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- Techniques: photometric (25)
- Techniques: radial velocities (18)
- Planetary systems (14)
- Stars: early-type (13)
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- Outflows (9)
- Stars: winds (9)
- X-rays: stars (9)
- Astrophysics - Earth and Planetary Astrophysics (8)
- Exoplanet astronomy (8)
- Stars: magnetic field (8)
- Stars: massive (8)
- Exoplanet systems (6)
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- Astrophysics - Solar and Stellar Astrophysics (4)
- Binaries: general (4)
- Planets and satellites: fundamental parameters (4)
- Protoplanetary disks (4)
- Stars: late-type (4)
- Stars: rotation (4)
- Exoplanet detection methods (3)
- Exoplanet evolution (3)
- M dwarf stars (3)
- Planets and satellites: composition (3)
- Planets and satellites: gaseous planets (3)
Articles 151 - 180 of 204
Full-Text Articles in Physical Sciences and Mathematics
Chandra X-Ray Spectroscopy Of The Very Early O Supergiant Hd 93129a: Constraints On Wind Shocks And The Mass-Loss Rate, David H. Cohen, M. Gagné, M. A. Leutenegger, James Paton Macarthur , '11, Emma Edwina Wollman , '09, J. O. Sundqvist, A. W. Fullerton, S. P. Owocki
Chandra X-Ray Spectroscopy Of The Very Early O Supergiant Hd 93129a: Constraints On Wind Shocks And The Mass-Loss Rate, David H. Cohen, M. Gagné, M. A. Leutenegger, James Paton Macarthur , '11, Emma Edwina Wollman , '09, J. O. Sundqvist, A. W. Fullerton, S. P. Owocki
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We present an analysis of both the resolved X-ray emission-line profiles and the broad-band X-ray spectrum of the O-2 If* star HD 93129A, measured with the Chandra High Energy Transmission Grating Spectrometer ( HETGS). This star is among the earliest and most massive stars in the Galaxy, and provides a test of the embedded wind-shock scenario in a very dense and powerful wind. A major new result is that continuum absorption by the dense wind is the primary cause of the hardness of the observed X-ray spectrum, while intrinsically hard emission from colliding wind shocks contributes less than 10 per …
The Young Exoplanet Transit Initiative (Yeti), R. Neuhäuser, R. Errmann, A. Berndt, G. Maciejewski, H. Takahashi, W. P. Chen, D. P. Dimitrov, T. Pribulla, E. H. Nikogossian, Eric L.N. Jensen, L. Marschall, Z.-Y. Wu, A. Kellerer, F. M. Walter, C. Briceño, R. Chini, M. Fernández, S. Raetz, G. Torres, D. W. Latham, S. N. Quinn, A. Niedzielski, L. Bukowiecki, G. Nowak, T. Tomov, K. Tachihara, S. C.-L. Hu, L. W. Hung, D. P. Kjurkchieva, V. S. Radeva, B. M. Mihov, L. Slavcheva-Mihova, I. N. Bozhinova, J. Budaj, M. Vanko, E. Kundra, L. Hambalek, V. Krushevska, T. Movsessian, H. Harutyunyan, J. J. Downes, J. Hernandez, V. H. Hoffmeister, David H. Cohen, Imoleayo Samson Abel , '14, Rebecca Ruby Ahmad , '14, Seth Walker Chapman , '14, Sierra Clare Eckert , '14, Jackson Goodman , '13, Adrien Charles Guerard , '14, H. M. Kim, Andrew Evan Koontharana , '11, Joshua Daniel Sokol , '11, Jennifer Trinh , '11, Yuwen Wang , '14, X. Zhou, R. Redmer, U. Kramm, N. Nettelmann, M. Mugrauer, J. Schmidt, M. Moualla, C. Ginski, C. Marka, C. Adam, M. Seeliger, S. Baar, T. Roell, T. O.B. Schmidt, L. Trepl, T. Eisenbeiss, S. Fiedler, N. Tetzlaff, E. Schmidt, M. M. Hohle, M. Kitze, N. Chakrova, C. Grafe, K. Schreyer, V. V. Hambaryan, C. H. Broeg, J. Koppenhoefer, A. K. Pandey
The Young Exoplanet Transit Initiative (Yeti), R. Neuhäuser, R. Errmann, A. Berndt, G. Maciejewski, H. Takahashi, W. P. Chen, D. P. Dimitrov, T. Pribulla, E. H. Nikogossian, Eric L.N. Jensen, L. Marschall, Z.-Y. Wu, A. Kellerer, F. M. Walter, C. Briceño, R. Chini, M. Fernández, S. Raetz, G. Torres, D. W. Latham, S. N. Quinn, A. Niedzielski, L. Bukowiecki, G. Nowak, T. Tomov, K. Tachihara, S. C.-L. Hu, L. W. Hung, D. P. Kjurkchieva, V. S. Radeva, B. M. Mihov, L. Slavcheva-Mihova, I. N. Bozhinova, J. Budaj, M. Vanko, E. Kundra, L. Hambalek, V. Krushevska, T. Movsessian, H. Harutyunyan, J. J. Downes, J. Hernandez, V. H. Hoffmeister, David H. Cohen, Imoleayo Samson Abel , '14, Rebecca Ruby Ahmad , '14, Seth Walker Chapman , '14, Sierra Clare Eckert , '14, Jackson Goodman , '13, Adrien Charles Guerard , '14, H. M. Kim, Andrew Evan Koontharana , '11, Joshua Daniel Sokol , '11, Jennifer Trinh , '11, Yuwen Wang , '14, X. Zhou, R. Redmer, U. Kramm, N. Nettelmann, M. Mugrauer, J. Schmidt, M. Moualla, C. Ginski, C. Marka, C. Adam, M. Seeliger, S. Baar, T. Roell, T. O.B. Schmidt, L. Trepl, T. Eisenbeiss, S. Fiedler, N. Tetzlaff, E. Schmidt, M. M. Hohle, M. Kitze, N. Chakrova, C. Grafe, K. Schreyer, V. V. Hambaryan, C. H. Broeg, J. Koppenhoefer, A. K. Pandey
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We present the Young Exoplanet Transit Initiative (YETI), in which we use several 0.2 to 2.6-m telescopes around the world to monitor continuously young (<= 100 Myr), nearby (<= 1 kpc) stellar clusters mainly to detect young transiting planets (and to study other variability phenomena on time-scales from minutes to years). The telescope network enables us to observe the targets continuously for several days in order not to miss any transit. The runs are typically one to two weeks long, about three runs per year per cluster in two or three subsequent years for about ten clusters. There are thousands of stars detectable in each field with several hundred known cluster members, e. g. in the first cluster observed, Tr-37, a typical cluster for the YETI survey, there are at least 469 known young stars detected in YETI data down to R = 16.5 mag with sufficient precision of 50 millimag rms (5 mmag rms down to R = 14.5 mag) to detect transits, so that we can expect at least about one young transiting object in this cluster. If we observe similar to 10 similar clusters, we can expect to detect similar to 10 young transiting planets with radius determinations. The precision given above is for a typical telescope of the YETI network, namely the 60/90-cm Jena telescope (similar brightness limit, namely within +/-1 mag, for the others) so that planetary transits can be detected. For targets with a periodic transit-like light curve, we obtain spectroscopy to ensure that the star is young and that the transiting object can be sub-stellar; then, we obtain Adaptive Optics infrared images and spectra, to exclude other bright eclipsing stars in the (larger) optical PSF; we carry out other observations as needed to rule out other false positive scenarios; finally, we also perform spectroscopy to determine the mass of the transiting companion. For planets with mass and radius determinations, we can calculate the mean density and probe the internal structure. We aim to constrain planet formation models and their time-scales by discovering planets younger than similar to 100 Myr and determining not only their orbital parameters, but also measuring their true masses and radii, which is possible so far only by the transit method. Here, we present an overview and first results. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
An Introduction To The Chandra Carina Complex Project, L. K. Townsley, P. S. Broos, M. F. Corcoran, E. D. Feigelson, M. Gagné, T. Montmerle, M. S. Oey, N. Smith, G. P. Garmire, K. V. Getman, M. S. Povich, N. R. Evans, Y. Nazé, E. R. Parkin, T. Preibisch, J. Wang, S. J. Wou, Y.-H. Chu, David H. Cohen, R. A. Gruendl, K. Hamaguchi, R. R. King, M.-M. M. Low, M. J. Mccaughrean, A. F.J. Moffat, L. M. Oskinova, J. M. Pittard, K. G. Stassun, A. Ud-Doula, N. R. Walborn, W. L. Waldron, E. Churchwell, J. S. Nictiols, S. P. Owocki, N. S. Schulz
An Introduction To The Chandra Carina Complex Project, L. K. Townsley, P. S. Broos, M. F. Corcoran, E. D. Feigelson, M. Gagné, T. Montmerle, M. S. Oey, N. Smith, G. P. Garmire, K. V. Getman, M. S. Povich, N. R. Evans, Y. Nazé, E. R. Parkin, T. Preibisch, J. Wang, S. J. Wou, Y.-H. Chu, David H. Cohen, R. A. Gruendl, K. Hamaguchi, R. R. King, M.-M. M. Low, M. J. Mccaughrean, A. F.J. Moffat, L. M. Oskinova, J. M. Pittard, K. G. Stassun, A. Ud-Doula, N. R. Walborn, W. L. Waldron, E. Churchwell, J. S. Nictiols, S. P. Owocki, N. S. Schulz
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The Great Nebula in Carina provides an exceptional view into the violent massive star formation and feedback that typifies giant H II regions and starburst galaxies. We have mapped the Carina star-forming complex in X-rays, using archival Chandra data and a mosaic of 20 new 60 ks pointings using the Chandra X-ray Observatory's Advanced CCD Imaging Spectrometer, as a testbed for understanding recent and ongoing star formation and to probe Carina's regions of bright diffuse X-ray emission. This study has yielded a catalog of properties of > 14,000 X-ray point sources;> 9800 of them have multiwavelength counterparts. Using Chandra's unsurpassed X-ray …
Carina Ob Stars: X-Ray Signatures Of Wind Shocks And Magnetic Fields, M. Gagné, G. Fehon, M. R. Savoy, David H. Cohen, L. K. Townsley, P. S. Broos, M. S. Povich, M. F. Corcoran, N. R. Walborn, N. R. Evans, A. F.J. Moffat, Y. Nazé, L. M. Oskinova
Carina Ob Stars: X-Ray Signatures Of Wind Shocks And Magnetic Fields, M. Gagné, G. Fehon, M. R. Savoy, David H. Cohen, L. K. Townsley, P. S. Broos, M. S. Povich, M. F. Corcoran, N. R. Walborn, N. R. Evans, A. F.J. Moffat, Y. Nazé, L. M. Oskinova
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The Chandra Carina Complex contains 200 known O- and B-type stars. The Chandra survey detected 68 of the 70 O stars and 61 of 127 known B0-B3 stars. We have assembled a publicly available optical/X-ray database to identify OB stars that depart from the canonical L-X/L-bol relation or whose average X-ray temperatures exceed 1 keV. Among the single O stars with high kT we identify two candidate magnetically confined wind shock sources: Tr16-22, O8.5 V, and LS 1865, O8.5 V((f)). The O4 III(fc) star HD 93250 exhibits strong, hard, variable X-rays, suggesting that it may be a massive binary with …
Global X-Ray Properties Of The O And B Stars In Carina, Y. Nazé, P. S. Broos, L. M. Oskinova, L. K. Townsley, David H. Cohen, M. F. Corcoran, N. R. Evans, M. Gagné, A. F.J. Moffat, J. M. Pittard, G. Rauw, A. Ud-Doula, N. R. Walborn
Global X-Ray Properties Of The O And B Stars In Carina, Y. Nazé, P. S. Broos, L. M. Oskinova, L. K. Townsley, David H. Cohen, M. F. Corcoran, N. R. Evans, M. Gagné, A. F.J. Moffat, J. M. Pittard, G. Rauw, A. Ud-Doula, N. R. Walborn
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The key empirical property of the X-ray emission from O stars is a strong correlation between the bolometric and X-ray luminosities. In the framework of the Chandra Carina Complex Project, 129 O and B stars have been detected as X-ray sources; 78 of those, all with spectral type earlier than B3, have enough counts for at least a rough X-ray spectral characterization. This leads to an estimate of the L-X-L-BOL ratio for an exceptional number of 60 O stars belonging to the same region and triples the number of Carina massive stars studied spectroscopically in X-rays. The derived log(L-X/L-BOL) is …
V474 Car: A Rare Halo Rs Cvn Binary In Retrograde Galactic Orbit, E. J. Bubar, E. E. Mamajek, Eric L.N. Jensen, F. M. Walter
V474 Car: A Rare Halo Rs Cvn Binary In Retrograde Galactic Orbit, E. J. Bubar, E. E. Mamajek, Eric L.N. Jensen, F. M. Walter
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We report the discovery that the star V474 Car is an extremely active, high velocity halo RS CVn system. The star was originally identified as a possible pre-main-sequence star in Carina, given its enhanced stellar activity, rapid rotation (10.3 days), enhanced Li, and absolute magnitude which places it above the main sequence (MS). However, its extreme radial velocity (264 km s–1) suggested that this system was unlike any previously known pre-MS system. Our detailed spectroscopic analysis of echelle spectra taken with the CTIO 4 m finds that V474 Car is both a spectroscopic binary with an orbital period …
X-Ray Spectral Diagnostics Of Activity In Massive Stars, David H. Cohen, Emma Edwina Wollman , '09, M. A. Leutenegger
X-Ray Spectral Diagnostics Of Activity In Massive Stars, David H. Cohen, Emma Edwina Wollman , '09, M. A. Leutenegger
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X-rays give direct evidence of instabilities, time-variable structure, and shock heating in the winds of O stars. The observed broad X-ray emission lines provide information about the kinematics of shock-heated wind plasma, enabling us to test wind-shock models. And their shapes provide information about wind absorption, and thus about the wind mass-loss rates. Mass-loss rates determined from X-ray line profiles are not sensitive to density-squared clumping effects, and indicate mass-loss rate reductions of factors of 3 to 6 over traditional diagnostics that suffer from density-squared effects. Broad-band X-ray spectral energy distributions also provide mass-loss rate information via soft X-ray absorption …
A Multiphase Suzaku Study Of X-Rays From Τ Sco, R. Ignace, L. M. Oskinova, M. Jardine, J. P. Cassinelli, David H. Cohen, J.-F. Donati, R. H.D. Townsend, A. Ud-Doula
A Multiphase Suzaku Study Of X-Rays From Τ Sco, R. Ignace, L. M. Oskinova, M. Jardine, J. P. Cassinelli, David H. Cohen, J.-F. Donati, R. H.D. Townsend, A. Ud-Doula
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We obtained relatively high signal-to-noise X-ray spectral data of the early massive star tau Sco (B0.2V) with the Suzaku X-ray Imaging Spectrometer (XIS) instrument. This source displays several unusual features that motivated our study: (1) redshifted absorption in UV P Cygni lines to approximately +250 km s(-1) suggestive of infalling gas, (2) unusually hard X-ray emission requiring hot plasma at temperatures in excess of 10 MK whereas most massive stars show relatively soft X-rays at a few MK, and (3) a complex photospheric magnetic field of open and closed field lines. In an attempt to understand the hard component better, …
Modeling Broadband X-Ray Absorption Of Massive Star Winds, M. A. Leutenegger, David H. Cohen, J. Zsargó, Erin Marie Martell , '09, James Paton Macarthur , '11, S. P. Owocki, M. Gagné, D. J. Hillier
Modeling Broadband X-Ray Absorption Of Massive Star Winds, M. A. Leutenegger, David H. Cohen, J. Zsargó, Erin Marie Martell , '09, James Paton Macarthur , '11, S. P. Owocki, M. Gagné, D. J. Hillier
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We present a method for computing the net transmission of X-rays emitted by shock-heated plasma distributed throughout a partially optically thick stellar wind from a massive star. We find the transmission by an exact integration of the formal solution, assuming that the emitting plasma and absorbing plasma are mixed at a constant mass ratio above some minimum radius, below which there is assumed to be no emission. This model is more realistic than either the slab absorption associated with a corona at the base of the wind or the exospheric approximation that assumes that all observed X-rays are emitted without …
A Mass-Loss Rate Determination For Zeta Puppis From The Quantitative Analysis Of X-Ray Emission-Line Profiles, David H. Cohen, M. A. Leutenegger, Emma Edwina Wollman , '09, J. Zsargó, D. J. Hillier, R. H.D. Townsend, S. P. Owocki
A Mass-Loss Rate Determination For Zeta Puppis From The Quantitative Analysis Of X-Ray Emission-Line Profiles, David H. Cohen, M. A. Leutenegger, Emma Edwina Wollman , '09, J. Zsargó, D. J. Hillier, R. H.D. Townsend, S. P. Owocki
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We fit every emission line in the high-resolution Chandra grating spectrum of. Pup with an empirical line profile model that accounts for the effects of Doppler broadening and attenuation by the bulk wind. For each of 16 lines or line complexes that can be reliably measured, we determine a best-fitting fiducial optical depth, tau(*) equivalent to kappa(M) over dot/4 pi R(*)upsilon(infinity), and place confidence limits on this parameter. These 16 lines include seven that have not previously been reported on in the literature. The extended wavelength range of these lines allows us to infer, for the first time, a clear …
Discovery Of Rotational Braking In The Magnetic Helium-Strong Star Sigma Orionis E, R. H.D. Townsend, M. E. Oksala, David H. Cohen, S. P. Owocki, A. Ud-Doula
Discovery Of Rotational Braking In The Magnetic Helium-Strong Star Sigma Orionis E, R. H.D. Townsend, M. E. Oksala, David H. Cohen, S. P. Owocki, A. Ud-Doula
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We present new U-band photometry of the magnetic helium-strong star sigma Ori E, obtained over 2004-2009 using the SMARTS 0.9 m telescope at Cerro Tololo Inter-American Observatory. When combined with historical measurements, these data constrain the evolution of the star's 1.19 day rotation period over the past three decades. We are able to rule out a constant period at the p(null) = 0.05% level, and instead find that the data are well described (p(null) = 99.3%) by a period increasing linearly at a rate of 77 ms per year. This corresponds to a characteristic spin-down time of 1.34 Myr, in …
A Test Of Pre-Main-Sequence Lithium Depletion Models, Jennifer Chun Ming Yee , '07, Eric L.N. Jensen
A Test Of Pre-Main-Sequence Lithium Depletion Models, Jennifer Chun Ming Yee , '07, Eric L.N. Jensen
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Despite the extensive study of lithium depletion during pre-main-sequence (PMS) contraction, studies of individual stars show discrepancies between ages determined from the Hertzsprung-Russell (H-R) diagram and ages determined from lithium depletion, indicating open questions in the PMS evolutionary models. To further test these models, we present high-resolution spectra for members of the beta Pictoris Moving Group (BPMG), which is young and nearby. We measure equivalent widths of the 6707.8 angstrom Li I line in these stars and use them to determine lithium abundances. We combine the lithium abundance with the predictions of PMS evolutionary models in order to calculate a …
Discovery Of A Magnetic Field In The O9 Sub-Giant Star Hd 57682 By The Mimes Collaboration, J. H. Grunhut, G. A. Wade, W. L.F. Marcolino, V. Petit, H. F. Henrichs, David H. Cohen, E. Alecian, D. A. Bohlender, J.-C. Bouret, O. Kochukhov, C. Neiner, N. St. Louis, R. H.D. Townsend
Discovery Of A Magnetic Field In The O9 Sub-Giant Star Hd 57682 By The Mimes Collaboration, J. H. Grunhut, G. A. Wade, W. L.F. Marcolino, V. Petit, H. F. Henrichs, David H. Cohen, E. Alecian, D. A. Bohlender, J.-C. Bouret, O. Kochukhov, C. Neiner, N. St. Louis, R. H.D. Townsend
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We report the detection of a strong, organized magnetic field in the O9IV star HD 57682, using spectropolarimetric observations obtained with ESPaDOnS at the 3.6-m Canada-France-Hawaii Telescope within the context of the Magnetism in Massive Stars (MiMeS) Large Programme. From the fitting of our spectra using non-local thermodynamic equilibrium model atmospheres, we determined that HD 57682 is a 17(-9)(+19)M(circle dot) star with a radius of 7.0(-1.8)(+2.4)R(circle dot) and a relatively low mass-loss rate of 1.4(-0.95)(+3.1) x 10(-9) M(circle dot) yr(-1). The photospheric absorption lines are narrow, and we use the Fourier transform technique to infer v sin i = 15 …
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
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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
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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 …
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 …
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 …
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
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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
Physics & Astronomy Faculty Works
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
Physics & Astronomy Faculty Works
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
Physics & Astronomy Faculty Works
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-Ray Emission From O Stars, David H. Cohen
X-Ray Emission From O Stars, David H. Cohen
Physics & Astronomy Faculty Works
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 …
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
Physics & Astronomy Faculty Works
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.
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
Physics & Astronomy Faculty Works
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
Physics & Astronomy Faculty Works
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 …