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uvby FCAPT Photometry of Six Small-Amplitude mCP Stars
If magnetic fields cause elemental abundances to change in the stellarphotosphere, then all magnetic chemically peculiar (mCP) stars shouldexhibit signatures of this effect in both their spectra and fluxdistributions. Given that all stars rotate, these stars should bemagnetic, spectrum, and photometric variables, albeit sometimes of lowamplitude. We study differential Strömgren observations from theFour College Automated Photoelectric Telescope (FCAPT) of α Psc,HR 5857, and HR 6709, which are small-amplitude mCP stars, and AB Cet,HD 15890, and HR 8240, which were thought to be near-constant mCP stars,to determine the periods and amplitudes of their photometricvariability. We construct the photometric amplitude distributionfunctions of the mCP stars studied by the first author to obtainstatistics on their photometric variability in Strömgrenphotometry. The star α Psc has a period of 0.74552 days, with itslargest amplitude in u of 0.030 mag. The largest amplitude (0.035 mag)for HR 5857 is also for u. We confirmed the period of 1.29957 days foundby Hatzes using Doppler imaging. For HR 6709 the largest amplitudeobserved (0.010 mag) is for u photometry. Its period is 1.20352 days. ABCet might be minimally variable. HD 15980 appears to be minimallyvariable, with a period of at least 5 yr. HR 8240 is variable with a bamplitude about 0.05 mag and a period that is several years long.Additional observations of these six stars would be useful to confirmtheir characteristics. Two apparently constant stars studied with FCAPTStrömgren data, HD 11187 and HD 50169, should be checked to seewhether they are long-period variables. If additional observations showtheir variability, then all mCP stars observed photometrically by thefirst author with the FCAPT will be found to be variable.
| The BOES Spectropolarimeter for Zeeman Measurements of Stellar Magnetic Fields
We introduce a new polarimeter installed on the high-resolutionfiber-fed echelle spectrograph (called BOES) of the 1.8 m telescope atthe Bohyunsan Optical Astronomy Observatory, Korea. The instrument isintended to measure stellar magnetic fields with high-resolution(R~60,000) spectropolarimetric observations of intrinsic polarization inspectral lines. In this paper we describe the spectropolarimeter andpresent test observations of the longitudinal magnetic fields in somewell-studied F-B main-sequence magnetic stars (mv<8.8mag). The results demonstrate that the instrument is able to detect thefields of these stars with high precision, with typical accuraciesranging from about 2 to a few tens of gauss.
| Are magnetic hot stars intrinsic X-ray sources?
Context: . X-ray surveys carried out with the Einstein and ROSATsatellites have resulted in rather unexpected detections of X-rayemission from late B-type and early A-type stars. These stars possessneither winds like early-type stars nor convective envelopes aslate-type stars, so that the origin and production mechanism of thisX-emission is unclear. Aims: . We investigate whether thepresence of large magnetic fields is related to the observed X-rayemission. Methods: . We carried out Chandra high-angularresolution observations of a sample of late B-type and A-type stars withmeasured magnetic fields in the range from 0.2-17 kG. Out of theselected 10 sample stars, 6 objects had been previously detected asX-ray sources, some of them, however, with high positional uncertaintyand a low signal-to-noise ratio, while 4 of our sample stars do havelarge magnetic fields but no previous detections of X-ray emission. Results: . Our Chandra data confirm all previous ROSAT detectionswith an extremely high significance, and the limits of the offsetsbetween X-ray and optical positions are greatly improved. In particular,HD 215441, known as Babcock's star with the strongestmagnetic field by far (17 kG) of our sample stars, a rather faint andsomewhat marginal ROSAT source, can clearly be detected. However, noneof the 4 ROSAT non-detections could be detected with the new Chandraobservations. Conclusions: . The pure existence of a magneticfield of kiloGauss strength on a late B-type or A-type star is thereforenot necessarily a prerequisite for finding X-ray emission among thesestars. Understanding the observed X-ray emission from Babcock's star isa challenge for observational and theoretical astrophysics.
| On-line database of photometric observations of magnetic chemically peculiar stars
We present our extensive project of the On-line database ofphotometric observations of magnetic chemically peculiar stars tocollect published data of photometric observations of magneticchemically peculiar (mCP) stars in the optical and near IR regions. Nowthe nascent database contains more than 107 000 photometric measurementsof 102 mCP stars and will be continually supplemented with published ornew photometric data on these and about 150 additional mCP stars. Thisreport describes the structure and organization of the database.Moreover, for the all included data we estimated the error ofmeasurements and the effective amplitudes of the light curves.
| What can X-rays tell us about accretion, mass loss, and magnetic fields in young stars?
Until recently, X-rays from low-mass young stars(105-106 yr) were thought to be a proxy formagnetic activity, enhanced by 3-4 orders of magnitude with respect tothe Sun, but otherwise similar in nature to all low-mass, late-typeconvective stars (including the Sun). However, there is now increasingevidence that specific X-ray emission mechanisms are at work when theyoung stars are still accreting from their circumstellar disk. The mostfrequently invoked mechanism is accretion shocks along magnetic fieldlines (``magnetic accretion"). In the case of the more massive A and Bstars, and their progenitors the Herbig AeBe stars, other, possibly moreexotic mechanisms can operate: star-disk magnetic reconnection,magnetically channeled shocked winds, etc. In any case, magnetic fields,both on small scale (surface activity) and on large scale (dipolarmagnetospheres), play a distinctive role in the emission of X-rays byyoung stars, probably throughout the IMF.
| 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
| 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
| Remarkable non-dipolar magnetic field of the Bp star HD 137509
The southern magnetic Bp star HD 137509 exhibits complex rotationalmodulation of the longitudinal field and other magnetic observables.Interpretation of this magnetic variability in the framework of thelow-order multipolar field models suggests a very strong quadrupolarcomponent to dominate the surface field topology of HD 137509. I haveexamined the high-quality VLT/UVES spectra of HD 137509 and discoveredresolved Zeeman split components in some of the spectral lines. Theinferred mean surface field modulus, < B >=29 kG, agrees with themultipolar model predictions. This confirms the presence of an extremelystrong non-dipolar magnetic field in HD 137509 and establishes this staras the object with the second-largest field among magnetic chemicallypeculiar stars.
| 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.
| The action of magnetic torques on the oblique rotator model for magnetic Ap stars
The oblique magnetic rotator is a satisfactory model describing thegeneral properties of the magnetic Ap stars. The inferred rotationperiods are almost always considerably longer than those of the normal Astars. The loss of angular momentum is plausibly the result of magneticcoupling with external gas during some epoch in the star's life. Thesame Maxwell stresses that brake the stellar rotation cause a precessionof the instantaneous axis of rotation, so reducing the obliquity anglebetween the rotational and magnetic axes. A star that has sufferedexcess braking in the pre-main-sequence Hayashi phase may bemagnetically spun up on the main sequence, with a simultaneous increasein the obliquity. This paper constructs the braking and precessionaltorques for an idealized illustrative model. The results may be relevantto recent observational inferences of a correlation between longrotation period and small obliquity.
| 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
| Magnetic fields in white dwarfs and stellar evolution
The surface magnetic field strengths observed in the magnetic Ap, Bpstars (100-20000 G) and in the high field magnetic white dwarfs(106-109 G) cover many decades but neverthelessthe range of magnetic fluxes observed in each of these stellar groups issimilar. An evolutionary link between them therefore appears plausible.For both groups of stars there is also information on field complexity.The magnetic white dwarfs in general show non-dipole field structureswhich can be best modelled if we assume contributions from higher ordermultipoles. The field structures of the Ap and Bp stars are similarlycomplex. We investigate the hypothesis that the magnetic fields of thewhite dwarfs could be fossil remnants from the main-sequence phase byfocussing on the problem of how field complexity may arise and bemaintained during evolution to the compact star state. We also addressthe question of to what extent magnetic fields seen in the early typestars could be fossil remnants from the pre-main-sequence phases ofstellar evolution dating back perhaps to the time of star formation.
| Extending the radio spectrum of magnetic chemically peculiar stars to the mm range
Magnetic chemically peculiar (MCP) stars can present radio emission atcentimetre wavelengths. The steep decrement of the dominant dipolarcomponent of the photospheric magnetic field results in each radiofrequency being mainly emitted in a well localised shell of thecircumstellar region. To explore the most internal regions of themagnetosphere, observations of a sample of eleven MCP stars known to beradio sources in the 1.4-22.5 GHz range were carried out at 87.7 GHzwith the IRAM interferometer. Millimeter emission, with a flux densityat about 4× the sensitivity limit of our observations, wasdetected towards two of the stars: HD 35298 and HD 124224. Combiningour mm-observations with previous cm-observations, it appears that MCPstars with a relatively weak magnetic field present a radio spectrumthat increases with frequency up to 22.5 GHz and then decreases towardsthe mm range. In presence of strong fields, the radio spectrum is alwaysdecreasing with frequency. A comparison of the observed cm-mm spectrumof HD 124224 with results of numerical simulations of thegyrosynchrotron emission suggests that circumstellar regions emitting inthe mm-range cannot present magnetic fields larger than 1-2 kG.Based on observations carried out with the IRAM Plateau de BureInterferometer. IRAM is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain).
| Horace Welcome Babcock (1912-2003)
Over a span of 40 years, Horace Babcock invented numerous astronomicalinstruments and procedures, the most important of which continue to beused after his death. Along the way he created a branch of astrophysicsand founded one of the world's premier astronomical observatories.
| Discovery of a huge magnetic field in the very young star NGC 2244-334 in the Rosette Nebula cluster
During a survey of field strengths in upper main sequence stars in openclusters, we observed the star NGC 2244-334 in the Rosette Nebulacluster and discovered an extraordinarily large mean longitudinal fieldof about -9 kG, the second largest longitudinal field known in anon-degenerate star. This star appears to be a typical Ap He-wk (Si)star of about 4 Mȯ. Spectrum synthesis using a linesynthesis code incorporating the effects of the strong magnetic fieldindicates that He is underabundant by about 1.5 dex, and C, O and Mg byabout 0.1-0.4 dex, while Si, Mn and Fe are overabundant by about 1 dex,and Cr and Ti are nearly 2 dex overabundant.Cluster membership for this star is secure, so its age is about 2×106 yr, which is less than 3% of its main sequence lifetime.This star is one of the very youngest magnetic upper main sequence starswith a well-determined age, and confirms that both magnetic fields andstrong chemical peculiarity can appear in stars which are both extremelyyoung and very close to the ZAMS.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 70.D-0352 and 270.D-5032.
| Catalogue of averaged stellar effective magnetic fields. I. Chemically peculiar A and B type stars
This paper presents the catalogue and the method of determination ofaveraged quadratic effective magnetic fields < B_e > for 596 mainsequence and giant stars. The catalogue is based on measurements of thestellar effective (or mean longitudinal) magnetic field strengths B_e,which were compiled from the existing literature.We analysed the properties of 352 chemically peculiar A and B stars inthe catalogue, including Am, ApSi, He-weak, He-rich, HgMn, ApSrCrEu, andall ApSr type stars. We have found that the number distribution of allchemically peculiar (CP) stars vs. averaged magnetic field strength isdescribed by a decreasing exponential function. Relations of this typehold also for stars of all the analysed subclasses of chemicalpeculiarity. The exponential form of the above distribution function canbreak down below about 100 G, the latter value representingapproximately the resolution of our analysis for A type stars.Table A.1 and its references are only available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/407/631 and Tables 3 to 9are only available in electronic form at http://www.edpsciences.org
| Discovery of a 14.5 kG magnetic field in the NGC 2516 star HD 66318
We have been searching for magnetic Ap stars in open clusters, in orderto clarify the time evolution of magnetic fields in middle main sequencestars from the ZAMS to the TAMS. We have discovered that the star HD66318 in the open cluster NGC 2516 has an extraordinarily large magneticfield: the measured mean longitudinal component Bl ~ 4.5 kG,and the mean field modulus Bs ~ 14.5 kG. This star thus hasone of the largest fields so far discovered in a non-degenerate star,and the largest field known in a current Ap star cluster member.We estimate that HD 66318 has completed about 16 +/- 5% of its mainsequence life. It thus appears to contradict the hypothesis of Hubrig etal. that magnetic fields are only found in stars that have completed atleast 30% of their main sequence lifetimes.There is no indication that the spectrum or brightness of the star isvariable, and the spectral lines are very sharp. The star probably has avery long rotation period (years).We have modelled some parts of the observed spectrum, assuming that thechemical composition is uniform both horizontally and vertically, andusing a simple multipolar expansion for the field structure; althoughour model does not reproduce exactly the observed spectrum, it is clearthat the atmospheric chemical composition of the star is very peculiar,with Ti, Cr and Fe overabundant by between 1.5 and 2.5 dex. Both La IIand III are apparently about 4 dex overabundant. In contrast, evidencefor the presence of other rare earths is difficult to find in thespectrum. It appears that Ce III, Pr III, Nd II and III, and Eu II aredetected with inferred overabundances ranging between 1.5 and 5 dex, butfor most of these elements, the abundance of the non-detected ionizationstate is significantly lower than that inferred for the detected state.HD 66318 thus seems to exhibit strong discrepancies between abundancesdeduced for different ionization states of rare earths, a phenomenon sofar found only in somewhat cooler stars.Even within one ionization state it has not been found possible to fitmost observed lines with a single value of abundance. For example, whenwe fit medium strength lines of Fe II, the calculated strong lines ofthis ion are deeper than observed, and the calculated weak lines areless deep than observed. This situation is probably due to strongvertical abundance stratification of most of the elements studied.Finally, HD 66318 also shows a new form of core-wing anomaly in Hαin which the observed line profile falls below the computed one in thecore. These characteristics clearly suggest that the atmosphericstructure of HD 66318 is not closely similar to that of a normal mainsequence A star of similar parameters.This paper is based on data obtained at the European SouthernObservatory VLT during observing runs 68.D-0403 and DDT-269.D-05022.
| Magnetic intensification of spectral lines
We present an in-depth investigation of the increase in equivalent widthof saturated lines under the influence of strong magnetic fields, aptlycalled ``magnetic intensification''. Assuming 20 different Zeemanpatterns (with the number of components ranging from 3 to 45) for afictitious Fe II line at 4500 Å in a 10 000 K, log g = 4.0 Kuruczatmosphere, we calculate equivalent widths as a function of magneticfield strength, field angle and line strength (abundance). The increaseover the zero field equivalent width is found to always fall below then/2 relation suggested by Babcock (\cite{bab49}), where n is the numberof Zeeman components. After a discussion of the behaviour of variousZeeman patterns we turn to the influence of anomalous dispersion onmagnetic intensification. Changes in line blanketing due to magneticdesaturation appear to be of negligible size. Finally we show thatmagnetic intensification leads to apparent enhanced abundances and tovirtual abundance variations in magnetic stars with dipolar surfacemagnetic field configurations.
| A statistical analysis of the magnetic structure of CP stars
We present the results of a statistical study of the magnetic structureof upper main sequence chemically peculiar stars. We have modelled asample of 34 stars, assuming that the magnetic morphology is describedby the superposition of a dipole and a quadrupole field, arbitrarilyoriented. In order to interpret the modelling results, we haveintroduced a novel set of angles that provides one with a convenient wayto represent the mutual orientation of the quadrupolar component, thedipolar component, and the rotation axis. Some of our results aresimilar to what has already been found in previous studies, e.g., thatthe inclination of the dipole axis to the rotation axis is usually largefor short-period stars and small for long-period ones - see Landstreet& Mathys (\cite{Landstreet2000}). We also found that forshort-period stars (approximately P<10 days) the plane containing thetwo unit vectors that characterise the quadrupole is almost coincidentwith the plane containing the stellar rotation axis and the dipole axis.Long-period stars seem to be preferentially characterised by aquadrupole orientation such that the planes just mentioned areperpendicular. There is also some loose indication of a continuoustransition between the two classes of stars with increasing rotationalperiod.
| On the Periods of the Magnetic CP Stars
An HR diagram annotated to show several ranges of photometericallydetermined periods has been constructed for the magnetic CP stars whoseperiods have been determined by the author and his collaborators. Thedistribution of periods reflects both the initial conditions as well asthe subsequent stellar histories. Since the stellar magnetic field doesnot penetrate the convective core, eventually a shear zone near thecore-radiative envelope boundary may develop which produces turbulenceand modifies the field. Many, but not all, of the most rapidly rotatingmCP stars are close to the ZAMS and some of the least rapidly rotatingmCP stars are the furthest from the ZAMS.
| The comparative accuracy of photographic observations of radio stars observed at the Engelhardt Astronomical Observatory
At the Engelhardt Astronomical Observatory (EAO), we observedphotographic positions of 113 Galactic Radio Sources (GRS) in the systemPPM catalogue (Rizvanov & Dautov 1998). Analysis of their accuracyis made by comparison with the Hipparcos catalogue (Perryman et al.1997) and astrometric catalogue of radio stars in the radio window fromthe article of Walter et al. (1991). Table 2 is only available inelectronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr(130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/375/670
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| Magnetic models of slowly rotating magnetic Ap stars: aligned magnetic and rotation axes
As a result of major surveys carried out during the past decade byMathys and collaborators, we now have measurements with full phasecoverage of several magnetic field moments, including the meanlongitudinal field B_l, the mean field modulus B_s, and in most casesthe mean quadratic field B_mq and mean crossover field B_xover, for asample of 24 chemically peculiar magnetic (Ap) stars. This represents anincrease of a factor of order five in the stellar sample with data ofthis quality, compared to the situation a decade ago. We exploit thisdataset to derive general and statistical properties of the stars in thesample, as follows. First, we fit the available field momentobservations assuming a simple, axisymmetric multipole magnetic fieldexpansion (with dipole, quadrupole, and octupole components) over eachstellar surface. We show that this representation, though not exact,gives an adequate description of the available data for all the stars inthis sample, although the fit parameters are in many cases not unique.We find that many of the stars require an important quadrupole and/oroctupole field component to satisfy the observations, and that some(usually small) deviations from our assumed axisymmetric fielddistributions are certainly present. We examine the inclination i (0<= i <= 90o) of the rotation axis to the line of sightand the obliquity beta (0 <= beta <= 90o) of themagnetic field with respect to the rotation axis, and show that thestars with periods of the order of a month or longer have systematicallysmall values of beta : slowly rotating magnetic stars generally havetheir magnetic and rotation axes aligned to within about 20o,unlike the short period magnetic Ap stars, in which beta is usuallylarge. This is a qualitatively new result, and one which is veryimportant for efforts to understand the evolution of magnetic fields andangular momentum in the magnetic Ap stars.
| Spectropolarimetric measurements of magnetic Ap and Bp stars in all four Stokes parameters
In this paper we begin an exploration of the potential of spectral lineZeeman linear and circular polarization signatures for reconstructingthe surface magnetic field topologies of magnetic Ap and Bp stars. Wepresent our first observational results, which include the very firsthigh-quality measurements of stellar Zeeman spectral line linearpolarization ever obtained. Using the new MuSiCoS spectropolarimeter atthe Pic du Midi Observatory, over 360 spectra were obtained, in circularor linear polarization, of 14 magnetic Ap/Bp stars and six calibrationobjects. Zeeman circular polarization signatures are detected in mostsingle lines in essentially all spectra of magnetic Ap stars, withtypical relative amplitudes of a few per cent. Linear polarizationZeeman signatures are unambiguously detected in individual strong,magnetically sensitive lines in outstanding spectra of five objects.However, linear polarization is generally not detected in individualstrong lines in our much more common moderate signal-to-noise ratio(S/N) spectra, and is essentially never detected in weak lines. In orderto overcome the limitations imposed by the S/N ratio and the inherentweakness of linear polarization Zeeman signatures, we exploit theinformation contained in the many lines in our spectra by using theleast-squares deconvolution (LSD) technique. Using LSD, mean linearpolarization signatures are consistently detected within the spectrallines of 10 of our 14 programme stars. These mean linear polarizationsignatures are very weak, with typical amplitudes 10-20 times smallerthan those of the associated mean circular polarization signatures. For11 stars full or partial rotational phase coverage has been obtained inthe Stokes I and V or the Stokes I, V, Q and U parameters. Therotational modulation of the LSD mean signatures is reported for theseobjects. Measurements of the longitudinal field and net linearpolarization obtained from these LSD profiles show they are consistentwith existing comparable data, and provide constraints on magnetic fieldmodels which are at least as powerful as any other data presentlyavailable. To illustrate the new information available from these datasets, we compare for four stars the observed Stokes profiles with thosepredicted by magnetic field models published previously in theliterature. Important and sometimes striking differences between theobserved and computed profiles indicate that the Zeeman signaturespresented here contain important new information about the structure ofthe magnetic fields of Ap and Bp stars capable of showing thelimitations of the best magnetic field models currently available.
| The First 50 Years at Palomar, 1949-1999 Another View: Instruments, Spectroscopy and Spectrophotometry and the Infrared
We review the research on a wide variety of topics using data obtainedwith the 200-inch Hale telescope. Using state-of-the-art spectrographs,photometers, spectrometers and infrared detectors, the Palomarastronomers investigated the spectra of stars, interstellar matter, AGNsand quasars in great detail. Spectral resolutions ranged from 1000 A forbroad-band photometry to 0.04 A using interferometric techniques.
| BD+40° 175: A visual binary with two magnetic components
The two components of the visual binary BD+40o175 were observedspectroscopically with a circular polarization analyzer attached to the6-m telescope. The two stars were found to possess strong magneticfields. The effective magnetic field of the brighter CP star BD+40o175Avaries from -2.0 to -3.4 kG. The second star, which lies at 3.7 arcsecfrom the first star, exhibits more intense spectral lines and has avariable magnetic field ranging from +0.8 to +2.6 kG. The rare-Earthelements, including Sr, Eu, Sm, Gd, Nd, and others, are appreciablyoverabundant in the two components. These stars have similar effectivetemperatures, Teff = 8000 +/- 400 K, as inferred from the H_gamma line.The projected rotational velocity v sin(i) was found from the profilesof FeII 4491 A with a low Lande factor to be no higher than 20 km/s foreach star.
| Long- and short-term variability in O-star winds. II. Quantitative analysis of DAC behaviour
A quantitative analysis of time series of ultraviolet spectra from asample of 10 bright O-type stars (cf. Kaper et al. \cite{KH96}, Paper I)is presented. Migrating discrete absorption components (DACs),responsible for the observed variability in the UV resonance doublets,are modeled. To isolate the DACs from the underlying P Cygni lines, amethod is developed to construct a template (``least-absorption'')spectrum for each star. The central velocity, central optical depth,width, and column density of each pair of DACs is measured and studiedas a function of time. It turns out that the column density of a DACfirst increases and subsequently decreases with time when the componentis approaching its asymptotic velocity. Sometimes a DAC vanishes beforethis velocity is reached. In some cases the asymptotic DAC velocitysystematically differs from event to event. In order to determine thecharacteristic timescale(s) of DAC variability, Fourier (CLEAN) analyseshave been performed on the time series. The recurrence timescale of DACsis derived for most targets, and consistent results are obtained fordifferent spectral lines. The DAC recurrence timescale is interpreted asan integer fraction of the stellar rotation period. In some datasets thevariability in the blue edge of the P Cygni lines exhibits a longerperiod than the DAC variability. This might be related to the systematicdifference in asymptotic velocity of successive DACs. The phaseinformation provided by the Fourier analysis confirms the expectedchange in phase with increasing velocity. This supports theinterpretation that the DACs are responsible for the detectedperiodicity. The phase diagram for the O giant xi Per shows clearevidence for so-called ``phase bowing'', which is an observationalindication for the presence of curved wind structures like corotatinginteraction regions in the stellar wind. An important difference withthe results obtained for the B supergiant HD 64760 (Fullerton et al.\cite{FM97}) is that in this O star the phase bowing can be associatedwith the DACs. No other O stars in our sample convincingly show phasebowing, but this could be simply due to the absence of periodic signaland hence coherent phase behaviour at low wind velocities. Based onobservations by the International Ultraviolet Explorer, collected atNASA Goddard Space Flight Center and Villafranca Satellite TrackingStation of the European Space Agency.
| Spectropolarimetry of Magnetospheric Accretion on the Classical T Tauri Star BP Tauri
High-resolution (R ~ 60,000) circular spectropolarimetry of theclassical T Tauri star BP Tauri is presented. No net polarization isdetected in photospheric absorption lines, placing strong limits (3sigma) on the mean longitudinal magnetic field (+/-200 G) that ispresent over the surface of the star. On the other hand, strong circularpolarization is measured in the He I lambda5876 emission line of BP Tau,indicating a mean longitudinal magnetic field of 2460+/-120 G in theline formation region. This implies that accretion occurs preferentiallyalong large-scale magnetic loops that occupy a small fraction of thestellar surface. These observations represent the first direct evidencefor magnetically controlled accretion onto classical T Tauri stars.
| On the HIPPARCOS photometry of chemically peculiar B, A, and F stars
The Hipparcos photometry of the Chemically Peculiar main sequence B, A,and F stars is examined for variability. Some non-magnetic CP stars,Mercury-Manganese and metallic-line stars, which according to canonicalwisdom should not be variable, may be variable and are identified forfurther study. Some potentially important magnetic CP stars are noted.Tables 1, 2, and 3 are available only in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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
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| özdevim Sað Açýklýk: | 5.6 |
| özdevim Yükselim: | -0.7 |
| B-T magnitude: | 8.862 |
| V-T magnitude: | 8.817 |
Kataloglar ve belirtme:
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