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Register a new userThe circumbinary disk of HD 98800 B: Evidence for disk warping
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The quadruple young stellar system HD 98800 consists of two spectroscopic binary pairs with a circumbinary disk around the B component. Recent work by Boden and collaborators using infrared interferometry and radial velocity data resulted in a determination of the physical orbit for HD 98800 B. We use the resulting inclination of the binary and the measured extinction toward the B component stars to constrain the distribution of circumbinary material. Although a standard optically and geometrically thick disk model can reproduce the spectral energy distribution, it can not account for the observed extinction if the binary and the disk are co-planar. We next constructed a dynamical model to investigate the influence of the A component, which is not in the Ba-Bb orbital plane, on the B disk. We find that these interactions have a substantial impact on the inclination of the B circumbinary disk with respect to the Ba-Bb orbital plane. The resulting warp would be sufficient to place material into the line of sight and the non-coplanar disk orientation may also cause the upper layers of the disk to intersect the line of sight if the disk is geometrically thick. These simulations also support that the dynamics of the Ba-Bb orbit clear the inner region to a radius of ~3 AU. We then discuss whether the somewhat unusual properties of the HD 98800 B disk are consistent with material remnant from the star formation process or with more recent creation by collisions from larger bodies.
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We present sub-arcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0.8 (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 microns show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess upon which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 microns and longer, peaks at 25 microns, and has a best-fit black-body temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most-likely values of disk properties in the ranges considered are R_in = 5.0 +/- 2.5 AU, DeltaR = 13+/-8 AU, lambda_0 = 2(+4/-1.5) microns, gamma = 0+/-2.5, and sigma_total = 16+/-3 AU^2, where R_in is the inner radius, DeltaR is the radial extent of the disk, lambda_0 is the effective grain size, gamma is the radial power-law exponent of the optical depth, tau, and sigma_total is the total cross-section of the grains. The range of implied disk masses is 0.001--0.1 times that of the moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.
We present the mid-infrared spectrum, obtained with the Spitzer Infrared Spectrograph (IRS), of HD 98800, a quadruple star system located in the 10-Myr-old TW Hydrae association. It has a known mid-infrared excess that arises from a circumbinary disk around the B components of the system. The IRS spectrum confirms that the disk around HD 98800 B displays no excess emission below about 5.5 micron, implying an optically thick disk wall at 5.9 AU and an inner, cleared-out region; however, some optically thin dust, consisting mainly of 3-micron-sized silicate dust grains, orbits the binary in a ring between 1.5 and 2 AU. The peculiar structure and apparent lack of gas in the HD 98800 B disk suggests that this system is likely already at the debris disks stage, with a tidally truncated circumbinary disk of larger dust particles and an inner, second-generation dust ring, possibly held up by the resonances of a planet. The unusually large infrared excess can be explained by gravitational perturbations of the Aa+Ab pair puffing up the outer dust ring and causing frequent collisions among the larger particles.
We have found a photoevaporated disk in the Orion Nebula that includes a wide binary. HST/ACS observations of the proplyd 124-132 show two point-like sources separated by 0.15, or about 60 AU at the distance of Orion. The two sources have nearly identical I and z magnitudes. We analyze the brightest component, Source N, comparing the observed magnitudes with those predicted using a 1 Myr Baraffe/NEXTGEN isochrone with different accretion luminosities and extinctions. We find that a low mass (simeq 0.04 M_odot) brown dwarf ~1 Myr old with mass accretion rate logdot{M}simeq -10.3, typical for objects of this mass, and about 2 magnitudes of visual extinction provides the best fit to the data. This is the first observation of a circumbinary disk undergoing photoevaporation and, if confirmed by spectroscopic observations, the first direct detection of a wide substellar pair still accreting and enshrouded in its circumbinary disk.
We present ALMA observations of the 0.88 millimeter dust continuum, 13CO, and C18O J=3-2 line emission of the circumbinary disk HD142527 at a spatial resolution of about 0.25. This system is characterized by a large central cavity of roughly 120 AU in radius, and asymmetric dust and gas emission. By comparing the observations with theoretical models, we find that the azimuthal variations in gas and dust density reach a contrast of 54 for dust grains and 3.75 for CO molecules, with an extreme gas-to-dust ratio of 1.7 on the dust crescent. We point out that caution is required in interpreting continuum subtracted maps of the line emission as this process might result in removing a large fraction of the line emission. Radially, we find that both the gas and dust surface densities can be described by Gaussians, centered at the same disk radius, and with gas profiles wider than for the dust. These results strongly support a scenario in which millimeter dust grains are radially and azimuthally trapped toward the center of a gas pressure bump. Finally, our observations reveal a compact source of continuum and CO emission inside the dust depleted cavity at about 50 AU from the primary star. The kinematics of the CO emission from this region is different from that expected from material in Keplerian rotation around the binary system, and might instead trace a compact disk around a third companion. Higher angular resolution observations are required to investigate the nature of this source.
The gas dissipation from a protoplanetary disk is one of the key processes affecting planet formation, and it is widely accepted that it happens on timescales of a few million years for disks around single stars. Over the last years, several protoplanetary disks have been discovered in multiple star systems, and despite the complex environment in which they find themselves, some of them seem to be quite old, a situation that may favor planet formation. A clear example of this is the disk around HD 98800 B, a binary in a hierarchical quadruple stellar system, which at a $sim$10 Myr age seems to still be holding significant amounts of gas. Here we present a 1D+1D model to compute the vertical structure and gas evolution of circumbinary disks in hierarchical triple star systems considering different stellar and disk parameters. We show that tidal torques due to the inner binary together with the truncation of the disk due to the external companion strongly reduce the viscous accretion and expansion of the disk. Even allowing viscous accretion by tidal streams, disks in these kind of environments can survive for more than 10 Myr, depending on their properties, with photoevaporation being the main gas dissipation mechanism. We particularly apply our model to the circumbinary disk around HD 98800 B and confirm that its longevity, along with the current non-existence of a disk around the companion binary HD 98800 A, can be explained with our model and by this mechanism.
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