259 research outputs found
Dust-enshrouded Asymptotic Giant Branch Stars in the Solar Neighbourhood
A study is made of a sample of 58 dust-enshrouded Asymptotic Giant Branch
(AGB) stars (including 2 possible post AGB stars), of which 27 are carbon-rich
and 31 are oxygen-rich. These objects were originally identified by Jura &
Kleinmann as nearby (within about 1 kpc of the sun) AGB stars with high
mass-loss rates, greater than 1E-6 solar masses per year. Ground-based
near-infrared photometry, data obtained by IRAS and kinematic data from the
literature are combined to investigate the properties of these stars. The light
amplitude in the near-infrared is found to be correlated with period, and this
amplitude decreases with increasing wavelength. Statistical tests show that
there is no reason to suspect any difference in the period distributions of the
carbon- and oxygen-rich stars for periods less than 1000 days, and no
carbon-rich star has a period longer than 1000 days. The colours are consistent
with those of cool stars with evolved circumstellar dust-shells. Luminosities
and distances are estimated using a period-luminosity relation. Mass-loss
rates, estimated from the 60 micron fluxes, show a correlation with pulsation
period and is tightly correlated with the K-[12] colour. The kinematics and
scale-height of the sample shows that the sources with periods less than 1000
days must have low mass main-sequence progenitors. It is argued that the three
oxygen-rich stars with periods over 1000 days probably had intermediate mass
main-sequence progenitors with remaining stars having an average progenitor
mass of about 1.3 solar masses. The average lifetime of stars in this phase is
estimated to be about 4.0E4 years, indicating they will undergo at most one
more thermal pulse before leaving the AGB.Comment: 27 pages, 20 figures, accepted for MNRA
Variability of eta Carinae III
Spectra (1951-78) of the central object in eta Car, taken by A.D. Thackeray,
reveal three previously unrecorded epochs of low excitation. Since 1948, at
least, these states have occurred regularly in the 2020 day cycle proposed by
Damineli et al. They last about 10 percent of each cycle. Early slit spectra
(1899-1919) suggest that at that time the object was always in a low state.
JHKL photometry is reported for the period 1994-2000. This shows that the
secular increase in brightness found in 1972-94 has continued and its rate has
increased at the shorter wavelengths. Modulation of the infrared brightness in
a period near 2020 days continues. There is a dip in the JHKL light curves near
1998.0, coincident with a dip in the X-ray light curve. Evidence is given that
this dip in the infrared repeats in the 2020 day cycle. As suggested by
Whitelock & Laney, the dip is best interpreted as an eclipse phenomenon in an
interacting binary system; the object eclipsed being a bright region (`hot
spot'), possibly on a circumstellar disc or produced by interacting stellar
winds. The eclipse coincides in phase and duration with the state of low
excitation. It is presumably caused by a plasma column and/or by one of the
stars in the system.Comment: 10 pages, 7 postscript figures, accepted for MNRA
Near-Infrared Photometry of Carbon Stars
Near-infrared, JHKL, photometry of 239 Galactic carbon-rich variable stars is
presented and discussed. From these and published data the stars were
classified as Mira or non-Mira variables and amplitudes and pulsation periods,
ranging from 222 to 948 days for the Miras, were determined for most of them. A
comparison of the colour and period relations with those of similar stars in
the Large Magellanic Cloud indicates minor differences, which may be the
consequence of sample selection effects. Apparent bolometric magnitudes were
determined by combining the mean JHKL fluxes with mid-infrared photometry from
IRAS and MSX. Then, using the Mira period luminosity relation to set the
absolute magnitudes, distances were determined -- to greater accuracy than has
hitherto been possible for this type of star. Bolometric corrections to the K
magnitude were calculated and prescriptions derived for calculating these from
various colours. Mass-loss rates were also calculated and compared to values in
the literature.
Approximately one third of the C-rich Miras and an unknown fraction of the
non-Miras exhibit apparently random obscuration events that are reminiscent of
the phenomena exhibited by the hydrogen deficient RCB stars. The underlying
cause of this is unclear, but it may be that mass loss, and consequently dust
formation, is very easily triggered from these very extended atmospheres.Comment: 35 pages, 21 figs, accepted for publication in MNRAS. Large data
table will be available on-line onl
The light curve of the semiregular variable L2 Puppis: I. A recent dimming event from dust
The nearby Mira-like variable L2 Pup is shown to be undergoing an
unprecedented dimming episode. The stability of the period rules out intrinsic
changes to the star, leaving dust formation along the line of sight as the most
likely explanation. Episodic dust obscuration events are fairly common in
carbon stars but have not been seen in oxygen-rich stars. We also present a
10-micron spectrum, taken with the Japanese IRTS satellite, showing strong
silicate emission which can be fitted with a detached, thin dust shell,
containing silicates and corundum.Comment: MNRAS (accepted
Near-IR variability properties of a selected sample of AGB stars
We present the results of a near-infrared monitoring programme of a selected
sample of stars, initially suspected to be Mira variables and OH/IR stars,
covering more than a decade of observations. The objects monitored cover the
typical range of IRAS colours shown by O-rich stars on the Asymptotic Giant
Branch and show a surprisingly large diversity of variability properties. 16
objects are confirmed as large-amplitude variables. Periods between 360 and
1800 days and typical amplitudes from 1 to 2 magnitudes could be determined for
nine of them. In three light curves we find a systematic decrease of the mean
brightness, two light curves show pronounced asymmetry. One source, IRAS
07222-2005, shows infrared colours typical of Mira variables but pulsates with
a much longer period (approx. 1200 days) than a normal Mira. Two objects are
ither close to (IRAS 03293+6010) or probably in (IRAS 18299-1705) the post-AGB
phase. In IRAS 16029-3041 we found a systematic increase of the H-K colour of
approximately 1 magnitude, which we interpret as evidence of a recent episode
of enhanced mass loss. IRAS 18576+0341, a heavily obscured Luminous Blue
Variable was also monitored. The star showed a continued decrease of brightness
over a period of 7 years (1995 - 2002).Comment: 9 pages + 3 appendix, 36 figures, photometry table, accepted in
Astronomy & Astrophysic
Carbon-Rich Mira Variables: Radial Velocities and Distances
Optical radial velocities have been measured for 38 C-type Mira variables.
These data together with others in the literature are used to study the
differences between optical and CO mm observations for C-Miras and the
necessary corrections to the optical velocities are derived in order to obtain
the true radial velocities of the variables. The difference between absorption
and emission line velocities is also examined. A particularly large difference
(+30 km\s) is found in the case of the H-alpha emission line. A catalogue is
given of 177 C-Miras with estimated distances and radial velocities. The
distances are based on bolometric magnitudes derived in Paper I using SAAO
observations or (for 60 of the stars) using non-SAAO photometry. In the latter
case the necessary transformations to the SAAO system are derived. These data
will be used in paper III to study the kinematics of the C-Miras.Comment: 11 pages, accepted for publication in MNRA
Modelling of intermediate-age stellar populations: III Effects of dust-shells around AGB stars
In this paper,we present single stellar population models of intermediate age
stellar populations where dust-enshrouded AGB stars are introduced. The
formation of carbon stars is also accounted for, and is taken to be a function
of both initial mass and metallicity. The effect of the dusty envelopes around
AGB stars on the optical/near-infrared spectral energy distribution were
introduced using semi-emipirical models where the mass-loss and the
photospheric chemistry determine the spectral properties of a star along the
AGB sequence. The spectral dichotomy between O-rich stars and C-rich stars is
taken into account in the modelling. We have investigated the AGB sequence
morphology in he near-infrared CMD as a function of time and metallicity. We
show that this diaggram is characterized by three morphological features,
occupied by optically bright O-rich stars, optically bright C-rich stars, and
dust-enshrouded O-rich and C-rich stars respectively. Our models are able to
reproduce the distribution of the three AGB subtype stellar populations in
colour-colour diagrams. Effects of dusty envelopes on the luminosity function
are also investigated (Abriged).Comment: 11 pages, 9 figures, accepted for publication in A&
Similarity of Recombinant Human Perlecan Domain 1 by Alternative Expression Systems Bioactive Heterogenous Recombinant Human Perlecan D1
<p>Abstract</p> <p>Background</p> <p>Heparan sulfate glycosaminoglycans are diverse components of certain proteoglycans and are known to interact with growth factors as a co-receptor necessary to induce signalling and growth factor activity. In this report we characterize heterogeneously glycosylated recombinant human perlecan domain 1 (HSPG2 abbreviated as rhPln.D1) synthesized in either HEK 293 cells or HUVECs by transient gene delivery using either adenoviral or expression plasmid technology.</p> <p>Results</p> <p>By SDS-PAGE analysis following anion exchange chromatography, the recombinant proteoglycans appeared to possess glycosaminoglycan chains ranging, in total, from 6 kDa to >90 kDa per recombinant. Immunoblot analysis of enzyme-digested high M<sub>r </sub>rhPln.D1 demonstrated that the rhPln.D1 was synthesized as either a chondroitin sulfate or heparan sulfate proteoglycan, in an approximately 2:1 ratio, with negligible hybrids. Secondary structure analysis suggested helices and sheets in both recombinant species. rhPln.D1 demonstrated binding to rhFGF-2 with an apparent k<sub>D </sub>of 2 ± 0.2 nM with almost complete susceptibility to digestion by heparinase III in ligand blot analysis but not to chondroitinase digestion. Additionally, we demonstrate HS-mediated binding of both rhPln.D1 species to several other GFs. Finally, we corroborate the augmentation of FGF-mediated cell activation by rhPln.D1 and demonstrate mitogenic signalling through the FGFR1c receptor.</p> <p>Conclusions</p> <p>With importance especially to the emerging field of DNA-based therapeutics, we have shown here that proteoglycan synthesis, in different cell lines where GAG profiles typically differ, can be directed by recombinant technology to produce populations of bioactive recombinants with highly similar GAG profiles.</p
An Infrared Census of DUST in Nearby Galaxies with Spitzer (DUSTiNGS). IV. Discovery of High-redshift AGB Analogs
The trigger of the AGB superwind: the importance of carbon
The driving mechanism of the AGB superwind has become controversial in recent
years. The efficacy of dust-driven mass loss has been queried. Spitzer
observation of AGB stars in Local Group Galaxies show the surprising result
that at low metallicity, AGB mass loss occurs at low luminosity, possibly lower
than in the Galaxy, but only for carbon-rich stars. Oxygen-rich stars in the
Galaxy and in lower metallicity galaxies have similar mass-loss rates only at
high luminosities. To explain this dichotomy, we propose that the superwind has
a dual trigger. The superwind starts either when sufficient excess carbon
builds up for efficient formation of carbonaceous dust (which we propose occurs
when ), or when the luminosity reaches
a value sufficient for a silicate-dust-driven wind (proposed at . We show that this dual trigger fits the current
observational constraints: the luminosity at which the superwind begins, and
the predominance of carbon superwind star at low metallicity. We use stellar
evolution models to check the consistency of our explanations and present
detailed predictions of the luminosities at which the superwind is triggered
for different metallicities and initial stellar masses.Comment: accepted for publications in MNRAS Letters, 5 pages, 4 figure
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