Zhao J.-H.,Harvard - Smithsonian Center for Astrophysics |
Morris M.R.,University of California at Los Angeles |
Astrophysical Journal | Year: 2013
We report the Very Large Array (VLA) detection of the radio counterpart of the X-ray object referred to as the "Cannonball," which has been proposed to be the remnant neutron star resulting from the creation of the Galactic center supernova remnant, Sagittarius A East. The radio object was detected both in our new VLA image from observations in 2012 at 5.5 GHz and in archival VLA images from observations in 1987 at 4.75 GHz and in the period from 1990 to 2002 at 8.31 GHz. The radio morphology of this object is characterized as a compact, partially resolved point source located at the northern tip of a radio "tongue" similar to the X-ray structure observed by Chandra. Behind the Cannonball, a radio counterpart to the X-ray plume is observed. This object consists of a broad radio plume with a size of 30″×15″, followed by a linear tail having a length of 30″. The compact head and broad plume sources appear to have relatively flat spectra (ν α) with mean values of α = -0.44 ± 0.08 and -0.10 ± 0.02, respectively, and the linear tail shows a steep spectrum with the mean value of -1.94 ± 0.05. The total radio luminosity integrated from these components is ∼8 × 1033 erg s-1, while the emission from the head and tongue amounts for only ∼1.5 × 10 31 erg s-1. Based on the images obtained from the two epochs' observations at 5 GHz, we infer the proper motion of the object: μα = 0.001 ± 0.003 arcsec yr-1 and μδ = 0.013 ± 0.003 arcsec yr-1. With an implied velocity of 500 km s-1, a plausible model can be constructed in which a runaway neutron star surrounded by a pulsar wind nebula was created in the event that produced Sgr A East. The inferred age of this object, assuming that its origin coincides with the center of Sgr A East, is approximately 9000 yr. © 2013. The American Astronomical Society. All rights reserved.
ALMA's best image of a protoplanetary disc to date. This picture of the nearby young star TW Hydrae reveals the classic rings and gaps that signify planets are in formation in this system. Credit: S. Andrews (Harvard-Smithsonian CfA); B. Saxton (NRAO/AUI/NSF); ALMA (ESO/NAOJ/NRAO) The star TW Hydrae is a popular target of study for astronomers because of its proximity to Earth (only about 175 light-years away) and its status as an infant star (about 10 million years old). It also has a face-on orientation as seen from Earth. This gives astronomers a rare, undistorted view of the complete protoplanetary disc around the star. "Previous studies with optical and radio telescopes confirm that TW Hydrae hosts a prominent disc with features that strongly suggest planets are beginning to coalesce," said Sean Andrews with the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, USA and lead author on a paper published today in the Astrophysical Journal Letters. "The new ALMA images show the disc in unprecedented detail, revealing a series of concentric dusty bright rings and dark gaps, including intriguing features that may indicate that a planet with an Earth-like orbit is forming there." Other pronounced gaps that show up in the new images are located three billion and six billion kilometres from the central star, similar to the average distances from the Sun to Uranus and Pluto in the Solar System. They too are likely to be the results of particles that came together to form planets, which then swept their orbits clear of dust and gas and shepherded the remaining material into well-defined bands. For the new TW Hydrae observations, astronomers imaged the faint radio emission from millimetre-sized dust grains in the disc, revealing details on the order of the distance between the Earth and the Sun (about 150 million kilometres). These detailed observations were made possible with ALMA 's high-resolution, long-baseline configuration. When ALMA's dishes are at their maximum separation, up to 15 kilometres apart, the telescope is able to resolve finer details. "This is the highest spatial resolution image ever of a protoplanetary disc from ALMA, and that won't be easily beaten in the future!" said Andrews. "TW Hydrae is quite special. It is the nearest known protoplanetary disc to Earth and it may closely resemble the Solar System when it was only 10 million years old," adds co-author David Wilner, also with the Harvard-Smithsonian Center for Astrophysics. Earlier ALMA observations of another system, HL Tauri, show that even younger protoplanetary discs—a mere 1 million years old—can display similar signatures of planet formation. By studying the older TW Hydrae disc, astronomers hope to better understand the evolution of our own planet and the prospects for similar systems throughout the Milky Way. The astronomers now want to find out how common these kinds of features are in discs around other young stars and how they might change with time or environment. Explore further: Discovery of multiple ring-like gaps in a protoplanetary disk More information: This research was presented in a paper "Ringed Substructure and a Gap at 1 AU in the Nearest Protoplanetary Disk", by S.M. Andrews et al., appearing in the Astrophysical Journal Letters. www.eso.org/public/archives/releases/sciencepapers/eso1611/eso1611a.pdf
Imagine taking the world's most powerful radio telescope, used by scientists around the globe, and piping a nearly continuous data stream into your research laboratory. That is exactly what scientists at the Naval Research Laboratory (NRL) in Washington, D.C. have done in collaboration with the National Radio Astronomy Observatory's Karl G. Jansky Very Large Array (NRAO VLA). The newly-completed VLA Low Band Ionospheric and Transient Experiment (VLITE for short) has been built to piggyback on the $300 million dollar infrastructure of the VLA. The primary scientific driver for VLITE is real-time monitoring of ionospheric weather conditions over the U.S. southwest. NRL ionospheric lead scientist Dr. Joseph Helmboldt says "This new system allows for continuous specification of ionospheric disturbances with remarkable precision. VLITE can detect and characterize density fluctuations as small as 30 parts per million within the total electron content along the line of sight to a cosmic source. This is akin to being at the bottom of Lake Superior and watching waves as small as 1-cm in height pass overhead. This will have a substantial impact on our understanding of ionospheric dynamics, especially the coupling between fine-scale irregularities within the lower ionosphere and larger disturbances higher up." Ionospheric disturbances represent one of the most significant limitations to the performance of many radio-frequency applications like satellite-based communication and navigation (including the GPS in your phone) as well as ground-based, over-the-horizon systems (think ham radio or AM radio). While the fine-scale irregularities that VLITE is especially sensitive to aren't large enough to make your smart phone think you are at your neighbor's house when you're really at home, they are quite problematic for vital remote sensing surveillance systems like over-the-horizon radar. The additional insights provided by VLITE into the nature of these ionospheric ripples will help us to better understand how to cope with their effects on such systems. "VLITE is also a powerful new tool in our arsenal for astrophysical research" says VLITE principle investigator Dr. Namir Kassim. He points out that "We know the Universe has many secrets including mysterious blips (so-called transients) that appear and vanish like fireflies in the night. Limited observing time at classical observatories hampers our ability to understand these intriguing objects. The power of VLITE is the nearly continual data stream over a large region of the sky. This opens up a new window on the transient Universe." At any given time, the region of the sky that VLITE peers at is so large that nearly 20 full moons would fit inside it. Astrophysics lead scientist Dr. Tracy Clarke of NRL describes VLITE as "a symbiotic instrument that piggybacks on world-class science at the VLA. It operates as a stand-alone tool for ionospheric and astrophysical studies while at the same time VLITE provides the opportunity for enhanced science in the research program running on the VLA." VLITE operations started with first light on July 17, 2014 but the real fun began two days before Thanksgiving, on November 25, 2014, when VLITE moved from a commissioning phase into full scientific operations. The system operates in real-time on 10 VLA antennas and provides 64 MHz of bandwidth centered on 352 MHz with a temporal resolution of 2s and a spectral resolution of 100 kHz. This powerful new instrument operates in parallel with the VLA and is essentially 'driven' around the sky by the primary science observer. Data streams off the telescope through dedicated systems that bypass normal VLA operations. The data then take two roads, one through real-time processing on computers located at the VLA site, and the other through off-line processing at NRL's facility in Washington. Due to the large volume of nearly continuous incoming data, all data must be analyzed by an automated pipeline that was custom designed for VLITE. Pipeline designer Dr. Wendy Lane Peters of NRL describes this process as being like "sitting in the passenger seat of a Google car and not knowing where it is taking you. VLITE is along for the ride wherever the primary science program takes us. We have to anticipate what they might do so that our pipeline is smart enough to understand the incoming data." Professor Bryan Gaensler, Director of the Dunlap Institute for Astronomy and Astrophysics at the University of Toronto, says that this is going to become the new way of doing astronomy. "It's a tragedy and a travesty that most of the information our telescopes gather from the sky is ignored and discarded. VLITE is part of a new generation of experiments that fully utilize the massive data torrents collected by the world's most powerful observatories." Over the first two months of science operations, VLITE has recorded observations of sources ranging from the Sun, nearby stars and galaxies, to some of the most distant sources in the Universe. NRL astronomers and their colleagues have been poring over the pipeline images, improving their analysis pipeline and exploring the scientific potential of the instrument. About the U.S. Naval Research Laboratory The U.S. Naval Research Laboratory provides the advanced scientific capabilities required to bolster our country's position of global naval leadership. The Laboratory, with a total complement of approximately 2,500 personnel, is located in southwest Washington, D.C., with other major sites at the Stennis Space Center, Miss., and Monterey, Calif. NRL has served the Navy and the nation for over 90 years and continues to advance research further than you can imagine. For more information, visit the NRL website or join the conversation on Twitter, Facebook, and YouTube.
The star and its disk were studied in 2014 with the Atacama Large Millimeter/submillimeter Array (ALMA), which produced what astronomers then called the best image ever of planet formation in progress. The ALMA image showed gaps in the disk, presumably caused by planet-like bodies sweeping out the dust along their orbits. This image, showing in real life what theorists had proposed for years, was surprising, however, because the star, called HL Tau, is only about a million years old—very young by stellar standards. The ALMA image showed details of the system in the outer portions of the disk, but in the inner portions of the disk, nearest to the young star, the thicker dust is opaque to the short radio wavelengths received by ALMA. To study this region, astronomers turned to the VLA, which receives longer wavelengths. Their VLA images show that region better than any previous studies. The new VLA images revealed a distinct clump of dust in the inner region of the disk. The clump, the scientists said, contains roughly 3 to 8 times the mass of the Earth. "We believe this clump of dust represents the earliest stage in the formation of protoplanets, and this is the first time we've seen that stage," said Thomas Henning, of the Max Planck Institute for Astronomy (MPIA). "This is an important discovery, because we have not yet been able to observe most stages in the process of planet formation," said Carlos Carrasco-Gonzalez from the Institute of Radio Astronomy and Astrophysics (IRyA) of the National Autonomous University of Mexico (UNAM). "This is quite different from the case of star formation, where, in different objects, we have seen stars in different stages of their life cycle. With planets, we haven't been so fortunate, so getting a look at this very early stage in planet formation is extremely valuable," he added. Analysis of the VLA data indicates that the inner region of the disk contains grains as large as one centimeter in diameter. This region, the scientists said, is presumably where Earth-like planets would form, as clumps of dust grow by pulling in material from their surroundings. Eventually, the clumps would gather enough mass to form solid bodies that would continue to grow into planets. The VLA observations, made in 2014 and 2015, received radio waves with a wavelength of 7 millimeters. The earlier ALMA observations of HL Tau were made at a wavelength of 1 millimeter. The VLA images showed a similar level of detail as the ALMA images. "These VLA observations are the most sensitive and show the most detail of any yet made of HL Tau's disk at these longer wavelengths," said Claire Chandler, of the National Radio Astronomy Observatory (NRAO). "The VLA's ability to produce such high-quality images in this region is very important to advancing our understanding of these initial stages of planet formation," Chandler added. The scientists are reporting their findings in the Astrophysical Journal Letters. Explore further: VLA images 18 years apart show dramatic difference in young stellar system
Astrophysical Journal | Year: 2010
In this paper, the effect of halo substructures on galaxy rotation curves is investigated using a simple model of dark matter clustering. A dark matter halo density profile is developed based only on the scale-free nature of clustering that leads to a statistically self-similar distribution of the substructures at the galactic scale. A semianalytical method is used to derive rotation curves for such a clumpy dark matter density profile. It is found that the halo substructures significantly affect the galaxy velocity field. Based on the fractal geometry of the halo, this self-consistent model predicts a Navarro-Frenk-White-like rotation curve and a scale-free power spectrum of the rotation velocity fluctuations. © 2010. The American Astronomical Society.