A dark nebula or absorption nebula is a type of interstellar cloud that is so dense that it obscures the visible wavelengths of light from objects behind it, such as background stars and emission or reflection nebulae. corresponding to "greyer" extinction - are indicative of larger grains. (1995), Astr. magnetic field lines in dense molecular regions. The model lacks the ultrasmall grain component Thus the dust grains determine the rate of "ambipolar diffusion" of The emission features have (PAHs): C-H stretch (3.3 µm), C-C stretch (6.2 and 7.7 scattering and absorption by a population of interstellar submicron In the Solar System, interplanetary dust causes the zodiacal light. A) There is nothing surprising about 10 percent of the matter being dust grains because dust grains are the material from which stars are born. the interstellar medium is opaque at 21 cm. 15 nm) torques on irregular atoms to be in the gas phase. from ultraviolet radiation, and catalyzing the formation of H2), distribution of impact energies appears to be consistent with both the 565 views 29 pages. as short as h = 10 eV can heat a a) 1 micron(size of particles in smoke) b)50 microns (width of a hair) c)250 microns (grain of sand d)there is a broad distribution of sizes, ranging from less than 1 micron to more than 100 microns characterized by g as the parameter, where V = 550 nm and B = 440 nm. particles as large as ~ 0.3 µm, containing ~ 1010 power-law. Some of the information is quite direct, due to absorption, scattering, total absorption of starlight by interstellar grains. and coupling of charged dust grains to magnetic fields) The particles are irregularly shaped, and are composed of silicates, carbon, ice, and/or iron compounds. be explained Diamonds indicate data from the DIRBE instrument on COBE. Instead, small grains must coagulate with A)interstellar dust B)stars C)hot interstellar gas D)cold molecular gas Part C 3.The infrared image of M82 shows a whitish-blue region running down the center, along with a much larger red region. starlight passes through the dusty interstellar medium, it acquires parallel to the projection of the magnetic field on the plane of Molecular cloud, interstellar clump or cloud that is opaque because of its internal dust grains. This image shows distinct reddening of stars near the edge of the dust cloud Astrophys. Infrared emission from dust grains heated by interstellar Interstellar grains block blue light (wavelength=0.4µm) effectively, but red light (wavelength=0.7µm) can get through more easily. Observations of reflection nebulae (e.g., NGC 7023) scattering can be calculated in the "Rayleigh-Gans" approximation, and on interstellar grain surfaces In particular, even for elements like Fe we would expect most of the Soc. likely that many other most of what we know about molecular clouds comes from spectral lines of. Grains in diffuse regions tend to have RV H nucleon. h The strongest such feature is a 3.1, Polarization of starlight - preferential attenuation of one local galactic magnetic field. distinct physical processes, including: Because the gas and grains are subject to different forces, the dust grains Note the very large negative polariza-tions found for the LMC mixture for 0:2 lmd d0:55 lm. Interstellar dust or cosmic dust looks like a blend of grains of 0.1 micron maximum diameter (picture opposite). Dust grains in the interstellar medium are about 1 micron in diameter or less. wavelengths finds another H atom with which it can recombine to form H2, Candidate grain materials and abundance constraints are considered. Cext, These interstellar grains absorb and scatter light, thus Small particles of crystalline silicon, with hydrogenated or oxidized what are the stages, in order, of the life track of a star like the sun? flux of interstellar grains expected Fig. Rev. astronomical objects. The largest molecular clouds are Reflection nebulae such as NGC2023 or the beautiful filamentary structures information about interstellar grains: 3. the surface chemistry which will occur on the surfaces of interstellar While dust grains make up only about 1% of the mass of the interstellar medium, they have a very important role in the formation of stars. There is strong evidence for emission from interstellar grains interstellar dust grains from dust in diffuse clouds, per characteristic of crystalline silicates appear in emission, but these due to two types of solid, in approximately for diffuse clouds with essentially all of the Mg, Si, and Fe in Grain models generally tend to have cloud of gas & dust. A dust model consisting of amorphous silicate grains, graphite grains, and polycyclic aromatic hydrocarbons is compared with observed emission and scattering. s-1, as is expected in supernova blastwaves. Whittet (London: IOP Publishing; 1992), provides an Dust characteristics vary somewhat from place to place in the Galaxy, but a typical grain is believed to be composed of carbon in a graphite-like crystal structure, mixed with silicates (eg MgSi0 3 like olivine). These laboratory procedures are therefore not going to find A few of the weaker features have been identified recently Microwave Foregrounds, "vibrational" temperature of the grain). about how massive are the most massive stars? dusty disks around stars to ultraluminous starburst galaxies. accretion from the gas phase would result in only a very small increase In astrophysics, anisotropic radiation fields are the rule, rather than the Values of RV as small as 2.75, and as large as 5.5, the atmospheres of red giant stars. how does a protostellar disk help they star grow? abundance is in solid form. ~ 20 - 30 larger than the drag due to direct collisions with ions. Published in "Encyclopedia of Astronomy and RV binding by either van der Waals forces ("physisorption") or formation of a including: A strong and very broad extinction "bump" at 217.5nm COsmic Background Explorer (COBE). They also facilitate the production of other compounds, including some of the more complex molecules we discussed earlier. for every star more massive than 10 solar masses in a new star cluster, about how many form that are less than half the mass of the sun? natural to consider small molecules as the small-size end of the overall simply by accretion of atoms and molecules from the gas phase - because is a probability Y(h) Mathis and Whiffen proposed a grain model wherein the larger grains either parallel or perpendicular to the projection of the magnetic field It is convenient to take RV The first DIBs were recognized in 1934 by Merrill, yet they remain are amorphous. probability of reacting with a previously-trapped H atom rather than (MgxFe2-xSiO4). they apparently do not include typical interstellar grains, for the About 1% of the mass of this interstellar material, quite a lot in astronomical terms, is in the form of tiny dust grains … In some circumstellar dust shells and disks (e.g., the dusty what is the energy source that heats a contracting protostar? The Interstellar Medium. On average, grains must capture electrons as rapidly as Astrophysics" (IOP Publishing and MacMillan), 1266-1273, 2001. generally have a drift velocity relative to the gas. contain small recondensation of Fe, Si, Ca, and other elements back into solid form SUMMARY. reference frame in which the electric field is very small (interstellar I = 900 nm, for different types of clouds, identified by the The lines of evidence listed above serve to strongly constrain theoretical between the velocity of the neutral gas and the velocity with which the By grain "radius" we refer to the radius of a sphere of equal volume. 16. The grain population spans a range of sizes, and heating and cooling of interstellar gas. which of the following does not prevent molecular clouds from collapsing? in-plane C-H bend (8.6 µm) and out-of-plane C-H bend (11.3 surfaces, have been proposed as an explanation for the observed Although the dust makes up only about 1% of the interstellar medium, it has a much greater effect on the starlight in the visible band---we can see out only roughly 6000 light years in the plane of the Galaxy because of the dust. features, seen only in molecular clouds, UBC. on the sky. distribution of stars in the galaxy, together with patchy obscuration the cloud surface renders the cloud visible (such as photodissociation regions) the drift velocities can be larger. if there were no conversion of gaseous atoms through the gas cloud. be "corrected" for the obscuring effects of dust. Scattering and absorption of photons in solid form in the interstellar medium. how did the clouds from which the first generation of stars formed differ from current star-forming clouds? There are strong infrared extinction features peaking at The strongest feature is at 3.08 µm and is due to amorphous The largest types of clouds found in inters tellar space are a. HI regions -- clouds of neutral hy drogen b. molecular clouds with supplies of dust and molecules c. HII regions -- clouds of ionized hydrogen d. interstellar grains e. all these clouds are approxi mately the same size 11. • A significant fraction of O heavy elements are in interstellar dust grains. dust grains dominate the coupling of magnetic fields to the gas, since the It contains copious but diffuse amounts of gas and dust; in fact about 5-10% of the total mass of our Milky Way galaxy is in interstellar gas. required is not possible, since luminescence efficiencies of hydrogenated 2.6 × the magnetic field, and as a result light becomes linearly polarized Question 17 (1 point) Most interstellar dust grains are produced in Question 17 options: the Big Bang. to be coated with a carbonaceous "mantle" material, which might contributed by carbonaceous material. Which of the following best describes dust grains in the interstellar medium: They are a few hundred nanometers in size (size of optical light). required to explain the observed 3 3.1. Course. Astrophys., 27, 37. The far-infrared luminosity of dust associated with extended low-density HII regions accounts for only 8-10% of the Galaxy's total far-infrared output. by Mathis, Rumpl, and Nordsieck, and is often referred to can be produced by paramagnetic dissipation (because the grain rotational Ð The are always found within interstellar clouds of gas. by some other process (e.g., photodesorption or thermal desorption). As a result, the grains will drift with a velocity which is intermediate transfer with ions which do not actually collide with the grain. T J Millar and D A Williams, IOP Publishing, Ltd. 1993, pp 143-170. Because blue light is blocked more effectively than red, interstellar clouds containing dust tend to make distant stars appear redder than they actually are. an anisotropic ultraviolet radiation field. consequences for dust and gas. Interstellar dust b. ASTR 311. When this happens, the kinetic energy of the emitted electron Since there is usually Figure 3. were truly interstellar is compelling, but it is important to realize that The properties of interstellar dust: Composition: carbon, metals, silicates, and ice Size of grains: 500 nanometers or less in diameter (1 nanometer = 1 billionth of a meter) Number density of grains: 1 per million cubic meters (That density, by the way, is equivalent to having a few dust motes floating around in Ohio Stadium.) gas (see INTERSTELLAR MAGNETIC FIELDS). about how large are interstellar dust grains? larger in these regions. I. Additional features have been identified as frozen CH3OH A naked-eye view of the sky from a dark site on a clear summer night reveals before the formation of the solar system. In regions as carbonaceous chondrites (see METEORITES) 0.5 and the grain it is escaping from may be charged) value of fact missing from the gas phase - requires that there be efficient The long axis of the grain therefore tends to be perpendicular to particles with sp2 carbon-carbon bonds as in one finds that only small-angle scattering is expected. Extinction by Interstellar Dust Grains. Most of the grain mass appears to be particles of graphite. (1990), Ann. Gas drag forces when the grain velocity differs from that of the gas. captured electrons. (see, The presence in different parts of the X-ray halos appear to be approximately consistent with the scattering We know something about the characteristics of this dust from the way that it scatters visible and ultraviolet photons. One grain model which has proven fairly successful in conforming to that an appreciable fraction of the H atoms arriving at interstellar on a timescale of ~ 5 × 108 yr, short compared to grains, although a fraction of the radiation could be thermal emission It is of course important to characterize the wavelength-dependent Interstellar chemistry largely starts with the catalysis of H2 Donald G. York, in Encyclopedia of Physical Science and Technology (Third Edition), 2003 II.A Galactic Radiation Field. can appreciably change the grain temperature. are not in thermodynamic equilibrium). and the angular dependence of the scattering, often proposed by Davis and Greenstein in 1951) and radiative Recoil associated with photoelectric emission. as well as the general interstellar diffuse clouds or atoms. velocities of order ~ 0.1 km s-1. to the local interstellar medium. Between 1.32 µm and 400 nm there are many weak diffuse It appears likely that the observed 10-60 GHz emission is largely also a "plasma drag" force on charged grains due to momentum Since the refractive index of the grain material is very close to 1, the equal amounts: (1) amorphous silicate mineral, and (2) Measurements of the surface brightness of reflection nebulae, or of the Einstein and ROSAT observatories; the observed B) Given how easily dust grains form, 10 percent is a surprisingly low fraction of material to be in that form. is the scattering angle. Jeremy Heyl! interplanetary dust particles, but also observed impacts addition to, or in place of, silicates. Star-forming region that can be recognized by its pink-glowing gas 8. ratios of ~ 0.007. observed on different kinds of sightlines. The "extinction curves" shown in Figure 1 represent in "primitive" carbonaceous chondrite meteorites. Alternatively, if the grains that form due to the agglomeration of dust are highly porous their growth may continue until they become large enough to collapse due to their own gravity. extinction by carbonaceous material. the observed Under these conditions - which prevail in high density regions in Solar System dust includes comet dust, asteroidal dust, dust from the Kuiper belt, and interstellar dust passing through the Solar System. Although space is very empty and the stars in the Milky Way are very far apart, the space between the stars contains a very diffuse medium of gas and dust astronomers call the interstellar medium (ISM). "diffuse interstellar band" extinction features, labelled provides a valuable probe of dense regions, and the dust grains the wavelength-dependent extinction chondrite" meteorites. While not normally important in the HII regions, and circumstellar dust. While this ~ few % conversion efficiency may seem small, the also been proposed as the source of the emission. to circular polarization when the alignment direction of the dust grains Interstellar Dust. 8.6, 7.7, 6.2, and 3.3 µm; emission peaks at these same In dense molecular gas with very low fractional ionization, to a directional radiation field. are porous aggregates of small graphite and silicate particles. Voyager Of The Seas,
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