Tiny Hi Clouds in the Local Ism
نویسنده
چکیده
We report deep, high spectral resolution WSRT H I 21cm observations of four high latitude compact radio sources, that have revealed a new population of tiny, discrete clouds in the diffuse ISM, with peak optical depths τ ∼ 0.1−2%, H I column densities of 0.4–8 ×1018 cm−2 and core temperatures of 20–80 K. Imaging detections confirm these low column densities and imply linear core dimensions of a few thousand AU, assuming a distance of 100 pc. The physical origin of these tiny H I structures and their distribution in the ISM is at present unknown. We measure small-scale opacity fluctuations two orders of magnitude larger than predicted from the structure function models of Deshpande (2000), suggesting significant injection of small-scale power into the ISM.
منابع مشابه
The Thinnest Cold Hi Clouds in the Diffuse Interstellar Medium?
We confirm and discuss recently discovered cold HI clouds with column densities among the lowest ever detected. The column densities of Cold Neutral Medium (CNM) towards 3C286 and 3C287 are ∼ 10 cm, below an observational lower limit, and also below tiny-scale-atomic clouds detected by VLBI and time-variable profiles against pulsars. These column densities are close to the minimum imposed by th...
متن کاملHow Long Can Tiny H I Clouds Survive?
We estimate the evaporation timescale for spherical H I clouds consisting of the cold neutral medium surrounded by the warm neutral medium. We focus on clouds smaller than 1pc, which corresponds to tiny H I clouds recently discovered by Braun & Kanekar and Stanimirović & Heiles. By performing one-dimensional spherically symmetric numerical simulations of the two-phase interstellar medium (ISM),...
متن کاملExotic clouds in the local interstellar medium
The neutral interstellar medium (ISM) inside the Local Bubble (LB) has been known to have properties typical of the warm neutral medium (WNM). However, several recent neutral hydrogen (HI) absorption experiments show evidence for the existence of at least several cold diffuse clouds inside or at the boundary of the LB, with properties highly unusual relative to the traditional cold neutral medi...
متن کاملPhotoelectric heating and [CII] cooling in translucent clouds: results for cloud models based on simulations of compressible MHD turbulence
Far–ultraviolet (FUV) photons expel electrons from interstellar dust grains and the excess kinetic energy of the electrons is converted into gas thermal energy through collisions. This photoelectric heating is believed to be the main heating mechanism in cool HI clouds. The heating rate cannot be directly measured, but it can be estimated through observations of the [CII] line emission, since t...
متن کاملLow-Velocity Halo Clouds
Models that reproduce the observed high-velocity clouds (HVCs) also predict clouds at lower radial velocities that may easily be confused with Galactic disk (|z| < 1 kpc) gas. We describe the first search for these low-velocity halo clouds (LVHCs) using IRAS data and the initial data from the Galactic Arecibo L-band Feed Array survey in Hi (GALFA-Hi). The technique is based upon the expectation...
متن کامل