463 research outputs found
Structure and Feedback in 30 Doradus I: Observations
We have completed a a new optical imaging and spectrophotometric survey of a
140 x 80 pc region of 30 Doradus centered on R136, covering key optical
diagnostic emission lines including \Ha, \Hb, \Hg, [O III]
4363, 4959, 5007, [N II] 6548, 6584, [S II]
6717, 6731 [S III] 6312 and in some locations [S III]
9069. We present maps of fluxes and intensity ratios for these lines,
and catalogs of isolated ionizing stars, elephant-trunk pillars, and edge-on
ionization fronts. The final science-quality spectroscopic data products are
available to the public. Our analysis of the new data finds that, while stellar
winds and supernovae undoubtedly produce shocks and are responsible for shaping
the nebula, there are no global spectral signatures to indicate that shocks are
currently an important source of ionization. We conclude that the considerable
region covered by our survey is well described by photoionization from the
central cluster where the ionizing continuum is dominated by the most massive O
stars. We show that if 30 Dor were viewed at a cosmological distance, its
integrated light would be dominated by its extensive regions of lower
surface-brightness rather than by the bright, eye-catching arcs.Comment: 42 pages, 16 figures, Accepted for publication in ApJ
Flame Evolution During Type Ia Supernovae and the Deflagration Phase in the Gravitationally Confined Detonation Scenario
We develop an improved method for tracking the nuclear flame during the
deflagration phase of a Type Ia supernova, and apply it to study the variation
in outcomes expected from the gravitationally confined detonation (GCD)
paradigm. A simplified 3-stage burning model and a non-static ash state are
integrated with an artificially thickened advection-diffusion-reaction (ADR)
flame front in order to provide an accurate but highly efficient representation
of the energy release and electron capture in and after the unresolvable flame.
We demonstrate that both our ADR and energy release methods do not generate
significant acoustic noise, as has been a problem with previous ADR-based
schemes. We proceed to model aspects of the deflagration, particularly the role
of buoyancy of the hot ash, and find that our methods are reasonably
well-behaved with respect to numerical resolution. We show that if a detonation
occurs in material swept up by the material ejected by the first rising bubble
but gravitationally confined to the white dwarf (WD) surface (the GCD
paradigm), the density structure of the WD at detonation is systematically
correlated with the distance of the deflagration ignition point from the center
of the star. Coupled to a suitably stochastic ignition process, this
correlation may provide a plausible explanation for the variety of nickel
masses seen in Type Ia Supernovae.Comment: 14 pages, 10 figures, accepted to the Astrophysical Journa
X-ray emission from the double-binary OB-star system QZ Car (HD 93206)
X-ray observations of the double-binary OB-star system QZ Car (HD 93206)
obtained with the Chandra X-ray Observatory over a period of roughly 2 years
are presented. The orbit of systems A (O9.7 I+b2 v, PA = 21 d) and B (O8 III+o9
v, PB = 6 d) are reasonably well sampled by the observations, allowing the
origin of the X-ray emission to be examined in detail. The X-ray spectra can be
well fitted by an attenuated three temperature thermal plasma model,
characterised by cool, moderate, and hot plasma components at kT ~ 0.2, 0.7,
and 2 keV, respectively, and a circumstellar absorption of ~ 0.2 x 10^22 cm-2.
Although the hot plasma component could be indicating the presence of wind-wind
collision shocks in the system, the model fluxes calculated from spectral fits,
with an average value of ~ 7 x 10^-13 erg s-1 cm-2, do not show a clear
correlation with the orbits of the two constituent binaries. A semi-analytical
model of QZ Car reveals that a stable momentum balance may not be established
in either system A or B. Yet, despite this, system B is expected to produce an
observed X-ray flux well in excess of the observations. If one considers the
wind of the O8 III star to be disrupted by mass transfer the model and
observations are in far better agreement, which lends support to the previous
suggestion of mass-transfer in the O8 III + o9 v binary. We conclude that the
X-ray emission from QZ Car can be reasonably well accounted for by a
combination of contributions mainly from the single stars and the mutual
wind-wind collision between systems A and B.Comment: 11 pages, 7 figures. Accepted for the ApJS Special Issue on the
Chandra Carina Complex Project (CCCP), scheduled for publication in May 2011.
All 16 CCCP Special Issue papers are available at
http://cochise.astro.psu.edu/Carina_public/special_issue.html through 2011 at
leas
51 Eri and GJ 3305: A 10-15 Myr old binary star system at 30 parsecs
Following the suggestion of Zuckerman et al. (2001, ApJ, 562, L87), we
consider the evidence that 51 Eri (spectral type F0) and GJ 3305 (M0),
historically classified as unrelated main sequence stars in the solar
neighborhood, are instead a wide physical binary system and members of the
young beta Pic moving group (BPMG). The BPMG is the nearest (d < 50 pc) of
several groups of young stars with ages around 10 Myr that are kinematically
convergent with the Oph-Sco-Cen Association (OSCA), the nearest OB star
association. Combining SAAO optical photometry, Hobby-Eberly Telescope
high-resolution spectroscopy, Chandra X-ray data, and UCAC2 catalog kinematics,
we confirm with high confidence that the system is indeed extremely young. GJ
3305 itself exhibits very strong magnetic activity but has rapidly depleted
most of its lithium. The 51 Eri/GJ 3305 system is the westernmost known member
of the OSCA, lying 110 pc from the main subgroups. The system is similar to the
BPMG wide binary HD 172555/CD -64d1208 and the HD 104237 quintet, suggesting
that dynamically fragile multiple systems can survive the turbulent
environments of their natal giant molecular cloud complexes, while still being
imparted high dispersion velocities. Nearby young systems such as these are
excellent targets for evolved circumstellar disk and planetary studies, having
stellar ages comparable to that of the late phases of planet formation.Comment: 27 pages, 7 figures. Accepted for publication in the Astronomical
Journal. For a version with high resolution figures, see
http://www.astro.psu.edu/users/edf/51Eri.pd
Editorial: crime patterns in time and space: the dynamics of crime opportunities in urban areas
The routine activity approach and associated crime pattern theory emphasise how crime emerges from spatio-temporal routines. In order to understand this crime should be studied in both space and time. However, the bulk of research into crime patterns and related activities has investigated the spatial distributions of crime, neglecting the temporal dimension. Specifically, disaggregation of crime by place and by time, for example hour of day, day of week, month of year, season, or school day versus none school day, is extremely relevant to theory. Modern data make such spatio-temporal disaggregation increasingly feasible, as exemplified in this special issue. First, much larger data files allow disaggregation of crime data into temporal and spatial slices. Second, new forms of data are generated by modern technologies, allowing innovative and new forms of analyses. Crime pattern analyses and routine activity inquiries are now able to explore avenues not previously available. The unique collection of nine papers in this thematic issue specifically examine spatio-temporal patterns of crime to; demonstrate the value of this approach for advancing knowledge in the field; consider how this informs our theoretical understanding of the manifestations of crime in time and space; to consider the prevention implications of this; and to raise awareness of the need for further spatio-temporal research into crime event
Spontaneous Initiation of Detonations in White Dwarf Environments: Determination of Critical Sizes
Some explosion models for Type Ia supernovae (SN Ia), such as the
gravitationally confined detonation (GCD) or the double detonation
sub-Chandrasekhar (DDSC) models, rely on the spontaneous initiation of a
detonation in the degenerate C/O material of a white dwarf. The length scales
pertinent to the initiation of the detonation are notoriously unresolved in
multi-dimensional stellar simulations, prompting the use of results of 1D
simulations at higher resolution, such as the ones performed for this work, as
guidelines for deciding whether or not conditions reached in the higher
dimensional full star simulations successfully would lead to the onset of a
detonation. Spontaneous initiation relies on the existence of a suitable
gradient in self-ignition (induction) times of the fuel, which we set up with a
spatially localized non-uniformity of temperature -- a hot spot. We determine
the critical (smallest) sizes of such hot spots that still marginally result in
a detonation in white dwarf matter by integrating the reactive Euler equations
with the hydrodynamics code FLASH. We quantify the dependences of the critical
sizes of such hot spots on composition, background temperature, peak
temperature, geometry, and functional form of the temperature disturbance, many
of which were hitherto largely unexplored in the literature. We discuss the
implications of our results in the context of modeling of SNe Ia.Comment: 43 pages, 12 figures, 12 table
X-ray Sources in the Hubble Deep Field Detected by Chandra
We present first results from an X-ray study of the Hubble Deep Field North
(HDF-N) and its environs obtained using 166 ks of data collected by the
Advanced CCD Imaging Spectrometer (ACIS) on board the Chandra X-ray
Observatory. This is the deepest X-ray observation ever reported, and in the
HDF-N itself we detect six X-ray sources down to a 0.5--8 keV flux limit of
4E-16 erg cm^-2 s^-1. Comparing these sources with objects seen in
multiwavelength HDF-N studies shows positional coincidences with the extremely
red object NICMOS J123651.74 +621221.4, an active galactic nucleus (AGN), three
elliptical galaxies, and one nearby spiral galaxy. The X-ray emission from the
ellipticals is consistent with that expected from a hot interstellar medium,
and the spiral galaxy emission may arise from a `super-Eddington' X-ray binary
or ultraluminous supernova remnant. Four of the X-ray sources have been
detected at radio wavelengths. We also place X-ray upper limits on AGN
candidates found in the HDF-N, and we present the tightest constraints yet on
X-ray emission from the SCUBA submillimeter source population. None of the 10
high-significance submillimeter sources reported in the HDF-N and its vicinity
is detected with Chandra ACIS. These sources appear to be dominated by star
formation or have AGN with Compton-thick tori and little circumnuclear X-ray
scattering.Comment: 11 pages, ApJ, in press, also available from
http://www.astro.psu.edu/users/niel/hdf/hdf-chandra.htm
Asymmetry and the Nucleosynthetic Signature of Nearly Edge-Lit Detonation in White Dwarf Cores
Most of the leading explosion scenarios for Type Ia supernovae involve the
nuclear incineration of a white dwarf star through a detonation wave. Several
scenarios have been proposed as to how this detonation may actually occur, but
the exact mechanism and environment in which it takes place remain unknown. We
explore the effects of an off-center initiated detonation on the spatial
distribution of the nucleosynthetic yield products in a toy model -- a
pre-expanded near Chandrasekhar-mass white dwarf. We find that a single-point
near edge-lit detonation results in asymmetries in the density and thermal
profiles, notably the expansion timescale, throughout the supernova ejecta. We
demonstrate that this asymmetry of the thermodynamic trajectories should be
common to off-center detonations where a small amount of the star is burned
prior to detonation. The sensitivity of the yields on the expansion timescale
results in an asymmetric distribution of the elements synthesized as reaction
products. We tabulate the shift in the center of mass of the various elements
produced in our model supernova and find an odd-even pattern for elements past
silicon. Our calculations show that off-center single-point detonations in
carbon-oxygen white dwarfs are marked by significant composition asymmetries in
their remnants which bear potentially observable signatures in both velocity
and coordinate space, including an elemental nickel mass fraction which varies
by a factor of two to three from one side of the remnant to the other.Comment: 7 pages, 7 figures, accepted for publication in the Astrophysical
Journa
X-ray emitting young stars in the Orion Nebula
The Orion Nebula Cluster and the molecular cloud in its vicinity have been
observed with the ACIS-I detector on board the Chandra X-ray Observatory with
23 hours exposure. We detect 1075 X-ray sources: 91% are spatially associated
with known stellar members of the cluster, and 7% are newly identified deeply
embedded cloud members. This provides the largest X-ray study of a pre-main
sequence stellar population. We examine here the X-ray properties of Orion
young stars as a function of mass. Results include: (a) the discovery of rapid
variability in the O9.5 31 M_o star \theta^2A Ori, and several early B stars,
inconsistent with the standard model of X-ray production in small wind shocks;
(b) support for the hypothesis that intermediate-mass mid-B through A type
stars do not themselves produce significant X-ray emission; (c) confirmation
that low-mass G- through M-type T Tauri stars exhibit powerful flaring but
typically at luminosities considerably below the `saturation' level; (d)
confirmation that the presence or absence of a circumstellar disk has no
discernable effect on X-ray emission; (e) evidence that T Tauri plasma
temperatures are often very high with T >= 100 MK, even when luminosities are
modest and flaring is not evident; and (f) detection of the largest sample of
pre-main sequence very low mass objects showing high flaring levels and a
decline in magnetic activity as they evolve into L- and T-type brown dwarfs.Comment: 82 pages, 16 figures, 6 tables. To appear in the Astrophysical
Journal. For a version with high quality images and electronic tables, see
ftp://ftp.astro.psu.edu/pub/edf/orion1
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