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S. Czesla

Publications and source records attributed to S. Czesla.

108 records · Page 6Linked to original sources
arXiv2015

The center-to-limb variation across the Fraunhofer lines of HD 189733; Sampling the stellar spectrum using a transiting planet

The center-to-limb variation (CLV) describes the brightness of the stellar disk as a function of the limb angle. Across strong absorption lines, the CLV can vary quite significantly. We obtained a densely sampled time series of high-resolution transit spectra of the active planet host star HD 189733 with UVES. Using the passing planetary disk of the hot Jupiter HD 189733 b as a probe, we study the CLV in the wings of the Ca II H and K and Na I D1 and D2 Fraunhofer lines, which are not strongly affected by activity-induced variability. In agreement with model predictions, our analysis shows that the wings of the studied Fraunhofer lines are limb brightened with respect to the (quasi-)continuum. The strength of the CLV-induced effect can be on the same order as signals found for hot Jupiter atmospheres. Therefore, a careful treatment of the wavelength dependence of the stellar CLV in strong absorption lines is highly relevant in the interpretation of planetary transit spectroscopy.

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arXiv2015

Characterising exoplanets and their environment with UV transmission spectroscopy

Exoplanet science is now in its full expansion, particularly after the CoRoT and Kepler space missions that led us to the discovery of thousands of extra-solar planets. The last decade has taught us that UV observations play a major role in advancing our understanding of planets and of their host stars, but the necessary UV observations can be carried out only by HST, and this is going to be the case for many years to come. It is therefore crucial to build a treasury data archive of UV exoplanet observations formed by a dozen "golden systems" for which observations will be available from the UV to the infrared. Only in this way we will be able to fully exploit JWST observations for exoplanet science, one of the key JWST science case.

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arXiv2015

TPCI: The PLUTO-CLOUDY Interface

We present an interface between the (magneto-) hydrodynamics code PLUTO and the plasma simulation and spectral synthesis code CLOUDY. By combining these codes, we constructed a new photoionization hydrodynamics solver: The PLUTO-CLOUDY Interface (TPCI), which is well suited to simulate photoevaporative flows under strong irradiation. The code includes the electromagnetic spectrum from X-rays to the radio range and solves the photoionization and chemical network of the 30 lightest elements. TPCI follows an iterative numerical scheme: First, the equilibrium state of the medium is solved for a given radiation field by CLOUDY, resulting in a net radiative heating or cooling. In the second step, the latter influences the (magneto-) hydrodynamic evolution calculated by PLUTO. Here, we validated the one-dimensional version of the code on the basis of four test problems: Photoevaporation of a cool hydrogen cloud, cooling of coronal plasma, formation of a Stroemgren sphere, and the evaporating atmosphere of a hot Jupiter. This combination of an equilibrium photoionization solver with a general MHD code provides an advanced simulation tool applicable to a variety of astrophysical problems.

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arXiv2015

High-energy irradiation and mass loss rates of hot Jupiters in the solar neighborhood

Giant gas planets in close proximity to their host stars experience strong irradiation. In extreme cases photoevaporation causes a transonic, planetary wind and the persistent mass loss can possibly affect the planetary evolution. We have identified nine hot Jupiter systems in the vicinity of the Sun, in which expanded planetary atmospheres should be detectable through Lyman alpha transit spectroscopy according to predictions. We use X-ray observations with Chandra and XMM-Newton of seven of these targets to derive the high-energy irradiation level of the planetary atmospheres and the resulting mass loss rates. We further derive improved Lyman alpha luminosity estimates for the host stars including interstellar absorption. According to our estimates WASP-80 b, WASP-77 b, and WASP-43 b experience the strongest mass loss rates, exceeding the mass loss rate of HD 209458 b, where an expanded atmosphere has been confirmed. Furthermore, seven out of nine targets might be amenable to Lyman alpha transit spectroscopy. Finally, we check the possibility of angular momentum transfer from the hot Jupiters to the host stars in the three binary systems among our sample, but find only weak indications for increased stellar rotation periods of WASP-77 and HAT-P-20.

astro-ph.EP↗
arXiv2014

A multiwavelength study of the hierarchical triple HD 181068: A test bed for studying star-planet-interaction?

HD 181068 is the only compact, triply eclipsing, hierarchical triple system containing a giant star known to date. With its central, highly-active G-type giant orbited by a close pair of main-sequence dwarfs, the system is ideal to study tidal interactions. We carried out a multiwavelength study to characterize the magnetic activity of the HD 181068 system. To this end, we obtained in- and out-of-eclipse X-ray snapshots with XMM-Newton and an optical spectrum, which we analyzed along with the Kepler light-curve. The primary giant shows strong quiescent X-ray emission at a level of 2e31 ergs, an S-index of 0.41 +/- 0.01, and marked white-light flares releasing up to 6e38 erg in the Kepler-band. During the second X-ray observation, we found a three-times elevated -- yet decaying -- level of X-ray emission, which might be due to an X-ray flare. The high level of magnetic activity is compatible with the previously reported absence of solar-like oscillations in the giant, whose atmosphere, however, undergoes tidally-induced oscillations imposed by the changing configuration of the dwarf-binary. We found that the driving force exciting these oscillations is comparable to the disturbances produced by a typical hot Jupiter, making the system a potential test bed to study the effects of tidal interactions also present in planetary systems.

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arXiv2014

X-ray emission from the super-Earth host GJ 1214

Stellar activity can produce large amounts of high-energy radiation, which is absorbed by the planetary atmosphere leading to irradiation-driven mass-loss. We present the detection and an investigation of high-energy emission in a transiting super-Earth host system, GJ 1214, based on an XMM-Newton observation. We derive an X-ray luminosity LX=7.4E25 erg/s and a corresponding activity level of log(LX/Lbol)~ -5.3. Further, we determine a coronal temperature of about -3.5 MK, which is typical for coronal emission of moderately active low-mass stars. We estimate that GJ 1214 b evaporates at a rate of 1.3E10 g/s and has lost a total of ~2-5.6 MEarth.

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arXiv2013

Pulsation analysis and its impact on primary transit modeling in WASP-33

To date, WASP-33 is the only delta Scuti star known to be orbited by a hot Jupiter. The pronounced stellar pulsations, showing periods comparable to the primary transit duration, interfere with the transit modeling. Therefore our main goal is to study the pulsation spectrum of the host star to redetermine the orbital parameters of the system by means of pulsation-cleaned primary transit light curves. Between August 2010 and October 2012 we obtained 457 hours of photometry of WASP-33 using small and middle-class telescopes located mostly in Spain and in Germany. Our observations comprise the wavelength range between the blue and the red, and provide full phase coverage of the planetary orbit. After a careful detrend, we focus our pulsation studies in the high frequency regime, where the pulsations that mostly deform the primary transit exist. The data allow us to identify, for the first time in the system, eight significant pulsation frequencies. The pulsations are likely associated with low-order p-modes. Furthermore, we find that pulsation phases evolve in time. We use our knowledge of the pulsations to clean the primary transit light curves and carry out an improved transit modeling. Surprisingly, taking into account the pulsations in the modeling has little influence on the derived orbital parameters. However, the uncertainties in the best-fit parameters decrease. Additionally, we find indications for a possible dependence between wavelength and transit depth, but only with marginal significance. A clear pulsation solution, in combination with an accurate orbital period, allows us to extend our studies and search for star-planet interactions (SPI). Although we find no conclusive evidence of SPI, we believe that the pulsation nature of the host star and the proximity between members make WASP-33 a promising system for further SPI studies.

astro-ph.EP↗
arXiv2012

The high-energy environment in the super-earth system CoRoT-7

High-energy irradiation of exoplanets has been identified to be a key influence on the stability of these planets' atmospheres. So far, irradiation-driven mass-loss has been observed only in two Hot Jupiters, and the observational data remain even more sparse in the super-earth regime. We present an investigation of the high-energy emission in the CoRoT-7 system, which hosts the first known transiting super-earth. To characterize the high-energy XUV radiation field into which the rocky planets CoRoT-7b and CoRoT-7c are immersed, we analyzed a 25 ks XMM-Newton observation of the host star. Our analysis yields the first clear (3.5 sigma) X-ray detection of CoRoT-7. We determine a coronal temperature of ca. 3 MK and an X-ray luminosity of 3*10^28 erg/s. The level of XUV irradiation on CoRoT-7b amounts to ca. 37000 erg/cm^2/s. Current theories for planetary evaporation can only provide an order-of-magnitude estimate for the planetary mass loss; assuming that CoRoT-7b has formed as a rocky planet, we estimate that CoRoT-7b evaporates at a rate of about 1.3*10^11 g/s and has lost ca. 4-10 earth masses in total.

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arXiv2011

The corona and companion of CoRoT-2A. Insights from X-rays and optical spectroscopy

CoRoT-2 is one of the most unusual planetary systems known to date. Its host star is exceptionally active, showing a pronounced, regular pattern of optical variability caused by magnetic activity. The transiting hot Jupiter, CoRoT-2b, shows one of the largest known radius anomalies. We analyze the properties and activity of CoRoT-2A in the optical and X-ray regime by means of a high-quality UVES spectrum and a 15 ks Chandra exposure both obtained during planetary transits. The UVES data are analyzed using various complementary methods of high-resolution stellar spectroscopy. We characterize the photosphere of the host star by deriving accurate stellar parameters such as effective temperature, surface gravity, and abundances. Signatures of stellar activity, Li abundance, and interstellar absorption are investigated to provide constraints on the age and distance of CoRoT-2. Furthermore, our UVES data confirm the presence of a late-type stellar companion to CoRoT-2A that is gravitationally bound to the system. The Chandra data provide a clear detection of coronal X-ray emission from CoRoT-2A, for which we obtain an X-ray luminosity of 1.9e29 erg/s. The potential stellar companion remains undetected in X-rays. Our results indicate that the distance to the CoRoT-2 system is approximately 270 pc, and the most likely age lies between 100 and 300 Ma. Our X-ray observations show that the planet is immersed in an intense field of high-energy radiation. Surprisingly, CoRoT-2A's likely coeval stellar companion, which we find to be of late-K spectral type, remains X-ray dark. Yet, as a potential third body in the system, the companion could account for CoRoT-2b's slightly eccentric orbit.

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arXiv2010

Planetary eclipse mapping of CoRoT-2a. Evolution, differential rotation, and spot migration

The lightcurve of CoRoT-2 shows substantial rotational modulation and deformations of the planet's transit profiles caused by starspots. We consistently model the entire lightcurve, including both rotational modulation and transits, stretching over approximately 30 stellar rotations and 79 transits. The spot distribution and its evolution on the noneclipsed and eclipsed surface sections are presented and analyzed, making use of the high resolution achievable under the transit path. We measure the average surface brightness on the eclipsed section to be (5\pm1) % lower than on the noneclipsed section. Adopting a solar spot contrast, the spot coverage on the entire surface reaches up to 19 % and a maximum of almost 40 % on the eclipsed section. Features under the transit path, i.e. close to the equator, rotate with a period close to 4.55 days. Significantly higher rotation periods are found for features on the noneclipsed section indicating a differential rotation of $ΔΩ> 0.1$. Spotted and unspotted regions in both surface sections concentrate on preferred longitudes separated by roughly 180 deg.

astro-ph.SR↗
arXiv2009

Detecting Planets Around Very Low Mass Stars with the Radial Velocity Method

The detection of planets around very low-mass stars with the radial velocity method is hampered by the fact that these stars are very faint at optical wavelengths where the most high-precision spectrometers operate. We investigate the precision that can be achieved in radial velocity measurements of low mass stars in the near infrared (nIR) Y-, J-, and H-bands, and we compare it to the precision achievable in the optical. For early-M stars, radial velocity measurements in the nIR offer no or only marginal advantage in comparison to optical measurements. Although they emit more flux in the nIR, the richness of spectral features in the optical outweighs the flux difference. We find that nIR measurement can be as precise than optical measurements in stars of spectral type ~M4, and from there the nIR gains in precision towards cooler objects. We studied potential calibration strategies in the nIR finding that a stable spectrograph with a ThAr calibration can offer enough wavelength stability for m/s precision. Furthermore, we simulate the wavelength-dependent influence of activity (cool spots) on radial velocity measurements from optical to nIR wavelengths. Our spot simulations reveal that the radial velocity jitter does not decrease as dramatically towards longer wavelengths as often thought. The jitter strongly depends on the details of the spots, i.e., on spot temperature and the spectral appearance of the spot. Forthcoming nIR spectrographs will allow the search for planets with a particular advantage in mid- and late-M stars. Activity will remain an issue, but simultaneous observations at optical and nIR wavelengths can provide strong constraints on spot properties in active stars.

astro-ph.SR↗
arXiv2009

A planetary eclipse map of CoRoT-2a. Comprehensive lightcurve modeling combining rotational-modulation and transits

We analyze the surface structure of the planet host star CoRoT-2a using a consistent model for both the `global' (i.e., rotationally modulated) lightcurve and the transit lightcurves, using data provided by the CoRoT mission. Selecting a time interval covering two stellar rotations and six transits of the planetary companion CoRoT-2b, we adopt a `strip' model of the surface to reproduce the photometric modulation inside and outside the transits simultaneously. Our reconstructions show that it is possible to achieve appropriate fits for the entire sub-interval using a low-resolution surface model with 36 strips. The surface reconstructions indicate that the brightness on the eclipsed section of the stellar surface is (6 +/- 1) % lower than the average brightness of the remaining surface. This result suggests a concentration of stellar activity in a band around the stellar equator similar to the behavior observed on the Sun.

astro-ph.SR↗
arXiv2009

Transit mapping of a starspot on CoRoT-2 - Probing a stellar surface by planetary transits

We analyze variations in the transit lightcurves of CoRoT-2b, a massive hot Jupiter orbiting a highly active G star. We use one transit lightcurve to eclipse-map a photospheric spot occulted by the planet. In this case study we determine the size and longitude of the eclipsed portion of the starspot and systematically study the corresponding uncertainties. We determine a spot radius between $4.5\degr$ and $10.5\degr$ on the stellar surface and the spot longitude with a precision of about $\pm 1$ degree. Given the well-known transit geometry of the CoRoT-2 system, this implies a reliable detection of spots on latitudes typically covered by sunspots; also regarding its size the modelled spot is comparable to large spot groups on the Sun. We discuss the future potential of eclipse mapping by planetary transits for the high-resolution analysis of stellar surface features.

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arXiv2009

How stellar activity affects the size estimates of extrasolar planets

Lightcurves have long been used to study stellar activity and have more recently become a major tool in the field of exoplanet research. We discuss the various ways in which stellar activity can influence transit lightcurves, and study the effects using the outstanding photometric data of the CoRoT-2 exoplanet system. We report a relation between the `global' lightcurve and the transit profiles which turn out to be shallower during high spot coverage on the stellar surface. Furthermore, our analysis reveals a color dependence of the transit lightcurve compatible with a wavelength-dependent limb darkening law as observed on the Sun. Taking into account activity-related effects, we re-determine the orbit inclination and planetary radius and find the planet to be approximately 3% larger than reported previously. Our findings also show that exoplanet research cannot generally ignore the effects of stellar activity.

astro-ph.SR↗
arXiv2009

Long-term stability of spotted regions and the activity-induced Rossiter-McLaughlin effect on V889 Herculis. A synergy of photometry, radial velocity measurements, and Doppler imaging

The young active G-dwarf star V889 Herculis (HD 171488) shows pronounced spots in Doppler images as well as large variations in photometry and radial velocity (RV) measurements. However, the lifetime and evolution of its active regions are not well known. We study the existence and stability of active regions on the star's surface using complementary data and methods. Furthermore, we analyze the correlation of spot-induced RV variations and Doppler images. Photometry and high-resolution spectroscopy are used to examine stellar activity. A CLEAN-like Doppler Imaging (DI) algorithm is used to derive surface reconstructions. We study high-precision RV curves to determine their modulation due to stellar activity in analogy to the Rossiter-McLaughlin effect. To this end we develop a measure for the shift of a line's center and compare it to RV measurements. We show that large spotted regions exist on V889 Her for more than one year remaining similar in their large scale structure and position. This applies to several time periods of our observations, which cover more than a decade. Furthermore we use DI line profile reconstructions to identify influences of long-lasting starspots on RV measurements. In this way we verify the RV curve's agreement with our Doppler images. Based on long-term RV data we confirm V889 Her's rotation period of $1.3371 \pm 0.0002$ days.

astro-ph.SR↗
arXiv2008

Discovery of X-ray emission from the eclipsing brown-dwarf binary 2MASS J05352184-0546085

The eclipsing brown-dwarf binary system 2MASS J05352184-0546085 is a case sui generis. For the first time, it allows a detailed analysis of the individual properties of young brown dwarfs, in particular, masses, and radii, and the temperature ratio of the system components can be determined accurately. The system shows a "temperature reversal" with the more massive component being the cooler one, and both components are found to be active. We analyze X-ray images obtained by Chandra and XMM-Newton containing 2MASS J05352184-0546085 in their respective field of view. The Chandra observatory data show a clear X-ray source at the position of 2MASS J05352184-0546085, whereas the XMM-Newton data suffer from contamination from other nearby sources, but are consistent with the Chandra detection. No indications of flaring activity are found in either of the observations (together about 70 ks), and we thus attribute the observed flux to quiescent emission. With an X-ray luminosity of 3*10^{28} erg/s we find an L_X/L_{bol}-ratio close to the saturation limit of 10^{-3} and an L_{X}/L_{Hα}-ratio consistent with values obtained from low-mass stars. The X-ray detection of 2MASS J05352184-0546085 reported here provides additional support for the interpretation of the temperature reversal in terms of magnetically suppressed convection, and suggests that the activity phenomena of young brown dwarfs resemble those of their more massive counterparts.

astro-ph↗
arXiv2007

The nature of the fluorescent iron line in V 1486 Ori

The fluorescent 6.4 keV iron line provides information on cool material in the vicinity of hard X-ray sources as well as on the characteristics of the X-ray sources themselves. First discovered in the X-ray spectra of the flaring Sun, X-ray binaries and active galactic nuclei (AGN), the fluorescent line was also observed in a number of stellar X-ray sources. The young stellar object (YSO) V1486 Ori was observed in the framework of the Chandra Ultra Deep Project (COUP) as the source COUP 331. We investigate its spectrum, with emphasis on the strength and time variability of the fluorescent iron K-alpha line, derive and analyze the light curve of COUP 331 and proceed with a time-resolved spectral analysis of the observation. The light curve of V 1486 Ori shows two major flares, the first one lasting for (approx) 20 ks with a peak X-ray luminosity of 2.6*10^{32} erg/s (dereddened in the 1-10 keV band) and the second one -- only partially observed -- for >60 ks with an average X-ray luminosity of 2.4*10^{31} erg/s (dereddened). The spectrum of the first flare is very well described by an absorbed thermal model at high temperature, with a pronounced 6.7 keV iron line complex, but without any fluorescent K-alpha line. The X-ray spectrum of the second flare is characterized by even higher temperatures (>= 10 keV) without any detectable 6.7 keV Fe XXV feature, but with a very strong fluorescent iron K-alpha line appearing predominantly in the 20 ks rise phase of the flare. Preliminary model calculations indicate that photoionization is unlikely to account for the entire fluorescent emission during the rise phase.

astro-ph↗
arXiv2007

Are magnetic hot stars intrinsic X-ray sources ?

Context: X-ray surveys carried out with the Einstein and ROSAT satellites have resulted in rather unexpected detections of X-ray emission from late B-type and early A-type stars. These stars possess neither winds like early-type stars nor convective envelopes as late-type stars, so that the origin and production mechanism of this X-emission is unclear. Aims: We investigate whether the presence of large magnetic fields is related to the observed X-ray emission. Methods: We carried out Chandra high-angular resolution observations of a sample of late B-type and A-type stars with measured magnetic fields in the range from 0.2 - 17 kG. Out of the selected 10 sample stars, 6 objects had been previously detected as X-ray sources, some of them, however, with high positional uncertainty and a low signal-to-noise ratio, while 4 of our sample stars do have large magnetic fields but no previous detections of X-ray emission. Results: Our Chandra data confirm all previous ROSAT detections with an extremely high significance, and the limits of the offsets between X-ray and optical positions are greatly improved. In particular, HD 215441, known as Babcock's star with the strongest magnetic field by far (17 kG) of our sample stars, a rather faint and somewhat marginal ROSAT source, can clearly be detected. However, none of the 4 ROSAT non-detections could be detected with the new Chandra observations. Conclusions: The pure existence of a magnetic field of kiloGauss strength on a late B-type or A-type star is therefore not necessarily a prerequisite for finding X-ray emission among these stars. Understanding the observed X-ray emission from Babcock's star is a challenge for observational and theoretical astrophysics.

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