arXiv subjects

K. Lange

Publications and source records attributed to K. Lange.

8 recordsLinked to original sources

arXiv2025

Dissociation and destruction of PAHs and PAH clusters induced by absorption of X-rays in protoplanetary discs around T Tauri stars

Only 8% of the protoplanetary discs orbiting a T Tauri star show emission features of polycyclic aromatic hydrocarbons (PAHs). As PAHs are strong absorbers of UV radiation, they contribute to the heating of the discs photosphere, shielding of UV radiation that drives photo-chemistry in the disc, and their abundance is a key parameter to determine the strength of photo-evaporative disc winds. We want to understand the photochemical evolution of PAHs in protoplanetary discs around T Tauri stars and thus explain the absence of PAH features. We want to determine whether PAHs are destroyed because of the X-ray emission from their host stars or whether PAHs can withstand these conditions. We developed a model for the absorption of X-rays by PAHs. X-rays with more energy than the K edge of carbon will double ionise PAHs and will vibrationally excite them by ~ 15-35 eV. With a Monte Carlo model, we modelled the dissociation of H, H2, and C2H2 from PAH monomers. Furthermore, we modelled the dissociation of PAH clusters and the desorption of PAH clusters from dust grains caused by X-ray excitation. We find that small PAH clusters will quickly desorb and dissociate into individual molecules. PAH molecules experience rapid loss of H and acetylene C2H2 by the high excitation and will lose C2H2 on average after three X-ray excitations. However, large PAH clusters can stay intact and frozen out on dust grains. Based on our results, we expect a gas-phase PAH abundance that is lower than 0.01 times the ISM abundance and will rapidly decrease over time due to the dissociation of small clusters that are subsequently destroyed. To maintain a higher abundance, replenishment processes must exist such as vertical mixing. Large PAH clusters remain in the disc, frozen out on dust grains, but barely emit PAH features because of their strong thermal coupling to dust grains.

astro-ph.EP↗

arXiv2023

The need for spatially resolved observations of PAHs in protoplanetary discs

The signatures of polycyclic aromatic hydrocarbons (PAHs) have been observed in protoplanetary discs, and their emission features obtained from spectral energy distributions (SED) have been used in the literature to characterise their size and determine their abundance. Two simple disc models (uniform PAH distribution against a PAH gap in the inner disc) are compared to investigate the difference of their SED and obtainable information. We used the radiative transfer code RADMC-3D to model the SED of two protoplanetary discs orbiting a typical Herbig star, one of which features a depletion of PAHs in the inner disc. We further created artificial images of the discs at face-on view to extract radial profiles of the PAH emission in the infrared. We find that the extracted PAH features from an SED provide limited information about the PAHs in protoplanetary disc environments, except for the ionisation state. The distribution of PAHs in a protoplanetary disc influences the total observed PAH luminosity in a non-linear fashion and alters the relative strength between the 3.3\,$\mu$m and 11.3\,$\mu$m features. Furthermore, we produced radial profiles at the 3\,$\mu$m, 6\,$\mu$m and, 11\,$\mu$m PAH emission features and find that they follow a double power-law profile where the slope reflects the radiative environment (single photon regime vs. multi-photon regime) in which the PAHs lie. Using spatially resolved techniques such as IFU or imaging in the era of the James Webb Space Telescope, we find that multi-wavelength radial emission profiles will not only provide information on the spatial distribution of the PAHs, but may also provide information on their size and underlying UV environment, which is crucial for photo-evaporative disc wind models.

astro-ph.EP↗

arXiv2023

Turbulent processing of PAHs in protoplanetary discs -- Coagulation and freeze-out leading to depletion of gas-phase PAH

Polycyclic aromatic hydrocarbons (PAHs) have been detected in numerous circumstellar discs. We propose the continuous processing of PAHs through clustering, adsorption on dust grains, and their reverse-processes as key mechanisms to reduce the emission-capable PAH abundance in protoplanetary discs. This cycle of processing is driven by vertical turbulence in the disc mixing PAHs between the disc midplane and the photosphere. We used a theoretical Monte Carlo model for photodesorption and a coagulation code in the disc midplane to estimate the relevance and timescale of these processes in a Herbig Ae/Be disc environment. By combining these components in a 1D vertical model, we calculated the gas-phase depletion of PAHs that stick as clusters on dust grains. Our results show that the clustering of gas-phase PAHs is very efficient, and that clusters with more than 100 monomers can grow for years before they are able to freeze out in the disc midplane. Once a PAH cluster is frozen on the dust grain surface, the large heat capacity of these clusters prevents them from evaporating off the grains in UV-rich environments such as the photosphere. Therefore, the clustering of PAHs followed by freeze-out can lead to a depletion of gas-phase PAHs in discs. Evaluated over the lifetime of protoplanetary discs, we find a depletion of PAHs by a factor that ranges between 50 and 1000 compared to the standard ISM abundance of PAHs in the inner disc through turbulent processing. Through these processes, we favour PAHs smaller than circumovalene as the major gas-phase emitters of the disc photosphere as larger PAH monomers cannot photodesorb from the grain surface. These gas-phase PAHs co-exist with large PAH clusters sticking on dust grains. We find a close relation between the amount of PAHs frozen out on dust grains and the dust population, as well as the strength of the vertical turbulence.

astro-ph.EP↗

arXiv2021

Stability of Polycyclic Aromatic Hydrocarbon Clusters in Protoplanetary Disks

The infrared signature of polycyclic aromatic hydrocarbons (PAHs) are present in many protostellar disks and these speciesare thought to play an important role in heating of the gas in the photosphere. We aim to consider PAH cluster formation as one possible cause for non-detections of PAH features in protoplanetary disks. We test the necessary conditions for cluster formation and cluster dissociation by stellar optical and FUV photons in protoplanetarydisks using a Herbig Ae/Be and a T Tauri star disk model. We perform Monte-Carlo (MC) and statistical calculations to determine dissociation rates for coronene, circumcoronene and circumcoronene clusters with sizes between 2 and 200 cluster members. By applying general disk models to our Herbig Ae/Be and T Tauri star model, we estimate the formation rate of PAH dimers and compare these with the dissociation rates. We show that the formation of PAH dimers can take place in the inner 100 AU of protoplanetary disks in sub-photospheric layers. Dimer formation takes seconds to years allowing them to grow beyond dimer size in a short time. We further demonstrate that PAH cluster increase their stability while they grow if they are located beyond a critical distance that depends on stellar properties and PAH species. The comparison with the local vertical mixing time scale allows a determination of the minimum cluster size necessaryfor survival of PAH clusters. Considering the PAH cluster formation sites, cluster survival in the photosphere of the inner disk of Herbig stars isunlikely because of the high UV radiation. For the T Tauri stars, survival of coronene, circumcoronene and circumcircumcoronene clusters is possible and cluster formation should be considered as one possible explanation for low PAH detection rates in T Tauri star disks.

astro-ph.EP↗

arXiv2020

Momentum Entanglement for Atom Interferometry

Compared to light interferometers, the flux in cold-atom interferometers is low and the associated shot noise large. Sensitivities beyond these limitations require the preparation of entangled atoms in different momentum modes. Here, we demonstrate a source of entangled atoms that is compatible with state-of-the-art interferometers. Entanglement is transferred from the spin degree of freedom of a Bose-Einstein condensate to well-separated momentum modes, witnessed by a squeezing parameter of -3.1(8) dB. Entanglement-enhanced atom interferometers open up unprecedented sensitivities for quantum gradiometers or gravitational wave detectors.

quant-ph↗

arXiv2018

Creation of entangled atomic states by an analogue of the Dynamical Casimir Effect

If the boundary conditions of the quantum vacuum are changed in time, quantum field theory predicts that real, observable particles can be created in the initially empty modes. Here, we realize this effect by changing the boundary conditions of a spinor Bose-Einstein condensate, which yields a population of initially unoccupied spatial and spin excitations. We prove that the excitations are created as entangled excitation pairs by certifying continuous-variable entanglement within the many-particle output state.

quant-ph↗

arXiv2016

0.75 atoms improve the clock signal of 10,000 atoms

Since the pioneering work of Ramsey, atom interferometers are employed for precision metrology, in particular to measure time and to realize the second. In a classical interferometer, an ensemble of atoms is prepared in one of the two input states, whereas the second one is left empty. In this case, the vacuum noise restricts the precision of the interferometer to the standard quantum limit (SQL). Here, we propose and experimentally demonstrate a novel clock configuration that surpasses the SQL by squeezing the vacuum in the empty input state. We create a squeezed vacuum state containing an average of 0.75 atoms to improve the clock sensitivity of 10,000 atoms by 2.05 dB. The SQL poses a significant limitation for today's microwave fountain clocks, which serve as the main time reference. We evaluate the major technical limitations and challenges for devising a next generation of fountain clocks based on atomic squeezed vacuum.

quant-ph↗

arXiv2015

Satisfying the Einstein-Podolsky-Rosen criterion with massive particles

In 1935, Einstein, Podolsky and Rosen (EPR) questioned the completeness of quantum mechanics by devising a quantum state of two massive particles with maximally correlated space and momentum coordinates. The EPR criterion qualifies such continuous-variable entangled states, where a measurement of one subsystem seemingly allows for a prediction of the second subsystem beyond the Heisenberg uncertainty relation. Up to now, continuous-variable EPR correlations have only been created with photons, while the demonstration of such strongly correlated states with massive particles is still outstanding. Here, we report on the creation of an EPR-correlated two-mode squeezed state in an ultracold atomic ensemble. The state shows an EPR entanglement parameter of 0.18(3), which is 2.4 standard deviations below the threshold 1/4 of the EPR criterion. We also present a full tomographic reconstruction of the underlying many-particle quantum state. The state presents a resource for tests of quantum nonlocality and a wide variety of applications in the field of continuous-variable quantum information and metrology.

quant-ph↗