ALBERT

Description: Context. EX Lup is a well-studied T Tauri star that represents the prototype of young eruptive stars known as EXors. They are characterized by repetitive outbursts that are due to enhanced accretion from the circumstellar disk onto the star. In this paper, we analyze new adaptive optics imaging and spectroscopic observations to study EX Lup and its circumstellar environment in near-infrared in its quiescent phase. Aims. We aim to provide a comprehensive understanding of the circumstellar environment around EX Lup in quiescence, building upon the vast store of data provided by the literature. Methods. We observed EX Lup in quiescence with the high contrast imager SPHERE/IRDIS in the dual-beam polarimetric imaging mode to resolve the circumstellar environment in near-infrared scattered light. We complemented the data with earlier SINFONI spectroscopy, which was also taken in quiescence. Results. We resolve, for the first time in scattered light, a compact feature around EX Lup azimuthally extending from ~280° to ~360° and radially extending from ~0.3′′ to ~0.55′′ in the plane of the disk. We explore two different scenarios for the detected emission. The first one accounts for the emission as coming from the brightened walls of the cavity excavated by the outflow whose presence was suggested by ALMA observations in the J = 3−2 line of 12CO. The second attributes the emission to an inclined disk. In this latter case, we detect, for the first time, a more extended circumstellar disk in scattered light, which shows that a region between ~10 and ~30 au is depleted of μm-size grains. We compare the J-, H-, and K-band spectra obtained with SINFONI in quiescence with the spectra taken during the outburst, showing that all the emission lines result from the episodic accretion event. Conclusions. Based on the morphology analysis, we favor the scenario that assumes the scattered light is coming from a circumstellar disk rather than the outflow around EX Lup. We determine the origin of the observed feature as either coming from a continuous circumstellar disk with a cavity, from the illuminated wall of the outer disk, or from a shadowed disk. Moreover, we discuss the potential origins of the depleted region of μm-size grains, exploring the possibility that a sub-stellar companion may be the source of this feature.

Description: Context. Direct imaging provides a steady flow of newly discovered giant planets and brown dwarf companions. These multi-object systems can provide information about the formation of low-mass companions in wide orbits and/or help us to speculate about possible migration scenarios. Accurate classification of companions is crucial for testing formation pathways. Aims. In this work we further characterise the recently discovered candidate for a planetary-mass companion CS Cha b and determine if it is still accreting. Methods. MUSE is a four-laser-adaptive-optics-assisted medium-resolution integral-field spectrograph in the optical part of the spectrum. We observed the CS Cha system to obtain the first spectrum of CS Cha b. The companion is characterised by modelling both the spectrum from 6300 Å to 9300 Å and the photometry using archival data from the visible to the near-infrared (NIR). Results. We find evidence of accretion and outflow signatures in Hα and OI emission. The atmospheric models with the highest likelihood indicate an effective temperature of 3450 ± 50 K with a log g of 3.6 ± 0.5 dex. Based on evolutionary models, we find that the majority of the object is obscured. We determine the mass of the faint companion with several methods to be between 0.07 M⊙ and 0.71 M⊙ with an accretion rate of Ṁ = 4 × 10−11±0.4 M⊙ yr−1. Conclusions. Our results show that CS Cha B is most likely a mid-M-type star that is obscured by a highly inclined disc, which has led to its previous classification using broadband NIR photometry as a planetary-mass companion. This shows that it is important and necessary to observe over a broad spectral range to constrain the nature of faint companions.

Description: Background: The discharge of the Cnidarian stinging organelle, the nematocyst, is one of the fastest processes in biology and involves volume changes of the highly pressurized (150 bar) capsule of up to 50%. Hitherto the molecular basis for the unusual biomechanical properties of nematocysts has been elusive, as their structure was mainly defined as a stress-resistant collagenous matrix. Results: Here, we characterize Cnidoin, a novel elastic protein identified as a structural component of Hydra nematocysts. Cnidoin is expressed in nematocytes of all types and immunostainings revealed incorporation into capsule walls and tubules concomitant with minicollagens. Similar to spider silk proteins, to which it is related at sequence level, Cnidoin possesses high elasticity and fast coiling propensity as predicted by Molecular Dynamics simulations and quantified by AFM force spectroscopy. Recombinant Cnidoin showed high tendency for spontaneous aggregation to bundles of fibrillar structures. Conclusion: Cnidoin represents the molecular factor involved in kinetic energy storage and release during the ultra-fast nematocyst discharge. Furthermore, it implies an early evolutionary origin of protein elastomers in basal metazoans.

Description: Background: It is generally the case that fast transmission at neural synapses is mediated by small molecule neurotransmitters. The simple nervous system of the cnidarian Hydra, however, contains a large repertoire of neuropeptides and it has been suggested that neuropeptides are the principal transmitters of Hydra. An ion channel directly gated by Hydra-RFamide neuropeptides has indeed been identified in Hydra – the Hydra Na+ channel (HyNaC) 2/3/5, which is expressed at the oral side of the tentacle base. Hydra-RFamides are more widely expressed, however, being found in neurons of the head and peduncle region. Here, we explore whether further peptide-gated HyNaCs exist, where in the animal they are expressed, and whether they are all gated by Hydra-RFamides. Results: We report molecular cloning of seven new HyNaC subunits – HyNaC6 to HyNaC12, all of which are members of the DEG/ENaC gene family. In Xenopus oocytes, these subunits assemble together with the four already known subunits into thirteen different ion channels that are directly gated by Hydra-RFamide neuropeptides with high affinity (up to 40 nM). In situ hybridization suggests that HyNaCs are expressed in epitheliomuscular cells at the oral and the aboral side of the tentacle base and at the peduncle. Moreover, diminazene, an inhibitor of HyNaCs, delayed tentacle movement in live Hydra. Conclusions: Our results show that Hydra has a large variety of peptide-gated ion channels that are activated by a restricted number of related neuropeptides. The existence and expression pattern of these channels, and behavioral effects induced by channel blockers, suggests that Hydra co-opted neuropeptides for fast neuromuscular transmission.

Description: Background: Wnt proteins present a family of secreted signaling molecules that regulate key developmental processes in metazoans. The molecular basis of Wnt binding to Frizzled and LRP5/6 co-receptors has long been unknown due to the lack of structural data on Wnt ligands. Only recently, the crystal structure of the Wnt8-Frizzled8-CRD complex was solved, but the significance of interaction sites that influence Wnt signaling has not been assessed. Results: Here, we present an extensive structure-function analysis of mouse Wnt3a in vitro and in vivo. We provide evidence for an essential role of Serine 209, Glycine 210 (site 1), and Tryptophan 333 (site 2) in Fz binding. Importantly, we discovered that Valine 337 in the site 2 binding loop is critical for signaling without contributing to binding. Mutations in the presumptive second CRD binding site (site 3) partly abolished Wnt binding. Intriguingly, most site 3 mutations increased Wnt signaling, probably by inhibiting Wnt-CRD oligomerization. In accordance, increasing amounts of soluble Frizzled8-CRD protein modulated Wnt3a signaling in a biphasic manner. Conclusion: We propose a concentration-dependent switch in Wnt-CRD complex formation from an inactive aggregation state to an activated high mobility state as a possible modulatory mechanism in Wnt signaling gradients.

Description: Context. Circumstellar disks and self-luminous giant exoplanets or companion brown dwarfs can be characterized through direct-imaging polarimetry at near-infrared wavelengths. SPHERE/IRDIS at the Very Large Telescope has the capabilities to perform such measurements, but uncalibrated instrumental polarization effects limit the attainable polarimetric accuracy. Aims. We aim to characterize and correct the instrumental polarization effects of the complete optical system, that is, the telescope and SPHERE/IRDIS. Methods. We created a detailed Mueller matrix model in the broadband filters Y, J, H, and Ks and calibrated the model using measurements with SPHERE’s internal light source and observations of two unpolarized stars. We developed a data-reduction method that uses the model to correct for the instrumental polarization effects, and applied it to observations of the circumstellar disk of T Cha. Results. The instrumental polarization is almost exclusively produced by the telescope and SPHERE’s first mirror and varies with telescope altitude angle. The crosstalk primarily originates from the image derotator (K-mirror). At some orientations, the derotator causes severe loss of signal (〉 90% loss in the H- and Ks-band) and strongly offsets the angle of linear polarization. With our correction method we reach, in all filters, a total polarimetric accuracy of ≲0.1% in the degree of linear polarization and an accuracy of a few degrees in angle of linear polarization. Conclusions. The correction method enables us to accurately measure the polarized intensity and angle of linear polarization of circumstellar disks, and is a vital tool for detecting spatially unresolved (inner) disks and measuring the polarization of substellar companions. We have incorporated the correction method in a highly-automated end-to-end data-reduction pipeline called IRDAP, which we made publicly available online.

Description: Context. Polarimetric imaging is one of the most effective techniques for high-contrast imaging and for the characterization of protoplanetary disks, and it has the potential of becoming instrumental in the characterization of exoplanets. The Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument installed on the Very Large Telescope (VLT) contains the InfraRed Dual-band Imager and Spectrograph (IRDIS) with a dual-beam polarimetric imaging (DPI) mode, which offers the capability of obtaining linear polarization images at high contrast and resolution. Aims. We aim to provide an overview of the polarimetric imaging mode of VLT/SPHERE/IRDIS and study its optical design to improve observing strategies and data reduction. Methods. For H-band observations of TW Hydrae, we compared two data reduction methods that correct for instrumental polarization effects in different ways: a minimization of the “noise” image (Uϕ), and a correction method based on a polarimetric model that we have developed, as presented in Paper II of this study. Results. We use observations of TW Hydrae to illustrate the data reduction. In the images of the protoplanetary disk around this star, we detect variability in the polarized intensity and angle of linear polarization that depend on the pointing-dependent instrument configuration. We explain these variations as instrumental polarization effects and correct for these effects using our model-based correction method. Conclusions. The polarimetric imaging mode of IRDIS has proven to be a very successful and productive high-contrast polarimetric imaging system. However, the instrument performance is strongly dependent on the specific instrument configuration. We suggest adjustments to future observing strategies to optimize polarimetric efficiency in field-tracking mode by avoiding unfavorable derotator angles. We recommend reducing on-sky data with the pipeline called IRDAP, which includes the model-based correction method (described in Paper II) to optimally account for the remaining telescope and instrumental polarization effects and to retrieve the true polarization state of the incident light.

Description: Context. Young circumstellar disks are the birthplaces of planets. Their study is of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features. Aims. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of an embedded young planet and search for disk structures that may be the result of disk–planet interactions and other evolutionary processes. Methods. We analyse new and archival near-infrared images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo, and Gemini/NICI instruments in polarimetric differential imaging and angular differential imaging modes. Results. We detect a point source within the gap of the disk at about 195 mas (~22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance. The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. The luminosity of the detected object is consistent with that of an L-type dwarf, but its IR colours are redder, possibly indicating the presence of warm surrounding material. Further, we confirm the detection of a large gap of ~54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than ~17 au in radius, and its position angle is consistent with that of the outer disk. The images of the outer disk show evidence of a complex azimuthal brightness distribution which is different at different wavelengths and may in part be explained by Rayleigh scattering from very small grains. Conclusions. The detection of a young protoplanet within the gap of the transition disk around PDS 70 opens the door to a so far observationally unexplored parameter space of planetary formation and evolution. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet–disk interactions, planetary atmospheres, and evolutionary models.