S. D. Horner

A Planet Orbiting the Star ρ Coronae Borealis

We report the discovery of near-sinusoidal radial velocity variations of the G0V star ρ CrB, with period 39.6 days and amplitude 67 m s-1. These variations are consistent with the existence of an orbital companion in a circular orbit. Adopting a mass of 1.0 M☉ for the primary, the companion has minimum mass about 1.1 Jupiter masses and orbital radius about 0.23 AU. Such an orbital radius is too large for tidal circularization of an initially eccentric orbit during the lifetime of the star, and hence we suggest that the low eccentricity is primordial, as would be expected for a planet formed in a dissipative circumstellar disk.

THE AFOE: A SPECTROGRAPH FOR PRECISION DOPPLER STUDIES

The Advanced Fiber Optic Echelle (AFOE) is a fiber-fed echelle spectrograph designed for the measurement of stellar Doppler shifts. Using a 2k x 2k CCD detector, it samples about 55% of the wavelength range between 450 nm and 700 nm (20 echelle orders) at a single shot, with spectral resolution R = 32000 to 70000 at 500 nm, depending on the slit width employed. The AFOE employs a number of devices to assure that the calibrations necessary for accurate Doppler measurements can be properly performed. The most important of these are: (1) coupling to the telescope via a double-scrambling optical fiber system; (2) continuous calibration of the wavelength scale and point-spread function by means of an atomic emission lamp entering the spectrograph via a separate fiber and/or a molecular iodine absorption cell; (3) availability of fiber-coupled sunlight for regular calibration against the solar spectrum; (4) appropriate mechanical design and active thermal control, yielding good mechanical stability. The AFOE is coupled to the Tillinghast 1.5-m telescope at the F. L. Whipple Observatory. It presently achieves S/N = 500 in the continuum near 500 nm in 60s when observing Arcturus (alpha-Boo, mV = -0.04). This noise level sets a limit of about 0.7 ms-1 to the Doppler precision attainable in this length of observing time. Currently, our actual frame-to-frame repeatability is worse than the photon noise limited value by about a factor of 3 for this bright star, and about 1.5 for stars with mV = 4. Work is continuing to refine data processing methods so that the ultimate noise limit may be approached more closely, and to improve the spectrographs relatively low efficiency.

A Radial Velocity Search for p-Mode Pulsations in η Bootis

The subgiant η Boo (G5 IV) has been reported to show p-mode pulsations, as evidenced by variations in the equivalent width of its hydrogen Balmer lines (reported by Kjeldsen et al.). In an attempt to confirm this report, we observed η Boos radial velocity with the AFOE spectrograph for a total of 22 hours spread over seven successive nights in 1995 March. We find no evidence for the presence of excess power at the frequencies reported by Kjeldsen et al.; our upper limit corresponds to typical mode amplitudes of 0.5 m s-1, about 3 times smaller than the velocity amplitudes they inferred. Signals with amplitudes larger than 0.5 m s-1 may be present at other frequencies within the 0-1000 μHz range, but evidence for such signals is scanty, and typical mode amplitudes greater than 1.5 m s-1 are clearly inconsistent with our observations.

Exoplanets or Dynamic Atmospheres? The Radial Velocity and Line Shape Variations of 51 Pegasi and τ Bootis

The stars 51 Pegasi and τ Bootis show radial velocity variations that have been interpreted as resulting from companions with roughly Jovian mass and orbital periods of a few days. Gray and Gray & Hatzes reported that the radial velocity signal of 51 Peg is synchronous with variations in the shape of the line λ6253 Fe I; thus, they argue that the velocity signal arises not from a companion of planetary mass but from dynamic processes in the atmosphere of the star, possibly nonradial pulsations. Here we seek confirming evidence for line shape or strength variations in both 51 Peg and τ Boo, using R = 50,000 observations taken with the Advanced Fiber Optic Echelle. Because of our relatively low spectral resolution, we compare our observations with Grays line bisector data by fitting observed line profiles to an expansion in terms of orthogonal (Hermite) functions. To obtain an accurate comparison, we model the emergent line profiles from rotating and pulsating stars, taking the instrumental point-spread function into account. We describe this modeling process in detail. We find no evidence for line profile or strength variations at the radial velocity period in either 51 Peg or in τ Boo. For 51 Peg, our upper limit for line shape variations with 4.23 day periodicity is small enough to exclude with 10 σ confidence the bisector curvature signal reported by Gray & Hatzes; the bisector span and relative line depth signals reported by Gray are also not seen, but in this case with marginal (2 σ) confidence. We cannot, however, exclude pulsations as the source of 51 Pegs radial velocity variation because our models imply that line shape variations associated with pulsations should be much smaller than those computed by Gray & Hatzes; these smaller signals are below the detection limits both for Gray & Hatzess data and for our own. τ Boos large radial velocity amplitude and v sin i make it easier to test for pulsations in this star. Again we find no evidence for periodic line shape changes, at a level that rules out pulsations as the source of the radial velocity variability. We conclude that the planet hypothesis remains the most likely explanation for the existing data.

The Oscillations of Tau Pegasi

We present extensive spectroscopic time series observations of the multiperiodic, rapidly rotating, ? Scuti star ? Pegasi. Information about the oscillations is contained within the patterns of line-profile variation of the stars blended absorption-line spectrum. We introduce the new technique of Doppler deconvolution with which to extract these patterns by modeling the intrinsic stellar spectrum and the broadening functions for each spectrum in the time series. Frequencies and modes of oscillation are identified from the variations using the technique of Fourier-Doppler imaging and a two-dimensional least-squares cleaning algorithm. We find a rich mode spectrum with degrees up to l = 20 and with frequencies below about 35 cycles day-1. Those modes with the largest amplitudes have frequencies that lie within a narrow band. We conclude that the observed spectrum can be explained if the modes of ? Peg propagate in the prograde direction with l |m| and with frequencies that are about equal in the corotating frame of the star. We discuss the implications of these results for the prospect of ? Scuti seismology.

A Search for Line Shape and Depth Variations in 51 Pegasi and τ Bootis

Spectroscopic observations of 51 Pegasi and τ Bootis show no periodic changes in the shapes of their line profiles; these results for 51 Peg are in significant conflict with those reported by Gray & Hatzes. Our detection limits are small enough to rule out nonradial pulsations as the cause of the variability in τ Boo, but not in 51 Peg. The absence of line shape changes is consistent with these stars radial velocity variability arising from planetary mass companions.

XMM Optical/UV Monitor Telescope

The optical/UV monitor (OM) on the ESA x-ray cornerstone mission XMM is designed to provide simultaneous optical and UV coverage of x-ray targets viewed by the observatory. The instrument consists of a 30 cm modified Ritchey-Chretien telescope. This feeds a compact photon counting detector operating in the blue part of the optical spectrum and the UV (1600 - 6000 angstrom). The OM has a square field of view of approximately 24 arcmin along the diagonal, and will cover the central region of the field of view of the EPIC x- ray cameras where the x-ray image quality is best. Because of the low sky background in space, the sensitivity of the OM for detecting stars will be comparable to that of a 4-m telescope at the Earths surface; it should detect a B equals 24th magnitude star in a 1000 s observation using unfiltered light. The pixel size of the detector corresponds to 0.5 arc seconds on the sky in normal operation. In front of each of two redundant detectors are filter wheels containing broad band filters. The filter wheels also contain Grisms for low resolution spectroscopy of brighter sources (lambda/Delta lambda 200) and a 4x field expander which will allow high spatial resolution images of the field center to be taken in optical light.

Breadboard phase of the XMM optical monitor

A multi-national consortium of research groups are developing the XMM (x-ray multi-mirror mission) optical monitor to provide a capability for optical identification and photometry of x-ray sources observed by the XMM observatory. This will be the first multi-wavelength facility dedicated to monitoring the variability of diverse sources from the optical through to x-ray wavelengths. Here we describe the system design and discuss progress in the breadboard phase of the development program.