OH / IR star
OH / IR stars are red giants hidden behind a dust cover , which emit radiation mainly in the infrared and sub-mm range . The OH in the name comes from the hydroxyl radical , whose typical emission line is contained in the spectrum of these stars.
development
OH / IR stars represent a strong mass ejection lasting 10 5 years at the end of the AGB phase from massive stars (3 to 8 solar masses ), which is referred to as super wind. The rate of mass loss reaches some 10 −5 solar masses per year and, like the Mira stars, is triggered by pulsations . The outflowing gas condenses to dust after a few star radii and completely absorbs the visible radiation. The dust heats up and radiates the heat in the infrared. In contrast, Mira stars only have 1 to 2 solar masses, which is not enough for the formation of optically thick shells.
observation
OH / IR stars were not discovered until the late 1960s with the help of radio and infrared astronomy . In the far infrared, there is a cyclical change of light with periods of 2 to 5 years and amplitudes similar to those of Mira stars. In addition, these stars show the far-infrared absorption lines at 10 and 20 microns, with the silicate dust accommodated in the case of the late giant compound. In the optical, OH / IR stars are hardly detectable, although they are among the brightest infrared sources in the sky.
Natural burl
In addition, non-thermal radiation from some molecular lines can be detected from the OH / IR stars. These are OH, water and silicon monoxide . These molecular lines show a typical U-shaped shape in the spectrum and are interpreted as a burl .
Usually only a maser radiation at 1612 MHz from the hydroxyl radical (OH) is detected. The distance between the lines in the U-profile corresponds to a speed of 5 to 25 km / s. One component comes from the front of the dust cover and the other from the back. The upper energy levels are filled by the absorption of infrared radiation from the warm dust. In order to achieve sufficient reinforcement of the line, the speed of the outflowing gas must be constant over long distances. The maser radiation therefore only comes from a small line of sight to the earth, while the other areas of the shell are not radiated sufficiently intensified in our direction. The maser radiation follows the brightness variations in the infrared with a delay of a few weeks.
Non-changeable OH / IR stars
In the case of the non-changeable OH / IR stars, no long-period change in brightness in the infrared is detectable. They have a dust cover that has already been separated from the star and is no longer fed by the Post-AGB star. The watermasers are only weakly or not at all detectable, while the OH-masers are still active. This short phase lasting about 1000 years is interpreted as the transition phase to the planetary nebulae .
Examples
- WX Piscium , AFGL 1686
See also
Web links
literature
- HJ Habing, H. Olofsson: Asymptotic Giant Branch Stars (Astronomy and Astrophysics Library) . Springer, Berlin 2003, ISBN 0-387-00880-2 .
- H. Scheffler, H. Elsässer: Physics of the sun and the stars . Bibliographisches Institut, Mannheim 1990, ISBN 3-411-14172-7 .
- K. Justtanont et al .: OH / IR stars and their superwinds as observed by the Herschel Space Observatory . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.1777v1 .