449 research outputs found
Properties of the close binary and circumbinary torus of the Red Rectangle
New diffraction-limited speckle images of the Red Rectangle in the wavelength
range 2.1--3.3 microns with angular resolutions of 44--68 mas and previous
speckle images at 0.7--2.2 microns revealed well-resolved bright bipolar
outflow lobes and long X-shaped spikes originating deep inside the outflow
cavities. This set of high-resolution images stimulated us to reanalyze all
infrared observations of the Red Rectangle using our two-dimensional radiative
transfer code. The new detailed modeling, together with estimates of the
interstellar extinction in the direction of the Red Rectangle enabled us to
more accurately determine one of the key parameters, the distance D=710 pc with
model uncertainties of 70 pc, which is twice as far as the commonly used
estimate of 330 pc. The central binary is surrounded by a compact, massive
(M=1.2 Msun), very dense dusty torus with hydrogen densities reaching
n_H=2.5x10^12 cm^-3 (dust-to-gas mass ratio rho_d/rho~0.01). The bright
component of the spectroscopic binary HD 44179 is a post-AGB star with mass
M*=0.57 Msun, luminosity L*=6000 Lsun, and effective temperature T*=7750 K.
Based on the orbital elements of the binary, we identify its invisible
component with a helium white dwarf with Mwd~0.35 Msun, Lwd~100 Lsun, and
Twd~6x10^4 K. The hot white dwarf ionizes the low-density bipolar outflow
cavities inside the dense torus, producing a small HII region observed at radio
wavelengths. We propose an evolutionary scenario for the formation of the Red
Rectangle nebula, in which the binary initially had 2.3 and 1.9 Msun components
at a separation of 130 Rsun. The nebula was formed in the ejection of a common
envelope after Roche lobe overflow by the present post-AGB star.Comment: 20 pages, 10 figures, accepted by Astronomy and Astrophysics, also
available at
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm
Direct Detection of the Brown Dwarf GJ 802B with Adaptive Optics Masking Interferometry
We have used the Palomar 200" Adaptive Optics (AO) system to directly detect
the astrometric brown dwarf GJ 802B reported by Pravdo et al. 2005. This
observation is achieved with a novel combination of aperture masking
interferometry and AO. The dynamical masses are 0.1750.021 M and
0.0640.032 M for the primary and secondary respectively. The
inferred absolute H band magnitude of GJ 802B is M=12.8 resulting in a
model-dependent T of 1850 50K and mass range of
0.057--0.074 M.Comment: 4 Pages, 5 figures, emulateapj format, submitted to ApJ
A close halo of large transparent grains around extreme red giant stars
Intermediate-mass stars end their lives by ejecting the bulk of their
envelope via a slow dense wind back into the interstellar medium, to form the
next generation of stars and planets. Stellar pulsations are thought to elevate
gas to an altitude cool enough for the condensation of dust, which is then
accelerated by radiation pressure from starlight, entraining the gas and
driving the wind. However accounting for the mass loss has been a problem due
to the difficulty in observing tenuous gas and dust tens of milliarcseconds
from the star, and there is accordingly no consensus on the way sufficient
momentum is transferred from the starlight to the outflow. Here, we present
spatially-resolved, multi-wavelength observations of circumstellar dust shells
of three stars on the asymptotic giant branch of the HR diagram. When imaged in
scattered light, dust shells were found at remarkably small radii (<~ 2 stellar
radii) and with unexpectedly large grains (~300 nm radius). This proximity to
the photosphere argues for dust species that are transparent to starlight and
therefore resistant to sublimation by the intense radiation field. While
transparency usually implies insufficient radiative pressure to drive a wind,
the radiation field can accelerate these large grains via photon scattering
rather than absorption - a plausible mass-loss mechanism for lower-amplitude
pulsating stars.Comment: 13 pages, 1 table, 6 figure
Masses of Astrometrically-Discovered and Imaged Binaries: G 78-28AB and GJ 231.1BC
The Stellar Planet Survey (STEPS) is an ongoing astrometric search for giant
planets and brown dwarfs around a sample of ~30 M-dwarfs. We have discovered
several low-mass companions by measuring the motion of our target stars
relative to their reference frames. The highest mass discovery thus far is G
78-28B, a companion to the M-dwarf G 78-28A. The orbital period is 4.18 +/-
0.03 y, the system mass is 0.565 +/- 0.055 Msolar, and the semi-major axis is
2.19 +/- 0.10 AU. Imaging observations with the Keck laser guide star adaptive
optics (LGSAO) and the Palomar AO instruments resolved the system and also
yielded JHK-band delta magnitudes. We use the orbital solution, light ratios,
and mass-luminosity relationships to derive component masses of MA = 0.370 +/-
0.034 Msolar and MB = 0.195 +/- 0.021 Msolar. G 78-28B is of type M4 V based
upon its colors and mass. We also discovered GJ 231.1C, a companion to GJ
231.1B, with STEPS and imaged the companion with LGSAO and Palomar AO, but the
orbital period is longer than our observing baseline; thus the system
parameters are less constrained. In GJ 231.1BC the masses are MB = 0.25 +/-
0.06 Msolar and MC =0.12 +/- 0.02 Msolar. The inferred spectral type of GJ
231.1C is M5 V. We demonstrate the results of the current state of mass
estimation techniques with our data.Comment: 25 pages, 8 figures, accepted for Ap
Long-Baseline Interferometric Multiplicity Survey of the Sco-Cen OB Association
We present the first multiplicity-dedicated long baseline optical
interferometric survey of the Scorpius-Centaurus-Lupus-Crux association. We
used the Sydney University Stellar Interferometer to undertake a survey for new
companions to 58 Sco-Cen B- type stars and have detected 24 companions at
separations ranging from 7-130mas, 14 of which are new detections. Furthermore,
we use a Bayesian analysis and all available information in the literature to
determine the multiplicity distribution of the 58 stars in our sample, showing
that the companion frequency is F = 1.35 and the mass ratio distribution is
best described as a power law with exponent equal to -0.46, agreeing with
previous Sco-Cen high mass work and differing significantly from lower-mass
stars in Tau-Aur. Based on our analysis, we estimate that among young B-type
stars in moving groups, up to 23% are apparently single stars. This has strong
implications for the understanding of high-mass star formation, which requires
angular momentum dispersal through some mechanism such as formation of multiple
systems.Comment: 7 figures, 5 tables, accepted for publication in MNRA
Two Wolf-Rayet stars at the heart of colliding-wind binary Apep
Infrared imaging of the colliding-wind binary Apep has revealed a spectacular dust plume with complicated internal dynamics that challenges standard colliding-wind binary physics. Such challenges can be potentially resolved if a rapidly-rotating Wolf-Rayet star is located at the heart of the system, implicating Apep as a Galactic progenitor system to long-duration gamma-ray bursts. One of the difficulties in interpreting the dynamics of Apep is that the spectral composition of the stars in the system was unclear. Here we present visual to near-infrared spectra that demonstrate that the central component of Apep is composed of two classical Wolf-Rayet stars of carbon- (WC8) and nitrogen-sequence (WN4-6b) subtypes. We argue that such an assignment represents the strongest case of a classical WR+WR binary system in the Milky Way. The terminal line-of-sight wind velocities of the WC8 and WN4-6b stars are measured to be 2100 ± 200 and 3500 ± 100 km sâ1, respectively. If the mass-loss rate of the two stars are typical for their spectral class, the momentum ratio of the colliding winds is expected to be â 0.4. Since the expansion velocity of the dust plume is significantly smaller than either of the measured terminal velocities, we explore the suggestion that one of the Wolf-Rayet winds is anisotropic. We can recover a shock-compressed wind velocity consistent with the observed dust expansion velocity if the WC8 star produces a significantly slow equatorial wind with a velocity of â530 km sâ1. Such slow wind speeds can be driven by near-critical rotation of a Wolf-Rayet star
IRC+10216 in Action: Present Episode of Intense Mass-Loss Reconstructed by Two-Dimensional Radiative Transfer Modeling
We present two-dimensional (2D) radiative transfer modeling of IRC+10216 at
selected moments of its evolution in 1995-2001, which correspond to three
epochs of our series of 8 near-infrared speckle images (Osterbart et al. 2000,
Weigelt et al. 2002). The high-resolution images obtained over the last 5.4
years revealed the dynamic evolution of the subarcsecond dusty environment of
IRC+10216 and our recent time-independent 2D radiative transfer modeling
reconstructed its physical properties at the single epoch of January 1997
(Men'shchikov et al. 2001). Having documented the complex changes in the
innermost bipolar shell of the carbon star, we incorporate the evolutionary
constraints into our new modeling to understand the physical reasons for the
observed changes. The new calculations imply that during the last 50 years, we
have been witnessing an episode of a steadily increasing mass loss from the
central star, from Mdot ~ 10^-5 Msun/yr to the rate of Mdot ~ 3x10^-4 Msun/yr
in 2001. The rapid increase of the mass loss of IRC+10216 and continuing
time-dependent dust formation and destruction caused the observed displacement
of the initially faint components C and D and of the bright cavity A from the
star which has almost disappeared in our images in 2001. Increasing dust
optical depths are causing strong backwarming that leads to higher temperatures
in the dust formation zone, displacing the latter outward with a velocity v_T ~
27 km/s due to the evaporation of the recently formed dust grains. This shift
of the dust density peak in the bipolar shell mimics a rapid radial expansion,
whereas the actual outflow has probably a lower speed v < v_inf ~ 15 km/s. The
model predicts that the star will remain obscured until Mdot starts to drop
back to lower values in the dust formation zone.Comment: 10 pages, 6 figures, accepted by Astronomy and Astrophysics, also
available at
http://www.mpifr-bonn.mpg.de/div/ir-interferometry/publications.htm
NACO/SAM observations of sources at the Galactic Center
Sparse aperture masking (SAM) interferometry combined with Adaptive Optics
(AO) is a technique that is uniquely suited to investigate structures near the
diffraction limit of large telescopes. The strengths of the technique are a
robust calibration of the Point Spread Function (PSF) while maintaining a
relatively high dynamic range. We used SAM+AO observations to investigate the
circumstellar environment of several bright sources with infrared excess in the
central parsec of the Galaxy. For our observations, unstable atmospheric
conditions as well as significant residuals after the background subtraction
presented serious problems for the standard approach of calibrating SAM data
via interspersed observations of reference stars. We circumvented these
difficulties by constructing a synthesized calibrator directly from sources
within the field-of-view. When observing crowded fields, this novel method can
boost the efficiency of SAM observations because it renders interspersed
calibrator observations unnecessary. Here, we presented the first NaCo/SAM
images reconstructed using this method.Comment: 8 pages, 10 figures, proceedings of the conference "Astrophysics at
High Angular Resolution" (AHAR-2011
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