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FY2014 Highlights

by Davis Murphy last modified Sep 28, 2016 by Amy Mioduszewski

View Highlights from previous years: FY2013FY2012
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An Accurate, Precise Pleiades Distance Measurement

The Pleiades hold an important place in our understanding of stellar evolution. Its hundreds of young stars help to define the zero-age main sequence, and serve as templates for determining the physical characteristics of more distant stars.  Because many derived physical properties of stars depend on their distance raised to some power, the distance to the Pleiades is an important measurement whose accuracy propagates throughout modern stellar physics.

It was of some concern, therefore, when results from the Hipparcos astrometric satellite suggested that the traditional distance to the Pleiades – about 135 pc via multiple techniques – was 10% too high and that the cluster was located only 120 pc away.  At the closer distance some stars in the Pleiades would be decidedly peculiar, possibly implying enhanced helium abundances and hinting of serious flaws in our understanding of fundamental stellar evolution.

To resolve the issue a research team led by Carl Melis (UC, San Diego) undertook a campaign of precise radio astrometry on several Pleiades stars that are weak radio sources and measured their annual stellar parallax with respect to a background quasar over ~18 months. Because of the weakness of the radio signal, a telescope network was used that included the VLBA, GBT, Arecibo and Effelsberg. The resultant distances to four stars had uncertainties of 0.5 to 1.1 pc and this a 99+% precision in the measured distance. After taking into account that the stars are likely at different depths in the cluster, an effect estimated from the cluster's angular size, the very long baseline interferometry measurements yield a distance for the combined sample of 136.2 ±1.2 pc. This is the most accurate and precise distance measurement for the Pleiades, and is compatible with most previous measurements, except for the Hipparcossatellite.

Figure: A summary of the distance measurements. It is now of considerable interest to understand the source of the systematic error in the Hipparcos results, as the astrometric satellite Gaia, a successor to Hipparcos with considerably enhanced capabilities, has similarities of design and data analysis, and is now mapping the Milky Way.

View Paper: Toward a VLBI resolution of the Pleiades distance controversy, Melis, C., Reid, M.J., Mioduszewski, A.J., Stauffer, J.R., Bower, G.C. 2014, Science, 345, 1029 (29 August 2014).

View 28 August 2014 Press Release

Added 02 October 2014

The Intrinsic Size of Sgr A*

Bower et al. have used the VLBA at a wavelength of 7mm to make the best measurement to date of the intrinsic size and two-dimensional structure of the radio source associated with the Galactic Center black hole, SgrA*. The intrinsic source is modeled as an elliptical Gaussian with major x minor axis size of 35.4 x 12.6 RS in position angle 95 deg East of North. This morphology can be interpreted in the context of both jet and accretion disk models for the radio emission. The authors have also placed a maximum peak-to-peak change of 15% in the intrinsic major axis size over five different epochs. Three observations were triggered by detection of near infrared (NIR) flares and one was simultaneous with a large X-ray flare detected by NuSTAR. The absence of simultaneous and quasi-simultaneous flares indicates that not all high-energy events produce variability at radio wavelengths. This supports the conclusion that NIR and X-ray flares are primarily due to electron excitation and not to an enhanced accretion rate onto the black hole.

Figure: Two-dimensional and one-dimensional intrinsic size estimates for Sgr A*. Two-dimensional solutions were obtained through deconvolution of all apparent sizes with the two-dimensional scattering size within the 3σ error surface. Colors show solutions that are <1σ (yellow), between 1 and 2σ (green), and between 2 and 3σ (red). Slices through the parameter space at the position of the best-fit position angle, minor axis, and major axis are shown (upper left, upper right, and lower left, respectively). The blue star indicates the best-fit size. The lower right panel illustrates the intrinsic size determined using only the measured major axis size. The same color-coding applies to this panel. The plots show that the maximum intrinsic minor-axis value never reaches the minimum permitted value for the major axis; that is, the intrinsic source must be elliptical.

View Publication: The Intrinsic Two-Dimensional Size of Sagittarius A*, Geoffrey C. Bower (Berkeley, ASIAA), Sera Markoff (Amsterdam), Andreas Brunthaler (MPIfR), Casey Law (Berkeley), Heino Falcke (MPIfR, Radboud, ASTRON), Dipankar Maitra (Wheaton), M. Clavel (Paris, CEA Saclay), A. Goldwurm (Paris, CEA Saclay), M.R. Morris (UCLA), Gunther Witzel (UCLA), Leo Meyer (UCLA), and A.M. Ghez (UCLA), 2014 ApJ, 790, 1 (20 July 2014).

Added 23 July 2014

Mapping the Milky Way’s Sagittarius Spiral Arm

As part of the continuing Bar and Spiral Structure Legacy Survey (BeSSeL Survey) with the VLBA, Wu et al. report measurements of parallaxes and proper motions of ten high-mass star-forming regions in the Sagittarius spiral arm of the Milky Way. Combining these results with eight others from the literature, the authors investigated the structure and kinematics of the Sagittarius arm between Galactocentric azimuths β ≈ −2° and 65°. They find that the spiral pitch angle is 7.3° ± 1.5°; the arm’s half-width, defined as the rms deviation from the fitted spiral, is ≈0.2 kpc; and the nearest portion of the Sagittarius arm is 1.4 ± 0.2 kpc from the Sun. Unlike for adjacent spiral arms, the authors find no evidence for significant peculiar motions of sources in the Sagittarius arm opposite to Galactic rotation.

Figure: [Blue, Green, Red points] Locations of high-mass star formation regions in the Milky Way’s Sagittarius, local, and Scutum spiral arms, respectively, as determined by trigonometric parallaxes (with error bars). The red star is the Galactic Center.

View Publication: Trigonometric Parallaxes of Star-forming Regions in the Sagittarius Spiral Arm, Y. Wu (MPIfR), M. Sato (MPIfR), M.J. Reid (CfA), L. Moscadelli (INAF), B. Zhang (MPIfR), Y. Xu (Purple Mountain), A. Brunthaler (MPIfR), K.M. Menten (MPIfR), T.M. Dame (CfA), X.W. Zheng (Nanjing), 2014 A&A, 566, A17 (June 2014).

Added 1 August 2014

Mapping the Milky Way in Unprecedented Detail

Over 100 trigonometric parallaxes and proper motions for masers associated with high-mass stars have been measured with the Bar and Spiral Structure Legacy Survey (BeSSeL) survey. This publication represents a major milestone in VLBA astrometry. These measurements provide strong evidence for the existence of spiral arms in the Milky Way, accurately locating many arm segments and yielding spiral pitch angles ranging from 7° to 20°. The widths of spiral arms increase with distance from the Galactic center.  The distance to the Galactic center is 8.34 ± 0.16 kpc, and the circular rotation speed at the Sun is 240 ± 8 km/sec. The rotation curve is nearly flat between Galactocentric radii of ≈ 5 and 16 kpc, and the solar motion component in the direction of Galactic rotation is 14.6 ± 5.0 km/sec.

Figure: Plan view of the Milky Way showing the locations of high-mass star forming regions with trigonometric parallaxes measured by the VLBA, VLBI Exploration of Radio Astrometry (VERA), and the European VLBI Network (EVN).

View Publication: Trigonometric Parallaxes of High Mass Star Forming Regions: The Structure and Kinematics of the Milky Way,M.J. Reid (CfA), K.M. Menten (MPIfR), A. Brunthaler (MPIfR), X.W. Zheng (Nanjing), T.M. Dame (CfA), Y. Xu (Purple Mountain), Y. Wu (MPIfR), B. Zhang (MPIfR), A. Sanna (MPIfR), and M. Sato (MPIfR), 2014 ApJ, 783, 130 (10 March 2014).

Added 2 May 2014


Probing the Inner Galaxy

Inner_Galaxy.pngThe pulsed radio emission from the Galactic Center (GC) magnetar SGR J1745-29 probes the turbulent, magnetized plasma of the GC hyperstrong scattering screen through both angular and temporal broadening. The angular size of SGR J1745-29 was obtained with the VLBA and VLA at 8.7 and 15.4 GHz. The source sizes are consistent with the scatter-broadened size of Sgr A*, demonstrating that SGR J1745-29 is also located behind the same hyperstrong scattering medium. The distance to the screen is 5.9 ± 0.3 kpc, consistent with being located in the Scutum spiral arm. This is a substantial revision of the previously held model in which the scattering screen is located very close to the GC. This result provides promise for pulsar searches at the GC at 10GHz.

Figure: VLBI mages of J1745-29 at 8 and 15GHz.

View Publication: The Angular Broadening of the Galactic Center Pulsar SGR J1745-29: A New Constraint on the Scattering Medium, Geoffrey C. Bower (Berkeley), Adam Deller (ASTRON), Paul Demorest (NRAO), Andreas Brunthaler (MPIfR), Ralph Eatough (MPIfR), Heino Falcke (ASTRON, MPIfR, Radboud), Michael Kramer (MPIfR), K.J. Lee (MPIfR), and Laura Spitler (MPIfR), 2014 ApJL,780, L2, (1 January 2014).

Added 21 Jan 2014


Measuring the Hubble Constant

Hubble_Constant_2.pngA new geometric maser distance has been estimated to the active galaxy NGC 4258 from 18 epochs of Very Long Baseline Interferometry observations, and 10 years monitoring the velocities of the 22 GHz masers in NGC 4258, including the VLBA. A new model includes both disk warping and confocal elliptical maser orbits with differential precession. The distance to NGC 4258 is determined to be 7.60 ± 0.17 ± 0.15 Mpc, a 3% uncertainty including formal fitting and systematic terms. The resulting Hubble Constant, based on the use of the Cepheid variables in NGC 4258 to recalibrate the Cepheid distance scale, is H0 = 72.0 ± 3.0 km s-1 Mpc-1.

Figure: Covariance plot of Ho and w from the NGC 4258 maser observations.

View Publication: Toward a New Geometric Distance to the Active Galaxy NGC 4258. III. Final Results and the Hubble Constant, Liz Humphreys (ESO, CfA), Mark Reid (CfA), Jim Moran (CfA), Lincoln Greenhill (CfA), and Alice Argon (CfA), 2013 ApJ, 775, 13 (1 September 2013).

Added 21 Jan 2014

View Highlights from previous years: FY2013FY2012