Paula Benaglia

Can the gamma-ray source 3EG J2033+4118 be produced by the stellar system Cygnus OB2 No. 5?

We discuss the possibility that the stellar system Cyg OB2 No. 5 can be a gamma-ray source in the light of recent EGRET and radio data. This system is formed by an O7 Ia + Ofpe/WN9 contact binary. A third star, probably a B0 V star also associated with the system, is located at ∼ 1700 AU from the primary. We estimate the expected gamma-ray luminosity from the colliding winds region, the terminal shock of the wind, and the unstable zone at the base of the wind, and conclude that, under very reasonable assumptions, Cyg OB2 No. 5 can generate about a half of the gamma-ray flux detected from the positionally coincident source 3EG J2033+4118. We suggest, then, that other O stars belonging to the association, also placed within the 95% probability EGRET location contour, could contribute to the observed gamma-ray flux.

The Interstellar Medium in the Environs of O-Type Stars

We analyze the distribution of the neutral gas in the neighborhood of four southern O-type stars based on observations of the 21 cm H I line. These data disclose H I bubbles probably associated with the stars HD 112244 [O8.5Iab(f)], HD 155913 [O5Vn((f))], HD 175754 [O8II((f))], and HD 175876 [O6.5III(n)(f)]. The dimensions of these bubbles are in the range 90 to 170 pc, and their expansion velocities are low, about 10 km s-1, implying dynamical ages of (3–6) × 106 yr. Only a few percent of the mechanical energy of the stellar wind is transformed into kinetic energy of the shells. Their origin and the contribution of other early-type stars to the formation of these bubbles are discussed. Particularly, it is suggested that HD 155913 belongs to an unknown OB association that could also have contributed in the formation of the H I bubble surrounding this star. These new H I bubbles resemble H I bubbles surrounding Wolf-Rayet (W-R) stars. The presence of stellar wind bubbles associated with O-type stars supports an interpretation where the massive progenitors of the W-R stars are also responsible for the formation of the H I bubbles detected around W-R stars.

The radio surroundings of the microquasar GRO J1655-40

We report the results of a study of the radio surroundings of the superluminal microquasar GRO J1655 40. We have searched for extended continuum structures that might be indicative of the pres- ence of a supernova remnant (SNR) associated with the formation of the compact object in the binary system. We also carried out HI-line observations of the region looking for a local minimum created by an explosive event. Our results indicate that there is, in fact, a bubble in the large-scale HI distribution around GRO J1655 40. We suggest that this structure might be created by the original supernova explosion occurred a few hundred thousand years ago and whose signatures can be traced by the overabundance of -elements recently found by Israelian et al. (1999) in the companion star.

A SEARCH FOR RADIO COUNTERPARTS OF SOUTHERN UNIDENTIFIED EGRET SOURCES

We present results of 1.42 GHz continuum and H I line observations of the environs of the unidentified EGRET sources 3EG J0724-4713 and 3EG J0725-5140. Using a filtering technique, we have eliminated from the radio continuum images the background diffuse radiation that originates in the Galactic disk. This enables us to study the presence of weak radio sources within the likelihood contours of the γ-ray detections. The H I line observations provide additional information about the structure of the interstellar medium in the line of sight to the sources, which is relevant in case of a Galactic origin. We discuss the nature of the individual sources in the light of the results.

Positional Correlation between Low-Latitude γ-Ray Sources and Supernova Remnants

We present the results of a spatial correlation analysis between unidentified gamma-ray sources in the 3EG catalog and candidates to supernova remnants. This work extends a previous study made by Romero, Benaglia & Torres (1999). We also consider the gamma-ray emission variability and the spectral index for the sources with positional coincidence.