Open Access. Powered by Scholars. Published by Universities.®

Physical Sciences and Mathematics Commons™

Open Access. Powered by Scholars. Published by Universities.®

Astrophysics and Astronomy

Swarthmore College

Articles 181 - 204 of 204

Full-Text Articles in Physical Sciences and Mathematics

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 Jun 2006

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 …

Go to article

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 Dec 2005

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

Physics & Astronomy Faculty Works

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 …

Go to article

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 Aug 2005

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

Physics & Astronomy Faculty Works

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 …

Go to article

Diagnostics Of Disks Around Hot Stars, David H. Cohen, M. H. Hanson, R. H.D. Townsend, K. S. Bjorkman, M. Gagné Jan 2005

Diagnostics Of Disks Around Hot Stars, David H. Cohen, M. H. Hanson, R. H.D. Townsend, K. S. Bjorkman, M. Gagné

Physics & Astronomy Faculty Works

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 …

Go to article

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 May 2004

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

Physics & Astronomy Faculty Works

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 …

Go to article

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 Jan 2004

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

Physics & Astronomy Faculty Works

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 …

Go to article

Testing Protoplanetary Disk Alignment In Young Binaries, Eric L.N. Jensen, R. D. Mathieu, A. X. Donar, Allyn Dullighan , '01 Jan 2004

Testing Protoplanetary Disk Alignment In Young Binaries, Eric L.N. Jensen, R. D. Mathieu, A. X. Donar, Allyn Dullighan , '01

Physics & Astronomy Faculty Works

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 …

Go to article

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 Jul 2003

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

Physics & Astronomy Faculty Works

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 …

Go to article

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 Mar 2003

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

Physics & Astronomy Faculty Works

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 …

Go to article

X-Ray Spectral Diagnostics Of Neon Photoionization Experiments On The Z-Machine, David H. Cohen, J. J. Macfarlane, J. E. Bailey, D. A. Liedahl Mar 2003

X-Ray Spectral Diagnostics Of Neon Photoionization Experiments On The Z-Machine, David H. Cohen, J. J. Macfarlane, J. E. Bailey, D. A. Liedahl

Physics & Astronomy Faculty Works

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 …

Go to article

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 Mar 2003

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

Physics & Astronomy Faculty Works

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 …

Go to article

Protoplanetary Disk Mass Distribution In Young Binaries, Eric L.N. Jensen, R. L. Akeson Feb 2003

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 …

Go to article

Analysis Of Doppler-Broadened X-Ray Emission Line Profiles From Hot Stars, David H. Cohen, Roban Hultman Kramer , '03, S. P. Owocki Dec 2002

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 …

Go to article

New Challenges For Wind Shock Models: The Chandra Spectrum Of The Hot Star Delta Orionis, N. A. Miller, J. P. Cassinelli, W. L. Waldron, J. J. Macfarlane, David H. Cohen Oct 2002

New Challenges For Wind Shock Models: The Chandra Spectrum Of The Hot Star Delta Orionis, N. A. Miller, J. P. Cassinelli, W. L. Waldron, J. J. Macfarlane, David H. Cohen

Physics & Astronomy Faculty Works

The Chandra spectrum of delta Ori A shows emission lines from hydrogen- and helium-like states of Si, Mg, Ne, and O, along with N VII Lyalpha and lines from ions in the range Fe XVII-Fe XXI In contrast to the broad lines seen in zeta Pup and zeta Ori (850 +/- 40 and 1000 +/- 240 km s(-1) half-width at half-maximum [HWHM], respectively), these lines are broadened to only 430 +/- 60 km s(-1) HWHM. This is much lower than the measured wind terminal velocity of 2000 km s(-1). The forbidden, intercombination, and resonance (fir) lines from He-like ions indicate …

Go to article

Radiation Science Using Z-Pinch X-Rays, J. E. Bailey, G. A. Chandler, David H. Cohen, M. E. Cuneo, M. E. Foord, R. F. Heeter, D. Jobe, P. W. Lake, J. J. Macfarlane, T. J. Nash, D. S. Nielson, R. Smelser, J. Torres May 2002

Radiation Science Using Z-Pinch X-Rays, J. E. Bailey, G. A. Chandler, David H. Cohen, M. E. Cuneo, M. E. Foord, R. F. Heeter, D. Jobe, P. W. Lake, J. J. Macfarlane, T. J. Nash, D. S. Nielson, R. Smelser, J. Torres

Physics & Astronomy Faculty Works

Present-day Z-pinch experiments generate 200 TW peak power, 5–10 ns duration x-ray bursts that provide new possibilities to advance radiation science. The experiments support both the underlying atomic and plasma physics, as well as inertial confinement fusion and astrophysics applications. A typical configuration consists of a sample located 1–10 cm away from the pinch, where it is heated to 10–100 eV temperatures by the pinch radiation. The spectrally-resolved sample-plasma absorption is measured by aiming x-ray spectrographs through the sample at the pinch. The pinch plasma thus both heats the sample and serves as a backlighter. Opacitymeasurements with this source are …

Go to article

Mass-Loss And Magnetospheres: X-Rays From Hot Stars And Young Stellar Objects, M. Gagné, David H. Cohen, S. P. Owocki, A. Ud-Doula Jan 2002

Mass-Loss And Magnetospheres: X-Rays From Hot Stars And Young Stellar Objects, M. Gagné, David H. Cohen, S. P. Owocki, A. Ud-Doula

Physics & Astronomy Faculty Works

High-resolution X-ray spectra of high-mass stars and low-mass T-Tauri stars obtained during the first year of the Chandra mission are providing important clues about the mechanisms which produce X-rays on very young stars. For zeta Puppis (O4 If) and zeta Ori (O9.5 I), the broad, blue-shifted line profiles, line ratios, and derived temperature distribution suggest that the X-rays are produced throughout the wind via instability-driven wind shocks. For some less luminous OB stars, like theta^1 Ori C (O7 V) and tau Sco (B0 V), the line profiles are symmetric and narrower. The presence of time-variable emission and very high-temperature lines …

Go to article

A Robotic Wide-Angle Hα Survey Of The Southern Sky, John E. Gaustad, P. R. Mccullough, W. Rosing, D. Van Buren Nov 2001

A Robotic Wide-Angle Hα Survey Of The Southern Sky, John E. Gaustad, P. R. Mccullough, W. Rosing, D. Van Buren

Physics & Astronomy Faculty Works

We have completed a robotic wide-angle imaging survey of the southern sky (delta = +15 degrees to -90 degrees) at 656.3 nm wavelength, the H alpha emission line of hydrogen. Each image of the resulting Southern H alpha Sky Survey Atlas (SHASSA) covers an area of the sky 13 degrees square at an angular resolution of approximately 0.'8 and reaches a sensitivity level of 2 R (1.2 x 10(-17) ergs cm(-2) s(-1) arcsec(-2)) pixel(-1), corresponding to an emission measure of 4 cm(-6) pc and to a brightness temperature for microwave free-free emission of 12 muK at 30 GHz. Smoothing over …

Go to article

X-Ray Line Profiles From Parameterized Emission Within An Accelerating Stellar Wind, S. P. Owocki, David H. Cohen Oct 2001

X-Ray Line Profiles From Parameterized Emission Within An Accelerating Stellar Wind, S. P. Owocki, David H. Cohen

Physics & Astronomy Faculty Works

Motivated by recent detections by the XMM and Chandra satellites of X-ray line emission from hot, luminous stars, we present synthetic line profiles for X-rays emitted within parameterized models of a hot-star wind. The X-ray line emission is taken to occur at a sharply defined comoving-frame resonance wavelength, which is Doppler-shifted by a stellar wind outflow parameterized by a "beta" velocity law, nu (r) = nu (infinity)(1-R-*/r)(beta). Above some initial onset radius for R-o X-ray emission, the radial variation of the emission filling factor is assumed to decline as a power law in radius, f (r) similar tor(-q). The computed …

Go to article

Chandra Detection Of Doppler-Shifted X-Ray Line Profiles From The Wind Of Zeta Puppis (O4f), J. P. Cassinelli, N. A. Miller, W. L. Waldron, J. J. Macfarlane, David H. Cohen Jun 2001

Chandra Detection Of Doppler-Shifted X-Ray Line Profiles From The Wind Of Zeta Puppis (O4f), J. P. Cassinelli, N. A. Miller, W. L. Waldron, J. J. Macfarlane, David H. Cohen

Physics & Astronomy Faculty Works

We report on a 67 ks High-Energy Transmission Grating observation of the optically brightest early O star zeta Puppis (O4 f). Many resolved X-ray lines are seen in the spectra over a wavelength range of 5-25 Angstrom. Chnndra has sufficient spectral resolution to study the velocity structure of isolated X-ray line profiles and to distinguish the individual forbidden, intercombination, and resonance (fir) emission lines in several He-like ions, even where the individual components are strongly Doppler-broadened. In contrast to X-ray line profiles in other hot stars, zeta Pup shows blueshifted and skewed line profiles, providing the dearest and most direct …

Go to article

Disk Clearing In The Young Binary Ak Sco, Eric L.N. Jensen Jan 2001

Disk Clearing In The Young Binary Ak Sco, Eric L.N. Jensen

Physics & Astronomy Faculty Works

The pre-main-sequence spectroscopic binary AK Sco is surrounded by a circumbinary disk. Yet while this disk must be somewhat disturbed by the binary at its center, there is no evidence from current data that it has a cleared central hole like those seen in some young binaries, nor that accretion from the outer disk has been cut off. Indeed, most of its spectral and photometric diagnostics are indistinguishable from those of young single stars. Why doesn't this system show any evidence of disturbance of its disk? It may be that the center of the disk is indeed largely cleared, but …

Go to article

Disks In Young Binary Systems: Unresolved Millimeter-Wave Observations, Eric L.N. Jensen Jan 2001

Disks In Young Binary Systems: Unresolved Millimeter-Wave Observations, Eric L.N. Jensen

Physics & Astronomy Faculty Works

Observations at millimeter and submillimeter wavelengths are sensitive to the total mass of circumstellar and circumbinary dust in a multiple system, and in some cases single-dish observations can help constrain the location of disk material in spite of their lack of spatial resolution. Young binary stars show a great diversity of disk properties, with a large part of the variation accounted for by binary separation. Many of the closest binaries (those with separations a less than a few AU) harbor massive circumbinary disks. Binaries with a greater than or equal to 100 AU tend to have massive circumstellar disks. In …

Go to article

Can Post T Tauri Stars Be Found? Yes!, Eric L.N. Jensen Jan 2001

Can Post T Tauri Stars Be Found? Yes!, Eric L.N. Jensen

Physics & Astronomy Faculty Works

I review the observational challenges of finding post T Tauri stars (PTTS), defined here as low-mass, pre-main-sequence stars with ages of 10(7)-10(8) yr. Such stars are difficult to find because they are less active than younger T Tauri stars, and they may not be associated with molecular gas. They are useful for studying the evolution of circumstellar disks and stellar activity between the 10(6)-yr ages of nearby star-forming regions and the main sequence. However, care must be taken in the search process so that the selection criteria used to locate such stars do not bias the sample used for subsequent …

Go to article

A Single Circumbinary Disk In The Hd 98800 Quadruple System, D. W. Koerner, Eric L.N. Jensen, K. L. Cruz, T. B. Guild, K. Gultekin Apr 2000

A Single Circumbinary Disk In The Hd 98800 Quadruple System, D. W. Koerner, Eric L.N. Jensen, K. L. Cruz, T. B. Guild, K. Gultekin

Physics & Astronomy Faculty Works

We present subarcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0".8 (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 mu m show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess on which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 mu m and longer, peaks at 25 mu m, and has a best-fit blackbody temperature of 150 K, indicating that most of the …

Go to article

The Three-Dimensional Structure Of The Infrared Cirrus, John E. Gaustad Jan 1994

The Three-Dimensional Structure Of The Infrared Cirrus, John E. Gaustad

Physics & Astronomy Faculty Works

This project was carried out over a period of four years, beginning 6/15/89 and continuing through 9/15/93. Intermediate results have been reported as poster papers at several meetings of the American Astronomical Society. A brief summary was presented in April 1993 at a symposium on the infrared cirrus. The final results were published in late 1993. The measurements have been deposited in NASA's Astronomical Data Center. Briefly, the results are as follows: Using the IRAS data base, we surveyed the 1808 06-B9.5 stars in the Bright Star Catalog for extended excess emission at 60 micrometers, indicating the presence of heated …

Go to article

First Prev