The presence of a Jupiter-mass companion to the star 51 Pegasi is inferred from observations of periodic variations in the star's radial velocity. The companion lies only about eight million kilometres from the star, which would be well inside the orbit of Mercury in our Solar System. This object might be a gas-giant planet that has migrated to this location through orbital evolution, or from the radiative stripping of a brown dwarf.

A Jupiter-Mass Companion to a Solar-Type Star

We present a uniform catalog of stellar properties for 1040 nearby F, G, and K stars that have been observed by the Keck, Lick, and AAT planet search programs. Fitting observed echelle spectra with synthetic spectra yielded effective temperature, surface gravity, metallicity, projected rotational velocity, and abundances of the elements Na, Si, Ti, Fe, and Ni, for every star in the catalog. Combining V-band photometry and Hipparcos parallaxes with a bolometric correction based on the spectroscopic results yielded stellar luminosity, radius, and mass. Interpolating Yonsei-Yale isochrones to the luminosity, effective temperature, metallicity, and α-element enhancement of each star yielded a theoretical mass, radius, gravity, and age range for most stars in the catalog. Automated tools provide uniform results and make analysis of such a large sample practical. Our analysis method differs from traditional abundance analyses in that we fit the observed spectrum directly, rather than trying to match equivalent widths, and we determine effective temperature and surface gravity from the spectrum itself, rather than adopting values based on measured photometry or parallax. As part of our analysis, we determined a new relationship between macroturbulence and effective temperature on the main sequence. Detailed error analysis revealed small systematic offsets with respect to the Sun and spurious abundance trends as a function of effective temperature that would be inobvious in smaller samples. We attempted to remove these errors by applying empirical corrections, achieving a precision per spectrum of 44 K in effective temperature, 0.03 dex in metallicity, 0.06 dex in the logarithm of gravity, and 0.5 km s-1 in projected rotational velocity. Comparisons with previous studies show only small discrepancies. Our spectroscopically determined masses have a median fractional precision of 15%, but they are systematically 10% higher than masses obtained by interpolating isochrones. Our spectroscopic radii have a median fractional precision of 3%. Our ages from isochrones have a precision that varies dramatically with location in the Hertzsprung-Russell diagram. We plan to extend the catalog by applying our automated analysis technique to other large stellar samples.

https://iopscience.iop.org/article/10.1086/430500/pdf

Spectroscopic properties of cool stars (spocs). i. 1040 f, g, and k dwarfs from keck, lick, and aat planet search programs

While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H
and K emission index (R'_(HK) = L_(HK)/L_(bol)) as an empirical age estimator for solar-type dwarfs, existing activity-age relations
produce implausible ages at both high and low activity levels.We have compiled R'_(HK)
HK data from the literature
for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the
cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity
age index,we derive an improved activity-age calibration for F7-K2 dwarfs (0:5 mag < B - V < 0.9 mag). We
also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_(HK) values through
the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that
our new activity-age calibration has typical age precision of ~0.2 dex for normal solar-type dwarfs aged between the
Hyades and the Sun (~0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent
effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal
activity as measured through the fractional X-ray luminosity (R_X = L_X/L_(bol)) has nearly the same age- and rotation inferring
capability as chromospheric activity measured through R'_(HK). As a first application of our calibrations, we
provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within
16 pc.

/pdf/improved-age-estimation-for-solar-type-dwarfs-using-activity-3a3h4zjus7.pdf

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics

Doppler measurements from Keck exhibit a sinusoidal periodicity in the velocities of the G0 dwarf HD 209458, having a semiamplitude of 81 m s 21 and a period of 3.5239 days, which is indicative of a “51 Peg‐like” planet with a minimum mass ( ) of 0.62 MJup and a semimajor axis of 0.046 AU. Follow-up photometry reveals M sin i a drop of 0.017 mag at the predicted time (within the errors) of transit by the companion based on the velocities. This is the first extrasolar planet observed to transit its star. The radius of the planet derived from the magnitude of the dimming is 1.42 RJup, which is consistent with models of irradiated Jupiter-mass planets. The transit implies that , leading to a true mass of 0.62 MJup for the planet. The resulting mean density of 0.27 g cm 23 sin i 1 0.993 implies that the companion is a gas giant. Subject headings: planetary systems — stars: individual (HD 209458)

https://iopscience.iop.org/article/10.1086/312458/pdf

A Transiting "51 Peg-like" Planet.

The G0 V star HD 166435 has been observed by the ber-fed spectrograph ELODIE as one of the targets in the large extra-solar planet survey that we are conducting at the Observatory of Haute-Provence. We detected coherent, low-amplitude, radial-velocity variations with a period of 3.7987 days, suggesting a possible close-in planetary companion. Subsequently, we initiated a series of high-precision photometric observations to search for possible planetary transits and an additional series of Ca II H and K observations to measure the level of surface magnetic activity and to look for possible rotational modulation. Surprisingly, we found the star to be photometrically variable and magnetically active. A detailed study of the phase stability of the radial-velocity signal revealed that the radial-velocity variability remains coherent only for durations of about 30 days. Analysis of the time variation of the spectroscopic line proles using line bisectors revealed a correlation between radial velocity and line-bisector orientation. All of these observations, along with a one-quarter cycle phase shift between the photometric and the radial-velocity variations, are well explained by the presence of dark photospheric spots on HD 166435. We conclude that the radial-velocity variations are not due to gravitational interaction with an orbiting planet but, instead, originate from line-prole changes stemming from star spots on the surface of the star. The quasi-coherence of the radial-velocity signal over more than two years, which allowed a fair t with a binary model, makes the stability of this star unusual among other active stars. It suggests a stable magnetic eld orientation where spots are always generated at about the same location on the surface of the star.

/pdf/no-planet-for-hd-166435-2bycgy3gh5.pdf

No planet for HD 166435

Rotation periods are reported for 14 main-sequence stars, bringing the total number of such stars with well-determined rotation periods to 41. It is found that the mean level of their Ca n H and K emission (averaged over 15 years) is correlated with rotation period, as expected. However, there is a further dependence of the emission on spectral type. When expressed as the ratio of chromospheric flux to total bolometric flux, the emission is well correlated with the parameter Pohs/Tc, where Pohs is the observed rotation period and tc(B—V) is a theoretically-derived convective overturn time, calculated assuming a mixing length to scale height ratio a ~ 2. This finding is consonant with general predictions of dynamo theory, if the relation between chromospheric emission and dynamo-generated magnetic fields is essentially independent of rotation rate and spectral type for the stars considered. The dependence of mean chromospheric emission on rotation and spectral type is essentially the same for stars above and below the Vaughan-Preston “gap,” thus casting doubt on explanations of the gap in terms of a discontinuity in dynamo characteristics. Subject headings: Ca n emission — convection — stai

Rotation, convection, and magnetic activity in lower main-sequence stars

Modele du cycle d'activite solaire. Niveaux d'activite chromospherique stellaire moyennes dans le temps. Fluctuations de l'activite a long terme d'etoiles froides

Stellar Activity Cycles

For a sample of 47 lower main-sequence stars, including the Sun, and eight evolved stars, the relative strength of the Ca n H and K emission cores has been measured daily over a nearly continuous interval during 1980 July through October at Mount Wilson. From these time series measurements of chromospheric emission, rotation rates have been inferred with quantitative estimates of both the reality and precision of the rotation periods. We find rotation rates easily for the main-sequence stars with strong emission or those later than about spectral type K0. With this technique, rotation rates can be measured precisely for the first time for equatorial velocities as slow as 1 km s", and independently of the aspect of the rotation axis. In a limited range of spectral type, a small sample of stars indicates that chromospheric emission decreases smoothly as a function of rotation period. No conclusion can be drawn on the question of the reality of a discontinuity in chromospheric emission as a function of time (the “Vaughan-Preston” gap for stars in the solar neighborhood). In our sample of giant stars, the G2 III star HD 218658 shows a persistent fluctuation of 4.6 days, a period that is inconsistent with stellar rotation. The GO III star HD 6903 is a previously unreported FK Comae-type star. For a few main-sequence stars, measurements continued beyond 1980 October suggest the presence of active longitudes (if not individual active regions) persisting through the observing season 1981. Subject headings: Ca ii emission — stars: chromospheres — stars: late-type — stars: rotation

Stellar rotation in lower main-sequence stars measured from time variations in H and K emission-line fluxes. II. Detailed analysis of the 1980 observing season data.

Enregistrements durant plusieurs saisons des raies Ca II H et K des cœurs de plusieurs etoiles froides, essentiellement des etoiles de la basse sequence principale ayant ete observees par Wilson pour des fluctuations d'activite chromospherique a long terme

Time-series measurements of chromospheric Ca II H and K emission in cool stars and the search for differential rotation

Etude des proprietes statistiques d'un releve de Voughan-Preston de l'emission chromospherique d'etoiles proches, a l'aide de modeles du declin de l'activite chromospherique avec un âge croissant

A. H. Vaughan

Papers

Rotation, convection, and magnetic activity in lower main-sequence stars

Stellar Activity Cycles

Stellar rotation in lower main-sequence stars measured from time variations in H and K emission-line fluxes. II. Detailed analysis of the 1980 observing season data.

Time-series measurements of chromospheric Ca II H and K emission in cool stars and the search for differential rotation

An analysis of the Vaughan-Preston survey of chromospheric emission