Browsing by Author "Hartigan, P."

Now showing 1 - 6 of 6
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    Irradiated interfaces in the Ara OB1, Carina, Eagle Nebula, and Cyg OB2 massive star formation regions
    (Elsevier, 2012) Hartigan, P.; Palmer, J.; Cleeves, L.I.
    Regions of massive star formation offer some of the best and most easily-observed examples of radiation hydrodynamics. Boundaries where fully-ionized H II regions transition to neutral/molecular photodissociation regions (PDRs) are of particular interest because marked temperature and density contrasts across the boundaries lead to evaporative flows and fluid dynamical instabilities that can evolve into spectacular pillar-like structures. When detached from their parent clouds, pillars become ionized globules that often harbor one or more young stars. H2 molecules at the interface between a PDR and an H II region absorb ultraviolet light from massive stars, and the resulting fluoresced infrared emission lines are an ideal way to trace this boundary independent of obscuring dust. This paper presents H2 images of four regions of massive star formation that illustrate different types of PDR boundaries. The Ara OB1 star formation region contains a striking long wall that has several wavy structures which are present in H2, but the emission is not particularly bright because the ambient UV fluxes are relatively low. In contrast, the Carina star formation region shows strong H2 fluorescence both along curved walls and at the edges of spectacular pillars that in some cases have become detached from their parent clouds. The less-spectacular but more well-known Eagle Nebula has two regions that have strong fluorescence in addition to its pillars. While somewhat older than the other regions, Cyg OB2 has the highest number of massive stars of the regions surveyed and contains many isolated, fluoresced globules that have head–tail morphologies which point towards the sources of ionizing radiation. These images provide a collection of potential astrophysical analogs that may relate to ablated interfaces observed in laser experiments of radiation hydrodynamics.
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    ON THE STRUCTURE AND STABILITY OF MAGNETIC TOWER JETS
    (The American Astronomical Society, 2012) Huarte-Espinosa, M.; Blackman, E.G.; Ciardi, A.; Hartigan, P.; Lebedev, S.V.; Chittenden, J.P.
    Modern theoretical models of astrophysical jets combine accretion, rotation, and magnetic fields to launch and collimate supersonic flows from a central source. Near the source, magnetic field strengths must be large enough to collimate the jet requiring that the Poynting flux exceeds the kinetic energy flux. The extent to which the Poynting flux dominates kinetic energy flux at large distances from the engine distinguishes two classes of models. In magneto-centrifugal launch models, magnetic fields dominate only at scales 100 engine radii, after which the jets become hydrodynamically dominated (HD). By contrast, in Poynting flux dominated (PFD) magnetic tower models, the field dominates even out to much larger scales. To compare the large distance propagation differences of these two paradigms, we perform three-dimensional ideal magnetohydrodynamic adaptive mesh refinement simulations of both HD and PFD stellar jets formed via the same energy flux.We also compare how thermal energy losses and rotation of the jet base affects the stability in these jets. For the conditions described, we show that PFD and HD exhibit observationally distinguishable features: PFD jets are lighter, slower, and less stable than HD jets. Unlike HD jets, PFD jets develop current-driven instabilities that are exacerbated as cooling and rotation increase, resulting in jets that are clumpier than those in the HD limit. Our PFD jet simulations also resemble the magnetic towers that have been recently created in laboratory astrophysical jet experiments.
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    Scaled laboratory experiments explain the kink behaviour of the Crab Nebula jet
    (Springer Nature, 2016) Li, C.K.; Tzeferacos, P.; Lamb, D.; Gregori, G.; Norreys, P.A.; Rosenberg, M.J.; Follett, R.K.; Froula, D.H.; Koenig, M.; Seguin, F.H.; Frenje, J.A.; Rinderknecht, H.G.; Sio, H.; Zylstra, A.B.; Petrasso, R.D.; Amendt, P.A.; Park, H.S.; Remington, B.A.; Ryutov, D.D.; Wilks, S.C.; Betti, R.; Frank, A.; Hu, S.X.; Sangster, T.C.; Hartigan, P.; Drake, R.P.; Kuranz, C.C.; Lebedev, S.V.; Woolsey, N.C.
    The remarkable discovery by the Chandra X-ray observatory that the Crab nebulaﾒs jet periodically changes direction provides a challenge to our understanding of astrophysical jet dynamics. It has been suggested that this phenomenon may be the consequence of magnetic fields and magnetohydrodynamic instabilities, but experimental demonstration in a controlled laboratory environment has remained elusive. Here we report experiments that use high-power lasers to create a plasma jet that can be directly compared with the Crab jet through well-defined physical scaling laws. The jet generates its own embedded toroidal magnetic fields; as it moves, plasma instabilities result in multiple deflections of the propagation direction, mimicking the kink behaviour of the Crab jet. The experiment is modelled with three-dimensional numerical simulations that show exactly how the instability develops and results in changes of direction of the jet.
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    Simulating radiative magnetohydrodynamical flows with ASTROBEAR: implementation and applications of non-equilibrium cooling
    (Oxford University Press, 2018) Hansen, E.C.; Hartigan, P.; Frank, A.; Wright, A.; Raymond, J.C.
    Radiative cooling plays a crucial role in the dynamics of many astrophysical flows, and is particularly important in the dense shocked gas within Herbig-Haro (HH) objects and stellar jets. Simulating cooling processes accurately is necessary to compare numerical simulations with existing and planned observations of HH objects, such as those from the Hubble Space Telescope and the James Webb Space Telescope. In this paper, we discuss a new, non-equilibrium cooling scheme we have implemented into the three-dimensional magnetohydrodynamic (MHD) code ASTROBEAR. The new cooling function includes ionization, recombination, and excitation of all the important atomic species that cool below 10 000 K. We tested the routine by comparing its predictions with those from the well-tested one-dimensional Cox–Raymond shock code (Raymond 1979). The results show that ASTROBEAR accurately tracks the ionization fraction, temperature, and other MHD variables for all low-velocity (≲90 km s−1) magnetized radiative shock waves. The new routine allows us to predict synthetic emission maps in all the bright forbidden and permitted lines observed in stellar jets, including H α, [N II], [O I], and [S II]. We present an example as to how these synthetic maps facilitate a direct comparison with narrowband images of HH objects.
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    A survey of irradiated pillars, globules, and jets in the Carina nebula
    (The American Astronomical Society, 2015) Hartigan, P.; Reiter, M.; Smith, N.; Bally, J.
    We present wide-field, deep narrowband H2, Brγ, Hα, [S ii], [O iii], and broadband I- and K-band images of the Carina star formation region. The new images provide a large-scale overview of all the H2 and Brγ emission present in over a square degree centered on this signature star-forming complex. By comparing these images with archival Hubble Space Telescope and Spitzer images we observe how intense UV radiation from O and B stars affects star formation in molecular clouds. We use the images to locate new candidate outflows and identify the principal shock waves and irradiated interfaces within dozens of distinct areas of star-forming activity. Shocked molecular gas in jets traces the parts of the flow that are most shielded from the intense UV radiation. Combining the H2 and optical images gives a more complete view of the jets, which are sometimes only visible in H2. The Carina region hosts several compact young clusters, and the gas within these clusters is affected by radiation from both the cluster stars and the massive stars nearby. The Carina Nebula is ideal for studying the physics of young H ii regions and photodissociation region (PDR), as it contains multiple examples of walls and irradiated pillars at various stages of development. Some of the pillars have detached from their host molecular clouds to form proplyds. Fluorescent H2 outlines the interfaces between the ionized and molecular gas, and after removing continuum, we detect spatial offsets between the Brγ and H2 emission along the irradiated interfaces. These spatial offsets can be used to test current models of PDRs once synthetic maps of these lines become available.
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    Young Stellar Objects, Accretion Disks, and Their Variability with Rubin Observatory LSST
    (IOP Publishing, 2023) Bonito, R.; Venuti, L.; Ustamujic, S.; Yoachim, P.; Street, R. A.; Prisinzano, L.; Hartigan, P.; Guarcello, M. G.; Stassun, K. G.; Giannini, T.; Feigelson, E. D.; Garatti, A. Caratti o; Orlando, S.; Clarkson, W. I.; McGehee, P.; Bellm, E. C.; Gizis, J. E.
    Vera C. Rubin Observatory, through the Legacy Survey of Space and Time (LSST), will allow us to derive a panchromatic view of variability in young stellar objects (YSOs) across all relevant timescales. Indeed, both short-term variability (on timescales of hours to days) and long-term variability (months to years), predominantly driven by the dynamics of accretion processes in disk-hosting YSOs, can be explored by taking advantage of the multiband filters option available in Rubin LSST, in particular the u, g, r, i filters that enable us to discriminate between photospheric stellar properties and accretion signatures. The homogeneity and depth of sky coverage that will be achieved with LSST will provide us with a unique opportunity to characterize the time evolution of disk accretion as a function of age and varying environmental conditions (e.g., field crowdedness, massive neighbors, metallicity) by targeting different star-forming regions. In this contribution to the Rubin LSST Survey Strategy Optimization Focus Issue, we discuss how implementing a dense observing cadence to explore short-term variability in YSOs represents a key complementary effort to the Wide–Fast–Deep observing mode that will be used to survey the sky over the full duration of the main survey (≈10 yr). The combination of these two modes will be vital to investigate the connection between the inner-disk dynamics and longer-term eruptive variability behaviors, such as those observed on EX Lupi–type objects.