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|Determinations of Temperature and Density for Solar-like Stars Using Si XI Soft X-Ray Emission Lines|
We study the temperature and density sensitivities of the ratios of SiXI soft X-ray emission lines in the wavelength range of 43-54 Å.The typical temperature of formation of the analyzed lines is around 1.6MK, which makes this analysis complementary to the analysis of He-liketriplets, which are sensitive to hotter plasma. We present theoreticalcalculations and compare them with ratios obtained from high-resolutionX-ray spectra of five solar-like stars: Procyon, α Cen A, αCen B, ɛ Eri, and Capella. We find that our results are in goodagreement with results obtained by other authors through differentdiagnostics, namely, the analysis of density- and temperature-sensitiveHe-like triplet lines. We further estimate the coronal pressure andfilling factors from Si XI lines in this study.
|Spectral Irradiance Calibration in the Infrared. XVI. Improved Accuracy in the Infrared Spectra of the Secondary and Tertiary Standard Calibration Stars|
We have increased the spectral resolution and accuracy of the absoluteinfrared spectral flux for a subset of stellar standards created byCohen and his collaborators in previous papers in this series. Wecombined the moderate-resolution (λ/Δλ~400)spectroscopy obtained on infrared standard stars by the Short WavelengthSpectrometer on the Infrared Space Observatory with high-qualityphotometry that is tied to our recent absolute calibration from theMidcourse Space Experiment. New spectra were created for 33 stars, 9 ofwhich are Cohen et al. secondary standards and another 20 of which aretertiary standards for which Cohen adopted spectral templates. Theeffective temperatures and angular diameters for the stars are derivedfrom the absolute spectra and compare favorably with independentmeasures of these quantities in the literature. The present spectradisplay systematic differences with those of Cohen and colleagues inthat they have 4%-7% lower fluxes in the 1-4 μm spectral region. Ourspectra remove the A-K star calibration bias recently noted in thecalibration of the Spitzer Space Telescope camera.
|The coronal Ne/O abundance of α Centauri|
Recent improvements in the modeling of solar convection and lineformation led to downward revisions of the solar photospheric abundancesof the lighter elements, which in turn led to changes in the radiativeopacity of the solar interior and hence to conflicts with the solarconvection zone depth as inferred from helioseismic oscillationfrequencies. An increase of the solar Ne/O abundance to values asobserved for nearby stars has been proposed as a solution. Because ofthe absence of strong neon lines in the optical, neon abundances aredifficult to measure and the correct solar and stellar Ne/O abundancesare currently hotly debated. Based on X-ray spectra obtained withXMM-Newton, we determine a reference value of Ne/O for the inactive,solar-like star α Cen (primarily α Cen B, which is thedominant component in X-rays), with three independent, line-basedmethods, using differential emission measure reconstruction and anemission measure-independent method. Our results indicate a value of≈ 0.28 for A_Ne/AO in α Cen, approximately twice thevalue measured for the Sun, but still below the average value obtainedfor other stars. The low Ne/O abundance of the Sun is peculiar whencompared to α Cen and other stars; our results emphasize thenecessity to obtain more and accurate Ne/O abundance measurements of lowactivity stars.
|Solar-like Oscillations in the Metal-poor Subgiant ν Indi: Constraining the Mass and Age Using Asteroseismology|
Asteroseismology is a powerful method for determining fundamentalproperties of stars. We report the first application to a metal-poorobject, namely the subgiant star ν Ind. We measured precisevelocities from two sites, allowing us to detect oscillations and infera large frequency separation of Δν=24.25+/-0.25 μHz.Combining this value with the location of the star in the H-R diagramand comparing with standard evolutionary models, we were able to placeconstraints on the stellar parameters. In particular, our resultsindicate that ν Ind has a low mass (0.85+/-0.04 Msolar)and is at least 9 Gyr old.
|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.
|First high spectral resolution VUV full-Sun spectrum compared to cool stars|
This paper reports the first full-Sun vacuum ultraviolet (VUV) emissionline profile originating from the transition region from thechromosphere to the corona. It is based on a raster scan of the wholesolar disk using SUMER/SOHO. The full-Sun spectrum has a spectralresolution which allows an investigation of details in the line profileas well as a thorough comparison to stellar spectra as obtained, e.g.with FUSE or STIS/HST. The full-Sun spectrum shows enhanced emission inthe wings, and is well described by a double Gaussian fit with a narrowand a broad component. It is shown that the broad component is due tostructures on the solar surface, especially those related to themagnetic chromospheric network. Thus it is proposed that the broadcomponents of other solar-like stars are also a consequence of themixture of surface structures, and not necessarily a signature ofsmall-scale heating processes like explosive events, as it is commonlyargued. A comparison to spectra of luminous cool stars shows that theline asymmetries of these stars might also be a surface structure effectand not or only partly due to opacity effects in their cool dense winds.These comparisons show the potential of high quality full-Sun VUVspectra and their value for the study of solar-stellar connections. Asan example, this study proposes that αCen A has a considerablyhigher amount of magnetic flux concentrated in the chromosphericmagnetic network than the Sun.
|Abundances of refractory elements in the atmospheres of stars with extrasolar planets|
Aims.This work presents a uniform and homogeneous study of chemicalabundances of refractory elements in 101 stars with and 93 without knownplanetary companions. We carry out an in-depth investigation of theabundances of Si, Ca, Sc, Ti, V, Cr, Mn, Co, Ni, Na, Mg and Al. The newcomparison sample, spanning the metallicity range -0.70< [Fe/H]<0.50, fills the gap that previously existed, mainly at highmetallicities, in the number of stars without known planets.Methods.Weused an enlarged set of data including new observations, especially forthe field "single" comparison stars . The line list previously studiedby other authors was improved: on average we analysed 90 spectral linesin every spectrum and carefully measured more than 16 600 equivalentwidths (EW) to calculate the abundances.Results.We investigate possibledifferences between the chemical abundances of the two groups of stars,both with and without planets. The results are globally comparable tothose obtained by other authors, and in most cases the abundance trendsof planet-host stars are very similar to those of the comparison sample.Conclusions.This work represents a step towards the comprehension ofrecently discovered planetary systems. These results could also beuseful for verifying galactic models at high metallicities andconsequently improve our knowledge of stellar nucleosynthesis andgalactic chemical evolution.
|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.
|Astrophysics in 2005|
We bring you, as usual, the Sun and Moon and stars, plus some galaxiesand a new section on astrobiology. Some highlights are short (the newlyidentified class of gamma-ray bursts, and the Deep Impact on Comet9P/Tempel 1), some long (the age of the universe, which will be found tohave the Earth at its center), and a few metonymic, for instance theterm ``down-sizing'' to describe the evolution of star formation rateswith redshift.
|Frequency, splitting, linewidth and amplitude estimates of low-l p modes of α Cen A: analysis of Wide-Field Infrared Explorer photometry|
We present results of fitting the 50-d time series of photometry ofα Cen A taken by the Wide-Field Infrared Explorer (WIRE) satellitein 1999. Both power spectrum and autocovariance function (ACF) fittingtechniques were used in an attempt to determine mode frequencies,rotational splittings, lifetimes and amplitudes of low-l p modes. Inall, using both techniques, we managed to fit 18 modes (seven l = 0,eight l = 1 and three l = 2) with frequencies determined to within 1-2μHz. These estimates are shown to be 0.6 +/- 0.3 μHz lower, onaverage, than the frequencies determined from two other more recentstudies, which used data gathered about 19 months after the WIREobservations. This could be indicative of an activity cycle, althoughdue to the large uncertainty, more data would be needed to confirm this.Over a range of 1700-2650 μHz, we were also able to use the ACFfitting to determine an average lifetime of 3.9 +/- 1.4 d, and anaverage rotational splitting of 0.54 +/- 0.22 μHz, which is the firstever reliable estimate of this parameter. In contrast to the ACF, thepower spectrum fitting was shown to return significantly biased resultsfor these parameters.
|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.
|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.
|The Coronae of γ Draconis|
The Chandra X-Ray Observatory has detected coronal (T>~106K) emission from the red giant γ Draconis (HD 164058: K5 III; d=45pc), now fully resolved from a stronger source 21" to the SE that hadconfused earlier measurements by Röntgensatellit (ROSAT). Thesecond source is coincident with the 13th magnitude visual component ADS10923B (γ Dra B), possibly a dM star in a wide orbit around thered giant. The 0.2-2 keV luminosity of γ Dra isLX~1.2+0.4-0.2×1027ergs s-1 (1 σ confidence interval), assuminglog(Tcor)>~6.5 K, while that of the faint opticalcompanion is 3.2+0.7-0.5×1027ergs s-1, assuming the same distance. Both sources have anintermediate spectral hardness within the range displayed by coronalstars. γ Dra has LX/Lbol an order of magnitude brighter than theother red giants previously imaged by Chandra, Arcturus (α Boo:K1.5 III) and Aldebaran (α Tau: K5 III), despite having an only 2× elevated LC IV/Lbol (T~105 K).
|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.
|Simulating observable comets. III. Real stellar perturbers of the Oort cloud and their output|
Context: .This is the third of a series of papers on simulating themechanisms acting currently on the Oort cloud and producing the observedlong-period comets.Aims.In this paper we investigate the influence ofcurrent stellar perturbers on the Oort cloud of comets under thesimultaneous galactic disk tide. We also analyse the past motion of theobserved long-period comets under the same dynamical model to verify thewidely used definition of dynamically new comets. Methods.The action ofnearby stars and the galactic disk tide on the Oort cloud was simulated.The original orbital elements of all 386 long-period comets of qualityclasses 1 and 2 were calculated, and their motion was followednumerically for one orbital revolution into the past, down to theprevious perihelion. We also simulated the output of the close futurepass of GJ 710 through the Oort cloud. Results.The simulated flux of theobservable comets resulting from the current stellar and galacticperturbations, as well as the distribution of perihelion direction, wasobtained. The same data are presented for the future passage of GJ 710.A detailed description is given of the past evolution of aphelion andperihelion distances of the observed long-period comets. Conclusions. Weobtained no fingerprints of the stellar perturbations in the simulatedflux and its directional structure. The mechanisms producing observablecomets are highly dominated by galactic disk tide because all currentstellar perturbers are too weak. Also the effect of the close passage ofthe star GJ 710 is very difficult to recognise on the background of theGalactic-driven observable comets. For the observed comets we found only45 to be really dynamically "new" according to our definition based onthe previous perihelion distance value.
|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.
|The limb darkening of α Centauri B. Matching 3D hydrodynamical models with interferometric measurements|
For the nearby dwarf star α Cen B (K1 V), we presentlimb-darkening predictions from a 3D hydrodynamical radiative transfermodel of its atmosphere. We first compared the results of this model toa standard Kurucz's atmosphere. Then we used both predictions to fit thenew interferometric visibility measurements of α Cen B obtainedwith the VINCI instrument of the VLT Interferometer. Part of these newvisibility measurements were obtained in the second lobe of thevisibility function, which is sensitive to stellar limb-darkening. Thebest agreement is found for the 3D atmosphere limb-darkening model and alimb-darkened angular diameter of θ_3D = 6.000± 0.021 mas,corresponding to a linear radius of 0.863 ± 0.003 R_ȯ(assuming π = 747.1 ± 1.2 mas). Our new linear radius agreeswell with the asteroseismic value predicted by Thévenin et al.(2002, A&A, 392, L9). In view of future observations of this starwith the VLTI/AMBER instrument, we also present limb-darkeningpredictions in the J, H, and K bands.
|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.
|Oxygen abundances in planet-harbouring stars. Comparison of different abundance indicators|
We present a detailed and uniform study of oxygen abundances in 155solar type stars, 96 of which are planet hosts and 59 of which form partof a volume-limited comparison sample with no known planets. EWmeasurements were carried out for the [O I] 6300 Å line and the OI triplet, and spectral synthesis was performed for several OH lines.NLTE corrections were calculated and applied to the LTE abundanceresults derived from the O I 7771-5 Å triplet. Abundances from [OI], the O I triplet and near-UV OH were obtained in 103, 87 and 77dwarfs, respectively. We present the first detailed and uniformcomparison of these three oxygen indicators in a large sample ofsolar-type stars. There is good agreement between the [O/H] ratios fromforbidden and OH lines, while the NLTE triplet shows a systematicallylower abundance. We found that discrepancies between OH, [O I] and the OI triplet do not exceed 0.2 dex in most cases. We have studied abundancetrends in planet host and comparison sample stars, and no obviousanomalies related to the presence of planets have been detected. Allthree indicators show that, on average, [O/Fe] decreases with [Fe/H] inthe metallicity range -0.8< [Fe/H] < 0.5. The planet host starspresent an average oxygen overabundance of 0.1-0.2 dex with respect tothe comparison sample.
|Fundamental parameters and granulation properties of Alpha Centauri A and B obtained from inversions of their spectra|
Properties of stellar granulation are obtained by inverting spectra ofthe late-type stars α Centauri A and B. Our inversions are basedon a multi-component model of the stellar photosphere and take intoaccount the center-to-limb variation and rotational broadening. Thedifferent atmospheric components describe the areas harboring up-, down-and horizontal flows. The inversions are constrained by fitting not onlythe flux profiles, but also their line bisectors, and by using a simplemass conservation scheme. The inversions return the properties ofconvection at the stellar surface, including the stratification of thethermodynamic parameters, as well as fundamental parameters such as thegravitational acceleration, v sin i and the element abundances. Forα Cen A (G2V) the derived stratifications of the temperature andconvective velocity are very similar to the Sun, while for α Cen B(K1V) we find similar up- and downflow velocities, but lower horizontalspeeds and a reduced overshoot. The latter is consistent with thesmaller scale height of the atmosphere, while mass conservationarguments taken with the lower horizontal speed imply that the granuleson α Cen B are smaller than on the Sun. Both these properties arein good agreement with the hydrodynamic simulation of Nordlund &Dravins (1990, A&A, 228, 155). The inversions also return thefundamental parameters (T_eff, log g, abundances, v sin i, etc.) of thetwo stars. These values are on the whole in good agreement withliterature values. Also, most of them do not strongly depend on thedetails of the inversion. However, importantly, the element abundancesare 0.1 to 0.15 dex lower when a 2- or 3-component inversion is carriedout than with a 1-component inversion.Based on observations collected at the European Southern Observatories, La Silla, Chile.
|X-rays from α Centauri - The darkening of the solar twin|
We present first results from five XMM-Newton observations of the binarysystem α Centauri, which has been observed in snapshot likeexposures of roughly two hours each during the last two years. In allour observations the X-ray emission of the system is dominated byα Cen B, a K1 star. The derived light curves of the individualcomponents reveal variability on short timescales and a flare wasdiscovered on α Cen B during one observation. A PSF fittingalgorithm is applied to the event distribution to determine thebrightness of each component during the observations. We perform aspectral analysis with multi-temperature models to calculate the X-rayluminosities. We investigate long term variability and possible activitycycles of both stars and find the optically brighter component αCen A, a G2 star very similar to our Sun, to have fainted in X-rays byat least an order of magnitude during the observation program, abehaviour never observed before on α Cen A, but rather similar tothe X-ray behaviour observed with XMM-Newton on HD 81809. We alsocompare our data with earlier spatially resolved observations performedover the last 25 years.
|Abundances of Mn, Co and Eu in a sample of 20 F-G disk stars: the influence of hyperfine structure splitting|
We present Mn, Co and Eu abundances for a sample of 20 disk F and Gdwarfs and subgiants with metallicities in the range-0.8≤[Fe/H]≤+0.3. We investigate the influence of hyperfinestructure (HFS) on the derived abundances of Mn and Co by using HFS datafrom different sources in the literature, as well as calculated HFS frominteraction factors A and B. Eu abundances were obtained from spectralsynthesis of one Eu II line that takes into account HFS from a series ofrecent laboratory measurements. For the lines analysed in this study, wefind that for manganese, the differences between abundances obtainedwith different HFSs are no greater than 0.10 dex. Our cobalt abundancesare even less sensitive to the choice of HFS than Mn, presenting a 0.07dex maximum difference between determinations with different HFSs.However, the cobalt HFS data from different sources are significantlydifferent. Our abundance results for Mn offer an independentconfirmation of literature results, favouring type Ia supernovae as themain nucleosynthesis site of Mn production, in contrast to trends of Mnversus metallicity previously reported in the literature. For Co, weobtain [Co/Fe]0.0 in the range -0.3<[Fe/H]<+0.3 and [Co/Fe]rising to a level of +0.2 when [Fe/H] decreases from -0.3 to -0.8, indisagreement with recent results in the literature. The observeddiscrepancies may be attributed to the lack of HFS in the works we usedfor comparison. Our results for Eu are in accordance with low-mass typeII supernovae being the main site of the r-process nucleosynthesis.
|Constraining fundamental stellar parameters using seismology. Application to α Centauri AB|
We apply the Levenberg-Marquardt minimization algorithm to seismic andclassical observables of the αCen binary system in order to derivethe fundamental parameters of αCenA+B, and to analyze thedependence of these parameters on the chosen observables, on theiruncertainty, and on the physics used in stellar modelling. We show thatwhile the fundamental stellar parameters do not depend on the treatmentof convection adopted (Mixing Length Theory - MLT - or “FullSpectrum of Turbulence” - FST), the age of the system depends onthe inclusion of gravitational settling, and is deeply biased by thesmall frequency separation of component B. We try to answer the questionof the universality of the mixing length parameter, and we find astatistically reliable dependence of the α-parameter on the HRdiagram location (with a trend similar to the predictions based on 2-Dsimulations). We propose the frequency separation ratios as betterobservables to determine the fundamental stellar parameters, and to usethe large frequency separation and frequencies to extract informationabout the stellar structure. The effects of diffusion and equation ofstate on the oscillation frequencies are also studied, but presentseismic data do not allow their determination.
|Detection of X-ray emission from β Pictoris with XMM-Newton: a cool corona, a boundary layer or what?|
β Pictoris (HR 2020) is the most prominentprototype of stars with circumstellar disks and has generated particularinterest in the framework of young planetary systems. Given its spectraltype A5, stellar activity is not expected. Nevertheless, resonance linesof C iii and O vi typical for a chromosphere and transition region havebeen unambiguously detected with FUSE. We present results from anXMM-Newton observation of β Pic and find evidence for X-rayemission. In particular, we detected an emission of O vii at 21.6Å with the MOS detectors. These findings present a challenge forthe development of both stellar activity and disk models. We discuss andinvestigate various models to explain the observed emission includingthe presence of a cool corona and a boundary layer.
|The light curve of the semiregular variable L2 Puppis - II. Evidence for solar-like excitation of the oscillations|
We analyse visual observations of the pulsations of the red giantvariable L2 Puppis (L2 Pup). The data cover 77 yrbetween 1927 and 2005, thus providing an extensive empirical base forcharacterizing properties of the oscillations. The power spectrum of thelight curve shows a single mode resolved into multiple peaks under anarrow envelope. We argue that this results from stochastic excitation,as seen in solar oscillations, with a mode lifetime of about 5 yr. Therandom fluctuations in phase also support this idea. A comparison with XCam, a true Mira star with the same pulsation period, and W Cyg, a truesemiregular star, illustrates the basic differences in phase behaviours.The Mira shows very stable phase, consistent with excitation by theκ-mechanism, whereas W Cyg shows large phase fluctuations thatimply stochastic excitation. We find L2 Pup to beintermediate, implying that both mechanisms play a role in itspulsation. Finally, we also checked the presence of low-dimensionalchaos and could safely exclude it.
|Rotation profile inversion in solar-like stars. In the COROT framework|
The observation of a few mixed modes on solar-like oscillating starswould enable their rotation profile to be inverted with success. Heresimulated data are used to show that it is possible to find models forsolar-like stars that present stochastically excited mixed modes withdetectable amplitudes. We take special care to build the mode set bycomputing the mode amplitudes and selecting those modes with amplitudescompatible with the performance of the forthcoming seismic spaceexperiment, COROT. The frequency set is inverted for various cases whereinput and trial stellar models differ and where random noise is added tothe splittings. We show it is possible to localize a rotation gradientand assess its magnitude. Moreover the use of inverse and forwardprocedures in parallel gives access to a large part of the profile. Weprovide several constraints to help the selection of such stars. Onelooks for a relatively evolved star (still on the main sequence) of≃ 1.5 ~ M_ȯ and the rotation rate on the surface should notbe too small.
|Spectroscopic Properties of Cool Stars (SPOCS). I. 1040 F, G, and K Dwarfs from Keck, Lick, and AAT Planet Search Programs|
We present a uniform catalog of stellar properties for 1040 nearby F, G,and K stars that have been observed by the Keck, Lick, and AAT planetsearch programs. Fitting observed echelle spectra with synthetic spectrayielded effective temperature, surface gravity, metallicity, projectedrotational velocity, and abundances of the elements Na, Si, Ti, Fe, andNi, for every star in the catalog. Combining V-band photometry andHipparcos parallaxes with a bolometric correction based on thespectroscopic results yielded stellar luminosity, radius, and mass.Interpolating Yonsei-Yale isochrones to the luminosity, effectivetemperature, metallicity, and α-element enhancement of each staryielded a theoretical mass, radius, gravity, and age range for moststars in the catalog. Automated tools provide uniform results and makeanalysis of such a large sample practical. Our analysis method differsfrom traditional abundance analyses in that we fit the observed spectrumdirectly, rather than trying to match equivalent widths, and wedetermine effective temperature and surface gravity from the spectrumitself, rather than adopting values based on measured photometry orparallax. As part of our analysis, we determined a new relationshipbetween macroturbulence and effective temperature on the main sequence.Detailed error analysis revealed small systematic offsets with respectto the Sun and spurious abundance trends as a function of effectivetemperature that would be inobvious in smaller samples. We attempted toremove these errors by applying empirical corrections, achieving aprecision per spectrum of 44 K in effective temperature, 0.03 dex inmetallicity, 0.06 dex in the logarithm of gravity, and 0.5 kms-1 in projected rotational velocity. Comparisons withprevious studies show only small discrepancies. Our spectroscopicallydetermined masses have a median fractional precision of 15%, but theyare systematically 10% higher than masses obtained by interpolatingisochrones. Our spectroscopic radii have a median fractional precisionof 3%. Our ages from isochrones have a precision that variesdramatically with location in the Hertzsprung-Russell diagram. We planto extend the catalog by applying our automated analysis technique toother large stellar samples.
|Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres|
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.