ABSTRACT
Data from Gaia DR2 and The Apache Point Observatory Galactic Evolution Experiment surveys revealed a relatively new component in the inner Galactic halo, which is likely the dynamical ...remnant of a disrupted dwarf galaxy named Gaia-Enceladus that collided with the Milky Way about 10 Gyr ago. This merging event offers an extraordinary opportunity to study chemical abundances of elements in a dwarf galaxy, since they are generally hampered in external galaxies. Here, we focus on 7Li and 9Be in dwarf stars that are out of reach even in Local Group galaxies. Searching in GALAH, Gaia-ESO survey and in literature, we found several existing 7Li abundance determinations of stars belonging to the Gaia-Enceladus galaxy. The 7Li abundances of stars at the low metallicity end overlap with those of the Galactic halo. These are effective extragalactic 7Li measurements, which suggest that the 7Li Spite plateau is universal, as is the cosmological 7Li problem. We found a 7Li-rich giant out of 101 stars, which suggests a small percentage similar to that of the Milky Way. We also collect 9Be abundance for a subsample of 25 Gaia-Enceladus stars from literature. Their abundances share the Galactic Be/H values at the low metallicity end but grow slower with Fe/H and show a reduced dispersion. This suggests that the scatter observed in the Milky Way could reflect the different 9Be evolution patterns of different stellar components that are mixed-up in the Galactic halo.
The English astronomer Thomas Harriot is a shadowy figure. Although an eclectic and brilliant scientist, he seldom published his work. The only picture of him, hanging in the President's room of ...Trinity College, Oxford, is unconfirmed to this day. In this article, we note a close resemblance between the Trinity College portrait and that of the mathematician in a painting on display at the National Gallery. The sitter is not only a mathematician but also characterized as a cartographer and astronomer as Harriot was. Moreover, the sitter shows certain psychological traits that match Harriot's personality. This association may offer new clues to confirming whether the Trinity College portrait is really of Harriot, and may help lift the veil of mystery from the figure of the first modern British scientist.
Context. Stellar radial velocities play a fundamental role in the discovery of extrasolar planets and the measurement of their physical parameters as well as in the study of stellar physical ...properties. Aims. We investigate the impact of the solar activity on the radial velocity of the Sun using the HARPS spectrograph to obtain measurements that can be directly compared with those acquired in the extrasolar planet search programmes. Methods. We used the Moon, the Galilean satellites, and several asteroids as reflectors to measure the radial velocity of the Sun as a star and correlated this velocity with disc-integrated chromospheric and magnetic indexes of solar activity that are similar to stellar activity indexes. We discuss in detail the systematic effects that affect our measurements and the methods to account for them. Results. We find that the radial velocity of the Sun as a star is positively correlated with the level of its chromospheric activity at ~95 percent significance level. The amplitude of the long-term variation measured in the 2006−2014 period is 4.98 ± 1.44 m/s, which is in good agreement with model predictions. The standard deviation of the residuals obtained by subtracting a linear best fit is 2.82 m/s and is due to the rotation of the reflecting bodies and the intrinsic variability of the Sun on timescales shorter than the activity cycle. A correlation with a lower significance is detected between the radial velocity and the mean absolute value of the line-of-sight photospheric magnetic field flux density. Conclusions. Our results confirm similar correlations found in other late-type main-sequence stars and provide support to the predictions of radial velocity variations induced by stellar activity based on current models.
Aims. One of the main scientific drivers for ESPRESSO, Échelle SPectrograph, is the detection and characterization of Earth-class exoplanets. With this goal in mind, the ESPRESSO guaranteed time ...observations (GTO) Catalog identifies the best target stars for a blind search for the radial velocity (RV) signals caused by Earth-class exoplanets. Methods. Using the most complete stellar catalogs available, we screened for the most suitable G, K, and M dwarf stars for the detection of Earth-class exoplanets with ESPRESSO. For most of the stars, we then gathered high-resolution spectra from new observations or from archival data. We used these spectra to spectroscopically investigate the existence of any stellar binaries, both bound or background stars. We derived the activity level using chromospheric activity indexes using log (R′HK) $(R' _{\textrm{HK}})$ (RHK′) , as well as the projected rotational velocity v sin i. For the cases where planet companions are already known, we also looked at the possibility that additional planets may exist in the host’s habitable zone using dynamical arguments. Results. We estimated the spectroscopic contamination level, v sin i, activity, stellar parameters and chemical abundances for 249 of the most promising targets. Using these data, we selected 45 stars that match our criteria for detectability of a planet like Earth. The stars presented and discussed in this paper constitute the ESPRESSO GTO catalog for the RV blind search for Earth-class planets. They can also be used for any other work requiring a detailed spectroscopic characterization of stars in the solar neighborhood.
TOPoS Bonifacio, P; Caffau, E; Spite, M ...
Astronomy and astrophysics (Berlin),
7/2015, Letnik:
579
Journal Article
Recenzirano
Odprti dostop
In the course of the Turn Off Primordial Stars (TOPoS) survey, aimed at discovering the lowest metallicity stars, we have found several carbon-enhanced metal-poor (CEMP) stars. These stars are very ...common among the stars of extremely low metallicity and provide important clues to the star formation processes. We here present our analysis of six CEMP stars. We want to provide the most complete chemical inventory for these six stars in order to constrain the nucleosynthesis processes responsible for the abundance patterns. We analyze both X-shooter and UVES spectra acquired at the VLT. We used a traditional abundance analysis based on OSMARCS 1D local thermodynamic equilibrium (LTE) model atmospheres and the turbo-spectrum line formation code. We propose an interpretation of this bi-modality according to which the stars on the high-carbon band are the result of mass transfer from an AGB companion, while the stars on the low-carbon band are genuine fossil records of a gas cloud that has also been enriched by a faint supernova (SN) providing carbon and the lighter elements.
Abstract
We present high-resolution spectroscopic observations of the fast nova ASASSN-16kt (V407 Lup). A close inspection of spectra obtained at early stages has revealed the presence of ...low-ionization lines, and among the others we have identified the presence of the ionized 7Be doublet in a region relatively free from possible contaminants. After studying their intensities, we have inferred that ASASSN-16kt has produced (5.9–7.7)× 10−9 M⊙ of 7Be. The identification of bright Ne lines may suggest that the nova progenitor is a massive (1.2 M⊙) oxygen–neon white dwarf. The high outburst frequency of oxygen–neon novae implies that they likely produce an amount of Be similar, if not larger, to that produced by carbon–oxygen novae, then confirming that classical novae are among the main factories of lithium in the Galaxy.
We present the results of observations taken with the X-shooter spectrograph devoted to the study of quasars at z ∼ 6. This paper focuses on the properties of metals at high redshift traced, in ...particular, by the C iv doublet absorption systems. Six objects were observed with resolutions 27 and 34 km s−1 in the visual, and 37.5 and 53.5 km s−1 in the near-infrared. We detected 102 C iv lines in the range: 4.35 < z < 6.2 of which 27 are above z ∼ 5. Thanks to the characteristics of resolution and spectral coverage of X-shooter, we could also detect 25 Si iv doublets associated with the C iv at z 5. The column density distribution function of the C iv line sample is observed to evolve in redshift for z 5.3, with respect to the normalization defined by low-redshift (1.5 < z < 4) C iv lines. This behaviour is reflected in the redshift evolution of the C iv cosmic mass density,
, of lines with column density in the range
, which is consistent with a drop of a factor of ∼2 for z 5.3. Considering only the stronger C iv lines (
),
gently rises by a factor of ∼10 between z 6.2 and z 1.5 with a possible flattening towards z ∼ 0. The increase is well fitted by a power law:
. An insight into the properties of the C iv absorbers and their evolution with redshift is obtained by comparing the observed column densities of associated C iv, Si iv and C ii absorptions with the output of a set of cloudy photoionization models. As already claimed by cosmological simulations, we find that C iv is a good tracer of the metallicity in the low-density intergalactic medium (IGM) gas at z ∼ 5-6 while at z ∼ 3 it arises in gas with overdensity δ ∼ 100.
Context.
Despite swift progress in the characterisation of exoplanet atmospheres in composition and structure, the study of atmospheric dynamics has not progressed at the same speed. While ...theoretical models have been developed to describe the lower layers of the atmosphere, and independently, the exosphere, little is known about the intermediate layers up to the thermosphere.
Aims.
We aim to provide a clearer picture of atmospheric dynamics for the class of ultra-hot Jupiters, which are highly irradiated gas giants, based on the example of WASP-76 b.
Methods.
We jointly analysed two datasets that were obtained with the HARPS and ESPRESSO spectrographs to interpret the resolved planetary sodium doublet. We then applied the MERC code, which retrieves wind patterns, speeds, and temperature profiles on the line shape of the sodium doublet. An updated version of MERC, with added planetary rotation, also provides the possibility of modelling the latitude dependence of the wind patterns.
Results.
We retrieve the highest Bayesian evidence for an isothermal atmosphere, interpreted as a mean temperature of 3389 ± 227 K, a uniform day- to nightside wind of 5.5
−2.0
+1.4
km s
−1
in the lower atmosphere with a vertical wind in the upper atmosphere of 22.7
−4.1
+4.9
km s
−1
, switching atmospheric wind patterns at 10
−3
bar above the reference surface pressure (10 bar).
Conclusions.
Our results for WASP-76 b are compatible with previous studies of the lower atmospheric dynamics of WASP-76 b and other ultra-hot Jupiters. They highlight the need for vertical winds in the intermediate atmosphere above the layers probed by global circulation model studies to explain the line broadening of the sodium doublet in this planet. This work demonstrates the capability of exploiting the resolved spectral line shapes to observationally constrain possible wind patterns in exoplanet atmospheres. This is an invaluable input to more sophisticated 3D atmospheric models in the future.
In the framework of the ESO Large Programme “First Stars”, very high-quality spectra of some 70 very metal-poor dwarfs and giants were obtained with the ESO VLT and UVES spectrograph. These stars are ...likely to have descended from the first generation(s) of stars formed after the Big Bang, and their detailed composition provides constraints on issues such as the nature of the first supernovae, the efficiency of mixing processes in the early Galaxy, the formation and evolution of the halo of the Galaxy, and the possible sources of reionization of the Universe. This paper presents the abundance analysis of an homogeneous sample of 35 giants selected from the HK survey of Beers et al. (CITE, CITE), emphasizing stars of extremely low metallicity: 30 of our 35 stars are in the range $-4.1 <{\rm Fe/H}< -2.7$, and 22 stars have ${\rm Fe/H} < -3.0$. Our new VLT/UVES spectra, at a resolving power of $R\sim45\,000$ and with signal-to-noise ratios of 100–200 per pixel over the wavelength range 330–1000 nm, are greatly superior to those of the classic studies of McWilliam et al. (CITE) and Ryan et al. (CITE). The immediate objective of the work is to determine precise, comprehensive, and homogeneous element abundances for this large sample of the most metal-poor giants presently known. In the analysis we combine the spectral line modeling code “Turbospectrum” with OSMARCS model atmospheres, which treat continuum scattering correctly and thus allow proper interpretation of the blue regions of the spectra, where scattering becomes important relative to continuous absorption ($\lambda < 400$ nm). We obtain detailed information on the trends of elemental abundance ratios and the star-to-star scatter around those trends, enabling us to separate the relative contributions of cosmic scatter and observational/analysis errors. Abundances of 17 elements from C to Zn have been measured in all stars, including K and Zn, which have not previously been detected in stars with Fe/H $< -$3.0. Among the key results, we discuss the oxygen abundance (from the forbidden OI line), the different and sometimes complex trends of the abundance ratios with metallicity, the very tight relationship between the abundances of certain elements (e.g., Fe and Cr), and the high Zn/Fe ratio in the most metal-poor stars. Within the error bars, the trends of the abundance ratios with metallicity are consistent with those found in earlier literature, but in many cases the scatter around the average trends is much smaller than found in earlier studies, which were limited to lower-quality spectra. We find that the cosmic scatter in several element ratios may be as low as 0.05 dex. The evolution of the abundance trends and scatter with declining metallicity provides strong constraints on the yields of the first supernovae and their mixing into the early ISM. The abundance ratios found in our sample do not match the predicted yields from pair-instability hypernovae, but are consistent with element production by supernovae with progenitor masses up to 100 $M_{\sun}$. Moreover, the composition of the ejecta that have enriched the matter
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