arXiv subjects

Timothy R. Bedding

Publications and source records attributed to Timothy R. Bedding.

At least 181 records · Page 10Linked to original sources

arXiv2011

Calculating asteroseismic diagrams for solar-like oscillations

With the success of the Kepler and CoRoT missions, the number of stars with detected solar-like oscillations has increased by several orders of magnitude, for the first time we are able to perform large-scale ensemble asteroseismology of these stars. In preparation for this golden age of asteroseismology we have computed expected values of various asteroseismic observables from models of varying mass and metallicity. The relationships between these asteroseismic observables, such as the separations between mode frequencies, are able to significantly constrain estimates of the ages and masses of these stars. We investigate the scaling relation between the large frequency separation, Delta nu, and mean stellar density. Furthermore we present model evolutionary tracks for several asteroseismic diagrams. We have extended the so-called C-D diagram beyond the main sequence to the subgiants and the red-giant branch. We also consider another asteroseismic diagram, the epsilon diagram, which is more sensitive to variations in stellar properties at the subgiant stages and can aid in determining the correct mode identification. The recent discovery of gravity-mode period spacings in red giants forms the basis for a third asteroseismic diagram. We compare the evolutionary model tracks in these asteroseismic diagrams with results from pre-Kepler studies of solar-like oscillations, and early results from Kepler.

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arXiv2011

Solar-like Oscillations: An Observational Perspective

Solar-like oscillations are excited stochastically by convection. They occur in stars with an outer convection zone, which includes the Sun and other stars on the lower main sequence, as well as cool subgiants and red giants. This chapter reviews the basic properties of solar-like oscillations and summarises some of the main observational results. Mixed modes in subgiants and red giants are discussed in some detail, including an introduction to the so-called p-g diagram for avoided crossings.

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arXiv2011

AAOmega spectroscopy of 29 351 stars in fields centered on ten Galactic globular clusters

Galactic globular clusters have been pivotal in our understanding of many astrophysical phenomena. Here we publish the extracted stellar parameters from a recent large spectroscopic survey of ten globular clusters. A brief review of the project is also presented. Stellar parameters have been extracted from individual stellar spectra using both a modified version of the Radial Velocity Experiment (RAVE) pipeline and a pipeline based on the parameter estimation method of RAVE. We publish here all parameters extracted from both pipelines. We calibrate the metallicity and convert this to [Fe/H] for each star and, furthermore, we compare the velocities and velocity dispersions of the Galactic stars in each field to the Besançon Galaxy model. We find that the model does not correspond well with the data, indicating that the model is probably of little use for comparisons with pencil beam survey data such as this.

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arXiv2011

Amplitudes of solar-like oscillations: a new scaling relation

Solar-like oscillations are excited by near-surface convection and are being observed in growing numbers of stars using ground and space-based telescopes. We have previously suggested an empirical scaling relation to predict their amplitudes (Kjeldsen and Bedding 1995). This relation has found widespread use but it predicts amplitudes in F-type stars that are higher than observed. Here we present a new scaling relation that is based on the postulate that the power in velocity fluctuations due to p-mode oscillations scales with stellar parameters in the same way as the power in velocity fluctuations due to granulation. The new relation includes a dependence on the damping rate via the mode lifetime and should be testable using observations from the CoRoT and Kepler missions. We also suggest scaling relations for the properties of the background power due to granulation and argue that both these and the amplitude relations should be applicable to red giant stars.

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arXiv2011

Gravity modes as a way to distinguish between hydrogen- and helium-burning red giant stars

Red giants are evolved stars that have exhausted the supply of hydrogen in their cores and instead burn hydrogen in a surrounding shell. Once a red giant is sufficiently evolved, the helium in the core also undergoes fusion. Outstanding issues in our understanding of red giants include uncertainties in the amount of mass lost at the surface before helium ignition and the amount of internal mixing from rotation and other processes. Progress is hampered by our inability to distinguish between red giants burning helium in the core and those still only burning hydrogen in a shell. Asteroseismology offers a way forward, being a powerful tool for probing the internal structures of stars using their natural oscillation frequencies. Here we report observations of gravity-mode period spacings in red giants that permit a distinction between evolutionary stages to be made. We use high-precision photometry obtained with the Kepler spacecraft over more than a year to measure oscillations in several hundred red giants. We find many stars whose dipole modes show sequences with approximately regular period spacings. These stars fall into two clear groups, allowing us to distinguish unambiguously between hydrogen-shell-burning stars (period spacing mostly about 50 seconds) and those that are also burning helium (period spacing about 100 to 300 seconds).

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arXiv2011

Solar-like oscillations and activity in Procyon: A comparison of the 2007 MOST and ground-based radial velocity campaigns

We compare the simultaneous 2007 space-based MOST photometry and ground-based radial velocity observations of the F5 star Procyon. We identify slow variations in the MOST data that are similar to those reported in the radial velocity (RV) time series, and confirm by comparison with the Sun that these variations are likely the signature of stellar activity. The MOST power spectrum yields clear evidence for individual oscillation frequencies that match those found in the radial velocity data by Bedding et al. (2010). We identify the same ridges due to modes of different spherical degree in both datasets, but are not able to confirm a definite ridge identification using the MOST data. We measure the luminosity amplitude per radial mode A_{l=0, phot} = 9.1 +/- 0.5 ppm. Combined with the estimate for the RV data by Arentoft et al. (2008) this gives a mean amplitude ratio of A_{l=0, phot}/A_{l=0, RV} = 0.24 +/- 0.02 ppm cm^{-1} s, considerably higher than expected from scaling relations but in reasonable agreement with theoretical models by Houdek (2010). We also compare the amplitude ratio as a function of frequency, and find that the maximum of the oscillation envelope is shifted to higher frequencies in photometry than in velocity.

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arXiv2011

Sounding open clusters: asteroseismic constraints from Kepler on the properties of NGC 6791 and NGC 6819

We present initial results on some of the properties of open clusters NGC 6791 and NGC 6819 derived from asteroseismic data obtained by NASA's Kepler mission. In addition to estimating the mass, radius and log g of stars on the red-giant branch of these clusters, we estimate the distance to the clusters and their ages. Our model-independent estimate of the distance modulus of NGC 6791 is (m-M)_0= 13.11\pm 0.06. We find (m-M)_0= 11.85\pm 0.05 for NGC 6819. The average mass of stars on the red-giant branch of NGC 6791 is 1.20 \pm 0.01 M_sun, while that of NGC 6819 is 1.68\pm 0.03M_sun. It should be noted that we do not have data that cover the entire red-giant branch and the actual mass will be somewhat lower. We have determined model-dependent estimates of ages of these clusters. We find ages between 6.8 and 8.6 Gyr for NGC 6791, however, most sets of models give ages around 7Gyr. We obtain ages between 2 and 2.4 Gyr for NGC 6819.

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arXiv2010

A comparison of Bayesian and Fourier methods for frequency determination in asteroseismology

Bayesian methods are becoming more widely used in asteroseismic analysis. In particular, they are being used to determine oscillation frequencies, which are also commonly found by Fourier analysis. It is important to establish whether the Bayesian methods provide an improvement on Fourier methods. We compare, using simulated data, the standard iterative sine-wave fitting method against a Markov Chain Monte Carlo (MCMC) code that has been introduced to infer purely the frequencies of oscillation modes (Brewer et al. 2007). A uniform prior probability distribution function is used for the MCMC method. We find the methods do equally well at determining the correct oscillation frequencies, although the Bayesian method is able to highlight the possibility of a misidentification due to aliasing, which can be useful. In general, we suggest that the least computationally intensive method is preferable.

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arXiv2010

Halo globular clusters observed with AAOmega: dark matter content, metallicity and tidal heating

Globular clusters have proven to be essential to our understanding of many important astrophysical phenomena. Here we analyse spectroscopic observations of ten Halo globular clusters to determine their dark matter content, their tidal heating by the Galactic disc and halo, describe their metallicities and the likelihood that Newtonian dynamics explain their kinematics. We analyse a large number of members in all clusters, allowing us to address all these issues together, and we have included NGC 288 and M30 to overlap with previous studies. We find that any flattening of the velocity dispersion profiles in the outer regions of our clusters can be explained by tidal heating. We also find that all our GCs have M/L_V < 5, therefore, we infer the observed dynamics do not require dark matter, or a modification of gravity. We suggest that the lack of tidal heating signatures in distant clusters indicates the Halo is not triaxial. The isothermal rotations of each cluster are measured, with M4 and NGC 288 exhibiting rotation at a level of 0.9 +/- 0.1 km/s and 0.25 +/- 0.15 km/s, respectively. We also indirectly measure the tidal radius of NGC 6752, determining a more realistic figure for this cluster than current literature values. Lastly, an unresolved and intriguing puzzle is uncovered with regard to the cooling of the outer regions of all ten clusters.

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arXiv2010

AAOmega Observations of 47 Tucanae: Evidence for a Past Merger?

The globular cluster 47 Tucanae is well studied but it has many characteristics that are unexplained, including a significant rise in the velocity dispersion profile at large radii, indicating the exciting possibility of two distinct kinematic populations. In this Letter we employ a Bayesian approach to the analysis of the largest available spectral dataset of 47 Tucanae to determine whether this apparently two-component population is real. Assuming the two models were equally likely before taking the data into account, we find that the evidence favours the two-component population model by a factor of ~3x10^7. Several possible explanations for this result are explored, namely the evaporation of low-mass stars, a hierarchical merger, extant remnants of two initially segregated populations, and multiple star formation epochs. We find the most compelling explanation for the two-component velocity distribution is that 47 Tuc formed as two separate populations arising from the same proto-cluster cloud which merged <7.3 +/- 1.5 Gyr ago. This may also explain the extreme rotation, low mass-to-light ratio and mixed stellar populations of this cluster.

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arXiv2010

Scaled oscillation frequencies and echelle diagrams as a tool for comparative asteroseismology

We describe a method for comparing the frequency spectra of oscillating stars. We focus on solar-like oscillations, in which mode frequencies generally follow a regular pattern. On the basis that oscillation frequencies of similar stars scale homologously, we show how to display two stars on a single echelle diagram. The result can be used to infer the ratio of their mean densities very precisely, without reference to theoretical models. In addition, data from the star with the better signal-to-noise ratio can be used to confirm weaker modes and reject sidelobes in data from the second star. Finally, we show that scaled echelle diagrams provide a solution to the problem of ridge identification in F-type stars, such as those observed by the CoRoT space mission.

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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

Automated extraction of oscillation parameters for Kepler observations of solar-type stars

The recent launch of the Kepler space telescope brings the opportunity to study oscillations systematically in large numbers of solar-like stars. In the framework of the asteroFLAG project, we have developed an automated pipeline to estimate global oscillation parameters, such as the frequency of maximum power (nu_max) and the large frequency spacing (Delta_nu), for a large number of time series. We present an effective method based on the autocorrelation function to find excess power and use a scaling relation to estimate granulation timescales as initial conditions for background modelling. We derive reliable uncertainties for nu_max and Delta_nu through extensive simulations. We have tested the pipeline on about 2000 simulated Kepler stars with magnitudes of V~7-12 and were able to correctly determine nu_max and Delta_nu for about half of the sample. For about 20%, the returned large frequency spacing is accurate enough to determine stellar radii to a 1% precision. We conclude that the methods presented here are a promising approach to process the large amount of data expected from Kepler.

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arXiv2009

Testing Newtonian Gravity with AAOmega: Mass-To-Light Profiles and Metallicity Calibrations From 47 Tuc and M55

Globular clusters are an important test bed for Newtonian gravity in the weak-acceleration regime, which is vital to our understanding of the nature of the gravitational interaction. Recent claims have been made that the velocity dispersion profiles of globular clusters flatten out at large radii, despite an apparent paucity of dark matter in such objects, indicating the need for a modification of gravitational theories. We continue our investigation of this claim, with the largest spectral samples ever obtained of 47 Tucanae and M55. Furthermore, this large sample allows for an accurate metallicity calibration based on the equivalent widths of the calcium triplet lines and K band magnitude of the Tip of the Red Giant Branch. Assuming an isothermal distribution, the rotations of each cluster are also measured with both clusters exhibiting clear rotation signatures. The global velocity dispersions of NGC 121 and Kron 3, two globular clusters in the Small Magellanic Cloud, are also calculated. By applying a simple dynamical model to the velocity dispersion profiles of 47 Tuc and M55, we calculate their mass-to-light profiles, total masses and central velocity dispersions. We find no statistically significant flattening of the velocity dispersion at large radii for M55, and a marked increase in the profile of 47 Tuc for radii greater than approximately half the tidal radius. We interpret this increase as an evaporation signature, indicating that 47 Tuc is undergoing, or has undergone, core-collapse, but find no requirement for dark matter or a modification of gravitational theories in either cluster.

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arXiv2009

Hipparcos calibration of the tip of the Red Giant Branch

We have detected the tip of the Red Giant Branch (TRGB) in the solar neighborhood using near infrared photometry from the 2MASS and DIRBE catalogs, and revised Hipparcos parallaxes. We confirm that the revised Hipparcos parallaxes are superior to the original ones, and that this improvement is necessary to detect the TRGB. We find a tip absolute magnitude of M_K=-6.85+/-0.03, in agreement with that expected from previous tip measurements of the Large Magellanic Cloud, Small Magellanic Cloud, and Bulge. This represents the first geometric calibration of the TRGB and extends previous calibrations, based on metal-poor globular clusters, to solar metallicities. We attempted to use the TRGB to confirm the presence of the Lutz-Kelker bias, with inconclusive results. Attempts to detect the tip in the I-band also produced inconsistent results, due to a lack of precise, homogeneous photometry for these bright stars.

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arXiv2009

Testing Newtonian Gravity with AAOmega: Mass-to-Light Profiles of Four Globular Clusters

Testing Newtonian gravity in the weak-acceleration regime is vital to our understanding of the nature of the gravitational interaction. It has recently been claimed that the velocity dispersion profiles of several globular clusters flatten out at large radii, reminiscent of galaxy rotation curves, even though globular clusters are thought to contain little or no dark matter. We investigate this claim, using AAOmega observations of four globular clusters, namely M22, M30, M53 and M68. M30, one such cluster that has had this claim made for its velocity dispersion, was included for comparison with previous studies. We find no statistically significant flattening of the velocity dispersion at large radii for any of our target clusters and therefore we infer the observed dynamics do not require that globular clusters are dark matter dominated, or a modification of gravity. Furthermore, by applying a simple dynamical model we determine the radial mass-to-light profiles for each cluster. The isothermal rotations of each cluster are also measured, with M22 exhibiting clear rotation, M68 possible rotation and M30 and M53 lacking any rotation, within the uncertainties.

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arXiv2008

A multi-site campaign to measure solar-like oscillations in Procyon. I. Observations, Data Reduction and Slow Variations

We have carried out a multi-site campaign to measure oscillations in the F5 star Procyon A. We obtained high-precision velocity observations over more than three weeks with eleven telescopes, with almost continuous coverage for the central ten days. This represents the most extensive campaign so far organized on any solar-type oscillator. We describe in detail the methods we used for processing and combining the data. These involved calculating weights for the velocity time series from the measurement uncertainties and adjusting them in order to minimize the noise level of the combined data. The time series of velocities for Procyon shows the clear signature of oscillations, with a plateau of excess power that is centred at 0.9 mHz and is broader than has been seen for other stars. The mean amplitude of the radial modes is 38.1 +/- 1.3 cm/s (2.0 times solar), which is consistent with previous detections from the ground and by the WIRE spacecraft, and also with the upper limit set by the MOST spacecraft. The variation of the amplitude during the observing campaign allows us to estimate the mode lifetime to be 1.5 d (+1.9/-0.8 d). We also find a slow variation in the radial velocity of Procyon, with good agreement between different telescopes. These variations are remarkably similar to those seen in the Sun, and we interpret them as being due to rotational modulation from active regions on the stellar surface. The variations appear to have a period of about 10 days, which presumably equals the stellar rotation period or, perhaps, half of it. The amount of power in these slow variations indicates that the fractional area of Procyon covered by active regions is slightly higher than for the Sun.

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arXiv2008

Correcting stellar oscillation frequencies for near-surface effects

In helioseismology, there is a well-known offset between observed and computed oscillation frequencies. This offset is known to arise from improper modeling of the near-surface layers of the Sun, and a similar effect must occur for models of other stars. Such an effect impedes progress in asteroseismology, which involves comparing observed oscillation frequencies with those calculated from theoretical models. Here, we use data for the Sun to derive an empirical correction for the near-surface offset, which we then apply three other stars (alpha Cen A, alpha Cen B and beta Hyi). The method appears to give good results, in particular providing an accurate estimate of the mean density of each star.

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