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J. Kovács

Publications and source records attributed to J. Kovács.

14 recordsLinked to original sources
arXiv2025

A high geometric albedo and small size of the Haumea cluster member (24835) 1995 SM55 from a stellar occultation and photometric observations

Trans-Neptunian objects (TNOs) are among the most ancient bodies of the solar system. Understanding their physical properties is key to constraining their origin and the evolution of the outer regions beyond Neptune. Stellar occultations provide highly accurate size and shape information. (24835) 1995 SM55 is one of the few members of the Haumea cluster and thus of particular interest. We aimed to determine its projected size, absolute magnitude, and geometric albedo, and to compare these with Haumea. A stellar occultation on 25 February 2024 was observed from five sites, with seven positive detections and 33 negative chords. An elliptical fit to the occultation chords yields semi-axes of $(104.3 \pm 0.4) \times (83.5 \pm 0.5)$ km, giving an area-equivalent diameter of $186.7 \pm 1.8$ km, smaller than the 250 km upper limit from Herschel thermal data. Photometry provides an absolute magnitude $H_V = 4.55 \pm 0.03$, a phase slope of $0.04 \pm 0.02$ mag/deg, and a $V-R = 0.37 \pm 0.05$. The rotational variability has an amplitude $Δm = 0.05$ mag, but the period remains uncertain. Combining occultation and photometry, we derive a geometric albedo $p_V = 0.80 \pm 0.04$, one of the highest values measured for a TNO. This value is slightly higher than that of Haumea, consistent with the interpretation that 1995 SM55 belongs to the Haumea cluster.

astro-ph.EP↗
arXiv2024

Atmospheric parameters and abundances of cool red giant stars

Understanding the atmospheric parameters of stars on the top of the RGB is essential to reveal the chemical composition of the Milky Way, as they can be used to probe the farthest parts of our Galaxy. Our goal is to determine the chemical composition of 21 RGB stars with $T_{\mathrm{eff}}<4200$K selected from the APOGEE-2 DR17 database using new observations carried out with the spectrograph mounted on the 1-m telescope of the Hungarian Piszkéstető Observatory and the SONG spectrograph (R=77000) on the Hertzsprung SONG telescope in the 4500$-$5800Å wavelength range. This is the first time the spectrograph (R=18000) on the 1-m telescope at Piszkéstető Observatory was used to measure the abundances of stars. We created a new LTE spectral library using MARCS model atmospheres and SYNSPEC by including the line list of 23 molecules to determine atmospheric parameters ($T_{\mathrm{eff}}$, $\log g$, [Fe/H], [$α$/Fe]) and abundances of Si, Ca, Ti, V, Cr, Mn, and Ni with FERRE. The resulting parameters were compared to that of APOGEE. We found a good agreement in general, the average difference is $-$11.2K in $T_{\mathrm{eff}}$, 0.11dex in $\log g$, 0.10dex in [Fe/H], and $-$0.01dex in [$α$/Fe]. Our analysis successfully demonstrates the ability of the spectrograph at Piszkéstető Observatory to reliably measure the abundance of bright stars.

astro-ph.SR↗
arXiv2023

Orbit determination for the binary Cepheid V1344 Aql

Context. Binary Cepheids play an important role in investigating the calibration of the classical Cepheid period-luminosity relationship. Therefore a thorough study of individual Cepheids belonging to binary systems is necessary. Aims. Our aim is to determine the orbit of the binary system V1344 Aql using newly observed and earlier published spectroscopic and photometric data. Methods. We collected new radial velocity observations using medium resolution (${R \approx 11000}$ and ${R \lessapprox 20000}$) spectrographs and we updated the pulsation period of the Cepheid based on available photometric observations using $O-C$ diagram. Separating the pulsational and orbital radial velocity variations for each observational season (year), we determined the orbital solution for the system using $χ^2$ minimisation. Results. The updated pulsation period of the Cepheid estimated for the epoch of HJD 2458955.83 is 7.476826 days. We determined orbital elements for the first time in the literature. The orbital period of the system is about 34.6 years, with an eccentricity e = 0.22.

astro-ph.SR↗
arXiv2023

Discovery of a substellar companion in the TESS light curve of the $δ$ Scuti/$γ$ Doradus hybrid pulsator HD 31221

Close-in, sub-stellar companions to $δ$ Scuti type stars present a highly suitable testbed for examining how planetary-mass objects can influence stellar pulsations. We aim to constrain the mass of HD 31221 b, probe its atmosphere, and demonstrate how it affects the pulsational pattern of its host, HD 31221. We made use of the available data from the short-cadence Transiting Exoplanet Survey Satellite (TESS). We modeled the nine observed transits and the out-of-phase variations, including Doppler beaming, ellipsoidal variations, and the reflection effect. We also incorporated ground-based photometry from the MuSCAT2 imager installed at the 1.52 m Telescopio Carlos Sanchez in the Teide Observatory, Spain. We found HD 31221 b to have an orbital period of $4.66631 \pm 0.00011$ days, with a radius of $1.32 \pm 0.14$ R$_J$ and a mass of $11.5 \pm 10.3$ M$_J$ (from the ellipsoidal effect), making it consistent with either a brown dwarf or a giant planet. As HD 31221 is a rapid rotator ($v \sin I_\star = 175.31 \pm 1.74$ km s$^{-1}$), we deduced the spin-orbit misalignment to be $λ= -121.6 \pm 14.4^\circ$ and $I_\star = 55.9 \pm 11.3^\circ$. The phase curve is dominated by the reflection effect, with a geometric albedo of $1.58 \pm 0.50$. We also found evidence that HD 31221 is a $δ$ Scuti/$γ$ Doradus hybrid pulsator. There are three cases for which the $3$rd, $85$th, and $221$st orbital harmonics almost exactly coincide with peaks in the Fourier spectrum of the star, hinting at tidally perturbed stellar oscillations. HD 31221 b is the third substellar object that is found to be disrupting the pulsations of its host, following HAT-P-2 and WASP-33. Additional photometric observations by CHEOPS and/or PLATO can be used to further constrain its mass and provide a more in-depth analysis of its atmosphere.

astro-ph.EP↗
arXiv2021

Entanglement and seniority

We study mode-entanglement in the seniority model, derive analytic formulas for the one-body reduced density matrix of states with seniority $ν= 0,\;1,\;2$, and $ν=3$, and also determine the particle number dependence of the one-body reduced density matrix for arbitrary seniority. We carry out numerical calculations for the lightest calcium isotopes and for $^{94}{\rm Ru}$ nucleus, and investigate the structure of their ground and low energy yrast states. We investigate the fulfillment of several predictions of the seniority model regarding the behavior of one-mode entropies, which we compare with the results of configuration interaction (CI) and density matrix renormalization group (DMRG) computations. For $^{94}{\rm Ru}$, the seniority model accounts for the $0g_{9/2}$ mode entropies, but seniority mixing is important for certain yrast states. Interaction induced quantum fluctuations decrease the occupation of the $0f_{5/2}$, $1p_{3/2}$ and $1p_{1/2}$ shells, and amount in finite mode entropies on these shells, too, clearly outside the scope of the simple $(0g_{9/2})^4$ seniority model. The $0f_{7/2}$ shell based seniority model is more accurate for the light ${\rm Ca}$ isotopes, but seniority mixing is substantial for some $^{44}{\rm Ca}$ yrast states, too.

nucl-th↗
arXiv2018

Description of two-electron atoms with correct cusp conditions

New sets of functions with arbitrary large finite cardinality are constructed for two-electron atoms. Functions from these sets exactly satisfy the Kato's cusp conditions. The new functions are special linear combinations of Hylleraas- and/or Kinoshita-type terms. Standard variational calculation, leading to matrix eigenvalue problem, can be carried out to calculate the energies of the system. There is no need for optimization with constraints to satisfy the cusp conditions. In the numerical examples the ground state energy of the He atom is considered.

physics.atom-ph↗
arXiv2016

V473 Lyr, a modulated, period-doubled Cepheid, and U TrA, a double-mode Cepheid observed by MOST

Space-based photometric measurements first revealed low-amplitude irregularities in the pulsations of Cepheid stars, but their origins and how commonly they occur remain uncertain. To investigate this phenomenon, we present MOST space telescope photometry of two Cepheids. V473 Lyrae is a second-overtone, strongly modulated Cepheid, while U Trianguli Australis is a Cepheid pulsating simultaneously in the fundamental mode and first overtone. The nearly continuous, high-precision photometry reveals alternations in the amplitudes of cycles in V473 Lyr, the first case of period doubling detected in a classical Cepheid. In U TrA, we tentatively identify one peak as the $f_X$ or 0.61-type mode often seen in conjunction with the first radial overtone in Cepheids, but given the short length of the data, we cannot rule out that it is a combination peak instead. Ground-based photometry and spectroscopy were obtained to follow two modulation cycles in V473 Lyr and to better specify its physical parameters. The simultaneous data yield the phase lag parameter (the phase difference between maxima in luminosity and radial velocity) of a second-overtone Cepheid for the first time. We find no evidence for a period change in U TrA or an energy exchange between the fundamental mode and the first overtone during the last 50 years, contrary to earlier indications. Period doubling in V473 Lyr provides a strong argument that mode interactions do occur in some Cepheids and we may hypothesise that it could be behind the amplitude modulation, as recently proposed for Blazhko RR Lyrae stars.

astro-ph.SR↗
arXiv2016

Investigating magnetic activity in very stable stellar magnetic fields: long-term photometric and spectroscopic study of the fully convective M4 dwarf V374 Peg

The ultrafast-rotating ($P_\mathrm{rot}\approx0.44 d$) fully convective single M4 dwarf V374 Peg is a well-known laboratory for studying intense stellar activity in a stable magnetic topology. As an observable proxy for the stellar magnetic field, we study the stability of the light curve, and thus the spot configuration. We also measure the occurrence rate of flares and coronal mass ejections (CMEs). We analyse spectroscopic observations, $BV(RI)_C$ photometry covering 5 years, and additional $R_C$ photometry that expands the temporal base over 16 years. The light curve suggests an almost rigid-body rotation, and a spot configuration that is stable over about 16 years, confirming the previous indications of a very stable magnetic field. We observed small changes on a nightly timescale, and frequent flaring, including a possible sympathetic flare. The strongest flares seem to be more concentrated around the phase where the light curve indicates a smaller active region. Spectral data suggest a complex CME with falling-back and re-ejected material, with a maximal projected velocity of $\approx$675km/s. We observed a CME rate much lower than expected from extrapolations of the solar flare-CME relation to active stars.

astro-ph.SR↗
arXiv2014

Discovery of the spectroscopic binary nature of the classical Cepheids FN Aql and V1344 Aql

We present the analysis of photometric and spectroscopic data of two classical Cepheids, FN Aquilae and V1344 Aquilae. Based on the joint treatment of the new and earlier radial velocity data, both Galactic Cepheids have been found to be a member in a spectroscopic binary system. To match the phases of the earlier radial velocity data correctly with the new ones, we also determined the temporal behaviour of the pulsation period of these Cepheids based on all available photometric data. The O-C graph covering about half century shows slight changes in the pulsation period due to stellar evolution for both Cepheids.

astro-ph.SR↗
arXiv2014

Affordable spectroscopy for 1m-class telescopes

Doppler observations of exoplanet systems have been a very expensive technique, mainly due to the high costs of high-resolution stable spectrographs. Recent advances in instrumentation enable affordable Doppler planet detections with surprisingly small optical telescopes. We investigate the possibility of measuring Doppler reflex motion of planet hosting stars with small-aperture telescopes that have traditionally been neglected for this kind of studies. After thoroughly testing the recently developed and commercially available Shelyak eShel echelle spectrograph, we demonstrated that it is routinely possible to achieve velocity precision at the $100 \mathrm{m\,s}^{-1}$ level, reaching down to $\pm 50~\mathrm{m\,s}^{-1}$ for the best cases. We describe our off-the-shelf instrumentation, including a new 0.5m RC telescope at the Gothard Astrophysical Observatory of Loránd Eötvös University equipped with an intermediate resolution fiber-fed echelle spectrograph. We present some follow-up radial velocity measurements of planet hosting stars and point out that updating the orbital solution of Doppler-planets is a very important task that can be fulfilled with sub-meter sized optical telescopes without requesting very expensive telescope times on 2--4~m (or larger) class telescopes.

astro-ph.SR↗
arXiv2013

Discovery of the spectroscopic binary nature of six southern Cepheids

We present the analysis of photometric and spectroscopic data of six bright Galactic Cepheids: GH Carinae, V419 Centauri, V898 Centauri, AD Puppis, AY Sagittarii, and ST Velorum. Based on new radial velocity data (in some cases supplemented with earlier data available in the literature), these Cepheids have been found to be members in spectroscopic binary systems. V898 Cen turned out to have one of the largest orbital radial velocity amplitude (> 40 km/s) among the known binary Cepheids. The data are insufficient to determine the orbital periods nor other orbital elements for these new spectroscopic binaries. These discoveries corroborate the statement on the high frequency of occurrence of binaries among the classical Cepheids, a fact to be taken into account when calibrating the period-luminosity relationship for Cepheids. We have also compiled all available photometric data that revealed that the pulsation period of AD Pup, the longest period Cepheid in this sample, is continuously increasing with Delta P = 0.004567 d/century, likely to be caused by stellar evolution. The wave-like pattern superimposed on the parabolic O-C graph of AD Pup may well be caused by the light-time effect in the binary system. ST Vel also pulsates with a continuously increasing period. The other four Cepheids are characterised with stable pulsation periods in the last half century.

astro-ph.SR↗
arXiv2011

Cepheid investigations using the Kepler space telescope

We report results of initial work done on selected candidate Cepheids to be observed with the Kepler space telescope. Prior to the launch 40 candidates were selected from previous surveys and databases. The analysis of the first 322 days of Kepler photometry, and recent ground-based follow-up multicolour photometry and spectroscopy allowed us to confirm that one of these stars, V1154 Cyg (KIC 7548061), is indeed a 4.9-d Cepheid. Using the phase lag method we show that this star pulsates in the fundamental mode. New radial velocity data are consistent with previous measurements, suggesting that a long-period binary component is unlikely. No evidence is seen in the ultra-precise, nearly uninterrupted Kepler photometry for nonradial or stochastically excited modes at the micromagnitude level. The other candidates are not Cepheids but an interesting mix of possible spotted stars, eclipsing systems and flare stars.

astro-ph.SR↗
arXiv2010

Structure and evolution of debris disks around F-type stars: I. Observations, database and basic evolutionary aspects

Although photometric and spectroscopic surveys with the Spitzer Space Telescope increased remarkably the number of well studied debris disks around A-type and Sun-like stars, detailed analyzes of debris disks around F-type stars remained less frequent. Using the MIPS camera and the IRS spectrograph we searched for debris dust around 82 F-type stars with Spitzer. We found 27 stars that harbor debris disks, nine of which are new discoveries. The dust distribution around two of our stars, HD 50571 and HD 170773, was found to be marginally extended on the 70um MIPS images. Combining the MIPS and IRS measurements with additional infrared and submillimeter data, we achieved excellent spectral coverage for most of our debris systems. We have modeled the excess emission of 22 debris disks using a single temperature dust ring model and of 5 debris systems with two-temperature models. The latter systems may contain two dust rings around the star. In accordance with the expected trends, the fractional luminosity of the disks declines with time, exhibiting a decay rate consistent with the range of model predictions. We found the distribution of radial dust distances as a function of age to be consistent with the predictions of both the self stirred and the planetary stirred disk evolution models. A more comprehensive investigation of the evolution of debris disks around F-type stars, partly based on the presented data set, will be the subject of an upcoming paper.

astro-ph.SR↗