High-resolution M-band Spectroscopy of CO toward the Massive Young Stellar Binary W3 IRS 5

Abstract

We present in this paper the results of high spectral resolution (R = 88,100) spectroscopy at 4.7 μm with iSHELL/IRTF of hot molecular gas close to the massive binary protostar W3 IRS 5. The binary was spatially resolved, and the spectra of the two sources (MIR1 and MIR2) were obtained simultaneously for the first time. Hundreds of 12CO ν = 0–1, ν = 1–2 lines, and ν = 0–1 transitions of the isotopes of 12CO were detected in absorption and are blueshifted compared to the cloud velocity v LSR = −38 km s−1. We decompose and identify kinematic components from the velocity profiles and apply rotation diagram and curve-of-growth analyses to determine their physical properties. The temperatures and column densities of the identified components range from 30 to 700 K and 1021 to 1022 cm−2, respectively. Our curve-of-growth analyses consider two scenarios. One assumes a foreground slab with a partial covering factor, which well reproduces the absorption of most of the components. The other assumes a circumstellar disk with an outward-decreasing temperature in the vertical direction and reproduces the absorption of all of the hot components. We attribute the physical origins of the identified components to the foreground envelope (<100 K), post-J-shock regions (200–300 K), and clumpy structures on the circumstellar disks (∼600 K). We propose that the components with a J-shock origin are akin to water maser spots in the same region and complement the physical information of water masers along the direction of their movements.

High-resolution SOFIA/EXES Spectroscopy of Water Absorption Lines in the Massive Young Binary W3 IRS 5

“…Previous studies of the MIR water absorption spectrum toward massive protostars have revealed a rich spectral content. High spectral resolution observations can provide much insight into the characteristics of these regions (e.g., Knez et al 2009;Indriolo et al 2015;Barr et al 2018;Dungee et al 2018;Indriolo et al 2018;Rangwala et al 2018;Goto et al 2019;Barr et al 2020;Indriolo et al 2020;Nickerson et al 2021;Barr et al 2022aBarr et al , 2022bLi et al 2022;Nickerson et al 2023). Water has also been studied at high spectral resolution via pure rotational transitions, using the heterodyne instruments on board SWAS, Odin, and Herschel Space Observatory (e.g., Snell et al 2000;Wilson et al 2003;Chavarría et al 2010;Karska et al 2014).…”

“…Millimeter studies reveal complex structures surrounding the binary on a large scale (10 3 -10 5 au), including a hot molecular core with outflows, jet lobes, shocks, and a circumbinary toroid (Imai et al 2000;Rodón et al 2008;Wang et al 2012Wang et al , 2013Purser et al 2021). In the infrared M-band (4.7 μm), high spectral resolution (R = 88,100) observations of rovibrational transitions of CO and its isotopologues (Li et al 2022) show various absorbing structures along the pencil beam line of weight against the pencil beam MIR background. Physical conditions such as temperature and column density have been derived from these CO observations.…”

“…Physical conditions such as temperature and column density have been derived from these CO observations. While MIR1 and MIR2 are spatially resolved in Li et al (2022), the absorbing components are identified as a shared envelope (∼50 K at −38.5 km s −1 ), several foreground clumps produced by either J-or C-shocks (200-300 K from −100 to −60 km s −1 ), and blobs that are likely associated with the circumstellar disks (∼500 K from −55 to −38.5 km s −1 ). The M-band study sets up the backdrop for understanding the origin of spatially unresolved water absorption lines in this paper, and we refer readers to Table 7 in Li et al (2022) for a complete list of properties of each decomposed CO component. This paper presents the rich spectrum of rovibrational (the ν 2 band) water lines observed in absorption toward W3 IRS 5.…”

“…While MIR1 and MIR2 are spatially resolved in Li et al (2022), the absorbing components are identified as a shared envelope (∼50 K at −38.5 km s −1 ), several foreground clumps produced by either J-or C-shocks (200-300 K from −100 to −60 km s −1 ), and blobs that are likely associated with the circumstellar disks (∼500 K from −55 to −38.5 km s −1 ). The M-band study sets up the backdrop for understanding the origin of spatially unresolved water absorption lines in this paper, and we refer readers to Table 7 in Li et al (2022) for a complete list of properties of each decomposed CO component. This paper presents the rich spectrum of rovibrational (the ν 2 band) water lines observed in absorption toward W3 IRS 5. The high spectral resolution spectroscopy allows us to separate and identify individual velocity components that are linked to different stars in the W3 IRS 5 binary and to derive the temperatures, the level-specific column densities, as well as the total column densities (and/or the abundances).…”

“…We describe our observations and data reduction in Section 2 and our analysis methods with both the rotation diagram and the curveof-growth in Section 3. We present in Section 4 the properties of multiple dynamical components and set up the connection between these water components to CO components that are identified in Li et al (2022). We discuss the chemical abundances along the line of sight based on the CO-to-H 2 O connection in Section 5.…”