arXiv subjects

E. W. Guenther

Publications and source records attributed to E. W. Guenther.

At least 109 records · Page 6Linked to original sources

arXiv2010

Transiting exoplanets from the CoRoT space mission XIV. CoRoT-11b: a transiting massive "hot-Jupiter" in a prograde orbit around a rapidly rotating F-type star

The CoRoT exoplanet science team announces the discovery of CoRoT-11b, a fairly massive hot-Jupiter transiting a V=12.9 mag F6 dwarf star (M*=1.27 +/- 0.05 Msun, R*=1.37 +/- 0.03 Rsun, Teff=6440 +/- 120 K), with an orbital period of P=2.994329 +/- 0.000011 days and semi-major axis a=0.0436 +/- 0.005 AU. The detection of part of the radial velocity anomaly caused by the Rossiter-McLaughlin effect shows that the transit-like events detected by CoRoT are caused by a planet-sized transiting object in a prograde orbit. The relatively high projected rotational velocity of the star (vsini=40+/-5 km/s) places CoRoT-11 among the most rapidly rotating planet host stars discovered so far. With a planetary mass of mp=2.33+/-0.34 Mjup and radius rp=1.43+/-0.03 Rjup, the resulting mean density of CoRoT-11b (rho=0.99+/-0.15 g/cm^3) can be explained with a model for an inflated hydrogen-planet with a solar composition and a high level of energy dissipation in its interior.

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arXiv2010

Pre-Main Sequence stars in the star forming complex Sh 2-284

Located at the galactic anticenter, Sh 2-284 is a HII region which harbors several young open clusters; Dolidze 25, a rare metal poor (Z~0.004) young cluster, is one of these. Given its association with Sh 2-284, it is reasonable to assume the low metallicity for the whole HII region. Sh~2-284 is expected to host a significant population of Pre-Main Sequence (PMS) stars of both low and intermediate mass stars (Herbig Ae stars). We aim at characterizing these stars by means of a spectroscopic and photometric survey conducted with VIMOS@VLT and complemented with additional optical and infrared observations. In this survey we selected and characterized 23 PMS objects. We derived the effective temperature, the spectral energy distribution and luminosity of these objects; using theoretical PMS evolutionary tracks, with the appropriate metallicity, we estimated the mass and the age of the studied objects. We also estimated a distance of 4 Kpc for Sh 2-284 by using spectroscopic parallax of 3 OB stars. From the age determination we concluded that triggered star formation is in act in this region. Our results show that a significant fraction of the young stellar objects (YSOs) may have preserved their disk/envelopes, in contrast with what is found in other recent studies of low-metallicity star forming regions in the Galaxy. Finally, among the 23 bona fide PMS stars, we identified 8 stars which are good candidates to pulsators of the delta Scuti type.

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arXiv2010

Transiting exoplanets from the CoRoT space mission. XIII. CoRoT-13b: a dense hot Jupiter in transit around a star with solar metallicity and super-solar lithium content

We announce the discovery of the transiting planet CoRoT-13b. Ground based follow-up in CFHT and IAC80 confirmed CoRoT's observations. The mass of the planet was measured with the HARPS spectrograph and the properties of the host star were obtained analyzing HIRES spectra from the Keck telescope. It is a hot Jupiter-like planet with an orbital period of 4.04 days, 1.3 Jupiter masses, 0.9 Jupiter radii, and a density of 2.34 g cm-3. It orbits a G0V star with Teff=5945K, M*=1.09 Msun, R*=1.01 Rsun, solar metallicity, a lithium content of +1.45 dex, and an estimated age between 0.12 and 3.15 Gyr. The lithium abundance of the star is consistent with its effective temperature, activity level, and age range derived from the stellar analysis. The density of the planet is extreme for its mass. It implies the existence of an amount of heavy elements with a mass between about 140 and 300 Mearth.

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arXiv2010

An algorithm for correcting CoRoT raw light curves

We introduce the CoRoT detrend algorithm (CDA) for detrending CoRoT stellar light curves. The algorithm CDA has the capability to remove random jumps and systematic trends encountered in typical CoRoT data in a fully automatic fashion. Since enormous jumps in flux can destroy the information content of a light curve, such an algorithm is essential. From a study of 1030 light curves in the CoRoT IRa01 field, we developed three simple assumptions which upon CDA is based. We describe the algorithm analytically and provide some examples of how it works. We demonstrate the functionality of the algorithm in the cases of CoRoT0102702789, CoRoT0102874481, CoRoT0102741994, and CoRoT0102729260. Using CDA in the specific case of CoRoT0102729260, we detect a candidate exoplanet around the host star of spectral type G5, which remains undetected in the raw light curve, and estimate the planetary parameters to be Rp=6.27Re and P=1.6986 days.

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arXiv2010

Eclipsing binaries with pulsating components: CoRoT 102918586

We present the preliminary results of the study of an interesting target in the first CoRoT exo-planet field (IRa1): CoRoT 102918586. Its light curve presents additional variability on the top of the eclipses, whose pattern suggests multi- frequency pulsations. The high accuracy CoRoT light curve was analyzed by applying an iterative scheme, devised to disentangle the effect of eclipses from the oscillatory pattern. In addition to the CoRoT photometry we obtained low resolution spectroscopy with the AAOmega multi-fiber facility at the Anglo Australian Observatory, which yielded a spectral classification as F0 V and allowed us to infer a value of the primary star effective temperature. The Fourier analysis of the residuals, after subtraction of the binary light curve, gave 35 clear frequencies. The highest amplitude frequency, of 1.22 c/d, is in the expected range for both γDor and SPB pulsators, but the spectral classification favors the first hypothesis. Apart from a few multiples of the orbital period, most frequencies can be interpreted as rotational splitting of the main frequency (an l = 2 mode) and of its overtones.

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arXiv2009

CARMENES: Calar Alto high-Resolution search for M dwarfs with Exo-earths with a Near-infrared Echelle Spectrograph

CARMENES, Calar Alto high-Resolution search for M dwarfs with Exo-earths with a Near-infrared Echelle Spectrograph, is a study for a next-generation instrument for the 3.5m Calar Alto Telescope to be designed, built, integrated, and operated by a consortium of nine German and Spanish institutions. Our main objective is finding habitable exoplanets around M dwarfs, which will be achieved by radial velocity measurements on the m/s level in the near-infrared, where low-mass stars emit the bulk of their radiation.

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arXiv2009

Transit timing analysis of CoRoT-1b

CoRoT, the pioneer space-based transit search, steadily provides thousands of high-precision light curves with continuous time sampling over periods of up to 5 months. The transits of a planet perturbed by an additional object are not strictly periodic. By studying the transit timing variations (TTVs), additional objects can be detected in the system. A transit timing analysis of CoRoT-1b is carried out to constrain the existence of additional planets in the system. We used data obtained by an improved version of the CoRoT data pipeline (version 2.0). Individual transits were fitted to determine the mid-transit times, and we analyzed the derived $O-C$ diagram. N-body integrations were used to place limits on secondary planets. No periodic timing variations with a period shorter than the observational window (55 days) are found. The presence of an Earth-mass Trojan is not likely. A planet of mass greater than $\sim 1$ Earth mass can be ruled out by the present data if the object is in a 2:1 (exterior) mean motion resonance with CoRoT-1b. Considering initially circular orbits: (i) super-Earths (less than 10 Earth-masses) are excluded for periods less than about 3.5 days, (ii) Saturn-like planets can be ruled out for periods less than about 5 days, (iii) Jupiter-like planets should have a minimum orbital period of about 6.5 days.

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arXiv2009

Spectroscopic properties of cool Ursa Major group members

Until now, most members of the Ursa Major (UMa) group of stars have been identified by means of kinematic criteria. However, in many cases kinematic criteria alone are insufficient to ascertain, whether an individual star is really a member of this group. Since photometric criteria are ineffective in the case of cool dwarf members, one must use spectroscopic criteria. Nevertheless, resulting membership criteria are inconclusive. We reanalyse spectroscopic properties of cool UMa group dwarfs. In particular, we study the distribution of iron abundance, the strength of the Li I absorption at 6708 A and the Li abundance, and the infilling of the core of the H alpha line. Twenty-five cool and northern bona-fide members are carefully selected from the literature. Homogeneously measured stellar parameters and iron abundances are given for all Sun-like stars selected, based on spectra of high resolution and high signal-to-noise ratio. In addition, we measure the Li equivalent width and abundance as well as the relative intensity of the H alpha core and the corresponding chromospheric flux. The studied stars infer an average Ursa Major group iron abundance of -0.03+-0.05 dex, which is higher by about 0.06 dex than determined elsewhere. The Li abundance derived of Ursa Major group dwarf stars is higher than in the Hyades at effective temperatures cooler than the Sun, but lower than in the younger Pleiades, a result which is independent of the exact value of the effective temperature adopted. The Sun-like and cooler dwarfs also display chromospheric infilling of the H alpha core. We present spectroscopic criteria that may be used to exclude non-members.

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arXiv2009

Planetary companions around the K giant stars 11 UMi and HD 32518

11 UMi and HD 32518 belong to a sample of 62 K giant stars that has been observed since February 2004 using the 2m Alfred Jensch telescope of the Thüringer Landessternwarte (TLS) to measure precise radial velocities (RVs). The aim of this survey is to investigate the dependence of planet formation on the mass of the host star by searching for planetary companions around intermediate-mass giants. An iodine absorption cell was used to obtain accurate RVs for this study. Our measurements reveal that the RVs of 11 UMi show a periodic variation of 516.22 days. The RV curve of HD 32518 shows sinusoidal variations with a period of 157.54 days. The HIPPARCOS photometry as well as our Hαcore flux measurements reveal no variability with the RV period. Thus, Keplerian motion is the most likely explanation for the observed RV variations for both giant stars. An exoplanet with a minimum mass of 10.5 Jupiter masses orbits the K giant 11 UMi. The K1 III giant HD 32518 hosts a planetary companion with a minimum mass of 3.0 Jupiter masses in a nearly circular orbit. These are the 4th and 5th planets published from this TLS survey.

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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

Variability of the transitional T Tauri star T Chamaeleontis

We characterize the physical properties of T Chamaeleontis, a transitional T Tauri star showing UX Ori-type variability, and of its associated disc, and probe possible effects of disc clearing processes. Different spectral diagnostics are examined, based on a rich collection of optical high- and low-resolution spectra. We determine radial and projected rotational velocities, and measure equivalent widths of the Li I (6708 A) line and of the most prominent emission lines (e.g. Ha, Hb and [OI] 6300A); we analyse shape changes of photospheric lines via bisector-method, while variability in Ha and Hb is inspected through line-profile correlation matrices. The strength of the Ha and Hb emission is highly variable and well correlated with that of the [OI] lines, as well as with Av variations of over three magnitudes. Variations up to nearly 10 km/s in the radial velocity of the star are measured on analogous time-scale, but with no apparent periodicity. SED modelling confirms the existence of a gap in the disc. Variable circumstellar extinction is pointed out as responsible for the conspicuous variations observed in the stellar continuum flux and for concomitant changes in the emission features by contrast effect. Clumpy structures, incorporating large dust grains and orbiting the star within a few tenths of AU, obscure episodically the star and, eventually, part of the inner circumstellar zone, while the bulk of the hydrogen lines emitting zone and outer low-density wind region traced by the [OI] remain unaffected. Coherently with this scenario, the detected radial velocity changes are also explainable in terms of clumpy materials transiting and partially obscuring the star.

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arXiv2009

Planetary companion candidates around the K giant stars 42 Dra and HD 139357

For the past 3 years we have been monitoring 62 K giant stars using precise stellar radial velocity (RV) measurements with the 2m Alfred Jensch telescope of the Thüringer Landessternwarte Tautenburg (TLS). To probe the dependence of planet formation on stellar mass by finding planets around intermediate-mass giant stars. We present high accuracy RV measurements of the K1.5 III star 42 Dra and the K4 III star HD 139357. The wavelength reference for the RV measurements was provided by an iodine absorption cell placed in the optical path of the spectrograph. Our measurements reveal that the time series of the radial velocity of 42 Dra shows a periodic variation of 479.1 days. HD 139357 shows periodic RV variations of 1125.7 days. Keplerian motion is the most likely explanation for the observed radial velocity variations for these stars. Thus the K giant stars 42 Dra and HD 139357 host extrasolar planets.

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arXiv2009

Short-term spectroscopic monitoring of two cool dwarfs with strong magnetic fields

Context: There is now growing evidence that some brown dwarfs (BDs) have very strong magnetic fields, and yet their surface temperatures are so low that the coupling is expected to be small between the matter and the magnetic field in the atmosphere. In the deeper layers, however, the coupling is expected to be much stronger. Aims: This raises the question of whether the magnetic field still leads to the formation of structures in the photosphere. Methods: We carried out a spectroscopic monitoring campaign of two ultracool dwarfs that have strong magnetic fields: the BD LP944-20 and 2MASSW J0036159+182110. LP944-20 was observed simultaneously in the optical and in the near infrared regime, 2MASSW J0036159+182110 only in the infrared. Results: Both dwarfs turned out to be remarkably constant. In the case of LP944-20, the Teff-variations are <50K, and the rms-variations in the equivalent widths of Halpha small. We also find that the equivalent widths of photospheric lines are remarkably constant. We did not find any significant variations in the case of 2MASSW J0036159+182110 either. Thus the most important result is that no significant variability was found at the time of our observations. When comparing our spectra with spectra taken over the past 11 years, we recognize significant changes during this time. Conclusions: We interpret these results as evidence that the photosphere of these objects are remarkably homogeneous, with only little structure in them, and despite the strong magnetic fields. Thus, unlike active stars, there are no prominent spots on these objects.

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arXiv2008

Testing evolutionary tracks of Pre-Main Sequence stars: the case of HD113449

Evolutionary tracks are of key importance for the understanding of star formation. Unfortunately, tracks published by various groups differ so that it is fundamental to have observational tests. In order to do this, we intend to measure the masses of the two components of the Pre-Main Sequence binary HD113449 by combining radial velocity measurements taken with HARPS, with infrared interferometric data using AMBER on the VLTI. The spectroscopic orbit that has already been determined, combined with the first AMBER measurement, allows us to obtain a very first estimation of the inclination of the binary system and from this the masses of the two stars. More AMBER measurements of HD 113449 are needed to improve the precision on the masses: in the ESO period P82 two new measurements are scheduled.

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arXiv2008

TW Hydrae: evidence of stellar spots instead of a Hot Jupiter

TW Hydrae shows significant radial-velocity variations in the optical regime. They have been attributed to a 10 Jupiter Mass planet orbiting the star at 0.04 AU. In this work, we have tested whether the observed RV variations can be caused by stellar spots. We have also analyzed new optical and infrared data to confirm the signal of the planet companion. We fitted the RV variations of TW Hya using a cool spot model. Our model shows that a cold spot covering 7% of the stellar surface and located at a latitude of 54 deg can reproduce the reported RV variations. The model also predicts a bisector semi-amplitude variation <10 m/s, which is less than the errors of the RV measurements discussed in an earlier publication. The analysis of new optical RV data, with typical errors of 10 m/s, shows a larger RV amplitude that varies depending on the correlation mask used. A slight correlation between the RV variation and the bisector is also observed, although not at a very significant level. The infrared H-band RV curve is almost flat, showing a small variation (<35 m/s) that is not consistent with the optical orbit. All these results support the spot scenario rather than the presence of a hot Jupiter around TW Hya.

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arXiv2007

Discovery of a planet around the K giant star 4 UMa

Context: For the past 3 years we have been monitoring a sample of 62 K giant stars using precise stellar radial velocity measurements taken at the Thueringer Landessternwarte Tautenburg. Aims: To search for sub-stellar companions to giant stars and to understand the nature of the diverse radial velocity variations exhibited by K giant stars. Methods: We present precise stellar radial velocity measurements of the K1III giant star 4 UMa (HD 73108). These were obtained using the coude echelle spectrograph of 2-m Alfred Jensch Telescope. The wavelength reference for the radial velocity measurements was provided by an iodine absorption cell. Results: Our measurements reveal that the radial velocity of 4 UMa exhibits a periodic variation of 269.3 days with a semiamplitude K = 216.8 m/s. A Keplerian orbit with an eccentricity, e = 0.43 +/- 0.02 is the most reasonable explanation for the radial velocity variations. The orbit yields a mass function, f(m) = (2.05 +/- 0.24) x 10^(- 7) M_sun. From our high resolution spectra we calculate a metallicity of -0.25 +/- 0.05 and derive a stellar mass of 1.23 M_sun +/- 0.15 for the host star. Conclusions: The K giant star 4 UMa hosts a substellar companion with minimum mass m sin i = 7.1 +/- 1.6 M_Jupiter.

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arXiv2007

Pre-main sequence spectroscopic binaries suitable for VLTI observations

A severe problem of the research in star-formation is that the masses of young stars are almost always estimated only from evolutionary tracks. Since the tracks published by different groups differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, with the VLTI it is now possible to set constrains on the tracks by determining the masses of many young binary stars precisely. In order to use the VLTI efficiently, a first step is to find suitable targets, which is the purpose of this work. Given the distance of nearby star-forming regions, suitable VLTI targets are binaries with orbital periods between at least 50 days, and few years. Although a number of surveys for detecting spectroscopic binaries have been carried out, most of the binaries found so far have periods which are too short. We thus surveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, rho Ophiuci star-forming regions in order to search for spectroscopic binaries with periods longer than 50 days, which are suitable for the VLTI observations. As a result of the 8 years campaign we discovered 8 binaries with orbital periods longer than 50 days. Amongst the newly discovered long period binaries is CS Cha, which is one of the few classical T Tauri stars with a circumbinary disk. The survey is limited to objects with masses higher than 0.1 to 0.2 Modot for periods between 1 and 8 years. We find that the frequency of binaries with orbital periods < 3000 days is of 20+/-5 percent. The frequency of long and short period pre-main sequence spectroscopic binaries is about the same as for stars in the solar neighbourhood. In total 14 young binaries are now known which are suitable for mass determination with the VLTI.

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arXiv2007

DE CVn: A bright, eclipsing red dwarf - white dwarf binary

Close white dwarf - red dwarf binaries must have gone through a common-envelope phase during their evolution. DE CVn is a detached white dwarf - red dwarf binary with a relatively short (~8.7 hours) orbital period. Its brightness and the presence of eclipses makes this system ideal for a more detailed study. From a study of photometric and spectroscopic observations of DE CVn we derive the system parameters which we discuss in the frame work of common-envelope evolution. Photometric observations of the eclipses are used to determine an accurate ephemeris. From a model fit to an average low-resolution spectrum of DE CVn we constrain the temperature of the white dwarf and the spectral type of the red dwarf. The eclipse light curve is analysed and combined with the radial velocity curve of the red dwarf determined from time-resolved spectroscopy to derive constraints on the inclination and the masses of the components in the system. The derived ephemeris is HJD_min = 2452784.5533(1) + 0.3641394(2) x E. The red dwarf in DE CVn has a spectral type of M3V and the white dwarf has an effective temperature of 8000 K. The inclination of the system is 86 (+3, -2) deg and the mass and radius of the red dwarf are 0.41 +/- 0.06 M_sun and 0.37 (+0.06, -0.007) R_sun, respectively, and the mass and radius of the white dwarf are 0.51 (+0.06, -0.02) M_sun and 0.0136 (+0.0008, -0.0002) R_sun, respectively. We found that the white dwarf has a hydrogen-rich atmosphere (DA-type). Given that DE CVn has experienced a common-envelope phase, we can reconstruct its evolution and we find that the progenitor of the white dwarf was a relatively low-mass star (M <= 1.6M_sun). The current age of this system is 3.3-7.3 x 10^9 years, while it will take longer than the Hubble time for DE CVn to evolve into a semi-detached system.

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