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Search Results: 1 - 10 of 9914 matches for " Stefan Dreizler "
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Kepler-9 revisited 60% the mass with six times more data
Stefan Dreizler,Aviv Ofir
Physics , 2014,
Abstract: Kepler-9 was the first case where transit timing variations have been used to confirm the planets in this system. Following predictions of dramatic TTVs - larger than a week - we re-analyse the system based on the full Kepler data set. We re-processed all available data for Kepler-9 removing short and long term trends, measured the times of mid-transit and used those for dynamical analysis of the system. The newly determined masses and radii of Kepler-9b and -9c change the nature of these planets relative to the one described in Holman et al. 2010 (hereafter H10) with very low, but relatively well charcterised (to better than 7%), bulk densities of 0.18 and 0.14 g cm$^3$ (about 1/3 of the H10 value). We constrain the masses (45.1 and 31.0 M$_\oplus$, for Kepler-9b and -9c respectively) from photometry alone, allowing us to see possible indications for an outer non-transiting planet in the radial velocity data. At $2R_\oplus$ Kepler-9d is determined to be larger than suggested before - suggesting that it is a low-mass low-density planet. The comparison between the H10 analysis and our new analysis suggests that small formal error in the TTV inversion may be misleading if the data does not cover a significant fraction of the interaction time scale.
An Independent Planet Search In The Kepler Dataset. I. A hundred new candidates and revised KOIs
Aviv Ofir,Stefan Dreizler
Physics , 2012, DOI: 10.1051/0004-6361/201219877
Abstract: Aims. We present our re-analyze the Kepler photometric dataset, searching for planetary transits using an alternative processing pipeline to the one used by the Kepler Mission. Methods. The SARS pipeline was tested extensively by processing all available CoRoT data. We used this pipeline to search for (additional) planetary transits only in the Kepler objects of interest (KOIs). Results. Although less than 1% of the Kepler dataset are KOIs we are able to significantly update the overall statistics of planetary multiplicity: we find 84 new transit signals on 64 systems on these light curves only, nearly doubling the number of transit signals in these systems. Forty-one of the systems were singly-transiting systems that are now multiply-transiting. Notable among the new discoveries are KOI 435 as a new six-candidate system (of which kind only Kepler-11 was known before), KOI 277 (which includes two candidates in a 6:7 resonance and anti-correlated transit timing variations) - all but validating the system, KOIs 719, 1574, and 1871 that have small planet candidates (1.15, 2.05 and 1.71R_Earth) in the habitable zone of their host star, and KOI 1843 that exhibits the shortest period (4.25hr) and among the smallest (0.63 R_Earth) of all planet candidates. We are also able to reject 11 KOIs as eclipsing binaries, update the ephemeris for five KOIs and otherwise discuss yet other objects. Interestingly, about 1/3 of the newly detected candidates participate in period commensurabilities. Finally, we discuss the possible overestimation of parameter errors in the current list of KOIs. Conclusions. Our results strengthen previous analyses of the multi-transiting ensemble. Nevertheless, we conclude that despite the phenomenal success of the Kepler mission, parallel analysis of the data by multiple teams is required to make full use of the data. [ABRIDGED]
Discovery of a nearby young brown dwarf binary candidate
Ansgar Reiners,Andreas Seifahrt,Stefan Dreizler
Physics , 2010, DOI: 10.1051/0004-6361/201014206
Abstract: In near-infrared NaCo observations of the young brown dwarf 2MASS J0041353-562112, we discovered a companion a little less than a magnitude fainter than the primary. The binary candidate has a separation of 143 mas, the spectral types are M6.5 and M9.0 for the two components. Colors and flux ratios are consistent with the components being located at the same distance minimizing the probability of the secondary being a background object. The brown dwarf is known to show Li absorption constraining the age to less than ~200 Myr, and it was suspected to show ongoing accretion, indicating an age as low as ~10 Myr. We estimate distance and orbital parameters of the binary as a function of age. For an age of 10 Myr, the distance to the system is 50 pc, the orbital period is 126 yr, and the masses of the components are ~30 and ~15 MJup. The binary brown dwarf fills a so far unoccupied region in the parameters mass and age; it is a valuable new benchmark object for brown dwarf atmospheric and evolutionary models.
The quest for companions to post-common envelope binaries IV: The 2:1 mean-motion resonance of the planets orbiting NN Serpentis
Klaus Beuermann,Stefan Dreizler,Frederic V. Hessman
Physics , 2013, DOI: 10.1051/0004-6361/201220510
Abstract: We present 69 new mid-eclipse times of the young post-common envelope binary (PCEB) NN Ser, which was previously suggested to possess two circumbinary planets. We have interpreted the observed eclipse-time variations in terms of the light-travel time effect caused by two planets, exhaustively covering the multi-dimensional parameter space by fits in the two binary and ten orbital parameters. We supplemented the fits by stability calculations for all models with an acceptable chi-square. An island of secularly stable 2:1 resonant solutions exists, which coincides with the global chi-square minimum. Our best-fit stable solution yields current orbital periods P_o = 15.47 yr and P_i = 7.65 yr and eccentricities e_o = 0.14 and e_i = 0.22 for the outer (o) and inner (i) planets, respectively. The companions qualify as giant planets, with masses of 7.0 M_Jup and 1.7 M_Jup for the case of orbits coplanar with that of the binary. The two-planet model that starts from the present system parameters has a lifetime greater than 10^8 yr, which significantly exceeds the age of NN Ser of 10^6 yr as a PCEB. The resonance is characterized by libration of the resonant variable Theta_1 and circulation of omega_i-omega_o, the difference between the arguments of periapse of the two planets. No stable non-resonant solutions were found, and the possibility of a 5:2 resonance suggested previously by us is now excluded at the 99.3% confidence level.
Planet formation from the ejecta of common envelopes
Dominik R. G. Schleicher,Stefan Dreizler
Physics , 2013, DOI: 10.1051/0004-6361/201322860
Abstract: The close binary system NN Serpentis must have gone through a common envelope phase before the formation of its white dwarf. During this phase, a substantial amount of mass was lost from the envelope. The recently detected orbits of circumbinary planets are likely inconsistent with planet formation before the mass loss.We explore whether new planets may have formed from the ejecta of the common envelope and derive the expected planetary mass as a function of radius.We employed the Kashi & Soker model to estimate the amount of mass that is retained during the ejection event and inferred the properties of the resulting disk from the conservation of mass and angular momentum. The resulting planetary masses were estimated from models with and without radiative feedback. We show that the observed planetary masses can be reproduced for appropriate model parameters. Photoheating can stabilize the disks in the interior, potentially explaining the observed planetary orbits on scales of a few AU. We compare the expected mass scale of planets for 11 additional systems with observational results and find hints of two populations, one consistent with planet formation from the ejecta of common envelopes and the other a separate population that may have formed earlier. The formation of the observed planets from the ejecta of common envelopes seems feasible. The model proposed here can be tested through refined observations of additional post-common envelope systems. While it appears observationally challenging to distinguish between the accretion on pre-existing planets and their growth from new fragments, it may be possible to further constrain the properties of the protoplanetary disk through additional observations of current planetary candidates and post-common envelope binary systems.
An Independent Planet Search In The Kepler Dataset. II. An extremely low-density super-Earth mass planet around Kepler-87
Aviv Ofir,Stefan Dreizler,Mathias Zechmeister,Tim-Oliver Husser
Physics , 2013, DOI: 10.1051/0004-6361/201220935
Abstract: [ABRIDGED]: Aims: two candidates in the KOI 1574 system are relatively long-period (about 114d and 191d) and in 5:3 resonance. We therefore search for TTVs in this particularly promising system. Methods: The full Kepler data was used, allowing to search for TTVs as well as for additional transit-like signals. Results: We detect strong anti-correlated TTVs of the 114d and 191d signals, dynamically confirming them as members of the same system. Dynamical simulations reproducing the observed TTVs allow us to also determine the masses of the planets. KOI 1574.01 (hereafter Kepler-87 b) was found to have a radius of 13.49 +/- 0.55 R_earth and a mass of 324.2 +/- 8.8M_earth, and KOI 1574.02 (Kepler-87 c) was found to have a radius of 6.14 +/- 0.29R_earth and a mass of 6.4 +/- 0.8M_earth. Both planets have low densities of 0.729 and 0.152 g cm^-3, respectively, which is non-trivial for such cold and old (7-8 Gyr) planets. Specifically, Kepler-87 c is the lowest- density planet in the super-Earth mass range. Both planets are thus particularly amenable to modeling and planetary structure studies, and also present an interesting case were ground-based photometric follow-up of Kepler planets is very desirable. Finally, we also detect two more short period super-Earth sized planetary (< 2R_earth) candidates in the system, making the relatively high multiplicity of this system notable against the general paucity of multiple systems in the presence of giant planets like Kepler-87 b.
NLTE Model Atmospheres for Central Stars of Planetary Nebulae
Thomas Rauch,Jochen L. Deetjen,Stefan Dreizler,Klaus Werner
Physics , 1999,
Abstract: Present observational techniques provide stellar spectra with high resolution at a high signal-to-noise ratio over the complete wavelength range -- from the far infrared to the X-ray. NLTE effects are particularly important for hot stars, hence the use of reliable NLTE stellar model atmosphere fluxes is required for an adequate spectral analysis. State-of-the-art NLTE model atmospheres include the metal-line blanketing of millions of lines of all elements from hydrogen up to the iron-group elements and thus permit precise analyses of extremely hot compact stars, e.g. central stars of planetary nebulae, PG 1159 stars, white dwarfs, and neutron stars. Their careful spectroscopic study is of great interest in several branches of modern astrophysics, e.g. stellar and galactic evolution, and interstellar matter.
Planet formation in post-common-envelope binaries
Dominik Schleicher,Stefan Dreizler,Marcel V?lschow,Robi Banerjee,Frederic V. Hessman
Physics , 2015, DOI: 10.1002/asna.201412184
Abstract: To understand the evolution of planetary systems, it is important to investigate planets in highly evolved stellar systems, and to explore the implications of their observed properties with respect to potential formation scenarios. Observations suggest the presence of giant planets in post-common-envelope binaries (PCEBs). A particularly well-studied system with planetary masses of 1.7 M_J and 7.0 M_J is NN Ser. We show here that a pure first-generation scenario where the planets form before the common envelope (CE) phase and the orbits evolve due to the changes in the gravitational potential is inconsistent with the current data. We propose a second-generation scenario where the planets are formed from the material that is ejected during the CE, which may naturally explain the observed planetary masses. In addition, hybrid scenarios where the planets form before the CE and evolve due to the accretion of the ejected gas appear as a realistic possibility.
The CRIRES Search for Planets Around the Lowest-Mass Stars. I. High-Precision Near-Infrared Radial Velocities with an Ammonia Gas Cell
Jacob L. Bean,Andreas Seifahrt,Henrik Hartman,Hampus Nilsson,Guenter Wiedemann,Ansgar Reiners,Stefan Dreizler,Todd J. Henry
Physics , 2009, DOI: 10.1088/0004-637X/713/1/410
Abstract: Radial velocities measured from near-infrared spectra are a potentially powerful tool to search for planets around cool stars and sub-stellar objects. However, no technique currently exists that yields near-infrared radial velocity precision comparable to that routinely obtained in the visible. We describe a method for measuring high-precision relative radial velocities of these stars from K-band spectra. The method makes use of a glass cell filled with ammonia gas to calibrate the spectrograph response similar to the "iodine cell" technique that has been used very successfully in the visible. Stellar spectra are obtained through the ammonia cell and modeled as the product of a Doppler-shifted template spectrum of the object and a spectrum of the cell, convolved with a variable instrumental profile model. A complicating factor is that a significant number of telluric absorption lines are present in the spectral regions containing useful stellar and ammonia lines. The telluric lines are modeled simultaneously as well using spectrum synthesis with a time-resolved model of the atmosphere over the observatory. The free parameters in the complete model are the wavelength scale of the spectrum, the instrumental profile, adjustments to the water and methane abundances in the atmospheric model, telluric spectrum Doppler shift, and stellar Doppler shift. Tests of the method based on the analysis of hundreds of spectra obtained for late M dwarfs over six months demonstrate that precisions of ~5 m/s are obtainable over long timescales, and precisions of better than 3 m/s can be obtained over timescales up to a week. The obtained precision is comparable to the predicted photon-limited errors, but primarily limited over long timescales by the imperfect modeling of the telluric lines.
The CRIRES Search for Planets Around the Lowest-Mass Stars. II. The Proposed Giant Planet Orbiting VB10 Does Not Exist
Jacob L. Bean,Andreas Seifahrt,Henrik Hartman,Hampus Nilsson,Ansgar Reiners,Stefan Dreizler,Todd J. Henry,Guenter Wiedemann
Physics , 2009,
Abstract: We present high-precision relative radial velocities of the very low-mass star VB10 that were obtained over a time span of 0.61 yr as part of an ongoing search for planets around stars at the end of the main sequence. The radial velocities were measured from high-resolution near-infrared spectra obtained using the CRIRES instrument on the VLT with an ammonia gas cell. The typical internal precision of the measurements is 10 m/s. These data do not exhibit significant variability and are essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not detect the radial velocity variations of VB10 expected due to the presence of an orbiting giant planet similar to that recently proposed by Pravdo and Shaklan based on apparent astrometric perturbations. In addition, we do not confirm the ~1 km/s radial velocity variability of the star tentatively detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements rule out planets with M_p > 3 M_Jup and the orbital period and inclination suggested by Pravdo and Shaklan at better than 5 sigma confidence. We conclude that the planet detection claimed by Pravdo and Shaklan is spurious on the basis of this result. Although the outcome of this work is a non-detection, it illustrates the potential of using ammonia cell radial velocities to detect planets around very low-mass stars.
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