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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.
| Spectroscopic Gravitational Lensing and Limits on the Dark Matter Substructure in Q2237+0305
Spatially resolved spectroscopic data from the CIRPASS integral fieldunit (IFU) on Gemini are used to measure the gravitational lensing ofthe four-image quasar Q2237+0305 on different size scales. A method formeasuring the substructure present in the lens using observations atmultiple wavelengths is demonstrated to be very effective andindependent of many of the degeneracies inherent in previous methods.The magnification ratios of the QSO's narrow-line region (NLR) andbroad-line region (BLR) are measured and found to disagree with eachother and with the published radio and mid-infrared magnificationratios. The disagreement between the BLR ratios and theradio/mid-infrared ratios is interpreted as the result of microlensingby stars in the lens galaxy of the BLR. This implies that themid-infrared emission region is larger than the BLR and that the BLR is<~0.1 pc in size. We find a small difference between the shape of theH? line in image A and that in the other images. We consider thisdifference too small and symmetric to be strong evidence for rotation orlarge-scale infall in the H? emission region. The disagreementbetween the radio/mid-infrared ratios and the NLR ratios is interpretedas a signature of substructure on a larger scale, possibly the missingsmall-scale structure predicted by the standard cold dark matter (CDM)model. Extensive lensing simulations are performed to obtain a lowerlimit on the amount of substructure that is required to cause thisdiscrepancy as a function of its mass and the radial profile of the hostlens. The substructure surface density is degenerate with the radialprofile of the host lens, but if the expectations of the CDM model aretaken into account, certain radial profiles and substructure surfacedensities can be ruled out. A substructure mass scale as large as108 Msolar is strongly disfavored, while104 Msolar is too small if the radio andmid-infrared emission regions have the expected sizes of ~10 pc. Thestandard elliptical isothermal lens mass profile is not compatible witha substructure surface density of ?sub<280Msolar pc-2, at the 95% confidence level. This is4%-7% of the galaxy's surface density (depending on which image positionis used to evaluate this). The required substructure surface density atthe required mass scale is high in comparison with the presentexpectations within the CDM model. Lens mass profiles that are flatterthan isothermal-where the surface density in dark matter is higher atthe image positions-are compatible with smaller quantities ofsubstructure.
| Ultraviolet observations in Puppis with the space telescope 'GLAZAR'
The results of observations of 159 stars at 1640 A in Puppis made withthe space telescope 'Glazar' are presented. It is shown that theobserved stars are members of different groups of stars situated atdistances of about 120, 370, 700, 1250, 2400, and 4000 pc. The last twogroups belong to the Pup OB1 and Pup OB2 stellar associations. There isrelatively little absorption in the observed region. The absorbingmatter is mainly concentrated in small clouds. It is suggested thatthree stars - CP - 27 deg 4197, HD 60479, and HD 61672 - are embedded ina dust envelope and that the star HD 60057 has a hot subdwarf companion.Forty-four of the observed stars are new OB or early A-type stars,spectral types of which were not known.
| Walraven photometry of nearby southern OB associations
Homogeneous Walraven (VBLUW) photometry is presented for 5260 stars inthe regions of five nearby southern OB associations: Scorpio Centaurus(Sco OB2), Orion OB1, Canis Major OB1, Monoceros OB1, and Scutum OB2.Derived V and (B - V) in the Johnson system are included.
| New optical positions and proper motions of late type stars associated with SiO masers
The 33 cm astrograph of the Bordeaux Observatory has been used to derivewith respect to AGK3 reference stars, accurate positions of late typevariable stars variable stars associated with strong SiO maser emission.It is suggested that selected maser stars can be used to link theHIPPARCOS system and the VLBI extragalactic reference frame. The propermotions of the program (SiO) and reference stars are derived and, forthe program stars, are compared to the McCormick catalog of propermotions.
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תצפית ומידע אסטרומטרי
| קבוצת-כוכבים: | כלב גדול |
| התרוממות ימנית: | 07h26m11.14s |
| סירוב: | -26°14'10.1" |
| גודל גלוי: | 8.872 |
| מרחק: | 10000000 פארסק |
| תנועה נכונה: | -4.8 |
| תנועה נכונה: | 2.7 |
| B-T magnitude: | 8.728 |
| V-T magnitude: | 8.861 |
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