374 research outputs found
4mu spectra of AGB stars I: Observations
We present times series of high resolution spectra of AGB variables at 4mu.
Line profiles from the major contributors to the spectra of oxygen rich stars
at 4mu, OH, HO, HCl and SiO, are examined. The velocity as well as shape
variations of these profiles with time are discussed. The line profiles
investigated frequently have emission and multiple absorption components. The
changes with time of the 4mu region lines do not always follow the cyclic
variability seen in NIR spectra and in the photometric light curve. We
interpret and discuss the results qualitatively considering comparing the
spectral variability with that of the well behaved 1.6mu region and of
dynamical model atmospheres. Miras and semiregular variables are compared. The
origins of non-periodic behavior are discussed, including the role of spatial
inhomogeneities in the stellar atmosphere.Comment: 14 pages, 12 figures, accepted for publication in A&
The time variation in infrared water-vapour bands in Mira variables
The time variation in the water-vapour bands in oxygen-rich Mira variables
has been investigated using multi-epoch ISO/SWS spectra of four Mira variables
in the 2.5-4.0 micron region. All four stars show H2O bands in absorption
around minimum in the visual light curve. At maximum, H2O emission features
appear in the ~3.5-4.0 micronm region, while the features at shorter
wavelengths remain in absorption. These H2O bands in the 2.5-4.0 micron region
originate from the extended atmosphere.
The analysis has been carried out with a disk shape, slab geometry model. The
observed H2O bands are reproduced by two layers; a `hot' layer with an
excitation temperature of 2000 K and a `cool' layer with an excitation
temperature of 1000-1400 K. The radii of the `hot' layer (R_hot) are ~1 R_* at
visual minimum and 2 R_* at maximum, where R_* is a radius of background source
of the model. The time variation of R_hot/R_* from 1 to 2 is attributed to the
actual variation in the radius of the H2O layer. A high H2O density shell
occurs near the surface of the star around minimum, and moves out with the
stellar pulsation. This shell gradually fades away after maximum, and a new
high H2O density shell is formed in the inner region again at the next minimum.
Due to large optical depth of H2O, the near-infrared variability is dominated
by the H2O layer, and the L'-band flux correlates with the area of the H2O
shell. The infrared molecular bands trace the structure of the extended
atmosphere and impose appreciable effects on near-infrared light curve of Mira
variables.Comment: 15 pages, 16 figures, accepted by A&
The K giant star Arcturus: the hybrid nature of its infrared spectrum
We study infrared spectrum of Arcturus to clarify the nature of the cool
component of its atmosphere, referred to as the CO-mosphere, with the use of
the IR spectral atlas by Hinkle et al.(1995). The nature of CO lines shows an
abrupt change at logW/nu = -4.75, and the lines stronger than this limit can no
longer be analyzed by the classical line-formation theory. A more simple
manifestation of this fact is that the curves-of-growth (CG) of CO lines show
an unpredictable upturn at logW/nu = -4.75. Similar unusual behaviors of
empirical CG are confirmed in other red (super)giant stars, and it looks as if
the CG is a hybrid of at least two components of different origins. Although
strong lines of the CO fundamentals observed in Arcturus show strengthening
compared with the predicted photospheric spectrum, the weaker lines show slight
weakening, and we interpret these results as due to absorption/emission by the
molecular clouds formed in the extended atmosphere. Now do clouds exist in
stellar atmospheres? It is by no means easy to answer this question by
spectroscopic observations alone, but we find several arguments in favor of
such a possibility in Arcturus by analyzing the CO lines. In cooler
(super)giant stars in which CO lines show similar unusual behaviors as in
Arcturus, the presence of molecular clouds in the outer atmospheres was
demonstrated by direct observations with spatial interferometry. We suggest
that the formation of molecular clouds is a general feature in cool luminous
stars from early K to late M (super)giant stars.Comment: 18 pages, 15 Postscript figures, 1 Table in electronic form, Accepted
for publication in Astronomy and Astrophysic
The A-X infrared bands of Aluminum Oxide in stars: search and new detections
We describe a search for the A-X infrared bands of AlO with a view to better
understand the characteristics of this radical. These bands are infrequently
encountered in astronomical sources but surprisingly were very prominent in the
spectra of two well-known, nova-like variables (V838 Mon and V4332 Sgr) thereby
motivating us to explore the physical conditions necessary for their
excitation. In this study, we present the detection of A-X bands in the spectra
of 13 out of 17 stars, selected on the basis of their J-K colors as potential
candidates for detection of these bands. The majority of the AlO detections are
in AGB stars viz. 9 OH/IR stars, 2 Mira variables and 2 bright infrared
sources. Our study shows that the A-X bands are fairly prevalent in sources
with low temperature and O-rich environments. Interesting variation in strength
of the AlO bands in one of the sources (IRAS 18530+0817) is reported and the
cause for this is examined. Possible applications of the present study are
discussed in terms of the role of AlO in alumina dust formation, the scope for
estimating the radioactive Al content in AGB stars from the A-X bands,
and providing possible targets for further mm/radio studies of AlO which has
recently been discovered at millimeter wavelengths.Comment: To appear in Astrophysical Journal (Letters), 14 pages, 2 figure
Red supergiants as potential Type IIn supernova progenitors: Spatially resolved 4.6 micron CO emission around VY CMa and Betelgeuse
We present high-resolution 4.6micron CO spectra of the circumstellar
environments of two RSGs that are potential SN progenitors: Betelgeuse and VY
CMa. Around Betelgeuse, 12CO emission within 3arcsec follows a mildly clumpy
but otherwise spherical shell, smaller than its 55arcsec shell in KI
lambda7699. In stark contrast, 4.6micron CO emission around VY CMa is
coincident with bright KI in its clumpy asymmetric reflection nebula, within
5arcsec of the star. Our CO data reveal redshifted features not seen in KI
spectra of VY CMa, indicating a more isotropic distribution of gas punctuated
by randomly distributed asymmetric clumps. The relative CO and KI distribution
in Betelgeuse arises from ionization effects within a steady wind, whereas in
VY CMa, KI is emitted from skins of CO cloudlets resulting from episodic mass
ejections 500--1000 yr ago. In both cases, CO and KI trace potential pre-SN
circumstellar matter: we conclude that an extreme RSG like VY CMa might produce
a Type IIn event like SN1988Z if it were to explode in its current state, but
Betelgeuse will not. VY CMa demonstrates that LBVs are not necessarily the only
progenitors of SNe IIn, but it underscores the requirement that SNe IIn suffer
enhanced episodic mass loss shortly before exploding.Comment: 16 pages, AJ accepte
Atomic hydrogen in AGB circumstellar environments. A case study: X Her
We report the detection of the HI line at 21 cm from the circumstellar shell
around the AGB star X Her using the position-switching technique with the
Nancay Radio Telescope. At the star position the line shows 2 components: (i) a
broad one (FWHM ~ 13 km/s) centered at -72.2 km/s, and (ii) a narrow one (FWHM
\~ 4 km/s) centered at ~ -70.6 km/s. Our map shows that the source associated
to the broad component is asymmetric with material flowing preferentially
towards the North-East. This source extends to ~ 10 arcmin. (~ 0.4 pc) from the
star in that direction. On the other hand, the narrow component is detected
only at the star position and indicates material flowing away from the
observer. The total mass of atomic hydrogen is ~ 6.5 10^{-3} solar mass which,
within a factor 2, agrees with the estimate obtained from IRAS data at 60
microns.Comment: accepted for publication in MNRA
Near-Infrared Classification Spectroscopy: H-band Spectra of Fundamental MK Standards
We present a catalogue of H-band spectra for 85 stars of approximately solar
abundance observed at a resolving power of 3000 with the KPNO Mayall 4m FTS.
The atlas covers spectral types O7-M5 and luminosity classes I-V as defined on
the MK system. We identify both atomic and molecular indices and line-ratios
which are temperature and luminosity sensitive allowing spectral classification
to be carried out in the H-band. The line ratios permit spectral classification
in the presence of continuum excess emission, which is commonly found in
pre-main sequence and evolved stars. We demonstrate that with spectra of R =
1000 obtained at SNR > 50 it is possible to derive spectral types within +- 2
subclasses for late-type stars. These data are available electronically through
the Astronomical Data Center in addition to being served on the World-Wide-Web.Comment: To appear in the November 20, 1998 issue of ApJ (Volume 508, #1
Kinematics and H_2 morphology of the multipolar Post-AGB star IRAS 16594-4656
context: The spectrum of IRAS 16594-4656 shows shock excited H_2 emission and
collisionally excited emission lines such as[O I],[C I],and [Fe II]. aim: The
goal is to determine the location of the H_2 and [Fe II] shock emission, to
determine the shock velocities,and constrain the physical properties in the
shock. methods: High resolution spectra of the H_2 1-0 S(1),H_2 2-1 S(1), [Fe
II], and Pa emission lines were obtained with the near infrared
spectrograph Phoenix on Gemini South. results: The position-velocity diagrams
of H_2 1-0 S(1), H_2 2-1 S(1), and [Fe II] are presented. The H_2 and [Fe II]
emission is spatially extended. The collisionally excited [O I] and [C I]
optical emission lines have a similar double peaked profile compared to the
extracted H_2 profile and appear to be produced in the same shock. They all
indicate an expansion velocity of ~8 km/s and the presence of a neutral, very
high density region with about 3 x 10^6 to 5 x10^7 cm. The
[Fe II] emission however is single peaked. It has a gaussian FWHM of 30 km/s
and a total width of 62 km/s at 1% of the peak. The Pa profile is even
wider with a gaussian FWHM of 48 km/s and a total width of 75 km/s at 1% of the
peak. conclusions: The H emission is excited in a slow 5 to 20 km/s shock
into dense material at the edge of the lobes, caused by the interaction of the
AGB ejecta and the post-AGB wind. The 3D representation of the H_2 data shows a
hollow structure with less H_2 emission in the equatorial region. The [Fe II]
emission is not present in the lobes, but originates close to the central star
in fast shocks in the post-AGB wind or in a disk. The Pa emission also
appears to originate close to the star.Comment: 11 pages and 8 figures; A&A in press; the paper includig high
resolution figures can be downloaded from
http://homepage.oma.be/gsteene/publications.htm
The Pulsation of Chi Cygni Imaged by Optical Interferometry; a Novel Technique to Derive Distance and Mass of Mira Stars
We present infrared interferometric imaging of the S-type Mira star Chi
Cygni. The object was observed at four different epochs in 2005-2006 with the
IOTA optical interferometer (H band). Images show up to 40% variation in the
stellar diameter, as well as significant changes in the limb darkening and
stellar inhomogeneities. Model fitting gave precise time-dependent values of
the stellar diameter, and reveals presence and displacement of a warm molecular
layer. The star radius, corrected for limb darkening, has a mean value of 12.1
mas and shows a 5.1mas amplitude pulsation. Minimum diameter was observed at
phase 0.94+/-0.01. Maximum temperature was observed several days later at phase
1.02+/-0.02. We also show that combining the angular acceleration of the
molecular layer with CO (Delta v = 3) radial velocity measurements yields a
5.9+/-1.5 mas parallax. The constant acceleration of the CO molecules -- during
80% of the pulsation cycle -- lead us to argument for a free-falling layer. The
acceleration is compatible with a gravitational field produced by a
2.1(+1.5/-0.7) solar mass star. This last value is in agreement with
fundamental mode pulsator models. We foresee increased development of
techniques consisting in combining radial velocity with interferometric angular
measurements, ultimately allowing total mapping of the speed, density, and
position of the diverse species in pulsation driven atmospheres.Comment: 36 pages, accepted in Ap
Nanoscale temperature measurements using non-equilibrium Brownian dynamics of a levitated nanosphere
Einstein realised that the fluctuations of a Brownian particle can be used to
ascertain properties of its environment. A large number of experiments have
since exploited the Brownian motion of colloidal particles for studies of
dissipative processes, providing insight into soft matter physics, and leading
to applications from energy harvesting to medical imaging. Here we use
optically levitated nanospheres that are heated to investigate the
non-equilibrium properties of the gas surrounding them. Analysing the sphere's
Brownian motion allows us to determine the temperature of the centre-of-mass
motion of the sphere, its surface temperature and the heated gas temperature in
two spatial dimensions. We observe asymmetric heating of the sphere and gas,
with temperatures reaching the melting point of the material. This method
offers new opportunities for accurate temperature measurements with spatial
resolution on the nanoscale, and a new means for testing non-equilibrium
thermodynamicsComment: 5 pages, 4 figures, supplementary material available upon reques
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