EWASS 2014 - SP2 : Helium (2-3 July 2014)

Wednesday 2nd of July
16.05-17.40   -   Chair : Joris De Ridder  
Gaia-Asteroseismology synergies
 Antonella Vallenari (I)
Bright stars in Gaia
 Andreas Korn Astrophysical parameters from Gaia
 Andrea Miglio (I) Synergy between Asteroseismology and Gaia
 Orlagh Creevey
Gaia, asteroseismology and initial helium abundance
 Open discussion

Thursday 3rd of July
11.00-12.45   -   Chair : Frederic Thevenin  
Asteroseismology of low mass stars
 Aldo Serenelli (I) Asteroseismic studies of the Sun and Main Sequence stars
11.30-11.45  Patrick Eggenberger
Transport processes in low mass stars constrained by asteroseismic measurements
 Laura Portinari
Helium abundance and DY/DZ in low Main Sequence stars
Low mass stellar models
 Santi Cassisi (I)
Low-mass Main Sequence Stars: the theoretical framework
 Stefano Pasetto
Theory of stellar convection: Removing the Mixing-Length Parameter
12:45-14:00  Lunch
14.00-15.30   -   Chair : Ulrike Heiter  
Stellar model atmospheres and chemical abundances
 Remo Collet (I)
Three-dimensional model stellar atmospheres: a review of the current status and a look into the next challenges
 Zazralt Magic
Steps towards improved stellar structure and evolutionary models
 Hans-G. Ludwig
The photospheric oxygen abundance of the Sun according to CO5BOLD
 Karin Lind
NLTE effects in the Sun and metal-poor stars
 Sofya Alexeeva Carbon abundances on the basis of 1D model atmospheres. Non-LTE line formation of C I
15:30-16:00 Coffee break
16.00-17.30   -   Chair : Corinne Charbonnel  
16.00-16.30  Maria Fernanda Nieva (I)
Present-day helium abundances in the Solar Vicinity
 Antonino Milone The helium abundance of multiple populations in Globular Clusters
 Anna Marino Second parameter problem on the horizontal branch morphology in globular clusters: the impact of helium
 Achim Weiss (I) Summary and Discussion


Invited talks

Antonella Vallenari (I)  -  INAF - Osservatorio Astronomico di Padova, Italy

Bright stars in  Gaia

The talk will present the expected  performances of Gaia concerning bright stars, in terms of photometry, astrometry, and spectroscopy, the data that will be available in the first data release. Special focus will be on the Gaia Catalog validation procedures, including the use of open clusters as validation tools and the contribution of asteroseismology.

Andrea Miglio (I)  -  University of Birmingham, UK

Synergy between Asteroseismology and Gaia

The Milky Way provides a unique opportunity to study in detail how a giant spiral galaxy is assembled and how it evolves. Our current understanding of Galactic formation and evolution is severely hampered by a lack of precise observations of basic stellar properties such as distances, masses, and ages. Gaia will shortly overcome current limitations associated with estimating distances to stars, however, accurate age determination of individual field stars will still be a major obstacle to our understanding the Milky Way. Asteroseismology provides the way forward.
I will here give an overview about the complementary role of astrometric and asteroseismic constraints which are/will be available for stars monitored by CoRoT, Kepler, K2, and, in the future, by PLATO.

Aldo Serenelli (I)  -  Institute of Space Sciences, Bellaterra, Spain

Asteroseismic studies of the Sun and Main Sequence stars

The talk will consist of two parts. In the first, I will first review the basic seismic techniques, and point out the weakest bonds of the chain, used to determine the helium abundance in the solar envelope and discuss what has been learned from these measurements in the context of solar (and stellar) evolution. I will then summarize recent helium determinations, based on a newly developed equation of state and solar composition.
The second part of the talk will discuss results from asteroseismic modeling of dwarf stars observed by Kepler. Direct fit to the observed oscillation frequencies currently lead to "best-fit" stellar models that show a clear deficit in the initial helium abundance, in many cases directly conflicting with primordial BBN values, and in many others leading to helium-metallicity relations that defy the logics of a common chemical evolution. Finally, a brief discussion of the status of more "direct techniques", analogous to that used for the Sun, will be presented.

Santi Cassisi (I)  - INAF - Osservatorio Astronomico di Teramo, Italy

Low-mass Main Sequence Stars: the theoretical framework

Reliable models for low-mass, core H-burning stars are, in principle, the most simple to be obtained due to the evidence that "Physics" at work in such stars is quite more simple in comparison with more advanced evolutionary stages such as the Red Giant and central He-burning phases. Notwithstanding, when comparing these stellar models with accurate empirical benchmarks as those provided by helio- and asteroseismology, there are some evident discrepancies. This is an unpleasant situation when also accounting for the fact that, at the dawn of the Gaia era, a huge amount of high-quality empirical data will be provided that requires a reliable and accurate theoretical scenario.
In this review we discuss the state-of-the-art of low-mass, Main Sequence stellar model computations, emphasizing the most important uncertainties still affecting the theoretical framework, and discussing the level of agreement between theory and observations.

Remo Collet  (I)  -  Australian National University, Canberra, Australia

Three-dimensional model stellar atmospheres: a review of the current status and a look into the next challenges

Recent years have seen a rapid development in the field of three-dimensional hydrodynamical models of stellar surface convection and stellar atmospheres. In this contribution, I will review the current status of three-dimensional modelling of stellar atmospheres and stellar spectra. I will give an overview of the physical assumptions behind realistic three-dimensional simulations of stellar surface convection and present their main characteristics and properties. In particular, I will highlight the differences with respect to classical one-dimensional models in terms of predicted physical stratifications and spectral features. I will also discuss the topic of implementing mean stratifications and other properties from three-dimensional simulations into one-dimensional stellar structure and evolution models, presenting the main resul ts in this area and examining the remaining challenges.

Maria Fernanda Nieva (I) - University of Innsbruck, Austria

Present-day helium abundances in the Solar Vicinity

I will present the current status of helium abundances in massive stars of the solar vicinity from the literature and own work. Some attention will be given to the target star selection and analysis in order to have meaningful results. Present-day helium abundances will be discussed within the frame of Galactic chemical evolution.

Contributed talks

Andreas Korn - Uppsala University, Sweden

Astrophysical parameters from Gaia

I will review the software tools under development in the Gaia Data Processing and Analysis Consortium's Coordination Unit 8 (DPAC CU8) that will derive astrophysical parameters of all single stars. A natural emphasis will be on stars of spectral type F, G and K. While a reasonable level of precision is attainable for parameters dominating the variation of stellar observables (mainly fluxes), constraints on some of the more subtle parameters (e.g. log g) will require parallax measurements and/or additional (non-Gaia) data. Parameter accuracy is to be safeguarded by means of extensive calibrations. Here asteroseismology may well play a decisive role.

Orlagh Creevey - Institut d'Astrophysique Spatiale, France

Gaia, asteroseismology and initial helium abundance

Distances from Gaia will give us access to luminosities and radii of stars, independently of stellar evolution models. These are very complementary to asteroseismic data which alone can probe stellar interiors and deliver precise stellar parameters such as radius and age. Combining these data, however, has the potential to disentangle some properties such as mass and initial helium abundance, which consequently leads to better determinations of mass and age. In this talk I discuss the possibility of Gaia shedding light on this issue.

Patrick Eggenberger - Observatoire de Geneve, Switzerland

Transport processes in low-mass stars constrained by asteroseismic measurements

The modelling of transport processes of angular momentum and chemicals in the radiative zones of stellar models is first presented. The effects of these processes on the global and internal properties of low-mass stars is then described and the observational constraints that are currently available to help us progress in our understanding of these mechanisms are discussed, with a focus on asteroseismic data.

Laura Portinari

co-authors : Luca Casagrande, Chris Flynn

Helium abundance and DY/DZ in low Main Sequence stars

The fine structure of the HR diagram in the low Main Sequence has been long used as a probe of the chemical composition of stars, as metallicity (Z) and helium content (Y) have opposing effects on the broadening of the low Main Sequence. We review the history of the subject, that ultimately highlighted the "sub-primordial helium problem", anticipating the latest results of asteroseismology. We briefly discuss possible solutions, including opacity changes and a possible connection to the helium problem in globular clusters.

Stefano Pasetto - University College London, UK

co-authors : Cesare Chiosi, Mark Cropper, Eva Grebel

Theory of stellar convection: Removing the Mixing-Length Parameter

Stellar convection is customarily described by the mixing-length theory, which makes use of the mixing-length scale to express the convective flux, velocity, and temperature gradients of the convective elements and stellar medium. The mixing-length scale is taken to be proportional to the local pressure scale height, and the proportionality factor (the mixing-length parameter) must be determined by comparing the stellar models to some calibrator, usually the Sun. No strong arguments exist to claim that the mixing-length parameter is the same in all stars and all evolutionary phases. Because of this, all stellar models in literature are hampered by this basic uncertainty.
We present a new theory of stellar convection that does not require the mixing length parameter. Our self-consistent analytical formulation of stellar convection determines all the properties of stellar convection as a function of the physical behaviour of the convective elements themselves and the surrounding medium.
The new theory of stellar convection is formulated starting from a conventional solution of the Navier-Stokes/Euler equations, i.e. the Bernoulli equation for a perfect fluid, but expressed in a non-inertial reference frame co-moving with the convective elements. In our formalism, the motion of stellar convective cells inside convective unstable layers is fully determined by a new system of equations for convection in a non-local and time dependent formalism.
We obtain an analytical, non-local, time-dependent solution for the convective energy transport that does not depend on any free parameter. The predictions of the new theory are compared with those from the standard mixing-length paradigm with very satisfactory results.

Zazralt Magic - Max-Planck Institute for Astrophysics, Garching, Germany

co-authors : Achim Weiss, Martin Asplund

Steps towards improved stellar structure and evolutionary models

The outermost layers in cool stars are notoriously difficult to model due to the convective energy transport, which is usually treated with the mixing-length theory (MLT) in one-dimensional (1D) models. These 1D MLT models require free parameters that are a priori unkown and also often neglect the turbulent pressure. Remedy is provided by three-dimensional (3D) hydrodynamical simulations of stellar subsurface convection, which solve the hydrodynamic conservation equations coupled with a realistic treatment of radiative transfer. We have computed the Stagger-grid, a large grid of 3D atmosphere models for late-type stars. In this contribution, we report on our efforts on improving stellar structure and evolutionary models with the aid of the Stagger-grid models. As a first step, we have calibrated the MLT free parameter, which will be useful to study the influence of its variation on stellar evolutionary tracks. Furthermore, we will discuss the differences in the stellar structures between 1D models and spatial and temporal averaged stratifications from 3D simulations.

Hans-G. Ludwig - Landesternwarte, Zentrum für Astronomie, Universität Heidelberg, Germany

co-authors : The Co5BOLD Team

The photospheric oxygen abundance of the Sun according to CO5BOLD

I will review the work which was conducted in the CO5BOLD Team to establish the solar photospheric oxygen abundance spectroscopically to high fidelity. I will report on present efforts, and point to lingering issues which still need to be settled. As time permits, I will put it into the broader context of Galactic chemical evolution.

Karin Lind - Uppsala University, Sweden

NLTE effects in the Sun and metal-poor stars

Departures from local thermodynamic equilibrium are unavoidable in the tenuous environments of stellar atmospheres. As such they are known to affect the determination of chemical abundances and stellar parameters for all types of stars to a small or large extent. I will discuss what we know about how non-LTE radiative transfer takes place in 3D hydrodynamical model atmospheres and future prospects and potential consequences of a fully consistent 3D, NLTE analysis of the Sun and metal-poor stars. Currently, this technique has only been tested for a handful of elements and stars.

Sofya Alexeeva - Institute of Astronomy RAS, Moscow, Russian Federation

co-authors : Lyudmila Mashonkina

Carbon abundances on the basis of 1D model atmospheres. Non-LTE line formation of C I

We present a non-LTE calculations for CI lines with using a comprehensive model atom of CI/II. As the first application of the model, carbon abundances are determined from atomic [CI], CI lines and also from molecular lines CH and C2 on the basis of classical plane-parallel 1D model atmospheres for Sun, metal-poor dwarf HD84937, and cool giant HD122563. Only high-resolution spectra with high S/N at visual and near-IR wavelengths were used. The non-LTE leads to stronger CI lines in comparison to LTE case. The non-LTE abundance corrections are negative, dNLTE do not exceed 0.41 in absolute value for CI lines in solar atmosphere and are less significant for HD84937. As for cool giant HD122563, dNLTE are close to zero due to weak C I lines, which form in deep atmospheric layers. The atomic and molecular lines give consistent abundances for the investigated stars. The derived solar carbon abundance from atomic lines is log EC = 8.43 +/- 0.04, while from molecular line is log EC = 8.42 +/- 0.04.

Antonino Milone
- Australian National University, Canberra, Australia

The helium abundance of multiple populations in Globular Clusters

Recent studies, mainly based on Hubble Space Telescope photometry, have provided a new view of the color-magnitude diagram of globular clusters (GCs). Appropriate combination of ultraviolet, visual, and near-infrared magnitudes have revealed that the CMD of any globular is made of intertwined sequences, from two in number up to five or more. These sequences can be followed continuously from the bottom of the main sequence up to the sub-giant branch, the red-giant branch and even the horizontal branch and correspond to generations of star with different abundance of helium and light elements. The helium range dramatically changes from one cluster to the other. In some cases stellar populations can approach Y~0.40. In this talk I will summarize the observational scenario and discuss the impact of multiple populations with different helium in the context of the star-formation history within GCs.

Anna Marino - Australian National University, Canberra, Australia

Second parameter problem on the horizontal branch morphology in globular clusters: the impact of helium

Globular clusters (GCs) exhibit a range of horizontal branch (HB) morphology, with some clusters showing red, blue, extended blue, or even multimodal HBs. While the overall metallicity is proposed as the first parameter governing the HB, this alone can not explain the wide range of observed structures.
The new findings on multiple stellar populations in globular clusters (GCs), allow us to look at the eluding second parameter problem from a new perspective. In this context, we have recently proven for the GC M4 that a connection exists between the
chemical composition of Na and O and the distribution of stars along the HB: HB stars located on the blue side of the RR-Lyrae gap are Na enhanced, and stars on the red side are Na-poor (Marino et al. 2011). If the material enriched in sodium, that formed stars on the blue side of the HB in M4, has undergone high-temperature H-burning, we expect that the same material is also enriched, on some degree, in helium. In fact, the multiple HB components of several GCs have been tentatively assigned to different stellar generations with different He by many authors (D'Antona et al. 2005; Piotto et al. 2007; Milone et al. 2008). The bottom line is that the He-enhanced stars are likely to populate the blue extreme of the HBs, and He would a fundamental parameter governing the HB morphology.
We have recently explored this idea. In this work, and we have provided empirical support for a connection between the colour extension of the HB and the intrinsic He variations. We found that helium is one of the parameters in play for the HB
morphology: specifically it is the non-global parameter that varies within cluster stars and governs the intra-cluster distribution of HB stars. By using FLAMES/GIRAFFE spectra we have recently provided direct spectroscopic evidence of the validity of this scenario in the case of NGC2808, whose BHB is populated exclusively by highly He-enriched stars (Marino et al. 2014).
I will summarize the observational scenario for the connection between helium and the morphology of the HB in GCs, and the impact of this connection on our the second parameter problem.