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Electron density diagnostic potential of the SiX ion and its application in Procyon
Theoretical electron density sensitive line ratiosR1-R6 of SiX soft X-ray emission lines arepresented. We found that these line ratios are sensitive to electrondensity ne, and the ratio R1 is insensitive toelectron temperature Te. For reliable determination of theelectron density of laboratory and astrophysical plasmas, atomic data,such as electron impact excitation rates, are very important. Ourresults reveal that the discrepancy of the line ratios from differentatomic data calculated with the distorted wave (DW) approximation andthe R-matrix method is up to 19 per cent at ne= 2 ×108cm-3. We applied the theoretical intensityratio R1 to the Low Energy Transmission Grating Spectrometer(LETGS) spectrum of the solar-like star Procyon. By comparing theobserved value (1.29) with the theoretical calculation, the derivedelectron density ne is 2.6 ×108cm-3, which is consistent with that derivedfrom (CV <8.3 × 108cm-3). When thetemperature structure of the Procyon corona is taken into account, thederived electron density increases from ne= 2.6 ×108 to 2.8 × 108cm-3.
| Reduction of time-resolved space-based CCD photometry developed for MOST Fabry Imaging data*
The MOST (Microvariability and Oscillations of Stars) satellite obtainsultraprecise photometry from space with high sampling rates and dutycycles. Astronomical photometry or imaging missions in low Earth orbits,like MOST, are especially sensitive to scattered light from Earthshine,and all these missions have a common need to extract target informationfrom voluminous data cubes. They consist of upwards of hundreds ofthousands of two-dimensional CCD frames (or subrasters) containing fromhundreds to millions of pixels each, where the target information,superposed on background and instrumental effects, is contained only ina subset of pixels (Fabry Images, defocused images, mini-spectra). Wedescribe a novel reduction technique for such data cubes: resolvinglinear correlations of target and background pixel intensities. Thisstep-wise multiple linear regression removes only those targetvariations which are also detected in the background. The advantage ofregression analysis versus background subtraction is the appropriatescaling, taking into account that the amount of contamination may differfrom pixel to pixel. The multivariate solution for all pairs oftarget/background pixels is minimally invasive of the raw photometrywhile being very effective in reducing contamination due to, e.g. straylight. The technique is tested and demonstrated with both simulatedoscillation signals and real MOST photometry.
| Hubble Space Telescope observations of SV Cam - II. First derivative light-curve modelling using PHOENIX and ATLAS model atmospheres
The variation of the specific intensity across the stellar disc is anessential input parameter in surface brightness reconstructiontechniques such as Doppler imaging, where the relative intensitycontributions of different surface elements are important in detectingstar-spots. We use PHOENIX and ATLAS model atmospheres to model lightcurves derived from high precision (signal-to-noise ratio ~= 5000)Hubble Space Telescope (HST) data of the eclipsing binary SV Cam(F9V+K4V), where the variation of specific intensity across the stellardisc will determine the contact points of the binary system light curve.For the first time, we use χ2 comparison fits to thefirst derivative profiles to determine the best-fitting modelatmosphere. We show the wavelength dependence of the limb darkening andthat the first derivative profile is sensitive to the limb-darkeningprofile very close to the limb of the primary star. It is concluded thatthere is only a marginal difference (<1σ) between theχ2 comparison fits of the two model atmospheres to theHST light curve at all wavelengths. The usefulness of the secondderivative of the light curve for measuring the sharpness of theprimary's limb is investigated, but we find that the data are too noisyto permit a quantitative analysis.
| About Procyon modeling
Models of Procyon satisfying the actual observational constraints,particularly the asteroseismic ones, are discussed. It is shown thatmuch more accurate frequencies must be obtained by asteroseismicobservations especially in the low frequency domain below 800 mu Hertzto determine the Procyon evolutionary stage.
| Seismological study of the solar-like star Procyon A
The F5 subgiant Procyon A was observed in January 2001 by the SARGspectrograph at TNG (Telescopio Nazionale Galileo) exploiting the iodinecell technique. The time series of about 950 spectra taken during 6observation nights and a preliminary data analysis were presented in\citet{Claudi}. These measurements showed a significant excess of powerbetween 0.5 and 1.5 mHz, with 0.98 ms-1 peak amplitude. Inthis work we present a more detailed analysis of the time series thanthat presented in \citet{Claudi}. An average large frequency spacing of55.66± 0.14 mu {Hz} and a small spacing of 5.7±0.3 mu {Hz}have been determined and twenty-one individual frequencies identified.
| Asteroseismology of solar-type stars with CORALIE and HARPS . II: Observations and modelling of binary stars
We present here the detection and characterization of solar-likeoscillations in visual binary stars such as alpha Centauri, Procyon and70 Ophiuchi obtained with the CORALIE and HARPS spectrographs at the ESOLa Silla Observatory. The strong additional constraints resulting fromthe binary nature of the systems (same age and initial chemicalcomposition) allow us to accurately determine their global parametersand to test the physics of the models.
| SARG@TNG: asteroseismology of solar like stars.
Since 1995, the extrasolar planet search has driven the high resolutionspectroscopy community to build more and more stable spectrograph inorder to reach the photon statistics limit in radial velocitymeasurements. This situation opened the possibility of asteroseismicobservations of stellar p-mode pulsations in solar - like stars. In thispaper we describe the possible performance of the SARG spectrographitself in asterosimological campaigns on fainter solar-like stars.
| Observing solar-like oscillations: recent results.
We review recent progress in observations of ground-based oscillations.Excellent observations now exist for a few stars (alpha Cen A{} and B,mu Ara), while there is some controversy over others (Procyon, etaBoo). We have reached the stage where single-site observations are oflimited value and where careful planning is needed to ensure the futureof asteroseismology.
| Progress in understanding and exploiting stellar oscillation spectra .
Rich oscillation spectra of dwarf-like pulsators contain a wealth ofinformation about the object interiors and, in particular, aboutmacroscopic transport processes, which is the most difficult aspect ofstellar physics. Examples of extracting such information from data onsolar-like and opacity driven pulsators are given. Problems inunderstanding new oscillation spectra are discussed. Importance ofemploying various data on excited mode is emphasized.
| Asteroseismology of solar-type stars with CORALIE and HARPS . I: Observations and modelling of single stars
We present here the detection and characterization of solar-likeoscillations in several targets such as beta Vir, eta Boo, delta Eri,chi Eri and the Am star HD 209625 obtained with the CORALIE and HARPSspectrographs based at the ESO La Silla Observatory. The measurement ofthe frequencies of p-mode oscillations provides an insight into theinternal structure and is nowadays the most powerful constraint on thetheory of stellar evolution.
| Observations of solar-like oscillations and asteroseismic models including rotation .
Since the success of helioseismology, numerous efforts have been made todetect solar-like oscillations on other stars. Thanks to newspectrographs developed for extra-solar planet searches, the accuracyneeded to detect such oscillations has recently been achieved. In thispaper, we present new asteroseismic measurements obtained with theCORALIE and HARPS spectrographs as well as new theoretical analysesbased on these observations. In particular, we focus on the effects ofrotation on the modelling of solar-type stars and on its influence onthe determination of fundamental stellar parameters.
| High-precision photometry with the WIRE satellite .
Around 200 bright stars (V<6) have been monitored with the two-inchstar tracker on the WIRE satellite since observations started in 1999.Here we present new results for the solar-like star Procyon A, the twodelta Scuti stars Altair and epsilon Cephei, and the triple systemlambda Scorpii which consist of two B-type stars - one of which we findto be an eclipsing binary.
| New Relativistic Atomic Data for Fe IX
A large-scale configuration interaction (CI) calculation using ProgramCIV3 of Hibbert is performed for the lowest 87 fine-structure levels ofargon-like iron ions. We have calculated energy levels, oscillatorstrengths, and transition probabilities for all electric-dipole-allowedand intercombination transitions between the levels of3s23p6, 3s23p53d,3s3p63d, 3s23p54s,3s23p54p, 3s23p54d,3s3p64s, 3s23p54f, 3s3p64p,3s23p55s, 3s23p55p, and3s23p55d states of Fe IX in the LSJ couplingscheme. Lifetimes of the 3d levels are also determined. The calculationsinclude the major correlation effects. We attempt to correct theinaccuracies in the CI coefficients in the wave functions, which wouldlead to inaccuracy in transition probabilities by applying a``fine-tuning'' technique. The relativistic effects are incorporated byadding mass correction, Darwin term, and spin-orbit interaction terms tothe nonrelativistic Hamiltonian in the Breit-Pauli approximations. Thepresent results are in good agreement with other available calculationsand experiments. We predict new data for several levels where no othertheoretical or experimental results are available. We hope our extensivecalculations will be useful to experimentalists in identifying thefine-structure levels in their future work.
| An Extended FUSE Survey of Diffuse O VI Emission in the Interstellar Medium
We present a survey of diffuse O VI emission in the interstellar medium(ISM) obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE).Spanning 5.5 yr of FUSE observations, from launch through 2004 December,our data set consists of 2925 exposures along 183 sight lines, includingall of those with previously published O VI detections. The data wereprocessed using an implementation of CalFUSE version 3.1 modified tooptimize the signal-to-noise ratio and velocity scale of spectra from anaperture-filling source. Of our 183 sight lines, 73 show O VIλ1032 emission, 29 at >3 σ significance. Six of the 3σ features have velocities |vLSR|>120 kms-1, while the others have |vLSR|<=50 kms-1. Measured intensities range from 1800 to 9100 LU (lineunit; 1 photon cm-2 s-1 sr-1), with amedian of 3300 LU. Combining our results with published O VI absorptiondata, we find that an O VI-bearing interface in the local ISM yields anelectron density ne=0.2-0.3 cm-3 and a path lengthof 0.1 pc, while O VI-emitting regions associated with high-velocityclouds in the Galactic halo have densities an order of magnitude lowerand path lengths 2 orders of magnitude longer. Although the O VIintensities along these sight lines are similar, the emission isproduced by gas with very different properties.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by Johns HopkinsUniversity under NASA contract NAS5-32985.
| An Upper Limit on the Albedo of HD 209458b: Direct Imaging Photometry with the MOST Satellite
We present space-based photometry of the transiting exoplanetary systemHD 209458 obtained with the Microvariablity and Oscillations of Stars(MOST) satellite, spanning 14 days and covering 4 transits and 4secondary eclipses. The HD 209458 photometry was obtained in MOST'slower precision direct imaging mode, which is used for targets in thebrightness range 6.5>=V>=13. We describe the photometric reductiontechniques for this mode of observing, in particular the corrections forstray earthshine. We do not detect the secondary eclipse in the MOSTdata, to a limit in depth of 0.053 mmag (1 σ). We set a 1 σupper limit on the planet-star flux ratio of 4.88×10-5corresponding to a geometric albedo upper limit in the MOST bandpass(400-700 nm) of 0.25. The corresponding numbers at the 3 σ levelare 1.34×10-4 and 0.68, respectively. HD 209458b ishalf as bright as Jupiter in the MOST bandpass. This low geometricalbedo value is an important constraint for theoretical models of the HD209458b atmosphere, in particular ruling out the presence of reflectiveclouds. A second MOST campaign on HD 209458 is expected to be sensitiveto an exoplanet albedo as low as 0.13 (1 σ), if the star does notbecome more intrinsically variable in the meantime.MOST is a Canadian Space Agency mission, operated jointly by Dynacon,Inc., and the Universities of Toronto and British Columbia, withassistance from the University of Vienna.
| On the Temperature-Emission Measure Distribution in Stellar Coronae
Strong peaks in the emission measure-temperature (EM-T ) distributionsin the coronae of some binary stars are associated with the presence ofhot (107 K), dense (up to 1013 cm -3)plasma. These peaks are very reminiscent of those predicted to arise inan impulsively heated solar corona. A coronal model comprised of manyimpulsively heated strands is adapted to stellar parameters. It is shownthat the properties of the EM-T distribution can be accounted for ingeneral terms provided the emission comes from many very small loops(length under 103 km) with intense magnetic fields (1 kG)distributed across part of the surface of the star. The heating requiresevents that generally dissipate between 1026 and 1028 ergs, which is in the range of solar microflares. This impliesthat such stars must be capable of generating regions of localizedintense magnetic fields.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| Investigating Disk Evolution: A High Spatial Resolution Mid-Infrared Survey of T Tauri Stars
We present a high spatial resolution, 10-20 μm survey of 65 T Tauribinary stars in Taurus, Ophiuchus, and Corona Australis using the Keck10 m telescopes. Designed to probe the inner ~1 AU region of thecircumstellar disks around the individual stellar components in thesebinary systems, this study increases the number of binaries withspatially resolved measurements at 10 μm by a factor of ~5. Combinedwith resolved near-infrared photometry and spectroscopic accretiondiagnostics, we find that ~10% of stars with a mid-infrared excess donot appear to be accreting. In contrast to an actively accreting disksystem, these passive disks have significantly lower near-infraredcolors that are, in most cases, consistent with photospheric emission,suggesting the presence of an inner disk hole. In addition, thereappears to be a spectral type/mass dependence associated with thepresence of a passive disk, with all passive disks occurring aroundM-type stars. The presence of a passive disk does not appear to berelated to the fact that these objects are in visual binary systems; thepassive disk systems span the entire range of binary separations presentin the sample, and a similar fraction of passive disks is observed in asample of single stars. The possibility that the passive disks arecaused by the presence of an as yet undetected companion at a smallseparation (0.3-3 AU) is possible for any individual system; however, itcannot account for the spectral type dependence of the passive disksample as a whole. We propose that these passive disks represent asubset of T Tauri stars that are undergoing significant disk evolution.The fraction of observed passive disks and the observed spectral typedependence can both be explained by models of disk evolution thatinclude disk photoevaporation from the central star.
| Coronal Density Diagnostics with Si X: Chandra LETGS Observations of Procyon, α Centauri A and B, Capella, and ɛ Eridani
Electron density diagnostics based on the line intensity ratio of Si Xare applied to the X-ray spectra of Procyon, α Cen A and B,Capella, and ɛ Eri measured with the Low Energy TransmissionGrating Spectrometer combined with the High Resolution Camera on boardthe Chandra X-Ray Observatory. The ratio R1 of theintensities of the Si X lines at 50.524 and 50.691 Å is adopted. Acertain temperature effect in R1 appears near the low-densitylimit region, which is due to the contamination of the Si X line at50.703 Å. Using the emission measure distribution model derived byAudard and coworkers for Capella and emissivities calculated with theAstrophysical Plasma Emission Code model by Smith and coworkers, wesuccessfully estimate the contributions of the Fe XVI lines at 50.367and 50.576 Å (73% and 62%, respectively). A comparison between theobserved ratios and theoretical predictions constrains the (logarithmic)electron densities for Procyon to be8.61+0.24-0.20 cm-3, while for αCen A and B, Capella, and ɛ Eri they are8.81+0.27-0.23,8.60+0.39-0.32, 9.30-0.48, and9.11+1.40-0.38 cm-3, respectively. Thecomparison of our results with those constrained by the triplet ofHe-like carbon shows good agreement. For normal stars, our resultsdisplay a narrow uncertainty, while for active stars, a relativelylarger uncertainty due to contamination from Fe XVI lines is found.Another possible reason may be the uncertainty of the continuum level,since the emission lines of Si X become weak for active stars. Forɛ Eri, an electron density in the C V-forming region was firstestimated through Si X emission.
| Detection Limits from the McDonald Observatory Planet Search Program
Based on the long-term radial velocity surveys carried out with theMcDonald Observatory 2.7 m Harlan J. Smith Telescope from 1988 to thepresent, we derive upper limits to long-period giant planet companionsfor 31 nearby stars. Data from three phases of the McDonald Observatory2.7 m planet-search program have been merged together, and for 17objects data from the pioneering Canada-France-Hawaii Telescope radialvelocity program have also been included in the companion-limitsdetermination. For those 17 objects, the baseline of observations is inexcess of 23 yr, enabling the detection or exclusion of giant planets inorbits beyond 8 AU. We also consider the possibility of eccentric orbitsin our computations. At an orbital separation of 5.2 AU, we can excludeon average planets of Msini>~(2.0+/-1.1)MJ (e=0) andMsini>~(4.0+/-2.8)MJ (e=0.6) for 25 of the 31 stars inthis survey. However, we are not yet able to rule out ``true Jupiters,''i.e., planets of Msini~1MJ in 5.2 AU orbits. These limits areof interest for the Space Interferometry Mission, Terrestrial PlanetFinder, and Darwin missions, which will search for terrestrial planetsorbiting nearby stars, many of which are included in this work.
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| A Mid-Infrared Study of the Class 0 Cluster in LDN 1448
We present ground-based mid-infrared observations of Class 0 protostarsin LDN 1448. Of the five known protostars in this cloud, we detectedtwo, L1448N:A and L1448C, at 12.5, 17.9, 20.8, and 24.5 μm, and athird, L1448 IRS 2, at 24.5 μm. We present high-resolution images ofthe detected sources and photometry or upper limits for all five Class 0sources in this cloud. With these data we are able to augment existingspectral energy distributions for all five objects and place them on anevolutionary status diagram.
| An Astrometric Study of Procyon
We report on the analysis of observations of Procyon obtained with theMultichannel Astrometric Photometer during the years 1986-2004. Whilethe results significantly increase the precision of the estimatedcharacteristics of the components of the system, they are generally ingood agreement with the most recent studies. The F5 IV primary is foundto have a mass of 1.43+/-0.034 Msolar, while the white dwarfcompanion's mass is 0.58+/-0.014 Msolar. The distance modulusis 2.277+/-0.005 mag, yielding an absolute visual magnitude for thecombined light of the system of 2.640+/-0.006 and an absolute visualmagnitude of the secondary of 13.20+/-0.05. The residuals of the fit ofthe positions of the photocenter to the excepted orbit support theparameters of that orbit, showing only a slight deviation in rightascension. The most significant improvements in our knowledge of thebinary system can be made with further measurements of the separation ofthe components. The masses found here tend to reaffirm the recent modelsof the nature of the primary and secondary stars.
| Computed Hβ indices from ATLAS9 model atmospheres
Aims.Grids of Hβ indices based on updated (new-ODF) ATLAS9 modelatmospheres were computed for solar and scaled solar metallicities[+0.5], [+0.2], [0.0], [ -0.5] , [ -1.0] , [ -1.5] , [ -2.0] , [ -2.5]and for α enhanced compositions [+0.5a], [0.0a], [ -0.5a] , [-1.0a] , [ -1.5a] , [ -2.0a] , [ -2.5a] , and [ -4.0a] . Methods:.Indices for T_eff > 5000 K were computed with the same methods asdescribed by Lester et al. (1986, LGK86) except for a differentnormalization of the computed natural system to the standard system.LGK86 used special ODFs to compute the fluxes. For T_eff ≤ 5000 K wecomputed the fluxes using the synthetic spectrum method. In order toassess the accuracy of the computed indices comparisons were made withthe indices computed by Smalley & Dworetsky (1995, A&A, 293,446, MD95) and with the empirical relations T_eff-Hβ given byAlonso et al. (1996, A&A, 313, 873) for several metallicities.Furthermore, for cool stars, temperatures inferred from the computedindices were compared with those of the fundamental stars listed byMD95. The same kind of comparison was made between gravities for B-typestars. Results: .The temperatures from the computed indices are ingood agreement, within the error limits, with the literature values for4750 K ≤ T_eff ≤ 8000 K, while the gravities agree for T_eff> 9000 K. The computed Hβ indices for the Sun and for Procyonare very close to the observed values. The comparison between theobserved and computed Hβ indices as function of the observedHβ has shown a very small trend which almost completely disappearswhen only stars hotter than 10 000 K are considered. The trend due tothe cool stars is probably related with the low accuracy of thefundamental T_eff which are affected by large errors for most of thestars.
| Radiative lifetimes, branching fractions and oscillator strengths in Pd I and the solar palladium abundance
Transition probabilities have been derived for 20 5s-5p transitions ofPd I from a combination of radiative lifetime measurements for 6odd-parity levels with time-resolved laser-induced fluorescencespectroscopy and of branching fraction determination using a hollowcathode discharge lamp. Additional oscillator strengths for 18transitions have been determined from measured lifetimes and theoreticalbranching fractions obtained from configuration interaction calculationswith core-polarization effects included. These new results have allowedus to refine the palladium abundance in the solar photosphere: A_Pd =1.66 ± 0.04, in the usual logarithmic scale, a result in closeagreement with the meteoritic value.
| Asteroseismology of the visual binary 70 Ophiuchi
Context: .Convection in stars excites resonant acoustic waves. Thefrequencies of these oscillations depend on the sound speed inside thestar, which in turn depends on density, temperature, gas motion, andother properties of the stellar interior. Therefore, analysis of theoscillations provides an unrivaled method to probe the internalstructure of a star. Aims: .Solar-like oscillations in the primaryof the visual binary 70 Ophiuchi are investigated. Methods: .70Ophiuchi A was observed with the Harps spectrograph mounted on the 3.6-mtelescope at the ESO La Silla Observatory (Chile) during 6 nights inJuly 2004 allowing us to collect 1758 radial velocity measurements witha standard deviation of about 1.39 m s-1. Results: .Thepower spectrum of the high precision velocity time series clearlypresents several identifiable peaks between 3 and 6 mHz showingregularity with a large spacing of Δν = 161.7 ± 0.3μHz. Fourteen individual modes were identified with amplitudes in therange 11 to 14 cm s-1.
| Modeling β Virginis using seismological data
This paper presents the modeling of the F9 V star βVirginis based on seismological measurements. Using the Genevaevolution code including rotation and atomic diffusion, we find that twodistinct solutions reproduce all existing asteroseismic andnon-asteroseismic observational constraints well: a main-sequence modelwith a mass of 1.28 ± 0.03~Mȯ and an age t=3.24± 0.20 Gyr, or a model in the post-main sequence phase ofevolution with a lower mass of 1.21 ± 0.02~Mȯ andan age t=4.01 ± 0.30 Gyr. The small spacings δν02 and the ratio r02 between small and largespacings are sensitive to the differences in the structure of thecentral layers between these two solutions and are also sensitive to thestructural changes due to the rotational mixing. They can therefore beused to unambiguously determine the evolutionary state ofβ Vir and to study the effects of rotation onthe inner structure of the star. Unfortunately, existing asteroseismicdata do not enable such precise determination. We also show that thescatter in frequencies introduced by the rotational splittings canaccount for the larger dispersion of the observed large spacings for thenon-radial modes than for the radial modes.
| Oscillation mode lifetimes in ξ Hydrae: will strong mode damping limit asteroseismology of red giant stars?
We introduce a new method to measure frequency separations and modelifetimes of stochastically excited and damped oscillations, so-calledsolar-like oscillations. Our method shows that velocity data of the redgiant star ξ Hya (Frandsen et al. 2002) support a large frequencyseparation between modes of roughly 7~μHz. We also conclude that thedata are consistent with a mode lifetime of 2 days, which is so shortrelative to its pulsation period that none of the observed frequenciesare unambiguous. Hence, we argue that the maximum asteroseismic outputthat can be obtained from these data is an average large frequencyseparation, the oscillation amplitude and the average mode lifetime.However, the significant discrepancy between the theoreticalcalculations of the mode lifetime (Houdek & Gough 2002) and ourresult based on the observations of ξ Hya, implies that red giantstars can help us better understand the damping and driving mechanismsof solar-like p-modes by convection.
| Hydrodynamical simulations of convection-related stellar micro-variability. I. Statistical relations for photometric and photocentric variability
Local-box hydrodynamical model atmospheres provide statisticalinformation about the spatial dependence, as well as temporal evolution,of a star's emergent radiation field. Here, we consider late-typestellar atmospheres for which temporal changes of the radiative outputare primarily related to convective (granular) surface flows. We derivedrelations for evaluating the granulation-induced, disk-integrated thusobservable fluctuations of the stellar brightness and location of thephotocenter from radiation intensities available from a local model.Apart from their application in the context of hydrodynamical stellaratmospheres, these formulae provide some broader insight into the natureof the fluctuations under consideration. Brightness fluctuations scaleinversely proportional to the square root of the number of convectivecells (the statistically independently radiating surface elements)present on the stellar surface and increase with more pronouncedlimb-darkening. Fluctuations of the stellar photocentric position do notdepend on the number of cells and are largely insensitive to the degreeof limb-darkening. They amount to a small fraction of the typical cellsize, and can become a limiting factor for high-precision astrometry inthe case of extreme giants. The temporal brightness and positionalfluctuations are statistically uncorrelated but closely related inmagnitude.
| Les tremblements de Procyon.
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