D. K. Witherick

University College London, London, ENG, United Kingdom

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Publications (11)42.32 Total impact

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    R. K. Prinja · K. S. Long · M. T. Richards · D. K. Witherick · L. W. Peck
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    ABSTRACT: Highly time-resolved time-tagged FUSE satellite spectroscopic data are analysed to establish the far-ultraviolet (FUV) absorption line characteristics of the nova-like cataclysmic variable binary, V3885 Sgr. We determine the temporal behaviour of low (Ly_beta, CIII, NIII) and high (SIV, PV, OVI) ion species, and highlight corresponding orbital phase modulated changes in these lines. On average the absorption troughs are blueshifted due to a low velocity disc wind outflow. Very rapid (~ 5 min) fluctuations in the absorption lines are isolated, which are indicative of stochastic density changes. Doppler tomograms of the FUV lines are calculated which provide evidence for structures where a gas stream interacts with the accretion disc. We conclude that the line depth and velocity changes as a function of orbital phase are consistent with an asymmetry that has its origin in a line-emitting, localised disc-stream interaction region.
    Full-text · Article · Oct 2011
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    R. K. Prinja · K. S. Long · M. T. Richards · D. K. Witherick · L. W. Peck
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    ABSTRACT: Highly time-resolved time-tagged FUSE satellite spectroscopic data are analysed to establish the far-ultraviolet (FUV) absorption line characteristics of the nova-like cataclysmic variable binary, V3885 Sgr. We determine the temporal behaviour of low (Lyβ, C iii, N iii) and high (S iv, P v, O vi) ion species, and highlight corresponding orbital phase modulated changes in these lines. On average, the absorption troughs are blueshifted due to a low-velocity disc wind outflow. Very rapid (∼5 min) fluctuations in the absorption lines are isolated, which are indicative of stochastic density changes. Doppler tomograms of the FUV lines are calculated which provide evidence for structures where a gas stream interacts with the accretion disc. We conclude that the line depth and velocity changes as a function of orbital phase are consistent with an asymmetry that has its origin in a line-emitting, localized disc–stream interaction region.
    Preview · Article · Jan 2011 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present far-infrared and submillimetre spectra of evolved objects observed with the SPIRE Fourier-transform spectrometer on board the Herschel Space Observatory. The observations cover wavelengths 195-670 μm, a region of the electromagnetic spectrum hitherto difficult to study. The far-infrared spectra of these objects are rich and complex. We determine physical conditions from observations of the rotational lines of several molecules, and present initial large velocity gradient models for AFGL 618. We detect water in AFGL 2688 for the first time, and confirm its presence in AFGL 618 in both ortho and para forms. In addition, we detect of the J = 1-0 line of CH<SUP>+</SUP> in NGC 7027. Finally, we present new Herschel-SPIRE spectroscopic observations of both O-rich and C-rich evolved sources, and discuss preliminary investigations into their physical conditions.
    No preview · Conference Paper · Jan 2011
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    ABSTRACT: The interstellar medium is enriched primarily by matter ejected from evolved low and intermediate mass stars. The outflows from these stars create a circumstellar envelope in which a rich gas-phase and dust-nucleation chemistry takes place. We observed the nearest carbon-rich evolved star, IRC+10216, using the PACS (55-210 {\mu}m) and SPIRE (194-672 {\mu}m) spectrometers on board Herschel. We find several tens of lines from SiS and SiO, including lines from the v=1 vibrational level. For SiS these transitions range up to J=124-123, corresponding to energies around 6700K, while the highest detectable transition is J=90-89 for SiO, which corresponds to an energy around 8400K. Both species trace the dust formation zone of IRC+10216, and the broad energy ranges involved in their detected transitions permit us to derive the physical properties of the gas and the particular zone in which each species has been formed. This allows us to check the accuracy of chemical thermodynamical equilibrium models and the suggested depletion of SiS and SiO due to accretion onto dust grains. Comment: 5 pages, 3 figures, 7 pages in online appendix, Astronomy & Astrophysics in press
    Full-text · Article · May 2010 · Astronomy and Astrophysics
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    ABSTRACT: We present far-infrared and submillimetre spectra of three carbon-rich evolved objects, AFGL 2688, AFGL 618 and NGC 7027. The spectra were obtained with the SPIRE Fourier transform spectrometer on board the Herschel Space Observatory, and cover wavelengths from 195-670 um, a region of the electromagnetic spectrum hitherto difficult to study in detail. The far infrared spectra of these objects are rich and complex, and we measure over 150 lines in each object. Lines due to 18 different species are detected. We determine physical conditions from observations of the rotational lines of several molecules, and present initial large velocity gradient models for AFGL 618. We detect water in AFGL 2688 for the first time, and confirm its presence in AFGL 618 in both ortho and para forms. In addition, we report the detection of the J=1-0 line of CH+ in NGC 7027. Comment: Accepted for A&A Herschel special issue. 4 tables, 2 figures.
    Full-text · Article · May 2010 · Astronomy and Astrophysics
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    ABSTRACT: With a luminosity >105 L&sun; and a mass-loss rate of ~2 × 10-4 M&sun; yr-1, the red supergiant VY CMa truly is a spectacular object. Because of its extreme evolutionary state, it could explode as supernova any time. Studying its circumstellar material, into which the supernova blast will run, provides interesting constraints on supernova explosions and on the rich chemistry taking place in such complex circumstellar envelopes. We have obtained spectroscopy of VY CMa over the full wavelength range offered by the PACS and SPIRE instruments of Herschel, i.e. 55-672 micron. The observations show the spectral fingerprints of more than 900 spectral lines, of which more than half belong to water. In total, we have identified 13 different molecules and some of their isotopologues. A first analysis shows that water is abundantly present, with an ortho-to-para ratio as low as ~1.3:1, and that chemical non-equilibrium processes determine the abundance fractions in the inner envelope. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.
    Full-text · Article · May 2010 · Astronomy and Astrophysics
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    A. Kaufer · O. Stahl · R. K. Prinja · D. Witherick
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    ABSTRACT: We report on the results of an extended optical spectroscopic monitoring campaign on the early-type B supergiant HD64760 (B0.5Ib) designed to probe the deep-seated origin of spatial wind structure. This new study is based on high-resolution echelle spectra obtained with the FEROS instrument at ESO La Silla. 279 spectra were collected over 10 consecutive nights in 2003. From the period analysis of the line-profile variability of the photospheric lines we identify three closely spaced periods around 4.810 hrs and a splitting of +/-3%. The velocity - phase diagrams of the line-profile variations for the distinct periods reveal characteristic prograde non-radial pulsation patterns of high order corresponding to pulsation modes with l and m in the range 6-10. The three pulsation modes have periods clearly shorter than the characteristic pulsation time scale and show small horizontal velocity fields and hence are identified as p-modes. The beating of the three pulsation modes leads to a retrograde beat pattern with two regions of constructive interference diametrically opposite on the stellar surface and a beat period of 162.8hrs (6.8days). This beat pattern is directly observed in the spectroscopic time series of the photospheric lines. The wind-sensitive lines display features of enhanced emission, which appear to follow the maxima of the photospheric beat pattern. Comment: 18 pages, 21 figures (reduced resolution)
    Full-text · Article · Oct 2005 · Astronomy and Astrophysics
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    R. K. Prinja · C. Knigge · D. K. Witherick · K. S. Long · G. Brammer
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    ABSTRACT: By exploiting Far Ultraviolet Spectroscopic Explorer (FUSE) satellite time‐series data, we have probed the relation between accretion disc geometry and bipolar outflow in the low‐inclination, nova‐like cataclysmic variable (CV) V592 Cas. Our results show that the outflow is strictly modulated on the orbital period and not on the (negative or positive) superhump periods of this system. This implies that the precession of either the disc eccentricity or its tilt is not directly affecting the structure and global dynamics of the outflow. The principal variability in the ultraviolet resonance lines is characterized by Doppler shifts of the entire blueward absorption troughs, in concert for a range of low‐ and high‐ionization cases. The repetitive behaviour is asymmetric and highly non‐sinusoidal over the orbital cycle of the system. The disc wind characteristics derived here allow V592 Cas to be added to a growing number of non‐magnetic CVs which exhibit orbitally modulated wind variability. The origin of the departure in axisymmetry, which has to be fixed in the binary star frame, remains to be established.
    Preview · Article · Oct 2004 · Monthly Notices of the Royal Astronomical Society
  • D. K. Witherick · R. K. Prinja · S. B. Howell · R. M. Wagner
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    ABSTRACT: We present results from time-series optical spectroscopy of the low-inclination, nova-like cataclysmic variable, V592 Cas. The data span the wavelength range from ∼4000 to 5000 Å, and include Balmer lines (Hβ to Hε) as well as He i and He ii. The Balmer lines are generally characterized by shallow absorption troughs with superimposed narrower central emission components. The absorption troughs are variable on time-scales of at least ∼20 min, but the fluctuations are asymmetric such that they are more dominant on the blueward side of the profile out to ∼−2000 km s−1. Fourier analysis reveals modulation periods for emission radial velocities and absorption changes of ∼0.114 and 0.103 d; we support earlier suggestions that the latter is likely to be 1-day alias of the former, which is essentially the system orbital period. However, there is a phase lag of ∼0.3 between absorption and emission changes. The Balmer absorption changes are compared to the predicted behaviour caused by the (stream overflowing) SW Sex phenomenon. There are several discrepancies in this scenario, including the highly asymmetric nature of blueward changes, the velocity amplitudes, and the phase relation to the emission changes. The role of a disc wind in V592 Cas is also discussed; in particular, with this data set, we cannot rule in or rule out the possibility that axisymmetry of the outflow is broken because it is seated on a warped/tilted inner disc, which is implied by previous evidence for negative superhumps in this system.
    No preview · Article · Dec 2003 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present Far Ultraviolet Spectroscopic Explorer (FUSE) satellite time-series data of the low-inclination, nova-like cataclysmic variable RW Sex. The data span the wavelength range between ∼905 and 1188 Å, which contains the resonance lines of C iii, N iii, S iv, P v, S vi and O vi. All these spectral lines are present in the form of blueshifted absorption components, with no evidence for the lines redward of rest velocity; the FUSE data probe the fast disc-wind of RW Sex. The time-variable nature of these features is established. Substantial line profile variability is observed, confined between ∼−1000 and 0 km s−1 , and modulated on the orbital period of the system (∼0.245 d) . The fundamental characteristics of the temporal behaviour are very similar between the low and high ionization lines. The wind-dominated lines exhibit a cyclic increase in absorption optical depth which is accompanied by a shift in the maximum absorption velocity from ∼−1000 km s−1 to near zero velocity. We argue that the empirical evidence from the FUSE analysis supports an interpretation of blueshifted absorptive changes as opposed to the action of a variable (in velocity) blueshifted (perhaps disc-formed) emission component. This conclusion is further bolstered by an analysis of archival Hubble Space Telescope (HST) Goddard High Resolution Spectrograph (GHRS) data of RW Sex. We discuss a phenomenological picture of the ultraviolet (UV) line profile variability where the symmetry of a confined bipolar wind in RW Sex is broken by assuming that the outflow is oblique, perhaps because it is seated on a warped or tilted disc. The temporal UV absorption properties of non-eclipsing nova-like systems are compared.
    Preview · Article · Apr 2003 · Monthly Notices of the Royal Astronomical Society
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    ABSTRACT: We present a compilation of spectroscopic observations of the sgB[e] star CI Cam. This includes data from before, during, and after its 1998 outburst. The object shows a rich emission line spectrum originating from circumstellar material, rendering it difficult to determine the nature of either star involved or the cause of the outburst. We collate pre-outburst data to determine the state of the system before this occurred and provide a baseline for comparison with later data. During the outburst all lines become stronger, and hydrogen and helium lines become significantly broader and asymmetric. After the outburst, spectral changes persist for at least three years, with FeII and [NII] lines still a factor of ~2 above the pre-outburst level and HeI, HeII, and NII lines suppressed by a factor of 2-10. We find that the spectral properties of CI Cam are similar to other sgB[e] stars and therefore suggest that the geometry of the circumstellar material is similar to that proposed for the other objects: a two component outflow, with a fast, hot, rarefied polar wind indistinguishable from that of a normal supergiant and a dense, cooler equatorial outflow with a much lower velocity. We suggest that CI Cam is among the hotter members of the class and is viewed nearly pole-on. The nature of the compact object and the mechanism for the outburst remain uncertain, although it is likely that the compact object is a black hole or neutron star, and that the outburst was precipitated by its passage through the equatorial material. We suggest that this prompted a burst of supercritical accretion resulting in ejection of much of the material, which was later seen as an expanding radio remnant. [Abbreviated] Comment: 25 pages including figures. Accepted for publication in A&A
    Full-text · Article · Jul 2002 · Astronomy and Astrophysics