Contents
Images
Upload your image
DSS Images Other Images
Related articles
New techniques for integral field spectroscopy - II. Performance of the GNIRS IFU
We present results on the performance of the integral field spectroscopy(IFS) capability of the Gemini Near-Infrared Spectrograph installed onthe Gemini-North telescope. This makes use of the innovative AdvancedImage Slicer optical concept described in Paper 1 and uses newdevelopments in diamond-machining to produce its complex micro-optics.The system delivers near-optimal performance for IFS in a small packagethat can be adapted to work with a wide range of spectrographs.In this paper, we present results of extensive tests carried out on thetelescope to verify the measurements of throughput, image quality andscattered light obtained in the laboratory and characterize theinstrument's geometry and spectroscopic performance. This shows that theperformance model, when fed with results of the component surfacemetrology, provides a good match to the throughput of the integral fieldunit measured on the telescope between 1 and 2.5 μm (65 and 90 percent, respectively). At longer wavelengths, the throughput exceeds 90per cent and the way that the design handles diffraction means that itsperformance can actually exceed that of the spectrograph alone with aslit of equivalent width.We also present examples of data obtained during commissioning andsystem verification and compare this with data obtained in other ways,to verify the system performance in recovering astrophysical data.Finally, we review the performance and its relevance to futureterrestrial and space observatories, in particular for large-scalemultiple-IFS applications. We make the case that our design is of greatrelevance to instrumentation for Extremely Large Telescopes. We showthat the throughput can be improved further by techniques to improve thequality of the optical surfaces.
| Statistical Constraints for Astrometric Binaries with Nonlinear Motion
Useful constraints on the orbits and mass ratios of astrometric binariesin the Hipparcos catalog are derived from the measured proper motiondifferences of Hipparcos and Tycho-2 (Δμ), accelerations ofproper motions (μ˙), and second derivatives of proper motions(μ̈). It is shown how, in some cases, statistical bounds can beestimated for the masses of the secondary components. Two catalogs ofastrometric binaries are generated, one of binaries with significantproper motion differences and the other of binaries with significantaccelerations of their proper motions. Mathematical relations betweenthe astrometric observables Δμ, μ˙, and μ̈ andthe orbital elements are derived in the appendices. We find a remarkabledifference between the distribution of spectral types of stars withlarge accelerations but small proper motion differences and that ofstars with large proper motion differences but insignificantaccelerations. The spectral type distribution for the former sample ofbinaries is the same as the general distribution of all stars in theHipparcos catalog, whereas the latter sample is clearly dominated bysolar-type stars, with an obvious dearth of blue stars. We point outthat the latter set includes mostly binaries with long periods (longerthan about 6 yr).
| 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)).
| Towards a fundamental calibration of stellar parameters of A, F, G, K dwarfs and giants
I report on the implementation of the empirical surface brightnesstechnique using the near-infrared Johnson broadband { (V-K)} colour assuitable sampling observable aimed at providing accurate effectivetemperatures of 537 dwarfs and giants of A-F-G-K spectral-type selectedfor a flux calibration of the Infrared Space Observatory (ISO). Thesurface brightness-colour correlation is carefully calibrated using aset of high-precision angular diameters measured by moderninterferometry techniques. The stellar sizes predicted by thiscorrelation are then combined with the bolometric flux measurementsavailable for a subset of 327 ISO standard stars in order to determineone-dimensional { (T, V-K)} temperature scales of dwarfs and giants. Theresulting very tight relationships show an intrinsic scatter induced byobservational photometry and bolometric flux measurements well below thetarget accuracy of +/- 1 % required for temperature determinations ofthe ISO standards. Major improvements related to the actual directcalibration are the high-precision broadband { K} magnitudes obtainedfor this purpose and the use of Hipparcos parallaxes for dereddeningphotometric data. The temperature scale of F-G-K dwarfs shows thesmallest random errors closely consistent with those affecting theobservational photometry alone, indicating a negligible contributionfrom the component due to the bolometric flux measurements despite thewide range in metallicity for these stars. A more detailed analysisusing a subset of selected dwarfs with large metallicity gradientsstrongly supports the actual bolometric fluxes as being practicallyunaffected by the metallicity of field stars, in contrast with recentresults claiming somewhat significant effects. The temperature scale ofF-G-K giants is affected by random errors much larger than those ofdwarfs, indicating that most of the relevant component of the scattercomes from the bolometric flux measurements. Since the giants have smallmetallicities, only gravity effects become likely responsible for theincreased level of scatter. The empirical stellar temperatures withsmall model-dependent corrections are compared with the semiempiricaldata by the Infrared Flux Method (IRFM) using the large sample of 327comparison stars. One major achievement is that all empirical andsemiempirical temperature estimates of F-G-K giants and dwarfs are foundto be closely consistent between each other to within +/- 1 %. However,there is also evidence for somewhat significant differential effects.These include an average systematic shift of (2.33 +/- 0.13) % affectingthe A-type stars, the semiempirical estimates being too low by thisamount, and an additional component of scatter as significant as +/- 1 %affecting all the comparison stars. The systematic effect confirms theresults from other investigations and indicates that previousdiscrepancies in applying the IRFM to A-type stars are not yet removedby using new LTE line-blanketed model atmospheres along with the updatedabsolute flux calibration, whereas the additional random component isfound to disappear in a broadband version of the IRFM using an infraredreference flux derived from wide rather than narrow band photometricdata. Table 1 and 2 are only available in the electronic form of thispaper
| Near-infrared imaging of the Seyfert 2 galaxy NGC 5252.
We present near-infrared (JHK) imaging of the Seyfert 2 galaxy NGC 5252,taken with the IRAC2 array on the 2.2m telescope at ESO, La Silla. Theunresolved and galactic components to the nuclear emission are separatedby a multicomponent fitting of the light profiles in the optical andnear-infrared bands. The nuclear flux in a 3" aperture is found to bedominated by the host galaxy with the unresolved component becoming lessevident toward shorter wavelengths, suggesting considerable reddening inthe nuclear region. The continuum energy distribution from the opticalto the radio wavelengths and the near-infrared nonstellar coloursconfirm a heavily reddened (A_V_>=6mag) nonstellar/intrinsic sourceand the presence of hot dust in the vicinity of the nucleus. On theother hand, the stellar component shows a progressive reddening towardthe center, consistent with an optical extinction of A_V_~1mag. Theinfrared luminosity, computed assuming reradiation by dust of UV/opticalnuclear photons, agrees within an order of magnitude with the observedinfrared luminosity. The near-infrared data are combined with opticalnarrow band continuum images to construct colour ratio maps. We find aband of red material in the optical-infrared ratio maps extending ~3kpcacross the nucleus and perpendicular to the biconical ionised structureof NGC 5252. If this infrared band is tracing the distribution of a ringor disk of dust in this galaxy, it may be getting denser in theproximity of the nucleus, significantly contributing to the obscurationof the Seyfert 1 nucleus.
| The MSSSO near-infrared photometric system
The JHKL photometric system currently used at the Mount Stromlo andSiding Spring Observatories (MSSSO) is described via an extensive listof standard-star values and filter transmission curves. At JHK thissystem is identical to the Mount Stromlo Observatory (MSO) systemdefined by Jones and Hyland (1982), except for small zero-pointdifferences which we impose here. Transformations are given between theMSSSO system and several near-infrared photometric systems in use inother observatories and the homogenized JHKL system proposed by Besselland Brett (1988).
| JHKLM standard stars in the ESO system
A list of 199 standard stars suitable for the ESO standard photometricsystem at JHKLM is given. Faint stars (although brighter than K = 7.7)to be used on larger telescopes are included. This list is based on ananalysis of all infrared photometric observations carried out at LaSilla from 1979 until 1989 inclusive. The accuracy of the data (about0.02 mag. at J, H, K, L, and M) is similar to the one achieved at SAAOand CTIO. Comparisons with these systems, as well as with the AAO andMSSO systems, are made: it is shown that the ESO system is very close tothe other ones, with the exception of CTIOs.
| 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.
| The luminosity function in NGC 2023
Sellgren (1983) has shown that the young stellar cluster within themolecular cloud associated with the reflection nebula NGC 2023 has aluminosity function consistent with the Salpeter IMF for stars brighterthan K = 12 mag. Extrapolation predicts that approximately 50 additionalstars brighter than K = 15 mag, arguably of spectral types G through K,should be present. However, images of the cluster using a sensitivetwo-dimensional IR array reveal no additional cluster members, whichsuggests that either no low-mass stars have formed or that they arestill heavily shrouded in molecular 'cocoons' left over from the starformation process. A straightforward experimental resolution of thealternatives is proposed.
| Southern JHKL standards
The basis for the current SAAO standard photometric system at JHKL isgiven. This depends on an extensive investigation involving 230 starsdistributed around the sky. The accuracy is estimated at + or - 0.02 magfor J, H and K and + or - 0.05 mag for L.
| Comparison of the Mt. Stromlo/AAO and Caltech/Tololo infrared photometric systems
The transformations between the Mt. Stromlo/AAO and the Caltech/Tololoinfrared photometric systems have been determined by means ofobservations of 21 stars with both systems. A small zero-pointdifference but no color term is found at K; color terms are found forthe J - K, J - H, and H - K colors. All scatter in the transformationscan be attributed to the random errors in the observations.
| Infrared standard stars
The results of an observational program aimed at setting up a network offaint near-infrared standards of sufficient accuracy are reported. Thenetwork covers both northern and southern hemispheres and includesstandards red enough to provide at least a limited check on colortransformations. The standards are set up at J (1.2 micron), H (1.6micron), K (2.2 microns), and L (3.5 microns), and their H2O and COmolecular absorption indices are determined. The problem of colortransformations between observatories is discussed briefly. Allmagnitudes presented are transformed to the natural system defined bythe CIT observations.
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
| Constellation: | Centaure |
| Right ascension: | 11h40m13.65s |
| Declination: | -39°08'47.7" |
| Apparent magnitude: | 7.524 |
| Distance: | 128.205 parsecs |
| Proper motion RA: | -30.1 |
| Proper motion Dec: | -22.6 |
| B-T magnitude: | 7.91 |
| V-T magnitude: | 7.556 |
Catalogs and designations:
|
