Using high-resolution infrared CO spectroscopy new results have been obtained on the environment of high-mass young stellar objects (YSOs). In particular, a new class of neutral, warm and dense outflows has been discovered. The infrared outflows appear a general phenomenon of the activity of high-mass YSOs. With well-defined and often multiple velocities they seem to correspond to episodic and violent mass-loss events from the central source. In addition, a shell of warm, quiescent gas, is formed near the massive star. All these dynamical elements influence the chemistry inside the giant molecular clouds. Beside CO in solid and in gas phase, detection in the infrared has been attempted of simple molecules like CH4, C2H2, H2O and ion H3+ toward few of these sources.

Recent results on infrared spectroscopy of the cluster of sources in the Orion molecular cloud are discussed. These results imply very high abundances of CO, C2H2, HCN, and OCS, suggesting that grain surface chemistry has been important.

We are mapping 29 rotational transitions of 21 chemical and isotopic molecular species in the central Orion molecular ridge with Nyquist sampling using the new 15-element focal plane array receiver QUARRY on the FCRAO 14 m telescope. Our goal is to obtain complete, unbiased data sets for a study of the interrelated physics and chemistry in GMC cores.

We present the results of 12 years of observations of the 8 km s−1 water maser in Orion. Four large flares in the flux density were observed, as well as variability in the degree and angle of polarization. The profile of the line is also variable, indicating the superposition of several sources.

We combine a sophisticated model of maser propagation with a simple model of an accelerating molecular outflow and show that the observation of different OH maser frequencies is consistent with emission from different parts of the outflow.

We assess the gas-phase abundances of Si, C, and Fe from our recent measurements of Si++, C++, and Fe++ in the Orion Nebula by expanding on our earlier “blister” models. The Fe++ 22.9 μm line measured with the KAO yields Fe/H ~ 3 × 10−6 - considerably larger than in the diffuse ISM, where relative to solar, Fe/H is down by ~ 100. However, in Orion, Fe/H is still lower than solar by a factor ~ 10. The C and Si abundances are derived from new IUE high dispersion spectra of the C++ 1907, 1909 Å and Si++ 1883, 1892 Å lines. Gas-phase Si/C = 0.016 in the Orion ionized volume and is particularly insensitive to uncertainties in extinction and temperature structure. The solar value is 0.098. Gas-phase C/H = 3 × 10−4 and Si/H = 4.8 × 10−6. Compared to solar, Si is depleted by 0.135 in the ionized region, while C is essentially undepleted. This suggests that most Si and Fe resides in dust grains even in the ionized volume.

The observational and theoretical information on the CO/H2 abundance in a variety of astrophysical regions including diffuse clouds, dense star-forming regions, shocked gas and circumstellar envelopes is discussed and reviewed.

At the interface between an HII region and a molecular cloud, lies a neutral gas layer which is subject to both an intense radiation field, and to shocks arising from the expansion of the ionisation front of the HII region. The gas in these regions is highly excited, hot, and may be fairly dense. We present the first high resolution images of atomic carbon towards a sample of ionisation front sources. This study has relevance to our understanding of shock induced star formation, the formation and destruction of molecular species under extreme conditions, shock processes in the ISM, and the energy balance in molecular clouds.

UV-radiation longward of the Lyman edge (912 Å) can escape from HII-regions. It photodissociates carbon monoxide (λ < 1118 Å) and photoionizes neutral atomic carbon (λ < 1101 Å). The resulting CII/CI/CO transition zone on the edges of molecular clouds (photodissociation region or photodominated region: PDR) has been studied in great detail theoretically (Tielens & Hollenbach, 1985; van Dishoek & Black, 1988; Sternberg & Dalgarno, 1989) and these investigations have recently been extended to cover a wide range of densities and UV-intensities (Burton, Hollenbach & Tielens, 1990; Hollenbach, Takahashi & Tielens, 1991; see also A. Sternberg, this volume).

Substantial CH+ abundances are found in at least three types of environment (Lambert and Danks, 1985): atomic regions with little H2, N(CH+) ~ 1012 cm−2; diffuse clouds such as that toward ζ Oph, N(CH+) ~ 1013 cm−2; reddened (2 ≲ Av ≲ 4) lines of sight to bright stars N(CH+) ~ 1014 cm−2. We explore the view that several different mechanisms operate.

Warm molecular gas is important in a large range of astronomical contexts. We discuss here determinations of the temperature and mass of warm material in protostellar disks and cores, photon dominated regions, and molecular material shocked by protostellar outflows. We then compare these results to heating and cooling models. The models of dense cores and photon dominated regions are not adequate to explain the large amounts of warm material observed. This conclusion raises the possibility that there may be other heating mechanisms at work in these regions which theorists have not yet included in their models.

The possibility that the observed abundances of several molecules in the Orion Hot Core are due to gas phase neutral chemistry has been examined.

Elevated temperatures in molecular clouds can result either from heating by ultraviolet photons or from the passage of shock waves. The effect that these processes have on the chemical abundances is not well established observationally, but is of great importance for the interpretation of molecular line observations not only in our own Galaxy, but also in external galaxies. We present here initial results from our study of two “template” regions: the photon-dominated region IC 63 and the shocked region IC 443.

The FIRAS instrument on COBE has mapped almost the entire sky with a 7° beam and a spectral resolution of about 1 cm−1 over the 1-100 cm−1 range. Maps showing the strengths of the three main components: dust continuum, [C II] and [N II] are presented. A mean spectrum of the galactic emission is found. These results are used to compare the Milky Way to other galaxies.

Comparing the data about the Cepheid periods and the star evolution calculations, the conclusion is made that inside 5 kpc distance from the Sun heavy element abundance Z changes between 0-01 and 0.04.

Theoretical models of photon-dominated regions (PDRs) of molecular cloud surfaces are described. Key aspects of the chemistry of dense PDRs are presented.

Detailed calculations are reported of the intensities of the near infrared forbidden lines of neutral carbon atoms at λ 985.0 nm, 982.3 nm and 872.7 nm emitted from dense clouds subjected to intense radiation fields. The metastable levels that produce the lines are excited by radiative recombination of the C+ ions produced by photoionization. Impacts of electrons with C atoms in the heated edge zones of the clouds contribute an insignificant part to the excitation. The lines observed in M42 and NGC 2024 can be interpreted as arising in gas with densities in excess of 105 cm−3 and radiation fields with intensities between 103 and 106 times the average interstellar field intensity. Radiative recombination of C+ ions may also be an important source of the emission lines detected in the planetary nebulae NGC 6270 and NGC 7027.

Water and molecular oxygen are two simple molecules which are not easily observed in the interstellar medium. O2 emissions have yet to be detected from interstellar space, and H2O has been observed only in rather special environments where the interstellar gas has been warmed by embedded infrared sources or by shock waves. This sorry state of affairs does not necessarily reflect a low interstellar abundance for these species, but rather is a consequence of the very high abundance of O2 and H2O within our own atmosphere: strong atmospheric absorption lines make it extremely difficult to carry out observations of water and molecular oxygen emissions using ground-based or even airborne telescopes. Future observations from two orbiting telescopes scheduled for launch in the coming five years - the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite (SWAS) - promise to improve radically the observational data on H2O and O2.

The formation conditions of the methanol masers in W3(OH) are studied separately. The lower limits of the densities of the A- and E-type maser regions are 105 and 106 cm−3. The A-type maser requires a stronger excitation from HII region, than the E-type maser. The relative abundance of methanol to molecular hydrogen is more than 5×10−5. The calculation indicates that there are maser series (J0-(J+1)-1) E, (J1-(J+1)0) E J=1,2 and (J2-(J+1)1) A+ J=7,8,9 in W3(OH).

We present the first VLBI measurement of interstellar ammonia (NH3) masers. Two masers were found toward the ultracompact Hii regions, W51-e1 and e2. The masers are unresolved in angle and smaller than 0.1 milliarcseconds. Unless these masers are highly beamed, they appear to be saturated.