Archive for protoplanetary disks

Complex structure at sub-arcsecond resolution in the gas-rich debris disk HD 141569A

Posted in astronomy with tags , , , on June 6, 2016 by Tim Kendall

28396PerrotHD 141569A is a transition disk that is still gas-rich but contains significant amounts of dust. In a new paper using the SPHERE coronagraph on the ESO VLT, Perrot et al. reveal that the inner 100 astronomical units (au) contains a series of concentric ringlets at physical separation of 47 au, 64 au, and 93 au. The paper is “Discovery of concentric broken rings at sub-arcsec separations in the HD 141569A gas-rich, debris disk with VLT/SPHERE”, 2016, A&A 590, L7. From the abstract:

Transition disks correspond to a short stage between the young protoplanetary phase and older debris phase. Along this evolutionary sequence, the gas component disappears leaving room for a dust-dominated environment where already-formed planets signpost their gravitational perturbations. We endeavor to study the very inner region of the well-known and complex debris, but still gas-rich disk, around HD 141569A using the exquisite high-contrast capability of SPHERE at the VLT. Recent near-infrared (IR) images suggest a relatively depleted cavity within ~200 au, while former mid-IR data indicate the presence of dust at separations shorter than ~100 au. We obtained multi-wavelength images in the near-IR in J, H2, H3 and Ks-bands with the IRDIS camera and a 0.95–1.35 μm spectral data cube with the IFS. Data were acquired in pupil-tracking mode, thus allowing for angular differential imaging. We discovered several new structures inside 1′′, of which the most prominent is a bright ring with sharp edges (semi-major axis: 0.4′′) featuring a strong north-south brightness asymmetry. Other faint structures are also detected from 0.4′′ to 1′′ in the form of concentric ringlets and at least one spiral arm. Finally, the VISIR data at 8.6 μm suggests the presence of an additional dust population closer in. Besides, we do not detect companions more massive than 1–3 mass of Jupiter. The performance of SPHERE allows us to resolve the extended dust component, which was previously detected at thermal and visible wavelengths, into very complex patterns with strong asymmetries; the nature of these asymmetries remains to be understood. Scenarios involving shepherding by planets or dust-gas interactions will have to be tested against these observations.

Young star TW Hydrae: the closest protoplanetary disk

Posted in astronomy with tags , , on July 24, 2013 by Tim Kendall

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Images (see links to credits): NASA/HST (left) and ESO/ALMA (right) images of the pole-on disk. A planet with between 6 and 28 Earth masses may be forming in the gap seen in the disk in the Hubble image, around 80 AU from TW Hydrae itself (the central hole in this image is a result of coronagraphic imaging, where the light from the central star is blocked out within the instrument itself). TW Hydrae is a K8 dwarf of 0.55 solar masses, rather more sun-like than the dim red M dwarfs, has an age less than 10 Myr, and lies 54 pc distant. The orbit of Neptune is just a little larger than the central hole in the ALMA image, whose radius is ~ 30 AU. This hole is real, and the inner edge marks the “snow-line” beyond which molecules, CO in this case, condense into solid ices. For a given ice – carbon monoxide, methane, water, carbon dioxide – the distance from the star where this occurs will depend on the temperature at which the bulk material becomes solid. In this case a rare radical, fragile diazenylium, is easily observed and traces CO:

Instead of looking for the snow — as it cannot be observed directly — they searched for a molecule known as diazenylium (N2H+), which shines brightly in the millimeter portion of the spectrum, and so is a perfect target for a telescope such as ALMA. The fragile molecule is easily destroyed in the presence of carbon monoxide gas, so would only appear in detectable amounts in regions where carbon monoxide had become snow and could no longer destroy it. In essence, the key to finding carbon monoxide snow lies in finding diazenylium.

TW Hydrae is the nearest accreting T Tauri star to the Sun. The T Tauri phase in the evolution of a protostar roughly corresponds to the appearance of the star in visible light, such that it can be placed on the Hertzsprung-Russell diagram. At earlier epochs, the protostar is embedded within the material from which it is forming, and it can only be observed in the infrared. Astronomers classify protostars in classes, Class 0 being deeply embedded and the T Tauri phase roughly coincident with Class III. T Tauri stars show emission lines in their spectra, which indicate the presence of a disk. At earlier stages, and depending on the changing amount of obscuration by circumstellar material as the star evolves (veiling) and the optical depth of this material, absorption lines from the photosphere itself may or may not be observable. If they are, the star may be classified by spectral type on the HR diagram, allowing comparison of its temperature and radius with theoretical models at different ages.

As I have noted before on this blog, recent Herschel observations of TW Hydrae, in the far-infrared, have been able to confirm that the disk is massive, indicating 50 Jupiter masses of potential planet-forming material. Even more recent observations at these wavelengths have revealed the debris disks around a sizeable fraction of nearby FGK stars. These disks are the remnant material left behind after planetary system formation, consist of cold dust which emits in the far IR, as well as grains and likely larger bodies, analogous to the Kuiper belt in the solar system.

Update: Inner system dust in 5 – 20 Myr young A stars