arXiv subjects

J. Christensen-Dalsgaard

Publications and source records attributed to J. Christensen-Dalsgaard.

At least 55 records · Page 3Linked to original sources

arXiv2016

Detection of Solar-Like Oscillations, Observational Constraints, and Stellar Models for $θ$ Cyg, the Brightest Star Observed by the {\it Kepler} Mission

$θ$ Cygni is an F3 spectral-type main-sequence star with visual magnitude V=4.48. This star was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were obtained during Quarter 6 (June-September 2010) and subsequently in Quarters 8 and 12-17. We present analyses of the solar-like oscillations based on Q6 and Q8 data, identifying angular degree $l$ = 0, 1, and 2 oscillations in the range 1000-2700 microHz, with a large frequency separation of 83.9 plus/minus 0.4 microHz, and frequency with maximum amplitude 1829 plus/minus 54 microHz. We also present analyses of new ground-based spectroscopic observations, which, when combined with angular diameter measurements from interferometry and Hipparcos parallax, give T_eff = 6697 plus/minus 78 K, radius 1.49 plus/minus 0.03 solar radii, [Fe/H] = -0.02 plus/minus 0.06 dex, and log g = 4.23 plus/minus 0.03. We calculate stellar models matching the constraints using several methods, including using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses 1.35-1.39 solar masses and ages 1.0-1.6 Gyr. theta Cyg's T_eff and log g place it cooler than the red edge of the gamma Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show gamma Dor gravity-mode pulsations driven by the convective-blocking mechanism, with frequencies of 1 to 3 cycles/day (11 to 33 microHz). However, gravity modes were not detected in the Kepler data, one signal at 1.776 cycles/day (20.56 microHz) may be attributable to a faint, possibly background, binary. Asteroseismic studies of theta Cyg and other A-F stars observed by Kepler and CoRoT, will help to improve stellar model physics and to test pulsation driving mechanisms.

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arXiv2016

SpaceInn hare-and-hounds exercise: Estimation of stellar properties using space-based asteroseismic data

Context: Detailed oscillation spectra comprising individual frequencies for numerous solar-type stars and red giants are or will become available. These data can lead to a precise characterisation of stars. Aims: Our goal is to test and compare different methods for obtaining stellar properties from oscillation frequencies and spectroscopic constraints, in order to evaluate their accuracy and the reliability of the error bars. Methods: In the context of the SpaceInn network, we carried out a hare-and-hounds exercise in which one group produced "observed" oscillation spectra for 10 artificial solar-type stars, and various groups characterised these stars using either forward modelling or acoustic glitch signatures. Results: Results based on the forward modelling approach were accurate to 1.5 % (radius), 3.9 % (mass), 23 % (age), 1.5 % (surface gravity), and 1.8 % (mean density). For the two 1 Msun stellar targets, the accuracy on the age is better than 10 % thereby satisfying PLATO 2.0 requirements. The average accuracies for the acoustic radii of the base of the convection zone, the He II ionisation, and the Gamma_1 peak were 17 %, 2.4 %, and 1.9 %, respectively. Glitch fitting analysis seemed to be affected by aliasing problems for some of the targets. Conclusions: Forward modelling is the most accurate approach, but needs to be complemented by model-independent results from, e.g., glitch analysis. Furthermore, global optimisation algorithms provide more robust error bars.

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arXiv2016

Hot super-Earths stripped by their host stars

Simulations predict that hot super-Earth sized exoplanets can have their envelopes stripped by photo-evaporation, which would present itself as a lack of these exoplanets. However, this absence in the exoplanet population has escaped a firm detection. Here we demonstrate, using asteroseismology on a sample of exoplanets and exoplanet candidates observed during the Kepler mission that, while there is an abundance of super-Earth sized exoplanets with low incident fluxes, none are found with high incident fluxes. We do not find any exoplanets with radii between 2.2 and 3.8 Earth radii with incident flux above 650 times the incident flux on Earth. This gap in the population of exoplanets is explained by evaporation of volatile elements and thus supports the predictions. The confirmation of a hot-super-Earth desert caused by evaporation will add an important constraint on simulations of planetary systems, since they must be able to reproduce the dearth of close-in super-Earths.

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arXiv2016

On the uncertain nature of the core of $\boldsymbolα$ Cen A

High-quality astrometric, spectroscopic, interferometric and, importantly, asteroseismic observations are available for $α$ Cen A, which is the closest binary star system to earth. Taking all these constraints into account, we study the internal structure of the star by means of theoretical modelling. Using the Aarhus STellar Evolution Code (ASTEC) and the tools of Computational Bayesian Statistics, in particular a Markov chain Monte Carlo algorithm, we perform statistical inferences for the physical characteristics of the star. We find that $α$ Cen A has a probability of approximately 40% of having a convective core. This probability drops to few percents if one considers reduced rates for the $^{14}$N(p,$γ$)$^{15}$O reaction. These convective cores have fractional radii less than 8% when overshoot is neglected. Including overshooting also leads to the possibility of a convective core mostly sustained by the ppII chain energy output. We finally show that roughly 30% of the stellar models describing $α$ Cen A are in the subgiant regime.

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arXiv2016

Asteroseismology with solar-like oscillations

Almost 100 years ago Sir Arthur Eddington noted that the interiors of stars were inaccessible to observations. The advent of helio- and asteroseismology has completely changed this assessment. Helioseismology has provided very detailed information about the interior structure and dynamics of the Sun, highlighting remaining issues in our understanding of the solar interior. In the last decade extensive observations of stellar oscillations, in particular from space photometry, have provided very detailed information about the global and internal properties of stars. Here I provide an overview of these developments, including the remarkable insight that has been obtained on the properties of evolved stars.

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arXiv2016

Testing the asymptotic relation for period spacings from mixed modes of red giants observed with the Kepler mission

Dipole mixed pulsation modes of consecutive radial order have been detected for thousands of low-mass red-giant stars with the NASA space telescope Kepler. Such modes have the potential to reveal information on the physics of the deep stellar interior. Different methods have been proposed to derive an observed value for the gravity-mode period spacing, the most prominent one relying on a relation derived from asymptotic pulsation theory applied to the gravity-mode character of the mixed modes. Our aim is to compare results based on this asymptotic relation with those derived from an empirical approach for three pulsating red-giant stars. We developed a data-driven method to perform frequency extraction and mode identification. Next, we used the identified dipole mixed modes to determine the gravity-mode period spacing by means of an empirical method and by means of the asymptotic relation. In our methodology, we consider the phase offset, $ε_{\mathrm{g}}$, of the asymptotic relation as a free parameter. Using the frequencies of the identified dipole mixed modes for each star in the sample, we derived a value for the gravity-mode period spacing using the two different methods. These differ by less than 5%. The average precision we achieved for the period spacing derived from the asymptotic relation is better than 1%, while that of our data-driven approach is 3%. Good agreement is found between values for the period spacing derived from the asymptotic relation and from the empirical method. Full abstract in PDF file.

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arXiv2016

Spin-orbit alignment of exoplanet systems: ensemble analysis using asteroseismology

The angle $ψ$ between a planet's orbital axis and the spin axis of its parent star is an important diagnostic of planet formation, migration, and tidal evolution. We seek empirical constraints on $ψ$ by measuring the stellar inclination $i_{\rm s}$ via asteroseismology for an ensemble of 25 solar-type hosts observed with NASA's Kepler satellite. Our results for $i_{\rm s}$ are consistent with alignment at the 2-$σ$ level for all stars in the sample, meaning that the system surrounding the red-giant star Kepler-56 remains as the only unambiguous misaligned multiple-planet system detected to date. The availability of a measurement of the projected spin-orbit angle $λ$ for two of the systems allows us to estimate $ψ$. We find that the orbit of the hot-Jupiter HAT-P-7b is likely to be retrograde ($ψ=116.4^{+30.2}_{-14.7}\:{\rm deg}$), whereas that of Kepler-25c seems to be well aligned with the stellar spin axis ($ψ=12.6^{+6.7}_{-11.0}\:{\rm deg}$). While the latter result is in apparent contradiction with a statement made previously in the literature that the multi-transiting system Kepler-25 is misaligned, we show that the results are consistent, given the large associated uncertainties. Finally, we perform a hierarchical Bayesian analysis based on the asteroseismic sample in order to recover the underlying distribution of $ψ$. The ensemble analysis suggests that the directions of the stellar spin and planetary orbital axes are correlated, as conveyed by a tendency of the host stars to display large inclination values.

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arXiv2015

Towards 21st Century Stellar Models: Star Clusters, Supercomputing, and Asteroseismology

Stellar models provide a vital basis for many aspects of astronomy and astrophysics. Recent advances in observational astronomy -- through asteroseismology, precision photometry, high-resolution spectroscopy, and large-scale surveys -- are placing stellar models under greater quantitative scrutiny than ever. The model limitations are being exposed and the next generation of stellar models is needed as soon as possible. The current uncertainties in the models propagate to the later phases of stellar evolution, hindering our understanding of stellar populations and chemical evolution. Here we give a brief overview of the evolution, importance, and substantial uncertainties of core helium burning stars in particular and then briefly discuss a range of methods, both theoretical and observational, that we are using to advance the modelling.

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arXiv2015

Internal rotation of the red-giant star KIC 4448777 by means of asteroseismic inversion

In this paper we study the dynamics of the stellar interior of the early red-giant star KIC 4448777 by asteroseismic inversion of 14 splittings of the dipole mixed modes obtained from {\it Kepler} observations. In order to overcome the complexity of the oscillation pattern typical of red-giant stars, we present a procedure which involves a combination of different methods to extract the rotational splittings from the power spectrum. We find not only that the core rotates faster than the surface, confirming previous inversion results generated for other red giants (Deheuvels et al. 2012,2014), but we also estimate the variation of the angular velocity within the helium core with a spatial resolution of $Δr=0.001R$ and verify the hypothesis of a sharp discontinuity in the inner stellar rotation (Deheuvels et al. 2014). The results show that the entire core rotates rigidly with an angular velocity of about $\langleΩ_c/2π\rangle=748\pm18$~nHz and provide evidence for an angular velocity decrease through a region between the helium core and part of the hydrogen burning shell; however we do not succeed to characterize the rotational slope, due to the intrinsic limits of the applied techniques. The angular velocity, from the edge of the core and through the hydrogen burning shell, appears to decrease with increasing distance from the center, reaching an average value in the convective envelope of $\langleΩ_s/2π\rangle=68\pm22$~nHz. Hence, the core in KIC~4448777 is rotating from a minimum of 8 to a maximum of 17 times faster than the envelope. We conclude that a set of data which includes only dipolar modes is sufficient to infer quite accurately the rotation of a red giant not only in the dense core but also, with a lower level of confidence, in part of the radiative region and in the convective envelope.

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arXiv2015

Oscillation frequencies for 35 \Kepler solar-type planet-hosting stars using Bayesian techniques and machine learning

\Kepler has revolutionised our understanding of both exoplanets and their host stars. Asteroseismology is a valuable tool in the characterisation of stars and \Kepler is an excellent observing facility to perform asteroseismology. Here we select a sample of 35 \Kepler solar-type stars which host transiting exoplanets (or planet candidates) with detected solar-like oscillations. Using available \Kepler short cadence data up to Quarter 16 we create power spectra optimised for asteroseismology of solar-type stars. We identify modes of oscillation and estimate mode frequencies by ``peak bagging'' using a Bayesian MCMC framework. In addition, we expand the methodology of quality assurance using a Bayesian unsupervised machine learning approach. We report the measured frequencies of the modes of oscillation for all 35 stars and frequency ratios commonly used in detailed asteroseismic modelling. Due to the high correlations associated with frequency ratios we report the covariance matrix of all frequencies measured and frequency ratios calculated. These frequencies, frequency ratios, and covariance matrices can be used to obtain tight constraint on the fundamental parameters of these planet-hosting stars.

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arXiv2015

IAU 2015 Resolution B2 on Recommended Zero Points for the Absolute and Apparent Bolometric Magnitude Scales

The XXIXth IAU General Assembly in Honolulu adopted IAU 2015 Resolution B2 on recommended zero points for the absolute and apparent bolometric magnitude scales. The resolution was proposed by the IAU Inter-Division A-G Working Group on Nominal Units for Stellar and Planetary Astronomy after consulting with a broad spectrum of researchers from the astronomical community. Resolution B2 resolves the long-standing absence of an internationally-adopted zero point for the absolute and apparent bolometric magnitude scales. Resolution B2 defines the zero point of the absolute bolometric magnitude scale such that a radiation source with $M_{\rm Bol}$ = 0 has luminosity L$_{\circ}$ = 3.0128e28 W. The zero point of the apparent bolometric magnitude scale ($m_{\rm Bol}$ = 0) corresponds to irradiance $f_{\circ}$ = 2.518021002e-8 W/m$^2$. The zero points were chosen so that the nominal solar luminosity (3.828e26 W) adopted by IAU 2015 Resolution B3 corresponds approximately to $M_{\rm Bol}$(Sun) = 4.74, the value most commonly adopted in recent literature. The nominal total solar irradiance (1361 W/m$^2$) adopted in IAU 2015 Resolution B3 corresponds approximately to apparent bolometric magnitude $m_{\rm bol}$(Sun) = -26.832. Implicit in the IAU 2015 Resolution B2 definition of the apparent bolometric magnitude scale is an exact definition for the parsec (648000/$π$ au) based on the IAU 2012 Resolution B2 definition of the astronomical unit.

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arXiv2015

IAU 2015 Resolution B3 on Recommended Nominal Conversion Constants for Selected Solar and Planetary Properties

Astronomers commonly quote the properties of celestial objects in units of parameters for the Sun, Jupiter, or the Earth. The resolution presented here was proposed by the IAU Inter-Division Working Group on Nominal Units for Stellar and Planetary Astronomy and passed by the XXIXth IAU General Assembly in Honolulu. IAU 2015 Resolution B3 adopts a set of nominal solar, terrestrial, and jovian conversion constants for stellar and (exo)planetary astronomy which are defined to be exact SI values. While the nominal constants are based on current best estimates (CBEs; which have uncertainties, are not secularly constant, and are updated regularly using new observations), they should be interpreted as standard values and not as CBEs. IAU 2015 Resolution B3 adopts five solar conversion constants (nominal solar radius, nominal total solar irradiance, nominal solar luminosity, nominal solar effective temperature, and nominal solar mass parameter) and six planetary conversion constants (nominal terrestrial equatorial radius, nominal terrestrial polar radius, nominal jovian equatorial radius, nominal jovian polar radius, nominal terrestrial mass parameter, and nominal jovian mass parameter).

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arXiv2015

On the red-giant luminosity bump

The increase in luminosity as a star evolves on the red-giant branch is interrupted briefly when the hydrogen-burning shell reaches the vicinity of the composition discontinuity left behind from the first convective dredge-up. The non-monotonic variation of luminosity causes an accumulation of stars, known as the `bump', in the distribution of stars in the colour-magnitude diagrams of stellar clusters, which has substantial diagnostic potential. Here I present numerical results on this behaviour and discuss the physical reason for the luminosity variation, with the goal of strengthening the understanding of origin of the phenomenon and hence of its diagnostic potential.

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arXiv2015

Asteroseismology of solar-type stars with K2

We present the first detections by the NASA K2 Mission of oscillations in solar-type stars, using short-cadence data collected during K2 Campaign\,1 (C1). We understand the asteroseismic detection thresholds for C1-like levels of photometric performance, and we can detect oscillations in subgiants having dominant oscillation frequencies around $1000\,\rm μHz$. Changes to the operation of the fine-guidance sensors are expected to give significant improvements in the high-frequency performance from C3 onwards. A reduction in the excess high-frequency noise by a factor of two-and-a-half in amplitude would bring main-sequence stars with dominant oscillation frequencies as high as ${\simeq 2500}\,\rm μHz$ into play as potential asteroseismic targets for K2.

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arXiv2015

Ages and fundamental properties of Kepler exoplanet host stars from asteroseismology

We present a study of 33 {\it Kepler} planet-candidate host stars for which asteroseismic observations have sufficiently high signal-to-noise ratio to allow extraction of individual pulsation frequencies. We implement a new Bayesian scheme that is flexible in its input to process individual oscillation frequencies, combinations of them, and average asteroseismic parameters, and derive robust fundamental properties for these targets. Applying this scheme to grids of evolutionary models yields stellar properties with median statistical uncertainties of 1.2\% (radius), 1.7\% (density), 3.3\% (mass), 4.4\% (distance), and 14\% (age), making this the exoplanet host-star sample with the most precise and uniformly determined fundamental parameters to date. We assess the systematics from changes in the solar abundances and mixing-length parameter, showing that they are smaller than the statistical errors. We also determine the stellar properties with three other fitting algorithms and explore the systematics arising from using different evolution and pulsation codes, resulting in 1\% in density and radius, and 2\% and 7\% in mass and age, respectively. We confirm previous findings of the initial helium abundance being a source of systematics comparable to our statistical uncertainties, and discuss future prospects for constraining this parameter by combining asteroseismology and data from space missions. Finally we compare our derived properties with those obtained using the global average asteroseismic observables along with effective temperature and metallicity, finding an excellent level of agreement. Owing to selection effects, our results show that the majority of the high signal-to-noise ratio asteroseismic {\it Kepler} host stars are older than the Sun.

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arXiv2015

Structural glitches near the cores of red giants revealed by oscillations in g-mode period spacings from stellar models

With recent advances in asteroseismology it is now possible to peer into the cores of red giants, potentially providing a way to study processes such as nuclear burning and mixing through their imprint as sharp structural variations -- glitches -- in the stellar cores. Here we show how such core glitches can affect the oscillations we observe in red giants. We derive an analytical expression describing the expected frequency pattern in the presence of a glitch. This formulation also accounts for the coupling between acoustic and gravity waves. From an extensive set of canonical stellar models we find glitch-induced variation in the period spacing and inertia of non-radial modes during several phases of red-giant evolution. Significant changes are seen in the appearance of mode amplitude and frequency patterns in asteroseismic diagrams such as the power spectrum and the échelle diagram. Interestingly, along the red-giant branch glitch-induced variation occurs only at the luminosity bump, potentially providing a direct seismic indicator of stars in that particular evolution stage. Similarly, we find the variation at only certain post-helium-ignition evolution stages, namely, in the early phases of helium-core burning and at the beginning of helium-shell burning signifying the asymptotic-giant-branch bump. Based on our results, we note that assuming stars to be glitch-free, while they are not, can result in an incorrect estimate of the period spacing. We further note that including diffusion and mixing beyond classical Schwarzschild, could affect the characteristics of the glitches, potentially providing a way to study these physical processes.

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arXiv2015

KOI-3158: The oldest known system of terrestrial-size planets

The first discoveries of exoplanets around Sun-like stars have fueled efforts to find ever smaller worlds evocative of Earth and other terrestrial planets in the Solar System. While gas-giant planets appear to form preferentially around metal-rich stars, small planets (with radii less than four Earth radii) can form under a wide range of metallicities. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the Universe's history when metals were far less abundant. We report Kepler spacecraft observations of KOI-3158, a metal-poor Sun-like star from the old population of the Galactic thick disk, which hosts five planets with sizes between Mercury and Venus. We used asteroseismology to directly measure a precise age of 11.2+/-1.0 Gyr for the host star, indicating that KOI-3158 formed when the Universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, providing scope for the existence of ancient life in the Galaxy.

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arXiv2015

An ancient extrasolar system with five sub-Earth-size planets

The chemical composition of stars hosting small exoplanets (with radii less than four Earth radii) appears to be more diverse than that of gas-giant hosts, which tend to be metal-rich. This implies that small, including Earth-size, planets may have readily formed at earlier epochs in the Universe's history when metals were more scarce. We report Kepler spacecraft observations of Kepler-444, a metal-poor Sun-like star from the old population of the Galactic thick disk and the host to a compact system of five transiting planets with sizes between those of Mercury and Venus. We validate this system as a true five-planet system orbiting the target star and provide a detailed characterization of its planetary and orbital parameters based on an analysis of the transit photometry. Kepler-444 is the densest star with detected solar-like oscillations. We use asteroseismology to directly measure a precise age of 11.2+/-1.0 Gyr for the host star, indicating that Kepler-444 formed when the Universe was less than 20% of its current age and making it the oldest known system of terrestrial-size planets. We thus show that Earth-size planets have formed throughout most of the Universe's 13.8-billion-year history, leaving open the possibility for the existence of ancient life in the Galaxy. The age of Kepler-444 not only suggests that thick-disk stars were among the hosts to the first Galactic planets, but may also help to pinpoint the beginning of the era of planet formation.

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