146 research outputs found
The Angular Diameter and Fundamental Parameters of Sirius A
The Sydney University Stellar Interferometer (SUSI) has been used to make a
new determination of the angular diameter of Sirius A. The observations were
made at an effective wavelength of 694.1 nm and the new value for the
limb-darkened angular diameter is 6.048 +/- 0.040mas (+/-0.66%). This new
result is compared with previous measurements and is found to be in excellent
agreement with a conventionally calibrated measurement made with the European
Southern Observatory's Very Large Telescope Interferometer (VLTI) at 2.176
microns (but not with a second globally calibrated VLTI measurement). A
weighted mean of the SUSI and first VLTI results gives the limb-darkened
angular diameter of Sirius A as 6.041 +/- 0.017mas (+/-0.28%). Combination with
the Hipparcos parallax gives the radius equal to 1.713 +/- 0.009R_sun. The
bolometric flux has been determined from published photometry and
spectrophotometry and, combined with the angular diameter, yields the emergent
flux at the stellar surface equal to (5.32+/- 0.14)x10^8 Wm^-2 and the
effective temperature equal to 9845 +/- 64 K. The luminosity is 24.7 +/- 0.7
L_sun.Comment: Accepted for publication in PAS
Dust Scattering in Miras R Car and RR Sco resolved by optical interferometric polarimetry
We present optical interferometric polarimetry measurements of the Mira-like
variables R Car and RR Sco, using the Sydney University Stellar Interferometer.
By making visibility measurements in two perpendicular polarisations, the
relatively low-surface brightness light scattered by atmospheric dust could be
spatially separated from the bright Mira photospheric flux. This is the first
reported successful use of long-baseline optical interferometric polarimetry.
Observations were able to place constraints on the distribution of
circumstellar material in R Car and RR Sco. The inner radius of dust formation
for both stars was found to be less than 3 stellar radii: much closer than the
expected innermost stable location for commonly-assumed astrophysical ``dirty
silicate'' dust in these systems (silicate dust with a significant iron
content). A model with the dust distributed over a shell which is geometrically
thin compared to the stellar radius was preferred over an outflow. We propose
dust components whose chemistry and opacity properties enable survival at these
extreme inner radii.Comment: 8 pages, 4 figures, accepted for MNRA
Bringing closure to microlensing mass measurement
Interferometers offer multiple methods for studying microlensing events and
determining the properties of the lenses. We investigate the study of
microlensing events with optical interferometers, focusing on narrow-angle
astrometry, visibility, and closure phase. After introducing the basics of
microlensing and interferometry, we derive expressions for the signals in each
of these three channels. For various forecasts of the instrumental performance,
we discuss which method provides the best means of measuring the lens angular
Einstein radius theta_E, a prerequisite for determining the lens mass. If the
upcoming generation of large-aperture, AO-corrected long baseline
interferometers (e.g. VLTI, Keck, OHANA) perform as well as expected, theta_E
may be determined with signal-to-noise greater than 10 for all bright events.
We estimate that roughly a dozen events per year will be sufficiciently bright
and have long enough durations to allow the measurement of the lens mass and
distance from the ground. We also consider the prospects for a VLTI survey of
all bright lensing events using a Fisher matrix analysis, and find that even
without individual masses, interesting constraints may be placed on the bulge
mass function, although large numbers of events would be required.Comment: 23 pages, aastex, submitted to Ap
\gamma^2 Velorum: Orbital Solution and Fundamental Parameter Determination with SUSI
The first complete orbital solution for the double-lined spectroscopic binary
system \gamma^2 Velorum, obtained from measurements with the Sydney University
Stellar Interferometer (SUSI), is presented. This system contains the closest
example of a Wolf-Rayet star and the promise of full characterisation of the
basic properties of this exotic high-mass system has subjected it to intense
study as an archetype for its class. In combination with the latest
radial-velocity results, our orbital solution produces a distance of
336^{+8}_{-7} pc, significantly more distant than the Hipparcos estimation
(Schaerer et al. 1997; van der Hucht 1997). The ability to fully specify the
orbital parameters has enabled us to significantly reduce uncertainties and our
result is consistent with the VLTI observational point (Millour et al. 2006),
but not with their derived distance. Our new distance, which is an order of
magnitude more precise than prior work, demands critical reassessment of all
distance-dependent fundamental parameters of this important system. In
particular, membership of the Vela OB2 association has been reestablished, and
the age and distance are also in good accord with the population of young stars
reported by Pozzo et al. (2000). We determine the O-star primary component
parameters to be M_V(O) = -5.63 \pm 0.10 mag, R(O) = 17 \pm 2 R_{\sun} and
{\cal M}(O) = 28.5 \pm 1.1 M_{\sun}. These values are consistent with
calibrations found in the literature if a luminosity class of II--III is
adopted. The parameters of the Wolf-Rayet component are M_v(WR) = -4.33 \pm
0.17 mag and {\cal M}(WR) = 9.0 \pm 0.6 M_{\sun}.Comment: 3 figures, accepted for publication in MNRA
A new embedded control system for SUSI
The Sydney University Stellar Interferometer uses embedded processors to control each siderostat station as well as other major components of the instrument. The maintenance of the original controllers has become a significant issue and we set out to design a new system that would be inexpensive, suitable for the relatively harsh operating environment and simple to maintain. We have demonstrated that the new system works satisfactorily and we are currently replacing the existing controllers with new ones.This research was supported under the Australian Research Council’s Discovery Projects funding scheme. Dr
Ireland was supported by an Australian Research Council Postdoctoral Fellowship
The radius and mass of the subgiant star bet Hyi from interferometry and asteroseismology
We have used the Sydney University Stellar Interferometer (SUSI) to measure
the angular diameter of beta Hydri. This star is a nearby G2 subgiant whose
mean density was recently measured with high precision using asteroseismology.
We determine the radius and effective temperature of the star to be
1.814+/-0.017 R_sun (0.9%) and 5872+/-44 K (0.7%) respectively. By combining
this value with the mean density, as estimated from asteroseismology, we make a
direct estimate of the stellar mass. We find a value of 1.07+/-0.03 M_sun
(2.8%), which agrees with published estimates based on fitting in the H-R
diagram, but has much higher precision. These results place valuable
constraints on theoretical models of beta Hyi and its oscillation frequencies.Comment: 3 figures, 3 tables, to appear in MNRAS Letter
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
Orbital parameters, masses and distance to Beta Centauri determined with the Sydney University Stellar Interferometer and high resolution spectroscopy
The bright southern binary star beta Centauri (HR 5267) has been observed
with the Sydney University Stellar Interferometer (SUSI) and spectroscopically
with the ESO CAT and Swiss Euler telescopes at La Silla. The interferometric
observations have confirmed the binary nature of the primary component and have
enabled the determination of the orbital parameters of the system. At the
observing wavelength of 442 nm the two components of the binary system have a
magnitude difference of 0.15. The combination of interferometric and
spectroscopic data gives the following results: orbital period 357 days,
semi-major axis 25.30 mas, inclination 67.4 degrees, eccentricity 0.821,
distance 102.3 pc, primary and secondary masses M1 = M2 = 9.1 solar masses and
absolute visual magnitudes of the primary and secondary M1V = -3.85 and M2V =
-3.70. The high accuracy of the results offers a fruitful starting point for
future asteroseismic modelling of the pulsating binary components.Comment: 10 pages, 4 figures. Accepted for publication in MNRA
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