Scattered Hα emission from a large translucent cloud G294-24^*

Observatory, Tähtitorninmäki, PO Box 14, 00014, University of Helsinki, Finland e-mail: kimmo.lehtinen@helsinki.fi

Received: 29 December 2009
Accepted: 25 March 2010

Abstract

Aims. We study an undocumented large translucent cloud, detected by means of its enhanced radiation in the SHASSA (Southern H-Alpha Sky Survey Atlas) survey. We consider whether its excess surface brightness can be explained by light scattered off the dust grains in the cloud, or whether emission from in situ ionized gas is required. In addition, we aim to determine the temperature of dust, the mass of the cloud, and its possible star formation activity.

Methods. We compare the observed Hα surface brightness of the cloud with predictions of a radiative transfer model. We use the WHAM (Wisconsin H-Alpha Mapper) survey as a source for the Galactic Hα interstellar radiation field illuminating the cloud. Visual extinction through the cloud is derived using 2MASS J, H, and K band photometry. We use far-IR ISOSS (ISO Serendipitous Survey), IRAS, and DIRBE data to study the thermal emission of dust. The LAB (The Leiden/Argentine/Bonn Galactic HI Survey) is used to study 21 cm HI emission associated with the cloud.

Results. Radiative transfer calculations of the Galactic diffuse Hα radiation indicate that the surface brightness of the cloud can be explained solely by radiation scattered off dust particles in the cloud. The maximum visual extinction through the cloud is about 1.2 mag. The cloud is found to be associated with 21 cm HI emission at a velocity ~-9 km s^-1. The total mass of the cloud is about 550–1000 . There is no sign of star formation in this cloud. The distance of the cloud is estimated from the Hipparcos data to be ~100 pc.