[Paper Review] ISO-LWS observations of C+ and O lines in absorption toward SgrB2

C. Vastel, P. Cox+4 more|CERN Bulletin|Aug 9, 2002

Astrophysics and Star Formation Studies1 references22 citations

TL;DR

This study uses high-resolution ISO-LWS Fabry-Pérot observations of [C II] 157.7 μm, [O I] 63.2 μm, and 145.5 μm lines toward Sgr B2 to disentangle multiple cloud layers along the line of sight. It reveals that ~70% of gaseous oxygen is in atomic form (O⁰), with an O⁰/CO ratio of ~2.5 in internal cloud cores, and demonstrates that the [C II] 157.7 μm line can be optically thick, explaining its deficiency in luminous galaxies.

ABSTRACT

High spectral resolution ISO-LWS observations of the [OI] 63.2 and 145.5 microns and [CII] 157.7 microns fine structure lines are presented for the center of the Sagittarius B2 complex (SgrB2). Both the [OI] 63.2 microns and the [CII] 157.7 microns lines are detected in absorption. Using observations of the CO isotopes and of the HI lines, absorption components can be associated with many clouds along the Sgr B2 line of sight. From these data, we were able to disentangle three different layers, as predicted by PDR models, which contain atomic oxygen. We derive lower limits for the column densities of C and O. The fact that the [CII] 157.7 microns line is detected in absorption implies that the main cooling line of the interstellar medium can be optically thick especially in the direction of large star-forming complexes or in the nuclei of galaxies. This could partially account for the deficiency in the [CII] 157.7 microns line which has been recently found toward infrared bright galaxies in ISO data.

Motivation & Objective

• To disentangle multiple cloud layers along the line of sight to Sgr B2 using high spectral resolution far-IR fine structure lines.
• To determine the distribution of atomic carbon and oxygen in different phases of interstellar clouds (C⁺, C⁰, CO).
• To quantify the O⁰/CO abundance ratio and assess the role of atomic oxygen in interstellar cooling.
• To investigate the optical depth of the [C II] 157.7 μm line and its implications for ISM cooling in star-forming regions.
• To test predictions of PDR models by comparing observed line profiles with cloud layer structure.

Proposed method

• High spectral resolution Fabry-Pérot observations of [C II] 157.7 μm, [O I] 63.2 μm, and 145.5 μm lines using the ISO-LWS instrument.
• Cross-identification of absorption components using H I and CO isotopic line data to trace cloud velocities and column densities.
• Modeling of line profiles to separate contributions from distinct cloud layers: C⁺-dominated outer layers, C⁰-transition zones, and CO-dominated inner cores.
• Derivation of lower limits for atomic carbon and oxygen column densities from line strengths and optical depth assumptions.
• Computation of O⁰/CO ratios by comparing O⁰ column densities with CO column densities derived from isotopic CO observations.
• Assessment of [C II] line optical depth based on the presence of absorption features in the profile.

Experimental results

Research questions

• RQ1 What fraction of gaseous oxygen is in atomic form (O⁰) rather than locked in CO in the dense clouds along the Sgr B2 line of sight?
• RQ2 How do the distributions of C⁺, C⁰, and CO vary across different layers of interstellar clouds in the Galactic center?
• RQ3 To what extent is the [C II] 157.7 μm line optically thick in regions with high column density, such as Sgr B2?
• RQ4 Can high-resolution far-IR observations resolve multiple absorption components and distinguish their physical origins?
• RQ5 How do the observed O⁰/CO ratios compare with predictions from standard PDR models?

Key findings

• The total column density of atomic oxygen (O⁰) along the line of sight to Sgr B2 is approximately 3.1 × 10¹⁹ cm⁻² for velocities between -120 km s⁻¹ and +10 km s⁻¹.
• The O⁰/CO ratio in the internal cores of the clouds is found to be approximately 2.5, indicating that about 70% of gaseous oxygen is in atomic form.
• The [C II] 157.7 μm line is detected in absorption at velocities below 20 km s⁻¹, indicating optical depth effects in the main cooling line of the ISM.
• The [O I] 63.2 μm line is in absorption across the full velocity range from -200 to 100 km s⁻¹, consistent with absorption by cold, atomic gas.
• The C⁰ column density is estimated at ~2.4 × 10¹⁷ cm⁻², in good agreement with independent [C I] 492 GHz observations.
• C⁺/C⁰ ratios of ~2.5 are derived for clouds at galactocentric distances of 3–4 kpc, decreasing to <0.7 in the galactic center region.