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  • richardmitnick 9:30 pm on October 2, 2018 Permalink | Reply
    Tags: , , , , , , , Protoplanetary disks,   

    From Science: “Cosmic conundrum: The disks of gas and dust that supposedly form planets don’t seem to have the goods” 

    From Science Magazine

    Artist’s illustration of the protoplanetary disk surrounding a young star. JPL-Caltech/NASA

    Astronomers have a problem on their hands: How can you make planets if you don’t have enough of the building blocks? A new study finds that protoplanetary disks—the envelopes of dust and gas around young stars that give rise to planets—seem to contain orders of magnitude too little material to produce the planets.

    “This work is telling us that we really have to rethink our planetary formation theories,” says astronomer Gijs Mulders of the University of Chicago in Illinois, who was not involved in the research.

    Stars are born from colossal clouds of gas and dust and, in their earliest stages, are surrounded by a thin disk of material. Dust grains within this halo collide, sometimes sticking together. The clumps build up into planetary cores, which are big enough to gravitationally attract additional dust and gas, eventually forming planets.

    But many details about this process remain unknown, such as just how quickly planets arise from the disk, and how efficient they are in capturing material. The disks, surrounded by an obscuring haze of gas and dust, are difficult to observe. But radio telescopes can penetrate the haze and investigate young stars. The brightness of radio waves emitted by dust in the disk can be used to give a reasonable estimate of its overall mass.

    ESO/NRAO/NAOJ ALMA Array in Chile in the Atacama at Chajnantor plateau, at 5,000 metres

    The Atacama Large Millimeter Array (ALMA), a radio observatory in the Atacama Desert in Chile, has made it far easier to study protoplanetary disks. In the new study, astronomers led by Carlo Manara of the European Southern Observatory in Munich, Germany, used ALMA to compare the masses of protoplanetary disks around young stars between 1 million and 3 million years old to the masses of confirmed exoplanets and exoplanetary systems around older stars of equivalent size. The disk masses were often much less than the total exoplanet mass—sometimes 10 or 100 times lower, the team will report in an upcoming paper in Astronomy & Astrophysics.

    Although such findings have been reported before for a few star systems, the study is the first to point out the mismatch over several hundred different systems. “I think what this work does is really set this as a fact,” Manara says.

    It is possible that astronomers are simply looking at the disks too late. Perhaps some planets form in the first million years, sucking up much of the gas and dust, Manara says. ALMA has found that some extremely young stars, such as the approximately 100,000-year-old HL Tauri, already have ringlike gaps in their disks, potentially indicating that protoplanets are sweeping up material inside of them.

    “But if you solve one problem, you end up with another,” says astronomer Jonathan Williams of the University of Hawaii Institute for Astronomy in Honolulu, who was also not involved in the work. If planetary cores form early, when so much material remains in the disk, nothing would stop them from ballooning into Jupiter-size behemoths. Yet the emerging census of exoplanets shows that most are Earth- or Neptune-size worlds.

    Williams favors the idea that current telescopes are simply missing some of the material. ALMA’s wavelengths are tuned to best see the smallest bits of dust. But a great deal of mass, perhaps as much as 10 times what’s been observed, could be hidden in the form of pebbles, which are slightly too big to show up in such investigations. A proposed upgrade to the Very Large Array, a radio telescope in New Mexico, should be able to spot such hidden debris, perhaps accounting for some of the missing material.

    NRAO/Karl V Jansky Expanded Very Large Array, on the Plains of San Agustin fifty miles west of Socorro, NM, USA, at an elevation of 6970 ft (2124 m)

    One final possibility is that protoplanetary disks are somehow sucking in additional material from the surrounding interstellar medium. Manara says some recent simulations show young stars drawing in fresh material for much longer periods of time than previously believed. He hopes that observations of the earliest stages of star formation from the upcoming Square Kilometer Array or James Webb Space Telescope will help researchers decide between these different hypotheses.

    NASA/ESA/CSA Webb Telescope annotated

    See the full article here .


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  • richardmitnick 6:30 am on June 17, 2017 Permalink | Reply
    Tags: , , , , , , Protoplanetary disks, Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line   

    From astrobites: “Watermelon Dust is the Best Dust: Forming Planetesimals Near the Snow Line” 

    Astrobites bloc


    June 16, 2017
    Michael Hammer

    Title: Planetesimal Formation near the snow line: in or out?
    Authors: Djoeke Schoonenberg and Chris Ormel
    First Author’s Institution: Anton Pannekoek Institute for Astronomy, University of Amsterdam
    Status: Published in A&A [open access]

    How is it possible for planets to exist? Even though we know planets must have formed from planetesimals that are tens of kilometers in size, the most basic models of protoplanetary disks have trouble forming planetesimals from the micron to centimeter-sized dust that populates these disks. For dust particles to grow into planetesimals, they need to be able to clump together enough to reach roughly the same level of concentration as the gas in the disk – which can be difficult since there is 100 times more gas than dust.

    A few months ago, I wrote an Astrobite describing a simple model that naturally achieves the conditions needed to form planetesimals in the inner disk, thereby offering a way to form planets in the inner solar system like Earth and Mars. However, this model leaves out the gas giant planets in the outer solar system like Jupiter!

    Fortunately, beyond the snow line at about 2 AU, the disk will get cold enough for water vapor to condense into solid ice. It is already widely accepted that this extra ice will enhance the concentration of solids enough to form planetesimals and in turn, planets in the outer disk. Yet, the most detailed models of this scenario leave out enough relevant effects that we cannot reliably determine the impact of this extra ice.

    Rough sketch which shows the sharp increase of solid surface density at the snow line, by a factor of ~3-4. [https://ay201b.wordpress.com/the-snow-line-in-protoplanetary-disks/]

    Djoeke Schoonenberg and Chris Ormel, the authors of today’s paper, set out to improve our understanding of whether the snow line can trigger the formation of planetesimals by creating a more rigorous model that better captures the dynamic structure of the disk and of the dust grains themselves.

    Model Upgrades with a Side of Fruit

    Schoonenberg and Ormel develop a steady-state model of how the ice beyond the snow line evaporates as it moves inwards and how some of the water vapor inside the snow line condenses as it moves outwards.

    My favorite part of their model is that they factor in the structure of individual “icy dust” grains. Many studies of protoplanetary disks often leave out what the mass in the disk is made of – instead, only tracking a distribution of density across the disk or nondescript particles. However, not only do Schoonenberg and Ormel describe each icy dust grain as 50% silicates (the typical composition of a regular dust grain) and 50% ice; but they also establish how the silicate “seeds” in each grain are divided up. As shown in Figure 1, a dust grain can either have a single silicate core like an avocado (their “single-seed” model), or it can have many smaller seeds evenly distributed through the grain like a watermelon (their “many-seeds” model).

    The authors run separate models for each type of dust grain. They then solve for the enhancement of ice and the solid-to-gas ratio to see if these values reach a high enough concentration to form planetesimals.

    Figure 1. Structures of icy dust grains. In the avocado model, each grain has a single silicate core surrounded by a shell of ice. In the watermelon model, each grain has a bunch of smaller silicate seeds evenly distributed through the ice. When an avocado dust grain evaporates, it slowly loses only its ice before leaving its core behind. When a watermelon dust grain evaporates, it slowly loses both its ice and its silicate seeds together as it drifts inward. Adapted from Figure 1.

    Forming Planetesimals Early and with Watermelon

    Besides varying the structure of the dust grains, the authors also experiment with different disk viscosities, dust particle sizes, and a few other variables. In particular, they find that higher viscosities are best-suited for producing planetesimals. Since these higher viscosities are more likely to occur early on in a disk’s lifetime, this suggests that disks can form planetesimals right away!

    In the single-seed “avocado” model with the optimal higher viscosities, Schoonenberg and Ormel find that the concentration of ice can be enhanced by a factor of 3 to 5 just beyond the snow line (see Figure 2). Interestingly with the many-seeds “watermelon dust” model, they find that this enhancement can double! This occurs because in the single-seed model, only the extra ice from inside the snow line contributes to the enhancement. However, in the watermelon model, the small size of the watermelon seeds plays a key role. Since the seeds are so small, they get carried by the gas that makes up most of the disk. When some of the gas from inside the snow line moves outward, some of the watermelon seeds follow it outward. As a result, many of these watermelon seeds end up captured in icy dust grains and also contribute to the extra ice, which doubles the enhancement.

    Ultimately, the authors expect the ice enhancement in an actual disk to be in-between the results from the avocado and watermelon models due to the fact that both structures of dust grains are plausible, and also because other dust grains may be a “hybrid” of both fruits and have a large silicate core as well as additional smaller silicate seeds in the outer icy shell.

    Figure 2. Solid-to-gas ratio near the ice line (blue vertical line). Solids are ice plus dust. The many-seeds model safely exceeds the threshold (orange horizontal line) to form planetesimals. Since the amount of solids is just an average at a given radius, the single-seeds model should also be able to form planetesimals. Adapted from Figure 5.


    The ice enhancement of about ~7 that the authors find is ten times lower than the enhancement of 75 found with a simpler model, emphasizing the importance of considering the more intricate details of the problem! Thankfully, this enhancement is still high enough for the concentration of large dust grains to reach the concentration of gas and produce planetesimals (see Figure 2). More importantly, the authors find that the best conditions for forming planetesimals happen soon after a star and its disk are born, supporting observational evidence that giant planets in the outer disk can form quickly.

    Lastly, Schoonenberg and Ormel expect the cm-sized ice pile-up near the snow line to be detectable by radio (cm-wavelength) telescopes. Seeing this feature in a real disk would be the best test for finding out how well we understand planetesimal formation near the snow line.

    Featured Image Credit: Luis Calçada

    See the full article here .

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  • richardmitnick 11:45 am on January 27, 2016 Permalink | Reply
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    From ALMA: “ALMA confirms predictions on the interaction between protoplanetary disks and planets” 

    ESO ALMA Array

    27 January 2016
    Héctor Cánovas
    Universidad de Valparaíso Valparaíso, Chile
    Tel: +56 032 – 299 5555
    Tel: +56 02 84144232
    E-mail: hector.canovas@uv.cl

    Valeria Foncea

    Education and Public Outreach Officer

    Joint ALMA Observatory

    Santiago, Chile

    Tel: +56 2 467 6258

    Cell: +56 9 75871963
    E-mail: vfoncea@alma.cl

    Charles E. Blue
    Public Information Officer
    National Radio Astronomy Observatory
    Charlottesville, Virginia, USA
    Tel: +1 434 296 0314
    Cell: +1 202 236 6324
    E-mail: cblue@nrao.edu

    Richard Hook
    Public Information Officer, ESO

    Garching bei München, Germany

    Tel: +49 89 3200 6655

    Cell: +49 151 1537 3591
    E-mail: rhook@eso.org

    Masaaki Hiramatsu

    Education and Public Outreach Officer, NAOJ Chile

    Tokyo, Japan

    Tel: +81 422 34 3630

    E-mail: hiramatsu.masaaki@nao.ac.jp

    Protoplanetary disc from ALMA
    Image taken by ALMA of the dust ring that surrounds the young star Sz 91. This ring is primarily made up of mm-sized dust particles. The interaction between several recently formed planets and the protoplanetary disk that still surrounds the star probably generate the dust ring observed by ALMA.

    New observations made with the Atacama Large Millimeter/submillimeter Array (ALMA) of the disk that surrounds a young star, less massive than the Sun, confirm theories about the interaction between recently formed planets and disks. A team of astronomers led by Héctor Cánovas from Universidad de Valparaíso and the Millennium ALMA Disk Nucleus (MAD) observed the dust ring possibly sculpted by planets in formation around the star Sz 91, at a distance roughly 650 light years from Earth.

    The results obtained show the first disk around a star that is less massive than ours – it has only half of the mass of our Sun – which simultaneously presents a migration of dust particles from the outermost zones and evident signs of interaction between young planets with the disk in the innermost zone.

    Planets are born in dust and gas disks that surround young stars and feed them with matter, leaving a “footprint” of this interaction in the structure of the disk. The theoretical models that study this interaction predict that the planets carve the protoplanetary disk, creating a “hole” in the innermost part of the disk, and preventing mm-sized dust particles (like grains of sand on a beach) from continuing their journey towards the central star. At the same time, dust particles in the outermost parts of the disk (the farthest from the star) are attracted by the gravitational force of the star.

    The combination of both effects should create dust structures in the form of a ring in disks that host recently formed giant planets on the inside.

    “The sharp image from ALMA shows a ring around the young star. And it is a surprisingly large ring, over three times the size of Neptune’s orbit (a radius of approximately 110 astronomical units (AU)” explains Héctor Cánovas.

    The image from ALMA only shows the ring, as the radio telescope detects the cold dust particles that make it up, and not the planets and the star, as these are primarily made up of hot gas.

    “Based on the current paradigm of planet-disk interactions, only giant planets orbiting the innermost parts of the disk can explain the presence of a ring with such a large radius,” indicates Antonio Hales, ALMA astronomer and member of the research team.

    The accumulation of dust particles in a narrow annular structure, as is the case with Sz91, can favor the formation of more planets, because the high density of dust particles in the ring would provide the ideal conditions for the dust particles to agglutinate and grow in size until they form small planetary nuclei.

    “The results of this investigation show that Sz91 is a highly important protoplanetary disk for the study of planetary formation, planet-disk interactions, and the evolution of these disks around stars of lower mass, as Sz91 shows evidence of all these processes simultaneously,” concludes Matthias Schreiber, coauthor of the study.

    Additional information

    This investigation was presented in an article entitled A ring-like concentration of mm-sized particles in Sz 91, written by Héctor Cánovas and collaborators, which will soon be published in the specialized journal Monthly Notices of the Royal Astronomical Society (MNRAS).

    The research team is made up of Héctor Cánovas, Claudio Cáceres, Matthias Schreiber, Adam Hardy (all from Universidad de Valparaíso and from the Millennium ALMA Disk Nucleus (MAD), Chile), Lucas Cieza (Universidad Diego Portales and MAD, Chile), Francois Ménard (Universidad de Chile) and Antonio Hales (JAO-ALMA, Chile).


    A ring-like concentration of mm-sized particles in Sz 91

    See the full article here .

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    The Atacama Large Millimeter/submillimeter Array (ALMA), an international astronomy facility, is a partnership of Europe, North America and East Asia in cooperation with the Republic of Chile. ALMA is funded in Europe by the European Organization for Astronomical Research in the Southern Hemisphere (ESO), in North America by the U.S. National Science Foundation (NSF) in cooperation with the National Research Council of Canada (NRC) and the National Science Council of Taiwan (NSC) and in East Asia by the National Institutes of Natural Sciences (NINS) of Japan in cooperation with the Academia Sinica (AS) in Taiwan.

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  • richardmitnick 3:58 pm on December 15, 2015 Permalink | Reply
    Tags: , , , Protoplanetary disks   

    From ESO: “Mind the Gap” 

    [I picked this up from 2008 because it relates to an ALMA release due out on 12.16.15.]

    European Southern Observatory

    8 September 2008
    Klaus Pontoppidan
    California Institute of Technology
    Pasadena, USA
    Tel: +1 626 395 4900
    Cell: +1 626 679 5793
    Email: pontoppi@gps.caltech.edu

    Ewine van Dishoeck
    Leiden University
    Leiden, Netherlands
    Tel: +31 71 527 58 14
    Email: ewine@strw.leidenuniv.nl

    VLT instrument hints at the presence of planets in young gas discs


    Astronomers have been able to study planet-forming discs around young Sun-like stars in unsurpassed detail, clearly revealing the motion and distribution of the gas in the inner parts of the disc. This result, which possibly implies the presence of giant planets, was made possible by the combination of a very clever method enabled by ESO’s Very Large Telescope.

    Planets could be home to other forms of life, so the study of exoplanets ranks very high in contemporary astronomy. More than 300 planets are already known to orbit stars other than the Sun, and these new worlds show an amazing diversity in their characteristics. But astronomers don’t just look at systems where planets have already formed – they can also get great insights by studying the discs around young stars where planets may currently be forming. “This is like going 4.6 billion years back in time to watch how the planets of our own Solar System formed,” says Klaus Pontoppidan from Caltech, who led the research.

    Pontoppidan and colleagues have analysed three young analogues of our Sun that are each surrounded by a disc of gas and dust from which planets could form. These three discs are just a few million years old and were known to have gaps or holes in them, indicating regions where the dust has been cleared and the possible presence of young planets.

    The new results not only confirm that gas is present in the gaps in the dust, but also enable astronomers to measure how the gas is distributed in the disc and how the disc is oriented. In regions where the dust appears to have been cleared out, molecular gas is still highly abundant. This can either mean that the dust has clumped together to form planetary embryos, or that a planet has already formed and is in the process of clearing the gas in the disc.

    For one of the stars, SR 21, a likely explanation is the presence of a massive giant planet orbiting at less than 3.5 times the distance between the Earth and the Sun, while for the second star, HD 135344B, a possible planet could be orbiting at 10 to 20 times the Earth-Sun distance. The observations of the third star, TW Hydrae, may also require the presence of one or two planets.

    “Our observations with the CRIRES instrument on ESO’s Very Large Telescope clearly reveal that the discs around these three young, Sun-like stars are all very different and will most likely result in very different planetary systems,” concludes Pontoppidan.


    “Nature certainly does not like to repeat herself” [1].

    “These kinds of observations complement the future work of the ALMA observatory, which will be imaging these discs in great detail and on a larger scale,” adds Ewine van Dishoeck, from Leiden Observatory, who works with Pontoppidan.

    To study the gaps in dust discs that are the size of the Solar System around stars that are located up to 400 light-years away is a daunting challenge that requires a clever solution and the best possible instruments [2].

    “Traditional imaging cannot hope to see details on the scale of planetary distances for objects located so far away,” explains van Dishoeck. “Interferometry can do better but won’t allow us to follow the motion of the gas.”

    Astronomers used a technique known as spectro-astrometric imaging to give them a window into the inner regions of the discs where Earth-like planets may be forming. They were able not only to measure distances as small as one-tenth the Earth-Sun distance, but to measure the velocity of the gas at the same time [3].

    “The particular configuration of the instrument and the use of adaptive optics allows astronomers to carry out observations with this technique in a very user-friendly way: as a consequence, spectro-astrometric imaging with CRIRES can now be routinely performed,” says team member Alain Smette, from ESO [4].


    Pontoppidan, K. M. et. al. 2008, Spectro-Astrometric Imaging of Molecular Gas Within Protoplanetary Disk Gaps, Astrophysical Journal, 684, 1323, 10 September 2008. Team members are Klaus M. Pontoppidan, Geoffrey A. Blake, and Michael J. Ireland (California Institute of Technology, Pasadena, USA), Ewine F. van Dishoeck (Leiden Observatory, The Netherlands, and Max-Planck-Institute for Extraterrestrial Physics, Garching, Germany – MPE), Alain Smette (ESO, Chile), and Joanna Brown (MPE).

    [1] The discs are about an hundred astronomical units (AU – the mean distance between the Earth and the Sun, or 149.6 million kilometres) across, but the stars are more than 200 light-years away (one light-year is 200 000 AU). To resolve structures on 1 AU scales in these systems corresponds to reading the license plate on a car at a distance of 2000 km – roughly the distance from Stockholm to Lisbon.

    [2] CRIRES, the near-infrared spectrograph attached to ESO’s Very Large Telescope, is fed from the telescope through an adaptive optics module which corrects for the blurring effect of the atmosphere and so makes it possible to have a very narrow slit with a high spectral dispersion: the slit width is 0.2 arcsecond and the spectral resolution is 100 000. Using spectro-astrometry, an ultimate spatial resolution of better than 1 milli-arcsecond is achieved.

    [3] The core of the spectro-astrometry imaging technique relies on the ability of CRIRES to be positioned very precisely on the sky, while retaining the ability to spread the light into a spectrum so that wavelength differences of 1 part in 100 000 can be detected. More precisely, the astronomers measure the centroid in the spatial direction of a spectrally resolved emission line: effectively, astronomers take a sharp emission line – a clear fingerprint of a molecule in the gas – and use data from several slit positions to locate the sources of particular emission lines, and hence to map the distribution of the gas with much greater precision than can be achieved by straightforward imaging. The astronomers have obtained spectra of the discs centred at wavelengths of 4.715 microns at 6 different position angles.

    [4] Alain Smette is the CRIRES Instrument Scientist.

    See the full article here .

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    ESO is the foremost intergovernmental astronomy organisation in Europe and the world’s most productive ground-based astronomical observatory by far. It is supported by 16 countries: Austria, Belgium, Brazil, the Czech Republic, Denmark, France, Finland, Germany, Italy, the Netherlands, Poland, Portugal, Spain, Sweden, Switzerland and the United Kingdom, along with the host state of Chile. ESO carries out an ambitious programme focused on the design, construction and operation of powerful ground-based observing facilities enabling astronomers to make important scientific discoveries. ESO also plays a leading role in promoting and organising cooperation in astronomical research. ESO operates three unique world-class observing sites in Chile: La Silla, Paranal and Chajnantor. At Paranal, ESO operates the Very Large Telescope, the world’s most advanced visible-light astronomical observatory and two survey telescopes. VISTA works in the infrared and is the world’s largest survey telescope and the VLT Survey Telescope is the largest telescope designed to exclusively survey the skies in visible light. ESO is a major partner in ALMA, the largest astronomical project in existence. And on Cerro Armazones, close to Paranal, ESO is building the 39-metre European Extremely Large Telescope, the E-ELT, which will become “the world’s biggest eye on the sky”.

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  • richardmitnick 3:39 pm on September 11, 2015 Permalink | Reply
    Tags: , , , Protoplanetary disks   

    From ASS NOVA: ” Water Vapor in an Unexpected Location” 


    Amercan Astronomical Society

    11 September 2015
    Susanna Kohler

    Artist’s impression of a pre-transitional disk. A recent study has, for the first time, found water vapor in the inner regions of such a disk. [NASA/JPL-CALTECH]

    The protoplanetary disk around DoAr 44 is fairly ordinary in most ways. But a recent study has found that this disk contains water vapor in its inner regions — the first such discovery for a disk of its type.

    Drying Out Disks

    DoAr 44 is a “transitional disk”: a type of protoplanetary disk that has been at least partially cleared of small dust grains in the inner regions of the disk. This process is thought to happen as a result of dynamical interactions with a protoplanet embedded in the disk; the planet clears out a gap as it orbits.

    A schematic of the differences between a full protoplanetary disk, a pre-transitional disk, and a transitional disk. [Catherine Espaillat]

    Classical protoplanetary disks surrounding young, low-mass stars often contain water vapor, but transitional disks are typically “dry” — no water vapor is detected from the disk inner regions. This is probably because water vapor is easily dissociated by far-UV radiation from the young, hot star. Once the dust is cleared out from the inner regions of the disk, the water vapor is no longer shielded from the UV radiation, so the disk dries out.

    Enter the exception: DoAr 44. The disk in this system doesn’t have a fully cleared inner region, which labels it “pre-transitional”. It’s composed of an inner ring out to 2 AU, a cleared gap between 2 and 36 AU, and then the outer disk. What makes DoAr 44 unusual, however, is that it’s the only disk with a large inner gap known to harbor detectable quantities of water vapor. The authors of this study ask a key question: where is this water vapor located?

    Unusual System

    Led by Colette Salyk (NOAO and Vassar College), the authors examined the system using the Texas Echelon Cross Echelle Spectrograph, a visiting instrument on the Gemini North telescope.

    Texas Echelon-Cross-Echelle Spectrograph
    Texas Echelon Cross Echelle Spectrograph

    Gemini North telescope
    Gemini North Interior
    NOAO/Gemini North

    They discovered that the water vapor emission originates from about 0.3 AU — the inner disk region, where terrestrial-type planets may well be forming.

    Both dust-shielding and water self-shielding seem to have protected this water vapor from the harsh radiation of the central star, and the authors model this shielding to place constraints on the composition of the disk’s inner regions. They conclude that DoAr 44 has maintained similar physical and chemical conditions to classical protoplanetary disks in its terrestrial-planet forming regions, in spite of having formed a large gap.

    Why has DoAr 44 succeeded at maintaining its water vapor, unlike other transition disks? The authors propose that gas might be migrating across the gap in the disk, replenishing the inner disk from the outer. Future observations are planned to help better understand the overall architecture of the gap, as well as the implications of these detections for any possible planets embedded in the disk.

    Colette Salyk et al 2015 ApJ 810 L24. doi:10.1088/2041-8205/810/2/L24

    See the full article here .

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