Solar-like oscillations in a metal-poor globular cluster with the HST

We present analyses of variability in the red giant stars in the metal-poor globular cluster NGC6397, based on data obtained with the Hubble Space Telescope. We use an non-standard data reduction approach to turn a 23-day observing run originally aimed at imaging the white dwarf population, into time-series photometry of the cluster's highly saturated red giant stars. With this technique we obtain noise levels in the final power spectra down to 50 parts per million, which allows us to search for low amplitude solar-like oscillations. We compare the observed excess power seen in the power spectra with estimates of the typical frequency range, frequency spacing and amplitude from scaling the solar oscillations. We see evidence that the detected variability is consistent with solar-like oscillations in at least one and perhaps up to four stars. With metallicities two orders of magnitude lower than of the Sun, these stars present so far the best evidence of solar-like oscillations in such a low metallicity environment.Comment: 7 pages, 6 figures, accepted by Ap

Oscillations in Arcturus from WIRE photometry

Observations of the red giant Arcturus (Alpha Boo) obtained with the star tracker on the Wide Field Infrared Explorer (WIRE) satellite during a baseline of 19 successive days in 2000 July-August are analysed. The amplitude spectrum has a significant excess of power at low-frequencies. The highest peak is at about 4.1 micro-Hz (2.8 d), which is in agreement with previous ground-based radial velocity studies. The variability of Arcturus can be explained by sound waves, but it is not clear whether these are coherent p-mode oscillations or a single mode with a short life-time.Comment: 6 pages, 1 Latex file, 4 .eps figures, 2 .sty files, ApJL, 591, L151 See erratum (astro-ph/0308424

Evidence for Granulation and Oscillations in Procyon from Photometry with the WIRE satellite

We report evidence for the granulation signal in the star Procyon A, based on two photometric time series from the star tracker on the WIRE satellite. The power spectra show evidence of excess power around 1 milliHz, consistent with the detection of p-modes reported from radial velocity measurements. We see a significant increase in the noise level below 3 milliHz, which we interpret as the granulation signal. We have made a large set of numerical simulations to constrain the amplitude and timescale of the granulation signal and the amplitude of the oscillations. We find that the timescale for granulation is T(gran) = 750(200) s, the granulation amplitude is 1.8(0.3) times solar, and the amplitude of the p-modes is 8(3) ppm. We found the distribution of peak heights in the observed power spectra to be consistent with that expected from p-mode oscillations. However, the quality of the data is not sufficient to measure the large separation or detect a comb-like structure, as seen in the p-modes of the Sun. Comparison with the recent negative result from the MOST satellite reveal that the MOST data must have an additional noise source that prevented the detection of oscillations.Comment: 23 pages, 12 figures, submitted to ApJ; v2 revisions: one reference corrected and a comment in Figure 7 correcte

Solar-like oscillations in the G9.5 subgiant beta Aquilae

An interesting asteroseismic target is the G9.5 IV solar-like star beta Aql. This is an ideal target for asteroseismic investigations, because precise astrometric measurements are available from Hipparcos that greatly help in constraining the theoretical interpretation of the results. The star was observed during six nights in August 2009 by means of the high-resolution \'echelle spectrograph SARG operating with the TNG 3.58 m Italian telescope on the Canary Islands, exploiting the iodine cell technique. We present the result and the detailed analysis of high-precision radial velocity measurements, where the possibility of detecting time individual p-mode frequencies for the first and deriving their corresponding asymptotic values will be discussed. The time-series analysis carried out from \sim 800 collected spectra shows the typical p-mode frequency pattern with a maximum centered at 416 \muHz. In the frequency range 300 - 600 \muHz we identified for the first time six high S/N (\gtrsim 3.5) modes with l = 0,2 and 11 < n < 16 and three possible candidates for mixed modes (l = 1), although the p-mode identification for this type of star appears to be quite difficult owing to a substantial presence of avoided crossings. The large frequency separation and the surface term from the set of identified modes by means of the asymptotic relation were derived for the first time. Their values are \Delta \nu = 29.56 \pm 0.10 \muHz and \epsilon = 1.29 \pm 0.04, consistent with expectations. The most likely value for the small separation is \delta\nu_{02} = 2.55 \pm 0.71 \muHz.Comment: 8 pages, 8 figures, 3 tables, accepted by A&

Asteroseismology of the Transiting Exoplanet Host HD 17156 with HST FGS

Observations conducted with the Fine Guidance Sensor on Hubble Space Telescope (HST) providing high cadence and precision time-series photometry were obtained over 10 consecutive days in December 2008 on the host star of the transiting exoplanet HD 17156b. During this time 10^12 photons (corrected for detector deadtime) were collected in which a noise level of 163 parts per million per 30 second sum resulted, thus providing excellent sensitivity to detection of the analog of the solar 5-minute p-mode oscillations. For HD 17156 robust detection of p-modes supports determination of the stellar mean density of 0.5301 +/- 0.0044 g/cm^3 from a detailed fit to the observed frequencies of modes of degree l = 0, 1, and 2. This is the first star for which direct determination of the mean stellar density has been possible using both asteroseismology and detailed analysis of a transiting planet light curve. Using the density constraint from asteroseismology, and stellar evolution modeling results in M_star = 1.285 +/- 0.026 solar, R_star = 1.507 +/- 0.012 solar, and a stellar age of 3.2 +/- 0.3 Gyr.Comment: Accepted by ApJ; 16 pages, 18 figure

Seismology of Procyon A: determination of mode frequencies, amplitudes, lifetimes, and granulation noise

The F5 IV-V star Procyon A (aCMi) was observed in January 2001 by means of the high resolution spectrograph SARG operating with the TNG 3.5m Italian telescope (Telescopio Nazionale Galileo) at Canary Islands, exploiting the iodine cell technique. The time-series of about 950 spectra carried out during 6 observation nights and a preliminary data analysis were presented in Claudi et al. 2005. These measurements showed a significant excess of power between 0.5 and 1.5 mHz, with ~ 1 m/s peak amplitude. Here we present a more detailed analysis of the time-series, based on both radial velocity and line equivalent width analyses. From the power spectrum we found a typical p-mode frequency comb-like structure, identified with a good margin of certainty 11 frequencies in the interval 0.5-1400 mHz of modes with l=0,1,2 and 7< n < 22, and determined large and small frequency separations, Dn = 55.90 \pm 0.08 muHz and dnu_02=7.1 \pm 1.3 muHz, respectively. The mean amplitude per mode (l=0,1) at peak power results to be 0.45 \pm 0.07 m/s, twice larger than the solar one, and the mode lifetime 2 \pm 0.4 d, that indicates a non-coherent, stochastic source of mode excitation. Line equivalent width measurements do not show a significant excess of power in the examined spectral region but allowed us to infer an upper limit to the granulation noise.Comment: 10 pages, 15 figures, 4 tables. Accepted for publication in A&

Solving the m-mixing problem for the three-dimensional time-dependent Schr\"{o}dinger equation by rotations: application to strong-field ionization of H2+

We present a very efficient technique for solving the three-dimensional time-dependent Schrodinger equation. Our method is applicable to a wide range of problems where a fullly three-dimensional solution is required, i.e., to cases where no symmetries exist that reduce the dimensionally of the problem. Examples include arbitrarily oriented molecules in external fields and atoms interacting with elliptically polarized light. We demonstrate that even in such cases, the three-dimensional problem can be decomposed exactly into two two-dimensional problems at the cost of introducing a trivial rotation transformation. We supplement the theoretical framework with numerical results on strong-field ionization of arbitrarily oriented H2+ molecules.Comment: 5 pages, 4 figure

Photoionization cross sections of O II, O III, O IV, and O V: benchmarking R-matrix theory and experiments

For crucial tests between theory and experiment, ab initio close coupling calculations are carried out for photoionization of O II, O III, O IV, O V. The relativistic fine structure and resonance effects are studied using the R-matrix and its relativistic variant the Breit Pauli R-matrix (BPRM) approximation. Detailed comparison is made with high resolution experimental measurements carried out in three different set-ups: Advanced Light Source at Berkeley, and synchrotron radiation experiments at University of Aarhus and University of Paris-Sud. The comparisons illustrate physical effects in photoionization such as (i) fine structure, (ii) resolution, and (iii) metastable components. Photoionization cross sections sigma{PI} of the ground and a few low lying excited states of these ions obtained in the experimental spectrum include combined features of these states. Theoretically calculated resonances need to be resolved with extremely fine energy mesh for precise comparison. In addition, prominent resonant features are observed in the measured spectra from transitions allowed with relativistic fine structure, but not in LS coupling. The sigma_{PI} are obtained for ground and metastable (i) 2s^22p^3(^4S^o, ^2D^o, ^2P^o) states of O II, (ii) 2s^22p^2(^3P,^1D,^1S) and 2s2p^3(^5S^o) states of O III, (iii) 2s^22p(^2P^o_J) and 2s2p^2(^4P_J) levels of O IV, and (iv) 2s^2(^1S) and 2s2p(^3P^o,^1P^o) states of O V. It is found that resonances in ground and metastable cross sections can be a diagnostic of experimental beam composition, with potential ap plications to astrophysical and laboratory plasma environments.Comment: 27 pages, 7 figs., submitted to Phys. Rev. A., text with high resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/Oions.p

Orientation-dependent ionization yields from strong-field ionization of fixed-in-space linear and asymmetric top molecules

The yield of strong-field ionization, by a linearly polarized probe pulse, is studied experimentally and theoretically, as a function of the relative orientation between the laser field and the molecule. Experimentally, carbonyl sulfide, benzonitrile and naphthalene molecules are aligned in one or three dimensions before being singly ionized by a 30 fs laser pulse centered at 800 nm. Theoretically, we address the behaviour of these three molecules. We consider the degree of alignment and orientation and model the angular dependence of the total ionization yield by molecular tunneling theory accounting for the Stark shift of the energy level of the ionizing orbital. For naphthalene and benzonitrile the orientational dependence of the ionization yield agrees well with the calculated results, in particular the observation that ionization is maximized when the probe laser is polarized along the most polarizable axis. For OCS the observation of maximum ionization yield when the probe is perpendicular to the internuclear axis contrasts the theoretical results.Comment: 14 pages, 4 figure