Paolo Molaro

The O-Na and Mg-Al anticorrelations in turn-off and early subgiants in globular clusters

High dispersion spectra (R > 40 000) for a quite large number of stars at the main sequence turn-o and at the base of the giant branch in NGC 6397 and NGC 6752 were obtained with the UVES on Kueyen (VLT UT2). The (Fe/H) values we found are 2:03 0:02 0:04 and 1:42 0:02 0:04 for NGC 6397 and NGC 6752 respectively, where the rst error bars refer to internal and the second ones to systematic errors (within the abundance scale dened by our analysis of 25 subdwarfs with good Hipparcos parallaxes). In both clusters the (Fe/H)s obtained for TO-stars agree perfectly (within a few percent) with that obtained for stars at the base of the RGB. The (O=Fe) = 0:21 0:05 value we obtain for NGC 6397 is quite low, but it agrees with previous results obtained for giants in this cluster. Moreover, the star-to-star scatter in both O and Fe is very small, indicating that this small mass cluster is chemically very homogenous. On the other hand, our results show clearly and for the rst time that the O-Na anticorrelation (up to now seen only for stars on the red giant branches of globular clusters) is present among unevolved stars in the globular cluster NGC 6752, a more massive cluster than NGC 6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very dicult to explain the observed Na-O, and Mg-Al anticorrelation in NGC 6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

Galactic evolution of nitrogen

We present detailed spectroscopic analysis of nitrogen abundances in 31 unevolved metal-poor stars analysed by spectral synthesis of the near-UV NH band at 3360 A observed at high resolution with various telescopes. We found that (N/Fe) scales with that of iron in the metallicity range −3.1 < (Fe/H) < 0 with the slope 0.01 ± 0.02. Furthermore, we derive uniform and accurate (N/O) ratios using oxygen abundances from near-UV OH lines obtained in our previous studies. We find that a primary component of nitrogen is required to explain the observations. The NH lines are discovered in the VLT/UVES spectra of the very metal-poor subdwarfs G64-12 and LP815-43 indicating that these stars are N rich. The results are compared with theoretical models and observations of extragalactic H II regions and Damped Lyα systems. This is the first direct comparison of the (N/O) ratios in these objects with those in Galactic stars.

Cu and Zn in the early Galaxy

We present Cu and Zn abundances for 38 FGK stars, mostly dwarfs, spanning a metallicity range between solar and (Fe/H) = −3. The abundances were obtained using Kuruczs local thermal equilibrium (LTE) model atmospheres and the near-UV lines of Cu  3273.95 A and Zn  3302.58 A observed at high spectral resolution. The trend of (Cu/Fe) versus (Fe/H) is almost solar for (Fe/H) > −1 and then decreases to a plateau � (Cu/Fe)� = −0.98 at (Fe/H) −2 and then slightly increases at lower metallicities to an average value of � (Zn/Fe)� =+ 0.18. We compare our results with previous work on these elements, and briefly discuss them in terms of nucleosynthesis processes. Predictions of halo chemical evolution fairly reproduce the trends, especially the (Cu/Fe) plateau at very low metallicities, but to a lesser extent the higher (Zn/Fe) ratios at low metallicities, indicating possibly missing yields.

A new constraint on cosmological variability of the proton‐to‐electron mass ratio

Exotic cosmologies predict the variability of the fundamental physical constants over the cosmic time. Using the Very Large Telescope/Ultraviolet-Visual Echelle Spectrograph (VLT/UVES) high-resolution spectra of the quasar Q0347-3819 and unblended electronic-vibrational-rotational lines of the H 2 molecule identified at Z a b s = 3.025 we test possible changes in the proton-to-electron mass ratio μ 0 = m p /m e over the period of ∼ 11 Gyr. We obtained a new constraint on the time-averaged variation rate of μ 0 of ‖μ/μ 0 ‖ < 5 × 10 - 1 5 yr - 1 (1σ confidence level). The estimated 1σ uncertainty interval of the ‖Δμ/μ 0 ‖ ratio of about 0.004 per cent implies that since the time when the H 2 spectrum was formed at z a b s = 3.025, μ 0 has not changed by more than a few thousandths of a per cent.

The INT Search for Metal-Poor Stars: Spectroscopic Observations and Classification via Artificial Neural Networks

With the dual aims of enlarging the list of extremely metal-poor stars identified in the Galaxy and boosting the numbers of moderately metal-deficient stars in directions that sample the rotational properties of the thick disk, we have used the 2.5 m Isaac Newton Telescope and the Intermediate Dispersion Spectrograph to carry out a survey of brighter (primarily northern hemisphere) metal-poor candidates selected from the HK objective–prism–interference-filter survey of Beers and collaborators. Over the course of only three observing runs (15 nights) we have obtained medium-resolution (λ/δλ 2000) spectra for 1203 objects (V 11–15). Spectral absorption-line indices and radial velocities have been measured for all the candidates. Metallicities, quantified by [Fe/H], and intrinsic (B-V)0 colors have been estimated for 731 stars with effective temperatures cooler than roughly 6500 K by using artificial neural networks (ANNs) trained with spectral indices. We show that this method performs as well as a previously explored Ca II K calibration technique, yet it presents some practical advantages. Among the candidates in our sample we identify 195 stars with [Fe/H] ≤ -1.0, 67 stars with [Fe/H] ≤ -2.0, and 12 new stars with [Fe/H] ≤ -3.0. Although the effective yield of metal-poor stars in our sample is not as large as that in previous HK survey follow-up programs, the rate of discovery per unit of telescope time is quite high. Further development of the ANN technique, with the networks being fed the entire spectrum, rather than just the spectral indices, holds the promise to produce fast, accurate, multidimensional spectral classifications (with the associated physical parameter estimates), as is required to process the large data flow provided by present and future instrumentation.

Abundances of metal-weak thick-disc candidates

High-resolution spectra of five candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen are analysed to determine their chemical abundances. The low abundance of all the objects has been confirmed, with metallicity reaching [Fe/H]=−2.9. However, for three objects the astrometric data from the Hipparcos catalogue suggest they are true halo members. The remaining two, for which proper-motion data are not available, may have disc-like kinematics. It is therefore clear that it is useful to address properties of putative metal-weak thick-disc stars only if they possess full kinematic data. For CS 22894−19 an abundance pattern similar to those of typical halo stars is found, suggesting that chemical composition is not a useful discriminant between thick-disc and halo stars. CS 29529−12 is found to be C-enhanced with [C/Fe]=+1.0; other chemical peculiarities involve the s-process elements: [Sr/Fe]=−0.65 and [Ba/Fe]=+0.62, leading to a high [Ba/Sr], considerably larger than that found in more metal-rich carbon-rich stars, but similar to those in LP 706-7 and LP 625-44, discussed by Norris et al. Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify three bona fide members, which support the existence of a metal-poor tail of the thick disc, at variance with a claim to the contrary by Ryan & Lambert.

High-Precision Measurements of Δα/αfrom QSO Absorption Spectra

Precise radial velocity measurements (δυ/c ∼ 10-7) of Fell lines in damped Lyα systems from very high quality VLT/UVES spectra of quasars HE 0515-4414 and Q 1101-264 are used to probe cosmological time dependence of the fine structure constant, α. It is found that between two redshifts z 1 = 1.15 and z 2 = 1.84 the value of Δα/α changes at the level of a few ppm: (αz2 — αz1)/α0 = 5.43 ± 2.52 ppm. Variations of α can be considered as one of the most reliable method to constrain the dark energy equation of state and improvements on the accuracy of the wavelength calibration of QSO spectra are of great importance.

LITHIUM IN VERY METAL-POOR THICK DISC STARS

A search for lithium is performed on seven metal poor dwarfs with metallicities ranging from [Fe/H]=-1.5 down to [Fe/H]=-3.0 but showing disk-like kinematics. These stars belong to the metal poor tail of the Galactic thick disk and they may be also the result of an accretion event (Beers and Sommer-Larsen 1995). The Li 6707.8 A line is present in all the seven dwarfs. The weighted average of the Li abundance for the stars is A(Li)=2.20 (+/-0.06) and is consistent within the errors with the plateau Li abundance of A(Li)=2.24(+/- 0.012) found in genuine halo stars in the same range of metallicities (Bonifacio and Molaro 1997). One of the stars, CS 22182-24, shows somewhat lower Li abundance (A(Li)=1.6(+/-0.40)) and is a candidate to being a Li-poor star. Whether this group of stars belongs to the oldest stars in the disk or to the old population of an external galaxy accreted by the Milky Way, the present observations provide support to the universality of a pre-Galactic Li abundance as is observed in the Galactic halo stars.

ESPRESSO data flow in operations: results of commissioning activities

ESPRESSO, the next generation ESO VLT high-resolution ultra-stable spectrograph, after the successful Preliminary Acceptance Europe held at the integration site of the Observatory of Geneva, has been re-integrated at Paranal and started its commissioning activities at the end of 2017. One critical aspect for ESPRESSO future operations, compared with other instruments currently running at ESO, is the way it will be operated which poses several constraints on its data flow. ESPRESSO has been conceived and developed as a “truly science-grade products generating machine” thanks to its fixed format and long-term stability. In addition to the Data Reduction Software (DRS), a Data Analysis Software (DAS), developed within the standard ESO Data Flow System, will be provided to the users – a novelty for the instruments at Paranal. Moreover, ESPRESSO will be fed either by the light of any of the UTs or by the incoherently combined light of up to four UTs, a feature which required a re-thinking of the current Paranal data handling injection schema. In this paper, after describing the main challenges and peculiarities of the ESPRESSO data flow system listed above, we will present the results of the first commissioning activities and the lessons learned to handle data produced by an instrument with such ambitious scientific requirements.

Calibration issues in Δα/α

Laser Comb Wavelength calibration shows that the ThAr one is locally unreliable with possible deviations of up to 100 m s−1within one order range, while delivering an overall 1 m s−1accuracy (Wilken et al 2009). Such deviation corresponds to δα/α≈ 7 · 10−6 for a Fe -Mg  pair. Comparison of line shifts among the 5 Fe  lines, with almost identical sensitivity to fine structure constant changes, offers a clean way to directly test the presence of possible local wavelength calibration errors of whatever origin. We analyzed 5 absorption systems, with zabs ranging from 1.15 to 2.19 towards 3 bright QSOs. The results show that while some lines are aligned within 20 m s−1, others reveal large deviations reaching 200 m s−1or higher and corresponding to a δα/α≥ 10−5 level. The origin of these deviations is not clearly identified but could be related to the adaptation of wavelength calibration to CCD manufacturing irregularities. These results suggest that to draw conclusions from δα/α analysis based on one or only few lines must be done with extreme care.