Examining the Inner Disk of GW Ori

GW Ori: Inner disk readjustments in a triple system

Authors:

Fang et al

Abstract:

We study the young stellar system GW Ori, concentrating on its accretion/wind activity by using our high-resolution optical spectra and U-band photometry. We also characterize the disk properties of GW Ori by modeling its spectral energy distribution (SED). By comparing our data to the synthetical spectra, we classify GW Ori as a G8 star. Based on the RVs derived from the spectra, we confirm the previous result as a close companion in GW Ori with a period of ~242 days and an orbital semi-major axis of ~1 AU. The RV residuals after the subtraction of the orbital solution with the equivalent widths of accretion-related emission lines vary with periods of 5-6.7 days during short time intervals, which are caused by the rotational modulation. The Hα and Hβ line profiles of GW Ori can be decomposed in two central-peaked emission components and one blue-shifted absorption component. The absorption components are due to a disk wind modulated by the orbital motion of the close companion. Therefore, the systems like GW Ori can be used to study the extent of disk winds. We find that the accretion rates of GW Ori are rather constant but can occasionally be enhanced by a factor of 2-3. We reproduce the SED of GW Ori by using disk models with gaps ~25-55 AU in size. A small population of tiny dust particles within the gap produces the excess emission at near-infrared bands and the strong and sharp silicate feature at 10 μm. The SED of GW Ori exhibits dramatic changes on timescales of ~20 yr in the near-infrared bands, which can be explained as the change in the amount and distribution of small dust grains in the gap. We collect a sample of binary/multiple systems with disks in the literature and find a strong positive correlation between their gap sizes and separations from the primaries to companions, which is generally consistent with the prediction from the theory.

Circumbinary Ring and Circumstellar Disks Detected in Binary System UY Aurigae

Circumbinary Ring, Circumstellar disks and accretion in the binary system UY Aurigae

Authors:

Tang et al

Abstract:

Recent exo-planetary surveys reveal that planets can orbit and survive around binary stars. This suggests that some fraction of young binary systems which possess massive circumbinary disks (CB) may be in the midst of planet formation. However, there are very few CB disks detected. We revisit one of the known CB disks, the UY Aurigae system, and probe 13CO 2-1, C18O 2-1, SO 5(6)-4(5) and 12CO 3-2 line emission and the thermal dust continuum. Our new results confirm the existence of the CB disk. In addition, the circumstellar (CS) disks are clearly resolved in dust continuum at 1.4 mm. The spectral indices between the wavelengths of 0.85 mm and 6 cm are found to be surprisingly low, being 1.6 for both CS disks. The deprojected separation of the binary is 1.26" based on our 1.4 mm continuum data. This is 0.07" (10 AU) larger than in earlier studies. Combining the fact of the variation of UY Aur B in R band, we propose that the CS disk of an undetected companion UY Aur Bb obscures UY Aur Ba. A very complex kinematical pattern inside the CB disk is observed due to a mixing of Keplerian rotation of the CB disk, the infall and outflow gas. The streaming gas accreting from the CB ring toward the CS disks and possible outflows are also identified and resolved. The SO emission is found to be at the bases of the streaming shocks. Our results suggest that the UY Aur system is undergoing an active accretion phase from the CB disk to the CS disks. The UY Aur B might also be a binary system, making the UY Aur a triple system.

DQ Tau Binary System has Anomalous Behavior in the Circumbinary Disk

Anomalous Accretion Activity and the Spotted Nature of the DQ Tau Binary System

Authors:

Bary et al

Abstract:

We report the detection of an anomalous accretion flare in the tight eccentric pre-main-sequence binary system DQ Tau. In a multi-epoch survey consisting of randomly acquired low to moderate resolution near-infrared spectra obtained over a period of almost ten years, we detect a significant and simultaneous brightening of four standard accretion indicators (CaII infrared triplet, the Paschen and Brackett series HI lines, and HeI 1.083 um), on back-to-back nights (phase = 0.372, 0.433) with the flare increasing in strength as the system approached apastron (phase = 0.5). The mass accretion rate measured for the anomalous flare is nearly an order of magnitude stronger than the average quiescent rate. While previous observations established that frequent, periodic accretion flares phased with periastron passages occur in this system, these data provide evidence that orbitally-modulated accretion flares occur near apastron, when the stars make their closest approach to the circumbinary disk. The timing of the flare suggests that this outburst is due to interactions of the stellar cores (or the highly truncated circumstellar disks) with material in non-axisymmetric structures located at the inner edge of the circumbinary disk. We also explore the optical/infrared spectral type mismatch previously observed for T Tauri stars and successfully model the shape of the spectra from 0.8 to 1.0 um and the strengths of the TiO and FeH bands as manifestations of large cool spots on the surfaces of the stellar companions in DQ Tau. These findings illustrate that a complete model of near-infrared spectra of many T Tauri stars must include parameters for spot filling factors and temperatures.

Exoplanets With Large Companions Will Remain Habitable Longer

Having a companion in old age is good for people — and, it turns out, might extend the chance for life on certain Earth-sized planets in the cosmos as well.

Planets cool as they age. Over time their molten cores solidify and inner heat-generating activity dwindles, becoming less able to keep the world habitable by regulating carbon dioxide to prevent runaway heating or cooling.

But astronomers at the University of Washington and the University of Arizona have found that for certain planets about the size of our own, the gravitational pull of an outer companion planet could generate enough heat — through a process called tidal heating — to effectively prevent that internal cooling, and extend the inner world's chance at hosting life.

UW astronomer Rory Barnes is second author of a paper published in the July issue of the Monthly Notices of the Royal Astronomical Society. The lead authors are graduate student Christa Van Laerhoven and planetary scientist Richard Greenberg at the University of Arizona.

Tidal heating results from the gravitational push and pull of the outer companion planet on its closer-in neighbor, Barnes said. The effect happens locally, so to speak, on Jupiter's moons Io and Europa. The researchers showed that this phenomenon can take place on exoplanets — those outside the solar system — as well.

Using computer models, the researchers found the effect can occur on older Earth-sized planets in noncircular orbits in the habitable zone of low-mass stars, or those less than one-quarter the mass of the Sun. The habitable zone is that swath of space around a star just right to allow an orbiting rocky planet to sustain liquid water on its surface, thus giving life a chance.

"When the planet is closer to the star, the gravitational field is stronger and the planet is deformed into an American football shape. When farther from the star, the field is weaker and the planet relaxes into a more spherical shape," Barnes said. "This constant flexing causes layers inside the planet to rub against each other, producing frictional heating."

The outer planet is necessary, Barnes added, to keep the potentially habitable planet's orbit noncircular. When a planet's orbit is circular, the gravitational pull from its host star is constant, so its shape never changes, and there is no tidal heating.

And so, the researchers conclude, any discoveries of Earth-sized planets in the habitable zone of old, small stars should be followed by searches for outer companion planets that might improve the inner world's chance at hosting life.

link.

The Distribution of Exoplanetary Radii Around Cool Stars

THE RADIUS DISTRIBUTION OF PLANETS AROUND COOL STARS

Authors:

Morton et al

Abstract:

We calculate an empirical, non-parametric estimate of the shape of the period-marginalized radius distribution of planets with periods less than 150 days using the small yet well-characterized sample of cool (T eff less than 4000 K) dwarf stars in the Kepler catalog. In particular, we present and validate a new procedure, based on weighted kernel density estimation, to reconstruct the shape of the planet radius function down to radii smaller than the completeness limit of the survey at the longest periods. Under the assumption that the period distribution of planets does not change dramatically with planet radius, we show that the occurrence of planets around these stars continues to increase to below 1 R ⊕, and that there is no strong evidence for a turnover in the planet radius function. In fact, we demonstrate using many iterations of simulated data that a spurious turnover may be inferred from data even when the true distribution continues to rise toward smaller radii. Finally, the sharp rise in the radius distribution below ~3 R ⊕ implies that a large number of planets await discovery around cool dwarfs as the sensitivities of ground-based transit surveys increase.

How to Directly Detect Exoplanets in Multi Stellar Systems

Simulation of a method to directly image exoplanets around multiple stars systems

Authors:

Thomas et al

Abstract:

Direct imaging of extra-solar planets has now become a reality, especially with the deployment and commissioning of the first generation of specialized ground-based instruments such as the GPI, SPHERE, P1640 and SCExAO. These systems will allow detection of planets 1e7 times fainter than their host star.

For space-based missions, such as EXCEDE, EXO-C, EXO-S, WFIRST-AFTA, different teams have shown in laboratories contrasts reaching 1e-10 within a few diffraction limits from the star using a combination of a coronagraph to suppress light coming from the host star and a wavefront control system. These demonstrations use a deformable mirror (DM) to remove residual starlight (speckles) created by the imperfections of telescope. However, all these current and future systems focus on detecting faint planets around a single host star or unresolved binaries/multiples, while several targets or planet candidates are located around nearby binary stars such as our neighbor star Alpha Centauri.

Until now, it has been thought that removing the light of a companion star is impossible with the current technology, excluding binary star systems from target lists of direct imaging missions. Direct imaging around binaries or multiples systems at a level of contrast allowing Earth-like planets detection is challenging because the region of interest, where a dark zone is essential, is contaminated by the light coming from the host star's companion. We propose a method to simultaneously correct aberration sand diffraction of light coming from the target star. This method works even if the companion star is outside the control region of the DM (beyond its half-Nyquist frequency), by taking advantage of aliasing effects.

Evidence From OTS44: The Coolest Stars are Rogue Planets

The coolest 'stars' are free-floating planets

Authors:

Joergens et al

Abstract:

We show that the coolest known object that is probably formed in a star-like mode is a free-floating planet. We discovered recently that the free-floating planetary mass object OTS,44 (M9.5, ~12 Jupiter masses, age ~2 Myr) has significant accretion and a substantial disk. This demonstrates that the processes that characterize the canonical star-like mode of formation apply to isolated objects down to a few Jupiter masses. We detected in VLT/SINFONI spectra that OTS44 has strong, broad, and variable Paschen beta emission. This is the first evidence for active accretion of a free-floating planet. The object allows us to study accretion and disk physics at the extreme and can be seen as free-floating analog of accreting planets that orbit stars. Our analysis of OTS44 shows that the mass-accretion rate decreases continuously from stars of several solar masses down to free-floating planets. We determined, furthermore, the disk mass (10 Earth masses) and further disk properties of OTS44 through modeling its SED including Herschel far-IR data. We find that objects between 14 and 0.01 solar masses have the same ratio of the disk-to-central-mass of about 1%. Our results suggest that OTS44 is formed like a star and that the increasing number of young free-floating planets and ultra-cool T and Y field dwarfs are the low-mass extension of the stellar population.