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An L'-band survey for circumstellar discs around low-mass stars in the young σ Orionis cluster
We present new K- and L'-band imaging of a representative sample ofmembers of the young 3-5 Myr old σ Orionis cluster. We identifiedobjects with (K-L') excess by analysing colour-colour diagrams andcomparing the observations with empirical main-sequence colours. Thederived disc frequency depends on the method used: (54 +/- 15) per centif measured directly from the JHKL' colour-colour diagram, or (46 +/-14) per cent if excesses are computed with respect to predictedphotospheric colours (according to the object spectral types, 2σexcess detections). We compare the (K-L') excess with other indicatorsand show that this is a robust and reliable disc indicator. We alsocompare the derived disc frequency with similarly aged clusters anddiscuss possible implications for disc lifetimes. The computed age ofthe σ Ori cluster is very important; a cluster age of 3 Myr wouldsupport the overall disc lifetime of 6 Myr proposed in the literature,while an age >4 Myr would point to a slower disc destruction rate.
| L' and M' standard stars for the Mauna Kea Observatories Near-Infrared system
We present L' and M' photometry, obtained at the United Kingdom InfraredTelescope (UKIRT) using the Mauna Kea Observatories Near-Infrared(MKO-NIR) filter set, for 46 and 31 standard stars, respectively. The L'standards include 25 from the in-house `UKIRT Bright Standards' withmagnitudes deriving from Elias et al. and observations at the InfraredTelescope Facility in the early 1980s, and 21 fainter stars. The M'magnitudes derive from the results of Sinton and Tittemore. We estimatethe average external error to be 0.015 mag for the bright L' standardsand 0.025 mag for the fainter L' standards, and 0.026 mag for the M'standards. The new results provide a network of homogeneously observedstandards, and establish reference stars for the MKO system, in thesebands. They also extend the available standards to magnitudes whichshould be faint enough to be accessible for observations with moderndetectors on large and very large telescopes.
| Star-forming regions in blue compact dwarf galaxies
We present near-infrared 2.1-micron continuum and Br gamma and 1-0S(1)line images of three blue compact dwarf galaxies: II Zw 40, NGC 5253,and He 2-10. Comparison of the morphologies of the emission-ine regionsand continuum together with the line ratios shows that in these casesthe starbursts are typified by strongly peaked Br gamma and weak diffuseH2 emission, indicating the presence of one or more compact nuclei, andalso tidal tails. We consider the possible origins of the H2 emissionand conclude that shock excitation in cloud collisions, and hencedynamical processes, such as interactions or mergers, are important. Bycomparing models of the equivalent width of Br gamma to other ageconstraints from the literature, we show that the star formation musthave occurred not only recently but in a short-duration burst. Thecurrent centers of star formation in all three galaxies are of a similarage but, whereas the hotspots in He 2-10 are coeval, we find phasedifferences between those in NGC 5253. We compare the masses and sizesof the star formation sites to those of present-day globular clusters.
| The Henry Draper Extension Charts: A catalogue of accurate positions, proper motions, magnitudes and spectral types of 86933 stars
The Henry Draper Extension Charts (HDEC), published in the form offinding charts, provide spectral classification for some 87000 starsmostly between 10th and 11th magnitude. This data, being highlyvaluable, as yet was practically unusable for modern computer-basedastronomy. An earlier pilot project (Roeser et al. 1991) demonstrated apossibility to convert this into a star catalogue, using measurements ofcartesian coordinates of stars on the charts and positions of theAstrographic Catalogue (AC) for subsequent identification. We presenthere a final HDEC catalogue comprising accurate positions, propermotions, magnitudes and spectral classes for 86933 stars of the HenryDraper Extension Charts.
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| Сазвежђа: | Орион |
| Ректацензија: | 05h19m17.16s |
| Deклинација: | +01°42'16.1" |
| Apparent магнитуда: | 10.021 |
| Proper motion RA: | 21.3 |
| Proper motion Dec: | -41 |
| B-T magnitude: | 10.685 |
| V-T magnitude: | 10.076 |
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