Apparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOs

Apparent disk-mass reduction and planetesimal formation in gravitationally unstable disks in Class 0/I YSOs

Authors:

Tsukamoto et al

Abstract:

We investigate the dust structure of gravitationally unstable gas disks undergoing mass accretion from the envelope, envisioning application to Class 0/I YSOs. We compute evolution of the surface density and dust size by taking into account dust collisional growth and radial drift. We find that the dust disk quickly settles into the steady state and the dust mass in the steady-state disk decreases by a factor of 1/2 to 1/3, while the radiative flux of the dust thermal emission also decreases by a factor of 1/3 to 1/5, both compared to that for a disk with ISM dust-to-gas mass ratio and micron-sized dust. We suggest that the disk mass in the Class 0/I YSOs is underestimated by factor of 1/3 to 1/5 when it is calculated from the dust thermal emission assuming an ISM dust-to-gas mass ratio and micron-sized dust opacity, and that a larger fraction of the disks in Class 0/I YSOs is gravitationally unstable than previously considered. We derive an empirical formula for the disk-mass reduction rate, which can be used to test whether or not the disks observed in Class 0/I YSOs are gravitationally unstable. We also investigate the orbital radius of planetesimal formation rP and show that rP becomes ∼20 AU. Because rP increases as the gas surface density increases and a gravitationally unstable disk has a theoretical maximum gas surface density, rP∼20 AU is the theoretical maximum radius for planetesimal formation. We find that the dust particles migrate inwardly in the form of "pebble", and we suggest that planet formation via pebble accretion in Class 0/I phase is preferable to that in the Class II phase because the dust is supplied by envelope accretion and a significant amount of dust particles (Mdust∼10−2M⊙) pass through the disk during the Class 0/I phase.

Cariable Radio Emissions From a Hot Jupiter Host Star V830 Tau

Variable Radio Emission from the Young Stellar Host of a Hot Jupiter

Authors:

Bower et al

Abstract:

We report the discovery of variable radio emission associated with the T Tauri star, V830 Tau, which was recently shown to host a hot Jupiter companion. Very Large Array observations at a frequency of 6 GHz reveal a detection on 01 May 2011 with a flux density 919±26 μJy, along with non-detections in two other epochs at <66 a="" additionally="" and="" array="" at="" baseline="" comparable="" consistent="" demonstrating="" detection="" emission="" field="" from="" gyro-synchrotron="" in="" include="" is="" long="" magnetic="" mechanism="" non-detection="" nonthermal="" observations="" one="" or="" origin.="" region="" sensitivity="" synchrotron="" that="" the="" very="" with="" y.="">30 G, and is likely driven by an energetic event such as magnetic reconnection that accelerated electrons. With the limited data we have, we are not able to place any constraint on the relationship between the radio emission and the rotational or orbital properties of V830 Tau. This is the first detection of radio emission from a non-degenerate star known to host an exoplanet.

Hunting for hot Jupiters in Young Stellar Associations

A Wide-Field Survey for Transiting Hot Jupiters and Eclipsing Pre-Main-Sequence Binaries in Young Stellar Associations

Authors:

Oelkers et al

Abstract:

The past two decades have seen a significant advancement in the detection, classification and understanding of exoplanets and binaries. This is due, in large part, to the increase in use of small-aperture telescopes (< 20 cm) to survey large areas of the sky to milli-mag precision with rapid cadence. The vast majority of the planetary and binary systems studied to date consist of main-sequence or evolved objects, leading to a dearth of knowledge of properties at early times (< 50 Myr). Only a dozen binaries and one candidate transiting Hot Jupiter are known among pre-main sequence objects, yet these are the systems that can provide the best constraints on stellar formation and planetary migration models. The deficiency in the number of well-characterized systems is driven by the inherent and aperiodic variability found in pre-main-sequence objects, which can mask and mimic eclipse signals. Hence, a dramatic increase in the number of young systems with high-quality observations is highly desirable to guide further theoretical developments. We have recently completed a photometric survey of 3 nearby (< 150 pc) and young (< 50 Myr) moving groups with a small aperture telescope. While our survey reached the requisite photometric precision, the temporal coverage was insufficient to detect Hot Jupiters. Nevertheless, we discovered 346 pre-main-sequence binary candidates, including 74 high-priority objects for further study.

Tabby's Star has a Sister

New research conducted by a team of astronomers, led by Simone Scaringi of the Max Planck Institute for Extraterrestrial Physics in Germany, examines peculiar dimming of a newly found young stellar object designated EPIC 204278916. The study tries to explain the nature of these dipping events observed in the object's light curve. The results were published Aug. 25 in a paper available on arXiv.org.

EPIC 204278916, is a young, pre-main-sequence star, about 5 million years old, of spectral type M1, located in the Upper Scorpius sub-group of the Scorpius-Centaurus OB association. It is the size of our sun in diameter, but has only 0.5 solar masses. This young stellar object was discovered by NASA Kepler spacecraft's prolonged mission known as K2, during its Campaign 2 between Aug. 23 and Nov. 13, 2014. Moreover, follow-up observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile revealed that EPIC 204278916 also has a resolved tilted disk.

In a recent paper, Scaringi and his colleagues analyze the data provided by K2 and ALMA regarding EPIC 204278916, available in the Ecliptic Plane Input Catalog (EPIC) and the Mikulski Archive for Space Telescope (MAST) archive, with the aim to examine the object's light curve and its irregular dimmings in detail.

"We examine the K2 light curve in detail and hypothesize that the irregular dimmings are caused by either a warped inner-disk edge or transiting cometary-like objects in either circular or eccentric orbits," the researchers wrote in the paper.

According to the data provided by K2, EPIC 204278916 exhibited irregular dimmings of up to 65 percent for about 25 consecutive days out of 79 days of observations. The researchers also noted that when it comes to the remaining days of observation, this variability is highly periodic and could be attributed to stellar rotation.

One of the two most plausible explanations offered by the astronomers to explain the irregular dips in the object's light curve is that they are caused by non-axisymmetric structures in the inner disk edge occulting EPIC 204278916. Due to the fact that these dimmings are at a level of up to 65 percent, the occulting material must have a large scale height comparable to the size of the object.

The researchers also noted that the dips in young stellar objects like EPIC 204278916 might be caused by transiting circumstellar objects. They emphasized that if transiting cometary-like bodies are responsible for the observed dips, the events are most likely occurring close to periastron passage.

However, more observations are definitely needed to fully understand the mysterious behavior of EPIC 204278916. Moreover, further continuous photometric and spectroscopic monitoring of this system for subsequent dipping events will help determine whether this behavior is periodic or not.

link.

paper link.

The SEEDS High Contrast Imaging Survey of Exoplanets around Young Stellar Objects

The SEEDS High Contrast Imaging Survey of Exoplanets around Young Stellar Objects

Authors:

Uyama et al

Abstract:

We present high-contrast observations of 68 young stellar objects (YSOs) taken as part of the SEEDS survey on the Subaru telescope. Our targets are very young (less than 10 Myr) stars, which often harbor protoplanetary disks where planets may be forming. We achieve typical contransts of ∼10−4-10−5.5 at the angular distance of 1$\arcsec$ from the central star, corresponding to typical mass sensitivities (assuming hot-start evolutionary models) of ∼10 MJ at 70 AU and ∼6 MJ at 140 AU. We have detected a stellar companion to HIP 79462 and confirmed the substellar objects GQ Lup b and ROXs 42B b. An additional six companion candidates await follow-up observations to check for common proper-motion. Our SEEDS YSO observations prove the population of planets and brown dwarfs at the very youngest ages, these may be compared to the results of surveys targeting somewhat older stars. We will present a detailed statistical analysis of our sample and its implications for giant planet formation in a future paper.

Confronting Standard Models of ProtoPlanetary Disks With New Mid Infrared Sizes from the Keck Interferometer

Confronting Standard Models of Proto--Planetary Disks With New Mid--Infrared Sizes from the Keck Interferometer

Authors:

Millan-Gabet et al

Abstract:

We present near and mid-infrared interferometric observations made with the Keck Interferometer Nuller and near-contemporaneous spectro-photometry from the IRTF of 11 well known young stellar objects, several observed for the first time in these spectral and spatial resolution regimes. With AU-level spatial resolution, we first establish characteristic sizes of the infrared emission using a simple geometrical model consisting of a hot inner rim and mid-infrared disk emission. We find a high degree of correlation between the stellar luminosity and the mid-infrared disk sizes after using near-infrared data to remove the contribution from the inner rim. We then use a semi-analytical physical model to also find that the very widely used "star + inner dust rim + flared disk" class of models strongly fails to reproduce the SED and spatially-resolved mid-infrared data simultaneously; specifically a more compact source of mid-infrared emission is required than results from the standard flared disk model. We explore the viability of a modification to the model whereby a second dust rim containing smaller dust grains is added, and find that the two-rim model leads to significantly improved fits in most cases. This complexity is largely missed when carrying out SED modelling alone, although detailed silicate feature fitting by McClure et al. 2013 recently came to a similar conclusion. As has been suggested recently by Menu et al. 2015, the difficulty in predicting mid-infrared sizes from the SED alone might hint at "transition disk"-like gaps in the inner AU; however, the relatively high correlation found in our mid-infrared disk size vs. stellar luminosity relation favors layered disk morphologies and points to missing disk model ingredients instead.

The evolution of self-gravitating accretion discs

The evolution of self-gravitating accretion discs

Authors:

Rice et al

Abstract:

It is quite likely that self-gravity will play an important role in the evolution of accretion discs, in particular those around young stars, and those around supermassive black holes. We summarise, here, our current understanding of the evolution of such discs, focussing more on discs in young stellar system, than on discs in active galactic nuclei. We consider the conditions under which such discs may fragment to form bound objects, and when they might, instead, be expected to settle into a quasi-steady, self-regulated state. We also discuss how this understanding may depend on the mass of the disc relative to the mass of the central object, and how it might depend on the presence of external irradiation. Additionally, we consider whether or not fragmentation might be stochastic, where we might expect it to occur in an actual protostellar disc, and if there is any evidence for fragmentation actually playing a role in the formation of planetary-mass bodies. Although there are still a number of outstanding issue, such as the convergence of simulations of self-gravitating discs, whether or not there is more than one mode of fragmentation, and quite what role self-gravitating discs may play in the planet formation process, our general understanding of these systems seems quite robust.

Understanding Binary T Tauri VV CrA System

Understanding discs in binary YSOs: detailed modelling of VV CrA

Authors:

Scicluna et al

Abstract:

Given that a majority of stars form in multiple systems, in order to fully understand the star- and planet-formation processes we must seek to understand them in multiple stellar systems. With this in mind, we present an analysis of the enigmatic binary T-Tauri system VV Corona Australis, in which both components host discs, but only one is visible at optical wavelengths. We seek to understand the peculiarities of this system by searching for a model for the binary which explains all the available continuum observations of the system. We present new mid-infrared interferometry and near-infrared spectroscopy along with archival millimetre-wave observations, which resolve the binary at 1.3mm for the first time. We compute a grid of pre-main-sequence radiative transfer models and calculate their posterior probabilities given the observed spectral energy distributions and mid-infrared interferometric visibilities of the binary components, beginning with the assumption that the only differences between the two components are their inclination and position angles. Our best-fitting solution corresponds to a relatively low luminosity T-tauri binary, with each component's disc having a large scale height and viewed at moderate inclination (∼50∘), with the infrared companion inclined by ∼5∘ degrees more than the primary. Comparing the results of our model to evolutionary models suggests stellar masses ∼1.7M⊙ and an age for the system of 3.5Myr, towards the upper end of previous estimates. Combining these results with accretion indicators from near-IR spectroscopy, we determine an accretion rate of 4.0×10−8M⊙ yr−1 for the primary. We suggest that future observations of VV~CrA and similar systems should prioritise high angular resolution sub-mm and near-IR imaging of the discs and high resolution optical/NIR spectroscopy of the central stars.

The Disks of Young Stellar Objects

Circumstellar Disks of the Most Vigorously Accreting Young Stars

Authors:

Liu et al

Abstract:

Young stellar objects (YSOs) may not accumulate their mass steadily, as was previously thought, but in a series of violent events manifesting themselves as sharp stellar brightening. These events can be caused by fragmentation due to gravitational instabilities in massive gaseous disks surrounding young stars, followed by migration of dense gaseous clumps onto the star. We report our high angular resolution, coronagraphic near-infrared polarization imaging observations using the High Contrast Instrument for the Subaru Next Generation Adaptive Optics (HiCIAO) of the Subaru 8.2 m Telescope, towards four YSOs which are undergoing luminous accretion outbursts. The obtained infrared images have verified the presence of several hundred AUs scale arms and arcs surrounding these YSOs. In addition, our hydrodynamics simulations and radiative transfer models further demonstrate that these observed structures can indeed be explained by strong gravitational instabilities occurring at the beginning of the disk formation phase. The effect of those tempestuous episodes of disk evolution on star and planet formation remains to be understood.

Absence of Significant Cool Disks in Young Stellar Objects Exhibiting Repetitive Optical Outbursts

Absence of Significant Cool Disks in Young Stellar Objects Exhibiting Repetitive Optical Outbursts

Authors:

Liu et al

Abstract:

We report Submillimeter Array (SMA) 1.3 mm high angular resolution observations towards the four EXor type outbursting young stellar objects (YSOs) VY Tau, V1118 Ori, V1143 Ori, and NY Ori. The data mostly show low dust masses Mdust in the associated circumstellar disks. Among the sources, NY Ori possesses a relatively massive disk with Mdust∼9×10−4 M⊙. V1118 Ori has a marginal detection equivalent to Mdust∼6×10−5 M⊙. V1143 Ori has a non-detection also equivalent to Mdust less than 6×10−5 M⊙. For the nearest source VY Tau, we get a surprising non-detection which provides a stringent upper limit Mdust less than 6×10−6 M⊙. We interpret our findings as suggesting that the gas and dust reservoirs that feed the short duration, repetitive optical outbursts seen in some EXors may be limited to the small scale, innermost region of their circumstellar disks. This hot dust may have escaped our detection limits. Follow-up, more sensitive millimeter observations are needed to improve our understanding of the triggering mechanisms of EXor type outbursts.

Impact of an inhomogeneous density distribution on selected observational characteristics of circumstellar disks

Impact of an inhomogeneous density distribution on selected observational characteristics of circumstellar disks

Authors:

Brauer et al

Abstract:

Context.

The analysis of observations of circumstellar disks around young stellar objects is often based on models with a smooth and continuous density distribution. However, spatially resolved observations with increasing angular resolution and dynamical models indicate that circumstellar disks are highlystructured.

Aims.

We investigate the influence of different clumpy density distributions on selected physical properties and observable characteristics of circumstellar disks.

Methods.

Based on radiative transfer modelling we calculate the temperature structure of the disk and simulate observational quantities in the thermal re-emission and scattering regime. We compare our results to those obtained for a smooth and continuous density distribution to quantify the influence of clumps on physical parameters and observable quantities of circumstellar disks.

Results.

Within the considered model space, the clumpiness has a significant impact on the disk temperature distribution. For instance, in the transition region from the upper disk layers to the disk interior, it causes a decrease of the mean temperature by up to 12 K. In addition, circumstellar disks with clumpy density distributions feature a lower spectral index in the submm/mm range of the SED. As a consequence of the lower spectral index, the dust grain size derived from the submm/mm-slope of the SED may be overestimated, if the inhomogeneity of the disk density distribution is not taken into account. Furthermore, the scattered light brightness distribution of clumpy disks shows a steeper radial decrease. Additionally, clumpy density distributions change the degree of polarization of the scattered light in the optical.

Accretion Disks in Luminous Young Stellar Objects

Accretion disks in luminous young stellar objects

Authors:

Beltran et al

Abstract:

An observational review is provided of the properties of accretion disks around young stars. It concerns the primordial disks of intermediate- and high-mass young stellar objects in embedded and optically revealed phases. The properties were derived from spatially resolved observations and therefore predominantly obtained with interferometric means, either in the radio/(sub)millimeter or in the optical/infrared wavelength regions. We make summaries and comparisons of the physical properties, kinematics, and dynamics of these circumstellar structures and delineate trends where possible. Amongst others, we report on a quadratic trend of mass accretion rates with mass from T Tauri stars to the highest mass young stellar objects and on the systematic difference in mass infall and accretion rates.

Young Stellar objects & Disk Mid-infrared Variability in IRAS 20050+2720 Cluster

YSOVAR: mid-infrared variability of young stellar objects and their disks in the cluster IRAS 20050+2720

Authors:

Poppenhaeger et al

Abstract:

We present a time-variability study of young stellar objects in the cluster IRAS 20050+2720, performed at 3.6 and 4.5 micron with the Spitzer Space Telescope; this study is part of the Young Stellar Object VARiability project (YSOVAR). We have collected light curves for 181 cluster members over 40 days. We find a high variability fraction among embedded cluster members of ca. 70%, whereas young stars without a detectable disk display variability less often (in ca. 50% of the cases) and with lower amplitudes. We detect periodic variability for 33 sources with periods primarily in the range of 2-6 days. Practically all embedded periodic sources display additional variability on top of their periodicity. Furthermore, we analyze the slopes of the tracks that our sources span in the color-magnitude diagram (CMD). We find that sources with long variability time scales tend to display CMD slopes that are at least partially influenced by accretion processes, while sources with short variability time scales tend to display extinction-dominated slopes. We find a tentative trend of X-ray detected cluster members to vary on longer time scales than the X-ray undetected members.

PTFO 8-8695: A 3 MYr Old T-Tauri Star Hosting a Potential hot Jupiter

Follow-Up Observations of PTFO 8-8695: A 3 MYr Old T-Tauri Star Hosting a Jupiter-mass Planetary Candidate

Authors:

Ciardi et al

Abstract:

We present Spitzer 4.5\micron\ light curve observations, Keck NIRSPEC radial velocity observations, and LCOGT optical light curve observations of PTFO~8-8695, which may host a Jupiter-sized planet in a very short orbital period (0.45 days). Previous work by \citet{vaneyken12} and \citet{barnes13} predicts that the stellar rotation axis and the planetary orbital plane should precess with a period of 300−600 days. As a consequence, the observed transits should change shape and depth, disappear, and reappear with the precession. Our observations indicate the long-term presence of the transit events (>3 years), and that the transits indeed do change depth, disappear and reappear. The Spitzer observations and the NIRSPEC radial velocity observations (with contemporaneous LCOGT optical light curve data) are consistent with the predicted transit times and depths for the M⋆=0.34 M⊙ precession model and demonstrate the disappearance of the transits. An LCOGT optical light curve shows that the transits do reappear approximately 1 year later. The observed transits occur at the times predicted by a straight-forward propagation of the transit ephemeris. The precession model correctly predicts the depth and time of the Spitzer transit and the lack of a transit at the time of the NIRSPEC radial velocity observations. However, the precession model predicts the return of the transits approximately 1 month later than observed by LCOGT. Overall, the data are suggestive that the planetary interpretation of the observed transit events may indeed be correct, but the precession model and data are currently insufficient to confirm firmly the planetary status of PTFO~8-8695b.

The Properties of Young Stellar Objects in the Perseus Cloud and Their Disks

Evolutionary Stages and Disk Properties of Young Stellar Objects in the Perseus Cloud

Authors:

Zhang et al

Abstract:

We investigated the evolutionary stages and disk properties of 211 Young stellar objects (YSOs) across the Perseus cloud by modeling the broadband optical to mid-infrared (IR) spectral energy distribution (SED). By exploring the relationships among the turnoff wave bands lambda_turnoff (longward of which significant IR excesses above the stellar photosphere are observed), the excess spectral index alpha_excess at lambda less than approximately 24 microns, and the disk inner radius R_in (from SED modeling) for YSOs of different evolutionary stages, we found that the median and standard deviation of alpha_excess of YSOs with optically thick disks tend to increase with lambda_turnoff, especially at lambda_turnoff greare than or equal to 5.8 microns, whereas the median fractional dust luminosities L_dust/L_star tend to decrease with lambda_turnoff. This points to an inside-out disk clearing of small dust grains. Moreover, a positive correlation between alpha_excess and R_in was found at alpha_excess greater than approximately 0 and R_in greater than approximately 10 × the dust sublimation radius R_sub, irrespective of lambda_turnoff, L_dust/L_star and disk flaring. This suggests that the outer disk flaring either does not evolve synchronously with the inside-out disk clearing or has little influence on alpha_excess shortward of 24 microns. About 23% of our YSO disks are classified as transitional disks, which have lambda_turnoff greater than or equal to 5.8 microns and L_dust/L_star greater than 10^(-3). The transitional disks and full disks occupy distinctly different regions on the L_dust/L_star vs. alpha_excess diagram. Taking L_dust/L_star as an approximate discriminator of disks with (greater than 0.1) and without (less than 0.1) considerable accretion activity, we found that 65% and 35% of the transitional disks may be consistent with being dominantly cleared by photoevaporation and dynamical interaction respectively. [abridged]