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The primordial binary population. I. A near-infrared adaptive optics search for close visual companions to A star members of Scorpius OB2
We present the results of a near-infrared adaptive optics survey withthe aim to detect close companions to Hipparcos members in the threesubgroups of the nearby OB association Sco OB2: Upper Scorpius (US),Upper Centaurus Lupus (UCL) and Lower Centaurus Crux (LCC). We havetargeted 199 A-type and late B-type stars in the KS band, anda subset also in the J and H band. We find 151 stellar components otherthan the target stars. A brightness criterion is used to separate thesecomponents into 77 background stars and 74 candidate physical companionstars. Out of these 74 candidate companions, 41 have not been reportedbefore (14 in US; 13 in UCL; 14 in LCC). The angular separation betweenprimaries and observed companion stars ranges from 0.22'' to 12.4''. Atthe mean distance of Sco OB2 (130 pc) this corresponds to a projectedseparation of 28.6 AU to 1612 AU. Absolute magnitudes are derived forall primaries and observed companions using the parallax andinterstellar extinction for each star individually. For each object wederive the mass from KS, assuming an age of 5 Myr for the USsubgroup, and 20 Myr for the UCL and LCC subgroups. Companion starmasses range from 0.10 Mȯ to 3.0 Mȯ. Themass ratio distribution follows f(q) = q-Γ withΓ=0.33, which excludes random pairing. No close (ρ ≤3.75'') companion stars or background stars are found in the magnituderange 12 mag≤ KS ≤ 14 mag. The lack of stars withthese properties cannot be explained by low-number statistics, and mayimply a lower limit on the companion mass of 0.1Mȯ. Close stellar components with KS >14mag are observed. If these components are very low-mass companion stars,a gap in the companion mass distribution might be present. The smallnumber of close low-mass companion stars could support theembryo-ejection formation scenario for brown dwarfs. Our findings arecompared with and complementary to visual, spectroscopic, andastrometric data on binarity in Sco OB2. We find an overall companionstar fraction of 0.52 in this association. This is a lower limit sincethe data from the observations and from literature are hampered byobservational biases and selection effects. This paper is the first steptoward our goal to derive the primordial binary population in Sco OB2.Full Table 1 is only available in electronic form athttp://www.edpsciences.orgBased on observations collected with the ADONIS instrument at theEuropean Southern Observatory, La Silla, Chile (65.H-0568(A) and67.D-0220(A)).
| The First Measurement of Spectral Lines in a Short-Period Star Bound to the Galaxy's Central Black Hole: A Paradox of Youth
We have obtained the first detection of spectral absorption lines in oneof the high-velocity stars in the vicinity of the Galaxy's centralsupermassive black hole. Both Brγ (2.1661 μm) and He I (2.1126μm) are seen in absorption in S0-2 with equivalent widths (2.8+/-0.3and 1.7+/-0.4 Å) and an inferred stellar rotational velocity(220+/-40 km s-1) that are consistent with that of an O8-B0dwarf, which suggests that it is a massive (~15 Msolar) young(less than 10 Myr) main-sequence star. This presents a major challengeto star formation theories, given the strong tidal forces that prevailover all distances reached by S0-2 in its current orbit (130-1900 AU)and the difficulty in migrating this star inward during its lifetimefrom farther out where tidal forces should no longer preclude starformation. The radial velocity measurements (z>=-510+/-40km s-1) and our reported proper motions for S0-2 stronglyconstrain its orbit, providing a direct measure of the black hole massof 4.1(+/-0.6)×106(R0/8 kpc)3Msolar. The Keplerian orbit parameters haveuncertainties that are reduced by a factor of 2-3 compared to previouslyreported values and include, for the first time, an independent solutionfor the dynamical center; this location, while consistent with thenominal infrared position of Sgr A*, is localized to a factor of 5 moreprecisely (+/-2 mas). Furthermore, the ambiguity in the inclination ofthe orbit is resolved with the addition of the radial velocitymeasurement, indicating that the star is behind the black hole at thetime of closest approach and counterrevolving against the Galaxy. Withfurther radial velocity measurements in the next few years, the orbit ofS0-2 will provide the most robust estimate of the distance to theGalactic center.
| Discovery of Close Companions to the Nearby Young Stars HD 199143 and HD 358623
Young stellar systems in the solar neighborhood provide valuablelaboratories for detailed studies of star and planet formation. Thebright F8 V star HD 199143 and the Li-rich late-type emission-line starHD 358623 are among the nearest young stars identified to date and maybe members of a young association in Capricornus. We presenthigh-resolution near-infrared images of these two sources, obtainedusing the adaptive optics system on the 3.6 m telescope at the EuropeanSouthern Observatory in La Silla, Chile. Our observations reveal thatboth are, in fact, close binary systems. The newly discovered companionat a separation of ~1" may account for the unusual characteristics of HD199143-rapid rotation, emission lines, ultraviolet variability, andexcess infrared emission-recently discussed by van den Ancker andcoworkers. HD 199143 may be a rare example of a close binary with only acircumsecondary disk. With the detection of a ~2" companion, HD 358623is now possibly one of the closest known T Tauri binaries. Both binarysystems are prime targets for follow-up spectroscopic and astrometricobservations. Based on observations collected at the European SouthernObservatory, Chile.
| A spectroscopic survey for lambda Bootis stars. II. The observational data
lambda Bootis stars comprise only a small number of all A-type stars andare characterized as nonmagnetic, Population i, late B to early F-typedwarfs which show significant underabundances of metals whereas thelight elements (C, N, O and S) are almost normal abundant compared tothe Sun. In the second paper on a spectroscopic survey for lambda Bootisstars, we present the spectral classifications of all program starsobserved. These stars were selected on the basis of their Strömgrenuvbybeta colors as lambda Bootis candidates. In total, 708 objects insix open clusters, the Orion OB1 association and the Galactic field wereclassified. In addition, 9 serendipity non-candidates in the vicinity ofour program stars as well as 15 Guide Star Catalogue stars were observedresulting in a total of 732 classified stars. The 15 objects from theGuide Star Catalogue are part of a program for the classification ofapparent variable stars from the Fine Guidance Sensors of the HubbleSpace Telescope. A grid of 105 MK standard as well as ``pathological''stars guarantees a precise classification. A comparison of our spectralclassification with the extensive work of Abt & Morrell(\cite{Abt95}) shows no significant differences. The derived types are0.23 +/- 0.09 (rms error per measurement) subclasses later and 0.30 +/-0.08 luminosity classes more luminous than those of Abt & Morrell(\cite{Abt95}) based on a sample of 160 objects in common. The estimatederrors of the means are +/- 0.1 subclasses. The characteristics of oursample are discussed in respect to the distribution on the sky, apparentvisual magnitudes and Strömgren uvbybeta colors. Based onobservations from the Observatoire de Haute-Provence, OsservatorioAstronomico di Padova-Asiago, Observatório do Pico dosDias-LNA/CNPq/MCT, Chews Ridge Observatory (MIRA) and University ofToronto Southern Observatory (Las Campanas).
| Sixth Catalogue of Fundamental Stars (FK6). Part III. Additional fundamental stars with direct solutions
The FK6 is a suitable combination of the results of the HIPPARCOSastrometry satellite with ground-based data, measured over a longinterval of time and summarized mainly in the FK5. Part III of the FK6(abbreviated FK6(III)) contains additional fundamental stars with directsolutions. Such direct solutions are appropriate for single stars or forobjects which can be treated like single stars. Part III of the FK6contains in total 3272 stars. Their ground-based data stem from thebright extension of the FK5 (735 stars), from the catalogue of remainingSup stars (RSup, 732 stars), and from the faint extension of the FK5(1805 stars). From the 3272 stars in Part III, we have selected 1928objects as "astrometrically excellent stars", since their instantaneousproper motions and their 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,354 of the stars in Part III 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(III) proper motion in the single-star mode is 0.59 mas/year. This isa factor of 1.34 better than the typical HIPPARCOS errors for thesestars of 0.79 mas/year. In the long-term prediction mode, in whichcosmic errors are taken into account, the FK6(III) proper motions have atypical mean error of 0.93 mas/year, which is by a factor of about 2better than the corresponding error for the HIPPARCOS values of 1.83mas/year (cosmic errors included).
| Near infrared light variations of CP stars. The SiSrCrEu stars
Twelve magnetic Chemically Peculiar (CP2) stars of the SiSrCrEu subgroupmostly brighter than the 7.5 visual magnitude have been investigated inthe infrared at 1.25, 1.6 and 2.2 mu . The stars HD 74521, HD 90044, HD119419, HD 125630, and HD 187473 are clearly variable in the nearinfrared with the same period as the visible light, spectrum, andmagnetic field variations. The stars HD 10783, HD 12447, HD 116458, HD147010, HD 166469, HD 170397, and HD 223640 do show a smaller amount ofvariability, although with quite large a dispersion of the data. Aremarkable result of the present investigation is that, at least for thestars for which contemporaneous observations are available, the observedinfrared variations appear to be in phase with the variations in thelong wavelength part of the visible. This may be an indication that themechanism of the infrared variations should be the same as for thevisible. Based on observations collected at the European SouthernObservatory, La Silla, Chile.
| SANTIAGO 91, a right ascension catalogue of 3387 stars (equinox J2000).
The positions in right ascension of 3387 stars belonging to the Santiago67 Catalogue, observed with the Repsold Meridian Circle at Cerro Calan,National Astronomical Observatory, during the period 1989 to 1994, aregiven. The average mean square error of a position, for the wholeCatalogue, is +/-0.009 s. The mean epoch of the catalogue is 1991.84.
| Fifth fundamental catalogue. Part 2: The FK5 extension - new fundamental stars
The mean positions and proper motions for 3117 new fundamental starsessentially in the magnitude range about 4.5 to 9.5 are given in thisFK5 extension. Mean apparent visual magnitude is 7.2 and is on average2.5 magnitudes fainter then the basic FK5 which has a mean magnitude of4.7. (The basic FK5 gives the mean positions and proper motions for theclassical 1535 fundamental stars). The following are discussed: theobservational material, reduction of observations, star selection, andthe system for the FK5 extension. An explanation and description of thecatalog are given. The catalog of 3117 fundamental stars for the equinoxand epoch J2000.0 and B1950.0 is presented. The parallaxes and radialvelocities for 22 extension stars with large forecasting effects aregiven. Catalogs used in the compilation of the FK5 fundamental catalogare listed.
| Photometry of silicon stars.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977A&A....54..443H&db_key=AST
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Observation and Astrometry data
| Constellation: | Sagittaire |
| Right ascension: | 20h05m12.01s |
| Declination: | -26°48'47.8" |
| Apparent magnitude: | 7.223 |
| Distance: | 164.745 parsecs |
| Proper motion RA: | 10.8 |
| Proper motion Dec: | -9.3 |
| B-T magnitude: | 7.336 |
| V-T magnitude: | 7.233 |
Catalogs and designations:
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INHABITED SKY
