Astronomy Research Works

Permanent URI for this collectionhttp://hdl.handle.net/1903/1587

Browse

Now showing 1 - 20 of 52

1620 GEOGRAPHOS AND 433 EROS: SHAPED BY PLANETARY TIDES?
(University of Chicago Press, 1999) BOTTKE, W. F. JR.; RICHARDSON, D. C.; MICHEL, P.; LOVE, S. G.
Until recently, most asteroids were thought to be solid bodies whose shapes were determined largely by collisions with other asteroids. Recent work by Burns and others has shown that many asteroids may be little more than rubble piles, held together by self-gravity ; this means that their shapes may be strongly distorted by tides during close encounters with planets. Here we report on numerical simulations of encounters between an ellipsoid-shaped rubble-pile asteroid and Earth. After an encounter, many of the simulated asteroids develop the same rotation rate and distinctive shape as 1620 Geographos (i.e., highly elongated with a single convex side, tapered ends, and small protuberances swept back against the rotation direction). Since our numerical studies show that these events occur with some frequency, we suggest that Geographos may be a tidally distorted object. In addition, our work shows that 433 Eros, which will be visited by the NEAR spacecraft in 1999, is much like Geographos, suggesting that it too may have been molded by tides in the past.
The AGN and Gas Disk in the Low Surface Brightness Galaxy PGC045080
(Blackwell, 2007) Das, M.; Kantharia, N.; Ramya, S.; Prabhu, T. P.; McGaugh, S. S.; Vogel, S. N.
We present radio observations and optical spectroscopy of the giant low surface brightness (LSB) galaxy PGC 045080 (or 1300+0144). PGC 045080 is a moderately distant galaxy having a highly inclined optical disk and massive HI gas content. Radio continuum observations of the galaxy were carried out at 320 MHz, 610 MHz and 1.4 GHz. Continuum emission was detected and mapped in the galaxy. The emission appears extended over the inner disk at all three frequencies. At 1.4 GHz and 610 MHz it appears to have two distinct lobes. We also did optical spectroscopy of the galaxy nucleus; the spectrum did not show any strong emission lines associated with AGN activity but the presence of a weak AGN cannot be ruled out. Furthermore, comparison of the H flux and radio continuum at 1.4 GHz suggests that a significant fraction of the emission is non-thermal in nature. Hence we conclude that a weak or hidden AGN may be present in PGC 045080. The extended radio emission represents lobes/jets from the AGN. These observations show that although LSB galaxies are metal poor and have very little star formation, their centers can host significant AGN activity. We also mapped the HI gas disk and velocity field in PGC 045080. The HI disk extends well beyond the optical disk and appears warped. In the HI intensity maps, the disk appears distinctly lopsided. The velocity field is disturbed on the lopsided side of the disk but is fairly uniform in the other half. We derived the HI rotation curve for the galaxy from the velocity field. The rotation curve has a flat rotation speed of 190km s−1.
Animated Sequences Showing the Ejecta Produced in the DART Impact of Asteroid (65803) Didymos
(2025-01-17) Farnham, Tony
This data collection contains animated sequences showing different aspects of the ejecta that were observed after the Double Asteroid Impact Test (DART) spacecraft crashed into Dimorphos, the moon of asteroid (65803) Didymos on September 26, 2022. The images comprising the sequences were obtained with the LICIACube Unit Key Explorer (LUKE) instrument on board the LICIACube spacecraft that flew by the Didymos system about 3 minutes after the impact event. Although the sequences are comprised of the same observations they are presented in different ways to emphasize various aspects of the ejecta field. These animations are intended as a supplement to the individual LUKE images, to provide insight and to help in the interpretation of the data in support of studies that address spatial and temporal changes in the DART ejecta field. Note that in some of the sequences, black sections may encroach in from the sides. These are gaps in the data where the asteroids moved to the edge of the camera's detector.
Astrophotonic Spectrographs
(MDPI, 2019-01-15) Gatkine, Pradip; Veilleux, Sylvain; Dagenais, Mario
Astrophotonics is the application of photonic technologies to channel, manipulate, and disperse light from one or more telescopes to achieve scientific objectives in astronomy in an efficient and cost-effective way. Utilizing photonic advantage for astronomical spectroscopy is a promising approach to miniaturizing the next generation of spectrometers for large telescopes. It can be primarily attained by leveraging the two-dimensional nature of photonic structures on a chip or a set of fibers, thus reducing the size of spectroscopic instrumentation to a few centimeters and the weight to a few hundred grams. A wide variety of astrophotonic spectrometers is currently being developed, including arrayed waveguide gratings (AWGs), photonic echelle gratings (PEGs), and Fourier-transform spectrometer (FTS). These astrophotonic devices are flexible, cheaper to mass produce, easier to control, and much less susceptible to vibrations and flexure than conventional astronomical spectrographs. The applications of these spectrographs range from astronomy to biomedical analysis. This paper provides a brief review of this new class of astronomical spectrographs.
The Balance of Dark and Luminous Mass in Rotating Galaxies
(Copyright 2005 The American Physical Society, 2005) McGaugh, Stacy S.
baryons to the total rotation velocity increases, the contribution of the dark matter decreases by a compensating amount. This poses a fine-tuning problem for CDM galaxy formation models, and may point to new physics for dark matter particles or even a modification of gravity.
The CARMA 3 mm Survey of the Inner 0.7 x 0.4 degrees of the Central Molecular Zone
(2017) Pound, Marc; Yusef-Zadeh, Farhad
The Central Molecular Zone (CMZ) of the Galactic Center has to date only been fully mapped at mm wavelengths with singledish telescopes, with resolution about 30 arcseconds (1.2 pc). Using the Combined Array for Research in Millimeter Astronomy (CARMA), we mapped the innermost 0.25 square degrees of the CMZ over the region between -0.2 < l < 0.5 degrees and -0.2 < b < 0.2 degrees (90 x 50 pc) with spatial and spectral resolution of 10 arcseconds (0.4 pc) and 2.5 km/s, respectively. We provide a catalog of 3 mm continuum sources as well as spectral line images of SiO(J=2-1), HCO+(J=1-0), HCN(J=1-0), N2H+(J=1-0), and CS(J=2-1) , with velocity coverage VLSR= -200 to 200 km/s To recover the large scale structure resolved out by the interferometer, the continuum-subtracted spectral line images were combined with data from the Mopra 22-m telescope survey, thus providing maps containing all spatial frequencies down to the resolution limit. We find that integrated intensity ratio of I(HCN)/I(HCO+) is anti-correlated with the intensity of the 6.4 keV Fe Kalpha, which is excited either by high energy photons or low energy cosmic rays, and the gas velocity dispersion as traced by HCO+ is correlated with Fe Kalpha intensity. The intensity ratio and velocity dispersion patterns are consistent with variation expected from the interaction of low energy cosmic rays with molecular gas.
A Catalog of Low Surface Brightness Galaxies: List II
(Copyright American Astronomical Society, 1992-04) SCHOMBERT, JAMES M.; BOTHUN, GREGORY D.; SCHNEIDER, STEPHEN E.; MCGAUGH, STACY S.
CN coma rotational sequences for Comet 46P/Wirtanen
(2020) Farnham, Tony
This data collection contains animations showing the rotational sequences observed in Comet 46P/Wirtanen during its 2018/2019 apparition. The images were obtained with a narrowband CN filter, and enhanced with three different techniques (see description) to reveal the detailed structure in the coma. The sequences were phased to the periods derived from our rotation analyses.
Code and Data for 'Generalized Time-Series Analysis for In-Situ Spacecraft Observations: Anomaly Detection and Data Prioritization using Principal Components Analysis and Unsupervised Clustering'
(2024) Finley, Matthew G.; Martinez-Ledesma, Miguel; Paterson, William R.; Argall, Matthew R.; Miles, David M.; Dorelli, John C.; Zesta, Eftyhia
Cross talk in 1872 Reticon diode arrays
(University of Chicago Press, 1990) Walker, G. A. H.; Johnson, R.; Richardson, D.; Campbell, B.; Irwin, A. W.; Yang, S.
The Dark and Baryonic Matter Content of Low Surface Brightness Galaxies
(Blackwell, 1997) de Blok, W.J.G.; McGaugh, S.S.
We present mass models of a sample of 19 low surface brightness (LSB) galaxies and compare the properties of their constituent mass components with those of a sample of high surface brightness (HSB) galaxies. We find that LSB galaxies are dark matter dominated. Their halo parameters are only slightly affected by assumptions on stellar mass-to-light ratios. Comparing LSB and HSB galaxies we find that mass models derived using the maximum disk hypothesis result in the disks of LSB galaxies having systematically higher stellar mass-to-light ratios than HSB galaxies of similar rotation velocity. This is inconsistent with all other available evidence on the evolution of LSB galaxies. We argue therefore that the maximum disk hypothesis does not provide a representative description of the LSB galaxies and their evolution. Mass models with stellar mass-to-light ratios determined by the colors and stellar velocity dispersions of galactic disks imply that LSB galaxies have dark matter halos that are more extended and less dense than those of HSB galaxies. Surface brightness is thus related to the halo properties. LSB galaxies are slowly evolving, low density and dark matter dominated galaxies.
Dataset for "Reconstruction of electron and ion distribution functions in magnetotail reconnection diffusion region"
(2020-02) Ng, Jonathan; Chen, Li-Jen; Hakim, Ammar; Bhattacharjee, Amitava
In the diffusion region of magnetotail reconnection, particle distributions are highly structured, exhibiting triangular shapes and multiple striations that deviate dramatically from the Maxwellian distribution. Fully kinetic simulations have been demonstrated to be capable of producing the essential structures of the observed distribution functions, yet are computationally not feasible for 3D global simulations. The fluid models used for large-scale simulations, on the other hand, do not have the kinetic physics necessary for describing reconnection accurately. Our study aims to bridge fully kinetic and fluid simulations by quantifying the information required to capture the non-Maxwellian features in the distributions underlying the closures used in the fluid code. We compare the results of fully kinetic simulations to observed electron velocity distributions in a magnetotail reconnection diffusion region, and use the maximum entropy model to reconstruct electron and ion distributions using various numbers of moments obtained from the simulation. Our results indicate that using only local moments, the maximum entropy model can reproduce many of the features of the distributions: (1) the anisotropic electron distributions inside the ion diffusion region but outside the current-sheet can be modelled with 10-14 moments, (2) the electron-outflow distribution with a tilted triangular structure is reproduced with 21-35 moments and (3) counterstreaming distributions can be captured with the 35-moment model when the separation in velocity space between the populations is large.
Dataset for "Stratigraphic Evidence for Early Martian Explosive Volcanism in Arabia Terra"
(2021) Richardson, Jacob; Whelley, Patrick; Matiella Novak, Alexandra
Seven Digital Elevation Models (DEMs) in the Arabia Terra region, Mars, were created using spatially overlapping images (i.e., "stereo images") from the Context Camera (CTX) aboard the Mars Reconnaissance Orbiter (MRO). Each DEM location was selected to spatially overlap hyperspectral data acquired by the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM) also aboard MRO. Elevation data from these DEMs were used to identify and measure geologic units that preserve minerals indicative of altered volcanic deposits. Stereophotogrammetry was performed using the Ames Stereo Pipeline toolkit v 2.6.0 and USGS ISIS Version 3.4.12.6662. Each DEM was aligned to the martian aeroid with Mars Orbiter Laser Altimeter Precision Experiment Data Records within the spatial footprint of the CTX DEM. DEM files are in the GeoTIFF raster format. Each raster has a sinusoidal projection coordinate system with a central longitude corresponding to the individual raster's center. For each CTX DEM, an ortho-rectified image was also produced. For naming conventions, references, and descriptions of each DEM site, see the enclosed readme.
Dynamics of the Storm Time Magnetopause and Magnetosheath Boundary Layers: An MMS-THEMIS Conjunction
(Wiley, 2024-02-13) Rice, Rachel C.; Chen, Li-Jen; Gershman, Dan; Fuselier, Stephen A.; Burkholder, Brandon L.; Gurram, Harsha; Beedle, Jason; Shuster, Jason; Petrinec, Steven M.; Pollock, Craig; Cohen, Ian; Gabrielse, Christine; Escoubet, Philippe; Burch, James
This letter uses simultaneous observations from Magnetosphere Multiscale (MMS) and Time History of Events and Macroscale Interactions during Substorms (THEMIS) to address the dynamics of the magnetopause and magnetosheath boundary layers during the main phase of a storm during which the interplanetary magnetic field (IMF) reverses from south to north. Near the dawn terminator, MMS observes two boundary layers comprising open and closed field lines and containing energetic electrons and ring current oxygen. Some closed field line regions exhibit sunward convection, presenting an avenue to replenish dayside magnetic flux lost during the storm. Meanwhile, THEMIS observes two boundary layers in the pre-noon sector which strongly resemble those observed at the flank by MMS. Observations from the three THEMIS spacecraft indicate the boundary layers are still evolving several hours after the IMF has turned northward. These observations advance our knowledge of the dynamic magnetopause and magnetosheath boundary layers under the combined effects of an ongoing storm and changing IMF.
Electron-Scale Reconnection in Three-Dimensional Shock Turbulence
(Wiley, 2022-08-11) Ng, J.; Chen, L.-J.; Bessho, N.; Shuster, J.; Burkholder, B.; Yoo, J.
Magnetic reconnection has been observed in the transition region of quasi-parallel shocks. In this work, the particle-in-cell method is used to simulate three-dimensional reconnection in a quasi-parallel shock. The shock transition region is turbulent, leading to the formation of reconnecting current sheets with various orientations. Two reconnection sites with weak and strong guide fields are studied, and it is shown that reconnection is fast and transient. Reconnection sites are characterized using diagnostics including electron flows and magnetic flux transport. In contrast to two-dimensional simulations, weak guide field reconnection is realized. Furthermore, the current sheets in these events form in a direction almost perpendicular to those found in two-dimensional simulations, where the reconnection geometry is constrained.
Example code and data for "Identifying physical structures in our Galaxy with Gaussian Mixture Models: An unsupervised machine learning technique"
(2023) Tiwari, Maitraiyee; Kievit, Rens; Kabanovic, Slawa; Bonne, Lars; Falasca, F.; Guevara, Cristian; Higgins, Ronan; Justen, M.; Karim, Ramsey; Pabst, Cornelia; Pound, Marc W.; Schneider, Nicola; Simon, R.; Stutzki, Jurgen; Wolfire, Mark; Tielens, Alexander G. G. M.
We present a python software repository implementing the PyGMMis (Melchior & Goudling 2018) method to astronomical data cubes of velocity resolved line observations. This implementation is described extensively in Tiwari et al. 2023, ApJ. An example is included in /example/ containing the SOFIA data of RCW120 used in Tiwari et al. 2023, ApJ, along with example scripts describing the full implementation of our code. The majority of parameter tweaking can be performed within 'rcw120-params.txt' which is continuously called during the procedure. A full description of the code and how to use it is in README.md (markdown file).
Example code and data for "SOFIA FEEDBACK Survey: The Pillars of Creation in [C II] and Molecular Lines"
(2023-09-07) Karim, Ramsey; Pound, Marc W.; Wolfire, Mark G.; Mundy, Lee; Tielens, Alexander G. G. M.
We present here the original observations used in the manuscript "SOFIA FEEDBACK Survey: The Pillars of Creation in [C II] and Molecular Lines" (Karim et al., Astronomical Journal, 2023). The data consist of FITS format images and datacubes of the rotational transitions of molecular lines CO, 13CO, C18O, CS, HCN, HCO+, and N2H+ in the 3mm spectral window, and the atomic spectral lines [C II] 158 micron and [O I] 63 micron. We also present a snapshot copy of the scoby (Spectra from Catalogs of OB Stars) software repository, some model data necessary for it, and some examples of how to run it.
Example code listings for the PhotoDissociation Region Toolbox
(2022-08-31) Pound, Marc; Wolfire, Mark
These are example code listings for the PhotoDissociation Region Toolbox (https://dustem.astro.umd.edu), and companion to the manuscript "The PhotoDissociation Region Toolbox: Software and Models for Astrophysical Analysis", by Pound & Wolfire (2022). These code snippets show typical ways to use the Toolbox and reproduce most of the figures in the manuscript. The code is written in Python 3 and demonstrate the pdrtpy Python package (https://pdrtpy.readthedocs.io).
From Sir Isaac Newton to the Sloan survey: calculating the structure and chaos owing to gravity in the universe
(Copyright: SIAM, 1997-01) Lake, George; Quinn, Thomas; Richardson, Derek C.
Gravitational instability and clustering in a disk of planetesimals
(EDP Sciences, 2004-08-05) Tanga, P.; Weidenschilling, S. J.; Michel, P.; Richardson, D. C.
For a long time, gravitational instability in the disk of planetesimals has been suspected to be the main engine responsible for the beginning of dust growth, its advantage being that it provides for rapid growth. Its real importance in planetary formation is still debated, mainly because the potential presence of turbulence can prevent the settling of particles into a gravitationally unstable layer. However, several mechanisms could yield strongly inhomogeneous distributions of solids in the disk: radial drift, trapping in vortices, perturbations by other massive bodies, etc. In this paper we present a numerical study of a gravitationally unstable layer. This allows us to go beyond the classical analytical study of linear perturbations, exploring a highly non-linear regime. A hierarchical growth of structure in the presence of dissipation (gas drag) can yield large, virialized clusters of planetesimals, the first time such clusters have been observed in the context of planetesimal disks.