OH Column Research Articles

Observations of hydroxyl ground state transitions in a complete sample of methanol sources

High sensitivity observations of all four transitions of the ground state 2 Π3/2, J = 3/ 2o f OH in both senses of circular polarization have been carried out with the Nancay radio telescope. The sample was a set of 100 star forming regions detected in a recent unbiased survey of 6668 MHz CH3OH masers. OH maser emission was found in 55 objects of which 31 were not previously catalogued. The 1665 MHz line was seen in almost all OH maser sources and was accompanied by the 1667 MHz line in about 75% of cases. Respectively 7% and 11% of OH 1665 MHz masers were accompanied by maser lines at 1612 and 1720 MHz. These two satellite line masers never occurred simultaneously in the same source nor at the same radial velocity, suggesting mutually exclusive physical conditions as predicted by models. OH maser emission usually shared the same velocity range as the 6668 MHz CH3OH maser. The intensity ratio of the 6668 MHz and 1665 MHz lines clearly divides the sample into methanol- and hydroxyl-rich sources and could be controlled by the abundance of maser molecules and the kinetic temperature. The OH maser emission was substantially polarized with a mean fractional circular polarization of 0.30 and Zeeman pair candidates were found in 15 targets. 36 sources were found in OH absorption at the main lines and 24 of them were also accompanied by OH maser emission. OH absorption features were blueshifted with regard to the related OH masers, indicating that they were formed in front of the central continuum sources. Absorption at 1720 MHz was always accompanied by emission at 1612 MHz and vice versa. The behaviour of stimulated emission and absorption in both satellite lines was thus conjugated and the 1720 MHz emission features seem to be signatures of regions of low hydrogen density and OH column density. The correlation of OH and CH3OH flux densities with the IRAS flux densities found for our sample appears to support pumping schemes of both molecules by infrared photons. Statistics of masers in the sample appear to be consistent with the scenario that the CH3OH masers appeared earlier than the OH masers.

OH column abundance over Table Mountain Facility, California: Intra‐annual variations and comparisons to model predictions for 1997–2001

Measurements of the OH column abundance over the Jet Propulsion Laboratory's Table Mountain Facility (TMF) have been made since July 1997 at 10°–80° solar zenith angle using a Fourier transform ultraviolet spectrometer. The measured OH column at any solar zenith angle is typically larger in the afternoon than in the morning. The variations observed in the OH column abundance appear to result from changes in atmospheric conditions on a daily or longer timescale. The larger observed variations are statistically significant. Sensitivity coefficients describing how the OH column abundance is expected to change in response to changes in the concentrations of H2O, O3, NO, CO, and CH4 have been calculated on the basis of an analytic model. On the basis of these sensitivity coefficients and Halogen Occultation Experiment observations of O3, the net sensitivity of the OH column abundance to variations in O3 should be close to zero. The observed OH column abundance over TMF increased by about 25% from July 1997 to December 2001. This interannual trend in OH column abundance is not consistent with calculations that incorporate observed trends in H2O and O3 and is at least a factor of 2 larger than the calculated difference between solar minimum and maximum. Comparisons between measured and calculated normalized OH column abundances suggest that the sensitivity of OH to variations in H2O may be a factor of 2 larger than predicted in present models and that there is some other major driver for the observed variability in the OH column abundance that was not included in the present analysis.

Detection of OH and wide H i absorption toward B0218+357

We present deep GMRT OH and WSRT HI absorption spectra of the $z = 0.6846$ gravitational lens toward B0218+357. Both the 1665 MHz and 1667 MHz OH lines are clearly detected for the first time while a new wide absorption component was detected (at low significance) in the HI spectrum; the OH spectra yielded an OH column density of $N_{OH} = 2.3 \times 10^{15}$ cm^{-2}. The ratio of 1667 and 1665 MHz equivalent widths is $\sim 1.8$ while the redshift of peak OH absorption is $z = 0.68468 \pm 0.000008$ for both lines; this redshift agrees with that obtained from the HI line. The velocity spread (between nulls) of the HI absorption is $\sim 140$ km/s, while that of both OH lines is $\sim 100$ km/s; the HI and OH spectra are broadly similar in that they each have two principal narrow components and a wide absorption trough. We argue that the wide absorption is likely to arise from source components in the Einstein ring and derive a rotation velocity of $\sim 150$ km/s at a distance of 1.5 kpc from the centre of the $z \sim 0.6846$ galaxy.

Seasonal and solar cycle variability of OH in the middle atmosphere

The photochemistry of the hydroxyl radical (OH) in the middle atmosphere involves the action of solar ultraviolet radiation in many respects. The production of stratospheric OH is initiated through the photodissociation of ozone, and in the mesosphere, OH is produced directly from the photolysis of water vapor. We present an analysis of measurements of the vertical column abundance of OH from Fritz Peak Observatory, Colorado, for seasonal and solar cycle variability. The OH column data span over two 11‐year solar cycles and include a wide range of diurnal and seasonal conditions. Hydroxyl abundances show distinct annual and semiannual cycles, with amplitudes of 4.4% and 2.5%, respectively. About half of the seasonal variation in OH can be explained by the annual cycle of ultraviolet intensity, and by annual and semiannual cycles in ozone and water vapor in the middle atmosphere. On longer time scales, a positive correlation with the NOAA MgII solar index is observed. The amplitude of 11‐year solar cycle variations in OH is 4.2 ± 2.4%, which is consistent with photochemical model calculations that consider changes in ozone and water vapor photodissociation rates. Changes in the vertical distribution of ozone over the solar cycle are found to have a smaller effect on the OH column.

Submillimeter Wave Astronomy Satelliteand Arecibo Observations of H2O and OH in a Diffuse Cloud along the Line of Sight to W51

Observations of W51 with the Submillimeter Wave Astronomy Satellite have yielded the first detection of water vapor in a diffuse molecular cloud. The water vapor lies in a foreground cloud that gives rise to an absorption feature at an LSR velocity of 6 km s-1. The inferred water column density is 2.5 × 1013 cm-2. Observations with the Arecibo radio telescope of hydroxyl molecules at 10 positions in W51 imply an OH column density of 8 × 1013 cm- 2 in the same diffuse cloud. The observed H2O/OH ratio of ~0.3 is significantly larger than an upper limit derived previously from ultraviolet observations of the similar diffuse molecular cloud lying in front of HD 154368. The observed variation in H2O/OH likely points to the presence in one or both of these clouds of a warm (T 400 K) gas component in which neutral-neutral reactions are important sources of OH and/or H2O.

OH column abundance over Table Mountain Facility, California: Annual average 1997–2000

The column abundance of OH over Table Mountain Facility, California, (TMF) has been measured regularly since July 1997 using the Fourier‐transform Ultraviolet Spectrometer. The annual average OH column observed for 1998–2000 is 10–20% larger than that observed by another group over Tokyo for 1992–1995; 30–65% smaller than that observed by other groups over Colorado and New Mexico for 1980–1996; and 15–30% smaller than calculated by photochemical models. The Tokyo annual average and model estimates are within the range of geophysical variability for the OH column observed over TMF but the Colorado and New Mexico annual averages are not. This suggests the possibility of systematic errors in one or more of the data sets.

The 2.5 THz heterodyne spectrometer THOMAS: Measurement of OH in the middle atmosphere and comparison with photochemical model results

The interpretation of recent odd hydrogen measurements in the stratosphere from balloons and in the mesosphere from space indicates a serious lack of understanding in atmospheric HOx chemistry. In order to resolve these persisting problems, coincident measurements of HOx molecules and/or measurements that cover both altitude regions are desirable. In this work, the airborne 2.5THz heterodyne spectrometer Terahertz OH Measurement Airborne Sounder (THOMAS) is introduced. Since the first THOMAS measurements in 1994/1995, the spectrometer was significantly improved by modification or replacement of individual components. The THOMAS instrumental setup and properties are presented together with a retrieval algorithm for atmospheric parameters based on a Phillips‐Tikhonov regularization scheme. Furthermore, the results of a complete error assessment are given. In August 1997, during the second CRISTA/MAHRSI campaign (Cryogenic Infrared Spectrometers and Telescopes for the Atmosphere / Middle Atmosphere High Resolution Spectrograph Investigation), OH observations were performed by THOMAS covering altitudes between about 30 and 90 km over a full diurnal cycle. Hydroxyl column densities derived from THOMAS measurements are presented and compared to photochemical model results. The model calculations using the standard HOx chemistry systematically show higher values by about 15% for the 40–90 km and 50–90 km OH columns. Moreover, a recently proposed change of an HOx chemistry reaction rate is included into the comparison which, for the same altitude intervals, yields OH column densities that are about 10% lower than the THOMAS measurements. A detailed comparison of the THOMAS and MAHRSI measurements is presented in a seperate publication [Englert et al., 2000].

The Interstellar Lithium Isotope Ratio Toward Per OB2

We are conducting a survey on the 7Li/6Li ratio in interstellar space in order to seek limits on the variation in this ratio. The analysis is based on the technique we adopted in extracting the 11B/10B ratio. This technique uses a line of comparable strength, from a species likely to occupy the same volume of the interstellar cloud, as a template for separating velocity components within the line profile. For our study of the 7Li/6Li ratio, the K I line at 4044 Å serves as the velocity template. Our initial focus is on the variation in the 7Li/6Li ratio around the star-forming region IC 348. Our high-resolution observations of the Li I lines toward o and ζ Per show remarkably different isotope ratios: 7Li/6Li = 2-4 and 11, respectively, where the Solar System ratio is 12.3 and cosmic ray spallation yields a ratio of about 2. The significance of the very low ratio toward o Per is that it is essentially the value predicted for cosmic rays through spallation reactions. The direction to o Per passes closer to IC 348, a site of massive star formation, than does the line of sight to ζ Per. Furthermore, our analysis of OH column densities (Federman, Weber, & Lambert 1996) showed that the cosmic ray flux through o Per's diffuse clouds is higher than average, presumably reflecting the nearby presence of IC 348.

CH3CN photochemistry at hot core margins

CH3CN is a species observed to be much more abundant in ultracompact hot molecular cores (UCHCs) than can be accounted for by dark cloud gas phase chemistry. The close proximity of UCHCs to newly formed OB stars suggests that an impinging far ultraviolet (FUV) radiation field might contribute to the chemical processing at the core margins. In this paper. the photochemistry of gas phase CH3CN formation is explored assuming an enhanced FUV radiation field interacts with dense neutral material evaporated from grain mantle ices. We are able to show that high CH3CN/CH3OH abundance ratios can be generated from CH3OH and NH3 precursors in such a photon-dominated margin as a realistic alternative to a grain surface origin. We show that the observed CH3 CN/CH3 OH column density ratio towards the UCHC in G34.3+0.15 can be interpreted as arising: from photochemical generation of CH3 CN in the gas phase.

OH 1720 Megahertz Masers in Supernova Remnants: C‐Shock Indicators

Recent observations show that the OH 1720 MHz maser is a powerful probe of the shocked region where a supernova remnant strikes a molecular cloud. We perform a thorough study of the pumping of this maser and find tight constraints on the physical conditions needed for its production. The presence of the maser implies moderate temperatures (50-125 K) and densities (~105 cm-3) and OH column densities of order 1016 cm-2. We show that these conditions can exist only if the shocks are of C-type. J-shocks fail by such a wide margin that the presence of this maser could become the most powerful indicator of C-shocks. These conditions also mean that the 1720 MHz maser will be inherently weak compared with the other ground-state OH masers. All the model predictions are in good agreement with the observations.

Further search for OH emission from IRAS sources

Results of a continued search for OH main line emission from IRAS sources are reported. Towards a sam- ple of 182 sources which consists of IRAS sources selected by the colour criterion for ultracompact HII region, and several IRAS cores, H 2O masers and bipolar outflows, we detected 19 new OH masers of Class I and 5 OH/IR can- didates. The correlation between IR and OH maser flux is conrmed. Also 51 thermal and 4 absorption sources were detected. The low radial velocities and derived OH column densities suggest that the thermal OH emission comes from nearby dark clouds. There is evidence that IRAS sources towards which thermal emission was de- tected are connected with these dark clouds.

Mercury's Polar Caps and the Generation of an OH Exosphere

We predict the OH column that will be present in the polar regions of the mercurian exosphere for physically realistic ice deposits at the poles, including both surface and buried ice. The probable rates of accretion by meteoritic, asteroidal, and cometary sources are computed and compared with loss rates. The rate of accretion of water at the poles from the nominal meteoritic infall is 2.5–8 × 10 8molecules cm −2sec −1. Including the uncertainty in meteoritic plus asteroidal infall rate, the accretion rate is 1–12 × 10 8cm −2sec −1. For T< 115 K, the limiting loss process for surface ice is vaporization by micrometeoroids. The loss rate due to meteoritic vaporization of freshly deposited ice, with subsequent dissociation to OH, is about 1–2 × 10 8cm −2sec −1. Thus, the net accretion rate from meteoritic impact is 0–4.5 m/4 × 10 9years. Since the steady-state influx of water from meteoroids equals or exceeds the loss rate from all processes, any additional water accreted from a comet or extinct comet nucleus would be retained. The most probable value of the depth of water at the pole from comet impacts is 3 m, with large uncertainties. The background zenith OH 3085-Å emission from vaporized meteoritic material in the absence of ice deposits is expected to be 12 R in the equatorial region and 0.3–0.7 R above the poles at aphelion. The background exceeds the emission from an outgassing source from buried ice deposits at <112–118 K. An OH exosphere resulting from buried ice deposits would be difficult to observe from ground-based or Earth-orbiting instruments, but fresh deposits would be easily observable. A UV spectrograph in orbit about Mercury that could determine both latitudinal variations and scale heights could be used to infer buried deposits at T> 112 K or surface ice deposits.

Fluorescent hydroxyl emissions from Saturn's ring atmosphere.

Just before earth passed through Saturn's ring plane on 10 August 1995, the Hubble Space Telescope Faint Object Spectrograph detected ultraviolet fluorescent emissions from a tenuous atmosphere of OH molecules enveloping the rings. Brightnesses decrease with increasing distance above the rings, implying a scale height of about 0.45 Saturn radii (Rs). A spatial scan 0.28Rs above the A and B rings indicates OH column densities of about 10(13) cm(-2) and number densities of up to 700 cm(-3). Saturn's rings must produce roughly 10(25) to 10(29) OH molecules per second to maintain the observed OH distribution.

Anomalously Excited OH and Competition between Maser Transitions toward Centaurus A

The ground-state radio transitions of OH have been observed toward the nucleus of the nearby active galaxy Centaurus A, using the Australia Telescope Compact Array. The main lines at rest frequencies 1667 and 1665 MHz show molecular absorption at velocities similar to those observed for other molecular species at millimeter wavelengths, which indicates that the lines of sight at radio and millimeter wavelengths sample the same material. The satellite transitions at 1612 and 1720 MHz exhibit stimulated emission and absorption, including the first extragalactic detection of maser emission in the 1720 MHz line. A remarkable feature of the two satellite transitions is that they are conjugate for all velocity components, with one in emission and the other equally strong in absorption. This is explained as the competition of the two lines for the same inverting infrared radiation in the regime in which the infrared lines are just becoming optically thick. The transition from 1720 MHz to 1612 MHz maser emission depends critically on the OH column density and therefore yields direct information on the distribution of molecular gas.

Enhanced production of stratospheric OH from methane oxidation at elevated reactive chlorine levels in northern midlatitudes

Examination of odd-hydrogen production and loss processes in the lower stratosphere reveals that the large abundances of midlatitude ClO recently measured requires a reassessment of the OH yield from methane oxidation due to a more rapid initiation of the methane oxidation sequence by atomic chlorine. Using a steady state calculation for OH and an iterative procedure for evaluating odd-hydrogen production from the methane oxidation sequence, we demonstrate that both OH and reactive chlorine are amplified below 25 km. The amplified values of OH and ClO are found to be consistent with recent measurements of ClO concentrations and of OH column abundances.

Analysis of hydroxyl earthlimb airglow emissions: Kinetic model for state‐to‐state dynamics of OH (υ,N)

Detailed spectroscopic analysis of hydroxyl fundamental vibration‐rotation and pure rotation emission lines has yielded OH(υ,N) absolute column densities for nighttime earthlimb spectra in the 20 to 110‐km tangent height region. High‐resolution spectra were obtained in the Cryogenic Infrared Radiance Instrumentation for Shuttle (CIRRIS 1A) experiment. Rotationally thermalized populations in υ = 1–9 have been derived from the fundamental bands between 2000 and 4000 cm−1. Highly rotationally excited populations with N ≤ 33 ( ≤ 2.3 eV rotational energy) have been inferred from the pure rotation spectra between 400 and 1000 cm−1. These emissions originate in the airglow region near 85–90 km altitude. Spectral fits of the pure rotation lines imply equal populations in the spinrotation states F1 and F2 but a ratio Π(A′):Π(A″) = 1.8±0.3 for the Λ‐doublet populations. A forward predicting, first‐principles kinetic model has been developed for the resultant OH(υ,N) limb column densities. The kinetic model incorporates a necessary and sufficient number of processes known to generate and quench OH(υ,N) in the mesopause region and includes recently calculated vibration‐rotation Einstein coefficients for the high‐N levels. The model reproduces both the thermal and the highly rotationally excited OH(υ,N) column densities. The tangent height dependence of the rotationally excited OH(υ,N) column densities is consistent with two possible formation mechanisms: (1) transfer of vibrational to rotational energy induced by collisions with O atoms or (2) direct chemical production via H + O3 → OH(υ,N) + O2.

An evaluation of O2(b¹Σg) as a possible source of OH and odd‐nitrogen in the stratosphere and mesosphere

A one dimensional photochemical model has been used to evaluate the hypothesis that the oxidation of H2 and N2O by O2(b¹Σg) could lead to significant production of OH and odd‐nitrogen in the middle atmosphere. For the reaction with H2, even if all the quenchings lead to oxidation, the change in the OH column is only 13%. This is still too small to resolve the discrepancy between the model and ground based measurements and furthermore could not account for the apparent change in the ground based column with time. For the reaction with N2O, it is possible to produce significant odd‐nitrogen in the lower stratosphere only if the oxidation yield is at least 5–10%. However, one then gets a surplus of odd‐nitrogen in the upper stratosphere unless one makes the additional assumption that the production of O2(b¹Σg) from O(¹D) quenching is small.

Evidence for non‐local‐thermodynamic‐equilibrium rotation in the OH nightglow

Improved ground‐based Fourier transform infrared spectroscopic measurements of the OH Meinel (M) Δν= 2 and 3 nightglow emissions revealed unexpectedly intense transitions from high rotational levels. The rotational development in the P branches of the OH M (3,1), (6,3), and (7,4) bands has been followed to the P1(N″) transitions with N″ = 10, 12, and 13, respectively. These measurements indicate that ∼10% of the OH(X, ν′ = 7) column rotational population is typically in rotational levels with N′ ≥7, in sharp contrast with the corresponding local thermodynamic equilibrium estimate of ∼1%. The excess high‐N′ population also persists in the lower vibrational states of OH(X), as evidenced by the readily detectable high‐P1(N″) transitions in the (3,1) and (6,3) bands and by the presence of observable R branch heads in the (4,2) and (5,3) bands. The R heads in the (4,2) and (5,3) bands form at moderately high N′ and are typically much more intense than expected on the assumption of complete rotational‐translational equilibration throughout the OH M source region. These new results provide strong evidence for incomplete R‐T equilibration of OH(X²Π,3≤ν′≤7,J′) on a column basis under typical nightglow conditions.

OH(ν,N) column densities from high‐resolution Earthlimb spectra

Individual OH(ν,N) rotational state population column densities have been derived from spectral analysis of CIRRIS 1A nighttime earthlimb airglow data. Both pure rotation and vibration‐rotation fundamental spectra have been examined, providing unique information on highly excited rotational states of OH(ν=0‐6). The relative populations of the four spin sublevels have also been determined. These findings provide important insights into OH dynamics at the mesopause.

The abundance of ammonia in Comet P/Halley derived from ultraviolet spectrophotometry of NH by ASTRON and IUE

From an analysis of the spatial profiles of both the NH and OH UV emissions observed by the ASTRON satellite, the ratio of ammonia-to-water production rates in Comet Halley on April 9, 1986 is derived and found to lie in the range of 0.44-0.94 percent. In order to compare this result with those based on both ground-based and in situ observations made on other dates during the 1985-1986 apparition of the comet, the IUE observational data base for December 1985 and March-April 1986 is used to evaluate the ratio of NH to OH column density in the IUE field of view and thus constrain the long-term behavior of this ratio. The IUE data base indicates that, to within a factor of 2, the ammonia-to-water production rate ratio is the same for a small sample of moderately bright comets observed recently.