4,673 research outputs found
An interferometric study of the post-AGB binary 89 Herculis I Spatially resolving the continuum circumstellar environment at optical and near-IR wavelengths with the VLTI, NPOI, IOTA, PTI, and the CHARA Array
Binary post-AGB stars are interesting laboratories to study both the
evolution of binaries as well as the structure of circumstellar disks. A
multiwavelength high angular resolution study of the prototypical object 89
Herculis is performed with the aim of identifying and locating the different
emission components seen in the SED. A large interferometric data set,
collected over the past decade and covering optical and near-IR wavelengths, is
analyzed with simple geometric models. Combining the interferometric
constraints with the photometry and the optical spectra, we reassess the energy
budget of the post-AGB star and its circumstellar environment. We report the
first (direct) detection of a large (35-40%) optical circumstellar flux
contribution and spatially resolve its emission region. Given this large amount
of reprocessed and/or redistributed optical light, the fitted size of the
emission region is rather compact and fits with(in) the inner rim of the
circumbinary dust disk. This rim dominates our K band data through thermal
emission and is rather compact, emitting significantly already at a radius of
twice the orbital separation. We interpret the circumstellar optical flux as
due to a scattering process, with the scatterers located in the extremely
puffed-up inner rim of the disk and possibly also in a bipolar outflow seen
pole-on. A non-LTE gaseous origin in an inner disk cannot be excluded but is
considered highly unlikely. This direct detection of a significant amount of
circumbinary light at optical wavelengths poses several significant questions
regarding our understanding of both post-AGB binaries and the physics in their
circumbinary disks. Although the identification of the source of
emission/scattering remains inconclusive without further study on this and
similar objects, the implications are manifold.Comment: Accepted for publication in A&A, 16 pages, 15 figure
Improving the surface brightness-color relation for early-type stars using optical interferometry
The aim of this work is to improve the SBC relation for early-type stars in
the color domain, using optical interferometry.
Observations of eight B- and A-type stars were secured with the VEGA/CHARA
instrument in the visible. The derived uniform disk angular diameters were
converted into limb darkened angular diameters and included in a larger sample
of 24 stars, already observed by interferometry, in order to derive a revised
empirical relation for O, B, A spectral type stars with a V-K color index
ranging from -1 to 0. We also took the opportunity to check the consistency of
the SBC relation up to using 100 additional measurements. We
determined the uniform disk angular diameter for the eight following stars:
Ori, Per, Cyg, Her, Aql, Peg,
Lyr, and Cyg with V-K color ranging from -0.70 to 0.02 and
typical precision of about . Using our total sample of 132 stars with
colors index ranging from about to , we provide a revised SBC
relation. For late-type stars (), the results are consistent
with previous studies. For early-type stars (), our new
VEGA/CHARA measurements combined with a careful selection of the stars
(rejecting stars with environment or stars with a strong variability), allows
us to reach an unprecedented precision of about 0.16 magnitude or
in terms of angular diameter.Comment: 13 pages, 5 figures, accepted for publication in A&
The fundamental parameters of the roAp star Equulei
Physical processes working in the stellar interiors as well as the evolution
of stars depend on some fundamental stellar properties, such as mass, radius,
luminosity, and chemical abundances. A classical way to test stellar interior
models is to compare the predicted and observed location of a star on
theoretical evolutionary tracks in a H-R diagram. This requires the best
possible determinations of stellar mass, radius, luminosity and abundances. To
derive its fundamental parameters, we observed the well-known rapidly
oscillating Ap star, Equ, using the visible spectro-interferometer
VEGA installed on the optical CHARA array. We computed the calibrated squared
visibility and derived the limb-darkened diameter. We used the whole energy
flux distribution, the parallax and this angular diameter to determine the
luminosity and the effective temperature of the star. We obtained a
limb-darkened angular diameter of 0.564~~0.017~mas and deduced a radius of
~=~2.20~~0.12~. Without considering the multiple
nature of the system, we derived a bolometric flux of erg~cm~s and an effective temperature of
7364~~235~K, which is below the effective temperature that has been
previously determined. Under the same conditions we found a luminosity of
~=~12.8~~1.4~. When the contribution of the closest
companion to the bolometric flux is considered, we found that the effective
temperature and luminosity of the primary star can be, respectively, up to
~100~K and up to ~0.8~L smaller than the values mentioned
above.These new values of the radius and effective temperature should bring
further constraints on the asteroseismic modelling of the star.Comment: Accepted by A&
The fundamental parameters of the roAp star 10 Aql
Due to the strong magnetic field and related abnormal surface layers existing
in rapidly oscillating Ap stars, systematic errors are likely to be present
when determining their effective temperatures, which potentially compromises
asteroseismic studies of these pulsators. Using long-baseline interferometry,
our goal is to determine accurate angular diameters of a number of roAp targets
to provide a temperature calibration for these stars. We obtained
interferometric observations of 10 Aql with the visible spectrograph VEGA at
the CHARA array. We determined a limb-darkened angular diameter of
0.275+/-0.009 mas and deduced a linear radius of 2.32+/-0.09 R_sun. We
estimated the star's bolometric flux and used it, in combination with its
parallax and angular diameter, to determine the star's luminosity and effective
temperature. For two data sets of bolometric flux we derived an effective
temperature of 7800+/-170 K and a luminosity of 18+/-1 L_sun or of 8000+/-210 K
and 19+/-2 L_sun. We used these fundamental parameters together with the large
frequency separation to constrain the mass and the age of 10 Aql, using the
CESAM stellar evolution code. Assuming a solar chemical composition and
ignoring all kinds of diffusion and settling of elements, we obtained a mass of
1.92 M_sun and an age of 780 Gy or a mass of 1.95 M_sun and an age of 740 Gy,
depending on the considered bolometric flux. For the first time, we managed to
determine an accurate angular diameter for a star smaller than 0.3 mas and to
derive its fundamental parameters. In particular, by only combining our
interferometric data and the bolometric flux, we derived an effective
temperature that can be compared to those derived from atmosphere models. Such
fundamental parameters can help for testing the mechanism responsible for the
excitation of the oscillations observed in the magnetic pulsating stars
Young people, crime and school exclusion: a case of some surprises
During the 1990s the number of young people being permanently excluded from schools in England and Wales increased dramatically from 2,910 (1990/91) to a peak of 12,700 (1996/97). Coinciding with this rise was a resurgence of the debate centring on lawless and delinquent youth. With the publication of Young People and Crime (Graham and Bowling 1995) and Misspent Youth (Audit Commission 1996) the 'common sense assumption' that exclusion from school inexorably promoted crime received wide support, with the school excludee portrayed as another latter day 'folk devil'. This article explores the link between school exclusion and juvenile crime, and offers some key findings from a research study undertaken with 56 young people who had experience of being excluded from school. Self-report interview questions reveal that whilst 40 of the young people had offended, 90% (36) reported that the onset of their offending commenced prior to their first exclusion. Moreover, 50 (89.2% of the total number of young people in the sample), stated that they were no more likely to offend subsequent to being excluded and 31 (55.4%) stated that they were less likely to offend during their exclusion period. Often, this was because on being excluded, they were 'grounded' by their parents
Time, spatial, and spectral resolution of the Halpha line-formation region of Deneb and Rigel with the VEGA/CHARA interferometer
BA-type supergiants are amongst the most optically-bright stars. They are
observable in extragalactic environments, hence potential accurate distance
indicators. Emission activity in the Halpha line of the BA supergiants Rigel
(B8Ia) and Deneb (A2Ia) is indicative of presence of localized time-dependent
mass ejections. Here, we employ optical interferometry to study the Halpha
line-formation region in these stellar environments. High spatial- (0.001
arcsec) and spectral- (R=30 000) resolution observations of Halpha were
obtained with the visible recombiner VEGA installed on the CHARA
interferometer, using the S1S2 array-baseline (34m). Six independent
observations were done on Deneb over the years 2008 and 2009, and two on Rigel
in 2009. We analyze this dataset with the 1D non-LTE radiative-transfer code
CMFGEN, and assess the impact of the wind on the visible and near-IR
interferometric signatures, using both Balmer-line and continuum photons. We
observe a visibility decrease in Halpha for both Rigel and Deneb, suggesting
that the line-formation region is extended (1.5-1.75 R*). We observe a
significant visibility decrease for Deneb in the SiII6371 line. We witness time
variations in the differential phase for Deneb, implying an inhomogeneous and
unsteady circumstellar environment, while no such variability is seen in
differential visibilities. Radiative-transfer modeling of Deneb, with allowance
for stellar-wind mass loss, accounts fairly well for the observed decrease in
the Halpha visibility. Based on the observed differential visibilities, we
estimate that the mass-loss rate of Deneb has changed by less than 5%
An investigation of the close environment of beta Cep with the VEGA/CHARA interferometer
High-precision interferometric measurements of pulsating stars help to
characterize their close environment. In 1974, a close companion was discovered
around the pulsating star beta Cep using the speckle interferometry technique
and features at the limit of resolution (20 milli-arcsecond or mas) of the
instrument were mentioned that may be due to circumstellar material. Beta Cep
has a magnetic field that might be responsible for a spherical shell or
ring-like structure around the star as described by the MHD models. Using the
visible recombiner VEGA installed on the CHARA long-baseline interferometer at
Mt. Wilson, we aim to determine the angular diameter of beta Cep and resolve
its close environment with a spatial resolution up to 1 mas level. Medium
spectral resolution (R=6000) observations of beta Cep were secured with the
VEGA instrument over the years 2008 and 2009. These observations were performed
with the S1S2 (30m) and W1W2 (100m) baselines of the array. We investigated
several models to reproduce our observations. A large-scale structure of a few
mas is clearly detected around the star with a typical flux relative
contribution of 0.23 +- 0.02. Our best model is a co-rotational geometrical
thin ring around the star as predicted by magnetically-confined wind shock
models. The ring inner diameter is 8.2 +- 0.8 mas and the width is 0.6 +- 0.7
mas. The orientation of the rotation axis on the plane of the sky is PA = 60 +-
1 deg, while the best fit of the mean angular diameter of beta Cep gives UD[V]
= 0.22 +- 0.05 mas. Our data are compatible with the predicted position of the
close companion of beta Cep. These results bring additional constraints on the
fundamental parameters and on the future MHD and asteroseismological models of
the star.Comment: Paper accepted for publication in A&A (in press
Chromosphere of K giant stars Geometrical extent and spatial structure detection
We aim to constrain the geometrical extent of the chromosphere of non-binary
K giant stars and detect any spatial structures in the chromosphere. We
performed observations with the CHARA interferometer and the VEGA beam combiner
at optical wavelengths. We observed seven non-binary K giant stars. We measured
the ratio of the radii of the photosphere to the chromosphere using the
interferometric measurements in the Halpha and the Ca II infrared triplet line
cores. For beta Ceti, spectro-interferometric observations are compared to an
non-local thermal equilibrium (NLTE) semi-empirical model atmosphere including
a chromosphere. The NLTE computations provide line intensities and contribution
functions that indicate the relative locations where the line cores are formed
and can constrain the size of the limb-darkened disk of the stars with
chromospheres. We measured the angular diameter of seven K giant stars and
deduced their fundamental parameters: effective temperatures, radii,
luminosities, and masses. We determined the geometrical extent of the
chromosphere for four giant stars. The chromosphere extents obtained range
between 16% to 47% of the stellar radius. The NLTE computations confirm that
the Ca II/849 nm line core is deeper in the chromosphere of ? Cet than either
of the Ca II/854 nm and Ca II/866 nm line cores. We present a modified version
of a semi-empirical model atmosphere derived by fitting the Ca II triplet line
cores of this star. In four of our targets, we also detect the signature of a
differential signal showing the presence of asymmetries in the chromospheres.
Conclusions. It is the first time that geometrical extents and structure in the
chromospheres of non-binary K giant stars are determined by interferometry.
These observations provide strong constrains on stellar atmosphere models.Comment: 10 pages, 12 figure
The Expanding Fireball of Nova Delphini 2013
A classical nova occurs when material accreting onto the surface of a white
dwarf in a close binary system ignites in a thermonuclear runaway. Complex
structures observed in the ejecta at late stages could result from interactions
with the companion during the common envelope phase. Alternatively, the
explosion could be intrinsically bipolar, resulting from a localized ignition
on the surface of the white dwarf or as a consequence of rotational distortion.
Studying the structure of novae during the earliest phases is challenging
because of the high spatial resolution needed to measure their small sizes.
Here we report near-infrared interferometric measurements of the angular size
of Nova Delphini 2013, starting from one day after the explosion and continuing
with extensive time coverage during the first 43 days. Changes in the apparent
expansion rate can be explained by an explosion model consisting of an
optically thick core surrounded by a diffuse envelope. The optical depth of the
ejected material changes as it expands. We detect an ellipticity in the light
distribution, suggesting a prolate or bipolar structure that develops as early
as the second day. Combining the angular expansion rate with radial velocity
measurements, we derive a geometric distance to the nova of 4.54 +/- 0.59 kpc
from the Sun.Comment: Published in Nature. 32 pages. Final version available at
http://www.nature.com/nature/journal/v515/n7526/full/nature13834.htm
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