397 research outputs found
Magellanic Cloud stars with TiO bands in emission: binary post-RGB/AGB stars or young stellar objects?
Fourteen stars from a sample of Magellanic Cloud objects selected to have a
mid-infrared flux excess have been found to also show TiO bands in emission.
The mid-infrared dust emission and the TiO band emission indicate that these
stars have large amounts of hot circumstellar dust and gas in close proximity
to the central star. The luminosities of the sources are typically several
thousand L_sun while the effective temperatures are 4000-8000 K. Such stars
could be post-AGB stars of mass 0.4-0.8 M_sun or pre-main-sequence stars (young
stellar objects) with masses of 7-19 M_sun. If the stars are pre-main-sequence
stars, they are substantially cooler and younger than stars at the birth line
where Galactic protostars are first supposed to become optically visible out of
their molecular clouds. They should therefore be hidden in their present
evolutionary state. The second explanation for these stars is that they are
post-AGB or post-RGB stars that have recently undergone a binary interaction
when the red giant of the binary system filled its Roche lobe. Being
oxygen-rich, they have gone through this process before becoming carbon stars.
Most of the stars vary slowly on timescales of 1000 days or more suggesting a
changing circumstellar environment. Apart from the slow variations, most stars
also show variability with periods of tens to hundreds of days. One star shows
a period that is rapidly decreasing and we speculate that this star may have
accreted a large blob of gas and dust onto a disk whose orbital radius is
shrinking rapidly. Another star has Cepheid-like pulsations of rapidly
increasing amplitude suggesting a rapid rate of evolution. Seven stars show
quasi-periodic variability and one star has a light curve similar to that of an
eclipsing binary.Comment: 15 pages, 2 tables, 8 figures, MNRAS, in pres
Optically visible post-AGB/RGB stars and young stellar objects in the Small Magellanic Cloud: candidate selection, spectral energy distributions and spectroscopic examination
We have carried out a search for optically visible post-AGB candidates in the
Small Magellanic Cloud (SMC). We used mid-IR observations from the Spitzer
Space Telescope to select optically visible candidates with a mid-IR excess. We
obtained low-resolution optical spectra for 801 candidates. After removing
contaminants and poor quality spectra, the final sample comprised of 63
post-AGB/RGB candidates of A - F spectral type. Using the spectra, we estimated
the stellar parameters: effective temperature, surface gravity and [Fe/H]. We
also estimated the reddening and deduced the luminosity using the stellar
parameters combined with photometry. Based on a luminosity criterion, 42 of
these 63 sources were classified as post-RGB candidates and the remaining as
post-AGB candidates. From the spectral energy distributions we found that 6 of
the 63 post-AGB/RGB candidates have a circumstellar shell suggesting that they
are single stars, while 27 of them have a surrounding disc, suggesting that
they are binaries. For the remaining candidates the nature of the circumstellar
environment was unclear. Variability is displayed by 38 post-AGB/RGB candidates
with common variability types being the Population II Cepheids (including
RV-Tauri stars) and semi-regular variables. This study has also revealed a new
s-process enriched RV Tauri star (J005107.19-734133.3). From the numbers of
post-AGB/RGB stars in the SMC, we were able to estimate evolutionary rates that
are in good agreement with the stellar evolution models with mass loss in the
post-AGB phase and re-accretion in the post-RGB phase. This study also resulted
in a new sample of 40 luminous young stellar objects (YSOs) of A - F spectral
type. Additionally, we also identified a group of 63 objects whose spectra are
dominated by emission lines and in some cases, a UV continuum. These objects
are likely to be either hot post-AGB/RGBs or luminous YSOs.Comment: 67 pages, 26 figures, 20 tables, 3 appendices + online supporting
information on CD
Chemical abundance study of two strongly s-process enriched post-AGB stars in the LMC: J051213.81-693537.1 and J051848.86-700246.9
Context: This paper is part of a larger project in which we study the
chemical abundances of extra-galactic post-AGB stars with the ultimate goal of
improving our knowledge of the poorly understood AGB third dredge-up mixing
processes and s-process nucleosynthesis. Aims: In this paper, we study two
carefully selected post-AGB stars in the LMC. The combination of favourable
atmospheric parameters for detailed abundance studies and their known distances
make these objects ideal probes of the internal AGB third dredge-up and
s-process nucleosynthesis in that they provide observational constraints for
theoretical AGB models. Methods: We use high-resolution optical UVES spectra to
determine accurate stellar parameters and perform detailed elemental abundance
studies. Additionally, we use available photometric data to construct SEDs for
reddening and luminosity determinations. We then estimate initial masses from
theoretical post-AGB tracks. Results: Both stars show extreme s-process
enrichment associated with relatively low C/O ratios of about 1.3. We could
only derive upper limits of the lead (Pb) abundance which indicate no strong Pb
overabundances with respect to other s-elements. Comparison with theoretical
post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low
initial masses between 1.0 and 1.5 Msun. Conclusion: This study adds to the
results obtained so far on a very limited number of s-process enriched post-AGB
stars in the Magellanic Clouds. We find an increasing discrepancy between
observed and predicted Pb abundances towards lower metallicities for all
studied Magellanic Cloud post-AGB stars found so far, as well as moderate C/O
ratios. We find that all s-process rich post-AGB stars in the LMC and SMC
studied so far, cluster in the same region of the HR-diagram and are associated
with low-mass stars with a low metallicity on average.Comment: 11 pages, 14 figure
A newly discovered stellar type: dusty post-red giant branch stars in the Magellanic Clouds
Context: We present a newly discovered class of low-luminosity, dusty,
evolved objects in the Magellanic Clouds. These objects have dust excesses,
stellar parameters, and spectral energy distributions similar to those of dusty
post-asymptotic giant branch (post-AGB) stars. However, they have lower
luminosities and hence lower masses. We suggest that they have evolved off the
red giant branch (RGB) instead of the AGB as a result of binary interaction.
Aims: In this study we aim to place these objects in an evolutionary context
and establish an evolutionary connection between RGB binaries (such as the
sequence E variables) and our new sample of objects. Methods: We compared the
theoretically predicted birthrates of the progeny of RGB binaries to the
observational birthrates of the new sample of objects. Results: We find that
there is order-of-magnitude agreement between the observed and predicted
birthrates of post-RGB stars. The sources of uncertainty in the birthrates are
discussed; the most important sources are probably the observational
incompleteness factor and the post-RGB evolution rates. We also note that
mergers are relatively common low on the RGB and that stars low on the RGB with
mid-IR excesses may recently have undergone a merger. Conclusions: Our sample
of dusty post-RGB stars most likely provides the first observational evidence
for a newly discovered phase in binary evolution: post-RGB binaries with
circumstellar dust.Comment: Accepted for publication in Astronomy and Astrophysics Letter
Eccentric Ellipsoidal Red Giant Binaries in the LMC: Complete Orbital Solutions and Comments on Interaction at Periastron
Modelling ellipsoidal variables with known distances can lead to exact
determination of the masses of both components, even in the absence of
eclipses. We present such modelling using light and radial velocity curves of
ellipsoidal red giant binaries in the LMC, where they are also known as
sequence E stars. Stars were selected as likely eccentric systems on the basis
of light curve shape alone. We have confirmed their eccentric nature and
obtained system parameters using the Wilson-Devinney code. Most stars in our
sample exhibit unequal light maxima as well as minima, a phenomenon not
observed in sequence E variables with circular orbits. We find evidence that
the shape of the red giant changes throughout the orbit due to the high
eccentricity and the varying influence of the companion. Brief intervals of
pulsation are apparent in two of the red giants. We determine pulsation modes
and comment on their placement in the period-luminosity plane. Defining the
parameters of these systems paves the way for modelling to determine by what
mechanism eccentricity is maintained in evolved binaries.Comment: 14 pages, 11 figures. Accepted to MNRAS 2012 January
V453 Oph: a s-process enriched, but carbon-deficient RV Tauri star of low intrinsic metallicity
This paper reports the detection of a heavy element enriched RV Tauri
variable with an abundance pattern that differs significantly from a standard
s-process enriched object: V453 Oph. Based on optical high-resolution spectra,
we determined that this object of low intrinsic metallicity ([Fe/H] = -2.2) has
a mild, but significant, enrichment ([s/Fe] ~ +0.5) of heavy elements for which
the distribution points to slow neutron capture nucleosynthesis. This result is
strengthened by a comparative analysis to the non-enriched RV Tauri star DS Aqr
([s/Fe] = 0.0). Although V453 Oph is the first RV Tauri star showing a strong
s-process signature, it is NOT accompanied by C enhancement, challenging our
current nucleosynthetic models of post-AGB stars that predict a simultaneous
enrichment in C and s-process elements. The low N abundance excludes CN cycling
as being responsible for the low C abundance. We explore three different
scenarios to explain the heavy element distribution in this evolved object: an
enrichment of the parental cloud, an accretion scenario in which the chemical
patterns were acquired by mass transfer in a binary system and an intrinsic
enrichment by dredge-up.Comment: Accepted for publication in A&
Resolving the compact dusty discs around binary post-AGB stars using N-band interferometry
We present the first mid-IR long baseline interferometric observations of the
circumstellar matter around binary post-AGB stars. Two objects, SX Cen and HD
52961, were observed using the VLTI/MIDI instrument during Science
Demonstration Time. Both objects are known binaries for which a stable
circumbinary disc is proposed to explain the SED characteristics. This is
corroborated by our N-band spectrum showing a crystallinity fraction of more
than 50 % for both objects, pointing to a stable environment where dust
processing can occur. Surprisingly, the dust surrounding SX Cen is not resolved
in the interferometric observations providing an upper limit of 11 mas (or 18
AU at the distance of this object) on the diameter of the dust emission. This
confirms the very compact nature of its circumstellar environment. The dust
emission around HD 52961 originates from a very small but resolved region,
estimated to be ~ 35 mas at 8 micron and ~ 55 mas at 13 micron. These results
confirm the disc interpretation of the SED of both stars. In HD 52961, the dust
is not homogeneous in its chemical composition: the crystallinity is clearly
concentrated in the hotter inner region. Whether this is a result of the
formation process of the disc, or due to annealing during the long storage time
in the disc is not clear.Comment: 12 pages, 10 figures, accepted for publication in A &
The orbits of subdwarf-B + main-sequence binaries. II. Three eccentric systems; BD+29 3070, BD +34 1543 and Feige 87
The predicted orbital-period distribution of the subdwarf-B (sdB) population
is bi-modal with a peak at short ( 250 days) periods.
Observationally, many short-period sdB systems are known, but the predicted
long period peak is missing as orbits have only been determined for a few
long-period systems. As these predictions are based on poorly understood
binary-interaction processes, it is of prime importance to confront the
predictions with reliable observational data. We therefore initiated a
monitoring program to find and characterize long-period sdB stars. In this
paper we aim to determine the orbital parameters of the three long-period
sdB+MS binaries BD+29 3070, BD+34 1543 and Feige 87, to constrain their
absolute dimensions and the physical parameters of the components.
High-resolution spectroscopic time series were obtained with HERMES at the
Mercator telescope on La Palma, and analyzed to determine the radial velocities
of both the sdB and MS components. Photometry from the literature was used to
construct the spectral-energy distribution (SED) of the binaries. Atmosphere
models were used to fit these SEDs and to determine the surface gravities and
temperatures of both components of all systems. Spectral analysis was used to
check the results of the SEDs. An orbital period of 1283 +- 63 d, a mass ratio
of q = 0.39 +- 0.04 and a significant non-zero eccentricity of e = 0.15 +- 0.01
were found for BD+29 3070. For BD+34 1543 we determined P = 972 +- 2 d, q =
0.57 +- 0.01 and again a clear non-zero eccentricity of e = 0.16 +- 0.01. Last,
for Feige 87 we found P = 936 +- 2 d, q = 0.55 +- 0.01 and e = 0.11 +- 0.01.
BD+29 3070, BD+34 1543 and Feige 87 are long period sdB + MS binaries on
clearly eccentric orbits. These results are in conflict with the predictions of
stable Roche-lobe overflow models.Comment: 15 pages, 6 figures, Accepted by A&
The Red Rectangle: Its Shaping Mechanism and its Source of Ultraviolet Photons
The proto-planetary Red Rectangle nebula is powered by HD 44179, a
spectroscopic binary (P = 318 d), in which a luminous post-AGB component is the
primary source of both luminosity and current mass loss. Here, we present the
results of a seven-year, eight-orbit spectroscopic monitoring program of HD
44179, designed to uncover new information about the source of the
Lyman/far-ultraviolet continuum in the system as well as the driving mechanism
for the bipolar outflow producing the current nebula. Our observations of the
H-alpha line profile around the orbital phase of superior conjunction reveal
the secondary component to be the origin of the fast (max. v~560^{-1}\sun_{max} \ge 17,0002 -
5\times10^{-5}\sun^{-1}\sun$, about 5% of the
luminosity of the entire system. (abridged)Comment: Accepted for publication in Ap
The perturbed sublimation rim of the dust disk around the post-AGB binary IRAS08544-4431
Context: Post-Asymptotic Giant Branch (AGB) binaries are surrounded by stable
dusty and gaseous disks similar to the ones around young stellar objects.
Whereas significant effort is spent on modeling observations of disks around
young stellar objects, the disks around post-AGB binaries receive significantly
less attention, even though they pose significant constraints on theories of
disk physics and binary evolution. Aims: We want to examine the structure of
and phenomena at play in circumbinary disks around post-AGB stars. We continue
the analysis of our near-infrared interferometric image of the inner rim of the
circumbinary disk around IRAS08544-4431. We want to understand the physics
governing this inner disk rim. Methods: We use a radiative transfer model of a
dusty disk to reproduce simultaneously the photometry as well as the
near-infrared interferometric dataset on IRAS08544-4431. The model assumes
hydrostatic equilibrium and takes dust settling self-consistently into account.
Results: The best-fit radiative transfer model shows excellent agreement with
the spectral energy distribution up to mm wavelengths as well as with the
PIONIER visibility data. It requires a rounded inner rim structure, starting at
a radius of 8.25 au. However, the model does not fully reproduce the detected
over-resolved flux nor the azimuthal flux distribution of the inner rim. While
the asymmetric inner disk rim structure is likely to be the consequence of
disk-binary interactions, the origin of the additional over-resolved flux
remains unclear. Conclusions: As in young stellar objects, the disk inner rim
of IRAS08544-4431 is ruled by dust sublimation physics. Additional observations
are needed to understand the origin of the extended flux and the azimuthal
perturbation at the inner rim of the disk.Comment: Accepted for publication in A&A, 13 figures, 13 page
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