1,720,965 research outputs found
Shocks and jets from the laboratory environment to the astrophysical regime: Transforming AstroBEAR into an all purpose MHD simulation package
No full textSupersonic jets and shocks play an important role in numerous astrophysical phenomena, ranging from stellar formation to active galactic nebulae (AGN). Laboratory astrophysics opens up new avenues for research into these jets and shocks, and computer simulations show great promise in linking laboratory and astronomical data. To date, the most effective codes for the laboratory environment are not readily available and lack magnetic fields, a key component in astrophysical jets and future magnetized laboratory experiments. Also no 3D simulation code has had its non-local thermodynamic equilibrium (LTE) cooling, essential for generating emission maps for comparison with astronomical observations, rigorously tested against an accepted baseline. The focus of this dissertation research was to improve an existing magneto-hydrodynamic code, AstroBEAR, to better model jets and shocks in laboratory and astrophysical environments, with the ultimate goal of developing a code that can link astronomical and laboratory data.
The work outlined in this dissertation facilitates the connection between astronomical and laboratory data in two areas. First, we added a multiple material and non-ideal equation of state capability into AstroBEAR to handle the high density ionized plasmas that characterize laboratory astrophysics experiments and now have the first working 3D MHD code capable of simulating the laboratory environment. We used AstroBEAR in 2.5 D hydrodynamic mode to simulate a series of experiments carried out on the OMEGA laser, and compared the simulations with experimental data.
Secondly, we improved AstroBEAR's handling of radiative cooling, specifically in the post-shock cooling zones prevalent in many astrophysical jets. The first ever validation tests of a 3D code against a fully non-LTE 1D radiative cooling atomic code show explicitly that AstroBEAR correctly models post-shock radiative cooling down to the resolution and micro-physics limits. We used this improved cooling to simulate the HH 110 jet and conclude from these simulations that any model of stellar jet formation must be able to produce processing and pulsing outflow. Overall the improvements of AstroBEAR's ability to handle jets and shocks in the laboratory and astrophysical environments position it to potentially link observational data with magnetized laboratory experiments
Infrared excesses from accretion streams in pre-main-sequence binaries
No full textWe used Smoothed Particle Hydrodynamics to study the formation and evolution of mass streams that transfer material from a circumbinary disk to the stars of a close binary. These streams form when the disk is hot enough and/or viscous enough (thickness-to-radius ratio c ≥ 0.05, for viscosity parameter alpha > 0.01).
We modeled binaries with different mass ratios and eccentricities and calculated the spectral energy distribution of the binary-disk system, for an optically thick disk. We focused on the variation of the excess in the N band (effective wavelength ∼11mum). The result of the mass streams is a considerable infrared excess that varies over the binary period. This variability is observable and can be used as a diagnostic element in the study of very close binary systems, because its presence is a sign of stream accretion. There is a periodicity in this variation, with a period equal to the binary period. The general pattern the excess follows is that it is larger at the periastron of the orbit.
We also examined the role of the temperature profile in the formation of the streams. We found that for a hotter disk temperature profile ( T ∼ 1/R1/2) streams form even for moderate values of thickness-to-radius ratio (c ≈ 0.03). Thus, systems like DQ Tau (c ≈ 0.03--0.045) may have such streams, something that previously was questioned
Internal velocities in the Orion Nebula
No full textThe Orion Nebula (NGC 1976, M 42) is an H II region composed of a slowly expanding thin zone of photoionized gas on the facing side of the Orion Molecular Cloud. The Orion Nebula is also a famous star formation region in which numerous jets and shocks arise from many young stars. Creating bipolar jets to shed excess angular momentum is an essential process in star formation. The jets interact with the interstellar medium or with wakes of previously passing jets and subsequently form shocks. These shocks can be observed with optical or near infrared emission lines and are called Herbig-Haro (HH) objects.
The purpose of the present study was to catalog and study the HH objects in the Orion Nebula, and hence, this will help us understand how stars form in a molecular cloud and then evolve in an H II region. We measured the proper motions (tangential velocities) and radial velocities of the HH objects with the highest possible accuracy. By combining the results of the proper-motion and radial velocity measurements, we could obtain the spatial (three-dimensional) motions of the HH objects, which gave us the opportunity to discover the true physical nature of the HH objects in the Orion Nebula.
We were able to measure the proper motions of the HH objects with a 10 km s-1 accuracy using the Hubble Space Telescope (HST) Wide Field Planetary Camera 2 (WFPC2) images in [S II], [N II], Halpha, and [O III], taken 4 to 6 years apart. This is the first study dedicated to measuring the proper motions of HH objects in the Orion Nebula covering the complete range of ionization states. A shock consists of a shock front followed by a collisional excitation zone and a cooling zone in which [O III], [N II], and [S II] emission layers form. Halpha emission comes directly from the collisional excitation zone. The presence of [O III], [N II], or [S II] emissions is a good indicator of the strength of a shock.
We identified all the HH flows in the northwest and southeast regions of the Orion Nebula and found two new bipolar objects. We confirmed that the major HH flows in the northwest region were created by an explosive event which took place approximately 1000 years ago. We also found a new HH flow moving toward the northeast in the southwest region of the Orion Nebula. The proper motion of HH 202 was measured with high accuracy for the first time, and the result indicates that HH 202 and HH 203/204 may have emanated from the same unknown origin. (Abstract shortened by UMI.
Forbidden line emission in the classical T Tauri spectroscopic binaries DQ Tau and UZ Tau-E monitored over an orbital period
No full textWe present high resolution, echelle spectroscopy of the close pre-main-sequence binary star systems DQ Tau and UZ Tau-E. Over a 16 day time interval we acquired 14 nights of spectra for DQ Tau and 12 nights of spectra for UZ Tau-E. This represents the entire orbital phase of DQ Tau, and 63 percent of that for UZ Tau-E. As expected, photospheric lines such as Li I lambda6707 clearly split into two components as the primary and secondary orbit one another, as did the permitted line He I lambda5876. Unlike the photospheric features, in both objects the forbidden lines of [O I] lambda6300, which has both a low-velocity and high-velocity component, and [O I] lambda5577, which has primarily a low-velocity component, retain the same shape throughout the orbit. Therefore these lines must originate outside of the immediate vicinity of the two stars and any gas that participates in the orbital motion of the stars
A search for low mass companions and a new determination of effective temperatures for T-Tauri stars
No full textI present an analysis of over 300 spectra of the photospheres of young stars in order to address three outstanding questions of current interest: (1) whether or not the youngest stars are accompanied by massive substellar companions, (2) the degree to which starspots influence radial velocity measurements in young stars, and (3) establishment of a scale to convert spectral observations to effective temperatures and determining effective temperature for a large sample of stars. The first question is essential for understanding the star and planet formation process, the second question sets fundamental limits on ability of radial velocity surveys to detect young planets, and the third is a key for getting accurate mass and age estimates.
Results from a new radial velocity survey of T Tauri stars focus on three specific stars---BP Tau, DN Tau, and LkCa19---with the largest number of observations. All the objects show far greater radial velocity variability than our radial velocity standards. Using a new simulation of the effect of star spots on the photometric and radial velocity variability of T Tauri star, I show the data for LkCa19 is fully consistent with variations caused by large star spots present on this rapidly rotating young star. In the case of BP Tau and DN Tau, the origin of the velocity variability is currently not well established.
I also present the results of a new method to determine the effective temperature for weak-lined T Tauri stars, from the direct fitting of TiO bands near 7000 to NextGen synthetic spectra. The new temperatures are consistently warmer (by as much as 400K) than previous determinations based on spectral type. For the lowest mass young stars, the revised effective temperature doubles the mass inferred from evolutionary models
Numerical Simulations of Radiative Shocked Clumps: Application to the Young Stellar Outflow HH 29
No full textHerbig-Haro objects are the nebulous emission regions arising from shocks associated with the bipolar outflows of young stellar objects. Observations of the emission lines of HH objects can reveal properties of these stellar jets, which are associated with star formation, and one tool to study stellar jets and HH objects is computer modeling. AstroBEAR is a 3D magnetohydrodynamic code with adaptive mesh refinement and parallelization capabilities. The goals of this research were (1) to improve the cooling in AstroBEAR and add the ability to create valid emission-line maps for HH objects, and (2) to model a jet overrunning and shocking clumps, and to explore a parameter space that may lead to morphologies like those of HH 29. The results generate emission-line maps with the correct spatial morphology in Hα and [SII], and begin to map out the regimes (shock velocities, resolution, physical scales) in which the code remains valid
Optical spectroscopy and numerical modeling of nonradiative shocks in supernova remnants
No full textThis thesis is an observational and theoretical study of the optical emission from nonradiative shocks in three supernova remnants: the Cygnus Loop, RCW 86 and Tycho, which together span a wide range of shock velocities (300 ≲ vS ≲ 2000 km s-1). The spectra are dominated by Balmer lines of H which have both a broad component caused by proton-neutral charge exchange and a narrow component produced by collisional excitation close to the shock front. The broad to narrow flux ratios observed in all three remnants are systematically smaller in Halpha than in Hbeta, and the narrow Balmer decrement is larger than the broad Balmer decrement. The broad component Halpha profiles of RCW 86 and Tycho are Gaussian, indicating that the postshock protons follow a Maxwellian velocity distribution.
To model the data, a new numerical shock code was developed which computes the broad and narrow Balmer line emission from a nonradiative shock in partially neutral gas. The models calculate the density, temperature and size of the postshock ionization layer for arbitrary electron-proton temperature equilibrations, and use a Monte Carlo simulation to compute narrow Balmer line enhancement from Lyman line trapping. The models constrain the shock velocity and equilibration of nonradiative shocks in each remnant using the observed Halpha and Hbeta broad to narrow ratios.
The models show that differences between the observed broad and narrow Balmer decrements can be explained by Lyman line trapping. The models also show that variations in electron-proton equilibration can reproduce the observed range of broad to narrow ratios. The results give 50--100% equilibration in nonradiative portions of the NE Cygnus Loop (vS ∼ 300 km s-1) and 40--50% equilibration in nonradiative portions of RCW 86 (vS ∼ 600 km s-1 ). In Tycho there are major discrepancies between the predicted and observed broad to narrow ratios, with only the Hbeta ratio matching the observations. The discrepancies may be due to additional narrow component emission that arises from a cosmic ray/fast neutral precursor. The observed Hbeta broad to narrow ratio implies ≲ 20% equilibration in Tycho. Hence, there is an inverse correlation between Mach number and equilibration for the three observed remnants. This correlation suggests there may be significant differences between collisionless shocks at low and high Mach numbers.
The spectroscopic observations also led to the serendipitous discovery of a photoionization precursor in Tycho. The precursor appears as a faint, diffuse region that extends several arcminutes ahead of the Balmer-dominated shocks in Tycho. A new photoionization model shows that the diffuse emission is mostly warm, neutral gas heated to ∼12,000 K by He II lambda304 photons from the nonradiative shock. High resolution spectra indicate that the upstream gas is further heated to ∼40,000 K just before entering the shock. This additional heating may arise from a second precursor produced either by cosmic rays or fast (broad component) neutrals from behind the hot postshock gas
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
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