Helium Ionization Zone Research Articles

A comparison of numerical methods for computing the reionization of intergalactic hydrogen and helium by a central radiating source

ABSTRACT We compare numerical methods for solving the radiative transfer equation in the context of the photoionization of intergalactic gaseous hydrogen and helium by a central radiating source. Direct integration of the radiative transfer equation and solutions using photon packets are examined, both for solutions to the time-dependent radiative transfer equation and in the infinite-speed-of-light approximation. The photon packet schemes are found to be more generally computationally efficient than a direct integration scheme. While all codes accurately describe the growth rate of hydrogen and helium ionization zones, it is shown that a fully time-dependent method is required to capture the gas temperature and ionization structure in the near zone of a source when an ionization front expands at a speed close to the speed of light. Applied to quasi-stellar objects in the Epoch of Reionization (EoR), temperature differences as high as 5 × 104 K result in the near zone for solutions of the time-dependent radiative transfer equation compared with solutions in the infinite-speed-of-light approximation. Smaller temperature differences are found following the nearly full photoionization of helium in gas in which the hydrogen was already ionized and the helium was singly ionized. Variations found in the temperature and ionization structure far from the source, where the gas is predominantly neutral, may affect some predictions for 21-cm EoR experiments.

Advanced asteroseismic modelling: breaking the degeneracy between stellar mass and initial helium abundance

ABSTRACT Current stellar model predictions of adiabatic oscillation frequencies differ significantly from the corresponding observed frequencies due to the non-adiabatic and poorly understood near-surface layers of stars. However, certain combinations of frequencies – known as frequency ratios – are largely unaffected by the uncertain physical processes as they are mostly sensitive to the stellar core. Furthermore, the seismic signature of helium ionization provides envelope properties while being almost independent of the outermost layers. We have developed an advanced stellar modelling approach in which we complement frequency ratios with parameters of the helium ionization zone while taking into account all possible correlations to put the most stringent constraints on the stellar internal structure. We have tested the method using the Keplerbenchmark star 16 Cyg A and have investigated the potential of the helium glitch parameters to constrain the basic stellar properties in detail. It has been explicitly shown that the initial helium abundance and mixing-length parameters are well constrained within our framework, reducing systematic uncertainties on stellar mass and age arising for instance from the well-known anticorrelation between the mass and initial helium abundance. The modelling of six additional Kepler stars including 16 Cyg B reinforces the above findings and also confirms that our approach is mostly independent of model uncertainties associated with the near-surface layers. Our method is relatively computationally expensive, however, it provides stellar masses, radii and ages precisely in an automated manner, paving the way for analysing numerous stars observed in the future during the ESA PLATO mission.

ON THE QUESTION OF THE HELIUM ABUNDANCE IN ORION A MEASURED BY RADIO RECOMBINATION LINES. AND THE PRIMORDIAL HELIUM ABUNDANCE

Recombination radio lines (RRLs)of hydrogen (H), helium (He-4), and carbon (C) area powerful tool for studying the interstellar medium(ISM) in space. The RRLs observations allow to obtainthe physical parameters of regions of ionized hydrogen(HII regions) as well as of the photo-dissociationregions; and to estimate the effective temperatureof stars that ionize the HII region. There is also animportant cosmological task for RRL – measuringPrimordial helium abundance produced at the stage ofPrimordial nucleosynthesis of the Universe. It turnedout that the Orion A nebula is an interesting objectfor the latter task. At the time, the recombinationradio lines observations of hydrogen, helium (H, He),and carbon (C) at a number of positions in the OrionA HII region were carry out with the RT22 radiotelescope (Pushchino) at wavelengths of 8 and 13 mm.The relative helium abundance, y + = n(He + )/n(H + ),in these positions was obtained. The behavior ofthis value over the nebula showed that the heliumionization zone is smaller than the hydrogen one withdifferent ratios for the core and halo. The locationwhere the maximum y + value is expected was alsodetermined. For the established ionization structure,it means that the actual helium abundance in OrionA, n(He)/n(H), will not be less than the maximumy + value. This allows to estimate the limitations onthe Primordial helium abundance. In this work, newH and He RRL observations were made at 13 mm inthe direction of the expected maximum of y + . RRLswere observed in two transitions - 65α and 66α. It wasfound that the maximum y + value is in the range of10 − 12%. Hence, we can expect that the Primordialhelium abundance (Yp, the ratio He/H by mass)lies in the range of ≈ 26.4 − 29%, and the numberof light neutrino-type particles during Primordialnucleosynthesis may exceed the standard value. Torefine the result the work will be continue.

Solar-cycle–related Changes in the Helium Ionization Zones of the Sun

Abstract Helioseismic data for solar cycles 23 and 24 have shown unequivocally that solar dynamics changes with solar activity. Changes in solar structure have been more difficult to detect. Basu & Mandel had claimed that the then-available data revealed changes in the He ii ionization zone of the Sun. The amount of change, however, indicated the need for larger than expected changes in the magnetic fields. Now that helioseismic data spanning two solar cycles are available, we have redone the analysis using improved fitting techniques. We find that there is indeed a change in the region around the He ii ionization zone that is correlated with activity. Because the data sets now cover two solar cycles, the time variation is easily discernible.

Detecting shock waves in non-fundamental mode RR Lyrae using large sample of spectra in SDSS and LAMOST

Steps toward the nature inside RR Lyrae variables can not only improve our understanding of variable stars but also innovate the precision when we use them as tracers to map the structure of the universe. In this work, we develop a hand-crafted one-dimensional pattern recognition pipeline to fetch out the "first apparitions", the most prominent observational characteristic of shock. We report the first detection of hydrogen emission lines in the first-overtone and multi-mode RR Lyrae variables. We find that there is an anti-correlation between the intensity and the radial velocity of the emission signal, which is possibly caused by opacity changing in the helium ionization zone. Moreover, we find one RRd star with hydrogen emission that possibly shows Blazhko-type modulations. According to our discoveries, with an enormous volume of upcoming data releases of variable stars and spectra, it may become possible to build up the bridge between shock waves and big problems like the Blazhko effect in non-fundamental mode RR Lyrae stars.

Asteroseismic Investigations of the Binary System HD 176465

Abstract HD 176465 is a binary system for which both components are solar-like pulsators and oscillation frequencies were observed by the Kepler mission. In this paper, we have modeled the asteroseismic and spectroscopic data of the stars, and have determined their convection-zone helium abundances using the signatures left by the He ii ionization zone on the mode frequencies. As expected, we find that the components of the binary are of the same age within uncertainties (3.087 ± 0.580 Gyr and 3.569 ± 0.912 Gyr); they also have the same initial helium abundance (Y init = 0.253 ± 0.006 and 0.254 ± 0.008). Their current metallicity ([Fe/H] = −0.275 ± 0.04 and −0.285 ± 0.04) is also the same within errors. Fits to the signature of the He ii acoustic glitch yield current helium abundances of Y A = 0.224 ± 0.006 and Y B = 0.233 ± 0.008 for the two components. Analyzing the complete ensemble of models generated for this investigation, we find that both the amplitude and acoustic depth of the glitch signature arising from the second helium ionization zone and the base of the convection zone (CZ) are functions of mass. We show that the acoustic depths of these glitches are positively correlated with each other. The analysis can help us to detect the internal structure and constrain the chemical compositions.

Seismic Measurement of the Locations of the Base of Convection Zone and Helium Ionization Zone for Stars in the Kepler Seismic LEGACY Sample

Abstract Acoustic glitches are regions inside a star where the sound speed or its derivatives change abruptly. These leave a small characteristic oscillatory signature in the stellar oscillation frequencies. With the precision achieved by Kepler seismic data, it is now possible to extract these small amplitude oscillatory signatures, and infer the locations of the glitches. We perform glitch analysis for all the 66 stars in the Kepler seismic LEGACY sample to derive the locations of the base of the envelope convection zone (CZ) and the helium ionization zone. The signature from helium ionization zone is found to be robust for all stars in the sample, whereas the CZ signature is found to be weak and problematic, particularly for relatively massive stars with large errorbars on the oscillation frequencies. We demonstrate that the helium glitch signature can be used to constrain the properties of the helium ionization layers and the helium abundance.

Large convective cells in the sun: a theoretical model

The geometric parameters and relationships of the horizontal and vertical velocities in a giant convective cylindrical cell were analyzed based on a continuity equation with the given vertical distribution of unperturbed gas density in the convective zone of the Sun. It has been found that the lower edge of such a convective cell must be positioned at a depth of 20 Mm in the secondary helium ionization zone. In addition, under the assumption that the plasma conductivity in the convective zone is caused by small-scale turbulence, a new stationary solution was obtained for the problem of magnetic field diffusion in the pattern of slow conductive plasma flows in scales of a convective cell.

ASTEROSEISMIC ANALYSIS OF THE INTERNAL STRUCTURE AND EVOLUTION OF RED GIANT BRANCH BUMP STARS

The is a prominent feature of the red giant branch (RGB) luminosity function of stellar clusters. Through constructing a grid of models with different masses and metallicities to study the feature of the RGB bump luminosity we find that the luminosity increases almost monotonically with increasing mass for a given metallicity and decreases monotonically with increasing metallicity. Moreover, different stars have different shapes of the RGB bump. It is correlated with the sharpness of the H discontinuity, which is left over by the convection envelope during the first dredge-up. Using the periodicity in the small separations d(01), d(10) to probe the internal structure, we find that, at about half the acoustic radius, the sound speed has a sharp variation that is caused by a local depression of the first adiabatic exponent Gamma(1) in the second helium ionization zone. It induces an oscillation modulation in d(01), d(10) with a period of 6.4 mu Hz. Meanwhile, in the same model, the base of the convective envelope is located at a relatively small acoustic radius t(envp)/T, which is about 10(-1). It is too deep to detect the exact location of the bottom of the outer convective envelope. In order to discriminate the evolutionary status of different stars, we calculate the asymptotic g-mode period spacing Delta Pi(1). We find that Delta Pi(1) decreases monotonically with evolution. It is a reliable parameter for distinguishing stars in different positions of the RGB bump.

Asteroseismic estimate of helium abundance of 16 Cyg A, B

The helium ionization zone in a star leaves a characteristic signature on its oscillation frequencies, which can be used to estimate the helium content in the envelope of the star. We use the oscillation frequencies of 16 Cyg A and B, obtained using 2.5 years of Kepler data, to estimate the envelope helium abundance of these stars. We find the envelope helium abundance to lie in the range 0.231–0.251 for 16 Cyg A and 0.218–0.266 for 16 Cyg B.

Physical parameters of the Orion Bar photodissociation region from radio recombination line observations at 8 mm

Observations of carbon (C), hydrogen and helium (H, He) radio recombination lines (RRLs) at four positions in the Orion Bar photodissociation region (PDR) and toward the center of Orion A have been performed with the RT-22 radio telescope (Pushchino) at 8 mm. The physical parameters of the PDR at these points have been estimated by comparing the carbon RRLs and infrared CII and OI lines. A hydrogen number density in the range 1.2–3.1 × 105 cm−3 and a mean size of the region along the line of sight (L) in the range 0.006–0.04 pc have been derived. The PDR temperature decreases with increasing distance from the exciting star (θ 1 C Ori) from 210–230 to 140–150 K (a distance of ≈5′). The data obtained confirm the increase in the PDR size along the line of sight toward the Orion Bar, where, however, L has turned out to be less than the available values in the literature, which can be explained by the presence of clumps in the PDR. A density jump is evident in the Orion Bar region. The PDR zone encompasses the core of the HII region by a thin layer and extends farther, delineating the boundary and the ionization front of the core of the HII region in the Orion Bar and further out the boundary between the halo of the HII region and the molecular cloud. The derived emission measure (EM) toward the Orion Bar has been compared with other C RRL observations. The EM measured from carbon RRLs is EM ≈ 100(±50%) pc cm−6, imposing constraints on the possible two-component PDR structure. Estimates show that the star θ 1 C Ori is quite sufficient as a carbon ionization source in the Orion Bar PDR. Some of the data on the ionized hot gas (HII) in this direction have been obtained from H and He RRLs. In particular, the radial velocities (V lsr) of the HII region are blueshifted with respect to V lsr of the PDR by 10–17 km s−1, while the relative ionized helium abundance decreases with increasing distance from the star, indicating that the helium ionization zone is smaller than the ionized hydrogen one.

A THEORETICAL STUDY OF ACOUSTIC GLITCHES IN LOW-MASS MAIN-SEQUENCE STARS

There are regions in stars, such as ionization zones and the interface between radiative and convective regions, that cause a localized sharp variation in the sound speed. These are known as "acoustic glitches". Acoustic glitches leave their signatures on the oscillation frequencies of stars, and hence these signature can be used as diagnostics of these regions. In particular, the signature of these glitches can be used as diagnostics of the position of the second helium ionization zone and that of the base of the envelope convection zone. With the help of stellar models we study the properties of these acoustic glitches in main-sequence stars. We find that the acoustic glitch due to the helium ionization zone does not correspond to the dip in the adiabatic index \Gamma_1 caused by the ionization of HeII, but to the peak in \Gamma_1 between the HeI and HeII ionization zones. We find that it is easiest to study the acoustic glitch due to the helium ionization zone in stars with masses in the range 0.9--1.2 M_\odot.

ASTEROSEISMIC ESTIMATE OF HELIUM ABUNDANCE OF A SOLAR ANALOG BINARY SYSTEM

16 Cyg A and B are among the brightest stars observed by Kepler. What makes these stars more interesting is that they are solar analogs. 16 Cyg A and B exhibit solar-like oscillations. In this work we use oscillation frequencies obtained using 2.5 years of Kepler data to determine the current helium abundance of these stars. For this we use the fact that the helium ionization zone leaves a signature on the oscillation frequencies and that this signature can be calibrated to determine the helium abundance of that layer. By calibrating the signature of the helium ionization zone against models of known helium abundance, the helium abundance in the envelope of 16 Cyg A is found to lie in the range 0.231 to 0.251 and that of 16 Cyg B lies in the range 0.218 to 0.266.

MEASUREMENT OF ACOUSTIC GLITCHES IN SOLAR-TYPE STARS FROM OSCILLATION FREQUENCIES OBSERVED BYKEPLER

For the very best and brightest asteroseismic solar-type targets observed by Kepler, the frequency precision is sufficient to determine the acoustic depths of the surface convective layer and the helium ionization zone. Such sharp features inside the acoustic cavity of the star, which we call acoustic glitches, create small oscillatory deviations from the uniform spacing of frequencies in a sequence of oscillation modes with the same spherical harmonic degree. We use these oscillatory signals to determine the acoustic locations of such features in 19 solar-type stars observed by the Kepler mission. Four independent groups of researchers utilized the oscillation frequencies themselves, the second differences of the frequencies and the ratio of the small and large separation to locate the base of the convection zone and the second helium ionization zone. Despite the significantly different methods of analysis, good agreement was found between the results of these four groups, barring a few cases. These results also agree reasonably well with the locations of these layers in representative models of the stars. These results firmly establish the presence of the oscillatory signals in the asteroseismic data and the viability of several techniques to determine the location of acoustic glitches inside stars.

Pulsation in extremely low mass helium stars

We explore the stability of extremely low-mass stars (M < 0.25 M⊙) across a wide range of composition, effective temperature, and luminosity. We identify the instability boundaries associated with radial oscillations. These are a strong function of both composition and radial order (0 ≤ n ≤ 13). The classical blue edge shifts to higher effective temperature and luminosity with decreasing hydrogen abundance. Higher-order modes are more easily excited, and small islands of instability develop. Short-period oscillations have been discovered in the low-mass pre-white dwarf component of the eclipsing binary J0247–25. If its envelope is depleted in hydrogen, J0247–25B is unstable to intermediate-order p modes. Driving is by the classical κ mechanism operating in the second helium ionization zone. The observed periods, temperature and luminosity of J0247–25B require an envelope hydrogen abundance 0.2 ≤ X ≤ 0.3.

Pulsation in extremely low-mass helium stars

AbstractWe explore the stability of extremely low-mass stars (M &lt; 0.25 M⊙) across a wide range of composition, effective temperature, and luminosity. We identify the instability boundaries associated with radial oscillations. These are a strong function of both composition and radial order (0 ≤ n ≤ 13). The classical blue edge shifts to higher effective temperature and luminosity with decreasing hydrogen abundance. Higher-order modes are more easily excited, and small islands of instability develop. Short-period oscillations have been discovered in the low-mass pre-white dwarf component of the eclipsing binary J0247–25. If its envelope is depleted in hydrogen, J0247–25B is unstable to intermediate-order p modes. Driving is by the classical κ mechanism operating in the second helium ionization zone. The observed periods, temperature and luminosity of J0247–25B require an envelope hydrogen abundance 0.2 ≤ X ≤ 0.3.

Nonlinear pulsations of red supergiants

Excitation of radial oscillations in population I (X=0.7, Z=0.02) red supergiants is investigated using the solution of the equations of radiation hydrodynamics and turbulent convection. The core helium burning stars with masses 8M_odot <= M <= 20M_odot end effective temperatures T_eff < 4000K are shown to be unstable against radial pulsations in the fundamental mode. The oscillation periods range between 45 and 1180 days. The pulsational instability is due to the kappa-mechanism in the hydrogen and helium ionization zones. Radial pulsations of stars with mass M < 15M_odot are strictly periodic with the light amplitude Delta M_bol <= 0.5 mag. The pulsation amplitude increases with increasing stellar mass and for M > 15M_odot the maximum expansion velocity of outer layers is as high as one third of the escape velocity. The mean radii of outer mass zones increase due to nonlinear oscillations by <= 30% in comparison with the initial equilibrium. The approximate method (with uncertainty of a factor of 1.5) to evaluate the mass of the pulsating red supergiant with the known period of radial oscillations is proposed. The approximation of the pulsation constant Q as a function of the mass-to-radius ratio is given. Masses of seven galactic red supergiants are evaluated using the period-mean density relation.

Pulsational instability of yellow hypergiants

Instability of population I (X=0.7, Y=0.02) massive stars against radial oscillations during the post-main sequence gravitational contraction of the helium core is investigated. Initial stellar masses are in the range from 65M_\odot to 90M_\odot. In hydrodynamic computations of self-exciting stellar oscillations we assumed that energy transfer in the envelope of the pulsating star is due to radiative heat conduction and convection. The convective heat transfer was treated in the framework of the theory of time-dependent turbulent convection. During evolutionary expansion of outer layers after hydrogen exhaustion in the stellar core the star is shown to be unstable against radial oscillations while its effective temperature is Teff > 6700K for Mzams=65M_\odot and Teff > 7200K for mzams=90M_\odot. Pulsational instability is due to the \kappa-mechanism in helium ionization zones and at lower effective temperature oscillations decay because of significantly increasing convection. The upper limit of the period of radial pulsations on this stage of evolution does not exceed 200 day. Radial oscillations of the hypergiant resume during evolutionary contraction of outer layers when the effective temperature is Teff > 7300K for Mzams=65M_\odot and Teff > 7600K for Mzams=90M_\odot. Initially radial oscillations are due to instability of the first overtone and transition to fundamental mode pulsations takes place at higher effective temperatures (Teff > 7700K for Mzams=65M_\odot and Teff > 8200K for Mzams=90M_\odot). The upper limit of the period of radial oscillations of evolving blueward yellow hypergiants does not exceed 130 day. Thus, yellow hypergiants are stable against radial stellar pulsations during the major part of their evolutionary stage.

A New Explanation of the Blazhko Effect in RR Lyrae Stars

An interpretation of the modulating Blazhko effect in RR Lyrae stars is presented here, in which turbulent convection inside the hydrogen and helium ionization zones becomes cyclically weakened and strengthened owing to the presence of a transient magnetic field that is generated in situ by either a turbulent or a rotational dynamo mechanism. Successful predictions, both qualitative and quantitative, are made for the small changes of the primary pulsation period, the fluctuations of light and velocity amplitudes, the slow periodicity and irregularity of the Blazhko cycles, the restricted ranges of effective temperature for the RRab and RRc Blazhko variables, and the complex correlations between the primary period change, amplitude change, and mean effective temperature. Characteristic features of the predicted light and velocity curves at high and low amplitudes, even though they are based on radiative stellar models, agree well with the observed features, for the most part. The present theory of the Blazhko effect is simple enough that it does not require any basic change in our current understanding of RRab and RRc stars as being purely fundamental-mode and first-overtone radial pulsators. It also accounts naturally for the observed fact that hotter and cooler classes of periodic variable stars do not exhibit the Blazhko effect.

BiSON Data Show Change in Solar Structure with Magnetic Activity

We find evidence for an activity-dependent change in solar structure at the second helium ionization zone (~0.98 R☉). We use low-degree p-mode frequencies obtained from 13 years of observations by the Birmingham Solar Oscillations Network (BiSON) to track changes in the amplitude of the rapid-variation signature imposed by the bump in the adiabatic exponent, Γ1, at the He II ionization zone. This is the first indication of structural change detected using only low-degree data and confirms the recent results of high-degree and local helioseismic studies.