10,454 research outputs found
Horizontal-branch morphology and multiple stellar populations in the anomalous globular cluster M22
M22 is an anomalous globular cluster that hosts two groups of stars with
different metallicity and s-element abundance. The star-to-star light-element
variations in both groups, with the presence of individual Na-O and C-N
anticorrelations, demonstrates that this Milky-Way satellite has experienced a
complex star-formation history. We have analysed FLAMES/UVES spectra for seven
stars covering a small color interval, on the reddest horizontal-branch (HB)
portion of this cluster and investigated possible relations between the
chemical composition of a star and its location along the HB. Our chemical
abundance analysis takes into account effects introduced by deviations from the
local-thermodynamic equilibrium (NLTE effects), that are significant for the
measured spectral lines in the atmospheric parameters range spanned by our
stars. We find that all the analysed stars are barium-poor and sodium-poor,
thus supporting the idea that the position of a star along the HB is strictly
related to the chemical composition, and that the HB-morphology is influenced
by the presence of different stellar populations.Comment: 21 pages, 3 figures, accepted for publication in Ap
Charge and Magnetic Flux Correlations in Chern-Simons Theory with Fermions
Charge and magnetic flux bearing operators are introduced in Chern-Simons
theory both in its pure form and when it is coupled to fermions. The magnetic
flux creation operator turns out to be the Wilson line. The euclidean
correlation functions of these operators are shown to be local and are
evaluated exactly in the pure case and through an expansion in the inverse
fermion mass whenever these are present. Physical states only occur in the
presence of fermions and consist of composite charge-magnetic flux carrying
states which are in general anyonic. The large distance behavior of the
correlation functions indicates the condensation of charge and magnetic flux.Comment: Latex, 17 page
The oxygen vs. sodium (anti)correlation(s) in omega Cen
Recent exam of large samples of omega Cen giants shows that it shares with
mono-metallic globular clusters the presence of the sodium versus oxygen
anticorrelation, within each subset of stars with iron content in the range
-1.9<~[Fe/H]<~-1.3. These findings suggest that, while the second generation
formation history in omega Cen is more complex than that of mono-metallic
clusters, it shares some key steps with those simpler cluster. In addition, the
giants in the range -1.3<[Fe/H]<~-0.7 show a direct O--Na correlation, at
moderately low O, but Na up to 20 times solar. These peculiar Na abundances are
not shared by stars in other environments often assumed to undergo a similar
chemical evolution, such as in the field of the Sagittarius dwarf galaxy. These
O and Na abundances match well the yields of the massive asymptotic giant
branch stars (AGB) in the same range of metallicity, suggesting that the stars
at [Fe/H]>-1.3 in omega Cen are likely to have formed directly from the pure
ejecta of massive AGBs of the same metallicities. This is possible if the
massive AGBs of [Fe/H]>-1.3 in the progenitor system evolve when all the
pristine gas surrounding the cluster has been exhausted by the previous star
formation events, or the proto--cluster interaction with the Galaxy caused the
loss of a significant fraction of its mass, or of its dark matter halo, and the
supernova ejecta have been able to clear the gas out of the system. The absence
of dilution in the metal richer populations lends further support to a scenario
of the formation of second generation stars in cooling flows from massive AGB
progenitors. We suggest that the entire formation of omega Cen took place in a
few 10^8yr, and discuss the problem of a prompt formation of s--process
elements.Comment: The Astrophysical Journal, in pres
A unique model for the variety of multiple populations formation(s) in globular clusters: a temporal sequence
We explain the multiple populations recently found in the 'prototype'
Globular Cluster (GC) NGC 2808 in the framework of the asymptotic giant branch
(AGB) scenario. The chemistry of the five -or more- populations is
approximately consistent with a sequence of star formation events, starting
after the supernovae type II epoch, lasting approximately until the time when
the third dredge up affects the AGB evolution (age ~90-120Myr), and ending when
the type Ia supernovae begin exploding in the cluster, eventually clearing it
from the gas. The formation of the different populations requires episodes of
star formation in AGB gas diluted with different amounts of pristine gas. In
the nitrogen-rich, helium-normal population identified in NGC 2808 by the UV
Legacy Survey of GCs, the nitrogen increase is due to the third dredge up in
the smallest mass AGB ejecta involved in the star formation of this population.
The possibly-iron-rich small population in NGC 2808 may be a result of
contamination by a single type Ia supernova. The NGC 2808 case is used to build
a general framework to understand the variety of 'second generation' stars
observed in GCs. Cluster-to-cluster variations are ascribed to differences in
the effects of the many processes and gas sources which may be involved in the
formation of the second generation. We discuss an evolutionary scheme, based on
pollution by delayed type II supernovae, which accounts for the properties of
s-Fe-anomalous clusters.Comment: 20 pages, 7 figures, in press on MNRA
Large-scale anisotropy in stably stratified rotating flows
We present results from direct numerical simulations of the Boussinesq
equations in the presence of rotation and/or stratification, both in the
vertical direction. The runs are forced isotropically and randomly at small
scales and have spatial resolutions of up to grid points and Reynolds
numbers of . We first show that solutions with negative energy
flux and inverse cascades develop in rotating turbulence, whether or not
stratification is present. However, the purely stratified case is characterized
instead by an early-time, highly anisotropic transfer to large scales with
almost zero net isotropic energy flux. This is consistent with previous studies
that observed the development of vertically sheared horizontal winds, although
only at substantially later times. However, and unlike previous works, when
sufficient scale separation is allowed between the forcing scale and the domain
size, the total energy displays a perpendicular (horizontal) spectrum with
power law behavior compatible with , including in the
absence of rotation. In this latter purely stratified case, such a spectrum is
the result of a direct cascade of the energy contained in the large-scale
horizontal wind, as is evidenced by a strong positive flux of energy in the
parallel direction at all scales including the largest resolved scales
Evidence for Bolgiano-Obukhov scaling in rotating stratified turbulence using high-resolution direct numerical simulations
We report results on rotating stratified turbulence in the absence of
forcing, with large-scale isotropic initial conditions, using direct numerical
simulations computed on grids of up to 4096^3 points. The Reynolds and Froude
numbers are respectively equal to Re=5.4 x 10^4 and Fr=0.0242. The ratio of the
Brunt-V\"ais\"al\"a to the inertial wave frequency, N/f, is taken to be equal
to 4.95, a choice appropriate to model the dynamics of the southern abyssal
ocean at mid latitudes. This gives a global buoyancy Reynolds number
R_B=ReFr^2=32, a value sufficient for some isotropy to be recovered in the
small scales beyond the Ozmidov scale, but still moderate enough that the
intermediate scales where waves are prevalent are well resolved. We concentrate
on the large-scale dynamics, for which we find a spectrum compatible with the
Bolgiano-Obukhov scaling, and confirm that the Froude number based on a typical
vertical length scale is of order unity, with strong gradients in the vertical.
Two characteristic scales emerge from this computation, and are identified from
sharp variations in the spectral distribution of either total energy or
helicity. A spectral break is also observed at a scale at which the partition
of energy between the kinetic and potential modes changes abruptly, and beyond
which a Kolmogorov-like spectrum recovers. Large slanted layers are ubiquitous
in the flow in the velocity and temperature fields, with local overturning
events indicated by small Richardson numbers, and a small large-scale
enhancement of energy directly attributable to the effect of rotation is also
observed.Comment: 19 pages, 9 figures (including compound figures
Four stellar populations and extreme helium variation in the massive outer-halo globular cluster NGC 2419
Recent work revealed that both the helium variation within globular clusters
(GCs) and the relative numbers of first and second-generation stars (1G, 2G)
depend on the mass of the host cluster. Precise determination of the internal
helium variations and of the fraction of 1G stars are crucial constraints to
the formation scenarios of multiple populations (MPs). We exploit multi-band
Hubble Space Telescope photometry to investigate MPs in NGC 2419, which is one
of the most-massive and distant GCs of the Galaxy, almost isolated from its
tidal influence. We find that the 1G hosts the ~37% of the analyzed stars, and
identified three populations of 2G stars, namely 2GA, 2GB, and 2GC, which
comprise the ~20%, ~31% and ~12% of stars, respectively. We compare the
observed colors of these four populations with the colors derived from
appropriate synthetic spectra to infer the relative helium abundances. We find
that 2GA, 2GB, and 2GC stars are enhanced in helium mass fraction by deltaY
~0.01, 0.06, and 0.19 with respectto 1G stars that have primordial helium
(Y=0.246). The high He enrichment of 2GC stars is hardly reconcilable with most
of the current scenarios for MPs. Furthermore, the relatively larger fraction
of 1G stars (~37%) compared to other massive GCs is noticeable. By exploiting
literature results, we find that the fractions of 1G stars of GCs with large
perigalactic distance are typically higher than in the other GCs with similar
masses. This suggests that NGC 2419, similarly to other distant GCs, lost a
lower fraction of 1G stars.Comment: 10 pages, 8 figures, submitted to MNRAS January 22n
Observing multiple stellar populations with FORS2@VLT - Main sequence photometry in outer regions of NGC 6752, NGC 6397, and NGC 6121 (M 4)
We present the photometric analysis of the external regions of three Galactic
Globular Clusters: NGC 6121, NGC 6397 and NGC 6752. The main goal is the
characterization of the multiple stellar populations along the main sequence
(MS) and the study of the radial trend of the different populations hosted by
the target clusters. The data have been collected using FORS2 mounted at the
ESO/VLT@UT1 telescope in UBVI filters. From these data sets we extracted
high-accuracy photometry and constructed color-magnitude diagrams. We exploit
appropriate combination of colors and magnitudes which are powerful tools to
identify multiple stellar populations, like B versus U-B and V versus
c_{U,B,I}=(U-B)-(B-I) CMDs. We confirm previous findings of a split MS in NGC
6752 and NGC 6121. Apart from the extreme case of omega Centauri, this is the
first detection of multiple MS from ground-based photometry. For NGC 6752 and
NGC 6121 we compare the number ratio of the blue MS to the red MS in the
cluster outskirts with the fraction of first and second generation stars
measured in the central regions. There is no evidence for significant radial
trend. The MS of NGC 6397 is consistent with a simple stellar population. We
propose that the lack of multiple sequences is due both to observational errors
and to the limited sensitivity of U,B,V,I photometry to multiple stellar
populations in metal-poor GCs. Finally, we compute the helium abundance for the
stellar populations hosted by NGC 6121 and NGC 6752, finding a mild (Delta Y ~
0.02) difference between stars in the two sequences.Comment: 16 pages, 5 tables, 17 figures, accepted for pubblication in A&
Different stellar rotation in the two main sequences of the young globular cluster NGC1818: first direct spectroscopic evidence
We present a spectroscopic analysis of main sequence (MS) stars in the young
globular cluster NGC1818 (age~40 Myrs) in the Large Magellanic Cloud. Our
photometric survey on Magellanic Clouds clusters has revealed that NGC1818,
similarly to the other young objects with age 600 Myrs, displays not only an
extended MS Turn-Off (eMSTO), as observed in intermediate-age clusters (age~1-2
Gyrs), but also a split MS. The most straightforward interpretation of the
double MS is the presence of two stellar populations: a sequence of
slowly-rotating stars lying on the blue-MS and a sequence of fast rotators,
with rotation close to the breaking speed, defining a red-MS. We report the
first direct spectroscopic measurements of projected rotational velocities
vsini for the double MS, eMSTO and Be stars of a young cluster. The analysis of
line profiles includes non-LTE effects, required for correctly deriving v sini
values. Our results suggest that: (i) the mean rotation for blue- and red-MS
stars is vsini=71\pm10 km/s (sigma=37 km/s) and vsini=202\pm23 km/s (sigma=91
km/s), respectively; (ii) eMSTO stars have different vsini, which are generally
lower than those inferred for red-MS stars, and (iii) as expected, Be stars
display the highest vsini values. This analyis supports the idea that distinct
rotational velocities play an important role in the appearence of multiple
stellar populations in the color-magnitude diagrams of young clusters, and
poses new constraints to the current scenarios.Comment: 16 pages, 1 table, 9 figures. Accepted for publication in AJ
(11/07/2018
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