arXiv subjects

K. von Braun

Publications and source records attributed to K. von Braun.

At least 19 recordsLinked to original sources

arXiv2022

A detailed analysis of the Gl 486 planetary system

The Gl 486 system consists of a very nearby, relatively bright, weakly active M3.5 V star at just 8 pc with a warm transiting rocky planet of about 1.3 R_Terra and 3.0 M_Terra that is ideal for both transmission and emission spectroscopy and for testing interior models of telluric planets. To prepare for future studies, we collected light curves of seven new transits observed with the CHEOPS space mission and new radial velocities obtained with MAROON-X/Gemini North and CARMENES/Calar Alto telescopes, together with previously published spectroscopic and photometric data from the two spectrographs and TESS. We also performed interferometric observations with the CHARA Array and new photometric monitoring with a suite of smaller telescopes. From interferometry, we measure a limb-darkened disc angular size of the star Gl 486. Together with a corrected Gaia EDR3 parallax, we obtain a stellar radius. We also measure a stellar rotation period at P_rot ~ 49.9 d, an upper limit to its XUV (5-920 AA) flux with new Hubble/STIS data, and, for the first time, a variety of element abundances (Fe, Mg, Si, V, Sr, Zr, Rb) and C/O ratio. Besides, we impose restrictive constraints on the presence of additional components, either stellar or substellar, in the system. With the input stellar parameters and the radial-velocity and transit data, we determine the radius and mass of the planet Gl 486 b at R_p = 1.343+/0.063 R_Terra and M_p = 3.00+/-0.13 M_Terra. From the planet parameters and the stellar element abundances, we infer the most probable models of planet internal structure and composition, which are consistent with a relatively small metallic core with respect to the Earth, a deep silicate mantle, and a thin volatile upper layer. With all these ingredients, we outline prospects for Gl 486 b atmospheric studies, especially with forthcoming James Webb Space Telescope observations (abridged).

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arXiv2016

Spitzer IRAC Sparsely Sampled Phase Curve of the Exoplanet WASP-14b

Motivated by a high Spitzer IRAC oversubscription rate, we present a new technique of randomly and sparsely sampling phase curves of hot Jupiters. Snapshot phase curves are enabled by technical advances in precision pointing as well as careful characterization of a portion of the central pixel on the array. This method allows for observations which are a factor of roughly two more efficient than full phase curve observations, and are furthermore easier to insert into the Spitzer observing schedule. We present our pilot study from this program using the exoplanet WASP-14b. Data of this system were taken both as a sparsely sampled phase curve as well as a staring mode phase curve. Both datasets as well as snapshot style observations of a calibration star are used to validate this technique. By fitting our WASP-14b phase snapshot dataset, we successfully recover physical parameters for the transit and eclipse depths as well as amplitude and maximum and minimum of the phase curve shape of this slightly eccentric hot Jupiter. We place a limit on the potential phase to phase variation of these parameters since our data are taken over many phases over the course of a year. We see no evidence for eclipse depth variations compared to other published WASP-14b eclipse depths over a 3.5 year baseline.

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arXiv2015

Follow-Up Observations of PTFO 8-8695: A 3 MYr Old T-Tauri Star Hosting a Jupiter-mass Planetary Candidate

We present Spitzer 4.5\micron\ light curve observations, Keck NIRSPEC radial velocity observations, and LCOGT optical light curve observations of PTFO~8-8695, which may host a Jupiter-sized planet in a very short orbital period (0.45 days). Previous work by \citet{vaneyken12} and \citet{barnes13} predicts that the stellar rotation axis and the planetary orbital plane should precess with a period of $300 - 600$ days. As a consequence, the observed transits should change shape and depth, disappear, and reappear with the precession. Our observations indicate the long-term presence of the transit events ($>3$ years), and that the transits indeed do change depth, disappear and reappear. The Spitzer observations and the NIRSPEC radial velocity observations (with contemporaneous LCOGT optical light curve data) are consistent with the predicted transit times and depths for the $M_\star = 0.34\ M_\odot$ precession model and demonstrate the disappearance of the transits. An LCOGT optical light curve shows that the transits do reappear approximately 1 year later. The observed transits occur at the times predicted by a straight-forward propagation of the transit ephemeris. The precession model correctly predicts the depth and time of the Spitzer transit and the lack of a transit at the time of the NIRSPEC radial velocity observations. However, the precession model predicts the return of the transits approximately 1 month later than observed by LCOGT. Overall, the data are suggestive that the planetary interpretation of the observed transit events may indeed be correct, but the precession model and data are currently insufficient to confirm firmly the planetary status of PTFO~8-8695b.

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arXiv2015

The Expanding Fireball of Nova Delphini 2013

A classical nova occurs when material accreting onto the surface of a white dwarf in a close binary system ignites in a thermonuclear runaway. Complex structures observed in the ejecta at late stages could result from interactions with the companion during the common envelope phase. Alternatively, the explosion could be intrinsically bipolar, resulting from a localized ignition on the surface of the white dwarf or as a consequence of rotational distortion. Studying the structure of novae during the earliest phases is challenging because of the high spatial resolution needed to measure their small sizes. Here we report near-infrared interferometric measurements of the angular size of Nova Delphini 2013, starting from one day after the explosion and continuing with extensive time coverage during the first 43 days. Changes in the apparent expansion rate can be explained by an explosion model consisting of an optically thick core surrounded by a diffuse envelope. The optical depth of the ejected material changes as it expands. We detect an ellipticity in the light distribution, suggesting a prolate or bipolar structure that develops as early as the second day. Combining the angular expansion rate with radial velocity measurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpc from the Sun.

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arXiv2014

Benchmark stars for Gaia: fundamental properties of the Population II star HD140283 from interferometric, spectroscopic and photometric data

We determined the fundamental properties of HD 140283 by obtaining new interferometric and spectroscopic measurements and combining them with photometry from the literature. The interferometric measurements were obtained using the visible interferometer VEGA on the CHARA array and we determined a 1D limb-darkened angular diameter of 0.353 +/- 0.013 milliarcseconds. Using photometry from the literature we derived the bolometric flux with two solutions: a zero-reddening one of Fbol = 3.890 +/- 0.066 1E-8 erg/s/cm2 and a solution with a maximum of Av = 0.1 mag, Fbol= 4.220 +/- 0.067 1E-8 erg/s/cm2. The interferometric Teff is thus 5534 +/- 103 K or 5647 +/- 105 K and its radius is R = 2.21 +/- 0.08 Rsol. Spectroscopic measurements of HD140283 were obtained using HARPS, NARVAL, and UVES and a 1D LTE analysis of H-alpha line wings yields Teff(Halpha) = 5626 +/- 75 K. Using fine-tuned stellar models including diffusion of elements we then determined the mass M and age t of HD140283. Once the metallicity has been fixed, the age of the star depends on M, initial helium abundance Yi and mixing-length parameter alpha, only two of which are independent. We need to adjust alpha to much lower values than the solar one (~2) in order to fit the observations, and if Av = 0.0 mag then 0.5 < alpha < 1. We give an equation to estimate t from M, Yi (alpha) and Av. Establishing a reference alpha = 1.00 and adopting Yi = 0.245 we derive a mass and age of HD140283: M = 0.780 +/- 0.010 Msol and t = 13.7 +/- 0.7 Gyr (Av = 0.0) or M = 0.805 +/- 0.010 Msol and t = 12.2 +/- 0.6 Gyr (Av=0.1 mag). Our stellar models yield an initial metallicity of [Z/X]i = -1.70 and logg = 3.65 +/- 0.03. Asteroseismic observations are critical for overcoming limitations in our results.

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arXiv2013

The NASA Exoplanet Archive: Data and Tools for Exoplanet Research

We describe the contents and functionality of the NASA Exoplanet Archive, a database and tool set funded by NASA to support astronomers in the exoplanet community. The current content of the database includes interactive tables containing properties of all published exoplanets, Kepler planet candidates, threshold-crossing events, data validation reports and target stellar parameters, light curves from the Kepler and CoRoT missions and from several ground-based surveys, and spectra and radial velocity measurements from the literature. Tools provided to work with these data include a transit ephemeris predictor, both for single planets and for observing locations, light curve viewing and normalization utilities, and a periodogram and phased light curve service. The archive can be accessed at http://exoplanetarchive.ipac.caltech.edu.

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arXiv2012

M dwarf stars in the light of (future) exoplanet searches

We present a brief overview of a splinter session on M dwarf stars as planet hosts that was organized as part of the Cool Stars 17 conference. The session was devoted to reviewing our current knowledge of M dwarf stars and exoplanets in order to prepare for current and future exoplanet searches focusing in low mass stars. We review the observational and theoretical challenges to characterize M dwarf stars and the importance of accurate fundamental parameters for the proper characterization of their exoplanets and our understanding on planet formation.

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arXiv2011

Recent Results form the CHARA Array

The CHARA Array is a six 1-m telescope optical and near infrared interferometer located at the Mount Wilson Observatory in southern California and operated by the Center for High Angular Resolution Astronomy of Georgia State University. The CHARA Array has been in regular scientific operation since 2005 and now has over 55 publications in the refereed literature, including two in Science and one in Nature. The Array now supports seven beam combiners ranging from 0.5 microns up to 2.3 microns and combing from 2 to 4 beams at a time. An upgrade to a full 6 beam combiner is now underway and fringes with all six telescopes were achieved soon after the meeting.We present some of the more recent results from the CHARA-Array.

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arXiv2011

The NStED Periodogram Service and Interface for Public CoRoT Data

As part of the NASA-CNES agreement, the NASA Star and Exoplanet Database (NStED) serves as the official US portal for the public CoRoT data products. NStED is a general purpose archive with the aim of providing support for NASA's planet finding and characterization goals. Consequently, the NASA Exoplanet Science Institute (NExScI) developed, and NStED adapted, a periodogram service for CoRoT data to determine periods of variability phenomena and create phased photometric light curves. Through the NStED periodogram interface, the user may choose three different period detection algorithms to use on any photometric time series product, or even upload and analyze their own data. Additionally, the NStED periodogram is remotely accessed by the CoRoT archive as part of its interface. NStED is available at {\bf http://nsted.ipac.caltech.edu}.

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arXiv2011

The 55 Cancri System: Fundamental Stellar Parameters, Habitable Zone Planet, and Super-Earth Diameter

The bright star 55 Cancri is known to host five planets, including a transiting super-Earth. We use the CHARA Array to directly determine the following of 55 Cnc's stellar astrophysical parameters: $R=0.943 \pm 0.010 R_{\odot}$, $T_{\rm EFF} = 5196 \pm 24$ K. Planet 55 Cnc f ($M \sin i = 0.155 M_{Jupiter}$) spends the majority of the duration of its elliptical orbit in the circumstellar habitable zone (0.67--1.32 AU) where, with moderate greenhouse heating, it could harbor liquid water. Our determination of 55 Cancri's stellar radius allows for a model-independent calculation of the physical diameter of the transiting super-Earth 55 Cnc e ($\simeq 2.1 R_{\earth}$), which, depending on the assumed literature value of planetary mass, implies a bulk density of 0.76 $ρ_{\earth}$ or 1.07 $ρ_{\earth}$.

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arXiv2010

Fundamental Properties of Cool Stars with Interferometry

We present measurements of fundamental astrophysical properties of nearby, low-mass, K- and M-dwarfs from our DISCOS survey (DIameterS of COol Stars). The principal goal of our study is the determination of linear radii and effective temperatures for these stars. We calculate their radii from angular diameter measurements using the CHARA Array and Hipparcos distances. Combined with bolometric flux measurements based on literature photometry, we use our angular diameter results to calculate their effective surface temperatures. We present preliminary results established on an assortment of empirical relations to the stellar effective temperature and radius that are based upon these measurements. We elaborate on the discrepancy seen between theoretical and observed stellar radii, previously claimed to be related to stellar activity and/or metallicity. Our preliminary conclusion, however, is that convection plays a larger role in the determination of radii of these late-type stars. Understanding the source of the radius disagreement is likely to impact other areas of study for low-mass stars, such as the detection and characterization of extrasolar planets in the habitable zones.

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arXiv2009

Exoplanet Characterization and the Search for Life

Over 300 extrasolar planets (exoplanets) have been detected orbiting nearby stars. We now hope to conduct a census of all planets around nearby stars and to characterize their atmospheres and surfaces with spectroscopy. Rocky planets within their star's habitable zones have the highest priority, as these have the potential to harbor life. Our science goal is to find and characterize all nearby exoplanets; this requires that we measure the mass, orbit, and spectroscopic signature of each one at visible and infrared wavelengths. The techniques for doing this are at hand today. Within the decade we could answer long-standing questions about the evolution and nature of other planetary systems, and we could search for clues as to whether life exists elsewhere in our galactic neighborhood.

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arXiv2009

The LAEX and NASA portals for CoRoT public data

* Aims. We describe here the main functionalities of the LAEX (Laboratorio de Astrofisica Estelar y Exoplanetas/Laboratory for Stellar Astrophysics and Exoplanets) and NASA portals for CoRoT Public Data. The CoRoT archive at LAEX was opened to the community in January 2009 and is managed in the framework of the Spanish Virtual Observatory. NStED (NASA Star and Exoplanet Database) serves as the CoRoT portal for the US astronomical community. NStED is a general purpose stellar and exoplanet archive with the aim of providing support for NASA planet finding and characterisation goals, and the planning and support of NASA and other space missions. CoRoT data at LAEX and NStED can be accessed at http://sdc.laeff.inta.es/corotfa/ and http://nsted.ipac.caltech.edu,respectively. * Methods. Based on considerable experience with astronomical archives, the aforementioned archives are designed with the aim of delivering science-quality data in a simple and efficient way. * Results. LAEX and NStED not only provide access to CoRoT Public Data but furthermore serve a variety of observed and calculated astrophysical data. In particular, NStED provides scientifically validated information on stellar and planetary data related to the search for and characterization of extrasolar planets, and LAEX makes any information from Virtual Observatory services available to the astronomical community.

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arXiv2009

The NASA/IPAC/NExScI Star and Exoplanet Database

The NASA/IPAC/NExScI Star and Exoplanet Database (NStED) is a general purpose stellar archive which supports NASA planet-finding and planet-characterization goals, stellar astrophysics, and the planning of NASA and other space missions. There are two principal components of NStED: a database of 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high precision photometric surveys for transiting exoplanets (NStED-ETSS). We present summaries of these components. The NStED stellar database currently serves published parameters for 140,000 stars. These parameters include coordinates, multiplicity, proper motion, parallax, spectral type, multiband photometry, radial velocity, metallicity, chromospheric and coronal activity index, rotation velocity/period, infrared excess. NStED-ETSS currently serves data from the TrES survey of the Kepler field as well as dedicated photometric surveys of four stellar clusters. NStED-ETSS aims to serve both the surveys and the broader astronomical community by archiving these data and making them available in a homogeneous format.

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arXiv2008

The NStED Stellar and Exoplanet Hosting Star Service

The NASA Star and Exoplanet Database (NStED) is a general purpose stellar archive with the aim of providing support for NASA's planet finding and characterization goals, stellar astrophysics, and the planning of NASA and other space missions. There are two principal components of NStED: a database of (currently) 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high precision photometric surveys for transiting exoplanets. We present a summary of the NStED stellar database, functionality, tools, and user interface. NStED currently serves the following kinds of data for 140,000 stars (where available): coordinates, multiplicity, proper motion, parallax, spectral type, multiband photometry, radial velocity, metallicity, chromospheric and coronal activity index, and rotation velocity/period. Furthermore, the following derived quantities are given wherever possible: distance, effective temperature, mass, radius, luminosity, space motions, and physical/angular dimensions of habitable zone. Queries to NStED can be made using constraints on any combination of the above parameters. In addition, NStED provides tools to derive specific inferred quantities for the stars in the database, cross-referenced with available extra-solar planetary data for those host stars. NStED can be accessed at http://nsted.ipac.caltech.edu

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arXiv2008

The NStED Exoplanet Transit Survey Service

The NASA Star and Exoplanet Database (NStED) is a general purpose stellar archive with the aim of providing support for NASA's planet finding and characterization goals, stellar astrophysics, and the planning of NASA and other space missions. There are two principal components of NStED: a database of (currently) 140,000 nearby stars and exoplanet-hosting stars, and an archive dedicated to high-precision photometric surveys for transiting exoplanets. We present a summary of the latter component: the NStED Exoplanet Transit Survey Service (NStED-ETSS), along with its content, functionality, tools, and user interface. NStED-ETSS currently serves data from the TrES Survey of the Kepler Field as well as dedicated photometric surveys of four stellar clusters. NStED-ETSS aims to serve both the surveys and the broader astronomical community by archiving these data and making them available in a homogeneous format. Examples of usability of ETSS include investigation of any time-variable phenomena in data sets not studied by the original survey team, application of different techniques or algorithms for planet transit detections, combination of data from different surveys for given objects, statistical studies, etc. NStED-ETSS can be accessed at \tt{http://nsted.ipac.caltech.edu}

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arXiv2006

The Astrobiology Primer: An Outline of General Knowledge - Version 1, 2006

Astrobiology, the study of life as a planetary phenomenon, aims to understand the fundamental nature of life on earth and the possibility of life elsewhere. To achieve this goal, astrobiologists have initiated unprecedented communication between the disciplines of astronomy, biology, chemistry, and geology. The Astrobiology Primer has been created as a reference tool for those who are interested in the interdisciplinary field of astrobiology. The field incorporates many diverse research endeavors, but it is our hope that this slim volume will present the reader with all he or she needs to know to become involved and to understand, at least at a fundamental level, the state of the art. Because of the great diversity of material, each section was written by a different author with a different expertise. The Primer was constructed collaboratively. Ninety researchers from around the world contributed information with regard to what they expected from other astrobiologists and what they would like to know themselves but still had difficulty understanding (see Contributors). Those submissions were read and considered by the Editors who produced a list of seven general categories of knowledge, represented by the seven chapters in the Primer: 1) Stellar Formation and Evolution, 2) Planetary Formation and Evolution, 3) Astrobiogeochemistry and the Origin of Life, 4) Evolution of Life through Time, 5) Planet Detection & Characterization, 6) Diversity of Life, and 7) Science in Space. No one volume, of course, can contain the vast amount of information brought to play in astrobiology, but we believe that the Primer will provide a forum and a language around which the community will have the opportunity to develop a consensus about central issues.

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arXiv2004

Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC

Open clusters potentially provide an ideal environment for the search for transiting extrasolar planets since they feature a relatively large number of stars of the same known age and metallicity at the same distance. With this motivation, over a dozen open clusters are now being monitored by four different groups. We review the motivations and challenges for open cluster transit surveys for short-period giant planets. Our photometric monitoring survey (EXPLORE/OC) of Galactic southern open clusters was designed with the goals of maximizing the chance of finding and characterizing planets, and of providing for a statistically valuable astrophysical result in the case of no detections. We use the EXPLORE/OC data from two open clusters NGC 2660 and NGC 6208 to illustrate some of the largely unrecognized issues facing open cluster surveys including severe contamination by Galactic field stars ($>$ 80%) and relatively low number of cluster members for which high precision photometry can be obtained. We discuss how a careful selection of open cluster targets under a wide range of criteria such as cluster richness, observability, distance, and age can meet the challenges, maximizing chances to detect planet transits. In addition, we present the EXPLORE/OC observing strategy to optimize planet detection which includes high-cadence observing and continuously observing individual clusters rather than alternating between targets.

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