35 research outputs found
The influence of nova nucleosynthesis on the chemical evolution of the Galaxy
We adopt up-to-date yields of 7Li, 13C, 15N from classical novae and use a
well tested model for the chemical evolution of the Milky Way in order to
predict the temporal evolution of these elemental species in the solar
neighborhood. In spite of major uncertainties due to our lack of knowledge of
metallicity effects on the final products of explosive nucleosynthesis in nova
outbursts, we find a satisfactory agreement between theoretical predictions and
observations for 7Li and 13C. On the contrary, 15N turns out to be overproduced
by about an order of magnitude.Comment: 8 pages, latex, 3 figures. To appear in "The Chemical Evolution of
the Milky Way: Stars versus Clusters", eds. F. Giovannelli and F. Matteucci
(Kluwer: Dordrecht
The ESO UVES Advanced Data Products Quasar Sample: Neutral Gas Mass and Metal Abundances in the Universe
Damped Lyα absorbers (DLAs), seen in the spectra of background quasars, are unique probes to select HI-rich galaxies. We selected a dataset of 250 quasars observed with the Ultraviolet Visual Echelle Spectrograph (UVES) and available through the ESO UVES Advanced Data Products (EUADP) archive, to study the gas and metal properties of 150 damped absorbers. These high-redshift absorbers contain information on the physical state and chemical composition of the interstellar medium and the neutral gas mass, a possible indicator of gas consumption as star formation proceeds. We find no evolution of the neutral gas mass density, with sub-DLAs contributing 8-20% (increasing with redshift). The EUADP dataset provides insights into the nucleosynthetic origin of nitrogen, confirming the bimodal behaviour of [N/α], and also confirms the deficiency of argon in DLAs. <P /
Early stages of Nitrogen enrichment in galaxies: Clues from measurements in damped Lyman alpha systems
We present 4 new measurements of nitrogen abundances and one upper limit in
damped Lyman alpha absorbers (DLAs) obtained by means of high resolution (FWHM
\~ 7 km/s) UVES/VLT spectra. In addition to these measurements we have compiled
data from all DLAs with measurements of nitrogen and alpha-capture elements (O,
S or Si) available in the literature, including all HIRES/Keck and UVES/VLT
data for a total of 33 systems, i.e. the largest sample investigated so far. We
find that [N/alpha] ratios are distributed in two groups: 75% of the DLAs show
a mean value of [N/alpha] = -0.87 with a scatter of 0.16 dex, while the
remaining 25% shows ratios clustered at [N/alpha] = -1.45 with an even lower
dispersion of 0.05 dex. The high [N/alpha] plateau is consistent with the one
observed in metal-poor HII regions of blue compact dwarf (BCD) galaxies
([N/alpha] = -0.73 +/- 0.13), while the [N/alpha] ~ -1.5 values are the lowest
ever observed in any astrophysical site. These low [N/alpha] ratios are real
and not due to ionization effects. They provide a crucial evidence against the
primary production of N by massive stars as responsible for the plateau at
-0.9/-0.7 dex observed in DLAs and BCD galaxies. The transition between the
low-N ([N/alpha] ~ -1.5) and high-N ([N/alpha] ~ -0.9) DLAs occurs at a
nitrogen abundance of [N/H] ~ -2.8, suggesting that the separation may result
from some peculiarity of the nitrogen enrichment history. The [N/alpha] ~ -1.5
values and their low dispersion are consistent with a modest production of
primary N in massive stars; however, due to the limited sample, specially for
the low-N DLAs, we cannot exclude a primary origin in intermediate mass stars
as responsible for the low N abundances observed.Comment: Accepted by A&A. 25 pages, including 9 tables and 10 figure
First Stars XIV. Sulfur abundances in extremely metal-poor (EMP) stars
Sulfur is important: the site of its formation is uncertain, and at very low
metallicity the trend of [S/Fe] against [Fe/H] is controversial. Below
[Fe/H]=-2.0, [S/Fe] remains constant or it decreases with [Fe/H], depending on
the author and the multiplet used in the analysis. Moreover, although sulfur is
not significantly bound in dust grains in the ISM, it seems to behave
differently in DLAs and in old metal-poor stars. We aim to determine precise S
abundance in a sample of extremely metal-poor stars taking into account NLTE
and 3D effects. NLTE profiles of the lines of the multiplet 1 of SI have been
computed using a new model atom for S. We find sulfur in EMP stars to behave
like the other alpha-elements, with [S/Fe] remaining approximately constant for
[Fe/H]<-3. However, [S/Mg] seems to decrease slightly as a function of [Mg/H].
The overall abundance patterns of O, Na, Mg, Al, S, and K are best matched by
the SN model yields by Heger & Woosley. The [S/Zn] ratio in EMP stars is solar,
as found also in DLAs. We obtain an upper limit on the abundance of sulfur,
[S/Fe] < +0.5, for the ultra metal-poor star CS 22949-037. This, along with a
previous reported measurement of zinc, argues against the conjecture that the
light-element abundances pattern in this star, and, by analogy, the hyper
metal-poor stars HE 0107-5240 and HE 1327-2326, are due to dust depletion.Comment: 10 pages, accepted for publication in A&
Clues to the nature of damped Lyman alpha systems from chemical evolution models
The evolution of the metallicity of damped Lyman alpha systems (DLAs) is
investigated in order to understand the nature of these systems. The
observational data on chemical abundances of DLAs are analysed with robust
statistical methods, and the abundances are corrected for dust depletion. The
results of this analysis are compared to predictions of several classes of
chemical evolution models: one-zone dwarf galaxy models, multizone disk models,
and chemodynamical models representing dwarf galaxies. We compare the
observational data on the [alpha/Fe] and [N/alpha] ratios to the predictions
from the models. In DLAs, these ratios are only partially reproduced by the
dwarf galaxy one-zone model and by the disk model. On the other hand, the
chemodynamical model for dwarf galaxies reproduces the properties of nearly all
DLAs. We derive the formation epoch of dwarf galaxies, and we find that dwarf
galaxies make a significant contribution to the total neutral gas density in
DLAs, and that this contribution is more important at high redshifts (z > 2-3).
We propose a scenario in which the DLA population is dominated by dwarf
galaxies at high redshifts and by disks at lower redshifts. We also find that
Lyman Break Galaxies (LBGs) may constitute a sequence rather than present a
sharp dichotomy between the two populations. We also arise the possibility that
we could be missing a whole population of high HI density column objects, with
metallicities intermediate between those of DLAs and LBGs. Finally, we discuss
the possibility that relying only on the observations of DLAs could lead to an
underestimate of the metal content of the high redshift Universe.Comment: 23 pages, 19 figures. Accepted for publication in MNRA
Spectral shape of the UV ionizing background and OVI absorbers at z ~ 1.5 towards HS0747+4259
We report on high resolution spectra of the bright QSO HS0747+4259 (zem =
1.90, V = 15.8) observed to search for intermediate redshift OVI absorption
systems. The spectra were obtained by means of the Space Telescope Imaging
Spectrograph (STIS) at the Hubble Space Telescope (HST) and the High Resolution
Echelle Spectrometer (HIRES) at the W. M. Keck telescope. We identify 16 OVI
systems in the range 1.07 <= z <= 1.87. Among them, six systems with zabs =
1.46-1.8 exhibit a sufficient number of lines of different ionic transitions to
estimate the shape of the ionizing radiation field in the range 1 Ryd < E < 10
Ryd. All recovered UV ionizing spectra are characterized by the enhanced
intensity at E > 3 Ryd compared to the model spectrum of Haardt and Madau
(1996). This is in line with the observational evidence of a deficiency of
strong Ly-alpha absorbers with N(HI) > 10^{15} cm^{-2}, at z < 2. The UV
background shows significant local variations: the spectral shape estimated at
z = 1.59 differs from that obtained at z = 1.81 and 1.73. A possible cause of
these variations is the presence of a QSO/AGN at z ~= 1.54-1.59 close to the
line of sight. No features favoring the input of stellar radiation to the
ionizing background are detected, limiting the escape fraction of the galactic
UV photons to f_esc < 0.05.Comment: 14 pages, 7 figures, 5 tables; accepted for publication in A&
Heavy elements abundances in turn-off stars and early subgiants in NGC 6752
High resolution spectra (R > 40 000) for 9 main sequence turn-off stars and 9
subgiants in the globular cluster NGC 6752 were acquired with UVES on the
VLT-Kueyen (UT2). These data have been used to determine the abundances of some
heavy elements (strontium, yttrium, barium and europium). This paper presents
for the first time accurate results for heavy elements in this globular
cluster. We did not find any systematic effect between the abundances found in
turn-off stars, subgiants, and giants. We obtain the following mean abundances
for these elements in our sample (turn-off stars and subgiants): [Sr/Fe] = 0.06
+/- 0.16, [Y/Fe] = -0.01 +/- 0.12, [Ba/Fe] = 0.18 +/- 0.11, and [Eu/Fe] = 0.41
+/- 0.09. The dispersion in the abundance ratios of the different elements is
low and can be totally explained by uncertainties in their derivation. These
ratios are in agreement with the values found in field halo stars with the same
metallicity. We did not observe any correlation between the [n-capture/Fe]
ratios and the star-to-star variations of the O and Na abundances. The [Ba/Eu]
and [Sr/Ba] ratios show clearly that this globular cluster has also been
uniformly enriched by r- and s-process synthesis.Comment: 10 pages, 8 figures. Accepted for publication in A&
Current and prospective pharmacological targets in relation to antimigraine action
Migraine is a recurrent incapacitating neurovascular disorder characterized by unilateral and throbbing headaches associated with photophobia, phonophobia, nausea, and vomiting. Current specific drugs used in the acute treatment of migraine interact with vascular receptors, a fact that has raised concerns about their cardiovascular safety. In the past, α-adrenoceptor agonists (ergotamine, dihydroergotamine, isometheptene) were used. The last two decades have witnessed the advent of 5-HT1B/1D receptor agonists (sumatriptan and second-generation triptans), which have a well-established efficacy in the acute treatment of migraine. Moreover, current prophylactic treatments of migraine include 5-HT2 receptor antagonists, Ca2+ channel blockers, and β-adrenoceptor antagonists. Despite the progress in migraine research and in view of its complex etiology, this disease still remains underdiagnosed, and available therapies are underused. In this review, we have discussed pharmacological targets in migraine, with special emphasis on compounds acting on 5-HT (5-HT1-7), adrenergic (α1, α2, and β), calcitonin gene-related peptide (CGRP 1 and CGRP2), adenosine (A1, A2, and A3), glutamate (NMDA, AMPA, kainate, and metabotropic), dopamine, endothelin, and female hormone (estrogen and progesterone) receptors. In addition, we have considered some other targets, including gamma-aminobutyric acid, angiotensin, bradykinin, histamine, and ionotropic receptors, in relation to antimigraine therapy. Finally, the cardiovascular safety of current and prospective antimigraine therapies is touched upon
Varying constants, Gravitation and Cosmology
Fundamental constants are a cornerstone of our physical laws. Any constant
varying in space and/or time would reflect the existence of an almost massless
field that couples to matter. This will induce a violation of the universality
of free fall. It is thus of utmost importance for our understanding of gravity
and of the domain of validity of general relativity to test for their
constancy. We thus detail the relations between the constants, the tests of the
local position invariance and of the universality of free fall. We then review
the main experimental and observational constraints that have been obtained
from atomic clocks, the Oklo phenomenon, Solar system observations, meteorites
dating, quasar absorption spectra, stellar physics, pulsar timing, the cosmic
microwave background and big bang nucleosynthesis. At each step we describe the
basics of each system, its dependence with respect to the constants, the known
systematic effects and the most recent constraints that have been obtained. We
then describe the main theoretical frameworks in which the low-energy constants
may actually be varying and we focus on the unification mechanisms and the
relations between the variation of different constants. To finish, we discuss
the more speculative possibility of understanding their numerical values and
the apparent fine-tuning that they confront us with.Comment: 145 pages, 10 figures, Review for Living Reviews in Relativit