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: http://star.arm.ac.uk/~csj/research/rcb_review/node11.html
Дата изменения: Fri Jan 25 15:33:06 2002 Дата индексирования: Tue Oct 2 09:45:56 2012 Кодировка: Поисковые слова: nebula |
Infrared observations of RCBs show that, even at maximum light, there are copious amounts of dust surrounding the star ([Kilkenny & Whittet 1984], [Walker 1985]). A very interesting result is that during a decline, when the V-band brightness is plummeting, the infrared L-band brightness shows no significant change and continues to show pulsational variations [Feast et al. 1997]. It seems that the V-band brightness is dominated by the obscured atmosphere, but the L-band brightness is dominated by dust surrounding the star at some distance. Evidently the dust continues to be illuminated by light from the photosphere, including its pulsations, whilst light coming directly from the photosphere to the observer is obstructed. Therefore the obscuring dust can only cover a fraction of the stellar surface.
The picture that emerges was first proposed by Loreta (1934) and O'Keefe (1939) (Fig. 2). It now seems clear that at a particular phase in the pulsation cycle, conditions in the photosphere can compel the carbon-rich gas to be expelled from a part of the stellar surface. This gas cools, condenses, and forms dust grains, which are very good at absorbing light from the star. Roughly once every twenty cycles, this `puff of soot' is directed into our line of sight, and obscures the photosphere from view. Another important observation associated with RCB declines is that, during the early part of the decline, a rich emission spectrum appears, consisting of many narrow lines of neutral and singly-ionized metals ([Payne-Gaposchkin 1963], [Alexander et al. 1972]). These lines (known as E1 lines) appear to be short-lived so that within two or three weeks, they are replaced by a simpler broad-line spectrum (BL), although a few narrow lines (E2) persist for an extended period. The E1 spectrum is probably due to hot gas close to the stellar surface which is visible only when the photosphere alone is obscured. As the dust cloud moves away from the stellar surface it expands, obscuring the E1 region. Since the E2 and BL regions remain visible, they must arise from cooler gas spread over a much larger volume around the star.
This picture of clouds of dust ejected by the pulsating stellar surface has been successful in explaining many properties of RCBs. There remain a number of difficult questions, the hardest being ``What is the origin of RCB stars?''