Statistical modelling of resolved debris discs

Es wurde eine Probe von 39 räumlich aufgelösten Trümmerschreiben mit der Absicht untersucht, Korrelationen zwischen den Zentralsternparametern und den Scheibenparametern zu finden. Eine Hürde war dabei die Entartung zwischen den Staubteilchengrößen und dem Ort des Staubs. Kleine Partikel in großer Entfernung lieferten gleiche Ergebnisse wie große Teilchen näher am Stern. Durch die Scheibenauflösung konnten die Radien mittels einer neuen Methode bestimmt und so die Entartung gebrochen werden. Die Modellierung der spektralen Energieverteilung war die Grundlage der Analyse, wobei 2 verschiedene Modelle genutzt wurden. Der Vergleich beider verdeutlichte, dass sie zu gleichen Korrelationen der Parameter führen, aber auch Unterschiede bei individuellen Scheiben sichtbar sind. Es wurde eine große Streuung der Scheibenradien über den gesamten Bereich der stellaren Leuchtkraft festgestellt. Daher scheint die entgültige Form der Scheibe nicht von temperaturabhängigen Prozessen abhängig zu sein. Der wahre Scheibenradius im Verhältnis zum Schwarzkörperradius zeigte einen Abfall mit zunehmender Leuchtkraft, was eine Schätzung des Radius von unaufgelösten Schreiben erlaubt. Die dominante Teilchengröße steigt leicht mit zunehmender Leuchtkraft an, jedoch ist der Anstieg auch mit einer konstanten Teilchengröße vereinbar. Erwartet wird aber ein steilerer Anstieg, bedingt durch wachsenden Strahlungsdruck. Daher ist ein Abfall der dominanten Teilchengröße in Ausstoßgrößen erkennbar, der hier auch die stärkste Korrelation darstellt. Es wurden mehrere Erklärungen für diesen Abfall untersucht, u.a. ein endliches Reservoir an Oberflächenenergie der kleinsten Staubteilchen und die Rolle der dynamischen Scheibenanregung. Erstere scheint nur von geringer Bedeutung zu sein. Letztere deutet darauf hin, dass die dynamische Anregung mit der Leuchtkraft zunimmt. Scheiben um leuchtkräftigere Sterne könnten daher massereichere Planeten/Planetesimale besitzen

Dust Spreading in Debris Discs: Do Small Grains Cling on to Their Birth Environment?

Debris discs are dusty belts of planetesimals around main-sequence stars, similar to the asteroid and Kuiper belts in our solar system. The planetesimals cannot be observed directly, yet they produce detectable dust in mutual collisions. Observing the dust, we can try to infer properties of invisible planetesimals. Here we address the question of what is the best way to measure the location of outer planetesimal belts that encompass extrasolar planetary systems. A standard method is using resolved images at mm-wavelengths, which reveal dust grains with sizes comparable to the observational wavelength. Smaller grains seen in the infrared (IR) are subject to several non-gravitational forces that drag them away from their birth rings, and so may not closely trace the parent bodies. In this study, we examine whether imaging of debris discs at shorter wavelengths might enable determining the spatial location of the exo-Kuiper belts with sufficient accuracy. We find that around M-type stars the dust best visible in the mid-IR is efficiently displaced inward from their birth location by stellar winds, causing the discs to look more compact in mid-IR images than they actually are. However, around earlier-type stars where the majority of debris discs is found, discs are still the brightest at the birth ring location in the mid-IR regime. Thus, sensitive IR facilities with good angular resolution, such as MIRI on JWST, will enable tracing exo-Kuiper belts in nearby debris disc systems.Comment: 16 page

Disk Radii and Grain Sizes in Herschel-Resolved Debris Disks

(Abridged) The radii of debris disks and the sizes of their dust grains are tracers of the formation mechanisms and physical processes operating in these systems. We use a sample of 34 debris disks spatially resolved in various Herschel programs to constrain them. While we modeled disks with both warm and cold components, we focus our analysis only on the cold outer disks, i.e. Kuiper-belt analogs. The disk radii derived from the resolved images reveal a large dispersion, but no significant trend with the stellar luminosity, which argues against ice lines as a dominant player in setting the debris disk sizes. Fixing the disk radii to those inferred from the resolved images, we model the spectral energy distributions to determine the dust temperatures and the grain size distributions. While the dust temperature systematically increases towards earlier spectral types, its ratio to the blackbody temperature at the disk radius decreases with the stellar luminosity. This is explained by an increase of typical grain sizes towards more luminous stars. The sizes are compared to the radiation pressure blowout limit $s_\text{blow}$ that is proportional to the stellar luminosity-to-mass ratio and thus also increases towards earlier spectral classes. The grain sizes in the disks of G- to A-stars are inferred to be several times $s_\text{blow}$ at all stellar luminosities, in agreement with collisional models of debris disks. The sizes, measured in the units of $s_\text{blow}$, appear to decrease with the luminosity, which may be suggestive of the disk's stirring level increasing towards earlier-type stars.Comment: accepted for publication in ApJ, 22 pages, 7 figure

Signs of late infall and possible planet formation around DR Tau using VLT/SPHERE and LBTI/LMIRCam

Context. Protoplanetary disks around young stars often contain substructures like rings, gaps, and spirals that could be caused by interactions between the disk and forming planets. Aims: We aim to study the young (1-3 Myr) star DR Tau in the near-infrared and characterize its disk, which was previously resolved through submillimeter interferometry with ALMA, and to search for possible substellar companions embedded into it. Methods: We observed DR Tau with VLT/SPHERE both in polarized light (H broad band) and total intensity (in Y, J, H, and K spectral bands). We also performed L' band observations with LBTI/LMIRCam on the Large Binocular Telescope (LBT). We applied differential imaging techniques to analyze both the polarized data, using dual beam polarization imaging, and the total intensity data, using angular and spectral differential imaging. Results: We found two previously undetected spirals extending north-east and south of the star, respectively. We further detected an arc-like structure north of the star. Finally a bright, compact and elongated structure was detected at a separation of 303 ± 10 mas and a position angle 21.2 ± 3.7 degrees, just at the root of the north-east spiral arm. Since this feature is visible both in polarized light and total intensity and has a blue spectrum, itis likely caused by stellar light scattered by dust. Conclusions: The two spiral arms are at different separations from the star, have very different pitch angles, and are separated by an apparent discontinuity, suggesting they might have a different origin. The very open southern spiral arm might be caused by infalling material from late encounters with cloudlets into the formation environment of the star itself. The compact feature could be caused by interaction with a planet in formation still embedded in its dust envelope and it could be responsible for launching the north-east spiral. We estimate a mass of the putative embedded object of the order of few MJup. Reduced images are also available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (ftp://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/658/A63 Based on observations made with European Southern Observatory (ESO) telescopes at Paranal Observatory in Chile, under programs ID 0102.C-0453(A) and 1104.C-0416(A). It also makes partial use of LBT/LMIRCam observations under program ID 74

New Millimeter CO Observations of the Gas-rich Debris Disks 49 Cet and HD 32297

Previous observations revealed the existence of CO gas at nearly protoplanetary level in several dust-rich debris disks around youn A-type stars. Here we used the Atacama Large Millimeter/submillimete Array (ALMA) 7 m Array to measure 13CO and C18 emission toward two debris disks, 49 Cet and HD 32297, and detecte similarly high CO content (>0.01 M ⊕). These high C masses imply a highly efficient shielding of CO molecules agains stellar and interstellar ultraviolet photons. Adapting a recen secondary gas disk model that considers both shielding by carbon atom and self-shielding of CO, we can explain the observed CO level in bot systems. Based on the derived gas densities we suggest that, in the H 32297 disk, dust and gas are coupled and the dynamics of small grains i affected by the gaseous component. For 49 Cet, the question of couplin remains undecided. We found that the main stellar and disk properties o 49 Cet and HD 32297 are very similar to those of previously identifie debris disks with high CO content. These objects constitute together th first known representatives of shielded debris disk

Characterizing the morphology of the debris disk around the low-mass star GSC 07396-00759

Context. Debris disks have commonly been studied around intermediate-mass stars. Their intense radiation fields are believed to efficiently remove the small dust grains that are constantly replenished by collisions. For lower-mass central objects, in particular M stars, the dust removal mechanism needs to be further investigated given the much weaker radiation field produced by these objects. Aims. We present new observations of the nearly edge-on disk around the pre-main-sequence M-type star GSC 07396-00759, taken with VLT/SPHERE IRDIS in dual-beam polarimetric imaging mode, with the aim to better understand the morphology of the disk, its dust properties, and the star-disk interaction via the stellar mass-loss rate.Methods. We model the polarimetric observations to characterize the location and properties of the dust grains using the Henyey-Greenstein approximation of the polarized phase function. We use the estimated phase function to evaluate the strength of the stellar winds.Results. We find that the polarized light observations are best described by an extended and highly inclined disk (i approximate to 84.3 degrees +/- 0.3) with a dust distribution centered at a radius r(0) approximate to 107 +/- 2 au. Our modeling suggests an anisotropic scattering factor g approximate to 0.6 to best reproduce the polarized phase function S-12. We also find that the phase function is reasonably well reproduced by small micron-sized dust grains with sizes s > 0.3 mu m. We discuss some of the caveats of the approach, mainly that our model probably does not fully recover the semimajor axis of the disk and that we cannot readily determine all dust properties due to a degeneracy between the grain size and the porosity.Conclusions. Even though the radius of the disk may be overestimated, our best-fit model not only reproduces the observations well but is also consistent with previous published data obtained in total intensity. Similarly to previous studies of debris disks, we suggest that using a given scattering theory might not be sufficient to fully explain key aspects, such as the shape of the phase function or the dust grain size. Taking into consideration the aforementioned caveats, we find that the average mass-loss rate of GSC 07396-00759 can be up to 500 times stronger than that of the Sun, supporting the idea that stellar winds from low-mass stars can evacuate small dust grains in an efficient way

MINDS. Abundant water and varying C/O across the disk of Sz 98 as seen by JWST/MIRI

MIRI/MRS on board the JWST allows us to probe the inner regions of protoplanetary disks. Here we examine the disk around the classical T Tauri star Sz 98, which has an unusually large dust disk in the millimetre with a compact core. We focus on the H$_2$O emission through both its ro-vibrational and pure rotational emission. Furthermore, we compare our chemical findings with those obtained for the outer disk from Atacama Large Millimeter/submillimeter Array (ALMA) observations. In order to model the molecular features in the spectrum, the continuum was subtracted and LTE slab models were fitted. The spectrum was divided into different wavelength regions corresponding to H$_2$O lines of different excitation conditions, and the slab model fits were performed individually per region. We confidently detect CO, H$_2$O, OH, CO$_2$, and HCN in the emitting layers. The isotopologue H$^{18}_2$O is not detected. Additionally, no other organics, including C$_2$H$_2$, are detected. This indicates that the C/O ratio could be substantially below unity, in contrast with the outer disk. The H$_2$O emission traces a large radial disk surface region, as evidenced by the gradually changing excitation temperatures and emitting radii. The OH and CO$_2$ emission are relatively weak. It is likely that H$_2$O is not significantly photodissociated; either due to self-shielding against the stellar irradiation, or UV-shielding from small dust particles. The relative emitting strength of the different identified molecular features point towards UV-shielding of H$_2$O in the inner disk of Sz 98, with a thin layer of OH on top. The majority of the organic molecules are either hidden below the dust continuum, or not present. In general, the inferred composition points to a sub-solar C/O ratio (<0.5) in the inner disk, in contrast with the larger than unity C/O ratio in the gas in the outer disk found with ALMA.Comment: Submitted to A&A on May 25 2023. 18 pages, 11 figure