Archive for astrometry

Parallaxes and proper motions of 134 southern late M, L and T dwarfs

Posted in astronomy with tags , , on April 22, 2016 by Tim Kendall

groombridge1830_720x956Stars in nearby solar space exhibit large proper motions because they are moving against a background of more distant stars. All stars have their own motion through space and most stars in the solar vicinity share approximately the velocity and direction of motion of the Sun. In the case where a parallax as well as the transverse velocity are known, the knowledge of the exact distance together with the radial velocity yields the precise motion of the star relative to the Sun. The image shows the motion of the nearby star Groombridge 1830, using two images taken a year apart. The ability to determine these basic facts is especially interesting for investigating young brown dwarfs near the Sun, as  a new paper by Weinberger et al., accepted to the Astronomical Journal, elaborates:

We report trigonometric parallaxes for 134 low mass stars and brown dwarfs, of which 38 have no previously published measurement and 79 more have improved uncertainties. Our survey targeted nearby targets, so 119 are closer than 30 pc. Of the 38 stars with new parallaxes, 14 are within 20 pc and seven are likely brown dwarfs (spectral types later than L0). These parallaxes are useful for studies of kinematics, multiplicity, and spectrophotometric calibration. Two objects with new parallaxes are confirmed as young stars with membership in nearby young moving groups: LP 870-65 in AB Doradus and G 161-71 in Argus. We also report the first parallax for the planet-hosting star GJ 3470; this allows us to refine the density of its Neptune-mass planet. One T-dwarf, 2MASS J12590470-4336243, previously thought to lie within 4 pc, is found to be at 7.8 pc, and the M-type star 2MASS J01392170-3936088 joins the ranks of nearby stars as it is found to be within 10 pc. Five stars that are over-luminous and/or too red for their spectral types are identified and deserve further study as possible young stars.

In other news the location of the still putative Planet Nine has been refined to within a twenty degree area centred on RA 2h 40m and declination -15°.

Direct imaging discovery of a planet around a young A8 star

Posted in astronomy with tags , , , , , , on June 3, 2013 by Tim Kendall

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Image credit: ESO/Digitized Sky Survey 2. Acknowledgement: Davide De Martin
eso1324aA new, 4 – 5 Jupiter mass probable planet discovery has been reported today around the young (10 – 17 Myr) dusty A8 star HD 95086. The companion is detected in the L’ band at 3.8 microns (left, image courtesy ESO/J. Rameau) using the NACO adaptive optics instrument at the ESO VLT. The best detection has a signal-to-noise ratio of 9. The projected separation is 56 AU and the planet is still warm, with surface temperature is around 1000K. The model-dependent mass obtained means this companion is the lightest planet yet directly imaged. The star lies close to the galactic plane as can easily be seen in the main image of the field around HD 95086, but contamination of the detection by a background object is unlikely, and the planetary status of the companion is reinforced by a non-detection in the Ks band (2.18 microns). The astrometry confirms it is co-moving with the star. The paper is “Discovery of a probable 4-5 Jupiter-mass exoplanet to HD 95086 by direct-imaging”, J. Rameau et al., arXiv preprint [pdf], and is accepted to ApJ letters.

In another new development, NASA have shown that the closest red dwarf, Proxima Centauri, will pass close to two background stars in alignments in 2014 and again in 2016 (below) which will allow possible planets around Proxima to be detected by microlensing, as well as yielding an accurate mass measurement for the star itself:

Microlensing occurs when a foreground star passes close to our line of sight to a more distant background star. These images of the background star may be distorted, brightened and multiplied depending on the alignment between the foreground lens and the background source. These microlensing events, ranging from a few hours to a few days in duration, will enable astronomers to measure precisely the mass of this isolated red dwarf. Getting a precise determination of mass is critical to understanding a star’s temperature, diameter, intrinsic brightness, and longevity. Astronomers will measure the mass by examining images of each of the background stars to see how far the stars are offset from their real positions in the sky. The offsets are the result of Proxima Centauri’s gravitational field warping space. The degree of offset can be used to measure Proxima Centauri’s mass. The greater the offset, the greater the mass of Proxima Centauri. If the red dwarf has any planets, their gravitational fields will produce a second small position shift.

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Image credit: NASA, ESA, K. Sahu and J. Anderson (STScI), H. Bond (STScI and Pennsylvania State University), M. Dominik (University of St. Andrews), and Digitized Sky Survey (STScI/AURA/UKSTU/AAO). The scalloped appearance of the path of the star is due to the Earth’s orbital motion.

There is more to read on highly accurate astrometry and microlensing, as well as direct imaging, in a new article from Space Review on the specifications and exoplanet detection capabilities of WFIRST-2.4, while for those who prefer more conceptual reading, there is another article about loop quantum gravity and the resolution of black hole singularities. The paper under discussion is Gambini & Pullin, “Loop quantisation of the Schwarzschild black hole”, in Phys. Rev. Lett., (arXiv), the essential idea being that LQG can provide a mathematical description applicable to both black holes and the early Universe which is able to remove (resolve) the problematic singularity introduced by general relativity in both cases.