〔C II〕Continuum Intensity Ratio in the Far-Infrared Toward the Central Kiloparsec of the Nearby Spiral Galaxy M31

We observed the nearby galaxy M31 in the 〔C II〕158 μm emission line. An extended component was detected over the central 1 5 kpc region with a line-to-continuum ratio of 〔C II〕/〔40-120μm〕6×10 -3 . This ratio is 3 times larger than that of the Galactic counterpart and is comparable to that in the general Galactic Plane. We expect that the difference between the two central regions are due to different gas densities; the self-shielding of CO molecules decreases the C + abundance at the higher density in the Galactic case.

Effects of adjacent bubble on spatiotemporal evolutions of mechanical stresses surrounding bubbles oscillating in tissues

The cavitation dynamics and mechanical stress in viscoelastic tissues, as the primary mechanisms of some ultrasound therapies, are extremely complex due to the interactions of cavitation bubble with adjacent bubbles and surrounding tissues.Therefore, the cavitation dynamics and resultant mechanical stress of two-interacting bubbles in the viscoelastic tissues are numerically investigated, especially focusing on the effects of the adjacent bubble. The results demonstrate that the mechanical stress is highly dependent on the bubble dynamics. The compressive stress and tensile stress are generated at the stage of bubble expansion and collapse stage, respectively. Furthermore, within the initial parameters examined in this paper, the effects of the adjacent bubble will distinctly suppress the radial expansion of the small bubble and consequently lead its associated stresses to decrease. Owing to the superimposition of two stress fields, the mechanical stresses surrounding the small bubble in the direction of the neighboring bubble are smaller than those in other directions. For two interacting cavitation bubbles, the suppression effects of the nearby bubble on both the cavitation dynamics and the stresses surrounding the small bubble increase as the ultrasound amplitude and the initial radius of the large bubble increase, whereas they decrease with the inter-bubble distance increasing. Moreover, increasing the tissue viscoelasticity will reduce the suppression effects of the nearby bubble, except in instances where the compressive stress and tensile stress first increase and then decrease with the tissue elasticity and viscosity increasing respectively. This study can provide a further understanding of the mechanisms of cavitation-associated mechanical damage to the adjacent tissues or cells.