Observations of Absorption by Water Vapor toward Sagittarius B2

Neufeld, David A.; Ashby, M. L. N.; Bergin, Edwin A.; Chin, G.; Erickson, N. R.; Goldsmith, Paul F.; Harwit, Martin; Howe, J. E.; Kleiner, S. C.; Koch, David; Patten, B. M.; Plume, R.; Schieder, R.; Snell, R. L.; Stauffer, J. R.; Tolls, Volker; Wang, Z.; Winnewisser, G.; Zhang, Y. F.; Melnick, Gary J.

doi:10.1086/312853

Cited by 51 publications

(45 citation statements)

References 15 publications

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“…The resulting column densities should be accurate to within a factor of 2. The H 2 O abundance in the various components is generally consistent with that derived by Neufeld et al (2000; 4−7 × 10 −7 ). The derived H 2 O abundance in the 5-20 km s −1 component, based on the H 18 2 O measurements, is a factor of 3-6 Table 1.…”

Section: Lines Of H 16supporting

confidence: 84%

“…These can be identified with the expanding molecular ring (< −50 km s −1 ), a transition between the 4 kpc arm and the Orion arm (−12 to −7 km s −1 ), the Sagittarius arm (5 to 20 km s −1 ), and the Scutum arm (27 to 35 km s −1 ; possibly blended with the Sagittarius B2 envelope; see e.g. Neufeld et al 2000). Assuming that the foreground absorption completely covers the continuum and all water molecules are in the ground state (a reasonable assumption for the diffuse foreground clouds given the very high spontaneous emission rate coefficients for the ground-state water lines, 3.458 × 10 −3 and 1.842 × 10 −2 s −1 for the ortho and para lines, respectively), we derive optical depths of the o-and p-water lines (τ = − ln I/I o ).…”

Section: Lines Of H 16mentioning

confidence: 99%

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Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

Lis¹,

Phillips²,

Goldsmith

et al. 2010

A&A

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We present Herschel/HIFI observations of the fundamental rotational transitions of ortho-and para-H 16 2 O and H 18 2 O in absorption towards Sagittarius B2(M) and W31C. The ortho/para ratio in water in the foreground clouds on the line of sight towards these bright continuum sources is generally consistent with the statistical high-temperature ratio of 3, within the observational uncertainties. However, somewhat unexpectedly, we derive a low ortho/para ratio of 2.35± 0.35, corresponding to a spin temperature of ∼27 K, towards Sagittarius B2(M) at velocities of the expanding molecular ring. Water molecules in this region appear to have formed with, or relaxed to, an ortho/para ratio close to the value corresponding to the local temperature of the gas and dust.

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Section: Lines Of H 16supporting

confidence: 84%

Section: Lines Of H 16mentioning

confidence: 99%

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

Lis¹,

Phillips²,

Goldsmith

et al. 2010

A&A

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“…column density of 1.6 × 10 23 cm −2 except for Sgr B2(M), which are roughly consistent with the values derived from 13 CO in spiral-arm clouds (Greaves & Nyman 1996;Neufeld et al 2000). Most of the gas components have a CH fractional abundance of a few times 10 −8 with an average abundance of 2.1 × 10 −8 , excluding the Sgr B2(M) cloud with a value of 9.6 × 10 −10 because the excitation there is not straightforward.…”

Section: Discussionsupporting

confidence: 89%

Herschelobservations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines

Qin¹,

Schilke

Comito

et al. 2010

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We have observed CH absorption lines (J = 3/2, N = 1 ← J = 1/2, N = 1) against the continuum source Sgr B2(M) using the Herschel/HIFI instrument. With the high spectral resolution and wide velocity coverage provided by HIFI, 31 CH absorption features with different radial velocities and line widths are detected and identified. The narrower line width and lower column density clouds show "spiral arm" cloud characteristics, while the absorption component with the broadest line width and highest column density corresponds to the gas from the Sgr B2 envelope. The observations show that each "spiral arm" harbors multiple velocity components, indicating that the clouds are not uniform and that they have internal structure. This line-of-sight through almost the entire Galaxy offers unique possibilities to study the basic chemistry of simple molecules in diffuse clouds, as a variety of different cloud classes are sampled simultaneously. We find that the linear relationship between CH and H 2 column densities found at lower A V by UV observations does not continue into the range of higher visual extinction. There, the curve flattens, which probably means that CH is depleted in the denser cores of these clouds.

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“…This interval lies slightly below the values presented for line-of-sight clouds towards Sgr B2 observed by S WAS (X[o-H 2 O]∼6 × 10 −7 ), but these results are not inconsistent since feature C is not clearly distinguishable in the S WAS H 18 2 O spectrum due to noise and possible baseline problems, and the S WAS column has been calculated over a much larger velocity range of 40 km s −1 (Neufeld et al 2000). Our result also agrees with the lower limit to the water abundance in the 3 kpc arm, X[H 2 O] > 2 × 10 −9 , obtained from the observed H 2 O absorption towards Sgr A by Odin (Sandqvist et al 2003).…”

Section: Watermentioning

confidence: 59%

Ground-state ammonia and water in absorption towards Sgr B2

Wirström

Bergman

Black

et al. 2010

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Context. Observations of transitions to the ground-state of a molecule are essential to obtain a complete picture of its excitation and chemistry in the interstellar medium, especially in diffuse and/or cold environments. For the important interstellar molecules H 2 O and NH 3 , these ground-state transitions are heavily absorbed by the terrestrial atmosphere, hence not observable from the ground. Aims. We attempt to understand the chemistry of nitrogen, oxygen, and their important molecular forms, NH 3 and H 2 O in the interstellar medium of the Galaxy. Methods. We have used the Odin submillimetre-wave satellite telescope to observe the ground state transitions of ortho-ammonia and ortho-water, including their 15 N, 18 O, and 17 O isotopologues, towards Sgr B2. The extensive simultaneous velocity coverage of the observations, >500 km s −1 , ensures that we can probe the conditions of both the warm, dense gas of the molecular cloud Sgr B2 near the Galactic centre, and the more diffuse gas in the Galactic disk clouds along the line-of-sight.Results. We present ground-state NH 3 absorption in seven distinct velocity features along the line-of-sight towards Sgr B2. We find a nearly linear correlation between the column densities of NH 3 and CS, and a square-root relation to N 2 H + . The ammonia abundance in these diffuse Galactic disk clouds is estimated to be about 0.5-1 × 10 −8 , similar to that observed for diffuse clouds in the outer Galaxy. On the basis of the detection of H 18 2 O absorption in the 3 kpc arm, and the absence of such a feature in the H 17 2 O spectrum, we conclude that the water abundance is around 10 −7 , compared to ∼10 −8 for NH 3 . The Sgr B2 molecular cloud itself is seen in absorption in NH 3 , 15 NH 3 , H 2 O, H 18 2 O, and H 17 2 O, with emission superimposed on the absorption in the main isotopologues. The non-LTE excitation of NH 3 in the environment of Sgr B2 can be explained without invoking an unusually hot (500 K) molecular layer. A hot layer is similarly not required to explain the line profiles of the 1 1,0 ←1 0,1 transition from H 2 O and its isotopologues. The relatively weak 15 NH 3 absorption in the Sgr B2 molecular cloud indicates a high [ 14 N/ 15 N] isotopic ratio >600. The abundance ratio of H 18 2 O and H 17 2 O is found to be relatively low, 2.5-3. These results together indicate that the dominant nucleosynthesis process in the Galactic centre is CNO hydrogen burning.

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Observations of Absorption by Water Vapor toward Sagittarius B2

Cited by 51 publications

References 15 publications

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

Herschel/HIFI measurements of the ortho/para ratio in water towards Sagittarius B2(M) and W31C

Herschelobservations of EXtra-Ordinary Sources (HEXOS): detecting spiral arm clouds by CH absorption lines

Ground-state ammonia and water in absorption towards Sgr B2

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