Tartalom
Képek
Kép feltöltése
DSS Images Other Images
Kapcsolódó cikkek
A Comparative Study of Flaring Loops in Active Stars
Dynamo activity in stars of different types is expected to generatemagnetic fields with different characteristics. As a result, adifferential study of the characteristics of magnetic loops in a broadsample of stars may yield information about dynamo systematics. In theabsence of direct imaging, certain physical parameters of a stellarmagnetic loop can be extracted if a flare occurs in that loop. In thispaper we employ a simple nonhydrodynamic approach introduced by Haisch,to analyze a homogeneous sample of all of the flares we could identifyin the EUVE DS database: a total of 134 flares that occurred on 44 starsranging in spectral type from F to M and in luminosity class from V toIII. All of the flare light curves that have been used in the presentstudy were obtained by a single instrument (EUVE DS). For each flare, wehave applied Haisch's simplified approach (HSA) in order to determineloop length, temperature, electron density, and magnetic field. For eachof our target stars, a literature survey has been performed to determinequantitatively the extent to which our results are consistent withindependent studies. The results obtained by HSA are found to be wellsupported by results obtained by other methods. Our survey suggeststhat, on the main sequence, short loops (with lengths<=0.5R*) may be found in stars of all classes, while thelargest loops (with lengths up to 2R*) appear to be confinedto M dwarfs. Based on EUVE data, the transition from small to largeloops on the main sequence appears to occur between spectral types K2and M0. We discuss the implications of this result for dynamo theories.
| Variations in D/H and D/O from New Far Ultraviolet Spectroscopic Explorer Observations
We use data obtained with the Far Ultraviolet Spectroscopic Explorer (FUSE) to determine the interstellar abundances of D I, N I, O I, Fe II,and H2 along the sight lines to WD 1034+001, BD +39 3226, andTD1 32709. Our main focus is on determining the D/H, N/H, O/H, and D/Oratios along these sight lines, with logN(H)>20.0, that probe gaswell outside of the Local Bubble. Hubble Space Telescope (HST) andInternational Ultraviolet Explorer (IUE) archival data are used todetermine the H I column densities along the WD 1034+001 and TD1 32709sight lines, respectively. For BD +39 3226, a previously published N(HI) is used. We find(D/H)×105=2.14+0.53-0.45,1.17+0.31-0.25, and1.86+0.53-0.43 and(D/O)×102=6.31+1.79-1.38,5.62+1.61-1.31, and7.59+2.17-1.76 for the WD 1034+001, BD +39 3226,and TD1 32709 sight lines, respectively (all 1 σ). The scatter inthese three D/H ratios exemplifies the scatter that has been found byother authors for sight lines with column densities in the range19.2
| A Large Millimeter Flare on the RS CVn Binary σ Geminorum
The flaring behavior of active RS CVn binaries has been studiedextensively at centimeter wavelengths, but very little has beenpublished concerning RS CVn flares in the millimeter region, where thenonthermal gyrosynchrotron emission is much more likely to be opticallythin. We present 3 mm (99 GHz) observations of a large flare on theactive binary σ Gem using the Owens Valley Millimeter Array. Two 3hr observations in 2004 February showed only 3 σ upper limits of 3mJy, while the third observation on 2004 April 22 showed a large flareoutburst that filled the 6 hr observation and reached a peak fluxdensity of 90 mJy. The strongest brightening was preceded by at least a4 hr period of significantly elevated emission at 20-40 mJy. Theseobservations demonstrate the feasibility of detailed multifrequencystudies of nonthermal electron distributions in active star coronae withthe next generation of millimeter and centimeter radio arrays.
| On temperature and abundance effects during an X-ray flare on σ Geminorum
We compare quiescent and flare X-ray spectra of the RS CVn binaryσ Gem obtained with the Chandra and XMM-Newton gratingspectrometers. We find that in addition to an overall 25% flux increase,which can be ascribed to variations in the system's quiescence activityover the 15 months that passed between the observations, there is a hotplasma component of kTe 3 keV that arises with theflare. The hot component is manifested primarily by emission from highcharge states of Fe and by a vast continuum. The cooler (kTe 2 keV) plasma remains undisturbed during the flare. We find nosignificant variations in the relative abundances during the flareexcept for a slight decrease (<30%) of O and Ne.
| Spurious `active longitudes' in parametric models of heavily spotted eclipsing binaries
In this paper, the size distributions of starspots extrapolated from thecase of the Sun are modelled on the eclipsing binary SV Cam tosynthesize images of stellar photospheres with high spot fillingfactors. These spot distributions pepper the primary's surface withspots, many of which are below the resolution capabilities ofeclipse-mapping and Doppler-imaging techniques. The light curvesresulting from these modelled distributions are used to determine thelimitations of image reconstruction from photometric data. Surfacebrightness distributions reconstructed from these light curves showdistinctive spots on the primary star at its quadrature points. It isconcluded that two-spot modelling or chi-squared minimization techniquesare more susceptible to spurious structures being generated bysystematic errors, arising from incorrect assumptions about photosphericsurface brightness, than simple Fourier analysis of the light curves.
| Stellar Lyα Emission Lines in the Hubble Space Telescope Archive: Intrinsic Line Fluxes and Absorption from the Heliosphere and Astrospheres
We search the Hubble Space Telescope (HST) archive for previouslyunanalyzed observations of stellar H I Lyα emission lines, ourprimary purpose being to look for new detections of Lyα absorptionfrom the outer heliosphere and to also search for analogous absorptionfrom the astrospheres surrounding the observed stars. The astrosphericabsorption is of particular interest because it can be used to studysolar-like stellar winds that are otherwise undetectable. We find andanalyze 33 HST Lyα spectra in the archive. All the spectra weretaken with the E140M grating of the Space Telescope Imaging Spectrograph(STIS) instrument on board HST. The HST STIS spectra yield four newdetections of heliospheric absorption (70 Oph, ξ Boo, 61 Vir, and HD165185) and seven new detections of astrospheric absorption (EV Lac, 70Oph, ξ Boo, 61 Vir, δ Eri, HD 128987, and DK UMa), doubling theprevious number of heliospheric and astrospheric detections. Whencombined with previous results, 10 of 17 lines of sight within 10 pcyield detections of astrospheric absorption. This high detectionfraction implies that most of the ISM within 10 pc must be at leastpartially neutral, since the presence of H I within the ISM surroundingthe observed star is necessary for an astrospheric detection. Incontrast, the detection percentage is only 9.7% (3 out of 31) for starsbeyond 10 pc. Our Lyα analyses provide measurements of ISM H I andD I column densities for all 33 lines of sight, and we discuss someimplications of these results. Finally, we measure chromosphericLyα fluxes from the observed stars. We use these fluxes todetermine how Lyα flux correlates with coronal X-ray andchromospheric Mg II emission, and we also study how Lyα emissiondepends on stellar rotation.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| Flip-flop phenomenon: observations and theory
In many active stars the spots concentrate on two permanent activelongitudes which are 180 ° apart. In some of these stars thedominant part of the spot activity changes the longitude every fewyears. This so-called flip-flop phenomenon has up to now been reportedin 11 stars, both single and binary alike, and including also the Sun.To explain this phenomenon, a non-axisymmetric dynamo mode, giving riseto two permanent active longitudes at opposite stellar hemispheres, isneeded together with an oscillating axisymmetric magnetic field. Here wediscuss the observed characteristics of the flip-flop phenomenon andpresent a dynamo solution to explain them.
| Deuterium Depletion and Magnesium Enhancement in the Local Disk
The local disk deuterium is known to be depleted in comparison to thelocal bubble. We used Hubble Space Telescope (HST) spectra to obtaincolumn densities of Si, Mg and Fe. We compared normalized columndensities of these elements in the directions with high and lowdeuterium abundances.We show, that the lines of sight that are depleted in deuterium, areenhanced in magnesium. This observation implicates that astration isresponsible for both deuterium depletion and magnesium enhancement.
| Photometric observations from theoretical flip-flop models
Some active stars show a so-called flip-flop phenomenon in which themain spot activity periodically switches between two active longitudesthat are 180° apart. In this paper we study the flip-flop phenomenonby converting results from dynamo calculations into long-term syntheticphotometric observations, which are then compared to the real stellarobservations. We show that similar activity patterns as obtained fromflip-flop dynamo calculations, can also be seen in the observations. Thelong-term light-curve behaviour seen in the synthesised data can be usedfor finding new stars exhibiting the flip-flop phenomenon.
| Flares observed with XMM-Newton and the VLA
We present lightcurves obtained in X-ray by the XMM-Newton EPIC camerasand simultaneous radio lightcurves obtained with the VLA for five activeM-type flare stars. A number of flare events were observed, and bycomparing radio with X-ray data, we consider various possible flaremechanisms. In cases where there seems to be a clear correlation betweenradio and X-ray activity, we use an energy budget argument to show thatthe heating which leads to the X-ray emission could be due to the sameparticles emitting in the radio. In cases where there is radio activitywithout corresponding X-ray activity, we argue that the radio emissionis likely to arise from coherent processes involving comparatively fewparticles. In one case, we are able to show from polarization of theradio emission that this is almost certainly the case. Cases for whichX-ray activity is seen without corresponding radio activity are moredifficult to explain. We suggest that the heating particles may beaccelerated to very high energy, and the resulting synchrotron radioemission may be beamed in directions other than the line of sight.
| 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
| Mg II chromospheric radiative loss rates in cool active and quiet stars
The Mg II k emission line is a good indicator of the level ofchromospheric activity in late-type stars. We investigate the dependenceof this activity indicator on fundamental stellar parameters. To thispurpose we use IUE observations of the Mg II k line in 225 late-typestars of luminosity classes I-V, with different levels of chromosphericactivity. We first re-analyse the relation between Mg II k lineluminosity and stellar absolute magnitude, performing linear fits to thepoints. The ratio of Mg II surface flux to total surface flux is foundto be independent of stellar luminosity for evolved stars and toincrease with decreasing luminosity for dwarfs. We also analyse the MgII k line surface flux-metallicity connection. The Mg II k emissionlevel turns out to be not dependent on metallicity. Finally, the Mg II kline surface flux-temperature relation is investigated by treatingseparately, for the first time, a large sample of very active and normalstars. The stellar surface fluxes in the k line of normal stars arefound to be strongly dependent on the temperature and slightly dependenton the gravity, thus confirming the validity of recently proposedmodels. In contrast, data relative to RS CVn binaries and BY Dra stars,which show very strong chromospheric activity, are not justified in theframework of a description based only on acoustic waves and uniformlydistributed magnetic flux tubes so that they require more detailedmodels.
| Local kinematics of K and M giants from CORAVEL/Hipparcos/Tycho-2 data. Revisiting the concept of superclusters
The availability of the Hipparcos Catalogue has triggered many kinematicand dynamical studies of the solar neighbourhood. Nevertheless, thosestudies generally lacked the third component of the space velocities,i.e., the radial velocities. This work presents the kinematic analysisof 5952 K and 739 M giants in the solar neighbourhood which includes forthe first time radial velocity data from a large survey performed withthe CORAVEL spectrovelocimeter. It also uses proper motions from theTycho-2 catalogue, which are expected to be more accurate than theHipparcos ones. An important by-product of this study is the observedfraction of only 5.7% of spectroscopic binaries among M giants ascompared to 13.7% for K giants. After excluding the binaries for whichno center-of-mass velocity could be estimated, 5311 K and 719 M giantsremain in the final sample. The UV-plane constructed from these datafor the stars with precise parallaxes (σπ/π≤20%) reveals a rich small-scale structure, with several clumpscorresponding to the Hercules stream, the Sirius moving group, and theHyades and Pleiades superclusters. A maximum-likelihood method, based ona Bayesian approach, has been applied to the data, in order to make fulluse of all the available stars (not only those with precise parallaxes)and to derive the kinematic properties of these subgroups. Isochrones inthe Hertzsprung-Russell diagram reveal a very wide range of ages forstars belonging to these groups. These groups are most probably relatedto the dynamical perturbation by transient spiral waves (as recentlymodelled by De Simone et al. \cite{Simone2004}) rather than to clusterremnants. A possible explanation for the presence of younggroup/clusters in the same area of the UV-plane is that they have beenput there by the spiral wave associated with their formation, while thekinematics of the older stars of our sample has also been disturbed bythe same wave. The emerging picture is thus one of dynamical streamspervading the solar neighbourhood and travelling in the Galaxy withsimilar space velocities. The term dynamical stream is more appropriatethan the traditional term supercluster since it involves stars ofdifferent ages, not born at the same place nor at the same time. Theposition of those streams in the UV-plane is responsible for the vertexdeviation of 16.2o ± 5.6o for the wholesample. Our study suggests that the vertex deviation for youngerpopulations could have the same dynamical origin. The underlyingvelocity ellipsoid, extracted by the maximum-likelihood method afterremoval of the streams, is not centered on the value commonly acceptedfor the radial antisolar motion: it is centered on < U > =-2.78±1.07 km s-1. However, the full data set(including the various streams) does yield the usual value for theradial solar motion, when properly accounting for the biases inherent tothis kind of analysis (namely, < U > = -10.25±0.15 kms-1). This discrepancy clearly raises the essential questionof how to derive the solar motion in the presence of dynamicalperturbations altering the kinematics of the solar neighbourhood: doesthere exist in the solar neighbourhood a subset of stars having no netradial motion which can be used as a reference against which to measurethe solar motion?Based on observations performed at the Swiss 1m-telescope at OHP,France, and on data from the ESA Hipparcos astrometry satellite.Full Table \ref{taba1} is only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/430/165}
| BVR photometry of a newly identified RS CVn binary star HD 61396
BVR photometry of a recently identified RS CVn binary star HD 61396,carried out during 2001, is presented. The new photometry revealsignificant evolution in the shape and amplitude of light curve whencompared with those reported earlier by Parihar et al. [MNRAS 314 (2000)733]. The traditional two-starspot model has been used to obtain thespot parameters from the observed light curve. Changes in the spot areaand their location on the stellar surface are discernible from theextracted parameters from the new photometry.
| Improved Baade-Wesselink surface brightness relations
Recent, and older accurate, data on (limb-darkened) angular diameters iscompiled for 221 stars, as well as BVRIJK[12][25] magnitudes for thoseobjects, when available. Nine stars (all M-giants or supergiants)showing excess in the [12-25] colour are excluded from the analysis asthis may indicate the presence of dust influencing the optical andnear-infrared colours as well. Based on this large sample,Baade-Wesselink surface brightness (SB) relations are presented fordwarfs, giants, supergiants and dwarfs in the optical and near-infrared.M-giants are found to follow different SB relations from non-M-giants,in particular in V versus V-R. The preferred relation for non-M-giantsis compared to the earlier relation by Fouqué and Gieren (basedon 10 stars) and Nordgren et al. (based on 57 stars). Increasing thesample size does not lead to a lower rms value. It is shown that theresiduals do not correlate with metallicity at a significant level. Thefinally adopted observed angular diameters are compared to thosepredicted by Cohen et al. for 45 stars in common, and there isreasonable overall, and good agreement when θ < 6 mas.Finally, I comment on the common practice in the literature to average,and then fix, the zero-point of the V versus V-K, V versus V-R and Kversus J-K relations, and then rederive the slopes. Such a commonzero-point at zero colour is not expected from model atmospheres for theV-R colour and depends on gravity. Relations derived in this way may bebiased.
| Kinematics of chromospherically active binaries and evidence of an orbital period decrease in binary evolution
The kinematics of 237 chromospherically active binaries (CABs) werestudied. The sample is heterogeneous with different orbits andphysically different components from F to M spectral-type main-sequencestars to G and K giants and supergiants. The computed U, V, W spacevelocities indicate that the sample is also heterogeneous in velocityspace. That is, both kinematically younger and older systems exist amongthe non-evolved main sequence and the evolved binaries containing giantsand subgiants. The kinematically young (0.95 Gyr) subsample (N= 95),which is formed according to the kinematical criteria of moving groups,was compared with the rest (N= 142) of the sample (3.86 Gyr) toinvestigate any observational clues of binary evolution. Comparing theorbital period histograms between the younger and older subsamples,evidence was found supporting the finding of Demircan that the CABs losemass (and angular momentum) and evolve towards shorter orbital periods.The evidence of mass loss is noticeable on the histograms of the totalmass (Mh+Mc), which is compared between theyounger (only N= 53 systems available) and older subsamples (only N= 66systems available). The orbital period decrease during binary evolutionis found to be clearly indicated by the kinematical ages of 6.69, 5.19and 3.02 Gyr which were found in the subsamples according to the periodranges of logP<= 0.8, 0.8 < logP<= 1.7 and 1.7 < logP<=3, respectively, among the binaries in the older subsample.
| Magnetic topology and surface differential rotation on the K1 subgiant of the RS CVn system HR 1099
We present here spectropolarimetric observations of the RS CVn system HR1099 (V711 Tau) secured from 1998 February to 2002 January with thespectropolarimeter MuSiCoS at the Télescope Bernard Lyot(Observatoire du Pic du Midi, France). We apply Zeeman-Doppler imagingand reconstruct surface brightness and magnetic topologies of the K1primary subgiant of the system, at five different epochs. We confirm thepresence of large, axisymmetric regions where the magnetic field ismainly azimuthal, providing further support to the hypothesis thatdynamo processes may be distributed throughout the whole convective zonein this star.We study the short-term evolution of surface structures from acomparison of our images with observations secured at close-by epochs byDonati et al. at the Anglo-Australian Telescope. We conclude that thesmall-scale brightness and magnetic patterns undergo major changeswithin a time-scale of 4-6 weeks, while the largest structures remainstable over several years.We report the detection of a weak surface differential rotation (bothfrom brightness and magnetic tracers) indicating that the equatorrotates faster than the pole with a difference in rotation rate betweenthe pole and the equator about four times smaller than that of the Sun.This result suggests that tidal forces also affect the global dynamicequilibrium of convective zones in cool active stars.
| Spot sizes on Sun-like stars
The total area coverage by starspots is of interest for a variety ofreasons, but direct techniques only provide estimates of this importantquantity. Sunspot areas exhibit a lognormal size distributionirrespective of the phase of the activity cycle, implying that mostsunspots are small. Here we explore the consequences if starspot areaswere similarly distributed. The solar data allow for an increase in thefraction of larger sunspots with increasing activity. Taking thisdifference between the size distribution at sunspot maximum and minimum,we extrapolate to higher activity levels, assuming different dependencesof the parameters of the lognormal distribution on total spot coverage.We find that, even for very heavily spotted (hypothetical) stars, alarge fraction of the spots are smaller than the current resolutionlimit of Doppler images and hence might be missed on traditional Dopplermaps.
| The Structure of the Local Interstellar Medium. III. Temperature and Turbulence
We present 50 individual measurements of the gas temperature andturbulent velocity in the local interstellar medium (LISM) within 100pc. By comparing the absorption line widths of many ions with differentatomic masses, we can satisfactorily discriminate between the twodominant broadening mechanisms, thermal broadening and macroscopicnonthermal, or turbulent, broadening. We find that the successful use ofthis technique requires a measurement of a light ion, such as D I, andan ion at least as heavy as Mg II. However, observations of more linesprovide an important consistency check and can also improve theprecision and accuracy of the measurement. Temperature and turbulentvelocity measurements are vital to understanding the physical propertiesof the gas in our local environment and can provide insight into thethree-dimensional morphological structure of the LISM. The weighted meangas temperature in the LISM warm clouds is 6680 K and the dispersionabout the mean is 1490 K. The weighted mean turbulent velocity is 2.24km s-1 and the dispersion about the mean is 1.03 kms-1. The ratio of the mean thermal pressure to the meanturbulent pressure is PT/Pξ~26. Turbulentpressure in LISM clouds cannot explain the difference in the apparentpressure imbalance between warm LISM clouds and the surrounding hot gasof the Local Bubble. Pressure equilibrium among the warm clouds may bethe source of a moderately negative correlation between temperature andturbulent velocity in these clouds. However, significant variations intemperature and turbulent velocity are observed. The turbulent motionsin the warm partially ionized clouds of the LISM are definitelysubsonic, and the weighted mean turbulent Mach number for clouds in theLISM is 0.19 with a dispersion of 0.11. These measurements provideimportant constraints on models of the evolution and origin of warmpartially ionized clouds in our local environment.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| Two New Low Galactic D/H Measurements from the Far Ultraviolet Spectroscopic Explorer
We analyze interstellar absorption observed toward two subdwarf O stars,JL 9 and LS 1274, using spectra taken by the Far UltravioletSpectroscopic Explorer (FUSE). Column densities are measured for manyatomic and molecular species (H I, D I, C I, N I, O I, P II, Ar I, FeII, and H2), but our main focus is on measuring the D/Hratios for these extended lines of sight, as D/H is an importantdiagnostic for both cosmology and Galactic chemical evolution. We findD/H=(1.00+/-0.37)×10-5 toward JL 9 andD/H=(0.76+/-0.36)×10-5 toward LS 1274 (2 σuncertainties). With distances of 590+/-160 and 580+/-100 pc,respectively, these two lines of sight are currently among the longestGalactic lines of sight with measured D/H. With the addition of thesemeasurements, we see a significant tendency for longer Galactic lines ofsight to yield low D/H values, consistent with previous inferences aboutthe deuterium abundance from D/O and D/N measurements. Short lines ofsight with H I column densities of logN(HI)<19.2 suggest that thegas-phase D/H value within the Local Bubble is(D/H)LBg=(1.56+/-0.04)×10-5. However, thefour longest Galactic lines of sight with measured D/H, which haved>500 pc and logN(HI)>20.5, suggest a significantly lower valuefor the true local disk gas-phase D/H value,(D/H)LDg=(0.85+/-0.09)×10-5. Oneinterpretation of these results is that D is preferentially depletedonto dust grains relative to H and that longer lines of sight thatextend beyond the Local Bubble sample more depleted material. In thisscenario, the higher Local Bubble D/H ratio is actually a betterestimate than (D/H)LDg for the true local disk D/H,(D/H)LD. However, if (D/H)LDg is different from(D/H)LBg simply because of variable astration and incompleteinterstellar medium mixing, then (D/H)LD=(D/H)LDg.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer (FUSE), which is operated for NASA by JohnsHopkins University under NASA contract NAS5-32985.
| The Structure of the Local Interstellar Medium. II. Observations of D I, C II, N I, O I, Al II, and Si II toward Stars within 100 Parsecs
Moderate- and high-resolution measurements(λ/Δλ>~40,000) of interstellar resonance lines ofD I, C II, N I, O I, Al II, and Si II (hereafter called light ions) arepresented for all available observed targets located within 100 pc thatalso have high-resolution observations of interstellar Fe II or Mg II(heavy ions) lines. All spectra were obtained with the Goddard HighResolution Spectrograph or the Space Telescope Imaging Spectrographinstrument aboard the Hubble Space Telescope. Currently, there are 41sight lines to targets within 100 pc with observations that include aheavy ion at high resolution and at least one light ion at moderate orhigh resolution. We present new measurements of light ions along 33 ofthese sight lines and collect from the literature results for theremaining sight lines that have already been analyzed. For all of thenew observations we provide measurements of the central velocity,Doppler width parameter, and column density for each absorptioncomponent. We greatly increase the number of sight lines with usefullocal interstellar medium (LISM) absorption-line measurements of lightions by using knowledge of the kinematic structure along a line of sightobtained from high-resolution observations of intrinsically narrowabsorption lines, such as Fe II and Mg II. We successfully fit theabsorption lines with this technique, even with moderate-resolutionspectra. Because high-resolution observations of heavy ions are criticalfor understanding the kinematic structure of local absorbers along theline of sight, we include 18 new measurements of Fe II and Mg II in anAppendix. We present a statistical analysis of the LISM absorptionmeasurements, which provides an overview of some physicalcharacteristics of warm clouds in the LISM, including temperature andturbulent velocity. This complete collection and reduction of allavailable LISM absorption measurements provides an important databasefor studying the structure of nearby warm clouds, including ionization,abundances, and depletions. Subsequent papers will present models forthe morphology and physical properties of individual structures (clouds)in the LISM.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS AR-09525.01A. Theseobservations are associated with program 9525.
| X-ray astronomy of stellar coronae
X-ray emission from stars in the cool half of the Hertzsprung-Russelldiagram is generally attributed to the presence of a magnetic coronathat contains plasma at temperatures exceeding 1 million K. Coronae areubiquitous among these stars, yet many fundamental mechanisms operatingin their magnetic fields still elude an interpretation through adetailed physical description. Stellar X-ray astronomy is thereforecontributing toward a deeper understanding of the generation of magneticfields in magnetohydrodynamic dynamos, the release of energy in tenuousastrophysical plasmas through various plasma-physical processes, and theinteractions of high-energy radiation with the stellar environment.Stellar X-ray emission also provides important diagnostics to study thestructure and evolution of stellar magnetic fields from the first daysof a protostellar life to the latest stages of stellar evolution amonggiants and supergiants. The discipline of stellar coronal X-rayastronomy has now reached a level of sophistication that makes tests ofadvanced theories in stellar physics possible. This development is basedon the rapidly advancing instrumental possibilities that today allow usto obtain images with sub-arcsecond resolution and spectra withresolving powers exceeding 1000. High-resolution X-ray spectroscopy has,in fact, opened new windows into astrophysical sources, and has played afundamental role in coronal research.
| Stellar Coronal Astronomy
Coronal astronomy is by now a fairly mature discipline, with a quartercentury having gone by since the detection of the first stellar X-raycoronal source (Capella), and having benefitted from a series of majororbiting observing facilities. Serveral observational characteristics ofcoronal X-ray and EUV emission have been solidly established throughextensive observations, and are by now common, almost text-book,knowledge. At the same time the implications of coronal astronomy forbroader astrophysical questions (e.g.Galactic structure, stellarformation, stellar structure, etc.) have become appreciated. Theinterpretation of stellar coronal properties is however still often opento debate, and will need qualitatively new observational data to bookfurther progress. In the present review we try to recapitulate our viewon the status of the field at the beginning of a new era, in which thehigh sensitivity and the high spectral resolution provided by Chandraand SMM-Newton will address new questions which were not accessiblebefore.
| Improved Astrometry and Photometry for the Luyten Catalog. II. Faint Stars and the Revised Catalog
We complete construction of a catalog containing improved astrometry andnew optical/infrared photometry for the vast majority of NLTT starslying in the overlap of regions covered by POSS I and by the secondincremental Two Micron All Sky Survey (2MASS) release, approximately 44%of the sky. The epoch 2000 positions are typically accurate to 130 mas,the proper motions to 5.5 mas yr-1, and the V-J colors to0.25 mag. Relative proper motions of binary components are measured to 3mas yr-1. The false-identification rate is ~1% for11<~V<~18 and substantially less at brighter magnitudes. Theseimprovements permit the construction of a reduced proper-motion diagramthat, for the first time, allows one to classify NLTT stars intomain-sequence (MS) stars, subdwarfs (SDs), and white dwarfs (WDs). We inturn use this diagram to analyze the properties of both our catalog andthe NLTT catalog on which it is based. In sharp contrast to popularbelief, we find that NLTT incompleteness in the plane is almostcompletely concentrated in MS stars, and that SDs and WDs are detectedalmost uniformly over the sky δ>-33deg. Our catalogwill therefore provide a powerful tool to probe these populationsstatistically, as well as to reliably identify individual SDs and WDs.
| Elemental Abundances and Mass Densities of Dust and Gas in the Local Interstellar Cloud
Observationally derived gas-phase abundances and appropriate assumptionsfor the total elemental abundances of dust and gas determine theelemental composition of dust and the elemental depletion from gas inthe interstellar medium (ISM). In addition to the elemental abundances,the total mass ratio of hydrogen atoms to dust grains per spatial volumeis a measure of the interaction between dust and gas in the ISM. Recentremote astronomical observations and in situ measurements provide theopportunity of estimating the elemental abundances and the hydrogengas-to-dust mass ratio of the Local Interstellar Cloud (LIC), in whichthe Sun is currently embedded. We show that the composition of dust inthe LIC is similar to that of cometary dust in the solar system,although the nitrogen abundance remains uncertain. Depletions ofelements from the LIC gas are consistent with measurements of warmneutral clouds in the Galactic disk, except for Mg and Si, which areheavily depleted in the LIC. Remote astronomical observations and insitu measurements give essentially the same value for the gas-to-dustmass ratio of the LIC, which is comparable to the average value of thediffuse ISM in the Galaxy. This indicates the association of dust withgas in the LIC, which is also inferred from the depletion pattern in theLIC. Neither the depletions of elements nor the gas-to-dust mass ratioshow evidence for severe grain destruction that would result from shockswith velocity ~1.5×107 cm s-1 as suggestedby a model that postulates the LIC to be a fragment of the expandingLoop I superbubble shell. Our results rather favor an alternative modelthat describes the origin of the LIC as being one of H I cloudletsexpelled from the interaction zone between the Loop I superbubble andthe Local Bubble, which encloses the LIC and similar clouds in the solarneighborhood.
| The 100 Brightest X-Ray Stars within 50 Parsecs of the Sun
Based on the Hipparcos and Tycho-2 astrometric catalogs and the ROSATsurveys, a sample of 100 stars most luminous in X-rays within or arounda distance of 50 pc is culled. The smallest X-ray luminosity in thesample, in units of 1029 ergs s-1, isLX=9.8 the strongest source in the solar neighborhood is IIPeg, a RS CVn star, at LX=175.8. With respect to the originof X-ray emission, the sample is divided into partly overlapping classesof pre-main-sequence, post-T Tauri, and very young ZAMS objects (typeXY), RS CVn-type binary stars (type RS), other active short-periodbinaries, including binary BY Dra-type objects (type XO), apparentlysingle or long-period binary active evolved stars (type XG), contactbinaries of WU UMa kind (type WU), apparently single or long-periodbinary variable stars of BY Dra kind (type BY), and objects of unknownnature (type X?). Chromospherically active, short-period binaries (RSand XO) make up 40% of the brightest X-ray emitters, followed by youngstars (XY) at 30% and unknown sources (X?) at 15%. The fraction ofspectroscopically single evolved X-ray emitters of spectral classes IVand III is quite large (10%). The sources identified as RS CVn-typestars (RS, 23 objects) are considerably stronger in X-ray than theXY-objects and the other active binaries (XO and WU, 20 objects). Sevenobjects have LX>100, all RS except one XY, viz., BO Mic. Onlyfive (22%) RS objects have LX<25, while only three (10%)XY stars have LX>25. Formally, the limit of LX=25could serve as a statistical criterion to differentiate RS and XY stars.However, the other short-period binaries (including eclipsing stars ofAlgol and β Lyr type) have a distribution of LX verysimilar to the XY objects. The contact binaries (WU) appear to be muchweaker in X-rays than their detached counterparts of RS type, but thesample of the former is too small (three objects) to reach a firmconclusion. Sources matched with giants (either single or in binaries)are found to be significantly harder, with only 7% of hardness ratiosbelow 0, than subgiants (66% of HR1<0) and dwarfs (59% of HR1<0).Almost all objects in the sample are binary or multiple stars; thefraction of components (FC), defined as the total number of componentsin all binary and multiple systems divided by the sum of the totalnumber of components and single stars, is at least 0.90. The FC for theXY objects reaches 0.81, and for the unknown type 0.89. About 70% of RSobjects have also visual or astrometric companions, which makes themhierarchical multiple systems. The RS objects (mostly old, evolvedstars) and the XY stars have quite different kinematics. While the RSobjects move at considerable velocities in apparently random directionswith respect to the local standard of rest, the young stars have smallerand orderly velocities and tend to comprise expanding mini-associationssuch as the β Pic and the Tucana groups. The majority of the youngX-ray active stars belong to the Pleiades stream with the meanheliocentric velocity (U,V,W)=(-9.6,-21.8,-7.7) km s-1.
| Doppler imaging of stellar surface structure. XX. The rapidly-rotating single K2-giant HD 31993 = V1192 Orionis
We present two Doppler images from two consecutive stellar rotations ofthe single K2-giant HD 31993. Each Doppler image is reconstructed fromspectra obtained within a single stellar rotation. With its 25-dayrotational period and a radius of ~18 solar radii, HD 31993 isconsidered a very rapidly rotating star and thus allows the applicationof the Doppler-imaging technique, despite the unusually long period. Allmaps reveal 7 isolated, predominantly low-latitude spots with atemperature difference, photosphere minus spot, of just ~200 K. No polarspot or high-latitude activity above, say, +60° is seen. A largewarm feature is detected at high latitude and is believed to be real.These spots act as tracers for a cross correlation analysis and yield aclear signature of anti-solar differential surface rotation, i.e. thepolar regions rotating faster than the equator, with alpha =0.125+/-0.05 corresponding to a lap time of ~200 days. A detailed parameterstudy is carried out to verify the reality of the HD 31993 maps.
| A study of the Mg II 2796.34 Å emission line in late-type normal and RS CVn stars
We carry out an analysis of the Mg II 2796.34 Å emission line inRS CVn stars and make a comparison with the normal stars studied in aprevious paper (Paper I). The sample of RS CVn stars consists of 34objects with known HIPPARCOS parallaxes and observed at high resolutionwith IUE. We confirm that RS CVn stars tend to possess wider Mg II linesthan normal stars having the same absolute visual magnitude. However, wecould not find any correlation between the logarithmic line width logWdeg and the absolute visual magnitude MV (theWilson-Bappu relationship) for these active stars, contrary to the caseof normal stars addressed in Paper I. On the contrary, we find that astrong correlation exists in the (MV, log LMg II)plane (LMg II is the absolute flux in the line). In thisplane, normal and RS CVn stars are distributed along two nearly parallelstraight lines with RS CVn stars being systematically brighter by ~1dex. Such a diagram provides an interesting tool to discriminate activefrom normal stars. We finally analyse the distribution of RS CVn and ofnormal stars in the (log LMg II, log Wdeg) plane,and find a strong linear correlation for normal stars, which can be usedfor distance determinations.
| Reprocessing the Hipparcos Intermediate Astrometric Data of spectroscopic binaries. II. Systems with a giant component
By reanalyzing the Hipparcos Intermediate Astrometric Data of a largesample of spectroscopic binaries containing a giant, we obtain a sampleof 29 systems fulfilling a carefully derived set of constraints andhence for which we can derive an accurate orbital solution. Of these,one is a double-lined spectroscopic binary and six were not listed inthe DMSA/O section of the catalogue. Using our solutions, we derive themasses of the components in these systems and statistically analyzethem. We also briefly discuss each system individually.Based on observations from the Hipparcos astrometric satellite operatedby the European Space Agency (ESA 1997) and on data collected with theSimbad database.
| Asymptotic dynamos in late-type stars
The behaviour of a simple thin-shell α-Ω dynamo model isconsidered in the asymptotic regime, characterized by dynamo numbersmuch larger than the critical ones, in order to derive scalingrelationships connecting the properties of dynamo waves with globalstellar parameters. The proposed approach is applied to stellar modelsof subgiant and giant stars from K0IV to K1III spectral types in theHertzsprung-Russell diagram, to predict some characteristics of activitycycles in very active stars. We found that the strength of the dynamoaction in such stars is higher than in the Sun. Therefore, largermagnetic field energy and larger spot filling factors are expected, inagreement with observations. The periods of stellar cycles are alsoestimated and compared with observations. The characteristic times ofmigration of the star-spot belts relative to the cycle period, namelythe Hale number, together with the ratio of toroidal to poloidal dynamomagnetic fields are estimated. From our simplified analysis we can onlyderive general trends, but cannot perform a direct comparison with theobserved properties of particular active stars. These general trendsindicate that the cycle periods have a large spread for stars with lowrotation rates (~1-5 times the solar one), while they tend to besaturated for stars with high rotation rates (~5-15 times solar), forwhich the periods range from 10 to 20 yr. For such stars, Hale numbersrange from 1.5 to approximately 4 (the Hale number for the Sun isapproximately 1.1), denoting the possible existence of cycles withdifferent periodicities present simultaneously, the ratio of toroidal topoloidal dynamo fields tends to become smaller for increasing rotationrates, indicating a transition from the α-Ω to theα2-Ω type of dynamo. Moreover, the magnetic fieldfilling factors tend to become larger for faster rotation rates, thoughthe effect of the convection zone depth should not be neglected. Ourresults show a reasonable agreement with available observations of asample of active stars we have considered.
|
Új cikk hozzáadása
Kapcsolódó hivatkozások
- - (nincs kapcsolódó hivatkozás) -
Új link hozzáadása
Besorolás csoportokba:
|
Pozíciós és asztrometriai adatok
| Csillagkép: | Ikrek |
| Rektaszcenzió: | 07h43m18.70s |
| Deklináció: | +28°53'01.0" |
| Vizuális fényesség: | 4.28 |
| Távolság: | 37.481 parszek |
| RA sajátmozgás: | 62.6 |
| Dec sajátmozgás: | -230.8 |
| B-T magnitude: | 5.675 |
| V-T magnitude: | 4.369 |
Katalógusok és elnevezések:
|
INHABITED SKY
