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Session 26 - Stars in the Visible & IR.
Display session, Tuesday, June 10
South Main Hall,

[26.10] Evidence for an Abrupt Change in the Period of the CP Star CU Virginis

D. P. Smith (UNLV), T. Ryabchikova (INASAN), V. Malanushenko (CRAO)

UBV and uvby photometry of the short-period (0.52 d) Magnetic Chemically Peculiar (MCP) star CU Vir obtained since 1987 with the Phoenix 10-inch and Four College 30-inch automatic telescopes cannot be plotted with the same period determined by investigations before 1984. The same is true of radial velocity and equivalent width variations from spectra obtained at the Haute Provence Observatory and at the Crimean Astrophysical Observatory from 1994 to 1996.

The automatic telescope photometry and earlier photometry going back to 1958 show evidence that the period of CU Vir may have changed about 1985. Prior to 1984, photometric and spectroscopic data can be represented by a period of 0.5206786 days; the 1987-89 data are best represented by a period of 0.5206987 days; and the 1991-96 data by a period of 0.5207085 days. Light, spectrum and magnetic variations in an MCP star are believed to be due to rigid rotation of an inhomogeneous atmosphere where a large magnetic field is present. The radial velocity and equivalent width variations in CU Vir can be explained by such a model. No reasonable spectroscopic binary orbit can be obtained from the radial velocity variations. The most obvious explanation of these data is that the rotational rate of the star is decreasing. Assuming a continuous period change, an analysis of all available data from 1955 to 1996 results in a period increase rate of 1.5E-09, but there are large residuals for several of the data sets. The photometry after 1985 shows a good fit with a period change of 2.0E-09. Pulsation, variable spots and precession models are also being examined as possible explanations. It is not clear whether the period is continuing to increase, so further monitoring of this star is necessary.

Partly supported by NSF Grant AST-9115114.

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