arXiv subjects

Jason T. Wright

Publications and source records attributed to Jason T. Wright.

225 records · Page 13Linked to original sources

arXiv2007

Retired A Stars and Their Companions: Exoplanets Orbiting Three Intermediate-Mass Subgiants

We report precision Doppler measurements of three intermediate-mass subgiants from Lick and Keck Observatories. All three stars show variability in their radial velocities consistent with planet-mass companions in Keplerian orbits. We find a planet with a minimum mass of 2.5 Mjup in a 351.5 day orbit around HD 192699, a planet with a minimum mass of 2.0 Mjup in a 341.1 day orbit around HD 210702, and a planet with a minimum mass of 0.61 Mjup in a 297.3 day orbit around HD 175541. Stellar mass estimates from evolutionary models indicate that all of these stars were formerly A-type dwarfs with masses ranging from 1.65 to 1.85 Msun. These three long-period planets would not have been detectable during their stars' main-sequence phases due to the large rotational velocities and stellar jitter exhibited by early-type dwarfs. There are now 9 "retired" (evolved) A-type stars (Mstar > 1.6 Msun) with known planets. All 9 planets orbit at distances a \geq 0.78 AU, which is significantly different than the semimajor axis distribution of planets around lower-mass stars. We examine the possibility that the observed lack of close-in planets is due to engulfment by their expanding host stars, but we find that this explanation is inadequate given the relatively small stellar radii of K giants (Rstar < 32 Rsun = 0.15 AU) and subgiants (Rstar < 7 Rsun = 0.03 AU). Instead, we conclude that planets around intermediate-mass stars reside preferentially beyond ~0.8 AU, which may be a reflection of different formation and migration histories of planets around A-type stars.

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arXiv2006

An Eccentric Hot Jupiter Orbiting the Subgiant HD 185269

We report the detection of a Jupiter-mass planet in a 6.838 day orbit around the 1.28 solar mass subgiant HD 185269. The eccentricity of HD 185269b (e = 0.30) is unusually large compared to other planets within 0.1 AU of their stars. Photometric observations demonstrate that the star is constant to +/-0.0001 mag on the radial velocity period, strengthening our interpretation of a planetary companion. This planet was detected as part of our radial velocity survey of evolved stars located on the subgiant branch of the H-R diagram--also known as the Hertzsprung Gap. These stars, which have masses between 1.2 and 2.5 solar masses, play an important role in the investigation of the frequency of extrasolar planets as a function of stellar mass.

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arXiv2006

A Long-Period Jupiter-Mass Planet Orbiting the Nearby M Dwarf GJ849

We report precise Doppler measurements of GJ849 (M3.5V) that reveal the presence of a planet with a minimum mass of 0.82 Mjup in a 5.16 year orbit. At a = 2.35 AU, GJ849b is the first Doppler-detected planet discovered around an M dwarf to orbit beyond 0.21 AU, and is only the second Jupiter mass planet discovered around a star less massive than 0.5 Msun. This detection brings to 4 the number of M stars known to harbor planets. Based on the results of our survey of 1300 FGKM main--sequence stars we find that giant planets within 2.5 AU are ~3 times more common around GK stars than around M stars. Due to the GJ849's proximity of 8.8 pc, the planet's angular separation is 0."27, making this system a prime target for high--resolution imaging using adaptive optics and future space--borne missions such as the Space Interferometry Mission. We also find evidence of a linear trend in the velocity time series, which may be indicative of an additional planetary companion.

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arXiv2006

The N2K Consortium VI: Doppler Shifts Without Templates and Three New Short-Period Planets

We present a modification to the iodine cell Doppler technique that eliminates the need for an observed stellar template spectrum. For a given target star, we iterate toward a synthetic template spectrum beginning with an existing spectrum of a similar star. We then perturb the shape of this first-guess template to match the program observation of the target star taken through an iodine cell. The elimination of a separate template observation saves valuable telescope time, a feature that is ideally suited for the quick-look strategy employed by the ``Next 2000 Stars'' (N2K) planet search program. Tests using Keck/HIRES spectra indicate that synthetic templates yield a short-term precision of 3 m/s and a long-term, run-to-run precision of 5 m/s. We used this new Doppler technique to discover three new planets: a 1.5 Mjup planet in a 2.1375 d orbit around HD 86081; a 0.71 Mjup planet in circular, 26.73 d orbit around HD 224693; and a Saturn-mass planet in an 18.179 d orbit around HD 33283. The remarkably short period of HD 86081b bridges the gap between the extremely short-period planets detected in the OGLE survey and the 16 Doppler-detected hot jupiters (P < 15 d), which have an orbital period distribution that piles up at about three days. We have acquired photometric observations of two of the planetary host stars with the automated photometric telescopes at Fairborn Observatory. HD 86081 and HD 224693 both lack detectable brightness variability on their radial velocity periods, supporting planetary-reflex motion as the cause of the radial velocity variability. HD 86081 shows no evidence of planetary transits in spite of a 17.6% transit probability.

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arXiv2005

Solar-like oscillations in alpha Centauri B

We have made velocity observations of the star alpha Cen B from two sites, allowing us to identify 37 oscillation modes with l=0-3. Fitting to these modes gives the large and small frequency separations as a function of frequency. The mode lifetime, as measured from the scatter of the oscillation frequencies about a smooth trend, is similar to that in the Sun. Limited observations of the star delta Pav show oscillations centred at 2.3 mHz with peak amplitudes close to solar. We introduce a new method of measuring oscillation amplitudes from heavily-smoothed power density spectra, from which we estimated amplitudes for alpha Cen A and B, beta Hyi, delta Pav and the Sun. We point out that the oscillation amplitudes may depend on which spectral lines are used for the velocity measurements.

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arXiv2004

A Neptune-Mass Planet Orbiting the Nearby M Dwarf GJ 436

We report precise Doppler measurements of GJ 436 (M2.5V) obtained at Keck Observatory. The velocities reveal a planetary companion with orbital period of 2.644 d, eccentricity of 0.12 (consistent with zero) and velocity semi-amplitude of $K =18.1$ \ms. The minimum mass (\msini) for the planet is 0.067 \mjup = 1.2 M$_{\rm NEP}$ = 21 M$_{\rm EARTH}$, making it the lowest mass exoplanet yet found around a main sequence star and the first candidate in the Neptune mass domain. GJ 436 (Mass = 0.41 \msune) is only the second M dwarf found to harbor a planet, joining the two--planet system around GJ 876. The low mass of the planet raises questions about its constitution, with possible compositions of primarily H and He gas, ice/rock, or rock--dominated. The implied semi--major axis is $a$ = 0.028 AU = 14 stellar radii, raising issues of planet formation, migration, and tidal coupling with the star. GJ 436 is $>3$ Gyr old, based on both kinematic and chromospheric diagnostics. The star exhibits no photometric variability on the 2.644-day Doppler period to a limiting amplitude of 0.0004 mag, supporting the planetary interpretation of the Doppler periodicity. Photometric transits of the planet across the star are ruled out for gas giant compositions and are also unlikely for solid compositions. As the third closest known planetary system, GJ 436 warrants follow--up observations by high resolution optical and IR

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arXiv2004

Do We Know of Any Maunder Minimum Stars?

Most stars previously identified as Maunder minimum stars are old stars evolved off of the main sequence. Analysis of activity measurements from the California and Carnegie Planet Search program stars and Hipparcos parallaxes implies that the canonical age-chromospheric activity relation breaks down for stars older than $\sim 6$ Gyr when activity is calculated from Mount Wilson S values. Stars only 1 magnitude above the main sequence exhibit significantly suppressed activity levels which have been mistaken for examples of Maunder minimum behavior.

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arXiv2004

Chromospheric CaII Emission in Nearby F, G, K, and M stars

We present chromospheric CaII activity measurements, rotation periods and ages for ~1200 F-, G-, K-, and M- type main-sequence stars from ~18,000 archival spectra taken at Keck and Lick Observatories as a part of the California and Carnegie Planet Search Project. We have calibrated our chromospheric S values against the Mount Wilson chromospheric activity data. From these measurements we have calculated median activity levels and derived R'HK, stellar ages, and rotation periods for 1228 stars, ~1000 of which have no previously published S values. We also present precise time series of activity measurements for these stars.

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arXiv2003

Ultra-high-precision velocity measurements of oscillations in alpha Cen A

We have made differential radial velocity measurements of the star alpha Cen A using two spectrographs, UVES and UCLES, both with iodine absorption cells for wavelength referencing. Stellar oscillations are clearly visible in the time series. After removing jumps and slow trends in the data, we show that the precision of the velocity measurements per minute of observing time is 0.42 m/s for UVES and 1.0 m/s for UCLES, while the noise level in the Fourier spectrum of the combined data is 1.9 cm/s. As such, these observations represent the most precise velocities ever measured on any star apart from the Sun.

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