arXiv subjects

Jan Budaj

Publications and source records attributed to Jan Budaj.

17 recordsLinked to original sources

arXiv2022

WD 1145+017: Alternative models of the atmosphere, dust clouds, and gas rings

WD 1145+017 is the first white dwarf known to be orbited by disintegrating exoasteroids. It is a DBZ-type white dwarf with strongly variable broad circumstellar lines and variable shallow UV transits. Various models of the dust clouds and gaseous rings have been proposed as an explanation for this behavior. Here we propose alternative models.The simple radiative transfer code Shellspec was modified for this purpose and used for testing the new dust cloud and gas disk models. We used modified TLUSTY and SYNSPEC codes to calculate atmosphere models assuming the LTE or NLTE, and to calculate the intrinsic spectrum of the star. We then used these atmosphere models to estimate the mass of the radiative and convective zones and NLTE spectrum synthesis to estimate their chemical composition. We offer an alternative explanation of some (not all) shallow UV transits. These may be naturally caused by the optical properties of the dust grains: opacities and mainly phase functions as a result of the forward scattering. The latter is much stronger in UV compared to the optical region, leaving more UV photons in the original direction during the transit. We also developed an alternative model of the gaseous disk, consisting of an inner, hotter, and almost circular disk and an outer, cooler, and eccentric disk. The structure precesses with a period of $3.83 \pm 0.12$ yr. We demonstrate that it fits the observed circumstellar lines reasonably well. These alternative models solve a few drawbacks that might be associated with the previous models, but they also have their own disadvantages. We confirm that the chemical composition of the atmosphere is similar to that of CI chondrites but C, N, and S are significantly underabundant and much closer to the bulk Earth composition. This is a strong argument that the star has recently encountered and accreted material from a body of Earth-like composition.

astro-ph.EP↗

arXiv2021

Looking into the cradle of the grave: J22564-5910, a young post-merger hot subdwarf?

We present the discovery of J22564-5910, a new type of hot subdwarf (sdB) which shows evidence of gas present in the system and has shallow, multi-peaked hydrogen and helium lines which vary in shape over time. All observational evidence points towards J22564-5910 being observed very shortly after the merger phase that formed it. Using high-resolution, high signal-to-noise spectroscopy, combined with multi-band photometry, Gaia astrometry, and TESS light curves, we aim to interpret these unusual spectral features. The photometry, spectra and light curves are all analyzed, and their results are combined in order to support our interpretation of the observations: the likely presence of a magnetic field combined with gas features around the sdB. Based on the triple-peaked H lines, the magnetic field strength is estimated and, by using the shellspec code, qualitative models of gas configurations are fitted to the observations. All observations can either be explained by a magnetic field of ~650 kG which enables the formation of a centrifugal magnetosphere, or a non-magnetic hot subdwarf surrounded by a circumstellar gas disk/torus. Both scenarios are not mutually exclusive and both can be explained by a recent merger. J22564-5910 is the first object of its kind. It is a rapidly spinning sdB with gas still present in the system. It is the first post-merger star observed this early after the merger event, and as such is very valuable system to test merger theories. If the magnetic field can be confirmed, it is not only the first magnetic sdB, but it hosts the strongest magnetic field ever found in a pre-white dwarf object. Thus, it could represent the long-sought for immediate ancestor of strongly magnetic WDs.

astro-ph.SR↗

arXiv2021

Shellspec39 -- a tool for modelling the spectra, light curves, and images of interacting binaries and exoplanets

Program SHELLSPEC is designed to calculate light curves, spectra, and images of interacting binaries and extra-solar planets immersed in a moving gaseous or dusty circumstellar matter. It solves simple radiative transfer along the line of sight in 3D moving media. The Roche model and synthetic spectra from the stellar atmosphere models such as TLUSTY from Ivan Hubeny can be used as a boundary condition for the radiative transfer. The latest publicly available version is Shellspec39. The code has been combined with other methods such as Doppler tomography and interferometry and used to analyze spectroscopic, photometric, and interferometric observations of binary stars and transiting exoplanets. A few examples are briefly mentioned.

astro-ph.EP↗

arXiv2020

Spatial distribution of exoplanet candidates based on Kepler and $Gaia$ data

The spatial distribution of exoplanets in the Galaxy is important for our understanding of planet formation and evolution. We aim to determine the spatial gradients of exoplanet occurrence in the Solar neighbourhood and in the vicinity of open clusters. We combined Kepler and Gaia DR2 data for this purpose, splitting the volume sampled by the Kepler mission into certain spatial bins. We determined an uncorrected and bias-corrected exoplanet frequency and metallicity for each bin. There is a clear drop in the uncorrected exoplanet frequency with distance for F-type stars, a decline with increasing distance along the Galactic longitude l=90 deg, and a drop with height above the Galactic plane. We find that the metallicity behaviour cannot be the reason for the drop of the exoplanet frequency around F stars with increasing distance. We argue that the above-mentioned gradients of uncorrected exoplanet frequency result from a single bias of undetected smaller planets around fainter stars. When we correct for observational biases, most of these gradients in exoplanet frequency become statistically insignificant. Only a slight decline of the planet occurrence with distance for F stars remains significant at the 3 sigma level. Apart from that, the spatial distribution of exoplanets in the Kepler field of view is compatible with a homogeneous one. At the same time, we do not find a significant change in the exoplanet frequency with increasing distance from open clusters. As a byproduct, we identified six exoplanet host star candidates that are members of open clusters. Four of them are in the NGC 6811 (KIC 9655005, KIC 9533489, Kepler-66, Kepler-67) and two belong to NGC 6866 (KIC 8396288, KIC 8331612). Two out of the six had already been known to be cluster members.

astro-ph.EP↗

arXiv2020

Extrasolar enigmas: from disintegrating exoplanets to exoasteroids

Thousands of transiting exoplanets have been discovered to date, thanks in great part to the {\em Kepler} space mission. As in all populations, and certainly in the case of exoplanets, one finds unique objects with distinct characteristics. Here we will describe the properties and behaviour of a small group of `disintegrating' exoplanets discovered over the last few years (KIC 12557548b, K2-22b, and others). They evaporate, lose mass unraveling their naked cores, produce spectacular dusty comet-like tails, and feature highly variable asymmetric transits. Apart from these exoplanets, there is observational evidence for even smaller `exo-'objects orbiting other stars: exoasteroids and exocomets. Most probably, such objects are also behind the mystery of Boyajian's star. Ongoing and upcoming space missions such as {\em TESS} and PLATO will hopefully discover more objects of this kind, and a new era of the exploration of small extrasolar systems bodies will be upon us.

astro-ph.EP↗

arXiv2018

$τ$ Ori and $τ$ Lib: Two new massive heartbeat binaries

We report the discovery of two massive eccentric systems with BRITE data, $τ$ Ori and $τ$ Lib, showing heartbeat effects close to the periastron passage. $τ$ Lib exhibits shallow eclipses that will soon vanish due to the apsidal motion in the system. In neither system, tidally excited oscillations were detected.

astro-ph.SR↗

arXiv2015

Tables of phase functions, opacities, albedos, equilibrium temperatures, and radiative accelerations of dust grains in exoplanets

There has been growing observational evidence for the presence of condensates in the atmospheres and/or comet-like tails of extrasolar planets. As a result, systematic and homogeneous tables of dust properties are useful in order to facilitate further observational and theoretical studies. In this paper we present calculations and analysis of non-isotropic phase functions, asymmetry parameter (mean cosine of the scattering angle), absorption and scattering opacities, single scattering albedos, equilibrium temperatures, and radiative accelerations of dust grains relevant for extrasolar planets. Our assumptions include spherical grain shape, Deirmendjian particle size distribution, and Mie theory. We consider several species: corundum/alumina, perovskite, olivines with 0\% and 50\% iron content, pyroxenes with 0\%, 20\% and 60\% iron content, pure iron, carbon at two different temperatures, water ice, liquid water, and ammonia. The presented tables cover the wavelength range of 0.2 to 500 micron and modal particle radii from 0.01 micron to 100 micron. Equilibrium temperatures and radiative accelerations assume irradiation by a non-black-body source of light with temperatures from 7000K to 700K seen at solid angles from 2$π$ to $10^{-6}$ sr. The tables are provided to the community together with a simple code which allows for an optional, finite, angular dimension of the source of light (star) in the phase function.

astro-ph.EP↗

arXiv2013

Spectroscopic metallicity determinations for W~UMa-type binary stars

This study is an attempt to determine the metallicities of WUMa-type binary stars using spectroscopy. ~4,500 spectra collected at the David Dunlap Observatory were subject to the same Broadening Function processing to determine the combined line strength in the spectral window centered on the MgI triplet (5080-5285A). Individual integrated BF's were phase averaged to derive a single line-strength indicator. The sample was limited to 90 EW binaries with the strict phase-constancy of colors and without spectral contamination by companions. The best defined results were obtained for a F-type sub-sample (0.32<(B-V)0<0.62) of 52 stars for which the BF strengths could be interpolated in the model predictions. The metallicities, [M/H], for the F-type sub-sample indicate abundances roughly similar to the solar [M/H], but with a large scatter which is partly due to combined random and systematic errors. Because of a color trend resulting from limitations in our approach, we set the scale of metallicities to correspond to that derived from the m_1 index of the Stromgren photometry for F-type binaries. The trend-adjusted [M/H]1 are distributed within -0.65<[M/H]1<+0.50, with the spread reflecting genuine metallicity differences between stars. One half of the F-sub-sample binaries have [M/H]1 within -0.37<[M/H]1 +0.10, a median of -0.04 and a mean of -0.10, with a tail towards low metallicities, and a possible bias against very high metallicities. A parallel study of kinematic data, utilizing the most reliable and recently obtained proper motion and radial velocity data for 78 stars of the full sample, shows that the F-type sub-sample binaries have similar kinematic properties to solar neighborhood, thin-disk dwarfs with ages about 3 - 5.5 Gyr. The F-type binaries which appear to be older than the rest tend to have systematically smaller mass-ratios than most of the EW binaries of the same period.

astro-ph.SR↗

arXiv2013

Modelling the light-curve of KIC 12557548b: an extrasolar planet with a comet like tail

An object with a very peculiar light-curve was discovered recently using Kepler data. Authors argue that this object may be a transiting disintegrating planet with a comet like dusty tail. We calculate the light-curves of stars with such planets and take into account the Mie absorption and scattering on spherical dust grains of various sizes assuming realistic dust opacities and phase functions and finite radius of the source of the scattered light. The planet light-curve is reanalysed using long and short cadence Kepler observations from the first 14 quarters. Orbital period of the planet was improved. We prove that the peculiar light-curve of this objects is in agreement with the idea of a planet with a comet like tail. There is an evidence of a quasi periodic long term evolution of the tail. Light-curve has a prominent pre-transit brightening and a less prominent post-transit brightening. Both are caused by the forward scattering and are a strong function of the particle size. This feature enabled us to estimate a typical particle size (radius) in the dust tail of about 0.1-1 micron. However, there is an indication that the particle size changes along the tail. Larger particles better reproduce the pre-transit brightening and transit core while smaller particles are more compatible with the egress and post-transit brightening. Dust density in the tail is a steep decreasing function of the distance from the planet which indicates a significant tail destruction caused by the star. We also argue that the 'planet' does not show uniform behaviour but may have at least two constituents. This light-curve with pre-transit brightening is analogous to the light-curve of $ε$ Aur with mid-eclipse brightening and forward scattering plays a significant role in such eclipsing systems.

astro-ph.EP↗

arXiv2012

Modeling the Accretion Structure of AU Mon

AU Mon is a long-period (11.113 d) Algol-type binary system with a persistent accretion disk that is apparent as double-peaked H-alpha emission. We present previously unpublished optical spectra of AU Mon which were obtained over several years with dense orbital phase coverage. We utilize these data, along with archival UV spectra, to model the temperature and structure of the accretion disk and the gas stream. Synthetic spectral profiles for lines including H-alpha, H-beta, and the Al III and Si IV doublets were computed with the Shellspec program. The best match between the model spectra and the observations is obtained for an accretion disk of inner/outer radius 5.1/23 R_sun, thickness of 5.2 R_sun, density of 1.0e-13 g/cm^3, and maximum temperature of 14000 K, along with a gas stream at a temperature of ~8000 K transferring ~2.4e-9 M_sun/yr. We show H-alpha Doppler tomograms of the velocity structure of the gas, constructed from difference profiles calculated through sequentially subtracting contributions from the stars and accretion structures. The tomograms provide independent support for the Shellspec modeling, while also illustrating that residual emission at sub-Keplerian velocities persists even after subtracting the disk and stream emission. Spectral variability in the H-alpha profile beyond that expected from either the orbital or the long-period cycle is present on both multi-week and multi-year timescales, and may reflect quasi-random changes in the mass transfer rate or the disk structure. Finally, a transient UV spectral absorption feature may be modeled as an occasional outflow launched from the vicinity of the disk-stream interaction region.

astro-ph.SR↗

arXiv2012

Evidence for enhanced chromospheric Ca II H & K emission in stars with close-in extrasolar planets

The planet-star interaction is manifested in many ways. It was found out that a close-in exoplanet causes small but measurable variability in the cores of a few lines in the spectra of several stars which corresponds to the orbital period of the exoplanet. Stars with and without exoplanets may have different properties. The main goal of our study is to search for influence which exoplanets might have on atmospheres of their host stars. Unlike the previous studies, we do not study changes in the spectrum of a host star or differences between stars with and without exoplanets. We aim to study a large number of stars with exoplanets, current level of their chromospheric activity and look for a possible correlation with the exoplanetary properties. To analyse the chromospheric activity of stars we exploit our own (2.2m ESO/MPG telescope) and publicly available archival spectra (Keck Observatory Archive), measure the equivalent widths of the cores of Ca II H and K lines and use them as a tracer of their activity. Subsequently, we search for their dependence on the orbital parameters and mass of the exoplanet. We found a statistically significant evidence that the equivalent width of the Ca II K line emission and log R'_{HK} activity parameter of the host star varies with the semi-major axis and mass of the exoplanet. Stars with T_eff <= 5500 K having exoplanets with semi-major axis a <= 0.15 AU (P_orb <= 20 days) have a broad range of Ca II K emissions and much stronger emission in general than stars at similar temperatures but with higher values of semi-major axes. Ca II K emission of cold stars (T_eff <= 5500 K) with close-in exoplanets (a <= 0.15 AU) is also more pronounced for more massive exoplanets. The overall level of the chromospheric activity of stars may be affected by their close-in exoplanets. Stars with massive close-in exoplanets may be more active.

astro-ph.EP↗

arXiv2011

Synthetic Spectra and Light Curves of Interacting Binaries and Exoplanets with Circumstellar Material: SHELLSPEC

Program SHELLSPEC is designed to calculate light-curves, spectra and images of interacting binaries and extrasolar planets immersed in a moving circumstellar environment which is optically thin. It solves simple radiative transfer along the line of sight in moving media. The assumptions include LTE and optional known state quantities and velocity fields in 3D. Optional (non)transparent objects such as a spot, disc, stream, jet, ufo, shell or stars may be defined (embedded) in 3D and their composite synthetic spectrum calculated. Roche model can be used as a boundary condition for the radiative transfer. Recently a new model of the reflection effect, dust and Mie scattering were incorporated into the code. $ε$ Aurigae is one of the most mysterious objects on the sky. Prior modeling of its light-curve assumed dark, inclined, disk of dust with the central hole to explain the light-curve with a sharp mid-eclipse brightening. Our model consists of two geometrically thick flared disks. Internal optically thick disk and external optically thin disk which absorbs and scatters radiation. Shallow mid-eclipse brightening may result from eclipses by nearly edge-on flared (dusty or gaseous) disks. Mid-eclipse brightening may also be due to strong forward scattering and optical properties of the dust which can have an important effect on the light-curves. There are many similarities between interacting binary stars and transiting extrasolar planets. Reflection effect which is briefly reviewed is one of them. The exact Roche shape and temperature distributions over the surface of all currently known transiting extrasolar planets have been determined. In some cases (HAT-P-32b, WASP-12b, WASP-19b) departures from the spherical shape can reach 7-15%.

astro-ph.SR↗

arXiv2011

Search for star-planet interaction

We analyse the chromospherical activity of stars with extrasolar planets and search for a possible correlation between the equivalent width of the core of Ca II K line and orbital parameters of the planet. We found a statistically significant evidence that the equivalent width of the Ca II K line reversal, which originates in the stellar chromosphere depends on the orbital period P_orb of the exoplanet. Planets orbiting stars with T_eff < 5500 K and with P_orb < 20 days generally have much stronger emission than planets at similar temperatures but at longer orbital periods. P_orb=20 days marks a sudden change in behaviour, which might be associated with a qualitative change in the star-planet interaction.

astro-ph.EP↗

arXiv2010

Photometric observation of transiting extrasolar planet WASP - 10b

Wasp-10b is a very interesting transiting extrasolar planet. Although its transit is very deep, about 40 mmag, there are very different estimates of its radius in the literature. We present new photometric observations of four complete transits of this planet. The whole event was detected for each transit and the final light curve consists of more than 1500 individual CCD exposures. We determine the following system parameters: planet to star radius ratio R_p/R_star=0.168 +- 0.001, star radius to semimajor axis ratio R_star/a = 0.094 +- 0.001 and inclination i=87.3 +- 0.1 deg. Assuming that the semimajor axis is 0.036 9 +- ^{0.0012}_{0.0014} AU (Christian et al. 2009), we obtain the following radius of the planet R_p=1.22 +- 0.05 R_J, and radius of the star R_star = 0.75 +- 0.03. The errors include the uncertainty in the stellar mass and the semimajor axis of the planet. Surprisingly, our estimate of the planet radius is significantly higher (by about 12 percent) than the most recent value of Johnson et al. (2009). We also improve the orbital period P_orb=3.092 731 +- 1x10^{-6} days and estimate the average transit duration T_D=0.097 4 +- 8x10^{-4} days.

astro-ph.EP↗

arXiv2010

A simple model of the reflection effect for the interacting binaries and extrasolar planets

Extrasolar planets are a natural extension of the interacting binaries towards the companions with very small masses and similar tools might be used to study them. Unfortunately, the generally accepted treatment of the reflection effect in interacting binaries is not very suitable to study cold objects irradiated by hot objects or extrasolar planets. Our simple model of the reflection effect takes into account the reflection (scattering), heating and heat redistribution over the surface of the irradiated object. The shape of the objects is described by the Roche potential and limb and gravity darkening can be taken into account. The orbital revolution and rotation of the planet with proper Doppler shifts for the scattered and thermal radiation are also accounted for. Subsequently, light-curves and/or spectra of exoplanets were modeled and the effects of the heat redistribution, limb darkening/brightening, (non-)grey albedo, and non-spherical shape were studied. Recent observations of HD189733b, WASP12b, and Wasp-19b were reproduced reasonably well. HD189733b has low Bond albedo and intense heat redistribution. Wasp-19b has low Bond albedo and low heat redistribution. We also calculate the exact Roche shapes and temperature distribution over the surface of all 78 transiting extrasolar planets known so far. It is found that the departures from the sphere vary considerably within the sample. Departures of about 1% are common. In some cases: WASP-12b, WASP-19b, WASP-33b departures can reach about 14, 12, and 8%, respectively. The mean temperatures of these planets also vary considerably from 300 K to 2600 K. The extreme cases are WASP-33b, WASP-12b, and WASP-18b with mean temperatures of about 2600, 2430, and 2330 K, respectively.

astro-ph.EP↗

arXiv2006

Revealing the Nature of Algol Disks through Optical and UV Spectroscopy, Synthetic Spectra, and Tomography of TT Hydrae

We have developed a systematic procedure to study the disks in Algol-type binaries using spectroscopic analysis, synthetic spectra, and tomography. We analyzed 119 H-alpha spectra of TT Hya, an Algol-type eclipsing interacting binary, collected from 1985-2001. The new radial velocities enabled us to derive reliable orbital elements, including a small non-zero eccentricity, and to improve the accuracy of the absolute dimensions of the system. High resolution IUE spectra were also analyzed to study the formation of the ultraviolet lines and continuum. Synthetic spectra of the iron curtain using our new shellspec program enabled us to derive a characteristic disk temperature of 7000K. We have demonstrated that the UV emission lines seen during total primary eclipse cannot originate from the accretion disk, but most likely arise from a hotter disk-stream interaction region. The synthetic spectra of the stars, disk, and stream allowed us to derive a lower limit to the mass transfer rate of 2e-10 solar masses per year. Doppler tomography of the observed H-alpha profiles revealed a distinct accretion disk. The difference spectra produced by subtracting the synthetic spectra of the stars resulted in an image of the disk, which virtually disappeared once the composite synthetic spectra of the stars and disk were used to calculate the difference spectra. An intensity enhancement of the resulting tomogram revealed images of the gas stream and an emission arc. We successfully modeled the gas stream using shellspec and associated the emission arc with an asymmetry in the accretion disk.

astro-ph↗

arXiv2005

A Study of Synthetic and Observed H_alpha Spectra of TT Hydrae

The formation and properties of accretion discs and circumstellar material in Algol-type systems is not very well understood. In order to study the underlying physics of these structures, we have calculated synthetic Halpha spectra of TT Hya, which is an Algol-type eclipsing binary with an accretion disc. Both the primary and secondary stars were considered in the calculations as well as a disc surrounding the primary. The Roche model for the secondary star was assumed. The synthetic spectra cover all the phases including primary eclipse and are compared with the observed spectra. The influence of various effects and free parameters of the disc on the emerging spectrum was studied. This enabled us to put some constraints on the geometry, temperature, density and velocity fields within the disc. Differences found between the observed and synthetic spectra unravel the existence of a gas stream as well as a hotter disc-gas interaction region. An additional cooler circumstellar region between the C1 and C2 Roche surfaces is suggested to account for various observed effects. A new computer code called {\sc{shellspec}} was created for this purpose and is briefly described in this work as well. It is designed to solve simple radiative transfer along the line of sight in 3D moving media. The scattered light from a central object is taken into account assuming an optically thin environment. Output intensities are then integrated through the 2D projection surface of a 3D object. The assumptions of the code include LTE and optional known state quantities and velocity fields in 3D.

astro-ph↗