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Hatch M.I.,Penn State Worthington Scranton | Smith R.J.,University of Scranton
Journal of Field Ornithology | Year: 2010

Hematocrits may provide information about the physiological condition of birds, but, to be a useful measure, information is needed concerning how hematocrits vary among individuals and over time. We examined the repeatability of hematocrits in a population of Gray Catbirds (Dumetella carolinensis) in Pennsylvania at several time scales and also examined the repeatability of body mass, another measure commonly used as an indicator of condition. Both hematocrit (r= 0.64) and mass (r= 0.65) were repeatable (P < 0.01) for first captures between years and between first and second captures within a year (r= 0.41 and r= 0.50, respectively; P < 0.01), but not repeatable (P > 0.05) between captures in different months within a year (r= 0.11 for both). Repeatability of both measures differed by sex and age. Females exhibited repeatability of hematocrit and body mass only between years, while male hematocrits were repeatable between years and between first and second captures within a season. Male mass was repeatable for all time periods. Hematocrits of younger birds were repeatable between captures within a season and their body mass was repeatable between months and weeks while hematocrits of older birds were not repeatable and their body mass was repeatable only between captures in a season. Our results indicate that hematocrits and body mass had similar repeatability coefficients overall, but that hematocrits of Gray Catbirds were a consistent trait of individuals only across years. Because repeatability between captures and months depended on sex and age, we conclude that the hematocrit is a useful measure of individual performance only in limited circumstances. ©2010 The Author(s). Journal compilation ©2010 Association of Field Ornithologists. Source

Ud-Doula A.,Penn State Worthington Scranton | Owocki S.,University of Delaware | Townsend R.,University of Wisconsin - Madison | Petit V.,University of Delaware | Cohen D.,Swarthmore College
Monthly Notices of the Royal Astronomical Society | Year: 2014

We use 2D magnetohydrodynamic (MHD) simulations to examine the effects of radiative cooling and inverse Compton (IC) cooling on X-ray emission from magnetically confined wind shocks (MCWS) in magnetic massive stars with radiatively driven stellar winds. For the standard dependence of mass-loss rate on luminosity Ṁ ~ L1.7, the scaling of IC cooling with L and radiative cooling with Ṁ means that IC cooling become formally more important for lower luminosity stars. However, because the sense of the trends is similar, we find the overall effect of including IC cooling is quite modest. More significantly, for stars with high enough mass-loss to keep the shocks radiative, the MHD simulations indicate a linear scaling of X-ray luminosity with mass-loss rate; but for lower luminosity stars with weak winds, X-ray emission is reduced and softened by a shock retreat resulting from the larger post-shock cooling length, which within the fixed length of a closed magnetic loop forces the shock back to lower pre-shock wind speeds. A semi-analytic scaling analysis that accounts both for the wind magnetic confinement and this shock retreat yields X-ray luminosities that have a similar scaling trend, but a factor few higher values, compared to time-averages computed from the MHD simulations. The simulation and scaling results here thus provide a good basis for interpreting available X-ray observations from the growing list of massive stars with confirmed large-scale magnetic fields. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Kee N.D.,University of Delaware | Owocki S.,University of Delaware | Ud-Doula A.,Penn State Worthington Scranton
Monthly Notices of the Royal Astronomical Society | Year: 2014

We examine X-rays from radiatively cooled shocks, focusing on how their thin-shell instability reduces X-ray emission. For 2D simulations of collision between equal expanding winds, we carry out a parameter study of such instability as a function of the ratio of radiative versus adiabatic-expansion cooling lengths. In the adiabatic regime, the extended cooling layer suppresses instability, leading to planar shock compression with X-ray luminosity that follows closely the expected (LX ~ M2) quadratic scaling with mass-loss rate M. In the strongly radiative limit, the X-ray emission now follows an expected linear scaling with mass-loss (LX ~ M), but the instability deforms the shock compression into extended shear layers with oblique shocks along fingers of cooled, dense material. The spatial dispersion of shock thermalization limits strong X-ray emission to the tips and troughs of the fingers, and so reduces the X-ray emission (here by about a factor 1/50) below what is expected from analytic radiative-shock models without unstable structure. Between these two limits, X-ray emission can switch between a high-state associated with extended shock compression, and a low-state characterized by extensive shear. Further study is needed to clarify the origin of this 'shear mixing reduction factor' in X-ray emission, and its dependence on parameters like the shock Mach number. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. Source

Sundqvist J.O.,University of Delaware | ud-Doula A.,Penn State Worthington Scranton | Owocki S.P.,University of Delaware | Townsend R.H.D.,University of Wisconsin - Madison | And 2 more authors.
Monthly Notices of the Royal Astronomical Society: Letters | Year: 2012

The magnetic O star HD191612 exhibits strongly variable, cyclic Balmer line emission on a 538-d period. We show here that its variable Hα emission can be well reproduced by the rotational phase variation of synthetic spectra computed directly from full radiation magnetohydrodynamical simulations of a magnetically confined wind. In slow rotators such as HD191612, wind material on closed magnetic field loops falls back to the star, but the transient suspension of material within the loops leads to a statistically overdense, low-velocity region around the magnetic equator, causing the spectral variations. We contrast such 'dynamical magnetospheres' (DMs) with the more steady-state 'centrifugal magnetospheres' of stars with rapid rotation, and discuss the prospects of using this DM paradigm to explain periodic line emission from also other non-rapidly rotating magnetic massive stars. © 2012 The Authors. Monthly Notices of the Royal Astronomical Society. © 2012 RAS. Source

Ud-Doula A.,Penn State Worthington Scranton | Sundqvist J.O.,University of Delaware | Owocki S.P.,University of Delaware | Petit V.,West Chester University | Townsend R.H.D.,University of Wisconsin - Madison
Monthly Notices of the Royal Astronomical Society | Year: 2013

We present the first fully 3D magnetohydrodynamic (MHD) simulation for magnetic channelling and confinement of a radiatively driven, massive-star wind. The specific parameters are chosen to represent the prototypical slowly rotating magnetic O star Θ1 Ori C, for which centrifugal and other dynamical effects of rotation are negligible. The computed global structure in latitude and radius resembles that found in previous 2D simulations, with unimpeded outflow along open field lines near the magnetic poles, and a complex equatorial belt of inner wind trapping by closed loops near the stellar surface, giving way to outflow above the Alfv́en radius. In contrast to this previous 2D work, the 3D simulation described here now also shows how this complex structure fragments in azimuth, forming distinct clumps of closed loop infall within the Alfv́en radius, transitioning in the outer wind to radial spokes of enhanced density with characteristic azimuthal separation of 15®-20®. Applying these results in a 3D code for line radiative transfer, we show that emission from the associated 3D 'dynamical magnetosphere' matches well the observed Hα emission seen from Θ1 Ori C, fitting both its dynamic spectrum over rotational phase and the observed level of cycle-to-cycle stochastic variation. Comparison with previously developed 2D models for the Balmer emission from a dynamical magnetosphere generally confirms that time averaging over 2D snapshots can be a good proxy for the spatial averaging over 3D azimuthal wind structure. Nevertheless, fully 3D simulations will still be needed to model the emission from magnetospheres with non-dipole field components, such as suggested by asymmetric features seen in the Hα equivalent-width curve of Θ1 Ori C. © 2012 The Authors. Source

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