[tel-00726959, v1] Caractériser le milieu interstellaire ... - HAL - INRIA

H. S. Liszt and J. Pety: Imaging diffuse clouds: bright and dark gas mapped in COtel-00726959, version 1 - 31 Aug 2012δ(J2000)δ(J2000)52˚00'51˚30'51˚00'51˚40'51˚30'51˚20'0.160.18 0.2 0.19 0.170.150.130.12B1954+5130.16 0.15 0.14 0.130.1220h00m0.1119h55mα (J2000)B1954+5130.140.220.14.2019h57m 19h56m 19h55m 19h54mα(J2000)T r* [Kelvin]e −τ -143210C 2Hx6.21OH1667x15.40HCO-1+West PeakEast CO PeakB1954-5 0 5V LSR (KM S -1 )Fig. 6. The sky field around the position of B1954+513, as in Fig. 2.Inthemapatlower left the grayscale represents the total integrated emissionat −1 ≤ v ≤ 2kms −1 and the red and blue contours show the individual distributions of red and blue-shifted components, respectively. Profiles atthe peak of the red and blue-shifted emission components are shown at upper right along with the profile toward the continuum source (shaded).however that absorption at the mean field velocity is not absenttoward the continuum source.The complexity of the emission distribution makes the divisioninto ranges based on HCO + absorption quite arbitrary.Moreover, the emission and absorption profiles show ratherdifferent structure even toward the continuum target. A verydetailed discussion of CO emission within a 90 ′′ field centeredon NRAO150 was given by Pety et al. (2008). Remarkably, thepeak emission brightness seen just 6 ′′ from the background continuumsource is almost 13 K. As the spatial resolution increases,the CO emission profile toward B0355 more nearly resemblesthe absorption and the blended emission at v ≈−10 km s −1resolves into two distinct components.Conversely, in the present dataset, the emission componentsseen toward B0355+508 lose their identity as the resolution degradesbelow a 4 ′ hpbw. Both the peak profile and the mean arebroad, largely unstructured and centrally peaked about velocitieslying between the two strong CO emission components seentoward the background.6. Statistical lessonsFaute de mieux, the first surveys for suitable absorption-line targetswere conducted in CO emission (Bania et al. 1991; Liszt &Wilson 1993; Liszt 1994) but the discovery of yet more commonHCO + absorption (Lucas & Liszt 1996) caused a reversal in thesearch strategy for diffuse molecular gas. Thus, all targets studiedhere were pre-selected to have absorption from HCO + butonly some were known to have CO emission. However any divisionbetween targets with and without CO emission is misleadingbecause the same sightline may have and lack CO emissionon a per-component basis.Stronger CO emission is always found somewhere else in themap when CO is present toward the continuum but comparablystrong CO emission was found, somewhere on the sky, from absorptioncomponents lacking CO emission counterparts towardthe continuum background. This is true with only one exception,B1928. The implication is that CO emission is somewhatmore ubiquitous than is presently believed to be the case becauseA58, page 9 of 23