V892 Tau Research Articles

Zodiacal exoplanets in time – XIII. Planet orbits and atmospheres in the V1298 Tau system, a keystone in studies of early planetary evolution

ABSTRACT Studies of planetary systems of stars in star-forming regions and young clusters open a window on the formative stages of planetary evolution. We obtained high-cadence high-resolution infrared spectroscopy of the solar-mass Taurus association-member V1298 Tau during a transit of its 10R⊕-size ‘b’ planet. We measured the systemic radial velocity (RV) and find that the kinematics of V1298 Tau suggest an affiliation with a ≳6 Myr-old subgroup. A comparison of V1298 Tau and the nearby, co-moving star 2M0405 with stellar evolution models suggests an age of ∼10–25 Myr. We measured the projected spin-orbit angle of ‘b’ as $\lambda =15_{-16}^{+15}$ and $\lambda = 2_{-4}^{+12}$ degrees using the apparent RV shift and change in line profile, respectively, induced by the transient occultation of the rotating star by the planet. These values indicate a prograde orbit like that of the interior ‘c’ planet of V1298 Tau and point to a co-planar multiplanet system that formed within a disc. We also measured variation in the strength of the 1083 nm triplet of neutral orthohelium as a probe of any extended/escaping atmosphere around ‘b’. We detect a steady decrease in absorption over the transit that appears to arise from the star or its planetary system. While this variation could be ascribed to ‘b’ or possibly to the immediately preceding transit of ‘d’, we cannot rule out that this is due to rapid variation in the stellar disc-integrated flux in the triplet. The amplitude of variation (∼0.04 nm) is consistent with moderate estimates of atmospheric escape driven by XUV radiation from the central star.

H-alpha and Ca ii Infrared Triplet Variations During a Transit of the 23 Myr Planet V1298 Tau c

Abstract Young transiting exoplanets (<100 Myr) provide crucial insight into atmospheric evolution via photoevaporation. However, transmission spectroscopy measurements to determine atmospheric composition and mass loss are challenging due to the activity and prominent stellar disk inhomogeneities present on young stars. We observed a full transit of V1298 Tau c, a 23 Myr, 5.59 R ⊕ planet orbiting a young K0-K1.5 solar analog with GRACES on Gemini North. We were able to measure the Doppler tomographic signal of V1298 Tau c using the Ca ii infrared triplet (IRT) and find a projected obliquity of λ = 5° ± 15°. The tomographic signal is only seen in the chromospherically driven core of the Ca ii IRT, which may be the result of star-planet interactions. Additionally, we find that excess absorption of the Hα line decreases smoothly during the transit. While this could be a tentative detection of hot gas escaping the planet, we find this variation is consistent with similar timescale observations of other young stars that lack transiting planets over similar timescales. We show this variation can also be explained by the presence of starspots with surrounding facular regions. More observations both in and out of the transits of V1298 Tau c are required to determine the nature of the Ca ii IRT and Hα line variations.

H-α Variability of V1298 Tau c

Abstract The 23 Myr system V1298 Tau hosts four transiting planets and is a valuable laboratory for exploring the early stages of planet evolution soon after formation of the star. We observe the innermost planet, V1298 Tau c, during transit using LBT PEPSI to obtain high spectral resolution characterization of escaping material near the H-α line. We find no strong evidence for atmospheric material escaping at the orbital velocity of the planet. Instead, we find a deep stellar feature that is variable on the few percent level, similar to a previous observation of the planet and can be explained by stellar activity. We attempted to monitor the broadband optical transit with LBT MODS but do not achieve the precision needed to characterize the atmosphere or improve the ephemeris.

The Architecture of the V892 Tau System: The Binary and Its Circumbinary Disk

Abstract We present high-resolution millimeter continuum and CO line observations for the circumbinary disk around V892 Tau to constrain the stellar and disk properties. The total mass of the two near-equal-mass A stars is estimated to be 6.0 ± 0.2 M ⊙ based on our models of the Keplerian-dominated gas disk rotation. The detection of strong ionized gas emission associated with the two stars at 8 mm, when combined with previous astrometric measurements in the near-infrared, provides an updated view of the binary orbit with a = 7.1 ± 0.1 au, e = 0.27 ± 0.1, and P = 7.7 ± 0.2 yr, which is about half of a previously reported orbital period. The binary orbital plane is proposed to be nearly coplanar to the circumbinary disk plane (with a mutual inclination of only Δ = 8 ± 4.°2; another solution with Δ = 113° is less likely given the short realignment timescale). An asymmetric dust disk ring peaking at a radius of 0.″2 is detected at 1.3 mm and its fainter counterparts are also detected at the longer 8 and 9.8 mm. The CO gas disk, though dominated by Keplerian rotation, presents a mild inner and outer disk misalignment. The radial extension of the disk, its asymmetric dust ring, and the presence of a disk warp could all be explained by the interaction between the eccentric binary and the circumbinary disk, which we assume were formed with nonzero mutual inclination. Some tentatively detected gas spirals in the outer disk are likely produced by interactions with the low-mass tertiary component located 4″ to the northeast. Our analyses demonstrate the promising usage of V892 Tau as an excellent benchmark system to study the details of binary–disk interactions.

Estimating the atmospheric mass loss of V1298 Tau's four young planets & the role of the host star in planet population studies

Estimating the atmospheric mass loss of V1298 Tau's four young planets & the role of the host star in planet population studies

Constraining the entropy of formation from young transiting planet

ABSTRACT Recently, K2 and TESS have discovered transiting planets with radii between ∼5 and 10 R⊕ around stars with ages <100 Myr. These young planets are likely to be the progenitors of the ubiquitous super-Earths/sub-Neptunes, which are well studied around stars with ages ≳1 Gyr. The formation and early evolution of super-Earths/sub-Neptunes are poorly understood. Various planetary origin scenarios predict a wide range of possible formation entropies. We show how the formation entropies of young (∼20–60 Myr), highly irradiated planets can be constrained if their mass, radius, and age are measured. This method works by determining how low-mass an H/He envelope a planet can retain against mass-loss, this lower bound on the H/He envelope mass can then be converted into an upper bound on the entropy. If planet mass measurements with errors ≲20 per cent can be achieved for the discovered young planets around DS Tuc A and V1298 Tau, then insights into their origins can be obtained. For these planets, higher measured planet masses would be consistent with the standard core-accretion theory. In contrast, lower planet masses (≲6–7 M⊕) would require a ‘boil-off’ phase during protoplanetary disc dispersal to explain.

X-ray irradiation and evaporation of the four young planets around V1298 Tau

ABSTRACT Planets around young stars are thought to undergo atmospheric evaporation due to the high magnetic activity of the host stars. Here we report on X-ray observations of V1298 Tau, a young star with four transiting exoplanets. We use X-ray observations of the host star with Chandra and ROSAT to measure the current high-energy irradiation level of the planets and employ a model for the stellar activity evolution together with exoplanetary mass-loss to estimate the possible evolution of the planets. We find that V1298 Tau is X-ray bright with log LX [erg s−1] = 30.1 and has a mean coronal temperature of ≈9 MK. This places the star amongst the more X-ray luminous ones at this stellar age. We estimate the radiation-driven mass-loss of the exoplanets and find that it depends sensitively on the possible evolutionary spin-down tracks of the star as well as on the current planetary densities. Assuming the planets are of low density due to their youth, we find that the innermost two planets can lose significant parts of their gaseous envelopes and could be evaporated down to their rocky cores depending on the stellar spin evolution. However, if the planets are heavier and follow the mass–radius relation of older planets, then even in the highest XUV irradiation scenario none of the planets is expected to cross the radius gap into the rocky regime until the system reaches an age of 5 Gyr.

Four Newborn Planets Transiting the Young Solar Analog V1298 Tau

Abstract Exoplanets orbiting pre-main-sequence stars are laboratories for studying planet evolution processes, including atmospheric loss, orbital migration, and radiative cooling. V1298 Tau, a young solar analog with an age of 23 ± 4 Myr, is one such laboratory. The star is already known to host a Jupiter-sized planet on a 24 day orbit. Here, we report the discovery of three additional planets—all between the sizes of Neptune and Saturn—based on our analysis of K2 Campaign 4 photometry. Planets c and d have sizes of 5.6 and 6.4 , respectively, and with orbital periods of 8.25 and 12.40 days reside 0.25% outside of the nominal 3:2 mean-motion resonance. Planet e is 8.7 in size but only transited once in the K2 time series and thus has a period longer than 36 days, but likely shorter than 223 days. The V1298 Tau system may be a precursor to the compact multiplanet systems found to be common by the Kepler mission. However, the large planet sizes stand in sharp contrast to the vast majority of Kepler multiplanet systems, which have planets smaller than 3 . Simple dynamical arguments suggest total masses of <28 and <120 for the c–d and d–b planet pairs, respectively. The implied low masses suggest that the planets may still be radiatively cooling and contracting, and perhaps losing atmosphere. The V1298 Tau system offers rich prospects for further follow-up including atmospheric characterization by transmission or eclipse spectroscopy, dynamical characterization through transit-timing variations, and measurements of planet masses and obliquities by radial velocities.

A Warm Jupiter-sized Planet Transiting the Pre-main-sequence Star V1298 Tau

Abstract We report the detection of V1298 Tau b, a warm Jupiter-sized planet (R P = 0.91 ± 0.05 R Jup, P = 24.1 days) transiting a young solar analog with an estimated age of 23 Myr. The star and its planet belong to Group 29, a young association in the foreground of the Taurus–Auriga star-forming region. While hot Jupiters have been previously reported around young stars, those planets are non-transiting and near-term atmospheric characterization is not feasible. The V1298 Tau system is a compelling target for follow-up study through transmission spectroscopy and Doppler tomography owing to the transit depth (0.5%), host star brightness (K s = 8.1 mag), and rapid stellar rotation ( = 23 km s−1). Although the planet is Jupiter-sized, its mass is currently unknown due to high-amplitude radial velocity jitter. Nevertheless, V1298 Tau b may help constrain formation scenarios for at least one class of close-in exoplanets, providing a window into the nascent evolution of planetary interiors and atmospheres.

On the origin of [Ne II] emission in young stars: mid-infrared and optical observations with the Very Large Telescope

{Abridged version for ArXiv}. We provide direct constraints on the origin of the [Ne II] emission in 15 young stars using high-spatial and spectral resolution observations with VISIR at the VLT that allow us to study the kinematics of the emitting gas. In addition we compare the [Ne II] line with optical forbidden lines observed for three stars with UVES. The [Ne II] line was detected in 7 stars, among them the first confirmed detection of [Ne II] in a Herbig Be star, V892 Tau. In four cases, the large blueshifted lines indicate an origin in a jet. In two stars, the small shifts and asymmetric profiles indicate an origin in a photo-evaporative wind. CoKu Tau 1, seen close to edge-on, shows a spatially unresolved line centered at the stellar rest velocity, although cross-dispersion centroids move within 10 AU from one side of the star to the other as a function of wavelength. The line profile is symmetric with wings extending up to about +-80 km/s. The origin of the [Ne II] line could either be due to the bipolar jet or to the disk. For the stars with VLT-UVES observations, in several cases, the optical forbidden line profiles and shifts are very similar to the profile of the [Ne II] line, suggesting that the lines are emitted in the same region. A general trend observed with VISIR is a lower line flux when compared with the fluxes obtained with Spitzer. We found no correlation between the line full-width at half maximum and the line peak velocity. The [Ne II] line remains undetected in a large part of the sample, an indication that the emission detected with Spitzer in those stars is likely extended.

APERTURE SYNTHESIS IMAGING OF V892 Tau AND PV Cep: DISK EVOLUTION

I present a study of two Herbig Ae stars that are in completely different evolutionary stages: V892 Tau and PV Cep. Using sub arc-second interferometric observations obtained with the Combined Array for Research in Millimeter-wave Astronomy (CARMA) at the 1.3 and 2.7 mm wavelengths, I have for the first time resolved their disks. I deduce that the 5 Myr old V892 Tau has a low dust opacity index $\beta$=1.1 and a disk mass of 0.03 $\msun$. These values correspond to the growth of its dust into large up to centimeters size structures. In contrast, the very young (a few 10$^5$yrs) PV Cep has a quite high opacity index $\beta$ =1.75 and a more massive disk 0.8 $\msun$. PV Cep has the youngest resolved disk around any Herbig Ae star. Unlike the youngest T Tauri and Class 0 stars, which contain large and processed grains, the young Herbig Ae star, PV Cep, disk contains ISM-like unprocessed dust. This suggests that PV Cep's dust evolution is slower than T Tauri stars'. I also present high spatial resolution interferometric observations of the PV Cep outflow. The outflow inclination is consistent with the orientation of the known Herbig-Haro flow in that region, HH315.

Discovery of a Circumbinary Disk around Herbig Ae/Be System V892 Tauri

We report the discovery of a circumbinary disk around the Herbig Ae/Be system V892 Tau. Our detailed mid-infrared images were made using segment-tilting interferometry on the Keck I telescope and reveal an asymmetric disk inclined at ~60° with an inner hole diameter of 250 mas (35 AU), approximately 5 times larger than the apparent separation of the binary components. In addition, we report a new measurement along the binary orbit using near-infrared Keck aperture masking, allowing a crude estimate of orbital parameters and the system mass for the first time. The size of the inner hole appears to be consistent with the minimum size prediction from tidal truncation theory, bearing a resemblance to the recently unmasked binary CoKu Tau/4. Our results have motivated a reanalysis of the system spectral energy distribution, concluding the luminosity of this system has been severely underestimated. With further study and monitoring, V892 Tau should prove a powerful testing ground for both predictions of dynamical models for disk-star interactions in young systems with gas-rich disks and for calibrations of pre-main-sequence tracks for intermediate-mass stars.

Observations of Herbig Ae Disks with Nulling Interferometry

We present the results of 10 micron nulling interferometric observations of 13 Herbig Ae stars using the Magellan I (Baade) and the MMT 6.5 m telescopes. A portion of the observations was completed with the adaptive secondary at the MMT. We have conclusively spatially resolved 3 of the 13 stars, HD 100546, AB Aur, and HD 179218, the latter two recently resolved using adaptive optics in combination with nulling interferometry. For the resolved objects we find that the 10 $\mu$m emitting regions have a spatial extent of 15-30 AU in diameter. We also have some evidence for resolved emission surrounding an additional 2 stars (V892 Tau and R CrA). For those objects in our study with mid-IR SEDs classifications from Meeus et al. (2001), we find that the Group I objects (those with constant to increasing mid-IR flux) are more likely to be resolved, within our limited sample. This trend is evident in correlations in the inferred disk sizes vs. the sub-millimeter SED slope and disk size vs. fractional infrared luminosity of the systems. We explore the spatial distribution and orientation of the warm dust in the resolved systems and constrain physical models which are consistent with their observational signatures.

Resolved Mid-Infrared Emission around AB Aurigae and V892 Tauri with Adaptive Optics Nulling Interferometric Observations

We present the results of adaptive optics nulling interferometric observations of two Herbig Ae stars, AB Aur and V892 Tau. Our observations at 10.3 μm show resolved circumstellar emission from both sources. Further analysis of the AB Aur emission suggests that there is an inclined disk surrounding the star. The diameter of the disk is derived to be 24-30 AU with an inclination of 45°-65° from face-on and a major-axis position angle of 30° ± 15° (east of north). Differences in the physical characteristics between the mid-IR emission and emission at other wavelengths (near-IR and millimeter), found in previous studies, suggest a complex structure for AB Aur's circumstellar environment, which may not be explained by a disk alone. The similarity in the observed size of AB Aur's resolved emission and that of another Herbig Ae star, HD 100546, is likely coincidental, as their respective evolutionary states and spectral energy distributions suggest significantly different circumstellar environments.

A large X-ray flare from the Herbig Ae star V892 Tau

We report the XMM-Newton observation of a large X-ray flare from the Herbig Ae star V892 Tau. The apparent low mass companion of V892 Tau, V892 Tau NE, is unresolved by XMM-Newton. Nevertheless there is compelling evidence from combined XMM-Newton and Chandra data that the origin of the flare is the Herbig Ae star V892 Tau. During the flare the X-ray luminosity of V892 Tau increases by a factor of ~15, while the temperature of the plasma increases from kT ~ 1.5 keV to kT ~ 8 keV. From the scaling of the flare event, based on hydrodynamic modeling, we conclude that a 500 G magnetic field is needed in order to confine the plasma. Under the assumptions that a dynamo mechanism is required to generate such a confining magnetic field and that surface convection is a necessary ingredient for a dynamo, our findings provide indirect evidence for the existence of a significant convection zone in the stellar envelope of Herbig Ae stars.