239 research outputs found
Predicting the locations of possible long-lived low-mass first stars: Importance of satellite dwarf galaxies
The search for metal-free stars has so far been unsuccessful, proving that if
there are surviving stars from the first generation, they are rare, they have
been polluted, or we have been looking in the wrong place. To predict the
likely location of Population~III (Pop~III) survivors, we semi-analytically
model early star formation in progenitors of Milky Way-like galaxies and their
environments. We base our model on merger trees from the high-resolution dark
matter only simulation suite \textit{Caterpillar}. Radiative and chemical
feedback are taken into account self-consistently, based on the spatial
distribution of the haloes. Our results are consistent with the non-detection
of Pop III survivors in the Milky Way today. We find that possible surviving
Population III stars are more common in Milky Way satellites than in the main
Galaxy. In particular, low mass Milky Way satellites contain a much larger
fraction of Pop~III stars than the Milky Way. Such nearby, low mass Milky Way
satellites are promising targets for future attempts to find Pop~III survivors,
especially for high-resolution, high signal-to-noise spectroscopic
observations. We provide the probabilities for finding a Pop~III survivor in
the red giant branch phase for all known Milky Way satellites to guide future
observations.Comment: 17 pages, 12 figures, 1 table, submitted to MNRA
Observational constraints on the survival of pristine stars
There is a longstanding discussion about whether low mass stars can form from
pristine gas in the early Universe. A particular point of interest is whether
we can find surviving pristine stars from the first generation in our local
neighbourhood. We present here a simple analytical estimate that puts tighter
constraints on the existence of such stars. In the conventional picture, should
these stars have formed in significant numbers and have preserved their
pristine chemical composition until today, we should have found them already.
With the presented method most current predictions for survivor counts larger
than zero can be ruled out.Comment: 5 pages, 2 figures, accepted for publication in MNRA
Descendants of the first stars: the distinct chemical signature of second generation stars
Extremely metal-poor (EMP) stars in the Milky Way (MW) allow us to infer the
properties of their progenitors by comparing their chemical composition to the
metal yields of the first supernovae. This method is most powerful when applied
to mono-enriched stars, i.e. stars that formed from gas that was enriched by
only one previous supernova. We present a novel diagnostic to identify this
subclass of EMP stars. We model the first generations of star formation
semi-analytically, based on dark matter halo merger trees that yield MW-like
halos at the present day. Radiative and chemical feedback are included
self-consistently and we trace all elements up to zinc. Mono-enriched stars
account for only of second generation stars in our fiducial model
and we provide an analytical formula for this probability. We also present a
novel analytical diagnostic to identify mono-enriched stars, based on the metal
yields of the first supernovae. This new diagnostic allows us to derive our
main results independently from the specific assumptions made regarding Pop III
star formation, and we apply it to a set of observed EMP stars to demonstrate
its strengths and limitations. Our results may provide selection criteria for
current and future surveys and therefore contribute to a deeper understanding
of EMP stars and their progenitors.Comment: 18 pages, 20 figures, published in MNRA
Bounds on neutrino masses from leptogenesis in type-II see-saw models
The presence of the triplet in left-right symmetric theories
leads to type-II see-saw mechanism for the neutrino masses. In these models,
assuming a normal mass hierarchy for the heavy Majorana neutrinos, we derive a
lower bound on the mass of the lightest of heavy Majorana neutrino from the
leptogenesis constraint. From this bound we establish a consistent picture for
the hierarchy of heavy Majorana neutrinos in a class of left right symmetric
models in which we identify the neutrino Dirac mass matrix with that of
Fritzsch type charged lepton mass matrix. It is shown that these values are
compatible with the current neutrino oscillation data.Comment: minor typos corrected, references added, match with published versio
The chemical composition of globular clusters in the Local Group
We present detailed abundance measurements for 45 globular clusters (GCs) in
galaxies in (and, in one case, beyond) the Local Group. The measurements are
based on new high-resolution integrated-light spectra of GCs in NGC 185, NGC
205, M31, M33, and NGC 2403, combined with reanalysis of previous observations
of GCs in the Fornax dSph, WLM, NGC 147, NGC 6822, and the Milky Way. The GCs
cover the range -2.8 < [Fe/H] < -0.1 and we determined abundances for Fe, Na,
Mg, Si, Ca, Sc, Ti, Cr, Mn, Ni, Cu, Zn, Zr, Ba, and Eu. Corrections for non
local thermodynamic equilibrium effects are included for Na, Mg, Ca, Ti, Mn,
Fe, Ni, and Ba. For several of the galaxies, our measurements provide the first
quantitative constraints on the detailed composition of their metal-poor
stellar populations. Overall, the GCs in different galaxies exhibit remarkably
uniform abundance patterns of the alpha-, iron-peak, and neutron-capture
elements, with a dispersion of less than 0.1 dex in [alpha/Fe] for the full
sample. There is a hint that GCs in dwarf galaxies are slightly less
alpha-enhanced (by about 0.04 dex on average) than those in larger galaxies.
One GC in M33 (HM33-B) resembles the most metal-rich GCs in the Fornax dSph
(Fornax 4) and NGC 6822 (SC7) by having alpha-element abundances closer to
scaled-solar values, possibly hinting at an accretion origin. We find that the
alpha-element abundances strongly correlate with those of Na, Sc, Ni, and Zn.
Several GCs with [Fe/H]<-1.5 are deficient in Mg compared to other
alpha-elements. We find no GCs with strongly enhanced r-process abundances as
reported for metal-poor stars in some ultra-faint dwarfs and the Magellanic
Clouds. The similarity of the abundance patterns for metal-poor GCs in
different environments points to similar early enrichment histories and only
allow for minor variations in the initial mass function.Comment: 34 pages + 6 appendices. Accepted for publication in Astronomy &
Astrophysic
A Minimum Dilution Scenario for Supernovae and Consequences for Extremely Metal-Poor Stars
© 2020 The Author(s) 2020 Published by Oxford University Press on behalf of the Royal Astronomical Society.To date no metal-free stars have been identified by direct observations. The most common method of constraining their properties is searching the spectra of the most metal-poor stars for the chemical elements created in the first stars and their supernova (SN). In this approach, modelled SN yields are compared to the observed abundance patterns in extremely metal-poor stars. The method typically only uses the abundance ratios, i.e. the yields are diluted to the observed level. Following the usual assumption of spherical symmetry we compute a simple lower limit of the mass an SN can mix with and find that it is consistent with all published simulations of early chemical enrichment in the interstellar medium. For three different cases, we demonstrate that this dilution limit can change the conclusions from the abundance fitting. There is a large discrepancy between the dilution found in simulations of SN explosions in minihaloes and the dilution assumed in many abundance fits. Limiting the dilution can significantly alter the likelihood of which supernovae are possible progenitors of observed CEMP-no stars. In particular, some of the faint, very low yield SNe, which have been suggested as models for the abundance pattern of SMSS0313-6708, cannot explain the measured metal abundances, as their predicted metal yields are too small by two orders of magnitude. Altogether, the new dilution model presented here emphasizes the need to better understand the mixing and dilution behaviour of aspherical SNe.Peer reviewedFinal Accepted Versio
Minimal Supersymmetric Pati-Salam Theory: Determination of Physical Scales
We systematically study the minimal supersymmetric Pati-Salam theory, paying
special attention to the unification constraints. We find that the SU(4)_c
scale M_c and the Left-Right scale M_R lie in the range 10^{10} GeV < M_c <
10^{14} GeV, 10^{3} GeV < M_R <10^{10} GeV (with single-step breaking at
10^{10} GeV), giving a potentially accessible scale of parity breaking. The
theory includes the possibility of having doubly-charged supermultiplets at the
supersymmetry breaking scale; color octet states with mass of order M_R^2/M_c;
magnetic monopoles of intermediate mass that do not conflict with cosmology,
and a 'clean' (type I) form for the see-saw mechanism of neutrino mass.Comment: 5 page
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