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A rival for Babcock's star: the extreme 30-kG variable magnetic field in the Ap star HD75049
The extraordinary magnetic Ap star HD75049 has been studied with dataobtained with the European Southern Observatory Very Large Telescope and2.2-m telescopes. Direct measurements reveal that the magnetic fieldmodulus at maximum reaches 30kG. The star shows photometric, spectraland magnetic variability with a rotation period of 4.049d. Variations ofthe mean longitudinal magnetic field can be described to first order bya centred dipole model with an inclination i = 25°, an obliquity? = 60° and a polar field Bp = 42kG. The combinationof the longitudinal and surface magnetic field measurements implies aradius of R = 1.7Rsolar, suggesting that the star is close tothe zero-age main sequence. HD75049 displays moderate overabundances ofSi, Ti, Cr, Fe and large overabundances of rare earth elements. Thisstar has the second strongest magnetic field of any main-sequence starafter Babcock's star, HD215441, which it rivals.Based on observations collected at the European Southern Observatory(ESO), Chile, in the programme 080.D-0170(A) and as part of programmes078.D-0080(A) and 078.D-0192(A).E-mail: velkin@uclan.ac.uk
| The discovery of a 21-kG magnetic field in the Ap star BD+0°4535
With high-resolution spectral observations using a 2.2-m telescope, wefound that the Ap star BD+0°4535 has a very strong magnetic field.From Zeeman components of rare earth element spectral lines, we detectedan average magnetic field modulus of about 21kG, although for lines ofdifferent chemical elements the estimated magnetic field ranges between15 and 23kG. This is a consequence of non-uniform surface distributionof these elements. Considering its possible variation with rotationperiod, the upper limit of magnetic field is potentially larger thanwhat we have detected. For rare earth elements, large overabundances andionization disequilibria were found.Based on observations collected at the European Southern Observatory, LaSilla, Chile, as part of programmes 083.D-0034(A) and 081.D-2002.E-mail: velkin@uclan.ac.uk
| The determination of the rotation period and magnetic field geometry of the strongly magnetic roAp star HD154708
We obtained 13 spectropolarimetric observations of the strongly magneticrapidly oscillating Ap star HD154708 over 3 months with the multimodeinstrument FORS1, installed at the 8-m Kueyen telescope of the VeryLarge Telescope. These observations have been used for the determinationof the rotation period of P = 5.3666 +/- 0.0007d. Using stellarfundamental parameters and the longitudinal magnetic field phase curve,we briefly discuss the magnetic field geometry. The star is observednearly pole-on and the magnetic field geometry can be described by acentred dipole with a surface polar magnetic field strengthBd between 26.1 and 28.8kG and an inclination of the magneticaxis to the rotation axis in the range to .Based on observations collected at ESO, Paranal, Chile (ESO programmesNos. 075.D-0145, 076.D-0169, 077.D-0150 and 079.D-0240).E-mail: shubrig@aip.de
| Studying the Magnetic Properties of Upper Main-sequence Stars with FORS1
We summarise the results of our recent magnetic field studies in uppermain-sequence stars, which have exploited the spectropolarimetriccapability of FORS1 at the VLT extensively.
| Discovery of very low amplitude 9-minute multiperiodic pulsations in the magnetic Ap star HD 75445
We present our discovery of pulsational radial-velocity variations inthe cool Ap star HD 75445, an object spectroscopically similar to thebright, rapidly-oscillating Ap (roAp) star ? Equ. Based onhigh-resolution time-series spectroscopy obtained with the HARPSspectrometer at the European Southern Observatory 3.6-m telescope, wedetected oscillations in Nd II and Nd III lines with a period close to 9min and amplitudes of 20-30 m s-1. Substantial variation inthe pulsational amplitude during our 3.8 h observing run reveals thepresence of at least three excited non-radial modes. The detection ofextremely low amplitude pulsations in HD 75445 indicates that the roApexcitation mechanism produces variability in the radial velocityamplitude of between a few tens m s-1 and several kms-1. This supports the idea that many, if not all, cool Apstars occupying the roAp instability strip may harbour non-radialpulsations, which currently remain undetected due to their smallphotometric and radial-velocity amplitudes.Based on observations collected at the European Southern Observatory,Chile (ESO programs 68.D-0254, 079.D-0118).
| A search for rapid pulsations in the magnetic cool chemically peculiar star HD3980
The Ap star HD3980 appears to be a promising roAp candidate based on itsfundamental parameters, leading us to search for rapid pulsations withthe VLT UV-Visual Echelle Spectrograph (UVES). A precise Hipparcosparallax and estimated temperature of 8100K place HD3980 in the middleof the theoretical instability strip for rapidly oscillating Ap stars,about halfway through its main-sequence evolution stage. The star has astrong, variable magnetic field, as is typical of the cool magnetic Apstars. Dipole model parameters were determined from VLT observationsusing Focal Reducer and low Dispersion Spectrograph (FORS)1. FromDoppler shift measurements for individual spectral lines of rare-earthelements and the H? line core, we find no pulsations above20-30ms-1. This result is corroborated by the inspection oflines of several other chemical elements, as well as withcross-correlation for long spectral regions with the average spectrum asa template. Abundances of chemical elements were determined and showlarger than solar abundances of rare-earth elements. Further, ionizationdisequilibria for the first two ionized states of Nd and Pr aredetected. We also find that the star has a strong overabundance ofmanganese, which is typical for much hotter HgMn and other Bp stars.Line profile variability with the rotation period was detected for themajority of chemical species.Based on observations collected at the European Southern Observatory(ESO), Paranal, Chile, as part of programme 077.D-0150(A) and part ofprogrammes 074.D-0392(A) and 076.D-0535(A) in the ESO archive.E-mail: velkin@uclan.ac.uk
| On the spectroscopic nature of the cool evolved Am star HD151878
Recently, Tiwari, Chaubey & Pandey detected the bright component ofthe visual binary HD151878 to exhibit rapid photometric oscillationsthrough a Johnson B filter with a period of 6min (2.78mHz) and a high,modulated amplitude up to 22mmag peak-to-peak, making this star by farthe highest amplitude rapidly oscillating Ap (roAp) star known. As a newroAp star, HD151878 is of additional particular interest as a scarceexample of the class in the northern sky, and only the second known caseof an evolved roAp star - the other being HD116114. We used theFIbre-fed Echelle Spectrograph at the Nordic Optical Telescope to obtainhigh time-resolution spectra at high dispersion to attempt to verify therapid oscillations. We show here that the star at this epoch isspectroscopically stable to rapid oscillations of no more than a fewtens of ms-1. The high-resolution spectra furthermore showthe star to be of type Am rather than Ap and we show the star lacks mostof the known characteristics for roAp stars. We conclude that this is anAm star that does not pulsate with a 6-min period. The originaldiscovery of pulsation is likely to be an instrumental artefact.Based on observations collected at the Nordic Optical Telescope as partof programme 36-418. E-mail: lfreyham@gmail.com
| Discovery of 17 new sharp-lined Ap stars with magnetically resolved lines
Chemically peculiar A stars (Ap) are extreme examples of the interactionof atomic element diffusion processes with magnetic fields in stellaratmospheres. The rapidly oscillating Ap stars provide a means forstudying these processes in three dimensions and are at the same timeimportant for studying the pulsation excitation mechanism in A stars. Aspart of the first comprehensive, uniform, high-resolution spectroscopicsurvey of Ap stars, which we are conducting in the Southern hemispherewith the Michigan Spectral Catalogues as the basis of target selection,we report here the discovery of 17 new magnetic Ap stars havingspectroscopically resolved Zeeman components from which we derivemagnetic field moduli in the range 3-30kG. Among these are (1) thecurrent second strongest known magnetic A star, (2) a double-lined Apbinary with a magnetic component and (3) an A star with particularlypeculiar and variable abundances. Polarimetry of these stars is neededto constrain their field geometries and to determine their rotationperiods. We have also obtained an additional measurement of the magneticfield of the Ap star HD92499.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 078.D-0080(A), 078.D-0192(A),072.D-0138(A).E-mail: lmfreyhammer@uclan.ac.uk
| The fundamental parameters of the roAp star ? Circini
We have used the Sydney University Stellar Interferometer to measure theangular diameter of ? Cir. This is the first detailedinterferometric study of a rapidly oscillating A (roAp) star, ?Cir being the brightest member of its class. We used the new and moreaccurate Hipparcos parallax to determine the radius to be 1.967 +/-0.066Rsolar. We have constrained the bolometric flux fromcalibrated spectra to determine an effective temperature of 7420 +/-170K. This is the first direct determination of the temperature of anroAp star. Our temperature is at the low end of previous estimates,which span over 1000 K and were based on either photometric indices orspectroscopic methods. In addition, we have analysed two high-qualityspectra of ? Cir, obtained at different rotational phases and wefind evidence for the presence of spots. In both spectra we find nearlysolar abundances of C, O, Si, Ca and Fe, high abundance of Cr and Mn,while Co, Y, Nd and Eu are overabundant by about 1 dex. The resultsreported here provide important observational constraints for futurestudies of the atmospheric structure and pulsation of ? Cir.Based on observations with the Sydney University Stellar Interferometerat the Paul Wild Observatory, Narrabri (Australia) and observationscollected at the European Southern Observatory, Paranal (Chile), as partof programmes 072.D-0138 and 077.D-0150.E-mail: bruntt@physics.usyd.edu.au
| Magnetic fields of chemically peculiar stars. I. The catalog of magnetic CP stars
This is the first paper of the series dedicated to the analysis of themagnetism of chemically peculiar (CP) stars of the upper Main Sequence.We use our own measurements and published data to compile a catalog ofmagnetic CP stars containing a total of 326 objects with confidentlydetected magnetic fields and 29 stars which are very likely to possessmagnetic field. We obtained the data on the magnetism of theoverwhelming majority of the stars solely based on the analysis oflongitudinal field component B e . The surface magneticfield, B s , has been measured for 49 objects. Our analysisshows that the number of magnetic CP stars decreases with increasingfield strength in accordance with exponential law, and stars with B e exceeding 5kG occur rarely (about 3% objects of ourlist).
| Pulsation properties of the rapidly oscillating Ap star 10Aquilae (HD176232)
We have studied the pulsation behaviour of radial velocities ofindividual spectral lines of different chemical elements for the wellknown, bright rapidly oscillating Ap (roAp) star 10Aql (HD176232) basedon high-quality Very Large Telescope spectra. We found that only linesof rare earth elements and the narrow H? core show detectablerapid variations of radial velocity with the known photometric pulsationperiod, 11.7min. The highest pulsation amplitudes of more than0.5kms-1 are found for rather weak lines of TbIII and DyIII.The pulsational phases obtained for lines of TbIII and DyIII differ by? rad, perhaps an indication of a radial node between theirline-forming layers. Weak lines of SmII also display amplitudes near0.5kms-1. Surprisingly, the lines of PrIII show the lowestdetected pulsation amplitude in strong contrast with the majority ofroAp stars for which this ion has some of the highest radial velocityamplitudes. Lines of NdIII with different intensities have pulsationamplitudes different to the weaker lines that are formed more deeply inthe atmosphere having higher amplitudes. Line bisectors for strong NdIIIline profiles show significant changes of phase, and even phase jumpsfor some lines, indicating complex variations in the pulsation phase asa function of atmospheric depth. The low-measured rotational velocity ofthis star indicates that there is little Doppler resolution of the lineprofiles, hence that the large range of pulsation amplitudes and phasesof radial velocities determined for line bisectors are primarily causedby atmospheric depth effects.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 077.D-0150(A).E-mail: velkin@uclan.ac.uk
| The Paschen-Back effect in the Li I 6708 Å line and the presence of lithium in cool magnetic Ap stars
Context: A number of cool magnetic Ap stars show a prominent feature atλ 6708 Å. Its identification with Li I remainscontroversial due to the lack of knowledge of the spectra of rare-earthelements that are strongly enhanced in peculiar stars so they canpotentially provide an alternative identification. Aims: We suggestinvestigating the 6708 Å line in Ap stars with strong magneticfields. In these objects, the magnetic broadening and splitting providesan additional, powerful criterium for line identification, allowing thewhole line profile to be used instead of depending on a mere coincidencein the observed and predicted wavelengths. Methods: The smallseparation of the Li I doublet components means that their magneticsplitting pattern deviates from the one expected for the Zeeman effect,even in relatively weak fields. We carried out detailed calculations ofthe transition between the Zeeman and Paschen-Back regimes in themagnetic splitting of the Li I line and computed polarised syntheticspectra for the range of field strength expected in Ap stars.Theoretical spectral synthesis is compared with the high-resolutionobservations of cool Ap stars HD 116114, HD 166473, and HD 154708, whichhave a mean field strength of 6.4, 8.6, and 24.5 kG, respectively, andshow a strong 6708 Å line. Results: High-resolution spectra forthe 6708 Å region were analysed for 17 magnetic Ap stars. Thepresence of the 6708 Å line is confirmed for 9 stars and reportedfor the first time for 6 stars. The strength of the Li I doublet doesnot correlate with the absorption features of any other element. Thestars HD 75445 and HD 201601 provide an extreme example of the twoobjects, which are dissimilar with respect to the 6708 Å line, butvery close in the atmospheric parameters and abundances of otherelements. We demonstrate that the observed profiles of the 6708 Åline in the strong field stars HD 116114, HD 166473, and HD 154708correspond fairly well to the theoretical calculations when assuming theLi I identification. Including the Paschen-Back effect improves theagreement with observations, especially for HD 154708. Conclusions:Results of our study confirm the Li I identification proposed for the6708 Å line in cool Ap stars.Based on observations collected at the European Southern Observatory,Paranal, Chile (ESO program 68.D-0254 and programs 072.D-0138,077.D-0150 retrieved through the ESO Archive).
| A search for rapid pulsations among nine luminous Ap stars
The rapidly oscillating Ap (roAp) stars are of importance for studyingthe atmospheric structure of stars where the process of chemical elementdiffusion is significant. We have performed a survey for rapidoscillations in a sample of nine luminous Ap stars, selected from theirlocation in the colour-magnitude diagram as more evolved main-sequenceAp stars that are inside the instability strip for roAp stars. Untilrecently this region was devoid of stars with observed rapid pulsations.We used the Very Large Telescope UV-Visual Echelle Spectrograph toobtain high time resolution spectroscopy to make the first systematicspectroscopic search for rapid oscillations in this region of the roApinstability strip. We report nine null detections with upper limits forradial velocity amplitudes of 20-65ms-1 and precisions of? = 7-20ms-1 for combinations of Nd and Pr lines.Cross-correlations confirm these null results. At least six stars aremagnetic and we provide magnetic field measurements for four of them, ofwhich three are newly discovered magnetic stars. It is found that fourstars have magnetic fields smaller than ~2kG, which according totheoretical predictions might be insufficient for suppressing envelopeconvection around the magnetic poles for more evolved Ap stars.Suppression of convection is expected to be essential for the opacitymechanism acting in the hydrogen ionization zone to drive thehigh-overtone roAp pulsations efficiently. Our null results suggest thatthe more evolved roAp stars may require particularly strong magneticfields to pulsate. Three of the studied stars do, however, have magneticfields stronger than 5kG.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programmes 075.D-0145 (A), 078.D-0080(A),072.D-0138(A) and 077.D-0150(A).E-mail: lmfreyhammer@uclan.ac.uk
| The discovery of high-amplitude, 10.9-minute oscillations in the cool magnetic Ap star HD 115226
We present the discovery of pulsational variations in the cool magneticAp star HD 115226 - the first high-amplitude rapidly oscillating Ap(roAp) star discovered with time-series spectroscopy. Usinghigh-resolution spectra obtained with the HARPS instrument at theEuropean Southern Observatory 3.6-m telescope, we detect radial velocityvariations with a period of 10.86 min in Pr III, Nd III, Dy III lines,and in the narrow cores of hydrogen lines. Pulsational amplitudes exceed1 km s-1 in individual lines of Nd III. The presence ofrunning waves in the stellar atmosphere is inferred from a phase shiftbetween the radial velocity maxima of rare-earth and hydrogen lines. Ourabundance analysis demonstrates that HD 115226 exhibits a typical roApspectroscopic signature, notably ionization anomaly of Pr, Nd, and Dy.We discuss the discovery of pulsations in HD 115226 in the context ofrecent spectroscopic studies of roAp stars and point to the existence ofa correlation between spectroscopic pulsational amplitude and thestellar rotation rate.Based on observations collected at the European Southern Observatory, LaSilla, Chile (ESO program 079.D-0118).
| Pulsation in the presence of a strong magnetic field: the roAp star HD166473
Phase-resolved high-resolution, high signal-to-noise ratio (S/N)observations of the strongly magnetic roAp star HD166473 are analysed.HD166473 was selected as the target of this study because it has one ofthe strongest magnetic fields of all the roAp stars known with resolvedmagnetically split lines. Indeed, we show that enhanced pulsationdiagnosis can be achieved from consideration of the different pulsationbehaviour of the π and σ Zeeman components of the resolvedspectral lines. This study is based on a time-series of high spectralresolution observations obtained with the Ultraviolet and Visual EchelleSpectrograph of the Very Large Telescope of the European SouthernObservatory. Radial velocity variations due to pulsation are observed inrare earth lines, with amplitudes up to 110ms-1. Thevariations occur with three frequencies, already detected in photometry,but which can in this work be determined with better precision: 1.833,1.886 and 1.928mHz. The pulsation amplitudes and phases observed in therare earth element lines vary with atmospheric height, as is the case inother roAp stars studied in detail. Lines of Fe and of other (mostlynon-rare earth) elements do not show any variation to very highprecision (1.5ms-1 in the case of Fe). The low amplitudes ofthe observed variations do not allow the original goal of studyingdifferences between the behaviour of the resolved Zeeman line componentsto be reached; the S/N achieved in the radial velocity determinations isinsufficient to establish definitely the possible existence of suchdifferences. Yet the analysis provides a tantalizing hint at theoccurrence of variations of the mean magnetic field modulus with thepulsation frequency, with an amplitude of 21 +/- 5G.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 067.D-0272.E-mail: gmathys@eso.org
| Two new chemically peculiar stars with resolved, magnetically split lines
We report the discovery of resolved, magnetically split lines in twochemically peculiar stars, the SrEuCr star HD92499 and the Bp SiCr starHD157751. From FEROS spectra, we have measured a mean magnetic fieldmodulus of 8.5kG for HD92499 and a mean magnetic field modulus of 6.6kGfor HD157751. Both stars have small projected rotational velocities:vsini = 3.0 kms-1 and 8.5 kms-1, respectively. Ourpreliminary abundance analysis reveals an ionization imbalance of rareearths in HD92499, indicating the abundance pattern typical of rapidlyoscillating Ap stars. Cr and Fe are found to be strongly overabundant inHD157751.Based on observations obtained at the European Southern Observatory, LaSilla, Chile [ESO programme 077.D-0477(A)].E-mail: shubrig@eso.org
| Heavy calcium in CP stars
Large wavelength shifts of infrared triplet lines of CaII have beenobserved in the spectra of HgMn and magnetic Ap stars. They have beenattributed to the heavy calcium isotopes, including 48Ca. Onemember of the triplet, ?8542, had been either unavailable, or ofpoor quality in earlier spectra. The present material shows conclusivelythat the stellar ?8542 shifts are consistent with aninterpretation in terms of 48Ca. We find no relation betweenisotopic shifts of the CaII triplet lines, and those of HgII?3984. There is a marginal indication that the shifts areanticorrelated with the surface field strengths of the magnetic stars.We see sparse evidence for 48Ca in other chemically peculiarstars, for example, Am stars, metal-poor stars or chemically peculiarred giants. However, the sample is still very small, and the wavelengthsof all three triplet lines, including those in the Sun, show slightpositive shifts with respect to terrestrial positions.Some profiles of the CaII infrared triplet in the magnetic stars showextensive wings beyond a well-defined core. We can obtain reasonablefits to these profiles using a stratified calcium abundance similar tothat used by previous workers. There is no indication that either thestratification or the Zeeman effect significantly disturbs themeasurement of isotope shifts.Based on observations obtained at the European Southern Observatory,Paranal and La Silla, Chile [ESO programmes 076.D-0169(A) and076.C-0172(A)].E-mail: cowley@umich.edu (CRC); shubrig@eso.org (SH);castelli@ts.astro.it (FC); fgonzalez@casleo.gov.ar (JFG); bwolff@eso.org(BW)
| New magnetic chemically peculiar stars
Spectropolarimetric observations of 96 chemically peculiar (CP)main-sequence stars have been carried out at the 6-m telescope at theSpecial Astrophysical Observatory of the Russian Academy of Sciences(SAO RAS) with the aim of searching for the presence of stellar magneticfields. The stars selected for investigation were CP stars known to havestrong anomalies in the wavelength region of the continuum fluxdepression around λ 5200Å. This selection was conductedwith the aid of low-resolution spectral observations, made with the SAORAS 1-m telescope, and of published differential photometric data.Magnetic fields have been successfully detected in 72 stars of whichonly three stars were previously known to have magnetic fields. For twostars, the longitudinal component of the magnetic field Beexceeds 5 kG: HD178892 - 7.4 kG, and HD258686 - 6.7 kG. We failed toreliably detect the magnetic field in the other 24 CP stars. These starsare mostly fast rotators, a feature which hampers accurate measurementsof Be. It is demonstrated in this paper that selectingcandidate magnetic stars by considering their photometric indices Z orΔa, or alternatively, by inspecting low-resolution spectra aroundthe λ5200Å flux depression, considerably increases thedetection rate.This paper is based on data obtained at the 6-m telescope of the RussianAcademy of Sciences.E-mail: dkudr@sao.ru
| On the roAp star status of β Coronae Borealis
β CrB is one of the best-studied of the magnetic Ap stars. Threeindependent investigations have suggested that this star is pulsatingwith a period of either 6.1 min, 11.5 min or 16.2 min, making this arapidly oscillating Ap star. The presence of pulsations in β CrBhas important implications for the understanding of pulsation drivingand damping in roAp stars, and each study has called for additionalobservations to confirm the suggested pulsations. New high timeresolution, high spectral resolution, high signal-to-noise spectra ofβ CrB obtained with the high resolution spectrograph SARG on the3.55-m Telescopio Nazionale Galileo are unable to confirm any of thesuggested periods. There is no indication of any variability with aperiod near 6.1 min. Studies of Fe i lines suggest that the 11.5-minperiod is spurious. Studies of Ce ii lines do not find the 16.2-minperiod suggested for one Ce ii line, but are not precise enough to testthe finding of 16.2-min oscillations for a large section of spectrum,hence the case for the 16.2-min period is still good. An extensiveinvestigation of β CrB is needed to resolve the issue.
| The discovery of 8.0-min radial velocity variations in the strongly magnetic cool Ap star HD154708, a new roAp star
HD154708 has an extraordinarily strong magnetic field of 24.5kG. Using2.5h of high time resolution Ultraviolet and Visual Echelle Spectrograph(UVES) spectra we have discovered this star to be an roAp star with apulsation period of 8min. The radial velocity amplitudes in the rareearth element lines of NdII, NdIII and PrIII are unusually low -~60ms-1 - for an roAp star. Some evidence suggests that roApstars with stronger magnetic fields have lower pulsation amplitudes.Given the central role that the magnetic field plays in the obliquepulsator model of the roAp stars, an extensive study of the relation ofmagnetic field strength to pulsation amplitude is desirable.Based on observations collected at the European Southern Observatory,Paranal, Chile, as part of programme 075.D-0145.E-mail: dwkurtz@uclan.ac.uk
| The Nainital-Cape Survey. II. Report for pulsation in five chemically peculiar A-type stars and presentation of 140 null results
Aims.We search for photometric variability in chemically peculiar A typestars in the northern hemisphere. Methods: .High-speed photometricobservations of Ap and Am star candidates have been carried out fromARIES (Manora Peak, Nainital) using a three-channel fast photometerattached to the ARIES 104-cm Sampurnanand telescope. Results:.This paper presents three new variables: HD 113878, HD 118660 and HD207561. During the time span of the survey (1999 December to 2004January) pulsations of the δ Sct type were also found for the twoevolved Am stars HD 102480 and HD 98851, as reported in Joshi et al.(2002, 2003). Additionally, we present 140 null results of the surveyfor this time span. Conclusions: .The star HD 113878 pulsates witha period of 2.31 h, which is typical of δ Sct stars. HD 118660exhibits multi-periodic variability with a prominent period of nearly 1h. These periods need to be investigated and make HD 118660 aparticularly interesting target for further observations. For HD 207561,a star classified as Am, a probable pulsation with a period of 6 min wasfound in the light curves obtained on two consecutive nights. Both HD102480 and HD 98851 exhibit unusual alternating high and low amplitudemaxima, with a period ratio of 2:1. The analysis of the null resultsconfirms the photometric quality of the Nainital site.
| Evolution of magnetic fields in stars across the upper main sequence: I. Catalogue of magnetic field measurements with FORS 1 at the VLT
To properly understand the physics of Ap and Bp stars it is particularlyimportant to identify the origin of their magnetic fields. For that, anaccurate knowledge of the evolutionary state of stars that have ameasured magnetic field is an important diagnostic. Previous resultsbased on a small and possibly biased sample suggest that thedistribution of magnetic stars with mass below 3 M_ȯ in the H-Rdiagram differs from that of normal stars in the same mass range (Hubriget al. 2000). In contrast, higher mass magnetic Bp stars may well occupythe whole main-sequence width (Hubrig, Schöller & North 2005b).In order to rediscuss the evolutionary state of upper main sequencemagnetic stars, we define a larger and bias-free sample of Ap and Bpstars with accurate Hipparcos parallaxes and reliably determinedlongitudinal magnetic fields. We used FORS 1 at the VLT in itsspectropolarimetric mode to measure the magnetic field in chemicallypeculiar stars where it was unknown or poorly known as yet. In thisfirst paper we present our results of the mean longitudinal magneticfield measurements in 136 stars. Our sample consists of 105 Ap and Bpstars, two PGa stars, 17 HgMn stars, three normal stars, and nine SPBstars. A magnetic field was for the first time detected in 57 Ap and Bpstars, in four HgMn stars, one PGa star, one normal B-type star and fourSPB stars.
| 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.
| Detection of an extraordinarily large magnetic field in the unique ultra-cool Ap star HD 154708
We have discovered an extraordinarily large mean longitudinal magneticfield of 7.5 kG in the ultra-cool low mass Ap starHD 154708 using FORS 1 in spectropolarimetric mode. FromUVES spectra, we have measured a mean magnetic field modulus of24.5 kG. This is the second-largest mean magnetic field modulusever measured in an Ap star. Furthermore, it is very likely thatthis star is one of the coolest and least massive among the Ap stars andis located in the H-R diagram in the same region in which rapidlyoscillating Ap stars have been detected. We note that all known roApstars have much smaller magnetic fields, by at least a factor of three.
| Rapidly oscillating Ap stars versus non-oscillating Ap stars
The positions in the HR diagram and the kinematic characteristics ofrapidly oscillating and non-oscillating chemically peculiar stars areobtained using new Hipparcos proper motions and parallaxes, and our ownradial velocity measurements. We find that rapidly oscillating stars, asa group, are (-0.47 +/- 0.34) mag above the zero-age main sequence(ZAMS), while the non-oscillating stars are (-1.20 +/- 0.65) mag abovethe ZAMS and so appear slightly more evolved on average. From thecomparison of the kinematical characteristics, we conclude that bothgroups are very similar. The results of radial velocity measurementsindicate that there is a real deficiency of binaries among rapidlyoscillating stars. Presently, no such star is known to be aspectroscopic binary.
| A search for rapid oscillations in chemically peculiar A-type stars
In 1995 we initiated a Northern Hemisphere survey for rapidlyoscillating Ap stars. This paper presents the results including one newroAp star (HD 122970), the confirmation of rapid oscillations of HD99563 and apparent null results for other stars. Using Hipparcos data astatistical analysis of the absolute magnitudes and galacticdistributions of all known roAp and noAp stars (also taken from theliterature) was made. A systematic trend for most of the program starsin a M_{ V} vs. beta (index of the Strömgren uvbybeta system)diagram was detected leading to the conclusion that beta issystematically influenced by the chemical peculiarity and/or magneticfield. Three roAp stars are outside the delta Scuti instability stripwhich implies that the driving mechanism of the two classes of pulsatingstar is different. This is also suggested by new pulsation models. Nostatistical difference between the galactic distribution of roAp andnoAp stars was found.
| The HR-diagram from HIPPARCOS data. Absolute magnitudes and kinematics of BP - AP stars
The HR-diagram of about 1000 Bp - Ap stars in the solar neighbourhoodhas been constructed using astrometric data from Hipparcos satellite aswell as photometric and radial velocity data. The LM method\cite{luri95,luri96} allows the use of proper motion and radial velocitydata in addition to the trigonometric parallaxes to obtain luminositycalibrations and improved distances estimates. Six types of Bp - Apstars have been examined: He-rich, He-weak, HgMn, Si, Si+ and SrCrEu.Most Bp - Ap stars lie on the main sequence occupying the whole width ofit (about 2 mag), just like normal stars in the same range of spectraltypes. Their kinematic behaviour is typical of thin disk stars youngerthan about 1 Gyr. A few stars found to be high above the galactic planeor to have a high velocity are briefly discussed. Based on data from theESA Hipparcos astrometry satellite and photometric data collected in theGeneva system at ESO, La Silla (Chile) and at Jungfraujoch andGornergrat Observatories (Switzerland). Tables 3 and 4 are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| The Cape rapidly oscillating AP star survey - III. Null results of searches for high-overtone pulsation.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1994MNRAS.271..129M&db_key=AST
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | מזבח |
| התרוממות ימנית: | 17h10m28.52s |
| סירוב: | -58°00'17.4" |
| גודל גלוי: | 8.772 |
| מרחק: | 140.845 פארסק |
| תנועה נכונה: | -20 |
| תנועה נכונה: | -35.1 |
| B-T magnitude: | 9.324 |
| V-T magnitude: | 8.818 |
קטלוגים וכינוים:
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