arXiv subjects

N. Alipour

Publications and source records attributed to N. Alipour.

6 recordsLinked to original sources

arXiv2025

A complex network approach to TESS light curves of $δ$ Sct stars

By using the horizontal visibility algorithm, the TESS light curves of 69 delta Sct stars are mapped to undirected horizontal visibility graphs (HVGs), where the graph nodes represent the light curve points. Then, the morphological characteristics of HVGs, such as the distribution of links between nodes (degree distribution) or the average fraction of triangles around each node (average clustering coefficient) are measured. The average clustering coefficients for HADS and LADS show two different linear correlations with the peak to peak amplitude of the TESS light curves is naturally separating them into two groups, thys, avoiding ad-hoc criteria for the first time. Exponential fits on HVG degree distributions for both HADS and LADS stars give indices suggesting correlated stochastic generating processes for delta Sct light curves. By applying the theoretical expression for the HVGs degree distribution of random time series we can distinguish significant pulsations from the background noise which might become a practical tool in frequency analysis of stars.

astro-ph.SR↗

arXiv2022

Automatic detection of small-scale EUV brightenings observed by the Solar Orbiter/EUI

Context. Accurate detections of frequent small-scale extreme ultraviolet (EUV) brightenings are essential to the investigation of the physical processes heating the corona. Aims. We detected small-scale brightenings, termed campfires, using their morphological and intensity structures as observed in coronal EUV imaging observations for statistical analysis. Methods. We applied a method based on Zernike moments and a support vector machine classifier to automatically identify and track campfires observed by Solar Orbiter/Extreme Ultraviolet Imager (EUI) and SDO/AIA. Results. This method detected 8678 campfires (with length scales between 400 km and 4000 km) from a sequence of 50 High Resolution EUV telescope (HRIEUV) 174Å images. From 21 near co-temporal AIA images covering the same field of view as EUI, we found 1131 campfires, 58% of which were also detected in HRIEUV images. In contrast, about 16% of campfires recognized in HRIEUV were detected by AIA. We obtain a campfire birthrate of 2*10-16m-2s-1. About 40% of campfires show a duration longer than 5 s, having been observed in at least two HRIEUV images. We find that 27% of campfires were found in coronal bright points and the remaining 73% have occurred out of coronal bright points. We detected 23 EUI campfires with a duration greater than 245 s. We found that about 80% of campfires are formed at supergranular boundaries, and the features with the highest total intensities are generated at network junctions and intense H I Lyman-α emission regions observed by EUI/HRILya. The probability distribution functions for the total intensity, peak intensity, and projected area of campfires follow a power law behavior with absolute indices between 2 and 3. This self-similar behavior is a possible signature of self-organization, or even self-organized criticality, in the campfire formation process.

astro-ph.SR↗

arXiv2018

Evolution of circumbinary protoplanetary disks with photoevaporative winds driven by External Far Ultraviolet Radiation

Lifetimes of protoplanetary disks (PPDs) are believed to be severely constrained by material depleting mechanisms, including photoevaporative winds due to the host star radiation or external radiation sources. Most previous studies focused on exploring the role of the winds in the exposed PPDs with a single star; however, exploring the evolution of the circumbinary disks with the photoevaporative winds driven by the host star radiation and external radiation sources deserves further investigation. In this study, we investigate the evolution of the circumbinary PPDs with the photoevaporative winds induced by external far ultraviolet (FUV) radiation field. We show that this mass-loss process can significantly constrain properties of a circumbinary PPD, including its lifetime, mass and radius. The lifetime of a circumbinary PPD, for instance, is found by a factor of about two longer than a similar circumstellar disk and this enhancement strongly depends on the viscosity parameter. But our model shows that viscosity dependence of the disk lifetime in the circumbinary case is more pronounced compared to the circumstellar case. We also show that dispersal of a circumbinary PPD occurs over a longer time as the disk temperature distribution becomes steeper. Our results also imply that dead zone in a photoevaporative circumbinary PPD extends over a larger radial range in comparison to a circumstellar disk counterpart. We also show that our calculations are in agreement with the observed circumbinary PPDs orbiting equal-mass binaries.

astro-ph.GA↗

arXiv2011

Characterization of Seven Ultra-Wide Trans-Neptunian Binaries

The low-inclination component of the Classical Kuiper Belt is host to a population of extremely widely-separated binaries. These systems are similar to other Trans-Neptunian binaries (TNBs) in that the primary and secondary components of each system are of roughly equal size. We have performed an astrometric monitoring campaign of a sample of seven wide-separation, long-period TNBs and present the first-ever well-characterized mutual orbits for each system. The sample contains the most eccentric (2006 CH69, e=0.9) and the most widely-separated, weakly bound (2001 QW322, a/Rh~0.22) binary minor planets known, and also contains the system with lowest-measured mass of any TNB (2000 CF105, M~1.85E17 kg). Four systems orbit in a prograde sense, and three in a retrograde sense. They have a different mutual inclination distribution compared to all other TNBs, preferring low mutual-inclination orbits. These systems have geometric r-band albedos in the range of 0.09-0.3, consistent with radiometric albedo estimates for larger solitary low-inclination Classical Kuiper Belt objects, and we limit the plausible distribution of albedos in this region of the Kuiper Belt. We find that gravitational collapse binary formation models produce a similar orbital distribution to that currently observed, which along with a confluence of other factors supports formation of the cold Classical Kuiper Belt in situ through relatively rapid gravitational collapse rather than slow hierarchical accretion. We show that these binary systems are sensitive to disruption via collisions, and their existence suggests that the size distribution of TNOs at small sizes remains relatively shallow.

astro-ph.EP↗

arXiv2010

Observational constraints on Chaplygin cosmology in a braneworld scenario with induced gravity and curvature effect

We study cosmological dynamics and late-time evolution of an extended induced gravity braneworld scenario. In this scenario, curvature effects are taken into account via the Gauss-Bonnet term in the bulk action and there is also a Chaplygin gas component on the brane. We show that this model mimics an effective phantom behavior in a relatively wider range of redshifts than previously formulated models. It also provides a natural framework for smooth crossing of the phantom-divide line due to presence of the Chaplygin gas component on the brane. We confront the model with observational data from type Ia Supernovae, Cosmic Microwave Background and Baryon Acoustic Oscillations to constraint the model parameters space.

astro-ph.CO↗

arXiv2009

Phantom Mimicry on the Normal Branch of a DGP-inspired Braneworld Scenario with Curvature Effect

It has been shown recently that phantom-like effect can be realized on the normal branch of the DGP setup without introduction of any phantom matter neither in the bulk nor on the brane and therefore without violation of the null energy condition. It has been shown also that inclusion of the Gauss-Bonnet term in the bulk action modifies this picture via curvature effects. Here, based on the Lue-Starkman conjecture on the dynamical screening of the brane cosmological constant in the DGP setup, we extend this proposal to a general DGP-inspired $f(R,ϕ)$ model that stringy effects in the ultra-violet sector of the theory are taken into account by inclusion of the Gauss-Bonnet term in the bulk action. We study cosmological dynamics of this setup, especially its phantom-like behavior and possible crossing of the phantom divide line especially with a non-minimally coupled quintessence field on the brane. In this setup, scalar field and curvature quintessence are treated in a unified framework.

gr-qc↗