Showing papers by "William D. Langer published in 2016"
TL;DR: In this article, the authors used the fine-structure lines of ionized nitrogen and ionized carbon to determine the properties of the highly ionized and neutral weakly ionized gas in the inner Galaxy.
Abstract: Context. The interstellar medium (ISM) consists of highly ionized and neutral atomic, as well as molecular, components. Knowledge of their distribution is important for tracing the structure and lifecycle of the ISM. Aims. To determine the properties of the highly ionized gas and neutral weakly ionized gas in the Galaxy traced by the fine-structure lines of ionized nitrogen, [N ii], and ionized carbon, [C ii]. Methods. We utilize observations of the [C ii] 158 μ m and [N ii] 205 μ m fine-structure lines taken with the high spectral resolution Heterodyne Instrument in the Far-Infrared (HIFI) on the Herschel Space Observatory along ten lines of sight towards the inner Galaxy to analyze the ionized ISM. The [N ii] emission can be used to estimate the contribution of the highly ionized gas to the [C ii] emission and separate the contributions from highly ionized and weakly ionized neutral gas. Results. We find that [N ii] has strong emission in distinct spectral features along all lines of sight associated with strong [C ii] emission. The [N ii] arises from moderate density extended H ii regions or ionized boundary layers of clouds. Comparison of the [N ii] and [C ii] spectra in 31 separate kinematic features shows that many of the [C ii] spectra are affected by absorption from low excitation gas associated with molecular clouds, sometimes strongly so. The apparent fraction of the [C ii] associated with the [N ii] gas is unrealistically large in many cases, most likely due to the reduction of [C ii] by absorption. In a few cases the foreground absorption can be modeled to determine the true source intensity. In these sources we find that the foreground absorbing gas layer has C + column densities of order 10 18 cm -2 . Conclusions. [C ii] emission arising from strong sources of [N ii] emission is frequently absorbed by low excitation foreground gas complicating the interpretation of the properties of the ionized and neutral gas components that give rise to [C ii] emission.
29 citations
TL;DR: In this article, the properties of the highly ionized and neutral weakly ionized gas in the Galaxy traced by the fine-structure lines of ionized nitrogen, [N II], and ionized carbon, [C II] were determined.
Abstract: The interstellar medium (ISM) consists of highly ionized and neutral atomic, as well as molecular, components. Knowledge of their distribution is important for tracing the structure and lifecycle of the ISM. Here we determine the properties of the highly ionized and neutral weakly ionized gas in the Galaxy traced by the fine-structure lines of ionized nitrogen, [N II], and ionized carbon, [C II]. To analyze the ionized ISM we utilize [C II] 158 micron and [N II] 205 micron lines taken with the high spectral resolution Heterodyne Instrument in the Far-Infrared (HIFI) on the Herschel Space Observatory along ten lines of sight towards the inner Galaxy. [N II] emission can be used to estimate the contribution of the highly ionized gas to the [C II] emission and separate the highly ionized and weakly ionized neutral gas. We find that [N II] has strong emission in distinct spectral features along all lines of sight associated with strong [C II] emission. The [N II] arises from moderate density extended HII regions or ionized boundary layers of clouds. Comparison of the [N II] and [C II] spectra in 31 separate kinematic features shows that many of the [C II] spectra are affected by absorption from low excitation gas associated with molecular clouds, sometimes strongly so. The apparent fraction of the [C II] associated with the [N II] gas is unrealistically large in many cases, most likely due to the reduction of [C II] by absorption. In a few cases the foreground absorption can be modeled to determine the true source intensity. In these sources we find that the foreground absorbing gas layer has C$^+$ column densities of order 10$^{18}$ cm$^{-2}$. [C II] emission arising from strong sources of [N II] emission is frequently absorbed by low excitation foreground gas complicating the interpretation of the properties of the ionized and neutral gas components that give rise to [C II] emission.
14 citations
TL;DR: In this paper, the effects of an asymmetric radiation field on the properties of a molecular cloud envelope were studied, and it was shown that the cloud may be undergoing differential radial motion.
Abstract: We study the effects of an asymmetric radiation field on the properties of a molecular cloud envelope. We employ observations of carbon monoxide (12CO and 13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the chemical and physical properties of the core and envelope of L1599B, a molecular cloud forming a portion of the ring at approximately 27 pc from the star Lambda Ori. The O III star provides an asymmetric radiation field that produces a moderate enhancement of the external radiation field. Observations of the [CII] fine structure line with the GREAT instrument on SOFIA indicate a significant enhanced emission on the side of the cloud facing the star, while the [Ci], 12CO and 13CO J = 1-0 and 2-1, and 12CO J = 3-2 data from the PMO and APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic, and molecular line centroid velocities track each other very closely, and indicate that the cloud may be undergoing differential radial motion. The HI data from the Arecibo GALFA survey and the SOFIA/GREAT [CII] data do not suggest any systematic motion of the halo gas, relative to the dense central portion of the cloud traced by 12CO and 13CO.
13 citations
TL;DR: In this paper, the effects of an asymmetric radiation field on the properties of a molecular cloud envelope were studied, and it was shown that the cloud may be undergoing differential radial motion.
Abstract: We study the effects of an asymmetric radiation field on the properties of a molecular cloud envelope. We employ observations of carbon monoxide (12CO and 13CO), atomic carbon, ionized carbon, and atomic hydrogen to analyze the chemical and physical properties of the core and envelope of L1599B, a molecular cloud forming a portion of the ring at approximately 27 pc from the star Lambda Ori. The O III star provides an asymmetric radiation field that produces a moderate enhancement of the external radiation field. Observations of the [CII] fine structure line with the GREAT instrument on SOFIA indicate a significant enhanced emission on the side of the cloud facing the star, while the [Ci], 12CO and 13CO J = 1-0 and 2-1, and 12CO J = 3-2 data from the PMO and APEX telescopes suggest a relatively typical cloud interior. The atomic, ionic, and molecular line centroid velocities track each other very closely, and indicate that the cloud may be undergoing differential radial motion. The HI data from the Arecibo GALFA survey and the SOFIA/GREAT [CII] data do not suggest any systematic motion of the halo gas, relative to the dense central portion of the cloud traced by 12CO and 13CO.