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Dive into the research topics where Robert F. Stellingwerf is active.

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Featured researches published by Robert F. Stellingwerf.


Astrophysical Journal Supplement Series | 1999

Classical Cepheid Pulsation Models. I. Physical Structure

G. Bono; M. Marconi; Robert F. Stellingwerf

The pulsation properties and modal stability of Cepheid models are investigated in both linear and nonlinear regimes. The linear survey is based on nonadiabatic, radiative models, whereas the nonlinear one relies on full-amplitude models that include a nonlocal and time-dependent treatment of stellar convection. To account for Cepheid pulsation characteristics over a substantial portion of the region in which they are expected to be pulsationally unstable, a wide range of stellar masses (5?M/M


The Astrophysical Journal | 1997

Second Overtone Pulsators among δ Scuti Stars

G. Bono; F. Caputo; Santi Cassisi; V. Castellani; M. Marconi; Robert F. Stellingwerf

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The Astrophysical Journal | 2003

CLASSICAL CEPHEID PULSATION MODELS. IX. NEW INPUT PHYSICS

Silvia Petroni; G. Bono; M. Marconi; Robert F. Stellingwerf

-->?11) and effective temperatures (4000?T -->e?7000 K) was adopted. The luminosity of each model was fixed according to the mass-luminosity (ML) relations predicted by evolutionary models that either neglect or take into account a mild convective core overshooting. Moreover, in order to estimate the effects of the helium and metal content on the limiting amplitude behavior of both Magellanic Clouds and Galactic Cepheids we adopted three different chemical compositions, namely, Y=0.25, Z=0.004; Y=0.25, Z=0.008; and Y=0.28, Z=0.02. For each set of input parameters we investigated the modal stability of both fundamental and first-overtone modes. The results of recent linear investigations are confirmed by our finding that linear observables such as periods and blue edges of the instability strip are only marginally affected by the chemical composition and that either an increase in metallicity or an increase in both the helium and metal content causes a mild shift of these edges toward lower effective temperatures. The approach to the nonlinear limit cycle stability, the physical structure, and the mechanisms that govern the pulsation instability are described in detail. The main results of this analysis are as follows: (1) At fixed chemical composition the width of the instability strip changes going from low- to high-mass Cepheids. (2) At fixed mass and luminosity an increase in metallicity shifts the instability strip toward lower effective temperatures. A thorough analysis of the total nonlinear work inside the instability strip points out that this effect is due to a decrease in the pulsation destabilization caused by the H ionization region. Therefore, the current theoretical scenario suggests that, at fixed period, metal-poor pulsators are brighter than metal-rich pulsators. (3) The dynamical structure of full-amplitude, first-overtone models supports the evidence that their nonlinear limit cycle behavior has been properly identified. The variations over a full pulsation cycle of the convective structure of fundamental and first-overtone pulsators located close to the blue and the red edges of the instability strip are discussed by taking into account the changes of the convective quantities across the convectively unstable region. As expected, we find that the main effect of convection on the limit cycle behavior is either to reduce the local radiative driving of the destabilizing regions, thus reducing the final amplitudes, or to damp the oscillations toward lower effective temperatures. We also find that the limiting amplitude behavior of high-mass, high-amplitude fundamental pulsators, and in particular the appearance of secondary features along their light and velocity curves, is tightly connected with the convection/pulsation interaction. By comparing the convective velocity perturbations close to the surface layers with the turbulent velocities obtained by spectroscopic measurements we find that toward lower effective temperatures both the absolute values of the convective velocity and its variation over the cycle agree reasonably well with observational data. However, the time behavior of the convective velocity in blue models and the strong decrease of this velocity predicted at low optical depths out of the maximum compression phases are presently not confirmed by observations. Both theoretical and observational shortcomings that could explain such a discrepancy are briefly discussed. The comparison between the linear periods currently adopted in the literature and the nonlinear periods obtained in this investigation shows a very good agreement in the mass range from 5 to 9 M?, whereas at 11 M? we find that linear, nonadiabatic, convective periods are systematically shorter than the nonlinear ones. Finally, the drawbacks of adopting linear observables for constraining the actual properties of Cepheids are discussed.


The Astrophysical Journal | 2005

Limit-Cycle Behavior in One-Zone Convective Models

Andreea Munteanu; G. Bono; Jordi Jose; Enrique García-Berro; Robert F. Stellingwerf

We investigate the modal stability of stellar models at masses and luminosity levels corresponding to post-main-sequence luminous δ Scuti pulsators. The envelope models have been computed at fixed mass value (M/M☉ = 2.0), luminosity level [log (L/L☉) = 1.7], and chemical composition (Y = 0.28, Z = 0.02). According to a nonlinear approach to radial oscillations, the present investigation for the first time predicts the occurrence of stable second overtone pulsators . More generally, we found that when moving inside the instability strip from lower to higher effective temperatures, the models show a stable limit cycle in three different pulsation modes: fundamental and first and second overtones. The shapes of both light and velocity curves are presented and discussed, providing a useful tool for the identification of second overtone pulsators among the known groups of radially pulsating stars. Comparison with observations shows that our nonlinear, nonlocal, and time-dependent convective models provide light curves in agreement with observed values, suggesting that second overtone pulsators have already been observed, though misclassified as fundamental pulsators. In a limited region of the instability strip we also found some models presenting mixed-mode features, i.e., radial pulsators that show a stable limit cycle in more than one pulsational mode. The period ratios of mixed-mode pulsators obtained by perturbing the first and the second overtone radial eigenfunctions are in agreement with observed values. This result is a crucial point for understanding the pulsation properties of δ Scuti stars, since it provides sound evidence that these variables during their evolution off the main sequence are pure or mixed-mode radial pulsators. Finally, the physical structure and the dynamical properties of second overtone pulsators are discussed in detail. The role played by the nodal lines in the destabilization of second overtone pulsators is also pointed out.


The Astrophysical Journal | 2018

On the Impact of Helium Content on the RR Lyrae Distance Scale

M. Marconi; G. Bono; A. Pietrinferni; V. F. Braga; M. Castellani; Robert F. Stellingwerf

We constructed several sequences of classical Cepheid envelope models at solar chemical composition (Y = 0.28, Z = 0.02) to investigate the dependence of the pulsation properties predicted by linear and nonlinear hydrodynamic models on input physics. To study the dependence on the equation of state (EOS) we performed several numerical experiments by using the simplified analytical EOS originally developed by Stellingwerf and the recent analytical EOS developed by Irwin. Current findings suggest that the pulsation amplitudes, as well as the topology of the instability strip, marginally depend on the adopted EOS. To compromise between accuracy and numerical complexity we computed new EOS tables using the Irwin analytical EOS. We found that the difference between analytical and tabular thermodynamic quantities and their derivatives are smaller than 2% when adopting suitable steps in temperature and density. To improve the numerical accuracy of physical quantities, we are now adopting bicubic splines to interpolate both opacity and EOS tables. The new approach presents a substantial advantage to avoiding numerical derivatives in both linear and nonlinear models. The EOS first- and second-order derivatives are estimated by means of the analytical EOS or by means of analytical derivatives of the interpolating function. The opacity first-order derivatives are evaluated by means of analytical derivatives of the interpolating function. We also investigated the dependence of observables predicted by theoretical models on the mass-luminosity (ML) relation and on the spatial resolution across the hydrogen and the helium partial ionization regions. We found that nonlinear models are marginally affected by these physical and numerical assumptions. In particular, the difference between new and old models in the location as well as in the temperature width of the instability strip is, on average, less than 200 K. However, the spatial resolution somehow affects the pulsation properties. The new fine models predict a period at the center of the Hertzsprung progression (PHP = 9.65-9.84 days) that reasonably agrees with empirical data based on light curves (PHP = 10.0 ± 0.5 days); and radial velocity curves (PHP = 9.95 ± 0.05 days); they improve previous predictions by Bono, Castellani, & Marconi.


Precision Asteroseismology: Proceedings of the International Astronomical Union, IAU Symposium | 2013

RR Lyrae Studies with Kepler: showcasing RR Lyr

K. Kolenberg; Robert L. Kurucz; Robert F. Stellingwerf; James M. Nemec; Pawel Moskalik; L. Fossati; Thomas G. Barnes

We present the results of a detailed set of one-zone models that account for the coupling between pulsation and convection following the original prescriptions of Stellingwerf. Motivated by the arbitrary nature of the input parameters adopted in this theoretical framework, we computed several sequences of models that cover a substantial fraction of the parameter space and a longer integration time. We found that our models show the same behavior as nonlinear, hydrodynamic models, that is, they approach either the limit-cycle stability (pulsational instability), the fixed point (pulsational stability), or their present vibrational instability. In agreement with Stellingwerf, we find that convection is the main quenching mechanism for pulsational models located across the Cepheid instability strip. Moreover, our one-zone models can mimic the pulsational behavior of both fundamental and first overtone Cepheids. We also included a turbulent pressure term and found that this physical mechanism plays a crucial role in the pulsation characteristics of the models by removing the sharp discontinuities along the light and the velocity curves shown by models that do not account for turbulent pressure. Finally, we investigated the vibrational and pulsational stability of completely convective models. We consider the most important finding of the present work to be the identification of a well-defined region in the parameter space where they approach limit-cycle stability. The inclusion of turbulent pressure widens this region, thus supporting original suggestions based on both linear and nonlinear models of long period variables (LPVs). Several numerical experiments performed by adopting different values of the adiabatic exponent and of the shell thickness indicate that the coupling between pulsation and convection is the key driving mechanism for LPVs, a finding supported by recent theoretical predictions.


Journal of the Acoustical Society of America | 1994

Acoustic modeling of plasma-induced vapor bubbles

Gary R. Hess; Arturo E. Rodriguez; Niels K. Winsor; Chris M. Young; Lawrence A. Crum; Robert F. Stellingwerf

We constructed new sets of He-enhanced (Y = 0.30, Y = 0.40) nonlinear, time-dependent convective hydrodynamical models of RR Lyrae (RRL) stars covering a broad range in metal abundances (Z from 0.0001 to 0.02). The increase in He content from the canonical value (Y = 0.245) to Y = 0.30 and 0.40 causes a simultaneous increase in stellar luminosity and in pulsation period. To investigate the dependence of the RRL distance scale on the He abundance, we computed new optical (RI) and near-infrared (JHK ) period luminosity metallicity helium relations. Interestingly enough, the increase in He content causes a minimal change in the coefficients of both period and metallicity terms, since canonical and He-enhanced models obey similar PLZ relations. On the contrary, the classical B and V band mean magnitude metallicity relations and the R band PLZ relation display a significant dependence on the He content. The He enhanced models are, at fixed metal content, 0.2 to 0.5 mag brighter than canonical ones. This variation is only marginally affected by evolutionary effects. The quoted distance diagnostics once calibrated with trigonometric parallaxes (Gaia) will provide the opportunity to estimate the He content of field and cluster RRLs. Moreover, the use of either spectroscopic or photometric metal abundances will pave the way to new empirical constraints on the universality of the helium to metal enrichment ratio in old stellar tracers.


Astrophysical Journal Supplement Series | 1995

Synthetic Mean Colors for RR Lyrae Variables

G. Bono; F. Caputo; Robert F. Stellingwerf

Four years into the Kepler mission, an updated review on the results for RR Lyrae stars is in order. More than 50 RR Lyrae stars in the Kepler field are observed with Kepler and each one of them can provide us with new insight into this class of pulsating stars. Ground-based spectroscopy of the Kepler targets allows us to narrow down their physical parameters. Previously, we already reported a 50% occurrence rate of modulation in the RRab stars, a large variety of modulation behavior, period doubling in several Blazhko stars, the detection of higher- overtone radial modes, probable non-radial modes and new types of multiple-mode RR Lyrae pulsators, among both the RRab and the RRc stars. In addition, the quasi-continuous photometry obtained over several years with Kepler allows one to observe changes in Blazhko behavior and additional longer cycles. These observations have sparked new theoretical modelling efforts. In this short paper we showcase RR Lyr itself. The star has been observed with Kepler in short cadence, and some remarkable features of its pulsation behavior are unveiled in this long-studied prototype, through the Kepler photometry and additional spectroscopic data.


The Astrophysical Journal | 2011

ON THE PERIOD DISTRIBUTION OF CLUSTER RR LYRAE STARS TO CONSTRAIN THEIR HELIUM CONTENT: THE CASE OF ω CENTAURI

M. Marconi; G. Bono; F. Caputo; A. M. Piersimoni; A. Pietrinferni; Robert F. Stellingwerf

Plasma‐induced bubbles respond to time‐dependent injection of energy which modifies the acoustic output relative to ‘‘classical’’ gas bubbles. Additionally, they consist primarily of hot vapor which complicates the thermodynamics, elevates the importance of some traditionally untreated variables, and requires additional physical process treatments beyond the basic hydrodynamics. A code that is based upon fundamental physical principles was developed to study the importance of many of these variables. The code is capable of treating the actual driving circuit, resultant plasma behavior, transition from cylindrical to spherical geometry early in the discharge, radiation production, plasma chemistry, thermal transport, and hydrodynamics in a modified Flynn formulation. Primary output consists of bubble wall acceleration, radius, etc., energy balance, and far field pressure. The code has been validated with experimental data and will be compared with ongoing hydrocode model development as well. Experimental a...


The Astrophysical Journal | 1994

Oosterhoff dichotomy in the Galaxy and globular clusters in the Large Magellanic Cloud

G. Bono; F. Caputo; Robert F. Stellingwerf

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