Southern Taurids to peak on Saturday 4 November 2017.

The Southern Taurid Meteor Shower will  peak on Saturday 4 November 2017, the first of two overlapping meteor showers associated with Comet 2P/Encke that peak in early November, the second being the Northern Taurids which will peak on Saturday 11 November. The showers take their name from the constellation of Taurus, where their radiant point (the point from which they appear to radiate) can be found. However viewing of the Southern Taurids is unlikely to be good this year, as the shower only produces about 5 meteors per hour at peak, and the peak coincides with the Full Moon on Friday 3 November this year.

The radiant points of the Northern and Southern Taurids. Starry Night/Space.com.

The Taurid Showers are caused by the Earth passing through the trail of Comet 2P/Encke, this is particularly spread out, so that the Earth takes several weeks to pass through it. This is thought to be because Encke is a remnant of a much larger Comet, which has broken up over the past 20 000 to 30 000 years, giving a long, spread-out debris stream that has been shepherded into two main substreams by tidal interactions with the Earth.

Image of 2P/Encke taken on 21 Februart 2017 with the Pearl Telescope at the Tenagra Observatory in Arizon. The image is a composite of six 60-second exposures. Gianluca Masi/Virtual Telescope/Michael Schwartz/Tenagra Observatory.

Comet 2P/Encke was first observed by French astronomer Pierre Méchain in 1786, but gets its name from the German astronomer Johann Franz Encke, who calculated its orbit in 1818. The designation 2/ implies that it was the second Periodic Comet (comet with an orbital period of less than 200 years) ever discovered.

The orbit and current position of Comet 2P/Encke. The Sky Live 3D Solar System Simulator.

Comet 2P/Encke completes one orbit every 1204 days (3.3 years) on an eccentric orbit tilted at 11.8° to the plane of the Solar System, that takes it from 0.34 AU from the Sun (34% of the average distance at which the Earth orbits the Sun, and inside the orbit of the planet Mercury) to 4.09 AU from the Sun (4.09 times as far from the Sun as the Earth,considerably more than twice the distance at which the planet Mars orbits the Sun).  

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Imaging the inner disk of LkCa 15.

LkCa 15 is a young (2-5 million-year-old) K5-type orange dwarf star, roughly 547 light years from Earth in the Taurus-Auriga star-forming region in the constellation of Taurus. It has approximately the same mass as the Sun but only about 74% of its luminosity, new material is still accreting onto the star at a rate of about one Earth mass every 23 years. The system has a one of the best known transition disks (a structure on the way from being a protoplanetary disk, a dense structure from which planets are thought to form, to a debris disk, a relict of earlier planet-formation, such as the Main Asteroid Belt and Kuiper Belt in our Solar System), which comprises an inner disk close to the star, a gap with three candidate planets and an outer disk which begins at about 50 AU from the star (i.e. 50 times as far from the star as the Earth is from the Sun). This is recognizably similar to our Solar System, with an outer disk in a similar position to our Kuiper Belt, three large candidate planets in a similar position to the four giant planets of our Solar System (Jupiter, Saturn, Uranus and Neptune) and an inner disk in the region occupied by our inner planets (Mercury, Venus, Earth and Mars), however while the outer disk of LkCa 15 has been well studied and the planets imaged several times, resolving the inner disk has proven problematic.

In a paper published on the arXiv database at Cornell University Library on 2 September 2016, a team of scientists led by Christian Thalmann of the Institute for Astronomy at ETH Zurich describe the results of a new study of the LkCa 15 system using the SPHERE (Spectro-Polarimetric High-contrast Exoplanet REsearch) instrument on the European Southern Observatory’s Very Large Telescope, and discuss the implications of this study.

Thalmann et al. made two rounds of observations with the instrument, which makes long exposure still images, the first making images with an exposure time of 32 seconds, using a coronagraph to block out the light from the star (the DEEP images), and the second making images with an exposure time of 0.85 seconds and not using a coronagraph (the FAST images.

The DEEP images were able to resolve both disks of LkCa 15 and the gap between them in much better detail than has previously been possible. These show the inner disk to be a roughly elliptical structure, comparable in shape and orientation to the outer disk, but approximately half the size. The FAST images show similar structures to the DEEP images, though at a lower resolution.

SPHERE IRDIS J-band imaging polarimetry of LkCa 15. Each panel shows the DEEP and FAST images side-by-side at the same scale, with insets showing the shape of the PSF core (a) Polarized flux of Deep at linear stretch (arb. units). The inner disk saturates the color scale. (b) The corresponding S/N map at a stretch of [-10σ , 10σ ]. (c) Polarized flux of DEEP after scaling with an inclined r² map to render the faint disk structures visible (arb. units). (d-f) The same three images for FAST. While overall sensitivity is lower in these data, they a fford an unobstructed view onto the inner disk. In all panels, the star’s location is marked with a white disk. The black wedges on the color scales mark the zero level. Thalmann et al. (2016).

The two arms of the inner disk appear to be asymmetrical, with the western arm trending outward and the eastward arm curling inward, and there appears to be a local brightening along the far side of the minor axis. There is also an apparent darkening on the inner part of the disk, possibly indicating a gap within it, though Thalmann et al. are cautious of over-interpreting these results, which are at the very limit of the telescope's operating capacity. The three candidate planets were also resolved, though again Thalmann et al. advise caution, but in this case they do feel the evidence for the best understood planet (LkCa 15b), is particularly strong, and note that such a planet could cause some of the apparent structures seen in the inner disk.

Thalmann et al. resolve the outer disk as being tilted at an angle of 60° seen by an Earth-based observer. They could not resolve any spirals or structural asymmetries within this outer disk, but did note four dimmed radial lines at 50° , 135° , 200° , and 325°. The nature of these lines is unclear, though they could be shadows cast by inner disk regions or magnetospheric accretion columns.

Analysis of the outer disk structure of LkCa 15. (a) Ellipse fits to the maximum gradient (solid blue line) and the flux minimum (dotted blue line) in the r²-scaled DEEP image. (b) Comparison of the best-fit gap edge in J-band (blue solid line) with those in RI-band (red long-dashed line) and sub-millimeter interferometry (green short-dashed line). (c) Full-intensity KLIP image (5 subtracted modes) of the Full data in the K1K2 filter for comparison. The gap edge derived from the DEEP image coincides very well with the edge of the bright crescent in the KLIP image. (d) The image in panel (a) at a harder stretch, emphasizing the surface brightness variations in the outer disk. Four position angles with reduced brightness are marked, possibly indicating transient shadowing from the inner disk. Thalmann et al. (2016).

See also...

Transition disks around LkCa 15.             Planets are thought to form in protoplanetary disks, which is to say disks of gas and dust around young stars. However not all the material in a protoplanetary disk is likely to be used up in the formation of planets, leaving one or more debris disks, such as the Main Asteroid Belt and Kuiper Belt in our own Solar System. These debris disks typically contain rocky and icy bodies, but not...

Searching for circumplanetary disks around LkCa 15.                                                        Just as young stars are typically surrounded by a disk of material that is accreting onto the star as well as potentially coalescing to form planets (circumstellar or protoplanetary disks), comets and other bodies; young planets, particularly very large ones, ought in theory to be surrounded by smaller disks of material, accreting onto the planet and potentially coalescing to form moons....

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Understanding the disk around HL Tauri.

HL Tauri is a young protostar (star which is still gaining mass by accretion from a surrounding disk, the accretion disk, and is emitting ionized material in jets from its poles) 140 parsecs (457 light years) from our Solar System in the constellation of Taurus. It's disk has been known and observed for a number of years, however recent observations by the Atacama Large Millimeter/submillimeter Array (ALMA) revealed this disk to be divided into a series of light and dark bands. This is surprising, as such banding is usually associated with planet formation in a more developed disk (a protoplanetary disk) with darker zones forming where (invisible) planets have formed and either accreted or tidally pushed away the (visible) dust in a the dark bands.
 

In a paper published on the arXiv database at Cornell University Library on 11 March 2016, a team of astronomers led by Carlos Carrasco-González of the Instituto de Radioastronomíay Astrofísica discuss a series of new observations of HL Tauri made with the Very Large Array (VLA) of the National Radio Astronomy Observatory at wavelengths of 6.7, 7.0 and 7.3 mm (observations of remote objects at different wavelengths tend to reveal different features, as different molecules reflect light at different wavelengths) between December 2014 and September 2015.

Atacama Large Millimeter/submillimeter Array of HL Tauri at a wavelength of 1.3 mm. The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) are shown. Carrasco-González et al. (2016).

Carrasco-González et al. were able to observe many of the features seen in the ALMA images of HL Tauri, including the innermost dark and light bands. However the much longer wavelengths used by the enabled a much more detailed study of the inner part of the disk, an area essentially opaque in the ALMA images. This enabled Carrasco-González et al. to make estimations of the density of particles and average grain sizes within different parts of the disk. 

Left: VLA image at 7.0 mm with an angular resolution of ~20 au (0.”15; tapered image). Right: Close-up to the center of the disk. VLA image at 7.0 mm with an angular resolution of ~10 au (~0.”7; natural weighting). The positions of the reported dark (D1-D7; dotted lines) and bright rings (B1-B7; dashed lines) from the ALMA images are shown. The inner disk and the first pair of dark (D1) and bright (B1) rings are clearly seen in the 7.0 mm images. Carrasco-González et al. (2016).

The VLA observations showed showed structure within the dust distribution in the innermost bright ring of HL Tauri. Here the dust was found to form clumps or knots, most of which were of a temporary nature, but with one particularly dense clump on the northeastern part of the limb being more permanent in nature. Carrasco-González et al. suggest that this is possibly a protoplanet in the earliest stages of formation, and that therefore planet formation around of HL Tauri is beginning in the brighter areas of the disk, not the lighter, with the bright areas representing parts of the disk with higher densities due to early stage planetary accretion rather than areas from which dust has been cleared by later stage planetary formation.

(a) Superposition of the VLA 7.0 mm image (contours; naturally weighted image; beam size ≈0:”067) over the ALMA 1.3 mm image (colour scale). Contour levels are -4, 4, 6, 8, 10, 15, 20, 25, 30, 40, 50, 100, and 150 times the rms of the 7.0 mm map, 3.5 μJy beam^-1. The two arrows mark the direction of the collimated jet at a P.A. of ~45º. (b) A close-up to the center of the disk. Colour scale is the ALMA 1.3 mm image and contours are from the high angular resolution VLA 7.0 mm image (robust 0 weighted image; beam size ≈0:”04). Contour levels are 3, 4, 5, 6, 7, 8, 12, 16, 20, 24, 28, 32, 40, 48, and 56 times 7 μJy beam^-1. (c) and (d): Comparison between sub-bands contour images at 7.3 mm and 6.7 mm (robust 0.4 weighting; beam sizes ≈0:”053) over the 7.0 mm colour scale image. Contours in both panels are 8, 9, 10, 11, 12, 13, 14, 15, and 16 times 6 μJy beam^-1.

See also...

The Keplerian Disk of Class I Protostar L1489 IRS.                                                Recent studies of the Keplerian Disks around other Protostars with the Submillimeter Array (SMA) have suggested that in the early Class 0 Protostar stage little rotation occurs within the Keplerian Disk and the rate of infalling (i.e. the rate at which material falls from...

Protoplanetary disks around Class I Protostars in the ρ Ophiuchi Star Forming Region.                                                        Stars are thought to form from the aggregation of material from vast clouds of molecules known as Stellar Nurseries or Star Forming...

Water and Hydroxides in the Circumstellar Disk around HD 163296.                                  HD 163296 is a young Herbig Ae star (a star producing heat by gravitational collapse, which is expected will fuse Hydrogen in the future, but which has not reached this stage yet) slightly under 400 light years from Earth. It is surrounded by a fairly well documented circumstellar disk, which reaches slightly over 900 AU from the star (i.e. over 900 times as far from the star as Earth is from the...

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Transition disks around LkCa 15.

Planets are thought to form in protoplanetary disks, which is to say disks of gas and dust around young stars. However not all the material in a protoplanetary disk is likely to be used up in the formation of planets, leaving one or more debris disks, such as the Main Asteroid Belt and Kuiper Belt in our own Solar System. These debris disks typically contain rocky and icy bodies, but not free gas. However this is not an instantaneous event, and a period exists where a planetary system is forming accompanied by one or more disks of dust and gas; these evolving disks in forming planetary systems are called 'transition disks.

In a paper published on the arXiv database at Cornell University Library on 5 January 2015 and accepted for publication in the Publications of the Astronomical Society of Japan, a team of scientists led by Daehyun Oh of the Department of Astronomical Science of the Graduate University for Advanced Studies and the National Astronomical Observatory of Japan discuss the results of a study of the LkCa 15 stellar system and its transition disks made with the Subaru Telescope.

LkCa 15 is a young (2-5 million-year-old) K5-type orange dwarf star, roughly 547 light years from Earth in the constellation of Taurus. It has approximately the same mass as the Sun but only about 74% of its luminosity, new material is still accreting onto the star at a rate of about one Earth mass every 23 years.  The system has a well documented transition disk as well as two candidate planets, with a second inner disk having been discovered in 2015.

Oh et al. were able to clearly resolve two elliptical disks around LkCa 15, these are apparently in the same plane, tilted at a angle of 44˚ from our perspective. However the gap separating these disks, which forms an apparent third component to the system, appears to be tilted at a greater angle, 52˚. Since this gap is only an apparent member of the disk system, not a genuine object, this apparent difference in angle can only be a product of the structure of the two actual rings.

PI and overlapped polarization vector map images (2.0′′ × 2.0′′) before (a) and after (b) halo subtraction. The saturated region is occulted by a software mask (r∼0.1′′ ), the vectors are binned with spatial resolution, and the lengths are arbitrary for presentation purposes. (a): The effect of a polarized halo appears to have a tendency toward the minor axis of the disk. (b): The polarization tendency to the minor axis was removed, and the disk-origin polarization along the disk surface was revealed. Bottom: The radial Stokes Qr (c) and Ur (d) images. In the Qr image, both the outer and inner disks are significantly detected as expected from the PI image. On the other hand, the Ur image shows no disk-like component. Oh et al. (2015).

In order to resolve this Oh et al. examined the brightness asymmetries of the disks in order to glean further information about their inclination. This works because the side of the disk behind the star is reflecting light directly back towards us, whereas light reaching us from the near side has to be scattered through the disk. Using this method they found that the inner portions of the disks were misaligned by about 13˚, the best explanation for this being that the inner disk is significantly warped.

Elliptical fitting results of the inner disk (purple), the gap (yellow), and the outer disk (red). The image has been smoothed by a gaussian with r=2 pixels to reduce the effects of speckles on the inferred structure of the disk. The central region is also shown in the right top panel. White star indicates the location of LkCa 15. Green and orange stars indicate where the planet candidates LkCa 15 b and c were detected in 2014, respectively (Sallum et al. 2015). Empty green and orange circles indicate the locations of two infrared sources seen in 2009-2010 (Kraus & Ireland 2012), which are assumed as LkCa 15 b and c, respectively. Oh et al. (2015).

This study greatly adds to the evidence for the presence of one or more planets or protoplanets in the LkCa 15 system. The warping of the inner disk observed would require the presence of a planet with a planet having a mass at least equivalent to that of Jupiter, while the gap between the two disks is about 27 AU (27 times the distance at which the Earth orbits the Sun), which would require multiple such large planets to clear.

See also...

Detecting debris disks around small nearby stars in old Hubble images.                        Debris disks are rings of dust, rock and icy material left surrounding stars after planet formation has occurred (unlike protoplanetary disks, which are present around very young stars only, and which are thought to be largely consumed by planetary formation). Our Solar System has two such debris disks, the Asteroid Belt and the Kuiper Belt, and in recent years...

The outer disk of T Chamaeleontis.                 T Chamaeleontis is a T Tauri star (a very young star which has not yet began to generate heat by hydrogen fusion, but which produces considerable energy through gravitational heating) estimated to be about 7 million years old, roughly 350 light years from Earth in the constellation of Chamaeleontis. It is known to be surrounded by two transition disks (disks of dust and gas surrounding very young stars, thought to...

 

The Keplerian Disk of Class I Protostar L1489 IRS.                                               Recent studies of the Keplerian Disks around other Protostars with the Submillimeter Array (SMA) have suggested that in the early Class 0 Protostar stage little rotation occurs within the Keplerian Disk and the rate of infalling (i.e. the rate at which material falls from the Disk onto the Protostar) is high. In late...

  

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Searching for circumplanetary disks around LkCa 15.

Just as young stars are typically surrounded by a disk of material that is accreting onto the star as well as potentially coalescing to form planets (circumstellar or protoplanetary disks), comets and other bodies; young planets, particularly very large ones, ought in theory to be surrounded by smaller disks of material, accreting onto the planet and potentially coalescing to form moons. Such disks should potentially be detectable, although as these disks are smaller and less massive than circumstellar disks, so they will be correspondingly hard to observe.

In a paper published on the arXiv database at Cornell University Library on 22 April 2014, a team of scientists led by Andrea Isella of the Department of Astronomy at the California Institute of Technology, describe the results of a search for circumplanetary disks around the young star LkCa 15, using the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array.

LkCa 15 is a young (2-5 million-year-old) K5-type orange dwarf star, roughly 547 light years from Earth in the constellation of Taurus. It has approximately the same mass as the Sun but only about 74% of its luminosity, new material is still accreting onto the star at a rate of about one Earth mass every 23 years. The system has an observed circumstellar disk with an inner margin about 45 AU from the star (i.e. about 45 times the average distance at which the Earth orbits the Sun), the area starward of this inner edge being thought to have coalesced into a number of planetary bodies. The material beyond 45 AU is unlikely to go on to form planets as it is too diffuse and scattered (hence circumstellar disk rather than protoplanetary disk), but may form comets or similar bodies. A single potential planet has been detected in the LkCa 15 system, LkCa 15b; if this observation is accurate the planet has a mass 6-10 times that of Jupiter and orbits at a distance of 16 AU. Such a large, young planet, or any other similar body in the system, would be likely to have a large circumplanetary disk, which would be amenable to detection.

Isella et al. were able to detect an inner ring of material about the LkCa 15, apparently made up of about 3 Earth masses of dust and fine grains within a few AU of the star. However they were not able to detect any circuplanetary disk about the candidate planet LkCa 15b, or any other body in the system. This non-detection does not mean such disks do not exist, but rather that if they do then they were below the limits for detection by the array. One or more disks comprising about 10% of the mass of Jupiter within 1 AU of a planet could still potentially exist in the LkCa 15 system, although this would imply that, despite the young age of the system and any potential planets, that the majority of planetary accretion has already taken place.

(Top) 1.6” x 1.6” map of the LkCa 15's continuum disk emission observed at the wavelength of 7 mm obtained by reducing the weights of the complex visibilities measured on the longest baselines to increase the sensitivity of the extended structures. The rms noise level in the map is 6.1 µJy beam-1 . The FWHM of the synthesized beam is 0.15”. (Center) Map of the 7 mm emission obtained by adopting natural weighting of the complex visibilities to maximize the angular resolution and the point source sensitivity. The rms noise level is 3.6 µJy beam-1 and the FWHM of the synthesized beam is 0.07”. The green ellipse corresponds to an orbital radius of 45 AU and traces the outer edge of the dust depleted cavity as measured from the observations at 1.3 mm. (Bottom) Map of the innermost 45 AU disk region. Contours are plotted at 2 and 4x the noise level. The white triangle shows the expected position of LkCa 15 b assuming that the star is located at the peak of the 7 mm emission. Isella et al. (2014).

See also…

 Discovering new debris disks in old Hubble images.

 The debris disk of 49 Ceti.

 Water and Hydroxides in the Circumstellar Disk around HD 163296.

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Three giant exoplanets in very wide orbits around young stars.

In the past two decades a large number of planets have been discovered orbiting other stars (exoplanets). The vast majority of these have been large planets orbiting close to their host stars, such planets being easier to detect due to the influence that their gravity has on the star. Planets further from their stars are harder to detect, as their gravity has less effect upon the star, and they have long orbital periods which will tend to mask this anyway. Such planets are more likely to be detected by direct imaging, though this will require separate observations over a long period of time to confirm the relationship with the host star.

In a paper published on the online arXiv database at Cornell University Library on 29 November 2013 and accepted for publication in The Astrophysics Journal, a team of scientists led by Adam Kraus of the Department of Astronomy at the The University of Texas at Austin and the Harvard-Smithsonian Center for Astrophysics describe three giant planets orbiting young stars at distances in excess of 100 AU (i.e. more than 100 times the distance at which the planet Earth orbits the Sun). All three bodies had previously been noted as potential planets over a decade ago, but can only now be confirmed as objects in orbit about their stars, due to follow-up observations that have tracked their movement. 

Large planets in very wide orbits about young stars presents a considerable challenge for conventional models of planet formation, since it should in theory take far longer for a planet to form this far from a star, potentially never accreting into a large body at all. Our own system contains considerable material beyond the orbit of Neptune (30 AU from the Sun), but this has apparently never accreted into a large planet, despite the 5 billion year age of the Solar System.

The first of the three new planets orbits the binary system FW Tau AB near the center of the Taurus- Auriga complex, 473 light years from Earth. The FW Tau system comprises two red dwarf stars  (FW Tau A and FW Tau B) each though to have a mass 28% of our Sun's orbiting one another at a distance of 11 AU (11 times the distance between the Sun and the Earth). The system is thought to be about 1.8 million years old. The planet, FW Tau b (when naming objects in other stellar systems stars are given upper case letters and planets lower case letters), is calculate to have a mass 10 times that of Jupiter and orbit at a distance of 330 AU (over 10 times the distance at which Neptune orbits the Sun). 

Infrared image of the FW Tau system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The second new planet orbits the binary star ROXs 42B, 440 light years from Earth in the constellation of Ophiuchus. The binary system comprises two stars with masses of 89% that of the Sun and 36% that of the Sun, orbiting at a distance of less than 10 AU. The system is thought to be 6.8 million years old. The new planet, ROXs 42B b is calculated to orbit this pair at a distance of 140 AU and have a mass 10 times that of Jupiter. A second, potential body in the system, provisionally dubbed ROXs 42B cc1, is thought to be a background star.

Infrared image of the ROXs 42B system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

The third new planet orbits ROXs 12, another star in the constellation of Ophiuchus, 391 light years from Earth. ROXs 12 is thought to have a mass 87% of that of the Sun and to be 7.6 million years old. ROXs 12b orbits this star at a distance of 210 AU, and has a mass 16 times that of Jupiter.

Infrared image of the ROXs 12 system. The image is not coronagraphic; most of the image is shown with a linear stretch that saturates at 110% of the peak brightness of the wide companion, while a box of size 0.5′′ is instead shown with a linear stretch that saturates at 110% of the peak brightness of the primary star in the close binary. North is up. The scale bar is 1 arc inch; i.e. 1/21 600th of the circumference of an imaginary sphere around the Earth. Kraus et al. (2013).

See also Kepler 63b; a giant planet in a polar orbit, A superjovian exoplanet directly imaged orbiting the A-class pre main-sequence star HD 95086, A fourth body in the KOI-13 system, Wasp-12b; slowly boiling away... and KOI-13b, a big, hot planet not a Brown Dwarf.

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