912 research outputs found
The Geometry and Ionization Structure of the Wind in the Eclipsing Nova-like Variables RW Tri and UX UMa
The UV spectra of nova-like variables are dominated by emission from the
accretion disk, modified by scattering in a wind emanating from the disk. Here
we model the spectra of RW Tri and UX UMa, the only two eclipsing nova-likes
which have been observed with the Hubble Space Telescope in the
far-ultraviolet, in an attempt to constrain the geometry and the ionization
structure of their winds. Using our Monte Carlo radiative transfer code we
computed spectra for simply-parameterized axisymmetric biconical outflow models
and were able to find plausible models for both systems. These reproduce the
primary UV resonance lines - N V, Si IV, and C IV - in the observed spectra in
and out of eclipse. The distribution of these ions in the wind models is
similar in both cases as is the extent of the primary scattering regions in
which these lines are formed. The inferred mass loss rates are 6% to 8% of the
mass accretion rates for the systems. We discuss the implication of our point
models for our understanding of accretion disk winds in cataclysmic variables.Comment: 13 pages, 15 figures and 4 tables. Published in Ap
Implications for the Formation of Blue Straggler Stars from HST Ultraviolet Observations of NGC 188
We present results of a Hubble Space Telescope far-ultraviolet (FUV) survey
searching for white dwarf (WD) companions to blue straggler stars (BSSs) in
open cluster NGC 188. The majority of NGC 188 BSSs (15 of 21) are single-lined
binaries with properties suggestive of mass-transfer formation via Roche lobe
overflow, specifically through an asymptotic giant branch star transferring
mass to a main sequence secondary, yielding a BSS binary with a WD companion.
In NGC 188, a BSS formed by this mechanism within the past 400 Myr will have a
WD companion hot and luminous enough to be directly detected as a FUV
photometric excess with HST. Comparing expected BSS FUV emission to observed
photometry reveals four BSSs with WD companions above 12,000 K (younger than
250 Myr) and three WD companions with temperatures between 11,000-12,000 K.
These BSS+WD binaries all formed through recent mass transfer. The location of
the young BSSs in an optical color-magnitude diagram (CMD) indicates that
distance from the zero-age main sequence does not necessarily correlate with
BSS age. There is no clear CMD separation between mass transfer-formed BSSs and
those likely formed through other mechanisms, such as collisions. The seven
detected WD companions place a lower limit on the mass-transfer formation
frequency of 33%. We consider other possible formation mechanisms by comparing
properties of the BSS population to theoretical predictions. We conclude that
14 BSS binaries likely formed from mass transfer, resulting in an inferred
mass-transfer formation frequency of approximately 67%.Comment: 13 pages, 6 figures, accepted to the Astrophysical Journa
Detection of white dwarf companions to blue stragglers in the open cluster NGC 188: direct evidence for recent mass transfer
Several possible formation pathways for blue straggler stars have been
developed recently, but no one pathway has yet been observationally confirmed
for a specific blue straggler. Here we report the first findings from a Hubble
Space Telescope ACS/SBC far-UV photometric program to search for white dwarf
companions to blue straggler stars. We find three hot and young white dwarf
companions to blue straggler stars in the 7-Gyr open cluster NGC 188,
indicating that mass transfer in these systems ended less than 300 Myr ago.
These companions are direct and secure observational evidence that these blue
straggler stars were formed through mass transfer in binary stars. Their
existence in a well-studied cluster environment allows for observational
constraints of both the current binary system and the progenitor binary system,
mapping the entire mass transfer history.Comment: 5 pages, 3 figures, accepted for publication in The Astrophysical
Journal Letter
Nuclear star cluster formation in energy-space
In a virialized stellar system, the mean-square velocity is a direct tracer
of the energy per unit mass of the system. Here, we exploit this to estimate
and compare root-mean-square velocities for a large sample of nuclear star
clusters and their host (late- or early-type) galaxies. Traditional
observables, such as the radial surface brightness and second-order velocity
moment profiles, are subject to short-term variations due to individual
episodes of matter infall and/or star formation. The total mass, energy and
angular momentum, on the other hand, are approximately conserved. Thus, the
total energy and angular momentum more directly probe the formation of galaxies
and their nuclear star clusters, by offering access to more fundamental
properties of the nuclear cluster-galaxy system than traditional observables.
We find that there is a strong correlation, in fact a near equality, between
the root-mean-square velocity of a nuclear star cluster and that of its host.
Thus, the energy per unit mass of a nuclear star cluster is always comparable
to that of its host galaxy. We interpret this as evidence that nuclear star
clusters do not form independently of their host galaxies, but rather that
their formation and subsequent evolution are coupled. We discuss how our
results can potentially be used to offer a clear and observationally testable
prediction to distinguish between the different nuclear star cluster formation
scenarios, and/or quantify their relative contributions.Comment: 12 pages, 3 figures, 4 tables; accepted for publication in MNRA
Three-dimensional simulations of rotationally-induced line variability from a Classical T Tauri star with a misaligned magnetic dipole
We present three-dimensional (3-D) simulations of rotationally induced line
variability arising from complex circumstellar environment of classical T Tauri
stars (CTTS) using the results of the 3-D magnetohydrodynamic (MHD) simulations
of Romanova et al., who considered accretion onto a CTTS with a misaligned
dipole magnetic axis with respect to the rotational axis. The density, velocity
and temperature structures of the MHD simulations are mapped on to the
radiative transfer grid, and corresponding line source function and the
observed profiles of neutral hydrogen lines (H-beta, Pa-beta and Br-gamma) are
computed using the Sobolev escape probability method. We study the dependency
of line variability on inclination angles (i) and magnetic axis misalignment
angles (Theta). By comparing our models with the Pa-beta profiles of 42 CTTS
observed by Folha & Emerson, we find that models with a smaller misaligngment
angle (Theta<~15 deg.) are more consistent with the observations which show
that majority of Pa-beta are rather symmetric around the line centre. For a
high inclination system with a small dipole misalignment angle (Theta ~ 15
deg.), only one accretion funnel (on the upper hemisphere) is visible to an
observer at any given rotational phase. This can cause an anti-correlation of
the line equivalent width in the blue wing (v0)
over a half of a rotational period, and a positive correlation over other half.
We find a good overall agreement of the line variability behaviour predicted by
our model and those from observations. (Abridged)Comment: 15 pages, 13 figures. Accepted for publication in MNRAS. A version
with full resolution figures can be downloaded from
http://www.physics.unlv.edu/~rk/preprint/inclined_dipole.pd
Is the slope of the intrinsic Baldwin effect constant?
We investigate the relationship between emission-line strength and continuum
luminosity in the best-studied nearby Seyfert 1 galaxy NGC5548. Our analysis of
13 years of ground-based optical monitoring data reveals significant
year-to-year variations in the observed H-beta emission-line response in this
source. More specifically, we confirm the result of Gilbert and Peterson (2003)
of a non-linear relationship between the continuum and H-beta emission-line
fluxes. Furthermore, we show that the slope of this relation is not constant,
but rather decreases as the continuum flux increases. Both effects are
consistent with photoionisation model predictions of a luminosity-dependent
response in this line.Comment: 9 pages, 6 figures, accepted for publication in MNRA
Deep near-IR observations of the Globular Cluster M4: Hunting for Brown Dwarfs
We present an analysis of deep HST/WFC3 near-IR (NIR) imaging data of the
globular cluster M4. The best-photometry NIR colour-magnitude diagram (CMD)
clearly shows the main sequence extending towards the expected end of the
Hydrogen-burning limit and going beyond this point towards fainter sources. The
white dwarf sequence can be identified. As such, this is the deepest NIR CMD of
a globular cluster to date. Archival HST optical data were used for
proper-motion cleaning of the CMD and for distinguishing the white dwarfs (WDs)
from brown dwarf (BD) candidates. Detection limits in the NIR are around F110W
approx 26.5 mag and F160W approx27 mag, and in the optical around F775W approx
28 mag. Comparing our observed CMDs with theoretical models, we conclude that
we have reached beyond the H-burning limit in our NIR CMD and are probably just
above or around this limit in our optical-NIR CMDs. Thus, any faint NIR sources
that have no optical counterpart are potential BD candidates, since the optical
data are not deep enough to detect them. We visually inspected the positions of
NIR sources which are fainter than the H-burning limit in F110W and for which
the optical photometry did not return a counterpart. We found in total five
sources for which we did not get an optical measurement. For four of these five
sources, a faint optical counterpart could be visually identified, and an upper
optical magnitude was estimated. Based on these upper optical magnitude limits,
we conclude that one source is likely a WD, one source could either be a WD or
BD candidate, and the remaining two sources agree with being BD candidates. For
only one source no optical counterpart could be detected, which makes this
source a good BD candidate. We conclude that we found in total four good BD
candidates.Comment: ApJ accepted, 28 pages including 16 figure
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