An Etymological Dictionary of Astronomy and Astrophysics
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A → molecular cloud that is physically or apparently related to a star formation region.

→ associate; → molecular; → cloud.

A vast, turbulent region encircling the → Milky Way's nucleus that contains a large fraction of the → Galaxy's dense → molecular clouds and → star formation regions. Spanning -1 to +1.5 degrees of → Sgr A*, the CMZ is about 400 pc × 100 pc in size and contains at least 10⁷→ solar masses of → giant molecular clouds, approximately 10% of the Galaxy's molecular gas. The gas in the CMZ is at higher temperature than typical giant molecular clouds and has high velocity dispersion reflecting the → turbulent nature of the gas in the area. Despite these extensive molecular reserves, the → star formation rate within the CMZ is actually lower than expected based on the analysis of nearby → star-forming regions in the quiescent → Galactic disk. A common assumption is that this is a result of the extreme conditions within the CMZ, where the density, pressure, temperature, → velocity dispersion and → radiation field are all significantly greater than elsewhere in the Milky Way (Clark et al. 2018, The Messenger 173, 22 and references therein).

→ central; → molecular; → zone.

A type of → interstellar medium cloud in which → carbon (C) becomes almost completely molecular due to relatively high → extinction. The chemistry is qualitatively different from that of → diffuse molecular clouds, as the → electron abundance is very low (→ cosmic-ray ionization being the dominant source) and the reactive C is replaced by the very stable → carbon monoxide (CO). This regime is found only in → sightlines with A_V > 5-10 mag; not all such sightlines will contain dense cloud material and if dense cloud material is present it is likely to be surrounded by → translucent material. These clouds are typically → self-gravitating, and are most often observed by → infrared absorption and → millimeter wave emission methods. Their densities are typically at least 10⁴ cm^-3, and their → kinetic temperatures are typically on the order of 10-50 K in the quiescent regions. Most of the more than 140 currently known → interstellar molecules were found through observations of → microwave→ rotational transitions in such clouds, starting with the discovery of OH, followed by a host of other new detections such as CO, NH₃, H₂O, and H₂CO (Snow & McCall, 2006, ARA&A 44, 367).

→ dense; → molecular; → cloud.

A type of → molecular cloud in which the → interstellar radiation field is sufficiently attenuated, so that the local fraction of → molecular hydrogen (H₂) becomes substantial (> 0.1). However, enough interstellar radiation is still present to → photoionize any atomic carbon, or to → photodissociate → carbon monoxide (CO) such that carbon is predominantly still in the form of C⁺ (> 0.5). In steady state, diffuse molecular clouds must necessarily be surrounded by diffuse atomic gas, in order to provide the → shielding of radiation. This means that most sightlines that cross a diffuse molecular cloud will also cross → diffuse atomic gas (Snow & McCall, 2006, ARA&A 44, 367).

→ diffuse; → molecular; → cloud.

A massive complex of → interstellar gas and → dust, consisting mostly of → molecular hydrogen, that typically stretches over 150 light-years and contains several hundred thousand → solar masses. Giant molecular clouds are the principal sites of star formation. → molecular cloud.

→ giant; → molecular; → cloud.

A relatively small, dense, and hot → molecular clump occurring in regions of → massive star formation. HMCs have diameters ≤ 0.1 pc, densities ≥ 10⁷ cm^-3, and temperatures ≥ 100 K. The densest hot cores are traced in → ammonia (NH₃) and possess densities of 10⁸ cm^-3, sizes down to 0.05 pc and temperatures of up to 250 K. Hot molecular cores are generally associated with → compact H II regions and → ultracompact H II regions. High angular resolution observations suggest that HMCs are internally heated by embedded sources, since temperature and density increases toward the center as expected if star formation is occurring close to the core center. Same as → hot core.

→ hot; → molecular; → core.

A Force acting between molecules.

→ inter-; → molecular; → force.

Within the molecule; occurring by a reaction between different parts of the same molecule.

→ intra-; → molecular.

The total atomic or molecular weight divided by the total number of particles. For instance, the mean molecular weight of a plasma of pure ionized ⁴He would be 4 (the atomic mass number) divided by 3, the total number of particles (1 nucleus plus 2 electrons), i.e. 4/3.

→ mean; → molecular; → weight.

Of or pertaining to or caused by molecules.

From → molecule + -ar variant of the adjective-forming suffix → -al, joined to words in which an l precedes the suffix.

Molekuli, from molekul→ molecule + -i adj. suffix.

A band of molecular origin present in a spectrum. See for example → cyanogen band, → S star.

→ molecular; → band.

A relatively dense, cold region of interstellar matter where the atoms are primarily bound together as molecules rather than free atoms or ionized particles. Molecular clouds represent the coldest and densest phase of the → interstellar medium. They consist primarily of → molecular hydrogen (H₂), with temperatures in the range 10-100 K. Molecular hydrogen is not directly observable under most conditions in molecular clouds. Therefore, almost all current knowledge about the properties of molecular clouds has been deduced from observations of molecules such as → carbon monoxide (CO), which have strong emission lines mainly in the → millimeter portion of the → electromagnetic spectrum. So far 129 molecular species have been detected in molecular clouds, among which complex organic molecules. → Dust grains in molecular clouds play a crucial role in the formation of molecules. Molecular clouds are the principal sites where stars form. → giant molecular cloud; → Orion molecular cloud.

→ molecular; → cloud.

One of many compact and dense sub-structures in a → molecular cloud. Their typical sizes are about 0.5-10 → parsecs and their masses range from about 50 to 10³ → solar masses. The gas temperatures are about 10-20 K, and the number densities from about 10³ to 10⁴ cm^-3.

→ molecular; → clump.

A mixing process in a → fluid caused by the → random relative motions due to → Brownian motion of the individual particles. See also → eddy diffusion.

→ molecular; → diffusion.

An → electromagnetic radiation emitted by → interstellar molecules through → transitions between → energy states of → molecules.

→ molecular; → emission.

The formula of a chemical compound, showing the kind and arrangement of atoms.

→ molecular; → formula.

A molecule consisting of two hydrogen atoms (H₂) which is the most abundant molecule in the Universe. Molecular hydrogen plays a fundamental role in many astrophysical contexts. It is found in all regions where → self-shielding against the ultraviolet photons, responsible for its → photodissociation, is sufficiently large. Containing two identical hydrogen atoms, H₂ is highly symmetric. Due to this property, the molecule has no → dipole moment and all → rotation-vibrational transitions within the electronic → ground state are → quadrupolar with low → spontaneous emission  → Einstein coefficient values. The molecule exists in two almost independent states, namely → orthohydrogen and → parahydrogen. H₂ may be excited through several mechanisms, including: 1) → far ultraviolet (FUV) induced → optical pumping and → collisional excitation in → photodissociation regions (PDRs) associated with → star formation; 2) → hard X-rays penetrating and heating regions within → molecular clouds, which in turn excite H₂ via collisions with electrons or hydrogen atoms; and 3) collisional excitation of H₂ due to acceleration produced by interstellar → shock waves. H₂ is thought to be chiefly produced via surface reactions on → interstellar grains, but the exact formation mechanism is not fully understood.

→ molecular; → hydrogen.

An outflow of molecular material, often → bipolar, observed in the regions of → star formation. Molecular outflows are thought to be driven by → bipolar jets from → protostars. They are probably → bow shocks which have had time to cool and be observable in molecular lines. Molecular outflows are poorly → collimated compared to the jets and tend to be slow moving (velocities 10-20 km s^-1). Some bipolar outflows may be driven by → stellar winds.

→ molecular; → outflow.

The ability of a molecular entity to be distorted from its normal shape by an external → electric field. When a molecule is subjected to an electric field there is a small displacement of electrical centers which induces a dipole in the molecule. More specifically, the molecular polarizability α is defined as the ratio of the induced → dipole moment (p) to the local electric field (E) that produces this dipole moment: α = p/E (in cgse units).

→ molecular; → polaizable; → -ity.

In → propositional logic, a → sentence containing at least one → connectives. See also → atomic proposition.

→ atomic; → proposition.