42 research outputs found
On the Origin of the Bimodal Rotational Velocity Distribution in Stellar Clusters: Rotation on the Pre-Main Sequence
We address the origin of the observed bimodal rotational distribution of stars in massive young and intermediate age stellar clusters. This bimodality is seen as split main sequences at young ages and also has been recently directly observed in the distribution of stars within massive young and intermediate age clusters. Previous models have invoked binary interactions as the origin of this bimodality, although these models are unable to reproduce all of the observational constraints on the problem. Here we suggest that such a bimodal rotational distribution is set up early within a cluster's life, i.e., within the first few Myr. Observations show that the period distribution of low-mass (\la 2 M_\odot) pre-main sequence (PMS) stars is bimodal in many young open clusters and we present a series of models to show that if such a bimodality exists for stars on the PMS that it is expected to manifest as a bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for stars with masses in excess of ~\msun. Such a bimodal period distribution of PMS stars may be caused by whether stars have lost (rapid rotators) or been able to retain (slow rotators) their circumstellar discs throughout their PMS lifetimes. We conclude with a series of predictions for observables based on our model
The IACOB project. VI. On the elusive detection of massive O-type stars close to the ZAMS
The apparent lack of massive O-type stars near the zero-age main sequence (at
ages < 2 Myr) is a topic widely discussed. Different explanations for this
elusive detection have been proposed, but no firm conclusions have been reached
yet. We reassess this empirical result benefiting from the high-quality
spectroscopic observations of >400 Galactic O-type stars gathered by the IACOB
and OWN surveys. We used temperatures and gravities from a iacob-gbat/fastwind
spectroscopic analysis to locate our sample in the Kiel and spectroscopic HR
diagrams. We evaluated the completeness of our sample of stars, observational
biases using information from the Galactic O star catalog (GOSC), systematics
of our methodology, and compare with other recent studies using smaller samples
of Galactic O-type stars. We base our discussion on the spectroscopic HR
diagram to avoid the use of uncertain distances. We performed a detailed study
of the young cluster Trumpler-14 as an example of how Gaia cluster distances
can help to construct the associated classical HR diagram. The apparent lack of
massive O-type stars near the ZAMS with masses between 30 and 70 Msol persist
even when spectroscopic results from a large, non-biased sample of stars are
used. We do not find correlation between the dearth of stars and observational
biases, limitations of our methodology, or the use of spectroscopic HR diagram
instead of the classical one. Investigating the efficiency of mass accretion
during the formation process we conclude that an adjustment of the accretion
rate towards lower values could reconcile the hotter boundary of detected
O-type stars and the theoretical birthline. Last, we discuss that the presence
of a small sample of O2-O3.5 stars found closer to the ZAMS might be explained
taking into account non-standard star evolution (e.g. binary interaction,
mergers, or homogeneous evolution).Comment: 20 pages, 15 figures, accepted for publication in Astronomy &
Astrophysic
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
Stellar models and isochrones from low-mass to massive stars including pre-main sequence phase with accretion
Grids of stellar models are useful tools to derive the properties of stellar
clusters, in particular young clusters hosting massive stars, and to provide
information on the star formation process in various mass ranges. Because of
their short evolutionary timescale, massive stars end their life while their
low-mass siblings are still on the pre-main sequence (pre-MS) phase. Thus the
study of young clusters requires consistent consideration of all the phases of
stellar evolution. But despite the large number of grids that are available in
the literature, a grid accounting for the evolution from the pre-MS accretion
phase to the post-MS phase in the whole stellar mass range is still lacking. We
build a grid of stellar models at solar metallicity with masses from 0.8
to 120 , including pre-MS phase with accretion. We use the
{\sc genec} code to run stellar models on this mass range. The accretion law is
chosen to match the observations of pre-MS objects on the Hertzsprung-Russell
diagram. We describe the evolutionary tracks and isochrones of our models. The
grid is connected to previous MS and post-MS grids computed with the same
numerical method and physical assumptions, which provides the widest grid in
mass and age to date. Numerical tables of our models and corresponding
isochrones are available online
Evolution of the rotational properties and nitrogen surface abundances of B-Type stellar populations
Stellar evolution models predict that rotation induces the mixing of chemical species, with the subsequent surface abundance anomalies relative to single non-rotating models, even during the main sequence (MS) evolution. The lack of measurable nitrogen surface enrichment in MS rotating stars, such as Be stars, has been interpreted as being in conflict with evolutionary models (e.g. Lennon et al. 2005; Hunter et al. 2008). In order to have an insight on the kind of ambient we do or we do not expect to find enriched rotating stars, we use our new population synthesis code, to produce synthetic intermediate-mass stellar populations fully accounting for stellar rotation effects, and study their evolution in tim
On the origin of the bimodal rotational velocity distribution in stellar clusters: rotation on the pre-main sequence
This is the final version. Available from Oxford University Press via the DOI in this recordWe address the origin of the observed bimodal rotational distribution of stars in massive young and intermediate age stellar clusters. This bimodality is seen as split main sequences at young ages and also has been recently directly observed in the Vsini distribution of stars within massive young and intermediate age clusters. Previous models have invoked binary interactions as the origin of this bimodality, although these models are unable to reproduce all of the observational constraints on the problem. Here, we suggest that such a bimodal rotational distribution is set-up early within a clusterâs life, i.e. within the first fewâMyr. Observations show that the period distribution of low-mass (â âČ2Mââ ) pre-main-sequence (PMS) stars is bimodal in many young open clusters, and we present a series of models to show that if such a bimodality exists for stars on the PMS that it is expected to manifest as a bimodal rotational velocity (at fixed mass/luminosity) on the main sequence for stars with masses in excess of âŒ1.5âMâ. Such a bimodal period distribution of PMS stars may be caused by whether stars have lost (rapid rotators) or been able to retain (slow rotators) their circumstellar discs throughout their PMS lifetimes. We conclude with a series of predictions for observables based on our model.European Research Council (ERC)Swiss National Science Foundatio
Grids of stellar models with rotation - III. Models from 0.8 to 120 Msun at a metallicity Z = 0.002
(shortened) We provide a grid of single star models covering a mass range from 0.8 to 120 Msun with an initial metallicity Z = 0.002 with and without rotation. We discuss the impact of a change in the metallicity by comparing the current tracks with models computed with exactly the same physical ingredients but with a metallicity Z = 0.014 (solar). We show that the width of the main-sequence (MS) band in the upper part of the Hertzsprung-Russell diagram (HRD), for luminosity above log(L/Lsun) > 5.5, is very sensitive to rotational mixing. Strong mixing significantly reduces the MS width. We confirm, but here for the first time on the whole mass range, that surface enrichments are stronger at low metallicity provided that comparisons are made for equivalent initial mass, rotation and evolutionary stage. We show that the enhancement factor due to a lowering of the metallicity (all other factors kept constant) increases when the initial mass decreases. Present models predict an upper luminosity for the red supergiants (RSG) of log (L/Lsun) around 5.5 at Z = 0.002 in agreement with the observed upper limit of RSG in the Small Magellanic Cloud. We show that models using shear diffusion coefficient calibrated to reproduce the surface enrichments observed for MS B-type stars at Z = 0.014 can also reproduce the stronger enrichments observed at low metallicity. In the framework of the present models, we discuss the factors governing the timescale of the first crossing of the Hertzsprung gap after the MS phase. We show that any process favouring a deep localisation of the H-burning shell (steep gradient at the border of the H-burning convective core, low CNO content) and/or the low opacity of the H-rich envelope favour a blue position in the HRD for the whole or at least a significant fraction of the core He-burning phase
The Physics of Star Cluster Formation and Evolution
© 2020 Springer-Verlag. The final publication is available at Springer via https://doi.org/10.1007/s11214-020-00689-4.Star clusters form in dense, hierarchically collapsing gas clouds. Bulk kinetic energy is transformed to turbulence with stars forming from cores fed by filaments. In the most compact regions, stellar feedback is least effective in removing the gas and stars may form very efficiently. These are also the regions where, in high-mass clusters, ejecta from some kind of high-mass stars are effectively captured during the formation phase of some of the low mass stars and effectively channeled into the latter to form multiple populations. Star formation epochs in star clusters are generally set by gas flows that determine the abundance of gas in the cluster. We argue that there is likely only one star formation epoch after which clusters remain essentially clear of gas by cluster winds. Collisional dynamics is important in this phase leading to core collapse, expansion and eventual dispersion of every cluster. We review recent developments in the field with a focus on theoretical work.Peer reviewe
General-relativistic instability in rapidly accreting supermassive stars: The impact of rotation
Context. Supermassive stars (SMSs) collapsing via the general-relativistic (GR) instability are invoked as the possible progenitors of supermassive black holes. Their mass and angular momentum at the onset of the instability are key in many respects, in particular regarding the possibility for observational signatures of direct collapse. Accretion dominates the evolution of SMSs and, similar to rotation, it has been shown to impact their final properties significantly. However, the combined effect of accretion and rotation on the stability of these objects is not known.
Aims. Here, we study the stability of rotating, rapidly accreting SMSs against GR perturbations and derive the properties of these stars at death.
Methods. On the basis of hylotropic structures, which are relevant for rapidly accreting SMSs, we define rotation profiles under the assumption of local angular momentum conservation in radiative regions, which allows for differential rotation. We account for rotation in the stability of the structure by adding a Newtonian rotation term in the relativistic equation of stellar pulsation, which is justified by the slow rotations imposed by the ΩÎ-limit.
Results. We find that rotation favours the stability of rapidly accreting SMSs as soon as the accreted angular momentum represents a fraction of fââłâ0.1% of the Keplerian angular momentum. For fââŒâ0.3â0.5%, the maximum masses consistent with GR stability are increased by an order of magnitude compared to the non-rotating case. For fââŒâ1%, the GR instability cannot be reached if the stellar mass does not exceed 107â
ââ
108 Mâ.
Conclusions. These results imply that, as in the non-rotating case, the final masses of the progenitors of direct collapse black holes range in distinct intervals depending on the scenario considered: 105âMâââČâMââČâ106 Mâ for primordial atomically cooled haloes and 106âMâââČâMââČâ109 Mâ for metal-rich galaxy mergers. The models suggest that the centrifugal barrier is inefficient to prevent the direct formation of a supermassive black hole at the collapse of a SMS. Moreover, the conditions of galaxy mergers appear to be more favourable than those of atomically cooled haloes for detectable gravitational wave emission and ultra-long gamma-ray bursts at black hole formation