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An accurate determination of the distance to the Pipe nebula
Aims.We seek an accurate distance to the Pipe nebula. Methods: TheB-band linear polarimetry collected for stars from the Hipparcoscatalogue is used to investigate the dependence of the measuredinterstellar polarization as a function of the star's trigonometricparallax. Results: The linear polarization obtained for 82 Hipparcosstars in the general direction of the Pipe nebula are presented andanalysed. The distribution of the obtained position angles suggests theexistence of two polarizing components. One of them has low averagecolumn density and seems to be closer than ~70 pc to the Sun, while theother component has a higher column density and seems to belong to avery extended interstellar structure. The obtained parallax-polarizationdiagram indicates a low degree of polarization for stars withπH > 8 mas, while a steep rise in polarization isobserved for stars with πH ≈ 7 mas, corresponding to adistance of approximately 140 pc. Conclusions: Our analysis suggests adistance of 145 ± 16 pc to de Pipe nebula, meaning that thiscloud is part of the Ophiuchus dark cloud complex. There is evidencethat the largest filament of the Pipe nebula has collapsed along themagnetic field lines, indicating that magnetic pressure plays animportant role in the evolution of this cloud.Based on observations collected at Observatório do Pico dos Dias,operated by Laboratório Nacional de Astrofísica (LNA/MCT,Brazil). Tables 1 and 2 are only available in electronic form athttp://www.aanda.org
| Two 2MASS-selected Young Stellar Clusters: Photometry, Spectroscopy, and the Initial Mass Function
We present near-IR (NIR) J, H, and Ks images and K-bandspectroscopy of two newly discovered stellar clusters at differentstages of evolution. Our spectra suggest the presence of massive youngstellar objects in the heavily embedded cluster in the star-formingregion near radio source G353.4-0.4 and an O5-O6 V star in the clusternear radio source G305+00.2. We determine a K-band luminosity function(KLF) for both clusters and an initial mass function (IMF) for thecluster near G305+00.2. The derived IMF slope is Γ=-1.5 if the KLFis used to derive the IMF and is Γ=-0.98 if the color-magnitudediagram (CMD) and spectra are used. The more reliable CMD-based slope isflatter than the Salpeter value usually found for stellar clusters. Wefind that using the KLF alone to derive an IMF is likely to produce anoverly steep slope in stellar clusters subject to variable extinction.
| A photometric catalogue of stars in the direction of the bright cloud B in Sagittarius
The present catalog includes photometric data on stars in a 5.5 deg x5.5 deg area in the direction of the bright cloud B in Sagittarius,centered at l equals 357.7 deg, b equals plus 5.6 deg. V magnitudes andB-V and U-B colors are reported for nearly all HD and HDE stars in thefield and also for a number of interesting stars (402 stars), 21 propermotion stars, and 45 stars in a 15 arcmin radius circular regioncentered on the galactic cluster Trumpler 26 (Tr 26). Furthermore,photographic B, V, R, and IR magnitudes are listed for 274 stars in thefield of Tr 26. These data will be used for further studies ofinterstellar absorption in the Galaxy's central direction in relation tospecific objects (red variable stars, star clusters, diffuse objects).
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