arXiv subjects

L. B. Lucy

Publications and source records attributed to L. B. Lucy.

At least 19 recordsLinked to original sources

arXiv2018

Projected distances to host galaxy reduce SNIa dispersion

We use multi-band imagery data from the Sloan Digital Sky Survey (SDSS) to measure projected distances of 302 supernova type Ia (SNIa) from the centre of their host galaxies, normalized to the galaxy's brightness scale length, with a Bayesian approach. We test the hypothesis that SNIas further away from the centre of their host galaxy are less subject to dust contamination (as the dust column density in their environment is smaller) and/or come from a more homogeneous environment. Using the Mann-Whitney U test, we find a statistically significant difference in the observed colour correction distribution between SNIas that are near and those that are far from the centre of their host. The local p-value is 3 x 10^{-3}, which is significant at the 5 per cent level after look-elsewhere effect correction. We estimate the residual scatter of the two subgroups to be 0.073 +/- 0.018 for the far SNIas, compared to 0.114 +/- 0.009 for the near SNIas -- an improvement of 30 per cent, albeit with a low statistical significance of 2sigma. This confirms the importance of host galaxy properties in correctly interpreting SNIa observations for cosmological inference.

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arXiv2017

Binary orbits from combined astrometric and spectroscopic data

An efficient Bayesian technique for estimation problems in fundamental stellar astronomy is tested on simulated data for a binary observed both astrometrically and spectroscopically. Posterior distributions are computed for the components' masses and for the binary's parallax. One thousand independent repetitions of the simulation demonstrate that the 1- and 2-$\!σ$ credibility intervals for these fundamental quantities have close to the correct coverage fractions. In addition, the simulations allow the investigation of the statistical properties of a Bayesian goodness-of-fit criterion and of the corresponding p-value. The criterion has closely similar properties to the traditional chi^{2} test for minimum-chi^{2} solutions.

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arXiv2017

Irradiated stars with convective envelopes

The observed radii of M dwarfs in eclipsing binaries exceed predicted radii by ~ 5%. To investigate this anomaly, the structure of low-mass stars irradiated by a close companion is considered. Irradiation modifies the surface boundary conditions and thereby also the adiabatic constants of their outer convection zones. This changes the models' radii and luminosities. For short-period M dwarf binaries with components of similar mass, radius inflation due to their mutual irradiation is found to be < 0.4%. This is an order of magnitude too small to explain the observed anomaly. Although stronger irradiation results in a monotonically increasing radius, a saturation effect limits the inflation to <~ 5\%.

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arXiv2016

Frequentist tests for Bayesian models

Analogues of the frequentist chi-square and F tests are proposed for testing goodness-of-fit and consistency for Bayesian models. Simple examples exhibit these tests' detection of inconsistency between consecutive experiments with identical parameters, when the first experiment provides the prior for the second. In a related analysis, a quantitative measure is derived for judging the degree of tension between two different experiments with partially overlapping parameter vectors.

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arXiv2015

Mass fluxes for O-type supergiants with metallicity Z = Z_{\sun}/5

A code used previously to predict O-star mass fluxes as a function of metallicity is used to compute a grid of models with the metallicity of the Small Magellanic Cloud (SMC). These models allow mass-loss rates to be derived by interpolation for all O-type supergiants in the SMC, with the possible exception of extremely massive stars close to the Eddington limit.

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arXiv2014

Analysing weak orbital signals in Gaia data

Anomalous orbits are found when minimum-chi^{2} estimation is applied to synthetic Gaia data for orbits with astrometric signatures comparable to the single-scan measurement error (Pourbaix 2002). These orbits are nearly parabolic, edge-on, and their major axes align with the line-of-sight to the observer. Such orbits violate the Copernican principle (CPr) and as such could be rejected. However, the preferred alternative is to develop a statistical technique that incorporates the CPr as a fundamental postulate. This can be achieved in a Bayesian context by defining a Copernican prior. Pourbaix's anomalous orbits then no longer arise. Instead, the selected orbits have a somewhat higher chi^{2} but do not violate the CPr. The problem of detecting a weak additional orbit in an astrometric binary with a well-determined orbit is also treated.

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arXiv2014

Frequentist confidence intervals for orbits

The problem of efficiently computing the orbital elements of a visual binary while still deriving confidence intervals with frequentist properties is treated. When formulated in terms of the Thiele-Innes elements, the known distribution of probability in Thiele-Innes space allows efficient grid-search plus Monte-Carlo-sampling schemes to be constructed for both the minimum-$\!χ^{2}$ and Bayesian approaches to parameter estimation. Numerical experiments with $10^{4}$ independent realizations of an observed orbit confirm that the $1-$ and $2σ$ confidence and credibility intervals have coverage fractions close to their frequentist values. \keywords{binaries: visual - stars: fundamental parameters - methods:statistical}

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arXiv2014

Mass estimates for visual binaries with incomplete orbits

The problem of estimating the total mass of a visual binary when its orbit is incomplete is treated with Bayesian methods. The posterior mean of a mass estimator is approximated by a triple integral over orbital period, time of periastron and orbital eccentricity. This reduction to 3-D from the 7-D space defined by the conventional Campbell parameters is achieved by adopting the Thiele-Innes elements and exploiting the linearity with respect to the four Thiele-Innes constants. The formalism is tested on synthetic observational data covering a variable fraction of a model binary's orbit. The posterior mean of the mass estimator is numerically found to be unbiased when the data cover > 40% of the orbit.

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arXiv2012

Bayesian inference for orbital eccentricities

Highest posterior density intervals (HPDI's) are derived for the true eccentricities of spectroscopic binaries with measured values e ~ 0. These yield upper limits when e is below the detection threshold e_th and seamlessly transform to upper and lower bounds when e > e_th. In the main text, HPDI's are computed with an informative eccentricity prior representing orbital decay due to tidal dissipation. In an appendix, the corresponding HPDI's are computed with a uniform prior and are the basis for a revised version of the Lucy-Sweeney test, with the previous outcome e = 0 now replaced by an upper limit. Sampling experiments with known prior confirm the validity of the HPDI's.

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arXiv2012

O-star mass-loss rates at low metallicity

Mass fluxes J are computed for the extragalactic O stars investigated by Tramper et al. (2011; TSKK). For one early-type O star, computed and observed rates agree within errors. However, for two late-type O stars, theoretical mass-loss rates underpredict observed rates by ~ 1.6 dex, far exceeding observational errors. A likely cause of the discrepancy is overestimated observed rates due to the neglect of wind-clumping. A less likely but intriguing possibility is that, in observing O stars with Z/Z_sun ~ 1/7, TSKK have serendipitously discovered an additional mass-loss mechanism not evident in the spectra of Galactic O stars with powerful radiation-driven winds. Constraints on this unknown mechanism are discussed. In establishing that the discrepancies, if real, are inescapable for purely radiation-driven winds, failed searches for high-J solutions are reported and the importance of a numerical technique that cannot spuriously create or destroy momentum stressed. The Z-dependences of the computed rates for Z/Z_sun in the interval (1/30, 2) show significant departures from a single power law, and these are attributed to curve-of-growth effects in the differentially-expanding reversing layers. The best-fitting power-law exponents range from 0.68-0.97.

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arXiv2012

Coronal winds powered by radiative driving

A two-component phenomenological model developed originally for zeta Puppis is revised in order to model the outflows of late-type O dwarfs that exhibit the weak-wind phenomenon. With the theory's standard parameters for a generic weak-wind star, the ambient gas is heated to coronal temperatures ~ 3 x 10^{6}K at radii > 1.4 R, with cool radiativly-driven gas being then confined to dense clumps with filling factor ~ 0.02. Radiative driving ceases at radius ~ 2.1R when the clumps are finally destroyed by heat conduction from the coronal gas. Thereafter, the outflow is a pure coronal wind, which cools and decelerates reaching infinity with terminal velocity ~ 980$ km/ s.

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arXiv2010

An extensive grid of mass fluxes for Galactic O stars

A previously-described code for constructing moving reversing layers (MRL) is improved by replacing a two-parameter model for the radiative acceleration due to lines with a flexible non-parametric description, thus allowing a greater degree of dynamical consistency to be achieved in modelling turbulent transonic flow in the outer atmospheric layers of O stars. With this new code, mass fluxes J are computed at fifty-seven points in (T_eff, g)-space. Specifically, J's are computed for all Lanz-Hubeny (2003) NLTE atmospheres with T_eff (kK) \in (27.5, 55) and log g \leq 4.5. Differences with widely-used mass-loss formulae are emphasized, and opportunities for differential spectroscopic tests identified.

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arXiv2010

Mass fluxes for hot stars

In an attempt to understand the extraordinarily small mass-loss rates of late-type O dwarfs, mass fluxes in the relevant part of (T_{eff}, g)-space are derived from first principles using a previously-described code for constructing moving reversing layers. From these mass fluxes, a weak-wind domain is identified within which a star's rate of mass loss by a radiatively-driven wind is less than that due to nuclear burning. The five weak-wind stars recently analysed by Marcolino et al. (2009) fall within or at the edge of this domain. But although the theoretical mass fluxes for these stars are ~ 1.4 dex lower than those derived with the formula of Vink et al. (2000), the observed rates are still not matched, a failure that may reflect our poor understanding of low-density supersonic outflows. Mass fluxes are also computed for two strong-wind O4 stars analysed by Bouret et al. (2005). The predictions agree with the sharply reduced mass loss rates found when Bouret et al. take wind clumping into account.

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arXiv2007

The structure of line-driven winds

Following procedures pioneered by Castor, Abbott & Klein (1975, [CAK]), spherically-symmetric supersonic winds for O stars are computed for matching to plane-parallel moving reversing layers (RL's) from Paper I (Lucy 2007). In contrast to a CAK wind, each of these solutions is singularity-free, thus allowing its mass-loss rate to be fixed by the regularity condition at the sonic point within the RL. Moreover, information propagation in these winds by radiative-acoustic waves is everywhere outwardly-directed, justifying the implicit assumption in Paper I that transonic flows are unaffected by inwardly-directed wave motions.

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arXiv2007

Mass fluxes for O stars

The theory of moving reversing layers for hot stars is updated to include an extensive line list, a radiative boundary condition from static model atmospheres, line transfer by scattering, and continuation to supersonic velocities. A Monte Carlo technique determines the theory's eigenvalue J, the mass flux, and the derived J's are in good agreement with the wind models of Pauldrach et al. (2001). The solutions' sensitivity to the photospheric microturbulent velocity reveals that this parameter has a throttling effect on J: turbulent line-broadening in the quasi-static layers reduces the radiation force available to accelerate matter through the sonic point. If photospheric turbulence approaches sonic velocities, this mechanism reduces mass loss rates by factors > 3, which would partly account for the reduced rates found observationally for clumpy winds.

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arXiv2006

Spectroscopic binaries with components of similar mass

The assertion that there is an intrinsic excess of binaries with mass ratios q \simeq 1 - the twin hypothesis - is investigated. A strong version of this hypothesis (H_s), due to Lucy & Ricco (1979) and Tokovinin (2000), refers to a narrow peak in the distribution function psi(q) for q \ga 0.95. A weak version (H_w), due to Halbwachs et al. (2003), refers to a broad peak for q \ga 0.8. Current data on SB2's is analysed and H_s is found to be statistically significant for a sample restricted to orbits of high precision. But claims that H_s is significant for binaries with special characteristics are not confirmed since the sample sizes are well below the minimum required for a reliable test. With regard to H_w, additional observational evidence is not presented, but evidence to the contrary in the form of Hogeveen's (1992b) model of biased sampling with psi \propto q^{-2} is criticized. Specifically, his success in thus fitting catalogued data depends on implausible assumptions about the research methodologies of binary-star spectroscopists.

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arXiv2005

Spectroscopic binaries with elliptical orbits

The radial velocity curves of many spectroscopic binaries (SBs) are perturbed by gas streams or proximity effects. For SBs with circular orbits, these perturbations can give rise to spurious orbital eccentricities of high statistical significance. But tests to identify such anomalous orbits can be constructed since perturbed velocity curves are in general no longer Keplerian. The derived tests are applied both to synthetic and to observed velocity curves.

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arXiv2004

Monte Carlo techniques for time-dependent radiative transfer in 3-D supernovae

Monte Carlo techniques based on indivisible energy packets are described for computing light curves and spectra for 3-D supernovae. The radiative transfer is time-dependent and includes all effects of O(v/c). Monte Carlo quantization is achieved by discretizing the initial distribution of 56Ni into radioactive pellets. Each pellet decays with the emission of a single energy packet comprising gamma-ray photons representing one line from either the 56Ni or the 56Co decay spectrum. Subsequently, these energy packets propagate through the homologously-expanding ejecta with appropriate changes in the nature of their contained energy as they undergo Compton scatterings and pure absorptions. The 3-D code is tested by applying it to a spherically-symmetric SN in which the transfer of optical radiation is treated with a grey absorption coefficient. This 1-D problem is separately solved using Castor's co-moving frame moment equations. Satisfactory agreement is obtained. The Monte Carlo code is a platform onto which more advanced treatments of the interactions of matter and radiation can be added. Some of these have already been developed and tested in previous papers and are summarized here.

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