechanism producing the polarized emission in β Pic. A comparison of timescales relevant to grain alignment also yields the same conclusion. For dust grains with realistic aspect ratios (i.e., s > 1.1), our models imply low grain-alignment efficiencies.

A Radiatively Driven Wind from the η Tel Debris Disk

Abstract We present far- and near-ultraviolet absorption spectroscopy of the ∼23 Myr edge-on debris disk surrounding the A0V star η Telescopii, obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph. We detect absorption lines from C i, C ii, O i, Mg ii, Al ii, Si ii, S ii, Mn ii, Fe ii, and marginally N i. The lines show two clear absorption components at −22.7 ± 0.5 km s−1 and −17.8 ± 0.7 km s−1, which we attribute to circumstellar (CS) and interstellar gas, respectively. CO absorption is not detected, and we find no evidence for star-grazing exocomets. The CS absorption components are blueshifted by −16.9 ± 2.6 km s−1 in the star’s reference frame, indicating that they are outflowing in a radiatively driven disk wind. We find that the C/Fe ratio in the η Tel CS gas is significantly higher than the solar ratio, as is the case in the β Pic and 49 Cet debris disks. Unlike those disks, however, the measured C/O ratio in the η Tel CS gas is consistent with the solar value. Our analysis shows that because η Tel is an earlier type star than β Pic and 49 Cet, with more substantial radiation pressure at the dominant C ii transitions, this species cannot bind the CS gas disk to the star as it does for β Pic and 49 Cet, resulting in the disk wind.

The far reaches of the β Pictoris debris disk

The nearby young star β Pictoris hosts a rich and complex planetary system, with at least two giant planets and a nearly edge-on debris disk that contains several dynamical subpopulations of planetesimals. While the inner ranges of the debris disk have been studied extensively, less information is known about the outer, fainter parts of the disk. Here we present an analysis of archival FORS V -band imaging data from 2003–2004, which have previously not been explored scientifically because the halo substructure of the bright stellar point spread function is complex. Through a high-contrast scheme based on angular differential imaging, with a forward-modelling approach to mitigate self-subtraction, we produced the deepest imaging yet for the outer range of the β Pic disk, and extracted its morphological characteristics. A brightness asymmetry between the two arms of the edge-on disk, which was previously noted in the inner disk, is even more pronounced at larger angular separations, reaching a factor ~10 around 1000 AU. Approaching 2000 AU, the brighter arm is visible at a surface brightness of 27–28 mag arcsec−2. Much like for the brightness asymmetry, a tilt angle asymmetry exists between the two arms that becomes increasingly extreme at large separations. The outer tilt angle of 7.2 deg can only be explained if the outer disk is farther from an edge-on inclination than the inner disk, or if its dust has a stronger scattering anisotropy, or (most likely) both. The strong asymmetries imply the presence of a highly eccentric kinematic disk component, which may have been caused by a disruptive event thought to have taken place at a closer-in location in the disk.

A planet within the debris disk around the pre-main-sequence star AU Microscopii.

AU Microscopii (AU Mic) is the second closest pre-main-sequence star, at a distance of 9.79parsecs and with an age of 22million years1. AU Mic possesses a relatively rare2 and spatially resolved3 edge-on debris disk extending from about 35 to 210 astronomical units from the star4, and with clumps exhibiting non-Keplerian motion5-7. Detection of newly formed planets around such a star is challenged by the presence of spots, plage, flares and other manifestations of magnetic 'activity' on the star8,9. Here we report observations of a planet transiting AU Mic. The transiting planet, AU Mic b, has an orbital period of 8.46days, an orbital distance of 0.07astronomical units, a radius of 0.4Jupiter radii, and a mass of less than 0.18 Jupiter masses at 3σ confidence. Our observations of a planet co-existing with a debris disk offer the opportunity to test the predictions of current models of planet formation and evolution.

The Gemini Planet Imager View of the HD 32297 Debris Disk

Abstract We present new H-band scattered light images of the HD 32297 edge-on debris disk obtained with the Gemini Planet Imager. The disk is detected in total and polarized intensity down to a projected angular separation of 0.″15, or 20 au. On the other hand, the large-scale swept-back halo remains undetected, likely a consequence of its markedly blue color relative to the parent body belt. We analyze the curvature of the disk spine and estimate a radius of ≈100 au for the parent body belt, smaller than past scattered light studies but consistent with thermal emission maps of the system. We employ three different flux-preserving post-processing methods to suppress the residual starlight and evaluate the surface brightness and polarization profile along the disk spine. Unlike past studies of the system, our high-fidelity images reveal the disk to be highly symmetric and devoid of morphological and surface brightness perturbations. We find the dust scattering properties of the system to be consistent with those observed in other debris disks, with the exception of HR 4796. Finally, we find no direct evidence for the presence of a planetary-mass object in the system.

High-fidelity Imaging of the Inner AU Mic Debris Disk: Evidence of Differential Wind Sculpting?

Abstract We present new high-fidelity optical coronagraphic imagery of the inner ∼50 au of AU Mic’s edge-on debris disk using the BAR5 occulter of the Hubble Space Telescope Imaging Spectrograph (HST/STIS) obtained on 2018 July 26–27. This new imagery reveals that “feature A,” residing at a projected stellocentric separation of 14.2 au on the southeast side of the disk, exhibits an apparent “loop-like” morphology at the time of our observations. The loop has a projected width of 1.5 au and rises 2.3 au above the disk midplane. We also explored Transiting Exoplanet Survey Satellite photometric observations of AU Mic that are consistent with evidence of two starspot complexes in the system. The likely co-alignment of the stellar and disk rotational axes breaks degeneracies in detailed spot modeling, indicating that AU Mic’s projected magnetic field axis is offset from its rotational axis. We speculate that small grains in AU Mic’s disk could be sculpted by a time-dependent wind that is influenced by this offset magnetic field axis, analogous to co-rotating solar interaction regions that sculpt and influence the inner and outer regions of our own Heliosphere. Alternatively, if the observed spot modulation is indicative of a significant misalignment of the stellar and disk rotational axes, we suggest that the disk could still be sculpted by the differential equatorial versus polar wind that it sees with every stellar rotation.

The HIP 79977 debris disk in polarized light

Context.Debris disks are observed around 10 to 20% of FGK main-sequence stars as infrared excess emission. They are important signposts for the presence of colliding planetesimals and therefore provide important information about the evolution of planetary systems. Direct imaging of such disks reveals their geometric structure and constrains their dust-particle properties.Aims.We present observations of the known edge-on debris disk around HIP 79977 (HD 146897) taken with the ZIMPOL differential polarimeter of the SPHERE instrument. We measure the observed polarization signal and investigate the diagnostic potential of such data with model simulations.Methods.SPHERE-ZIMPOL polarimetric data of the 15 Myr-old F star HIP 79977 (Upper Sco, 123 pc) were taken in the Very Broad Band (VBB) filter (λc= 735 nm, Δλ= 290 nm) with a spatial resolution of about 25 mas. Imaging polarimetry efficiently suppresses the residual speckle noise from the AO system and provides a differential signal with relatively small systematic measuring uncertainties. We measure the polarization flux along and perpendicular to the disk spine of the highly inclined disk for projected separations between 0.2′′ (25 AU) and 1.6′′ (200 AU). We perform model calculations for the polarized flux of an optically thin debris disk which are used to determine or constrain the disk parameters of HIP 79977.Results.We measure a polarized flux contrast ratio for the disk of (Fpol)disk/F∗= (5.5 ± 0.9) × 10-4in the VBB filter. The surface brightness of the polarized flux reaches a maximum of SBmax= 16.2 mag arcsec-2at a separation of 0.2′′–0.5′′ along the disk spine with a maximum surface brightness contrast of 7.64 mag arcsec-2. The polarized flux has a minimum near the star <0.2′′ because no or only little polarization is produced by forward or backward scattering in the disk section lying in front of or behind the star. The width of the disk perpendicular to the spine shows a systematic increase in FWHM from 0.1′′ (12 AU) to 0.3′′−0.5′′, when going from a separation of 0.2′′ to >1′′. This can be explained by a radial blow-out of small grains. The data are modelled as a circular dust belt with a well defined disk inclinationi= 85( ± 1.5)° and a radius betweenr0= 60 and 90 AU. The radial density dependence is described by (r/r0)αwith a steep (positive) power law indexα= 5 insider0and a more shallow (negative) indexα= −2.5 outsider0. The scattering asymmetry factor lies betweeng= 0.2 and 0.6 (forward scattering) adopting a scattering-angle dependence for the fractional polarization such as that for Rayleigh scattering.Conclusions.Polarimetric imaging with SPHERE-ZIMPOL of the edge-on debris disk around HIP 79977 provides accurate profiles for the polarized flux. Our data are qualitatively very similar to the case of AU Mic and they confirm that edge-on debris disks have a polarization minimum at a position near the star and a maximum near the projected separation of the main debris belt. The comparison of the polarized flux contrast ratio (Fpol)disk/F∗with the fractional infrared excess provides strong constraints on the scattering albedo of the dust.

Expelled grains from an unseen parent body around AU Microscopii

Context. Recent observations of the edge-on debris disk of AU Mic have revealed asymmetric, fast outward-moving arch-like structures above the disk midplane. Although asymmetries are frequent in debris disks, no model can readily explain the characteristics of these features. Aims. We present a model aiming to reproduce the dynamics of these structures, more specifically their high projected speeds and their apparent position. We test the hypothesis of dust emitted by a point source and then expelled from the system by the strong stellar wind of this young M-type star. In this model we make the assumption that the dust grains follow the same dynamics as the structures, i.e., they are not local density enhancements. Methods. We perform numerical simulations of test particle trajectories to explore the available parameter space, in particular the radial location R0 of the dust producing parent body and the size of the dust grains as parameterized by the value of β (ratio of stellar wind and radiation pressure forces over gravitation). We consider the cases of a static and of an orbiting parent body. Results. We find that for all considered scenarios (static or moving parent body), there is always a set of (R0,β) parameters able to fit the observed features. The common characteristics of these solutions is that they all require a high value of β, of around 6. This means that the star is probably very active, and the grains composing the structures are submicronic in order for observable grains to reach such high β values. We find that the location of the hypothetical parent body is closer in than the planetesimal belt, around 8 ± 2 au (orbiting case) or 28 ± 7 au (static case). A nearly periodic process of dust emission appears, of 2 yr in the orbiting scenarios and 7 yr in the static case. Conclusions. We show that the scenario of sequential dust releases by an unseen point-source parent body is able to explain the radial behavior of the observed structures. We predict the evolution of the structures to help future observations discriminate between the different parent body configurations that have been considered. In the orbiting parent body scenario, we expect new structures to appear on the northwest side of the disk in the coming years.

Stellar Winds and Dust Avalanches in the AU Mic Debris Disk

Abstract We explain the fast-moving, ripple-like features in the edge-on debris disk orbiting the young M dwarf AU Mic. The bright features are clouds of submicron dust repelled by the host star’s wind. The clouds are produced by avalanches: radial outflows of dust that gain exponentially more mass as they shatter background disk particles in collisional chain reactions. The avalanches are triggered from a region a few au across—the “avalanche zone”—located on AU Mic’s primary “birth” ring at a true distance of ∼35 au from the star but at a projected distance more than a factor of 10 smaller: the avalanche zone sits directly along the line of sight to the star, on the side of the ring nearest Earth, launching clouds that disk rotation sends wholly to the southeast, as observed. The avalanche zone marks where the primary ring intersects a secondary ring of debris left by the catastrophic disruption of a progenitor up to Varuna in size, less than tens of thousands of years ago. Only where the rings intersect are particle collisions sufficiently violent to spawn the submicron dust needed to seed the avalanches. We show that this picture works quantitatively, reproducing the masses, sizes, and velocities of the observed escaping clouds. The Lorentz force exerted by the wind’s magnetic field, whose polarity reverses periodically according to the stellar magnetic cycle, promises to explain the observed vertical undulations. The timescale between avalanches, about 10 yr, might be set by time variability of the wind mass loss rate or, more speculatively, by some self-regulating limit cycle.

Cometary grains in the HD 32297 debris disk

HD 32297 is a young A-type star with a bright edge-on debris disk. The dust thermal emission spectral energy distribution and scattered starlight spectrum are simultaneously modeled in terms of porous cometary grains. Our modeling suggests that, similar to the solar system, the debris disk around HD 32297 may have an inner warm ring and an outer cold disk which are seen in other young debris disks as well.

DIRECT IMAGING OF AN ASYMMETRIC DEBRIS DISK IN THE HD 106906 PLANETARY SYSTEM

Author(s): Kalas, PG; Rajan, A; Wang, JJ; Millar-Blanchaer, MA; Duchene, G; Chen, C; Fitzgerald, MP; Dong, R; Graham, JR; Patience, J; Macintosh, B; Murray-Clay, R; Matthews, B; Rameau, J; Marois, C; Chilcote, J; Rosa, RJD; Doyon, R; Draper, ZH; Lawler, S; Ammons, SM; Arriaga, P; Bulger, J; Cotten, T; Follette, KB; Goodsell, S; Greenbaum, A; Hibon, P; Hinkley, S; Hung, LW; Ingraham, P; Konapacky, Q; Lafreniere, D; Larkin, JE; Long, D; Maire, J; Marchis, F; Metchev, S; Morzinski, KM; Nielsen, EL; Oppenheimer, R; Perrin, MD; Pueyo, L; Rantakyro, FT; Ruffio, JB; Saddlemyer, L; Savransky, D; Schneider, AC; Sivaramakrishnan, A; Soummer, R; Song, I; Thomas, S; Vasisht, G; Ward-Duong, K; Wiktorowicz, SJ; Wolff, SG | Abstract: We present the first scattered light detections of the HD 106906 debris disk using the Gemini/Gemini Planet Imager in the infrared and Hubble Space Telescope (HST)/Advanced Camera for Surveys in the optical. HD 106906 is a 13 Myr old F5V star in the Sco-Cen association, with a previously detected planet-mass candidate HD 106906b projected 650 AU from the host star. Our observations reveal a near edge-on debris disk that has a central cleared region with radius ∼50 AU, and an outer extent g500 AU. The HST data show that the outer regions are highly asymmetric, resembling the needle morphology seen for the HD 15115 debris disk. The planet candidate is oriented ∼21° away from the position angle of the primary's debris disk, strongly suggesting non-coplanarity with the system. We hypothesize that HD 106906b could be dynamically involved in the perturbation of the primary's disk, and investigate whether or not there is evidence for a circumplanetary dust disk or cloud that is either primordial or captured from the primary. We show that both the existing optical properties and near-infrared colors of HD 106906b are weakly consistent with this possibility, motivating future work to test for the observational signatures of dust surrounding the planet.

DISCOVERY OF AN EDGE-ON DEBRIS DISK WITH A DUST RING AND AN OUTER DISK WING-TILT ASYMMETRY

Using VLT/SPHERE near-infrared dual-band imaging and integral field spectroscopy we discovered an edge-on debris disk around the 17\,Myr old A-type member of the Scorpius-Centaurus OB association HD 110058. The edge-on disk can be traced to about 0.6" or 65 AU projected separation. In its northern and southern wings, the disk shows at all wavelengths two prominent, bright and symmetrically placed knots at 0.3" or 32 AU from the star. We interpret these knots as a ring of planetesimals whose collisions may produce most of the dust observed in the disk. We find no evidence for a bow in the disk, but we identify a pair of symmetric, hook-like features in both wings. Based on similar features in the Beta Pictoris disk we propose that this wing-tilt asymmetry traces either an outer planetesimal belt that is inclined with respect to the disk midplane or radiation-pressure-driven dust blown out from a yet unseen, inner belt which is inclined with respect to the disk midplane. The misaligned inner or outer disk may be a result of interaction with a yet unseen planet. Overall, the disk geometry resembles the nearby disk around Beta Pictoris, albeit seen at smaller radial scales.

Fast-moving features in the debris disk around AU Microscopii.

In the 1980s, excess infrared emission was discovered around main-sequence stars; subsequent direct-imaging observations revealed orbiting disks of cold dust to be the source. These 'debris disks' were thought to be by-products of planet formation because they often exhibited morphological and brightness asymmetries that may result from gravitational perturbation by planets. This was proved to be true for the β Pictoris system, in which the known planet generates an observable warp in the disk. The nearby, young, unusually active late-type star AU Microscopii hosts a well-studied edge-on debris disk; earlier observations in the visible and near-infrared found asymmetric localized structures in the form of intensity variations along the midplane of the disk beyond a distance of 20 astronomical units. Here we report high-contrast imaging that reveals a series of five large-scale features in the southeast side of the disk, at projected separations of 10-60 astronomical units, persisting over intervals of 1-4 years. All these features appear to move away from the star at projected speeds of 4-10 kilometres per second, suggesting highly eccentric or unbound trajectories if they are associated with physical entities. The origin, localization, morphology and rapid evolution of these features are difficult to reconcile with current theories.

Near-infrared image of the debris disk around HD 15115

We present a Subaru/Infrared Camera and Spectrograph H-band image of the edge-on debris disk around the F2V star HD 15115. We detected the debris disk, which has a bow shape and an asymmetric surface brightness, at a projected separation of 1′′–3′′ (∼ 50–150 au). The disk surface brightness is ∼ 0.5–1.5 mag brighter on the western side than on the eastern side. We use an inclined annulus disk model to probe the disk geometry. The model fitting suggests that the disk has an inner hole with a radius of 86 au and an eccentricity of 0.06. The disk model also indicates that the amount of dust on the western side is 2.2 times larger than that on the eastern side. A several Jupiter-mass planet may exist at ≳ 45 au and capture grains at the Lagrangian points to open the eccentric gap. This scenario can explain both the eccentric gap and the difference in the amount of dust. In case of the stellar age of several 100 Myr, a dramatic planetesimal collision possibly causes the dust to increase in the western side. Interstellar medium interaction is also considered as a possible explanation of the asymmetric surface brightness, however, it hardly affects large grains in the vicinity of the inner hole.

RESOLVED MILLIMETER EMISSION FROM THE HD 15115 DEBRIS DISK

We have used the Submillimeter Array (SMA) to make 1.3 millimeter observations of the debris disk surrounding HD 15115, an F-type star with a putative membership in the beta Pictoris moving group. This nearly edge-on debris disk shows an extreme asymmetry in optical scattered light, with an extent almost two times larger to the west of the star than to the east (originally dubbed the "Blue Needle"). The SMA observations reveal resolved emission that we model as a circumstellar belt of thermal dust emission. This belt extends to a radius of ~110 AU, coincident with the break in the scattered light profile convincingly seen on the western side of the disk. This outer edge location is consistent with the presence of an underlying population of dust-producing planetesimals undergoing a collisional cascade, as hypothesized in "birth ring" theory. In addition, the millimeter emission shows a ~3 sigma feature aligned with the asymmetric western extension of the scattered light disk. If this millimeter extension is real, then mechanisms for asymmetry that affect only small grains, such as interactions with interstellar gas, are disfavored. This tentative feature might be explained by secular perturbations to grain orbits introduced by neutral gas drag, as previously invoked to explain asymmetric morphologies of other, similar debris disks.

Is the HD 15115 inner disk really asymmetrical?

Context. Debris disks are intrinsically connected to the planetary system's formation and evolution. The development of high-contrast imaging techniques in the past 20 years is now allowing the detection of faint material around bright stars with high angular resolution, hence opening an avenue to study in detail the structures of circumstellar disks and their relation to planetary formation. Aims. The purpose of this paper is to revisit the morphology of the almost edge-on debris disk around HD 15115. Methods. We analyzed data from the Gemini science archive obtained in 2009 and 2011 with the Near-Infrared Coronagraphic Imager instrument in the H and Ks bands using coronagraphy and angular differential imaging tech- niques. Results. We resolved the disk in both the H and Ks bands. We confirmed the position angles inferred by previous authors, as well as the brightness asymmetry, which is the origin of the object's nickname, the blue needle. We were able to detect the bow-like shape of the disk suspected from other observations. However, these new NICI images suggest the presence of a highly inclined ring-like disk of which we see the brighter side and the ansae located at 90 AU symmetrically about the star. The inner part is likely depleted of dust. The fainter side of the disk is suspected but not firmly detected, which also indicates a large anisotropic scattering factor. Conclusions. The morphological symmetry of the disk contrasts with the obvious brightness asymmetry. This asymmetry may be explained by the coexistence of several types of grains in this disk and/or variable dust density. Interaction with the interstellar medium was invoked by previous authors as a possible explanation but other mechanisms may account for the brightness asymmetry, for instance a recent collision in the disk