Terms Developed in Context
molecular hydrogen (H2)
The dominant molecule in cold molecular gas, composed of two hydrogen atoms but difficult to detect directly at typical cloud temperatures. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. Because CO and H2 form and are destroyed together under similar physical conditions (both require sufficient dust/gas shielding from dissociating ultraviolet radiation), CO’s velocity-integrated line intensity W_CO (in K km/s) is empirically found to correlate tightly with the H2 column density along the same line of sight, N(H2) = X_CO * W_CO, with a nearly constant conversion factor X_CO across most of the Milky Way disk’s molecular gas (Bolatto, Wolfire & Leroy 2013).
It is tied to the lecture’s observables or evidence in this way: Gas-phase formation of H2 (two hydrogen atoms colliding directly) is far too slow to explain the observed molecular fractions in clouds, because the newly formed H2 molecule has no way to shed its formation energy fast enough via gas-phase collisions alone; grain-surface catalysis -- H atoms sticking to and diffusing across a cold dust-grain surface (the same dust grains from Unit 3) until they meet and combine, with the grain itself absorbing the excess binding energy -- dominates H2 formation throughout the ISM (Hollenbach & Salpeter 1971; Gould & Salpeter 1963). A second connection is important for using the term in physical reasoning: Multiplying this column-density relation by the observed area of a cloud (or, equivalently, treating the product W_CO*Area as the cloud’s "CO luminosity" L_CO) converts an entirely observational quantity (an integrated CO line flux) into a molecular hydrogen mass via M(H2) = 2 m_H X_CO L_CO (the factor of 2 accounts for two H atoms per H2 molecule). The observational interpretation is constrained by H2’s lowest rotational transitions require temperatures of hundreds to thousands of kelvin to be significantly populated (because H2’s small moment of inertia gives it widely spaced rotational energy levels), far above the ~10-25 K typical of the bulk of molecular-cloud gas, so H2 itself is essentially undetectable in the cold gas that dominates a cloud’s mass.
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
grain-surface catalysis
Formation of molecules on dust surfaces, where adsorbed atoms can meet and the grain can accept excess reaction energy. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. Multiplying this column-density relation by the observed area of a cloud (or, equivalently, treating the product W_CO*Area as the cloud’s "CO luminosity" L_CO) converts an entirely observational quantity (an integrated CO line flux) into a molecular hydrogen mass via M(H2) = 2 m_H X_CO L_CO (the factor of 2 accounts for two H atoms per H2 molecule).
It is tied to the lecture’s observables or evidence in this way: H2’s lowest rotational transitions require temperatures of hundreds to thousands of kelvin to be significantly populated (because H2’s small moment of inertia gives it widely spaced rotational energy levels), far above the ~10-25 K typical of the bulk of molecular-cloud gas, so H2 itself is essentially undetectable in the cold gas that dominates a cloud’s mass. A second connection is important for using the term in physical reasoning: This CO-derived mass can be independently cross-checked against a direct mass estimate (density times volume, using an independently determined H2 density and cloud size, e.g., from dust continuum emission or extinction mapping) and against a virial mass estimate (Lecture 08) -- exactly the three-way cross-check this lecture’s worked example performs for the real Orion Molecular Cloud. The observational interpretation is constrained by Carbon monoxide (CO), despite being roughly 10^4 times less abundant than H2, has closely spaced rotational energy levels (because of its larger moment of inertia) that are easily excited even at 10-25 K, and its rotational transitions (particularly the J=1-0 line at 2.6 mm) are the standard, nearly universal tracer of molecular gas throughout the Galaxy and in external galaxies (Bolatto, Wolfire & Leroy 2013, ARA&A 51, 207, the definitive X_CO review).
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
carbon monoxide (CO)
A trace molecule with readily excited rotational lines that is widely used to locate and kinematically map molecular gas. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. This CO-derived mass can be independently cross-checked against a direct mass estimate (density times volume, using an independently determined H2 density and cloud size, e.g., from dust continuum emission or extinction mapping) and against a virial mass estimate (Lecture 08) -- exactly the three-way cross-check this lecture’s worked example performs for the real Orion Molecular Cloud.
It is tied to the lecture’s observables or evidence in this way: Carbon monoxide (CO), despite being roughly 10^4 times less abundant than H2, has closely spaced rotational energy levels (because of its larger moment of inertia) that are easily excited even at 10-25 K, and its rotational transitions (particularly the J=1-0 line at 2.6 mm) are the standard, nearly universal tracer of molecular gas throughout the Galaxy and in external galaxies (Bolatto, Wolfire & Leroy 2013, ARA&A 51, 207, the definitive X_CO review). A second connection is important for using the term in physical reasoning: Because CO and H2 form and are destroyed together under similar physical conditions (both require sufficient dust/gas shielding from dissociating ultraviolet radiation), CO’s velocity-integrated line intensity W_CO (in K km/s) is empirically found to correlate tightly with the H2 column density along the same line of sight, N(H2) = X_CO * W_CO, with a nearly constant conversion factor X_CO across most of the Milky Way disk’s molecular gas (Bolatto, Wolfire & Leroy 2013). The observational interpretation is constrained by Gas-phase formation of H2 (two hydrogen atoms colliding directly) is far too slow to explain the observed molecular fractions in clouds, because the newly formed H2 molecule has no way to shed its formation energy fast enough via gas-phase collisions alone; grain-surface catalysis -- H atoms sticking to and diffusing across a cold dust-grain surface (the same dust grains from Unit 3) until they meet and combine, with the grain itself absorbing the excess binding energy -- dominates H2 formation throughout the ISM (Hollenbach & Salpeter 1971; Gould & Salpeter 1963).
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
X_CO conversion factor
The empirical proportionality between velocity-integrated CO brightness and inferred H2 column density. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. Because CO and H2 form and are destroyed together under similar physical conditions (both require sufficient dust/gas shielding from dissociating ultraviolet radiation), CO’s velocity-integrated line intensity W_CO (in K km/s) is empirically found to correlate tightly with the H2 column density along the same line of sight, N(H2) = X_CO * W_CO, with a nearly constant conversion factor X_CO across most of the Milky Way disk’s molecular gas (Bolatto, Wolfire & Leroy 2013).
It is tied to the lecture’s observables or evidence in this way: Gas-phase formation of H2 (two hydrogen atoms colliding directly) is far too slow to explain the observed molecular fractions in clouds, because the newly formed H2 molecule has no way to shed its formation energy fast enough via gas-phase collisions alone; grain-surface catalysis -- H atoms sticking to and diffusing across a cold dust-grain surface (the same dust grains from Unit 3) until they meet and combine, with the grain itself absorbing the excess binding energy -- dominates H2 formation throughout the ISM (Hollenbach & Salpeter 1971; Gould & Salpeter 1963). A second connection is important for using the term in physical reasoning: Multiplying this column-density relation by the observed area of a cloud (or, equivalently, treating the product W_CO*Area as the cloud’s "CO luminosity" L_CO) converts an entirely observational quantity (an integrated CO line flux) into a molecular hydrogen mass via M(H2) = 2 m_H X_CO L_CO (the factor of 2 accounts for two H atoms per H2 molecule). The observational interpretation is constrained by H2’s lowest rotational transitions require temperatures of hundreds to thousands of kelvin to be significantly populated (because H2’s small moment of inertia gives it widely spaced rotational energy levels), far above the ~10-25 K typical of the bulk of molecular-cloud gas, so H2 itself is essentially undetectable in the cold gas that dominates a cloud’s mass.
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
CO luminosity
CO velocity-integrated brightness multiplied by projected emitting area, commonly used with a conversion factor to estimate molecular mass. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. Multiplying this column-density relation by the observed area of a cloud (or, equivalently, treating the product W_CO*Area as the cloud’s "CO luminosity" L_CO) converts an entirely observational quantity (an integrated CO line flux) into a molecular hydrogen mass via M(H2) = 2 m_H X_CO L_CO (the factor of 2 accounts for two H atoms per H2 molecule).
It is tied to the lecture’s observables or evidence in this way: H2’s lowest rotational transitions require temperatures of hundreds to thousands of kelvin to be significantly populated (because H2’s small moment of inertia gives it widely spaced rotational energy levels), far above the ~10-25 K typical of the bulk of molecular-cloud gas, so H2 itself is essentially undetectable in the cold gas that dominates a cloud’s mass. A second connection is important for using the term in physical reasoning: This CO-derived mass can be independently cross-checked against a direct mass estimate (density times volume, using an independently determined H2 density and cloud size, e.g., from dust continuum emission or extinction mapping) and against a virial mass estimate (Lecture 08) -- exactly the three-way cross-check this lecture’s worked example performs for the real Orion Molecular Cloud. The observational interpretation is constrained by Carbon monoxide (CO), despite being roughly 10^4 times less abundant than H2, has closely spaced rotational energy levels (because of its larger moment of inertia) that are easily excited even at 10-25 K, and its rotational transitions (particularly the J=1-0 line at 2.6 mm) are the standard, nearly universal tracer of molecular gas throughout the Galaxy and in external galaxies (Bolatto, Wolfire & Leroy 2013, ARA&A 51, 207, the definitive X_CO review).
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.
self-shielding
Protection of a molecule from dissociating radiation when its own absorption lines become optically thick along a column. In this lecture, the term is not an isolated label: This concept is needed to reason about the molecular interstellar medium: h2 formation and co as a tracer. This CO-derived mass can be independently cross-checked against a direct mass estimate (density times volume, using an independently determined H2 density and cloud size, e.g., from dust continuum emission or extinction mapping) and against a virial mass estimate (Lecture 08) -- exactly the three-way cross-check this lecture’s worked example performs for the real Orion Molecular Cloud.
It is tied to the lecture’s observables or evidence in this way: Carbon monoxide (CO), despite being roughly 10^4 times less abundant than H2, has closely spaced rotational energy levels (because of its larger moment of inertia) that are easily excited even at 10-25 K, and its rotational transitions (particularly the J=1-0 line at 2.6 mm) are the standard, nearly universal tracer of molecular gas throughout the Galaxy and in external galaxies (Bolatto, Wolfire & Leroy 2013, ARA&A 51, 207, the definitive X_CO review). A second connection is important for using the term in physical reasoning: Because CO and H2 form and are destroyed together under similar physical conditions (both require sufficient dust/gas shielding from dissociating ultraviolet radiation), CO’s velocity-integrated line intensity W_CO (in K km/s) is empirically found to correlate tightly with the H2 column density along the same line of sight, N(H2) = X_CO * W_CO, with a nearly constant conversion factor X_CO across most of the Milky Way disk’s molecular gas (Bolatto, Wolfire & Leroy 2013). The observational interpretation is constrained by Gas-phase formation of H2 (two hydrogen atoms colliding directly) is far too slow to explain the observed molecular fractions in clouds, because the newly formed H2 molecule has no way to shed its formation energy fast enough via gas-phase collisions alone; grain-surface catalysis -- H atoms sticking to and diffusing across a cold dust-grain surface (the same dust grains from Unit 3) until they meet and combine, with the grain itself absorbing the excess binding energy -- dominates H2 formation throughout the ISM (Hollenbach & Salpeter 1971; Gould & Salpeter 1963).
Boundary and common confusion. Treating X_CO as a universal constant applicable to any galaxy or environment. X_CO is calibrated for typical Milky Way-disk conditions and is known to vary substantially in low-metallicity dwarf galaxies (where less dust shields CO from photodissociation, suppressing CO relative to H2) and in the Galactic center’s more extreme conditions (Bolatto, Wolfire & Leroy 2013); applying the disk-average X_CO outside its calibrated regime can introduce order-of-magnitude mass errors. The term should therefore be applied only after identifying the measured quantity, the model assumption that links it to the desired physical quantity, and the regime in which that link remains valid.