The early Universe had a chemical composition consisting of hydrogen, helium and traces of lithium; almost all other elements were subsequently created in stars and supernovae. The mass fraction of ...elements more massive than helium, Z, is known as 'metallicity'. A number of very metal-poor stars has been found, some of which have a low iron abundance but are rich in carbon, nitrogen and oxygen. For theoretical reasons and because of an observed absence of stars with Z < 1.5 × 10(-5), it has been suggested that low-mass stars cannot form from the primitive interstellar medium until it has been enriched above a critical value of Z, estimated to lie in the range 1.5 × 10(-8) to 1.5 × 10(-6) (ref. 8), although competing theories claiming the contrary do exist. (We use 'low-mass' here to mean a stellar mass of less than 0.8 solar masses, the stars that survive to the present day.) Here we report the chemical composition of a star in the Galactic halo with a very low Z (≤ 6.9 × 10(-7), which is 4.5 × 10(-5) times that of the Sun) and a chemical pattern typical of classical extremely metal-poor stars--that is, without enrichment of carbon, nitrogen and oxygen. This shows that low-mass stars can be formed at very low metallicity, that is, below the critical value of Z. Lithium is not detected, suggesting a low-metallicity extension of the previously observed trend in lithium depletion. Such lithium depletion implies that the stellar material must have experienced temperatures above two million kelvin in its history, given that this is necessary to destroy lithium.
Galileo’s Self-Portrait Molaro, Paolo
Colle di Galileo,
01/2016, Letnik:
5, Številka:
1
Journal Article
Recenzirano
Before becoming the great scientist that everyone knows and the founding father of modern science, the young Galileo Galilei was nurtured within a typically Renaissance context, acquiring a marked ...artistic sensibility which transpires in his literary style and even in his scientific work. Galileo was indeed a musician, an intellectual and also a painter, as consensually recorded in his first biographies and as illustrated by watercolours of his first observations of the Moon and his drawings of the sunspots. However there is also a somewhat astonishing reference to the existence of an actual self-portrait, as recorded by Thomas Salusbury (1664) in what continues to be the first biography of Galileo to have been published. Recently a possible new youthful portrait of Galileo Galilei has been proposed. The remarkable similarity of this new portrait with those of Domenico Tintoretto (circa 1604), Santi di Tito (1601, via Calendi) and Furini (1612) has recently been confirmed using sophisticated face recognition techniques (Srinivasan et al 2015, Rudolph et al 2015). Were the identity to be definitively ascertained, other elements which emerged from the infrared analysis of the painting – such as the young age or the restitched canvas – would appear to suggest a connection with the very work mentioned by Thomas Salusbury.
The strong intervening absorption system at redshift 1.15 towards the very bright quasar HE 0515−4414 is the most studied absorber for measuring possible cosmological variations in the fine-structure ...constant,
α
. We observed HE 0515−4414 for 16.1 h with the Very Large Telescope and present here the first constraint on relative variations in
α
with parts-per-million (ppm) precision from the new ESPRESSO spectrograph: Δ
α
/
α
= 1.3 ± 1.3
stat
± 0.4
sys
ppm. The statistical uncertainty (1
σ
) is similar to the ensemble precision of previous large samples of absorbers and derives from the high signal-to-noise ratio achieved (≈105 per 0.4 km s
−1
pixel). ESPRESSO’s design, and the calibration of our observations with its laser frequency comb, effectively removed wavelength calibration errors from our measurement. The high resolving power of our ESPRESSO spectrum (
R
= 145 000) enabled the identification of very narrow components within the absorption profile, allowing a more robust analysis of Δ
α
/
α
. The evidence for the narrow components is corroborated by their correspondence with previously detected molecular hydrogen and neutral carbon. The main remaining systematic errors arise from ambiguities in the absorption profile modelling, effects from redispersing the individual quasar exposures, and convergence of the parameter estimation algorithm. All analyses of the spectrum, including systematic error estimates, were initially blinded to avoid human biases. We make our reduced ESPRESSO spectrum of HE 0515−4414 publicly available for further analysis. Combining our ESPRESSO result with 28 measurements, from other spectrographs, in which wavelength calibration errors have been mitigated yields a weighted mean Δ
α
/
α
= −0.5 ± 0.5
stat
± 0.4
sys
ppm at redshifts 0.6−2.4.
In his biography of Titian Giorgio Vasari reports the existence of a self-portrait made by the painter before 1541 which is now lost. Recently, the sitter of an ancient painting has been noted to ...show resemblance with Titian’s lineaments. A study by means of Macro X-Ray Fluorescence and UV-IR imaging of this painting has been conducted seeking elements useful to verify whether it could be the lost portrait mentioned by Vasari. The analysis provided consistency in terms of pigments and absence of underlying drawings with Titian’s style, but also revealed extensive restoration which prevents firm attribution. Stringent analogies of the painting with the portrait of Pietro Aretino made by Titian in 1537 are also noted.
Cosmological evolution of the nitrogen abundance Vangioni, Elisabeth; Dvorkin, Irina; Olive, Keith A ...
Monthly notices of the Royal Astronomical Society,
06/2018, Letnik:
477, Številka:
1
Journal Article
Recenzirano
Odprti dostop
Abstract
The abundance of nitrogen in the interstellar medium is a powerful probe of star formation processes over cosmological time-scales. Since nitrogen can be produced both in massive and ...intermediate-mass stars with metallicity-dependent yields, its evolution is challenging to model, as evidenced by the differences between theoretical predictions and observations. In this work, we attempt to identify the sources of these discrepancies using a cosmic evolution model. To further complicate matters, there is considerable dispersion in the abundances from observations of damped Lyα absorbers (DLAs) at z ∼ 2–3. We study the evolution of nitrogen with a detailed cosmic chemical evolution model and find good agreement with these observations, including the relative abundances of (N/O) and (N/Si). We find that the principal contribution of nitrogen comes from intermediate-mass stars, with the exception of systems with the lowest N/H, where nitrogen production might possibly be dominated by massive stars. This last result could be strengthened if stellar rotation which is important at low metallicity can produce significant amounts of nitrogen. Moreover, these systems likely reside in host galaxies with stellar masses below 108.5 M⊙. We also study the origin of the observed dispersion in nitrogen abundances using the cosmological hydrodynamical simulations Horizon-AGN. We conclude that this dispersion can originate from two effects: difference in the masses of the DLA host galaxies, and difference in their position inside the galaxy.
Using a high-resolution spectrum of the secondary star in the neutron star binary Cen X-4, we have derived the stellar parameters and veiling caused by the accretion disk in a consistent way. We have ...used a j super(2) minimization procedure to explore a grid of 1,500,000 LTE synthetic spectra computed for a plausible range of both stellar and veiling parameters. Adopting the best model parameters found, we have determined atmospheric abundances of Fe, Ca, Ti, Ni, and Al. These element abundances are supersolar (Fe/H = 0.23 c 0.10), but only the abundances of Ti and Ni appear to be moderately enhanced (.1 s) as compared with the average values of stars of similar iron content. These element abundances can be explained if the secondary star captured a significant amount of matter ejected from a spherically symmetric supernova (SN) explosion of a 4 M sub(z) He core progenitor and if we assume solar abundances as primordial abundances in the secondary star. The kinematic properties of the system indicate that the neutron star received a natal kick velocity through an aspherical SN and/or an asymmetric neutrino emission. The former scenario might be ruled out, since our model computations cannot produce acceptable fits to the observed abundances. We have also examined whether this system could have formed in the Galactic halo, and our simulations show that this possibility seems unlikely. We also report a new determination of the Li abundance, consistent with previous studies, that is unusually high and close to the cosmic Li abundance in the Galactic disk.
Detection of 7Be ii in the Small Magellanic Cloud Izzo, Luca; Molaro, Paolo; Cescutti, Gabriele ...
Monthly notices of the Royal Astronomical Society,
01/2022, Letnik:
510, Številka:
4
Journal Article
Recenzirano
Odprti dostop
ABSTRACT
We analyse high-resolution spectra of two classical novae that exploded in the Small Magellanic Cloud (SMC). 7Be ii resonance transitions are detected in both ASASSN-19qv and ASASSN-20ni ...novae. This is the first detection outside the Galaxy and confirms that thermo-nuclear runaway reactions, leading to the 7Be formation, are effective also in the low-metallicity regime, characteristic of the SMC. Derived yields are of N(7Be = 7Li)/N(H) = (5.3 ± 0.2) × 10−6 which are a factor 4 lower than the typical values of the Galaxy. Inspection of two historical novae in the Large Magellanic Cloud observed with IUE in 1991 and 1992 showed also the possible presence of 7Be and similar yields. For an ejecta of MH, ej = 10−5 M⊙, the amount of 7Li produced is of $M_{^7 Li} = (3.7 \pm 0.6) \times 10^{-10}$ M⊙ per nova event. Detailed chemical evolutionary model for the SMC shows that novae could have made an amount of lithium in the SMC corresponding to a fractional abundance of A(Li) ≈ 2.6. Therefore, it is argued that a comparison with the abundance of Li in the SMC, as measured by its interstellar medium, could effectively constrain the amount of the initial abundance of primordial Li, which is currently controversial.
We report here a study of nitrogen and α-capture element (O, S, and Si) abundances in 18 damped Lyα absorbers (DLAs) and sub-DLAs drawn from the European Southern Observatory's Ultraviolet Visual ...Echelle Spectrograph (ESO-UVES) Advanced Data Products (EUADP) data base. We report nine new measurements, five upper and four lower limits of nitrogen that when compiled with available nitrogen measurements from the literature makes a sample of 108 systems. The extended sample presented here confirms the N/α bimodal behaviour suggested in previous studies. Three-quarter of the systems show 〈N/α〉 = −0.85 (±0.20 dex) and one-quarter of the systems show that ratios are clustered at 〈N/α〉 = −1.41 (±0.14 dex). The high N/α plateau is consistent with the H ii regions of dwarf irregular and blue compact dwarf galaxies although extended to lower metallicities and could be interpreted as the result of a primary nitrogen production by intermediate mass stars. The low N/α values are the lowest ever observed in any astrophysical site. In spite of this fact, even lower values could be measured with the present instrumentation, but we do not find them below N/α ≈ −1.7. This suggests the presence of a floor in N/α abundances, which along with the lockstep increase of N and Si may indicate a primary nitrogen production from fast rotating, massive stars in relatively young or unevolved systems.
The redshifts of all cosmologically distant sources are expected to experience a small, systematic drift as a function of time due to the evolution of the Universe's expansion rate. A measurement of ...this effect would represent a direct and entirely model-independent determination of the expansion history of the Universe over a redshift range that is inaccessible to other methods. Here we investigate the impact of the next generation of Extremely Large Telescopes on the feasibility of detecting and characterizing the cosmological redshift drift. We consider the Lyα forest in the redshift range 2 < z < 5 and other absorption lines in the spectra of high-redshift QSOs as the most suitable targets for a redshift drift experiment. Assuming photon-noise-limited observations and using extensive Monte Carlo simulations we determine the accuracy to which the redshift drift can be measured from the Lyα forest as a function of signal-to-noise ratio and redshift. Based on this relation and using the brightness and redshift distributions of known QSOs we find that a 42-m telescope is capable of unambiguously detecting the redshift drift over a period of ∼20 yr using 4000 h of observing time. Such an experiment would provide independent evidence for the existence of dark energy without assuming spatial flatness, using any other cosmological constraints or making any other astrophysical assumption.
ABSTRACT
Primordial abundances of light elements are sensitive to the physics of the early Universe and can directly constrain cosmological quantities, such as the baryon-to-photon ratio $\eta ..._{10}$, the baryon density, and the number of neutrino families. Deuterium is especially suited for these studies: its primordial abundance is sensitive and monotonically dependent on $\eta _{10}$, allowing an independent measurement of the cosmic baryon density that can be compared, for instance, against the Planck satellite data. The primordial deuterium abundance can be measured in high H i column density absorption systems towards distant quasars. We report here a new measurement, based on high-resolution ESPRESSO data, of the primordial D i abundance of a system at redshift $z \sim 3.572$, towards PKS1937-101. Using only ESPRESSO data, we find a D /H ratio of $2.638\pm 10^{-5}$, while including the available UVES data improves the precision, leading to a ratio of $2.608 \pm 10^{-5}$. The results of this analysis agree with those of the most precise existing measurements. We find that the relatively low column density of this system ($\log {N_{\rm H_I}/ {\rm cm}^{-2}}\sim 18$) introduces modelling uncertainties, which become the main contributor to the error budget.
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