Analysis of Spatial Structure of the SPica H II Region

Far ultraviolet (FUV) spectral images of the Spica H II region are first presented here for the Si II* 1533.4A and Al II 1670.8A lines and then compared with the optical Halpha image. The H alpha and Si II* images show enhanced emissions in the southern part of the H II region where H I density increases outwards. This high density region, which we identify as part of the "interaction ring" of the Loop I superbubble and the Local Bubble, seems to bound the southern H II region. On the other hand, the observed profile of Al II shows a broad central peak, without much difference between the northern and southern parts, which we suspect results from multiple resonant scattering. The extended tails seen in the radial profiles of the FUV intensities suggest that the nebula may be embedded in a warm ionized gas. Simulation with a spectral synthesis code yields the values of the Lyman continuum luminosity and the effective temperature of the central star similar to previous estimates with 10^46.2 photons s^-1 and 26,000 K, respectively, but the density of the northern H II region, 0.22 cm^-3, is much smaller than previous estimates for the H alpha brightest region.Comment: 15 pages, 5 figures, accepted for Ap

Kinematics of the Young Stellar Objects associated with the Cometary Globules in the Gum Nebula

An analysis of proper motion measurements of the Young Stellar Objects (YSOs) associated with the Cometary Globules (CGs) in the Gum Nebula is presented. While earlier studies based on the radial velocity measurements of the CGs suggested expansion of the system of the CGs, the observed proper motion of the YSOs shows no evidence for expansion. In particular the kinematics of two YSOs embedded in CGs is inconsistent with the supernova explosion of the companion of $\zeta$ Pup about 1.5 Myr ago as the cause of the expansion of the CG system. YSOs associated with the CGs share the average proper motion of the member stars of the Vela OB2 association. A few YSOs that have relatively large proper motions are found to show relatively low infrared excesses.Comment: 19 pages, 6 figures, 4 tables, Accepted for publication in MNRA

Interstellar reddening towards six small areas in Puppis-Vela

We investigate the distribution of the interstellar dust towards six small volumes of the sky in the region of the Gum nebula. New high-quality four-colour uvby and H\beta\ Str\"omgren photometry obtained for 352 stars in six selected areas of Kapteyn, complemented with data obtained in a previous investigation for two of these areas, were used to estimate the colour excess and distance to these objects. The obtained colour excess versus distance diagrams, complemented with other information, when available, were analysed in order to infer the properties of the interstellar medium permeating the observed volumes. On the basis of the overall standard deviation in the photometric measurements, we estimate that colour excesses and distances are determined with an accuracy of 0.010 mag and better than 30%, respectively, for a sample of 520 stars. A comparison with 37 stars in common with the new Hipparcos catalogue attests to the high quality of the photometric distance determination. The obtained colour excess versus distance diagrams testify to the low density volume towards the observed lines-of-sight. Very few stars out to distances of 1 kpc from the Sun have colour excesses larger than E(b-y) = 0.1 mag. In spite of the low density character of the interstellar medium towards the Puppis-Vela direction, the obtained reddening as a function of the distance indicates that two or more interstellar structures are crossed towards the observed lines-of-sight. One of these structures may be associated with the very low density wall of the Local Cavity, which has a distance of 100-150 pc from the Sun. Another structure might be related to the Gum nebula, and if so, its front face would be located at about 350 pc from the Sun.Comment: Accepted by Astronomy and Astrophysic

Scatter broadening of pulsars in the direction of the Gum nebula

We have measured the scatter broadening of pulsars in the direction of the Gum nebula. For the first time, our observations show clear variations of scattering properties across the Gum nebula. The IRAS-Vela shell is shown to be a high scattering region. Our revised estimations of distances to these pulsars are consistently less by a factor of 2--3, which has very important consequences for the deduced values of radio luminosity and transverse velocity of pulsars.Comment: Astronomy & Astrophysics, 2001, 370, 58

The discovery of an evolving dust scattered X-ray halo around GRB 031203

We report the first detection of a time-dependent, dust-scattered X-ray halo around a gamma-ray burst. GRB 031203 was observed by XMM-Newton starting six hours after the burst. The halo appeared as concentric ring-like structures centered on the GRB location. The radii of these structures increased with time as t^{1/2}, consistent with small-angle X-ray scattering caused by a large column of dust along the line of sight to a cosmologically distant GRB. The rings are due to dust concentrated in two distinct slabs in the Galaxy located at distances of 880 and 1390 pc, consistent with known Galactic features. The halo brightness implies an initial soft X-ray pulse consistent with the observed GRB.Comment: 4 pages. 4 figures. Accepted for publication in ApJ Letter

The Origin of Massive O-type Field Stars. Part I: A Search for Clusters

We present a study aimed at clarifying the birthplace for 43 massive O-type field stars. In this first paper we present the observational part: a search for stellar clusters near the target stars. We derive stellar density maps at two different resolving scales, viz. 0.25pc and 1.0pc from NTT and TNG imaging and the 2MASS catalogue. These scales are typical for cluster sizes. The main result is that the large majority of the O-type field population are isolated stars: only 12% (5 out of 43) of the O-type field stars is found to harbour a small-scale stellar cluster. We review the literature and aim at characterizing the stellar field of each O-type field star with the emphasis on star formation and the presence of known young stellar clusters. An analysis of the result of this paper and a discussion of the O-type field population as products of a dynamical ejection event is presented in an accompanying paper.Comment: 22 pages, accepted for publication in A&

Sourires et pleurs, 187.-1883 / Édouard Chanot

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