The motion and distribution of the gas in the central 300 pc of the galaxy as revealed by spectra of H3.

Spectroscopy of absorption lines of H3+ in the central molecular zone (CMZ) of the Galaxy show that a previously largely unknown component of the interstellar medium there, warm (T∼200 K) and diffuse (n ≲ 102 cm-3) gas, makes up a large fraction of the volume of the CMZ, and that this gas is moving radially outward from the centre. These discoveries upend the generally accepted understanding that the interstellar environment of the CMZ comprises almost entirely an ultra-hot plasma and dense molecular clouds. The radial momentum associated with the diffuse gas in the CMZ exceeds that of the ejecta of thousands of core-collapse supernovae and implies some extraordinary past activity in the centre, possibly associated with the supermassive black hole, Sgr A*. We speculate that within approximately 106 years, gravity could halt the expansion of the diffuse gas and that contraction towards the centre could then commence. This article is part of a discussion meeting issue 'Advances in hydrogen molecular ions: H3+, H5+ and beyond'.

H3(+) spectroscopy and the ionization rate of molecular hydrogen in the central few parsecs of the galaxy.

We report observations and analysis of infrared spectra of H3(+) and CO lines in the Galactic center, within a few parsecs of the central black hole, Sgr A*. We find a cosmic ray ionization rate typically an order of magnitude higher than outside the Galactic center. Notwithstanding, the elevated cosmic ray ionization rate is 4 orders of magnitude too short to match the proton energy spectrum, as inferred from the recent discovery of the TeV γ-ray source in the vicinity of Sgr A*.

Exploring the central molecular zone of the Galaxy using spectroscopy of H3+ and CO.

The central 400 parsecs of the Milky Way, a region known as the central molecular zone (CMZ), contains interstellar gas in a wide range of physical environments, from ultra-hot, rarified and highly ionized to warm, dense and molecular. The combination of infrared spectroscopy of H(3)(+) and CO is a powerful way to determine the basic properties of molecular interstellar gas, because the abundance ratio of H(3)(+) to CO in 'dense' clouds is quite different from that in 'diffuse' clouds. Moreover, the energy-level structure and the radiative properties of H(3)(+) combined with the unusually warm temperatures of molecular gas in the CMZ make H(3)(+) a unique probe of the physical conditions there. This paper describes how, using infrared absorption spectroscopy of H(3)(+) and CO, it has been discovered that a large fraction of the volume of the CMZ is filled with warm, diffuse and partially molecular gas moving at speeds of up to approximately 200 km s(-1) and that the mean cosmic ray ionization rate in the CMZ exceeds by roughly an order of magnitude values found in diffuse molecular clouds elsewhere in the Galaxy.

Infrared diffuse interstellar bands in the Galactic Centre region.

The spectrum of any star viewed through a sufficient quantity of diffuse interstellar material reveals a number of absorption features collectively called 'diffuse interstellar bands' (DIBs). The first DIBs were reported about 90 years ago, and currently well over 500 are known. None of them has been convincingly identified with any specific element or molecule, although recent studies suggest that the DIB carriers are polyatomic molecules containing carbon. Most of the DIBs currently known are at visible and very near-infrared wavelengths, with only two previously known at wavelengths beyond one micrometre (10,000 ångströms), the longer of which is at 1.318 micrometres (ref. 6). Here we report 13 diffuse interstellar bands in the 1.5-1.8 micrometre interval on high-extinction sightlines towards stars in the Galactic Centre. We argue that they originate almost entirely in the Galactic Centre region, a considerably warmer and harsher environment than where DIBs have been observed previously. The relative strengths of these DIBs towards the Galactic Centre and the Cygnus OB2 diffuse cloud are consistent with their strengths scaling mainly with the extinction by diffuse material.

H3+ towards and within the Galactic centre.

High-resolution spectroscopy of bright infrared sources in the centre of the Galaxy has resulted in the detection of H3+ in a remarkable array of dense and diffuse clouds along the 8000 parsec long line of sight, at a wide range of distances from the centre. Most prominent among these is a previously undetected, but very large amount of warm (T approximately 250 K) and diffuse (n approximately 100 cm2) gas within a few hundred parsecs of the centre. The key to understanding the environment of the H3+ in this region is an H3+ absorption line at 3.53 microm from the metastable (3,3) rotational level, which has not been detected in dense or diffuse clouds outside of the Galactic centre (GC). We have used spectroscopy of this line along with other lines of H3+ and CO to characterize all of the clouds along the line of sight to the GC. The high abundance of H3+ in the central few hundred parsecs implies an ionization rate there that is several times larger than estimated for diffuse clouds outside the GC, and nearly two orders of magnitude greater than originally predicted for diffuse clouds.

An enhanced cosmic-ray flux towards zeta Persei inferred from a laboratory study of the H3+-e- recombination rate.

The H3+ molecular ion plays a fundamental role in interstellar chemistry, as it initiates a network of chemical reactions that produce many molecules. In dense interstellar clouds, the H3+ abundance is understood using a simple chemical model, from which observations of H3+ yield valuable estimates of cloud path length, density and temperature. But observations of diffuse clouds have suggested that H3+ is considerably more abundant than expected from the chemical models. Models of diffuse clouds have, however, been hampered by the uncertain values of three key parameters: the rate of H3+ destruction by electrons (e-), the electron fraction, and the cosmic-ray ionization rate. Here we report a direct experimental measurement of the H3+ destruction rate under nearly interstellar conditions. We also report the observation of H3+ in a diffuse cloud (towards Persei) where the electron fraction is already known. From these, we find that the cosmic-ray ionization rate along this line of sight is 40 times faster than previously assumed. If such a high cosmic-ray flux is ubiquitous in diffuse clouds, the discrepancy between chemical models and the previous observations of H3+ can be resolved.

Detection of daily clouds on Titan.

We have discovered frequent variations in the near-infrared spectrum of Titan, Saturn's largest moon, which are indicative of the daily presence of sparse clouds covering less than 1% of the area of the satellite. The thermodynamics of Titan's atmosphere and the clouds' altitudes suggest that convection governs their evolutions. Their short lives point to the presence of rain. We propose that Titan's atmosphere resembles Earth's, with clouds, rain, and an active weather cycle, driven by latent heat release from the primary condensible species.

H3+ in dense and diffuse clouds.

Interstellar H3+ has been detected in dense as well as diffuse clouds using three 3.7 microns infrared spectral lines of the nu 2 fundamental band. Column densities of H3+ from (1.7-5.5) x 10(14) cm-2 have been measured in dense clouds in absorption against the infrared continua of the deeply embedded young stellar objects GL2136, W33A, MonR2 IRS 3, GL961E, and GL2591. Strong and broad H3+ absorptions have been detected in dense and diffuse clouds towards GC IRS 3 and GCS3-2 in the region of the galactic center. A large column density of H3+, comparable to that of a dense cloud, has been detected towards the visible star Cygnus OB2 No. 12, which has a line of sight that crosses mostly diffuse clouds. The H3+ chemistry of dense and diffuse clouds are discussed using a very simple model. Some future projects and problems are discussed.

Detection of H3+ in the diffuse interstellar medium toward Cygnus OB2 No. 12.

The molecular ion H3+ is considered the cornerstone of interstellar chemistry because it initiates the reactions responsible for the production of many larger molecules. Recently discovered in dense molecular clouds, H3+ has now been observed in the diffuse interstellar medium toward Cygnus OB2 No. 12. Analysis of H3+ chemistry suggests that the high H3+ column density (3.8 x 10(14) per square centimeter) is due not to a high H3+ concentration but to a long absorption path. This and other work demonstrate the ubiquity of H3+ and its potential as a probe of the physical and chemical conditions in the interstellar medium.

Near-infrared spectroscopy of the proto-planetary nebula CRL 618 and the origin of the hydrocarbon dust component in the interstellar medium.

A new 2.8-3.8 micrometers spectrum of the carbon-rich protoplanetary nebula CRL 618 confirms the previous detection of a circumstellar 3.4 micrometers absorption feature in this object (Lequeux & Jourdain de Muizon). The high resolution and high signal-to-noise ratio of our spectrum allow us to derive the detailed profile of this absorption feature, which is very similar to that observed in the spectrum of the Galactic center and also resembles the strong 3.4 micrometers emission feature in some post-asymptotic giant branch stars. A weak 3.3 micrometers unidentified infrared band, marginally detected in the CRL 618 spectrum of Lequeux & Jourdain de Muizon, is present in our spectrum. The existence of the 3.4 micrometers feature implies the presence of relatively short-chained, aliphatic hydrocarbon materials (-CH2-/-CH3 approximately = 2-2.5) in the circumstellar environment around CRL 618. It also implies that the carriers of the interstellar 3.4 micrometers feature are produced at least in part in circumstellar material, and it calls into question whether any are produced by the processing of interstellar ices in dense interstellar clouds, as has been previously proposed. Other features in the spectrum are recombination lines of hydrogen, rotational and vibration-rotation lines of molecular hydrogen, and a broad absorption probably due to a blend of HCN and C2H2 bands.

Variations in the 3 micron spectrum across the Orion Bar: polycyclic aromatic hydrocarbons and related molecules.

Long-slit spectra across the Orion Bar reveal significant differences in the spatial behavior of the components of the 3 microns polycyclic aromatic hydrocarbon (PAH) spectrum. The strong PAH band at 3.29 microns generally decreases exponentially with distance from the ionization front into the molecular cloud (scale height approximately 12"), although excesses appear approximately 10" and 20" behind the ionization front, close to layers of H2 and CO emission, respectively. The 3.40 microns PAH feature separates into two components with very different spatial distributions. The main component (at 3.395 microns), along with the 3.51 microns band and the PAH plateau (3.3-3.6 microns), shows excess emission approximately 10" and approximately 20" behind the ionization front, stronger than the excesses in the 3.29 microns band. The extra component of the 3.40 microns band, which peaks at approximately 3.405 microns, has a spatial distribution very similar to the H2 emission. Aromatic C-H stretches in PAHs most likely produce the 3.29 microns feature. Aliphatic C-H stretches in either attached methyl side-groups or superhydrogenated PAHs, or perhaps both, could produce the complicated spectral and spatial structure at 3.40 microns.

Titan's 5 micrometers spectral window: carbon monoxide and the albedo of the surface.

We have measured the spectrum of Titan near 5 micrometers and have found it to be dominated by absorption from the carbon monoxide 1-0 vibration-rotation band. The position of the band edge allows us to constrain the abundance of CO in the atmosphere and/or the location of the reflecting layer in the atmosphere. In the most likely case, 5 micrometers radiation is reflected from the surface and the mole fraction of CO in the atmosphere is qCO=10(+10/-5) ppm, significantly lower than previous estimates for tropospheric CO. The albedo of the reflecting layer is approximately 0.07(+0.02/-0.01) in the 5 micrometers continuum outside the CO band. The 5 micrometers albedo is consistent with a surface of mixed ice and silicates similar to the icy Galilean satellites. Organic solids formed in simulated Titan conditions can also produce similar albedos at 5 micrometers.

Detection of H3+ in interstellar space.

The H3+ ion is widely believed to play an important role in interstellar chemistry, by initiating the chains of reactions that lead to the production of many of the complex molecular species observed in the interstellar medium. The presence of H3+ in the interstellar medium was first suggested in 1961, and its infrared spectrum was measured in the laboratory in 1980. But attempts to detect it in interstellar space have hitherto proved unsuccessful. Here we report the detection of H3+ absorption in the spectra of two molecular clouds. Although the present results do not permit an accurate determination of the H3+ abundances, these ions appear nevertheless to be present in sufficient quantities to drive much of the chemistry in molecular clouds. It should soon be possible to obtain more accurate measurements, and thus better quantify the role of ion-neutral reactions in the chemical evolution of molecular clouds.

Spatial variation of the 3.29 and 3.40 micron emission bands within reflection nebulae and the photochemical evolution of methylated polycyclic aromatic hydrocarbons.

Spectra of 3 micrometers emission features have been obtained at several positions within the reflection nebulae NGC 1333 SVS3 and NGC 2023. Strong variations of the relative intensities of the 3.29 micrometers feature and its most prominent satellite band at 3.40 micrometers are found. It is shown that (i) the 3.40 micrometers band is too intense with respect to the 3.29 micrometers band at certain positions to arise from hot band emission alone, (ii) the 3.40 micrometers band can be reasonably well matched by new laboratory spectra of gas-phase polycyclic aromatic hydrocarbons (PAHs) with alkyl (-CH3) side groups, and (iii) the variations in the 3.40 micrometers to 3.29 micrometers band intensity ratios are consistent with the photochemical erosion of alkylated PAHs. We conclude that the 3.40 micrometers emission feature is attributable to -CH3 side groups on PAH molecules. We predict a value of 0.5 for the peak intensity ratio of the 3.40 and 3.29 micrometers emission bands from free PAHs in the diffuse interstellar medium, which would correspond to a proportion of one methyl group for four peripheral hydrogens. We also compare the 3 micrometers spectrum of the proto-planetary nebula IRAS 05341+0852 with the spectrum of the planetary nebula IRAS 21282+5050. We suggest that a photochemical evolution of the initial aliphatic and aromatic hydrocarbon mixture formed in the outflow is responsible for the changes observed in the 3 micrometers emission spectra of these objects.

Spectroscopy of Mars from 2.04 to 2.44 micrometers during the 1993 opposition: absolute calibration and atmospheric vs mineralogic origin of narrow absorption features.

We present moderate-resolution (lambda/delta lambda = 300 to 370) reflectance spectra of Mars from 2.04 to 2.44 micrometers that were obtained at UKIRT during the 1993 opposition. Seven narrow absorption features were detected and found to have a Mars origin. By comparison with solar and Mars atmospheric spectra, five of these features were attributed all or in part to Mars atmospheric CO2 or CO(2.052 +/- 0.003, 2.114 +/- 0.002, 2.150 +/- 0.003, 2.331 +/- 0.001, and 2.357 +/- 0.002 micrometers). Two of the bands (2.331 +/- 0.001 and 2.357 +/- 0.002 micrometers) appear to have widths and depths that are consistent with additional, nonatmospheric absorptions, although a solar contribution cannot be entirely ruled out. Two other weak bands centered at 2.278 +/- 0.002 and 2.296 +/- 0.002 micrometers may be at least partially mineralogic in origin. The data provide no conclusive identification of the mineralogy responsible for these absorption features. However, examination of terrestrial spectral libraries and previous moderate spectral resolution mineral studies indicates that the most likely origin of these features is either (bi)carbonate or (bi)sulfate anions in framework silicates or (Fe, Mg)-OH bonds in sheet silicates. If the bands are caused by phyllosilicate minerals, then an explanation must be found for the extremely narrow widths of the cation-OH features in the Mars spectra as compared to terrestrial minerals.

Temperature of nitrogen ice on Pluto and its implications for flux measurements.

Previous work by K.A. Tryka et al. (Science 261, 751-754, 1993) has shown that the profile of the 2.148-micrometers band of solid nitrogen can be used as a "thermometer" and determined the temperature of nitrogen ice on Triton to be 38(+2)-1 K. Here we reevaluate that data and refine the temperature value to 38 +/- 1 K. Applying the same technique to Pluto we determine that the temperature of the N2 ice on that body is 40 +/- 2 K. Using this result we have created a nonisothermal flux model of the Pluto-Charon system. The model treats Pluto as a body with symmetric N2 polar caps and an equatorial region devoid of N2. Comparison with the infrared and millimeter flux measurements shows that the published fluxes are consistent with models incorporating extensive N2 polar caps (down to +/- 15 degrees or +/- 20 degrees latitude) and an equatorial region with a bolometric albedo < or = 0.2.

Spectroscopic detection of molecular hydrogen frozen in interstellar ices.

A weak infrared absorption feature near 4141 wavenumbers (2.415 micrometers) in the spectrum of WL5, an infrared source in the rho Ophiuchus cloud complex, has been detected. It is attributed to molecular hydrogen created by irradiation and frozen in situ into water-rich ices. A second, broader absorption at 4125 wavenumbers centimeters (2.424 micrometers) is probably due to methanol in the ices. The column densities of frozen molecular hydrogen and methanol are inferred to be about 2.5 x 10(18) and 3.0 x 10(19), respectively. There is about three times more frozen molecular hydrogen than frozen carbon monoxide along this line of sight.

Ices on the surface of triton.

The near-infrared spectrum of Triton reveals ices of nitrogen, methane, carbon monoxide, and carbon dioxide, of which nitrogen is the dominant component. Carbon dioxide ice may be spatially segregated from the other more volatile ices, covering about 10 percent of Triton's surface. The absence of ices of other hydrocarbons and nitriles challenges existing models of methane and nitrogen photochemistry on Triton.