404 research outputs found
Analysis of the infrared spectra of the peculiar post-AGB stars EPLyr and HD52961
Aim: We aim to study in detail the peculiar mineralogy and structure of the
circumstellar environment of two binary post-AGB stars, EPLyr and HD52961. Both
stars were selected from a larger sample of evolved disc sources observed with
Spitzer and show unique solid-state and gas features in their infrared spectra.
Moreover, they show a very small infrared excess in comparison with the other
sample stars. Methods: The different dust and gas species are identified on the
basis of high-resolution Spitzer-IRS spectra. We fit the full spectrum to
constrain grain sizes and temperature distributions in the discs. This,
combined with our broad-band spectral energy distribution and interferometric
measurements, allows us to study the physical structure of the disc, using a
self-consistent 2D radiative-transfer disc model. Results: We find that both
stars have strong emission features due to CO_2 gas, dominated by
^{12}C^{16}O_2, but with clear ^{13}C^{16}O_2 and even ^{16}O^{12}C^{18}O
isotopic signatures. Crystalline silicates are apparent in both sources but
proved very hard to model. EP Lyr also shows evidence of mixed chemistry, with
emission features of the rare class-C PAHs. Whether these PAHs reside in the
oxygen-rich disc or in a carbon-rich outflow is still unclear. With the
strongly processed silicates, the mixed chemistry and the low ^{12}C/^{13}C
ratio, EP Lyr resembles some silicate J-type stars, although the depleted
photosphere makes nucleosynthetic signatures difficult to probe. We find that
the disc environment of both sources is, to a first approximation, well
modelled with a passive disc, but additional physics such as grain settling,
radial dust distributions, and an outflow component must be included to explain
the details of the observed spectral energy distributions in both stars.Comment: 14 pages, accepted for publication by A&
Ground-based NIR emission spectroscopy of HD189733b
We investigate the K and L band dayside emission of the hot-Jupiter HD
189733b with three nights of secondary eclipse data obtained with the SpeX
instrument on the NASA IRTF. The observations for each of these three nights
use equivalent instrument settings and the data from one of the nights has
previously reported by Swain et al (2010). We describe an improved data
analysis method that, in conjunction with the multi-night data set, allows
increased spectral resolution (R~175) leading to high-confidence identification
of spectral features. We confirm the previously reported strong emission at
~3.3 microns and, by assuming a 5% vibrational temperature excess for methane,
we show that non-LTE emission from the methane nu3 branch is a physically
plausible source of this emission. We consider two possible energy sources that
could power non-LTE emission and additional modelling is needed to obtain a
detailed understanding of the physics of the emission mechanism. The validity
of the data analysis method and the presence of strong 3.3 microns emission is
independently confirmed by simultaneous, long-slit, L band spectroscopy of HD
189733b and a comparison star.Comment: ApJ accepte
IRAS\,11472-0800: an extremely depleted pulsating binary post-AGB star
We focus here on one particular and poorly studied object, IRAS11472-0800. It
is a highly evolved post-Asymptotic Giant Branch (post-AGB) star of spectral
type F, with a large infrared excess produced by thermal emission of
circumstellar dust. We deploy a multi-wavelength study which includes the
analyses of optical and IR spectra as well as a variability study based on
photometric and spectroscopic time-series. The spectral energy distribution
(SED) properties as well as the highly processed silicate N-band emission show
that the dust in IRAS11472-0800 is likely trapped in a stable disc. The
energetics of the SED and the colour variability show that our viewing angle is
close to edge-on and that the optical flux is dominated by scattered light.
With photospheric abundances of [Fe/H] = -2.7 and [Sc/H]=-4.2, we discovered
that IRAS11472-0800 is one of the most chemically-depleted objects known to
date. Moreover, IRAS11472-0800 is a pulsating star with a period of 31.16 days
and a peak-to-peak amplitude of 0.6 mag in V. The radial velocity variability
is strongly influenced by the pulsations, but the significant cycle-to-cycle
variability is systematic on a longer time scale, which we interpret as
evidence for binary motion. We conclude that IRAS11472-0800 is a pulsating
binary star surrounded by a circumbinary disc. The line-of-sight towards the
object lies close the the orbital plane making that the optical light is
dominated by scattered light. IRAS11472-0800 is one of the most
chemically-depleted objects known so far and links the dusty RV\,Tauri stars to
the non-pulsating class of strongly depleted objects.Comment: 12 pages, 14 figures Accepted for publication in A&A Main Journa
The geometry of the close environment of SV Psc as probed by VLTI/MIDI
Context. SV Psc is an asymptotic giant branch (AGB) star surrounded by an
oxygen-rich dust envelope. The mm-CO line profile of the object's outflow shows
a clear double-component structure. Because of the high angular resolution,
mid-IR interferometry may give strong constraints on the origin of this
composite profile.
Aims. The aim of this work is to investigate the morphology of the
environment around SV Psc using high-angular resolution interferometry
observations in the mid-IR with the Very Large Telescope MID-infrared
Interferometric instrument (VLTI/MIDI).
Methods. Interferometric data in the N-band taken at different baseline
lengths (ranging from 32-64 m) and position angles (73- 142{\deg}) allow a
study of the morphology of the circumstellar environment close to the star. The
data are interpreted on the basis of 2-dimensional, chromatic geometrical
models using the fitting software tool GEM-FIND developed for this purpose.
Results. The results favor two scenarios: (i) the presence of a highly
inclined, optically thin, dusty disk surrounding the central star; (ii) the
presence of an unresolved binary companion at a separation of 13.7 AU and a
position angle of 121.8{\deg} NE. The derived orbital period of the binary is
38.1 yr. This detection is in good agreement with hydrodynamic simulations
showing that a close companion could be responsible for the entrainment of the
gas and dust into a circumbinary structure.Comment: 10 pages, 12 figure
Post-AGB stars with hot circumstellar dust: binarity of the low-amplitude pulsators
While the first binary post-AGB stars were serendipitously discovered, the
distinct characteristics of their Spectral Energy Distribution (SED) allowed us
to launch a more systematic search for binaries. We selected post-AGB objects
which show a broad dust excess often starting already at H or K, pointing to
the presence of a gravitationally bound dusty disc in the system. We started a
very extensive multi-wavelength study of those systems and here we report on
our radial velocity and photometric monitoring results for six stars of early F
type, which are pulsators of small amplitude. To determine the radial velocity
of low signal-to-noise time-series, we constructed dedicated auto-correlation
masks. The radial velocity variations were subjected to detailed analysis to
differentiate between pulsational variability and variability due to orbital
motion. Finally orbital minimalisation was performed to constrain the orbital
elements. All of the six objects are binaries, with orbital periods ranging
from 120 to 1800 days. Five systems have non-circular orbits. The mass
functions range from 0.004 to 0.57 solar mass and the companions are likely
unevolved objects of (very) low initial mass. We argue that these binaries must
have been subject to severe binary interaction when the primary was a cool
supergiant. Although the origin of the circumstellar disc is not well
understood, the disc is generally believed to be formed during this strong
interaction phase. The eccentric orbits of these highly evolved objects remain
poorly understood. With the measured orbits and mass functions we conclude that
the circumbinary discs seem to have a major impact on the evolution of a
significant fraction of binary systems.Comment: 13 pages, 15 figures, accepted for Astronomy and Astrophysic
Carbonaceous molecules in the oxygen-rich circumstellar environment of binary post-AGB stars: C_{60} fullerenes and polycyclic aromatic hydrocarbons
Context. The circumstellar environment of evolved stars is generally rich in
molecular gas and dust. Typically, the entire environment is either oxygen-rich
or carbon-rich, depending on the evolution of the central star. Aims. In this
paper we discuss three evolved disc sources with evidence of atypical emission
lines in their infrared spectra. The stars were taken from a larger sample of
post-AGB binaries for which we have Spitzer infrared spectra, characterised by
the presence of a stable oxygen-rich circumbinary disc. Our previous studies
have shown that the infrared spectra of post-AGB disc sources are dominated by
silicate dust emission, often with an extremely high crystallinity fraction.
However, the three sources described here are selected because they show a
peculiar molecular chemistry. Methods. Using Spitzer infrared spectroscopy, we
study in detail the peculiar mineralogy of the three sample stars. Using the
observed emission features, we identify the different observed dust, molecular
and gas species. Results. The infrared spectra show emission features due to
various oxygen-rich dust components, as well as CO2 gas. All three sources show
the strong infrared bands generally ascribed to polycyclic aromatic
hydrocarbons. Furthermore, two sample sources show C60 fullerene bands.
Conclusions. Even though the majority of post-AGB disc sources are dominated by
silicate dust in their circumstellar environment, we do find evidence that, for
some sources at least, additional processing must occur to explain the presence
of large carbonaceous molecules. There is evidence that some of these sources
are still oxygen-rich, which makes the detection of these molecules even more
surprising.Comment: Accepted for publication in Astronomy and Astrophysics, 10 pages, 7
figure
Angular Misalignment Measurements for an Off-Plane Reflection Grating Module
We present an analysis of an alignment technique used for an off-plane reflection grating system that, if proven to be feasible, would ideally be utilized for future astronomical x-ray spectrometers. The use of reflection gratings allows for the production of both high throughput and spectral resolution. As such, they are a candidate grating technology for future soft X-ray spectroscopy missions. To be viable for these missions, however, a low-cost optical technique for co-aligning multiple gratings into a module for use in a spectrograph must be demonstrated. The off-plane grating module was built to contain fifteen gratings with proper relative alignment to one another for a converging X-ray beam. The module was coupled with a silicon pore optic mirror to produce a spectrum of reflected and diffracted light onto a CCD camera at the focal plane. The alignment performance of the module’s grating system was assessed both before and after a series of vibrational and thermal tests were conducted at the NASA Marshall Space Flight Center. Data reduction was done in order to identify the number and position of photon events from the diffraction spots for each grating, and raytracing analysis was conducted in order to calculate the induced grating-to-grating angular misalignments. Finally, these measurements were compared to theoretical alignment tolerances derived using analytical techniques. The grating system yielded misalignments within a factor of 2-3 of the analytical tolerances, which is very encouraging for a first attempt. Further refinement and troubleshooting is required to see whether or not this alignment technique can be used in the future
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