We have studied molecular hydrogen emission in a sample of 21 YSOs using spectra obtained with the Infrared Space Observatory (ISO). H2 emission was detected in 12 sources and can be explained as arising in either a shock, caused by the interaction of an outflow from an embedded YSO with the surrounding molecular cloud, or in a PDR surrounding an exposed young earlytype star. The distinction between these two mechanisms can not always be made from the pure rotational H2 lines alone. Other tracers, such as PAH emission or [SI] 25.25 µm emission, are needed to identify the H2 heating mechanism. No deviations from a 3:1 ortho/para ratio of H2 were found. Both shocks and PDRs show a warm and a hot component in H2, which we explain as thermal emission from warm molecular gas (warm component), or UV-pumped infrared fluorescence in the case of PDRs and the re-formation of H2 for shocks (hot component).

Introduction

Molecular hydrogen is expected to be ubiquitous in the circumstellar environment of Young Stellar Objects (YSOs). It is the main constituent of the molecular cloud from which the young star has formed and is also expected to be the main component of the circumstellar disk. Most of this material will be at temperatures of 20–30 K and difficult to observe. However, some regions may be heated to temperatures of a few hundred K and produce observable H2 emission.

Observations of the near-infrared spectrum of molecular hydrogen in photo-dissociation regions has become a standard tool for revealing the detailed physical conditions and complex density structures of molecular clouds. Most recently, consideration has been give to the detailed behaviour of the ratio of ortho-to-para excited states, and the information that this ratio may contain regarding the history of the molecular cloud (Draine & Bertoldi 1996, Sternberg & Neufeld 1999). This paper will review NIR observations of the H2 spectrum with particular reference to the ortho-para ratios observed. Recent spectroscopy of both galactic and extragalactic sources provide some interesting constraints on the models.

Introduction

Modelling of the H2 emission from photodissociation regions (PDRs) has reached a very high level of sophistication a decade after the first observations of H2 fluorescent emission, from the planetary nebula NGC2023. The earliest models, which predicted the response of low density H2 gas to a moderate intensity UV field (Black & van Dishoeck 1987, Sternberg & Dalgarno 1998) have been expanded to include the effects of collisional excitation of the lowest H2 energy levels (Burton, Hollenbach & Tielens 1990, Sternberg 1991) and of self-shielding of dense H2 (Draine & Bertoldi 1996). Observations of the H2 far-red and near-infrared spectrum confirm the model results for emission arising in energy levels as high as Ek > 40,000K (Draine 2000). Recently, theoretical attention has turned to the observed ortho-para ratio of H2 and the potential that this measure may hold for furthering our understanding of the past and present physical conditions in the PDR.

Present day laser technology has advanced such that multiple resonance excitation can be performed using several lasers of various wavelengths. Also narrowband tunable extreme ultraviolet laser radiation is readily available, to bridge the gap between the low-lying electronic ground state and the excited singlet states in molecular hydrogen. These methods have been employed to investigate a new class of excited states of H2 that are confined to large internuclear separation.

Introduction

Molecular hydrogen, the smallest neutral chemical entity, is often considered to be the simplest molecule. For a spectroscopist, however, H2 brings about a number of complications which make it a difficult object to study. First of all, from an experimental perspective, the electronic ground state is separated from the excited states by a large energy gap, which can be bridged only by photons in the domain of the extreme ultraviolet (XUV). Furthermore hydrogen is a light molecule with a very open rotational structure; the rotational lines are often so widely spaced that it is not obvious that they form a progression. Also, as a consequence of the small mass, deviations from the Born-Oppenheimer are most prominent and strongest in H2. Non-adiabatic interactions shift the energy levels over several tens of cm−1, so that the rovibronic structure becomes confused. As a result assignment of observed spectra, even with rotational quantum numbers only, is not straightforward. This point is illustrated by the Dieke atlas (Crosswhite 1972), a compilation of spectra pertaining to transitions between excited states, recorded in the visible domain with a classical spectrometer.

This paper reports the theoretical and experimental work on H2 formation on interstellar dust mimics. These studies are being carried out under the auspices of the UCL Centre for Cosmic Chemistry and Physics.

Introduction

The purpose of this article is to report on the current state of work at the UCL Centre for Cosmic Chemistry and Physics, a consortium of scientists at University College London addressing problems of chemistry arising in astronomy. All the work currently in progress in this consortium is concerned with H2 formation on surfaces, and it consists of both theoretical and experimental programmes.

The Centre was formed a few years ago when it was realised that advances in both experimental and theoretical techniques now make it possible to address in a realistic manner some problems of longstanding and fundamental interest in astronomy. The expertise at UCL, both in theory and experiment, is very strong on surface reactions; the current motivation from astronomy also emphasises the gas/dust interaction (Williams 1998). It was decided, therefore, to undertake a long-term and coordinated programme on surface processes of relevance to astronomy. Of course, the most fundamental interaction is that leading to H2 formation on dust. There is currently some important experimental and theoretical work being carried out in this particular area, and much of this work has been reported at this meeting. Nevertheless, it was felt that the UCL consortium could make a useful contribution without simply replicating the experiments and calculations of others.

Cool fronts originated by H2 formation and supported by non saturated thermal conduction in the pregalactic gas, are analyzed. The pressure (p2), number density(n2), temperature (T2) and flow velocity (v2) behind the front are found as functions of the temperature ahead the cool front T1 and the intake Mach number M1. Compression behind the cool front occur for both, supersonic and subsonic intake flows providing that M1 is larger than a threshold value, the exact value of which depends on T1. But strongly compressed subsonic flows are left for larger values of M1. Quasi-isobaric cool fronts (p2/p1 ≈ 1) occur when the ratio n1/n2 is closed to the maximum value, where the compressional branch just emerges, beyond which the pressure of the flow behind the front increases when n1/n2 decreases, i.e. for denser subsonic flows behind the cool front. Implications of the above results on the formation of cool condensations in the primordial gas are outlined.

Introduction

Previous studies (Field 1965, Yoneyama 1973, Ibáñez & Parravano 1983, Fall & Rees 1985, Corbelli & Ferrara 1995, Puy et al. 1998) have showed that thermal instability can originate cool condensations in hot plasmas. Also it is believed that at large scales such cold structures are the precursors of the gravitational instability, because if a thermal instability is triggered, in cool regions the temperature decreases and the density increases, i.e. the Jeans mass (∼ T3/2ρ−1/2) could decrease below the value of the actual mass and therefore such regions should gravitationally collapse likely forming stars, globular clusters and galaxies.

Observations and interpretation of extragalactic rotational and rovibrational H2 emission are reviewed. Direct observations of H2 lines do not trace bulk H2 mass, but excitation rate. As such, the H2 lines are unique diagnostics, if the excitation mechanism can be determined, which generally requires high-quality spectroscopy and suitable additional data. The diagnostic power of the H2 lines is illustrated by two cases studies: H2 purely rotational line emission from the disk of the nearby spiral galaxy NGC891 and high resolution imaging and spectroscopy of H2 vibrational line emission from the luminous merger NGC6240.

Introduction

Direct observations of H2 emission from external galaxies have become standard practice in the past decade through the revolution in ground-based near-infrared instrumentation. As a result, the near-infrared H2 rovibrational lines are now readily detectable throughout the local universe (e.g., Moorwood & Oliva 1988, 1990; Puxley et al. 1988, 1990; Goldader et al. 1995, 1997; Vanzi et al. 1998). More recently, the Short Wavelength Spectrograph (SWS) on the Infrared Space Observatory (ISO) has for the first time allowed detection of the purely rotational H2 lines in the mid-infrared spectral regime. For instance, the first detection (outside the solar system) of the H2S(0) line at 28.21 µm was reported by Valentijn et al. (1996) from the star forming nucleus of the nearby spiral galaxy NGC 6946.

The ultraviolet Lyman and Werner absorption lines of H2 have been searched for in a number of high redshift quasar spectra, and detected unambiguously in at least 3 systems at redshifts z∼2. The lack of detectable H2 in most absorbers results from the strong selection in quasar studies against lines-of-sight with significant dust extinction. At high redshift, the ultraviolet radiation field is inferred to be higher than that observed in the local solar neighborhood, suggesting that vigorous star-formation is underway in these galaxies.

Introduction

Recent observations of the high redshift Universe, interpreted in the context of a new generation of computer simulated model Universes, are providing a clear picture of how large galaxies like the Milky Way formed. A number of different observations suggest that large galaxies were assembled from what appear at z = 2 – 3 to be several star-forming proto-galactic fragments (PGF's), widely distributed in space (Windhorst et al. 1994, Pascarelle et al. 1996ab, 1998; Steidel et al. 1996ab, Bechtold et al. 1998). Computer simulations suggest that initially small clumps of material collapsed at the intersection of sheets and filaments in the intergalactic medium, and began forming stars, and that eventually these clumps merged to form large galaxies (Haehnelt, Steinmetz & Rauch 1998, Steinmetz 1998 and references therein). Searches for the galaxies associated with damped Ly-α quasar absorbers show that at z ∼ 2 they are the same population of objects seen in the Hubble Deep Field faint galaxies and the Lyman dropout galaxies (Steidel et al. 1996ab; Bechtold et al. 1998).

We review the relevance of H2 molecules for structure formation in cosmology. Molecules are important at high–redshifts, when the first collapsed structures appear with typical temperatures of a few hundred Kelvin. In these chemically pristine clouds, radiative cooling is dominated by H2 molecules. As a result, H2 “astro–Chemistry” is likely to determine the epoch when the first astrophysical objects appear. We summarize results of recent three–dimensional simulations. A discussion of the effects of feedback, and implications for the reionization of the universe is also given.

Introduction

In current “best–fit” cosmological models, cold dark matter (CDM) dominates the dynamics of structure formation, and processes the initial density fluctuation power spectrum P(K) ∝ kn with n = 1 to predict n = 1 on large scales and n ≈ −3 on small scales (Peebles 1982). The r.m.s. density fluctuation σM then varies inversely with the mass-scale (σM ∝ M−2/3 for M » 1012M⊙, while the dependence is only logarithmic for M « 1012M⊙). The more overdense a region, the earlier it collapses, implying that the present structure was built from the bottom up, with smaller objects appearing first, and subsequently merging and/or clustering together to assemble the larger objects (Peebles 1980). The predicted formation epochs of “objects” (i.e. collapsed dark matter halos) with various masses in the so-called standard CDM cosmology (Bardeen et al. 1986) are shown in Figure 1.

Currently there are three quite different views about galaxy evolution, each one improving the previous state of knowledge:

1. (1) The older one (“ELS”) in which galaxies form by collapse early, quickly, and synchronously (during the “galaxy formation epoch”), ending the dynamically active period; subsequent galaxy evolution is merely a matter of stellar formation processes in a rigid potential.

2. (2) An alternative one (“SZ”) in which disks are viewed as forming inside out over an extended period of time. Galaxy evolution occurs without important internal dynamical instabilities.

3. (3) The slowly emerging picture, after 40 years of N-body simulations and the obvious evidences from recent high-z observations: galaxies evolve both dynamically and chemically over most of the Hubble time in a widely asynchronous way at different speeds, depending on the environment. The Hubble sequence, from late to early types, appears to represent a broad description of the general aging process.

Thus galaxies appear now as evolving structures over typical time-scales of order of 1 Gyr. A fundamental aspect of the micro-physics in galaxies is star formation and gas processes in which the H2 molecule must play a key role: indeed interstellar gas must first form H2 before being able to form stars, so star forming regions do trace molecules, although CO might not have been detected.

Most of the H2 in our Galaxy resides in the cold interiors of molecular clouds. The most reliable way to trace the H2 content of a molecular cloud is, in principle, to measure the distribution of dust through it. In this contribution we present a new observational approach that uses infrared dust extinction of starlight to construct high resolution maps of the distribution of dust (H2) inside molecular clouds over unprecedented ranges of cloud depth: 1 < Av < 40 magnitudes. We also present a comparison of our results with conventional molecular-line column density tracer C18O and conclude that for cloud depths of Av > 10 magnitudes this species is a very poor tracer of H2.

Introduction

Molecular clouds are the reservoirs of H2 in the Galaxy. They contain about half of the mass of the Interstellar Medium and hence an important fraction of the mass of the Galaxy. By far the most important characteristic of molecular clouds is that they are the nurseries out of which stars like our Sun were born. This creation process not only determines the origins of stars and planetary systems in our Galaxy but also regulates the structure and evolution of galaxies on the large scale. To understand star and planet formation is to understand how cold H2 clouds evolve.

We present near-infrared imaging of IC443, covering entire supernova remnant (50 diameter) from the Two Micron All Sky Survey (2MASS), which images are taken simultaneously in the J (1.25µm), H (1.65µm) and Ks (2.17µm) bands. Emission from IC443 was detected in all 3 bands from most of the optically bright parts of the remnant, revealing a shell-like morphology. These are the first near-infrared images that covers entire remnant. The color and structure are very different between the northeastern and southern parts. Bright J and H band emission from the northeast rim can be explained mostly by [Fe II] and the rest by hydrogen lines of Pβ and Br10. We also report ISO LWS observation of [O I] (63µm) for 11 positions in the northeast. Strong lines were detected and the strongest line is in the northeastern shell, where 2MASS image showed filamentary structure in J and H. In contrast, the southern ridge is dominated by Ks band light with knotty structure, and has weak J and H band emission. The shocked H2 line emission is well known from the sinus ridge produced by an interaction with dense molecular clouds. The large field of view and color of the 2MASS images show that the H2 emission extends to the east and the northeast. This H2 emission suggests that the interaction with the molecular clouds extends to the front side in the northeast.

Observations of the interstellar medium within 1 kpc of the Sun with the Copernicus satellite showed a value of the gas to dust ratio that varies by less than a factor of two from its average. The fraction of hydrogen that is molecular is well described by a steady state model that balances formation on grains with photo-destruction. However, in contrast to the local interstellar medium, both in quasar absorption line systems and in circumstellar disks around young stars, there appears to be relatively little H2. We particularly focus on estimating the amount of H2 in circumstellar disks around main sequence stars – the environment where planets form.

Introduction

One of the main results achieved with the Copernicus satellite was the systematic measurement of interstellar H and H2 within about 1 kpc of the Sun. It was found (Savage et al. 1977, Bohlin, Savage & Drake 1978) that the dust to gas ratio is uniform to within a factor of 2 of its average value with the mass in gas being approximately 100 times larger than the mass in dust. Also, the fraction of hydrogen that is molecular, [2N(H2)]/[N(H) + 2N(H2)], is well described by a standard steady state model (Hollenbach, Werner & Salpeter 1971). In this standard model, the H2 is formed on the surface of grains with a rate of about 3 × 1017 cm3 s−1 (Jura 1975) and destroyed by the absorption of ultraviolet photons with a rate near 5 × 10−11 s−1 when the gas is optically thin (Jura 1974).

We summarize the results of recent quantum mechanical calculations of cross sections and rate coefficients for the rovibrational excitation of H2 and HD by the principal perturbers, H, He, and H2. These results have been used to evaluate the rate of cooling of astrophysical media by H2 and HD molecules; these calculations are also described. The cooling of the primordial gas by rotational transitions of H2 is considered as a special case.

All the numerical results and related software are available from http://ccp7.dur.ac.uk/.

Introduction

Molecular hydrogen is recognized as a major contributor to the cooling of astrophysical media. Its role is all the more significant under conditions, such as those which prevailed in the primordial gas, where few other coolants were present; but H2 is also an important, sometimes the dominant coolant of low density interstellar gas, for kinetic temperatures T > 100 K. Interstellar gas can be heated to such temperatures by shock waves, by the dissipation of turbulence, or by absorbing energy from the local ultraviolet radiation field, as in photon-dominated regions.

Although the elemental abundance of deuterium is approximately 5 orders of magnitude less than that of hydrogen, it turns out that cooling by HD must often be taken into account, essentially for two reasons. First, chemical fractionation can, in media which are only partially molecular, enhance the abundance of HD, relative to that of H2.

We review recent H2 absorption line measurements in the diffuse interstellar medium, using FUV spectra from the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS). We investigate molecular hydrogen gas along lines of sight toward 5 stars in the Magellanic Clouds and toward 3 stars within the Milky Way. Molecular fractions in gas within the Magellanic Clouds are significantly lower than typically found in gas in the Milky Way, most likely caused by the lower dust content. The finding of H2 in a Galactic high-velocity cloud led us to speculate that the high-velocity gas in front of the Magellanic Clouds is part of the Galactic fountain. Sight lines toward the Galactic stars show well defined absorption by molecular hydrogen, deuterium and metals, allowing the study of physical and chemical conditions in the local interstellar gas in great detail.

Introduction

Molecular hydrogen is by far the most abundant molecule in the interstellar medium. Its measurement, however, is difficult: H2 has no permanent dipole moment and no radio emission is seen from H2, in striking contrast to the second most abundant molecule in the ISM, carbon monoxide (CO). For the study of the diffuse interstellar medium the FUV absorption spectroscopy is the only method to obtain information about the molecular hydrogen content, but satellites are required for this method, since the earth's atmosphere is opaque for radiation in the FUV domain.

Molecular hydrogen is a key species for the formation of the first luminous objects in the early universe. It is therefore crucial to understand the various physical processes leading to its formation and destruction and the feedbacks regulating this chemical network. Here we review both the radiative and SN-induced feedbacks and we assess the role of the objects relying on H2 for their collapse in the evolution of the reionization of the universe.

Introduction

At z ≈ 1100 the intergalactic medium (IGM) is expected to recombine and remain neutral until the first sources of ionizing radiation form and reionize it. Until recently, QSOs were thought to be the main source of ionizing photons, but observational constraints suggest the existence of an early population of pregalactic objects (Pop III hereafter) which could have contributed to the reheating, reionization and metal enrichment of the IGM at high redshift. In order to virialize in the potential well of dark matter halos, the gas must have a mass greater than the Jeans mass (Mb > MJ), which, at z ∼ 20 – 30 corresponds to very low virial temperatures (Tvir < 104 K). To have a further collapse and fragmentation of the gas, and to ignite star formation, additional cooling is required. It is well known that in these conditions the only efficient coolant for a plasma of primordial composition, is molecular hydrogen (Abel et al. 1997; Tegmark et al. 1997; Ciardi, Ferrara & Abel 2000 [CFA]).