arXiv subjects

Alexios Liakos

Publications and source records attributed to Alexios Liakos.

At least 19 recordsLinked to original sources

arXiv2026

IZ Tel and UW Vir: Southern oscillating eclipsing Algol systems with active mass transfer

This study is an in-depth examination of IZ Tel and UW Vir which are semi-detached oscillating Eclipsing Algol binary systems (oEA stars). The radial velocities of both components of each system were derived using spectra observed with the Australian National University's 2.3 m telescope. The spectral types of the IZ Tel primary and secondary components were determined as F2V and K2IV; and those of UW Vir were determined as A7V and K6IV, respectively. Spectroscopy revealed mass transfer in progress which was confirmed by the photometric analysis for both cases and also by Eclipse-Timing Variation analysis in the case of UW Vir. Data from the Transiting Exoplanet Survey Satellite (TESS) and ground-based observations enabled detailed light-curve modelling and pulsation analysis. We determined component masses of $M_1=1.48$ M_sun and $M_2=0.33$ M_sun for IZ Tel, and $M_1=2.39$ M_sun and $M_2=0.67$ M_sun for UW Vir from the simultaneous solution of the light and radial velocity curves. Spectra during total eclipses of the primary components revealed H$\alpha$ emission was present. Both primary components are $\delta$ Sct stars. That of IZ Tel pulsates with a dominant frequency of 13.56 d$^{-1}$, which is revealed as non-radial pressure mode, as well as in another 16 combination frequencies. The primary component of UW Vir oscillates in three main frequencies within the range 34.9-43.3 d$^{-1}$ and in more than 50 another combination frequencies. Mode coupling was detected in the three main frequencies, which showed amplitude and phase modulations within a time span of approximately four years. The physical and pulsational properties of the $\delta$ Sct stars of both systems were compared with other members of oEA stars.

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arXiv2025

ASASSN-24fw: Candidate circumplanetary disk occultation of a main-sequence star

Dusty disks around planetary and substellar companions in outer reaches of exo-planetary systems can be detected as long-lasting occultations, provided the observer is close to the secondary's orbital plane. Here we report optical spectroscopy with KOSMOS (APO), MagE (Magellan) and GHOST (Gemini-S) of ASASSN-24fw (Gaia 07:05:18.97+06:12:19.4), a 4-magnitude dimming event of a main-sequence star which lasted 8.5 months. We discover multiple low-ionization metal emission lines with velocity dispersion $\lesssim 10$ km/s blue-shifted by 27 km/s with respect to the star, as well as kinematically complex Na D absorption. If associated with the occulter, these detections suggest that the occulter is gas-rich. Further, we detect blue-shifted and broad ($\sim 200$ km/s) H$\alpha$ line, which likely originates in the inner circumstellar disk. We confirm the previously reported occultations in 1981 and 1937 seen in historic data, yielding a semi-major axis of the occulter's orbital motion around the star of 14 AU. If the occulter is a circumsecondary disk filling 30-100% of the Hill radius, we estimate the minimum mass of the secondary to be a few Jupiter masses and a disk mass of 1% of the mass of the Moon. Given the age of the star ($>2$ Gyr), the disk is unlikely to be a survivor of the planet formation stage and may be a result of a planetary collision. If Na D absorption and/or metal emission lines originate in the disk, the observations presented here are the first discovery of a circumsecondary disk wind or rotation.

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arXiv2025

Asteroseismology of four eccentric double-lined spectroscopic eclipsing binaries

Photometric data from the Transiting Exoplanet Survey Satellite (TESS) mission and radial velocities from the Gaia mission and ground-based observations were used to model the light curves and calculate the physical parameters of the eccentric eclipsing systems CH Ind, V577 Oph, CX Phe, and TIC 35481236. The components of these systems have temperatures between 6450 and 7500 K, masses between 1.4 and 1.85 solar masses, and radii between 1.49 and 3.05 solar radii. The residuals of these models were further analyzed using the Fourier method to reveal the pulsational frequencies of their oscillating components. Due to the similarity of the components of each system, the eclipses were used as spatial filters in order to determine which member is the pulsating star. CH Ind was found to pulsate in 46 frequencies; its primary component is a $\gamma$ Dor star and the secondary a $\delta$ Sct star. The primary component of V577 Oph oscillates in both the regimes of $\gamma$ Dor and $\delta$ Sct stars. Moreover, using past timings of minima, an eclipse timing variation analysis was also performed for V577 Oph, resulting in the calculation of the apsidal motion parameters and the existence of a third body around the system. Both components of CX Phe were found to be $\delta$ Sct stars; its primary has three independent frequencies in the range of 14.5-17.4 d$^{-1}$ and its secondary has two main modes of 5.19 and 7.22 d$^{-1}$. The analysis of TIC 35481236 indicates the hybrid $\delta$ Sct-$\gamma$ Dor nature of its secondary component. The physical and pulsational properties of the $\delta$ Sct stars of these systems were compared with those of other $\delta$ Sct stars-members of binaries in evolutionary diagrams.

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arXiv2025

Is the Symbiotic Recurrent Nova T CrB Late? Recent Photometric Evolution and Comparison with Past Pre-Outburst Behaviour

T CrB is a symbiotic recurrent nova that last erupted in 1946. Given its recurrence timescale of approximately 80 years, the next outburst is eagerly anticipated by the astronomical community. In this work, we analyse the optical light curves of T CrB, comparing recent photometric evolution with historical data to evaluate potential predictive indicators of nova eruptions. Although the "super-active" phases preceding both the 1946 and anticipated eruptions are strikingly similar, the subsequent photometric behaviour differs. We find that the decline in brightness observed in 2023, interpreted by some as a "pre-eruption dip", deviates from the deep minimum recorded prior to the 1946 event and does not reliably predict the eruption timing. Recent photometric and spectroscopic observations indicate that the system is returning to a high-accretion state. Given this, an eruption may be imminent, even without distinct precursors. While the next eruption of T CrB will be a major scientific event, its expected peak brightness of $V \sim 2$ mag highlights the importance of setting realistic public expectations for what will be a visually modest, yet astrophysically very significant, celestial event.

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arXiv2024

The catalogue of $\delta$ Sct pulsators in binary systems in 2024

The updated catalogue of $\delta$ Sct stars in binary systems as well as their statistical properties are presented. Thanks to the Kepler, K2 and TESS space missions, this version of the catalogue contains more than 1000 $\delta$ Sct pulsators in binaries. The sample is divided according to the Roche Geometry of the binary systems in order to check for any systematic differences in the pulsators' evolution due to the proximity of the companion star. Statistics, demographics, and distributions of these pulsating stars within the H-R and mass-radius diagrams are provided. We notice that the absolute parameters have been accurately determined for only approximately 10% of the whole sample. Additionally, updated correlations between pulsation and orbital periods and evolutionary status are presented. This work aims to motivate the researchers for systematic analyses of such objects in order to increase the sample of systems with well known physical properties.

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arXiv2024

Modelling of eclipsing binary systems with pulsating components and tertiary companions: BF Vel and RR Lep

This paper presents a comprehensive analysis of RR Lep and BF Vel, two short-period semi-detached oscillating Algols (oEA stars), which are shown to be triple systems. Spectral types of their primaries were determined and radial velocities calculated from spectra observed with the Australian National University's 2.3 m telescope and Wide Field Spectrograph. Spectra of the Na I D doublet confirmed the presence of tertiary components which were apparent in the broadening function analyses and, with H_a spectra during primary eclipses, indicated chromospherical activity in their secondaries. Ground-based telescopes were used for observations in several pass bands for photometric analyses. These data were complemented by data from the TESS mission to enable the modelling of the light curves, followed by a detailed analysis of pulsations. Eclipse-timing variation (ETV) analyses of both systems were used to determine the most likely mechanisms modulating the orbital period. We found mass values M1 = 2.9 M_sun and M2 = 0.75 M_sun for the components of RR Lep, and M1 = 1.93 M_sun and M2 = 0.97 M_sun for those of BF Vel. By integrating information from photometry, spectroscopy and ETV analysis, we found that tertiary components revolve around both systems. The primary star of RR Lep pulsates in 36 frequencies, of which five were identified as independent modes, with the dominant one being 32.28 d^-1. The pulsating component of BF Vel oscillates in 37 frequencies, with the frequency 46.73 d^-1 revealed as the only independent mode. For both systems, many frequencies were found to be related to the orbital frequency. Their physical properties were compared with other oEA stars in Mass-Radius and H-R diagrams, and the pulsational properties of their delta Sct components were compared with currently known systems of this type within the orbital-pulsation period and logg-pulsation period diagrams.

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arXiv2024

NELIOTA: New results and updated statistics after 6.5 years of lunar impact flashes monitoring

We present results of the NELIOTA campaign for lunar impact flashes observed with the 1.2 m Kryoneri telescope. From August 2019 to August 2023, we report 113 validated and 70 suspected flashes. For the validated flashes, we calculate the physical parameters of the corresponding projectiles, the temperatures developed during the impacts, and the expected crater sizes. For the multiframe flashes we present light curves and thermal evolution plots. Using the whole sample of NELIOTA that encompasses 192 validated flashes in total from 2017, the statistics of the physical parameters of the meteoroids, the peak temperatures of the impacts and the expected crater sizes has been updated. Using this large sample, empirical relations correlating the luminous energies per photometric band were derived and used to roughly estimate the parameters of 92 suspected flashes of the NELIOTA archive. For a typical value of the luminous efficiency, we found that more than the 75% of the impacting meteoroids have masses between 1-200 g, radii between 0.5-3 cm and produced craters up to 3.5 m. 85% of the peak temperatures of the impacts range between 2000 and 4500 K. Statistics regarding the magnitude decline and the cooling rates of the multiframe flashes are also presented. The recalculation of the appearance frequency of meteoroids, lying within the aforementioned ranges of physical parameters, on the Moon yields that the total lunar surface is bombarded with 7.4 sporadic meteoroids/ hour and up to 12.6 meteoroids/hour when the Earth-Moon system passes through a strong meteoroid stream. By extrapolating these rates on Earth, the respective rates for various distances from its surface are calculated and used to estimate the probability of an impact of a meteoroid with a hypothetical infrastructure on the Moon, or with a satellite orbiting Earth for various impact surfaces and duration times of the missions.

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arXiv2023

Lunar impact flash results and space surveillance activities at Kryoneri Observatory

We present current and future activities regarding lunar impact flash and NEO observations and satellite tracking from Kryoneri Observatory. In particular, we present results from the ESA-funded NELIOTA program, which has been monitoring the Moon for impact flashes since early 2017. Using the 1.2 m Kryoneri telescope, which is equipped with two high frame-rate cameras recording simultaneously in two optical bands, NELIOTA has recorded over 170 validated lunar impact flashes, while another ~90 have been characterized as suspected. We present statistical results concerning the sizes, the masses and the appearance frequency of the meteoroids in the vicinity of the Earth, as well as the temperatures developed during the impacts. Moreover, we present the capabilities of the Kryoneri telescope as a sensor for satellite tracking and the future plans regarding the provision of high-quality services for both the Planetary Defense activities of ESA (S2P/PDO) and the European Union's Space Surveillance and Tracking programme (EU/SST).

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arXiv2021

On the pulsations of the $\delta$ Scuti star of the binary system KIC 6629588

This work includes a comprehensive analysis of the Kepler detached eclipsing binary system KIC 6629588 that aims to the detailed study of the oscillation properties of its pulsating component. Ground-based spectroscopic observations were obtained and used to classify the components of the system. The spectroscopic results were used as constrains for the modelling of the short-cadence Kepler light curves and for the estimation of the absolute parameters of the components. Furthermore, the light curves residuals are analyzed using Fourier transformation techniques in order to search for pulsation frequencies. The primary component of the system is identified as a $\delta$ Scuti star that pulsates in seven eigenfrequencies in the range 13-22 d$^{-1}$, while more than 270 combination frequencies were also detected. The absolute and the oscillation parameters of this pulsating star are compared with those of other $\delta$ Scuti stars-members of detached binary systems using evolutionary and correlation diagrams. Finally, the distance of the system is also estimated.

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arXiv2020

Asteroseismic Analysis of $δ$ Scuti Components of Binary systems: The Case of KIC 8504570

The present work concerns the Asteroseismology of the Kepler detached eclipsing binary KIC 8504570. Particularly, it focuses on the pulsational behaviour of the oscillating component of this system and the estimation of its physical parameters in order to enrich the so far poor sample of this kind of systems. Using spectroscopic observations, the spectral type of the primary component was determined and used to create accurate light curves models and estimate its absolute parameters. The light curve residuals were subsequently analysed using Fourier transformation techniques to obtain the pulsation models. Theoretical models of $δ$ Scuti stars were employed to identify the oscillation modes of the six detected independent frequencies of the pulsator. In addition, more than 385 combination frequencies were also detected. The absolute and the pulsational properties of the $δ$ Scuti star of this system are discussed and compared with all the currently known similar cases. Moreover, using a recent (empirical) luminosity-pulsation period relationship for $δ$ Scuti stars, the distance of the system was estimated.

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arXiv2020

Asteroseismology of two Kepler detached eclipsing binaries

The present work contains light curve, spectroscopic and asteroseismic analyses for KIC 04851217 and KIC 10686876. These systems are detached eclipsing binaries hosting a pulsating component of delta Scuti type and have been observed with the unprecedented accuracy of the Kepler space telescope. Using ground based spectroscopic observations, the spectral types of the primary components of the systems were estimated as A6V and A5V for KIC 04851217 and KIC 10686876, respectively, with an uncertainty of one subclass. The present spectral classification together with literature radial velocities curves were used to model the light curves of the systems and, therefore, to calculate the absolute parameters of their components with higher certainty. The photometric data were analysed using standard eclipsing binary modeling techniques, while their residuals were further analysed using Fourier transformation techniques in order to extract the pulsation frequencies of their host delta Scuti stars. The oscillation modes of the independent frequencies were identified using theoretical models of delta Scuti stars. The distances of the systems were calculated using the relation between the luminosity and the pulsation period for delta Scuti stars. The physical and the oscillation properties of the pulsating components of these systems are discussed and compared with others of the same type. Moreover, using all the currently known cases of delta Scuti stars in detached binaries, updated correlations between orbital and dominant pulsation periods and between log g and pulsation periods were derived. It was found that the proximity of the companion plays significant role to the evolution of the pulsational frequencies.

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arXiv2020

Two transiting hot Jupiters from the WASP survey: WASP-150b and WASP-176b

We report the discovery of two transiting exoplanets from the WASP survey, WASP-150b and WASP-176b. WASP-150b is an eccentric ($e$ = 0.38) hot Jupiter on a 5.6 day orbit around a $V$ = 12.03, F8 main-sequence host. The host star has a mass and radius of 1.4 $\rm M_{\odot}$ and 1.7 $\rm R_{\odot}$ respectively. WASP-150b has a mass and radius of 8.5 $\rm M_J$ and 1.1 $\rm R_J$, leading to a large planetary bulk density of 6.4 $\rm ρ_J$. WASP-150b is found to be $\sim3$ Gyr old, well below its circularisation timescale, supporting the eccentric nature of the planet. WASP-176b is a hot Jupiter planet on a 3.9 day orbit around a $V$ = 12.01, F9 sub-giant host. The host star has a mass and radius of 1.3 $\rm M_{\odot}$ and 1.9 $\rm R_{\odot}$. WASP-176b has a mass and radius of 0.86 $\rm M_J$ and 1.5 $\rm R_J$ respectively, leading to a planetary bulk density of 0.23 $\rm ρ_J$.

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arXiv2020

Spitzer Observations of the Predicted Eddington Flare from Blazar OJ 287

Binary black hole (BH) central engine description for the unique blazar OJ 287 predicted that the next secondary BH impact-induced bremsstrahlung flare should peak on 2019 July 31. This prediction was based on detailed general relativistic modeling of the secondary BH trajectory around the primary BH and its accretion disk. The expected flare was termed the Eddington flare to commemorate the centennial celebrations of now-famous solar eclipse observations to test general relativity by Sir Arthur Eddington. We analyze the multi-epoch Spitzer observations of the expected flare between 2019 July 31 and 2019 September 6, as well as baseline observations during 2019 February-March. Observed Spitzer flux density variations during the predicted outburst time display a strong similarity with the observed optical pericenter flare from OJ 287 during 2007 September. The predicted flare appears comparable to the 2007 flare after subtracting the expected higher base-level Spitzer flux densities at 3.55 and 4.49 $μ$m compared to the optical R-band. Comparing the 2019 and 2007 outburst lightcurves and the previously calculated predictions, we find that the Eddington flare arrived within 4 hours of the predicted time. Our Spitzer observations are well consistent with the presence of a nano-Hertz gravitational wave emitting spinning massive binary BH that inspirals along a general relativistic eccentric orbit in OJ 287. These multi-epoch Spitzer observations provide a parametric constraint on the celebrated BH no-hair theorem.

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arXiv2019

NELIOTA: Methods, statistics and results for meteoroids impacting the Moon

This paper contains the results from the first 30 months of the NELIOTA project for impacts of NEOs/meteoroids on the lunar surface. Our analysis on the statistics concerning the efficiency of the campaign and the parameters of the projectiles and the impacts is presented. The parameters of the lunar impact flashes based on simultaneous observations in two wavelength bands are used to estimate the distributions of the masses, sizes and frequency of the impactors. These statistics can be used both in space engineering and science. The photometric fluxes of the flashes are measured using aperture photometry and their apparent magnitudes are calculated using standard stars. Assuming that the flashes follow a black body law of irradiation, the temperatures can be derived analytically, while the parameters of the projectiles are estimated using fair assumptions on their velocity and luminous efficiency of the impacts. 79 lunar impact flashes have been observed with the 1.2 m Kryoneri telescope in Greece. The masses of the meteoroids range between 0.7 g and 8 kg and their respective sizes between 1-20 cm depending on their assumed density, impact velocity, and luminous efficiency. We find a strong correlation between the observed magnitudes of the flashes and the masses of the meteoroids. Moreover, an empirical relation between the emitted energies of each band has been derived allowing the estimation of the physical parameters of the meteoroids that produce low energy impact flashes. The NELIOTA project has so far the highest detection rate and the faintest limiting magnitude for lunar impacts compared to other ongoing programs. Based on the impact frequency distribution on Moon, we estimate that sporadic meteoroids with typical masses less than 100 g and sizes less than 5 cm enter the mesosphere of the Earth with a rate ~108 meteoroids/hr and also impact Moon with a rate of ~8 meteoroids/hr.

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arXiv2019

NELIOTA Lunar Impact Flash Detection and Event Validation

NELIOTA (NEO Lunar Impacts and Optical TrAnsients) is an ESA-funded lunar monitoring project, which aims to determine the size-frequency distribution of small Near-Earth Objects (NEOs) via detection of impact flashes on the surface of the Moon. A prime focus, high-speed, twin-camera system providing simultaneous observations in two photometric bands at a rate of 30 frames-per-second on the 1.2 m Kryoneri telescope of the National Observatory of Athens was commissioned for this purpose. A dedicated software processes the images and automatically detects candidate lunar impact flashes, which are then validated by an expert user. The four year observing campaign began in February 2017 and has so far detected more than 40 lunar impact events. The software routinely detects satellites, which typically appear as streaks or dots crossing the lunar disk. To avoid confusing these events with real flashes, we check different available catalogs with spacecraft orbital information and exclude spacecraft identifications.

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arXiv2018

KIC 8553788: A pulsating Algol with an extreme mass ratio

The present paper focuses on the eclipsing binary KIC 8553788 which belongs to two different types of binary systems regarding its physical properties. In particular, it is one of the 71 oscillating stars of Algol type that have been discovered so far and one of the six that have been published based on high cadence data of the Kepler mission. In addition, it is one of the four semi-detached binaries of the group of R CMa type systems, while its pulsating component has the fourth fastest frequency among the delta Scuti stars-members of semi-detached binaries. Detailed light curves, spectroscopic and pulsation analyses are presented, while possible explanation scenarios for the evolution of the system involving past mass transfer, mass loss and/or angular momentum loss due to the presence of a tertiary component are discussed. The goal of the study is to extract the pulsational characteristics of the oscillating star of the system, to estimate the absolute parameters of its components and to provide possible explanation for its extreme evolutionary status. The results show that the primary component of the system is of A8 spectral type, has a mass of 1.6 M_sun, and a radius of 2 R_sun. It is a relatively fast pulsator that oscillates in 89 frequency modes with the dominant one to be 58.26 c/d. The secondary component has a mass of only 0.07 M_sun, a radius of 1 R_sun, and a temperature of 4400 K. KIC 8553788 according to its geometrical configuration and its pulsational properties belongs to the group of the oscillating stars of Algol type, while according to its very low mass ratio and its relatively short orbital period belongs also to the group of R CMa stars. If confirmed by radial velocity data of the secondary component, the system would have the lowest mass ratio that has ever been found in semi-detached systems and it can be considered as one of the most extreme cases.

astro-ph.SR↗

arXiv2017

Asteroseismology of Kepler Algol type oscillating eclipsing binaries

Context. This research paper contains light curve modelling, spectroscopy and detailed asteroseismic studies for four out of five in total semidetached eclipsing binaries with a delta Scuti component, that have been detected to date through Kepler mission, namely KIC 06669809, KIC 10581918, KIC 10619109 and KIC 11175495. Aims. The goal is to study the pulsational characteristics of the oscillating stars of the systems as well as to estimate their absolute parameters and enrich the so far poor sample of this kind of systems. Methods. Ground based spectroscopic observations provide the means to estimate the spectral types of the primary components and to model the light curves with higher certainty. The photometric data are analysed using eclipsing binary modeling techniques, while Fourier analysis is applied on their residuals in order to reveal the pulsation frequency modes. Results. The results of analyses show that the primaries are pulsating stars of delta Scuti type and that all systems belong to the group of Algol type binaries with an oscillating star, namely oEA stars. The primaries of KIC 06669809, KIC 10581918, and KIC 10619109 pulsate in three, two, and five frequencies, respectively and in more than 200 other detected as combinations. The delta Scuti star of KIC 11175495 is the youngest and the fastest pulsator in binary systems that has ever been found and it oscillates in three main non radial frequencies, while other 153 are also found as depended ones. Moreover, a comparison of their properties with other systems of the same type as well as with theoretical models of pulsating stars are also presented and discussed.

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arXiv2016

Catalogue and Properties of δ Scuti Stars in Binaries

The catalogue contains 199 confirmed cases of binary systems containing at least one pulsating component of δ Scuti type. The sample is divided into subgroups in order to describe the properties and characteristics of the δ Sct type stars in binaries according to their Roche geometry. Demographics describing quantitatively our knowledge for these systems as well as the distributions of their pulsating components in the Mass-Radius, Colour-Magnitude and Evolutionary Status-Temperature diagrams are presented and discussed. It is shown that a threshold of ~13 days of the orbital period regarding the influence of binarity on the pulsations is established. Finally, the correlations between the pulsation periods and the orbital periods, evolutionary status, and companion's gravity influence are updated based on the largest sample to date.

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