Садржај
Слике
| - Није пронађена ниједна слика - |
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Accurate magnetic field measurements of Vega-like stars and Herbig Ae/Be stars
We obtained accurate circular spectropolarimetric observations of asample of Vega-like and Herbig Ae/Be stars with FORS 1 at the VLT in anattempt to detect their magnetic fields. No magnetic field could bediagnosed in any Vega-like star. The most accurate determination of amagnetic field, at 2.6 σ level, was performed for the Vega-likestar ι Cen, for which we measured =-77±30 G. Inthe prototype of Vega-like stars, the star β Pictoris, which showsconspicuous signs of chromospheric activity, a longitudinal magneticfield is measured only at ~1.5 σ level. We diagnosed alongitudinal magnetic field for the first time at a level higher than 3σ for the two Herbig Ae stars HD 31648 and HD 144432 and confirmthe existence of a previously detected magnetic field in a third HerbigAe star, HD 139614. Finally, we discuss the discovery of distinctiveZeeman features in the unusual Herbig Ae star HD 190073, where the Ca IIdoublet displays several components in both H and K lines. From themeasurement of circular polarization in all Balmer lines from Hβ toH8, we obtain =+26±34 G. However, using only the Ca IIH and K lines for the measurement of circular polarization, we are ableto diagnose a longitudinal magnetic field at 2.8 σ level,=+84±30 G.
| Probable nonradial g-mode pulsation in early A-type stars
Context: . Aims: .Asteroseismology of early A-type stars could bea new tool to test stellar convection theories. Methods: .A surveyfor line profile variability in early A-type stars has been performed inorder to detect nonradial pulsation signatures. Results: .The starHR 6139, with spectral type A2V and estimated T{eff}=8800 K,shows evident line profile variations that can be explained byoscillations in prograde g-modes. This feature and the known photometricvariability are similar to those observed in the Slowly Pulsating B-typestars. However HR 6139 is much cooler than the cool border of theinstability strip of such variables, and it is hotter than the blue edgeof the δ Scuti instability strip. There are indications of a tinyvariability also in other four objects, whose nature is not yetclear. Conclusions: .
| Decay of Planetary Debris Disks
We report new Spitzer 24 μm photometry of 76 main-sequence A-typestars. We combine these results with previously reported Spitzer 24μm data and 24 and 25 μm photometry from the Infrared SpaceObservatory and the Infrared Astronomy Satellite. The result is a sampleof 266 stars with mass close to 2.5 Msolar, all detected toat least the ~7 σ level relative to their photospheric emission.We culled ages for the entire sample from the literature and/orestimated them using the H-R diagram and isochrones; they range from 5to 850 Myr. We identified excess thermal emission using an internallyderived K-24 (or 25) μm photospheric color and then compared allstars in the sample to that color. Because we have excluded stars withstrong emission lines or extended emission (associated with nearbyinterstellar gas), these excesses are likely to be generated by debrisdisks. Younger stars in the sample exhibit excess thermal emission morefrequently and with higher fractional excess than do the older stars.However, as many as 50% of the younger stars do not show excessemission. The decline in the magnitude of excess emission, for thosestars that show it, has a roughly t0/time dependence, witht0~150 Myr. If anything, stars in binary systems (includingAlgol-type stars) and λ Boo stars show less excess emission thanthe other members of the sample. Our results indicate that (1) there issubstantial variety among debris disks, including that a significantnumber of stars emerge from the protoplanetary stage of evolution withlittle remaining disk in the 10-60 AU region and (2) in addition, it islikely that much of the dust we detect is generated episodically bycollisions of large planetesimals during the planet accretion end game,and that individual events often dominate the radiometric properties ofa debris system. This latter behavior agrees generally with what we knowabout the evolution of the solar system, and also with theoreticalmodels of planetary system formation.
| A near-infrared stellar spectral library: I. H-band spectra.
This paper presents the H band near-infrared (NIR) spectral library of135 solar type stars covering spectral types O5-M3 and luminosityclasses I-V as per MK classification. The observations were carried outwith 1.2 meter Gurushikhar Infrared Telescope (GIRT), at Mt. Abu, Indiausing a NICMOS3 HgCdTe 256 x 256 NIR array based spectrometer. Thespectra have a moderate resolution of 1000 (about 16 A) at the H bandand have been continuum shape corrected to their respective effectivetemperatures. This library and the remaining ones in J and K bands oncereleased will serve as an important database for stellar populationsynthesis and other applications in conjunction with the newly formedlarge optical coude feed stellar spectral library of Valdes et al.(2004). The complete H-Band library is available online at: http://vo.iucaa.ernet.in/~voi/NIR_Header.html
| Quasars and active galaxies.
Not Available
| Some anomalies in the occurrence of debris discs around main-sequence A and G stars
Debris discs consist of large dust grains that are generated bycollisions of comets or asteroids around main-sequence stars, and thequantity and distribution of debris may be used to detect the presenceof perturbing planets akin to Neptune. We use stellar and disc surveysto compare the material seen around A- and G-type main-sequence stars.Debris is detected much more commonly towards A stars, even when acomparison is made only with G stars of comparable age. Detection ratesare consistent with disc durations of ~0.5 Gyr, which may occur at anytime during the main sequence. The higher detection rate for A stars canresult from this duration being a larger fraction of the main-sequencelifetime, possibly boosted by a globally slightly larger disc mass thanfor the G-type counterparts. The disc mass range at any given age is afactor of at least ~100 and any systematic decline with time is slow,with a power law estimated to not be steeper than t-1/2.Comparison with models shows that dust can be expected as late as a fewGyr when perturbing planetesimals form slowly at large orbital radii.Currently, the Solar system has little dust because the radius of theKuiper Belt is small and hence the time-scale to produce planetesimalswas less than 1 Gyr. However, the apparently constant duration of ~0.5Gyr when dust is visible is not predicted by the models.
| 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.
| 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.
| Limits on Chromospheres and Convection among the Main-Sequence A Stars
In deeply convective stars, the nonthermal energy required to heat thechromosphere ultimately is supplied by turbulent magnetoconvection.Because the early and middle A stars have very shallow convectivelayers, they are not expected to produce enough magnetoconvective powerto sustain luminous chromospheres or hot coronae. Here we describe asearch for chromospheric emission in the far-ultraviolet (905-1185Å) spectra of seven main-sequence A stars, based on observationsfrom the Far Ultraviolet Spectroscopic Explorer (FUSE) telescope. Oursurvey spans the interval in effective temperature along the mainsequence over which powerful subsurface convection zones and hencechromospheric emission are expected to vanish. The presence or absenceof high-temperature emissions in our FUSE spectra therefore can be usedto identify the locus for the transition from convective to radiativeenvelopes-a change in stellar structure that is difficult to assess byother means. We present our observations and analysis of the subcoronalemission lines of C III λλ977, 1175 and O VIλλ1032, 1037, which bracket a range in formationtemperatures from 50,000 to 300,000 K. To supplement our FUSEobservations, we also report Goddard High Resolution Spectrographmeasurements of Si III λ1206 and H I Lyα λ1215,obtained from archival observations of the Hubble Space Telescope, aswell as X-ray measurements from previous ROSAT survey and pointedobservations. We detected C III and O VI emission features in the FUSEspectra of the coolest stars of our sample, at Teff<~8200K. When normalized to the bolometric luminosities, the detectedemission-line fluxes are comparable to solar values. We detected none ofthe hotter stars in our survey at Teff>=8300 K. Upperlimits on the normalized flux in some instances approach 40 times lessthan solar. Within an uncertainty in the effective temperature scale ofup to several hundred kelvins, our FUSE observations indicate that thetransition between convective and radiative stellar envelopes takesplace at, or very near, the point along the main sequence where stellarstructure models predict and, moreover, that the changeover occurs veryabruptly, over a temperature interval no greater than ~100 K in width.Our FUSE sample also includes two binary stars. In both cases, thenarrow UV line profiles we have observed suggest that thehigh-temperature emission is most likely associated with the late-typecompanions rather than the A stars themselves. Based on observationsmade with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer,operated for NASA by Johns Hopkins University under NASA contractNAS5-32985.
| The Velocity Distribution of the Nearest Interstellar Gas
The bulk flow velocity for the cluster of interstellar cloudlets within~30 pc of the Sun is determined from optical and ultraviolet absorptionline data, after omitting from the sample stars with circumstellar disksor variable emission lines and the active variable HR 1099. A total of96 velocity components toward the remaining 60 stars yield a streamingvelocity through the local standard of rest of -17.0+/-4.6 kms-1, with an upstream direction of l=2.3d, b=-5.2d (usingHipparcos values for the solar apex motion). The velocity dispersion ofthe interstellar matter (ISM) within 30 pc is consistent with that ofnearby diffuse clouds, but present statistics are inadequate todistinguish between a Gaussian or exponential distribution about thebulk flow velocity. The upstream direction of the bulk flow vectorsuggests an origin associated with the Loop I supernova remnant.Groupings of component velocities by region are seen, indicatingregional departures from the bulk flow velocity or possibly separateclouds. The absorption components from the cloudlet feeding ISM into thesolar system form one of the regional features. The nominal gradientbetween the velocities of upstream and downstream gas may be an artifactof the Sun's location near the edge of the local cloud complex. The Sunmay emerge from the surrounding gas patch within several thousand years.
| Catalogue of Apparent Diameters and Absolute Radii of Stars (CADARS) - Third edition - Comments and statistics
The Catalogue, available at the Centre de Données Stellaires deStrasbourg, consists of 13 573 records concerning the results obtainedfrom different methods for 7778 stars, reported in the literature. Thefollowing data are listed for each star: identifications, apparentmagnitude, spectral type, apparent diameter in arcsec, absolute radiusin solar units, method of determination, reference, remarks. Commentsand statistics obtained from CADARS are given. The Catalogue isavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcar?J/A+A/367/521
| The proper motions of fundamental stars. I. 1535 stars from the Basic FK5
A direct combination of the positions given in the HIPPARCOS cataloguewith astrometric ground-based catalogues having epochs later than 1939allows us to obtain new proper motions for the 1535 stars of the BasicFK5. The results are presented as the catalogue Proper Motions ofFundamental Stars (PMFS), Part I. The median precision of the propermotions is 0.5 mas/year for mu alpha cos delta and 0.7mas/year for mu delta . The non-linear motions of thephotocentres of a few hundred astrometric binaries are separated intotheir linear and elliptic motions. Since the PMFS proper motions do notinclude the information given by the proper motions from othercatalogues (HIPPARCOS, FK5, FK6, etc.) this catalogue can be used as anindependent source of the proper motions of the fundamental stars.Catalogue (Table 3) is only available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/222
| Atmospheric sodium monitor for Laser Guide Star Adaptive Optics
Laser Guide Star Adaptive Optics (AO) makes use of a laser to excite themesospheric sodium (Na) layer and thereby creates an artificial starthat can be used as reference for the AO wavefront sensor. The intensityof the created star depends strongly on the amount of Na present in theatmosphere. Experiments to measure the column density and details of theexcitation and scattering properties of sodium atoms in thecorresponding atmospheric layer are thus very important to refine thedesign parameters of laser and assess the power requirement. Thisarticle reviews the current knowledge on the mesospheric sodium derivedfrom both astronomical and atmospherical studies. It then presentspossible methods to study the sodium layer characteristics relevant foradaptive optics laser guide star systems.
| Polarization measurements of Vega-like stars
Optical linear polarization measurements are presented for about 30Vega-like stars. These are then compared with the polarization observedfor normal field stars. A significant fraction of the Vega-like starsare found to show polarization much in excess of that expected to be dueto interstellar matter along the line of sight to the star. The excesspolarization must be intrinsic to the star, caused by circumstellarscattering material that is distributed in a flattened disk. Acorrelation between infrared excess and optical polarization is foundfor the Vega-like stars.
| Sixth Catalogue of Fundamental Stars (FK6). Part I. Basic fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over more than twocenturies and summarized in the FK5. Part I of the FK6 (abbreviatedFK6(I)) contains 878 basic fundamental stars with direct solutions. Suchdirect solutions are appropriate for single stars or for objects whichcan be treated like single stars. From the 878 stars in Part I, we haveselected 340 objects as "astrometrically excellent stars", since theirinstantaneous proper motions and mean (time-averaged) ones do not differsignificantly. Hence most of the astrometrically excellent stars arewell-behaving "single-star candidates" with good astrometric data. Thesestars are most suited for high-precision astrometry. On the other hand,199 of the stars in Part I are Δμ binaries in the sense ofWielen et al. (1999). Many of them are newly discovered probablebinaries with no other hitherto known indication of binarity. The FK6gives, besides the classical "single-star mode" solutions (SI mode),other solutions which take into account the fact that hidden astrometricbinaries among "apparently single-stars" introduce sizable "cosmicerrors" into the quasi-instantaneously measured HIPPARCOS proper motionsand positions. The FK6 gives in addition to the SI mode the "long-termprediction (LTP) mode" and the "short-term prediction (STP) mode". TheseLTP and STP modes are on average the most precise solutions forapparently single stars, depending on the epoch difference with respectto the HIPPARCOS epoch of about 1991. The typical mean error of anFK6(I) proper motion in the single-star mode is 0.35 mas/year. This isabout a factor of two better than the typical HIPPARCOS errors for thesestars of 0.67 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(I) proper motions have atypical mean error of 0.50 mas/year, which is by a factor of more than 4better than the corresponding error for the HIPPARCOS values of 2.21mas/year (cosmic errors included).
| Monitoring of the atmospheric sodium above La Silla.
Not Available
| A search for circumstellar gas around normal A stars and Lambda Bootis stars
We have searched for interstellar or circumstellar absorption lines inthe center of Ca II K towards bright A-type stars that are mostly within80 pc of the Sun. Narrow absorption features are found in about 30 % ofthe 28 normal main-sequence A stars and 18 metal-deficient lambda Bootisstars studied. We have determined surface gravities and projectedrotational velocities. Most of the stars with detectable Ca K featureshave comparatively low gravities and high projected rotationalvelocities. This correlation with stellar properties implies that mostof the narrow absorption features are of circumstellar rather thaninterstellar origin. The preference of low gravity and rapid rotationfurthermore suggests that most of the gas shells around A stars developin the pre-main-sequence phase of evolution, and disappear largelybefore the star arrives at the ZAMS. Among the normal A stars studied,about 50 % are known to have dust disks. Unlike A stars withcircumstellar gas, these dusty stars do not prefer low log g and high vsin i. This results in an apparent lack of correlation between gas anddust, and indicates that normal A stars with gas shells and those withdust disks are not in the same evolutionary stage. We conjecture thatdust disks tend to develop after most of the gas has disappeared. Basedon observations collected at the European Southern Observatory, LaSilla, Chile
| Dusty and dust-free A stars
We present preliminary results of our search for circumstellarabsorption features in the Ca K lines based on high S/N observationsobtained with the ESO CAT/CES system.
| Candidate Main-Sequence Stars with Debris Disks: A New Sample of Vega-like Sources
Vega-like sources are main-sequence stars that exhibit IR fluxes inexcess of expectations for stellar photospheres, most likely due toreradiation of stellar emission intercepted by orbiting dust grains. Wehave identified a large sample of main-sequence stars with possibleexcess IR radiation by cross-correlating the Michigan Catalog ofTwo-dimensional Spectral Types for the HD Stars with the IRAS FaintSource Survey Catalog. Some 60 of these Vega-like sources were not foundduring previous surveys of the IRAS database, the majority of whichemployed the lower sensitivity Point Source Catalog. Here, we providedetails of our search strategy, together with a preliminary examinationof the full sample of Vega-like sources.
| Determination of the temperatures of selected ISO flux calibration stars using the Infrared Flux Method
Effective temperatures for 420 stars with spectral types between A0 andK3, and luminosity classes between II and V, selected for a fluxcalibration of the Infrared Space Observatory, ISO, have been determinedusing the Infrared Flux Method (IRFM). The determinations are based onnarrow and wide band photometric data obtained for this purpose, andtake into account previously published narrow-band measures oftemperature. Regression coefficients are given for relations between thedetermined temperatures and the photometric parameters (B2-V1), (b-y)and (B-V), corrected for interstellar extinction through use ofHipparcos parallaxes. A correction for the effect of metallicity on thedetermination of integrated flux is proposed. The importance of aknowledge of metallicity in the representation of derived temperaturesfor Class V, IV and III stars by empirical functions is discussed andformulae given. An estimate is given for the probable error of eachtemperature determination. Based on data from the ESA HipparcosAstrometry Satellite.
| Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants
I report on the implementation of the empirical surface brightnesstechnique using the near-infrared Johnson broadband { (V-K)} colour assuitable sampling observable aimed at providing accurate effectivetemperatures of 537 dwarfs and giants of A-F-G-K spectral-type selectedfor a flux calibration of the Infrared Space Observatory (ISO). Thesurface brightness-colour correlation is carefully calibrated using aset of high-precision angular diameters measured by moderninterferometry techniques. The stellar sizes predicted by thiscorrelation are then combined with the bolometric flux measurementsavailable for a subset of 327 ISO standard stars in order to determineone-dimensional { (T, V-K)} temperature scales of dwarfs and giants. Theresulting very tight relationships show an intrinsic scatter induced byobservational photometry and bolometric flux measurements well below thetarget accuracy of +/- 1 % required for temperature determinations ofthe ISO standards. Major improvements related to the actual directcalibration are the high-precision broadband { K} magnitudes obtainedfor this purpose and the use of Hipparcos parallaxes for dereddeningphotometric data. The temperature scale of F-G-K dwarfs shows thesmallest random errors closely consistent with those affecting theobservational photometry alone, indicating a negligible contributionfrom the component due to the bolometric flux measurements despite thewide range in metallicity for these stars. A more detailed analysisusing a subset of selected dwarfs with large metallicity gradientsstrongly supports the actual bolometric fluxes as being practicallyunaffected by the metallicity of field stars, in contrast with recentresults claiming somewhat significant effects. The temperature scale ofF-G-K giants is affected by random errors much larger than those ofdwarfs, indicating that most of the relevant component of the scattercomes from the bolometric flux measurements. Since the giants have smallmetallicities, only gravity effects become likely responsible for theincreased level of scatter. The empirical stellar temperatures withsmall model-dependent corrections are compared with the semiempiricaldata by the Infrared Flux Method (IRFM) using the large sample of 327comparison stars. One major achievement is that all empirical andsemiempirical temperature estimates of F-G-K giants and dwarfs are foundto be closely consistent between each other to within +/- 1 %. However,there is also evidence for somewhat significant differential effects.These include an average systematic shift of (2.33 +/- 0.13) % affectingthe A-type stars, the semiempirical estimates being too low by thisamount, and an additional component of scatter as significant as +/- 1 %affecting all the comparison stars. The systematic effect confirms theresults from other investigations and indicates that previousdiscrepancies in applying the IRFM to A-type stars are not yet removedby using new LTE line-blanketed model atmospheres along with the updatedabsolute flux calibration, whereas the additional random component isfound to disappear in a broadband version of the IRFM using an infraredreference flux derived from wide rather than narrow band photometricdata. Table 1 and 2 are only available in the electronic form of thispaper
| The Tokyo PMC catalog 90-93: Catalog of positions of 6649 stars observed in 1990 through 1993 with Tokyo photoelectric meridian circle
The sixth annual catalog of the Tokyo Photoelectric Meridian Circle(PMC) is presented for 6649 stars which were observed at least two timesin January 1990 through March 1993. The mean positions of the starsobserved are given in the catalog at the corresponding mean epochs ofobservations of individual stars. The coordinates of the catalog arebased on the FK5 system, and referred to the equinox and equator ofJ2000.0. The mean local deviations of the observed positions from theFK5 catalog positions are constructed for the basic FK5 stars to comparewith those of the Tokyo PMC Catalog 89 and preliminary Hipparcos resultsof H30.
| The Angular Momentum of Main Sequence Stars and Its Relation to Stellar Activity
Rotational velocities are reported for intermediate-mass main sequencestars it the field. The measurements are based on new, high S/N CCDspectra from the Coudé Feed Telescope of the Kitt Peak NationalObservatory. We analyze these rotation rates for a dependence on bothmass and age. We compare the average rotation speeds of the field starswith mean velocities for young stars in Orion, the Alpha Persei cluster,the Pleiades, and the Hyades. The average rotation speeds of stars moremassive than $\sim1.6$ \msun\experience little or no change during theevolutionary lifetimes of these stars on the zero age main sequence orwithin the main sequence band. Less massive stars in the range betwee n1.6\msun\ and 1.3\msun\ also show little decline in mean rotation ratewhile they are on the main sequence, and at most a factor of 2 decreasein velocity as they evolve off the main sequence. The {\it e}-foldingtime for the loss of angular momentum b y the latter group of stars isat least 1--2 billion years. This inferred characteristic time scale forspindown is far longer than the established rotational braking time forsolar-type stars with masses below $\sim1.3$ \msun. We conclude from acomparison of the trends in rotation with trends in chromospheric andcoronal activity that the overall decline in mean rotation speed alongthe main sequence, from $\sim2$ \msun\ down to $\sim1.3$ \msun, isimposed during the pre-main sequence phase of evolution, and that thispattern changes little thereafter while the star resides on the mainsequence. The magnetic activity implicated in the rotational spindown ofthe Sun and of similar stars during their main sequence lifetimes mus ttherefore play only a minor role in determining the rotation rates ofthe intermediate mass stars, either because a solar-like dynamo is weakor absent, or else the geometry of the magnetic field is appreciablyless effective in removing angular momentu m from these stars. (SECTION:Stars)
| Dust around Main-Sequence Stars: Nature or Nurture by the Interstellar Medium?
Dust from the interstellar medium (ISM) can collide with and destroyparticles in the circumstellar dust disks around main-sequence stars(Vega/ beta Pic stars). Two current theories tying the occurrence of theVega/ beta Pic phenomenon to the erosive influence of the ISM arecritically reconsidered here. Using the local standard of rest frame, wefind little evidence for a correlated motion (streaming) of prominentdisk systems, which one theory suggests would result from a passageabout 107 yr ago of these stars, but not the control A-type stars,through the nearby Lupus-Centaurus interstellar cloud complex. Moreover,the prototype system of beta Pic could not have retained dust producedin such a passage for much longer than 104 yr. We show theoreticallythat the ISM sandblasting of disks has minor importance for thestructure and evolution of circumstellar disks, except perhaps in theiroutskirts (usually >400 AU from the stars), where under favorableconditions it may cause asymmetries in observed brightness and color.The ISM neither produces the disks (as in one theory) nor depletes andeliminates them with time (as in another theory), because typical ISMgrains are subject to strong radiative repulsion from A- and F-typedwarfs (a few to 100 times stronger than gravity). Atypically large ISMgrains are not repelled strongly, but are unimportant on account oftheir small number density. Dust production and destruction in betaPic-type disks results mainly from their collisional nature enhanced bythe radiatively produced eccentricities of particle orbits, rather thanfrom nurture in a hostile ISM. The residence times of the few-microndust grains predominant in the densest part of the beta Pic disk is only104 yr, or a few dozen orbital periods. Submicronic debris is blown outas beta meteoroids, carrying away from this system an equivalent of thesolar system's total mass in solids (~120 Earth masses) in only ~65 Myr.This rate of collisional erosion exceeds almost 108 times that of thezodiacal light disk of our own system. A massive and relatively young(<~102 Myr) planetesimal disk appears to surround beta Pic, destinedto decline in dust density over time comparable to its age. Other dustdisks, like those around Fomalhaut and Vega, contain much less dust andmay be much older than the beta Pic disk, but like the beta Pic diskthey are also derived from and replenished many times during theirlifetimes by unseen parent bodies.
| Local interstellar cloud electron density from magnesium and sodium ionization: a comparison.
The ambient interstellar plasma density (i.e. the plasma density of theinterstellar medium surrounding the Sun) directly governs the structureand the size of our heliosphere. Information on this density can bederived from the ionization states of the interstellar species which canbe detected in absorption along the paths to the nearby stars, and whichcan be shown to belong to the Local Interstellar Cloud (LIC). Echellespectra around the resonance lines of neutral and singly ionizedmagnesium have been obtained for the nearby star δ Cas with theGoddard High-Resolution Spectrograph (GHRS) on board the Hubble SpaceTelescope. While apparently a unique velocity component (a uniquecloudlet) is detected in both lines of the MgII λλ2800resonance doublet, at the expected Doppler shift for the LIC, anextremely small λλ2853 Mg line is also detected at aDoppler shift compatible with the LIC motion, allowing a measurement ofthe LIC MgII/MgI ratio, here found to be 400 (-130,+190). This ratioimplies a mean electron density of about 0.28 (-0.14, +0.34)cm^-3^ alongthis line-of-sight, if equilibrium conditions prevail, and if T=7000K,when using the most recent recombination and charge-exchange rates. ThisMgII/MgI ratio is larger than for Sirius (R=~220), which lies at 110^o^from δ Cas, providing some evidence for an ionization gradient inthe local cloud. Such an electron density implies a surprisingly largeionization degree, and the upper range of the interval is incompatiblewith the minimum size of our heliosphere. A second and independent wayto derive the electron density along the path to δ Cas uses thesimplicity of the LIC geometry in the sky region surrounding the star,which allows an estimate of the H column-density to the star, as well asprevious ground-based CaII data, and the measured NaI/CaII ratio andcalcium depletion in the LIC. The resulting most probable electrondensity at 7000K, 0.05cm^-3^, provides a new evidence for a significantionization degree of the LIC, but is a factor of four to five smallerthan the value based on magnesium. The upper limit of 0.19cm^-3^ remainsconsistent with the minimum size of our heliosphere. The existence of acommon interval to the two determinations: n_e_=0.14-0.19cm^-3^ impliesthat ionization equilibrium within the LIC is not totally precluded.However, the lack of a real convergence deserves further observations,involving other interstellar species. The common interval is compatiblewith the result of Frisch (1994Sci...265.1443F), from anomalous C and Ocosmic rays abundances, if carbon is not too much filtered at theheliospheric interface. On the other hand, the sodium-based mostprobable value is in agreement with neutral hydrogen deceleration at theheliospheric interface for the Baranov two-shocks model, as well as withthe ionization degree of hydrogen implied by local EUV sources.
| The velocity structure of the local interstellar medium probed by ultra-high-resolution spectroscopy.
We present ultra-high-resolution (0.35km/s FWHM) observations of theinterstellar Ca K line towards eight nearby stars (six of which arecloser than 30pc). The spectral resolution is sufficient to resolve theline profiles fully, thereby enabling us to detect hitherto unresolvedvelocity components, and to obtain accurate measurements of the velocitydispersions (b-values). Absorption components due to the LocalInterstellar Cloud (LIC) and/or the closely associated `G Cloud' areidentified towards all but one star (γ Oph), but only in one case(51 Oph) are both clouds reliably detected towards the same star. Mostof these nearby clouds have velocity dispersions (b=~2km/s) whichsuggest physical conditions similar to those inferred for the LIC(T_k_=~7000K, v_t_=~1km/s), although at least three lines of sight(towards γ Aqr, β Cen and ρ Cen) also sample coolerand/or less turbulent material. The spectrum of the nearby Vega-excessstar 51 Oph is of particular interest, owing to evidence that several ofthe absorption components arise in the circumstellar environment.
| The Pulkovo Spectrophotometric Catalog of Bright Stars in the Range from 320 TO 1080 NM
A spectrophotometric catalog is presented, combining results of numerousobservations made by Pulkovo astronomers at different observing sites.The catalog consists of three parts: the first contains the data for 602stars in the spectral range of 320--735 nm with a resolution of 5 nm,the second one contains 285 stars in the spectral range of 500--1080 nmwith a resolution of 10 nm and the third one contains 278 stars combinedfrom the preceding catalogs in the spectral range of 320--1080 nm with aresolution of 10 nm. The data are presented in absolute energy unitsW/m(2) m, with a step of 2.5 nm and with an accuracy not lower than1.5--2.0%.
| Characteristics of Nearby Interstellar Matter
There is a warm tenuous partially ionized cloud (T~104 K,n(HI)~0.1 cm-3, n(HII)~0.22-0.44 cm-3) surroundingthe solar system which regulates the environment of the solar system,determines the structure of the heliopause region, and feeds neutralinterstellar gas into the inner solar system. The velocity (V ~ -20 kms-1 from l~335 deg, b~0 deg in the local standard of rest)and enhanced CaII and FeII abundances of this cloud suggest an origin asevaporated gas from cloud surfaces in the Scorpius-CentaurusAssociation. Although the soft X-ray emission attributed to the 'LocalBubble' is enigmatic, optical and ultraviolet data are consistent withbubble formation caused by star formation epochs in theScorpius-Centaurus Association as regulated by the nearby spiral armconfiguration. The cloud surrounding the solar system (the 'localfluff') appears to be the leading region of an expanding interstellarstructure (the 'squall line') which contains a magnetic field causingpolarization of the light of nearby stars, and also absorption featuresin nearby upwind stars. The velocity vectors of the solar system andlocal fluff are perpendicular in the local standard of rest. Combiningthis information with the low column densities seen toward Sirius in theanti-apex direction, and the assumption that the cloud velocity vectoris parallel to the surface normal, suggests that the Sun entered thelocal fluff within the historical past (less than 10,000 years ago) andis skimming the surface of the cloud. Comparison of magnesiumabsorption lines toward Sirius and anomalous cosmic-ray data suggest thelocal fluff is in ionization equilibrium. [Note that a typographicalerror on page 532 incorrectly gives the age of the squall line shell as~400,000 years; the correct age is ~4 Myrs.]
| A microwave survey of southern early-type stars
A multi-epoch survey with the Parkes telescope of a completedistance-limited sample of 57 stars earlier than F6 has detectedpossible 8.4-GHz emission from 16 stars. Single-epoch partial synthesisobservations with the Australia Telescope Compact Array (ATCA) at 4.8GHz on 27 stars from the same sample (including the possible Parkesdetections) found no emission at the stellar positions above a fluxdensity limit of 1.2-1.9 mJy, but the maps show that the Parkesdetections are not merely the results of confusion of sources within theParkes beam. Three early F stars with UV and/or X-ray emission wereobserved simultaneously at 4.8 and 8.4 GHz in 12-h syntheses with the6-element ATCA. Two of these stars were from the above sample and thethird was the supergiant Alpha Carinae. We detected only alphaCar withflux densities of 300+/-65 and 140+/-65 muJy at 4.8 and 8.6 GHz(S~nu^-1.3+/-1.3). We discuss the legitimacy of the Parkes 3-6sigmadetections and show that, although none has been detected by synthesisobservations, there is no compelling reason for rejecting them on theinternal evidence. The power emitted by the supergiant alphaCar issimilar to that of the 16 possible Parkes detections, although itsactivity index is orders of magnitude lower. We show that this emissioncannot be thermal bremsstrahlung from the 10^7.2-K corona of the starbut is probably synchrotron emission from a magnetically maintainedcorona.
| SAO 206462 - a solar-type star with a dusty, organically rich environment
We report photometric, polarimetric and spectroscopic observations ofSAO 206462 in the optical, near-infrared and submillimetre. The systemhas near- and far-infrared excesses, and emissions from atomic hydrogenand PAH molecules. We interpret these data and the IRAS far-infraredphotometry in terms of a dusty environment around the star. SAO 206462is unique in being a solar-type, Vega-excess star with an organiccomponent to its surrounding dust.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Кентаур |
| Ректацензија: | 13h20m35.80s |
| Deклинација: | -36°42'44.0" |
| Apparent магнитуда: | 2.75 |
| Даљина: | 17.973 parsecs |
| Proper motion RA: | -341.9 |
| Proper motion Dec: | -87.4 |
| B-T magnitude: | 2.814 |
| V-T magnitude: | 2.732 |
Каталог и designations:
|
INHABITED SKY
