Lidar atmospheric measurements on Mars and Earth

The LIDAR instrument operating from the surface of Mars on the Phoenix Mission measured vertical profiles of atmospheric dust and water ice clouds at temperatures around −65 °C. An equivalent lidar system was utilized for measurements in the atmosphere of Earth where dust and cloud conditions are similar to Mars. Coordinated aircraft in situ sampling provided a verification of lidar measurement and analysis methods and also insight for interpretation of lidar derived optical parameters in terms of the dust and cloud microphysical properties. It was found that the vertical distribution of airborne dust above the Australian desert is quite similar to what is observed in the planetary boundary layer above Mars. Comparison with the in situ sampling is used to demonstrate how the lidar derived optical extinction coefficient is related to the dust particle size distribution. The lidar measurement placed a constraint on the model size distribution that has been used for Mars. Airborne lidar measurements were also conducted to study cirrus clouds that form in the Earth’s atmosphere at a similar temperature and humidity as the clouds observed with the lidar on Mars. Comparison with the in situ sampling provides a method to derive the cloud ice water content (IWC) from the Mars lidar measurements.     

Interstellar extinction in the vicinity of the reflection nebula NGC 1333 in Perseus

The dependence of interstellar extinction on distance in the direction of a dark cloud around the reflection nebula NGC 1333 is determined on the basis of photoelectric Vilnius photometry and photometric classification of 78 stars. Two dust clouds are noted at distances 160 and 220 pc. The first one with mean extinction of 0.4 mag is concluded to belong to the Taurus cloud complex and the second cloud with mean extinction of 1.8 mag belongs to the chain of dark clouds and other young objects which is almost perpendicular to the spiral arm but lies 80 pc below the galactic plane. The star BD +30°549 which illuminates the NGC 1333 nebula is at distance 212 pc from the Sun. No extinction increase behind the Perseus cloud is detected.     

Surface distribution of dark clouds in M31 absorption gradient along the major axis,I

The distribution of dust clouds in M31 was studied based on a photocopy of this galaxy taken from Hubble'sAtlas. The picture was divided into strips starting from the center of the galaxy on both sides along the major and the minor axis, respectively. The number of dark clouds per square kiloparsec was estimated as a function of the distance from the center of the galaxy along the major axis. If we assume the validity of the standard cloud model in M31, the filling factor introduced by the model was found to vary with distance from the center. It was found furthermore, that the filling factor introduced by the model may be an upper limit. The counts of dark clouds indicate a periodical change in the absorption gradient along the major axis with a very slight systematic variation assumed by the exponential part of the representation.     

UV radiation fields in dark clouds

How dark is it inside a dark cloud? If—as is currently believed-interstellar extinction at UV wavelengths is mainly due to scattering with a strongly forward throwing phase-function, the interior of a dark cloud may be much better illuminated at UV wavelengths than its measured extinction would suggest. We consider the penetration of radiation into a dark cloud against scattering and absorption by grains; and we define a new group property for interstellar grains, the exclusion optical depth τ_d. τ_d is a measure of the ability of the grains to exclude radiation from the interior of an externally illuminated cloud. Radiation—as measured by the radiation energy density-penetrates the cloud approximately as if against pure absorption only, with effective optical depth τ_d. Thus τ_d is a conceptually and numerically useful quantity when estimating the role of UV radiation in the thermal and chemical balance within a dark cloud. Computations are made of the radiation fields in (1200, 4500) Å, at the centres of dark clouds with measured visual extinctions. It is found that even in very dark clouds, the radiation energy density in (1200, 1800) Å may be significant, due to the high grain albedo at these short wavelengths.     

Studies of dust grain properties in infrared reflection nebulae

We have developed a model for reflection nebulae around luminous infrared sources embedded in dense dust clouds. The aim of this study is to determine the sizes of the scattering grains. In our analysis, we have adopted an MRN-like power-law size distribution (Mathis, Rumpl, and Nordsieck) of graphite and silicate grains, but other current dust models would give results which were substantially the same. In the optically thin limit, the intensity of the scattered light is proportional to the dust column density, while in the optically thick limit, it reflects the grain albedo. The results show that the shape of the infrared spectrum is the result of a combination of the scattering properties of the dust, the spectrum of the illuminating source, and foreground extinction, while geometry plays a minor role. Comparison of our model results with infrared observations of the reflection nebula surrounding OMC-2/IRS 1 shows that either a grain size distribution like that found in the diffuse interstellar medium, or one consisting of larger grains, can explain the observed shape of the spectrum. However, the absolute intensity level of the scattered light, as well as the observed polarization, requires large grains (approximately 5000 angstroms). By adding water ice mantles to the silicate and graphite cores, we have modeled the 3.08 micrometers ice band feature, which has been observed in the spectra of several infrared reflection nebulae. We show that this ice band arises naturally in optically thick reflection nebulae containing ice-coated grains. We show that the shape of the ice band is diagnostic of the presence of large grains, as previously suggested by Knacke and McCorkle. Comparison with observations of the BN/KL reflection nebula in the OMC-1 cloud shows that large ice grains (approximately 5000 angstroms) contribute substantially to the scattered light.     

Protoplanetary dust clouds in disks of Herbig Ae/Be stars

The very large brightness decrease of late-type Herbig Ae/Be stars is believed to be caused by obscuring dust clouds orbiting in the outer parts of their circumstellar disks. The distances of the dust clouds to the central stars have been estimated using the wavelength at maximum flux of the excess near-IR radiation, Wien's displacement law, and a formula derived by Rowan-Robinson (1980). The critical masses of these clouds were calculated employing Chandrasekhar's (1943) formula. The minimum size of the dust grains in the obscuring clouds was estimated using Aumannet al.'s (1984) formula they had applied to the star Lyr. However, it can be about ten times smaller if the dust grains are situated at the back of the cloud. The average size of these grains has been determined by assuming a size distribution similar to that in the asteroidal belt (Dohnanyi, 1969) and in the interstellar medium (Mathiset al., 1977). Their number density was determined by means of the extinction power of the dust cloud at theV pass-band. The results of our calculations show that above parameters are similar to those in our solar system. Therefore, we believe that most probably (a) the formation of planetesimals in the circumstellar disks of Herbig Ae/Be stars is on-going; and (b) the obscuring clouds will, in the long run, become planet-like objects.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

A study of the kinematics of the local dark clouds

Lack of reliable estimates of distances to most of the local dark clouds has, so far, prevented a quantitative study of their kinematics. Using a statistical approach, we have been able to extract the average spatial distribution as well as the kinematical behaviour of the local dark clouds from their measured radial velocities. For this purpose, we have obtained radial velocities for 115 southern clouds and used the data from the literature for the northern ones. In this paper we present this new data, analyse the combined data and compare our results with those arrived at by earlier studies. The local clouds are found to be expanding at a speed of ∼ 4 kms^-1 which is in general agreement with the estimates from optical and HI studies. However, it is found that the kinematics of the local clouds is not described by the model proposed for the local HI gas where a ring of gas expanding from a point gets sheared by the galactic rotation. Rather, the observed distribution of their radial velocities is best understood in terms of a model in which the local clouds are participating in circular rotation appropriate to their present positions with a small expansion also superimposed. This possibly implies that cloud-cloud collisions are important. The spatial distribution of clouds derived using such a model is in good agreement with the local dust distribution obtained from measurements of reddening and extinction towards nearby stars. In particular, a region of size ∼ 350 pc in diameter enclosing the Sun is found to be devoid of clouds. Intriguingly, most clouds in the longitude range 100‡ to 145‡ appear to have negative radial velocities implying that they are approaching us. Carried out under the auspices of the Joint Astronomy Program, Department of Physics, Indian Institute of Science, Bangalore in partial fulfillment of the requirements for the Degree of Doctor of Philosophy.     

Shattering and coagulation of dust grains in interstellar turbulence

We investigate shattering and coagulation of dust grains in turbulent interstellar medium (ISM). The typical velocity of dust grain as a function of grain size has been calculated for various ISM phases based on a theory of grain dynamics in compressible magnetohydrodynamic turbulence. In this paper, we develop a scheme of grain shattering and coagulation and apply it to turbulent ISM by using the grain velocities predicted by the above turbulence theory. Since large grains tend to acquire large velocity dispersions as shown by earlier studies, large grains tend to be shattered. Large shattering effects are indeed seen in warm ionized medium within a few Myr for grains with radius   a ≳ 10⁻⁶  cm. We also show that shattering in warm neutral medium can limit the largest grain size in ISM  ( a ∼ 2 × 10⁻⁵ cm)  . On the other hand, coagulation tends to modify small grains since it only occurs when the grain velocity is small enough. Coagulation significantly modifies the grain size distribution in dense clouds (DC), where a large fraction of the grains with   a < 10⁻⁶ cm  coagulate in 10 Myr. In fact, the correlation among   R_V   , the carbon bump strength and the ultraviolet slope in the observed Milky Way extinction curves can be explained by the coagulation in DC. It is possible that the grain size distribution in the Milky Way is determined by a combination of all the above effects of shattering and coagulation. Considering that shattering and coagulation in turbulence are effective if dust-to-gas ratio is typically more than ∼1/10 of the Galactic value, the regulation mechanism of grain size distribution should be different between metal-poor and metal-rich environments.     

Distance estimate of M31 based on the size distribution of Hii regions and dark clouds

The size distribution of dark clouds and of H II regions were used to derive a distance of 440±22 kpc for M31. Some galactic H II regions and dark clouds were combined with the same ingredients in M31 to study the mass spectrum of both phenomena. A gas to dust ratio of 10001 has been derived for M31. Radii of the classical H II regions as derived in this work using distance of 440 kpc agrees with the galactic classical H II region radii.     

Ice in the Southern Coalsack

To better understand the conditions under which ice mantles form on grains in molecular clouds, three globules in the Southern Coalsack have been searched for the presence of H₂O ice. Given the total lack of star formation in the Coalsack, it is an ideal site for studying unprocessed icy molecular mantles. In our sample of eight field stars lying behind the Coalsack we detect strong H₂O ice absorption in the lines of sight to two stars and possible weak absorption in four others. We estimate H₂O ice column densities or upper limits for these lines of sight. Compared to dark clouds such as Taurus, the Coalsack H₂O ice column densities are lower than expected given the quiescent nature of the Coalsack region. It is possible that the chemical evolution of the Coalsack may simply be at too early a stage for significant ice mantles to appear on the grains, except perhaps in the densest parts of some of the globules. Alternatively, the presence or absence of ice absorption may be related to the distribution of dust along each line of sight, specifically, the relative contributions of dense globules and a more extended diffuse component. For example, our observations are consistent with an ice threshold extinction similar to that observed in the Taurus dark cloud if extinction amounting to   A _V∼5  towards Globules 2 and 3 arises in the extended component. Globule 1 appears to have no extended component.     

The Amount of Dust and the Distance of NGC 253

The areas of 267 dark clouds in the galaxy NGC 253 were measured. It is shown that the size distribution of these clouds obeys the same exponential formula found for the galaxies NGC 3031, 5128, 5194, and 5457. From the apparent size distribution of the dark clouds the amount of dust and the distance of NGC 253 is found to 4.4 · 10⁷ M_⊙ and 2.06 Mpc, respectively.     

Electrostatic forces and shadow effects in planetary dust rings

The vertical dust distribution of dust clouds around planets, resulting from electrostatic forces, is calculated as a function of dust and plasma parameters. Photoelectric charging is included and differences between clouds on the illuminated side and in the shadow zone are examined. We compute ring structures for conditions which may apply in the spoke-forming regions and study at what dust and plasma conditions the shadow has a significant effect on the vertical dust cloud structure.     

Mars: Topographic control of clouds, 1907–1973

Mariner 9 high-resolution photos and topographic information were used to make a topographic analysis of “blue” and “red” cloud positions reported over a 66-year period in Lowell Observatory records. A sample of 77 “blue” cloud sites lay preferentially at the highest Martian elevations; 60% centered precisely on the seven major volcanic mountain peaks (unknown when the clouds were observed); another 16% lay on substantial slopes or contacts between cratered terrain and lower plains. The median altitude of blue cloud sites was 2.1 km above the global topographic median. These results agree with other evidence that most Earth-detected blue clouds are orographic uplift clouds, composed of condensates. Over half of 131 sporadic yellowish or red clouds were associated with blue clouds or volcanoes, and thus probably did not represent dust storm phenomena, contrary to a commonly held belief. Of 88 “possible dust clouds” (chosen by additional criteria), about two-thirds occur at borders between light and dark areas, in the light regions. These sites may have thin veneers of dust, and current depositional or denudational activity. Median altitude of “possible dust cloud” sites was 0.5 km below the global topographic median. Major dust storms begin in a few “core areas,” two of which associate with major basins Hellas and Isidis, probable reservoirs of mobile dust; but exact topographic control and causes of dust storms are unclear.     

Systematic variations of interstellar linear polarization and growth of dust grains

The observed relation between the interstellar linear polarization curve parameters K and λ _max characterizing the width and the wavelength of the polarization maximum, respectively, is interpreted quantitatively. We have considered 57 stars located in four dark clouds with evidence of star formation: in Taurus, Chamaeleon, around the stars ρ Oph and R CrA. In our modeling we have used the spheroidal dust grain model applied previously to simultaneously interpret the interstellar extinction and polarization curves in a wide wavelength range. The observed trend K ≈ 1.7λ _max is shown to be most likely related to the growth of dust grains due to coagulation rather than accretion. The relationship of the interstellar polarization curve parameters K and λ _max to the mean dust grain size is discussed.     

Surface distribution of dark clouds in M31 absorption gradient along the minor axis and in theR andθ directions,II

Absorption and absorption gradient is studied along the minor axis of M31 and as a function of the angle measured from the minor axis with the center of the galaxy as origin. The radial distribution of these phenomena is studied also. The counts of dark clouds averaged over all in each case assumes periodical changes. In some cases systematic changes are shown also. The fraction of the volume occupied by these clouds is smaller by an order than that introduced by the standard cloud model.     

Diffuse interstellar band formation in dense clouds

Measurements of the strengths of the diffuse interstellar bands at 4430, 5780 and 5797 Å show that the bands tend to be week with respect to extinction in dense interstellar clouds. Data on 10 stars in the ? Ophiuchi cloud complex show further that the diffuse band-producing efficiency of the grains decreases systematically with increasing grain size. It is concluded that the diffuse bands are not formed in the mantles which accrete on the grains in interstellar clouds, but that they could be produced in the cores of grains or in some molecular species.     

A SCUBA survey of compact dark Lynds clouds

We present the first results of a submillimetre continuum survey of Lynds dark clouds. Submillimetre surveys of star-forming regions are an important tool with which to obtain representative samples of the very first phases of star formation. Maps of 24 small clouds were obtained with SCUBA, the bolometer array receiver at the James Clerk Maxwell Telescope, and 19 clouds were detected. The total dark cloud area surveyed was ∼130 arcmin², and a total gas mass of 90 M_⊙ was detected. The dust emission is in general in good agreement with the extinction of optical starlight. The observed clouds contain a newly discovered protostar in L944, and a previously known protostar IRAS 23228+4320 in L1246. Another eight starless cores, either gravitationally unbound or pre-stellar in nature, were also detected. All starless cores and protostars were detected in only seven clouds, and the remaining 17 clouds seem quiescent and do not show any signs of recent star formation activity. The 850-μm images of all detected clouds are presented, as well as 450-μm images of L328, L944, L1014 and L1262. The outflows of the protostars in L944 and L1246 were also discovered and were mapped in ¹²CO J =2→1. The detection of the young protostar in L944, which is not present in the IRAS Point Source Catalog, shows the capacity of submillimetre surveys to detect unknown protostars.     

Spectral reflectance of Martian areas during the 1973 opposition: Photoelectric filter photometry 0.33–1.10 μm

Reflectance spectra of 26 Martian areas (200–400 km in diameter) that were measured during the 1973 opposition are presented. They were measured through 25 narrow-band interference filters between 0.33 and 1.10 μm, using a photoelectric filter photometer at the Mauna Kea 230-cm telescope. There were many more bright and dark areas observed than during previous oppositions, and for the first time spectra were obtained of dust clouds and areas of mixed and intermediate albedo. The bright areas and dust clouds were all apparently composed of the same mineralogic unit. The dark area spectra differed substantially from the bright area and dust cloud spectra, and they showed major regional variations. The spectra of mixed and intermediate albedo areas had absorption bands seen in both bright and dark area spectra, and did not display any unique new features: thus they were apparently not compositionally unique, but rather were probably composed of mixtures of high albedo dust and dark area soils.     

Cloudy circumstellar dust shells around young variable stars

A number of variable stars of the Orion population has been identified with IRAS point sources by us. This finding supports the conclusion that the prominent Algol-like minima in the lightcurves of these stars originate from obscurations by dust clouds in a circumstellar shell. The discussion of the existingUBVR data leads to the remarkable conclusion that the extinction properties of the grain populations contained in individual dust clouds moving in one and the same circumstellar shell are quite different.From the multicolour photometric data of the different Algol-like minima we derived individual values of the reddening parameterR = A _v /E(B - V). It covers a remarkable wide range of values from that one typical of the interstellar extinction law up to 7. In the case of SV Cep one of the grain populations produces a virtually neutral extinction. The large values ofR speak in favour of larger than normal (interstellar) dust grains, which may have grown by coagulation processes. The cloudy circumstellar dust shell provides a natural explanation for the observed infrared excess. The properties derived from the optical light variations are fully compatible with the properties deduced from the infrared radiation. The irregularity of the light variations indicates that many clouds are involved and may sometimes superimpose themselves.Paper presented at the Conference onPlanetary Systems: Formation, Evolution, and Detection held 7–10 December, 1992 at CalTech, Pasadena, California, U.S.A.     

Three-component dust models for interstellar extinction

Interstellar extinction curves obtained from the ‘extinction without standard’ method were used to constrain the dust characteristics in the mean ISM (R _V = 3.1), along the lines of sight through a high latitude diffuse molecular cloud towards HD 210121 (R _V = 2.1) and in a dense interstellar environment towards the cluster NGC 1977 (R _V = 6.42). We have used three-component dust models comprising silicate, graphite and very small carbonaceous grains (polycyclic aromatic hydrocarbons) following the grain size distributions introduced by Li & Draine in 2001. It is shown that oxygen, carbon and silicon abundances derived from our models are closer with the available elemental abundances for the dust grains in the ISM if F & G type stars atmospheric abundances are taken for the ISM than the solar. The importance of very small grains in modelling the variation of interstellar extinction curves has been investigated. Grain size distributions and elemental abundances locked up in dust are studied and compared at different interstellar environments using these three extinction curves. We present the albedo and the scattering asymmetry parameter evaluated from optical to extreme-UV wavelengths for the proposed dust models.