arXiv subjects

Joyce A. Guzik

Publications and source records attributed to Joyce A. Guzik.

27 records · Page 2Linked to original sources

arXiv2016

Sound speed and oscillation frequencies for solar models evolved with Los Alamos ATOMIC opacities

Los Alamos National Laboratory has calculated a new generation of radiative opacities (OPLIB data using the ATOMIC code) for elements with atomic number Z=1-30 with improved physics input, updated atomic data, and finer temperature grid to replace the Los Alamos LEDCOP opacities released in the year 2000. We calculate the evolution of standard solar models including these new opacities, and compare with models evolved using the Lawrence Livermore National Laboratory OPAL (Iglesias and Rogers 1996) opacities. We use the solar abundance mixture of Asplund et al. (2009). The new Los Alamos ATOMIC opacities have steeper opacity derivatives than those of OPAL for temperatures and densities of the solar interior radiative zone. We compare the calculated nonadiabatic solar oscillation frequencies and solar interior sound speed to observed frequencies and helioseismic inferences. The calculated sound-speed profiles are similar for models evolved using either the updated Iben evolution code (see \cite{Guzik2010}), or the MESA evolution code (Paxton et al., 2015). The LANL ATOMIC opacities partially mitigate the "solar abundance problem".

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arXiv2016

2-D and 3-D Models of Convective Turbulence and Oscillations in Intermediate-Mass Main-Sequence Stars

We present multidimensional modeling of convection and oscillations in main-sequence stars somewhat more massive than the Sun, using three separate approaches: 1) Using the 3-D planar StellarBox radiation hydrodynamics code to model the envelope convection zone and part of the radiative zone. Our goals are to examine the interaction of stellar pulsations with turbulent convection in the envelope, excitation of acoustic modes, and the role of convective overshooting; 2) Applying the spherical 3-D MHD ASH (Anelastic Spherical Harmonics) code to simulate the core convection and radiative zone. Our goal is to determine whether core convection can excite low-frequency gravity modes, and thereby explain the presence of low frequencies for some hybrid gamma Doradus/delta Scuti variables for which the envelope convection zone is too shallow for the convective blocking mechanism to drive gravity modes; 3) Applying the ROTORC 2-D stellar evolution and dynamics code to calculate evolution with a variety of initial rotation rates and extents of core convective overshooting. The nonradial adiabatic pulsation frequencies of these nonspherical models are calculated using the 2-D pulsation code NRO. We present new insights into pulsations for stars of one to two solar masses gained by multidimensional modeling.

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arXiv2015

Results of a Search for gamma Dor and delta Sct Stars with the Kepler Spacecraft

The light curves of 2768 stars with effective temperatures and surface gravities placing them near the gamma Doradus/delta Scuti instability region were observed as part of the Kepler Guest Observer program from Cycles 1 through 5. The light curves were analyzed in a uniform manner to search for gamma Doradus, delta Scuti, and hybrid star pulsations. The gamma Doradus, delta Scuti, and hybrid star pulsations extend asteroseismology to stars slightly more massive (1.4 to 2.5 solar masses) than our Sun. We find 207 gamma Doradus, 84 delta Scuti, and 32 hybrid candidate stars. Many of these stars are cooler than the red edge of the gamma Doradus instability strip as determined from ground-based observations made before Kepler. A few of our gamma Doradus candidate stars lie on the hot side of the ground-based gamma Doradus instability strip. The hybrid candidate stars cover the entire region between 6200 K and the blue edge of the ground-based delta Scuti instability strip. None of our candidate stars are hotter than the hot edge of the ground-based delta Scuti instability strip. Our discoveries, coupled with the work of others, shows that Kepler has discovered over 2000 gamma Doradus, delta Scuti, and hybrid star candidates in the 116 square degree Kepler field of view. We found relatively few variable stars fainter than magnitude 15, which may be because they are far enough away to lie between spiral arms in our Galaxy, where there would be fewer stars.

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arXiv2015

The Occurrence of Non-Pulsating Stars in the gamma Dor and delta Sct Pulsation Instability Regions: Results from Kepler Quarter 14-17 Data

In our 2013 Astronomical Review article, we discussed the statistics of variability for 633 faint spectral type A-F stars observed by the Kepler spacecraft during Quarters 6-13. We found six stars that showed no variability with amplitude 20 ppm or greater in the range 0.2 to 24.4 cycles/day, but whose positions in the log g--Teff diagram place them in the delta Sct or gamma Dor pulsation instability regions established from pre-Kepler ground-based observations. Here we present results for 2137 additional stars observed during Quarters 14-17, and find 34 stars that lie within the instability regions. In Paper I, we included a +229 K offset to the Kepler Input Catalog Teff to take into account an average systematic difference between the KIC values and the Teff derived from SDSS color photometry for main-sequence F stars (Pinsonneault et al. 2012). Here we compare the KIC Teff value and the Teff derived from spectroscopy taken by the LAMOST instrument (Molenda-Zakowicz et al. 2013, 2014) for 54 stars common to both samples. We find no trend to support applying the offset, but instead find that a small average temperature decrease relative to the KIC Teff may be more appropriate for the stars in our spectral-type range. If the offset is omitted, only 17 of our 34 `constant' stars fall within the instability regions. For the two `constant' stars also observed by LAMOST, the LAMOST Teff values are cooler than the KIC Teff by several hundred K, and would move these stars out of the instability regions. It is possible that a more accurate determination of their Teff and log g would move some of the other `constant' stars out of the instability regions. However, if average (random) errors in Teff are taken into account, 15 to 52 stars may still persist within the instability regions. Explanations for these `constant' stars, both theoretical and observational, remain to be investigated.

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arXiv2014

The Occurrence of Non-Pulsating Stars in the gamma Doradus/delta Scuti Pulsation Instability Region

As part of the NASA Kepler Guest Observer program, we requested and obtained long-cadence data on about 2700 faint (magnitude 14-16) Kepler stars with effective temperatures and surface gravities that lie near or within the pulsation instability region for main-sequence gamma Doradus and delta Scuti pulsating variables. These variables are of spectral type A-F with masses of 1.4 to 2.5 solar masses. The delta Scuti stars pulsate in radial and non-radial acoustic modes, with periods of a few hours (frequencies around 10 cycles/day), while gamma Doradus variables pulsate in nonradial gravity modes with periods 0.3 to 3 days (frequencies around 1 cycle/day). Here we consider the light curves and Fourier transforms of 633 stars in an unbiased sample observed by Kepler in Quarters 6-13 (June 2010-June 2012). We show the location of these stars in the log surface gravity--effective temperature diagram compared to the instability region limits established from ground-based observations, and taking into account uncertainties and biases in the Kepler Input Catalog T_eff values. While hundreds of variables have been discovered in the Kepler data, about 60% of the stars in our sample do not show any frequencies between 0.2 and 24.4 cycles per day with amplitude above 20 parts per million. We find that six of these apparently constant stars lie within the pulsation instability region. We discuss some possible reasons that these stars do not show photometric variability in the Kepler data. We also comment on the non-constant stars, and on 26 variable-star candidates, many of which also do not lie within the expected instability regions.

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arXiv2014

Pulsations and Hydrodynamics of Luminous Blue Variable Stars

The Luminous Blue Variable stars exhibit behavior ranging from light curve `microvariations' on timescales of tens of days, to `outbursts' accompanied by mass loss of up to 10e-03 solar masses per year, occurring decades apart, to `giant eruptions' such as seen in Eta Carinae ejecting one or more solar masses and recurring on timescales of centuries. Here we review the work of the Los Alamos group since 1993 to investigate pulsations and instabilities in massive stars using linear pulsation models and non-linear hydrodynamic models. The models predict pulsational variability that may be associated with the microvariations. Using a nonlinear pulsation hydrodynamics code with a time-dependent convection treatment, we show that, in some circumstances, the Eddington limit is exceeded periodically in the pulsation driving region of the stellar envelope, accelerating the outer layers, and perhaps initiating mass loss or LBV outbursts. We discuss how pulsations and mass loss may be responsible for the location of the Humphreys-Davidson Limit in the H-R diagram. The `giant eruptions', however, must involve much deeper regions in the stellar core to cause such large amounts of mass to be ejected. We review and suggest some possible explanations, including mixing from gravity modes, secular instabilities, the epsilon mechanism, or the SASI instability as proposed for Type II supernovae. We outline future work and required stellar modeling capabilities to investigate these possibilities.

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arXiv2014

Challenges in Solar and Stellar Model Physics

We are reaching relative maturity and standardization in one-dimensional single-star stellar evolution and pulsation modeling, and are making advances in binary and 2D and 3D models. However, many physical inputs are still uncertain or neglected in models of the Sun and of other stars. Thanks to the the Kepler, CoRoT, and MOST spacecraft, for example, as well as to ground-based networks, we now have pulsation data for stars that are of comparable quality to that for the Sun to constrain models and test physical assumptions. Here I will focus on main sequence (core H-burning) or slightly post-main sequence (shell H-burning) stellar models, and some of the unsolved problems for these stars. I will revisit the solar abundance problem, and show the effects of modified electron screening, dark matter, and early mass loss on solar models. I will discuss the gamma Dor/ delta Sct hybrid stars, the mismatch between predicted and observed frequencies for delta Sct stars, and how seismology of stars more massive than the Sun, e.g. the brightest Kepler target theta Cyg, could help us constrain physical processes such as diffusive settling, test pulsation driving mechanisms, and provide clues to the solar abundance problem.

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arXiv2013

Observing Open Clusters with a Sequence of Ages with Kepler

We propose to observe with Kepler an age sequence of nearby uncrowded open clusters. The current Kepler field contains very few (only 4) somewhat distant and/or old clusters. Nearby open clusters are already well characterized from ground- and many space-based observations. Our proposal focuses on mid and upper main-sequence variables (main sequence and pre-main sequence gamma Dor or delta Scuti stars, SPB/beta Cep stars, Cepheids, or yellow supergiants), having periods of hours to days accessible by longer cadence observations. Asteroseismology for these objects to date is limited by the number of modes observable from the ground, difficulty in obtaining spectroscopic or photometric mode ID for the fainter targets that have been observed by Kepler, uncertainties in interior and initial abundances, especially for stars with abundance anomalies, uncertainties in distance/luminosity, and lack of knowledge of prior evolution history. The additional constraints of common age, distance and initial abundances in clusters will place these variables in their evolutionary context, and help unlock some of the science and reduce uncertainties for understanding star formation and stellar evolution.

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arXiv2008

Pulsation-Initiated Mass Loss in Luminous Blue Variables: A Parameter Study

Luminous blue variables (LBVs) are characterized by semi-periodic episodes of enhanced mass-loss, or outburst. The cause of these outbursts has thus far been a mystery. One explanation is that they are initiated by kappa-effect pulsations in the atmosphere caused by an increase in luminosity at temperatures near the so-called ``iron bump'' (T ~ 200,000 K), where the Fe opacity suddenly increases. Due to a lag in the onset of convection, this luminosity can build until it exceeds the Eddington limit locally, seeding pulsations and possibly driving some mass from the star. We present some preliminary results from a parameter study focusing on the conditions necessary to trigger normal S-Dor type (as opposed to extreme eta-Car type) outbursts. We find that as Y increases or Z decreases, the pulsational amplitude decreases and outburst-like behavior, indicated by a large, sudden increase in photospheric velocity, becomes likes likely.

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