Pvtel Pv Telescopii stars

- These are helium supergiant Bp stars with weak hydrogen lines and enhanced lines of He and C. They pulsate with periods of ff! 1 to or vary in brightness with an amplitude ^ff'.'l in V during a time interval of about a year. GCVS

PV Telescopii stars in the past were called helium stars, helium horizontal branch stars, extreme helium (EHe) stars, and hydrogen-deficient binary (HdB) stars. Small amplitude light and radial velocity variations of the R Coronae Borealis (RCB) variable RY Sgr have long been identified with radial pulsation. Thereafter, small-amplitude light variations were observed in other RCB stars as well as in extreme helium (EHe) stars and in hydrogen-deficient binary (HdB) stars.

All three groups of stars are characterized by an extremely low surface abundance of hydrogen and low surface gravity. It has been known for some time that the behavior of the small-amplitude variations in some RCB stars is roughly periodic and as a result has been attributed to stellar pulsation. Evidence for periodic light variation of the HdB star KS Per was found in 1963 and the EHe star HD 160641 in 1975. In recent years more substantial evidence for periodic light variation of EHe and HdB stars has become available, beginning with the detection of a - 21 day period in the EHe BD +lc 4382 in 1985.

Although similar in many respects, RCB, EHe, and HdB stars represent quite distinct groups. EHe and RCB stars are apparently single stars with a surface abundance of C (carbon) and N (nitrogen). RCB stars are distinguished by the presence of a large infrared excess and by an occasional deep light minimum. HdB

stars have low surface abundance of C and are all single-lined spectroscopic binaries.

As a simple consequence of their energy budget, the evolution of luminous blue stars invariably occurs on time-scales of a few hundreds to a few thousands of years. Extreme helium stars can be no exception. Their surfaces are predominately helium, with a few percent of carbon and nitrogen and in general a negligible contamination by hydrogen. They are almost certainly of low mass. The majority show small-amplitude variations on time-scales of 1-20 or more days, and should correctly be classified as PV Tel variables.

The principal question posed by the extreme helium stars is in regard to their evolutionary origin. Two principal hypotheses have emerged with the chief difference between the two being whether the progenitor is a single or a binary white dwarf. The general properties of both hypotheses are simply referred to as the late thermal pulse (LTP) model and the merged binary white dwarf (MBWD) model.

Being rare and unusual, several extreme helium stars were observed within the first two years of operation of the International Ultraviolet Explorer (IUE: 1978-79) and nearly all had been observed by the mid 1980s. Upon recognizing the great sensitivity of their fluxes to temperature, a series of second epoch observations of extreme helium stars was made with IUE in the early 1990s.

In the meantime it had been recognized that many extreme helium stars show photometric variability on time-scales of hours to weeks and the most luminous are likely to pulsate. Clearly, cyclic changes in the flux due to pulsation would be as easy to measure but could mask any secular changes due to evolution. Such cyclic changes would, however, be extremely useful for measuring the temperature and radius variations associated with the pulsation and could, in conjunction with radial velocity measurements, help to ascertain the radius of a pulsating helium star, independent of distance. Should the secular change be large compared with the cyclic change, then a measurement of the cyclic changes at one end of the secular vector would indicate the overall uncertainty in the length and direction of that vector.

When examining the observed properties of the hydrogen-deficient stars, it will be found that the periods of the variable hydrogen-deficient stars decrease as the effective temperature increases and the non-variable hydrogen-deficient stars tend to have small luminosity-to-mass ratios.

Observation

^ Bright stars jjjj Mixed amplitudes A. Mixed periods

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