scholarly journals Evolution of D and 3He in the Galaxy

2000 ◽  
Vol 198 ◽  
pp. 525-534 ◽  
Author(s):  
Monica Tosi
Keyword(s):  
Big Bang ◽  
Isotopic Ratios ◽  
Galactic Chemical Evolution ◽  
Low Mass Stars ◽  
Carbon Isotopic ◽  
The Galaxy ◽  
Low Mass ◽  
Red Giant ◽  
The Big Bang ◽  
Self Consistency

The predictions of Galactic chemical evolution models for D and 3He are described in connection with those on the other Galactic quantities for which observational constraints are available.Models in agreement with the largest set of data predict deuterium depletions from the Big Bang to the present epoch smaller than a factor of 3 and do not allow for D/H primordial abundances larger than ∼ 4 × 10—5. Models predicting higher D consumption do not reproduce other observed features of our Galaxy.If both the primordial D and 3He are low, models assuming that 90% of low-mass stars experience an extra-mixing during the red giant phase reproduce all the 3He observed abundances. The same percentage allows to fit also the observed carbon isotopic ratios, thus supporting the self-consistency of the extra-mixing mechanism.

scholarly journals The destruction of 3He by Rayleigh-Taylor instability on the first giant branch

2006 ◽  
Vol 2 (S239) ◽  
pp. 286-293
Author(s):  
Peter P. Eggleton ◽  
David S. P. Dearborn ◽  
John C. Lattanzio
Keyword(s):  
Local Minimum ◽  
Big Bang ◽  
Taylor Instability ◽  
Big Bang Nucleosynthesis ◽  
Low Mass Stars ◽  
Hydrogen Burning ◽  
Convective Envelope ◽  
Low Mass ◽  
Rayleigh Taylor Instability ◽  
Red Giant

AbstractLow-mass stars, ∼ 1–2 solar masses, near the Main Sequence are efficient at producing 3He, which they mix into the convective envelope on the giant branch and distribute into the Galaxy by way of envelope loss. This process is so efficient that it is difficult to reconcile the observed cosmic abundance of 3He with the predictions of Big Bang nucleosynthesis. In this paper we find, by modeling a red giant with a fully three-dimensional hydrodynamic code and a full nucleosynthetic network, that mixing arises in the supposedly stable and radiative zone between the hydrogen-burning shell and the base of the convective envelope. This mixing is due to Rayleigh-Taylor instability within a zone just above the hydrogen-burning shell. In this zone the burning of the 3He left behind by the retreating convective envelope is predominantly by the reaction 3He + 3He → 4He + 1H + 1H, a reaction which, untypically for stellar nuclear reactions, lowers the mean molecular weight, leading to a local minimum. This local minimum leads to Rayleigh-Taylor instability, and turbulent motion is generated which will continue ultimately up into the normal convective envelope. Consequently material from the envelope is dragged down sufficiently close to the burning shell that the He in it is progressively destroyed. Thus we are able to remove the threat that He production in low-mass stars poses to the Big Bang nucleosynthesis of 3He.Some slow mixing mechanism has long been suspected, that connects the convective envelope of a red giant to the burning shell. It appears to be necessary to account for progressive changes in the C/C and N/C ratios on the First Giant Branch. We suggest that these phenomena are also due to the Rayleigh-Taylor-unstable character of the He-burning region.

scholarly journals Possible Constituents of Halos

1987 ◽  
Vol 117 ◽  
pp. 395-409
Author(s):  
Martin J. Rees
Keyword(s):  
Galaxy Formation ◽  
Massive Stars ◽  
Critical Density ◽  
Big Bang ◽  
Galactic Halos ◽  
Low Mass Stars ◽  
Groups Of Galaxies ◽  
Low Mass ◽  
The Big Bang ◽  
The Universe

There still seem to be three serious contenders for the dark matter in galactic halos and groups of galaxies: (i) very low mass stars, (ii) black hole remnants of very massive stars or (iii) some species of particle (e.g. axions, photinos, etc.) surviving from the big bang. There are genuine prospects of detecting individual objects in all three of these categories, and thereby narrowing down the present range of options. If the Universe has the critical density (Ω = 1), rather than the lower value (Ω = 0.1–0.2) inferred from dynamical evidence, then the galaxies must be more clustered than the overall distribution even on scales 10–20 Mpc. “Biased” galaxy formation could account for this.

scholarly journals Galactic evolution of D, 3He and 4He

2009 ◽  
Vol 5 (S268) ◽  
pp. 431-440
Author(s):  
Donatella Romano
Keyword(s):  
Magnetic Fields ◽  
Chemical Evolution ◽  
Globular Clusters ◽  
Milky Way ◽  
Galactic Evolution ◽  
Galactic Chemical Evolution ◽  
Low Mass Stars ◽  
Local Abundance ◽  
The Galaxy ◽  
Low Mass

AbstractThe uncertainties which still plague our understanding of the evolution of the light nuclides D, 3He and 4He in the Galaxy are described. Measurements of the local abundance of deuterium range over a factor of 3. The observed dispersion can be reconciled with the predictions on deuterium evolution from standard Galactic chemical evolution models, if the true local abundance of deuterium proves to be high, but not too high, and lower observed values are due to depletion onto dust grains. The nearly constancy of the 3He abundance with both time and position within the Galaxy implies a negligible production of this element in stars, at variance with predictions from standard stellar models which, however, do agree with the (few) measurements of 3He in planetary nebulae. Thermohaline mixing, inhibited by magnetic fields in a small fraction of low-mass stars, could in principle explain the complexity of the overall scenario. However, complete grids of stellar yields taking this mechanism into account are not available for use in chemical evolution models yet. Much effort has been devoted to unravel the origin of the extreme helium-rich stars which seem to inhabit the most massive Galactic globular clusters. Yet, the issue of 4He evolution is far from being fully settled even in the disc of the Milky Way.

scholarly journals Spiral morphology in an intensely star-forming disk galaxy more than 12 billion years ago

Science ◽  
2021 ◽  
pp. eabe9680
Author(s):  
Takafumi Tsukui ◽  
Satoru Iguchi
Keyword(s):  
Big Bang ◽  
Compact Structure ◽  
Spiral Arms ◽  
Star Forming ◽  
Gas Kinematics ◽  
Internal Structures ◽  
The Galaxy ◽  
Galaxy Center ◽  
The Big Bang ◽  
Tidal Tails

Spiral galaxies have distinct internal structures including a stellar bulge, disk and spiral arms. It is unknown when in cosmic history these structures formed. We analyze observations of BRI 1335–0417, an intensely star-forming galaxy in the distant Universe, at redshift 4.41. The [C ii] gas kinematics show a steep velocity rise near the galaxy center and have a two-armed spiral morphology, which extends from about 2 to 5 kiloparsecs in radius. We interpret these features as due to a central compact structure, such as a bulge; a rotating gas disk; and either spiral arms or tidal tails. These features had formed within 1.4 billion years after the Big Bang, long before the peak of cosmic star formation.

scholarly journals Li isotopes in metal-poor halo dwarfs: a more and more complicated story

2009 ◽  
Vol 5 (S268) ◽  
pp. 201-210
Author(s):  
Monique Spite ◽  
François Spite
Keyword(s):  
Cosmic Rays ◽  
Big Bang ◽  
The Other ◽  
Big Bang Nucleosynthesis ◽  
Lithium Abundance ◽  
The Galaxy ◽  
Li Isotope ◽  
Low Metallicity ◽  
The Big Bang ◽  
Early Galaxy

AbstractThe nuclei of the lithium isotopes are fragile, easily destroyed, so that, at variance with most of the other elements, they cannot be formed in stars through steady hydrostatic nucleosynthesis.The 7Li isotope is synthesized during primordial nucleosynthesis in the first minutes after the Big Bang and later by cosmic rays, by novae and in pulsations of AGB stars (possibly also by the ν process). 6Li is mainly formed by cosmic rays. The oldest (most metal-deficient) warm galactic stars should retain the signature of these processes if, (as it had been often expected) lithium is not depleted in these stars. The existence of a “plateau” of the abundance of 7Li (and of its slope) in the warm metal-poor stars is discussed. At very low metallicity ([Fe/H] < −2.7dex) the star to star scatter increases significantly towards low Li abundances. The highest value of the lithium abundance in the early stellar matter of the Galaxy (logϵ(Li) = A(7Li) = 2.2 dex) is much lower than the the value (logϵ(Li) = 2.72) predicted by the standard Big Bang nucleosynthesis, according to the specifications found by the satellite WMAP. After gathering a homogeneous stellar sample, and analysing its behaviour, possible explanations of the disagreement between Big Bang and stellar abundances are discussed (including early astration and diffusion). On the other hand, possibilities of lower productions of 7Li in the standard and/or non-standard Big Bang nucleosyntheses are briefly evoked.A surprisingly high value (A(6Li)=0.8 dex) of the abundance of the 6Li isotope has been found in a few warm metal-poor stars. Such a high abundance of 6Li independent of the mean metallicity in the early Galaxy cannot be easily explained. But are we really observing 6Li?

scholarly journals The Evolution of 3He, 4He and D in the Galaxy

2000 ◽  
Vol 198 ◽  
pp. 540-546 ◽  
Author(s):  
Cristina Chiappini ◽  
Francesca Matteucci
Keyword(s):  
Chemical Evolution ◽  
Present Model ◽  
Galactic Disk ◽  
The Sun ◽  
Mixing Process ◽  
Low Mass Stars ◽  
A Value ◽  
The Galaxy ◽  
Low Mass ◽  
Standing Problem

In this work we present the predictions of a modified version of the ‘two-infall model’ (Chiappini et al. 1997 - CMG) for the evolution of 3He, 4He and D in the solar vicinity, as well as their distributions along the Galactic disk. In particular, we show that when allowing for extra-mixing process in low mass stars (M < 2.5 M⊙), as predicted by Charbonnel and do Nascimento (1998), a long standing problem in chemical evolution is solved, namely: the overproduction of 3He by the chemical evolution models as compared to the observed values in the sun and in the interstellar medium. Moreover, we show that chemical evolution models can constrain the primordial value of the deuterium abundance and that a value of (D/H)p < 3 × 10—5 is suggested by the present model. Finally, adopting the primordial 4He abundance suggested by Viegas et al. (1999), we obtain a value for ΔY/ΔZ ≃ 2 and a better agreement with the solar 4He abundance.

scholarly journals Observations of low mass stars in clusters: Some constraints and puzzles for stellar evolution theory

1984 ◽  
Vol 105 ◽  
pp. 123-138
Author(s):  
R.D. Cannon
Keyword(s):  
Globular Clusters ◽  
Main Sequence ◽  
Star Clusters ◽  
Variable Stars ◽  
Open Clusters ◽  
Low Mass Stars ◽  
Theory Of Evolution ◽  
Theoretical Predictions ◽  
Low Mass ◽  
Red Giant

This review will attempt to do two things: (i) discuss some of the data which are available for testing the theory of evolution of low mass stars, and (ii) point out some problem areas where observations and theory do not seem to agree very well. This is of course too vast a field of research to be covered in one brief review, so I shall concentrate on one particular aspect, namely the study of star clusters and especially their colour-magnitude (CM) diagrams. Star clusters provide large samples of stars at the same distance and with the same age, and the CM diagram gives the easiest way of comparing theoretical predictions with observations, although crucial evidence is also provided by spectroscopic abundance analyses and studies of variable stars. Since this is primarily a review of observational data it is natural to divide it into two parts: (i) galactic globular clusters, and (ii) old and intermediate-age open clusters. Some additional evidence comes from Local Group galaxies, especially now that CM diagrams which reach the old main sequence are becoming available. For each class of cluster I shall consider successive stages of evolution from the main sequence, up the hydrogen-burning red giant branch, and through the helium-burning giant phase.

Deuterium in the Galaxy

1977 ◽  
Vol 3 (2) ◽  
pp. 100-101 ◽  
Author(s):  
R. D. Brown
Keyword(s):  
Great Majority ◽  
Big Bang ◽  
Baryon Density ◽  
The Galaxy ◽  
Mass Fractions ◽  
Expansion Of The Universe ◽  
Sensitive Function ◽  
The Big Bang ◽  
Deuterium Production ◽  
The Universe

There have been a number of attempts made in the last decade or two to observe deuterium in parts of the universe other than here in Earth. It is of interest merely to detect deuterium elsewhere just as it is to detect the occurrence of any nuclide. However in the case of deuterium there is a special interest because in big-bang cosmologies the great majority of deuterium in the universe is considered to have been formed in the initial fireball (Wagoner, 1973). Any observation of the present abundance of deuterium thus might give information about the very early stages of the creation of the universe. Detailed studies of nucleosynthesis during the early expansion of hot big-bang universes have however indicated a particular feature of deuterium production. (Fig. 1) The mass fraction produced X(D) is a very sensitive function of the size of the universe, as measured say by the present baryon density ϱb. Other nuclides that are mainly produced in the early expansion, such as 4He, have mass fractions less dependent on ϱb. Thus if we adopt the big-bang model for our universe we can determine ϱb from observations of X(D). Apart from any intrinsic interest in the present density of the’universe, there is considerable interest in whether the value is great enough for the present expansion to halt and go over to a collapse — or so small that the expansion of the universe will go on forever.

scholarly journals Magnetic activity in the HARPS M dwarf sample

2017 ◽  
Vol 600 ◽  
pp. A13 ◽  
Author(s):  
N. Astudillo-Defru ◽  
X. Delfosse ◽  
X. Bonfils ◽  
T. Forveille ◽  
C. Lovis ◽  
...  
Keyword(s):  
Monitoring Program ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Spectral Lines ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
The Galaxy ◽  
Low Mass ◽  
M Dwarf ◽  
The Relationship

Context. Atmospheric magnetic fields in stars with convective envelopes heat stellar chromospheres, and thus increase the observed flux in the Ca ii H and K doublet. Starting with the historical Mount Wilson monitoring program, these two spectral lines have been widely used to trace stellar magnetic activity, and as a proxy for rotation period (Prot) and consequently for stellar age. Monitoring stellar activity has also become essential in filtering out false-positives due to magnetic activity in extra-solar planet surveys. The Ca ii emission is traditionally quantified through the R'HK-index, which compares the chromospheric flux in the doublet to the overall bolometric flux of the star. Much work has been done to characterize this index for FGK-dwarfs, but M dwarfs – the most numerous stars of the Galaxy – were left out of these analyses and no calibration of their Ca ii H and K emission to an R'HK exists to date. Aims. We set out to characterize the magnetic activity of the low- and very-low-mass stars by providing a calibration of the R'HK-index that extends to the realm of M dwarfs, and by evaluating the relationship between R'HK and the rotation period. Methods. We calibrated the bolometric and photospheric factors for M dwarfs to properly transform the S-index (which compares the flux in the Ca ii H and K lines to a close spectral continuum) into the R'HK. We monitored magnetic activity through the Ca ii H and K emission lines in the HARPS M dwarf sample. Results. The R'HK index, like the fractional X-ray luminosity LX/Lbol, shows a saturated correlation with rotation, with saturation setting in around a ten days rotation period. Above that period, slower rotators show weaker Ca ii activity, as expected. Under that period, the R'HK index saturates to approximately 10-4. Stellar mass modulates the Ca ii activity, with R'HK showing a constant basal activity above 0.6 M⊙ and then decreasing with mass between 0.6 M⊙ and the fully-convective limit of 0.35 M⊙. Short-term variability of the activity correlates with its mean level and stars with higher R'HK indexes show larger R'HK variability, as previously observed for earlier spectral types.

scholarly journals Dark Matter in the Universe

1986 ◽  
Vol 7 ◽  
pp. 27-38 ◽  
Author(s):  
Vera C. Rubin
Keyword(s):  
Dark Matter ◽  
Deceleration Parameter ◽  
Big Bang ◽  
Theoretical Cosmology ◽  
Observational Cosmology ◽  
The Past ◽  
The Galaxy ◽  
Expansion Of The Universe ◽  
The Big Bang ◽  
The Universe

Thirty years ago, observational cosmology consisted of the search for two numbers: Ho, the rate of expansion of the universe at the position of the Galaxy; and qo, the deceleration parameter. Twenty years ago, the discovery of the relic radiation from the Big Bang produced another number, 3oK. But it is the past decade which has seen the enormous development in both observational and theoretical cosmology. The universe is known to be immeasurably richer and more varied than we had thought. There is growing acceptance of a universe in which most of the matter is not luminous. Nature has played a trick on astronomers, for we thought we were studying the universe. We now know that we were studying only the small fraction of it that is luminous. I suspect that this talk this evening is the first IAU Discourse devoted to something that astronomers cannot see at any wavelength: Dark Matter in the Universe.

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