12 research outputs found
An extreme paucity of second population AGB stars in the normal globular cluster M4
Galactic Globular clusters (GCs) are now known to harbour multiple stellar
populations, which are chemically distinct in many light element abundances. It
is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs
show different abundance distributions in light elements compared to those in
the red giant branch (RGB) and other phases, skewing toward more primordial,
field-star-like abundances, which we refer to as subpopulation one (SP1). As
part of a larger program targeting giants in GCs, we obtained high-resolution
spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using
the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we
report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which
contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB
abundance distribution is consistent with all AGB stars being from SP1. This
result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor)
avoid the AGB phase. This is an unexpected result given M4's horizontal branch
morphology -- it does not have an extended blue horizontal branch. This is the
first abundance study to be performed utilising the HERMES spectrograph.Comment: 5 pages, 2 figures, 4 tables (full Table 1 online). Accepted for
publication in MNRAS Letter
AGB subpopulations in the nearby globular cluster NGC 6397
It has been well established that Galactic Globular clusters (GCs) harbour
more than one stellar population, distinguishable by the anti-correlations of
light element abundances (C-N, Na-O, and Mg-Al). These studies have been
extended recently to the asymptotic giant branch (AGB). Here we investigate the
AGB of NGC 6397 for the first time. We have performed an abundance analysis of
high-resolution spectra of 47 RGB and 8 AGB stars, deriving Fe, Na, O, Mg and
Al abundances. We find that NGC 6397 shows no evidence of a deficit in Na-rich
AGB stars, as reported for some other GCs - the subpopulation ratios of the AGB
and RGB in NGC 6397 are identical, within uncertainties. This agrees with
expectations from stellar theory. This GC acts as a control for our earlier
work on the AGB of M 4 (with contrasting results), since the same tools and
methods were used.Comment: 10 pages, 7 figures, 8 tables (2 online-only). Accepted for
publication in MNRA
On the AGB stars of M 4: A robust disagreement between spectroscopic observations and theory
Several recent spectroscopic investigations have presented conflicting
results on the existence of Na-rich asymptotic giant branch (AGB) stars in the
Galactic globular cluster M4 (NGC6121). The studies disagree on whether or not
Na-rich red giant branch (RGB) stars evolve to the AGB. For a sample of
previously published HER- MES/AAT AGB and RGB stellar spectra we present a
re-analysis of O, Na, and Fe abundances, and a new analysis of Mg and Al
abundances; we also present CN band strengths for this sample, derived from
low-resolution AAOmega spectra. Following a detailed literature comparison, we
find that the AGB samples of all studies consistently show lower abundances of
Na and Al, and are weaker in CN, than RGB stars in the cluster. This is similar
to recent observations of AGB stars in NGC 6752 and M 62. In an attempt to
explain this result, we present new theoretical stellar evolutionary models for
M 4; however, these predict that all stars, including Na-rich RGB stars, evolve
onto the AGB. We test the robustness of our abundance results using a variety
of atmospheric models and spectroscopic methods; however, we do not find
evidence that systematic modelling uncertainties can explain the apparent lack
of Na- rich AGB stars in M4. We conclude that an unexplained, but robust,
discordance between observations and theory remains for the AGB stars in M 4.Comment: 25 pages, 21 figures, 15 tables, accepted for publication in MNRA
Stellar Population Astrophysics (SPA) with TNG Atmospheric parameters of members of 16 unstudied open clusters
Thanks to modern understanding of stellar evolution, we can accurately
measure the age of Open Clusters (OCs). Given their position, they are ideal
tracers of the Galactic disc. Gaia data release 2, besides providing precise
parallaxes, led to the detection of many new clusters, opening a new era for
the study of the Galactic disc. However, detailed information on the chemical
abundance for OCs is necessary to accurately date them and to efficiently use
them to probe the evolution of the disc.Mapping and exploring the Milky Way
structure %to combine accurate chemical information of OCs is the main aim of
the Stellar Population Astrophysics (SPA) project. Part of this work involves
the use of OCs and the derivation of their precise and accurate chemical
composition.We analyze here a sample of OCs located within about 2 kpc from the
Sun, with ages from about 50 Myr to a few Gyr.We used HARPS-N at the Telescopio
Nazionale Gaileo and collected very high-resolution spectra (R = 115\,000) of
40 red giant/red clump stars in 18 OCs (16 never or scarcely studied plus two
comparison clusters). We measured their radial velocities and derived the
stellar parameters.We discussed the relationship between metallicity and
Galactocentric distance, adding literature data to our results to enlarge the
sample and taking also age into account. We compared the result of
observational data with that from chemo-dynamical models. These models
generally reproduce the metallicity gradient well. However, at young ages we
found a large dispersion in metallicity, not reproduced by models. Several
possible explanations are explored, including uncertainties in the derived
metallicity. We confirm the difficulties in determining parameters for young
stars (age < 200 Myr), due to a combination of intrinsic factors which
atmospheric models can not easily reproduce and which affect the parameters
uncertaintyComment: 21 pages 9 figures Astronomy & Astrophysic
Characterization of very wide companion candidates to young stars with planets and disks
Discovering wide companions of stellar systems allows us to constrain the dynamical environment and age of the latter. We studied four probable wide companions of four different stellar systems. The candidates were selected mainly based on their similar kinematic properties to the central star using Gaia DR2. The central stars are V4046 Sgr, HIP 74865, HIP 65426, and HIP 73145, and their probable wide companions are 2MASS J18152222-3249329, 2MASS J15174874-3028484, 2MASS J13242119-5129503, and 2MASS J14571503-3543505 respectively. V4046 Sgr is a member of β-Pictoris Moving Group while the rest of the stellar systems are acknowledged as members of the Scorpius-Centaurus association. The selected stellar systems are particularly interesting because all of them are already known to possess a low-mass companion and/or a spatially resolved disk. Identifying wider companions of these stars can improve their eligibility as benchmarks for understanding the formation channels of various triple systems, and can help us to determine the orbits of their possibly undiscovered inner, wider companions in case of higher multiplicity. By analyzing the X-shooter spectra of the wide companion candidates of these stars, we obtained their stellar parameters and determined their ages. We find that 2MASS J15174874-3028484 (0.11 M, 7.4 ± 0.5 Myr), an already recognized pre-main sequence (PMS) member of Scorpius-Centaurus association, is a highly probable wide companion of HIP 74865. 2MASS J13242119-5129503 (0.04 M, 16 ± 2.2 Myr) is ruled out as a plausible wide companion of HIP 65426, but confirmed to be a new sub-stellar member of the Scorpius-Centaurus association. 2MASS J14571503-3543505 (0.02 M, 17.75 ± 4.15 Myr) is a probable sub-stellar member of the same association, but we cannot confirm whether or not it is gravitationally bound to HIP 73145. 2MASS J18152222-3249329 (0.3 M, older than 150 Myr) is determined to be a mildly active main sequence star, much older than members of β-Pictoris Moving Group, and unbound to V4046 Sgr despite their similar kinematic features. PMS wide companions such as 2MASS J15174874-3028484 might have formed through cascade fragmentation of their natal molecular core, hinting at high multiplicity in shorter separations which can be confirmed with future observations
Post-conjunction detection of β Pictoris b with VLT/SPHERE
Context. With an orbital distance comparable to that of Saturn in the solar system, β Pictoris b is the closest (semi-major axis ≃9 au) exoplanet that has been imaged to orbit a star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope (VLT), all following astrometric measurements relative to β Pictoris have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet’s orbital parameters. Aims. We aimed at further constraining β Pictoris b orbital properties using more data, and, in particular, data taken in the northeastern part of the orbit. Methods. We used SPHERE at the VLT to precisely monitor the orbital motion of beta β Pictoris b since first light of the instrument in 2014. Results. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6 au) and prevented further detection. We redetected β Pictoris b on the northeast side of the disk at a separation of 139 mas and a PA of 30° in September 2018. The planetary orbit is now well constrained. With a semi-major axis (sma) of a = 9.0 ± 0.5 au (1σ), it definitely excludes previously reported possible long orbital periods, and excludes β Pictoris b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbit at less than 2 au from a star that is 20 pc away
Post-conjunction detection of β Pictoris b with VLT/SPHERE
Context. With an orbital distance comparable to that of Saturn in the solar system, β Pictoris b is the closest (semi-major axis ≃9 au) exoplanet that has been imaged to orbit a star. Thus it offers unique opportunities for detailed studies of its orbital, physical, and atmospheric properties, and of disk-planet interactions. With the exception of the discovery observations in 2003 with NaCo at the Very Large Telescope (VLT), all following astrometric measurements relative to β Pictoris have been obtained in the southwestern part of the orbit, which severely limits the determination of the planet’s orbital parameters.
Aims. We aimed at further constraining β Pictoris b orbital properties using more data, and, in particular, data taken in the northeastern part of the orbit.
Methods. We used SPHERE at the VLT to precisely monitor the orbital motion of beta β Pictoris b since first light of the instrument in 2014.
Results. We were able to monitor the planet until November 2016, when its angular separation became too small (125 mas, i.e., 1.6 au) and prevented further detection. We redetected β Pictoris b on the northeast side of the disk at a separation of 139 mas and a PA of 30° in September 2018. The planetary orbit is now well constrained. With a semi-major axis (sma) of a = 9.0 ± 0.5 au (1σ), it definitely excludes previously reported possible long orbital periods, and excludes β Pictoris b as the origin of photometric variations that took place in 1981. We also refine the eccentricity and inclination of the planet. From an instrumental point of view, these data demonstrate that it is possible to detect, if they exist, young massive Jupiters that orbit at less than 2 au from a star that is 20 pc away