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Hohle M.M.,Universitats Sternwarte Jena | Haberl F.,Max Planck Institute for Extraterrestrial Physics | Vink J.,University of Amsterdam | de Vries C.P.,SRON Netherlands Institute for Space Research | And 4 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

RXJ0720.4-3125 is the most peculiar object among a group of seven isolated X-ray pulsars (the so-called Magnificent Seven), since it shows long-term variations of its spectral and temporal properties on time-scales of years. This behaviour was explained by different authors either by free precession (with a 7 or 14 yr period) or possibly a glitch that occurred around MJD = 52866 ± 73d. We analysed our most recent XMM-Newton and Chandra observations in order to further monitor the behaviour of this neutron star. With the new data sets, the timing behaviour of RXJ0720.4-3125 suggests a single (sudden) event (e.g. a glitch) rather than a cyclic pattern as expected by free precession. The spectral parameters changed significantly around the proposed glitch time, but more gradual variations occurred already before the (putative) event. Since MJD ≈ 53000d the spectra indicate a very slow cooling by ∼2eV over 7 yr. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Hohle M.M.,Max Planck Institute for Extraterrestrial Physics | Hohle M.M.,Universitats Sternwarte Jena | Haberl F.,Max Planck Institute for Extraterrestrial Physics | Vink J.,University Utrecht | And 5 more authors.
Astronomy and Astrophysics | Year: 2010

Aims. Since the last phase coherent timing solution of the nearby radio-quiet isolated neutron star RX  J0720.4-3125 six new XMM-Newton and three Chandra observations were carried out. The phase coherent timing solutions from previous authors were performed without restricting to a fixed energy band. However, we recently showed that the phase residuals are energy dependent, and thus phase coherent solutions must be computed referring always to the same energy band. Methods. We updated the phase coherent timing solution for RX J0720.4-3125 by including the recent XMM-Newton EPIC-pn, MOS1, MOS2 and Chandra ACIS data in the energy range 400-1000 eV. Altogether these observations cover a time span of almost 10 yrs. A further timing solution was obtained including the ROSAT pointed data. In this case, observations cover a time span of ≈16 yrs. To illustrate the timing differences between the soft band (120-400 eV) and the hard band (400-1000 eV) a timing solution for the soft band is also presented and the results are verified using a Zn 2 test. Results. In contrast to previous work, we obtain almost identical solutions whether or not we include the ROSAT or Chandra data. Thanks to the restriction to the hard band, the data points from EPIC-pn are in better agreement with those from MOS1, MOS2 and Chandra than in previous works. In general the phase residuals are still large and vary with time. In particular, the latest XMM-Newton and Chandra data show that the phase residuals have attained relatively large and negative values. Using this and previous timing solutions, the residuals indicate a cyclic behaviour with a period ≈ 7-9 yrs if the variations follow a sinusoid, or twice this value in case the residuals are modulated by an abs(sine) probably approaching a new minimum around MJD = 55 240 days (February 2010). As an alternative interpretation, the phase residuals can be fitted with a glitch that occured around MJD = 53 000 days. © 2010 ESO.

Mugrauer M.,Universitats Sternwarte Jena | Ginski C.,Universitats Sternwarte Jena | Seeliger M.,Universitats Sternwarte Jena
Monthly Notices of the Royal Astronomical Society | Year: 2014

We present seven new widely separated companions of exoplanet host stars, detected via common proper motion, whose companionship could be confirmed with photometry, and in two cases also with follow-up spectroscopy. We derive the masses and projected separations of all detected companions, among them HD 107148 B, a further wide white dwarf companion of an exoplanet host star. The current multiplicity rate of the exoplanet host stars is found to be about 13 per cent. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society.

Mugrauer M.,Universitats Sternwarte Jena | Roll T.,Universitats Sternwarte Jena | Ginski C.,Universitats Sternwarte Jena | Vogt N.,University of Valparaíso | And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2012

We present follow-up high-contrast imaging data of PZTelB, the substellar companion of a solar analogue pre-main-sequence star and member of the approximately 12-Myr-old βPic moving group. Between 2010 October and 2011 June, we observed PZTel in three observing epochs with NACO/ESO-VLT, which proves the companionship of PZTelB on a very high significance level and shows that the photometry of the PZTel system is stable on the 10per cent level. The orbital motion of PZTelB relative to its primary is clearly detected between all observing epochs, and we even find evidence for deceleration of its orbital motion, as expected for an object on a Keplerian orbit moving towards its apastron. We also present new photometric measurements of PZTel just recently obtained by WISE, which clearly confirm that there is no excess emission in the spectral energy distribution (SED) of the star up to 24 μ m, while a prominent excess is detected at 70μm with MIPS/Spitzer. To quantify the orbit elements of the PZTel system, we applied a least-squares Monte Carlo (LSMC) fit on all astrometric measures taken with NACO. The results of the performed LSMC fit are that one cannot yet determine a single 'best'-fitting orbital solution, due to the poor astrometric coverage of the orbit. However, the obtained good LSMC solutions (χ red 2≤2) give an overview of the distributions and correlations of all orbit elements. According to our LSMC fit, PZTel is an eccentric binary (e ≳ 0.6), and the orbit elements degenerate with growing eccentricity. For e < 0.95, the PZTel system is seen nearly edge-on (92° ≲ i ≲ 110°), has a longitude of the ascending node of 50° ≲ Ω ≲ 65°, an argument of periastron 170° ≲ ω ≲ 290° and exhibits a semimajor axis a larger than about 20au, with the highest peak in the a-histogram located at about 25au (or about 110yr of orbital period), up to a few tens of au, as orbital solutions with a significantly larger semimajor axis would not be able to harbour a circumbinary disc which excites the observed excess in the SED of PZTelA. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS.

Tachihara K.,Joint ALMA Observatory | Tachihara K.,Japan National Astronomical Observatory | Saigo K.,Japan National Astronomical Observatory | Higuchi A.E.,Joint ALMA Observatory | And 6 more authors.
Astrophysical Journal | Year: 2012

In order to investigate the origin of the interstellar turbulence, detailed observations in the CO J = 1-0 and 3-2 lines have been carried out in an interacting region of a molecular cloud with an H II region. As a result, several 1000-10,000AU scale cloudlets with small velocity dispersion are detected, whose systemic velocities have a relatively large scatter of a fewkms-1. It is suggested that the cloud is composed of small-scale dense and cold structures and their overlapping effect makes it appear to be a turbulent entity as a whole. This picture strongly supports the two-phase model of a turbulent medium driven by thermal instability proposed previously. On the surface of the present cloud, the turbulence is likely to be driven by thermal instability following ionization shock compression and UV irradiation. Those small-scale structures with line widths of 0.6kms-1 have a relatively high CO line ratio of J = 3-2 to 1-0, 1 ≲ R 3 - 2/1 - 0 ≲ 2. The large velocity gradient analysis implies that the 0.6kms-1 width component cloudlets have an average density of 103-10 4 cm-3, which is relatively high at cloud edges, but their masses are only ≲ 0.05 M. © 2012. The American Astronomical Society. All rights reserved..

Masda S.G.,Universitats Sternwarte Jena | Al-Wardat M.A.,Al al-Bayt University | Neuhauser R.,Universitats Sternwarte Jena | Al-Naimiy H.M.,University of Sharjah
Research in Astronomy and Astrophysics | Year: 2016

We present the physical and geometrical parameters of the individual components of the close visual double-lined spectroscopic binary system Gliese 762.1, which were estimated using Al-Wardat's complex method for analyzing close visual binary systems. The estimated parameters of the individual components of the system are as follows: radius RA = 0.845 ± 0.09 R o, RB = 0.795 ± 0.10 R o, effective temperature TA eff = 5300 ± 50 K, TB eff = 5150 ± 50 K, surface gravity log gA = 4.52 ± 0.10, log gB = 4.54±0.15 and luminosity LA = 0.51±0.08 L o, LB = 0.40±0.07 L o. New orbital elements are presented with a semi-major axis of 0.0865 ± 0.010 arcsec using the Hippracos parallax π = 58.96 ± 0.65 mas, and an accurate total mass and individual masses of the system are determined as M = 1.72 ± 0.60 Mo, MA = 0.89 ± 0.08 M o and MB = 0.83 ± 0.07 Mo. Finally, the spectral types and luminosity classes of both components are assigned as K0V and K1.5V for the primary and secondary components respectively, and their positions on the H-R diagram and evolutionary tracks are given. © 2016 National Astronomical Observatories, Chinese Academy of Sciences and IOP Publishing Ltd.

Seeiger M.,Universitats Sternwarte Jena | Neuhauser R.,Universitats Sternwarte Jena | Eisenbeiss T.,Universitats Sternwarte Jena
Astronomische Nachrichten | Year: 2011

We report on the results of the spectroscopy of 10 objects previously classified as brown dwarf candidates via RIJHK colors by Eisenbeiss et al. (2009), who performed deep imaging observations on a ~0.4 sq.deg. field at the edge of the Pleiades. We describe and judge on classification techniques in the region of M-type stars. To classify and characterise the objects, visual and near infrared spectra have been obtained with VLT FORS and ISAAC. The spectral classification was performed using the shape of the spectra as well as spectral indices that are sensitive to the spectral type and luminosity class of M-type stars and late M-type brown dwarfs. Furthermore a spectrophotometric distance was calculated and compared the distance of the Pleiades to investigate the membership probability. As a second argument we analyzed the proper motion. The brown dwarf candidates were found not to be brown dwarfs, but late-K to mid-M-type dwarf stars. Based on the obtained distance and tabulated proper motions we conclude that all objects are background dwarf stars. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Tetzlaff N.,Universitats Sternwarte Jena | Neuhauser R.,Universitats Sternwarte Jena | Hohle M.M.,Universitats Sternwarte Jena | Hohle M.M.,Max Planck Institute for Extraterrestrial Physics
Monthly Notices of the Royal Astronomical Society | Year: 2011

Traditionally, runaway stars are O- and B-type stars with large peculiar velocities. We would like to extend this definition to young stars (up to ≈50 Myr) of any spectral type and to identify those present in the Hipparcos catalogue by applying different selection criteria, such as peculiar space velocities or peculiar one-dimensional velocities. Runaway stars are important for studying the evolution of multiple star systems or star clusters, as well as for identifying the origins of neutron stars. We compile the distances, proper motions, spectral types, luminosity classes, V magnitudes and B-V colours, and we utilize evolutionary models from different authors to obtain star ages. We study a sample of 7663 young Hipparcos stars within 3 kpc from the Sun. The radial velocities are obtained from the literature. We investigate the distributions of the peculiar spatial velocity and the peculiar radial velocity as well as the peculiar tangential velocity and its one-dimensional components and we obtain runaway star probabilities for each star in the sample. In addition, we look for stars that are situated outside any OB association or OB cluster and the Galactic plane as well as stars for which the velocity vector points away from the median velocity vector of neighbouring stars or the surrounding local OB association/cluster (although the absolute velocity might be small). We find a total of 2547 runaway star candidates (with a contamination of normal Population I stars of 20 per cent at most). Thus, after subtracting these 20 per cent, the runaway frequency among young stars is about 27 per cent. We compile a catalogue of runaway stars, which is available via VizieR. © 2010 The Authors. Journal compilation © 2010 RAS.

Mugrauer M.,Universitats Sternwarte Jena | Vogt N.,University of Valparaíso | Vogt N.,Católica del Norte University | Neuhauser R.,Universitats Sternwarte Jena | Schmidt T.O.B.,Universitats Sternwarte Jena
Astronomy and Astrophysics | Year: 2010

Aims. We study the formation of substellar objects (exoplanets and brown dwarfs) that are companions to young nearby stars. Methods. With high-contrast AO imaging obtained with NACO at ESO VLT, we search for faint companion candidates around our targets, whose companionship can be confirmed with astrometry. Results. We identified a faint substellar companion to the nearby pre-main sequence star PZTel, a member of the β Pic moving group. The companion is 5-6 mag fainter than its host star in JHK and is located at a separation of only 0.3 arcsec (or 15AU of projected separation) northeast of PZ Tel. Within three NACO observing epochs, we can confirm the common proper motion (>39) and detected orbital motion of PZ TelB around its primary (>37). The photometry of the newly found companion is consistent with a brown dwarf with a mass between 24 and 40 MJup, at the distance (50 pc) and age (8.20 Myr) of PZ Tel. The effective temperature of the companion, derived from its photometry, ranges between 2500 and 2700 K, which corresponds to a spectral type between M6 and M8. After β Pic b, PZ TelB is the second closest substellar companion imaged directly around a young star. © 2010 ESO.

Suleimanov V.,University of Tübingen | Suleimanov V.,Kazan Federal University | Hambaryan V.,Universitats Sternwarte Jena | Potekhin A.Y.,Ecole Normale Superieure de Lyon | And 4 more authors.
Astronomy and Astrophysics | Year: 2010

Context. In the X-ray spectra of most X-ray dim isolated neutron stars (XDINSs), absorption features with equivalent widths (EWs) of 50-200 eV are observed. These features are usually connected with the proton cyclotron line, but their nature is not yet well known. Aims. We theoretically investigate different models to explain these absorption features and compare their properties with observations to obtain a clearer understanding of the radiation properties of magnetized neutron star surfaces. Based on these models, we create a fast and flexible code to fit observed spectra of isolated neutron stars. Methods. We consider various theoretical models of the magnetized neutron star surface, including naked condensed iron surfaces and partially ionized hydrogen model atmospheres, with semi-infinite and thin atmospheres above a condensed surface. Spectra of condensed iron surfaces are represented by a simple analytical approximation. The condensed surface radiation properties are considered as the inner atmosphere boundary condition for the thin atmosphere. The properties of the absorption features (especially equivalent widths) and the angular distributions of the emergent radiation are described for all models. A code for computing light curves and integral emergent spectra of magnetized neutron stars is developed. We assume a dipole surface magnetic field distribution with a possible toroidal component and corresponding temperature distribution. A model with two uniform hot spots at the magnetic poles may also be employed. Results. Light curves and spectra of highly magnetized neutron stars with parameters typical of XDINSs are computed using different surface temperature distributions and various local surface models. Spectra of magnetized model atmospheres are approximated by diluted black-body spectra with one or two Gaussian lines having parameters, which allow us to describe the model absorption features. The EWs of the absorption features in the integral spectra cannot significantly exceed 100 eV, if a local surface model assumes either a semi-infinite magnetic atmosphere or a naked condensed surface. A thin atmosphere above a condensed surface can have an absorption feature whose EW exceeds 200 eV in the integrated spectrum. If the toroidal component of the magnetic field on the neutron star atmosphere is 3-7 times higher than the poloidal component, the absorption feature in the integral spectrum is too wide and shallow to be detectable. Conclusions. To explain the prominent absorption features in the soft X-ray spectra of XDINSs, we infer that a thin atmosphere above the condensed surface must be present, whereas a strong toroidal magnetic field component on the XDINS surfaces can be excluded. © 2010 ESO.

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