Engineering topics

Marcy, G. W.

Publications and source records attributed to Marcy, G. W..

6 records

NASA NTRS2003

Long baseline interferometric observations of HD 195019: no K dwarf companion detected

Radial velocity measurements of the G3V/IV star HD 195019 revealed the presence of an orbiting companion with m sin(i) = 3.5 Jupiter masses and a period of 18 days. Here we present new visability measurements obtained at the Palomar Testbed Interferometer which rule out any companion in an orbit consistent with the spectroscopic data and having more than 1% of the flux of the primary star in the near-infrared K band.

extrasolar planets interferometry radial velocity↗

NASA NTRS1984

Magnetic field observations of evolved stars

A search has been made for Zeeman broadening in high resolution spectra of eight G and K type, evolved stars which have active chromospheres and coronae. No Zeeman broadening was found. It is likely that the fraction of the stellar surface that contains magnetic fields is similar in active giants and dwarfs, based on their comparable chromospheric line strengths and plage surface brightnesses (deduced from Mg II/C IV ratios) and on their comparable requirements for coronal confinement. Thus, since Zeeman broadening is detectable on active dwarfs, the nondetections of Zeeman broadening on these evolved stars are probably due to systematically lower field strengths. That the lower field strengths do not result in lower chromospheric surface brightnesses on giants is consistent with the theoretial 'slow mode' MHD wave generation rates of Ulmschneider and Stein (1982). The low field strengths can be explained by requiring that the fields be in equipartition with either the thermal or turbulent energy densities, both of which are reduced in late-type evolved stars compared with the sun.

Marcy, G. W.↗

NASA NTRS1984

Observations of magnetic fields on solar-type stars

Magnetic-field observations were carried out for 29 G and K main-sequence stars. The area covering-factors of magnetic regions tends to be greater in the K darfs than in the G dwarfs. However, no spectral-type dependence is found for the field strengths, contrary to predictions that pressure equilibrium with the ambient photospheric gas pressure would determine the surface field strengths. Coronal soft X-ray fluxes from the G and K dwarfs correlate well with the fraction of the stellar surface covered by magnetic regions. The dependence of coronal soft X-ray fluxes on photospheric field strengths is consistent with Stein's predicted generation-rates for Alfven waves. These dependences are inconsistent with the one dynamo model for which a specific prediction is offered. Finally, time variability of magnetic fields is seen on the two active stars that have been extensively monitored. Significant changes in magnetic fields are seen to occur on timescales as short as one day. Previously announced in STAR as N83-24435

Marcy, G. W.↗

NASA NTRS1983

Observations of magnetic fields on late-type stars

The 'Robinson' method for measuring magnetic fields on solarand late-type stars is reviewed. The results of such measurements for a sample of 29 G and K main-sequence stars are presented. The area covering-factors of magnetic regions are greater in the K dwarfs than in the G dwarfs, but no spectral-type dependence is found for the field strengths, contrary to expectations of some flux-tube models. The dependence of Ca II H and K emission on magnetic fields and T(eff) is consistent with theoretical expections for 'slow-mode' mhd wave-generation rates, but inconsistent with those of other mhd modes. Coronal soft X-ray fluxes correlate well with the magnetic fields, and it is argued that Alfven waves are the likely energy-transport mechanism. Surface magnetic fluxes vary with rotation as V(0.5-1.0 rot) depending on spectral type.

Marcy, G. W.↗

NASA NTRS1982

A technique for measuring magnetic fields on solar-type stars

A method is developed and tested by which magnetic-field strengths and area filling-factors of magnetic regions on late-type stars may be inferred from high-resolution, absorption-line profiles that have been broadened by the Zeeman effect. The technique involves fitting such profiles with a triplet of components, the shape of which is derived from profiles of lines of low Zeeman sensitivity. Tests of the systematic and random errors indicate that such magnetic flux measurements have an uncertainty of 20% for stars with field strengths of 2000 gauss if at least 10% of the stellar surface contains magnetic regions.

Marcy, G. W.↗

NASA NTRS1982

Observations of magnetic fields on solar-type stars

Magnetic-field observations were carried out for 29 G and K main-sequence stars. The area covering-factors of magnetic regions tends to be greater in the K dwarfs than in the G dwarfs. However, no spectral-type dependence is found for the field strengths, contrary to predictions that pressure equilibrium with the ambient photospheric gas pressure would determine the surface field strengths. Coronal soft X-ray fluxes from the G and K dwarfs correlate well with the fraction of the stellar surface covered by magnetic regions. The dependence of coronal soft X-ray fluxes on photospheric field strengths is consistent with Stein's predicted generation-rates for Alfven waves. These dependences are inconsistent with the one dynamo model for which a specific prediction is offered. Finally, time variability of magnetic fields is seen on the two active stars that have been extensively monitored. Significant changes in magnetic fields are seen to occur on timescales as short as one day.

Marcy, G. W.↗