Nearby Massive Stars Research Articles

The galactic center black hole: Clues for the evolution of black holes in Galactic nuclei

The evidence for a black hole located at the dynamical center of the Milky Way and identified with the unusual radio source, Sgr A ∗, is now very compelling. Proper motion and radial velocity surveys of stars clearly demonstrate the presence of a non-luminous concentration of 2.6 × 10 6 M ⊙ within a volume of radius ∼0.01 pc centered on Sgr A ∗. At present, the accretion rate onto this object is rather small, leading to a total accretion luminosity at radio through far-IR wavelengths < 10 3 L ⊙. The accreted material apparently originates in the winds of nearby massive stars. However, neither the stellar nor the gaseous environments are static. The surrounding cluster of massive stars, most lying well within a parsec, is only a few million years old, and is destined to fade substantially within another 10 7 years. How did such a cluster form in the immediate and tidally stressed vicinity of a supermassive black hole? The circumnuclear disk of gas, which presently has an inner radius of 1 pc, seems destined to migrate inwards and eventually cause a much higher accretion rate onto Sgr A ∗, with a consequent flurry of new activity. Because the young stars and gas in the vicinity of the black hole interact with each other, the episodes of recurrent activity there can be described in terms of a limit cycle, which effectively controls the growth of the central black hole. In addition to describing the steps of this cycle, we identify several key observations which serve as potential clues to the past activity not only of our Galactic center, but to the activity of gas-rich nuclei in general.

Photoevaporation of Disks and Clumps by Nearby Massive Stars: Application to Disk Destruction in the Orion Nebula

We present a model for the photoevaporation of circumstellar disks or dense clumps of gas by an external source of ultraviolet radiation. Our model includes the thermal and dynamic effects of 6-13.6 eV far-ultraviolet (FUV) photons and Lyman continuum EUV photons incident upon disks or clumps idealized as spheres of radius rd and enclosed mass M*. For sufficiently large values of rd/M*, the radiation field evaporates the surface gas and dust. Analytical and numerical approximations to the resulting flows are presented; the model depends on rd, M*, the flux of FUV and EUV photons, and the column density of neutral gas heated by FUV photons to high temperatures. Application of this model shows that the circumstellar disks (rd ~ 1014-1015 cm) in the Orion Nebula ("proplyds") are rapidly destroyed by the external UV radiation field.

The Formation of Globular Clusters and of The Stars Within Them

We propose that proto-globular cluster clouds form in a collapsing protogalactic cloud as a consequence of thermal instability. The clouds are photoionized and heated by nearby massive stars. Most are not self-gravitating, but are confined by the residual hot gas in the protogalactic cloud. Their masses evolve as they undergo cohesive collisions with each other and erosion due to interaction with the residual halo gas. Collisions may also trigger thermal instability and fragmentation within protocluster clouds. The resulting cloudlets are pressure confined, and fall toward the center of the protocluster cloud due to inverse buoyancy. Their mass distribution is also regulated by coagulation and erosion. While most cloudlets have substellar masses, the largest become self-gravitating, and collapse to form protostellar cores without further fragmentation. The initial stellar mass function is established as these cores capture additional residual cloudlets. Energy dissipation from the mergers ensures that the cluster will remain bound in the limit of low star formation efficiency. Dissipation also promotes the formation and retention of the most massive stars in the cluster center.

Distribution of plasma turbulence in our Galaxy derived from radio scintillation maps

THE scattering of radio waves by plasma turbulence in the Galaxy is analogous to the scintillation of starlight due to the Earth's atmosphere, and similarly leads to a broadening of the apparent angular sizes of extragalactic radio sources1–3, as well as the smearing out of pulsar pulse profiles. Previous investigations of the galactic plasma turbulence using these effects have suggested a monotonic increase in plasma turbulence towards the galactic plane4–6 and towards the centre7,8. Here I present a new map of galactic plasma turbulence derived from the results of a recent interplanetary scintillation radio survey9, using a method of analysis based on the angular size distribution of radio sources. The map reveals that structure persists to high galactic latitudes, and that it agrees with the morphology of the soft X-ray background. These results suggest that the solar neighbourhood is encapsulated in an envelope of plasma turbulence, most probably the relic of the supernova explosion of a nearby massive star.

Formation of stars by implosion of a gas cloud

The evolution of the molecular gas cloud embedded in anHii region is studied. We consider a spherical, uniform, and stable molecular gas cloud, which is exposed to strong ionizing radiation from nearby massive stars. The gas cloud is strongly compressed by the surrounding high-pressure ionized gas and a shock wave is formed. The shock compressed gas forms a shell which converges toward the center of the cloud sweeping up the inner material. As the shell accumulates the swept up gas, it becomes gravitationally unstable. The growth time of the instability is less than the time-scale of the whole collapse of the cloud. Thus, the shell will break into fragments before it reaches the center of the cloud, leading to formation of a stellar association or a cluster.