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A revisit to agglomerates of early-type Hipparcos stars
% We study the spatial structure and sub-structure of regions rich in{Hipparcos} stars with blue B_T-V_T colours. These regions, whichcomprise large stellar complexes, OB associations, and young openclusters, are tracers of on-going star formation in the Galaxy. TheDBSCAN (Density-Based Spatial Clustering of Applications with Noise)data clustering algorithm is used to look for spatial overdensities ofearly-type stars. Once an overdensity, ``agglomerate'', is identified,we carry out a data and bibliographic compilation of their star membercandidates. The actual membership in agglomerate of each early-type staris studied based on its heliocentric distance, proper motion, andprevious spectro-photometric information. We identify 35 agglomerates ofearly-type {Hipparcos} stars. Most of them are associated to previouslyknown clusters and OB associations. The previously unknown P Puppisagglomerate is subject of a dedicated study with Virtual Observatorytools. It is actually a new, nearby, young open cluster (d ˜ 470pc, age ˜ 20 Ma) with a clear radial density gradient. We list PPuppis and other six agglomerates (including NGC 2451 A, vdBH 23, andTrumpler 10) as new sites for substellar searches because of theiryouth, closeness, and spatial density. We investigate in detail thesub-structure in the Orion, CMa-Pup and Pup-Vel OB complexes(``super-agglomerates''). We confirm or discover some stellaroverdensities in the Orion complex, like the 25 Ori group, the Horseheadregion (including the σ Orionis cluster), and the η Orionisagglomerate. Finally, we derive accurate parallactic distances to thePleiades, NGC 2451 A, and IC 2391, describe several field early-typestars at d < 200 pc, and discuss the incompleteness of our search.
| K-band spectroscopy of pre-cataclysmic variables
Aims. There exists now substantial evidence for abundance anomalies in anumber of cataclysmic variables (CVs), indicating that the photosphereof the secondary star incorporates thermonuclear processed material.However, the spectral energy distribution in CVs is usually dominated bythe radiation produced by the accretion process, severely hindering aninvestigation of the stellar components. On the other hand, depending onhow the secondary star has acquired such material, the above mentionedabundance anomalies could also be present in pre-CVs, i.e. detachedwhite/red dwarf binaries that will eventually evolve into CVs, but havenot yet started mass transfer, and therefore allow for an unobstructedview on the secondary star at infrared wavelengths. Methods: We havetaken K-band spectroscopy of a sample of 13 pre-CVs in order to examinethem for anomalous chemical abundances. In particular, we study thestrength of the 12CO and 13CO absorption bandsthat have been found diminished and enhanced, respectively, in similarstudies of CVs. Results: All our systems show CO abundances that arewithin the range observed for single stars. The weakest 12CObands with respect to the spectral type are found in the pre-CV BPM71214, although on a much smaller scale than observed in CVs.Furthermore there is no evidence for enhanced 13CO. Takinginto account that our sample is subject to the present observationalbias that favours the discovery of young pre-CVs with secondary stars oflate spectral types, we can conclude the following: 1) our studyprovides observational proof that the CO anomalies discovered in certainCVs are not due to any material acquired during the common envelopephase, and 2) if the CO anomalies in certain CVs are not due toaccretion of processed material during nova outburst, then theprogenitors of these CVs are of a significantly different type than thecurrently known sample of pre-CVs.Based on observations made at ESO-Paranal, proposals 075.D-0012 and076.D-0538.
| Investigation of a Milky Way field in Canis Major
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1974AJ.....79.1022C&db_key=AST
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| Takýmyýldýz: | Tekboynuz |
| Sað Açýklýk: | 07h10m46.96s |
| Yükselim: | -09°20'09.6" |
| Görünürdeki Parlaklýk: | 7.412 |
| Uzaklýk: | 423.729 parsek |
| özdevim Sað Açýklýk: | -4.1 |
| özdevim Yükselim: | 3.4 |
| B-T magnitude: | 7.276 |
| V-T magnitude: | 7.401 |
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