Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F- to G-type main-sequence stars polluted with heavy elements by their binary companions when the ...companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung–Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-transfer objects. Combining our spectroscopic orbits with the H
IPPARCOS
astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming
Gaia
data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants.
Context
. Barium stars are
s
-process enriched giants. They owe their chemical peculiarities to a past mass transfer phase. During this phase they were polluted by their binary companion, which at ...the time was an asymptotic giant branch (AGB) star, but is now an extinct white dwarf. Barium stars are thus ideal targets for understanding and constraining the
s
-process in low- and intermediate-mass AGB stars.
Aims
. We derive the abundances of a large number of heavy elements in order to shed light on the conditions of operation of the neutron source responsible for the production of
s
-elements in the former companions of the barium stars.
Methods
. Adopting a recently used methodology, we analyse a sample of eighteen highly enriched barium stars observed with the high-resolution HERMES spectrograph mounted on the
Mercator
telescope (La Palma). We determine the stellar parameters and abundances using MARCS model atmospheres. In particular, we derive the Nb–Zr ratio which was previously shown to be a sensitive thermometer for the
s
-process nucleosynthesis. Indeed, in barium stars,
93
Zr has fully decayed into mono-isotopic
93
Nb, so Nb/Zr is a measure of the temperature-sensitive
93
Zr/Zr isotopic ratio.
Results
.
HD 28159
, previously classified as K5III and initially selected to serve as a reference cool K star for our abundance analysis, turns out to be enriched in
s
-process elements, and as such is a new barium star. Four stars are characterised by high nitrogen abundances, and among those three have high Nb/Zr and hs/ls ratios. The derived Zr and Nb abundances provide more accurate constraints on the
s
-process neutron source, identified to be
13
C(
α
, n)
16
O for barium stars. The comparison with stellar evolution and nucleosynthesis models shows that the investigated barium stars were polluted by a low-mass (
M
∼ 2 − 3
M
⊙
) AGB star.
HD 100503
is potentially identified as a high metallicity analogue of carbon-enhanced metal-poor star enriched in both
r
- and
s
-process elements (CEMP-rs).
We present observations proving the close binary nature of the central stars belonging to the planetary nebulae (PNe) NGC 6326 and NGC 6778. Photometric monitoring reveals irradiated lightcurves with ...orbital periods of 0.372 and 0.1534 days, respectively, constituting firm evidence that they passed through a common-envelope (CE) phase. Unlike most surveys for close binary central stars (CSPN) however, the binary nature of NGC 6326 was first revealed spectroscopically and only later did photometry obtain an orbital period. Gemini South observations revealed a large 160 km s-1 shift between the nebula and emission lines of C III and N III well known to originate from irradiated atmospheres of main-sequence companions. These so-called weak emission lines are fairly common in PNe and measurement of their radial velocity shifts in spectroscopic surveys could facilitate the construction of a statistically significant sample of post-CE nebulae. There is growing evidence that this process can be further accelerated by pre-selecting nebulae with traits of known post-CE nebulae. Both NGC 6326 and NGC 6778 were selected for their rich attribution of low-ionisation filaments and collimated outflows, thereby strengthening the connection between these traits and post-CE CSPN.
Context.
Tidal tails of star clusters are commonly understood to be populated symmetrically. Recently, the analysis of
Gaia
data revealed large asymmetries between the leading and trailing tidal tail ...arms of the four open star clusters Hyades, Praesepe, Coma Berenices, and NGC 752.
Aims.
As the evaporation of stars from star clusters into the tidal tails is a stochastic process, the degree of stochastic asymmetry is quantified in this work.
Methods.
For each star cluster, 1000 configurations of test particles were integrated in the combined potential of a Plummer sphere and the Galactic tidal field over the lifetime of the particular star cluster. For each of the four star clusters, the distribution function of the stochastic asymmetry was determined and compared with the observed asymmetry.
Results.
The probabilities for a stochastic origin of the observed asymmetry of the four star clusters are ≈1.7
σ
for Praesepe, ≈2.4
σ
for Coma Berenices, ≈6.7
σ
for Hyades, and ≈1.6
σ
for NGC 752.
Conclusions.
In the case of Praesepe, Coma Berenices, and NGC 752, the observed asymmetry can be interpreted as a stochastic evaporation event. However, for the formation of the asymmetric tidal tails of the Hyades, additional dynamical processes beyond a pure statistical evaporation effect are required.
Using FORS2 on the Very Large Telescope, we have astrometrically monitored over a period of two months the two components of the brown dwarf system WISE J104915.57-531906.1, the closest one to the ...Sun. Our astrometric measurements – with a relative precision at the milli-arcsecond scale – allowed us to detect the orbital motion and derive more precisely the parallax of the system, leading to a distance of 2.020 ± 0.019 pc. The relative orbital motion of the two objects is found to be perturbed, which leads us to suspect the presence of a substellar companion around one of the two components. We also performed VRIz photometry of the two components and compared this with models. We confirm the flux reversal of the T dwarf.
WISE J104915.57-531906.1 is a L/T brown dwarf binary located 2 pc from the Sun. The pair contains the closest known brown dwarfs and is the third closest known system, stellar or sub-stellar. We ...report comprehensive follow-up observations of this newly uncovered system. We have determined the spectral types of both components (L8 + or - 1, for the primary, agreeing with the discovery paper; T1.5 + or - 2 for the secondary, which was lacking spectroscopic type determination in the discovery paper) and, for the first time, their radial velocities (Vrad ~ 23.1, 19.5 km s super(-1) using optical spectra obtained at the Southern African Large Telescope and other facilities located at the South African Astronomical Observatory (SAAO). The relative radial velocity of the two components is smaller than the range of orbital velocities for theoretically predicted masses, implying that they form a gravitationally bound system. We report resolved near-infrared JHKS photometry from the Infrared Survey Facility telescope at the SAAO which yields colors consistent with the spectroscopically derived spectral types. The available kinematic and photometric information excludes the possibility that the object belongs to any of the known nearby young moving groups or associations. Simultaneous optical polarimetry observations taken at the SAAO 1.9 m give a non-detection with an upper limit of 0.07%. For the given spectral types and absolute magnitudes, 1 Gyr theoretical models predict masses of 0.04-0.05 M sub(middot in circle) for the primary, and 0.03-0.05 M sub(middot in circle) for the secondary.
The TROY project Lillo-Box, J.; Leleu, A.; Parviainen, H. ...
Astronomy and astrophysics (Berlin),
10/2018, Letnik:
618
Journal Article
Recenzirano
Odprti dostop
Context.
Co-orbital bodies are the byproduct of planet formation and evolution, as we know from the solar system. Although planet-size co-orbitals do not exists in our planetary system, dynamical ...studies show that they can remain stable for long periods of time in the gravitational well of massive planets. Should they exist, their detection is feasible with the current instrumentation.
Aims.
In this paper, we present new ground-based observations searching for these bodies co-orbiting with nine close-in (
P
< 5 days) planets, using various observing techniques. The combination of all of these techniques allows us to restrict the parameter space of any possible trojan in the system.
Methods.
We used multi-technique observations, comprised of radial velocity, precision photometry, and transit timing variations, both newly acquired in the context of the TROY project and publicly available, to constrain the presence of planet-size trojans in the Lagrangian points of nine known exoplanets.
Results.
We find no clear evidence of trojans in these nine systems through any of the techniques used down to the precision of the observations. However, this allows us to constrain the presence of any potential trojan in the system, especially in the trojan mass or radius vs. libration amplitude plane. In particular, we can set upper mass limits in the super-Earth mass regime for six of the studied systems.
The TROY project Lillo-Box, J.; Leleu, A.; Parviainen, H. ...
Astronomy and astrophysics (Berlin),
10/2018, Letnik:
618
Journal Article
Recenzirano
Odprti dostop
Context. Co-orbital bodies are the byproduct of planet formation and evolution, as we know from the solar system. Although planet-size co-orbitals do not exists in our planetary system, dynamical ...studies show that they can remain stable for long periods of time in the gravitational well of massive planets. Should they exist, their detection is feasible with the current instrumentation. Aims. In this paper, we present new ground-based observations searching for these bodies co-orbiting with nine close-in (P < 5 days) planets, using various observing techniques. The combination of all of these techniques allows us to restrict the parameter space of any possible trojan in the system. Methods. We used multi-technique observations, comprised of radial velocity, precision photometry, and transit timing variations, both newly acquired in the context of the TROY project and publicly available, to constrain the presence of planet-size trojans in the Lagrangian points of nine known exoplanets. Results. We find no clear evidence of trojans in these nine systems through any of the techniques used down to the precision of the observations. However, this allows us to constrain the presence of any potential trojan in the system, especially in the trojan mass or radius vs. libration amplitude plane. In particular, we can set upper mass limits in the super-Earth mass regime for six of the studied systems.
Witnessing the emergence of a carbon star Guzman-Ramirez, L; Lagadec, E; Wesson, R ...
Monthly notices of the Royal Astronomical Society. Letters,
07/2015, Letnik:
451, Številka:
1
Journal Article
Recenzirano
Odprti dostop
During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Most of the carbon in the Universe is believed to originate from stars with masses up ...to a few solar masses. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. Two models have been proposed to explain this dual chemistry: one postulates that a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch changed the surface chemistry of the star. The other model postulates that oxygen-rich material exists in stable keplerian rotation around the central star. The two models make contradictory, testable, predictions on the location of the oxygen-rich material, either located further from the star than the carbon-rich gas, or very close to the star in a stable disc. Using the Faint Object InfraRed CAmera (FORCAST) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) Telescope, we obtained images of the carbon-rich planetary nebula BD +30° 3639 which trace both carbon-rich polycyclic aromatic hydrocarbons and oxygen-rich silicate dust. With the superior spectral coverage of SOFIA, and using a 3D photoionization and dust radiative transfer model we prove that the O-rich material is distributed in a shell in the outer parts of the nebula, while the C-rich material is located in the inner parts of the nebula. These observations combined with the model, suggest a recent change in stellar surface composition for the double chemistry in this object. This is evidence for dredge-up occurring ∼103 yr ago.
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