Kawka A.,Astronomicky Ustav |
Vennes S.,Astronomicky Ustav |
Vennes S.,Astronomer |
Vaccaro T.R.,Astronomer |
Vaccaro T.R.,Francis Marion University
Astronomy and Astrophysics | Year: 2010
We show that the extremely low-mass white dwarf NLTT 11748 (0.17 Modot) is in a close binary with a fainter companion. We obtained a series of radial velocity measurements of the low-mass white dwarf using the Hα core and determined an orbital period of 5.64 h. The velocity semi-amplitude (K = 274.8 kms-1) and orbital period imply that it is a degenerate star, and that the minimum mass for the companion is 0.75 Modot; (assuming a mass of 0.167 Modot; for the primary). Our analysis of Balmer line profiles shows that a 0.75 Modot white dwarf companion does not contribute more than 2% or 5% of the flux (V-band) for helium-or hydrogen-rich surfaces, respectively. The kinematics of the system suggest that it belongs to the Galactic halo. © 2010 ESO.
Environmental Law and Management | Year: 2013
Population trends beyond 2050 will depend on what people now in their teens and 20s decide about the number and spacing of their children. Hundreds of millions of women are denied such a choice. There is a valuable organization called the 'Optimum Population Trust' which highlights the problems of overpopulation. And there is the key issue of the discount rate. Commercial decisions apply a discount that assigns minimal importance to what happens at the end of the century. But surely we should not value the grandchildren's welfare less than the own. It is ironic that there is only one area of policy where we do implicitly apply a zero discount rate: radioactive waste disposal, where people talk with a straight face about whether the repository would be safe for ten thousand years. Geoengineering would be an utter political nightmare: not all nations would want to adjust the thermostat the same way. Very elaborate climatic modeling would be needed in order to calculate the regional impacts of such an intervention.
Prochter G.E.,University of California at Santa Cruz |
Prochaska J.X.,University of California at Santa Cruz |
O'Meara J.M.,Saint Michael's College |
O'Meara J.M.,Astronomer |
And 2 more authors.
Astrophysical Journal | Year: 2010
We present an absorption line analysis of the Lyman limit system (LLS) at z ≈ 3.55 in our Magellan/MIKE spectrum of PKS2000 - 330. Our analysis of the Lyman limit and full H I Lyman series constrains the total H I column density of the LLS (N HI = 1018.0 ± 0.25 cm-2 for b HI ≥ 20 km s-1) and also the N HI values of the velocity subsystems comprising the absorber. We measure ionic column densities for metal-line transitions associated with the subsystems and use these values to constrain the ionization state (>90% ionized) and relative abundances of the gas. We find an order of magnitude dispersion in the metallicities of the subsystems, marking the first detailed analysis of metallicity variations in an optically thick absorber. The results indicate that metals are not well mixed within the gas surrounding high z galaxies. Assuming a single-phase photoionization model, we also derive an N H-weighted metallicity, 〈[Si/H]〉 = -1.66 ±0.25, which matches the mean metallicity in the neutral interstellar medium in high z damped Lyα systems (DLAs). Because the line density of LLSs is over 10× higher than the DLAs, we propose that the former dominate the metal mass-density at z ∼ 3 and that these metals reside in the galaxy/intergalactic medium interface. Considerations of a multi-phase model do not qualitatively change these conclusions. Finally, we comment on an anomalously large O0/Si + ratio in the LLS that suggests an ionizing radiation field dominated by soft UV sources (e.g., a starburst galaxy). Additional abundance analysis is performed on the super-LLS systems at z ≈ 3.19. © 2010. The American Astronomical Society.
Horch E.P.,Southern Connecticut State University |
Proceedings of SPIE - The International Society for Optical Engineering | Year: 2015
Intensity interferometry, which was first used for obtaining ultra-high-resolution image information in astronomy in the 1960's and 1970's, is now being revived using modern detectors and electronics. This paper explores the possibility of wireless optical interferometry made possible by technological advancements in timing correlation, signal processing, and detector technology. If this can be achieved, then baselines of one to several kilometers may be possible in optical interferometry in the coming years. This would improve the resolution over the current generation of amplitude-based optical interferometers by a factor of at least ten. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only. © 2015 SPIE.
Advances in the Astronautical Sciences | Year: 2014
This paper presents a method of initial orbit determination (IOD) for use with short arcs of deep-space, angles-only observations. The method is then used to determine the Earth flyby path of the asteroid 2012 DA14 on 2013 February 15, as an extreme example. (The asteroid was inbound and 150-116 Earth radii distant from Earth during the time span of the observations.) The solution is obtained in geocentric coordinates, rather than heliocentric, to emphasize the significance, for Earth, of close-Earth asteroid encounters. Being founded in the work of Paul Herget, the method uses all of the available observations in the arc, not just three, as with classical IOD methods. But when only three suitably-spaced observations are available, it provides an exact, two-body fit. The method is modular with respect to the Lambert solution. It is therefore a schema or framework for preliminary orbit determination, in that any of the Lambert solutions of Gauss, Battin, Gooding, or der could be incorporated. But for the asteroid example at hand, a Lambert solution attributable Gauss, with some modifications, is used to illustrate the method.
Belton M.J.S.,Belton Space Exploration Initiatives |
Icarus | Year: 2015
I show that the size-distribution of small scattered-disk trans-neptunian objects when derived from the observed size-distribution of Jupiter Family comets (JFCs) and other observational constraints implies that a large percentage (94-97%) of newly arrived active comets within a range of 0.2-15.4km effective radius must physically disrupt, i.e., macroscopically disintegrate, within their median dynamical lifetime. Additional observational constraints include the numbers of dormant and active nuclei in the near-Earth object (NEO) population and the slope of their size distributions. I show that the cumulative power-law slope (-2.86 to -3.15) of the scattered-disk TNO hot population between 0.2 and 15.4km effective radius is only weakly dependent on the size-dependence of the otherwise unknown disruption mechanism. Evidently, as JFC nuclei from the scattered disk evolve into the inner Solar System only a fraction achieve dormancy while the vast majority of small nuclei (e.g., primarily those with effective radius <2km) break-up. The percentage disruption rate appears to be comparable with that of the dynamically distinct Oort cloud and Halley type comets (Levison, H.F., Morbidelli, A., Dones, L., Jedicke, R., Wiegert, P.A., Bottke Jr., W.F. . Science 296, 2212-2215) suggesting that all types of comet nuclei may have similar structural characteristics even though they may have different source regions and thermal histories. The typical disruption rate for a 1km radius active nucleus is ~5×10-5disruptions/year and the dormancy rate is typically 3 times less. We also estimate that average fragmentation rates range from 0.01 to 0.04events/year/comet, somewhat above the lower limit of 0.01events/year/comet observed by Chen and Jewitt (Chen, J., Jewitt, D.C. . Icarus 108, 265-271). © 2014 Elsevier Inc.
Crocker A.F.,University of Massachusetts Amherst |
Crocker A.F.,University of Oxford |
Bureau M.,University of Oxford |
Young L.M.,New Mexico Institute of Mining and Technology |
And 2 more authors.
Monthly Notices of the Royal Astronomical Society | Year: 2011
We present new mm interferometric and optical integral-field unit (IFU) observations and construct a sample of 12 elliptical (E) and lenticular (S0) galaxies with molecular gas which have both CO and optical maps. The galaxies contain 2 × 107 to 5 × 109 M of molecular gas distributed primarily in central discs or rings (radii 0.5-4 kpc). The molecular gas distributions are always coincident with distributions of optically obscuring dust that reveal tightly wound spiral structures in many cases. The ionized gas always approximately corotates with the molecular gas, evidencing a link between these two gas components, yet star formation is not always the dominant ionization source. The galaxies with less molecular gas tend to have [O iii]/Hβ emission-line ratios at high values not expected for star formation. Most E/S0s with molecular gas have young or intermediate-age stellar populations based on optical colours, ultraviolet colours and absorption linestrengths. The few that appear purely old lie close to the limit where such populations would be undetectable based on the mass fractions of expected young to observed old stars. The 8 μm polycyclic aromatic hydrocarbon (PAH) and 24 μm emission yield similar star formation rate (SFR) estimates of E/S0s, but the total infrared overpredicts the rate due to a contribution to dust heating from older stars. The radio-far-infrared relation also has much more scatter than for other star-forming galaxies. However, despite these biases and additional scatter, the derived star formation rates locate the E/S0 galaxies within the large range of the Schmidt-Kennicutt and constant efficiency star formation laws. Thus, the star formation process in E/S0s is not overwhelmingly different than in other star-forming galaxies, although one of the more reliable tracers (24 μm) points to a possible lower star formation efficiency at a given gas surface density. © 2010 The Authors. Journal compilation © 2010 RAS.
Eilek J.A.,New Mexico Institute of Mining and Technology |
New Journal of Physics | Year: 2014
In this paper I present dynamic models of the radio source Centaurus A, and critique possible models of in situ particle reacceleration (ISR) within the radio lobes. The radio and γ-ray data require neither homogeneous plasma nor quasi-equipartition between the plasma and magnetic field; inhomogeneous models containing both high-field and low-field regions are equally likely. Cen A cannot be as young as the radiative lifetimes of its relativistic electrons, which range from a few to several tens of Myr. Two classes of dynamic models - flow driven and magnetically driven - are consistent with current observations; each requires Cen A to be on the order of a Gyr old. Thus, ongoing ISR must be occuring within the radio source. Alfven-wave ISR is probably occuring throughout the source, and may be responsible for maintaining the γ-ray-loud electrons. This is likely to be supplemented by shock or reconnection ISR, which maintains the radio-loud electrons in high-field regions. © 2014 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.
Anderson L.D.,West Virginia University |
Anderson L.D.,Astronomer |
Bania T.M.,Boston University |
Balser D.S.,U.S. National Radio Astronomy Observatory |
And 5 more authors.
Astrophysical Journal, Supplement Series | Year: 2014
Using data from the all-sky Wide-Field Infrared Survey Explorer (WISE) satellite, we made a catalog of over 8000 Galactic H II regions and H II region candidates by searching for their characteristic mid-infrared (MIR) morphology. WISE has sufficient sensitivity to detect the MIR emission from H II regions located anywhere in the Galactic disk. We believe this is the most complete catalog yet of regions forming massive stars in the Milky Way. Of the ∼8000 cataloged sources, ∼1500 have measured radio recombination line (RRL) or Hα emission, and are thus known to be H II regions. This sample improves on previous efforts by resolving H II region complexes into multiple sources and by removing duplicate entries. There are ∼2500 candidate H II regions in the catalog that are spatially coincident with radio continuum emission. Our group's previous RRL studies show that ∼95% of such targets are H II regions. We find that ∼500 of these candidates are also positionally associated with known H II region complexes, so the probability of their being bona fide H II regions is even higher. At the sensitivity limits of existing surveys, ∼4000 catalog sources show no radio continuum emission. Using data from the literature, we find distances for ∼1500 catalog sources, and molecular velocities for ∼1500H II region candidates. © 2014. The American Astronomical Society. All rights reserved.