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A Gravitational Redshift Determination Of The Mean Mass Of White Dwarfs: DBA And DB Stars
Get PDFWe measure apparent velocities (nu(app)) of absorption lines for 36 white dwarfs (WDs) with helium-dominated atmospheres-16 dbAs and 20 dbs-using optical spectra taken for the European Southern Observatory SN Ia progenitor survey. We find a difference of 6.9 +/- 6.9 kms(-1) in the average apparent velocity of the H alpha lines versus that of the He I 5876 angstrom lines for our dbAs. This is a measure of the blueshift of this He line due to pressure effects. By using this as a correction, we extend the gravitational redshift method employed by Falcon et al. to use the apparent velocity of the He I 5876 angstrom line and conduct the first gravitational redshift investigation of a group of WDs without visible hydrogen lines. We use biweight estimators to find an average apparent velocity, <nu(app)>(BI), (and hence average gravitational redshift, <nu(g)>(BI)) for our WDs; from that we derive an average mass, < M >(BI). For the dbAs, we find <nu(app)>(BI) = 40.8 +/- 4.7 kms(-1) and derive < M >(BI) = 0.71(-0.05)(+0.04) M-circle dot. Though different from <nu(app)> of DAs (32.57 km s(-1)) at the 91% confidence level and suggestive of a larger dbA mean mass than that for normal DAs derived using the same method (0.647(-0.014)(+0.013) M-circle dot; Falcon et al.), we do not claim this as a stringent detection. Rather, we emphasize that the difference between <nu(app)>(BI) of the dbAs and <nu(app)> of normal DAs is no larger than 9.2 kms(-1), at the 95% confidence level; this corresponds to roughly 0.10 M-circle dot. For the dbs, we find <nu(He)(app)>(BI) = 42.9 +/- 8.49 km s(-1) after applying the blueshift correction and determine < M >(BI) = 0.74(-0.09)(+0.08) M-circle dot. The difference between <nu(He)(app)>(BI) of the dbs and <nu(app)> of DAs is <= 11.5 kms(-1) (similar to 0.12 M-circle dot), at the 95% confidence level. The gravitational redshift method indicates much larger mean masses than the spectroscopic determinations of the same sample by Voss et al. Given the small sample sizes, it is possible that systematic uncertainties are skewing our results due to the potential of kinematic substructures that may not average out. We estimate this to be unlikely, but a larger sample size is necessary to rule out these systematics.National Science Foundation AST-0909107, AST-0602288Norman Hackerman Advanced Research Program 003658-255-2007, 003658-0252-2009Institute for High Energy Density ScienceUniversity of Texas SystemSandia National LaboratoriesNational Physical Science ConsortiumDelaware Asteroseismic Research CenterNASA NNX11AG82G, HST-GO-11141McDonald Observator
Hubble Space Telescope and ground-based observations of V455 Andromedae post-outburst
Get PDFHubble Space Telescope spectra obtained in 2010 and 2011, 3 and 4 yr after the large amplitude dwarf nova outburst of V455 And, were combined with optical photometry and spectra to study the cooling of the white dwarf, its spin, and possible pulsation periods after the outburst. The modeling of the ultraviolet (UV) spectra shows that the white dwarf temperature remains ~600 K hotter than its quiescent value at 3 yr post-outburst, and still a few hundred degrees hotter at 4 yr post-outburst. The white dwarf spin at 67.6 s and its second harmonic at 33.8 s are visible in the optical within a month of outburst and are obvious in the later UV observations in the shortest wavelength continuum and the UV emission lines, indicating an origin in high-temperature regions near the accretion curtains. The UV light curves folded on the spin period show a double-humped modulation consistent with two-pole accretion. The optical photometry 2 yr after outburst shows a group of frequencies present at shorter periods (250-263 s) than the periods ascribed to pulsation at quiescence, and these gradually shift toward the quiescent frequencies (300-360 s) as time progresses past outburst. The most surprising result is that the frequencies near this period in the UV data are only prominent in the emission lines, not the UV continuum, implying an origin away from the white dwarf photosphere. Thus, the connection of this group of periods with non-radial pulsations of the white dwarf remains elusive
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A Gravitational Redshift Determination Of The Mean Mass Of DBA White Dwarfs
Get PDFWe measure apparent velocities (v(app)) of the H alpha and H beta Balmer line cores for 16 helium-dominated white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Following the gravitational redshift method employed by Falcon et al. [1], we find a mean apparent velocity of (v(app)) = 39.58 +/- 4.41 km s(-1) and use it to derive a mean mass of < M > = 0.701(-0.046)(+0.042) M(circle dot). Though the sample is small, the mean mass appears to be larger than the mean mass of DAs derived using the same method [0.647(-0.014)(+0.013) M(circle dot), 1]Astronom
A gravitational redshift determination of the mean mass of white dwarfs. DA stars.
Get PDFtextWe measure apparent velocities (v_app) of the H alpha and H beta Balmer line cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical spectra taken for the European Southern Observatory SN Ia progenitor survey (SPY). Assuming these WDs are nearby and comoving, we correct our velocities to the local standard of rest so that the remaining stellar motions are random. By averaging over the sample, we are left with the mean gravitational redshift, <v_g>: we find <v_g> = <v_app> = 32.57+/-1.17 km/s. Using the mass-radius relation from evolutionary models, this translates to a mean mass of 0.647+0.013-0.014 Msun. We interpret this as the mean mass for all DAs. Our results are in agreement with previous gravitational redshift studies but are significantly higher than all previous spectroscopic determinations except the recent findings of Tremblay & Bergeron. Since the gravitational redshift method is independent of surface gravity from atmosphere models, we investigate the mean mass of DAs with spectroscopic Teff both above and below 12,000 K; fits to line profiles give a rapid increase in the mean mass with decreasing Teff. Our results are consistent with no significant change in mean mass: <M>^hot = 0.640+/-0.014 Msun and <M>^cool = 0.686+0.035-0.039 Msun.Astronom
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Laboratory Measurements Of White Dwarf Photospheric Spectral Lines: H Beta
Get PDFWe spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. Here we present time-resolved measurements of H beta and fit this line using different theoretical line profiles to diagnose electron density, n(e), and n = 2 level population, n2. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n(e) similar to 4 to similar to 30 x 10(16) cm(-3) throughout a 120-ns evolution of our plasma. Also, we observe n(2) to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within similar to 55 ns to become consistent with LTE. This supports our electrontemperature determination of T-e similar to 1.3 eV (similar to 15,000 K) after this time. At n(e) greater than or similar to 10(17) cm(-3), we find that computer-simulation-based line-profile calculations provide better fits (lower reduced chi(2)) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.Laboratory Directed Research and Development programUnited States Department of Energy DE-AC04-94AL85000, DE-SC0010623National Science Foundation DGE-1110007Astronom
Measuring the evolutionary rate of cooling of ZZ Ceti
Get PDFWe have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 ± 1.4) × 10–15 s s–1 employing the O – C method and (5.45 ± 0.79) × 10–15 s s–1 using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 ± 1.0) × 10–15 s s–1. After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 ± 1.1) × 10–15 s s–1. This value is consistent within uncertainties with the measurement of (4.19 ± 0.73) × 10–15 s s–1 for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.Fil: Mukadam, Anjum S. . University of Washington; Estados UnidosFil: Bischoff Kim, Agnes . Georgia State University; Estados UnidosFil: Fraser, Oliver. University of Washington; Estados UnidosFil: Corsico, Alejandro Hugo. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico La Plata. Instituto de Astrofísica de La Plata; ArgentinaFil: Montgomery, M. H. . University of Texas at Austin; Estados UnidosFil: Kepler, S. O. . Universidade Federal do Rio Grande do Sul; BrasilFil: Romero, A. D. . Universidade Federal do Rio Grande do Sul; BrasilFil: Winget, D. E. . University of Texas at Austin; Estados UnidosFil: Hermes, J. J. . University of Texas at Austin; Estados UnidosFil: Riecken, T. S. . Washington State University; Estados UnidosFil: Kronberg, M. E. . University of Washington; Estados UnidosFil: Winget, K. I. . University of Texas at Austin; Estados UnidosFil: Falcon, Ross E. . University of Texas at Austin; Estados UnidosFil: Chandler, D. W. . Central Texas Astronomical Society. Meyer Observatory; Estados UnidosFil: Kuehne, J. W. . McDonald Observatory; Estados UnidosFil: Sullivan, D. J. . Victoria University of Wellington; Nueva ZelandaFil: Reaves, D. . University of Texas at Austin; Estados UnidosFil: von Hippel, T. . Embry-Riddle Aeronautical University; Estados UnidosFil: Mullally, F. . National Aeronautics And Space Administration; Estados UnidosFil: Shipman,H. . Delaware Asteroseismic Research Center; Estados UnidosFil: Thompson, S. E. . National Aeronautics And Space Administration; Estados UnidosFil: Silvestri, N. M. . University of Washington; Estados UnidosFil: Hynes, R. I. . State University Of Louisiana; Estados Unido
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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