2,943 research outputs found
Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
We model the detailed time-evolution of Discrete Absorption Components (DACs)
observed in P Cygni profiles of the Si IV lam1400 resonance doublet lines of
the fast-rotating supergiant HD 64760 (B0.5 Ib). We adopt the common assumption
that the DACs are caused by Co-rotating Interaction Regions (CIRs) in the
stellar wind. We perform 3D radiative transfer calculations with hydrodynamic
models of the stellar wind that incorporate these large-scale density- and
velocity-structures. We develop the 3D transfer code Wind3D to investigate the
physical properties of CIRs with detailed fits to the DAC shape and morphology.
The CIRs are caused by irregularities on the stellar surface that change the
radiative force in the stellar wind. In our hydrodynamic model we approximate
these irregularities by circular symmetric spots on the stellar surface. We use
the Zeus3D code to model the stellar wind and the CIRs, limited to the
equatorial plane. We constrain the properties of large-scale wind structures
with detailed fits to DACs observed in HD 64760. A model with two spots of
unequal brightness and size on opposite sides of the equator, with opening
angles of 20 +/- 5 degr and 30 +/- 5 degr diameter, and that are 20 +/- 5 % and
8 +/- 5 % brighter than the stellar surface, respectively, provides the best
fit to the observed DACs. The recurrence time of the DACs compared to the
estimated rotational period corresponds to spot velocities that are 5 times
slower than the rotational velocity. The mass-loss rate of the structured wind
model for HD 64760 does not exceed the rate of the spherically symmetric smooth
wind model by more than 1 %. The fact that DACs are observed in a large number
of hot stars constrains the clumping that can be present in their winds, as
substantial amounts of clumping would tend to destroy the CIRs.Comment: 58 pages, 16 figures, 1 animation. Accepted for publication in The
Astrophysical Journal, Main Journal. More information and animations are
available at http://alobel.freeshell.org/hotstars.htm
Estimating Be Star Disk Radii using H-alpha Emission Equivalent Widths
We present numerical models of the circumstellar disks of Be stars, and we
describe the resulting synthetic H-alpha emission lines and maps of the
wavelength-integrated emission flux projected onto the sky. We demonstrate that
there are monotonic relationships between the emission line equivalent width
and the ratio of the angular half-width at half maximum of the projected disk
major axis to the radius of the star. These relationships depend mainly upon
the temperatures of the disk and star, the inclination of the disk normal to
the line of sight, and the adopted outer boundary for the disk radius. We show
that the predicted H-alpha disk radii are consistent with those observed
directly through long baseline interferometry of nearby Be stars (especially
once allowance is made for disk truncation in binaries and for dilution of the
observed H-alpha equivalent width by continuum disk flux in the V-band).Comment: 12 pages, 2 figures, ApJL in pres
Electromagnetic Scattering from Relativistic Bound States
The quasipotential formalism for elastic scattering from relativistic bound
states is formulated based on the instant constraint in the Breit frame. The
quasipotential electromagnetic current is derived from Mandelstam's five-point
kernel and obeys a two-body Ward identity. Breit-frame wave functions are
obtained directly by solving integral equations with nonzero total
three-momentum, thus accomplishing a dynamical boost. Calculations of
electron-deuteron elastic form factors illustrate the importance of the
dynamical boost versus kinematic boosts of the rest frame wave functions.Comment: RevTeX 3.0 manuscript, 9 pages. UU-file is a single PostScript file
of the manuscript including figures. U. MD PP #93-17
Induced star formation in interacting galaxies
Measurements of H alpha emission line fluxes and FIR fluxes in approx. 100 interacting spirals were used to investigate the effects of close tidal interactions on the disk and nuclear star formation rates in galaxies. Two samples of interacting spirals were studied, a complete sample of close pairs, and a set of strongly perturbed systems from the Arp atlas. Both the integrated H alpha luminosities and FIR luminosities are enhanced in the interacting galaxies, indicating that the encounters indeed trigger massive star formation in many cases. The response of individual galaxies is highly variable, however. A majority of the interacting spirals exhibit normal star formation rates, while a small fraction are undergoing bursts with luminosities which are rarely, if ever, observed in noninteracting systems. Virtually all of the latter are in the Arp sample, indicating that the Arp atlas is heavily biased to the most active star forming systems
Line Forces in Keplerian Circumstellar Disks and Precession of Nearly Circular Orbits
We examine the effects of optically thick line forces on orbiting
circumstellar disks, such as occur around Be stars. For radially streaming
radiation, line forces are only effective if there is a strong radial velocity
gradient, as occurs, for example, in a line-driven stellar wind. However,
within an orbiting disk, the radial shear of the azimuthal velocity leads to
strong line-of-sight velocity velocity gradients along nonradial directions. As
such, in the proximity of a stellar surface extending over a substantial cone
angle, the nonradial stellar radiation can impart a significant line force,
even in the case of purely circular orbits. Given the highly supersonic nature
of orbital velocity variations, we use the Sobolev approximation, thereby
extending to the disk case the standard CAK formalism developed for line-driven
winds. We delineate the parameter regimes for which radiative forces might
alter disk properties; but even when radiative forces are small, we
analytically quantify higher-order effects in the linear limit, including the
precession of weakly elliptical orbits. We find that optically thick line
forces can have observable implications for the dynamics of disks around Be
stars, including the generation of either prograde or retrograde precession in
slightly eccentric orbits. However, our analysis suggests a net retrograde
effect, in apparent contradiction with observed long-term variations of
violet/red line profile asymmetries from Be stars, which are generally thought
to result from prograde propagation of a so-called ``one arm mode''. We also
conclude that radiative forces may alter the dynamical properties at the
surface of the disk where disk winds originate, and may even make low-density
disks vulnerable to being blown away.Comment: 31 pages, Latex, aaspp4 macro, 4 figure
Cicrumnuclear Supernova Remnants and HII Regions in NGC 253
Archival VLA data has been used to produce arcsecond-resolution 6- and 20-cm
images of the region surrounding the nuclear 200-pc (~15") starburst in NGC
253. Twenty-two discrete sources stronger than 0.4 mJy have been detected
within ~2 kpc (~3') of the galaxy nucleus; almost all these sources must be
associated with the galaxy. None of the radio sources coincides with a detected
X-ray binary, so they appear to be due to supernova remnants and H II regions.
The region outside the central starburst has a derived radio supernova rate of
<~0.1/yr, and may account for at least 20% of the recent star formation in NGC
253. Most of the newly identified sources have steep, nonthermal radio spectra,
but several relatively strong thermal sources also exist, containing the
equivalent of tens of O5 stars. These stars are spread over tens of parsecs,
and are embedded in regions having average ionized gas densities of
20-200/cm^3, much lower than in the most active nuclear star-forming regions in
NGC 253 or in the super star clusters seen in other galaxies. The strongest
region of thermal emission coincides with a highly reddened area seen at
near-infrared wavelengths, possibly containing optically obscured H II regions.Comment: 17 pages, 3 postscript figures, AASTeX format, in press for
Astronomical Journal, July 200
Radio Sources in Low-Luminosity Active Galactic Nuclei. I. VLA Detections of Compact, Flat-Spectrum Cores
We report a 0.2" resolution, 15 GHz survey of a sample of 48 low-luminosity
active galactic nuclei with the Very Large Array. Compact radio emission has
been detected in 57% (17 of 30) of LINERs and low-luminosity Seyferts, at least
15 of which have a flat to inverted radio spectrum (alpha > -0.3). The compact
radio cores are found in both type 1 (i.e. with broad Halpha) and type 2
(without broad Halpha) nuclei. The 2 cm radio power is significantly correlated
with the emission-line ([OI] lambda6300) luminosity. While the present
observations are consistent with the radio emission originating in star-forming
regions, higher resolution radio observations of 10 of the detected sources,
reported in a companion paper (Falcke et al. 2000), show that the cores are
very compact (= 10^8K) and
probably synchrotron self-absorbed, ruling out a starburst origin. Thus, our
results suggest that at least 50% of low-luminosity Seyferts and LINERs in the
sample are accretion powered, with the radio emission presumably coming from
jets or advection-dominated accretion flows. We have detected only 1 of 18
`transition' (i.e. LINER + HII) nuclei observed, indicating their radio cores
are significantly weaker than those of `pure' LINERs.Comment: To appear in the Astrophysical Journal, October 20, 200
The Pearson-Readhead Survey of Compact Extragalactic Radio Sources From Space. II. Analysis of Source Properties
We have performed a multi-dimensional correlation analysis on the observed
properties of a statistically complete core-selected sample of compact
radio-loud active galactic nuclei, based on data from the VLBI Space Observing
Programme (Paper I) and previously published studies. Our sample is drawn from
the well-studied Pearson-Readhead (PR) survey, and is ideally suited for
investigating the general effects of relativistic beaming in compact radio
sources. In addition to confirming many previously known correlations, we have
discovered several new trends that lend additional support to the beaming
model. These trends suggest that the most highly beamed sources in
core-selected samples tend to have a) high optical polarizations; b) large
pc/kpc-scale jet misalignments; c) prominent VLBI core components; d)
one-sided, core, or halo radio morphology on kiloparsec scales; e) narrow
emission line equivalent widths; and f) a strong tendency for intraday
variability at radio wavelengths. We have used higher resolution space and
ground-based VLBI maps to confirm the bi-modality of the jet misalignment
distribution for the PR survey, and find that the sources with aligned parsec-
and kiloparsec-scale jets generally have arcsecond-scale radio emission on both
sides of the core. The aligned sources also have broader emission line widths.
We find evidence that the BL Lacertae objects in the PR survey are all highly
beamed, and have very similar properties to the high-optically polarized
quasars, with the exception of smaller redshifts. A cluster analysis on our
data shows that after partialing out the effects of redshift, the luminosities
of our sample objects in various wave bands are generally well-correlated with
each other, but not with other source properties.Comment: 21 pages, 10 figures. Accepted for publication in the Astrophysical
Journal. Part I can be found at astro-ph/010227
Stellar parameters of Be stars observed with X-shooter
Aims. The X-shooter archive of several thousand telluric star spectra was
skimmed for Be and Be-shell stars to derive the stellar fundamental parameters
and statistical properties, in particular for the less investigated late type
Be stars, and the extension of the Be phenomenon into early A stars. Methods.
An adapted version of the BCD method is used, utilizing the Balmer
discontinuity parameters to determine effective temperature and surface
gravity. This method is optimally suited for late B stars. The projected
rotational velocity was obtained by profile fitting to the Mg ii lines of the
targets, and the spectra were inspected visually for the presence of peculiar
features such as the infrared Ca ii triplet or the presence of a double Balmer
discontinuity. The Balmer line equivalent widths were measured, but due to
uncertainties in determining the photospheric contribution are useful only in a
subsample of Be stars for determining the pure emission contribution. Results.
A total of 78 Be stars, mostly late type ones, were identified in the X-shooter
telluric standard star archive, out of which 48 had not been reported before.
The general trend of late type Be stars having more tenuous disks and being
less variable than early type ones is confirmed. The relatively large number
(48) of relatively bright (V > 8.5) additional Be stars casts some doubt on the
statistics of late type Be stars; they are more common than currently thought:
The Be/B star fraction may not strongly depend on spectral subtype.Comment: Accepted for publication in A&
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