A catalogue of oxygen-rich pulsating giants in the Galactic halo and the Sagittarius stream

To construct a catalogue of oxygen-rich (M) asymptotic giant branch (AGB) stars in the halo, complementing the catalogues of carbon-rich (C) stars, previous lists of Miras and SRa semi-regulars located in the northern hemisphere are merged and cleaned of various defects. After putting aside known C stars, characteristics such as colours and periods indicate that most of the remaining objects are M stars. Distances are obtained through the period-luminosity relation. By considering their position in the sky, stars lying at |Z| > 5 kpc are confirmed to be in majority in the Sgr tidal arms. The M stars are more numerous than C ones. Our distance scale is supported by two cool variables located in the Pal 4 globular cluster. Along the Sgr arms, there is reasonable agreement on distances of our objects with recent RR Lyrae distances. A few stars may be as distant as 150 kpc, with possibly four at the trailing arm apocentre, and two in the A16 sub-structure, angularly close to two C stars. Ninety radial velocities are collected from Gaia and other sources. A catalogue with 417 M pulsating AGB stars is provided. This catalogue contains ∼260 stars in the halo with |Z| > 5 kpc. Their Ks magnitudes range from 8 up to 13. For comparison, the catalogue also provides ∼150 stars in the disc having 5 < Ks < 8.

Download Full-text

Observational evidence of third dredge-up occurrence in S-type stars with initial masses around 1 M⊙

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935296 ◽

2019 ◽

Vol 625 ◽

pp. L1 ◽

Cited By ~ 3

Author(s):

S. Shetye ◽

S. Goriely ◽

L. Siess ◽

S. Van Eck ◽

A. Jorissen ◽

...

Keyword(s):

Stellar Evolution ◽

Reasonable Agreement ◽

Direct Evidence ◽

Observational Evidence ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Data Release ◽

Low Mass ◽

M Stars ◽

Hr Diagram

Context. S stars are late-type giants with spectra showing characteristic molecular bands of ZrO in addition to the TiO bands typical of M stars. Their overabundance pattern shows the signature of s-process nucleosynthesis. Intrinsic, technetium (Tc)-rich S stars are the first objects on the asymptotic giant branch (AGB) to undergo third dredge-up (TDU) events. Exquisite Gaia parallaxes now allow for these stars to be precisely located in the Hertzsprung–Russell (HR) diagram. Here we report on a population of low-mass, Tc-rich S stars previously unaccounted for by stellar evolution models. Aims. Our aim is to derive parameters for a sample of low-mass, Tc-rich S stars and then, by comparing their location in the HR diagram with stellar evolution tracks, to derive their masses and to compare their measured s-process abundance profiles with recently derived STAREVOL nucleosynthetic predictions for low-mass AGB stars. Methods. Stellar parameters were obtained using a combination of HERMES high-resolution spectra, accurate Gaia Data Release 2 (Gaia-DR2) parallaxes, stellar-evolution models, and newly designed MARCS model atmospheres for S-type stars. Results. We report on six Tc-rich S stars lying close to the 1 M⊙ (initial mass) tracks of AGB stars of the corresponding metallicity and above the predicted onset of TDU, as expected. This provides direct evidence for TDUs occurring in AGB stars with initial masses as low as ∼1 M⊙ and at low luminosity, that is, at the start of the thermally pulsing AGB. We present AGB models producing TDU in those stars with [Fe/H] in the range −0.25 to −0.5. There is reasonable agreement between the measured and predicted s-process abundance profiles. For two objects however, CD −29°5912 and BD +34°1698, the predicted C/O ratio and s-process enhancements do not simultaneously match the measured ones.

Download Full-text

Dust Production around Carbon-Rich Stars: The Role of Metallicity

Universe ◽

10.3390/universe7070233 ◽

2021 ◽

Vol 7 (7) ◽

pp. 233

Author(s):

Ambra Nanni ◽

Sergio Cristallo ◽

Jacco Th. van Loon ◽

Martin A. T. Groenewegen

Keyword(s):

Wind Speed ◽

Galactic Halo ◽

Magellanic Clouds ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Circumstellar Envelopes ◽

Chemical Enrichment ◽

Wide Range ◽

The Universe

Background: Most of the stars in the Universe will end their evolution by losing their envelope during the thermally pulsing asymptotic giant branch (TP-AGB) phase, enriching the interstellar medium of galaxies with heavy elements, partially condensed into dust grains formed in their extended circumstellar envelopes. Among these stars, carbon-rich TP-AGB stars (C-stars) are particularly relevant for the chemical enrichment of galaxies. We here investigated the role of the metallicity in the dust formation process from a theoretical viewpoint. Methods: We coupled an up-to-date description of dust growth and dust-driven wind, which included the time-averaged effect of shocks, with FRUITY stellar evolutionary tracks. We compared our predictions with observations of C-stars in our Galaxy, in the Magellanic Clouds (LMC and SMC) and in the Galactic Halo, characterised by metallicity between solar and 1/10 of solar. Results: Our models explained the variation of the gas and dust content around C-stars derived from the IRS Spitzer spectra. The wind speed of the C-stars at varying metallicity was well reproduced by our description. We predicted the wind speed at metallicity down to 1/10 of solar in a wide range of mass-loss rates.

Download Full-text

Hydrodynamic simulations of second generation star formation in a young globular cluster

Proceedings of the International Astronomical Union ◽

10.1017/s1743921319006732 ◽

2019 ◽

Vol 14 (S351) ◽

pp. 269-272

Author(s):

Francesco Calura

Keyword(s):

Star Formation ◽

Globular Cluster ◽

Second Generation ◽

Three Dimensional ◽

Asymptotic Giant Branch ◽

Central Density ◽

Agb Stars ◽

Hydrodynamic Simulations ◽

Numerical Studies

AbstractI will present results obtained by means of three-dimensional hydrodynamic simulations of the formation of second generation (SG) stars in a young globular cluster (GC). Our setup includes the mass return from Asymptotic Giant branch (AGB) stars, the accretion of pristine gas as well as star formation of SG stars, three ingredients which have never been simultaneously taken into account in previous 3D numerical studies of GC formation. The cluster is set in motion with respect to a distribution of gas and allowed to accrete mass from it. Formation of SG stars occurs out of the gas shed by AGB stars and from the gas accreted during the motion of the cluster. We consider two models characterised by different densities of the external gas. In both cases, we find that a very compact SG subsystem with central density > 105M⊙/pc3 forms in the innermost regions of the cluster.

Download Full-text

Dynamical implications of multiple stellar populations

Proceedings of the International Astronomical Union ◽

10.1017/s1743921307011258 ◽

2006 ◽

Vol 2 (14) ◽

pp. 436-437

Author(s):

Alison I. Sills ◽

Jonathan M. Downing

Keyword(s):

Star Formation ◽

Globular Cluster ◽

Globular Clusters ◽

Stellar Populations ◽

The Self ◽

Asymptotic Giant Branch ◽

Agb Stars ◽

Early Generation ◽

Single Burst

AbstractWe investigate some implications of having two star formation episodes in globular clusters, rather than the traditional single-burst approximation. Evidence for more than one stellar generation is accumulating in observations of abundances of elements lighter than iron in globular cluster stars, and is thought to imply some self-enrichment of the globular cluster gas. In particular, we explore models based on the assumption that the self-enrichment comes from an early generation of asymptotic giant branch (AGB) stars.

Download Full-text

CCD Photometry of the Globular Cluster NGC 5986 and Its Post–Asymptotic Giant Branch and RR Lyrae Stars

The Astronomical Journal ◽

10.1086/318030 ◽

2001 ◽

Vol 121 (1) ◽

pp. 318-326 ◽

Cited By ~ 20

Author(s):

David R. Alves ◽

Howard E. Bond ◽

Christopher Onken

Keyword(s):

Globular Cluster ◽

Asymptotic Giant Branch ◽

Ccd Photometry ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

Lyrae Stars

Download Full-text

Abundances and kinematics of carbon-enhanced metal-poor stars in the Galactic halo

Astronomy and Astrophysics ◽

10.1051/0004-6361/201834601 ◽

2019 ◽

Vol 623 ◽

pp. A128 ◽

Cited By ~ 15

Author(s):

C. J. Hansen ◽

T. T. Hansen ◽

A. Koch ◽

T. C. Beers ◽

B. Nordström ◽

...

Keyword(s):

Neutron Star ◽

Neutron Capture ◽

Galactic Halo ◽

Asymptotic Giant Branch ◽

Elemental Abundance ◽

High Signal ◽

Galactic Centre ◽

Agb Stars ◽

Orbital Parameters ◽

Wide Range

Carbon-enhanced metal-poor (CEMP) stars span a wide range of stellar populations, from bona fide second-generation stars to later-forming stars that provide excellent probes of binary mass transfer and stellar evolution. Here we analyse 11 metal-poor stars (8 of which are new to the literature), and demonstrate that 10 are CEMP stars. Based on high signal-to-noise ratio (S/N) X-shooter spectra, we derive abundances of 20 elements (C, N, O, Na, Mg, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Sr, Y, Ba, La, Ce, Pr, Nd, and Eu). From the high-S/N spectra, we were able to trace the chemical contribution of the rare earth elements (REE) from various possible production sites, finding a preference for metal-poor low-mass asymptotic giant branch (AGB) stars of 1.5 M⊙ in CEMP-s stars, while CEMP-r/s stars may indicate a more massive AGB contribution (2–5 M⊙). A contribution from the r-process – possibly from neutron star–neutron star mergers (NSM) – is also detectable in the REE stellar abundances, especially in the CEMP-r/s sub-group rich in both slow(s) and rapid(r) neutron-capture elements. Combining spectroscopic data with Gaia DR2 astrometric data provides a powerful chemodynamical tool for placing CEMP stars in the various Galactic components, and classifying CEMP stars into the four major elemental-abundance sub-groups, which are dictated by their neutron-capture element content. The derived orbital parameters indicate that all but one star in our sample (and the majority of the selected literature stars) belong to the Galactic halo. These stars exhibit a median orbital eccentricity of 0.7, and are found on both prograde and retrograde orbits. We find that the orbital parameters of CEMP-no and CEMP-s stars are remarkably similar in the 98 stars we study. A special case is the CEMP-no star HE 0020−1741, with very low Sr and Ba content, which possesses the most eccentric orbit among the stars in our sample, passing close to the Galactic centre. Finally, we propose an improved scheme to sub-classify the CEMP stars, making use of the Sr/Ba ratio, which can also be used to separate very metal-poor stars from CEMP stars. We explore the use of [Sr/Ba] versus [Ba/Fe] in 93 stars in the metallicity range −4.2 ≲ [Fe/H] < −2. We show that the Sr/Ba ratio can also be successfully used for distinguishing CEMP-s, CEMP-r/s, and CEMP-no stars. Additionally, the Sr/Ba ratio is found to be a powerful astro-nuclear indicator, since the metal-poor AGB stars exhibit very different Sr/Ba ratios compared to fast-rotating massive stars and NSM, and is also reasonably unbiased by NLTE and 3D corrections.

Download Full-text

THE IMPACT OF CONTAMINATED RR LYRAE/GLOBULAR CLUSTER PHOTOMETRY ON THE DISTANCE SCALE

The Astrophysical Journal ◽

10.1088/2041-8205/752/1/l10 ◽

2012 ◽

Vol 752 (1) ◽

pp. L10 ◽

Cited By ~ 12

Author(s):

D. Majaess ◽

D. Turner ◽

W. Gieren ◽

D. Lane

Keyword(s):

Globular Cluster ◽

Distance Scale ◽

Rr Lyrae ◽

The Impact

Download Full-text

Globular cluster abundances: the role of asymptotic giant branch stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309991013 ◽

2009 ◽

Vol 5 (S266) ◽

pp. 161-168

Author(s):

Amanda I. Karakas

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Asymptotic Giant Branch ◽

Asymptotic Giant Branch Stars ◽

Agb Stars ◽

Modelling Uncertainties ◽

Low Metallicity ◽

Abundance Anomalies ◽

Giant Branch Stars

AbstractOne of the more popular theories to account for the abundance anomalies in globular cluster stars is the ‘self-pollution scenario,’ where the polluters were a previous generation of intermediate-mass asymptotic giant branch (AGB) stars. This idea has proved attractive because: (i) the hot-bottom burning experienced by these objects qualitatively provides an ideal proton-capture environment to produce helium and convert C and O to N, Ne to Na and Mg to Al, and (ii) the slow winds from these stars allow their retention by the cluster's gravitational potential. New stellar yields from low-metallicity AGB models are presented and compared to abundances derived in globular clusters. We also discuss external pollution and inhomogeneous-pollution models that use AGB stars as polluters. Current models of AGB stars cannot match all observational features of globular cluster stars. However, stellar modelling uncertainties are considerable and suggest AGB stars should not be ruled out just yet.

Download Full-text

RR Lyrae variables in globular clusters and nearby galaxies

International Astronomical Union Colloquium ◽

10.1017/s0252921100010459 ◽

2004 ◽

Vol 193 ◽

pp. 113-123

Author(s):

M. Catelan

Keyword(s):

Globular Cluster ◽

Globular Clusters ◽

Galactic Halo ◽

Light Curves ◽

Rr Lyrae Stars ◽

Rr Lyrae ◽

Nearby Galaxies ◽

Lyrae Stars

AbstractI point out that the Oosterhoff dichotomy for globular cluster and field RR Lyrae stars may place the strongest constraints so far on the number of dwarf spheroidal-like protogalactic fragments that may have contributed to the formation of the Galactic halo. The first calibration of the RR Lyrae period-luminosity relation in I, J, H, K taking evolutionary effects into account is provided. Problems in the interpretation of RR Lyrae light curves and evolutionary properties are briefly reviewed.

Download Full-text