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Dartmouth Scholarship

Articles 241 - 270 of 430

Full-Text Articles in Astrophysics and Astronomy

The Effect Of Cluster Environment On Galaxy Evolution In The Pegasus I Cluster, Lorenza Levy, James A. Rose, Jacqueline H. Van Gorkom, Brian Chaboyer Feb 2007

The Effect Of Cluster Environment On Galaxy Evolution In The Pegasus I Cluster, Lorenza Levy, James A. Rose, Jacqueline H. Van Gorkom, Brian Chaboyer

Dartmouth Scholarship

We present neutral hydrogen observations of 54 galaxies in the Pegasus Cluster. The observations include single-dish H I measurements, obtained with the Arecibo telescope for all 54 galaxies in the sample, as well as H I images obtained with the Very Large Array (VLA) for 10 of these. The Arecibo profiles reveal an overall H I deficiency in the cluster, with similar to 40% of the galaxies in the core of the cluster showing modest deficiencies of typically a factor of 2-3. The HI morphology of so me galaxies shows that the HI disk is smaller than the optical disk …

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The Wind‐Ism Interaction Of Α Tauri, Brian E. Wood, Graham M. Harper, Hans-Reinhard Muller, Jacob Heerikhuisen, Gary P. Zank Feb 2007

The Wind‐Ism Interaction Of Α Tauri, Brian E. Wood, Graham M. Harper, Hans-Reinhard Muller, Jacob Heerikhuisen, Gary P. Zank

Dartmouth Scholarship

Ultraviolet spectra of α Tau (K5 III) obtained by the Hubble Space Telescope (HST) show many emission lines affected by broad absorption from the strong wind of this red giant star. For the Mg II h and k lines there is also a narrow absorption feature in the midst of the wind absorption that has been interpreted as being from α Tau's wind-interstellar medium (ISM) interaction region (i.e., its "astrosphere"). We try to reproduce this absorption using hydrodynamic models of the α Tau astrosphere, which show that stellar wind material heated, compressed, and decelerated at the wind's termination …

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Discovery Of Extensive Optical Emission Associated With The X-Ray Bright, Radio Faint Galactic Snr G156.2+5.7, Christopher L. Gerardy, Robert A. Fesen Dec 2006

Discovery Of Extensive Optical Emission Associated With The X-Ray Bright, Radio Faint Galactic Snr G156.2+5.7, Christopher L. Gerardy, Robert A. Fesen

Dartmouth Scholarship

We present wide‐field Hα images of the Galactic supernova remnant (SNR) G156.2+5.7 which reveal the presence of considerable faint Hα line emission coincident with the remnant's X‐ray emission. We also present low‐resolution optical spectra for a few representative emission regions. The outermost Hα emission consists largely of long and thin (unresolved), smoothly curved filaments of Balmer‐dominated emission presumably associated with the remnant's forward shock front. Patches of brighter Hα emission along the western, south‐central, and north‐eastern regions appear to be radiative shocked interstellar medium filaments like those commonly seen in SNRs, with relatively strong [O i]λλ6300, 6364 and [S ii]λλ6716, …

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Small-Scale Structures In Three-Dimensional Magnetohydrodynamic Turbulence, P. D. Mininni, A. G. Pouquet, D. C. Montgomery Dec 2006

Small-Scale Structures In Three-Dimensional Magnetohydrodynamic Turbulence, P. D. Mininni, A. G. Pouquet, D. C. Montgomery

Dartmouth Scholarship

We investigate using direct numerical simulations with grids up to 15363 points, the rate at which small scales develop in a decaying three-dimensional MHD flow both for deterministic and random initial conditions. Parallel current and vorticity sheets form at the same spatial locations, and further destabilize and fold or roll up after an initial exponential phase. At high Reynolds numbers, a self-similar evolution of the current and vorticity maxima is found, in which they grow as a cubic power of time; the flow then reaches a finite dissipation rate independent of the Reynolds number.

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Faint X‐Ray Structure In The Crab Pulsar Wind Nebula, F. D. Seward, W. H. Tucker, R. A. Fesen Dec 2006

Faint X‐Ray Structure In The Crab Pulsar Wind Nebula, F. D. Seward, W. H. Tucker, R. A. Fesen

Dartmouth Scholarship

We report on a Chandra observation of the Crab Nebula that gives the first clear view of the faint boundary of the Crab's X-ray-emitting pulsar wind nebula. There is structure in all directions. Fingers, loops, bays, and the south pulsar jet all indicate that either filamentary material or the magnetic field is controlling the relativistic electrons. In general, spectra soften as distance from the pulsar increases but do not change rapidly along linear features. This is particularly true for the pulsar jet. The termination of the jet is abrupt; the east side is close to an [O III] optical filament, …

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The Asymmetric Explosion Of Type Ia Supernovae As Seen From Near-Infrared Observations, Kentaro Motohara, Keiichi Maeda, Christopher L. Gerardy, Ken'ichi Nomoto, Masaomi Tanaka, Nozomu Tominaga, Takuya Ohkubo, Paolo A. Mazzali, Robert A. Fesen, Peter Hoflich, J. Craig Wheeler Nov 2006

The Asymmetric Explosion Of Type Ia Supernovae As Seen From Near-Infrared Observations, Kentaro Motohara, Keiichi Maeda, Christopher L. Gerardy, Ken'ichi Nomoto, Masaomi Tanaka, Nozomu Tominaga, Takuya Ohkubo, Paolo A. Mazzali, Robert A. Fesen, Peter Hoflich, J. Craig Wheeler

Dartmouth Scholarship

We present near-infrared spectra of late-phase (>200 days) Type Ia supernovae (SNe Ia) taken at the Subaru Telescope. The [Fe II] line of SN 2003hv shows a clear flat-topped feature, while that of SN 2005W shows a less prominent flatness. In addition, a large shift in their line center, varying from -3000 to 1000 km s-1 with respect to the host galaxies, is seen. Such a shift suggests the occurrence of an off-center, nonspherical explosion in the central region and provides important, new constraints on the explosion models of SNe Ia.

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Spitzer Measurements Of Atomic And Molecular Abundances In The Type Iip Sn 2005af, Rubina Kotak, Peter Meikle, Monica Pozzo, Schuyler D. Van Dyk, Duncan Farrah, Robert Fesen, Alexei V. Filippenko, Ryan J. Foley, Claes Fransson, Christopher L. Gerardy, Peter A. Hoflich, Peter Lundqvist, Seppo Mattila, Jesper Sollerman, J. Craig Wheeler Oct 2006

Spitzer Measurements Of Atomic And Molecular Abundances In The Type Iip Sn 2005af, Rubina Kotak, Peter Meikle, Monica Pozzo, Schuyler D. Van Dyk, Duncan Farrah, Robert Fesen, Alexei V. Filippenko, Ryan J. Foley, Claes Fransson, Christopher L. Gerardy, Peter A. Hoflich, Peter Lundqvist, Seppo Mattila, Jesper Sollerman, J. Craig Wheeler

Dartmouth Scholarship

We present results based on mid-infrared (3.6-30 μm) observations with the Spitzer Space Telescope of the nearby Type IIP supernova 2005af. We report the first ever detection of the SiO molecule in a Type IIP supernova. Together with the detection of the CO fundamental, this is an exciting finding as it may signal the onset of dust condensation in the ejecta. From a wealth of fine-structure lines we provide abundance estimates for stable Ni, Ar, and Ne that, via spectral synthesis, may be used to constrain nucleosynthesis models.

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Casimir Forces And Non-Newtonian Gravitation, Roberto Onofrio Oct 2006

Casimir Forces And Non-Newtonian Gravitation, Roberto Onofrio

Dartmouth Scholarship

The search for non-relativistic deviations from Newtonian gravitation can lead to new phenomena signalling the unification of gravity with the other fundamental interactions. Various recent theoretical frameworks indicate a possible window for non-Newtonian forces with gravitational coupling strength in the micrometre range. The major expected background in the same range is attributable to the Casimir force or variants of it if dielectric materials, rather than conducting ones, are considered. Here we review the measurements of the Casimir force performed so far in the micrometre range and how they determine constraints on non-Newtonian gravitation, also discussing the dominant sources of false …

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The Young Open Cluster Ngc 2129, Giovanni Carraro, Brian Chaboyer, James Perencevich Oct 2006

The Young Open Cluster Ngc 2129, Giovanni Carraro, Brian Chaboyer, James Perencevich

Dartmouth Scholarship

The first charge‐coupled device UBV(RI)C photometric study in the area of the doubtful open cluster NGC 2129 is presented. Photometry of a field offset 15 arcmin northwards is also provided, to probe the Galactic disc population towards the cluster. Using star counts, proper motions from the UCAC2 catalogue, colour–magnitude and colour–colour diagrams, we demonstrate that NGC 2129 is a young open cluster. The cluster radius is 2.5 arcmin, and across this region we find evidence of significant differential reddening, although the reddening law seems to be normal towards its direction. Updated estimates of the cluster fundamental …

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A Spitzer Space Telescope Study Of Sn 2002hh: An Infrared Echo From A Type Iip Supernova, W. P. S. Meikle, S. Mattila, C. L. Gerardy, R. Kotak, M. Pozzo, S. D. Van Dyk, D. Farrah, R. A. Fesen Sep 2006

A Spitzer Space Telescope Study Of Sn 2002hh: An Infrared Echo From A Type Iip Supernova, W. P. S. Meikle, S. Mattila, C. L. Gerardy, R. Kotak, M. Pozzo, S. D. Van Dyk, D. Farrah, R. A. Fesen

Dartmouth Scholarship

We present late-time (590-994 days) mid-IR photometry of the normal but highly reddened Type IIP supernova SN 2002hh. Bright, cool, slowly fading emission is detected from the direction of the supernova. Most of this flux appears not to be driven by the supernova event but instead probably originates in a cool, obscured star formation region or molecular cloud along the line of sight. We also show, however, that the declining component of the flux is consistent with an SN-powered IR echo from a dusty progenitor CSM. Mid-IR emission could also be coming from newly condensed dust and/or an ejecta/CSM impact, …

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Heliospheric Response To Different Possible Interstellar Environments, Hans-Reinhard Muller, Priscilla C. Frisch, Vladimir Florinski, Gary P. Zank Aug 2006

Heliospheric Response To Different Possible Interstellar Environments, Hans-Reinhard Muller, Priscilla C. Frisch, Vladimir Florinski, Gary P. Zank

Dartmouth Scholarship

At present, the heliosphere is embedded in a warm, low-density interstellar cloud that belongs to a cloud system flowing through the local standard of rest with a velocity near ~18 km s-1. The velocity structure of the nearest interstellar material (ISM), combined with theoretical models of the local interstellar cloud (LIC), suggest that the Sun passes through cloudlets on timescales of ≤103-104 yr, so the heliosphere has been, and will be, exposed to different interstellar environments over time. By means of a multifluid model that treats plasma and neutral hydrogen self-consistently, the interaction of the …

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Grb 060218/Sn 2006aj: A Gamma-Ray Burst And Prompt Supernova Atz= 0.0335, N. Mirabal, J. P. Halpern, D. An, J. R. Thorstensen, D. M. Terndrup Jun 2006

Grb 060218/Sn 2006aj: A Gamma-Ray Burst And Prompt Supernova Atz= 0.0335, N. Mirabal, J. P. Halpern, D. An, J. R. Thorstensen, D. M. Terndrup

Dartmouth Scholarship

We report the imaging and spectroscopic localization of GRB 060218 to a low-metallicity dwarf starburst galaxy at z = 0.03345 ± 0.00006. In addition to making it the second nearest gamma-ray burst known, optical spectroscopy reveals the earliest detection of weak, supernova-like Si II near 5720 Å (~0.1c), starting 1.95 days after the burst trigger. UBVRI photometry obtained between 1 and 26 days postburst confirms the early rise of supernova light, and suggests a short time delay between the gamma-ray burst and the onset of SN 2006aj if the early appearance of a soft component in the X-ray …

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Theoretical Uncertainties In Red Giant Branch Evolution: The Red Giant Branch Bump, Stephan R. R. Bjork, Brian Chaboyer Apr 2006

Theoretical Uncertainties In Red Giant Branch Evolution: The Red Giant Branch Bump, Stephan R. R. Bjork, Brian Chaboyer

Dartmouth Scholarship

A Monte Carlo simulation exploring uncertainties in standard stellar evolution theory on the red giant branch of metal-poor globular clusters has been conducted. Confidence limits are derived on the absolute V-band magnitude of the bump in the red giant branch luminosity function (MV,b) and the excess number of stars in the bump, Rb. The analysis takes into account uncertainties in the primordial helium abundance, abundance of α-capture elements, radiative and conductive opacities, nuclear reaction rates, neutrino energy losses, the treatments of diffusion and convection, the surface boundary conditions, and color transformations. The uncertainty in …

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From Canonical To Enhanced Extra Mixing In Low‐Mass Red Giants: Tidally Locked Binaries, Pavel A. Denissenkov, Brian Chaboyer, Ke Li Apr 2006

From Canonical To Enhanced Extra Mixing In Low‐Mass Red Giants: Tidally Locked Binaries, Pavel A. Denissenkov, Brian Chaboyer, Ke Li

Dartmouth Scholarship

Stellar models that incorporate simple diffusion or shear-induced mixing are used to describe canonical extra mixing in low-mass red giants of low and solar metallicity. These models are able to simultaneously explain the observed Li and CN abundance changes along the upper red giant branch (RGB) in field low-metallicity stars and match photometry, rotation, and 12C/13C ratios for stars in the old open cluster M67. The shear mixing model requires that main-sequence (MS) progenitors of upper RGB stars possessed rapidly rotating radiative cores and that specific angular momentum was conserved in each of their mass shells during …

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The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models For Active And Inactive Solar-Type Stars, Guillermo Torres, Claud H. Lacy, L. A. Marshall, Holly A. Sheets, Jeff A. Mader Apr 2006

The Eclipsing Binary V1061 Cygni: Confronting Stellar Evolution Models For Active And Inactive Solar-Type Stars, Guillermo Torres, Claud H. Lacy, L. A. Marshall, Holly A. Sheets, Jeff A. Mader

Dartmouth Scholarship

We present spectroscopic and photometric observations of the eclipsing system V1061 Cyg (P = 2.35 days). A third star is visible in the spectrum, and the system is a hierarchical triple. We combine the radial velocities for the three stars, times of eclipse, and intermediate astrometric data from the Hipparcos mission (abscissa residuals) to establish the elements of the outer orbit, which is eccentric and has a period of 15.8 yr. We determine accurate values for the masses, radii, and effective temperatures of the binary components: MAa = 1.282 ± 0.015 M☉, RAa = 1.615 …

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The Structure Of Flux Transfer Events Recovered From Cluster Data, H Hasegawa, B U. Ö Sonnerup, C J. Owen, B Klecker, G Paschmann, A Balogh, H Re`Me Mar 2006

The Structure Of Flux Transfer Events Recovered From Cluster Data, H Hasegawa, B U. Ö Sonnerup, C J. Owen, B Klecker, G Paschmann, A Balogh, H Re`Me

Dartmouth Scholarship

The structure and formation mechanism of a to- tal of five Flux Transfer Events (FTEs), encountered on the equatorward side of the northern cusp by the Cluster space- craft, with separation of ∼5000 km, are studied by apply- ing the Grad-Shafranov (GS) reconstruction technique to the events. The technique generates a magnetic field/plasma map of the FTE cross section, using combined magnetic field and plasma data from all four spacecraft, under the assump- tion that the structure is two-dimensional (2-D) and time- independent. The reconstructed FTEs consist of one or more magnetic flux ropes embedded in the magnetopause, suggest- ing …

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The Noao Fundamental Plane Survey - Iii. Variations In The Stellar Populations Of Red-Sequence Galaxies From The Cluster Core To The Virial Radius, Russell J. S. Smith, Michael J. Hudson, John R. Lucey, Jenica E. Nelan, Gary A. Wegner Feb 2006

The Noao Fundamental Plane Survey - Iii. Variations In The Stellar Populations Of Red-Sequence Galaxies From The Cluster Core To The Virial Radius, Russell J. S. Smith, Michael J. Hudson, John R. Lucey, Jenica E. Nelan, Gary A. Wegner

Dartmouth Scholarship

We analyse absorption line-strength indices for ~3000 red-sequence galaxies in 94 nearby clusters, to investigate systematic variations of their stellar content with location in the host cluster. The data are drawn from the NOAO Fundamental Plane Survey. Our adopted method is a generalization of that introduced by Nelan et al. to determine the global age-mass and metallicity-mass relations from the same survey. We find strong evidence for a change in galaxy properties, at fixed mass, over a range from the cluster centre to the virial radius, R_200. For example, red-sequence galaxies further out in the clusters have weaker Mgb5177 (at …

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Sudden Gravitational Transition, Robert R. Caldwell, William Komp, Leonard Parker, Daniel A. T. Vanzella Jan 2006

Sudden Gravitational Transition, Robert R. Caldwell, William Komp, Leonard Parker, Daniel A. T. Vanzella

Dartmouth Scholarship

We investigate the properties of a cosmological scenario which undergoes a gravitational phase transition at late times. In this scenario, the Universe evolves according to general relativity in the standard, hot big bang picture until a redshift z≲1. Nonperturbative phenomena associated with a minimally-coupled scalar field catalyzes a transition, whereby an order parameter consisting of curvature quantities such as R2, RabRab, RabcdRabcd acquires a constant expectation value. The ensuing cosmic acceleration appears driven by a dark-energy component with an equation-of-state w<−1. We evaluate the constraints from type 1a supernovae, the cosmic microwave background, and other cosmological observations. We find that a range of models making a sharp transition to cosmic acceleration are consistent with observations.

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Near‐Infrared And Optical Limits For The Central X‐Ray Point Source In The Cassiopeia A Supernova Remnant, R. A. Fesen, G. G. Pavlov, D. Sanwal Jan 2006

Near‐Infrared And Optical Limits For The Central X‐Ray Point Source In The Cassiopeia A Supernova Remnant, R. A. Fesen, G. G. Pavlov, D. Sanwal

Dartmouth Scholarship

We set new near-infrared and optical magnitude limits for the central X-ray point source (XPS) in the Cassiopeia A supernova remnant based on HST images. Near-infrared images of the center of Cas A taken with the NICMOS 2 camera in combination with the F110W and F160W filters (~J and H bands) have magnitude limits ≥26.2 and ≥24.6, respectively. These images reveal no sources within a 12 radius (corresponding to a 99% confidence limit) of the Chandra XPS position. The NICMOS data, taken together with broadband optical magnitude limits (R ~ 28 mag) obtained from a deep STIS CCD …

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A New Color-Magnitude Diagram For Palomar 11, Matthew S. Lewis, W. M. Liu, N. E. Q. Paust, Brian Chaboyer Jan 2006

A New Color-Magnitude Diagram For Palomar 11, Matthew S. Lewis, W. M. Liu, N. E. Q. Paust, Brian Chaboyer

Dartmouth Scholarship

We present new photometry for the Galactic thick-disk globular cluster Palomar 11 extending well past the main-sequence turnoff in the V and I bands. This photometry shows noticeable, but depleted, red giant and subgiant branches. The difference in magnitude between the red horizontal branch (red clump) and the subgiant branch is used to determine that Palomar 11 has an age of 10.4 ± 0.5 Gyr. The red clump is used to derive a distance d⊙ = 14.3 ± 0.4 kpc and a mean cluster reddening of E(V - I) = 0.40 ± 0.03. There is differential …

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Ubvi Ccd Photometry Of The Old Open Cluster Berkeley 17, Zosia A. C. Krusberg, Brian Chaboyer Jan 2006

Ubvi Ccd Photometry Of The Old Open Cluster Berkeley 17, Zosia A. C. Krusberg, Brian Chaboyer

Dartmouth Scholarship

Photometric UBVI CCD photometry is presented for NGC 188 and Berkeley 17. Color-magnitude diagrams (CMDs) are constructed and reach well past the main-sequence turnoff for both clusters. Cluster ages are determined by means of isochrone fitting to the cluster CMDs. These fits are constrained to agree with spectroscopic metallicity and reddening estimates. Cluster ages are determined to be 7.0 ± 0.5 Gyr for NGC 188 and 10.0 ± 1.0 Gyr for Berkeley 17, where the errors refer to uncertainties in the relative age determinations. These ages are compared to the ages of relatively metal-rich inner halo/thick-disk globular clusters and other …

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Discovery Of Outlying High-Velocity Oxygen-Rich Ejecta In Cassiopeia A, Robert A. Fesen, Molly C. Hammell, Jon Morse, Roger A. Chevalier, Kazimierz J. Borkowski, Michael A. Dopita, Christopher L. Gerardy, Stephen S. Lawrence, John C. Raymond, Sydney Van Den Bergh Jan 2006

Discovery Of Outlying High-Velocity Oxygen-Rich Ejecta In Cassiopeia A, Robert A. Fesen, Molly C. Hammell, Jon Morse, Roger A. Chevalier, Kazimierz J. Borkowski, Michael A. Dopita, Christopher L. Gerardy, Stephen S. Lawrence, John C. Raymond, Sydney Van Den Bergh

Dartmouth Scholarship

Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. ACS filte r flux ratios along with follow-up, ground-based spectra are used to investigate some of the kinema tic and chemical properties of these outermost ejecta. In this paper, we concentrate on a ≃ 1.5 sq arcmin region located along the eastern limb of the remnant where numerous outer emission knots are optica lly visible due to an interaction with a local circumstellar …

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Noao Fundamental Plane Survey. Ii. Age And Metallicity Along The Red Sequence From Line‐Strength Data, Jenica E. Nelan, Russell J. Smith, Michael J. Hudson, Gary A. Wegner Oct 2005

Noao Fundamental Plane Survey. Ii. Age And Metallicity Along The Red Sequence From Line‐Strength Data, Jenica E. Nelan, Russell J. Smith, Michael J. Hudson, Gary A. Wegner

Dartmouth Scholarship

We present spectroscopic line-strength data for 4097 red-sequence galaxies in 93 low-redshift galaxy clusters and use these to investigate variations in average stellar populations as a function of galaxy mass. Our analysis includes an improved treatment of nebular emission contamination, which affects ~10% of the sample galaxies. Using the stellar population models of D. Thomas and collaborators, we simultaneously fit 12 observed line-strength-σ relations in terms of common underlying trends of age, [Z/H] (total metallicity), and [α/Fe] (α-element enhancement). We find that the observed line-strength-σ relations can be explained only if higher mass red-sequence galaxies are, on average, …

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Limits Of Quintessence, R. R. Caldwell, Eric V. Linder Sep 2005

Limits Of Quintessence, R. R. Caldwell, Eric V. Linder

Dartmouth Scholarship

We present evidence that the simplest particle-physics scalar-field models of dynamical dark energy can be separated into distinct behaviors based on the acceleration or deceleration of the field as it evolves down its potential towards a zero minimum. We show that these models occupy narrow regions in the phase plane of w and w′, the dark energy equation of state and its time derivative in units of the Hubble time. Restricting an energy scale of the dark energy microphysics limits how closely a scalar field can resemble a cosmological constant. These results, indicating a desired measurement resolution of order σ(w′)≈(1+w), …

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Hs 2331+3905: The Cataclysmic Variable That Has It All, S. Araujo-Betancor, B. T. Gänsicke, H.-J. Hagen, T. R. Marsh, E T. Harlaftis, J Thorstensen Sep 2005

Hs 2331+3905: The Cataclysmic Variable That Has It All, S. Araujo-Betancor, B. T. Gänsicke, H.-J. Hagen, T. R. Marsh, E T. Harlaftis, J Thorstensen

Dartmouth Scholarship

We report detailed follow-up observations of the cataclysmic variable HS 2331+3905, identified as an emission- line object in the Hamburg Quasar Survey. An orbital period of 81.08 min is unambiguously determined from the detection of eclipses in the light curves of HS 2331+3905. A second photometric period is consistently detected at P ≃ 83.38 min, ∼2.8% longer than Porb, which we tentatively relate to the presence of permanent superhumps. High time resolution photometry exhibits short-timescale variability on time scales of ≃5−6 min which we interpret as non-radial white dwarf pulsations, as well as a coherent signal at 1.12 min, which …

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Dark-Energy Evolution Across The Cosmological-Constant Boundary, Robert R. Caldwell, Michael Doran Aug 2005

Dark-Energy Evolution Across The Cosmological-Constant Boundary, Robert R. Caldwell, Michael Doran

Dartmouth Scholarship

We explore the properties of dark-energy models for which the equation of state, w, defined as the ratio of pressure to energy density, crosses the cosmological-constant boundary w=−1. We adopt an empirical approach, treating the dark energy as an uncoupled fluid or a generalized scalar field. We describe the requirements for a viable model, in terms of the equation of state and sound speed. A generalized scalar field cannot safely traverse w=−1, although a pair of scalars with w>−1 and w<−1 will work. A fluid description with a well-defined sound speed can also cross the boundary. Contrary to expectations, such a crossing model does not instantaneously resemble a cosmological constant at the moment w=−1 since the density and pressure perturbations do not necessarily vanish. But because a dark energy with w<−1 dominates only at very late times, and because the dark energy is not generally prone to gravitational clustering, then crossing the cosmological-constant boundary leaves no distinct imprint.

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Is First J102347.6+003841 Really A Cataclysmic Binary?, John R. Thorstensen, Eve Armstrong Aug 2005

Is First J102347.6+003841 Really A Cataclysmic Binary?, John R. Thorstensen, Eve Armstrong

Dartmouth Scholarship

The radio source FIRST J102347.6+003841 was presented as the first radio-selected cataclysmic variable star. In the discovery paper, Bond et al. (2002) show a spectrum consistent with a magnetic AM Her–type system, or polar, featuring strong Balmer lines, He I and He II emission lines, and a light curve with rapid, irregular flickering. In contrast, Woudt, Warner, and Pretorius found a smoothly varying light curve with a period near 4.75 hr and one minimum per orbit, indicating that the state of the system had changed dramatically. We present time-resolved spectra showing a superficially normal, mid-G type photosphere, with no detectable …

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Dynamo Regimes With A Nonhelical Forcing, Pablo D. Mininni, Yannick Ponty, David C. Montgomery, Jean-Francois Pinton Jun 2005

Dynamo Regimes With A Nonhelical Forcing, Pablo D. Mininni, Yannick Ponty, David C. Montgomery, Jean-Francois Pinton

Dartmouth Scholarship

A three-dimensional numerical computation of magnetohydrodynamic dynamo behavior is described. The dynamo is mechanically forced with a driving term of the Taylor-Green type. The magnetic field development is followed from negligibly small levels to saturated values that occur at magnetic energies comparable to the kinetic energies. Although there is locally a nonzero helicity density, there is no overall integrated helicity in the system. Persistent oscillations are observed in the saturated state for not-too-large mechanical Reynolds numbers, oscillations in which the kinetic and magnetic energies vary out of phase but with no reversal of the magnetic field. The flow pattern exhibits …

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Late‐Time X‐Ray, Uv, And Optical Monitoring Of Supernova 1979c, Stefan Immler, Robert A. Fesen, Schuyler D. Van Dyk, Kurt W. Weiler, Robert Petre, Walter H.G Lewin, David Pooley, Wolfgang Pietsch, Bernd Aschenbach, Molly C. Hammell, Gwen C. Rudie Jun 2005

Late‐Time X‐Ray, Uv, And Optical Monitoring Of Supernova 1979c, Stefan Immler, Robert A. Fesen, Schuyler D. Van Dyk, Kurt W. Weiler, Robert Petre, Walter H.G Lewin, David Pooley, Wolfgang Pietsch, Bernd Aschenbach, Molly C. Hammell, Gwen C. Rudie

Dartmouth Scholarship

We present results from observations of supernova (SN) 1979C with the Newton X-Ray Multi-Mirror (XMM-Newton) mission in X-rays and in UV, archival X-ray, and Hubble Space Telescope (HST) data, and follow-up ground-based optical imaging. The XMM-Newton MOS spectrum shows the best-fit two-temperature thermal plasma emission characteristics of both the forward (kThigh = 4.1 keV) and reverse shock (kTlow = 0.79 keV) with no intrinsic absorption. The long-term X-ray light curve, constructed from all X-ray data available, reveals that SN 1979C is still radiating at a flux level similar to that detected by …

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Probing Multiple Sight Lines Through The Sn 1006 Remnant By Ultraviolet Absorption Spectroscopy, P. Frank Winkler, Knox S. Long, Andrew S. Hamilton, Robert A. Fesen May 2005

Probing Multiple Sight Lines Through The Sn 1006 Remnant By Ultraviolet Absorption Spectroscopy, P. Frank Winkler, Knox S. Long, Andrew S. Hamilton, Robert A. Fesen

Dartmouth Scholarship

Absorption-line spectroscopy is an effective probe for cold ejecta within a supernova remnant (SNR), provided that suitable background UV sources can be identified. For the SN 1006 remnant we have identified four such sources, in addition to the much-studied Schweitzer-Middleditch (SM) star. We have used STIS on the Hubble Space Telescope to obtain UV spectra of all four sources, to study "core samples" of the SN 1006 interior. The line of sight closest to the center of the SNR shell, passing only 20 away, is to a V = 19.5 QSO at z = 1.026. Its spectrum shows broad Fe …

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