내용
사진
사진 업로드
DSS Images Other Images
관련 글
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.
| Secular variability of the longitudinal magnetic field of the Ap star γ Equ
We present an analysis of the secular variability of the longitudinalmagnetic field Be in the roAp star γ Equ (HD 201601).Measurements of the stellar magnetic field Be were mostlycompiled from the literature, and we appended also our 33 newBe measurements which were obtained with the 1-m opticaltelescope of the Special Astrophysical Observatory (Russia). All theavailable data cover the time period of 58 yr, and include both phasesof the maximum and minimum Be. We determined that the periodof the long-term magnetic Be variations equals 91.1 +/- 3.6yr, with Be(max) =+577 +/- 31 G and Be(min) =-1101+/- 31 G.
| Pulsational variability of Li I 6708 Åline profile in the spectra of roAp star gamma Equ
In the framework of the Project "Lithium in CP stars" the task ofpulsational line profile variations (LPV) for Li I 6708 Å iscarried out. The high spectral and time resolution observations wereobtained for typical roAp(CP2) star gamma Equ. Analysis of two night'sobservations shows a definite blue-to-red LPV of Li I 6708 A duringpulsational period, that could be explained by two ways: the firstsupposes the formation of shock wave in the most upper layers nearmagnetic poles and red shifts due to matter falling on star; the second- a red asymmetry of Li I line profile is explained by high isotopicratio 6Li/7Li (about 0.5) due to spallationprocesses in polar Li spot.
| An Atlas of K-Line Spectra for Cool Magnetic CP Stars: The Wing-Nib Anomaly (WNA)
We present a short atlas illustrating the unusual Ca II K-line profilesin upper main-sequence stars with anomalous abundances. Slopes of theprofiles for 10 cool, magnetic chemically peculiar (CP) stars changeabruptly at the very core, forming a deep ``nib.'' The nibs show thesame or nearly the same radial velocity as the other atomic lines. Thenear wings are generally more shallow than in normal stars. In threemagnetic CP stars, the K lines are too weak to show this shape, althoughthe nibs themselves are arguably present. The Ca II H lines also showdeep nibs, but the profiles are complicated by the nearby, strongHɛ absorption. The K-line structure is nearly unchanged withphase in β CrB and α Cir. Calculations, including NLTE, showthat other possibilities in addition to chemical stratification mayyield niblike cores.
| 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.
| Evolutionary state of magnetic chemically peculiar stars
Context: .The photospheres of about 5-10% of the upper main sequencestars exhibit remarkable chemical anomalies. Many of these chemicallypeculiar (CP) stars have a global magnetic field, the origin of which isstill a matter of debate. Aims: .We present a comprehensivestatistical investigation of the evolution of magnetic CP stars, aimedat providing constraints to the theories that deal with the origin ofthe magnetic field in these stars. Methods: .We have collectedfrom the literature data for 150 magnetic CP stars with accurateHipparcos parallaxes. We have retrieved from the ESO archive 142 FORS1observations of circularly polarized spectra for 100 stars. From thesespectra we have measured the mean longitudinal magnetic field, anddiscovered 48 new magnetic CP stars (five of which belonging to the rareclass of rapidly oscillating Ap stars). We have determined effectivetemperature and luminosity, then mass and position in the H-R diagramfor a final sample of 194 magnetic CP stars. Results: .We foundthat magnetic stars with M > 3 ~M_ȯ are homogeneouslydistributed along the main sequence. Instead, there are statisticalindications that lower mass stars (especially those with M ≤2~M_ȯ) tend to concentrate in the centre of the main sequence band.We show that this inhomogeneous age distribution cannot be attributed tothe effects of random errors and small number statistics. Our datasuggest also that the surface magnetic flux of CP stars increases withstellar age and mass, and correlates with the rotation period. For starswith M > 3~M_ȯ, rotation periods decrease with age in a wayconsistent with the conservation of the angular momentum, while for lessmassive magnetic CP stars an angular momentum loss cannot be ruledout. Conclusions: .The mechanism that originates and sustains themagnetic field in the upper main sequence stars may be different in CPstars of different mass.
| Search for radial velocity and magnetic field pulsational variations in the roAp star γ Equulei
Aims.A new continuous 3.5 h series of high-resolution spectroscopicobservations of the roAp star γ Equ, with about 12 points perpulsation cycle, is presented to study the mean magnetic field modulusvariability with the pulsation period.Methods.Radial velocity variationswith amplitudes of up to approximately 400 m s-1 are clearlydetected in the rare earth lines present in the covered spectral range.Two frequencies are resolved in their analysis, which are consistentwith frequencies previously observed in the star. No significant radialvelocity variation is detected for the line Fe II λ 6149.2. Themean magnetic field modulus derived from measurement of the wavelengthseparation of the magnetically resolved components of this line does notshow any variation, with a 3σ upper limit of ~10 G. Results.Whileour results are consistent with the conclusion of the previous studies,difficulties to reconcile this result with those of other works may beattributed, in part at least, to the use of different diagnostic linesin different works and to the failure of all studies so far to fullyresolve all the pulsation frequencies of γ Equ.
| Pulsational line profile variation of the roAp star HR 3831
We report the first comprehensive investigation of the line profilevariation caused by non-radial pulsation in an oscillating magneticchemically peculiar star. Spectrum variation of the well-known roAp starHR 3831 was detected using very high-resolution high signal-to-noisespectroscopic time-series observations and was followed through thewhole rotation cycle of the star. We confirmed the outstanding diversityof pulsational behaviour of different lines in the HR 3831 spectrum andattributed this phenomenon to an interplay between extreme verticalchemical inhomogeneity of the HR 3831 atmosphere and a running pulsationwave, that is propagating towards the upper photospheric layers withincreasing amplitude. The rapid profile variation of the Nd III 6145Å line, which shows the maximum pulsational disturbances in thestudied wavelength region, is characterised by measuring changes of itsequivalent width and the first three moments. Each of these observablesexhibits almost purely sinusoidal variation through the pulsation cycle,with the amplitude and phase clearly modulated by the stellar rotation.In contrast to previous photometric pulsational measurements of roApstars, our spectroscopic observational material admits straightforwardastrophysical interpretation and, hence, opens new and excitingpossibilities for direct and accurate analysis of the roAp pulsations.We demonstrate that rotational modulation of the radial velocityoscillations cannot be fully explained by an oblique axisymmetric dipole(ℓ=1, m=0) mode, implied by the classical oblique pulsator model ofroAp stars. Pulsational variation of the higher moments, in particularthe line width, reveals substantial contribution of the high-order(ℓ=3) spherical harmonics that appear due to distortion ofpulsations in HR 3831 by the global magnetic field. We interpretedobservations with a novel numerical model of the pulsational variationand rotational modulation of the line profile moments in roAp stars.Comparison of the observed and computed radial velocity and line widthvariation was used to establish parameters of the oblique pulsator modelof HR 3831. Furthermore, definite detection of pulsational variation inlines of light and iron-peak elements allows the first 3-D mapping ofpulsations in a non-radially oscillating star.
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| The discovery of remarkable 5kms-1 pulsational radial velocity variations in the roAp star HD99563*
In a high-resolution spectral survey of nearly half the 34 known rapidlyoscillating Ap (roAp) stars, using the Ultraviolet-Visual EchelleSpectrograph on the Very Large Telescope, we have discovered remarkablylarge amplitude pulsations in the roAp star HD99563 with some spectrallines showing radial velocity amplitudes up to 5kms-1(10kms-1 peak-to-peak) with a pulsation period of 10.7min. Asfor many other roAp stars, we find the largest pulsation amplitudes forlines of some rare earth elements and in the core of the Hα line.The highest amplitudes of 5kms-1 are seen in rather weaklines of EuII and TmII. Stronger lines of PrIII and NdIII have pulsationamplitudes in the range 0.7 to 3.5kms-1 for different lines.In the narrow Hα core, the average amplitude is2.6kms-1, but, as is the case for other lines, the amplitudeand phase vary strongly with line depth (atmospheric height), with theamplitude of the radial velocity variations of the line bisectorreaching a maximum of 4.3kms-1 at the bottom of the core.Some other elements show pulsation amplitudes 0.1 to0.7kms-1. Variations in velocity amplitude and phase forseveral spectral lines were studied using line-bisector measurements toobtain information about the vertical structure of the pulsation modesand the stellar atmosphere.
| A non-adiabatic analysis for axisymmetric pulsations of magnetic stars
This paper presents the results of a non-adiabatic analysis foraxisymmetric non-radial pulsations including the effect of a dipolemagnetic field. Convection is assumed to be suppressed in the stellarenvelope, and the diffusion approximation is used to radiativetransport. As in a previous adiabatic analysis, the eigenfunctions areexpanded in a series of spherical harmonics. The analysis is applied toa 1.9-Msolar, main-sequence model (logTeff=3.913). The presence of a magnetic field always stabilizes low-orderacoustic modes. All the low-order modes of the model that are excited bythe κ-mechanism in the HeII ionization zone in the absence of amagnetic field are found to be stabilized if the polar strength of thedipole magnetic field is larger than about 1 kG. For high-order p modes,on the other hand, distorted dipole and quadrupole modes excited by theκ-mechanism in the H ionization zone remain overstable, even inthe presence of a strong magnetic field. It is found, however, that allthe distorted radial high-order modes are stabilized by the effect ofthe magnetic field. Thus, our non-adiabatic analysis suggests thatdistorted dipole modes and distorted quadrupole modes are most likelyexcited in rapidly oscillating Ap stars. The latitudinal amplitudedependence is found to be in reasonable agreement with theobservationally determined one for HR 3831. Finally, the expectedamplitude of magnetic perturbations at the surface is found to be verysmall.
| Probing the magnetoacoustic boundary layer in the peculiar magnetic star 33 Lib (HD 137949)*
We show unprecedented resolution of the amplitudes and phases of theprincipal 2.015-mHz mode and its 4.030-mHz harmonic as a function ofatmospheric depth in the roAp star 33 Lib using high-resolution VeryLarge Telescope (VLT) Ultraviolet-Visual Echelle Spectrograph (UVES)spectra. We show that the pulsation amplitude increases upwards into theNd III line-forming layer away from an atmospheric node, and increasesdownwards from the same node in the Nd II line-forming layer. We suggestthat the Nd III layer, which arises at, or above, τ~10-3, overlaps with the magnetoacoustic boundary layer, andthat our detailed observations of pulsation amplitude and phasevariations with depth provide a first probe of this layer.
| Pushing the ground-based limit: 14-μmag photometric precision with the definitive Whole Earth Telescope asteroseismic data set for the rapidly oscillating Ap star HR1217
HR1217 is one of the best-studied rapidly oscillating Ap (roAp) stars,with a frequency spectrum of alternating even- and odd-l modes that aredistorted by the presence of a strong, global magnetic field. Severalrecent theoretical studies have found that within the observableatmospheres of roAp stars the pulsation modes are magneto-acoustic withsignificant frequency perturbations that are cyclic with increasingfrequency. To test these theories a Whole Earth Telescope extendedcoverage campaign obtained 342 h of Johnson B data at 10-s timeresolution for the roAp star HR1217 over 35 d with a 36 per cent dutycycle in 2000 November-December. The precision of the derived amplitudesis 14 μmag, making this one of the highest precision ground-basedphotometric studies ever undertaken. Substantial support has been foundfor the new theories of the interaction of pulsation with the strongmagnetic field. In particular, the frequency jump expected as themagnetic and acoustic components cycle through 2π rad in phase hasbeen found. Additionally, comparison of the new 2000 data with anearlier 1986 multisite study shows clear amplitude modulation for somemodes between 1986 and 2000. The unique geometry of the roAp starsallows their pulsation modes to be viewed from varying aspect withrotation, yielding mode identification information in the rotationalsidelobes that is available for no other type of pulsating star. Thoserotational sidelobes in HR1217 confirm that two of the modes aredipolar, or close to dipolar; based on the frequency spacings andHipparcos parallax, three other modes must be either l= 0 or 2 modes,either distorted by the magnetic field, or a mix of m-modes of given lwhere the mixture is the result of magnetic and rotational effects. Astudy of all high-speed photometric Johnson B data from 1981 to 2000gives a rotation period Prot= 12.4572 d, as found in previouspulsation and photometric studies, but inconsistent with a differentrotation period found in magnetic studies. We suggest that this rotationperiod is correct and that zero-point shifts between magnetic data setsdetermined from different spectral lines are the probable cause of thecontroversy over the rotation period. This WET data set is likely tostand as the definitive ground-based study of HR1217. It will be thebaseline for comparison for future space studies of HR1217, particularlythe MOST satellite observations.
| An elemental abundance analysis of the magnetic chemically peculiar star HR 8216
An elemental abundance analysis of the cool magnetic CP star HR 8216 (=HD204411) was performed using 2.4 Åmm-1 DominionAstrophysical Observatory Reticon exposures coveringλλ3830-4770 with a typical signal-to-noise ratio of 200taken with the long camera of the 1.22-m telescope. The spectrogramswere measured interactively with the graphics computer program REDUCE.The fine analysis used an ATLAS9 metal-rich model atmosphere, thepredictions of which best-matched the optical region fluxes and theHγ profile. The anomalies of HR 8216 are primarily an enhancementof many iron peak elements with the rare earths elements much lessenhanced than in many similar cool magnetic CP stars. Using the resultsof the fine analysis the spectrum was synthesized. Comparison with theobserved spectrum showed that the agreement is good but did not fullyaccount for all of the observed line spectrum.
| High Resolution Stellar Spectroscopy with VBT Echelle Spectrometer
The optical design and performance of the recently commissioned fiberfed echelle spectrometer of 2.34 meter Vainu Bappu Telescope aredescribed. The use of it for stellar spectroscopic studies is discussed.
| Magnetic Field and Chemical Composition of the Peculiar Star HD 10221
We analyzed the chemical composition of the chemically peculiar (CP)star HD 0221=43 Cas using spectra taken with the NES spectrograph of the6-m telescope with a spectral resolution of 45 000. The Hβ lineprofile corresponds most closely to T eff = 11 900 K and log g = 3.9.The rotational velocity is v e sin i = 27 ± 2 km s-1, and themicroturbulence is ξ t = 1 km s-1. The results of our abundancedetermination by the method of synthetic spectra show that the star haschemical anomalies typical of SrCrEu stars, although its effectivemagnetic field is weak, B e < 100 G. For silicon, we obtained anabundance distribution in atmospheric depth with a sharp jump of 1.5 dexat an optical depth of log τ 5000 = -0.3 and with siliconconcentration in deep atmospheric layers. Similar distributions werefound in the atmospheres of cooler stars with strong and weak magneticfields. A comparison of the chemical peculiarities in HD 10221 withknown CP stars with magnetic fields of various strengths leads us toconclude that a low rotational velocity rather than amagnetic field isthe determining factor in the formation mechanism of chemical anomaliesin the atmospheres of CP stars.
| Model of the Magnetic Field of HD 187474
A model is constructed for the magnetic field of the star HD 187474,which has a very long axial rotation period P = 2345d. It turns out thatthe structure of the magnetic field is best described by a model of adisplaced (Δα = 0.1) dipole inclined to the axis of rotationby an angle β = 24°. The star is inclined to the line of sightby an angle i = 86°. Because of the displaced dipole the magnitudeof the magnetic field differs at the poles: Bp = +6300 and 11600 G. AMercator map of the distribution of the magnetic field over the surfaceis obtained. The 7 slowly rotating CP stars studied thus far have anaverage angle β = 62°, which equals the average value for arandom orientation of dipoles.
| Investigation of the Rapid Magnetic Field Variability in gamma Equulei
We investigated magnetic variability of the roAp star gamma Equ (HD201601) with high time resolution. Measurements of the stellarlongitudinal magnetic field B_e were performed with the hydrogen linepolarimeter and the 6-m optical telescope of the Special AstrophysicalObservatory (Russia) on 20/21 August 1989. We obtained a single 3-hourseries of 1720 rapid B_e measurements for this star with the averagetime resolution of 6.6 sec. The averaged value of on thisnight equals to -750+/-22 G. Both the power spectrum and the amplitudespectrum of the B_e time series are essentially flat. However, they showthe presence of a marginally significant magnetic period at P_B=3.596min which is real with the probability of 67%. The amplitude of themagnetic field variations with this period equals 347+/-31 G. We did notdetect variations of B_e field in gamma Equ with the well-knownphotometric period, P_phot=12.44 min.
| NLTE ionization equilibrium of Nd II and Nd III in cool A and Ap stars
We investigate the formation of Nd ii- iii lines in the atmospheres ofA-type stars with a comprehensive atomic model including 1651 levels ofNd ii, 607 levels of Nd iii and the ground state of Nd iv. NLTE leads tooverionization of Nd ii which weakens the Nd ii lines relative to thecorresponding LTE line strengths at mild neodymium overabundance ([Nd/H]< 2.5) and amplifies them at higher [Nd/H] values. NLTE abundancecorrections grow with the effective temperature and reach 0.6 dex atT_eff = 9500 K for [Nd/H] = 2.5. The Nd iii lines are strengthenedcompared with LTE in all cases, and NLTE abundance corrections liebetween -0.3 dex and -0.2 dex for T_eff between 7500 K and 9500 K. NLTEeffects are larger for an inhomogeneous vertical abundance distributioncompared with a homogeneous one resulting in positive NLTE abundancecorrection up to 1.3 dex for the Nd ii lines and in negative ones downto -0.5 dex for the Nd iii lines. The neodymium distribution in theatmospheres of roAp stars γ Equ and HD 24712 is deduced from NLTEanalysis of the Nd ii and Nd iii lines and a strong evidence is foundfor the existence of enhanced Nd abundance layers abovelogτ5000 = -3.
| Abundances and chemical stratification analysis in the atmosphere of Cr-type Ap star HD 204411
We present results of an abundance and stratification analysis of theweakly magnetic chemically peculiar star HD 204411 based on the echellespectrum obtained with the high resolution spectrograph at the 3.55-mTelescopio Nazionale Galileo at the Observatorio del Roque de losMuchachos (La Palma, Spain). Atmospheric parameters obtained from thespectroscopy and spectrophotometry together with the Hipparcos parallaxshow that this star has already left the Main Sequence band. The upperlimit for the surface magnetic field derived from the differentialbroadening of the spectral lines with different magnetic sensitivity is750 G, which agrees with the recent detection of the weak effectivemagnetic field in this star. The best fit to the observed spectral lineprofiles was obtained with a combination of the rotational velocity v_esin i = 5.4 km s-1 and the radial-tangential macroturbulenceof 4.8 km s-1. The average abundances of HD 204411 aretypical for an Ap star of the Cr-type: C and O are deficient, Cr and Feare strongly overabundant. Sr, Y, Zr and the rare-earths, which usuallyhave large overabundances in cool Ap stars with strong magnetic fields,are either normal (Y, Ce) or only +0.5 dex overabundant in the weaklymagnetic star HD 204411. The chemical stratification analysis wasperformed for 5 elements, Mg, Si, Ca, Cr and Fe. Si, Ca and Fe show atendency to be concentrated below logτ5000=-1, while forMg we found marginal evidence for concentration in the upper atmosphere.This behaviour of Mg may be an artifact caused by the limited sample ofspectral lines and poor atomic data available for the Mg II lines usedin our analysis. Chromium, the most anomalous Fe-peak element, does notshow significant abundance gradients in the line-forming region.
| Variation of the line profile moments for stars pulsating in distorted oblique non-radial modes
We derive expressions and develop a numerical technique for the analysisof the line profile moment variations for stars pulsating in obliquenon-radial modes. This represents an extension of the widely usedspectroscopic moment mode identification method to the oblique distortedpulsations observed in rapidly oscillating Ap stars. We demonstrate thata non-axisymmetric superposition of the pulsation and rotation velocityfields results in a qualitatively new behaviour of some of the lineprofile characteristics. It is found that for the majority of roAp starsthe second moment varies with the pulsation frequency rather than withits first harmonic even for axisymmetric modes. We also identifypulsation observables which do not change during pulsation cycle but aremodulated by the stellar rotation and can contribute to the variabilityof the stellar spectra averaged over many pulsation cycles. As anillustration of the new version of the moment technique, we computerotational modulation of the pulsational changes of the line profilemoments for the oblique axisymmetric dipolar pulsation modes withdifferent parameters. It is also shown that a distortion of the obliquedipolar modes predicted by the recent theoretical studies of the stellarmagneto-acoustic oscillations can be readily diagnosed through themoment analysis. In particular, the shape of the pulsation phasemodulation for the radial velocity and the second moment is verysensitive to non-axisymmetric pulsation components, whereas therotational modulation of the second moment amplitude is best suited torevealing axisymmetric magnetically induced distortion of pulsations.
| On the influence of Stark broadening on Cr I lines in stellar atmospheres
Using the semiclassical perturbation method, electron-, proton-, andionized helium-impact line widths and shifts for the nine Cr i spectrallines from the 4p^7P^0{-}4d^7D multiplet were calculated for a perturberdensity of 1014 cm-3 and for temperatures T = 2500{-} 50 000 K. The results were used to investigate the influence ofStark broadening effect in the Cr-rich Ap star β CrB atmosphere online shapes of these lines. It was found that the contribution of protonand He ii collisions to the line width and shift is significant andcomparable, and is sometimes even larger than electron-impactcontribution depending of the electron temperature. Moreover, not onlythe Stark line width, but also the Stark shift may contribute to theblue as well as to the red asymmetry of the same line depending on theelectron-, proton-, and He ii density in stellar atmosphere. The resultswere used to investigate the influence of Stark broadening effect on Cri line shapes in the atmosphere of the Cr-rich Ap star β CrB.
| A catalog of stellar magnetic rotational phase curves
Magnetized stars usually exhibit periodic variations of the effective(longitudinal) magnetic field Be caused by their rotation. Wepresent a catalog of magnetic rotational phase curves, Be vs.the rotational phase φ, and tables of their parameters for 136stars on the main sequence and above it. Phase curves were obtained bythe least squares fitting of sine wave or double wave functions to theavailable Be measurements, which were compiled from theexisting literature. Most of the catalogued objects are chemicallypeculiar A and B type stars (127 stars). For some stars we also improvedor determined periods of their rotation. We discuss the distribution ofparameters describing magnetic rotational phase curves in our sample.All tables and Appendix A are only available in electronic form athttp://www.edpsciences.org
| Radial velocity variations in pulsating Ap stars. V. 10 Aquilae
We have used precise stellar radial velocities to study the pulsationalmotion of the rapidly oscillating Ap star 10 Aql.Observations were made on three separate nights using the highresolution spectrograph of the Harlan J. Smith 2.7 m telescope atMcDonald Observatory. A high radial velocity precision was achieved byusing an iodine gas absorption cell. The integrated radial velocitymeasurements using the full wavelength region covered by iodineabsorption lines (5000-6300 Å) failed to detect any coherentpulsational velocity variations to a level of 2.5-5 m s-1. Ananalysis over a much narrower wavelength range revealed that pulsationalradial velocity variations are indeed present in 10Aql, but only in 5 spectral lines. The amplitude of thesevariations ranged from about 100 m s-1 to as high as 398 ms-1 for an unidentified feature at λ5471.40 Å.Other spectral features showing pulsational radial velocity variationsare tentatively identified as Sm II and Tm II.Based on observations collected at McDonald Observatory.
| The 5200-Åflux depression of chemically peculiar stars - II. The cool chemically peculiar and λ Bootis stars
After establishing the synthetic Δa photometric system in thefirst paper of this series, we now present model atmospheres computedwith individual abundances for a representative sample of chemicallypeculiar (CP) stars and either confirm or redetermine their inputparameters through comparisons with photometric, spectrophotometric andhigh-resolution spectroscopic data. The final models obtained from thisprocedure were used to compute synthetic Δa indices which werecompared with observations. The observed behaviour of Δa isreproduced for several types of CP stars: models for Am stars shownegligible (or marginally positive) values of a few mmag, while forλ Bootis stars - and for metal deficient A stars in general - weobtain negative values (as low as -12 mmag in one case). For the coolestCP2 stars with effective temperatures below about 8500 K, we obtain mild(~+10 mmag) to moderately large (~+30 mmag) flux depressions inagreement with observations. However, Δa values for slightlyhotter members of the CP2 group (for which still Teff <10000 K) are underestimated from these new models. The effect of themicroturbulence parameter on the Δa index is revisited and itsdifferent role in various types of CP stars for reproducing the fluxdepression at 5200 Åis explained. We also provide reasons whymodels based on enhanced microturbulence and scaled solar abundancecould not explain the observed flux depression for all types of CPstars. We discuss potential improvements of the current models includingthe possibility of still missing line opacities (unidentified andautoionization lines), modifications due to an explicit account of aglobal stellar magnetic field, and the effect of vertical abundancestratification.
| The null result of a search for pulsational variations of the surface magnetic field in the roAp star γ Equulei
We describe an analysis of the time-resolved measurements of thesurface magnetic field in the roAp star γEqu. We have obtained ahigh-resolution and high signal-to-noise (S/N) spectroscopic timeseries, and the magnetic field was determined using Zeeman-resolvedprofiles of the FeII 6149.25 Åand FeI 6173.34 Ålines.Contrary to recent reports, we do not find any evidence of magneticvariability with pulsation phase, and derive an upper limit of 5-10 Gfor pulsational modulation of the surface magnetic field in γEqu.
| Radial velocity variations in pulsating Ap stars - III. The discovery of 16.21-min oscillations in β CrB
We present the analysis of 3 h of a rapid time series of precisestellar radial velocity (RV) measurements (σ= 4.5 ms-1) of the cool Ap star β CrB. The integrated RVmeasurements spanning the wavelength interval 5000-6000 Åshowsignificant variations (false alarm probability = 10-5) witha period of 16.21 min (ν= 1028.17 μHz) and an amplitude of 3.54+/- 0.56 m s-1. The RV measured over a much narrowerwavelength interval reveals one spectral feature at λ6272.0Åpulsating with the same 16.21-min period and an amplitude of 138+/- 23 m s-1. These observations establish β CrB to be alow-amplitude rapidly oscillating Ap star.
| No magnetic field variation with pulsation phase in the roAp star γ Equulei
We present an analysis of 210 high-resolution time-resolvedspectropolarimetric observations of the roAp star γ Equ obtainedover three nights in August and September 2003. Radial velocityvariations due to p-mode non-radial pulsations are clearly detected inthe lines of rare-earth elements, in particular Pr III, Nd II and NdIII. In contrast, we find absolutely no evidence for the variation ofthe mean longitudinal magnetic field over the pulsation period inγ Equ at the level of 110-240 G which was recently reported byLeone & Kurtz (\cite{LK03}). Our investigation of the variability ofcircularly polarized profiles of 13 Nd III lines demonstrates that, atthe 3σ confidence level, no magnetic field variation with anamplitude above ≈40-60 G was present in γ Equ during ourmonitoring of this star.
| Measurements of magnetic fields over the pulsation cycle in six roAp stars with FORS 1 at the VLT
With FORS 1 at the VLT we have tried for the first time to measure themagnetic field variation over the pulsation cycle in six roAp stars tobegin the study of how the magnetic field and pulsation interact. Forthe star HD 101065, which has one of the highest photometric pulsationamplitudes of any roAp star, we found a signal at the known photometricpulsation frequency at the 3σ level in one data set; however thiscould not be confirmed by later observations. A preliminary simplecalculation of the expected magnetic variations over the pulsation cyclesuggests that they are of the same order as our current noise levels,leading us to expect that further observations with increased S/N have agood chance of achieving an unequivocal detection.Based on observations obtained at the European Southern Observatory,Paranal, Chile (ESO programmes Nos. 69.D-0210 and 270.D-5023).
| New heights in asteroseismology: VLT spectroscopy of the roAp star HD 166473
We show for the first time, for any star other than the Sun, thevertical resolution of pulsation modes into standing waves in theatmosphere and running waves in the magnetoacoustic reflective boundarylayer of the upper atmosphere. This has only become possible with thenovel use of the Very Large Telescope (VLT) on a bright, exceedinglypeculiar magnetic star, HD 166473. These are the first observations thatmay directly test recent theoretical developments for magnetoacousticstellar pulsation modes in rapidly oscillating Ap stars.
|
새 글 등록
관련 링크
새 링크 등록
다음 그룹에 속해있음:
|
관측 및 측정 데이터
천체목록:
|
INHABITED SKY
