Search

We carried out a new attempt to check for the presence promethium lines in the spectrum of HD101065 (Przybylski's star). The neutron capture element promethium does not have stable isotopes and the maximum half-life time is about 18 years. Thus its presence in this peculiar star would indicate an ongoing process of irradiation of its surface layers with free neutrons. Unfortunately, almost all promethium lines are heavily blended with lines of other neutron capture elements and other species. We selected and analysed three lines of promethium (Pm I and Pm II) and came to the conclusion that at present it is impossible to definitely claim the presence of this element in Przybylski's star atmosphere., Accepted for publication in Astronomische Nachrichten

We report initial results from an ongoing spectroscopic survey of central stars of faint planetary nebulae (PNe), obtained with the Low-Resolution Spectrograph on the Hobby-Eberly Telescope. The six PN nuclei (PNNi) discussed here all have strong emission at the O VI 3811-3834 A doublet, indicative of very high temperatures. Five of them--the nuclei of Ou 2, Kn 61, Kn 15, Abell 72, and Kn 130--belong to the hydrogen-deficient PG 1159 class, showing a strong absorption feature of He II and C IV at 4650-4690 A. Based on exploratory comparisons with synthetic model-atmosphere spectra, and the presence of Ne VIII emission lines, we estimate them to have effective temperatures of order 170,000 K. The central star of Kn 15 has a Wolf-Rayet-like spectrum, with strong and broad emission lines of He II, C IV, N V, and O V-VI. We classify it [WO2], but we note that the N V 4604-4620 A emission doublet is extremely strong, indicating a relatively high nitrogen abundance. Several of the emission lines in Kn 15 vary in equivalent width by factors as large as 1.5 among our four observations from 2019 to 2022, implying significant variations in the stellar mass-loss rate. We encourage spectroscopic monitoring. Follow-up high-time-resolution photometry of these stars would be of interest, given the large fraction of pulsating variables seen among PG 1159 and [WO] PNNi., Comment: Accepted by MNRAS

Geometry and dynamical structure of emission regions in accreting pulsars are shaped by the interplay between gravity, radiation, and strong magnetic field, which significantly affects the opacities of a plasma and radiative pressure under such extreme conditions. Quantitative consideration of magnetic plasma opacities is, therefore, an essential ingredient of any self-consistent modeling of emission region structure of X-ray pulsars. We present results of computations of the Rosseland and Planck mean opacities of a strongly magnetized plasma with a simple chemical composition,namely the solar hydrogen/helium mix. We consider all relevant specific opacities of the magnetized plasma including vacuum polarization effect and contribution of electron-positron pairs where the pair number density is computed in the thermodynamic equilibrium approximation. The magnetic Planck mean opacity determines the radiative cooling of an optically thin strongly magnetized plasma. It is by factor of three smaller than non-magnetic Planck opacity at $k_{\rm B}T < 0.1\,E_{\rm cyc}$ and increases by a factor of $10^2 - 10^4$ at $k_{\rm B}T > 0.3\,E_{\rm cyc}$ due to cyclotron thermal processes. We propose a simple approximate expression which has sufficient accuracy for the magnetic Planck opacity description. We provide the Rosseland opacity in a tabular form computed in the temperature range 1 - 300 keV, magnetic field range $3 \times 10^{10} - 10^{15}$ G, and a broad range of plasma densities. We demonstrate that the scattering on the electron-positron pairs increases the Rosseland opacity drastically at temperatures >50 keV in the case of mass densities typical for accretion channel in X-ray pulsars., 14 pages, 12 figures, accepted for publication in MNRAS, added with data files and an interpolation code together with their description

Central compact objects (CCOs) are neutron stars found close to the center of some supernova remnants. A certain number of them are presumably covered by carbon envelopes. Their unpulsed thermal X-ray emission can originate either from the entire surface covered by a carbon atmosphere or alternatively from a nonuniformly emitting hydrogen atmosphere. However, the latter scenario appears unlikely given the available upper limits on the amplitude of pulsations. Here we explore a possibility to further discriminate between the two scenarios using X-ray polarimetric observations. We compute the polarization degree (PD) for nonmagnetized pure-carbon and pure-hydrogen atmospheres with effective temperatures of between 1 and 6 MK and find that it can reach up to 25% and 40% for hydrogen and carbon atmospheres, respectively, in the photon energy band 1-10 keV. However, given the available constraints on possible inhomogeneities of the temperature distribution deduced from models of the X-ray spectrum of the CCO in HESS J1731-347, the integrated PD appears to be very low for both carbon (, 10 pages, 16 figures. accepted to be published by A&A, corrected after the language Editor remarks, misprints in some numbers were corrected, a new version of Fig.15

We present two new GW Vir-type pulsating white dwarf stars, TIC\,0403800675 (WD\,J115727.68-280349.64) and TIC\,1989122424 (WD J211738.38-552801.18) discovered in the Transiting Exoplanet Survey Satellite (TESS) photometric data. For both stars, the TESS light curves reveal the presence of oscillations with periods in a narrow range between 400 and 410\,s, which are associated with typical gravity ($g$)-modes. Follow-up ground-based spectroscopy shows that both stars have similar effective temperature ($T_\mathrm{eff} = 110,000 \pm 10,000$\,K) and surface gravity ($\log g = 7.5 \pm 0.5$), but different He/C composition (mass fractions): He\,=\,0.75 and C\,=\,0.25 for TIC\,0403800675, and He\,=\,0.50 and C\,=\,0.50 for TIC\,1989122424. By performing a fit to their spectral energy distributions, we found for both stars radii and luminosities of $R=0.019\pm0.002\,R_\odot$ and $\log(L/L_\odot)=1.68^{+0.15}_{-0.24}$, respectively. By employing evolutionary tracks of PG~1159 stars, we find the masses of both stars to be $0.56\pm0.18 M_{\odot}$ from the $\log g$-$T_\mathrm{eff}$ diagram and $0.60^{+0.11}_{-0.09} M_{\odot}$ from the Hertzsprung Russell diagram., 8 Pages. arXiv admin note: text overlap with arXiv:2108.11093

One of the models explaining the high luminosity of pulsing ultra-luminous X-ray sources (pULXs) was suggested by Mushtukov et al. (2015). They showed that the accretion columns on the surfaces of highly magnetized neutron stars can be very luminous due to opacity reduction in the high magnetic field. However, a strong magnetic field leads also to amplification of the electron-positron pairs creation. Therefore, increasing of the electron and positron number densities compensates the cross-section reduction, and the electron scattering opacity does not decrease with the magnetic field magnification. As a result, the maximum possible luminosity of the accretion column does not increase with the magnetic field. It ranges between 10$^{40} - 10^{41}$ erg s$^{-1}$ depending only slightly on the magnetic field strength., Comment: 3 pages, 2 figures, subm. to Proc. IAU Symp. 363, poster presentation

Many intermediate polars are hard X-ray sources. The theory of their hard X-ray radiation is well developed and allows us to determine white dwarf masse in this kind of cataclysmic variables. Here we present the results of determination the masses of 35 white dwarfs in the intermediate polars observed by observatories NuSTAR (10 sources) and Swift/BAT (25 sources). The corresponding mass accrerion rates and the luminosity function were also derived due to accurate distance to the sources well known now after Gaia DR2.

(abridged version) We present a detailed spectroscopic and timing analysis of X-ray observations of the bright radio-to-gamma-ray emitting pulsar PSR B0656+14, which were obtained simultaneously with eROSITA and XMM-Newton during the Calibration and Performance Verification phase of the Spektrum-Roentgen-Gamma mission (SRG) for 100 ks. Using XMM-Newton and NICER we firstly established an X-ray ephemeris for the time interval 2015 to 2020, which connects all X-ray observations in this period without cycle count alias and phase shifts. The mean eROSITA spectrum clearly reveals an absorption feature originating from the star at 570 eV with a Gaussian sigma of about 70 eV, tentatively identified earlier in a long XMM-Newton observation (Arumugasamy et al. 2018). A second absorption feature, described here as an absorption edge, occurs at 260-265 eV. It could be of atmospheric or of instrumental origin. These absorption features are superposed on various emission components, phenomenologically described as the sum of hot (120 eV) and cold (65 eV) blackbody components, both of photospheric origin, and a power-law with photon index Gamma=2. The phase-resolved spectroscopy reveals that the Gaussian absorption line at 570 eV is clearly present throughout ~60% of the spin cycle. The visibility of the line strength coincides in phase with the maximum flux of the hot blackbody. We also present three families of model atmospheres: a magnetised atmosphere, a condensed surface, and a mixed model, which were applied to the mean observed spectrum and whose continuum fit the observed data well. The atmosphere model, however, predicts too short distances. For the mixed model, the Gaussian absorption may be interpreted as proton cyclotron absorption in a field as high as 10^14 G, which is significantly higher than that derived from the moderate observed spin-down., Comment: Submitted to A&A for the Special Issue: The Early Data Release of eROSITA and Mikhail Pavlinsky ART-XC on the SRG Mission. 21 pages, 13 figures, 13 tables

Context. The Transiting Exoplanet Survey Satellite (TESS) mission is revolutionizing the blossoming area of asteroseismology, particularly of pulsating white dwarfs and pre-white dwarfs, thus continuing the impulse of its predecessor, the Kepler mission. Aims. In this paper, we present the observations from the extended TESS mission in both 120 s short-cadence and 20 s ultra-short-cadence mode of two pre-white dwarf stars showing hydrogen deficiency. We identify them as two new GW Vir stars, TIC 333432673 and TIC 095332541. We apply the tools of asteroseismology with the aim of deriving their structural parameters and seismological distances. Methods. We carried out a spectroscopic analysis and a spectral fitting of TIC 333432673 and TIC 095332541. We also processed and analyzed the high-precision TESS photometric light curves of the two target stars, and derived their oscillation frequencies. We performed an asteroseismological analysis of these stars on the basis of PG 1159 evolutionary models that take into account the complete evolution of the progenitor stars. We searched for patterns of uniform period spacings in order to constrain the stellar mass of the stars. We employed the individual observed periods to search for a representative seismological model. Results. The analysis of the TESS light curves of TIC 333432673 and TIC 095332541 reveals the presence of several oscillations with periods ranging from 350 to 500 s associated to typical gravity (g)-modes. From follow-up ground-based spectroscopy, we find that both stars have a similar effective temperature (Teff = 120 000 ± 10 000 K) and surface gravity (log g = 7.5 ± 0.5), but a different He/C composition of their atmosphere. On the basis of PG 1159 evolutionary tracks, we derived a spectroscopic mass of M⋆ = 0.58−0.08+0.16 M⊙ for both stars. Our asteroseismological analysis of TIC 333432673 allowed us to find a constant period spacing compatible with a stellar mass M⋆ ∼ 0.60 − 0.61 M⊙, and an asteroseismological model for this star with a stellar mass M⋆ = 0.589 ± 0.020 M⊙, as well as a seismological distance of d = 459−156+188 pc. For this star, we find an excellent agreement between the different methods to infer the stellar mass, and also between the seismological distance and that measured with Gaia (dGaia = 389−5.2+5.6 pc). For TIC 095332541, we have found a possible period spacing that suggests a stellar mass of M⋆ ∼ 0.55 − 0.57 M⊙. Unfortunately, we have not been able to find an asteroseismological model for this star. Conclusions. Using the high-quality data collected by the TESS space mission and follow-up spectroscopy, we have been able to discover and characterize two new GW Vir stars. The TESS mission is having, and will continue to have, an unprecedented impact on the area of white-dwarf asteroseismology.

Hot, compact, hydrogen-deficient pre-white dwarfs (pre-WDs) with effective temperatures of Teff > 70,000 K and a surface gravity of 5.0 < log g < 7.0 are rather rare objects despite recent and ongoing surveys. It is believed that they are the outcome of either single star evolution (late helium-shell flash or late helium-core flash) or binary star evolution (double WD merger). Their study is interesting because the surface elemental abundances reflect the physics of thermonuclear flashes and merger events. Spectroscopically they are divided in three different classes, namely PG1159, O(He), or He-sdO. We present a spectroscopic analysis of five such stars that turned out to have atmospheric parameters in the range Teff = 70,000-80,000 K and log g = 5.2-6.3. The three investigated He-sdOs have a relatively high hydrogen mass fraction (10%) that is unexplained by both single (He core flash) and binary evolution (He-WD merger) scenarios. The O(He) star JL9 is probably a binary helium-WD merger, but its hydrogen content (6%) is also at odds with merger models. We found that RL 104 is the 'coolest' (Teff = 80,000 K) member of the PG1159 class in a pre-WD stage. Its optical spectrum is remarkable because it exhibits C IV lines involving Rydberg states with principal quantum numbers up to n = 22. Its rather low mass (0.48 +0.03/-0.02 Msun) is difficult to reconcile with the common evolutionary scenario for PG1159 stars due to it being the outcome of a (very) late He-shell flash. The same mass-problem faces a merger model of a close He-sdO plus CO WD binary that predicts PG1159-like abundances. Perhaps RL 104 originates from a very late He-shell flash in a CO/He WD formed by a merger of two low-mass He-WDs., Accepted for publication in A&A

Stellar post asymptotic giant branch (post-AGB) evolution can be completely altered by a final thermal pulse (FTP) which may occur when the star is still leaving the AGB (AFTP), at the departure from the AGB at still constant luminosity (late TP, LTP) or after the entry to the white-dwarf cooling sequence (very late TP, VLTP). Then convection mixes the He-rich material with the H-rich envelope. According to stellar evolution models the result is a star with a surface composition of $\mathrm{H}\approx\,20\,$% by mass (AFTP), $\approx 1\,$% (LTP), or (almost) no H (VLTP). Since FTP stars exhibit intershell material at their surface, spectral analyses establish constraints for AGB nucleosynthesis and stellar evolution. We performed a spectral analysis of the so-called hybrid PG 1159-type central stars (CS) of the planetary nebulae Abell 43 and NGC7094 by means of non-local thermodynamical equilibrium models. We confirm the previously determined effective temperatures of $T_\mathrm{eff} = 115\,000\pm 5\,000\,$K and determine surface gravities of $\log (g\,/\,\mathrm{cm/s^2}) = 5.6\pm 0.1$ for both. From a comparison with AFTP evolutionary tracks, we derive stellar masses of $0.57^{+0.07}_{-0.04}\,M_\odot$ and determine the abundances of H, He, and metals up to Xe. Both CS are likely AFTP stars with a surface H mass fraction of $0.25 \pm 0.03$ and $0.15 \pm 0.03$, respectively, and a Fe deficiency indicating subsolar initial metallicities. The light metals show typical PG 1159-type abundances and the elemental composition is in good agreement with predictions from AFTP evolutionary models. However, the expansion ages do not agree with evolution timescales expected from the AFTP scenario and alternatives should be explored., 27 pages, 22 figures

We present a systematic search for periodically varying quasar and supermassive black hole binary (SMBHB) candidates in the Pan-STARRS1 Medium Deep Survey. From $\sim9,000$ color-selected quasars in a $\sim50$ deg$^{2}$ sky area, we initially identify $26$ candidates with more than $1.5$ cycles of variation. We extend the baseline of observations via our imaging campaign with the Discovery Channel Telescope and the Las Cumbres Observatory network and reevaluate the candidates using a more rigorous, maximum likelihood method. Using a range of statistical criteria and assuming the Damped Random Walk model for normal quasar variability, we identify one statistically significant periodic candidate. We also investigate the capabilities of detecting SMBHBs by the Large Synoptic Survey Telescope using our study with MDS as a benchmark and explore any complementary, multiwavelength evidence for SMBHBs in our sample., Comment: Accepted to ApJ. 26 pages, 12 figures, 11 tables, 2 appendices

We have identified 484 lines of the trans-iron elements (TIEs) Zn, Ga, Ge, Se, Br, Kr, Sr, Zr, Mo, In, Te, I, Xe, and Ba, for the first time in the ultraviolet spectrum of a DAO-type WD, namely BD$-22{^\circ}3467$, surrounded by the ionized nebula Abell 35. Our TIE abundance determination shows extremely high overabundances of up to five dex -- a similar effect is already known from hot, H-deficient (DO-type) white dwarfs. In contrast to these where a pulse-driven convection zone has enriched the photosphere with TIEs during a final thermal pulse and radiative levitation has established the extreme TIE overabundances, {here the extreme TIE overabundances are exclusively driven by radiative levitation on the initial stellar metallicity. The very low mass ($0.533^{+0.040}_{-0.025}\,M_\odot$) of BD$-22{^\circ}3467$ implies that a third dredge-up with enrichment of s-process elements in the photosphere did not occur in the AGB precursor., Comment: 34 pages, 19 Figures

Context. The recent arrival of continuous photometric observations of unprecedented quality from space missions has strongly promoted the study of pulsating stars and caused great interest in the stellar astrophysics community. In the particular case of pulsating white dwarfs, the TESS mission is taking asteroseismology of these compact stars to a higher level, emulating or even surpassing the performance of its predecessor, the Kepler mission. Aims. We present a detailed asteroseismological analysis of six GW Vir stars that includes the observations collected by the TESS mission. Methods. We processed and analyzed TESS observations of RX J2117+3412 (TIC 117070953), HS 2324+3944 (TIC 352444061), NGC 6905 (TIC 402913811), NGC 1501 (TIC 084306468), NGC 2371 (TIC 446005482), and K 1−16 (TIC 233689607). We carried out a detailed asteroseismological analysis of these stars on the basis of PG 1159 evolutionary models that take into account the complete evolution of the progenitor stars. We constrained the stellar mass of these stars by comparing the observed period spacing with the average of the computed period spacings, and we employed the individual observed periods to search for a representative seismological model when possible. Results. In total, we extracted 58 periodicities from the TESS light curves of these GW Vir stars using a standard prewhitening procedure to derive the potential pulsation frequencies. All the oscillation frequencies that we found are associated with g-mode pulsations, with periods spanning from ∼817 s to ∼2682 s. We find constant period spacings for all but one star (K 1−16), which allowed us to infer their stellar masses and constrain the harmonic degree ℓ of the modes. Based on rotational frequency splittings, we derive the rotation period of RX J2117+3412, obtaining a value in agreement with previous determinations. We performed period-to-period fit analyses on five of the six analyzed stars. For four stars (RX J2117+3412, HS 2324+3944, NGC 1501, and NGC 2371), we were able to find an asteroseismological model with masses that agree with the stellar mass values inferred from the period spacings and are generally compatible with the spectroscopic masses. Obtaining seismological models allowed us to estimate the seismological distance and compare it with the precise astrometric distance measured with Gaia. Finally, we find that the period spectrum of K 1−16 exhibits dramatic changes in frequency and amplitude that together with the scarcity of modes prevented us from meaningful seismological modeling of this star. Conclusions. The high-quality data collected by the TESS space mission, considered simultaneously with ground-based observations, provide very valuable input to the asteroseismology of GW Vir stars, similar to the case of other classes of pulsating white dwarf stars. The TESS mission, in conjunction with future space missions and upcoming surveys, will make impressive progress in white dwarf asteroseismology., Instituto de Astrofísica de La Plata

The article presents a review of scientific problems and methods of ultraviolet astronomy, focusing on perspective scientific problems (directions) whose solution requires UV space observatories. These include reionization and the history of star formation in the Universe, searches for dark baryonic matter, physical and chemical processes in the interstellar medium and protoplanetary disks, the physics of accretion and outflows in astrophysical objects, from Active Galactic Nuclei to close binary stars, stellar activity (for both low-mass and high-mass stars), and processes occurring in the atmospheres of both planets in the solar system and exoplanets. Technological progress in UV astronomy achieved in recent years is also considered. The well advanced, international, Russian-led Spektr-UV (World Space Observatory—Ultraviolet) project is described in more detail. This project is directed at creating a major space observatory operational in the ultraviolet (115–310 nm). This observatory will provide an effective, and possibly the only, powerful means of observing in this spectral range over the next ten years, and will be an powerful tool for resolving many topical scientific problems.

The NASA Kepler mission has revolutionised time-domain astronomy and has massively expanded the number of known extrasolar planets. However, the effect of wide multiplicity on exoplanet occurrence has not been tested with this dataset. We present a sample of 401 wide multiple systems containing at least one Kepler target star. Our method uses Pan-STARRS1 and archival data to produce an accurate proper motion catalogue of the Kepler field. Combined with Pan-STARRS1 SED fits and archival proper motions for bright stars, we use a newly developed probabilistic algorithm to identify likely wide binary pairs which are not chance associations. As by-products of this we present stellar SED templates in the Pan-STARRS1 photometric system and conversions from this system to Kepler magnitudes. We find that Kepler target stars in our binary sample with separations above 6 arcseconds are no more or less likely to be identified as confirmed or candidate planet hosts than a weighted comparison sample of Kepler targets of similar brightness and spectral type. Therefore we find no evidence that binaries with projected separations greater than 3,000AU affect the occurrence rate of planets with P, 15 figures, 6 tables, MNRAS accepted, Tables 1 and 4 available from http://www.star.herts.ac.uk/~ndeacon/PS1Kepler.html . Replacement updates Figure 11 and the text referring to it

The subgiant UCAC2 46706450 is a late-type star with an ultraviolet (UV) excess. It was considered as a candidate to establish a sample of stars of spectral type F, G, and K with white dwarf (WD) companions that could be used to test binary evolution models. To verify the WD nature of the companion, UV spectroscopy has previously been performed by other authors. Via a detailed model-atmosphere analysis, we show that the UV source is an extremely hot WD with an effective temperature of Teff = 105 000 ± 5000 K, mass of M M· = 0.54 ± 0.02, radius of R/R⊙ = 0.040-0.004+0.005, and luminosity of L/L⊙ = 176-49+55, meaning that the compact object is just about to enter the WD cooling sequence. Investigating spectra of the cool star (Teff = 4945 ± 250 K), we found that it is a K-type subgiant with M M⊙ = 0.8-2.4, R/R⊙ = 5.9-0.5+0.7, and L/L⊙ = 19-5+5 that is rapidly rotating with vsin(i) = 81 km s-1. Optical light curves reveal a period of two days and an o-band peak-to-peak amplitude of 0.06 mag. We suggest that it is caused by stellar rotation in connection with star spots. With the radius, we infer an extremely high rotational velocity of vrot = 151-13+18 km s-1, thus marking the star as one of the most rapidly rotating subgiants known. This explains chromospheric activity observed by H α emission and emission-line cores in Ca※ II H and K as well as NUV flux excess. From equal and constant radial velocities of the WD and the K subgiant as well as from a fit to the spectral energy distribution, we infer that they form a physical, wide (though unresolved) binary system. Both components exhibit similar metal abundances and show iron-group elements with slightly oversolar (up to 0.6 dex) abundance, meaning that atomic diffusion in the WD atmosphere is not yet active due to a residual, weak radiation-driven wind. Kinematically and from its height above the Galactic plane, the system belongs to the Galactic thick disk, indicating that it is an old system and that the initial masses of both stars were close to 1 M⊙., We thank Matti Dorsch and Boris Gänsicke for pointing out this system to us and useful discussions. A.R.M. acknowledges support from the MINECO under the Ramón y Cajal programme (RYC-2016-20254) and the AYA2017-86274-P grant, and the AGAUR grant SGR-661/2017. The TMAD tool (http://astro. uni-tuebingen.de/~TMAD) used for this paper was constructed as part of the activities of the German Astrophysical Virtual Observatory. This work has made use of data obtained at the Thai National Observatory on Doi Inthanon, operated by NARIT. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). This research has made use of NASA’s Astrophysics Data System and the SIMBAD database, operated at CDS, Strasbourg, France. This research made use of Lightkurve, a Python package for Kepler and TESS data analysis (Lightkurve Collaboration 2018). This work has made use of data from the European Space Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/ web/gaia/dpac/consortium). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work has made use of BaSTI web tools. This work includes data from the Asteroid Terrestrial-impact Last Alert System (ATLAS) project. ATLAS is primarily funded to search for near Earth asteroids through NASA grants NN12AR55G, 80NSSC18K0284, and 80NSSC18K1575; by products of the NEO search include images and catalogs from the survey area. The ATLAS science products have been made possible through the contributions of the University of Hawaii Institute for Astronomy, the Queen’s University Belfast, the Space Telescope Science Institute, and the South African Astronomical Observatory. Guoshoujing Telescope (the Large Sky Area Multi-Object Fiber Spectroscopic Telescope LAMOST) is a National Major Scientific Project built by the Chinese Academy of Sciences. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences. Funding for the Sloan Digital Sky Survey IV has been provided by the Alfred P. Sloan Foundation, the U.S. Department of Energy Office of Science, and the Participating Institutions. SDSS-IV acknowledges support and resources from the Center for High-Performance Computing at the University of Utah. The SDSS web site is www.sdss.org. SDSS-IV is managed by the Astrophysical Research Consortium for the Participating Institutions of the SDSS Collaboration including the Brazilian Participation Group, the Carnegie Institution for Science, Carnegie Mellon University, the Chilean Participation Group, the French Participation Group, Harvard-Smithsonian Center for Astrophysics, Instituto de Astrofísica de Canarias, The Johns Hopkins University, Kavli Institute for the Physics and Mathematics of the Universe (IPMU) / University of Tokyo, the Korean Participation Group, Lawrence Berkeley National Laboratory, Leibniz Institut für Astrophysik Potsdam (AIP), Max-Planck-Institut für Astronomie (MPIA Heidelberg), Max-Planck-Institut für Astrophysik (MPA Garching), Max-Planck-Institut für Extraterrestrische Physik (MPE), National Astronomical Observatories of China, New Mexico State University, New York University, University of Notre Dame, Observatário Nacional / MCTI, The Ohio State University, Pennsylvania State University, Shanghai Astronomical Observatory, United Kingdom Participation Group, Universidad Nacional Autónoma de México, University of Arizona, University of Colorado Boulder, University of Oxford, University of Portsmouth, University of Utah, University of Virginia, University of Washington, University of Wisconsin, Vanderbilt University, and Yale University.

Neutron stars (NSs) in low-mass X-ray binaries rotate at frequencies high enough to significantly deviate from sphericity ($\nu_* \sim$ 200--600 Hz). We investigate the effects of rapid rotation on the observational appearance of a NS. We propose analytical formulae relating gravitational mass and equatorial radius of the rapidly rotating NS to the mass $M$ and radius $R$ of a non-rotating NS of the same baryonic mass using accurate fully relativistic computations. We compute spectra from an oblate rotating NS observed at different inclination angles using the modified oblate Schwarzschild (MOS) approximation, where light bending is computed in Schwarzschild metric, but frame dragging and quadrupole moment of a NS are approximately accounted for in the photon redshift calculations. We generalize the cooling tail method to the case of a rapidly rotating NS to obtain the most probable values of $M$ and $R$ of the corresponding non-rotating NS with the same baryonic mass. We approximate the local spectra from the NS surface by a diluted blackbody using previously computed NS atmosphere models. We show that the NS radius could be overestimated by 3--3.5 km for face-on stars of $R\approx 11$ km rotating at $\nu_* =$ 700 Hz if the version of the cooling tail method for a non-rotating NS is used. We apply the method to an X-ray burst observed from the NS rotating at $\nu_* \approx$ 532 Hz in SAX J1810.8$-$2609. The resulting radius of the non-rotating NS (assuming $M=1.5 M_\odot$) becomes $11.8\pm0.5$ km if it is viewed at inclination i=60 deg and $R=11.2\pm0.5$ km for a face-on view, which are smaller by 0.6 and 1.2 km than the radius obtained using standard cooling tail method ignoring rotation. The corresponding equatorial radii of these rapidly rotating NSs are 12.3$\pm 0.6$ km (for i=60 deg) and 11.6$\pm 0.6$\,km (for i=0 deg)., Comment: 17 pages, 16 figures, accepted for publication in Astronomy and Astrophysics

Accurate atomic data is an essential ingredient for the calculation of reliable non-local thermodynamic equilibrium (NLTE) model atmospheres that are mandatory for the spectral analysis of hot stars. We aim to search for and identify for the first time spectral lines of copper (atomic number Z = 29) and indium (Z = 49) in hot white dwarf (WD) stars and to subsequently determine their photospheric abundances. Oscillator strengths of Cu IV - VII were calculated to include radiative and collisional bound-bound transitions of Cu in our NLTE model-atmosphere calculations. Oscillator strengths of In IV - VI were compiled from the literature. We newly identified 1 Cu IV, 51 Cu V, 2 Cu VI, and 5 In Vlines in the ultraviolet (UV) spectrum of DO-type WD RE 0503-289. We determined the photospheric abundances of 9.3 X 10**-5 (mass fraction, 132 times solar) and 3.0 X 10**-5 (56 600 times solar), respectively; we also found Cu overabundances in the DA-type WD G191-B2B (6.3 X 10**-6, 9 times solar). All identified Cu IV - VI lines in the UV spectrum of RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths. With the detection of Cu and In in RE 0503-289, the total number of trans-iron elements (Z > 28) in this extraordinary WD reaches an unprecedented number of 18., 32 pages, 7 figures

The presence of ultra-high excitation (UHE) absorption lines (e.g., O VIII) in the optical spectra of several of the hottest white dwarfs poses a decades-long mystery and is something that has never been observed in any other astrophysical object. The occurrence of such features requires a dense environment with temperatures near $10^6$K, by far exceeding the stellar effective temperature. Here we report the discovery of a new hot wind white dwarf, GALEXJ014636.8+323615. Astonishingly, we found for the first time rapid changes of the equivalent widths of the UHE features, which are correlated to the rotational period of the star ($P=0.242035$d). We explain this with the presence of a wind-fed circumstellar magnetosphere in which magnetically confined wind shocks heat up the material to the high temperatures required for the creation of the UHE lines. The photometric and spectroscopic variability of GALEXJ014636.8+323615 can then be understood as consequence of the obliquity of the magnetic axis with respect to the rotation axis of the white dwarf. This is the first time a wind-fed circumstellar magnetosphere around an apparently isolated white dwarf has been discovered and finally offers a plausible explanation of the ultra hot wind phenomenon., Published in MNRAS Letters

The hardness of the X-ray spectra of intermediate polars (IPs) is determined mainly by the white dwarf (WD) compactness (mass-radius ratio, M/R) and, thus, hard X-ray spectra can be used to constrain the WD mass. An accurate mass estimate requires the finite size of the WD magnetosphere R_m to be taken into the account. We suggested to derive it either directly from the observed break frequency in power spectrum of X-ray or optical lightcurves of a polar, or assuming the corotation. Here we apply this method to all IPs observed by NuSTAR (10 objects) and Swift/BAT (35 objects). For the dwarf nova GK Per we also observe a change of the break frequency with flux, which allows to constrain the dependence of the magnetosphere radius on the mass-accretion rate. For our analysis we calculated an additional grid of two-parameter (M and R_m/R) model spectra assuming a fixed, tall height of the accretion column H_sh/R=0.25, which is appropriate to determine WD masses in low mass-accretion IPs like EX\,Hya. Using the Gaia Data Release 2 we obtain for the first time reliable estimates of the mass-accretion rate and the magnetic field strength at the WD surface for a large fraction of objects in our sample. We find that most IPs accrete at rate of ~10^{-9} M_Sun/yr, and have magnetic fields in the range 1--10 MG. The resulting WD mass average of our sample is 0.79 +/- 0.16 M_Sun, which is consistent with earlier estimates., 14 pages, 16 figures, 4 tables, submitted to MNRAS

Some thermonuclear (type I) X-ray bursts at the neutron star surfaces in low-mass X-ray binaries take place during hard persistent states of the systems. Spectral evolution of these bursts is well described by the atmosphere model of a passively cooling neutron star when the burst luminosity is high enough. The observed spectral evolution deviates from the model predictions when the burst luminosity drops below a critical value of 20-70% of the maximum luminosity. We suggest that these deviations are induced by the additional heating of the accreted particles. We present a method for computation of the neutron star atmosphere models heated by accreted particles assuming that their energy is released via Coulomb interactions with electrons. We compute the temperature structures and the emergent spectra of the atmospheres of various chemical compositions and investigate the dependence of the results on the other model parameters. We show that the heated atmosphere develops the hot (20--100 keV) corona-like surface layer cooled by Compton scattering, and the deeper, almost isothermal optically thick region with a temperature of a few keV. The emergent spectra deviate strongly from those of undisturbed neutron star atmospheres, with the main differences being the presence of a high-energy tail and a strong excess in the low-energy part of the spectrum. They also lack the iron absorption edge, which is visible in the spectra of undisturbed low-luminosity atmospheres with solar chemical composition. Using the computed spectra, we obtained the dependences of the dilution and color-correction factors as functions of relative luminosities for pure helium and solar abundance atmospheres. We show that the helium model atmosphere heated by accretion corresponding to 5% of the Eddington luminosity describes well the late stages of the X-ray bursts in 4U 1820-30., 14 pages, 19 figures, published version, some misprints were fixed

EGB6 is an extended, faint old planetary nebula (PN) with an enigmatic nucleus. The central star (PG0950+139) is a hot DAOZ-type white dwarf (WD). An unresolved, compact emission knot was discovered to be located 0.166" away from the WD and it was shown to be centered around a dust-enshrouded low-luminosity star. It was argued that the dust disk and evaporated gas (photoionized by the hot WD) around the companion are remnants of a disk formed by wind material captured from the WD progenitor when it was an asymptotic giant branch (AGB) star. In this paper, we assess the hot WD to determine its atmospheric and stellar parameters. We performed a model-atmosphere analysis of ultraviolet (UV) and optical spectra. We found Teff = 105,000 +/- 5000 K, log g = 7.4 +/- 0.4, and a solar helium abundance (He = 0.25 +/- 0.1, mass fraction). We measured the abundances of ten more species (C, N, O, F, Si, P, S, Ar, Fe, Ni) and found essentially solar abundance values, indicating that radiation-driven wind mass-loss, with a theoretical rate of log(dot-M/M_sun/yr) = -11.0 (+1.1)(-0.8) prevents the gravitational separation of elements in the photosphere. The WD has a mass of M/M_sun = 0.58 (+0.12)(-0.04) and its post-AGB age (log(t_evol/yr) = 3.60 (+1.26)(-0.09)) is compatible with the PN kinematical age of log(t_PN}/yr) = 4.2. In addition, we examined the UV spectrum of the hot nucleus of a similar object with a compact emission region, TOL26 (PN G298.0+34.8), and found that it is a slightly cooler DAOZ WD (Teff about 85,000 K), but this WD shows signatures of gravitational settling of heavy elements., A&A accepted

We present the largest Cepheid sample in M31 based on the complete Pan-STARRS1 survey of Andromeda (PAndromeda) in the $r_{\mathrm{P1}}$ , $i_{\mathrm{P1}}$ and $g_{\mathrm{P1}}$ bands. We find 2686 Cepheids with 1662 fundamental mode Cepheids, 307 first-overtone Cepheids, 278 type II Cepheids and 439 Cepheids with undetermined Cepheid type. Using the method developed by Kodric et al. (2013) we identify Cepheids by using a three dimensional parameter space of Fourier parameters of the Cepheid light curves combined with a color cut and other selection criteria. This is an unbiased approach to identify Cepheids and results in a homogeneous Cepheid sample. The Period-Luminosity relations obtained for our sample have smaller dispersions than in our previous work. We find a broken slope that we previously observed with HST data in Kodric et al. (2015), albeit with a lower significance., 79 pages, 39 figures, 8 tables, accepted for publication in AJ, K18b is submittted to ApJ, electronic data will be available on CDS

We are conducting a proper-motion survey for young brown dwarfs in the Taurus-Auriga molecular cloud based on the Pan-STARRS1 3$��$ Survey. Our search uses multi-band photometry and astrometry to select candidates, and is wider (370 deg$^{2}$) and deeper (down to $\approx$3 M$_{\rm Jup}$) than previous searches. We present here our search methods and spectroscopic follow-up of our high-priority candidates. Since extinction complicates spectral classification, we have developed a new approach using low-resolution ($R \approx 100$) near-infrared spectra to quantify reddening-free spectral types, extinctions, and gravity classifications for mid-M to late-L ultracool dwarfs ($\approx 100-3$ M$_{\rm Jup}$ in Taurus). We have discovered 25 low-gravity (VL-G) and the first 11 intermediate-gravity (INT-G) substellar (M6-L1) members of Taurus, constituting the largest single increase of Taurus brown dwarfs to date. We have also discovered 1 new Pleiades member and 13 new members of the Perseus OB2 association, including a candidate very wide separation (58 kAU) binary. We homogeneously reclassify the spectral types and extinctions of all previously known Taurus brown dwarfs. Altogether our discoveries have thus far increased the substellar census in Taurus by $\approx 40\%$ and added three more L-type members ($\approx 5-10$ M$_{\rm Jup}$). Most notably, our discoveries reveal an older ($>$10 Myr) low-mass population in Taurus, in accord with recent studies of the higher-mass stellar members. The mass function appears to differ between the younger and older Taurus populations, possibly due to incompleteness of the older stellar members or different star formation processes., ApJ, in press. 95 pages, 40 figures, 14 tables. Machine-readable tables are available. For a brief video explaining about this paper, see https://youtu.be/yMd1OknJj7w

The German Astrophysical Virtual Observatory (GAVO) developed the registered service TheoSSA (theoretical stellar spectra access) and the supporting registered VO tool TMAW (Tuebingen Model-Atmosphere WWW interface). These allow individual spectral analyses of hot, compact stars with state-of-the-art non-local thermodynamical equilibrium (NLTE) stellar-atmosphere models that presently consider opacities of the elements H, He, C, N, O, Ne, Na, and Mg, without requiring detailed knowledge about the involved background codes and procedures. Presently, TheoSSA provides easy access to about 150000 pre-calculated stellar SEDs and is intended to ingest SEDs calculated by any model-atmosphere code. In the case of the exciting star of PRTM 1, we demonstrate the easy way to calculate individual NLTE stellar model-atmospheres to reproduce an observed optical spectrum. We measured Teff = 98000 +/- 5000 K, log (g / cm/s**2) = 5.0 (+0.3/-0.2) and photospheric mass fractions of H = 7.5 x 10**-1 (1.02 times solar), He = 2.4 x 10**-1 (0.96), C = 2.0 x 10**-3 (0.84), N = 3.2 x 10**-4 (0.46), O = 8.5 x 10**-3 (1.48) with uncertainties of +/- 0.2 dex. We determined the stellar mass and luminosity of 0.73 (+0.16/-0.15) Msun and log (L / Lsun) = 4.2 +/- 0.4, respectively., 15 pages, 12 figures

We are developing imaging and photon counting UV-MCP detectors, which are sensitive in the wavelength range from far ultraviolet to near ultraviolet. A good quantum efficiency, solar blindness and high spatial resolution is the aim of our development. The sealed detector has a Cs-activated photoactive layer of GaN (or similarly advanced photocathode), which is operated in semitransparent mode on (001)-MgF 2 . The detector comprises a stack of two long-life MCPs and a coplanar cross strip anode with advanced readout electronics. The main challenge is the flawless growth of the GaN photocathode layer as well as the requirements for the sealing of the detector, to prevent a degradation of the photocathode. We present here the detector concept and the experimental setup, examine in detail the status in the production and describe the current status of the readout electronics development.

The metal abundances in the atmospheres of hot white dwarfs (WDs) entering the cooling sequence are determined by the preceding Asymptotic Giant Branch (AGB) evolutionary phase and, subsequently, by the onset of gravitational settling and radiative levitation. In this paper, we investigate three hot He-rich WDs, which are believed to result from a late He-shell flash. During such a flash, the He-rich intershell matter is dredged up and dominates the surface chemistry. Hence, in contrast to the usual H-rich WDs, their spectra allow direct access to s-process element abundances in the intershell that were synthesized during the AGB stage. In order to look for trans-iron group elements (atomic number Z>29), we performed a non-local thermodynamic equilibrium model atmosphere analysis of new ultraviolet spectra taken with the Cosmic Origins Spectrograph aboard the Hubble Space Telescope. One of our program stars is of PG1159 spectral type; this star, PG1707+427, has effective temperature Teff=85,000 K, and surface gravity logg=7.5. The two other stars are DO white dwarfs: WD0111+002 has Teff=58,000 K and logg=7.7, and PG0109+111 has Teff=70,000 K and logg=8.0. These stars trace the onset of element diffusion during early WD evolution. While zinc is the only trans-iron element we could detect in the PG1159 star, both DOs exhibit lines from Zn, Ga, Ge, Se; one additionally exhibits lines from Sr, Sn, Te, and I and the other from As. Generally, the trans-iron elements are very abundant in the DOs, meaning that radiative levitation must be acting. Most extreme is the almost six orders of magnitude oversolar abundance of tellurium in PG0109+111. In terms of mass fraction, it is the most abundant metal in the atmosphere. The two DOs join the hitherto unique hot DO RE0503-289, in which 14 trans-iron elements had even been identified., A&A accepted

There is a striking paucity of hydrogen-rich (DA) white dwarfs (WDs) relative to their hydrogen-deficient (non-DA) counterparts at the very hot end of the WD cooling sequence. The three hottest known DAs (surface gravity log g $\geq$ 7.0) have effective temperatures around Teff = 140,000 K, followed by only five objects in the range 104,000 - 120,000 K. They are by far outnumbered by forty non-DAs with Teff = 100,000 - 250,000 K, giving a DA/non-DA ratio of 0.2. In contrast, this ratio is the inverse of that for the cooler WDs. One reason for this discrepancy could be uncertainties in the temperature determination of hot DAs using Balmer-line spectroscopy. Recent investigations involving metal-ionization balances in ultraviolet (UV) spectra indeed showed that the temperatures of some DAs were underestimated, but the paucity of extremely hot DAs prevailed. Here we present the results of a UV spectral analysis of one of the three hottest DAs, PG0948+534. We find that its temperature was strongly overestimated by recent Balmer line analyses. We correct it downward to 105,000 $\pm$ 5000 K, aggravating the hot-DA paucity. The Balmer-line problem encountered previously is not resolved by our non-LTE line-blanketed model atmospheres. We speculate that it might be related to the possible presence of a magnetosphere. This is supported by the V-band variability that shows a period of P=3.45 d (amplitude 0.19 mag), which we interpret as the star's rotation period. The metal abundances in PG0948+534 are affected by atomic diffusion and we conclude that the onset of diffusion in hot DAs occurs when they cool below Teff about 105,000 K. We discuss the possibility that the paucity of very hot DAs is a consequence of their fast evolutionary rate., Comment: Accepted for publication in MNRAS

The ultracompact low-mass X-ray binary 4U 1820-30 situated in the globular cluster NGC 6624 has an orbital period of only $\approx$11.4 min which likely implies a white dwarf companion. The observed X-ray bursts demonstrate a photospheric radius expansion phase and therefore are believed to reach the Eddington luminosity allowing us to estimate the mass and the radius of the neutron star (NS) in this binary. Here we re-analyse all Rossi X-ray Timing Explorer observations of the system and confirm that almost all the bursts took place during the hard persistent state of the system. This allows us to use the recently developed direct cooling tail method to estimate the NS mass and radius. However, because of the very short, about a second, duration of the cooling tail phases that can be described by the theoretical atmosphere models, the obtained constraints on the NS radius are not very strict. Assuming a pure helium NS atmosphere we found that the NS radius is in the range 10-12 km, if the NS mass is below 1.7 $M_\odot$, and in a wider range of 8-12 km for a higher 1.7-2.0 $M_\odot$ NS mass. The method also constrains the distance to the system to be 6.5$\pm$0.5 kpc, which is consistent with the distance to the cluster. For the solar composition atmosphere, the NS parameters are in strong contradiction with the generally accepted range of possible NS masses and radii., 9 pages, 9 figures, 2 tables, accepted for publication in MNRAS

The data from all observations of RX J0720.4-3125 conducted by XMM-Newton EPIC-pn with the same instrumental setup in 2000-2012 were reprocessed to form a homogenous data set of solar barycenter corrected photon arrival times registered from RX J0720.4-3125. A Bayesian method for the search, detection, and estimation of the parameters of an unknown-shaped periodic signal was employed as developed by Gregory & Loredo (1992). A number of complex models (single and double peaked) of light curves from pulsating neutron stars were statistically analyzed. The distribution of phases for the registered photons was calculated by folding the arrival times with the derived spin-period and the resulting distribution of phases approximated with a mixed von Mises distribution, and its parameters were estimated by using the Expected Maximization method. Spin phase-resolved spectra were extracted, and a number of highly magnetized atmosphere models of an INS were used to fit simultaneously, the results were verified via an MCMC approach. The phase-folded light curves in different energy bands with high S/N ratio show a high complexity and variations depending on time and energy. They can be parameterized with a mixed von Mises distribution, i.e. with double-peaked light curve profile showing a dependence of the estimated parameters (mean directions, concentrations, and proportion) upon the energy band, indicating that radiation emerges from at least two emitting areas. The genuine spin-period of the isolated neutron star RX J0720-3125 derived as more likely is twice of that reported in the literature (16.78s instead of 8.39s). The gravitational redshift of RX J0720.4-3125 was determined to $z=0.205_{-0.003}^{+0.006}$ and the compactness was estimated to $(M/M_{Sun})/R(km)=0.105 \pm 0.002 $., Comments: 19 pages, 15 figures and 5 tables, Astronomy and Astrophysics accepted. arXiv admin note: text overlap with arXiv:1108.3897

The X-ray spectra of the neutron stars located in the centers of supernova remnants Cas A and HESS J1731-347 are well fit with carbon atmosphere models. These fits yield plausible neutron star sizes for the known or estimated distances to these supernova remnants. The evidence in favor of the presence of a pure carbon envelope at the neutron star surface is rather indirect and is based on the assumption that the emission is generated uniformly by the entire stellar surface. Although this assumption is supported by the absence of pulsations, the observational upper limit on the pulsed fraction is not very stringent. In an attempt to quantify this evidence, we investigate the possibility that the observed spectrum of the neutron star in HESS J1731-347 is a combination of the spectra produced in a hydrogen atmosphere of the hotspots and of the cooler remaining part of the neutron star surface. The lack of pulsations in this case has to be explained either by a sufficiently small angle between the neutron star spin axis and the line of sight, or by a sufficiently small angular distance between the hotspots and the neutron star rotation poles. As the observed flux from a non-uniformly emitting neutron star depends on the angular distribution of the radiation emerging from the atmosphere, we have computed two new grids of pure carbon and pure hydrogen atmosphere model spectra accounting for Compton scattering. Using new hydrogen models, we have evaluated the probability of a geometry that leads to a pulsed fraction below the observed upper limit to be about 8.2 %. Such a geometry thus seems to be rather improbable but cannot be excluded at this stage., 8 pages, 14 figures. Accepted for publication in A&A

Determining neutron star (NS) radii and masses can help to understand the properties of matter at supra-nuclear densities. Thermal emission during thermonuclear X-ray bursts from NSs in low-mass X-ray binaries provides a unique opportunity to study NS parameters, because of the high fluxes, large luminosity variations and the related changes in the spectral properties. The standard cooling tail method uses hot NS atmosphere models to convert the observed spectral evolution during cooling stages of X-ray bursts to the Eddington flux F_Edd and the stellar angular size \Omega. These are then translated to the constraints on the NS mass M and radius R. Here we present the improved, direct cooling tail method that generalises the standard approach. First, we adjust the cooling tail method to account for the bolometric correction to the flux. Then, we fit the observed dependence of the blackbody normalization on flux with a theoretical model directly on the M-R plane by interpolating theoretical dependences to a given gravity, hence ensuring only weakly informative priors for M and R instead of F_Edd and \Omega. The direct cooling method is demonstrated using a photospheric radius expansion burst from SAX J1810.8--2609, which has happened when the system was in the hard state. Comparing to the standard cooling tail method, the confidence regions are shifted by 1 sigma towards larger radii, giving R=11.5-13.0 km at M=1.3-1.8 M_sun for this NS., Comment: 8 pages, 5 figures. Accepted for publications in MNRAS

SAO 244567 is a rare example of a star that allows us to witness stellar evolution in real time. Between 1971 and 1990, it changed from a B-type star into the hot central star of the Stingray Nebula. This observed rapid heating has been a mystery for decades, since it is in strong contradiction with the low mass of the star and canonical post-asymptotic giant branch (AGB) evolution. We speculated that SAO 244567 might have suffered from a late thermal pulse (LTP) and obtained new observations with Hubble Space Telescope (HST)/COS to follow the evolution of the surface properties of SAO 244567 and to verify the LTP hypothesis. Our non-LTE spectral analysis reveals that the star cooled significantly since 2002 and that its envelope is now expanding. Therefore, we conclude that SAO 244567 is currently on its way back towards the AGB, which strongly supports the LTP hypothesis. A comparison with state-of-the-art LTP evolutionary calculations shows that these models cannot fully reproduce the evolution of all surface parameters simultaneously, pointing out possible shortcomings of stellar evolution models. Thereby, SAO 244567 keeps on challenging stellar evolution theory and we highly encourage further investigations., Instituto de Astrofísica de La Plata

We present three dimensional maps in monochromatic extinction $A_{\rm 0}$ and the extinction parameter $R_0$ within a few degrees of the Galactic plane. These are inferred using photometry from the Pan-STARRS1 and Spitzer Glimpse surveys of nearly $20$ million stars located in the region $l = 0-250^{\circ}$ and from $b = -4.5^{\circ}$ to $b=4.5^{\circ}$. Given the available stellar number density, we use an angular resolution of $7' \times 7'$ and steps of $1{\rm mag}$ in distance modulus. We simultaneously estimate distance modulus and effective temperature $T_{\rm eff}$ alongside the other parameters for stars individually using the method of \citet{Hanson2014} before combining these estimates to a complete map. The full maps are available via the MNRAS website., Accepted for publication in MNRAS. Free-access version available at http://mnras.oxfordjournals.org/cgi/content/abstract/stw2240?ijkey=8TdulFJR3q3ppl1&keytype=ref - data temporarily available at http://www.rhanson.de/gpdust

Gaseous and dust debris disks around white dwarfs (WDs) are formed from tidally disrupted planetary bodies. This offers an opportunity to determine the composition of exoplanetary material by measuring element abundances in the accreting WD's atmosphere. A more direct way to do this is through spectral analysis of the disks themselves. Currently, the number of chemical elements detected through disk emission-lines is smaller than that of species detected through lines in the WD atmospheres. We assess the far-ultraviolet (FUV) spectrum of one well-studied object (SDSS J122859.93+104032.9) to search for disk signatures at wavelengths, 5 pages, 5 figures, accepted for publication in A&A

For the spectral analysis of high-resolution and high-signal-to-noise (S/N) spectra of hot stars, state-of-the-art non-local thermodynamic equilibrium (NLTE) model atmospheres are mandatory. These are strongly dependent on the reliability of the atomic data that is used for their calculation. New of Kr IV - VII oscillator strengths for a large number of lines allow to construct more detailed model atoms for our NLTE model-atmosphere calculations. This enables us to search for additional Kr lines in observed spectra and to improve Kr abundance determinations. We calculated Kr IV - VII oscillator strengths to consider radiative and collisional bound-bound transitions in detail in our NLTE stellar-atmosphere models for the analysis of Kr lines exhibited in high-resolution and high-S/N ultraviolet (UV) observations of the hot white dwarf RE 0503-289. We reanalyzed the effective temperature and surface gravity and determined Teff = 70 000 +/- 2000 K and log (g / cm/s**2) = 7.5 +/- 0.1. We newly identified ten Kr V lines and one Kr VI line in the spectrum of RE 0503-289. We measured a Kr abundance of -3.3 +/- 0.3 (logarithmic mass fraction). We discovered that the interstellar absorption toward RE 0503-289 has a multi-velocity structure within a radial-velocity interval of -40 km/s < vrad < +18 km/s. Reliable measurements and calculations of atomic data are a prerequisite for state-of-the-art NLTE stellar-atmosphere modeling. Observed Kr V - VII line profiles in the UV spectrum of the white dwarf RE 0503-289 were simultaneously well reproduced with our newly calculated oscillator strengths., 30 pages, 17 figures

The Pan-Planets survey observed an area of 42 sq deg. in the galactic disk for about 165 hours. The main scientific goal of the project is the detection of transiting planets around M dwarfs. We establish an efficient procedure for determining the stellar parameters $T_{eff}$ and log$g$ of all sources using a method based on SED fitting, utilizing a three-dimensional dust map and proper motion information. In this way we identify more than 60000 M dwarfs, which is by far the largest sample of low-mass stars observed in a transit survey to date. We present several planet candidates around M dwarfs and hotter stars that are currently being followed up. Using Monte-Carlo simulations we calculate the detection efficiency of the Pan-Planets survey for different stellar and planetary populations. We expect to find $3.0^{+3.3}_{-1.6}$ hot Jupiters around F, G, and K dwarfs with periods lower than 10 days based on the planet occurrence rates derived in previous surveys. For M dwarfs, the percentage of stars with a hot Jupiter is under debate. Theoretical models expect a lower occurrence rate than for larger main sequence stars. However, radial velocity surveys find upper limits of about 1\% due to their small sample, while the Kepler survey finds a occurrence rate that we estimate to be at least $0.17(^{+0.67}_{-0.04})$%, making it even higher than the determined fraction from OGLE-III for F, G and K stellar types, $0.14(^{+0.15}_{-0.076})\%$. With the large sample size of Pan-Planets, we are able to determine an occurrence rate of $0.11(^{+0.37}_{-0.02})$% in case one of our candidates turns out to be a real detection. If, however, none of our candidates turn out to be true planets, we are able to put an upper limit of 0.34% with a 95% confidence on the hot Jupiter occurrence rate of M dwarfs. Therefore we cannot yet confirm the theoretical prediction of a lower occurrence rate for cool stars., Comment: 15 pages, 29 figures, accepted to A&A

We report the first identification of forbidden line components from an element heavier than helium in the spectrum of astrophysical plasmas. As yet, these components were identified only in laboratory plasmas and not in astrophysical objects. Forbidden components are well known for neutral helium lines in hot stars, particularly in helium-rich post-AGB stars and white dwarfs. We discovered that two hitherto unidentified lines in the ultraviolet spectra of hot hydrogen-deficient (pre-) white dwarfs can be identified as forbidden line components of triply ionized carbon (C IV). The forbidden components (3p-4f and 3d-4d) appear in the blue and red wings of the strong, Stark broadened 3p-4d and 3d-4f lines at 1108 Angstroem and 1169 Angstroem, respectively. They are visible over a wide effective temperature range (60,000-200,000 K) in helium-rich (DO) white dwarfs and PG1159 stars that have strongly oversolar carbon abundances., Comment: Accepted for publication in ApJ Letters

In the line-of-sight toward the DO-type white dwarf RX J0503.9-2854, the density of the interstellar medium (ISM) is very low, and thus the contamination of the stellar spectrum almost negligible. This allows us to identify many metal lines in a wide wavelength range from the extreme ultraviolet to the near infrared. In previous spectral analyses, many metal lines in the ultraviolet spectrum of RX J0503.9-2854 have been identified. A complete line list of observed and identified lines is presented here. We compared synthetic spectra that had been calculated from model atmospheres in non-local thermodynamical equilibrium, with observations. In total, we identified 1272 lines (279 of them were newly assigned) in the wavelength range from the extreme ultraviolet to the near infrared. 287 lines remain unidentified. A close inspection of the EUV shows that still no good fit to the observed shape of the stellar continuum flux can be achieved although He, C, N, O, Al, Si, P, S, Ca, Sc, Ti, V, Cr, Mn, Fe, Cr, Ni Zn, Ga, Ge, As, Kr, Zr, Mo, Sn, Xe, and Ba are included in the stellar atmosphere models. There are two possible reasons for the deviation between observed and synthetic flux in the EUV. Opacities from hitherto unconsidered elements in the model-atmosphere calculation may be missing and/or the effective temperature is slightly lower than previously determined., Comment: 92 pages, 45 figures

Observed hard X-ray spectra of intermediate polars are determined mainly by the accretion flow velocity at the white dwarf surface, which is normally close to the free-fall velocity. This allows to estimate the white dwarf masses as the white dwarf mass-radius relation M-R and the expected free-fall velocities at the surface are well known. This method is widely used, however, derived white dwarf masses M can be systematically underestimated because the accretion flow is stopped at and re-accelerates from the magnetospheric boundary R_m, and therefore, its velocity at the surface will be lower than free-fall.To avoid this problem we computed a two-parameter set of model hard X-ray spectra, which allows to constrain a degenerate M - R_m dependence. On the other hand, previous works showed that power spectra of accreting X-ray pulsars and intermediate polars exhibit breaks at the frequencies corresponding to the Keplerian frequencies at the magnetospheric boundary. Therefore, the break frequency \nu_b in an intermediate polar power spectrum gives another relation in the M - R_m plane. The intersection of the two dependences allows, therefore, to determine simultaneously the white dwarf mass and the magnetospheric radius. To verify the method we analyzed the archival Suzaku observation of EX Hya obtaining M /M_sun= 0.73 \pm 0.06 and R_ m / R = 2.6 \pm 0.4 consistent with the values determined by other authors. Subsequently, we applied the same method to a recent NuSTAR observation of another intermediate polar GK~Per performed during an outburst and found M/M_sun = 0.86 \pm 0.02 and R_ m / R = 2.8 \pm 0.2. The long duration observations of GK Per in quiescence performed by Swift/BAT and INTEGRAL observatories indicate increase of magnetosphere radius R_m at lower accretion rates., Comment: 12 pages, 17 figures, submitted to Astronomy and Astrophysics

In the framework of the Virtual Observatory, the newly developed service TheoSSA provides access to theoretical stellar spectral-energy distributions. In a pilot phase, this service is based on the well established Tuebingen NLTE Model-Atmosphere Package for hot, compact stars. We demonstrate its present capabilities and future extensions., Proceedings of JENAM-2011, European Week of Astronomy and Space Science, 8 pages, 6 figures

We present results of a (F)UV spectral analysis of 15 hot, hydrogen-rich central stars of planetary nebulae (CSPNe) of DAO-type (A 7, A 31, A 35, A 39, NGC 3587, NGC 6720, NGC 6853, NGC 7293, PuWe 1, Sh 2-174) and O(H)-type (A 36, Lo 1, LSS 1362, NGC 1360, NGC 4361). The sample covers a wide range of parameters (Teff ≈ 70–130 kK, log g = 5.4–7.4). It represents different stages of post-AGB evolution. The derived stellar parameters are crucial constraints for AGB nucleosynthesis and stellar evolutionary calculations. Detailed spectral analyses using fully line-blanketed NLTE model atmospheres including 23 elements from hydrogen to nickel are performed. Additional modeling of the ISM line absorption enables to unambigiously identify nearly all observed lines and to improve both, the photospheric as well as the ISM model.

Continuous access to the UV domain has been considered of importance to astrophysicists and planetary scientists since the mid-sixties. However, the future of UV missions for the post-HST era is believed by a significant part of astronomical community to be less encouraging. We argue that key science problems of the coming years will require further development of UV observational technologies. Among these hot astrophysical issues are: the search for missing baryons, revealing the nature of astronomical engines, properties of atmospheres of exoplanets as well as of the planets of the Solar System etc. We give a brief review of UV-missions both in the past and in the future. We conclude that UV astronomy has a great future but the epoch of very large and efficient space UV facilities seems to be a prospect for the next decades. As to the current state of the UV instrumentation we think that this decade will be dominated by the HST and coming World Space Observatory-Ultraviolet (WSO-UV) with a 1.7 m UV-telescope onboard. The international WSO-UV mission is briefly described. It will allow high resolution/high sensitivity imaging and high/low resolution spectroscopy from the middle of the decade.

The World Space Observatory Ultraviolet (WSO-UV) is a multi-national project lead by the Russian Federal Space Agency (Roscosmos) with the objective of high performance observations in the ultraviolet range. The 1.7 m WSO-UV telescope is equipped with UV spectrographs (responsibility of Russia and Germany) and UV imagers (responsibility of Spain). The UV spectroscopic instrumentation comprises two high resolution echelle spectrographs operating in wavelength ranges of 102–176 nm and 174–310 nm respectively, and a Long Slit Spectrograph designed to operate in the range of 102–310 nm. All three spectrographs represent individual instruments. In order to save mass while maintaining high stiffness, the instruments are combined to a monoblock, World Space Observatory Ultraviolet Spectrographs (WUVS). Due to strict technical requirements stated by the customer the material CeSiC (provided by the company ECM) has been selected for the design of the spectrograph structure. In contrast to aluminium, the stable structure of CeSiC is significantly less sensitive to thermal gradients. No further mechanism for focus correction with high functional, technical and operational complexity and corresponding additional System costs are necessary. Using CeSiC also relaxes the thermal control requirements of ±5°C, which represents a considerable cost driver for the S/C design. The phase B2 study of the WUVS instrument finished in December 2010 in collaboration with Russia and with industrial support of the Kayser–Threde company. It included construction of a Structural Thermal Model (STM) for verification of thermal and mechanical loads, stability with respect to thermal distortions and CeSiC manufacturing feasibility.

We present new ultraviolet spectra of the hottest known, peculiar white dwarf H1504+65, obtained with the Cosmic Origins Spectrograph on the Hubble Space Telescope. H1504+65 is the hottest known white dwarf (Teff=200 000 K) and has an atmosphere mainly composed by carbon and oxygen, augmented with high amounts of neon and magnesium. This object is unique and the origin of its surface chemistry is completely unclear. We probably see the naked core of either a C–O white dwarf or even a O–Ne–Mg white dwarf. In the latter case, this would be the first proof that such white dwarfs can be the outcome of single-star evolution. The new observations were performed to shed light on the origin of this mysterious object.

We present the early UV and optical light curve of Type IIP supernova (SN) 2010aq at z=0.0862, and compare it to analytical models for thermal emission following SN shock breakout in a red supergiant star. SN 2010aq was discovered in joint monitoring between the Galaxy Evolution Explorer (GALEX) Time Domain Survey (TDS) in the NUV and the Pan-STARRS1 Medium Deep Survey (PS1 MDS) in the g, r, i, and z bands. The GALEX and Pan-STARRS1 observations detect the SN less than 1 day after shock breakout, measure a diluted blackbody temperature of 31,000 +/- 6,000 K 1 day later, and follow the rise in the UV/optical light curve over the next 2 days caused by the expansion and cooling of the SN ejecta. The high signal-to-noise ratio of the simultaneous UV and optical photometry allows us to fit for a progenitor star radius of 700 +/- 200 R_sun, the size of a red supergiant star. An excess in UV emission two weeks after shock breakout compared to SNe well fitted by model atmosphere-code synthetic spectra with solar metallicity, is best explained by suppressed line blanketing due to a lower metallicity progenitor star in SN 2010aq. Continued monitoring of PS1 MDS fields by the GALEX TDS will increase the sample of early UV detections of Type II SNe by an order of magnitude, and probe the diversity of SN progenitor star properties., 4 pages, 3 color figures. Accepted for publication in ApJ Letters, corrections from proofs added

Up to now, iron had not been found in any hydrogen-deficient (pre-) white dwarf of spectral type PG1159, despite intense searching in a number of objects. Consequently, an iron deficiency was claimed, in some cases at least 1 dex. The primary indicators were UV lines of Fe VII. Therefore, the search was confined to relatively cool PG1159 stars (T eff 150,000 K), otherwise Fe is too strongly ionized for a significant population of Fe VII. In this Letter, we announce the discovery of iron in the very hottest PG1159 stars (T eff = 150,000-200,000 K; RX J2117.1+3412, K 1–16, Longmore 4, NGC 246, H1504+65), based on the identification of the Fe X λλ979.3, 1022.9 lines in spectra obtained with the Far Ultraviolet Spectroscopic Explorer. Surprisingly, our analysis results in a solar iron abundance for these stars. It is conspicuous that they are among the most massive PG1159 stars (0.71-0.82 M ☉), in contrast to those objects for which strongest Fe deficiency was claimed (0.53-0.56 M ☉).

Bromine (atomic number Z=35) and antimony (Z=51) are extremely difficult to detect in stars. In very few instances, weak and mostly uncertain identifications of Br I, Br II, and Sb II in relatively cool, chemically peculiar stars were successful. Adopted solar abundance values rely on meteoritic determinations. Here, we announce the first identification of these species in far-ultraviolet spectra of hot stars (with effective temperatures of 49,500-70,000 K), namely in helium-rich (spectral type DO) white dwarfs. We identify the Br VI resonance line at 945.96 A. A previous claim of Br detection based on this line is incorrect because its wavelength position is inaccurate by about 7 A in atomic databases. Taking advantage of precise laboratory measurements, we identify this line as well as two other, subordinate Br VI lines. Antimony is detected by the Sb V resonance doublet at 1104.23/1225.98 A, as well as two subordinate Sb VI lines. A model-atmosphere analysis reveals strongly oversolar Br and Sb abundances that are caused by radiative-levitation dominated atomic diffusion., Accepted for publication in A&A

Some isolated neutron stars show harmonically spaced absorption features in their thermal soft X-ray spectra. The interpretation of the features as a cyclotron line and its harmonics has been suggested, but the usual explanation of the harmonics as caused by relativistic effects fails because the relativistic corrections are extremely small in this case. We suggest that the features correspond to the peaks in the energy dependence of the free-free opacity in a quantizing magnetic field, known as quantum oscillations. The peaks arise when the transitions to new Landau levels become allowed with increasing the photon energy; they are strongly enhanced by the square-root singularities in the phase-space density of quantum states in the case when the free (non-quantized) motion is effectively one-dimensional. To explore observable properties of these quantum oscillations, we calculate models of hydrogen neutron star atmospheres with B \sim 10^{10} - 10^{11} G (i.e., electron cyclotron energy E_{c,e} = 0.1 - 1 keV) and T_{eff} = 1 - 3 MK. Such conditions are thought to be typical for the so-called central compact objects in supernova remnants, such as 1E 1207.4-5209 in PKS 1209-51/52. We show that observable features at the electron cyclotron harmonics form at moderately large values of the quantization parameter, b_{eff} = E_{c,e}/kT_{eff} = 0.5 - 20. The equivalent widths of the features can reach 100 - 200 eV; they grow with increasing b_{eff} and are lower for higher harmonics., Comment: 6 pages; shortened, references updated; published in ApJ