Introduction

Red dwarfs are among the least massive stellar objects in the Universe. Among them, M-type stars are in the mass range from around 0.1 to 0.5 solar masses. At lower values we only find brown dwarfs or non-stellar planetary bodies. Nevertheless, low-mass stars are probably the most common type in our Galaxy, with a high potential influence in the definition of its mass function. They also provide the connection between objects with radiation generated through nuclear reactions and those which are not able to do so because of insuficiently high internal temperatures. From the structural point of view, M-type stars are very important because they are dominated by convective energy transport, and the treatment of convection is still one of the least known parts of the theory of stellar structure.

We discuss ISO observations of infrared ionic and H2 lines toward molecular shocks in the supernova remnants 3C 391, W 28, and W 44. The total surface brightness of the H2 lines toward these lines of sight exceeds that of atomic fine structure lines, showing that these lines of sight are dominated by dense molecular shocks. The H2 excitation and the presence of bright ionic lines require that there are multiple shocks into gas with a range of pre-shock densities from 10–103 cm−3.

Introduction

Massive stars end their lives in supernova explosions, and they do not live long enough to travel far from their parent molecular clouds. Therefore, supernovae frequently occur inside molecular clouds, providing compression, turbulence, cosmic rays, radiation, and heat. Using the Infrared Space Observatory, we performed a set of observations designed to search for infrared emission from the gas and dust that gets excited in molecular shock fronts. When the shock front passes through a molecular cloud, the gas cools via the most ‘convenient’ transitions available to it: low-density gas cools via atomic fine structure lines from the abundant ions, while molecular gas cools via the large number of rotational and/or vibrational transitions available. The first results of our project were the detection of bright [O I] 63 µm lines (Reach & Rho 1996), proving that abundant energy was being pumped into the gas by the shock fronts.

Shock waves in outflows are generated by the impact of jets, associated with low-mass star formation, on the surrounding molecular gas. These shocks give rise to a strong H2 rovibrational emission spectrum which has been observed by the ISO satellite in several star formation regions. The dynamical time scales associated with these outflows are estimated to be a few thousand years and can be, in some regions, as short as a few hundred years. On the other hand, the time required to reach steady state for a C-shock is about 104 years. Under such circumstances, the shocks are unlikely to have attained a state of equilibrium, and a time dependent approach has to be considered. Non stationary C-shocks are found to exhibit both C-and J-type characteristics. The H2 rotational excitation diagram can provide a measure of the age of the shock; in the case of the outflow observed in Cepheus A West by the ISO satellite, the shock age is estimated to be approximately 1.5 × 103 yr.

Time scales

Steady state shocks

Shocks propagating in the interstellar medium are expected to modify profoundly the local physical and chemical conditions. Even in the simplest case of planar shocks, the structure of the shock can take a number of different forms, from ‘jump’ or J-type structure, in which changes in density, velocity and temperature occur quasi-discontinuously, to ‘continuous’ or C-type, where the variations take place smoothly over a much larger distance scale.

The Lyman and Werner band systems of deuterated molecular hydrogen (HD) occur in the far UV range below 120 nm. This spectral window is now open at moderate resolution and high sensitivity with the FUSE satellite. FUSE spectra of hot stars with high extinction through translucent clouds will give access to the deuterium abundance inside molecular clouds where D is essentially in the form of HD. Measurement of HD/H2 ratio becomes thus a new powerful method to evaluate the D/H ratio in the interstellar medium.

An example is given with the FUSE spectrum of the high extinction O9III star HD 73882 (EB–V = 0.7). Very preliminary analysis and an estimate of the HD/H2 ratio are presented.

Introduction

It has long been recognized that the primordial abundance of deuterium represents the most sensitive probe of the baryonic density Ωb of the Universe (see, e.g., Schramm & Turner 1998; Olive et al. 1999). On the other hand, abundance of deuterium at any epoch is a lower limit to its primordial abundance, since deuterium is destroyed, not created, in stars of any mass. For this reason, deuterium abundance is also an efficient tracer of the universal star formation rate. Unfortunately, the evolution of deuterium abundance from the primordial to the solar metallicity is still unclear.

Measurements of the atomic D/H ratio have been performed in different astrophysical sites, namely in moderate to high redshift quasar absorbers, in the presolar nebula and in the local interstellar medium (for reviews see, e.g., Ferlet & Lemoine 1996; Linsky 1998; Vidal-Madjar et al. 1998a; Lemoine et al. 1999).

We present spectroscopic and imaging observations of dust and gas emission from the western edge of the ρ Ophiuchus molecular cloud facing the B2 III/IV star HD 147889. The emissions from dust heated by the external UV radiation, from collisionally excited and fluorescent H2 are resolved and observed to coincide spatially. The spectroscopic data allows to estimate the gas temperature to 350 ± 30 K in the H2 emitting layer. In the framework of a steady state model of the photo-dissociation region, a high formation rate: 210−16cm3s−1 at 350 K, seems to be required to account for this temperature. For smaller formation rates the H2 emitting layer moves into the cloud where the gas is colder due to radiation attenuation.

Introduction

ISO observations of the dust emission and H2 rotational lines are bringing a new perspective on the structure and physical conditions in regions of H2 photodissociation (PDRs) at the surface of molecular clouds illuminated by hot stars. Spectroscopic observations of bright PDRs such as NGC 2023 have allowed to build detailed excitation diagrams of H2 with numerous lines to test physical models (Draine this conference). In this paper, we present observations of a fainter PDR on the western edge of the ρ Ophiuchus molecular cloud heated by the B2III/IV star HD 147889. This is a nearby PDR (d = 135±15 pc from the star parallax) with an edge-on geometry where the observations allow to spatially resolve the layer of UV light penetration and of H2 photo-dissociation.

Using the Short-Wavelength-Spectrometer on the Infrared Satellite Observatory (ISO), we obtained near- and mid-infrared spectra toward the brightest H2 emission peak of the Orion OMC-1 outflow. A wealth of emission and absorption features were detected, dominated by 60 H2 ro-vibrational and pure rotational lines reaching from H2 0–0 S(1) to 0–0 S(25).

The total H2 luminosity in the ISO-SWS aperture is (17 ± 5) L⊙, and extrapolated to the entire outflow, (120 ± 60) L⊙. The H2 level column density distribution shows no signs of fluorescent excitation or a deviation from an ortho-to-para ratio of three. It shows an excitation temperature which increases from about 600 K for the lowest rotational and vibrational levels to about 3200 K at level energies E(v, J)/k > 14 000 K.

Introduction

The Orion molecular cloud, OMC-1, located behind the Orion M42 Nebula at a distance of ∼450 pc (Genzel & Stutzki 1989), is the best-studied massive star forming region. This cloud embeds a spectacular outflow arising from some embedded young stellar object, which can possibly be identified as the radio source “I” 0.49 arcsec south of the infrared source IRc2-A (Menten & Reid 1995; Dougados et al. 1993). The outflow shocks the surrounding molecular gas, thereby giving rise to the strongest H2 infrared line emission appearing in the sky. Peak 1 (Beckwith et al. 1978) is the brighter of the two H2 emission lobes of the outflow. Although the outflow has been studied extensively for nearly two decades, the nature of the emission mechanism remains unclear.

A review is presented of ISO observations of molecular hydrogen, H2, toward various Galactic source types, such as shocks and photon dominated regions. In so doing I examine the similarities and differences in the H2 spectrum found under these different excitation conditions and mechanisms, and how the observations impact on some of the latest models.

Introduction

Before the launch of the Infrared Space Observatory (ISO, Kessler et al. 1996) observations of H2 were restricted to a hot component with an excitation temperature of about 2000 K in shock excited sources, and a non-thermally (i.e. fluorescently) excited component in photon dominated regions (PDRs). These components were typically probed with the 1–0 S(1) line at 2.12 µm as well as several other near-infrared ro-vibrational transitions, and in some cases pure rotational transitions from high J levels (e.g. Gredel 1994, Knacke & Young 1981). Only a few observations, principally toward the Orion star forming region, of lower energy pure rotational transitions existed, e.g. the 0–0 S(2) and 0–0 S(1) lines at 12.2786 and 17.0348 µm, but which already pointed to the existence of a lower temperature component in such sources (e.g. Beck, Lacy & Geballe 1979; Parmar, Lacy & Achtermann 1991, 1994; Richter et al. 1995; Burton & Haas 1997).

From the broadest perspective, the hydrogen molecule is found virtually everywhere in the Universe. Some issues concerning H2 in space clearly deserve more attention. For example, can traces of the formation process of H2 in the interstellar medium be observed? Is it possible for large quantities of very cold H2 to escape detection? How can H2 be used to probe gas at high redshift and in the centers of active galaxies?

Introduction

The hydrogen molecule plays myriad roles on the cosmic stage. During this conference, we have been reminded how the study of H2 ranges widely in space, time, and energy: from the microcosm of molecular processes to the giant molecular clouds, from the origin of structure in the early universe to places where a star will form tomorrow. We marvel at speculations about clumpuscules of H2 that might be as cold as 3 K yet contribute measurably to gamma radiation from the Galactic halo. In trying to offer a forward-looking perspective on H2 in space, it seems best to concentrate on a few topics where rapid progress in observation is taking place and where the interpretation of existing results is inadequate.

On the interpretation of astronomical spectra of H2

Dilute matter in space generally exists in a chemical and physical state far out of thermodynamic equilibrium. The state of such dilute matter reflects a competition among microscopic processes, which often operate in contact with several thermal reservoirs at different effective temperatures.

High spatial and spectral resolution observations are reported of H2 infrared emission from the reflection nebulæ NGC2023 and NGC7023. The local molecular gas is strongly perturbed by the presence of the massive stars which power these nebulae. Data yield information on the small-scale structure, the temperature and density and the dynamics of the excited gas. Excited material is found to be hot (400-500K), dense (105-106 cm−3) and clumped containing substantial flows and velocity fields.

Introduction

The two reflection nebulæ NGC2023 and NGC7023 are prototypes of regions in which recently formed massive stars are interacting strongly with their parent gas. The outcome of these interactions is important in understanding the cycle of star formation in which massive stars are created and, by perturbing their surroundings, influence the nature of the gas in which future stars may form. The goal of our work is to examine in detail the perturbed gas around massive young stars. Some of the observations of infrared (IR) emission of molecular hydrogen in NGC2023 and NGC7023, performed in recent years in our group, are described below.

Nebulosity in NGC2023 and NGC7023 is excited by B-stars of temperatures respectively 22,000K and 20,400K. The distance between the star and the illuminated surrounding gas is ∼ 0.1 pc in both nebulæ. NGC2023 shows a strong IR excess with emission from small dust particles plus extended red emission, and has an associated molecular cloud with OH, HCHO, HCN, CO, CH, CH+ and other detections (see Field et al. 1994).

Observations of H2 line emission have revealed higher-than-expected gas temperatures in a number of photodissociation fronts. We discuss the heating and cooling processes in photodissociation regions. Observations of NGC 2023 are compared to a theoretical model in which there is substantial gas at temperatures T = 500 – 1000K heated by photoelectric emission and collisional de-excitation of H2. In general the model successfully reproduces the observed H2 line emission from a wide range of energy levels. The observed [SiII]34.8µm emission appears to indicate substantial depletion of Si in NGC 2023.

Introduction

A significant fraction of the ultraviolet radiation emitted by massive stars impinges on the molecular gas associated with star formation. The resulting photodissociation regions (PDRs) therefore play an important role in re-processing the energy flow in star-forming galaxies. Modelling these PDRs is therefore an important theoretical challenge, both to test our understanding of the physical processes in interstellar gas, and to interpret observations of star-forming galaxies.

It is frequently the case that the illuminating star is hot enough to produce an H II region, in which case the photodissociation region is bounded on one side by an ionization front, and on the other by cold molecular gas which has not yet been appreciably affected by ultraviolet radiation. The hv < 13.6eV photons propagating beyond the ionization front raise the fractional ionization, photo-excite and photo-dissociate the H2, and heat the gas via photoemission from dust and collisional de-excitation of vibrationally-excited H2.

Most of the diffuse interstellar medium is cold, but it must harbor pockets of hot gas to explain the large observed abundances of molecules like CH+, OH and HCO+. Because they dissipate locally large amounts of kinetic energy, MHD shocks and coherent vortices in turbulence can drive endothermic chemical reactions or reactions with large activation barriers. We predict the spectroscopic signatures in the H2 rotational lines of MHD shocks and vortices and compare them to those observed with the ISO-SWS along a line of sight through the Galaxy which samples 20 magnitudes of mostly diffuse gas.

The trigger of hot chemistry in the cold diffuse medium

The large observed abundances of CH, CH+, HCO+, and OH in the (mainly cold) diffuse medium (T ≈ 50 K) imply that activation barriers and endothermicities of several 103 K are overcome. Pockets of hot gas must therefore exist. Large ion-neutral drift speeds, of several km s−1, can equally contribute to triggering certain ion-neutral reactions in cold gas, such as the endothermic reaction C+ + H2 which forms CH+ (ΔE/k=4640 K). Two phenomena, operating at very different scales, are able to reproduce the observed abundances. These are MHD shocks (Flower & Pineau des Forêts, 1998 and references therein) and intense vortices, thought to be responsible for a large fraction of the viscous dissipation of supersonic turbulence (Joulain et al. 1998).

Molecular hydrogen is the most abundant constituent in interstellar space, but is difficult to observe in its most common form. Because H2 has no dipole moment, it does not have allowed rotational or vibrational transitions and therefore in its cold, unexcited state it has no radio or infrared spectral features. Therefore the most sensitive method for detecting cold H2 is through its allowed electronic bands, which lie in the far-ultraviolet portion of the spectrum. Previous UV instruments have provided some information on far-UV spectra of cold H2, but all were limited in either throughput or spectral resolving power, or both. The current FUSE mission has a combination of high throughput and moderately high spectral resolution, and is providing information on molecular hydrogen in interstellar regimes that have been previously unexplored. This review summarizes previous UV observations of H2 and then gives an overview of early FUSE results, with an emphasis on H2 in translucent clouds.

Introduction

The mass in the galactic interstellar medium is dominated by molecular hydrogen, which begins to outweigh atomic hydrogen in diffuse clouds and is expected to become completely dominant for translucent and dense clouds; i.e. clouds having visual extinctions greater than about Av = 2 magnitudes. Even in diffuse clouds, H2 is important, playing a crucial role in cloud chemical and physical processes.

Despite the importance of understanding H2 and its distribution, physics, and chemistry, relatively little direct information is available because of the obtuse spectroscopic properties of the molecule.

Two observable quantities have been calculated by using the data blocks which provide all details of interstellar UV absorption in H2 gas from the electronic ground state of H2 into 6 electronically excited states and fluorescence emission back into bound and continuum states of the electronic ground state. Both quantities describe details happening in the edges of interstellar H2 abundances where UV radiation is very efficiently shielded and HI gas is produced by fluorescent radiative dissociation. The first is the fluorescence spectrum of H2 in the wavelength range between 1350 and 1700 Å. Comparisons with published spectra show very nice agreement of the resolved features. The second is the velocity distribution of HI produced in fluorescent radiative dissociation. Subject of this comparison are the observed 21 cm spin-flip line profiles of the CNM and WNM (Cold and Warm Neutral Medium) HI species. The profile of the HI velocity dispersion, compared with the “narrow” WNM component, has been obtained at ≈ 9 km/s FWHM which is slightly below the statistical average, and this profile was found to be a molecular property of the H2 gas rather than a function of intensity of the incident UV field or the temperature. The required long tail (“wide” Gaussian component) is provided by all LTE models.

The rest of the paper is devoted to an outline of a dark matter model which fulfills the condition of statistical equilibrium in an active gaseous interphase between the interstellar UV field and the constituent dark matter mass.

We present a complete study of the H2 infrared emission, including the pure rotational lines, of the proto Planetary Nebulae CRL 618 with the ISO SWS. A large number of lines are detected. The analysis of our observations shows: (i) an OTP ratio very different from the classical value of 3, probably around 1.76-1.87; (ii) a stratification of the emitting region, and more precisely different regions of emission, plausibly located in the lobes, in an intermediate zone, and close to the torus; (iii) different excitation mechanisms, collisions and fluorescence.

Introduction

CRL 618 is one of the few clear examples of an AGB star in the transition phase to the Planetary Nebula stage: a Proto Planetary Nebula (PPN). It has a compact HII region created by a hot central C-rich star, and is observed as a bipolar nebula at optical, radio and infrared wavelengths. The expansion velocity of the envelope is around 20 kms−1, but CO observations show the presence of a high-velocity outflow with velocities up to 300 kms−1 (Cernicharo et al.1989). High-velocity emission in H2 is also detected (Burton & Geballe 1986). The high velocity wind and the UV photons from the star perturb the circumstellar envelope producing shocks and photodissociation regions (PDRs) which modify the physical and chemical conditions of the gas (Cernicharo et al.1989, and Neri et al.1992). Clumpiness within the visible lobes and low-velocity shocks being the remnant of the AGB circumstellar envelope are also proposed by Latter et al.(1992).

A brief history of observations of H2 and HD molecules is presented. The properties of H2 and HD that make observations of them a uniquely powerful diagnostic probe are pointed out. The interpretations of the observations in the ultraviolet, near infrared and infrared made of a diverse range of astrophysical objects are discussed and the influence of H2 and HD on their evolution is described.

Introduction

The hydrogen molecule occupies a central place in astrophysics. There are many reasons. The hydrogen molecule was the first neutral molecule to be formed in the Universe and it played a crucial role in the collapse of the first cosmological objects. It is the most abundant molecular species in the Universe, able to survive in hostile environments and found to exist in diverse astronomical objects ranging from planets to quasars. Hydrogen molecules have been detected by the absorptions and emissions at ultraviolet and infrared wavelengths to which they give rise. Molecular hydrogen has specific radiative and collisional properties that make it a diagnostic probe of unique capability.

Ultraviolet absorption

The first detection of H2 beyond the solar system was accomplished by Carruthers (1970) who employed a rocket-borne ultraviolet spectrometer and detected absorption bands of the Lyman system of H2 looking towards the star ξ Persei. More extensive data at much higher spectral resolution were obtained with a spectrometer aboard the Copernicus satellite (Spitzer and Jenkins 1975).

We present recent results from a program devoted to the study of small scale structure in translucent molecular clouds, using dust as a tracer. Several methods have been employed: i) statistical analysis of stellar fields, ii) studies of background galaxies; iii) searches for time variations of the extinction and reddening. For each method, we summarize the principles, the type of constraints provided (scales, sensitivity) and present the results obtained so far. We conclude by some prospects concerning direct studies of the distribution of H2 itself.

Motivations

Originally, we wished to constrain in a direct way the level at which the penetration of the stellar UV and visible radiation is enhanced by structure effects (cf Boissé 1990). Studies of spatial variations of dust extinction and reddening offer a powerful way to address this question. Indeed, in the presence of density fluctuations, the analysis of extinction in stellar fields directly provides a measure of the effective opacity (Thoraval et al. 1997). Further, in the visible, the required observations are easy to perform and provide an excellent spatial resolution.

Another motivation comes from the detection of very small scale structure both in atomic (Frail et al. 1994) and molecular gas (Moore & Marscher 1995). In the assumption of a uniform dust to gas ratio, one should observe corresponding variations of the amount of dust, resulting in local variations of the extinction.

Polycyclic aromatic hydrocarbons (PAHs) could play an important role in interstellar chemistry. In particular, it is important to evaluate their possible contribution to the formation of H2. To address this question, recent laboratory results and new observations are presented. Although still preliminary, these first results are very encouraging. First, the photodissociation of PAHs isolated in ion traps and exposed to UV light involves the loss of pairs of hydrogen atoms which are likely to form H2 molecules. Second, the PAH and H2 emission observed in the photodissociation region associated with the young stellar object S106-IR was found to coincide at some positions. This suggests a coupling between the interstellar PAH and H2 populations. More results are expected in the near future.

Introduction

The Infrared Space Observatory (ISO) has recently showed the ubiquity of the emission features at 3.3, 6.2, 7.7, 8.6, 11.3 and 12.7 µm in the interstellar medium (First ISO Results 1996, Boulanger 1999). Amongst the carriers for these features, polycyclic aromatic hydrocarbons (PAHs) are the best candidates as far as an excitation mechanism and a reasonable spectral agreement are concerned. A lot of infrared spectroscopy has been performed in the laboratory since the initial proposal by Léger & Puget (1984) and Allamandola et al. (1985) to find the laboratory species whose spectrum match the interstellar spectrum (Szczepanski & Vala 1993, Joblin et al. 1995, Cook et al. 1998, Allamandola & Hudgins 1999, Hudgins & Allamandola 1999, etc…).