2,145 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
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
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
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
Optically visible post-AGB stars, post-RGB stars and young stellar objects in the Large Magellanic Cloud
We have carried out a search for optically visible post-asymptotic giant branch (post-AGB) stars in the Large Magellanic Cloud (LMC). First, we selected candidates with a mid-IR excess and then obtained their optical spectra. We disentangled contaminants with unique spectra such as M stars, C stars, planetary nebulae, quasi-stellar objects and background galaxies. Subsequently, we performed a detailed spectroscopic analysis of the remaining candidates to estimate their stellar parameters such as effective temperature, surface gravity (log g), metallicity ([Fe/H]), reddening and their luminosities. This resulted in a sample of 35 likely post-AGB candidates with late-G to late-A spectral types, low log g, and [Fe/H] < -0.5. Furthermore, our study confirmed the existence of the dusty post-red giant branch (post-RGB) stars, discovered previously in our Small Magellanic Cloud survey, by revealing 119 such objects in the LMC. These objects have mid-IR excesses and stellar parameters (Teff, log g, [Fe/H]) similar to those of post-AGB stars except that their luminosities (< 2500 L&sun;), and hence masses and radii, are lower. These post-RGB stars are likely to be products of binary interaction on the RGB. The post-AGB and post-RGB objects show spectral energy distribution properties similar to the Galactic post-AGB stars, where some have a surrounding circumstellar shell, while some others have a surrounding stable disc similar to the Galactic post-AGB binaries. This study also resulted in a new sample of 162 young stellar objects, identified based on a robust log g criterion. Other interesting outcomes include objects with an UV continuum and an emission line spectrum; luminous supergiants; hot main-sequence stars; and 15 B[e] star candidates, 12 of which are newly discovered in this study
Comparison of Partial Recording Protocols in Disease Assessment among Periodontitis Patients in a Central Indian Population
Objectives: Partial recording protocols may be used for the purpose of assessing periodontal disease extent and
severity in epidemiological studies. As there is very little data at present regarding the reliability of any partial examination
methods among Indian populations, as an initial study, an attempt was made to determine the reliability
of 9 different protocols in estimating the extent and severity of periodontal disease among periodontitis patients in
a central Indian population.
Study Design: Probing depths (PD) and clinical attachment levels (CAL) were recorded in 85 periodontitis patients
on 6 sites on all the teeth excluding the third molars. Nine partial recording protocols (PRP) were compared with
the full-mouth examination. Intra-class correlation coefficients (ICC) were calculated for mean PD, mean CAL,
and percentage of sites with various thresholds of PD and CAL to determine the agreement between the PRP and
the full-mouth examination. The sensitivity of different PRP for defining prevalence of sites with PD ≥6 and ≥7 mm
and CAL ≥7 and ≥8 mm was also determined.
Results: For all the tested PRP, the ICCs were consistently >0.9. The methods involving examination of 4 sites/
tooth slightly over-estimated the disease extent and severity in comparison with the full-mouth examination as well
as PRP involving examination of 6 sites/tooth.
Conclusions: The findings of the present study suggest that the tested PRP are in good agreement with the fullmouth
examination. However, further studies need to be conducted with an improved methodology in a larger
sample of subjects from the general population
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