OH as an Alternate Tracer for Molecular Gas: Excitation Temperatures of the OH 18-cm Main Lines in W5

Abstract

We present excitation temperatures $T_{ex}$ for the OH 18-cm main lines at 1665 and 1667 MHz measured directly in front of the W5 star-forming region, using observations from the Green Bank Telescope and the Very Large Array. We find unequivocally that $T_{ex}$ at 1665 MHz is greater than $T_{ex}$ at 1667 MHz. Our method exploits variations in the continuum emission from W5, and the fact that the continuum brightness temperatures $T_C$ in this nebula are close to the excitation temperatures of the OH lines in the foreground gas. The result is that an OH line can appear in emission in one location and in absorption in a neighboring location, and the value of $T_C$ where the profiles switch from emission to absorption indicates $T_{ex}$. Absolute measurements of $T_{ex}$ for the main lines were subject to greater uncertainty because of unknown effects of geometry of the OH features. We also employed the traditional "expected profile" method for comparison with our "continuum background" method, and found that the continuum background method provided more precise results, and was the one to definitively show the $T_{ex}$ difference. Our best estimate values are: $T_{ex}^{65}=6.0\pm 0.5$ K, $T_{ex}^{67}=5.1\pm 0.2$ K, and $T_{ex}^{65}-T_{ex}^{67}=0.9\pm 0.5$ K. The $T_{ex}$ values we have measured for the ISM in front of W5 are similar to those found in the quiescent ISM, indicating that proximity to massive star-forming regions does not generally result in widespread anomalous excitation of OH emission.

arXiv commentAccepted for publication in the Astrophysical Journal

References in corpus (7)

• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• Where is OH and Does It Trace the Dark Molecular Gas (DMG)?
• Fermi-LAT Observations of High- and Intermediate-Velocity Clouds: Tracing Cosmic Rays in the Halo of the Milky Way
• SWAS and Arecibo observations of H2O and OH in a diffuse cloud along the line-of-sight to W51
• The Structure of Dark Molecular Gas in the Galaxy - I: A Pilot Survey for 18-cm OH Emission Towards $l\approx 105^{°},b\approx +1^{°}$
• Faint Extended OH Emission from the Local Interstellar Medium in the Direction l \approx 108\circ, b \approx 5\circ
• OH Survey along Sightlines of Galactic Observations of Terahertz C+

Cited by in corpus (10)

• Dust-Gas Scaling Relations and OH Abundance in the Galactic ISM
• Observational Evidence for a Thick Disk of Dark Molecular Gas in the Outer Galaxy
• HyGAL: Characterizing the Galactic ISM with observations of hydrides and other small molecules -- I. Survey description and a first look toward W3(OH), W3 IRS5 and NGC 7538 IRS1
• Fingerprinting the effects of hyperfine structure on CH and OH far infrared spectra using Wiener filter deconvolution
• The Structure of Dark Molecular Gas in the Galaxy -- II. Physical State of "CO-Dark" Gas in the Perseus Arm
• OH Evolution in Molecular Clouds
• SPLASH: The Southern Parkes Large-Area Survey in Hydroxyl -- Data Description & Release
• OH as an Alternate Tracer for Molecular Gas: Quantity and Structure of Molecular Gas in W5
• GNOMES II: Analysis of the Galactic diffuse molecular ISM in all four ground state hydroxyl transitions using Amoeba
• AMOEBA: Automated Molecular Excitation Bayesian Line-Fitting Algorithm