Circumstellar clouds derived from the 2800 Mgii data |
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| Authors: | G A Gurzadyan |
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| Institution: | (1) Garny Space Astronomy Laboratory, Armenian SSR, U.S.S.R. |
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| Abstract: | As a result of the analysis of the observed interstellar 2800 Mgii absorption line data, an empirical relationship — a positive correlation — between the equivalent widthW(2800) and the effective temperature of the starT was discovered (Figure 1). However, in the case when this doublet is of stellar (photospheric) origin, only a negative correlation betweenW(2800) andT exists. Hence, the existence itself of such a positive correlation betweenW(2800) andT may be viewed as incomprehensible for the present influence of the star on the strength of the absorption line 2800 Mgii of nonstellar origin.On the other hand, we have evidence that the ionizing radiation of hot stars cannot provide for the observed very high degree of ionization of the interstellar magnesium. In particular, the observations give for interstellar magnesium the ratioN
+/N
1  1000, while in the case of ionization under the action of stellar radiation only we haveN
+/N
1 10.The assumption that circumstellar clouds surround hot stars can naturally explain these and other similar facts. A method for the determination of the general parameters-size, concentration, mass etc. — of the circumstellar clouds is developed. The main results of the application of this method to the relation of more than 20 hot stars are:(1) The circumstellar clouds surround almost (70%) all hot giants and subgiants. In the remaining (30%) cases, the absence of circumstellar envelopes requires additional evidence. (2) The linear sizes of circumstellar clouds vary within wide ranges — from 0.002 pc up to 1 pc. Most frequent are clouds with size of 0.1 pc. (3) The main concentration of hydrogen atoms (electrons) in circumstellar clouds is of the order of 100 cm–3; the minimum value is 20–30 cm–3, the maximum 104 cm–3. In one case (Deneb) the electron concentration rises up to 105 cm–3 for the size of the cloud 0.001 pc=3×1015 cm. (4) Stars of the same spectral and luminosity classes may possess circumstellar clouds characterized by quite different parameters. (5) Hydrogen in circumstellar clouds is completely ionized; for these clouds the optical depth 
c
 1; on the average,T
c 0.005. (6) The integrated brightness of circumstellar clouds is substantially fainter (by 8–10m) than that of the central star. This is the reason why these clouds cannot be detected by ground-based observations. (7) The masses of individual circumstellar clouds vary from 1  down to 10–4  . This gives for the mass ejection rate from 10–10 to 10–6 per year in case if these clouds are formed by the braking and accumulation of the ejected mass.The  method of 2800 Mgii seems very convenient, fruitful and promising for the detection and study of circumstellar envelopes. Also, this method is very sensitive for a determination of the general parameters of such clouds, and concerns practically all their geometric, physical, kinematic and other properties. |
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