Dreizler, Stefan

[["dc.bibliographiccitation.firstpage","1039"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","1046"],["dc.bibliographiccitation.volume","486"],["dc.contributor.author","Bean, Jacob L."],["dc.contributor.author","Benedict, G. Fritz"],["dc.contributor.author","Charbonneau, David"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Taylor, D. C."],["dc.contributor.author","McArthur, Barbara E."],["dc.contributor.author","Seifahrt, Andreas"],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Reiners, Ansgar"],["dc.date.accessioned","2018-11-07T11:12:17Z"],["dc.date.available","2018-11-07T11:12:17Z"],["dc.date.issued","2008"],["dc.description.abstract","We present time series photometry for six partial transits of GJ 436b obtained with the Fine Guidance Sensor instrument on the Hubble Space Telescope (HST). Our analysis of these data yields independent estimates of the host star's radius R- = 0.505(-0.029) (+0.020) R-circle dot, and the planet's orbital period P = 2.643882(-0.000058)(+0.000060) d, orbital inclination i = 85.80 degrees(-0.25 degrees),(+ 0.21 degrees), mean central transit time T-c = 2 454 455.279241(-0.00025)(+0.00026) HJD, and radius Rp = 4.90(-0.33)(+ 0.45) R-circle plus. The radius we determine for the planet is larger than the previous findings from analyses of an infrared light curve obtained with the Spitzer Space Telescope. Although this discrepancy has a 92% formal significance (1.7s), it might be indicative of systematic errors that still influence the analyses of even the highest-precision transit light curves. Comparisons of all the measured radii to theoretical models suggest that GJ 436b has a H/ He envelope of similar to 10% by mass. We point out the similarities in structure between this planet and Uranus and Neptune and discuss possible parallels between these planets' formation environments and dynamical evolution. We also find that the transit times for GJ 436b are constant to within 10s over the 11 planetary orbits that the HST data span. However, the ensemble of published values exhibits a long-term drift and our mean transit time is 128 s later than that expected from the Spitzer ephemeris. The sparseness of the currently available data hinders distinguishing between an error in the orbital period or perturbations arising from an additional object in the system as the cause of the apparent trend. Assuming the drift is due to an error in the orbital period we obtain an improved estimate for it of P = 2.643904 +/- 0.000005 d. This value and our measured transit times will serve as important benchmarks in future studies of the GJ 436 system."],["dc.identifier.doi","10.1051/0004-6361:200810013"],["dc.identifier.fs","513377"],["dc.identifier.isi","000258326500046"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9395"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/53628"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","A Hubble Space Telescope transit light curve for GJ 436b"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.artnumber","A6"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.volume","553"],["dc.contributor.author","Husser, T.-O."],["dc.contributor.author","Wende-von Berg, S."],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Barman, T."],["dc.contributor.author","Hauschildt, P. H."],["dc.date.accessioned","2018-11-07T09:24:54Z"],["dc.date.available","2018-11-07T09:24:54Z"],["dc.date.issued","2013"],["dc.description.abstract","Aims. We present a new library of high-resolution synthetic spectra based on the stellar atmosphere code PHOENIX that can be used for a wide range of applications of spectral analysis and stellar parameter synthesis. Methods. The spherical mode of PHOENIX was used to create model atmospheres and to derive detailed synthetic stellar spectra from them. We present a new self-consistent way of describing micro-turbulence for our model atmospheres. Results. The synthetic spectra cover the wavelength range from 500 angstrom to 5.5 mu m with resolutions of R = 500 000 in the optical and near IR, R = 100 000 in the IR and Delta lambda = 0.1 angstrom in the UV. The parameter space covers 2300 K <= T-eff <= 12 000K, 0.0 <= log g <= +6.0, -4.0 <= [Fe/H] <= +1.0, and -0.2 <= [alpha/Fe] <= +1.2. The library is a work in progress and we expect to extend it up to T-eff = 25 000K."],["dc.identifier.doi","10.1051/0004-6361/201219058"],["dc.identifier.isi","000319858700006"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/10567"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/29944"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","A new extensive library of PHOENIX stellar atmospheres and synthetic spectra"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","191"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","205"],["dc.bibliographiccitation.volume","496"],["dc.contributor.author","Heller, R."],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Ostensen, Roy H."],["dc.date.accessioned","2018-11-07T08:31:50Z"],["dc.date.available","2018-11-07T08:31:50Z"],["dc.date.issued","2009"],["dc.description.abstract","Context. We present a catalog of 857 white dwarf (WD)-M binaries from the sixth data release (DR6) of the Sloan Digital Sky Survey (SDSS), most of which were previously identified. For 636 of them, we complete a spectral analysis and derive the basic parameters of their stellar constituents and their distances from Earth. Aims. We attempt to measure fundamental parameters of these systems by completing spectral analyses. We propose to test models typically applied in fitting procedures and constrain likely and appropriate evolutionary scenarios for the systems. Methods. We use chi(2) minimization technique to decompose each combined spectrum and derive independent parameter estimates for its components. The possibility of alignment by chance is demoted to statistical insignificance, hence, we use physical interaction of the binary constituents as input parameter. Additionally, we check the corresponding photometric data from the SDSS to find optically resolved systems. Results. Forty-one of the stellar duets in our spectroscopic sample are optically resolved in their respective SDSS images. For these systems, we also derive a minimum true spatial separation and a lower limit to their orbital periods, typically which are some 10(4) yr. Spectra of 167 stellar duets show significant hydrogen emission and in most cases no additional He I or He II features. We also find that 20 of the 636 WDs are fitted to be DOs, with 16 measured to have T(eff)(WD) around 40 000 K. Furthermore, we identify 70 very low-mass objects (VLMOs), which are secondaries of masses smaller than about 0.1 M(circle dot), to be candidate substellar companions. Conclusions. Although various selection effects may play a role, the fraction 6.4% of WD-M star binaries with orbital separations of around 500 AU is a criterion for evolutionary models of stellar binary systems. Of the 167 spectra with hydrogen emission, 8 had already been found to be post-common envelope binaries (PCEBs) and 4 are systems with strong irradiation processes on the Mdwarf. The remaining 155 Balmer-emitting binaries probably harbor an active Mdwarf (dM), corresponding to a fraction of 24.4%. The excess of cool DOs is most likely due to additional WDs in the DB-DO T(eff) range, for which no detailed fitting was completed. The trend of the M stars being closer to Earth than the WD component is probably due to an underestimation of the theoretical M star radii."],["dc.identifier.doi","10.1051/0004-6361:200810632"],["dc.identifier.fs","573283"],["dc.identifier.isi","000264404800017"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9400"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/17207"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Spectral analysis of 636 white dwarf-M star binaries from the sloan digital sky survey"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","793"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","800"],["dc.bibliographiccitation.volume","498"],["dc.contributor.author","Huegelmeyer, S. D."],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Hauschildt, P. H."],["dc.contributor.author","Seifahrt, Andreas"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Barman, T."],["dc.date.accessioned","2018-11-07T08:30:07Z"],["dc.date.available","2018-11-07T08:30:07Z"],["dc.date.issued","2009"],["dc.description.abstract","We present a new code for the calculation of the 1D structure and synthetic spectra of accretion disks. The code is an extension of the general purpose stellar atmosphere code PHOENIX and is therefore capable of including extensive lists of atomic and molecular lines as well as dust in the calculations. We assume that the average viscosity can be represented by a critical Reynolds number in a geometrically thin disk and solve the structure and radiative transfer equations for a number of disk rings in the vertical direction. The combination of these rings provides the total disk structure and spectrum. Since the warm inner regions of protoplanetary disks show a rich molecular spectrum, they are well suited for a spectral analysis with our models. In this paper we test our code by comparing our models with high-resolution VLT CRIRES spectra of the T Tauri star GQ Lup."],["dc.identifier.doi","10.1051/0004-6361/200811536"],["dc.identifier.isi","000266121800014"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9390"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/16817"],["dc.notes.intern","In goescholar not merged with http://resolver.sub.uni-goettingen.de/purl?gs-1/9694 but duplicate"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.rights","Goescholar"],["dc.rights.uri","https://goedoc.uni-goettingen.de/licenses"],["dc.title","Radiative transfer in circumstellar disks I. 1D models for GQ Lupi"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","367"],["dc.bibliographiccitation.issue","1"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","372"],["dc.bibliographiccitation.volume","464"],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Schuh, Sonja"],["dc.contributor.author","Homeier, Derek"],["dc.date.accessioned","2018-11-07T11:04:13Z"],["dc.date.available","2018-11-07T11:04:13Z"],["dc.date.issued","2007"],["dc.description.abstract","Context. The Optical Gravitational Lensing Experiment project has recently provided the OGLE III list of low-luminosity object transits from campaigns # 3 and # 4, reporting 40 new objects exhibiting the low-amplitude photometric eclipses expected for exoplanets. Compared to previous OGLE targets, these OGLE III candidates have been more restrictively selected and may contain low-mass planets. Aims. We have secured follow-up low-resolution spectroscopy for 28 candidates out of this list ( and one from the OGLE Carina fields) to obtain an independent characterization of the primary stars by spectral classification and thus better constrain the parameters of their companions. Methods. We fed the constraints from these results back into an improved light curve solution. Together with the radius ratios from the transit measurements, we derived the radii of the low-luminosity companions. This allows us to examine the possible sub-stellar nature of these objects. Results. Sixteen of the companions can be clearly identified as low-mass stars orbiting a main sequence primary, while 10 more objects are likely to have red giant primaries and therefore also host a stellar companion; 3 possibly have a sub-stellar nature (R <= 0.15 R-circle dot). Conclusions. The planetary nature of these objects should therefore be confirmed by dynamical mass determinations."],["dc.identifier.doi","10.1051/0004-6361:20054362"],["dc.identifier.fs","297835"],["dc.identifier.isi","000244407900038"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/7677"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/51787"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Spectral types of planetary host star candidates from OGLE III"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","615"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","621"],["dc.bibliographiccitation.volume","499"],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Reiners, Ansgar"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Noll, M."],["dc.date.accessioned","2018-11-07T08:30:07Z"],["dc.date.available","2018-11-07T08:30:07Z"],["dc.date.issued","2009"],["dc.description.abstract","Context. The detection of extrasolar planets' atmospheres requires very demanding observations. For planets that cannot be spatially separated from their host stars, i.e. the vast majority of planets, the transiting planets are the only ones to allow their atmospheres to be probed. This is possible from transmission spectroscopy or from measurements taken during the secondary eclipse. An alternative is to measure of the Rossiter-McLaughlin effect, which is sensitive to the size of the planetary radius. Since the radius is wavelength-dependent due to contributions of strong planetary absorption lines, this opens a path toward also probing planetary atmospheres with ground-based high-resolution spectroscopy. Aims. The major goal of our numerical simulations is to provide a reliable estimate of the amplitude of the wavelength-dependent Rossiter-McLaughlin effect. Methods. Our numerical simulations provide detailed phase-resolved synthetic spectra modeling the partly eclipsed stellar surface during the transit. With these spectra we can obtain Rossiter-McLaughlin curves for different wavelength regions and for a wavelength-dependent planetary radius. Curves from regions with high and low contributions of absorption lines within the planetary atmosphere can be compared. Observable quantities are derived from these differential effects. Results. We applied our simulations to HD209458. Our numerical simulations show that a detailed treatment of the limb-darkening for the synthetic spectra is important for a precise analysis. Compared to a parameterized limb-darkening law, systematic errors of 6 m s(-1) occur. The wavelength dependency of the planetary atmospheres over the NaD-doublet produces a differential effect in the Rossiter-McLaughlin curve of 1.5 m s(-1) for a star with a rotation velocity of 4.5 km s(-1), which increases to 4 m s(-1) for twice the rotation velocity. Conclusions. As a tool for probing planetary atmospheres the Rossiter-McLaughlin effect requires phase-resolved, high signal-tonoise, high-resolution spectra taken with a stabilized spectrograph in order to obtain reliable results for slowly rotating (< 10 m s(-1)) planet host stars. Stars with spectral type earlier than about F5 are a bit less demanding since the typically higher rotation velocity increases the amplitude of the effect to about 15 m s(-1) for a star with v sin i = 25 km s(-1)."],["dc.description.sponsorship","Deutsche Forschungsgemeinschaft [DFG RE 1664/4-1]"],["dc.identifier.doi","10.1051/0004-6361/200810656"],["dc.identifier.fs","573806"],["dc.identifier.isi","000266730500025"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9695"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/16816"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","1432-0746"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","On the possibility of detecting extrasolar planets' atmospheres with the Rossiter-McLaughlin effect"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","617"],["dc.bibliographiccitation.issue","2"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","624"],["dc.bibliographiccitation.volume","454"],["dc.contributor.author","Huegelmeyer, S. D."],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Krzesinski, J."],["dc.contributor.author","Werner, Katharina"],["dc.contributor.author","Nitta, A."],["dc.contributor.author","Kleinman, S. J."],["dc.date.accessioned","2018-11-07T09:26:58Z"],["dc.date.available","2018-11-07T09:26:58Z"],["dc.date.issued","2006"],["dc.description.abstract","Context. The Sloan Digital Sky Survey Data Release 4 has provided spectra of several new PG 1159 stars and DO white dwarfs. This increase in known hot H-deficient compact objects significantly improves the statistics and helps to investigate late stages of stellar evolution. Aims: From the optical SDSS spectra, effective temperatures and surface gravities are derived in order to place the observed objects in an evolutionary context. Especially the connection between PG 1159 stars and DO white dwarfs shall be investigated. Methods. Using our non-LTE model atmospheres and applying.2-fitting techniques, we determine stellar parameters and their errors. We derive total stellar masses for the DO white dwarfs using model evolutionary tracks. Results. We confirm three PG 1159 stars, with one showing ultra-high excitation ion features, and one sdO which we originally classified as a PG 1159 star. Additionally, we re-analysed the known PG 1159 star, PG 1424+535, with our new models. Furthermore, we present the first spectral analyses of thirteen DO white dwarfs, three of which show M-star features in their spectra, while two display ultra-high excitation ion features."],["dc.identifier.doi","10.1051/0004-6361:20064869"],["dc.identifier.fs","45031"],["dc.identifier.isi","000239004300052"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9877"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/30424"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Spectral analyses of eighteen hot H-deficient (pre-) white dwarfs from the Sloan Digital Sky Survey Data Release 4"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]

[["dc.bibliographiccitation.firstpage","1163"],["dc.bibliographiccitation.issue","3"],["dc.bibliographiccitation.journal","Astronomy and Astrophysics"],["dc.bibliographiccitation.lastpage","1168"],["dc.bibliographiccitation.volume","469"],["dc.contributor.author","Huegelmeyer, S. D."],["dc.contributor.author","Dreizler, Stefan"],["dc.contributor.author","Homeier, Derek"],["dc.contributor.author","Reiners, Ansgar"],["dc.date.accessioned","2018-11-07T11:00:51Z"],["dc.date.available","2018-11-07T11:00:51Z"],["dc.date.issued","2007"],["dc.description.abstract","Context. Planets outside our solar system transiting their host star, i.e. those with an orbital inclination near 90 degrees, are of special interest to derive physical properties of extrasolar planets. With the knowledge of the host star's physical parameters, the planetary radius can be determined. Combined with spectroscopic observations the mass and therefore the density can be derived from Doppler-measurements. Depending on the brightness of the host star, additional information, e. g. about the spin-orbit alignment between the host star and planetary orbit, can be obtained. Aims. The last few years have witnessed a growing success of transit surveys. Among other surveys, the MACHO project provided nine potential transiting planets, several of them with relatively bright parent stars. The photometric signature of a transit event is, however, insufficient to confirm the planetary nature of the faint companion. The aim of this paper therefore is a determination of the spectroscopic parameters of the host stars as well as a dynamical mass determination through Doppler-measurements. Methods. We obtained follow-up high-resolution spectra for five stars selected from the MACHO sample, which are consistent with transits of low-luminosity objects. Radial velocities were determined by means of cross-correlation with model spectra. The MACHO light-curves were compared to simulations based on the physical parameters of the system derived from the radial velocities and spectral analyses. Results. We show that all transit light-curves of the exoplanet candidates analysed in this work can be explained by eclipses of stellar objects, hence none of the five transiting objects is a planet."],["dc.identifier.doi","10.1051/0004-6361:20077147"],["dc.identifier.fs","297796"],["dc.identifier.isi","000247740200039"],["dc.identifier.purl","https://resolver.sub.uni-goettingen.de/purl?gs-1/9935"],["dc.identifier.uri","https://resolver.sub.uni-goettingen.de/purl?gro-2/51020"],["dc.notes.intern","Merged from goescholar"],["dc.notes.status","zu prüfen"],["dc.notes.submitter","Najko"],["dc.publisher","Edp Sciences S A"],["dc.relation.issn","0004-6361"],["dc.relation.orgunit","Fakultät für Physik"],["dc.title","Investigation of transit-selected exoplanet candidates from the MACHO survey"],["dc.type","journal_article"],["dc.type.internalPublication","yes"],["dc.type.peerReviewed","yes"],["dc.type.status","published"],["dc.type.version","published_version"],["dspace.entity.type","Publication"]]