Dieterich S.B.,Georgia State University |
Henry T.J.,Georgia State University |
Jao W.-C.,Georgia State University |
Winters J.G.,Georgia State University |
And 3 more authors.
We construct a Hertzsprung-Russell diagram for the stellar/substellar boundary based on a sample of 63 objects ranging in spectral type from M6V to L4. We report newly observed VRI photometry for all 63 objects and new trigonometric parallaxes for 37 objects. The remaining 26 objects have trigonometric parallaxes from the literature. We combine our optical photometry and trigonometric parallaxes with 2MASS and WISE photometry and employ a novel spectral energy distribution fitting algorithm to determine effective temperatures, bolometric luminosities, and radii. Our uncertainties range from ∼20 K to ∼150 K in temperature, ∼0.01 to ∼0.06 in log (L/L) and ∼3% to ∼10% in radius. We check our methodology by comparing our calculated radii to radii directly measured via long baseline optical interferometry. We find evidence for the local minimum in the radius-temperature and radius-luminosity trends that signals the end of the stellar main sequence and the start of the brown dwarf sequence at T eff∼ 2075 K, log (L/L) ∼ -3.9, and (R/R) ∼ 0.086. The existence of this local minimum is predicted by evolutionary models, but at temperatures ∼400 K cooler. The minimum radius happens near the locus of 2MASS J0523-1403, an L2.5 dwarf with V-K = 9.42. We make qualitative arguments as to why the effects of the recent revision in solar abundances accounts for the discrepancy between our findings and the evolutionary models. We also report new color-absolute magnitude relations for optical and infrared colors which are useful for estimating photometric distances. We study the optical variability of all 63 targets and find an overall variability fraction of 36% at a threshold of 15 mmag in the I band, which is in agreement with previous studies. © 2014. The American Astronomical Society. All rights reserved.. Source
Makarov V.V.,United States Naval Observatory
The dynamical evolution of terrestrial planets resembling Mercury in the vicinity of spin-orbit resonances is investigated using comprehensive harmonic expansions of the tidal torque taking into account the frequency-dependent quality factors and Love numbers. The torque equations are integrated numerically with a small step in time, including the oscillating triaxial torque components but neglecting the layered structure of the planet and assuming a zero obliquity. We find that a Mercury-like planet with a current value of orbital eccentricity (0.2056) is always captured in 3:2 resonance. The probability of capture in the higher 2:1 resonance is approximately 0.23. These results are confirmed by a semi-analytical estimation of capture probabilities as functions of eccentricity for both prograde and retrograde evolutions of spin rate. As follows from analysis of equilibrium torques, entrapment in 3:2 resonance is inevitable at eccentricities between 0.2 and 0.41. Considering the phase space parameters at the times of periastron, the range of spin rates and phase angles for which an immediate resonance passage is triggered is very narrow, and yet a planet like Mercury rarely fails to align itself into this state of unstable equilibrium before it traverses 2:1 resonance. © 2012. The American Astronomical Society. All rights reserved.. Source
Chesley S.R.,Jet Propulsion Laboratory |
Baer J.,James Cook University |
Monet D.G.,United States Naval Observatory
In this paper, we discuss the detection of systematic biases in star positions of the USNO A1.0, A2.0, and B1.0 catalogs, as deduced from the residuals of numbered asteroid observations. We present a technique for the removal of these biases, and validate this technique by illustrating the resulting improvements in numbered asteroid residuals, and by establishing that debiased orbits predict omitted observations more accurately than do orbits derived from non-debiased observations. We also illustrate the benefits of debiasing to high-precision astrometric applications such as asteroid mass determination and collision analysis, including a refined prediction of the impact probability of 99942 Apophis. Specifically, we find the IP of Apophis to be lowered by nearly an order of magnitude to 4.5×10-6 for the 2036 close approach. © 2010 Elsevier Inc. Source
Crawled News Article
A partial map of the distribution of galaxies in the Sloan Digital Sky Survey, going out to a distance of 7 billion light years. The amount of galaxy clustering that we observe today is a signature of how gravity acted over cosmic time, and all More Don Lincoln is a senior scientist at the U.S. Department of Energy's Fermilab, America's largest Large Hadron Collider research institution. He also writes about science for the public, including his recent "The Large Hadron Collider: The Extraordinary Story of the Higgs Boson and Other Things That Will Blow Your Mind" (Johns Hopkins University Press, 2014). You can follow him on Facebook. Lincoln contributed this article to Space.com's Expert Voices: Op-Ed & Insights. November marks the beginning of the holiday season, but this year, there's even more reason to throw a party: It's the 100th birthday of Einstein's theory of general relativity. Forget the turkey this year — we science enthusiasts can instead celebrate the invention of a paradigm that completely overthrew our fundamental understanding of the very meaning of space and time. In November 1915, Albert Einstein published four papers — each separated by a week, followed by a summary paper in March 1916 — in which he laid out his theory of general relativity and blew humanity's collective mind. Einstein's earlier theory of special relativity (1905) was already confusing enough, because of how it inextricably linked space and time. But at least in that theory, space was comfortably familiar to people who had learned Euclidean geometry: Parallel lines never crossed, the sum of the angles of a triangle was 180 degrees and space was flat. Special relativity might have taken a little work to understand — but Einstein's general relativity was, well, twisted. The new theory showed a changing and dynamic space-time that was tied to the energy and mass density of the universe. Space itself could be bent and warped by the presence of matter. This new vision of the universe was not immediately accepted. It wasn't until 1919 that the scientific community embraced the idea, when Sir Arthur Eddington's naval expedition to the then-Portuguese island of Príncipe showed that the sun bent the path of light emitted by distant stars passing near it. The day after Eddington's Nov. 6 presentation to the Royal Society, Einstein and his theory were instant scientific rock stars, trumpeted worldwide on the front page of major newspapers. Even prior to the hubbub of 1919, scientists were exploring the consequences of the proposed new paradigm. In one of his papers of 1915, Einstein had compared classical Newtonian gravity and his own theory, and found differences in how they predicted the precession of the orbit of Mercury. While both calculations predicted a precession, the theory of general relativity agreed with data, while Newton's did not. Another early explorer of the consequences of Einstein's idea was Karl Schwarzschild. Already a well-respected scientist, Schwarzschild joined the German army during World War I. While in the trenches on the Russian Front, he contracted a rare autoimmune skin disease, from which he eventually died. Sent home with the intent that he convalesce, Schwarzschild returned to his love of science. Within a month of the flurry of papers in 1915, he explored the consequences of Einstein's theory. As he lay in his bed, afflicted by painful sores, Schwarzschild worked out a solution to the new equations for an extreme bending of space, which we now call a black hole. In this centennial of Einstein's successful year, we can now look back and see the impact of general relativity on how we understand the universe. In contrast to the halting acceptance of a century ago, the scientific community has now firmly embraced the idea, which has a range of testable consequences. One such prediction is relevant to our modern life. Einstein predicted that, in addition to the familiar (and nonintuitive!) changes in space and time that occur when one approaches the speed of light, the passage of time also depends on the strength of the gravitational field. This implies that clocks that experience stronger gravity tick more slowly than those in a weaker gravitational environment. This prediction was first tested in 1971, when Joseph C. Hafele of Washington University in St. Louis and Richard E. Keating of the United States Naval Observatory flew four very precise atomic clocks around the world and compared them to clocks left stationary in their laboratory. When the clocks were reunited, they reported a different elapsed time, in exact agreement with the predictions of general relativity. Modern relativity skeptics love to dispute the Hafele-Keating measurement, but the experiment has been repeated many times over the past 44 years. In fact, modern strontium clocks built by JILA — a joint institute of the University of Colorado, Boulder, and the National Institute of Standards and Technology — are so precise that they can measure a shift in time if one clock is lifted a mere 2 centimeters (less than an inch) higher than its twin. More practically, general relativity has a real implication for the GPS system built into your phone. Because the system works by comparing both orbiting and Earth-bound clocks, the fact that the clocks in satellites tick more quickly than their terrestrial cousins must be taken into account. If general relativity were not accounted for, the difference in the clocks would lead the GPS system to tell you that you were in the wrong place. And the effect isn't small. Each day, the offset would be about 6 miles (10 kilometers)! Very quickly, the GPS system would be totally useless. Another triumph of Einstein's theory of general relativity employs the same technique as Eddington's measurement of the deflection of light by distant stars. By using improved versions of the same methods, scientists can use distortions of distant galaxies to literally measure the mass of the universe. There is one prediction of general relativity that has not yet been confirmed directly. If mass can distort space, then moving mass can set up vibrations of space — what scientists call gravitational waves. In 1974, Russell A. Hulse and Joseph H. Taylor Jr. of the University of Massachusetts Amherst discovered a binary pulsar. A pulsar is a rapidly rotating neutron star that emits regular radio signals. In the case of Hulse and Taylor, the pulsar was co-orbiting another very dense stellar object. By watching the binary system, they saw that the orbital period was decreasing very slowly over the years — specifically, 75 millionths of a second per year. This decline is thought to be caused by the loss of energy by gravitational radiation. The observation is persuasive enough for the two men to have been awarded the 1993 Nobel Prize in physics, but it would be valuable to directly observe gravitational waves. A series of experiments conducted here on Earth employing a range of technologies are underway. With these experiments, scientists hope to observe these gravitational ripples as they pass over the planet. These waves are thought to be created in extremely violent astronomical events, like the merging of two black holes. When observed, the achievement will be a crowning confirmation of Einstein's theory. There is absolutely no question that Einstein's theory of general relativity is one of the most impressive intellectual achievements of all time. Our familiar understanding of space and time were shown to be quite wrong. Space can bend and twist under the influence of matter. Mass and energy are inextricably intertwined with the shape of space and time. It is, indeed, Einstein's greatest triumph. Follow all of the Expert Voices issues and debates — and become part of the discussion — on Facebook, Twitter and Google+. The views expressed are those of the author and do not necessarily reflect the views of the publisher. This version of the article was originally published on Space.com. Einstein Is Right About General Relativity — Again Proof Is in the Cosmos: Einstein's General Relativity Confirmed Copyright 2015 SPACE.com, a Purch company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.
Berghea C.T.,United States Naval Observatory |
Dudik R.P.,United States Naval Observatory |
Tincher J.,St Johns College |
Winter L.M.,Atmospheric and Environmental Research Inc.
We have used XMM-Newton's Optical Monitor (OM) images to study the local environment of a sample of 27 ultraluminous X-ray sources (ULXs) in nearby galaxies. UVW1 fluxes were extracted from 100 pc regions centered on the ULX positions. We find that at least 4 ULXs (out of 10 published) have spectral types that are consistent with previous literature values. In addition, the colors are similar to those of young stars. For the highest-luminosity ULXs, the UVW1 fluxes may have an important contribution from the accretion disk. We find that the majority of ULXs are associated with recent star formation. Many of the ULXs in our sample are located inside young OB associations or star-forming regions (SFRs). Based on their colors, we estimated ages and masses for SFRs located within 1 kpc from the ULXs in our sample. The resolution of the OM was insufficient to detect young dense superclusters, but some of these SFRs are massive enough to contain such clusters. Only three ULXs have no associated SFRs younger than ∼50 Myr. The age and mass estimates for clusters were used to test runaway scenarios. The data are, in general, compatible with stellar-mass binaries accreting at super-Eddington rates and ejected by natal kicks. We also tested the hypothesis that ULXs are sub-Eddington accreting intermediate mass black holes ejected by three-body interactions; however, this is not supported well by the data. © 2013. The American Astronomical Society. All rights reserved. Source