Image Credit: NASA, ESA, and the Hubble Heritage Team (STScI/AURA) – ESA/Hubble Collaboration
Otherwise known as N90, this is a classic region for the study of star formation, boasting a well-defined pre-main sequence. More than 5,500 cluster members have been detected with HST, down to the 28th magnitude.
The region, with Chandra X-ray (purple) and Spizter infrared (red) overlain on the Hubble optical image.
Image credit: HST
Sharpless 2-106, Sh2-106 or S106 for short, lies nearly 2,000 light-years from us. The nebula measures several light-years in length. It appears in a relatively isolated region of the Milky Way galaxy. A massive, young star, IRS 4 (Infrared Source 4), is responsible for the furious activity we see in the nebula. Twin lobes of super-hot gas, glowing blue in this image, stretch outward from the central star. A ring of dust and gas orbiting the star acts like a belt, cinching the expanding nebula into an “hourglass” shape. Hubble’s sharp resolution reveals ripples and ridges in the gas as it interacts with the cooler interstellar medium. Dusky red veins surround the blue emission from the nebula. The faint light emanating from the central star reflects off of tiny dust particles. This illuminates the environment around the star, showing darker filaments of dust winding beneath the blue lobes. Detailed studies of the nebula have also uncovered several hundred brown dwarfs. At purely infrared wavelengths, more than 600 of these sub-stellar objects appear. These “failed” stars weigh less than a tenth of our Sun. Because of their low mass, they cannot produce sustained energy through nuclear fusion like our Sun does. They encompass the nebula in a small cluster.
An alternative image from the Gran Telescopio Canarias (GTC) is here and a press release from the Subaru telescope concerning the brown dwarf discoveries is here, with the 2006 paper from the Astronomical Journal. At left is the infrared imaging from Subaru. The least massive and faintest brown dwarfs observed are only a few times more massive than Jupiter. Examples are highlighted in a supplementary image.
Image Credit: Gemini Observatory/AURA
This image, obtained during the late commissioning phase of the GeMS adaptive optics system, with the Gemini South AO Imager (GSAOI) on the night of December 28, 2012, reveals exquisite details in the outskirts of the Orion Nebula. The large adaptive optics field-of-view (85 arcseconds across) demonstrates the system’s extreme resolution and uniform correction across the entire field. The three filters used for this composite color image include [Fe II], H2, and, K(short)-continuum (2.093 microns) for blue, orange, and white layers respectively. The natural seeing while these data were taken ranged from about 0.8 to 1.1 arcseconds, with AO corrected images ranging from 0.084 to 0.103 arcsecond. Each filter had a total integration (exposure) of 600 seconds. In this image, the blue spots are clouds of gaseous iron “bullets” being propelled at supersonic speeds from a region of massive star formation outside, and below, this image’s field-of-view. As these “bullets” pass through neutral hydrogen gas it heats up the hydrogen and produces the pillars that trace the passage of the iron clouds.
Image source and more, including an animation of this image together with a 2007 image from an earlier AO instrument, showing the movement of the bullets over a five year baseline.
It is said that the astronomer Copernicus never saw the elusive planet Mercury, so perhaps it is not so surprising that modern day astronomers had to wait a long time before the first observational confirmation of both brown dwarfs and extrasolar planets in 1995, although while planets around other stars had been postulated for centuries, the idea of a brown dwarf first hit the science stands only in 1963.
Today I want to mention two ongoing research efforts, one using the radial velocity method and the other an unusual direct imaging technique. The MARVELS survey is one of four efforts which form the third phase of the Sloan Digital Sky Survey (SDSS III). This ongoing project will monitor the radial velocities of 11,000 bright stars over six years and be able to detect giant planets in orbits with periods of two years or less. One aim of the project is to investigate the so-called ‘brown dwarf desert’, an apparent paucity of these objects in close binary orbit around main sequence stars when compared to low-mass stellar companions. One brown dwarf companion has a minimum mass around 28 Jupiter masses and orbits a slightly evolved F9 star roughly every six days. A further similar discovery was announced today, a forty Jupiter mass object in an eccentric 13 day orbit around a G0 subgiant.
The Lucky imaging technique exploits extremely short exposure times to ‘freeze’ atmospheric (seeing) disturbances to obtain resolutions near the diffraction limit from ground-based telescopes. In a recent paper new low mass companions have been found in a sample of 451 late K and M stars, constraining the binary fraction in this regime to around 20%.
Those interested in the pre-1995 history of seeking brown dwarfs should refer to this presentation by Rafael Rebolo, one of the leaders in the field, and review articles by Gibor Basri (2000), Oppenheimer et al. (2000) and Kumar (2002).
The Planets around Low Mass Stars (PALMS) collaboration has published two discoveries relevant to the ‘brown dwarf desert’ question, using high contrast adaptive optics imaging on Keck and Subaru. The second is a good candidate for a near-future dynamical mass measurement.
Update: MARVELS-6b is a newly discovered 32 Jupiter mass object in a 47 day orbit around a solar-type star with an age less than 6 Gyr.
Image credit: ESO
An evocative new image from ESO shows a dark cloud where new stars are forming, along with a cluster of brilliant stars that have already emerged from their dusty stellar nursery. The new picture was taken with the MPG/ESO 2.2-metre telescope at the La Silla Observatory in Chile and is the best image ever taken in visible light of this little-known object. In the centre of this new image there is a dark column resembling a cloud of smoke. Within this column shines a small group of brilliant stars. At first glance these two features could not be more different, but they are in fact closely linked. The cloud contains huge amounts of cool cosmic dust and is a nursery where new stars are being born. It is likely that the Sun formed in a similar star formation region more than four billion years ago. This cloud is known as Lupus 3 and it lies about 600 light-years from Earth in the constellation of Scorpius. As the denser parts of such clouds contract under the effects of gravity they heat up and start to shine. At first this radiation is blocked by the dusty clouds and can only be seen by telescopes observing at longer wavelengths than visible light, such as the infrared. But as the stars get hotter and brighter their intense radiation and stellar winds gradually clear the clouds around them until they emerge in all their glory.
Image credit: Digital Drew Space Art’s photostream as featured by Universe Today.
The WD 0137-349 system is an odd ultra close binary approximately 200 light years away. The smaller but more massive component is a small massive white dwarf, formerly the core region of a normal star. The larger component is a close low mass brown dwarf which orbits its white dwarf companion in a tight two hour orbit. It glows with dull reddish color, and largely from this angle the brown dwarf is illuminated by the white dwarf.
This binary, discovered in 2006, is proof of the survival of a brown dwarf companion through engulfment during the red giant phase. The brown dwarf has been detected in the near-infrared. The period is 116 minutes. An even shorter period binary, NLTT 5306, has been discovered very recently. Wider, spatially resolved binaries consisting of a white dwarf and a proven substellar companion are known, but rare. Unresolved L dwarf companions to white dwarfs are also few in number. Prior to the discovery of the Y dwarf companion to WD 0806-661 to which I drew attention a few days ago, one wide T dwarf companion is also in the literature, the companion to the high proper motion white dwarf LSPM 1459+0857. Lastly, one interacting white dwarf binary with a substellar secondary has been confirmed spectroscopically using the 8 meter Gemini instrument, SDSS 1212.
Image credit and information APOD. Cometary globules, where dust grains are swept back and made to glow by ultraviolet radiation from nearby hot stars, were only recognised as a phenomenon in 1976. This image represents several hours exposure time with a 14.5 inch telescope, and can be compared to an older view from the Anglo-Australian Observatory, which I learn today has, along with everyone associated with Siding Spring, survived the bush fires. The exception is the observatory Lodge.
This image shows just how difficult it is to pick out faint brown dwarf companions against the stellar background, in this case by measurement of proper motion to confirm the association of the companion with the white dwarf, over a five year baseline. Lead author Kevin Luhman on this 2011 discovery:
“This planet-like companion is the coldest object ever directly photographed outside our solar system,” said Luhman, who led the discovery team. “Its mass is about the same as many of the known extra-solar planets — about six to nine times the mass of Jupiter — but in other ways it is more like a star. Essentially, what we have found is a very small star with an atmospheric temperature about cool as the Earth’s.”
Two further papers further clarify the nature of the companion, which is also known as GJ 3483b. It has an effective temperature in the range 300 – 345K, and a mass less than 10 – 13 Jupiter masses. It is also the reddest brown dwarf known to date.
Today a new, similar object, WISE 1828+2650, has been reported, a Y2 dwarf with an estimated temperature of 250 – 400K. The two objects are compared in the paper. Unlike the companion to WD 0806-661, where the age is known from the age of the white dwarf (about 1.5 Gyr) there is no age constraint, so the mass could lie anywhere between 0.5 and 20 Jupiter masses for ages 0.1 – 10 Gyr. Comparison with theoretical models gives a mass 3 – 6 Jupiter masses for a plausible age between 2 and 4 Gyr. This estimate is backed by the high tangential motion of the object. Whether WISE 1828+2650 sits at the low mass end of the brown dwarf population or is the first example of a large number of “free-floating” planets is not yet known. If such a population exists, it may yet be discoverable at lower signal-to-noise ratios in the WISE data. I caution that the status of these objects is still uncertain. From the new paper:
The absolute magnitude of WISE 1828+2650 is brighter by several magnitudes (depending on wavelength) than extrapolated from other Y dwarfs making the true nature of this source somewhat of a mystery. WISE 1828+2650 is similar in color and absolute magnitude to the cool object orbiting the nearby white dwarf WD 0806-661 B. The exact nature and evolutionary state, including its mass and age, of WISE 1828+2650 will require further observation and theoretical investigation.
On the subject of a population of isolated planetary mass objects, the young (3 Myr) sigma Orionis cluster was one of the first hunting grounds for such objects, which are brighter at young ages for a given mass. As reported in another new paper today, the number of planetary mass objects (4 – 12 Jupiter masses) known in the region has increased by 23 to 37, together with 69 brown dwarfs.
This system*, discovered in 2010 and published in Nature in 2011, contains six planets orbiting with periods between 10 and 118 days. The planets are super-Earths, with masses of 4.3, 13.5, 6.1, 8.4 and 2.3 Earth masses (the mass of the outermost planet is not well constrained) and radii between 2 and 4.5 Earth radii. The star is very Sun-like, spectral type G6V. Compared to the Solar system, the orbits would all be contained within the orbit of Venus. Lead author Jack J. Lissauer states:
By measuring the sizes and masses of the five inner planets, we have determined they are among the smallest confirmed exoplanets, or planets beyond our solar system. These planets are mixtures of rock and gases, possibly including water. The rocky material accounts for most of the planets’ mass, while the gas takes up most of their volume.
More discoveries of this nature may soon be made from the ground at Paranal with the Next Generation Transit Survey (NGTS), which is a cost-effective venture using off-the-shelf telescopes, publicized today and led by Don Pollacco at the University of Warwick. The wide-field NGTS will survey nearby stars brighter than thirteenth magnitude, concentrating on spectral types between early M and late F. This covers the types of star thought most likely to harbour habitable planets, and will naturally feed instruments like HARPS for ground-based radial velocity follow-up observations. The figure (left) shows the detection space:
Parameter space for transit detection with the yellow area indicating the prime science parameter space search of NGTS. Transit depth is indicated as a function of planet and star radius. Approximate spectral types of stars are also indicated, as well as the radii of representative Solar system planets. Known transiting systems are marked in green where they were discovered in ground-based transit surveys, blue if they were originally identified in radial velocity surveys, and red if they were discovered from space.
The special geometry of transit observations allows masses and radii of planets to be accurately constrained, which is sought after to allow statistical comparison of bulk planetary composition with theoretical models. Moreover, the transit geometry also allows to probe the atmosphere during secondary eclipse. NGTS will make available targets for high precision transmission spectroscopy of starlight passing through the planetary atmosphere, which is only possible using large telescopes, or from space.
*Note: planet masses and radii in this link are in units of Jupiter masses and radii.
White dwarf stars can shred and vaporise any asteroids or rocky planets in their vicinity, if these small bodies are flung inward by gravitational interaction with a larger outer body. The debris disk reveals itself spectroscopically via double peaked emission lines. From the New Scientist article:
The spectra indicate that a disc containing calcium, magnesium, and iron gas is orbiting the white dwarf at a distance 100 times closer than Mercury’s orbit around the Sun (below right). At this distance, intense radiation from the white dwarf heats the gas to 5000 Kelvin. The spectra also show that the white dwarf’s atmosphere is enriched in magnesium. That indicates material from the disc is falling onto the star, since the star’s own surface gravity is so great that its own heavy elements should have already sunk towards its centre – and out of sight.
In another, early postulation of a tidally destroyed asteroid calcium was found to be abundant in the white dwarf G29-38. There is now a diversity of up to eleven different metals observed, with carbon-to-silicon ratios consistent with bulk Earth material, implying rocky material made up of iron- and magnesium- rich silicates.
Observationally, disk material is sought using wide area surveys via detection of infrared excesses. Cool DZ white dwarfs, with metal lines, have been found in the Sloan Digital Sky Survey. Rather than originating in the interstellar medium, as was thought for many years, the source of the metal lines is rocky planetesimals. Since white dwarfs are the end point of evolution of the vast majority of stars, observations of metal-rich material associated with them give a robust lower limit on the number of stars like the Sun, as well as those rather more massive, which form planetary systems.
Main image credit. Note this is a simulation of an accretion disk around a compact object, such as a white dwarf, rather than a bona-fide debris disk illustration.
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