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Klokocnik J.,Academy of Sciences of the Czech Republic | Klokocnik J.,Astronomical Institute | Bezdek A.,Academy of Sciences of the Czech Republic | Bezdek A.,Astronomical Institute | And 3 more authors.
Journal of Guidance, Control, and Dynamics | Year: 2010

When measurements from the Earth artificial satellites Gravity Recovery and Climate Experiment (devoted to the study of Earth's gravity field) were analyzed, it was discovered that there is a direct relationship between the density of satellite ground tracks D and the accuracy A of the gravity-field parameters derived from the satellite orbit(s) (in particular, of the monthly solutions for variable geopotential); the lower the D, the lower the A. This is due to passage of the satellite orbit through a low-order orbital resonance. A lesson learned from the Gravity Recovery and Climate Experiment was applied to the Gravity-Field and Steady-State Ocean Circulation Explorer; namely, the necessity of avoiding close proximity of its orbit (kept, in this case, at constant altitudes for the measurement phases with a gradiometer on board) to the 16:1 resonance. To avoid the decrease in A, we have to choose the orbit in such a way that order B of the lowest-order resonance, which will occur, will be higher than the highest degree L max of spherical harmonic expansion of the potential already known for the particular body. For the Earth, L max is now 150 [European improved gravity model of the Earth by new techniques (EIGEN-5S)] for satellite-only solutions and 2190 (Earth gravitational model 2008) for combination models. We extend these findings for a hypothetical future low-flying Mars orbiter; small changes in the semimajor axis can result in a dramatic improvement in accuracy of the derived gravity-field parameters, without any additional costs. The situation for Mars is similar to that of Earth. However, slowly rotating bodies like the moon, Mercury, or Venus do not yet suffer from similar problems because, for them, we obtain L max < B. Copyright © 2009.

Marscher A.,Institute for Astrophysical Research | Jorstad S.G.,Institute for Astrophysical Research | Jorstad S.G.,Astronomical Institute | Larionov V.M.,Astronomical Institute | And 2 more authors.
Journal of Astrophysics and Astronomy | Year: 2011

We are leading a comprehensive multi-waveband monitoring program of 34 γ-ray bright blazars designed to locate the emission regions of blazars from radio to γ-ray frequencies. The 'maps' are anchored by sequences of images in both total and polarized intensity obtained with the VLBA at an angular resolution of ~0. 1 milliarcseconds. The time-variable linear polarization at radio to optical wavelengths and radio to γ-ray light curves allow us to specify the locations of flares relative to bright stationary features seen in the images and to infer the geometry of the magnetic field in different regions of the jet. Our data reveal that some flares occur simultaneously at different wavebands and others are only seen at some of the frequencies. The flares are often triggered by a superluminal knot passing through the stationary 'core' on the VLBA images. Other flares occur upstream or even parsecs downstream of the core. © 2011 Indian Academy of Sciences.

Latour M.,University of Montréal | Latour M.,Astronomical Institute | Randall S.K.,ESO | Fontaine G.,University of Montréal | And 4 more authors.
Astrophysical Journal | Year: 2014

Taking advantage of a recent FORS2/VLT spectroscopic sample of extreme horizontal branch (EHB) stars in ω Cen, we isolate 38 spectra well suited for detailed atmospheric studies and determine their fundamental parameters (T eff, log g, and log N(He)/N(H)) using NLTE, metal line-blanketed models. We find that our targets can be divided into three groups: 6 stars are hot (T eff ≳ 45,000 K) H-rich subdwarf O stars, 7 stars are typical H-rich sdB stars (T eff ≲ 35,000 K), and the remaining 25 targets at intermediate effective temperatures are He-rich (log N(He)/N(H) ≳ -1.0) subdwarfs. Surprisingly, and quite interestingly, these He-rich hot subdwarfs in ω Cen cluster in a narrow temperature range (35,000 K to 40,000 K). We additionally measure the atmospheric carbon abundance and find a most interesting positive correlation between the carbon and helium atmospheric abundances. This correlation certainly bears the signature of diffusion processes - most likely gravitational settling impeded by stellar winds or internal turbulence - but also constrains possible formation scenarios proposed for EHB stars in ω Cen. For the He-rich objects in particular, the clear link between helium and carbon enhancement points toward a late hot flasher evolutionary history. © 2014. The American Astronomical Society. All rights reserved.

Zboril M.,Astronomical Institute
Astrophysics and Space Science | Year: 2010

We present new results on the extended atmosphere of the chromospherically active system UZ Lib. Doppler tomography confirms that the atmosphere of primary is extended to about 3.0 times its radius. Significant downfalls and probably plage regions are present in the atmosphere and we find a rather good correlation between the hydrogen integrated emission and the light curve. We also fix stellar parameters and distance to the system. An unseen companion revolves close to co-rotation radius, as well as close to the Roche L1 point, and in the corona of the primary. At this distance, tidal effects are significant. Despite the fact that the UV spectra are contaminated by the active giant atmosphere, we speculate that the unseen companion might be (a yellow-type) white dwarf (evidence for accretion, however, was not found), or alternatively an object like a brown dwarf. Finally, the IR observations indicate hydrogen emission without a recourse to extra-atmospheric material. © Springer Science+Business Media B.V. 2010.

Fruh C.,University of Bern | Fruh C.,Astronomical Institute | Schildknecht T.,University of Bern | Schildknecht T.,Astronomical Institute
Journal of Guidance, Control, and Dynamics | Year: 2012

The accuracy of two-line-element (TLE) data, provided by the U.S. Strategic Command (USSTRATCOM) in the geostationary orbit (GEO) and high-eccentricity orbit (HEO) regime, is investigated by a comparison with optical observations of the Zimmerwald Laser and Astrometry Telescope and the Zimmerwald Small Robotic Telescope (ZimSMART), both located in Zimmerwald, Switzerland, and the European Space Agency Space Debris Telescope (ESASDT) in Tenerife, Spain. The accuracy of the astrometric optical observations of all three telescopes is below 1 arcsec, as the comparison of optical observations to high-precision ephemerides to Global Navigation Satellite System (GNSS) satellites provided by the International GNSS Service shows. The differences of TLE ephemerides and the observations of the investigatedGEOobjects are on average (one-sigma confidence region) 0.04 deg or 25km in along-track and 10kmin the cross-track direction. For the investigated objects inHEOthe differences are 0.08 deg or 35kmin along-track and 25kmin the cross-track direction. The selection of the propagator (SDP4 or SDP8) is not significant for GEO objects and SDP4 shows only slightly higher accuracies for HEO objects. The determined accuracies are used to successfully implement a catalog correlation algorithm in real-time processing of observations of ZimSMART and the ESASDT. Copyright © 2012 by the American Institute of Aeronautics and Astronautics, Inc.

Vuckovic M.,University of Valparaíso | Vuckovic M.,Astronomical Observatory of Belgrade | Ostensen R.H.,Catholic University of Leuven | Nemeth P.,Catholic University of Leuven | And 4 more authors.
Astronomy and Astrophysics | Year: 2016

The effects of irradiation on the secondary stars of close binary systems are crucial for reliably determining the system parameters and for understanding the close binary evolution. They affect the stellar structure of the irradiated star and are reflected in the appearance of characteristic features in the spectroscopic and photometric data of these systems. We aim to study the light that originates from the irradiated side of the low-mass component of a close binary eclipsing system, which comprises a hot subdwarf primary and a low mass companion, to precisely interpret their high precision photometric and spectroscopic data, and accurately determine their system and surface parameters. We reanalyse the archival high-resolution time-resolved VLT/UVES spectra of AA Dor system, where irradiation features have already been detected. After removing the predominant contribution of the hot subdwarf primary, the residual spectra reveal more than 100 emission lines from the heated side of the secondary, which show maximum intensity close to the phases around the secondary eclipse. We analyse the residual spectrum to model the irradiation of the low-mass secondary. We perform a detailed analysis of 22 narrow emission lines of the irradiated secondary, mainly of O II, with a few significant C II lines. Their phase profiles constrain the emission region of the heated side to a radius ≥95% of the radius of the secondary, while the shape of their velocity profiles reveals two distinct asymmetry features, one at the quadrature and the other at the secondary eclipse. In addition, we identify weaker emission signatures originating from more than 70 lines, including lines from He I, N II, Si III, Ca II, and Mg II. From the emission lines of the heated side of the secondary star, we determine the radial velocity semi-amplitude of the centre-of-light and correct it to the centre-of-mass of the secondary which, in turn, gives accurate masses of both components of the AA Dor system. The resulting masses M1 = 0.46 ± 0.01 M⊙ and M2 = 0.079 ± 0.002 M⊙ are in perfect accordance with those of a canonical hot subdwarf primary and a low mass that is just at the substellar limit for the companion. We also compute a first generation atmosphere model of the low mass secondary, which includes irradiation effects and matches the observed spectrum well. We find an indication of an extended atmosphere of the irradiated secondary star. © ESO, 2016.

Budaj J.,Astronomical Institute
Proceedings of the International Astronomical Union | Year: 2011

Program SHELLSPEC is designed to calculate light-curves, spectra and images of interacting binaries and extrasolar planets immersed in a moving circumstellar environment which is optically thin. It solves simple radiative transfer along the line of sight in moving media. The assumptions include LTE and optional known state quantities and velocity fields in 3D. Optional (non)transparent objects such as a spot, disc, stream, jet, shell or stars may be defined (embedded) in 3D and their composite synthetic spectrum calculated. The Roche model can be used as a boundary condition for the radiative transfer. Recently, a new model of the reflection effect, dust and Mie scattering were incorporated into the code. Ï Aurigae is one of the most mysterious objects on the sky. Prior modeling of its light-curve assumed a dark, inclined, disk of dust with a central hole to explain the light-curve with a sharp mid-eclipse brightening. Our model consists of two geometrically thick flared disks: an internal optically thick disk and an external optically thin disk which absorbs and scatters radiation. Shallow mid-eclipse brightening may result from eclipses by nearly edge-on flared (dusty or gaseous) disks. Mid-eclipse brightening may also be due to strong forward scattering and optical properties of the dust which can have an important effect on the light-curves. There are many similarities between interacting binary stars and transiting extrasolar planets. The reflection effect which is briefly reviewed is one of them. The exact Roche shape and temperature distributions over the surface of all currently known transiting extrasolar planets have been determined. In some cases (HAT-P-32b, WASP-12b, WASP-19b), departures from the spherical shape can reach 7-15%. © International Astronomical Union 2012.

Ostensen R.H.,Catholic University of Leuven | Geier S.,European Southern Observatory | Geier S.,Astronomical Institute | Schaffenroth V.,Astronomical Institute | And 19 more authors.
Astronomy and Astrophysics | Year: 2013

The project Massive Unseen Companions to Hot Faint Underluminous Stars from SDSS (MUCHFUSS) aims to discover subdwarf-B stars with massive compact companions such as overmassive white dwarfs (M > 1.0 M ⊙), neutron stars or black holes. From the 127 subdwarfs with substantial radial-velocity variations discovered in the initial survey, a number of interesting objects have been selected for extensive follow-up. After an initial photometry run with BUSCA revealed that FBS 0117+396 is photometrically variable both on long and short timescales, we chose it as an auxiliary target during a 6-night multi-color photometry run with Ultracam. Spectroscopy was obtained at a number of observatories in order to determine the binary period and obtain a radial-velocity amplitude. After establishing an orbital period of P = 0.252 d, and removing the signal associated with the irradiated hemisphere of the M-dwarf companion, we were able to detect ten pulsation periods in the Fourier spectrum of the light curve. Two pulsation modes are found to have short periods of 337 and 379 s, and at least eight modes are found with periods between 45 min and 2.5 h. This establishes that FBS 0117+396 is an sdB+dM reflection binary, in which the primary is a hybrid pulsator, and the first one found with this particular mélange of flavours. © ESO, 2013.

News Article | February 20, 2017

V501 Aur was detected as an X-ray source in 1996 by the ROSAT space observatory. After the discovery, the source was classified as a possible new weak-lined T-Tauri star (WTTS) belonging to the Taurus-Auriga star-forming region. However, subsequent studies of this star provided evidence of its unseen stellar companion, suggesting that V501 Aur could be a single-lined spectroscopic binary. In the latest study, a team of astronomers led by Martin Vaňko of the Astronomical Institute of Slovak Academy of Sciences in Tatranská Lomnica, Slovakia, has found that V501 Aur is a single-lined spectroscopic binary star. "The spectroscopic observations reveal that V501 Aur is a single-lined spectroscopic binary system with a 68.8-day orbital period, a slightly eccentric orbit (e ∼ 0.03), and a systemic velocity discrepant from the mean of Taurus-Auriga," the paper reads. The findings are based on a series of observational campaigns carried out between 1996 and 2016, utilizing telescopes and spectrographs worldwide. These observations allowed the team to reveal more details about the nature of this star. "Information gathered since, as well as observations reported here, paint a rather different picture of the nature of the system that we now describe," the authors wrote. According to the paper, V501 Aur is a rapidly rotating early K star with a radius of more than 26.3 solar radii. The spectroscopic observations also indicate that V501 Aur has a fairly massive unseen companion, making it a binary star. Moreover, the new evidence shows that V501 Aur is not a T-Tauri star, but is instead a field binary far behind the Taurus-Auriga star-forming region. "The scenario that emerges for V501 Aur, aided by a comparison with stellar evolution models that succeed in matching all observational constraints, is one in which it is a background, non-eclipsing spectroscopic binary projected onto the Taurus-Auriga star-forming region, with a luminous, spotted, and fairly rapidly rotating giant star as the primary, and a likely much more rapidly rotating early-type star as the secondary," the researchers wrote in the paper. Furthermore, the scientists estimated the mass of this binary system. Their calculations show that the primary star is about four times more massive than the sun, while its companion has a mass between 1.63 to 2.3 solar masses. The findings also indicate that the system is roughly 180 million years old and is located approximately 2,600 light years away. The team concluded that in order to uncover more details about V501 Aur, a detailed chemical analysis of this system should be performed. It could help us reveal its other parameters and improve our knowledge about its evolutionary state. Explore further: Water detected in the atmosphere of hot Jupiter exoplanet 51 Pegasi b More information: On the nature of the candidate T-Tauri star V501 Aurigae, arXiv:1702.04512 [astro-ph.SR] Abstract We report new multi-colour photometry and high-resolution spectroscopic observations of the long-period variable V501 Aur, previously considered to be a weak-lined T-Tauri star belonging to the Taurus-Auriga star-forming region. The spectroscopic observations reveal that V501 Aur is a single-lined spectroscopic binary system with a 68.8-day orbital period, a slightly eccentric orbit (e ~ 0.03), and a systemic velocity discrepant from the mean of Taurus-Auriga. The photometry shows quasi-periodic variations on a different, ~55-day timescale that we attribute to rotational modulation by spots. No eclipses are seen. The visible object is a rapidly rotating (vsini ~ 25 km/s) early K star, which along with the rotation period implies it must be large (R > 26.3 Rsun), as suggested also by spectroscopic estimates indicating a low surface gravity. The parallax from the Gaia mission and other independent estimates imply a distance much greater than the Taurus-Auriga region, consistent with the giant interpretation. Taken together, this evidence together with a re-evaluation of the LiI~λ6707 and Hα lines shows that V501 Aur is not a T-Tauri star, but is instead a field binary with a giant primary far behind the Taurus-Auriga star-forming region. The large mass function from the spectroscopic orbit and a comparison with stellar evolution models suggest the secondary may be an early-type main-sequence star.

News Article | February 21, 2017

ALBUQUERQUE, N.M. -- When a meteor is about to conk your neighborhood and gives fair warning by emitting sizzling, rustling and hissing sounds as it descends, you might think that the universe is being sporting. But these auditory warnings, which do occur, seem contrary to the laws of physics if they are caused by the friction of the fast-moving meteor or asteroid plunging into Earth's atmosphere. Because sound travels far slower than light, the sounds should arrive several minutes after the meteor hits, rather than accompany or even precede it. So maybe atmospheric shock waves from the meteors are not the cause of the spooky noises. Another theory is that the sounds are created by radio frequency emissions. That seems unlikely without designated receivers. But what if the sounds are caused by the brilliant, pulsating light emitted by the asteroid as it burns up in Earth's atmosphere? In an article published Feb. 1 in the journal Scientific Reports, the late Sandia National Laboratories researcher Richard Spalding reasoned that such intense light could suddenly heat the surface of objects many miles away, which in turn heats the surrounding air. This could create sounds near the observer. Colleagues John Tencer, William Sweatt, Ben Conley, Roy Hogan, Mark Boslough and Gigi Gonzales, along with Pavel Spurny from the Astronomical Institute of the Czech Republic, experimentally demonstrated and analyzed that effect. They found that objects with low conductivity, such as leaves, grass, dark paint and even hair, could rapidly warm and transmit heat into nearby air and generate pressure waves by subtle oscillations that create a variety of sounds. The process is called photoacoustic coupling. Sounds concurrent with a meteor's arrival "must be associated with some form of electromagnetic energy generated by the meteor, propagated to the vicinity of the observer and transduced into acoustic waves," according to the article. "A succession of light-pulse-produced pressure waves can then manifest as sound to a nearby observer." The experimenters exposed several materials, including dark cloths and a wig, to intense pulsing light akin to that produced by a fireball. The process produced faint sounds similar to rustling leaves or faint whispers. Computer models bear out the results. A less extreme version of the photoacoustic effect had been observed in 1880 by Alexander Graham Bell when, testing the possibilities of light for long-distance phone transmissions, he intermittently interrupted sunlight shining on a variety of materials and noted the sounds produced. Sandia National Laboratories is a multimission laboratory operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corp., for the U.S. Department of Energy's National Nuclear Security Administration. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major R&D responsibilities in national security, energy and environmental technologies and economic competitiveness.

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