Direct imaging of exoplanetary systems is a powerful technique that can reveal Jupiter-like planets in wide orbits, can enable detailed characterization of planetary atmospheres, and is a key step toward imaging Earth-like planets. Imaging detections are challenging because of the combined effect of small angular separation and large luminosity contrast between a planet and its host star. High-contrast observations with the Keck and Gemini telescopes have revealed three planets orbiting the star HR 8799, with projected separations of 24, 38, and 68 astronomical units. Multi-epoch data show counter clockwise orbital motion for all three imaged planets. The low luminosity of the companions and the estimated age of the system imply planetary masses between 5 and 13 times that of Jupiter. This system resembles a scaled-up version of the outer portion of our solar system.

https://science.sciencemag.org/content/sci/322/5906/1348.full.pdf

Direct imaging of multiple planets orbiting the star HR 8799.

Context. The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. Aims. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars, and observationally investigate the relation between γ Doradus (γ Dor), δ Scuti (δ Sct), and hybrid stars. Methods. We compile a database of physical parameters for the sample stars from the literature and new ground-based observations. We analyse the Kepler light curve of each star and extract the pulsational frequencies using different frequency analysis methods. We construct two new observables, “energy ”a nd “efficiency”, related to the driving energy of the pulsation mode and the convective efficiency of the outer convective zone, respectively. Results. We propose three main groups to describe the observed variety in pulsating A-F type stars: γ Dor, δ Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much higher fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the γ Dor and δ Sct range, also between 5 and 10 d −1 , which is a challenge for the current models. We find indications for the existence of δ Sct and γ Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the δ Sct and γ Dor instability strips and beyond. Non-variable stars seem to exist within the instability strips. The location of γ Dor and δ Sct classes in the (Teff ,l ogg)-diagram has been extended. We investigate two newly constructed variables, “efficiency ”a nd “energy”, as a means to explore the relation between γ Dor and δ Sct stars. Conclusions. Our results suggest a revision of the current observational instability strips of δ Sct and γ Dor stars and imply an investigation of pulsation mechanisms to supplement the κ mechanism and convective blocking effect to drive hybrid pulsations. Accurate physical parameters for all stars are needed to confirm these findings.

/pdf/the-kepler-characterization-of-the-variability-among-a-and-f-y9gptafczn.pdf

The Kepler characterization of the variability among A- and F-type stars I. General overview

The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars. We propose three main groups to describe the observed variety in pulsating A-F type stars: gamma Dor, delta Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the gamma Dor and delta Sct range, also between 5 and 10 c/d, which is a challenge for the current models. We find indications for the existence of delta Sct and gamma Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the delta Sct and gamma Dor instability strips, and beyond. Non-variable stars seem to exist within the instability strips. The location of gamma Dor and delta Sct classes in the (Teff,logg)-diagram has been extended. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between gamma Dor and delta Sct stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of other pulsation mechanisms to supplement the kappa mechanism and convective blocking effect to drive hybrid pulsations.

/pdf/the-kepler-characterization-of-the-variability-among-a-and-f-2au724c8rd.pdf

The Kepler characterization of the variability among A- and F-type stars. I. General overview

In this paper we describe a new class of pulsating stars, the prototype of which is the bright, early, F‐type dwarf γ Doradus These stars typically have between 1 and 5 periods ranging from 04 to 3 days with photometric amplitudes up to 01 mag in Johnson V The mechanism for these observed variations is high‐order, low‐degree, nonradial, gravity‐mode pulsation

https://iopscience.iop.org/article/10.1086/316399/pdf

γ Doradus Stars: Defining a New Class of Pulsating Variables

By studying the oscillations of a star's surface, it is possible to extract information about stellar strata, age, and dynamics. Though one might guess that observation of surface oscillatory motions would be limited to our Sun, high-precision brightness measurements of distant stars, performed over many years, have enabled the field of asteroseismology. This comprehensive review covers the recent development of this field, the necessary blending of numerical simulation and data, and the way in which this new information enhances our understanding of stellar evolution.

/pdf/probing-the-interior-physics-of-stars-through-2dqd8qjata.pdf

Probing the interior physics of stars through asteroseismology

/pdf/the-g-doradus-type-variable-9-aurigae-results-from-a-multi-2ctsdlx4is.pdf

The γ Doradus-type variable 9 Aurigae: results from a multi-site campaign

Photometry of alpha Orionis (October 1994 to April 1996)

/pdf/the-g-dor-variable-hd-164615-results-from-a-multisite-47a0r3wxkm.pdf

The γ Dor variable HD 164615: results from a multisite photometric campaign

We present further photometric observations of the unusual F0 V star 9 Aurigae and present evidence that this star’s radial velocity, spectroscopic line widths and line depths are also variable with the same frequencies as the photometric data (f1 � 0.795 and f2 � 0.345 d 1 ). The phases of these sinusoids are stable over time scales of longer than one year, though the amplitudes can vary, making the prediction of photometric behavior impossible. Given that a variety of other explanations have already been discounted (e.g. interactions with a close companion, the existence of a lumpy, orbiting ring of dust, or star spots) and that these variations occur on time scales an order of magnitude slower than the fundamental radial pulsation period, we have very strong evidence that 9 Aurigae exhibits non-radial g-mode pulsations. Since the power spectrum of the radial velocity data shows frequency f2 but does not clearly show f1, the present data suggest that f2 is associated with a low degree spherical harmonic (l = 1 or 2), while f1 is associated with a higher degree harmonic. 9 Aurigae, along with such stars as γ Doradus, HD 224638, HD 224945, and HD 164615, appear to constitute a new class of pulsating variables. These stars are to be found at or beyond the cool edge of the Cepheid instability strip in the HR Diagram. Prior to this, only much hotter stars have been shown to exhibit non-radial g-modes.

9 Aurigae: strong evidence for non-radial pulsations

$\gamma$ Doradus stars are photometrically variable early F-type stars on or just above the main sequence in the Hertzsprung-Russell Diagram. They vary by as much as 0.1 mag on time scales an order of magnitude slower than the fundamental radial pulsation period. These brightness variations are presumably due to non-radial gravity-mode pulsations. We obtained high precision $V$-band photometry of 8 F0 to F5 stars in the Hyades (BS 1319, BS 1354, BS 1385, BS 1408, BS 1430, BS 1432, BS 1459, and BS 1472) and found that none of them showed strong evidence of $\gamma$ Dor-type variability. Since $\gamma$ Dor-type candidates are found in the Pleiades and in NGC 2516 (having ages of 78 and 137 Myr, repectively) but apparently not in the Hyades (age $>$ 600 Myr), we hypothesize that the $\gamma$ Dor phenomenon is a characteristic of relatively young F stars. 
Two of the stars investigated showed marginal evidence of low-amplitude variability. The $\pm$3 mmag variability of the F5 star BS 1319 is most likely due to rotational modulation of star spots, though it is not impossible that it is a $\gamma$ Dor star. Another F5 star BS 1459 ($\Delta V = \pm 2$ mmag) has a possible period similar to $\delta$ Scuti stars, but no firm conclusions should be made concerning its behavior unless and until its variability is confirmed.

A search for γ Doradus-type variable stars in the Hyades

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