arXiv subjects

Aldo M. Serenelli

Publications and source records attributed to Aldo M. Serenelli.

34 records · Page 2Linked to original sources

arXiv2013

Multi-periodic pulsations of a stripped red giant star in an eclipsing binary

Low mass white dwarfs are the remnants of disrupted red giant stars in binary millisecond pulsars and other exotic binary star systems. Some low mass white dwarfs cool rapidly, while others stay bright for millions of years due to stable fusion in thick surface hydrogen layers. This dichotomy is not well understood so their potential use as independent clocks to test the spin-down ages of pulsars or as probes of the extreme environments in which low mass white dwarfs form cannot be fully exploited. Here we present precise mass and radius measurements for the precursor to a low mass white dwarf. We find that only models in which this star has a thick hydrogen envelope can match the strong constraints provided by our new observations. Very cool low mass white dwarfs must therefore have lost their thick hydrogen envelopes by irradiation from pulsar companions or by episodes of unstable hydrogen fusion (shell flashes). We also find that this low mass white dwarf precursor is a new type of pulsating star. The observed pulsation frequencies are sensitive to internal processes that determine whether this star will undergo shell flashes.

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arXiv2012

Solar Neutrinos: Status and Prospects

We describe the current status of solar neutrino measurements and of the theory -- both neutrino physics and solar astrophysics -- employed in interpreting measurements. Important recent developments include Super-Kamiokande's determination of the neutrino-electron elastic scattering rate for 8B neutrinos to 3%; the latest SNO global analysis in which the inclusion of low-energy data from SNO I and II significantly narrowed the range of allowed values for the neutrino mixing angle theta12; Borexino results for both the 7Be and pep neutrino fluxes, the first direct measurements constraining the rate of ppI and ppII burning in the Sun; global reanalyses of solar neutrino data that take into account new reactor results on theta13; a new decadal evaluation of the nuclear physics of the pp chain and CNO cycle defining best values and uncertainties in the nuclear microphysics input to solar models; recognition of an emerging discrepancy between two tests of solar metallicity, helioseismological mappings of the sound speed in the solar interior, and analyses of the metal photoabsorption lines based on our best current description of the Sun's photosphere; a new round of standard solar model calculations optimized to agree either with helioseismology or with the new photospheric analysis; and, motivated by the solar abundance problem, the development of nonstandard, accreting solar models, in order to investigate possible consequences of the metal segregation that occurred in the proto-solar disk. We review this progress and describe how new experiments such as SNO+ could help us further exploit neutrinos as a unique probe of stellar interiors.

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arXiv2011

Solar models with accretion. I. Application to the solar abundance problem

We generate new standard solar models using newly analyzed nuclear fusion cross sections and present results for helioseismic quantities and solar neutrino fluxes. We discuss the status of the solar abundance problem and investigate whether nonstandard solar models with accretion from the protoplanetary disk might alleviate the problem. We examine a broad range of possibilities, analyzing both metal-enriched and metal-depleted accretion models and exploring three scenarios for the timing of the accretion. Only partial solutions are found: one can bring either the depth of the convective zone or the surface helium abundance into agreement with helioseismic results, but not both simultaneously. In addition, detailed results for solar neutrino fluxes show that neutrinos are a competitive source of information about the solar core and can help constrain possible accretion histories of the Sun. Finally, we briefly discuss how measurements of solar neutrinos from the CN-cycle could shed light on the interaction between the early Sun and its protoplanetary disk.

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arXiv2009

Detection of solar-like oscillations from Kepler photometry of the open cluster NGC 6819

Asteroseismology of stars in clusters has been a long-sought goal because the assumption of a common age, distance and initial chemical composition allows strong tests of the theory of stellar evolution. We report results from the first 34 days of science data from the Kepler Mission for the open cluster NGC 6819 -- one of four clusters in the field of view. We obtain the first clear detections of solar-like oscillations in the cluster red giants and are able to measure the large frequency separation and the frequency of maximum oscillation power. We find that the asteroseismic parameters allow us to test cluster-membership of the stars, and even with the limited seismic data in hand, we can already identify four possible non-members despite their having a better than 80% membership probability from radial velocity measurements. We are also able to determine the oscillation amplitudes for stars that span about two orders of magnitude in luminosity and find good agreement with the prediction that oscillation amplitudes scale as the luminosity to the power of 0.7. These early results demonstrate the unique potential of asteroseismology of the stellar clusters observed by Kepler.

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arXiv2009

New Results on Standard Solar Models

We describe the current status of solar modelling and focus on the problems originated with the introduction of solar abundance determinations with low CNO abundance values. We use models computed with solar abundance compilations obtained during the last decade, including the newest published abundances by Asplund and collaborators. Results presented here make focus both on helioseismic properties and the models as well as in the neutrino fluxes predictions. We also discuss changes in radiative opacities to restore agreement between helioseismology, solar models, and solar abundances and show the effect of such modifications on solar neutrino fluxes.

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arXiv2009

Fresh insights on the structure of the solar core

We present new results on the structure of the solar core, obtained with new sets of frequencies of solar low-degree p modes obtained from the BiSON network. We find that different methods used in extracting the different sets of frequencies cause shifts in frequencies, but the shifts are not large enough to affect solar structure results. We find that the BiSON frequencies show that the solar sound speed in the core is slightly larger than that inferred from data from MDI low-degree modes, and the uncertainties on the inversion results are smaller. Density results also change by a larger amount, and we find that solar models now tend to show smaller differences in density compared to the Sun. The result is seen at all radii, a result of the fact that conservation of mass implies that density differences in one region have to cancel out density differences in others, since our models are constructed to have the same mass as the Sun. The uncertainties on the density results are much smaller too. We attribute the change in results to having more, and lower frequency, low-degree mode frequencies available. These modes provide greater sensitivity to conditions in the core.

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arXiv2007

Solar heavy element abundance: constraints from frequency separation ratios of low-degree p modes

We use very precise frequencies of low-degree solar-oscillation modes measured from 4752 days of data collected by the Birmingham Solar-Oscillations Network (BiSON) to derive seismic information on the solar core. We compare these observations to results from a large Monte Carlo simulation of standard solar models, and use the results to constrain the mean molecular weight of the solar core, and the metallicity of the solar convection zone. We find that only a high value of solar metallicity is consistent with the seismic observations. We can determine the mean molecular weight of the solar core to a very high precision, and, dependent on the sequence of Monte Carlo models used, find that the average mean molecular weight in the inner 20% by radius of the Sun ranges from 0.7209 to 0.7231, with uncertainties of less than 0.5% on each value. Our lowest seismic estimate of solar metallicity is Z=0.0187 and our highest is Z=0.0239, with uncertainties in the range of 12--19%. Our results indicate that the discrepancies between solar models constructed with low metallicity and the helioseismic observations extend to the solar core and thus cannot be attributed to deficiencies in the modeling of the solar convection zone.

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arXiv2006

Solar abundances and helioseismology: fine structure spacings and separation ratios of low-degree p modes

We have used 4752 days of data collected by the Birmingham Solar-Oscillations Network (BiSON) to determine very precise oscillation frequencies of acoustic low-degree modes that probe the solar core. We compare the fine (small frequency) spacings and frequency separation ratios formed from these data with those of different solar models. We find that models constructed with low metallicity are incompatible with the observations. The results provide strong support for lowering the theoretical uncertainties on the neutrino fluxes. These uncertainties had recently been raised due to the controversy over the solar abundances.

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arXiv2006

Life Products of Stars

We attempt to document complete energetic transactions of stars in their life. We calculate photon and neutrino energies that are produced from stars in their each phase of evolution from 1 to 8 M_sun, using the state-of-the-art stellar evolution code, tracing the evolution continuously from pre-main sequence gravitational contraction to white dwarfs. We also catalogue gravitational and thermal energies and helium, and heavier elements that are stored in stars and those ejected into interstellar space in each evolutionary phase.

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arXiv2005

10,000 Standard Solar Models: a Monte Carlo Simulation

We have evolved 10,000 solar models using 21 input parameters that are randomly drawn for each model from separate probability distributions for every parameter. We use the results of these models to determine the theoretical uncertainties in the predicted surface helium abundance, the profile of the sound speed versus radius, the profile of the density versus radius, the depth of the solar convective zone, the eight principal solar neutrino fluxes, and the fractions of nuclear reactions that occur in the CNO cycle or in the three branches of the p-p chains. We also determine the correlation coefficients of the neutrino fluxes for use in analysis of solar neutrino oscillations. Our calculations include the most accurate available input parameters, including radiative opacity, equation of state, and nuclear cross sections. We incorporate both the recently determined heavy element abundances recommended by Asplund, Grevesse & Sauval (2005) and the older (higher) heavy element abundances recommended by Grevesse & Sauval (1998). We present best-estimates of many characteristics of the standard solar model for both sets of recommended heavy element compositions.

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arXiv2005

What Is The Neon Abundance Of The Sun?

We have evolved a series of thirteen complete solar models that utilize different assumed heavy element compositions. Models that are based upon the heavy element abundances recently determined by Asplund, Grevesse, and Sauval (2005) are inconsistent with helioseismological measurements. However, models in which the neon abundance is increased by 0.4-0.5 dex to log N(Ne) = 8.29 +- 0.05 (on the scale in which log N(H) = 12) are consistent with the helioseismological measurements even though the other heavy element abundances are in agreement with the determinations of Asplund et al. (2005). These results sharpen and strengthen an earlier study by Antia and Basu (2005). The predicted solar neutrino fluxes are affected by the uncertainties in the composition by less than their 1sigma theoretical uncertainties.

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arXiv2005

How do Uncertainties in the Surface Chemical Abundances of the Sun Affect the Predicted Solar Neutrino Fluxes?

We show that uncertainties in the values of the surface heavy element abundances of the Sun are the largest source of the theoretical uncertainty in calculating the p-p, pep, 8B, 13N, 15O, and 17F solar neutrino fluxes. We evaluate for the first time the sensitivity (partial derivative) of each solar neutrino flux with respect to the surface abundance of each element. We then calculate the uncertainties in each neutrino flux using `conservative (preferred)' and `optimistic' estimates for the uncertainties in the element abundances. The total conservative (optimistic) composition uncertainty in the predicted 8B neutrino flux is 11.6% (5.0%) when sensitivities to individual element abundances are used. The traditional method that lumps all abundances into a single quantity (total heavy element to hydrogen ratio, Z/X) yields a larger uncertainty, 20%. The uncertainties in the carbon, oxygen, neon, silicon, sulphur, and iron abundances all make significant contributions to the uncertainties in calculating solar neutrino fluxes; the uncertainties of different elements are most important for different neutrino fluxes. The uncertainty in the iron abundance is the largest source of the estimated composition uncertainties of the important 7Be and 8B solar neutrinos. Carbon is the largest contributor to the uncertainty in the calculation of the p-p, 13N, and 15O neutrino fluxes. However, for all neutrino fluxes, several elements contribute comparable amounts to the total composition uncertainty.

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arXiv2005

New solar opacities, abundances, helioseismology, and neutrino fluxes

We construct solar models with the newly calculated radiative opacities from the Opacity Project (OP) and recently determined (lower) heavy element abundances. We compare results from the new models with predictions of a series of models that use OPAL radiative opacities, older determinations of the surface heavy element abundances, and refinements of nuclear reaction rates. For all the variations we consider, solar models that are constructed with the newer and lower heavy element abundances advocated by Asplund et al. (2005) disagree by much more than the estimated measuring errors with helioseismological determinations of the depth of the solar convective zone, the surface helium composition, the internal sound speeds, and the density profile. Using the new OP radiative opacities, the ratio of the 8B neutrino flux calculated with the older and larger heavy element abundances (or with the newer and lower heavy element abundances) to the total neutrino flux measured by the Sudbury Neutrino Observatory is 1.09 (0.87) with a 9% experimental uncertainty and a 16% theoretical uncertainty, 1 sigma errors.

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arXiv2004

Helioseismological Implications of Recent Solar Abundance Determinations

We show that standard solar models are in good agreement with the helioseismologically determined sound speed and density as a function of solar radius, the depth of the convective zone, and the surface helium abundance, as long as those models do not incorporate the most recent heavy element abundance determinations. However, sophisticated new analyses of the solar atmosphere infer lower abundances of the lighter metals (like C, N, O, Ne, and Ar) than the previously widely used surface abundances. We show that solar models that include the lower heavy element abundances disagree with the solar profiles of sound speed and density as well as the depth of the convective zone and the helium abundance. The disagreements for models with the new abundances range from factors of several to many times the quoted uncertainties in the helioseismological measurements. The disagreements are at temperatures below what is required for solar interior fusion reactions and therefore do not significantly affect solar neutrino emission. If errors in thecalculated OPAL opacities are solely responsible for the disagreements, then the corrections in the opacity must extend from 2 times 10^6 K (R = 0.7R_Sun)to 5 times 10^6 K (R = 0.4 R_Sun), with opacity increases of order 10%.

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arXiv2004

Supernova 1993J: a veiled pulsar in a binary system?

The recent report of a binary companion to the progenitor of supernova 1993J may provide important clues for identifying the nature of the nascent compact object. Given the estimates of the progenitor mass, the potential power source is probably a pulsar rather than an accreting black hole. If there is a pulsar, one would expect the rotational luminosity to be stored and reprocessed by the supernova remnant, but no pulsar nebula has yet been seen. The lack of detection of an X-ray synchrotron nebula should be taken as strong evidence against the presence of a bright pulsar. This is because absorption by the surrounding supernova gas should be negligible for the light of the companion star to have been detected. A model is developed here for the luminosity of the pulsar nebula in SN 1993J, which is then used to predict the spin-down power of the putative pulsar. If one exists, it can be providing no more than about 6 x 10^{39} erg/s. With an initial rotation period of 10-30 ms, as extrapolated from young galactic pulsars, the nascent neutron star can have either a weak magnetic field, B < 10^{11} G, or one so strong, B > 10^{15} G, that its spin was rapidly slowed down. The companion star, if bound to the neutron star, should provide ample targets for the pulsar wind to interact and produce high-energy gamma-rays. The expected non-pulsed, GeV signal is calculated; it could be detected by current and future experiments provided that the pulsar wind velocity is similar to that of the Crab Nebula.

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arXiv2004

How Accurately Can We Calculate the Depth of the Solar Convective Zone?

We evaluate the logarithmic derivative of the depth of the solar convective zone with respect to the logarithm of the radiative opacity. We use this expression to show that the radiative opacity near the base of the solar convective zone (CZ) must be known to an accuracy of +- 1% in order to calculate the CZ depth to the accuracy of the helioseismological measurement, R(CZ) = (0.713 +- 0.001)R(Sun). The radiative opacity near the base of the CZ that is obtained from OPAL tables must be increased by about 21% in the Bahcall-Pinsonneault (2004) solar model if one wants to invoke opacity errors in order to reconcile recent solar heavy abundance determinations with the helioseismological measurement of R(CZ). We show that the radiative opacity near the base of the convective zone depends sensitively upon the assumed heavy element mass fraction, Z. The uncertainty in the measured value of Z is currently the limiting factor in our ability to calculate the depth of the CZ. Different state-of-the-art interpolation schemes using the existing OPAL tables yield opacity values that differ by 4% . We describe the finer grid spacings that are necessary to interpolate the radiative opacity to 1%. Uncertainties due to the equation of state do not significantly affect the calculated depth of the convective zone.

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