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| First Surface-resolved Results with the Infrared Optical Telescope Array Imaging Interferometer: Detection of Asymmetries in Asymptotic Giant Branch Stars
We have measured nonzero closure phases for about 29% of our sample of56 nearby asymptotic giant branch (AGB) stars, using the three-telescopeInfrared Optical Telescope Array (IOTA) interferometer at near-infraredwavelengths (H band) and with angular resolutions in the range 5-10 mas.These nonzero closure phases can only be generated by asymmetricbrightness distributions of the target stars or their surroundings. Wediscuss how these results were obtained and how they might beinterpreted in terms of structures on or near the target stars. We alsoreport measured angular sizes and hypothesize that most Mira stars wouldshow detectable asymmetry if observed with adequate angular resolution.
| Optical Spectropolarimetry of Asymptotic Giant Branch and Post-Asymptotic Giant Branch Stars
Spectropolarimetric observations are presented for 21 AGB stars, 13proto-planetary nebulae (PPNs), and two R CrB-type stars. The spectracover the wavelength range from ~4200 to 8400 Å with 16 Åresolution. Among the AGB stars, 8 of 14 M giants, five of six carbonstars, and zero of one S star showed intrinsic polarization. At least 9of 13 PPNs exhibited intrinsic polarization, while the R CrB-type starsshow intrinsic polarization during fading episodes. There is astatistical correlation between mean polarization, , and IRcolor, K-[12], among the AGB stars such that redder stars tend to bemore polarized. The PPN sample is significantly redder and morepolarized, on average, than the AGB stars. This increase in with increased reddening is consistent with an evolutionary sequence inwhich AGB stars undergo increasing mass loss, with growing asymmetriesin the dust distribution as they evolve up and then off the AGB into theshort-lived PPN phase. A related trend is found between polarization andmass-loss rate in gas, M˙gas. The detectability ofpolarization increases with mass-loss rate, and probably all AGB starslosing mass at >10-6 Msolar yr-1have detectable polarization. Multiple observations of three polarizedAGB stars show that in some cases increases withmV, and in others it decreases. If polarization arises fromscattering of starlight off an aysmmetric distribution of grains, thenthe distribution varies with time. Polarized features are detected inthe TiO bands of three M-type Mira variables, in the CN bands of thecarbon stars R Lep and V384 Per, and in the Swan bands of C2in R CrB and two PPNs. Polarization effects in the molecular bandsappear to be more common and the effects are larger in O-rich thanC-rich objects.
| A self-consistent model of a 22 GHz water maser in a dusty environment near late-type stars
We study the conditions for operation of the 22 GHz ortho-water maser ina dusty medium near late-type stars. The main physical processes, suchas exchange of energy between dust and gas in the radiation field of astar, radiative cooling by water molecules and pumping of water masersare described self-consistently. We show that the presence of dustgrains of various types (or of one type with size distribution) stronglyaffects the maser action. The pumping mechanism based on the presence ofthe dust of different optical properties is able to explain water masersin the silicate carbon star V778 Cyg. However, the masers in the windsfrom asymptotic giant branch stars require an additional source ofheating, for instance due to the dust drift through the gas.
| A catalogue of eclipsing variables
A new catalogue of 6330 eclipsing variable stars is presented. Thecatalogue was developed from the General Catalogue of Variable Stars(GCVS) and its textual remarks by including recently publishedinformation about classification of 843 systems and making correspondingcorrections of GCVS data. The catalogue1 represents thelargest list of eclipsing binaries classified from observations.
| Automatic classification of eclipsing binaries light curves using neural networks
In this work we present a system for the automatic classification of thelight curves of eclipsing binaries. This system is based on aclassification scheme that aims to separate eclipsing binary systemsaccording to their geometrical configuration in a modified version ofthe traditional classification scheme. The classification is performedby a Bayesian ensemble of neural networks trained with Hipparcos data ofseven different categories including eccentric binary systems and twotypes of pulsating light curve morphologies.
| Magnetic fields around late-type stars using H_2O maser observations .
We present the analysis of the circular polarization, due to Zeemansplitting, of the H_2O masers around a sample of late-type stars todetermine the magnetic fields in their circumstellar envelopes (CSEs).The magnetic field strengths in the H_2O maser regions around the Miravariable stars U Ori and U Her are shown to be several Gauss while thoseof the supergiants S Per, NML Cyg and VY CMa are several hundred mG. Wealso show that large scale magnetic fields permeate the CSE of anevolved star; the polarization of the H_2O masers around VX Sgr revealsa dipole field structure. We shortly discuss the coupling of themagnetic field with the stellar outflow, as such fields could possiblybe the cause of distinctly aspherical mass-loss and the resultingaspherical planetary nebulae.
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| Secular Evolution in Mira Variable Pulsations
Stellar evolution theory predicts that asymptotic giant branch (AGB)stars undergo a series of short thermal pulses that significantly changetheir luminosity and mass on timescales of hundreds to thousands ofyears. These pulses are confirmed observationally by the existence ofthe short-lived radioisotope technetium in the spectra of some of thesestars, but other observational consequences of thermal pulses are subtleand may only be detected over many years of observations. Secularchanges in these stars resulting from thermal pulses can be detected asmeasurable changes in period if the star is undergoing Mira pulsations.It is known that a small fraction of Mira variables exhibit largesecular period changes, and the detection of these changes among alarger sample of stars could therefore be useful in evolutionary studiesof these stars. The American Association of Variable Star Observers(AAVSO) International Database currently contains visual data for over1500 Mira variables. Light curves for these stars span nearly a centuryin some cases, making it possible to study the secular evolution of thepulsation behavior on these timescales. In this paper we present theresults of our study of period change in 547 Mira variables using datafrom the AAVSO. We use wavelet analysis to measure the period changes inindividual Mira stars over the span of available data. By making linearfits to the period versus time measurements, we determine the averagerates of period change, dlnP/dt, for each of these stars. We findnonzero dlnP/dt at the 2 σ significance level in 57 of the 547stars, at the 3 σ level in 21 stars, and at the level of 6 σor greater in eight stars. The latter eight stars have been previouslynoted in the literature, and our derived rates of period change largelyagree with published values. The largest and most statisticallysignificant dlnP/dt are consistent with the rates of period changeexpected during thermal pulses on the AGB. A number of other starsexhibit nonmonotonic period change on decades-long timescales, the causeof which is not yet known. In the majority of stars, the periodvariations are smaller than our detection threshold, meaning theavailable data are not sufficient to unambiguously measure slowevolutionary changes in the pulsation period. It is unlikely that morestars with large period changes will be found among heretoforewell-observed Mira stars in the short term, but continued monitoring ofthese and other Mira stars may reveal new and serendipitous candidatesin the future.
| The magnetic field around late-type stars revealed by the circumstellar H_2O masers
Through polarization observations, circumstellar masers are excellentprobes of the magnetic field in the envelopes of late-type stars.Whereas observations of the polarization of the SiO masers close to thestar and on the OH masers much further out were fairly commonplace,observations of the magnetic field strength in the intermediate densityand temperature region where the 22 GHz H2O masers occur have onlyrecently become possible. Here we present the analysis of the circularpolarization, due to Zeeman splitting, of the H2O masers around the Miravariable stars U Her and U Ori and the supergiant VX Sgr. We present anupper limit of the field around U Her that is lower but consistent withprevious measurements, reflecting possible changes in the circumstellarenvelope. The field strengths around U Ori and VX Sgr are shown to be ofthe order of several Gauss. Moreover, we show for the first time thatlarge scale magnetic fields permeate the circumstellar envelopes of anevolved star; the polarization of the H2O masers around VX Sgr reveals adipole field structure. We discuss the coupling of the magnetic fieldwith the stellar outflow, as such fields could possibly be the cause ofdistinctly aspherical mass-loss.
| Astrophysical water masers: Line profiles analysis
The changes in the spectral line profile of the 22 GHz H2O maser arecalculated as a function of emerging maser flux. We address not only thenarrowing and re-broadening of the maser lines, but also the deviationsof Gaussian symmetry as a result of the various hyperfine components ofthe 22 GHz maser line. Non-LTE models of the H2O maser transition,including hyperfine structure and cross-relaxation, are compared withhigh spectral resolution Very Long Baseline Interferometry (VLBI)observations of the H2O masers in the circumstellar envelopes (CSEs) ofa sample of late-type stars. This yields estimates on the thermal widthin the maser region as well as the emerging maser flux and thus thelevel of saturation. We also discuss the effect of a velocity gradientalong the maser path on the line widths and shapes of the line profileand the effect of the geometry of the maser region. We find that thetypical velocity shift along the maser path is of the order of 1.0 kms-1. The effect of this shift on the shape of the maserspectrum is difficult to distinguish from the effect of the hyperfinecomponents.
| Search for radiative pumping lines of OH masers. I. The 34.6 m absorption line towards 1612 MHz OH maser sources
The 1612 MHz hydroxyl maser in circumstellar envelopes has long beenthought to be pumped by 34.6 μm photons. Only recently, the InfraredSpace Observatory has made possible spectroscopic observations whichenable the direct confirmation of this pumping mechanism in a few cases.To look for the presence of this pumping line, we have searched theInfrared Space Observatory Data Archive and found 178 spectra with dataaround 34.6 μm for 87 galactic 1612 MHz masers. The analysisperformed showed that the noise level and the spectral resolution of thespectra are the most important factors affecting the detection of the34.6 μm absorption line. Only 5 objects from the sample (3 redsupergiants and 2 galactic center sources) are found to show clear 34.6μm absorption (all of them already known) while two additionalobjects only tentatively show this line. The 3 supergiants show similarpump rates and their masers might be purely radiatively pumped. The pumprates of OH masers in late type stars are found to be about 0.05, only1/5 of the theoretical value of 0.25 derived by Elitzur (1992). We havealso found 16 maser sources which, according to the analysis assumingElitzur's pump rate, should show the 34.6 μm absorption line but donot. These non-detections can be tentatively explained by far-infraredphoton pumping, clumpy nature of the OH masing region or a limb-fillingemission effect in the OH shell.
| SiO masers in TX Cam. Simultaneous VLBA observations of two 43 GHz masers at four epochs
We present the results of simultaneous high resolution observations of v= 1 and v = 2, J = 1-0 SiO masers toward TX Cam at four epochs coveringa stellar cycle. We used a new observing technique to determine therelative positions of the two maser maps. Near maser maximum (Epochs IIIand IV), the individual components of both masers are distributed inring-like structures but the ring is severely disrupted near stellarmaser minimum (Epochs I and II). In Epochs III and IV there is a largeoverlap between the radii at which the two maser transitions occur.However in both epochs the average radius of the v = 2 maser ring issmaller than for the v = 1 maser ring, the difference being larger forEpoch IV. The observed relative ring radii in the two transitions, andthe trends on the ring thickness, are close to those predicted by themodel of Humphreys et al. ([CITE], A&A, 386, 256). In manyindividual features there is an almost exact overlap in space andvelocity of emission from the two transitions, arguing against pureradiative pumping. At both Epochs III and IV in many spectral featuresonly 50% of the flux density is recovered in our images, implyingsignificant smooth maser structure. For both transitions we find thatred- and blue-shifted masers occur in all parts of the rings, withrelatively few masers at the systemic velocity. Thus there is noevidence for rotation, although the blue-shifted masers are somewhatmore prominent to the west. At all four epochs red-shifted componentsare generally brighter than blue-shifted ones. Blue-shifted masersbecome very weak at some stellar phases but never completely disappear.At Epochs III and IV, we see many filamentary or spoke-like features inboth v = 1 and v = 2 masers, especially in the red-shifted gas. Thesespokes show systematic velocity gradients consistent with a deceleratingoutward flow with increasing radius. We outline a possible model toexplain why, given the presence of these spokes, there is a deficit ofmaser features at the systemic velocity. The breaking of sphericalsymmetry by spoke-like features may explain the high-velocity wings seenin SiO maser single dish spectra.
| An infrared study of galactic OH/IR stars. I. An optical/near-IR atlas of the Arecibo sample
In this paper we present optical and near-infrared finding charts,accurate astrometry (≈1 arcsec) and single-epoch near-infraredphotometry for 371 IRAS sources, 96% of those included in the so-called``Arecibo sample of OH/IR stars'' (Eder et al. \cite{Eder88}, ApJS, 66,183; Lewis et al. \cite{Lewis90a}, ApJ, 362, 634; Chengalur et al.\cite{Chengalur93}, ApJS, 89, 189). The main photometric properties ofthe stars in the sample are presented and discussed as well as theproblems found during the process of identification of theoptical/near-infrared counterparts. In addition, we also identifysuitable reference stars in each field to be used for differentialphotometry purposes in the future.We find that 39% of the sources (144 in number) have no opticalcounterpart, 8 of them being invisible even at near infraredwavelengths. The relative distribution of sources with and withoutoptical counterpart in the IRAS two-colour diagram and theircharacteristic near infrared colours are interpreted as the consequenceof the increasing thickness of their circumstellar shells. Among theobjects not detected at near infrared wavelengths four non-variablesources are proposed to be heavily obscured post-AGB stars which havejust very recently left the AGB. Eight additional objects withunusually bright and/or blue near-infrared colours are identified ascandidate post-AGB stars and/or proto-planetary nebulae.Based on observations collected at the German-Spanish AstronomicalCentre, Calar Alto, operated jointly by Max-Planck-Institut füerAstronomie and Instituto de Astrofísica de Andalucía(CSIC).Appendix A (Tables A1 and A2) is 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/431/779Finding charts are only available in electronic form athttp://www.edpsciences.org
| CHARM2: An updated Catalog of High Angular Resolution Measurements
We present an update of the Catalog of High Angular ResolutionMeasurements (CHARM, Richichi & Percheron \cite{CHARM}, A&A,386, 492), which includes results available until July 2004. CHARM2 is acompilation of direct measurements by high angular resolution methods,as well as indirect estimates of stellar diameters. Its main goal is toprovide a reference list of sources which can be used for calibrationand verification observations with long-baseline optical and near-IRinterferometers. Single and binary stars are included, as are complexobjects from circumstellar shells to extragalactic sources. The presentupdate provides an increase of almost a factor of two over the previousedition. Additionally, it includes several corrections and improvements,as well as a cross-check with the valuable public release observationsof the ESO Very Large Telescope Interferometer (VLTI). A total of 8231entries for 3238 unique sources are now present in CHARM2. Thisrepresents an increase of a factor of 3.4 and 2.0, respectively, overthe contents of the previous version of CHARM.The catalog is only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/431/773
| First mm-VLBI Observations between the TRAO 14-m and the NRO 45-m Telescopes: Observations of 86 GHz SiO Masers in VY Canis Majoris
We have made VLBI observations at 86GHz using a 1000-km baseline betweenKorea and Japan with successful detections of SiO v = 1, J = 2 ‑ 1maser emissions from VY CMa and Orion KL in 2001 June. This was thefirst VLBI result for this baseline and the first astronomical VLBIobservation for the Korean telescope. Since then, we observed SiO v = 1,J = 2 ‑ 1 maser emission in VY CMa in 2002 January and 2003February and derived the distributions of the maser emissions. Ourresults show that the maser emissions extend over 2-4 stellar radii, andwere within the inner radius of the dust shell. We observed other SiOmaser sources and continuum sources, and 86-GHz continuum emissions weredetected from three continuum sources. It was verified that thisbaseline has a performance comparable to the most sensitive baseline inthe VLBA and the CMVA, and is capable of investigating the propermotions of maser features in circumstellar envelopes using monitoringobservations.
| First polarimetric images of NML Cyg at 1612 and 1665 MHz
We present the first view of the magnetic field structure in the OHshell of the supergiant NML Cygni (NML Cyg). MERLIN interferometricobservations of this object were obtained in 1993 October in fullpolarization, at 1612 and 1665 MHz. They reveal a complex structure inboth total intensity and polarization. At 1612 MHz, the majority of thecomponents lie along a north-west-south-east (NW-SE) geometrical axisalong which a velocity gradient is clearly evident. The polarizationvectors associated with these maser spots are roughly aligned on thatsame NW-SE axis. The distribution of the external and fainter componentsis spherical and the associated polarization vectors have a tangentialdistribution. The total extent of the maser emission at 1612 MHz isabout 5.3 arcsec. At 1665 MHz, the maser spot distribution reveals anincomplete spherical shell of total extent about 2.5 arcsec. As observedfor the 1612-MHz spherical emission, the polarization vectors associatedwith the 1665-MHz spherically distributed maser spots show a tangentialdistribution. This dichotomy strongly suggests the existence of twodistinct OH shells; an older shell, exhibiting the spherical maser spotdistribution and a tangential distribution of the polarization vectors,and a more recent shell showing a NW-SE maser spot distribution withassociated polarization vectors along that same axis.
| Searches for OH masers in the circumstellar envelopes of red giant irregular variable stars
Irregular variables of type Lb share many characteristics withasymptotic giant branch (AGB) stars such as Miras and semiregulars(SRs). Despite their large number, irregulars are poorly understood.Using the large Nançay radio telescope, we searched for 18-cm OHmaser emission from these stars. We selected two samples of Lbs thatpresent a noticeable circumstellar envelope. In the first one, 44high-latitude objects with infrared colours of dusty mass-losing starswere observed in a wide-bandwidth velocity-search mode. In addition, 16objects (including six from the first sample), in which othercircumstellar molecular lines had recently been found, were observedwith higher resolution and sensitivity. We did not detect any new OHmaser emission from these samples down to 3σ sensitivity limitsranging from 30 to 200 mJy. We derived upper limits for the efficiencyof a typical pump using far-infrared photons. When compared with knownOH masers in AGB stars, we show that a significant gap is leftunexplored below the threshold that we determined for Lbs. Lowerefficiencies exist in some OH Miras and in the OH SRs. We cannot discardthe possibility of faint OH emission from certain Lbs, based on the sameprocesses as encountered in those sources. We briefly discuss physicalconditions allowing OH emission in the envelope, and the evolutionarystage of objects classified as Lb variables with respect to the AGB.
| Letter to the Editor: Brighter Maxima of 30 Selected Mira-Type Variable Stars for the Period 1978-1977
Letter to the Editor
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| Automated Classification of 2000 Bright IRAS Sources
An artificial neural network (ANN) scheme has been employed that uses asupervised back-propagation algorithm to classify 2000 bright sourcesfrom the Calgary database of Infrared Astronomical Satellite (IRAS)spectra in the region 8-23 μm. The database has been classified into17 predefined classes based on the spectral morphology. We have beenable to classify over 80% of the sources correctly in the firstinstance. The speed and robustness of the scheme will allow us toclassify the whole of the Low Resolution Spectrometer database,containing more than 50,000 sources, in the near future.
| 28SiO v = 1 and v = 2, J = 1-0 maser variability in evolved stars. Eleven years of short spaced monitoring
This paper presents and discusses the final data set of a long-term andshort-spaced monitoring of 21 SiO maser sources, mostly evolved stars,carried out in two SiO maser lines at 43 GHz with the ObservatorioAstronómico Nacional 13.7 m telescope at the CentroAstronómico de Yebes (Guadalajara, Spain). In most objects, morethan 80 spectra per transition over a period of 11 years have beenrecorded. The new data presented here, previously unpublished, representnearly 50% of the total SiO data collected in the project. In addition,the availability of optical light curves from the AAVSO for most of theobjects during the whole period of the SiO monitoring, ground-basednear-IR data for four sources overlapping with 3 to 5 observed SiOperiods, and DIRBE near-IR data covering a significant portion of an SiOperiod in 10 sources, make this data set a unique reference forcomparing optical, NIR and SiO variability in order to elucidate thephysical mechanisms that pump SiO masers in evolved stars. The basis forthe conclusions obtained in this work comes from a numerical time seriesanalysis of the suitable SiO, optical and NIR light curves in regularvariables to obtain precise values of the periods and phase lags betweenthe different curves. This analysis shows evidence that in regularvariable evolved stars the three types of emission have the same periodand that the SiO maxima happen in phase with NIR maxima and with a phaselag typically between 0.05 and 0.20 with respect to optical maxima. Weconclude that in these objects the observational evidence presented inthis work favors the radiative pumping of SiO masers against thecollisional pumping.Figures 1-4 and 11-21 are only available in electronic form athttp://www.edpsciences.org
| Catalogue of Algol type binary stars
A catalogue of (411) Algol-type (semi-detached) binary stars ispresented in the form of five separate tables of information. Thecatalogue has developed from an earlier version by including more recentinformation and an improved layout. A sixth table lists (1872) candidateAlgols, about which fewer details are known at present. Some issuesrelating to the classification and interpretation of Algol-like binariesare also discussed.Catalogue is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/417/263
| VLBA observations of SiO masers towards Mira variable stars
We present new total intensity and linear polarization VLBA observationsof the ν=2 and ν=1 J=1-0 maser transitions of SiO at 42.8 and 43.1GHz in a number of Mira variable stars over a substantial fraction oftheir pulsation periods. These observations were part of an observingprogram that also includes interferometric measurements at 2.2 and 3.6micron \citep{Mennesson2002}; comparison of the results from differentwavelengths allows studying the envelope independently of the poorlyknown distances to these stars. Nine stars were observed at from one tofour epochs during 2001. The SiO emission is largely confined to ringswhich are smaller than the inner radius of the dust shells reported by\citet{Danchi1994}. Two stars (U Orionis, R Aquarii) have maser ringswith diameters corresponding to the size of the hot molecular layer asmeasured at 3.6 micron; in the other cases, the SiO rings aresubstantially larger. Variations of ring diameter for most, but not allstars, had an rms amplitude in agreement with the models of\citet{Humphreys2002} although the expected relationship between thediameter and pulsation phase was not seen. The ring diameter in UOrionis shows remarkably small variation. A correlation between the2.2/3.6 μm diameter ratio with that of the SiO/3.6 μm diameterratio is likely due to differences in the opacities at 2.2 and 3.6 μmin a molecular layer. A further correlation with the inner size of thedust shell reported by \citet{Danchi1994} suggest some differences inthe temperature structure. Clear evidence is seen in R Aquarii for anequatorial disk similar to that reported by \citet{Hollis2001}; rotationis possibly also detected in S Coronae Boralis.
| J - K DENIS photometry of a VLTI-selected sample of bright southern stars
We present a photometric survey of bright southern stars carried outusing the DENIS instrument equipped with attenuating filters. Theobservations were carried out not using the survey mode of DENIS, butwith individual target pointings. This project was stimulated by theneed to obtain near-infrared photometry of stars to be used in earlycommissioning observations of the ESO Very Large TelescopeInterferometer, and in particular to establish a network of brightcalibrator sources.We stress that near-infrared photometry is peculiarly lacking for manybright stars. These stars are saturated in 2MASS as well as in regularDENIS observations. The only other observations available for brightinfrared stars are those of the Two Micron Sky Survey dating from overthirty years ago. These were restricted to declinations above≈-30°, and thus cover only about half of the sky accessible fromthe VLTI site.We note that the final 2MASS data release includes photometry of brightstars, obtained by means of point-spread function fitting. However, thismethod only achieves about 30% accuracy, which is not sufficient formost applications.In this work, we present photometry for over 600 stars, each with atleast one and up to eight measurements, in the J and K filters. Typicalaccuracy is at the level of 0\fm05 and 0\fm04 in the J and K_s bands,respectively.Based on observations collected at the European Southern Observatory, LaSilla.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/413/1037
| Long periodic variable stars
The information on Mira-type stars and stars adjacent to them at theHertzsprung -- Russel diagram is presented. A detailed description oftheir observational characteristics is given. We give a survey ofimportant observational works concerning: multicolor photometry withspecial attention to the IR emission, maser emission, shock waves, massloss, binarity, the problem of the pulsational mode, direct measurementsof angular and linear dimensions, statistic investigations, study ofkinematic characteristics etc. The most interesting problems regardinglong periodic variable stars are specified. Some attention is given tothe classification and evolutionary stage of these objects.
| Astral magnetic fields-as observed in starforming nurseries, in stars, and in the Solar system
``Cherchez le champ magnétique'' is trendy in astronomy. Themagnetic field takes the hourglass shape in gravitationally contractingyoung stellar objects, helically wraps up the jets of protostars, guidesparticles in stellar coronae, explodes in stellar flares, causes unrestin pulsar quakes and creates planetary aurorae. In stars and pulsars,dipolar dynamo magnetic fields play a dominant rôle in the gasdynamics. In planetary disks, the field is toroidal or archimedeanspiral. Remanent magnetism is found in meteorites and asteroids. Theastral magnetic fields can reach 1018 G in magnetars,dwarfing anything we can do on Earth in stable conditions(107 G) or in pulsed conditions (1010 G).Magnetism plays a physical rôle in starforming clouds and itsconcomittent structures, from stellar nurseries to protostars, down toplanets and asteroids. Starting with star-forming clouds (~10 pc=32light-years=31×1016 m), this reviews coversprotostellar systems (~1 cpc), circumstellar space (~1 mpc), masers,interplanetary space (~1 μpc), pulsars, stars, planets (~1 npc),asteroids (~30 km; ~1 ppc), and meteorites (~0.3 m; ~10 apc).
| Sub-au imaging of water vapour clouds around four asymptotic giant branch stars
We present MERLIN maps of the 22-GHz H2O masers around fourlow-mass late-type stars (IK Tau, U Ori, RT Vir and U Her), made with anangular resolution of ~15 milliarcsec and a velocity resolution of 0.1km s-1. The H2O masers are found in thickexpanding shells with inner radii ~6 to 16 au and outer radii four timeslarger. The expansion velocity increases radially through theH2O maser regions, with logarithmic velocity gradients of0.5-0.9. IK Tau and RT Vir have well-filled H2O maser shellswith a spatial offset between the near and far sides of the shell, whichsuggests that the masers are distributed in oblate spheroids inclined tothe line of sight. U Ori and U Her have elongated poorly filled shellswith indications that the masers at the inner edge have been compressedby shocks; these stars also show OH maser flares. MERLIN resolvesindividual maser clouds, which have diameters of 2-4 au and fillingfactors of only ~0.01 with respect to the whole H2O masershells. The circumstellar envelope velocity structure gives additionalevidence the maser clouds are density-bounded. Masing clouds can beidentified over a similar time-scale to their sound crossing time (~2yr) but not longer. The sizes and observed lifetimes of these clouds arean order of magnitude smaller than for those around red supergiants,similar to the ratio of low-mass:high-mass stellar masses and sizes.This suggests that cloud size is determined by stellar properties, notlocal physical phenomena in the wind.
| Infrared investigation from earth and space on the evolutionary state of a sample of LPV
We selected a sample of highly reddened AGB stars among the sourcesobserved with the SWS instrument on the ISO satellite. These SWS dataallow us to compute the source's photometry in the mid-IR filters of thecamera TIRCAM at the TIRGO telescope. Our photometric data, supplementedwith other measurements taken from the literature, permit to select thecarbon-rich sources in the sample. For these stars, a linear relationholds between dust mass loss and the color index [8.8]-[12.5]. One maythen, from photometric data alone, evaluate the total mass loss (forwhich we used the estimate of \citet{loup}, based on radio data). Theoxygen-rich sources, on the other hand, are distributed in two branches,of which the upper one appears superimposed with carbon stars; the starsin this group have both high luminosity and high wind velocity andtherefore higher masses. Finally S stars lie between the carbon-starbranch and the low-mass oxygen-rich stars, in agreement with theirintermediate evolutionary status.
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| Csillagkép: | Herkules |
| Rektaszcenzió: | 16h25m47.70s |
| Deklináció: | +18°53'33.0" |
| Vizuális fényesség: | 6.7 |
| Távolság: | 609.756 parszek |
| RA sajátmozgás: | 0 |
| Dec sajátmozgás: | 0 |
| B-T magnitude: | 10.757 |
| V-T magnitude: | 9.545 |
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