Contenuti
Immagini
Carica la tua immagine
DSS Images Other Images
Articoli relazionati
Coronal Emission Measures and Abundances for Moderately Active K Dwarfs Observed by Chandra
We have used Chandra to resolve the nearby 70 Oph (K0 V+K5 V) and 36 Oph(K1 V+K1 V) binary systems for the first time in X-rays. The LETG/HRC-Sspectra of all four of these stars are presented and compared with anarchival LETG spectrum of another moderately active K dwarf, ɛEri. Coronal densities are estimated from O VII line ratios and emissionmeasure distributions are computed for all five of these stars. We seeno substantial differences in coronal density or temperature among thesestars, which is not surprising considering that they are all early Kdwarfs with similar activity levels. However, we do see significantdifferences in coronal abundance patterns. Coronal abundance anomaliesare generally associated with the first ionization potential (FIP) ofthe elements. On the Sun, low-FIP elements are enhanced in the coronarelative to high-FIP elements, the so-called FIP effect. Differentlevels of FIP effect are seen for our stellar sample, ranging from 70Oph A, which shows a prominent solar-like FIP effect, to 70 Oph B, whichhas no FIP bias at all or possibly even a weak inverse FIP effect. Thestrong abundance difference exhibited by the two 70 Oph stars isunexpected considering how similar these stars are in all other respects(spectral type, age, rotation period, X-ray flux). It will be difficultfor any theoretical explanation for the FIP effect to explain how twostars so similar in all other respects can have coronae with differentdegrees of FIP bias. Finally, for the stars in our sample exhibiting aFIP effect, a curious difference from the solar version of thephenomenon is that the data seem to be more consistent with the high-FIPelements being depleted in the corona rather than with a low-FIPenhancement.
| 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
| Dwarfs in the Local Region
We present lithium, carbon, and oxygen abundance data for a sample ofnearby dwarfs-a total of 216 stars-including samples within 15 pc of theSun, as well as a sample of local close giant planet (CGP) hosts (55stars) and comparison stars. The spectroscopic data for this work have aresolution of R~60,000, a signal-to-noise ratio >150, and spectralcoverage from 475 to 685 nm. We have redetermined parameters and derivedadditional abundances (Z>10) for the CGP host and comparison samples.From our abundances for elements with Z>6 we determine the meanabundance of all elements in the CGP hosts to range from 0.1 to 0.2 dexhigher than nonhosts. However, when relative abundances ([x/Fe]) areconsidered we detect no differences in the samples. We find nodifference in the lithium contents of the hosts versus the nonhosts. Theplanet hosts appear to be the metal-rich extension of local regionabundances, and overall trends in the abundances are dominated byGalactic chemical evolution. A consideration of the kinematics of thesample shows that the planet hosts are spread through velocity space;they are not exclusively stars of the thin disk.
| Abundance difference between components of wide binaries. II. The southern sample
Aims.We present high-precision iron abundance differences for 33 widebinaries with similar components. They were observed with the FEROSspectrograph at ESO, looking for abundance anomalies due to theingestion of metal rich material of a planetary origin. Methods:.An optimized data analysis technique and the high quality of thespectra allowed us to achieve an error of about 0.02 dex for pairs withsmall temperaure differences. Results: . We found one case (HIP64030 = HD 113984) with a large (0.27 dex) abundance difference. Theprimary of this binary appears to be a blue straggler, and the abundancedifference might be due to the peculiar evolution of the star. A fewother pairs show small abundance differences (≤0.09 dex). In a fewcases these differences suggest the ingestion of a small amount of metalrich material, but in others they are likely spurious, because of thelarge temperature difference, high level of magnetic activity, ordifferent evolutionary phases between the components. Some cases ofabundance differences involving pairs with warm (Teff ≥6000 K) primaries might be due to the diffusion of heavy elements;dedicated theoretical models for the stellar parameters of the targetswould be welcome. Conclusions: . This study confirms ourpreliminary result based on analysis of 23 pairs (Desidera et al. 2004)that the occurrence of large alterations in stellar abundances caused bythe ingestion of metal rich, rocky material is not a common event. Forat least 65% of the pairs with components warmer than 5500 K, the limitson the amount of rocky material accreted by the program stars arecomparable to the estimates of rocky material accreted by the Sun duringits main-sequence lifetime.
| Spectroscopic characterization of a sample of southern visual binaries
Aims.We present the spectroscopic characterization of 56 pairs of visualbinaries with similar components, based on high resolution spectraacquired with FEROS at ESO La Silla. Methods: .For all stars, wemeasured radial and rotational velocities and CaII H&K emission. Results: .Five previously unknown double lined spectroscopic binarieswere found. Six other pairs show velocity differences that are notcompatible with the orbital motion of the wide pair, indicating thepresence of further companion(s) in the system. The fraction of visualbinaries that contain additional spectroscopic components is27±10%, compatible with other literature estimates. The ages ofthe components of the pairs derived from chromospheric activitytypically show apparent differences of about 0.2 dex. A few pairs show arather large difference in activity level, but in most cases this isconsistent with the variability of chromospheric emission observed forthe Sun along its magnetic cycle.
| Deflection of the Interstellar Neutral Hydrogen Flow Across the Heliospheric Interface
Using an absorption cell, we measured the Doppler shifts of theinterstellar hydrogen resonance glow to show the direction of theneutral hydrogen flow as it enters the inner heliosphere. The neutralhydrogen flow is found to be deflected relative to the helium flow byabout 4°. The most likely explanation of this deflection is adistortion of the heliosphere under the action of an ambientinterstellar magnetic field. In this case, the helium flow vector andthe hydrogen flow vector constrain the direction of the magnetic fieldand act as an interstellar magnetic compass.
| Tentative Identification of Interstellar Dust in the Magnetic Wall of the Heliosphere
Observations of the weak polarization of light from nearby stars,reported by Tinbergen, are consistent with polarization by small (radius<0.14 μm), interstellar dust grains entrained in the magnetic wallof the heliosphere. The region of maximum polarization is towardecliptic coordinates (λ, β)~(295deg,0deg), corresponding to (l, b) = (20°, -21°). Thedirection of maximum polarization is offset along the ecliptic longitudeby ~35° from the nose of the heliosphere and extends to low eclipticlatitudes. An offset is also seen between the region with thebest-aligned dust grains, λ~281deg-330deg,and the upwind direction of the undeflected large grains,λ~259deg, β~+8deg, which are observedby Ulysses and Galileo to be flowing into the heliosphere. In thealigned-grain region, the strength of polarization anticorrelates withecliptic latitude, indicating that the magnetic wall is predominantly atnegative ecliptic latitudes. An extension of the magnetic wall toβ<0deg, formed by the interstellar magnetic fieldBIS draped over the heliosphere, is consistent withpredictions by Linde (1998). A consistent interpretation follows if themaximum-polarization region traces the heliosphere magnetic wall in adirection approximately perpendicular to BIS, while theregion of best-aligned dust samples the region where BISdrapes smoothly over the heliosphere with maximum compression. Thesedata are consistent with BIS being tilted by 60° withrespect to the ecliptic plane and parallel to the Galactic plane.Interstellar dust grains captured in the heliosheath may also introducea weak, but important, large-scale contaminant for the cosmic microwavebackground signal with a symmetry consistent with the relative tilts ofBIS and the ecliptic.
| New Mass-Loss Measurements from Astrospheric Lyα Absorption
Measurements of stellar mass-loss rates are used to assess how windstrength varies with coronal activity and age for solar-like stars. Massloss generally increases with activity, but we find evidence that windssuddenly weaken at a certain activity threshold. Very active stars areoften observed to have polar starspots, and we speculate that themagnetic field geometry associated with these spots may be inhibitingthe winds. Our inferred mass-loss/age relation represents an empiricalestimate of the history of the solar wind. This result is important forplanetary studies as well as solar/stellar astronomy, since solar winderosion may have played an important role in the evolution of planetaryatmospheres.
| Abundance trends in kinematical groups of the Milky Way's disk
We have compiled a large catalogue of metallicities and abundance ratiosfrom the literature in order to investigate abundance trends of severalalpha and iron peak elements in the thin disk and the thick disk of theGalaxy. The catalogue includes 743 stars with abundances of Fe, O, Mg,Ca, Ti, Si, Na, Ni and Al in the metallicity range -1.30 < [Fe/H]< +0.50. We have checked that systematic differences betweenabundances measured in the different studies were lower than randomerrors before combining them. Accurate distances and proper motions fromHipparcos and radial velocities from several sources have been retreivedfor 639 stars and their velocities (U, V, W) and galactic orbits havebeen computed. Ages of 322 stars have been estimated with a Bayesianmethod of isochrone fitting. Two samples kinematically representative ofthe thin and thick disks have been selected, taking into account theHercules stream which is intermediate in kinematics, but with a probabledynamical origin. Our results show that the two disks are chemicallywell separated, they overlap greatly in metallicity and both showparallel decreasing alpha elements with increasing metallicity, in theinterval -0.80 < [Fe/H] < -0.30. The Mg enhancement with respectto Fe of the thick disk is measured to be 0.14 dex. An even largerenhancement is observed for Al. The thick disk is clearly older than thethin disk with tentative evidence of an AMR over 2-3 Gyr and a hiatus instar formation before the formation of the thin disk. We do not observea vertical gradient in the metallicity of the thick disk. The Herculesstream has properties similar to that of the thin disk, with a widerrange of metallicity. Metal-rich stars assigned to the thick disk andsuper-metal-rich stars assigned to the thin disk appear as outliers inall their properties.
| New Hipparcos-based Parallaxes for 424 Faint Stars
We present a catalog of 424 common proper-motion companions to Hipparcosstars with good (>3 σ) parallaxes, thereby effectively providingnew parallaxes for these companions. Compared with typical stars in theHipparcos catalog, these stars are substantially dimmer. The catalogincludes 20 white dwarfs and an additional 29 stars withMV>14, the great majority of the latter being M dwarfs.
| 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.
| On the Possibility of a Strong Magnetic Field in the Local Interstellar Medium
We analyze the consequences of the local interstellar magnetic fieldbeing almost 3 times larger than the Galactic average (ordered) field onthe structure of the heliospheric interface in the axisymmetric casewhen the field is parallel to the relative direction of motion betweenthe local interstellar medium (LISM) and the Sun. A field of suchstrength is expected to exist in the Local Interstellar Cloud, if thelatter condensed from material inside a magnetic flux tube reboundingfrom the wall of the Local Bubble cavity. The analysis is performedusing a newly developed multifluid neutral MHD model. We show that thebow shock ahead of the heliopause still exists for supersonic andsub-Alfvénic LISM parameters. Our results agree well with theobservations of the Lyα absorption spectra and yield positions ofthe termination shock and the heliopause similar to those obtained fromthe standard super-Alfvénic model.
| S4N: A spectroscopic survey of stars in the solar neighborhood. The Nearest 15 pc
We report the results of a high-resolution spectroscopic survey of allthe stars more luminous than M_V = 6.5 mag within 14.5 pc from the Sun.The Hipparcos catalog's completeness limits guarantee that our survey iscomprehensive and free from some of the selection effects in othersamples of nearby stars. The resulting spectroscopic database, which wehave made publicly available, includes spectra for 118 stars obtainedwith a resolving power of R ≃ 50 000, continuous spectral coveragebetween 362-921 nm, and typical signal-to-noise ratios in therange 150-600. We derive stellar parameters and perform a preliminaryabundance and kinematic analysis of the F-G-K stars in the sample. Theinferred metallicity ([Fe/H]) distribution is centered at about -0.1dex, and shows a standard deviation of 0.2 dex. A comparison with largersamples of Hipparcos stars, some of which have been part of previousabundance studies, suggests that our limited sample is representative ofa larger volume of the local thin disk. We identify a number ofmetal-rich K-type stars which appear to be very old, confirming theclaims for the existence of such stars in the solar neighborhood. Withatmospheric effective temperatures and gravities derived independentlyof the spectra, we find that our classical LTE model-atmosphere analysisof metal-rich (and mainly K-type) stars provides discrepant abundancesfrom neutral and ionized lines of several metals. This ionizationimbalance could be a sign of departures from LTE or inhomogeneousstructure, which are ignored in the interpretation of the spectra.Alternatively, but seemingly unlikely, the mismatch could be explainedby systematic errors in the scale of effective temperatures. Based ontransitions of majority species, we discuss abundances of 16 chemicalelements. In agreement with earlier studies we find that the abundanceratios to iron of Si, Sc, Ti, Co, and Zn become smaller as the ironabundance increases until approaching the solar values, but the trendsreverse for higher iron abundances. At any given metallicity, stars witha low galactic rotational velocity tend to have high abundances of Mg,Si, Ca, Sc, Ti, Co, Zn, and Eu, but low abundances of Ba, Ce, and Nd.The Sun appears deficient by roughly 0.1 dex in O, Si, Ca, Sc, Ti, Y,Ce, Nd, and Eu, compared to its immediate neighbors with similar ironabundances.Based on observations made with the 2.7 m telescope at the McDonaldObservatory of the University of Texas at Austin (Texas), and the 1.52 mtelescope at the European Southern Observatory (La Silla, Chile) underthe agreement with the CNPq/Observatorio Nacional (Brazil).Tables 3-5 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/420/183
| Atomic Deuterium/Hydrogen in the Galaxy
An accurate value of the D/H ratio in the local interstellar medium(LISM) and a better understanding of the D/H variations with position inthe Galactic disk and halo are vitally important questions as theyprovide information on the primordial D/H ratio in the Galaxy at thetime of the protosolar nebula, and the amount of astration and mixing inthe Galaxy over time. Recent measurements have been obtained with UVspectrographs on FUSE, HST, and IMAPS using hot white dwarfs, OB stars,and late-type stars as background light sources against which to measureabsorption by D and H in the interstellar medium along the lines ofsight. Recent analyses of FUSE observations of seven white dwarfs andsubdwarfs provide a weighted mean value of D/H = (1.52±0.08)× 10‑5 (15.2 ± 0.8 ppm), consistent withthe value of (1.50 ± 0.10) × 10‑5 (15.0± 1.0 ppm) obtained from analysis of lines of sight toward nearbylate-type stars. Both numbers refer to the ISM within about 100 pc ofthe Sun, which samples warm clouds located within the Local Bubble.Outside of the Local Bubble at distances of 200 to 500 pc, analyses offar-UV spectra obtained with the IMAPS instrument indicate a much widerrange of D/H ratios between 0.8 to 2.2 ppm. This portion of the Galacticdisk provides information on inhomogeneous astration in the Galaxy.
| Local Interstellar Matter: The Apex Cloud
Several nearby individual low column density interstellar cloudlets havebeen identified previously on the basis of kinematical features evidentin high-resolution Ca+ observations near the Sun. One ofthese cloudlets, the ``Apex Cloud'' (AC), is within 5 pc of the Sun inthe solar apex direction. The question of which interstellar cloud willconstitute the next Galactic environment of the Sun can, in principle,be determined from cloudlet velocities. The interstellar absorptionlines toward α Cen (the nearest star) are consistent withinmeasurement uncertainties with the projected ``G'' cloud (GC) and ACvelocities, and also with the velocity of the cloud inside of the solarsystem (the local interstellar cloud [LIC]), provided a small velocitygradient is present in the LIC. The high GC column density towardα Oph compared to α Aql suggests that α Aql may beembedded in the GC so that the AC would be closer to the Sun than theGC. This scenario favors the AC as the next cloud to be encountered bythe Sun, and the AC would have a supersonic velocity with respect to theLIC. The weak feature at the AC velocity toward 36 Oph suggests that theAC cloud is either patchy or does not extend to this direction.Alternatively, if the GC is the cloud that is foreground to α Cen,the similar values for N(H0) in the GC components towardα Cen and 36 Oph indicate this cloud is entirely contained withinthe nearest ~1.3 pc, and the Ca+ GC data toward α Ophwould then imply a cloud volume density of ~5 cm-3, withdramatic consequences for the heliosphere in the near future.
| Selection criteria for targets of asteroseismic campaigns
Various dedicated satellite projects are underway or in advanced stagesof planning to perform high-precision, long duration time seriesphotometry of stars, with the purpose of using the frequencies ofstellar oscillations to put new constraints on the internal structure ofstars. It is known (cf. \cite{Bro+94}) that the effectiveness ofoscillation frequencies in constraining stellar model parameters issignificantly higher if classical parameters such as effectivetemperature and luminosity are known with high precision. In order tooptimize asteroseismic campaigns it is therefore useful to selecttargets from among candidates for which good spectroscopic andastrometric data already exists. This paper presents selection criteria,as well as redeterminations of stellar luminosity and reddening forstars satisfying these criteria.
| Kinematics of Hipparcos Visual Binaries. II. Stars with Ground-Based Orbital Solutions
This paper continues kinematical investigations of the Hipparcos visualbinaries with known orbits. A sample, consisting of 804 binary systemswith orbital elements determined from ground-based observations, isselected. The mean relative error of their parallaxes is about 12% andthe mean relative error of proper motions is about 4%. However, even 41%of the sample stars lack radial velocity measurements. The computedGalactic velocity components and other kinematical parameters are usedto divide the stars with known radial velocities into kinematical agegroups. The majority (92%) of binaries from the sample are thin diskstars, 7.6% have thick disk kinematics and only two binaries have halokinematics. Among them, the long-period variable Mira Ceti has a verydiscordant {Hipparcos} and ground-based parallax values. From the wholesample, 60 stars are ascribed to the thick disk and halo population.There is an urgent need to increase the number of the identified halobinaries with known orbits and substantially improve the situation withradial velocity data for stars with known orbits.
| Elemental Abundances and Ionization States within the Local Interstellar Cloud Derived from Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer Observations of the Capella Line of Sight
We use ultraviolet spectra of Capella from the Hubble Space Telescopeand Far Ultraviolet Spectroscopic Explorer satellites to studyinterstellar absorption lines from the Local Interstellar Cloud (LIC).Measurements of these lines are used to empirically determine theionization states of carbon, nitrogen, and silicon in the LIC, forcomparison with the predictions of theoretical photoionization models.We find that the observed ionization states are consistent withpreviously published photoionization predictions. Total abundances aredetermined for the elements mentioned above, and others, for comparisonwith solar abundances. Magnesium, aluminum, silicon, and iron are alldepleted by at least a factor of 10 toward Capella. The abundances ofcarbon, nitrogen, and oxygen are essentially solar, although the errorbars are large enough to also allow depletions of about a factor of 2for these elements. Based on observations made with the NASA-CNES-CSAFar Ultraviolet Spectroscopic Explorer. FUSE is operated for NASA by theJohns Hopkins University under NASA contract NAS5-32985. Also based onobservations with the NASA/ESA Hubble Space Telescope, obtained from theData Archive at the Space Telescope Science Institute, which is operatedby the Association of Universities for Research in Astronomy, Inc.,under NASA contract NAS5-26555.
| Measured Mass-Loss Rates of Solar-like Stars as a Function of Age and Activity
Collisions between the winds of solar-like stars and the localinterstellar medium result in a population of hot hydrogen gassurrounding these stars. Absorption from this hot H I can be detected inhigh-resolution Lyα spectra of these stars from the Hubble SpaceTelescope. The amount of absorption can be used as a diagnostic for thestellar mass-loss rate. We present new mass-loss rate measurementsderived in this fashion for four stars (ɛ Eri, 61 Cyg A, 36 Oph AB,and 40 Eri A). Combining these measurements with others, we study howmass loss varies with stellar activity. We find that for the solar-likeGK dwarfs, the mass loss per unit surface area is correlated with X-raysurface flux. Fitting a power law to this relation yieldsM~F1.15+/-0.20X. The active M dwarf Proxima Cenand the very active RS CVn system λ And appear to be inconsistentwith this relation. Since activity is known to decrease with age, theabove power-law relation for solar-like stars suggests that mass lossdecreases with time. We infer a power-law relation ofM~t-2.00+/-0.52. This suggests that the solar wind may havebeen as much as 1000 times more massive in the distant past, which mayhave had important ramifications for the history of planetaryatmospheres in our solar system, that of Mars in particular. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contractNAS5-26555.
| A Coronagraphic Survey for Companions of Stars within 8 Parsecs
We present the technique and results of a survey of stars within 8 pc ofthe Sun with declinations δ>-35° (J2000.00). The survey,designed to find without color bias faint companions, consists ofoptical coronagraphic images of the 1' field of view centered on eachstar and infrared direct images with a 32" field of view. The imageswere obtained through the optical Gunn r and z filters and the infraredJ and K filters. The survey achieves sensitivities up to 4 absolutemagnitudes fainter than the prototype brown dwarf, Gliese 229B. However,this sensitivity varies with the seeing conditions, the intrinsicbrightness of the star observed, and the angular distance from the star.As a result, we tabulate sensitivity limits for each star in the survey.We used the criterion of common proper motion to distinguish companionsand to determine their luminosities. In addition to the brown dwarf Gl229B, we have identified six new stellar companions of the sample stars.Since the survey began, accurate trigonometric parallax measurements formost of the stars have become available. As a result, some of the starswe originally included should no longer be included in the 8 pc sample.In addition, the 8 pc sample is incomplete at the faint end of the mainsequence, complicating our calculation of the binary fraction of browndwarfs. We assess the sensitivity of the survey to stellar companionsand to brown dwarf companions of different masses and ages.
| AB Dor in '94. I. Hubble Space Telescope Goddard High Resolution Spectrogaph Observations of the Quiescent Chromosphere of an Active Star
We analyze Hubble Space Telescope/Goddard High Resolution Spectrographspectra of AB Doradus, the prototypical, ultrarapidly rotating K dwarf.We observed chromospheric (Mg II) and transition-region (C II, Si IV, CIV, and N V) lines periodically throughout the stellar rotation periodand provide a low-dispersion stellar atlas of 78 emission lines. Thequiescent line profiles of the chromospheric and transition-region linesshow narrow cores superposed on very broad wings. The broad wings of theMg II k and h lines and of the transition-region lines can be explainedby emission from gas corotating with the star and extending out to nearthe Keplerian corotation radius (2.8 stellar radii). While this is not aunique solution, it is consistent with previous studies of Hαemission, which are naturally explained by large corotating prominences.We find no evidence for rotational modulation of the emission-linefluxes. The density diagnostics suggest that the transition region isformed at constant pressure, with an electron density of2-3×1012 cm-3 at a temperature of3×104 K. The electron pressure is about 100 timeslarger than that for the quiet Sun. The emission-measure distributionshows a minimum between logT=5 and 5.5. The Mg II line exhibits threeinterstellar absorption components along the 15 pc line of sight. Weidentify the lowest velocity component with the G Cloud, but the othercomponents are not identified with any interstellar clouds previouslydetected from other lines of sight.
| The long-period companions of multiple stars tend to have moderate eccentricities
We examined the statistics of an angle gamma between the radius vectorof a visual companion of a multiple star and the vector of its apparentrelative motion in the system. Its distribution f(gamma ) is related tothe orbital eccentricity distribution in the investigated sample. Wefound that for the wide physical subsystems of the 174 objects from theMultiple Star Catalogue f(gamma ) is bell-shaped. The Monte-Carlosimulations have shown that our f(gamma ) corresponds to the populationof the moderate-eccentricity orbits and is not compatible with thelinear distribution f(e)=2e which follows from stellar dynamics andseems to hold for wide binaries. This points to the absence of highlyelongated orbits among the outer subsystems of multiple stars. Theconstraint of dynamical stability of triple systems is not sufficient toexplain the ``rounded-off'' outer orbits; instead, we speculate that itcan result from the angular momentum exchange in multiple systems duringtheir early evolution.
| Hydrogen Lyα Absorption Predictions by Boltzmann Models of the Heliosphere
We use self-consistent kinetic/hydrodynamic models of the heliosphere topredict H I Lyα absorption profiles for various lines of sightthrough the heliosphere. These results are compared with Lyαabsorption lines of six nearby stars observed by the Hubble SpaceTelescope. The directions of these lines of sight range from nearlyupwind (36 Oph) to nearly downwind (ɛ Eri). Only three of theLyα spectra (36 Oph, α Cen, and Sirius) actually showevidence for the presence of heliospheric absorption, which is blendedwith the ubiquitous interstellar absorption, but the other three spectrastill provide useful upper limits for the amount of heliosphericabsorption for those lines of sight. Most of our models use a Boltzmannparticle code for the neutrals, allowing us to estimate neutral velocitydistributions throughout the heliosphere, from which we compute modelLyα absorption profiles. In comparing these models with the data,we find they predict too much absorption in sidewind and downwinddirections, especially when higher Mach numbers are assumed for theinterstellar wind. Models created assuming different values of theinterstellar temperature and proton density fail to improve theagreement. Somewhat surprisingly, a model that uses a multifluidtreatment of the neutrals rather than the Boltzmann particle code ismore consistent with the data, and we speculate as to why this may bethe case.
| Heliospheric, Astrospheric, and Interstellar Lyα Absorption toward 36 Ophiuchi
We use high-resolution UV spectra taken by the Space Telescope ImagingSpectrograph instrument on board the Hubble Space Telescope to study the5.5 pc line of sight to the K0 V star 36 Oph A. The one interstellarcomponent detected for this line of sight has a velocity inconsistentwith the local interstellar cloud (LIC) flow vector but consistent withthe flow vector of the so-called G cloud, a very nearby warm cloud inthe Galactic center direction. From our data, we measure the followingvalues for the interstellar temperature, nonthermal velocity, H I columndensity, and D/H value: T=5900+/-500 K, ξ=2.2+/-0.2 kms-1, logNH=17.85+/-0.15, andD/H=(1.5+/-0.5)×10-5. The temperature of the G cloud issomewhat lower than that of the LIC, and Mg and Fe depletions are alsolower, but the D/H value appears to be the same. Based on upper limitsfor the LIC absorption, we estimate the distance to the edge of the LICto be dedge<0.19 pc, which the Sun will reach intedge<7400 yr. The H I Lyα absorption line hasproperties inconsistent with those of the other absorption lines,indicating the presence of one or more absorption components not seen inthe other lines. We present evidence that hot neutral hydrogen local toboth the Sun and the star is responsible for the excess Lyαabsorption. This hot H I is created by the interaction between the ISMand the winds of the Sun and 36 Oph A. The observed line of sight liesonly 12° from the upwind direction of the LIC flow vector, wherehydrodynamic models of the heliosphere suggest that heliospheric H Iabsorption should be particularly prominent. The properties of theheliospheric absorption (T=38,000+/-8000 K andlogNH=14.6+/-0.3) are consistent with previous measurementsof this absorption for the α Cen line of sight 52° from theupwind direction. Based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS5-26555.
| CCD and micrometric observations of visual double stars
Relative positions and separations for 65 visual binaries are given. CCDframes were taken at the 1-m coudetelescope and micrometric observationswere carried out at the 65-cm refractor, both at the Venezuelan NationalObservatory. The visual double stars observed were selected from theRamon Maria Aller Observatory program list. In addition, we alsoobserved several long period double stars with poorly determined orbitalelements.
| One- versus Two-Shock Heliosphere: Constraining Models with Goddard High Resolution Spectrograph Ly alpha Spectra toward alpha Centauri
Redshifted Ly alpha absorption toward alpha Cen has been interpreted byLinsky & Wood and Frisch et al. as evidence for deceleratedinterstellar hydrogen piled up on the upstream side of the heliosphere.We utilize newly developed two-dimensional multifluid models of thesolar wind interaction with the interstellar material to corroboratethis interpretation by synthesizing the Ly alpha absorption profilepredicted for this "hydrogen wall." Both subsonic and supersonic inflowinto the heliosphere are considered, corresponding to one-shock andtwo-shock global morphologies, respectively. We find that these twoextremes give observably different redward absorption characteristics inthe Ly alpha profiles, and our preliminary conclusion is that the Lyalpha profiles seen toward alpha Cen favor a barely subsonic model (Machnumber 0.9). For such a model to hold, additional interstellar pressureterms, such as cosmic-ray or magnetic pressures, must contribute. Tomake this conclusion more certain, an extended-model parameter survey isrequired, coupled with Ly alpha data along other lines of sight.
| Photovisual Magnitude Differences for 169 Double Stars
Photovisual magnitude differences determined from multi-exposurephotographic plates for 169 double stars are presented. The separationsrange from 1.5'' to 113\arcsec, and the photovisual magnitudedifferences vary from 0.03 to 6.14 magnitudes. The internal mean errorof a single magnitude difference estimate is +/-0.064 magnitude.
| Photographic observations of visual double stars.
We present the results of photographic observations of 399 visual doublestars of the Hipparcos Input Catalogue (HIC), observed in the years 1980and 1990-1991 with the 60-cm double-refractor of the Bosscha Observatoryat Lembang, Java.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
| Infrared sources in the neighbourhood of the Sun.
Not Available
|
Sottometti un nuovo articolo
Link relazionati
Sottometti un nuovo link
Membro dei seguenti gruppi:
|
Osservazione e dati astrometrici
Cataloghi e designazioni:
|
