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تسجيل مستخدم جديدPlanets around Giant Stars
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We present results from a radial-velocity survey of 373 giant stars at Lick Observatory, which started in 1999. The previously announced planets iota Dra b and Pollux b are confirmed by continued monitoring. The frequency of detected planetary companions appears to increase with metallicity. The star nu Oph is orbited by two brown dwarf companions with masses of 22.3 M_Jup and 24.5 M_Jup in orbits with a period ratio close to 6:1. It is likely that the two companions to nu Oph formed in a disk around the star.
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We report the discovery of planetary companions orbiting four low-luminosity giant stars with M$_star$ between 1.04 and 1.39 M$_odot$. All four host stars have been independently observed by the EXoPlanets aRound Evolved StarS (EXPRESS) program and t
he Pan-Pacific Planet Search (PPPS). The companion signals were revealed by multi-epoch precision radial velocities obtained during nearly a decade. The planetary companions exhibit orbital periods between $sim$ 1.2 and 7.1 years, minimum masses of m$_{rm p}$sini $sim$ 1.8-3.7 M$_{jup}$ and eccentricities between 0.08 and 0.42. Including these four new systems, we have detected planetary companions to 11 out of the 37 giant stars that are common targets between the EXPRESS and PPPS. After excluding four compact binaries from the common sample, we obtained a fraction of giant planets (m$_{rm p} gtrsim$ 1-2 M$_{jup}$) orbiting within 5 AU from their parent star of $f = 33.3^{+9.0}_{-7.1} %$. This fraction is significantly higher than that previously reported in the literature by different radial velocity surveys. Similarly, planet formation models under predict the fraction of gas giant around stars more massive than the Sun.
We present evidence for a new two-planet system around the giant star HD202696 (= HIP105056, BD+26 4118). The discovery is based on public HIRES radial velocity measurements taken at Keck Observatory between July 2007 and September 2014. We estimate
a stellar mass of 1.91$^{+0.09}_{-0.14}M_odot$ for HD202696, which is located close to the base of the red giant branch. A two-planet self-consistent dynamical modeling MCMC scheme of the radial velocity data followed by a long-term stability test suggests planetary orbital periods of $P_{rm b}$ = 517.8$_{-3.9}^{+8.9}$ days and $P_{rm c}$ = 946.6$_{-20.9}^{+20.7}$ days, eccentricities of $e_{rm b}$ = 0.011$_{-0.011}^{+0.078}$ and $e_{rm c}$ = 0.028$_{-0.012}^{+0.065}$ , and minimum dynamical masses of $m_{rm b}$ = 2.00$_{-0.10}^{+0.22}$,$M_{mathrm{Jup}}$ and $m_{rm c}$ = 1.86$_{-0.23}^{+0.18}$,$M_{mathrm{Jup}}$, respectively. Our stable MCMC samples are consistent with orbital configurations predominantly in a mean period ratio of 11:6 and its close-by high order mean-motion commensurabilities with low eccentricities. For the majority of the stable configurations we find an aligned or anti-aligned apsidal libration (i.e. $Deltaomega$ librating around 0$^circ$ or 180$^circ$), suggesting that the HD202696 system is likely dominated by secular perturbations near the high-order 11:6 mean-motion resonance. The HD202696 system is yet another Jovian mass pair around an intermediate mass star with a period ratio below the 2:1 mean motion resonance. Therefore, the HD202696 system is an important discovery, which may shed light on the primordial disk-planet properties needed for giant planets to break the strong 2:1 mean motion resonance and settle in more compact orbits.
The recent high spatial/spectral resolution observations have enabled constraining formation mechanisms of giant planets, especially at the final stages. The current interpretation of such observations is that these planets undergo magnetospheric acc
retion, suggesting the importance of planetary magnetic fields. We explore the properties of accreting, magnetized giant planets surrounded by their circumplanetary disks, using the physical parameters inferred for PDS 70 b/c. We compute the magnetic field strength and the resulting spin rate of giant planets, and find that these planets may possess dipole magnetic fields of either a few 10 G or a few 100 G; the former is the natural outcome of planetary growth and radius evolution, while the resulting spin rate cannot reproduce the observations. For the latter, a consistent picture can be drawn, where strong magnetic fields induced by hot planetary interiors lead both to magnetospheric accretion and to spin-down due to disk locking. We also compute the properties of circumplanetary disks in the vicinity of these planets, taking into account planetary magnetic fields. The resulting surface density becomes very low, compared with the canonical models, implying the importance of radial movement of satellite-forming materials. Our model predicts a positive gradient of the surface density, which invokes the traps for both satellite migration and radially drifting dust particles. This work thus concludes that the final formation stages of giant planets are similar to those of low-mass stars such as brown dwarfs, as suggested by recent studies.
We have carried out high contrast imaging of 70 young, nearby B and A stars to search for brown dwarf and planetary companions as part of the Gemini NICI Planet-Finding Campaign. Our survey represents the largest, deepest survey for planets around hi
gh-mass stars (~1.5-2.5 M_sun) conducted to date and includes the planet hosts beta Pic and Fomalhaut. We obtained follow-up astrometry of all candidate companions within 400 AU projected separation for stars in uncrowded fields and identified new low-mass companions to HD 1160 and HIP 79797. We have found that the previously known young brown dwarf companion to HIP 79797 is itself a tight (3 AU) binary, composed of brown dwarfs with masses 58 (+21, -20) M_Jup and 55 (+20, -19) M_Jup, making this system one of the rare substellar binaries in orbit around a star. Considering the contrast limits of our NICI data and the fact that we did not detect any planets, we use high-fidelity Monte Carlo simulations to show that fewer than 20% of 2 M_sun stars can have giant planets greater than 4 M_Jup between 59 and 460 AU at 95% confidence, and fewer than 10% of these stars can have a planet more massive than 10 M_Jup between 38 and 650 AU. Overall, we find that large-separation giant planets are not common around B and A stars: fewer than 10% of B and A stars can have an analog to the HR 8799 b (7 M_Jup, 68 AU) planet at 95% confidence. We also describe a new Bayesian technique for determining the ages of field B and A stars from photometry and theoretical isochrones. Our method produces more plausible ages for high-mass stars than previous age-dating techniques, which tend to underestimate stellar ages and their uncertainties.
Despite more than 20 years since the discovery of the first gas giant planet with an anomalously large radius, the mechanism for planet inflation remains unknown. Here, we report the discovery of EPIC228754001.01, an inflated gas giant planet found w
ith the NASA K2 Mission, and a revised mass for another inflated planet, K2-97b. These planets reside on ~9 day orbits around host stars which recently evolved into red giants. We constrain the irradiation history of these planets using models constrained by asteroseismology and Keck/HIRES spectroscopy and radial velocity measurements. We measure planet radii of 1.31 +- 0.11 Rjup and and 1.30 +- 0.07 Rjup, respectively. These radii are typical for planets receiving the current irradiation, but not the former, zero age main sequence irradiation of these planets. This suggests that the current sizes of these planets are directly correlated to their current irradiation. Our precise constraints of the masses and radii of the stars and planets in these systems allow us to constrain the planetary heating efficiency of both systems as 0.03% +0.03%/-0.02%. These results are consistent with a planet re-inflation scenario, but suggest the efficiency of planet re-inflation may be lower than previously theorized. Finally, we discuss the agreement within 10% of stellar masses and radii, and planet masses, radii, and orbital periods of both systems and speculate that this may be due to selection bias in searching for planets around evolved stars.
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