Context. The nearby and young M star AU Mic is surrounded by a debris disk in which we previously identified a series of large-scale arch-like structures that have never been seen before in any other ...debris disk and that move outward at high velocities. Aims. We initiated a monitoring program with the following objectives: (1) track the location of the structures and better constrain their projected speeds, (2) search for new features emerging closer in, and ultimately (3) understand the mechanism responsible for the motion and production of the disk features. Methods. AU Mic was observed at 11 different epochs between August 2014 and October 2017 with the IR camera and spectrograph of SPHERE. These high-contrast imaging data were processed with a variety of angular, spectral, and polarimetric differential imaging techniques to reveal the faintest structures in the disk. We measured the projected separations of the features in a systematic way for all epochs. We also applied the very same measurements to older observations from the Hubble Space Telescope (HST) with the visible cameras STIS and ACS. Results. The main outcomes of this work are (1) the recovery of the five southeastern broad arch-like structures we identified in our first study, and confirmation of their fast motion (projected speed in the range 4–12 km s−1); (2) the confirmation that the very first structures observed in 2004 with ACS are indeed connected to those observed later with STIS and now SPHERE; (3) the discovery of two new very compact structures at the northwest side of the disk (at 0.40′′ and 0.55′′ in May 2015) that move to the southeast at low speed; and (4) the identification of a new arch-like structure that might be emerging at the southeast side at about 0.4′′ from the star (as of May 2016). Conclusions. Although the exquisite sensitivity of SPHERE allows one to follow the evolution not only of the projected separation, but also of the specific morphology of each individual feature, it remains difficult to distinguish between possible dynamical scenarios that may explain the observations. Understanding the exact origin of these features, the way they are generated, and their evolution over time is certainly a significant challenge in the context of planetary system formation around M stars.
Context. Scattered light images of circumstellar disks play an important role in characterizing the planet forming environments around young stars. The characteristic size of the scattering dust ...grains can be estimated from the observed brightness asymmetry between the near and far side of the disk, for example using standard Mie theory. Such models, however, often overpredict the brightness of the disk by one or two orders of magnitude, and have difficulty explaining very red disk colors. Aims. We aim to develop a dust model that simultaneously explains the observed disk surface brightness, colors, and asymmetry in scattered light, focusing on constraining grain sizes. Methods. We use the 2D radiative transfer code MCMax with anisotropic scattering to explore the effects of grain size on synthetic scattered light images of circumstellar disks. We compare the results with surface brightness profiles of the protoplanetary disk HD 100546 in scattered light at wavelengths from 0.4 to 2.2 microns. Results. We find that extreme forward scattering by micron-sized particles lowers the effective dust albedo and creates a faint, red disk that appears only slightly forward scattering. For the outer (≳100 AU) disk of HD 100546 we derive a minimum grain size of 2.5 microns, likely present in the form of aggregates. Intermediate-sized grains are too bright, whereas smaller grains are faint and scatter more isotropically, but also produce disk colors that are too blue. Conclusions. Observed surface brightness asymmetries alone are not sufficient to constrain the grain size in circumstellar disks. Additional information, such as the brightness and colors of the disk, are needed to provide additional constraints.
ABSTRACT
We present Spitzer observations at 3.6 and 4.5 µm and a near-infrared IRTF SpeX spectrum of the irradiated brown dwarf NLTT5306B. We determine that the brown dwarf has a spectral type of L5 ...and is likely inflated, despite the low effective temperature of the white dwarf primary star. We calculate brightness temperatures in the Spitzer wavebands for both the model radius, and Roche Lobe radius of the brown dwarf, and conclude that there is very little day–night side temperature difference. We discuss various mechanisms by which NLTT5306B may be inflated, and determine that while low-mass brown dwarfs (M < 35 MJup) are easily inflated by irradiation from their host star, very few higher mass brown dwarfs are inflated. The higher mass brown dwarfs that are inflated may be inflated by magnetic interactions or may have thicker clouds.
We present the first scattered-light images of two debris disks around the F8 star HD 104860 and the F0V star HD 192758, respectively ∼45 and ∼67 pc away. We detected these systems in the F110W and ...F160W filters through our reanalysis of archival Hubble Space Telescope (HST) NICMOS data with modern starlight-subtraction techniques. Our image of HD 104860 confirms the morphology previously observed by Herschel in thermal emission with a well-defined ring at a radius of ∼114 au inclined by ∼58°. Although the outer edge profile is consistent with dynamical evolution models, the sharp inner edge suggests sculpting by unseen perturbers. Our images of HD 192758 reveal a disk at radius ∼95 au inclined by ∼59°, never resolved so far. These disks have low scattering albedos of 10% and 13%, respectively, inconsistent with water ice grain compositions. They are reminiscent of several other disks with similar inclination and scattering albedos: Fomalhaut, HD 92945, HD 202628, and HD 207129. They are also very distinct from brighter disks in the same inclination bin, which point to different compositions between these two populations. Varying scattering albedo values can be explained by different grain porosities, chemical compositions, or grain size distributions, which may indicate distinct formation mechanisms or dynamical processes at work in these systems. Finally, these faint disks with large infrared excesses may be representative of an underlying population of systems with low albedo values. Searches with more sensitive instruments on HST or on the James Webb Space Telescope and using state-of-the art starlight-subtraction methods may help discover more of such faint systems.
ABSTRACT
We present the discovery of only the third brown dwarf known to eclipse a non-accreting white dwarf. Gaia parallax information and multicolour photometry confirm that the white dwarf is cool ...(9950 ± 150 K) and has a low mass (0.45 ± 0.05 M⊙), and spectra and light curves suggest the brown dwarf has a mass of 0.067 ± 0.006 M⊙ (70MJup) and a spectral type of L5 ± 1. The kinematics of the system show that the binary is likely to be a member of the thick disc and therefore at least 5-Gyr old. The high-cadence light curves show that the brown dwarf is inflated, making it the first brown dwarf in an eclipsing white dwarf-brown dwarf binary to be so.
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 (STIS) ...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 northeast and southwest 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 less than 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.
The Eroding Disk of AU Mic Grady, C. A.; Wisniewski, J. P.; Schneider, G. ...
Astrophysical journal. Letters,
01/2020, Volume:
889, Issue:
1
Journal Article
Peer reviewed
Open access
We report Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph imaging of AU Mic's debris disk from 2017 and archival data. Outward motion of the features in the southeast arm continues. ...At least three features have reached or exceeded projected escape velocity in the past decade, yielding a combined feature mass-loss rate of ∼1.2 × 10−7 MEarth yr−1, or ∼1.2 × 10−13 M yr−1, ∼10% of AU Mic's stellar wind mass-loss rate, and similar to the ratio of coronal mass ejection mass loss to the stellar wind mass-loss rate. We confirm the 2018 finding of feature height changes for one feature (B/SE4), but the HST data are too sparse to compare (yet) with the stellar activity cycle. Detection of what appears to be a chain of features in a second system suggests that the disk of AU Mic is not unique, although a larger sample of disks is required to quantify how common the phenomenon is.
ABSTRACT We present new results on PHL 5038AB, a widely separated binary system composed of a white dwarf and a brown dwarf, refining the white and brown dwarf parameters and determining the binary ...separation to be $66^{+12}_{-24}$ au. New spectra of the white dwarf show calcium absorption lines suggesting that the hydrogen-rich atmosphere is weakly polluted, inferring the presence of planetesimals in the system, which we determine are in an S-type orbit around the white dwarf in orbits closer than 17–32 au. We do not detect any infrared excess that would indicate the presence of a disc, suggesting that all dust present either has been totally accreted or is optically thin. In this system, we suggest that the metal pollution in the white dwarf atmosphere can be directly attributed to the presence of the brown dwarf companion disrupting the orbits of planetesimals within the system.
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