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The Pre-Main-Sequence Population of L988
L988 is a large (~0.5d×0.7d) dark cloud complex at about 600 pcthat contains several bright pre-main-sequence objects (such as V1331Cyg and LkHα 321), but this paper deals in detail only with asmall region on its eastern edge, near the HAeBe star LkHα 324.That star and its distant companion LkHα 324SE lie at the apex ofa V-shaped area apparently excavated from the edge of L988, and are thebrightest members of a small cluster containing about 50Hα-emission stars. A median age of about 0.6 Myr (with largedispersion) is inferred from its color-magnitude diagram, constructedfrom VRI photometry to V=22. Keck HIRES spectra show that LkHα324SE is probably also an HAeBe. Its image is nonstellar, and within 3"to the northwest are three condensations having complex [S II] and [O I]profiles and radial velocities up to -200 km s-1. Theyprobably originate in an outflow from LkHα 324SE. A bright Ap starwith strong Si II lines is embedded in the heavy obscuration 8' to thewest. It illuminates a small reflection nebulosity, has several faintHα emitters nearby, and shares the radial velocity of L988, soclearly it was formed in that cloud. It demonstrates again that suchchemical peculiarities can be established very early in young stars ofmoderate mass.
| Accretion rates in Herbig Ae stars
Aims.Accretion rates from disks around pre-main sequence stars are ofimportance for our understanding of planetary formation and diskevolution. We provide in this paper estimates of the mass accretionrates in the disks around a large sample of Herbig Ae stars.Methods: .We obtained medium resolution 2 μm spectra and used theresults to compute values of dot M_acc from the measured luminosity ofthe Brγ emission line, using a well established correlationbetween L(Brγ) and the accretion luminosity L_acc. Results:.We find that 80% of the stars, all of which have evidence of anassociated circumstellar disk, are accreting matter, with rates 3×10-9 dot M_acc 10-6 M_ȯ/yr; for 7objects, 6 of which are located on the ZAMS in the HR diagram, we do notdetect any line emission. Few HAe stars (25%) have dotM_acc>10-7 M_ȯ/yr. Conclusions: .In most HAestars the accretion rate is sufficiently low that the gas in the innerdisk, inside the dust evaporation radius, is optically thin and does notprevent the formation of a puffed-up rim, where dust is directly exposedto the stellar radiation. When compared to the dot M_acc values foundfor lower-mass stars in the star forming regions Taurus and Ophiuchus,HAe stars have on average higher accretion rates than solar-mass stars;however, there is a lack of very strong accretors among them, probablydue to the fact that they are on average older.
| Modeling of PMS Ae/Fe stars using UV spectra
Context: .Spectral classification of AeFe stars, based on visualobservations, may lead to ambiguous conclusions. Aims: . We aimto reduce these ambiguities by using UV spectra for the classificationof these stars, because the rise of the continuum in the UV is highlysensitive to the stellar spectral type of A/F-type stars. Methods: . We analyse the low-resolution UV spectra in terms of a3-component model, that consists of spectra of a central star, of anoptically-thick accretion disc, and of a boundary-layer between the discand star. The disc-component was calculated as a juxtaposition of Planckspectra, while the 2 other components were simulated by thelow-resolution UV spectra of well-classified standard stars (taken fromthe IUE spectral atlases). The hot boundary-layer shows strongsimilarities to the spectra of late-B type supergiants (see Appendix A). Results: . We modeled the low-resolution UV spectra of 37 AeFestars. Each spectral match provides 8 model parameters: spectral typeand luminosity-class of photosphere and boundary-layer, temperature andwidth of the boundary-layer, disc-inclination and circumstellarextinction. From the results of these analyses, combined with availabletheoretical PMS evolutionary tracks, we could estimate their masses andages and derive their mass-accretion rates. For a number of analysed PMSstars we calculated the corresponding SEDs and compared these with theobserved SEDs. Conclusions: . All stars (except βPic) showindications of accretion, that affect the resulting spectral type of thestellar photosphere. Formerly this led to ambiguities in classificatonof PMS stars as the boundary-layer was not taken into consideration. Wegive evidence for an increase of the mass-accretion rate with stellarmass and for a decreases of this rate with stellar age.
| HD 65949: the highest known mercury excess of any CP star?
ESO spectra of HD 65949 show it to be unlike any of the well-known typeswithin its temperature range ≈13 600 K. It is neither a silicon, nora mercury-manganese star, though it has a huge Hg II line atλ3984. We estimate log (Hg/H) + 12.0 ≈ 7.4. This is higherthan any published stellar mercury abundance. HD 65949 is a member of anearby open cluster, NGC 2516, which is only slightly older than thePleiades, and has been of recent interest because of its numerous X-rayemission stars, including HD 65949 itself, or a close companion. Alongitudinal magnetic field of the order of -290 Gauss at the 4.7σlevel was very recently diagnosed from accurate circularspectropolarimetric observations with FORS 1 at the VLT. The spectrallines are sharp, allowing a thorough identification study. Secondspectra of Ti, Cr, and Fe are rich. Mn II is well identified but notunusually strong. Numerous lines of S II and P II are found, but not GaII. The resonance lines of Sr II are strong. While many Y II lines areidentified, and Nb II is very likely present, no Zr II lines were found.Xe II is well identified. Strong absorptions from the third spectra ofthe lanthanides Pr, Nd, and Ho are present, but lines from the secondspectra of lanthanides are extremely weak or absent. Among lines fromthe heavier elements, those of Pt II are clearly present, and theheaviest isotope, 198Pt, is indicated. The uncommon spectrumof Re II is certain, while Os II and Te II are highly probable. Severalof the noted anomalies are unusual for a star as hot as HD 65949.
| X-ray emission from T Tauri stars in the Lupus 3 star-forming region
Aims.In this paper, I present analysis results of an {XMM-Newton}observation of the Lupus 3 region that contains a high proportion ofyoung low mass (M < 0.3 Mȯ) T Tauri stars in theLupus star-forming complex. Methods: .The detection of X-raysources in 0.5 to 4.5 keV images of the Lupus 3 core was performed usingthe standard source detection method of the {XMM-Newton} ScienceAnalysis Software. The detected sources were correlated with a list ofHerbig-Haro objects and Hα emission stars that contains mainlyclassical T Tauri stars, with a catalogue of weak-line T Tauri Stars andwith a recent list of new low-mass members of the Lupus 3 dark cloudfound in a visible-light spectroscopic survey at the center of the Lupus3 star-forming core. The light curves and spectra of the brightest X-raysources with known T Tauri star counterparts were analysed.Results: .One hundred and two X-ray sources were detected in the30´ diameter field-of-view of the EPIC cameras, of which 25 havevisible or near-IR counterparts that are known as pre-main sequencestars. Their X-ray luminosity ranges from 3 × 1028 to 3× 1030 erg s-1. Two of these objects withmass estimates lower than 0.075 Mȯ have an X-rayluminosity of about 4-7 × 1028 erg s-1,comparable with that of flaring young brown dwarfs. A linear correlationis found between the X-ray luminosity and the mass or volume of thestars that is qualitatively expected from some models of distributedturbulent dynamos. The EPIC spectra of the X-ray brightest sources canbe fitted using optically thin plasma emission models with twocomponents at temperatures in the ranges 3-9 × 106 Kand 1-50 × 107 K, respectively. The large emissionmeasure of hot plasma may be caused by disruptions of magnetic fieldsassociated with an intense flaring activity, while the X-ray emissionfrom the "cool" plasma components may result from solar-type activeregions. The emission measures of the plasma components are of the orderof 1052 cm-3, typical of the values expected fromcoronal plasmas in T Tauri stars, post-T Tauri stars, and activelate-type dwarfs in close binary systems. One property of the X-raybrightest stars in Lupus 3 that seems common among pre-main sequencestars is the low abundance of Fe.
| Multi-aperture photometry of extended IR sources with ISOPHOT. I. The nature of extended IR emission of planetary Nebulae
Context: .ISOPHOT multi-aperture photometry is an efficient method toresolve compact sources or to detect extended emission down torelatively faint levels with single detectors in the wavelength range 3to 100 μm. Aims: .Using ISOPHOT multi-aperture photometry andcomplementary ISO spectra and IR spectral energy distributions wediscuss the nature of the extended IR emission of the two PNe NGC 6543and NGC 7008. Methods: .In the on-line appendix we describe thedata reduction, calibration and interpretation methods based on asimultaneous determination of the IR source and background contributionsfrom the on-source multi-aperture sequences. Normalized profiles enabledirect comparison with point source and flat-sky references. Modellingthe intensity distribution offers a quantitative method to assess sourceextent and angular scales of the main structures and is helpful inreconstructing the total source flux, if the source extends beyond aradius of 1 arcmin. The photometric calibration is described and typicalaccuracies are derived. General uncertainty, quality and reliabilityissues are addressed, too. Transient fitting to non-stabilised signaltime series, by means of combinations of exponential functions withdifferent time constants, improves the actual average signals andreduces their uncertainty. Results: .The emission of NGC 6543 inthe 3.6 μm band coincides with the core region of the optical nebulaand is homogeneously distributed. It is comprised of 65% continuum and35% atomic hydrogen line emission. In the 12 μm band a resolved butcompact double source is surrounded by a fainter ring structure with allemission confined to the optical core region. Strong line emission of[ArIII] at 8.99 μm and in particular [SIV] at 10.51 μm shapes thisspatial profile. The unresolved 60 μm emission originates from dust.It is described by a modified (emissivity index β = 1.5) blackbodywith a temperature of 85 K, suggesting that warm dust with a mass of 6.4× 10-4 Mȯ is mixed with the ionisedgas. The gas-to-dust mass ratio is about 220. The 25 μm emission ofNGC 7008 is characterised by a FWHM of about 50´´ with anadditional spot-like or ring-like enhancement at the bright rim of theoptical nebula. The 60 μm emission exhibits a similar shape, but isabout twice as extended. Analysis of the spectral energy distributionsuggests that the 25 μm emission is associated with 120 K warm dust,while the 60 μm emission is dominated by a second dust component with55 K. The dust mass associated with this latter component amounts to 1.2× 10-3 Mȯ, significantly higher thanpreviously derived. The gas-to-dust mass ratio is 59 which, compared tothe average value of 160 for the Milky Way, hints at dust enrichment bythis object.
| Coronagraphic Imaging of Pre-Main-Sequence Stars with the Hubble Space Telescope Space Telescope Imaging Spectrograph. I. The Herbig Ae Stars
STIS white-light coronagraphic imaging has been carried out for 14nearby, lightly reddened Herbig Ae stars, providing data on theenvironments and disks associated with these stars. No disks aredetected in our data when the Herbig Ae star is accompanied by a stellarcompanion at r<=2''. We find that the optical visibilityof protoplanetary disks associated with Herbig Ae stars at r>=50-70AU from the star is correlated with the strength of the mid-IR PAHfeatures, particularly 6.2 μm. These features, like the FUVfluorescent H2 emission, trace the presence of materialsufficiently far above the disk midplane that it is directly illuminatedby the star's FUV radiation. In contrast, measures of the bulkproperties of the disk, including ongoing accretion activity, mass, andthe submillimeter slope of the SED, do not correlate with the surfacebrightness of the optical nebulosity. Modelers have interpreted theappearance of the IR SED and the presence of emission from warm silicategrains at 10 μm as a measure of geometrical shadowing by material inthe disk near the dust sublimation radius of 0.5 AU. Geometricalshadowing sufficient to render a disk dark to distances as large as 500AU from a star would require that the star be optically visible only ifviewed essentially pole-on, in disagreement with our program star systeminclinations. Rather than invoking shadowing to account for theoptically dark disks, the correlation of the STIS detections with PAHemission features suggests a correlation with disk flaring and ananticorrelation with the degree of dust settling toward the midplane. Ifthis correlation continues to lower levels, the STIS data suggest thatimprovements in coronagraph performance that suppress the residualscattered and diffracted stellar light by an additional factor of>=10 should render the majority of disks associated with nearbyHerbig Ae stars detectable.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations made with the NASA-CNES-CSA Far UltravioletSpectroscopic Explorer. FUSE is operated for NASA by The Johns HopkinsUniversity under NASA contract NAS5-32985.
| From Dusty Filaments to Cores to Stars: An Infrared Extinction Study of Lupus 3
We present deep near-infrared observations of a dense region of theLupus 3 cloud obtained with the ESO NTT and VLT. Using the NICE method,we construct a detailed high angular resolution dust extinction map ofthe cloud. The dust extinction map reveals embedded globules, a densefilament, and a dense ring structure. We derive dust column densitiesand masses for the entire cloud and for the individual structurestherein. We construct radial extinction profiles for the embeddedglobules and find a range of profile shapes from relatively shallowprofiles for cores with low peak extinctions, to relatively steepprofiles for cores with high extinction. Overall, the profiles aresimilar to those of pressure-truncated isothermal spheres of varyingcenter-to-edge density contrast. We apply Bonnor-Ebert analysis tocompare the density profiles of the embedded cores in a quantitativemanner and derive physical parameters such as temperatures, centraldensities, and external pressures. We examine the stability of the coresand find that two cores are likely stable and two are likely unstable.One of these latter cores is known to harbor an active protostar.Finally, we discuss the relation between an emerging cluster in theLupus 3 cloud and the ring structure identified in our extinction map.Assuming that the ring is the remnant of the core within which thecluster originally formed, we estimate that a star formation efficiencyof ~30% characterized the formation of the small cluster. Ourobservations of the Lupus 3 cloud suggest an intimate link between thestructure of a dense core and its state of star-forming activity. Thedense cores in this cloud are found to span the entire range ofevolution from a stable, starless core of modest central concentration,to an unstable, star-forming core that is highly centrally concentrated,to a significantly disrupted core from which a cluster of young stars isemerging.Based on observations carried out at ESO, La Silla and Paranal, Chile.
| X-Ray Study of Herbig Ae/Be Stars
We present ASCA results of intermediate-mass pre-main-sequence (PMS)stars, or Herbig Ae/Be (HAeBe) stars. Among the 35 ASCA pointed sources,we detect 11 plausible X-ray counterparts. X-ray luminosities of thedetected sources in the 0.5-10 keV band are in the range oflogLX~30-32 ergs s-1, which is systematicallyhigher than those of low-mass PMS stars. This fact suggests that thecontribution of a possible low-mass companion is not large. Most of thebright sources show significant time variation; in particular, two HAeBestars-MWC 297 and TY CrA-exhibit flarelike events with long decaytimescales (e-folding time ~10-60 ks). These flare shapes are similar tothose of low-mass PMS stars. The X-ray spectra are successfullyreproduced by an absorbed one- or two-temperature thin-thermal plasmamodel. The temperatures are in the range of kT~1-5 keV, significantlyhigher than those of main-sequence OB stars (kT<1 keV). These X-rayproperties are not explained by wind-driven shocks, but are more likelydue to magnetic activity. On the other hand, the plasma temperaturerises as absorption column density increases or as HAeBe stars ascend toearlier phases. The X-ray luminosity reduces after stellar age of a fewtimes 106 yr. X-ray activity may be related to stellarevolution. The age of the activity decay is apparently near thetermination of jet or outflow activity. We thus hypothesize thatmagnetic activity originates from the interaction of the large-scalemagnetic fields coupled to the circumstellar disk. We also discussdifferences in X-ray properties between HAeBe stars and main-sequence OBstars.
| Very low-mass members of the Lupus 3 cloud
We report on a multi-band survey for very low-mass stars and browndwarfs in the Lupus 3 cloud with the Wide Field Imager (WFI) at theESO/MPG 2.2 m telescope on La Silla Observatory (Chile). Our multibandoptical photometry is combined with available 2MASS JHK photometry toidentify 19 new young stars and 3 brown dwarf candidates as probablemembers of this star forming region. Our objects are mostly clusteredaround the cloud core. Stars and brown dwarfs have similar levels ofHα emission, probably a signature of accretion. One object, abrown dwarf candidate, exhibits a near-infrared excess, which mayindicate the presence of a disk, but its Hα emission cannot beconfirmed due to its faintness in the optical passbands. We also findtwo visual pairs of probable Lupus 3 members that may be wide binaries.
| Balmer lines as Teff and log g indicators for non-solar composition atmospheres. An application to the extremely helium-weak star HR 6000
Although the importance of a correct abundance assumption in thedetermination of effective temperature and surface gravity of a star hasbeen demonstrated in the literature, this determination is often stillextremely simplified, neglecting the effects of non-solar chemicalabundances. In this paper we show how the modeling of the profiles ofHδ and Hγ, commonly used as T_eff andlog g indicators, is affected when the chemical composition is far fromthe standard one. As a target for our study we selected the chemicallypeculiar star HR 6000. Comparing the observed and synthetic profiles ofHδ and Hγ we obtained T_eff = 12 950 Kand log g = 4.05; the atmospheric model has been computed with a metalopacity scale evaluated for [M/H] = -0.5 and He/H = 0. A number of Fe IIlines have been used to infer the rotational velocity (ve sini = 0 km s-1) and the heliocentric radial velocity (RV = 0.67km s-1). By requiring that the abundance of iron isindependent of the 96 measured equivalent widths, we determined themicroturubulence velocity (ξ = 0 km s-1). The abundancepattern coming from our study is similar to the one inferred from UVlines by Castelli et al. (\cite{castelli85}), with the exception of O,Al, Si, Sc and Ni. Possible causes for these descrepancies arediscussed. With respect to the Sun, we found the iron peak elements tobe normal or overabundant and the light elements, with the exception ofNa and P, to be extremely underabundant. We find that HR 6000 is one ofthe most He-underabundant among the chemically peculiar stars.Based on observations collected at European Southern Observatory (ESO),La Silla, Chile, proposal ID 69.D-0537.
| Implementing TOPbase/Iron Project: continuous absorption from FeII
We discuss implementation of TOPbase and Iron Project opacities forstellar spectral codes. We use a technique employed by Peach, where aBoltzmann-averaged cross-section is calculated for selectedtemperatures, and the opacity obtained from double interpolation intemperature and wavelength. It is straightforward to include all levelsfor which cross-sections have been calculated. Boltzmann-averagedcross-sections for FeII show a local maximum between 1700 and 2000Å. We suggest this feature arises from 3d54snl->3d54pnl transitions within FeII. IUE spectra of iron-rich CPstars show local minima in this region. Theoretical calculations of arepresentative stellar continuum demonstrate that FeII photoionizationcontributes significantly to the observed minima.
| Brown Dwarfs in Southern Star Forming Regions
Most of the star forming regions within 200 pc from the Sun are locatedSouth of the celestial equator. Brown dwarfs or strong candidates havebeen discovered in all of them, and have been often studied at multiplewavelengths. This paper summarizes some highlights of the research onyoung brown dwarfs in these regions.
| New low-mass members of the Lupus 3 dark cloud: Further indications of pre-main-sequence evolution strongly affected by accretion
A spectroscopic survey of a small area at the center of the Lupus 3star-forming core has revealed four new mid-to-late M-type members,including a M7.5 brown dwarf. One of the new members, classified as M5,displays prominent forbidden lines and strong Hα emission(EW(Hα ) = 410 Å), in addition to other permitted lines, andits luminosity is far below that of other members of the region withsimilar or later spectral types. We estimate a mass accretion rate rateof ~ 1.4 x 10-9 Msun yr-1 for thisobject, although with uncertainties that do not exclude values as low as10-10 Msun yr-1. Based on the Hα/ [SII] ratio, the detection of HeI, and the CaII infrared triplet, weargue that most of the Hα emission is produced near the surfaceof the object, probably in accretion columns or at the base of jets,rather than in a low-density extended region. The strong emission-linespectrum superimposed on an unusually faint photospheric continuum thusseems to be a real, intrinsic feature rather than a result of theviewing geometry caused by an edge-on disk blocking the light from thecentral object. Other Lupus 3 late-type members also display noticeableunderluminosity, all of them having EW(Hα ) > 100 Å as aresult of the faint underlying continuum. We tentatively interpret thesefindings as evidence for the pre-main sequence evolution of objects withvery low (possibly substellar) initial masses being significantlymodified by accretion.Based on observations obtained at the European Southern Observatoryusing the Very Large Telescope in Cerro Paranal (program 67.C-0549(A)),the ESO New Technology Telescope in La Silla (program 69.B-0126(A)), andthe 1.5 Danish telescope, also in La Silla (program 69.C-0423(B)).
| Rotational velocities of A-type stars in the northern hemisphere. II. Measurement of v sin i
This work is the second part of the set of measurements of v sin i forA-type stars, begun by Royer et al. (\cite{Ror_02a}). Spectra of 249 B8to F2-type stars brighter than V=7 have been collected at Observatoirede Haute-Provence (OHP). Fourier transforms of several line profiles inthe range 4200-4600 Å are used to derive v sin i from thefrequency of the first zero. Statistical analysis of the sampleindicates that measurement error mainly depends on v sin i and thisrelative error of the rotational velocity is found to be about 5% onaverage. The systematic shift with respect to standard values fromSlettebak et al. (\cite{Slk_75}), previously found in the first paper,is here confirmed. Comparisons with data from the literature agree withour findings: v sin i values from Slettebak et al. are underestimatedand the relation between both scales follows a linear law ensuremath vsin inew = 1.03 v sin iold+7.7. Finally, thesedata are combined with those from the previous paper (Royer et al.\cite{Ror_02a}), together with the catalogue of Abt & Morrell(\cite{AbtMol95}). The resulting sample includes some 2150 stars withhomogenized rotational velocities. Based on observations made atObservatoire de Haute Provence (CNRS), France. Tables \ref{results} and\ref{merging} are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/393/897
| Rotational velocities of A-type stars. I. Measurement of v sin i in the southern hemisphere
Within the scope of a Key Programme determining fundamental parametersof stars observed by HIPPARCOS, spectra of 525 B8 to F2-type starsbrighter than V=8 have been collected at ESO. Fourier transforms ofseveral line profiles in the range 4200-4500 Å are used to derivev sin i from the frequency of the first zero. Statistical analysis ofthe sample indicates that measurement error is a function of v sin i andthis relative error of the rotational velocity is found to be about 6%on average. The results obtained are compared with data from theliterature. There is a systematic shift from standard values from\citet{Slk_75}, which are 10 to 12% lower than our findings. Comparisonswith other independent v sin i values tend to prove that those fromSlettebak et al. are underestimated. This effect is attributed to thepresence of binaries in the standard sample of Slettebak et al., and tothe model atmosphere they used. Based on observations made at theEuropean Southern Observatory (ESO), La Silla, Chile, in the frameworkof the Key Programme 5-004-43K. Table 4 is only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.125.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/381/105
| A photometric catalogue of southern emission-line stars
We present a catalogue of previously unpublished optical and infraredphotometry for a sample of 162 emission-line objects and shell starsvisible from the southern hemisphere. The data were obtained between1978 and 1997 in the Walraven (WULBV), Johnson/Cousins(UBV(RI)c) and ESO and SAAO near-infrared (JHKLM) photometricsystems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars orHAeBe candidates appearing in the list of HAeBe candidates of Théet al. (1994), although several B[e] stars, LBVs and T Tauri stars arealso included in our sample. For many of the stars the data presentedhere are the first photo-electric measurements in the literature. Theresulting catalogue consists of 1809 photometric measurements. Opticalvariability was detected in 66 out of the 116 sources that were observedmore than once. 15 out of the 50 stars observed multiple times in theinfrared showed variability at 2.2 mu m (K band). Based on observationscollected at the European Southern Observatory, La Silla, Chile and onobservations collected at the South African Astronomical Observatory.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/380/609
| 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.
| On the delta Scuti variability in the pre-main sequence Herbig Ae star HR 5999
A reanalysis of photometric data for the bright, southern Herbig Ae starHR 5999 obtained by Praderie et al. (\cite{r17}) at ESO in 1985 showsdelta Scuti pulsation with the same period, 4.812 d-1, andsemi-amplitude, 6 mmag, found by Kurtz & Marang (\cite{r10}) in dataobtained from SAAO in 1994. HR 5999 is, therefore, a potential candidateto test Breger & Pamyatnykh's (\cite{r4}) predictions ofevolutionary period changes 10 to 100 times faster in pre-main sequencedelta Scuti stars than in post-main sequence stars. We argue that thelow frequency, low amplitude alpha 2 CVn variability of HR6000 with a period near 2 d, and its long-term variability are nohindrance to using it as a comparison star for study of the delta Scutivariability of HR 5999. With a separation of 44 arcsec between HR 5999and HR 6000, and similar brightnesses for the two stars, we urge the useof small telescopes with CCD photometers to obtain the long-term datanecessary for the study of period change. We point out that this can bedone under less-than-ideal photometric conditions and urban lightingwith CCD photometers on small telescopes which are widely available atsmall observatories, at universities and at the home observatories ofserious amateur astronomers.
| A study of interstellar Nai D absorption lines towards the Lupus molecular clouds
Intermediate-resolution (60000<=R<=120000) observations ofinterstellar Nai lines towards 29 stars in the general direction of theLupus molecular clouds (330°<~l<~350°0°<~b<~25°) are presented. Previously published spectratowards an additional seven stars are also included. Based on theHipparcos distances to these stars, and the minimum distance at whichstrong interstellar Nai lines appear in the spectra, I obtain a distanceof ~150+/-10pc to the Lupus molecular complex. While in agreement with anumber of other independent estimates, this result is at odds with thevalue of 100pc recently obtained by Knude & Høg from aHipparcos-based study of interstellar extinction. A possible explanationfor this discrepancy is discussed, and it is concluded that the value of150+/-10pc obtained here is to be preferred. In addition, theseobservations have some other implications for the structure of theinterstellar medium in this direction, and these are briefly considered.
| Dense Cores of Dark Clouds. XII. 13CO and C18O in Lupus, Corona Australis, Vela, and Scorpius
More than 110 dense condensations of the dark clouds in Lupus, CoronaAustralis, Norma, Vela, and Scorpius were observed in the13CO and C18O (J=1-0) transitions. Thecondensations of dark clouds with high star formation activity like theOphiuchus, Taurus, and Cepheus have average C18O andH2 column densities of 1.8x1015 and1.1x1022 cm-2. If we take the average size of thecondensations to be 0.2 pc, a condensation must have averageH2 volumetric densities >=2x104 cm-3in order to be a good candidate to form stars. The four Lupus filamentshave similar radial velocities and velocity dispersions, suggesting thatthey originated from the same parental cloud. Among these filaments,Lupus 1 is unique in having recent star formation activity, despite thehigh number of T Tauri stars observed toward the others. Lupus 1 alsoshows a complex velocity gradient along its main axis. The distributionof radial velocities of the condensations observed toward Scorpius arein good agreement with the hypothesis that they are in a region withexpansion velocity smaller than or equal to 18 km s-1. TheCorona Australis cloud has velocity gradients ranging from -0.5 kms-1 pc-1 at one extreme to 0.1 km s-1pc-1 at the other.
| A Near-Infrared Imaging Survey of the Lupus 3 Dark Cloud: A Modest Cluster of Low-Mass, Pre-Main-Sequence Stars
We present the first report on results of a near-infrared imaging surveyof the Lupus 3 dark cloud. This cloud is known to be associated with amodest cluster of T Tauri stars from a previous optical Hαemission-line star survey. The survey covers7'x11', which corresponds to a projected area of~0.35x0.55 pc at a distance of 150 pc. Mapping was carried out at J, H,and Ks, to 10 σ limiting magnitudes of J=17.0, H=16.5,and Ks=15.5. A total of 229 sources brighter thanKs<15.8 were detected at all bands with a 90% completenesslimit. Source classification is performed based on the near-infraredcolors. Ten sources are candidates of Lada's Class II pre-main-sequence(PMS) stars, as they have a color excess that cannot be explained byreddening resulting from interstellar dust. We also identified 11 ClassI-like candidates that were not detected at J and have a large colorexcess (H-Ks>=2), which is unlikely to arise fromextinction in the Lupus dark cloud. There are four subclusters in thissurvey area of which three are embedded and mainly consist of the ClassI-like candidates. The average density of PMS stars is around 500pc-3, suggesting the presence of a modest cluster of embeddedPMS stars. We estimate masses of the Class II candidates with aid of anevolutionary model of PMS stars. Ten of them have masses less than 0.08Msolar if we assume their age to be 106 yr. Hence,we consider them to be young brown dwarf (YBD) candidates. The relativepopulation of YBDs in the Lupus 3 dark cloud is larger than in theTaurus.
| Mid infrared emission of nearby Herbig Ae/Be stars
We present mid IR spectro-photometric imaging of a sample of eightnearby (D <= 240 pc) Herbig Ae/Be stars. The spectra are dominatedby photospheric emission (HR6000), featureless infrared excess emission(T Cha), broad silicate emission feature (HR5999) and the infraredemission bands (HD 97048, HD 97300, TY CrA, HD 176386). The spectrum ofHD179218 shows both silicate emission and infrared emission bands (IEB).All stars of our sample where the spectrum is entirely dominated by IEBhave an extended emission on scales of a few thousand AU ( ~ 10''). Weverify the derived source extension found with ISOCAM by multi-aperturephotometry with ISOPHT and compare our ISOCAM spectral photometry withISOSWS spectra. Based on observations with ISO, an ESA project withinstruments funded by ESA Member States (especially the PI countries:France, Germany, the Netherlands and the United Kingdom) with theparticipation of ISAS and NASA.
| A-shell stars in the Geneva system
Among the various kinds of A stars having a peculiar spectrum, we findthe A-shell stars. Many questions are still open concerning these stars,including their evolutionary status. In the present study we have useddata from the Hipparcos catalogue to examine this point. We have foundthat the majority of A-shell stars are well above the main sequence. Nodifferences could be established between A-shell stars in luminosityclasses III and I and those in luminosity class V as regardsvariability, duplicity, or the importance of the shell feature.
| Manganese abundances in mercury-manganese stars
We use exact curve-of-growth analysis and spectral synthesis to deducethe abundance of Mn from high signal-to-noise ratio visible-regionechelle spectra of selected Mn i and MnII lines in 24 HgMn stars. Theresults are compared with the Mn abundances derived from UV resonancelines by Smith & Dworetsky. We find excellent agreement for severalunblended Mn lines and confirm the temperature dependence of the Mnabundance found by Smith & Dworetsky. The MnII lines at lambdalambda 4206 and 4326 are much stronger than one would predict from themean Mn abundances. The lack of agreement is greatest for stars with thestrongest MnII lines. Using ad hoc multicomponent fits to the profilesof sharp-lined stars, we show that most of the discrepancies can beexplained by hyperfine structure that desaturates the lines, with fullwidths of the order of 0.06-0.09 A.
| A HIPPARCOS Census of the Nearby OB Associations
A comprehensive census of the stellar content of the OB associationswithin 1 kpc from the Sun is presented, based on Hipparcos positions,proper motions, and parallaxes. It is a key part of a long-term projectto study the formation, structure, and evolution of nearby young stellargroups and related star-forming regions. OB associations are unbound``moving groups,'' which can be detected kinematically because of theirsmall internal velocity dispersion. The nearby associations have a largeextent on the sky, which traditionally has limited astrometricmembership determination to bright stars (V<~6 mag), with spectraltypes earlier than ~B5. The Hipparcos measurements allow a majorimprovement in this situation. Moving groups are identified in theHipparcos Catalog by combining de Bruijne's refurbished convergent pointmethod with the ``Spaghetti method'' of Hoogerwerf & Aguilar.Astrometric members are listed for 12 young stellar groups, out to adistance of ~650 pc. These are the three subgroups Upper Scorpius, UpperCentaurus Lupus, and Lower Centaurus Crux of Sco OB2, as well as VelOB2, Tr 10, Col 121, Per OB2, alpha Persei (Per OB3), Cas-Tau, Lac OB1,Cep OB2, and a new group in Cepheus, designated as Cep OB6. Theselection procedure corrects the list of previously known astrometricand photometric B- and A-type members in these groups and identifiesmany new members, including a significant number of F stars, as well asevolved stars, e.g., the Wolf-Rayet stars gamma^2 Vel (WR 11) in Vel OB2and EZ CMa (WR 6) in Col 121, and the classical Cepheid delta Cep in CepOB6. Membership probabilities are given for all selected stars. MonteCarlo simulations are used to estimate the expected number of interloperfield stars. In the nearest associations, notably in Sco OB2, thelater-type members include T Tauri objects and other stars in the finalpre-main-sequence phase. This provides a firm link between the classicalhigh-mass stellar content and ongoing low-mass star formation. Detailedstudies of these 12 groups, and their relation to the surroundinginterstellar medium, will be presented elsewhere. Astrometric evidencefor moving groups in the fields of R CrA, CMa OB1, Mon OB1, Ori OB1, CamOB1, Cep OB3, Cep OB4, Cyg OB4, Cyg OB7, and Sct OB2, is inconclusive.OB associations do exist in many of these regions, but they are eitherat distances beyond ~500 pc where the Hipparcos parallaxes are oflimited use, or they have unfavorable kinematics, so that the groupproper motion does not distinguish it from the field stars in theGalactic disk. The mean distances of the well-established groups aresystematically smaller than the pre-Hipparcos photometric estimates.While part of this may be caused by the improved membership lists, arecalibration of the upper main sequence in the Hertzsprung-Russelldiagram may be called for. The mean motions display a systematicpattern, which is discussed in relation to the Gould Belt. Six of the 12detected moving groups do not appear in the classical list of nearby OBassociations. This is sometimes caused by the absence of O stars, but inother cases a previously known open cluster turns out to be (part of) anextended OB association. The number of unbound young stellar groups inthe solar neighborhood may be significantly larger than thoughtpreviously.
| Radial velocities of HIPPARCOS southern B8-F2 type stars
Radial velocities have been determined for a sample of B8-F2 type starsobserved by the Hipparcos satellite. Observations were obtained withinthe framework of an ESO key-program. Radial velocities have beenmeasured using a cross-correlation method, the templates being a grid ofsynthetic spectra. The obtained precision depends on effectivetemperature and projected rotational velocity of the star as well as ona possible asymmetry of the correlation peak generally due to secondarycomponents. New spectroscopic binaries have been detected from theseasymmetries and the variability of the measured radial velocity.Simulations of binary and triple systems have been performed. Forbinaries our results have been compared with Hipparcos binary data.Adding the variable radial velocities, the minimum binary fraction hasbeen found 60% for physical systems. Radial velocities have beendetermined for 581 B8-F2 stars, 159 being new. Taking into accountpublished radial velocities, 39% south A-type stars with V magnitudelower than 7.5 have a radial velocity. Based on observations obtained atthe European Southern Observatory (ESO, La Silla, Chile) and on datafrom the ESA Hipparcos astrometry satellite.}\fnmsep \thanks{Tables 7, 8and 9 are only available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| Revisiting Hipparcos data for pre-main sequence stars
We cross-correlate the Herbig & Bell and Hipparcos Catalogues inorder to extract the results for young stellar objects (YSOs). Wecompare the distances of individual young stars and the distance oftheir presumably associated molecular clouds, taking into accountpost-Hipparcos distances to the relevant associations and usingHipparcos intermediate astrometric data to derive new parallaxes of thepre-main sequence stars based on their grouping. We confirm that YSOsare located in their associated clouds, as anticipated by a large bodyof work, and discuss reasons which make the individual parallaxes ofsome YSOs doubtful. We find in particular that the distance of TaurusYSOs as a group is entirely consistent with the molecular clouddistance, although Hipparcos distances of some faint Taurus-Auriga starsmust be viewed with caution. We then improve some of the solutions forthe binary and multiple pre-main sequence stars. In particular, weconfirm three new astrometric young binaries discovered by Hipparcos:RY Tau, UX Ori, and IXOph. Based on observations made with the ESA Hipparcosastrometry satellite
| Elemental abundance analyses with DAO spectrograms - XVIII. The double-lined spectroscopic binary 46 Draconis
We present a study of the double-lined spectroscopic binary star 46Draconis (46 Dra). The variations of the gamma-velocity indicate thepresence of a third star which revolves around the mutual centre-of-massof the close binary system with a period of 33.6 yr. Perhaps this thirdstar is responsible for the observed X-ray radiation of 46 Dra. With atemperature difference of only 600 K, both components of the closebinary are mercury-manganese stars. This system demonstrates thechemical evolution of two apparently metal-rich objects with the sameinitial composition, but with slightly different masses and resultingeffective temperatures. Significant abundance differences are found forHe, Al, S, Sc, Mn, Sr, Ga, Xe and Pt. These provide tests for thetheories which purport to explain the anomalies.
| The Scorpius OB2 Complex
The Sco OB2 complex is a member of the Local Association. Theassociation contains pre-main-sequence stars in addition to objects some3 x 10^7 yr old. If it is assumed that stars in wide binaries arecoeval, then the He-weak variables in the supercluster arepre-main-sequence, elevated above that sequence by an amount similar tothat of the 3 x 10^7 yr old, normal stars but contracting toward thesequence, not evolving from it. The apparent great depth of theassociation is probably a result of some foreground superclustermembers, superposed on Sco OB2, and a rift in the absorption cloudbetween Scorpius and Ophiuchus. The adjoining Chamaeleon complex alsoappears to be a member of the supercluster.
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הכנס מאמר חדש
לינקים קשורים
הכנס לינק חדש
משמש של הקבוצה הבאה
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | עקרב |
| התרוממות ימנית: | 16h08m34.40s |
| סירוב: | -39°05'35.0" |
| גודל גלוי: | 6.65 |
| מרחק: | 240.964 פארסק |
| תנועה נכונה: | -6.4 |
| תנועה נכונה: | -23.9 |
| B-T magnitude: | 6.536 |
| V-T magnitude: | 6.617 |
קטלוגים וכינוים:
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INHABITED SKY
