Садржај
Слике
Уплоадјуј своје слике
DSS Images Other Images
Везани чланци
Is There Enhanced Depletion of Gas-Phase Nitrogen in Moderately Reddened Lines of Sight?
We report on the abundance of interstellar neutral nitrogen (N I) for 30sight lines, using data from the Far Ultraviolet Spectroscopic Explorer(FUSE) and the Hubble Space Telescope (HST). N I column densities arederived by measuring the equivalent widths of several ultravioletabsorption lines and subsequently fitting those to a curve of growth. Wefind a mean interstellar N/H of 51+/-4 ppm. This is below the mean foundby Meyer et al. of 62+4-3 ppm (adjusted for adifference in f-values). Our mean N/H is similar, however, to the(f-value adjusted) mean of 51+/-3 ppm found by Knauth et al. for alarger sample of sight lines with larger hydrogen column densitiescomparable to those in this study. We discuss the question of whether ornot nitrogen shows increased gas-phase depletion in lines of sight withcolumn densities logN(Htot)>~21, as claimed by Knauth etal. The nitrogen abundance in the line of sight toward HD 152236 isparticularly interesting. We derive very small N/H and N/O ratios forthis line of sight that may support a previous suggestion that membersof the Sco OB1 association formed from an N-deficient region.
| Multiple Outflows and Protostars near IC 348 and the Flying Ghost Nebula
Using optical (Hα, [S II], and i'), near-IR (H2, J, H,and KS), mid-IR (Spitzer Space Telescope IRAC 4.5 μm), andsubmillimeter (850 and 450 μm) data, we have examined the regionsurrounding the IC 348 cluster and the neighboring ``Flying GhostNebula'' (FGN) and found a multitude of shocks from protostellaroutflows including HH 211, which had previously not been detected invisible wavelength images. We have identified 13 protostars in theregion that drive protostellar outflows. The region surrounding the FGNis rich in ongoing star formation, with a number of outflows similar tothose found in other sites of moderate star formation in Perseus (e.g.,L1448, L1455, and Barnard 1). We have also found a candidate bent jet inthis region. The axis defined by the bending angle suggests that thissource may have been ejected from a multiple star system near the IC 348IR source.
| Surface density of the young cluster IC 348 in the Perseus molecular cloud
The IC 348 young star cluster contains more than 300confirmed members. It is embedded in the Perseusmolecular cloud, making any clustering analysis subject to an extinctionbias. In this work, we derive the extinction map of the cloud andrevisit the content of IC 348 through a statisticalapproach that uses the 2MASS data. Our goal was to address the questionof the completeness of IC 348 and of young clustersin general. We performed a combined analysis of the star color anddensity in this region, in order to establish the surface density map ofthe cluster. We reached the conclusion that IC 348has structures up to 25' from the cluster center, and we estimate thatabout 40 members brighter than K_s=13 mag are still unidentified.Although we cannot use our statistical method to identify these newmembers individually, the surface density map gives a strong indicationof their actual location. They are distributed in the outer regions ofthe cluster, where very few dedicated observations have been made sofar, which is probably why they escaped previous identification. Inaddition, we propose the existence of a new embedded cluster associatedto the infrared source MSX 6C G160.2784-18.4216,about 38' south of IC 348.
| Upper limit on the gas density in the β Pictoris system
We investigate the effect of gas drag on the dynamics of the dustparticles in the edge-on β Pictoris disc to derive an upper limiton the mass of gas in this system. Our study is motivated by the largeuncertainties on the amount of gas in the β Pictoris disc currentlyfound in the literature. The dust particles are assumed to originatefrom a colliding annulus of planetesimals peaking around 100 AU from thecentral star. We consider the various gas densities that have beeninferred from independent observing techniques and we discuss theirimpact on dust dynamics and on the disc profile in scattered light alongthe midplane. We show that the observed scattered light profile of thedisc cannot be properly reproduced if the hygrogen gas number density at117 AU exceeds 104 cm-3. This corresponds to anupper limit on the total gas mass of about 0.4 M⊕ andthus to a gas to dust mass ratio smaller than 1. Our approach thereforeprovides an independent diagnostic of gas depletion in the βPictoris system relative to the dust disc. Such an approach could alsobe used to constrain the gas content of recently identified systems likethe edge-on disc around AU Mic.
| Abundances and Depletions of Interstellar Oxygen
We report on the abundance of interstellar neutral oxygen (O I) for 26sight lines, using data from the Far Ultraviolet Spectroscopic Explorer,the International Spectroscopic Explorer, and the Hubble SpaceTelescope. O I column densities are derived by measuring the equivalentwidths of several ultraviolet absorption lines and subsequently fittingthose to a curve of growth. We consider both our general sample of 26sight lines and a more restrictive sample of 10 sight lines that utilizeHST data for a measurement of the weak 1355 Å line of oxygen andare thus better constrained owing to our sampling of all three sectionsof the curve of growth. The column densities of our HST sample showratios of O/H that agree with the current best solar value if dust isconsidered, with the possible exception of one sight line (HD 37903). Wenote some very limited evidence in the HST sample for trends ofincreasing depletion with respect to RV and f(H2),but the trends are not conclusive. Unlike a recent result from Cartledgeet al., we do not see evidence for increasing depletion with respect to, but our HST sample contains only two points moredense than the critical density determined in that paper. The columndensities of our more general sample show some scatter in O/H, but mostagree with the solar value to within errors. We discuss these results inthe context of establishing the best method for determining interstellarabundances, the unresolved question of the best value for O/H in theinterstellar medium, the O/H ratios observed in Galactic stars, and thedepletion of gas-phase oxygen onto dust grains.
| A Multiyear Photometric Study of IC 348
The extremely young cluster IC 348 has been monitored photometricallyover five observing seasons from 1998 December to 2003 March in CousinsI with a 0.6 m telescope at Van Vleck Observatory. Twenty-eight periodicvariables and 16 irregular variables have been identified. Thevariability study is most sensitive for stars with I<14.3 mag; atthat brightness level, we find that 24 of the 27 known pre-main-sequence(PMS) cluster members in the monitored field are variables, illustratingthe value of photometric monitoring for identifying PMS cluster members.Among this brighter sample, 14 of the 16 known K or M-type weak-line TTauri stars (WTTS) were found to be periodic variables, while all fiveof the known classical T Tauri stars (CTTS) were found to be irregularvariables. In the full sample, which includes 150 stars with I as faintas 18, we find that 40% of the 63 WTTS are detected as variables, nearlyall of them periodic, while 55% of the 20 CTTS are also detected asvariable, with none of them periodic. Our study suggests that 80%-90% ofall WTTS in young clusters will be detected as periodic variables givensufficiently precise and extended monitoring, whereas CTTS will revealthemselves primarily or solely as irregular variables. This has clearconsequences for PMS rotational studies based on photometric periods,suggesting that any such sample may be biased against stars that arecurrently actively accreting (i.e., CTTS). We examine the stability ofthe periodic light curves from season to season. All periodic stars showmodulations of their amplitude, mean brightness, and light-curve shapeon timescales of less than 1 yr, presumably due to changes in spotconfigurations and/or physical characteristics. In no case, however, canwe find definitive evidence of a change in period, indicating thatdifferential rotation is probably much less in WTTS than it is in theSun. While some stars show a hint of what could be cyclic behavioranalogous to the sunspot cycle, no clear cycles could be found. Itappears that most of the variation in light-curve shape is caused byredistribution of spots on the surface rather than by an increase ordecrease in the areal spot coverage. While most of the variables are ofK or M spectral class, we do confirm the existence of three,low-amplitude, periodic G stars. The rotation periods of these moremassive stars are short compared to the bulk of the sample; it appearsthat mid-K to early M (i.e., ~0.5 Msolar) represents aminimum in mean rotation rate for extremely young stars. Among thenonperiodic stars, we report the detection of two possible UX Orionisstars as well as a pre-main-sequence star, HMW 15, which apparentlyundergoes an eclipse with a duration exceeding 3 yr.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Constraints on the IMF and the brown dwarf population of the young cluster IC 348
We use a deep near-infrared census of the young stellar cluster IC 348to construct and analyze its luminosity function. Our mosaic image of IC348 covers the full extent of the cluster with a completeness limit of J~ 19.5 and is therefore sensitive for 2 Myr old cluster members withmasses as low as M >=0.005 Msun for the mean extinction ofthe known cluster members (AV ~ 3.5 mag). By usinginformation on stellar ages, extinctions, and the binary population inIC 348 from several recent studies, we can derive statisticalconstraints on the stellar and sub-stellar mass function of the clusterby modeling the observed luminosity function. We find that the stellarpart of the mass function in IC 348 is well described by the galacticfield star IMF. While several brown dwarfs have recently been identifiedin IC 348, our data show that the cluster harbors only a relativelysmall population of sub-stellar objects. We find that brown dwarfs inthe mass range 0.02-0.075 Msun constitute at most ~ 10% ofthe total cluster population, in contrast to recent results suggestingmuch larger brown dwarf populations in other young clusters and also thegalactic field. Our results suggest that IC 348 has ~ 2 x fewer browndwarfs than the Orion Trapezium cluster. A similar brown dwarf``deficit'' was recently found in the Taurus star forming region. Wespeculate about the possible causes for this result, including thepresence or absence of nearby massive stars and their influence on theformation of low-mass young stellar objects.Based on observations obtained at the German-Spanish AstronomicalCentre, Calar Alto, operated by the Max-Planck-Instiute for Astronomy,Heidelberg, jointly with the Spanish National Commission for Astronomy.Table 1 is only available in electronic form athttp://www.edpsciences.org
| Observations of Star-Forming Regions with the Midcourse Space Experiment
We have imaged seven nearby star-forming regions, the Rosette Nebula,the Orion Nebula, W3, the Pleiades, G300.2-16.8, S263, and G159.6-18.5,with the Spatial Infrared Imaging Telescope on the Midcourse SpaceExperiment (MSX) satellite at 18" resolution at 8.3, 12.1, 14.7, and21.3 μm. The large angular scale of the regions imaged (~7.2-50deg2) makes these data unique in terms of the combination ofsize and resolution. In addition to the star-forming regions, twocirrus-free fields (MSXBG 160 and MSXBG 161) and a field near the southGalactic pole (MSXBG 239) were also imaged. Point sources have beenextracted from each region, resulting in the identification over 500 newsources (i.e., no identified counterparts at other wavelengths), as wellas over 1300 with prior identifications. The extended emission from thestar-forming regions is described, and prominent structures areidentified, particularly in W3 and Orion. The Rosette Nebula isdiscussed in detail. The bulk of the mid-infrared emission is consistentwith that of photon-dominated regions, including the elephant trunkcomplex. The central clump, however, and a line of site toward thenorthern edge of the cavity show significantly redder colors than therest of the Rosette complex.
| A Census of the Young Cluster IC 348
We present a new census of the stellar and substellar members of theyoung cluster IC 348. We have obtained images at I and Z for a42'×28' field encompassing the cluster andhave combined these measurements with previous optical and near-infraredphotometry. From spectroscopy of candidate cluster members appearing inthese data, we have identified 122 new members, 15 of which havespectral types of M6.5-M9, corresponding to masses of ~0.08-0.015Msolar by recent evolutionary models. The latest census forIC 348 now contains a total of 288 members, 23 of which are later thanM6 and thus are likely to be brown dwarfs. From an extinction-limitedsample of members (AV<=4) for a16'×14' field centered on the cluster, weconstruct an initial mass function (IMF) that is unbiased in mass andnearly complete for M/Msolar>=0.03 (<~M8). Inlogarithmic units where the Salpeter slope is 1.35, the mass functionfor IC 348 rises from high masses down to a solar mass, rises moreslowly down to a maximum at 0.1-0.2 Msolar, and then declinesinto the substellar regime. In comparison, the similarly derived IMF forTaurus from Briceño et al. and Luhman et al. rises quickly to apeak near 0.8 Msolar and steadily declines to lower masses.The distinctive shapes of the IMFs in IC 348 and Taurus are reflected inthe distributions of spectral types, which peak at M5 and K7,respectively. These data provide compelling, model-independent evidencefor a significant variation of the IMF with star-forming conditions.Based on observations obtained at Keck Observatory, Steward Observatory,the MMT Observatory, and the Canada-France-Hawaii Telescope. Thispublication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
| High-Resolution Observations of Interstellar Ca I Absorption-Implications for Depletions and Electron Densities in Diffuse Clouds
We present high-resolution (FWHM~0.3-1.5 km s-1) spectra,obtained with the AAT UHRF, the McDonald Observatory 2.7 m coudéspectrograph, and/or the KPNO coudé feed, of interstellar Ca Iabsorption toward 30 Galactic stars. Comparisons of the column densitiesof Ca I, Ca II, K I, and other species-for individual componentsidentified in the line profiles and also when integrated over entirelines of sight-yield information on relative electron densities anddepletions (dependent on assumptions regarding the ionizationequilibrium). There is no obvious relationship between the ratio N(CaI)/N(Ca II) [equal to ne/(Γ/αr) forphotoionization equilibrium] and the fraction of hydrogen in molecularform f(H2) (often taken to be indicative of the local densitynH). For a smaller sample of sight lines for which thethermal pressure (nHT) and local density can be estimated viaanalysis of the C I fine-structure excitation, the average electrondensity inferred from C, Na, and K (assuming photoionizationequilibrium) seems to be independent of nH andnHT. While the electron density (ne) obtained fromthe ratio N(Ca I)/N(Ca II) is often significantly higher than the valuesderived from other elements, the patterns of relative nederived from different elements show both similarities and differencesfor different lines of sight-suggesting that additional processesbesides photoionization and radiative recombination commonly andsignificantly affect the ionization balance of heavy elements in diffuseinterstellar clouds. Such additional processes may also contribute tothe (apparently) larger than expected fractional ionizations(ne/nH) found for some lines of sight withindependent determinations of nH. In general, inclusion of``grain-assisted'' recombination does reduce the inferred ne,but it does not reconcile the ne estimated from differentelements; it may, however, suggest some dependence of ne onnH. The depletion of calcium may have a much weakerdependence on density than was suggested by earlier comparisons with CHand CN. Two appendices present similar high-resolution spectra of Fe Ifor a few stars and give a compilation of column density data for Ca I,Ca II, Fe I, and S I.
| A 13CO and C18O Survey of the Molecular Gas Around Young Stellar Clusters within 1 Kiloparsec of the Sun
As the first step of a multiwavelength investigation into therelationship between young stellar clusters and their environment, wepresent fully sampled maps in the J=1-0 lines of 13CO andC18O and the J=2-1 line of C18O for a selectedgroup of 30 young stellar groups and clusters within 1 kpc of the Sun.This is the first systematic survey of these regions to date. Theclusters range in size from several stars to a few hundred stars. Thirtyfields ranging in size from 8'×8' to 30'×60' were mappedwith 47" resolution simultaneously in the two J=1-0 lines at the FiveCollege Radio Astronomy Observatory. Seventeen sources were mapped overfields ranging in size from 3'×3' to 13'×13' in the J=2-1line with 35" resolution at the Submillimeter Telescope Observatory. Wecompare the cloud properties derived from each of the three tracers inorder to better understand systematic uncertainties in determiningmasses and line widths. Cloud masses are determined independently usingthe 13CO and C18O transitions; these masses rangefrom 30 to 4000 Msolar. Finally, we present a simplemorphological classification scheme, which may serve as a roughindicator of cloud evolution.
| Radial velocities of early-type stars in the Perseus OB2 association
We present radial velocities for 29 B- and A-type stars in the field ofthe nearby association Perseus OB2. The velocities are derived fromspectra obtained with AURELIE, via cross correlation with radialvelocity standards matched as closely as possible in spectral type. Theresulting accuracy is ~ 2-3 km s-1. We use thesemeasurements, together with published values for a few other early-typestars, to study membership of the association. The mean radial velocity(and measured velocity dispersion) of Per OB2 is 23.5 +/- 3.9 kms-1, and lies ~ 15 km s-1 away from the meanvelocity of the local disk field stars. We identify a number ofinterlopers in the list of possible late-B- and A-type members which wasbased on Hipparcos parallaxes and proper motions, and discuss thecolour-magnitude diagram of the association.Based on observations made at the Observatoire de Haute-Provence (CNRS),France.
| Planetary orbits in the elliptic restricted problem. V.. The ADS 11060 system
Numerical simulations are made within the framework of the ellipticplane restricted three-body problem, in order to determine whetherstable orbits exist for planets around one of the two components indouble stars. The ADS 11060 system is investigated here. Large stableplanetary orbits, already known to exist (more precisely, known to bepossible from the dynamical point of view) through a systematicexploration of the circular model and for several cases of the ellipticmodel (Sun-Jupiter, alpha Centauri, Sirius, eta Coronae Borealis andADS 12033), are found to exist around ADS 11060 A and ADS 11060 B up todistances from each star of the order of more than half the binary'speriastron separation. But, in this case, nearly circular stableplanetary orbits are not found to exist either in the so-called``habitable zone" around each star or anywhere else; this is very likelydue to the high value of the eccentricity of the binary's orbit.
| Some Diffuse Interstellar Bands Related to Interstellar C2 Molecules
We have investigated the correlations between the equivalent widths of21 selected diffuse interstellar bands (DIBs) and the correspondinginterstellar column densities N(C2), N(CN), and N(CH), toward53 stars with color excesses 0.11<=E(B-V)<=1.99. The observationaldata were derived primarily from echelle spectra acquired at R=38,000 aspart of our extensive, continuing survey of the bands. All but six ofthe 53 final spectra show signal-to-noise ratios >=800 at 5780Å. The principal result presented here is that seven of the 21bands prove to be examples of ``the C2 DIBs,'' a class ofweak, narrow bands whose normalized equivalent widthsWλ(X)/Wλ (λ6196) are wellcorrelated specifically with N(C2)/E(B-V) via power laws. Incontrast, the similarly normalized equivalent widths of the 14 other,well-known DIBs analyzed here are uncorrelated, or weaklyanticorrelated, with N(C2)/E(B-V), to within theobservational uncertainties. Thus, the polyatomic molecule(s) presumedto cause these seven C2 DIBs may bear a direct chemicalrelation to C2 that is not shared by the polyatomic moleculesputatively responsible for the other 14 bands. The C2 DIBsalso show positive correlations with N(CN)/E(B-V) and N(CH)/E(B-V) inour particular sample of light paths, although generally with shallowerslopes in the case of N(CN) and with greater scatter in the case ofN(CH). Eleven additional C2 DIBs are also identified but arenot analyzed here. Among the 18 C2 DIBs identified, fourapparently have not been previously detected. The λ4963 band isgenerally the strongest of the 18 C2 DIBs, while theλ4734 band shows the most sensitive correlation withN(C2).Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Merged catalogue of reflection nebulae
Several catalogues of reflection nebulae are merged to create a uniformcatalogue of 913 objects. It contains revised coordinates,cross-identifications of nebulae and stars, as well as identificationswith IRAS point sources.The catalogue is 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/399/141
| The total-to-selective extinction ratio determined from near IR photometry of OB stars
The paper presents an extensive list of the total to selectiveextinction ratios R calculated from the infrared magnitudes of 597 O andB stars using the extrapolation method. The IR magnitudes of these starswere taken from the literature. The IR colour excesses are determinedwith the aid of "artificial standards" - Wegner (1994). The individualand mean values of total to selective extinction ratios R differ in mostcases from the average value R=3.10 +/-0.05 - Wegner (1993) in differentOB associations. The relation between total to selective extinctionratios R determined in this paper and those calculated using the "methodof variable extinction" and the Cardelli et al. (1989) formulae isdiscussed. The R values presented in this paper can be used to determineindividual absolute magnitudes of reddened OB stars with knowntrigonometric parallaxes.
| A Far Ultraviolet Spectroscopic Explorer Survey of Interstellar Molecular Hydrogen in Translucent Clouds
We report the first ensemble results from the Far UltravioletSpectroscopic Explorer survey of molecular hydrogen in lines of sightwith AV>~1 mag. We have developed techniques for fittingcomputed profiles to the low-J lines of H2, and thusdetermining column densities for J=0 and J=1, which contain >~99% ofthe total H2. From these column densities and ancillary datawe have derived the total H2 column densities, hydrogenmolecular fractions, and kinetic temperatures for 23 lines of sight.This is the first significant sample of molecular hydrogen columndensities of ~1021 cm-2, measured through UVabsorption bands. We have also compiled a set of extinction data forthese lines of sight, which sample a wide range of environments. We havesearched for correlations of our H2-related quantities withpreviously published column densities of other molecules and extinctionparameters. We find strong correlations between H2 andmolecules such as CH, CN, and CO, in general agreement with predictionsof chemical models. We also find the expected correlations betweenhydrogen molecular fraction and various density indicators such askinetic temperature, CN abundance, the steepness of the far-UVextinction rise, and the width of the 2175 Å bump. Despite therelatively large molecular fractions, we do not see the values greaterthan 0.8 expected in translucent clouds. With the exception of a fewlines of sight, we see little evidence for the presence of individualtranslucent clouds in our sample. We conclude that most of the lines ofsight are actually composed of two or more diffuse clouds similar tothose found toward targets like ζ Oph. We suggest a modification interminology to distinguish between a ``translucent line of sight'' and a``translucent cloud.''
| Formation of Planetary Systems and Brown Dwarfs around Single Stars
The conditions for the formation of planets and brown dwarfs aroundsingle main-sequence stars are considered in two scenarios. Theformation of planets and brown dwarfs requires that the initial specificangular momentum of a solar-mass protostar be (0.32)×1018 cm2/s.The accreted matter of the protostar envelope forms a compact gas ring(disk) around the young star. If the viscosity of the matter in thisring (disk) is small, increasing its mass above a certain limit resultsin gravitational instability and the formation of a brown dwarf. If theviscosity of the gas is sufficiently large, the bulk of the protostarenvelope material will be accreted by the young star, and the gas diskwill grow considerably to the size of a protoplanetary dust disk due tothe conservation of angular momentum. The formation of dust in the coolpart of the extended disk and its subsequent collisional coalescenceultimately results in the formation of solar-type planetary systems.
| Collisional Cascades in Planetesimal Disks. I. Stellar Flybys
We use a new multiannulus planetesimal accretion code to investigate theevolution of a planetesimal disk following a moderately close encounterwith a passing star. The calculations include fragmentation, gas andPoynting-Robertson drag, and velocity evolution from dynamical frictionand viscous stirring. We assume that the stellar encounter increasesplanetesimal velocities to the shattering velocity, initiating acollisional cascade in the disk. During the early stages of ourcalculations, erosive collisions damp particle velocities and producesubstantial amounts of dust. For a wide range of initial conditions andinput parameters, the time evolution of the dust luminosity follows asimple relation,Ld/L*=L0/[α+(t/td)β]. The maximum dust luminosity L0 andthe damping time td depend on the disk mass, withL0~Md andtd~M-1d. For disks with dust masses of1%-100% of the ``minimum-mass solar nebula'' (1-100 M⊕at 30-150 AU), our calculations yield td~1-10 Myr,α~1-2, β=1, and dust luminosities similar to the rangeobserved in known ``debris disk'' systems, L0~10-3to 10-5. Less massive disks produce smaller dust luminositiesand damp on longer timescales. Because encounters with field stars arerare, these results imply that moderately close stellar flybys cannotexplain collisional cascades in debris disk systems with stellar ages of~100 Myr or longer.
| EXPORT: Optical photometry and polarimetry of Vega-type and pre-main sequence stars
This paper presents optical UBVRI broadband photo-polarimetry of theEXPORT sample obtained at the 2.5 m Nordic Optical Telescope. Thedatabase consists of multi-epoch photo-polarimetry of 68pre-main-sequence and main-sequence stars. An investigation of thepolarization variability indicates that 22 objects are variable at the3sigma level in our data. All these objects are pre-main sequence stars,consisting of both T Tauri and Herbig Ae/Be objects while the mainsequence, Vega type and post-T Tauri type objects are not variable. Thepolarization properties of the variable sources are mostly indicative ofthe UXOR-type behaviour; the objects show highest polarization when thebrightness is at minimum. We add seven new objects to the class of UXORvariables (BH Cep, VX Cas, DK Tau, HK Ori, LkHα 234, KK Oph and RYOri). The main reason for their discovery is the fact that our data-setis the largest in its kind, indicating that many more young UXOR-typepre-main sequence stars remain to be discovered. The set of Vega-likesystems has been investigated for the presence of intrinsicpolarization. As they lack variability, this was done using indirectmethods, and apart from the known case of BD+31o643, thefollowing stars were found to be strong candidates to exhibitpolarization due to the presence of circumstellar disks: 51 Oph,BD+31o643C, HD 58647 and HD 233517. Table A1 is onlyavailable 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/qcat?J/A+A/379/564
| EXPORT: Spectral classification and projected rotational velocities of Vega-type and pre-main sequence stars
In this paper we present the first comprehensive results extracted fromthe spectroscopic campaigns carried out by the EXPORT (EXoPlanetaryObservational Research Team) consortium. During 1998-1999, EXPORTcarried out an intensive observational effort in the framework of theorigin and evolution of protoplanetary systems in order to obtain clueson the evolutionary path from the early stages of the pre-main sequenceto stars with planets already formed. The spectral types of 70 stars,and the projected rotational velocities, v sin i, of 45 stars, mainlyVega-type and pre-main sequence, have been determined from intermediate-and high-resolution spectroscopy, respectively. The first part of thework is of fundamental importance in order to accurately place the starsin the HR diagram and determine the evolutionary sequences; the secondpart provides information on the kinematics and dynamics of the starsand the evolution of their angular momentum. The advantage of using thesame observational configuration and methodology for all the stars isthe homogeneity of the set of parameters obtained. Results from previouswork are revised, leading in some cases to completely new determinationsof spectral types and projected rotational velocities; for some stars noprevious studies were available. Tables 1 and 2 are only, and Table 6also, available 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/qcat?J/A+A/378/116 Based onobservations made with the Isaac Newton and the William Herscheltelescopes operated on the island of La Palma by the Isaac Newton Groupin the Spanish Observatorio del Roque de los Muchachos of the Institutode Astrofísica de Canarias.
| Far-ultraviolet extinction and diffuse interstellar bands
We relate the equivalent widths of the major diffuse interstellar bands(DIBs) near 5797 and 5780Å with different colour excesses,normalized by E(B-V), which characterize the growth of interstellarextinction in different wavelength ranges. It is demonstrated that thetwo DIBs correlate best with different parts of the extinction curve,and the ratio of these diffuse bands is best correlated with thefar-ultraviolet (UV) rise. A number of peculiar lines of sight are alsofound, indicating that the carriers of some DIBs and the far-UVextinction can be separated in certain environments, e.g. towards thePer OB2 association.
| Deep Chandra X-Ray Observatory Imaging Study of the Very Young Stellar Cluster IC 348
We have obtained a deep (53 ks) X-ray image of the very young stellarcluster IC 348 with the Advanced CCD Imaging Spectrometer on board theChandra X-Ray Observatory. In our image with a sensitivity limit of~1×1028 ergs s-1 (more than 10 times deeperthan our ROSAT images of IC 348), 215 X-ray sources are detected. While115 of these sources can be identified with known cluster members, 58X-ray sources are most likely new, still unidentified cluster members.About 80% of all known cluster members with masses between ~0.15 and 2Msolar are visible as X-ray sources in our image. We discoverX-ray emission at levels of ~1028 ergs s-1 fromfour of 13 known brown dwarfs and from three of 12 brown dwarfcandidates in IC 348. We also detect X-ray emission from two deeplyembedded objects, presumably class I protostars, south of the clustercenter.
| Circumstellar Disks in the IC 348 Cluster
We report the results of the first sensitive L-band (3.4 μm) imagingsurvey of the young IC 348 cluster in Perseus. In conjunction withpreviously acquired JHK (1.25, 1.65, and 2.2 μm) observations, we useL-band data to obtain a census of the circumstellar disk population tomK=mL<=12.0 in the central ~110arcmin2 region of the cluster. An analysis of the JHKL colorsof 107 sources indicates that 65%+/-8% of the cluster membershippossesses (inner) circumstellar disks. This fraction is lower than those(86%+/-8% and 80%+/-7%) obtained from similar JHKL surveys of theyounger NGC 2024 and Trapezium clusters, suggesting that the diskfraction in clusters decreases with cluster age. Sources withcircumstellar disks in IC 348 have a median age of 0.9 Myr, while thediskless sources have a median age of 1.4 Myr, for a cluster distance of320 pc. Although the difference in the median ages between the twopopulations is only marginally significant, our results suggest thatover a timescale of ~2-3 Myr more than a third of the disks in the IC348 cluster disappear. Moreover, we find that at a very high confidencelevel the disk fraction is a function of spectral type. All starsearlier than G appear diskless, while stars with spectral types G andlater have a disk fraction ranging between 50%-67%, with the latest-typestars having the higher disk fraction. This suggests that the disksaround stars with spectral types G and earlier have evolved more rapidlythan those with later spectral types. The L-band disk fraction forsources with similar ages in both IC 348 and Taurus is the same withinthe errors, suggesting that at least in clusters with no O stars thedisk lifetime is independent of environment.
| 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
| EXPORT: Near-IR observations of Vega-type and pre-main sequence stars
We present near-IR JHK photometric data of a sample of 58 main-sequence,mainly Vega-type, and pre-main sequence stars. The data were takenduring four observing runs in the period May 1998 to January 1999 andform part of a coordinated effort with simultaneous optical spectroscopyand photo-polarimetry. The near-IR colors of the MS stars correspond inmost cases to photospheric colors, although noticeable reddening ispresent towards a few objects, and these stars show no brightnessvariability within the observational errors. On the other hand, the PMSstars show near-IR excesses and variability consistent with previousdata. Tables 1 and 2 are only available in electronic form at the CDSvia anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strastg.fr/cgi-bin/qcat?J/A+A/365/110
| Absolute proper motions of open clusters. I. Observational data
Mean proper motions and parallaxes of 205 open clusters were determinedfrom their member stars found in the Hipparcos Catalogue. 360 clusterswere searched for possible members, excluding nearby clusters withdistances D < 200 pc. Members were selected using ground basedinformation (photometry, radial velocity, proper motion, distance fromthe cluster centre) and information provided by Hipparcos (propermotion, parallax). Altogether 630 certain and 100 possible members werefound. A comparison of the Hipparcos parallaxes with photometricdistances of open clusters shows good agreement. The Hipparcos dataconfirm or reject the membership of several Cepheids in the studiedclusters. Tables 1 and 2 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Size distributions of dust in circumstellar debris discs
The size distribution of particles in a dust disc is determined by, andholds the key to, sources, sinks and dynamics of grains. Here we derivethe size distribution in circumstellar debris discs, exemplified by thedisc of begin {equation}ta Pictoris, by modelling the dynamicalevolution of the circumstellar dust, dominated by collisions. The wholedisc is considered as consisting of two dust populations: larger grainsmoving in bound orbits (alpha -meteoroids) and smaller ones blown awayby radiation pressure (begin {equation}ta-meteoroids). Although begin{equation}ta-meteoroids leave the disc in hyperbolic trajectories, theyare continuously replenished by collisions, so that at any time the disccontains a substantial population of small particles. As a consequence,the fragmentation of alpha -meteoroids not only by mutual collisions,but also by impacts of begin {equation}ta-meteoroids becomessignificant. This flattens the distribution of alpha -meteoroids in thesize regime adjacent to the blow-out limit and shifts the crosssection-dominating sizes from a few micrometres to ga 10 mum . Theoverall distribution shows essentially three different slopes: steeperones for both begin {equation}ta-meteoroids and large alpha -meteoroidsand a gentler one for alpha -meteoroids with sizes just above theblow-out limit. This resembles the size distribution of interplanetarydust particles in the Solar system which, however, is shaped bydifferent mechanisms. The basic features of the modelled sizedistribution (the presence of a substantial population of smallhyperbolic particles in the disc, the dominance of grains ~ 10 mum insize) well agree with the observational data available. Althoughparticular calculations were made for the begin {equation}ta Pic disc,our basic qualitative conclusions directly apply to the debris discsaround other Vega-type stars with low gas contentsand similar or somewhat lower optical depths.
| 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.
|
Додај нови чланак
Линкови у сродству са темом
Додај нови линк
Чланови следећих група \:
|
Посматрања и Астрометриски подаци
| Сазвежђа: | Персеј |
| Ректацензија: | 03h44m34.19s |
| Deклинација: | +32°09'46.4" |
| Apparent магнитуда: | 8.734 |
| Даљина: | 221.239 parsecs |
| Proper motion RA: | 4.7 |
| Proper motion Dec: | -7.6 |
| B-T magnitude: | 9.509 |
| V-T magnitude: | 8.798 |
Каталог и designations:
|
