On the interpretation of the long-term cyclic period variations in RR Lyrae stars

Many RR Lyrae stars show long-term variations of their pulsation period, some of them in a cyclic way. Such behaviour can be attributed to the light-travel time effect (LTTE) caused by an unseen companion. Solutions of the LTTE often suggest very eccentric orbits and minimal mass of the companion on the order of several solar masses, thus, in the black hole range. We discuss the possibility of the occurrence of the RR Lyr-black hole pairs and on the case of Z CVn demonstrate that the LTTE hypothesis can be false in some of the binary candidates.Comment: 5 pages, 2 figures, published in the proceedings of "The RR Lyrae 2017 Conference", Niepolomice, Poland, 17-21 September 201

A cautionary tale of interpreting O-C diagrams: period instability in a classical RR Lyr Star Z CVn mimicking as a distant companion

We present a comprehensive study of Z CVn, an RR Lyrae star that shows long-term cyclic variations of its pulsation period. A possible explanation suggested from the shape of the O-C diagram is the light travel-time effect, which we thoroughly examine. We used original photometric and spectroscopic measurements and investigated the period evolution using available maximum times spanning more than one century. If the binary hypothesis is valid, Z CVn orbits around a black hole with minimal mass of $56.5$ $\mathfrak{M}_{\odot}$ on a very wide ($P_{\rm orbit}=78.3$ years) and eccentric orbit ($e=0.63$). We discuss the probability of a formation of a black hole-RR Lyrae pair and, although we found it possible, there is no observational evidence of the black hole in the direction to Z CVn. However, the main objection against the binary hypothesis is the comparison of the systemic radial velocity curve model and spectroscopic observations that clearly show that Z CVn cannot be bound in such a binary. Therefore, the variations of pulsation period are likely intrinsic to the star. This finding represents a discovery/confirmation of a new type of cyclic period changes in RR Lyrae stars. By the analysis of our photometric data, we found that the Blazhko modulation with period of 22.931 d is strongly dominant in amplitude. The strength of the phase modulation varies and is currently almost undetectable. We also estimated photometric physical parameters of Z CVn and investigated their variations during the Blazhko cycle using the Inverse Baade-Wesselink method.Comment: 15 pages, 8 tables, 9 figures, accepted for publication in MNRA

Fractional Brownian Motion in a Finite Interval: Correlations Effect Depletion or Accretion Zones of Particles Near Boundaries

Fractional Brownian motion (FBM) is a Gaussian stochastic process with stationary, long-time correlated increments and is frequently used to model anomalous diffusion processes. We study numerically FBM confined to a finite interval with reflecting boundary conditions. The probability density function of this reflected FBM at long times converges to a stationary distribution showing distinct deviations from the fully flat distribution of amplitude 1/L in an interval of length L found for reflected normal Brownian motion. While for superdiffusion, corresponding to a mean squared displacement (MSD) ⟨X2(t)⟩ ≃ tα with 1 \u3c α \u3c 2, the probability density function is lowered in the centre of the interval and rises towards the boundaries, for subdiffusion (0 \u3c α \u3c 1) this behaviour is reversed and the particle density is depleted close to the boundaries. The MSD in these cases at long times converges to a stationary value, which is, remarkably, monotonically increasing with the anomalous diffusion exponent α. Our a priori surprising results may have interesting consequences for the application of FBM for processes such as molecule or tracer diffusion in the confines of living biological cells or organelles, or other viscoelastic environments such as dense liquids in microfluidic chambers

Fractional Brownian Motion in a Finite Interval: Correlations Effect Depletion or Accretion Zones of Particles Near Boundaries

Fractional Brownian motion (FBM) is a Gaussian stochastic process with stationary, long-time correlated increments and is frequently used to model anomalous diffusion processes. We study numerically FBM confined to a finite interval with reflecting boundary conditions. The probability density function of this reflected FBM at long times converges to a stationary distribution showing distinct deviations from the fully flat distribution of amplitude 1/L in an interval of length L found for reflected normal Brownian motion. While for superdiffusion, corresponding to a mean squared displacement (MSD) ⟨X2(t)⟩ ≃ tα with 1 \u3c α \u3c 2, the probability density function is lowered in the centre of the interval and rises towards the boundaries, for subdiffusion (0 \u3c α \u3c 1) this behaviour is reversed and the particle density is depleted close to the boundaries. The MSD in these cases at long times converges to a stationary value, which is, remarkably, monotonically increasing with the anomalous diffusion exponent α. Our a priori surprising results may have interesting consequences for the application of FBM for processes such as molecule or tracer diffusion in the confines of living biological cells or organelles, or other viscoelastic environments such as dense liquids in microfluidic chambers

Particle size as controlling factor of soil microaggregate formation

Aggregates are formed when soil particles connect to larger secondary units. Stable microaggregates in soils are supposed to consist of close associations of Fe-oxides and clay minerals with both components being attracted by electrostatic forces between the oppositely charged particles. However, the geometric preconditions for the formation of stable associations between Fe oxides and clay minerals are poorly known. Therefore, our goal was to determine geometrical constraints resulting from particle size and morphology likely impeding optimum arrangement of particles for shielding of charges during aggregate formation. Aggregation kinetics was determined for nine combinations of each three particle size fractions of goethite and mica in a Zetasizer at pH 6. Experiments were conducted using needle-shape goethites synthesized at 4, 20, and 60°C (lengths of 0.42, 0.46 and 0.84 µm, specific surface areas (SSA) of 87, 75, and 60 m²/g, respectively) and ground platy muscovite separated in fine, medium and coarse clay (diameters of 0.16, 0.80, and 2.9 µm, SSA of 182, 100, and 27 m²/g, respectively). For five combinations even smallest additions of goethite to muscovite facilitated aggregation. By further additions of goethite maximum aggregate sizes up to 5.6 µm were obtained, the respective mixing ratio strongly depending on the type of combination. After that sizes declined. For medium and coarse-sized muscovite, goethite amendments >18% did not facilitate aggregation, indicating the dominance of repulsive forces. In contrast, for fine-sized muscovite aggregation was facilitated up to an addition of 63% fine-sized goethite and of 90% coarse-sized goethite. Here also biggest aggregate sizes were obtained. Based on all examined size fraction combinations, our results suggest a strong impact of particle size on aggregation. Whereas all combinations with fine-sized muscovite facilitated aggregation at very different mixing ratios, the amendment of the finest fraction of goethite to medium- and coarse-sized muscovite facilitated aggregation at small additions only. Aggregation was favored for evenly sized combinations. The quantification of surface charge density of minerals and calculation of charge balances of the combinations is in progress and will help interpreting the observed aggregation patterns. For soils it is likely that aggregation by electrostatic interactions occurs only at certain mineral mixing ratios highly depending on particle morphology

Mobilization of phosphorus from secondary minerals by the arbuscular mycorrhiza Rhizophagus irregularis and consequences for carbon sequestration in soils

Phosphorus can be a major limiting factor for plant growth due to its slow diffusion and high degree of immobilization in soils. Understanding the strategies evolved by plant-symbiont couples increasing P uptake is crucial, under the aim of adopting the involved mechanisms by modern sustainable agriculture. This study aims to explore whether tomato plants mycorrhized with the arbuscular mycorrhizal (AM) fungi Rhizophagus irregularis have the ability to mobilize P from secondary minerals and organic sources. Our hypothesis was that AM-bearing plants will invest more carbon to their fungal symbiont in case P must be exploited from less accessible P sources. For this, we carried out a time course experiment (91 days) with split-chamber mesocosms ensuring the mobilization of P by the mycorrhizal partner only. Orthophosphate (OP) and phytic acid (PA) in their free state and adsorbed to goethite (GOE-OP; GOE-PA) have been offered to the host plant. According to our knowledge, this is the first report where an organic P source bonded to a secondary mineral has been tested as a plant P source via the mycorrhizal P uptake pathway. The PLFA 16:1ω5c is known to be part of the membrane constituents and it is considered a good AM biomass estimator (Olsson and Wilhelmsson 2000). In our study it correlated positively with incorporated P and the AM plant root activity (arbuscules %) for all provided P sources. Additionally, those AM plants which accessed OP and GOE-OP also showed a positive significant correlation of the arbuscules percentages, with the incorporated P, the PLFA 18:1ω7c, and in case of GOE-PA also with the PLFA 18:2ω6,9. These two PLFA biomarkers have been previously found in R. irregularis hyphae (Olsson et al. 2002) and might indicate that AM fungi modified their fatty acid composition in the hyphae during the mobilization of P from the different P sources. As fungal energy storage we also measured the NLFA 16:1ω5c. It was significantly higher for both P sources bonded to goethite compared to free OP and PA. These results point towards different C investment to uptake of P though the mycorrhizal pathway having a direct consequence for the carbon sequestration in soils

Greening boosts soil formation and soil organic matter accumulation in Maritime Antarctica

Global warming in the Antarctic Peninsula, Maritime Antarctica, within the past 45 years has accelerated rapid glacier retreatment, forming temporal gradients of soil development that concurs with the colonization of the ice-free soils by phototrophs. In the past decade the paradigm emerged that above- and belowground processes are interconnected, e.g. recently gained carbon fuels microbial activity and thus drives soil organic matter built-up and decomposition as well as mineral weathering. Studies of carbon allocation for Antarctic ecosystems, occurring in harsh conditions are lacking. Little is also known about the contribution of bacteria and fungi to decomposition of different soil carbon pools with different turnover rates in these soils, which is of utmost importance for the prediction of the future feedback of the Antarctic carbon balance to climate change. We followed soil horizon formation, soil organic carbon accumulation and carbon exchange with the atmosphere along a gradient of phototrophs of different trophic complexity level at King George Island by combining soil chemical analyses, field CO2 flux measurements, C-13 in situ labeling and molecular methods (PLFA and metabolomics). Our study revealed that colonization of the ice-free soils by vascular plant (Deschampsia antarctica) was leading to the formation of well-developed soil, with high contents of organic carbon and with a relatively high rates of photosynthesis and CO2 soil efflux. The soils sampled under D. antarctica showed the impact of this higher plant on the soil organic matter, containing significantly higher amounts of carbohydrates and amines, presumably as a result of root exudation. As determined by the C-13 labeling experiment more than 15% of the carbon recently assimilated by D. antarctica was transferred belowground, with a major flow into soil fungi. This suggests that not bacteria, but rather fungi preferentially and faster utilize the recently assimilated low molecular compounds allocated to the soil. Probably, successful performance of vascular plants in Maritime Antarctica may significantly foster biological weathering via enhanced microbial activity

Revisiting CoRoT RR Lyrae stars: detection of period doubling and temporal variation of additional frequencies

We search for signs of period doubling in CoRoT RR Lyrae stars. The occurrence of this dynamical effect in modulated RR Lyrae stars might help us to gain more information about the mysterious Blazhko effect. The temporal variability of the additional frequencies in representatives of all subtypes of RR Lyrae stars is also investigated. We pre-process CoRoT light curves by applying trend and jump correction and outlier removal. Standard Fourier technique is used to analyze the frequency content of our targets and follow the time dependent phenomena. The most comprehensive collection of CoRoT RR Lyrae stars, including new discoveries is presented and analyzed. We found alternating maxima and in some cases half-integer frequencies in four CoRoT Blazhko RR Lyrae stars, as clear signs of the presence of period doubling. This reinforces that period doubling is an important ingredient to understand the Blazhko effect - a premise we derived previously from the Kepler RR Lyrae sample. As expected, period doubling is detectable only for short time intervals in most modulated RRab stars. Our results show that the temporal variability of the additional frequencies in all RR Lyrae sub-types is ubiquitous. The ephemeral nature and the highly variable amplitude of these variations suggest a complex underlying dynamics of and an intricate interplay between radial and possibly nonradial modes in RR Lyrae stars. The omnipresence of additional modes in all types of RR Lyrae - except in non-modulated RRab stars - implies that asteroseismology of these objects should be feasible in the near future (Abridged).Comment: 20 pages, 13 figures, accepted for publication in A&