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Articles 181 - 210 of 221
Full-Text Articles in Astrophysics and Astronomy
No Transition Disk? Infrared Excess, Pah, H-2, And X-Rays From The Weak-Lined T Tauri Star Doar 21, Eric L.N. Jensen, David H. Cohen, M. Gagné
No Transition Disk? Infrared Excess, Pah, H-2, And X-Rays From The Weak-Lined T Tauri Star Doar 21, Eric L.N. Jensen, David H. Cohen, M. Gagné
Physics & Astronomy Faculty Works
As part of a program to understand disk dispersal and the interplay between circumstellar disks and X-ray emission, we present new high-resolution mid-infrared (IR) imaging, high-resolution optical spectroscopy, and Chandra grating X-ray spectroscopy of the weak-lined T Tauri star DoAr 21. DoAr 21 (age < 10(6) yr and mass similar to 2.2M(circle dot) based on evolutionary tracks) is a strong X-ray emitter, with conflicting evidence in the literature about its disk properties. It shows weak but broad H alpha emission (reported here for the first time since the 1950s); polarimetric variability; polycyclic aromatic hydrocarbon (PAH) and H-2 emission; and a strong, spatially resolved 24 mu m excess in archival Spitzer photometry. Gemini sub-arcsecond-resolution 9-18 mu m images show that there is little or no excess mid-IR emission within 100 AU of the star; the excess emission is extended over several arcseconds and is quite asymmetric. The extended emission is bright in the ultraviolet (UV)-excited lambda = 11.3 mu m PAH emission feature. A new high-resolution X-ray grating spectrum from Chandra shows that the stellar X-ray emission is very hard and dominated by continuum emission; it is well fit by a multi-temperature thermal model, typical of hard coronal sources, and shows no evidence of unusually high densities. A flare during the X-ray observation shows a temperature approaching 10(8) K. We argue that the far-UV emission from the transition region is sufficient to excite the observed extended PAH and continuum emission, and that the H-2 emission may be similarly extended and excited. While this extended emission may be a disk in the final stages of clearing, it also could be more akin to a small-scale photodissociation region than a protoplanetary disk, highlighting both the very young ages (<10(6) yr) at which some stars are found without disks and the extreme radiation environment around even late-type pre-main-sequence stars.
Modelling, Design And Diagnostics For A Photoionised Plasma Experiment, I. M. Hall, T. Turmaz, R. C. Mancini, J. E. Bailey, G. Rochau, Michael Jonathan Rosenberg , '08, David H. Cohen, I. Golovkin, J. J. Macfarlane, M. E. Sherril, J. Abdallah, R. F. Heeter, M. E. Foord, S. H. Glenzer, H. A. Scott
Modelling, Design And Diagnostics For A Photoionised Plasma Experiment, I. M. Hall, T. Turmaz, R. C. Mancini, J. E. Bailey, G. Rochau, Michael Jonathan Rosenberg , '08, David H. Cohen, I. Golovkin, J. J. Macfarlane, M. E. Sherril, J. Abdallah, R. F. Heeter, M. E. Foord, S. H. Glenzer, H. A. Scott
Physics & Astronomy Faculty Works
Photoionised plasmas are common in astrophysical environments and new high resolution spectra from such sources have been recorded in recent years by the Chandra and XMM-Newton satellites. These provide a wealth of spectroscopic information and have motivated recent efforts aimed at obtaining a detailed understanding of the atomic-kinetic and radiative characteristics of photoionised plasmas. The Z-pinch facility at the Sandia National Laboratories is the most powerful terrestrial source of X-rays and provides an opportunity to produce photoionised plasmas in a well characterised radiation environment. We present modelling work and experimental design considerations for a forthcoming experiment at Sandia in which …
Spectroscopic Measurements Of Temperature And Plasma Impurity Concentration During Magnetic Reconnection At The Swarthmore Spheromak Experiment, Vernon Hampden Chaplin , '07, Michael R. Brown, David H. Cohen, T. Gray, C. D. Cothran
Spectroscopic Measurements Of Temperature And Plasma Impurity Concentration During Magnetic Reconnection At The Swarthmore Spheromak Experiment, Vernon Hampden Chaplin , '07, Michael R. Brown, David H. Cohen, T. Gray, C. D. Cothran
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Electron temperature measurements during counterhelicity spheromak merging studies at the Swarthmore Spheromak Experiment (SSX) [M. R. Brown, Phys. Plasmas 6, 1717 (1999)] are presented. VUV monochromator measurements of impurity emission lines are compared with model spectra produced by the non-LTE excitation kinematics code PRISMSPECT [J. J. MacFarlane et al., in Proceedings of the Third Conference on Inertial Fusion Science and Applications (2004)] to yield the electron temperature in the plasma with 1 mu s time resolution. Average T(e) is seen to increase from 12 to 19 eV during spheromak merging. Average C III ion temperature, measured with a new ion …
3d Reconnection And Flow Dynamics In The Ssx Experiment, Michael R. Brown, C. D. Cothran, David H. Cohen, Jason Alexander Horwitz , '07, Vernon Hampden Chaplin , '07
3d Reconnection And Flow Dynamics In The Ssx Experiment, Michael R. Brown, C. D. Cothran, David H. Cohen, Jason Alexander Horwitz , '07, Vernon Hampden Chaplin , '07
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Several new experimental results are reported from plasma merging studies at the Swarthmore Spheromak Experiment (SSX) with relevance to collisionless three-dimensional magnetic reconnection in laboratory and space plasmas. First, recent high-resolution velocity measurements of impurity ions using ion Doppler spectroscopy (IDS) show bi-directional outflow jets at 40 km/s (nearly the Alfven speed). The SSX IDS instrument measures with 1 mu s or better time resolution the width and Doppler shift of the C-III impurity (H plasma) 229.7 nm line to determine the temperature and line-averaged flow velocity during spheromak merging events. High flow speeds are corroborated using an in situ …
X-Ray Spectroscopy Of The Radiation-Driven Winds Of Massive Stars: Line Profile And Line Ratio Diagnostics, David H. Cohen
X-Ray Spectroscopy Of The Radiation-Driven Winds Of Massive Stars: Line Profile And Line Ratio Diagnostics, David H. Cohen
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Massive stars drive powerful, supersonic winds via the radiative momentum associated with the thermal UV emission from their photospheres. Shock phenomena are ubiquitous in these winds, heating them to millions, and sometimes tens of millions, of degrees. The emission line spectra from the shock-heated plasma provide powerful diagnostics of the winds' physical conditions, which in turn provide constraints on models of wind shock heating. Here I show how x-ray line transfer is affected by photoelectric absorption in the partially ionized component of the wind and how it can be modeled to determine the astrophysically important mass-loss rates of these stellar …
On The Importance Of The Interclump Medium For Superionization: O Vi Formation In The Wind Of Zeta Puppis, J. Zsargó, D. J. Hillier, J.-C. Bouret, T. Lanz, M. A. Leutenegger, David H. Cohen
On The Importance Of The Interclump Medium For Superionization: O Vi Formation In The Wind Of Zeta Puppis, J. Zsargó, D. J. Hillier, J.-C. Bouret, T. Lanz, M. A. Leutenegger, David H. Cohen
Physics & Astronomy Faculty Works
We have studied superionization and X-ray line formation in the spectra of zeta Pup using our new stellar atmosphere code (XCMFGEN) that can be used to simultaneously analyze optical, UV, and X-ray observations. Here, we present results on the formation of the O VI lambda lambda 1032, 1038 doublet. Our simulations, supported by simple theoretical calculations, show that clumped wind models that assume void in the interclump space cannot reproduce the observed O vi profiles. However, enough O vi can be produced if the voids are filled by a low-density gas. The recombination of O vi is very efficient in …
High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster, S. L. Skinner, K. R. Sokal, David H. Cohen, M. Gagné, S. P. Owocki, R. H.D. Townsend
High-Resolution Chandra X-Ray Imaging And Spectroscopy Of The Sigma Orionis Cluster, S. L. Skinner, K. R. Sokal, David H. Cohen, M. Gagné, S. P. Owocki, R. H.D. Townsend
Physics & Astronomy Faculty Works
We present results of a 90 ks Chandra X-ray observation of the young sigma Orionis cluster ( age similar to 3 Myr) obtained with the HETGS. We use the high-resolution grating spectrum and moderate-resolution CCD spectrum of the massive central star sigma Ori AB (O9.5 V + B0.5 V) to test wind shock theories of X-ray emission and also analyze the high spatial resolution zero-order ACIS-S image of the central cluster region. Chandra detected 42 X-ray sources on the primary CCD (ACIS-S3). All but five have near-IR or optical counterparts and about one-fourth are variable. Notable high-mass stellar detections are …
Chandra Spectroscopy Of The Hot Star Β Crucis And The Discovery Of A Pre-Main-Sequence Companion, David H. Cohen, Michael Ascher Kuhn , '07, M. Gagné, Eric L.N. Jensen, N. A. Miller
Chandra Spectroscopy Of The Hot Star Β Crucis And The Discovery Of A Pre-Main-Sequence Companion, David H. Cohen, Michael Ascher Kuhn , '07, M. Gagné, Eric L.N. Jensen, N. A. Miller
Physics & Astronomy Faculty Works
In order to test the O star wind-shock scenario for X-ray production in less luminous stars with weaker winds, we made a pointed 74-ks observation of the nearby early B giant, beta Crucis (beta Cru; B0.5 III), with the Chandra High Energy Transmission Grating Spectrometer. We find that the X-ray spectrum is quite soft, with a dominant thermal component near 3 million K, and that the emission lines are resolved but quite narrow, with half widths of 150 km s(-1). The forbidden-to-intercombination line ratios of Ne IX and Mg XI indicate that the hot plasma is distributed in the wind, …
Flow Dynamics And Plasma Heating Of Spheromaks In Ssx, Michael R. Brown, C. D. Cothran, David H. Cohen, Jason Alexander Horwitz , '07, Vernon Hampden Chaplin , '07
Flow Dynamics And Plasma Heating Of Spheromaks In Ssx, Michael R. Brown, C. D. Cothran, David H. Cohen, Jason Alexander Horwitz , '07, Vernon Hampden Chaplin , '07
Physics & Astronomy Faculty Works
We report several new experimental results related to flow dynamics and heating from single dipole-trapped spheromaks and spheromak merging studies at SSX. Single spheromaks (stabilized with a pair of external coils, see Brown, Phys. Plasmas 13 102503 (2006)) and merged FRC-like configurations (see Brown, Phys. Plasmas 13, 056503 (2006)) are trapped in our prolate (R = 0.2 m, L = 0.6 m) copper flux conserver. Local spheromak flow is studied with two Mach probes (r(1) = rho(i) ) calibrated by time-of-flight with a fast set of magnetic probes at the edge of the device. Both Mach probes feature six ion …
The Evolution Of Circumstellar Disks In Ophiuchus Binaries, J. Patience, R. L. Akeson, Eric L.N. Jensen
The Evolution Of Circumstellar Disks In Ophiuchus Binaries, J. Patience, R. L. Akeson, Eric L.N. Jensen
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Four Ophiuchus binaries, two Class I systems and two Class II systems, with separations of similar to 450-1100 AU, were observed with the Owens Valley Radio Observatory (OVRO) millimeter interferometer. In each system, the 3 mm continuum maps show dust emission at the location of the primary star, but no emission at the position of the secondary. This result is different from observations of less evolved Class 0 binaries, in which dust emission is detected from both sources. The nondetection of secondary disks is, however, similar to the dust distribution seen in wide Class II Taurus binaries. The combined OVRO …
Resonance Scattering In The X-Ray Emission Lines Profiles Of Ζ Puppis, M. A. Leutenegger, David H. Cohen, S. M. Kahn, S. P. Owocki, F. B.S. Paerels
Resonance Scattering In The X-Ray Emission Lines Profiles Of Ζ Puppis, M. A. Leutenegger, David H. Cohen, S. M. Kahn, S. P. Owocki, F. B.S. Paerels
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We present XMM-Newton Reflection Grating Spectrometer observations of pairs of X-ray emission line profiles from the O star ζ Pup that originate from the same He-like ion. The two profiles in each pair have different shapes and cannot both be consistently fit by models assuming the same wind parameters. We show that the differences in profile shape can be accounted for in a model including the effects of resonance scattering, which affects the resonance line in the pair but not the intercombination line. This implies that resonance scattering is also important in single resonance lines, where its effect is difficult …
Quantitative Analysis Of Resolved X-Ray Emission Line Profiles Of O Stars, David H. Cohen, M. A. Leutenegger, R. H.D. Townsend
Quantitative Analysis Of Resolved X-Ray Emission Line Profiles Of O Stars, David H. Cohen, M. A. Leutenegger, R. H.D. Townsend
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By quantitatively fitting simple emission line profile models that include both atomic opacity and porosity to the Chandra X-ray spectrum of ζ Pup, we are able to explore the trade-offs between reduced mass-loss rates and wind porosity. We find that reducing the mass-loss rate of ζ Pup by roughly a factor of four, to 1.5 \times 10^{-6} M_sun/yr, enables simple non-porous wind models to provide good fits to the data. If, on the other hand, we take the literature mass-loss rate of 6 \times 10^{-6} M_sun/yr, then to produce X-ray line profiles that fit the data, extreme porosity lengths -- …
X-Rays From Magnetically Channeled Winds Of Ob Stars, In 'Magnetic Massive Stars', David H. Cohen
X-Rays From Magnetically Channeled Winds Of Ob Stars, In 'Magnetic Massive Stars', David H. Cohen
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OB stars with strong radiation-driven stellar winds and large-scale magnetic fields generate strong and hard X-ray emission via the Magnetically Channeled Wind Shock (MCWS) mechanism. In this brief paper, I describe four separate X-ray diagnostics of the MCWS mechanism in OB stars, with applications to the prototype young O star, theta-1 Ori C.
X-Ray Emission From O Stars, David H. Cohen
X-Ray Emission From O Stars, David H. Cohen
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Young O stars are strong, hard, and variable X-ray sources, properties which strongly affect their circumstellar and galactic environments. After ~1 Myr, these stars settle down to become steady sources of soft X-rays. I use high-resolution X-ray spectroscopy and MHD modeling to show that young O stars like theta-1 Ori C are well explained by the magnetically channeled wind shock scenario. After their magnetic fields dissipate, older O stars produce X-rays via shock heating in their unstable stellar winds. Here too I use X-ray spectroscopy and numerical modeling to confirm this scenario. In addition to elucidating the nature and cause …
Periodic Accretion From A Circumbinary Disk In The Young Binary Uz Tau E, Eric L.N. Jensen, Saurav Dhital , '06, K. G. Stassun, J. Patience, W. Herbst, F. M. Walter, M. Simon, G. Basri
Periodic Accretion From A Circumbinary Disk In The Young Binary Uz Tau E, Eric L.N. Jensen, Saurav Dhital , '06, K. G. Stassun, J. Patience, W. Herbst, F. M. Walter, M. Simon, G. Basri
Physics & Astronomy Faculty Works
Close pre-main-sequence binary stars are expected to clear central holes in their protoplanetary disks, but the extent to which material can flow from the circumbinary disk across the gap onto the individual circumstellar disks has been unclear. In binaries with eccentric orbits, periodic perturbation of the outer disk is predicted to induce mass flow across the gap, resulting in accretion that varies with the binary period. This accretion may manifest itself observationally as periodic changes in luminosity. Here we present a search for such periodic accretion in the pre-main-sequence spectroscopic binary UZ Tau E. We present BVRI photometry spanning 3 …
Measurements And Analysis Of Helium-Like Triplet Ratios In The X-Ray Spectra Of O-Type Stars, M. A. Leutenegger, F. B.S. Paerels, S. M. Kahn, David H. Cohen
Measurements And Analysis Of Helium-Like Triplet Ratios In The X-Ray Spectra Of O-Type Stars, M. A. Leutenegger, F. B.S. Paerels, S. M. Kahn, David H. Cohen
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We discuss new methods of measuring and interpreting the forbidden-to-intercombination line ratios of heliumlike triplets in the X-ray spectra of O- type stars, including accounting for the spatial distribution of the X-ray-emitting plasma and using the detailed photospheric UV spectrum. Measurements are made for four O stars using archival Chandra HETGS data. We assume an X- ray-emitting plasma spatially distributed in the wind above some minimum radius R-0. We find minimum radii of formation typically in the range of 1.25 < R-0 / R-* < 1.67, which is consistent with results obtained independently from line profile fits. We find no evidence for anomalously low f / i ratios, and we do not require the existence of X-ray-emitting plasmas at radii that are too small to generate sufficiently strong shocks.
The Effect Of Porosity On X-Ray Emission-Line Profiles From Hot-Star Winds, S. P. Owocki, David H. Cohen
The Effect Of Porosity On X-Ray Emission-Line Profiles From Hot-Star Winds, S. P. Owocki, David H. Cohen
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We investigate the degree to which the nearly symmetric form of X- ray emission lines seen in Chandra spectra of early- type supergiant stars could be explained by the possibly porous nature of their spatially structured stellar winds. Such porosity could effectively reduce the bound- free absorption of X- rays emitted by embedded wind shocks, and thus allow a more similar transmission of redshifted and blueshifted emission from the back and front hemispheres, respectively. To obtain the localized self- shielding that is central to this porosity effect, it is necessary that the individual clumps be optically thick. In a medium …
Wind Signatures In The X-Ray Emission-Line Profiles Of The Late-O Supergiant Zeta Orionis, David H. Cohen, M. A. Leutenegger, K. T. Grizzard, Catherine Lois Reed , '05, Roban Hultman Kramer , '03, S. P. Owocki
Wind Signatures In The X-Ray Emission-Line Profiles Of The Late-O Supergiant Zeta Orionis, David H. Cohen, M. A. Leutenegger, K. T. Grizzard, Catherine Lois Reed , '05, Roban Hultman Kramer , '03, S. P. Owocki
Physics & Astronomy Faculty Works
X-ray line-profile analysis has proved to be the most direct diagnostic of the kinematics and spatial distribution of the very hot plasma around O stars. The Doppler-broadened line profiles provide information about the velocity distribution of the hot plasma, while the wavelength-dependent attenuation across a line profile provides information about the absorption to the hot plasma, thus providing a strong constraint on its physical location. In this paper, we apply several analysis techniques to the emission lines in the Chandra High Energy Transmission Grating Spectrometer (HETGS) spectrum of the late-O supergiant zeta Ori (O9.7 Ib), including the fitting of a …
Numerical Modeling Of Hohlraum Radiation Conditions: Spatial And Spectral Variations Due To Sample Position, Beam Pointing, And Hohlraum Geometry, David H. Cohen, O. L. Landen, J. J. Macfarlane
Numerical Modeling Of Hohlraum Radiation Conditions: Spatial And Spectral Variations Due To Sample Position, Beam Pointing, And Hohlraum Geometry, David H. Cohen, O. L. Landen, J. J. Macfarlane
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View-factor simulations are presented of the spatially varying radiation conditions inside double-ended gold Hohlraums and single-ended gold Hohlraums ('' halfraums '') used in inertial confinement fusion and high-energy density physics experiments [J. Lindl, Phys. Plasmas 11, 339 (2004); M. D. Rosen, Phys. Plasmas 3, 1803 (1996)]. It is shown that in many circumstances, the common assumption that the Hohlraum '' drive '' can be characterized by a single temperature is too simplistic. Specifically, the radiation conditions seen by an experimental package can differ significantly from the wall reemission measured through diagnostic holes or laser entrance holes (LEHs) by absolutely calibrated …
Chandra Hetgs Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C, M. Gagné, M. E. Oksala, David H. Cohen, Stephanie Korinne Tonnesen , '03, A. Ud-Doula, S. P. Owocki, R. H.D. Townsend, J. J. Macfarlane
Chandra Hetgs Multiphase Spectroscopy Of The Young Magnetic O Star Theta(1) Orionis C, M. Gagné, M. E. Oksala, David H. Cohen, Stephanie Korinne Tonnesen , '03, A. Ud-Doula, S. P. Owocki, R. H.D. Townsend, J. J. Macfarlane
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We report on four Chandra grating observations of the oblique magnetic rotator theta(1) Ori C (O5.5 V), covering a wide range of viewing angles with respect to the star's 1060 G dipole magnetic field. We employ line-width and centroid analyses to study the dynamics of the X-ray - emitting plasma in the circumstellar environment, as well as line-ratio diagnostics to constrain the spatial location, and global spectral modeling to constrain the temperature distribution and abundances of the very hot plasma. We investigate these diagnostics as a function of viewing angle and analyze them in conjunction with new MHD simulations of …
Diagnostics Of Disks Around Hot Stars, David H. Cohen, M. H. Hanson, R. H.D. Townsend, K. S. Bjorkman, M. Gagné
Diagnostics Of Disks Around Hot Stars, David H. Cohen, M. H. Hanson, R. H.D. Townsend, K. S. Bjorkman, M. Gagné
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We discuss three different observational diagnostics related to disks around hot stars: absorption line determinations of rotational velocities of Be stars; polarization diagnostics of circumstellar disks; and X-ray line diagnostics of one specific magnetized hot star, theta(1) Ori C. Some common themes that emerge from these studies include (a) the benefits of having a specific physical model as a framework for interpreting diagnostic data; (b) the importance of combining several different types of observational diagnostics of the same objects; and (c) that while there is often the need to reinterpret traditional diagnostics in light of new theoretical advances, there are …
Tracer Spectroscopy Diagnostics Of Doped Ablators In Inertial Confinement Fusion Experiments On Omega, David H. Cohen, J. J. Macfarlane, P. Jaanimagi, O. L. Landen, D. A. Haynes, David Scott Conners , '03, Katherine Lenore Penrose , '04, Nathan Cooper Shupe , '05
Tracer Spectroscopy Diagnostics Of Doped Ablators In Inertial Confinement Fusion Experiments On Omega, David H. Cohen, J. J. Macfarlane, P. Jaanimagi, O. L. Landen, D. A. Haynes, David Scott Conners , '03, Katherine Lenore Penrose , '04, Nathan Cooper Shupe , '05
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A technique has been developed for studying the time-dependent, local physical conditions in ablator samples in an inertial confinement fusion(ICF)hohlraum environment. This technique involves backlit point-projection absorption spectroscopy of thin tracer layers buried in the interior of solid samples mounted on laser-driven hohlraums. It is shown how detailed view-factor, atomic, hydrodynamics, and radiation-transport modeling can be used to infer time-dependent physical conditions in the interiors of these samples from the observed absorption spectra. This modeling is applied to the results of an experimental campaign on the OMEGA laser [T. R. Boehly et al., Opt. Commun. 133, 495 (1997)] designed to …
Testing Protoplanetary Disk Alignment In Young Binaries, Eric L.N. Jensen, R. D. Mathieu, A. X. Donar, Allyn Dullighan , '01
Testing Protoplanetary Disk Alignment In Young Binaries, Eric L.N. Jensen, R. D. Mathieu, A. X. Donar, Allyn Dullighan , '01
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We present K-band (2.2 mum) imaging polarimetry that resolves 19 T Tauri binary and multiple systems in the Taurus-Auriga and Scorpius-Ophiuchus star-forming regions. We observed systems with projected separations 1."5 - 7."2 (similar to200 - 1000 AU) in order to determine the relative orientation of the circumstellar disks in each binary system. Scattered light from these disks is polarized, allowing us to deduce the position angle of the disk on the sky from the position angle of polarization even though our observations do not resolve the disks themselves. We detected measurable polarization ( typically 0.5% - 2%, with typical uncertainty …
High-Resolution Chandra Spectroscopy Of Gamma Cassiopeiae (B0.5e), M. A. Smith, David H. Cohen, M. F. Gu, R. D. Robinson, N. R. Evans, Prudence G. Schran
High-Resolution Chandra Spectroscopy Of Gamma Cassiopeiae (B0.5e), M. A. Smith, David H. Cohen, M. F. Gu, R. D. Robinson, N. R. Evans, Prudence G. Schran
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gamma Cas is the prototypical classical B0.5e star and is now known to be the primary in a wide binary system. It has long been famous for its unique hard X-ray characteristics, among which are variations that correlate with changes in a number of optical light and UV line and continuum properties. These peculiarities have led to a picture in which processes on or near the Be star produce the observed X-ray emission. In this paper we report on a 53 ks Chandra High Energy Transmission Grating Spectrometer observation of this target. An inspection of our spectrum shows that it …
X-Ray Emission-Line Profile Modeling Of O Stars: Fitting A Spherically Symmetric Analytic Wind-Shock Model To The Chandra Spectrum Of Zeta Puppis, Roban Hultman Kramer , '03, David H. Cohen, S. P. Owocki
X-Ray Emission-Line Profile Modeling Of O Stars: Fitting A Spherically Symmetric Analytic Wind-Shock Model To The Chandra Spectrum Of Zeta Puppis, Roban Hultman Kramer , '03, David H. Cohen, S. P. Owocki
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X-ray emission-line profiles provide the most direct insight into the dynamics and spatial distribution of the hot, X-ray-emitting plasma above the surfaces of OB stars. The O supergiant zeta Puppis shows broad, blueshifted, and asymmetric line profiles, generally consistent with the wind-shock picture of OB star X-ray production. We model the profiles of eight lines in the Chandra HETGS spectrum of this prototypical hot star. The fitted lines indicate that the plasma is distributed throughout the wind starting close to the photosphere, that there is significantly less attenuation of the X-rays by the overlying wind than is generally supposed, and …
High-Resolution Chandra Spectroscopy Of Tau Scorpii: A Narrow-Line X-Ray Spectrum From A Hot Star, David H. Cohen, Genevieve Escande De Messières , '04, J. J. Macfarlane, N. A. Miller, J. P. Cassinelli, S. P. Owocki, D. A. Liedahl
High-Resolution Chandra Spectroscopy Of Tau Scorpii: A Narrow-Line X-Ray Spectrum From A Hot Star, David H. Cohen, Genevieve Escande De Messières , '04, J. J. Macfarlane, N. A. Miller, J. P. Cassinelli, S. P. Owocki, D. A. Liedahl
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Long known to be an unusual early-type star by virtue of its hard and strong X-ray emission, tau Scorpii poses a severe challenge to the standard picture of O-star wind-shock X-ray emission. The Chandra HETGS spectrum now provides significant direct evidence that this B0.2 star does not fit this standard wind-shock framework. The many emission lines detected with the Chandra gratings are significantly narrower than what would be expected from a star with the known wind properties of tau Sco, although they are broader than the corresponding lines seen in late-type coronal sources. While line ratios are consistent with the …
X-Ray Spectral Diagnostics Of Neon Photoionization Experiments On The Z-Machine, David H. Cohen, J. J. Macfarlane, J. E. Bailey, D. A. Liedahl
X-Ray Spectral Diagnostics Of Neon Photoionization Experiments On The Z-Machine, David H. Cohen, J. J. Macfarlane, J. E. Bailey, D. A. Liedahl
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We report on an initial spectroscopic study of low-density, x-ray photoionized neon with x-ray spectroscopy. These experiments, carried out on the Z-machine at Sandia, are optimized to produce a gradient-free, collisionless plasma, and to explore issues related to the rapid x-ray photoionization of relatively cold, low-density plasmas. The initial experiments used time-integrated absorption spectroscopy, backlit by the pinch radiation, to determine the ionization balance in the gas cell. Future experiments will use time-resolved spectroscopy in both absorption and emission. The emission spectra are expected to be similar to those seen from photoionized astrophysical sources, such as x-ray binaries. Indeed, in …
X-Ray Emission Line Profile Modeling Of Hot Stars, Roban Hultman Kramer , '03, Stephanie Korinne Tonnesen , '03, David H. Cohen, S. P. Owocki, A. Ud-Doula, J. J. Macfarlane
X-Ray Emission Line Profile Modeling Of Hot Stars, Roban Hultman Kramer , '03, Stephanie Korinne Tonnesen , '03, David H. Cohen, S. P. Owocki, A. Ud-Doula, J. J. Macfarlane
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The launch of high-spectral-resolution x-ray telescopes (Chandra, XMM) has provided a host of new spectralline diagnostics for the astrophysics community. In this paper we discuss Doppler-broadened emission line profiles from highly supersonic outflows of massive stars. These outflows, or winds, are driven by radiation pressure and carry a tremendous amount of kinetic energy, which can be converted to x rays by shock-heating even a small fraction of the wind plasma. The unshocked, cold wind is a source of continuum opacity to the x rays generated in the shock-heated portion of the wind. Thus the emergent line profiles are affected by …
Protoplanetary Disk Mass Distribution In Young Binaries, Eric L.N. Jensen, R. L. Akeson
Protoplanetary Disk Mass Distribution In Young Binaries, Eric L.N. Jensen, R. L. Akeson
Physics & Astronomy Faculty Works
We present millimeter-wave continuum images of four wide (separations 210-800 AU), young stellar binary systems in the Taurus-Auriga star-forming region. For all four sources, the resolution of our observations is sufficient to determine the millimeter emission from each of the components. In all four systems, the primary star's disk has stronger millimeter emission than the secondary's, and in three of the four the secondary is undetected; this is consistent with predictions of recent models of binary formation by fragmentation. The primaries circumstellar disk masses inferred from these observations are comparable to those found for young single stars, confirming that the …
Analysis Of Doppler-Broadened X-Ray Emission Line Profiles From Hot Stars, David H. Cohen, Roban Hultman Kramer , '03, S. P. Owocki
Analysis Of Doppler-Broadened X-Ray Emission Line Profiles From Hot Stars, David H. Cohen, Roban Hultman Kramer , '03, S. P. Owocki
Physics & Astronomy Faculty Works
We show how X-ray emission arising within an accelerating, expanding medium that also contains a source of continuum absorption generates line profiles of a characteristic shape. A simple, spherical wind model based on this picture provides good fits to the Chandra HETGS spectrum of the prototypical O star, Zeta Pup. We discuss the model, the fitting procedure and the determination of confidence limits on the model parameters, and our initial results for this star. The derived fit parameters are consistent with a generic wind-shock scenario for Zeta Pup, but there are several surprising aspects of the results, including a lower-than-expected …