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  • richardmitnick 3:49 pm on December 1, 2016 Permalink | Reply
    Tags: , , , Does the Galactic Bulge Have Fewer Planets?, Exoplanetary systems, , Planet formation   

    From AAS NOVA: “Does the Galactic Bulge Have Fewer Planets?” 

    AASNOVA

    American Astronomical Society

    1
    The Milky Way’s disk and bulge are very different environments. Are the relative abundances of exoplanets in each the same? [ESO/S. Brunier]

    The Milky Way’s dense central bulge is a very different environment than the surrounding galactic disk in which we live. Do the differences affect the ability of planets to form in the bulge?

    Exploring Galactic Planets

    2
    Schematic illustrating how gravitational microlensing by an extrasolar planet works. [NASA]

    Planet formation is a complex process with many aspects that we don’t yet understand. Do environmental properties like host star metallicity, the density of nearby stars, or the intensity of the ambient radiation field affect the ability of planets to form? To answer these questions, we will ultimately need to search for planets around stars in a large variety of different environments in our galaxy.

    One way to detect recently formed, distant planets is by gravitational microlensing. In this process, light from a distant source star is bent by a lens star that is briefly located between us and the source. As the Earth moves, this momentary alignment causes a blip in the source’s light curve that we can detect — and planets hosted by the lens star can cause an additional observable bump.

    3
    Artist’s impression of the Milky Way galaxy. The central bulge is much denser than the surrounding disk. [ESO/NASA/JPL-Caltech/M. Kornmesser/R. Hurt]

    Relative Abundances

    Most source stars reside in the galactic bulge, so microlensing events can probe planetary systems at any distance between the Earth and the galactic bulge. This means that planet detections from microlensing could potentially be used to measure the relative abundances of exoplanets in different parts of our galaxy.

    A team of scientists led by Matthew Penny, a Sagan postdoctoral fellow at Ohio State University, set out to do just that. The group considered a sample of 31 exoplanetary systems detected by microlensing and asked the following question: are the planet abundances in the galactic bulge and the galactic disk the same?

    A Paucity of Planets

    To answer this question, Penny and collaborators derived the expected distribution of host distances from a simulated microlensing survey, correcting for dominant selection effects. They then compared the distribution of distances in this model sample to the distribution of distances measured for the actual, observed systems.

    4
    Histogram and cumulative distribution (black lines) of distance estimates for microlensing planet hosts. Red lines show the distributions predicted by the model if the disk and bulge abundances were the same. [Penny et al. 2016]

    Intriguingly, the two distributions don’t match when you assume that the planet abundances in the disk and the bulge are the same. The relative abundances appear to be higher in the disk than in the bulge, according to the team’s results: the observations agree with a model in which the bulge/disk abundance ratio is less than 0.54.

    What’s to Blame?

    There are a few ways to interpret this result: 1) distance measurements for the sample of planets discovered by microlensing have errors, 2) the model is too simplified; it needs to also include dependence of planet abundance and detection sensitivity on properties like host mass and metallicity, or 3) the galactic bulge actually has fewer planets than the disk.

    Penny and collaborators suspect some combination of the first two interpretations is most likely, but an actual paucity of planets in the galactic bulge can’t be ruled out. Performing similar analysis on a larger sample of microlensing planets — expected from upcoming, second-generation microlensing searches — and obtaining more accurate distance measurements will help us to address this puzzle more definitively in the future.

    Citation

    Matthew T. Penny et al 2016 ApJ 830 150. doi:10.3847/0004-637X/830/2/150

    See the full article here .

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  • richardmitnick 1:06 pm on July 1, 2016 Permalink | Reply
    Tags: , , , Planet formation   

    From AAS NOVA: “Forming Spirals From Shadows” 

    AASNOVA

    Amercan Astronomical Society

    1 July 2016
    Susanna Kohler

    1
    In this image by the Subaru Telescope, the gas-rich disk (roughly 14 billion miles across, or twice the size of Pluto’s orbit) around SAO 206462 clearly exhibits two spiral arms. A recent study presents a new way such spirals could form. [NAOJ/Subaru]

    NAOJ/Subaru Telescope at Mauna Kea Hawaii, USA
    NAOJ Subaru Telescope interior
    NAOJ/Subaru

    What causes the large-scale spiral structures found in some protoplanetary disks? Most models assume they’re created by newly-forming planets, but a new study suggests that planets might have nothing to do with it.

    Perturbations from Planets?

    In some transition disks — protoplanetary disks with gaps in their inner regions — we’ve directly imaged large-scale spiral arms. Many theories currently attribute the formation of these structures to young planets: either the direct perturbations of a planet embedded in the disk cause the spirals, or they’re indirectly caused by the orbit of a planetary body outside of the arms.

    2
    Another example of spiral arms detected in a protoplanetary disk, MWC 758. [NASA/ESA/ESO/M. Benisty et al.]

    But what if you could get spirals without any planets? A team of scientists led by Matías Montesinos (University of Chile) have recently published a study in which they examine what happens to a shadowed protoplanetary disk.

    Casting Shadows with Warps

    In the team’s setup, they envision a protoplanetary disk that is warped: the inner region is slightly tilted relative to the outer region. As the central star casts light out over its protoplanetary disk, this disk warping would cause some regions of the disk to be shaded in a way that isn’t axially symmetric — with potentially interesting implications.

    Montesinos and collaborators ran 2D hydrodynamics simulations to determine what happens to the motion of particles within the disk when they pass in and out of the shadowed regions. Since the shadowed regions are significantly colder than the illuminated disk, the pressure in these regions is much lower. Particles are therefore accelerated and decelerated as they pass through these regions, and the lack of axial symmetry causes spiral density waves to form in the disk as a result.

    3
    Initial profile for the stellar heating rate per unit area for one of the authors’ simulations. The regions shadowed as a result of the disk warp subtend 0.5 radians each (shown on the left and right sides of the disks here). [Montesinos et al. 2016]

    Observations of Shadow Spirals

    In the authors’ models, two shadowed regions result in the formation of two spiral arms. The arms that develop start at a pitch angle of 15°–22°, and gradually evolve to a shallower 11°–14° pitch at distances of ~65–150 AU.

    The more luminous the central star, the more quickly the spiral arms form, due to the greater contrast between illuminated and shadowed disk regions: for a 0.25 solar-mass disk illuminated by a 1 solar-luminosity star, arms start to form after about 2500 orbits. If we increase the star’s brightness to 100 solar luminosities, the arms form after only 150 orbits.

    Montesinos and collaborators conclude by testing whether or not such spiral structures would be observable. They use a 3D radiative transfer code to produce scattered-light predictions of what the disk would look like to direct-imaging telescopes. They find that these shadow-induced spirals should be detectable.

    This first study clearly demonstrates that large-scale spiral density waves can form in protoplanetary disks without the presence of planets. The authors now plan to add more detailed physics to their models to better understand what we might observe when looking at systems that were shaped in this way.

    4
    Density evolution in two shadowed disks. Top row: disk illuminated by a 100 L⊙ star, at 150, 250, and 500 orbits (from left to right). Bottom row: disk illuminated by a 1 L⊙ star, at 2500, 3500, and 4000 orbits. The rightmost top and bottom panels show control simulations (no shadows were present on the disk) after 1000 and 6000 orbits. (A different type of spiral starts to develop in the bottom control simulation as a result of a gravitational instability, but it never extends to the edges of the disk.) [Montesinos et al. 2016]

    Citation

    Matías Montesinos et al 2016 ApJ 823 L8. doi:10.3847/2041-8205/823/1/L8

    See the full article here .

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  • richardmitnick 11:57 am on December 16, 2015 Permalink | Reply
    Tags: , , , Planet formation,   

    From ALMA: “ALMA Reveals Planetary Construction Sites” 

    ESO ALMA Array
    ALMA

    16 December 2015
    Valeria Foncea
    Education and Public Outreach Officer
    Joint ALMA Observatory
    Santiago, Chile
    Tel: +56 2 467 6258
    Cell: +56 9 75871963
    Email: vfoncea@alma.cl

    Richard Hook
    Public Information Officer, ESO
    Garching bei München, Germany
    Tel: +49 89 3200 6655
    Cell: +49 151 1537 3591
    Email: rhook@eso.org

    Charles E. Blue
    Public Information Officer
    National Radio Astronomy Observatory
    Charlottesville, Virginia, USA
    Tel: +1 434 296 0314
    Cell: +1 434.242.9559
    E-mail: cblue@nrao.edu

    Masaaki Hiramatsu
    Education and Public Outreach Officer, NAOJ Chile
    Observatory Tokyo, Japan
    Tel: +81 422 34 3630
    E-mail: hiramatsu.masaaki@nao.ac.jp

    1
    Schematic view of a transitional disc around a young star
    This schematic diagram shows how the dust (brown) and gas (blue) is distributed around the star, and how a young planet is clearing the central gap. Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser

    2
    ALMA imaging of the transitional disc HD 135344B. This ALMA image combines a view of the dust around the young star HD 135344B (orange) with a view of the gaseous material (blue). The smaller hole in the inner gas is a telltale sign of the presence of a young planet clearing the disc. Credit ALMA (ESO/NOAJ/NRAO)

    3
    ALMA imaging of the transitional disc DoAr 44. This ALMA image combines a view of the dust around the young star DoAr 44 (orange) with a view of the gaseous material (blue). The smaller hole in the inner gas is a telltale sign of the presence of a young planet clearing the disc. Credit ALMA (ESO/NOAJ/NRAO)


    download mp4 video here.
    Animated artist’s impression of a transitional disc around a young star.
    Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found telltale differences between the gaps in the gas and the dust in discs around four young stars. These new observations are the clearest indications yet that planets with masses several times that of Jupiter have recently formed in these discs. Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser


    download mp4 video here.
    Animated schematic view of a transitional disc around a young star
    This schematic diagram shows how the dust (brown) and gas (blue) is distributed around the star, and how a young planet is clearing the central gap. Credit: ALMA (ESO/NAOJ/NRAO)/M. Kornmesser

    Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found telltale differences between the gaps in the gas and the dust in discs around four young stars. These new observations are the clearest indications yet that planets with masses several times that of Jupiter have recently formed in these discs. Measurements of the gas around the stars also provide additional clues about the properties of those planets.

    Planets are found around nearly every star, but astronomers still do not fully understand how — and under what conditions — they form. To answer such questions, they study the rotating discs of gas and dust present around young stars from which planets are built. But these discs are small and far from Earth, and the power of ALMA was needed for them to reveal their secrets.

    A special class of discs, called transitional discs, has a surprising absence of dust in their centers, in the region around the star. Two main ideas have been put forward to explain these mysterious gaps. Firstly, the strong stellar winds and intense radiation could have blown away or destroyed the encircling gas and dust [1]. Alternatively, massive young planets in the process of formation could have cleared the material as they orbit the star [2].

    The unparalleled sensitivity and image sharpness of ALMA have now allowed the team of astronomers, led by Nienke van der Marel from the Leiden Observatory in the Netherlands to map the distribution of gas and dust in four of these transitional discs better than ever before [3]. This in turn has allowed them to choose between the two options — photoevaporation or young planets — as the cause of the gaps for the first time.

    The new images show that there are significant amounts of gas within the dust gap [4]. But to the team’s surprise, this disc of gas also possessed a gap, up to three times smaller than that of the dust. This could only be explained by the scenario in which the newly formed massive planet cleared the gas as it travelled around their orbit, but trapped the dust particles further out [5].

    “Previous observations already hinted at the presence of gas inside the dust gaps,” explains Nienke van der Marel. “But as ALMA can image the material in the entire disc in much greater detail than other facilities, we could rule out the alternative photoevaporation scenario. The deep gap points clearly to the presence of planets with several times the mass of Jupiter, creating these caverns as they sweep through the disc.”

    Remarkably, these observations were conducted utilising just one tenth of the current resolving power of ALMA, as they were performed whilst half of the array was still under construction on the Chajnantor Plateau in northern Chile. Further studies are now needed to determine whether more transitional discs also point towards this planet-clearing scenario, although ALMA’s observations have, in the meantime, provided astronomers with a valuable new insight into the complex process of planetary formation.

    “All the transitional discs studied so far that have large dust cavities also have gas cavities. So, with ALMA, we can now find out where and when giant planets are being born in these discs, and compare these results with planet formation models,” says Ewine van Dishoeck, also of Leiden University and the Max Planck Institute for Extraterrestrial Physics in Garching [6]. “Direct planetary detection is just within reach of current instruments, and the next generation telescopes currently under construction, such as the European Extremely Large Telescope, will be able to go much further. ALMA is pointing out where they will need to look.”

    Notes

    [1] This process, which clears the dust and gas from the inside out, is known as photoevaporation.

    [2] Such planets are difficult to observe directly and previous studies at millimeter wavelengths have failed to achieve a sharp view of their inner, planet-forming zones where these different explanations could be put to the test. Other studies could not measure the bulk of the gas in these discs.

    [3] The four targets of these investigations were SR 21, HD 135344B (also known as SAO 206462), DoAr 44 and Oph IRS 48.

    [4] The gas present in transitional discs consists primarily of hydrogen, and is traced through observations of the carbon monoxide — or CO — molecule.

    [5] The process of dust trapping is explained in an earlier release.

    [6] Other examples include the HD 142527 (eso1301 and here) and J1604-2130 transitional discs.

    Additional information

    This research was presented in a paper entitled Resolved gas cavities in transitional disks inferred from CO isotopologs with ALMA, by N. van der Marel, et al., to appear in Astronomy & Astrophysics in December 2015.

    The team is composed of N. van der Marel (Leiden University, Leiden, the Netherlands; Institute for Astronomy, University of Hawaii, Honolulu, USA), E. F. van Dishoeck (Leiden University, Leiden, the Netherlands; Max Planck Institute for Extraterrestrial Physics in Garching, Germany), S. Bruderer (Max-Planck Institute for Extraterrestrial Physics, Garching, Germany), S. M. Andrews (Harvard-Smithsonian Center for Astrophysics, Massachusetts, USA), K. M. Pontoppidan (Space Telescope Science Institute, Baltimore, Maryland, USA), G. J. Herczeg (Peking University, Beijing, China), T. van Kempen (Leiden University, Leiden, the Netherlands) and A. Miotello (Leiden University, Leiden, the Netherlands).

    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.

    ALMA construction and operations are led on behalf of Europe by ESO, on behalf of North America by the National Radio Astronomy Observatory (NRAO), which is managed by Associated Universities, Inc. (AUI) and on behalf of East Asia by the National Astronomical Observatory of Japan (NAOJ). The Joint ALMA Observatory (JAO) provides the unified leadership and management of the construction, commissioning and operation of ALMA.

    NRAO Small

    ESO 50

    NAOJ

     
  • richardmitnick 5:38 pm on November 19, 2015 Permalink | Reply
    Tags: , , Planet formation,   

    From Stanford: “Stanford astronomers observe the birth of an alien planet” 

    Stanford University Name
    Stanford University

    November 18, 2015
    Bjorn Carey

    1
    Artist’s illustration shows how planets could form in a transition disk around a star. Astronomers at Stanford and the University of Arizona have been able to identify a planet in the early stages of formation. NASA/JPL-Caltech

    Since prehistory, humans have speculated about how the planets in our solar system were formed. Today, astronomers announce evidence of an exoplanet being born that could move us one step closer to understanding the process of planet formation around other stars.

    The alien planet, called LkCa 15 b, orbits a star 450 light years away and appears to be on its way to growing into a world similar to Jupiter.

    “This is the first incontrovertible detection of a planet still in the process of forming – a so-called protoplanet,” said Kate Follette, a postdoctoral researcher at Stanford and co-lead author on the study in the Nov. 19 issue of Nature. Follette’s work produced a digital picture of LkCa 15 b glowing in the light of ultra-hot hydrogen gas, a prediction of planet formation theories verified directly by Follette and her co-authors.

    The observation was combined in the paper with data from Steph Sallum, the co-lead author and a graduate student at the University of Arizona, who independently observed the same star with a complementary technique.

    The planet is forming in a transition disk, a doughnut-like ring of dust and rocky debris orbiting its parent star, LkCa 15. The central clearings within transition disks are believed to be created by the formation of planets, which sweep up dust and gas from the disk as they orbit the star. Astronomers have long speculated that investigating these gaps could lead to the discovery of protoplanets, but getting a good look at these infant worlds has been challenging.

    Follette and her colleagues took a new tack, and designed an imaging instrument to look for a characteristic planet formation signature. The process by which a planet grows from a rocky or icy core to a full-fledged gas giant is incredibly energetic. As hydrogen gas falls from the disk onto the core of the protoplanet, it heats up and glows like a fluorescent light bulb, emitting a characteristic wavelength of visible light called “Hydrogen-alpha,” or H-alpha.

    Using the University of Arizona’s Magellan Telescope in Chile, Follette, her adviser at Stanford, Professor Bruce Macintosh, and their co-authors at the University of Arizona were able to hone in on this particular shade of red H-alpha light emanating from LkCa 15 b.

    Magellan 6.5 meter telescopes
    Magellan 6.5 meter Interior
    Magellan Telescope in Chile

    “I was pretty excited as soon as I processed the data, but I wanted to be cautious,” said Follette, who began the research as a graduate student at Arizona. “I was pretty sure I had found something interesting, but in this field we’re always chasing objects that are just at the edge of what we can detect. The really cool thing is that it survived all of our tests to make sure it was real.”

    2
    By isolating the hydrogen-alpha light from the vicinity of a star, astronomers at Stanford and the University of Arizona were able to identify a planet in the early stages of formation. Kate Follette

    To make the discovery, the scientists processed the images to remove the light from the host star, allowing them to isolate light from the much fainter planet. The protoplanet is very close to its parent star, Follette said, and if it were much closer or fainter, Lk Ca 15 would have washed out its light completely.

    “The difference in brightness between a star and a young exoplanet is usually comparable to the difference between a firefly and a lighthouse,” Follette said. “It’s very hard to isolate the light from the planet when it is so faint and so close to the star from our point of view. But, because we could focus on a special color of light where the planet is glowing very brightly, the signal was significantly stronger than what we normally look for.”

    The images were sharpened using adaptive optics to correct for the bending of light as it passes through Earth’s atmosphere. The Magellan adaptive optics system is the first telescope with a visible light camera behind it capable of imaging at H-alpha, and will be an attractive technique for future planet-hunting work.

    Professor Macintosh, who led the recent discovery of the slightly older planet 51 Eridani b, said that adaptive optics imaging is allowing astronomers to fill in the birth cycle of planets.

    “51 Eri b is an adolescent – about 20 million years old, already full-grown and still cooling off from the energy released during its formation,” he said. “Kate’s planet is a baby, still heating up and growing.”

    The team will continue monitoring LkCa 15 b to better understand the planet formation process, as well as the fingerprint it leaves on the transition disk. If this planet is responsible for the gap in the disk, it may indicate that similar holes in other disks are also signposts of developing planets.

    Follette said that this type of work will ultimately provide better understanding of how planets form, and is driven by the desire to know whether the mechanisms by which we think our own solar system formed are the exception or the norm in the universe.

    “One of the fundamental human questions is whether we’re alone or unique,” Follette said. “It’s cool to look at Jupiter-like exoplanets like LkCa 15 b, but ultimately we’re trying to push the technology to be able to detect Earth-like exoplanets. I’ve always been inspired by the famous ‘pale blue dot’ image of Earth taken by Voyager as it passed Saturn. We’d really like to do that some day for a planet around another star, and this sort of work is moving us in that direction.”

    See the full article here .

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    Leland and Jane Stanford founded the University to “promote the public welfare by exercising an influence on behalf of humanity and civilization.” Stanford opened its doors in 1891, and more than a century later, it remains dedicated to finding solutions to the great challenges of the day and to preparing our students for leadership in today’s complex world. Stanford, is an American private research university located in Stanford, California on an 8,180-acre (3,310 ha) campus near Palo Alto. Since 1952, more than 54 Stanford faculty, staff, and alumni have won the Nobel Prize, including 19 current faculty members

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