arXiv subjects

Gordon A. H. Walker

Publications and source records attributed to Gordon A. H. Walker.

24 records · Page 2Linked to original sources

arXiv2003

The Radial Velocity Precision of Fiber-fed Spectrographs

We have measured the radial velocities of five 51 Peg-type stars and one star with constant velocity. Our measurements, on 20 Åcentered at 3947 Å, were conventional using Th/Ar comparison spectra taken every 20 or 40 minutes between the stellar exposures. Existing IRAF routines were used for the reduction. We find $σ_{RV}$ $\leq$ 20 m s$^{-1}$, provided 4 measurements (out of 72) with residuals $>5σ_{RV}$ are neglected. The observations were made with the CFHT Gecko spectrograph, fiber-fed with the CAFE system (R$\sim$110,000). $σ_{RV}$ $\leq$10 m s$^{-1}$ seems possible with additional care. This study was incidental to the main program and so not exhaustive but the small value of $σ_{RV}$ implies that the fiber feed/image slicer system on Gecko + CAFE, essentially eliminates the long standing problem of guiding errors in radial velocity measurements. We are not suggesting this conventional approach for serious Doppler planet searches (especially with Gecko which has such a small multiplex gain), but the precision is valuable for observations made in spectral regions remote from telluric lines or captive-gas fiducials. Instrument builders might consider the advantages of the CAFE optics which incorporate agitation and invert the object and pupil for slit and grating illumination in future spectrograph designs.

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arXiv2002

Modulation of Ca II H & K Emission by Short-Period Planets

We have detected modulation of the Ca II H&K reversal structure in four out of five 51 Peg-type stars whose planets have orbital periods between 3 and 4 days. We observe two effects in the K-core: (1) a broad 3-A variation at ~1% level and (2) changes on a scale of 0.5 A (~1-3%) in each of the three reversal features. The nightly variations are coherent in both H and K. From differential radial velocities measured to better than 20 m/s, up-to-date phases were extracted. The enhancements in the reversals tend to be greatest at the sub-planetary points which may imply that there is a magnetic interaction between the star's outer layers and the magnetosphere of the planet. These high-S/N (500 per pixel in the continuum) and high-resolution (R = 110,000) data are too few to confirm orbital synchronization.

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arXiv2001

Limits to Interstellar C_4 and C_5 Towards zeta Ophiuchi

We have made a sensitive search for the origin bands in the known electronic transitions of the linear carbon chains C_4 and C_5 at 3789 and 5109 A towards zeta Oph (A_V <= 1). The incentive was a recent detection of C_3 in this interstellar cloud with a column density of 1.6 x 10^12 cm^-2 plus the availability of laboratory gas phase spectra of C_4 and C_5. Further, some models of diffuse interstellar clouds predict that the abundance of these latter species should be within an order of magnitude of C_3. Despite achieving S/N of 2300 to 2600 per pixel at a resolution of ~110,000, the searches were negative, leading to 3 sigma upper limits to the column density of N(C_5) = 2 x 10^11 cm^-2 and N(C_4) = 4 x 10^12-13 cm^-2 where these values rely on theoretically calculated oscillator strengths. The implication of these limits are discussed on the choice of molecules for study in future attempts to identify the carriers of the stronger diffuse interstellar bands.

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arXiv2000

Evidence for a Long-period Planet Orbiting Epsilon Eridani

High precision radial velocity (RV) measurements spanning the years 1980.8--2000.0 are presented for the nearby (3.22 pc) K2 V star $ε$ Eri. These data, which represent a combination of six independent data sets taken with four different telescopes, show convincing variations with a period of $\approx$ 7 yrs. A least squares orbital solution using robust estimation yields orbital parameters of period, $P$ = 6.9 yrs, velocity $K$-amplitude $=$ 19 {\ms}, eccentricity $e$ $=$ 0.6, projected companion mass $M$ sin $i$ = 0.86 $M_{Jupiter}$, and semi-major axis $a_2$ $=$ 3.3 AU. Ca II H&K S-index measurements spanning the same time interval show significant variations with periods of 3 and 20 yrs, yet none at the RV period. If magnetic activity were responsible for the RV variations then it produces a significantly different period than is seen in the Ca II data. Given the lack of Ca II variation with the same period as that found in the RV measurements, the long-lived and coherent nature of these variations, and the high eccentricity of the implied orbit, Keplerian motion due to a planetary companion seems to be the most likely explanation for the observed RV variations. The wide angular separation of the planet from the star (approximately 1 arc-second) and the long orbital period make this planet a prime candidate for both direct imaging and space-based astrometric measurements.

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arXiv1999

Near-IR adaptive optics imaging of nuclear spiral structure in the Seyfert galaxy, NGC3227

We present high spatial resolution, near-IR images in J, H, K of the nucleus of NGC3227, obtained with the Adaptive Optics bonnette on CFHT. The ~0.15 arcsec (17pc) resolution allows structures to be probed in the core region. Dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core region. We are able to identify two main features in our maps: (i) a spiraling association of knots with a counterpart in an HST F606W image; (ii) a smaller scale annulus, orthogonal to the spiral of knots. These features may provide a means to transport material inwards to fuel the active nucleus.

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arXiv1998

The core structure of AGN: perils of adaptive optics artifacts

As part of a project to map nearby Seyfert galaxies in the near-IR with adaptive optics, we present high spatial resolution, near-IR images in J,H, and K of the nuclei of NGC 3227 and NGC 2992, obtained with the Adaptive Optics Bonnette (AOB) on CFHT. The ~0.15" resolution allows us to probe structures in the core region at unprecedented scales. With NGC 3227, we are able to identify an inwards spiraling starburst in all three near-IR bands. NGC 2992 shows evidence for emission along a radio loop. Compared with HST optical images, dust obscuration becomes significantly less pronounced at longer wavelengths, revealing the true geometry of the core regions. The observed structures may help to elucidate fueling mechanisms for the central engine, as well as providing insight into the unification paradigms. The results are tempered with the discovery of AOB-related artifacts in the central arcsec of observed AGN galaxies which take on the appearance of spiral/disk structures.

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