Young Circumstellar Disks Research Articles

HST Survey of the Orion Nebula Cluster in ACS/Visible and WFC3/IR Bands. IV. A Bayesian Multiwavelength Study of Stellar Parameters in the Orion Nebula Cluster

We have performed a comprehensive study of the Orion Nebula Cluster (ONC) combining the photometric data obtained by the two Hubble Space Telescope Treasury programs that targeted this region. To consistently analyze the rich data set obtained in a wide variety of filters, we adopted a Bayesian approach to fit the spectral energy distribution of the sources, deriving mass, age, extinction, distance, and accretion for each source in the region. The three-dimensional study of mass distribution for bona fide cluster members shows that mass segregation in the ONC extends to subsolar masses, while the age distribution strongly supports the idea that star formation in the ONC is best described by a major episode of star formation that happened ∼1 Myr ago. For masses ≳0.1 M ⊙, our derived empirical initial mass function (IMF) is in good agreement with a Chabrier system IMF. Both the accretion luminosity (L acc) and mass accretion rates ( Ṁacc ) are best described by broken power-law relations. This suggests that for the majority of young circumstellar disks in this cluster the excess emission may be dominated by X-ray-driven photoevaporation by the central star rather than external photoevaporation. If this is the case, the slopes of the power-law relations may be largely determined by the initial conditions set at the onset of the star formation process, which may be quite similar between regions that eventually form clusters of different sizes.

Gravitoturbulent dynamo in global simulations of gaseous disks

Context. The turbulence driven by gravitational instabilities (GIs) can amplify magnetic fields in massive gaseous disks. This GI dynamo may appear in young circumstellar disks, whose weak ionization challenges other amplification routes, as well as in active galactic nuclei. Although regarded as a large-scale dynamo, only local simulations have so far described its kinematic regime. Aims. We study the GI dynamo in global magnetohydrodynamic (MHD) models of accretion disks, focusing on its kinematic phase. Methods. We perform resistive MHD simulations with the PLUTO code for different radiative cooling times and electrical resistivities. A weak magnetic field seeds the dynamo, and we adopt mean-field and heuristic models to capture its essence. Results. We recover the same induction process leading to magnetic field amplification as previously identified in local simulations. The dynamo is, however, global in nature, connecting distant annuli of the disk via a large-scale dynamo mode of a fixed growth rate. This large-scale amplification can be described by a mean-field model that does not rely on conventional α-Ω effects. When varying the disk parameters we find an optimal resistivity that facilitates magnetic amplification, whose magnetic Reynolds number, ℛm ≲ 10, is substantially smaller than in local simulations. Unlike local simulations, we find an optimal cooling rate and the existence of global oscillating dynamo modes. The nonlinear saturation of the dynamo puts the disk in a strongly magnetized turbulent state on the margins of the effective range of GI. In our simulations, the accretion power eventually exceeds the threshold required by local thermal balance against cooling, leaving the long-term nonlinear outcome of the GI dynamo uncertain.

Direct Formation of Planetary Embryos in Self-gravitating Disks

Giant planets have been discovered at large separations from the central star. Moreover, a striking number of young circumstellar disks have gas and/or dust gaps at large orbital separations, potentially driven by embedded planetary objects. To form massive planets at large orbital separations through core accretion within the disk lifetime, however, an early solid body to seed pebble and gas accretion is desirable. Young protoplanetary disks are likely self-gravitating, and these gravitoturbulent disks may efficiently concentrate solid material at the midplane driven by spiral waves. We run 3D local hydrodynamical simulations of gravitoturbulent disks with Lagrangian dust particles to determine whether particle and gas self-gravity can lead to the formation of dense solid bodies, seeding later planet formation. When self-gravity between dust particles is included, solids of size St = 0.1–1 concentrate within the gravitoturbulent spiral features and collapse under their own self-gravity into dense clumps up to several M ⊕ in mass at wide orbits. Simulations with dust that drift most efficiently, St = 1, form the most massive clouds of particles, while simulations with smaller dust particles, St = 0.1, have clumps with masses an order of magnitude lower. When the effect of dust backreaction onto the gas is included, dust clumps become smaller by a factor of a few but more numerous. The existence of large solid bodies at an early stage of the disk can accelerate the planet formation process, particularly at wide orbital separations, and potentially explain planets distant from the central stars and young protoplanetary disks with substructures.

Radiative Scale Height and Shadows in Protoplanetary Disks

Abstract Planets form in young circumstellar disks called protoplanetary disks. However, it is still difficult to catch planet formation in situ. Nevertheless, from recent ALMA/SPHERE data, encouraging evidence of the direct and indirect presence of embedded planets has been identified in disks around young stars: co-moving point sources, gravitational perturbations, rings, cavities, and emission dips or shadows cast on disks. The interpretation of these observations needs a robust physical framework to deduce the complex disk geometry. In particular, protoplanetary disk models usually assume the gas pressure scale height given by the ratio of the sound speed over the azimuthal velocity H/r = c s /v k. By doing so, radiative pressure fields are often ignored, which could lead to a misinterpretation of the real vertical structure of such disks. We follow the evolution of a gaseous disk with an embedded Jupiter-mass planet through hydrodynamical simulations, computing the disk scale height including radiative pressure, which was derived from a generalization of the stellar atmosphere theory. We focus on the vertical impact of the radiative pressure in the vicinity of circumplanetary disks, where temperatures can reach ≳1000 K for an accreting planet and radiative forces can overcome gravitational forces from the planet. The radiation pressure effects create a vertical, optically thick column of gas and dust at the protoplanet location, casting a shadow in scattered light. This mechanism could explain the peculiar illumination patterns observed in some disks around young stars such as HD 169142 where a moving shadow has been detected or the extremely high aspect ratio H/r ∼ 0.2 observed in systems like AB Aur and CT Cha.

Dynamical signatures of Rossby vortices in cavity-hosting disks

Context. Planets are formed amidst young circumstellar disks of gas and dust. The latter is traced by thermal radiation, where strong asymmetric clumps have been observed in a handful of cases. These dust traps could be key to understanding the early stages of planet formation, when solids grow from micron-size to planetesimals. Aims. Vortices are among the few known asymmetric dust trapping scenarios. The present work aims to predict their characteristics in a complementary observable. Namely, line-of-sight velocities are well suited to trace the presence of a vortex. Moreover, the dynamics of disks is subject to recent developments. Methods. Two-dimensional hydro simulations were performed in which a vortex forms at the edge of a gas-depleted region. We derived idealized line-of-sight velocity maps, varying disk temperature and orientation relative to the observer. The signal of interest, as a small perturbation to the dominant axisymmetric component in velocity, may be isolated in observational data using a proxy for the dominant quasi-Keplerian velocity. We propose that the velocity curve on the observational major axis be such a proxy. Results. Applying our method to the disk around HD 142527 as a study case, we predict that line-of-sight velocities are barely detectable by currently available facilities, depending on disk temperature. We show that corresponding spirals patterns can also be detected with similar spectral resolutions, which will help to test against alternative explanations.

Validating scattering-induced (sub)millimetre disc polarization through the spectral index, wavelength-dependent polarization pattern, and polarization spectrum: the case of HD 163296

ABSTRACT A number of young circumstellar discs show strikingly ordered (sub)millimetre polarization orientations along the minor axis, which is strong evidence for polarization due to scattering by ∼0.1 mm-sized grains. To test this mechanism further, we model the ALMA dust continuum and polarization data of HD 163296 using radmc-3d. We find that scattering by grains with a maximum size of 90 μm simultaneously reproduces the polarization observed at Band 7 and the unusually low spectral index (α ∼ 1.5) between Bands 7 and 6 in the optically thick inner disc as a result of more efficient scattering at the shorter wavelength. The low spectral index of ∼2.5 inferred for the optically thin gaps is reproduced by the same grains, as a result of telescope beam averaging of the gaps (with an intrinsic α ∼ 4) and their adjacent optically thick rings (where α ≲ 2). The tension between the grain sizes inferred from polarization and spectral index disappears because the low α values do not require large mm-sized grains. In addition, the polarization fraction has a unique azimuthal variation: higher along the major axis than the minor axis in the gaps, but vice versa in the rings. We find a rapidly declining polarization spectrum (with p ∝ λ−3 approximately) in the gaps, which becomes flattened or even inverted towards short wavelengths in the optically thick rings. These contrasting behaviours in the rings and gaps provide further tests for scattering-induced polarization via resolved multiwavelength observations.

Cooling in the shade of warped transition discs

ABSTRACT The mass of the gaseous reservoir in young circumstellar discs is a crucial initial condition for the formation of planetary systems, but estimates vary by orders of magnitude. In some discs with resolvable cavities, sharp inner disc warps cast two-sided shadows on the outer rings; can the cooling of the gas as it crosses the shadows bring constraints on its mass? The finite cooling time-scale should result in dust temperature decrements shifted ahead of the optical/IR shadows in the direction of rotation. However, some systems show temperature drops, while others do not. The depth of the drops and the amplitude of the shift depend on the outer disc surface density Σ through the extent of cooling during the shadow crossing time, and also on the efficiency of radiative diffusion. These phenomena may bear observational counterparts, which we describe with a simple 1D model. An application to the HD 142527 disc suggests an asymmetry in its shadows, and predicts a ≳10 deg shift for a massive gaseous disc, with peak Σ > 8.3 g cm−2. Another application to the DoAr 44 disc limits the peak surface density to Σ < 13 g cm−2.

Pebble accretion in self-gravitating protostellar discs

Abstract Pebble accretion has become a popular component to core accretion models of planet formation, and is especially relevant to the formation of compact, resonant terrestrial planetary systems. Pebbles initially form in the inner protoplanetary disc, sweeping outwards in a radially expanding front, potentially forming planetesimals and planetary cores via migration and the streaming instability. This pebble front appears at early times, in what is typically assumed to be a low-mass disc. We argue this picture is in conflict with the reality of young circumstellar discs, which are massive and self-gravitating. We apply standard pebble accretion and streaming instability formulae to self-gravitating protostellar disc models. Fragments will open a gap in the pebble disc, but they will likely fail to open a gap in the gas, and continue rapid inward migration. If this does not strongly perturb the pebble disc, our results show that disc fragments will accrete pebbles efficiently. We find that in general the pebble-to-gas-density ratio fails to exceed 0.01, suggesting that the streaming instability will struggle to operate. It may be possible to activate the instability if 10 cm grains are available, and spiral structures can effectively concentrate them in regions of low gravito-turbulence. If this occurs, lunar mass cores might be assembled on time-scales of a few thousand years, but this is likely to be rare, and is far from proven. In any case, this work highlights the need for study of how self-gravitating protostellar discs define the distribution and properties of solid bodies, for future planet formation by core accretion.

Protoplanetary Disk Sizes and Angular Momentum Transport

Abstract In young circumstellar disks, accretion—the inspiral of disk material onto the central star—is important for both the buildup of stellar masses and the outcome of planet formation. Although the existence of accretion is well documented, understanding the angular momentum transport mechanism that enables disk accretion has proven to be an enduring challenge. The leading theory to date, the magnetorotational instability, which redistributes angular momentum within the disk, is increasingly questioned, and magnetothermal disk winds, which remove angular momentum from the disk, have emerged as an alternative theoretical solution. Here we investigate whether measurements of disk radii can provide useful insights into which, if either, of these mechanisms drives disk accretion, by searching for evidence of viscous spreading in gaseous disks, a potential signature of “in-disk” angular momentum transport. We find that the large sizes of most Class II (T Tauri) gas disks compared to those of their earlier evolutionary counterparts, Class I gas disks, are consistent with expectations for viscous spreading in the Class II phase. There is, however, a large spread in the sizes of Class II gas disks at any age, including a population of very small Class II gas disks. Their small sizes may result from processes such as photoevaporation, disk winds, or truncation by orbiting low-mass companions.

Discovery of a planetary-mass companion within the gap of the transition disk around PDS 70

Context. Young circumstellar disks are the birthplaces of planets. Their study is of prime interest to understand the physical and chemical conditions under which planet formation takes place. Only very few detections of planet candidates within these disks exist, and most of them are currently suspected to be disk features. Aims. In this context, the transition disk around the young star PDS 70 is of particular interest, due to its large gap identified in previous observations, indicative of ongoing planet formation. We aim to search for the presence of an embedded young planet and search for disk structures that may be the result of disk–planet interactions and other evolutionary processes. Methods. We analyse new and archival near-infrared images of the transition disk PDS 70 obtained with the VLT/SPHERE, VLT/NaCo, and Gemini/NICI instruments in polarimetric differential imaging and angular differential imaging modes. Results. We detect a point source within the gap of the disk at about 195 mas (~22 au) projected separation. The detection is confirmed at five different epochs, in three filter bands and using different instruments. The astrometry results in an object of bound nature, with high significance. The comparison of the measured magnitudes and colours to evolutionary tracks suggests that the detection is a companion of planetary mass. The luminosity of the detected object is consistent with that of an L-type dwarf, but its IR colours are redder, possibly indicating the presence of warm surrounding material. Further, we confirm the detection of a large gap of ~54 au in size within the disk in our scattered light images, and detect a signal from an inner disk component. We find that its spatial extent is very likely smaller than ~17 au in radius, and its position angle is consistent with that of the outer disk. The images of the outer disk show evidence of a complex azimuthal brightness distribution which is different at different wavelengths and may in part be explained by Rayleigh scattering from very small grains. Conclusions. The detection of a young protoplanet within the gap of the transition disk around PDS 70 opens the door to a so far observationally unexplored parameter space of planetary formation and evolution. Future observations of this system at different wavelengths and continuing astrometry will allow us to test theoretical predictions regarding planet–disk interactions, planetary atmospheres, and evolutionary models.

How to identify magnetic field activity in young circumstellar disks

AbstractRecent advanced simulations of protoplanetary disks allow us to search for observational constraints to identify the magnetic field activity in protoplanetary disks. With our 3D radiation non-ideal magneto-hydrodynamical (MHD) models including irradiation from an Herbig type star we are able to model the thermal and dynamical evolution in a so far never reached detail (Flock et al. 2017). The activity of the magneto-rotational instability in the inner hot ionized regions comes along with a magnetic dynamo. The oscillations in the mean toroidal magnetic field with a timescale of 10 local orbits can slightly bend the inner dust rim and so the irradiation surface. This causes a clear variability pattern in the near infrared (NIR) emission at the dust inner rim surface. Another way to identify the presence of magnetic fields are to search for polarization signatures. Using 3D non-ideal MHD simulations of the outer disk regions (Flock et al. 2015) we calculate synthetic images of the intrinsically polarized continuum emitted by aspherical grain aligned with the dominantly toroidal magnetic field (Bertrang et al. 2017). Our results show a clear radial polarization pattern for face-on observed disk, similar to recent observations by Ohashi et al. (2018). Additionally, we are even able to see the change of the polarization pattern inside the vortex as the poloidal magnetic field dominates therein.

Evidence for the start of planet formation in a young circumstellar disk

The growth of dust grains in protoplanetary disks is a necessary first step towards planet formation. This growth has been inferred via observations of thermal dust emission towards mature protoplanetary systems (age >2 million years) with masses that are, on average, similar to Neptune3. In contrast, the majority of confirmed exoplanets are heavier than Neptune. Given that young protoplanetary disks are more massive than their mature counterparts, this suggests that planet formation starts early, but evidence for grain growth that is spatially and temporally coincident with a massive reservoir in young disks remains scarce. Here, we report observations on a lack of emission of carbon monoxide isotopologues within the inner ~15 au of a very young (age ~100,000 years) disk around the Solar-type protostar TMC1A. By using the absence of spatially resolved molecular line emission to infer the gas and dust content of the disk, we conclude that shielding by millimeter-size grains is responsible for the lack of emission. This suggests that grain growth and millimeter-size dust grains can be spatially and temporally coincident with a mass reservoir sufficient for giant planet formation. Hence, planet formation starts during the earliest, embedded phases in the life of young stars.

On the inner disc structure of MWC480: evidence for asymmetries?

Studying the physical conditions structuring the young circumstellar disks is required for understanding the onset of planet formation. Of particular interest is the protoplanetary disk surrounding the Herbig star MWC480. The structure and properties of the circumstellar disk of MWC480 are studied by infrared interferometry and interpreted from a modeling approach. New observations are driving this study, in particular some recent Very Large Telescope Interferometer (VLTI)/MIDI data acquired in December 2013. Our one-component disk model could not reproduce simultaneously all our data: the Spectral Energy Distribution, the near-infrared Keck Interferometer data and the mid-infrared data obtained with the MIDI instrument. In order to explain all measurements, one possibility is to add an asymmetry in our one-component disk model with the assumption that the structure of the disk of MWC480 has not varied with time. Several scenarios are tested, and the one considering the presence of an azimuthal bright feature in the inner component of the disk model provides a better fit of the data. (In this study, we assumed that the size of the outer disk of MWC480 to be 20 au since most of the near and mid-IR emissions come from below 20 au. In our previous study (Jamialahmadi et al. 2015), we adopted an outer radius of 80 au, which is consistent with the value found by previous studies based on mm observations).

A Resolved and Asymmetric Ring of PAHs within the Young Circumstellar Disk of IRS 48

Abstract For one decade, the spectral type and age of the ρ Oph object IRS-48 were subject to debate and mystery. Modeling its disk with mid-infrared to millimeter observations led to various explanations to account for the complex intricacy of dust holes and gas-depleted regions. We present multi-epoch high-angular-resolution interferometric near-infrared data of spatially resolved emissions in the first 15 au of IRS-48, known to have very strong polycyclic aromatic hydrocarbon (PAH) emissions within this dust-depleted region. We make use of new Sparse-Aperture-Masking data to instruct a revised radiative-transfer model, where spectral energy distribution fluxes and interferometry are jointly fitted. Neutral and ionized PAH, very small grains (VSG), and classical silicates are incorporated into the model; new stellar parameters and extinction laws are explored. A bright (42 L ⊙) and hence large (2.5 R ⊙) central star with A v = 12.5 mag and R v = 6.5 requires less near-infrared excess: the inner-most disk at ≈1 au is incompatible with the interferometric data. The revised stellar parameters place this system on a 4 Myr evolutionary track, four times younger than the previous estimations, which is in better agreement with the surrounding ρ Oph region and disk-lifetime observations. The disk-structure solution converges to a classical-grain outer disk from 55 au combined with an unsettled and fully resolved VSG and PAH ring, between 11 and 26 au. We find two overluminosities in the PAH ring at color-temperatures consistent with the radiative transfer simulations; one follows a Keplerian circular orbit at 14 au. We show a depletion of a factor of ≈5 of classical dust grains up to 0.3 mm compared to very small particles: the IRS-48 disk is nearly void of dust grains in the first 55 au. A 3.5 M Jup planet on a 40 au orbit can qualitatively explain the new disk structure.

Grand-design Spiral Arms in a Young Forming Circumstellar Disk

Abstract We study formation and long-term evolution of a circumstellar disk in a collapsing molecular cloud core using a resistive magnetohydrodynamic simulation. While the formed circumstellar disk is initially small, it grows as accretion continues, and its radius becomes as large as 200 au toward the end of the Class-I phase. A pair of grand-design spiral arms form due to gravitational instability in the disk, and they transfer angular momentum in the highly resistive disk. Although the spiral arms disappear in a few rotations as expected in a classical theory, new spiral arms form recurrently as the disk, soon becoming unstable again by gas accretion. Such recurrent spiral arms persist throughout the Class-0 and I phases. We then perform synthetic observations and compare our model with a recent high-resolution observation of a young stellar object Elias 2–27, whose circumstellar disk has grand-design spiral arms. We find good agreement between our theoretical model and the observation. Our model suggests that the grand-design spiral arms around Elias 2–27 are consistent with material arms formed by gravitational instability. If such spiral arms commonly exist in young circumstellar disks, it implies that young circumstellar disks are considerably massive and gravitational instability is the key process of angular momentum transport.

Gaps, rings, and non-axisymmetric structures in protoplanetary disks: Emission from large grains

Dust grains with sizes around (sub)mm are expected to couple only weakly to the gas motion in regions beyond 10 au of circumstellar disks. In this work, we investigate the influence of the spatial distribution of such grains on the (sub)mm appearance of magnetized protoplanetary disks. We perform non-ideal global 3D magneto-hydrodynamic stratified disk simulations including particles of different sizes (50 micron to 1 cm), using a Lagrangian particle solver. We calculate the spatial dust temperature distribution, including the dynamically coupled submicron-sized dust grains, and derive ideal continuum re-emission maps of the disk through radiative transfer simulations. Finally, we investigate the feasibility to observe specific structures in the thermal re-emission maps with the Atacama Large Millimeter/submillimeter Array (ALMA). The pressure bump close to the outer edge of the dead-zone leads to particle trapping in ring structures. More specifically, vortices in the disk concentrate the dust and create an inhomogeneous distribution of the solid material in the azimuthal direction. The large-scale disk perturbations are preserved in the (sub)mm re-emission maps. The observable structures are very similar to those expected to result from planet-disk interaction. The larger dust particles increase the brightness contrast between gap and ring structures. We find that rings, gaps and the dust accumulation in the vortex could be traced with ALMA down to a scale of a few astronomical units in circumstellar disks located in nearby star-forming regions. Finally, we present a brief comparison of these structures with those recently found with ALMA in the young circumstellar disks of HL Tau and Oph IRS 48.

Observations of Solids in Protoplanetary Disks

This review addresses the state of research that employs astronomical (remote sensing) observations of solids (“dust”) in young circumstellar disks to learn about planet formation. The intention is for it to serve as an accessible, introductory, pedagogical resource for junior scientists interested in the subject. After some historical background and a basic observational primer, the focus is shifted to the three fundamental topics that broadly define the field: (1) demographics—the relationships between disk properties and the characteristics of their environments and hosts; (2) structure—the spatial distribution of disk material and its associated physical conditions and composition; and (3) evolution—the signposts of key changes in disk properties, including the growth and migration of solids and the impact of dynamical interactions with young planetary systems. Based on the state-of-the-art results in these areas, suggestions are made for potentially fruitful lines of work in the near future.

Growth of a Protostar and a Young Circumstellar Disk with a High Mass-Accretion Rate onto the Disk

Abstract The growing process of both a young protostar and a circumstellar disk is investigated. The viscous evolution of a disk around a single star is considered with a model where a disk increases its mass by a dynamically accreting envelope, and simultaneously loses its mass via viscous accretion onto the central star. We focus on a circumstellar disk with a high mass-accretion rate onto the disk, $\dot{M} =$ 8.512 $c_{\rm s}^3/G$, as the result of a dynamical collapse of the rotating molecular cloud core. We consider the origin of the surface density distribution as well as that of the disk-to-star mass ratio by means of numerical calculations of an unsteady viscous accretion disk in a one-dimensional axisymmetric model. It is shown that the radial profiles of the surface density, $\Sigma$, azimuthal velocity, $v_{\phi}$, and mass-accretion rate, $\dot{M}$, in the inner region approach to the quasi-steady state. The profile of the surface density distribution in a quasi-steady state is determined as to be a result of angular momentum transport, rather than its original distribution of angular momentum in the cloud core. It is also shown that the disk mass becomes larger than the central star in the long-time limit as long as temporally constant mass flux onto the disk is assumed. After the mass infall rate onto the disk declines owing to a depletion of the parent cloud core, the disk-to-star mass ratio, $M_{\rm disk}/M_*$, decreases. The disk-to-star mass ratio becomes smaller than unity after $t \gt 10^{5}$ yr and $t \gt 10^{6}$ yr from the beginning of the accretion phase in the case with $\alpha_{0} =$ 1 and 0.1, respectively, where $\alpha_{0}$ is the constant part of the viscous parameter. In the case with $\alpha_{0} \leq 10^{-2}, M_{\rm disk}/M_*$ is still larger than unity at 2 Myr from the beginning of the accretion phase.

Detection of a Proto-planetary Clump in the Habitable Zone of GM Cephei

AbstractGM Cephei is an active T Tauri star in the young open cluster Trumpler 37, showing abrupt UX Orionis type of photometric variability. Its light curves exhibit frequent, sporadic brightening events, each of <0.5 mag and lasting for days, which must have been originated from unsteady circumstellar accretion. In addition, the star undergoes a brightness drop up to ~1 mag lasting for about a month, during which the star became bluer when fainter. Moreover, the brightness drops seem to have a recurrence timescale of about 300 days. It is proposed that the brightness drop arises from obscuration of the central star by an orbiting dust concentration, exemplifying disk inhomogeneity in transition between grain coagulation and planetesimal formation in a young circumstellar disk. GM Cep was found to show a few percent polarization in the optical wavelengths, and an enhanced level of polarization during the occultation phase.

A POSSIBLE DETECTION OF OCCULTATION BY A PROTO-PLANETARY CLUMP IN GM Cephei

GM Cep in the young (~4 Myr) open cluster Trumpler 37 has been known to be an abrupt variable and to have a circumstellar disk with very active accretion. Our monitoring observations in 2009-2011 revealed the star to show sporadic flare events, each with brightening of < 0.5 mag lasting for days. These brightening events, associated with a color change toward the blue, should originate from an increased accretion activity. Moreover, the star also underwent a brightness drop of ~1 mag lasting for about a month, during which the star became bluer when fainter. Such brightness drops seem to have a recurrence time scale of a year, as evidenced in our data and the photometric behavior of GM Cep over a century. Between consecutive drops, the star brightened gradually by about 1 mag and became blue at peak luminosity. We propose that the drop is caused by obscuration of the central star by an orbiting dust concentration. The UX Orionis type of activity in GM Cep therefore exemplifies the disk inhomogeneity process in transition between grain coagulation and planetesimal formation in a young circumstellar disk.