Circumstellar Dust Research Articles

Mid-infrared evolution of η Carinae from 1968 to 2018

η Car is one of the most luminous and massive stars in our Galaxy and is the brightest mid-IR source in the sky outside our solar system. Since the late 1990s, the central source has dramatically brightened at UV and optical wavelengths. This might be explained by a decrease in circumstellar dust extinction. We aim to establish the mid-IR flux evolution and further our understanding of the star’s UV and optical brightening. Mid-IR images from 8−20 μm were obtained in 2018 with VISIR at the Very Large Telescope. Archival data from 2003 and 2005 were retrieved from the ESO Science Archive Facility, and historical records were collected from publications. We present mid-IR images of η Car with the highest angular resolution to date at the corresponding wavelengths (≥0.22″). We reconstruct the mid-IR evolution of the spectral energy distribution of the spatially integrated Homunculus nebula from 1968 to 2018 and find no long-term changes. The bolometric luminosity of η Car has been stable over the past five decades. We do not observe a long-term decrease in the mid-IR flux densities that could be associated with the brightening at UV and optical wavelengths, but circumstellar dust must be declining in our line of sight alone. Short-term flux variations within about 25% of the mean levels could be present.

The Cold and Dusty Circumstellar Matter around Fast-expanding Type Ia Supernovae

Abstract SNe Ia play key roles in revealing the accelerating expansion of the universe, but our knowledge of their progenitors is still very limited. Here we report the discovery of a rigid dichotomy in circumstellar (CS) environments around two subclasses of SNe Ia as defined by their distinct photospheric velocities. For the SNe Ia with high photospheric velocities (HVs), we found a significant excess flux in blue light 60–100 days past maximum, while this phenomenon is absent for SNe with normal photospheric velocity. This blue excess can be attributed to light echoes by circumstellar dust located at a distance of about (1–2) × 1017 cm from the HV subclass. Moreover, we also found that the HV SNe Ia show systematically evolving Na i absorption line by performing a systematic search of variable Na i absorption lines in spectra of all SNe Ia, whereas this evolution is rarely seen in normal ones. The evolving Na i absorption can be modeled in terms of photoionization model, with the location of the gas clouds at a distance of about 2 × 1017 cm, in striking agreement with the location of CS dust inferred from B-band light-curve excess. These observations show clearly that the progenitors of HV subclass are likely from single-degenerate progenitor system (i.e., symbiotic binary), while the NV subclass may arise from double-degenerate system.

The Duration of Star Formation in Galactic Giant Molecular Clouds. I. The Great Nebula in Carina

Abstract We present a novel infrared spectral energy distribution (SED) modeling methodology that uses likelihood-based weighting of the model fitting results to construct probabilistic Hertzsprung–Russell diagrams (pHRD) for X-ray-identified, intermediate-mass (2–8 M ⊙), pre-main-sequence young stellar populations. This methodology is designed specifically for application to young stellar populations suffering strong, differential extinction (ΔA V > 10 mag), typical of Galactic massive star-forming regions. We pilot this technique in the Carina Nebula Complex (CNC) by modeling the 1–8 μm SEDs of 2269 likely stellar members that exhibit no excess emission from circumstellar dust disks at 4.5 μm or shorter wavelengths. A subset of ∼100 intermediate-mass stars in the lightly obscured Trumpler 14 and 16 clusters have available spectroscopic T eff, measured from the Gaia-ESO survey. We correctly identify the stellar temperature in 85% of cases, and the aggregate pHRD for all sources returns the same peak in the stellar age distribution as obtained using the spectroscopic T eff. The SED model parameter distributions of stellar mass and evolutionary age reveal significant variation in the duration of star formation among four large-scale stellar overdensities within the CNC and a large distributed stellar population. Star formation began ∼10 Myr ago and continues to the present day, with the star formation rate peaking ≲3 Myr ago when the massive Trumpler 14 and 16 clusters formed. We make public the set of 100,000 SED models generated from standard pre-main-sequence evolutionary tracks and our custom software package for generating pHRDs and mass–age distributions from the SED fitting results.

GG Tauri A: dark shadows on the ringworld

Context. With its high complexity, large size, and close distance, the ringworld around GG Tau A is an appealing case to study the formation and evolution of protoplanetary disks around multiple star systems. However, investigations with radiative transfer models usually neglect the influence of the circumstellar dust around the individual stars. Aims. We investigate how circumstellar disks around the stars of GG Tau A influence the emission that is scattered at the circumbinary disk and if constraints on these circumstellar disks can be derived. Methods. We performed radiative transfer simulations with the POLArized RadIation Simulator (POLARIS) to obtain spectral energy distributions and emission maps in the H-Band (near-infrared). Subsequently, we compared them with observations to achieve our aims. Results. We studied the ratio of polarized intensity at different locations in the circumbinary disk. We conclude that the observed scattered-light near-infrared emission is best reproduced if the circumbinary disk lies in the shadow of at least two coplanar circumstellar disks surrounding the central stars. This implies that the inner wall of the circumbinary disk is strongly obscured around the midplane, while the observed emission is actually dominated by the upper-most disk layers. In addition, the inclined dark lane (“gap”) on the western side of the circumbinary disk, which has been a stable, nonrotating, feature for approximately 20 yr, can only be explained by the self-shadowing of a misaligned circumstellar disk surrounding one of the two components of the secondary close-binary star GG Tau Ab.

The 2018 eruption and long term evolution of the new high-mass Herbig Ae/Be object Gaia-18azl = VES 263

Abstract We have been monitoring, at high cadence, the photometric and spectroscopic evolution of VES 263 following the discovery in 2018 of a brightening labeled as event Gaia-18azl. VES 263 is so far a neglected emission-line object discovered in the 1960s on objective prism plates, tentatively classified as a semi-regular AGB cool giant by automated analysis of ASASSN lightcurves. We have discovered that VES 263 is a bonafide massive pre-Main Sequence object (∼12 M⊙), of the Herbig AeBe type. It is located at 1.68±0.07 kpc distance, within the Cyg OB2 star-forming region, and it is highly reddened (EB − V=1.80±0.05) by interstellar extinction. In quiescence, the spectral energy distribution is dominated by the ∼20,000 K photospheric emission from the central B1II star, and at λ≥6μm by emission from circumstellar warm dust (T≤400○K). The 2018-19 eruption was caused by a marked brightening of the accretion disk around the B1II star as traced by the evolution with time of the integrated flux and the double-peaked profile of emission lines. At the peak of the eruption, the disk has a bulk temperature of ∼7500 K and a luminosity L ≥860 L⊙, corresponding to a mass accretion rate M≥1.1 × 10−5 M⊙ yr−1. Spectroscopic signature of possible bipolar jets (at −700 and +700 km s−1) of variable intensity are found. We have reconstructed from Harvard, Moscow and Sonneberg photographic plates the photometric history of VES 263 from 1896 to 1995.

Discovery of an Intermediate-luminosity Red Transient in M51 and Its Likely Dust-obscured, Infrared-variable Progenitor

Abstract We present the discovery of an optical transient (OT) in Messier 51, designated M51 OT2019-1 (also ZTF 19aadyppr, AT 2019abn, ATLAS19bzl), by the Zwicky Transient Facility (ZTF). The OT rose over 15 days to an observed luminosity of M r = −13 (νL ν = 9 × 106 L ⊙), in the luminosity gap between novae and typical supernovae (SNe). Spectra during the outburst show a red continuum, Balmer emission with a velocity width of ≈400 km s−1, Ca ii and [Ca ii] emission, and absorption features characteristic of an F-type supergiant. The spectra and multiband light curves are similar to the so-called “SN impostors” and intermediate-luminosity red transients (ILRTs). We directly identify the likely progenitor in archival Spitzer Space Telescope imaging with a 4.5 μm luminosity of M [4.5] ≈ −12.2 mag and a [3.6]–[4.5] color redder than 0.74 mag, similar to those of the prototype ILRTs SN 2008S and NGC 300 OT2008-1. Intensive monitoring of M51 with Spitzer further reveals evidence for variability of the progenitor candidate at [4.5] in the years before the OT. The progenitor is not detected in pre-outburst Hubble Space Telescope optical and near-IR images. The optical colors during outburst combined with spectroscopic temperature constraints imply a higher reddening of E(B − V) ≈ 0.7 mag and higher intrinsic luminosity of M r ≈ −14.9 mag (νL ν = 5.3 × 107 L ⊙) near peak than seen in previous ILRT candidates. Moreover, the extinction estimate is higher on the rise than on the plateau, suggestive of an extended phase of circumstellar dust destruction. These results, enabled by the early discovery of M51 OT2019-1 and extensive pre-outburst archival coverage, offer new clues about the debated origins of ILRTs and may challenge the hypothesis that they arise from the electron-capture induced collapse of extreme asymptotic giant branch stars.

GK Car and GZ Nor: two low-luminous, depleted RV Tauri stars

ABSTRACT We performed a photometric and spectroscopic analysis of two RV Tauri stars: GK Car and GZ Nor. Both objects are surrounded by hot circumstellar dust. Their pulsation periods, derived from ASAS (All Sky Automated Survey) photometric time-series, have been used to derive their luminosities and distances via the period–luminosity–distance (PLC) relation. In addition, for both objects, Gaia distances are available. The Gaia distances and luminosities are consistent with the values obtained from the PLC relationship. GK Car is at distance of 4.5 ± 1.3 kpc and has a luminosity of 1520 ± 840 L⊙, while GZ Nor is at distance of 8.4 ± 2.3 kpc and has a luminosity of 1240 ± 690 L⊙. Our abundance analysis reveals that both stars show depletion of refractory elements with [Fe/H] = −1.3 and [Zn/Ti] = +1.2 for GK Car and [Fe/H] = −2.0 and [Zn/Ti] = +0.8 for GZ Nor. In the WISE(Wide-Field Infrared Survey Explorer) colour–colour diagram, GK Car is located in the RV Tauri box as originally defined by Lloyd Evans and updated by Gezer et al., while GZ Nor is not. Despite this, we argue that both objects are surrounded by a gravitationally bound disc. As depletion is observed in binaries, we postulate that both stars are binaries as well. RV Tauri stars are generally acknowledged to be post-asymptotic giant branch (post-AGB) stars. Recent studies show that they might be either indeed post-AGB or post-red giant branch (post-RGB) objects depending on their luminosity. For both objects, the derived luminosities are relatively low for post-AGB objects, however, the uncertainties are quite large. We conclude that they could be either post-RGB or post-AGB objects.

Planetesimals around stars with TESS (PAST) – I. Transient dimming of a binary solar analogue at the end of the planet accretion era

ABSTRACT We report detection of quasi-periodic (1.5-d) dimming of HD 240779, the solar-mass primary in a 5 arcsec visual binary (also TIC 284730577), by the Transiting Exoplanet Survey Satellite. This dimming, as has been shown for other ‘dipper’ stars, is likely due to occultation by circumstellar dust. The barycentric space motion, lithium abundance, rotation, and chromospheric emission of the stars in this system point to an age of ≈125 Myr, and possible membership in the AB Doradus moving group. As such it occupies an important but poorly explored intermediate regime of stars with transient dimming between young stellar objects in star-forming regions and main-sequence stars, and between UX Orionis-type Ae/Be stars and M-type ‘dippers’. HD 240779, but not its companion BD+10 714B, has Wide-field Infrared Survey Explorer (WISE)-detected excess infrared emission at 12 and 22 μm indicative of circumstellar dust. We propose that infrared emission is produced by collisions of planetesimals during clearing of a residual disc at the end of rocky planet formation, and that quasi-periodic dimming is produced by the rapid disintegration of a ≳100 km planetesimal near the silicate evaporation radius. Further studies of this and similar systems will illuminate a poorly understood final phase of rocky planet formation like that which produced the inner Solar system.

2018 Nier Prize for Aki Takigawa

The Nier Prize of the Meteoritical Society recognizes early career members of the Society whose achievements extend the frontiers of meteoritics. The 2019 Nier Prize winner, Dr. Aki Takigawa, is a true scientific pioneer, who has extended meteoritics across multiple disciplinary boundaries. She began her research as an undergraduate researcher in the lab of Dr. Shogo Tachibana at the University of Tokyo in 2006. Her first project was to develop a model to explain the abundances of short-lived radionuclides in the early solar system, which not only led to a first author publication (Takigawa et al. 2008), but remains in use today (Wasserburg et al. 2017). Notably, even in this first project, she made connections with researchers in a related field, in this case astrophysicists working on supernova nucleosynthesis, so that she could draw on cross-disciplinary knowledge to make progress on an important problem in meteoritics. For her second undergraduate project, Aki worked with both Dr. Tachibana and Dr. Hiroko Nagahara. She again considered important questions about early solar system materials that link astronomy and meteoritics, this time using the effects of shape anisotropy on the infrared spectra of forsterite crystals to constrain the temperature and gas conditions of circumstellar grain condensation, similar to the presolar grain building blocks of our solar system. To complete this project, she performed laboratory experiments on the anisotropic evaporation of forsterite, measured the optical properties of the heated grains, and applied the results to the interpretation of astronomical spectra (Takigawa et al. 2009). This work ultimately became the basis of her master’s thesis, and is an important scientific milestone. The results are among the first demonstrations that the features of individual mineral grains can be used to constrain their condensation histories, providing a potential link from presolar silicates found in meteorites to the conditions of the circumstellar envelopes in which they formed (Takigawa and Tachibana 2012). For her PhD thesis also under the guidance of Dr. Tachibana, Dr. Takigawa turned from silicates to oxides, focusing on corundum, which similar to forsterite, exhibits crystal anisotropy effects in infrared spectra that can be linked to condensation conditions. This anisotropy is important for interpreting the features of some O-rich Red Giant stars, because it can explain the relative intensities of the “13-micron” and “22-micron” features as being consistent with variations in the shape of corundum grains, and thus settled a debate among astronomers about which mineral to assign the 13-micron feature (Takigawa et al. 2015). At the end of her PhD, Dr. Takigawa was awarded a Japanese Society for the Promotion of Science Ikushi Prize, one of only 20 awarded across all academic fields for the entire country in 2012. This prestigious postdoctoral fellowship provided 3 years of support, including 18 months to be spent conducting research outside Japan. Most of these 18 months were spent in Washington, D.C., working jointly at the Carnegie Institution of Washington with Drs. Larry Nittler and Conel Alexander, and in my laboratory at the Naval Research Laboratory. During that time, Dr. Takigawa expanded her work on presolar Al2O3 to include oxygen isotope measurements with secondary ion mass spectrometry, transmission electron microscopy, cathodoluminescence, and electron backscatter diffraction (Takigawa et al. 2018). Working directly with Dr. Takigawa, I was greatly impressed with her drive to learn to make each of the measurements herself, and to build a comprehensive understanding of her samples that could be used to draw broad conclusions linking cosmochemistry, astromineralogy, materials science, and astronomy. Furthermore, during her postdoctoral fellowship, she also developed a proposal for time on the newly commissioned ALMA telescope, in order to directly measure circumstellar dust features in O-rich stars that could link to her modeling and laboratory studies. Despite the intense competition for telescope time, her proposal was selected and resulted in two published papers and one in press from her observations of silicate and alumina dust (Vlemmings et al. 2017; Takigawa et al. 2017; Tachibana et al. 2019). Now an assistant professor at the Hakubi Center for Advanced Research at Kyoto University, Dr. Takigawa is a co-investigator on the NASA CAESAR mission proposal, and has begun laboratory studies looking at the interactions between amorphous silicates and cometary ice analogs. Dr. Takigawa's unique synthesis of laboratory experiments, coordinated microanalysis of meteoritic materials, astronomical observations, and modeling make her an outstanding example of the interdisciplinary nature of meteoritical science. Her results have greatly improved our understanding of the formation of refractory dust in stars and the early solar system. She has worked as an international and intellectual ambassador for the meteoritic community. It is a great honor to present her as the 2019 Nier Prize awardee.

Variable dust emission by WC type Wolf–Rayet stars observed in the NEOWISE-R survey

ABSTRACT Photometry at 3.4 and 4.6 ${\mu m}$ of 128 Population I WC type Wolf–Rayet stars in the Galaxy and 12 in the Large Magellanic Cloud (LMC) observed in the WISE NEOWISE-R survey was searched for evidence of circumstellar dust emission and its variation. Infrared spectral energy distributions (SEDs) were assembled, making use of archival r, i, Z, and Y photometry to determine reddening and stellar wind levels for the WC stars found in recent IR surveys and lacking optical photometry. From their SEDs, 10 apparently non-variable stars were newly identified as dust makers, including three, WR 102-22, WR 110-10, and WR 124-10, having subtype earlier than WC8–9, the first such stars to show this phenomenon. The 11 stars found to show variable dust emission include six new episodic dust makers, WR 47c, WR 75-11, WR 91-1, WR 122-14, and WR 125-1 in the Galaxy and HD 38030 in the LMC. Of previously known dust makers, NEOWISE-R photometry of WR 19 captured its rise to maximum in 2018 confirming the 10.1-yr period, that of WR 125 the beginning of a new episode of dust formation suggesting a period near 28.3 yr. while that of HD 36402 covered almost a whole period and forced revision of it to 5.1 yr.

ROAD† and K2 photometry of V348 Sgr: probing the pulsation dust connection

ABSTRACTV348 Sgr is simultaneously an active hot R Coronae Borealis (RCB) variable, a peculiar extreme helium star, and the hydrogen-deficient central star of a planetary nebula. Explaining the RCB-type variability has been difficult since the star spends much of its time at minimum light. We present new ground-based multicolour photometry covering five observing seasons and 80 d of continuous photometry from space. The latter demonstrate small-amplitude (<0.01 mag) variability at maximum light on time-scales typical for strange-mode pulsation in hot helium supergiants. These could provide a trigger for frequent dust-production episodes; other mechanisms must also be considered. Multicolour photometry probes the reddening properties of extinction events from minimum to maximum light. The latter are comparable with extinction events due to carbonaceous grains seen in cooler RCB stars. Minimal reddening at minimum light is indicative that starlight scattered from circumstellar dust into the line of sight dominates transmitted light.

Uncovering Red and Dusty Ultraluminous X-Ray Sources with Spitzer

Abstract We present a mid-infrared (IR) sample study of nearby ultraluminous X-ray sources (ULXs) using multiepoch observations with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. Spitzer/IRAC observations taken after 2014 were obtained as part of the Spitzer Infrared Intensive Transients Survey. Our sample includes 96 ULXs located within 10 Mpc. Of the 96 ULXs, 12 have candidate counterparts consistent with absolute mid-IR magnitudes of supergiants, and 16 counterparts exceeded the mid-IR brightness of single supergiants and are thus more consistent with star clusters or non-ULX background active galactic nuclei. The supergiant candidate counterparts exhibit a bimodal color distribution in a Spitzer/IRAC color–magnitude diagram, where “red” and “‘blue” ULXs fall in IRAC colors [3.6] – [4.5] ∼ 0.7 and [3.6] – [4.5] ∼ 0.0, respectively. The mid-IR colors and absolute magnitudes of four “red” and five “blue” ULXs are consistent with those of supergiant B[e] (sgB[e]) and red supergiant (RSG) stars, respectively. Although “blue,” RSG-like mid-IR ULX counterparts likely host RSG mass donors; we propose that “red” counterparts are ULXs exhibiting the “B[e] phenomenon” rather than hosts of sgB[e] mass donors. We show that the mid-IR excess from the “red” ULXs is likely due to thermal emission from circumstellar or circumbinary dust. Using dust as a probe for total mass, we estimate mass-loss rates of M ˙ ∼ 1 × 10 − 4 M ⊙ yr−1 in dust-forming outflows of red ULXs. Based on the transient mid-IR behavior and its relatively flat spectral index, α = −0.19 ± 0.1, we suggest that the mid-IR emission from Holmberg IX X-1 originates from a variable jet.

Influence of the Disk Wind on the Intrinsic Polarization of Young Stars

The behavior of the linear polarization parameters of UX Ori stars during their eclipses by circumstellar dust clouds is studied. A circumstellar disk with a disk wind creating a puffing in the dust sublimation zone is considered. We show that the disk puffing can strongly affect the degree of polarization and color index of the star during its eclipse. A strong wind can change the orientation of the plane of linear polarization. The scattered radiation from a thin disk is polarized perpendicularly to its plane, but the radiation from a disk with a strong wind can be polarized along the disk plane. A situation where the disk-scattered radiation is not polarized in a certain spectral band is possible owing to the disk puffing. There can be different orientations of the linear polarization of the disk radiation in different spectral bands.

Questions about the evolution of ices, from diffuse molecular clouds to comets

AbstractThe surfaces of interstellar and circumstellar dust grains are the sites of molecule formation, most of which, except H2, stick and form ice mantles. The study of ice evolution thus seems directly relevant for understanding our own origins, although the relation between interstellar and solar system ices remains a key question. The comparison of interstellar and solar system ices relies evidently on an accurate understanding of the composition and processes in both environments. With the accurate in situ measurements available for the comet 67P/Churyumov-Gerasimenko with the Rosetta mission, improving our understanding of interstellar ices is the more important. Here, I will address three specific questions. First, while laboratory experiments have made much progress in understanding complex organic molecule (COM) formation in the ices, the question remains, how does COM formation depend on environment and time? Second, what is the carrier of sulfur in the ices? And third, can ice absorption bands trace the processing history of the ices? Laboratory experiments, ranging from infrared spectroscopy to identify interstellar ice species, to surface experiments to determine reaction parameters in ice formation scenarios, to heating and irradiation experiments to simulate space environments, are essential to address these questions and analyze the flood of new observational data that will become available with new facilities in the next 2-10 years.

Variable Circumstellar Extinction in UX Ori Stars of Late Spectral Types: V695 Per

Eclipses of young stars by dust fragments in protoplanetary disks are the main reason for stochastic variability in the luminosity of UX Ori stars. In analyses and modeling of these kinds of eclipses, the star is often treated as a point light source. This approach is fully justified in the case of hot stars in this family, since the sublimation radius for the dust in their circumstellar disk is much greater than the star’s radius. This approach may not apply to the case of cold stars, since the evaporation zone for the circumstellar dust is close to the star. Here it is shown that accounting for the finite size of the radiation source leads to a change in the shape of a color track on a “color-magnitude” diagram. Calculations for the star V695 Per simultaneously illustrate the variability of the brightness of UX Ori and AA Tau stars for the coldest currently known star of this type.

The Breakthrough Listen Search for Intelligent Life: Searching Boyajian's Star for Laser Line Emission

Boyajian's Star (KIC 8462852) has received significant attention, due to its unusual periodic brightness fluctuations detected by the Kepler spacecraft and subsequent ground-based observations. Possible explanations for the dips in the photometric measurements include interstellar or circumstellar dust, and it has been speculated that an artificial megastructure could be responsible. We analyze 177 high-resolution spectra of Boyajian’s Star in an effort to detect potential laser signals from extraterrestrial civilizations. The spectra were obtained by the Lick Observatory’s Automated Planet Finder (APF) telescope as part of the Breakthrough Listen Project, and cover the wavelength range of visible light from 374 to 970 nm. We calculate that the APF would be capable of detecting lasers of power greater than approximately 24 MW at the distance of Boyajian’s Star, d = 1470 ly. The top candidates from the analysis can all be explained as either cosmic-ray hits, stellar emission lines or atmospheric airglow emission lines.

The ice composition in the disk around V883 Ori revealed by its stellar outburst

Complex organic molecules (COMs), which are the seeds of prebiotic material and precursors of amino acids and sugars, form in the icy mantles of circumstellar dust grains but cannot be detected remotely unless they are heated and released to the gas phase. Around solar-mass stars, water and COMs only sublimate in the inner few au of the disk, making them extremely difficult to spatially resolve and study. Sudden increases in the luminosity of the central star will quickly expand the sublimation front (so-called snow line) to larger radii, as seen previously in the FU Ori outburst of the young star V883 Ori. In this paper, we take advantage of the rapid increase in disk temperature of V883 Ori to detect and analyze five different COMs, methanol, acetone, acetonitrile, acetaldehyde, and methyl formate, in spatially-resolved submillimeter observations. The COMs abundances in V883 Ori is in reasonable agreement with cometary values. This result suggests that outbursting young stars can provide a special opportunity to study the ice composition of material directly related to planet formation.

Diagnosing the Clumpy Protoplanetary Disk of the UXor Type Young Star GM Cephei

Abstract UX Orionis stars (UXors) are Herbig Ae/Be or T Tauri stars exhibiting sporadic occultation of stellar light by circumstellar dust. GM Cephei is such a UXor in the young (∼4 Myr) open cluster Trumpler 37, showing prominent infrared excess, emission-line spectra, and flare activity. Our photometric monitoring (2008–2018) detects (1) an ∼3.43 day period, likely arising from rotational modulation by surface starspots, (2) sporadic brightening on timescales of days due to accretion, (3) irregular minor flux drops due to circumstellar dust extinction, and (4) major flux drops, each lasting for a couple of months with a recurrence time, though not exactly periodic, of about two years. The star experiences normal reddening by large grains, i.e., redder when dimmer, but exhibits an unusual “blueing” phenomenon in that the star turns blue near brightness minima. The maximum extinction during relatively short (lasting ≤50 days) events, is proportional to the duration, a consequence of varying clump sizes. For longer events, the extinction is independent of duration, suggestive of a transverse string distribution of clumps. Polarization monitoring indicates an optical polarization varying ∼3%–8%, with the level anticorrelated with the slow brightness change. Temporal variation of the unpolarized and polarized light sets constraints on the size and orbital distance of the circumstellar clumps in the interplay with the young star and scattering envelope. These transiting clumps are edge-on manifestations of the ring- or spiral-like structures found recently in young stars with imaging in infrared of scattered light, or in submillimeter of thermalized dust emission.

Clumpy dust rings around non-accreting young stars

We investigate four young, but non-accreting, very low mass stars in Orion, which show irregular eclipses by circumstellar dust. The eclipses are not recurring periodically, are variable in depth, lack a flat bottom, and their duration is comparable to the typical timescale between eclipses. The dimming is associated with reddening consistent with dust extinction. Taken together this implies the presence of rings around these four stars, with radii ranging from 0.01 to 40 AU, comprised of optically thin dust clouds. The stars also show IR excess indicating the presence of evolved circumstellar disks, with orders of magnitude more material than needed for the eclipses. However, the rings need to cover an opening angle of about 20 degrees to explain how common these variable stars are in the coeval population in the same region, which is more extended than a typical disk. Thus, we propose that the rings may not be part of the disks, but instead separate structures with larger scale heights. To be sustained over years, the rings need to be replenished by dust from the disk or gravitationally bound to an object (e.g., planets or planetesimals). These four stars belong to a growing and diverse class of post-T Tauri stars with dips or eclipses in their lightcurves. Dusty rings with scale heights exceeding those of disks may be a common phenomenon at stellar ages between 5 and 10 Myr, in the transition from accretion disks to debris disks. These structures could be caused by migrating planets and may be signposts for the presence of young planetary systems.

Circumstellar Dust Distribution in Systems with Two Planets in Resonance

Abstract We investigate via numerical modeling the effects of two planets locked in resonance, and migrating outward, on the dust distribution of the natal circumstellar disk. We aim to test whether the dust distribution exhibits peculiar features arising from the interplay among the gravitational perturbations of the planets in resonance, the evolution of the gas, and its influence on the dust grain dynamics. We focus on the 3:2 and 2:1 resonance, where the trapping may be caused by the convergent migration of a Jupiter- and Saturn-mass planet, preceding the common gap formation and ensuing outward (or inward) migration. Models show that a common gap also forms in the dust component similarly to what a single, more massive planet would generate and that outward migration leads to a progressive widening of the dust gap and to a decoupling from the gas gap. As the system evolves, a significantly wider gap is observed in the dust distribution, which ceases to overlap with the gas gap in the inner disk regions. At the outer edge of the gas gap, outward migration of the planets produces an overdensity of dust particles, which evolve differently in the 3:2 and 2:1 resonances. For the 3:2 resonance, the dust trap at the gap’s outer edge is partly efficient, and a significant fraction of the grains filters through the gap. For the 2:1 resonance, the trap is more efficient, and very few grains cross the gap, while the vast majority accumulate at the outer edge of the gap.