T. H. Dall

Bio: T. H. Dall is an academic researcher from European Southern Observatory. The author has contributed to research in topics: Stars & Solar-like oscillations. The author has an hindex of 21, co-authored 47 publications receiving 1627 citations.

The Amplitude of Solar Oscillations Using Stellar Techniques

Abstract: The amplitudes of solar-like oscillations depend on the excitation and damping, both of which are controlled by convection. Comparing observations with theory should therefore improve our understanding of the underlying physics. However, theoretical models invariably compute oscillation amplitudes relative to the Sun, and it is therefore vital to have a good calibration of the solar amplitude using stellar techniques. We have used daytime spectra of the Sun, obtained with HARPS and UCLES, to measure the solar oscillations and made a detailed comparison with observations using the BiSON helioseismology instrument. We find that the mean solar amplitude measured using stellar techniques, averaged over one full solar cycle, is 18.7 ± 0.7 cm s−1 for the strongest radial modes (l = 0) and 25.2 ± 0.9 cm s−1 for l = 1. In addition, we use simulations to establish an equation that estimates the uncertainty of amplitude measurements that are made of other stars, given that the mode lifetime is known. Finally, we also give amplitudes of solar-like oscillations for three stars that we measured from a series of short observations with HARPS (γ Ser, β Aql, and α For), together with revised amplitudes for five other stars for which we have previously published results (α Cen A, α Cen B, β Hyi, ν Ind, and δ Pav).

A Multisite Campaign to Measure Solar-like Oscillations in Procyon. I. Observations, Data Reduction, and Slow Variations

Abstract: We have carried out a multisite campaign to measure oscillations in the F5 star Procyon A. We obtained high-precision velocity observations over more than three weeks with 11 telescopes, with almost continuous coverage for the central 10 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 centered at 0.9 mHz and is broader than has been seen for other stars. The mean amplitude of the radial modes is 38:1 AE 1:3 cm s A1 (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 þ1:9 A0:8 days. 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.

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

Abstract: 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.

Solar-like Oscillations in the G2 Subgiant β Hydri from Dual-Site Observations

Abstract: We have observed oscillations in the nearby G2 subgiant starHyi using high-precision velocity observations obtained over more than a week with the HARPS and UCLES spectrographs. The oscillation frequencies show a regularcombstructure,asexpectedforsolar-likeoscillations,butwithseverall ¼ 1modesbeingstronglyaffectedby avoided crossings. These data, combined with those we obtained five years earlier, allow us to identify 28 oscillation modes.Byscalingthelarge-frequencyseparationfromtheSun,wemeasurethemeandensityofHyitoanaccuracy of 0.6%. The amplitudes of the oscillations are about 2.5 times solar and the mode lifetime is 2.3 days. A detailed comparison of the mixed l ¼ 1 modes with theoretical models should allow a precise estimate of the age of the star.

A multi-site campaign to measure solar-like oscillations in procyon. ii. mode frequencies

Abstract: We have analyzed data from a multi-site campaign to observe oscillations in the F5 star Procyon. The data consist of high-precision velocities that we obtained over more than three weeks with 11 telescopes. A new method for adjusting the data weights allows us to suppress the sidelobes in the power spectrum. Stacking the power spectrum in a so-called echelle diagram reveals two clear ridges, which we identify with even and odd values of the angular degree (l = 0 and 2, and l = 1 and 3, respectively). We interpret a strong, narrow peak at 446 μHz that lies close to the l = 1 ridge as a mode with mixed character. We show that the frequencies of the ridge centroids and their separations are useful diagnostics for asteroseismology. In particular, variations in the large separation appear to indicate a glitch in the sound-speed profile at an acoustic depth of ~1000 s. We list frequencies for 55 modes extracted from the data spanning 20 radial orders, a range comparable to the best solar data, which will provide valuable constraints for theoretical models. A preliminary comparison with published models shows that the offset between observed and calculated frequencies for the radial modes is very different for Procyon than for the Sun and other cool stars. We find the mean lifetime of the modes in Procyon to be 1.29+0.55 -0.49 days, which is significantly shorter than the 2-4 days seen in the Sun.

Properties of Relativistic Jets in Gamma-Ray Burst Afterglows

Abstract: We extend our calculation of physical parameters of gamma-ray burst (GRB) jets by modeling the broadband emission of the afterglows 970508, 980519, 991208, 000926, 000418, and 010222. Together with 990123, 990510, 991216, and 000301c, there are 10 well-observed afterglows for which the initial opening angle of the GRB jet can be constrained. The jet energies (after the GRB phase) obtained for this set of afterglows are within one decade around 5 × 1050 ergs. With the exception of 000418, which requires a jet wider than rad, the jet initial half-angle in the other cases ranges from 2? to 20?. We find that in half of the cases, a homogeneous ambient medium accommodates the afterglow emission better than the windlike r-2 profile medium expected around massive stars. The two types of media give fits of comparable quality in four cases, with a wind medium providing a better description only for 970508. The circumburst densities we obtain are in the 0.1-100 cm-3 range, with the exception of 990123, for which it is below 10-2 cm-3. If in all 10 cases the observed GRB durations are a good measure of the ejecta deceleration timescale, then the parameters obtained here lead to jet Lorentz factors at the deceleration radius between 70 and 300, anticorrelated with the jet initial aperture, and jet masses around 10-6 M? . Our results on the jet energy, opening Lorentz factor, and evacuation of material until breakout provide constraints on theoretical models of GRB jets.

The Ages of Stars

Abstract: The age of an individual star cannot be measured, only estimated through mostly model-dependent or empirical methods, and no single method works well for a broad range of stellar types or for a full range in age. This review presents a summary of the available techniques for age-dating stars and ensembles of stars, their realms of applicability, and their strengths and weaknesses. My emphasis is on low-mass stars because they are present from all epochs of star formation in the Galaxy and because they present both special opportunities and problems. The ages of open clusters are important for understanding the limitations of stellar models and for calibrating empirical age indicators. For individual stars, a hierarchy of quality for the available age-dating methods is described. Although our present ability to determine the ages of even the nearest stars is mediocre, the next few years hold great promise as asteroseismology probes beyond stellar surfaces and starts to provide precise interior properties o...

The Exoplanet Handbook

Abstract: 1. Introduction 2. Radial velocities 3. Astrometry 4. Timing 5. Microlensing 6. Transits 7. Imaging 8. Host stars 9. Brown dwarfs and free-floating planets 10. Formation and evolution 11. Interiors and atmospheres 12. The Solar System Appendixes References Index.

Asteroseismology of Solar-Type and Red-Giant Stars

Abstract: We are entering a golden era for stellar physics driven by satellite and telescope observations of unprecedented quality and scope. New insights on stellar evolution and stellar interiors physics are being made possible by asteroseismology, the study of stars by the observation of natural, resonant oscillations. Asteroseismology is proving to be particularly significant for the study of solar-type and red-giant stars. These stars show rich spectra of solar-like oscillations, which are excited and intrinsically damped by turbulence in the outermost layers of the convective envelopes. In this review we discuss the current state of the field, with a particular emphasis on recent advances provided by the Kepler and COROT (Convection, Rotation & Planetary Transits) space missions and the wider significance to astronomy of the results from asteroseismology, such as stellar populations studies and exoplanet studies.

Transmission Spectra as Diagnostics of Extrasolar Giant Planet Atmospheres

Abstract: Atmospheres of transiting extrasolar giant planets (EGPs) such as HD 209458b must impose features on the spectra of their parent stars during transits; these features contain information about the physical conditions and chemical composition of the atmospheres. The most convenient observational index showing these features is the "spectrum ratio" (λ), defined as the wavelength-dependent ratio of spectra taken in and out of transit. The principal source of structure in is the variation with wavelength of the height at which the EGP atmosphere first becomes opaque to tangential rays—one may think of the planet as having different radii, and hence different transit depths, at each wavelength. The characteristic depth of absorption lines in scales with the atmospheric scale height and with the logarithm of the opacity ratio between continuum and strong lines. For close-in EGPs, line depths of 10-3 relative to the stellar continuum can occur. The atmospheres of EGPs probably consist mostly of molecular species, including H2, CO, H2O, and CH4, while the illuminating flux is characteristic of a Sun-like star. Thus, the most useful diagnostics are likely to be the near-infrared bands of these molecules, and the visible/near-IR resonance lines of the alkali metals. I describe a model that estimates (λ) for EGPs with prescribed radius, mass, temperature structure, chemical composition, and cloud properties. This model assumes hydrostatic and chemical equilibrium in an atmosphere with chemistry involving only H, C, N, and O. Other elements (He, Na, K, Si) are included as nonreacting minor constituents. Opacity sources include Rayleigh scattering, the strongest lines of Na and K, collision-induced absorption by H2, scattering by cloud particles, and molecular lines of CO, H2O, and CH4. The model simulates Doppler shifts from height-dependent winds and from planetary rotation, and deals in a schematic way with photoionization of Na and K by the stellar UV flux. Using this model, I investigated the diagnostic potential of various spectral features for planets similar to HD 209458b. Clouds are the most important determinants of the depth of features in ; they decrease the strength of all features as they reach higher in the atmosphere. The relative strengths of molecular lines provide diagnostics for the heavy-element abundance, temperature, and the vertical temperature structure, although diagnostics for different physical properties tend to be somewhat degenerate. Planetary rotation with likely periods leaves a clear signature on the line profiles, as do winds with speeds comparable to that of rotation. Successful use of these diagnostics will require spectral observations with signal-to-noise ratio (S/N) of 103 or better and resolving power R = λ/δλ ranging from 103 to 106, depending on the application. Because of these stringent demands, it will be important to evolve analysis methods that combine information from many lines into a few definitive diagnostic indices.