Visual extinction

Fig. 4 - Visual light curve during the first two days. Time zero is defined by the Kamiokande - IMB neutrino signal. Light curves from four models (Table 1) are shown as solid lines. A distance modulus of 18.5 and visual extinction of 0.5m have been adopted. Shown for comparison axe observational data points.

PAAR TURBIDIMETER. A visual-extinction device for measurement of solution turbidity. The length of the column of liquid suspension is adjnsted until the light filament can no longer be seen. 

Fig. 12. Contour map of the visual extinction in the Taurus cloud. The shaded areas represent dark clouds of small diameter with visual extinction ranging from 5m to 8m. Also indicated are the positions of T-Tauri stars and quasi-thermal OH emission...

Fig. 21a and b. Lifetime of interstellar molecules as a function of the thickness of the shielding dust layer, expressed in magnitudes of visual extinction (after Stief, 1971)... 

The time elapsed between hitting the grain surface and molecule formation is of the order of seconds. The lifetime of the molecule in free space after ejection from the surface depends upon the region where it is released. In a cloud with a visual extinction of Av = 3 corresponding to a hydrogen column density of TVh = 5 x 1021 cm-2 the lifetime of a typical molecule will be in the order of 1013 sec. During this time the probability for photodissociation is unity. 

Dense molecular clouds, often also called dark clouds, block entirely the light of stars which lie behind them, and can therefore be studied observationally only by radio astronomy or infrared techniques. These clouds have a visual extinction in excess of A 10 which corresponds to a gas density of n lO cm" and a kinetic temperature usually well below T 100 K, typically between 10 and 25 K. Within the last ten years, the investigation of these dark molecular clouds has become almost entirely the domain of radio astronomy although now the first very promising results by infrared astronomy reveal the power of this new branch of spectroscopy. 

It is usually assumed that the contributions of line and continuum attenuation are separable. The depth dependence of the photodestruction rates due to grain attenuation can be computed by solving the equations of radiative transfer for specified grain properties and geometry of the cloud. If the cloud is assumed to have a plane-parallel geometry with radiation Incident on both sides of the cloud, the depth-dependent rates depend strongly on the total extent of the cloud. The results for clouds with a total visual extinction are usually represented by single exponential decays to the centers... 

Figure 9. Photodissoda.tion rates of CO and its principal isotopes as functions of depth into a cloud with a total visual extinction of 5 mag (from van Dishoeck and Black 1988).

The rate coefficients for photodissociation caused by external UV photons depend on the visual extinction Ay ... 

Such reduction methods have been applied by a number of groups for different cases. To the best of our knowledge. Ruffle et al. [35] were the first to apply such a technique (in a more complex version) to determine the minimum network for the computation of the gas-phase CO abundance in interstellar clouds of different density, temperature and visual extinction. Their network still contains more than two hundred reactions and more than 60 species. Reduced networks to compute the ionisation fraction in dense clouds [36] and in proto-planetary disks [37], and reduced networks for dense clouds [38] have also been proposed. 

Paar turbidimeter analychem A visual-extinction device for measurement of solution turbidity the length of the column of liquid suspension is adjusted until the light filament can no longer be seen. par, tar-ba dim-3d-ar) paired electron phys chem One of two electrons that form a valence bond between two atoms. perd I lek.tran ... 

One of the most fruitful application of laboratory microwave spectroscopy over the last twenty years is the analysis of the molecular content of interstellar clouds. These clouds contain gas (99% in mass) which has been mostly studied by radioastronomy, and dust, whose content has been analysed mostly by IR astronomy. The clouds rich in molecular content are dense or dark clouds (they present a large visual extinction), with a gas density of 10 -10 molecules cm", and temperatures of T < 50K. At these low temperatures only the low-lying quantum states of molecules can be thermally (or collisionally) excited, i.e. rotational levels. Spontaneous emission from these excited states occurs at microwave wavelengths. In some warm regions of dense clouds (star formation cores) the absorption of IR radiation produces rotational emission in excited vibrational states. Other rich chemical sources are the molecular clouds surrounding evolved old stars, such as IRC-i-10216, and called circumstellar clouds. 

X is the enhancement of the UV flux around 160 nm above the general interstellar field and the visual extinction. For a more detailed discussion see the paper by Elitzur, 1979 [117]. 

By comparing the flux ratios between the H II recombination lines Br-7 and B a we may estimate the intervening dust extinction. Li Figure 1 we show the Br-7 emission image (on the left) and the Ha line emission image (on the right). The individual HII regions are clearly detected in Ha. Most of the Ha clumps also have Br-7 feattires with at least >2flux ratios for the 15 - 20 dumps indicate visual extinctions of i4y = 0 - 3. This result is consistent with NGC 4713 s face-on spiral-disk inclination, as well as its relative defldency in gas and dust (log Xit/Xb f j... 

The mean color excess is proportional to the extinction and colrunn density of dust at each point where it is determined. The color excess can be directly converted to an extinction at some specified wavelength via the reddening law. For example, the mean color excess can be scaled to a mean visual extinction using the normal reddening law (Rieke and Lebosfky 1985) ... 

Fig. 2. Map of the equivalent mean visual extinction in the Northern Streamer cloud derived from H-K color excess measurements of more than 1200 stars observed in this field. The lowest contour corresponds to a visual extinction of 2 magnitudes and subsequent contours increase in steps of 2 magnitudes.

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