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P. Molaro

Publications and source records attributed to P. Molaro.

At least 163 records · Page 9Linked to original sources
arXiv2000

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-off and at the base of the subgiant branch in NGC6397 and NGC6752 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 NGC6397 and NGC6752 respectively, where the first error bars refer to internal errors and the second ones to systematic errors. In both clusters the [Fe/H] obtained for TO-stars agree perfectly (within a few per cents) with that obtained for stars at the base of the RGB. The [O/Fe]=0.21+/-0.05 value we obtain for NGC6397 is quite low, but it agrees with previous results obtained for giants in this cluster; also, 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 side, our results show clearly and for the first 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 NGC6752, a more massive cluster than NGC6397. A similar anticorrelation is present also for Mg and Al, and C and N. It is very difficult to explain the observed Na-O, and Mg-Al anticorrelation in NGC6752 stars by a deep mixing scenario; we think it requires some non internal mechanism.

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arXiv2000

Molecular hydrogen abundance in the dust-free damped Ly-alpha galaxy at z = 3.4

New results from the search for H2 absorption in the damped Ly-alpha galaxy at redshift z = 3.4 toward QSO 0000-2620 (z_em = 4.1) are reported. The high-resolution (lambda/Delta lambda = 48,000) spectra of Q0000-2620 were obtained using the Ultraviolet - Visual Echelle Spectrograph (UVES) on the 8.2m ESO Kueyen telescope. The ortho-H2 column density is found to be N(J=1) = (5.55 +/- 1.35) 10^{13} cm^{-2} (2sigma C.L.). The combination of N(J=1) with the limits available for other low rotational levels restricts the excitation temperature T_ex in the range (290-540) K. This gives the total H2 column density of N(H2) = (8.75 +/- 1.25) 10^{13} cm^{-2} and the corresponding fraction of hydrogen atoms bound in molecules of f(H2) = (6.8 +/- 2.0) 10^{-8}.

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arXiv2000

UVES observations of QSO 0000-2620: Molecular hydrogen abundance in the damped Ly-alpha system at z_abs = 3.3901

We have discovered molecular hydrogen in a fourth quasar damped Ly-alpha system (hereafter DLA). The UVES spectrograph on the 8.2m ESO Kueyen telescope has allowed the detection of H2 in gas with low metallicity, Z/Z_solar ~= 10^{-2}, and high neutral hydrogen column density, N(HI) ~= 2.6*10^{21} cm^{-2}, at redshift z_abs = 3.3901 toward QSO 0000-2620. The measured H2 fractional abundance of f(H2) ~= 4*10^{-8} is lower than a typical value for Galactic interstellar clouds of high N(HI) column density by a factor of (2-3)*10^6. Since H2 molecules are formed efficiently on dust grains, it implies that the dust condensation in this DLA is negligible, and hence the abundances derived from metal absorption lines are the actual ones. The obtained f(H2) value leads to an estimate of the dust number density of < n_d >_DLA ~= 10^{-3}*< n_d >_ISM, which is consistent with the dust-to-gas ratio k <= 1.6 10^{-3} derived independently from the [Cr/Zn] and [Fe/Zn] ratios.

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arXiv2000

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 2.5m 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 (resolving power ~ 2000) spectra for 1203 objects (V ~ 11-15). Spectral absorption-line indices and radial velocities have been measured for all of the candidates. Metallicities, quantified by [Fe/H], and intrinsic (B-V)o colors have been estimated for 731 stars with effective temperatures cooler than roughly 6500 K, making use of 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 EFECTIVE YIELD of metal-poor stars in our sample is not as large as previous HK survey follow-up programs, the rate of discovery per unit of telescope time is quite high.

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arXiv1999

The Galactic Lithium Evolution Revisited

The evolution of the 7Li abundance in the Galaxy has been computed by taking into account several 7Li sources: novae, massive AGB stars, C-stars, Type II SNe and GCRs. The theoretical predictions for the evolution of the 7Li abundance in the solar neighborhood have been compared to a new compilation of data. A critical analysis of extant observations revealed a possible extension of the Li plateau towards higher metallicities (up to [Fe/H] = -0.5 or even -0.3) with a steep rise afterwards. We conclude that 1) the 7Li contribution from novae is required in order to reproduce the shape of the growth of A(Li) versus [Fe/H], 2) the contribution from Type II SNe should be lowered by at least a factor of two, and 3) the 7Li production from GCRs is probably more important than previously estimated, in particular at the highest metallicities.

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arXiv1999

The 1999 outburst of the eclipsing and recurrent nova U Scorpii

The spectroscopic and photometric evolution of the 1999 outburst of the eclipsing and recurrent nova U Sco is presented. The photometric evolution closely matches that of the previous events. The FWZI=10,000 km/sec for emission lines at maximum has decreased to 4000 km/sec by day +23, with continuous and dramatic changes in the line profiles. No nebular line has become visible and the ionization degree has increased during the brightness decline. A not previously reported and quite puzzling splitting of the emission lines into three components after the first two weeks is outstanding in our spectra. The radiated luminosity is found to be a tiny fraction of that of classical novae for any reasonable distance to U Sco.

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arXiv1999

Abundances of metal-weak thick-disc candidates

High resolution spectra of 5 candidate metal-weak thick-disc stars suggested by Beers & Sommer-Larsen (1995) are analyzed 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 suggests 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 the 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 what is found in more metal-rich carbon-rich stars, but similar to LP 706-7 and LP 625-44 discussed by Norris et al (1997a). Hipparcos data have been used to calculate the space velocities of 25 candidate metal-weak thick-disc stars, thus allowing us to identify 3 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 (1995).

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arXiv1998

Spectroscopy of red giants of the Sagittarius dwarf galaxy

The photometric study of the Sagittarius dwarf galaxy by Marconi et al (1998) has suggested the presence of a spread in metallicity (-0.7< [Fe/H] < -1.6), which may result from different bursts of star formation. We present the results from a Multi Object Slit spectroscopy program carried out at the NTT with the aim to determine spectroscopic abundances of Sagittarius giants selected from the CCD photometry. From our intermediate resolution (R about 1500) spectra radial velocities are determined to confirm the membership and metallicities are derived by using spectral synthesis codes for stars down to V about18, i.e. below the RGB clump. Out of 57 observed stars 23 have a radial velocity consistent with Sagittarius membership, here we present results for 8 of these. No star with [Fe/H]<-1.0 is found, three stars are found to be metal-rich.

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arXiv1998

Spectroscopy of Giants of the Sagittarius Dwarf Galaxy

In this paper we present the first results of the analysis of intermediate resolution ($Δλ\sim 3.5 Å$) spectra of giants of the Sagittarius dwarf galaxy acquired using the ESO NTT telescope. From the deep CCD photometry of Marconi et al (1998a) we have selected a sample of giants representative of the metallicity spread suggested by the comparison of the colour-magnitude diagram of Sagittarius with those of galactic globular clusters. The spectra have been used to measure radial velocities, to confirm the membership to Sagittarius, and to provide a metallicity estimate by using spectral synthesis techniques. The analyzed stars show a spread in metallicities in the range -1.0<[Fe/H]<+0.7, some 0.5 dex more metal-rich than the photometric estimates.

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arXiv1998

Light and Heavy Elements in the Galactic Bulge

The chemical evolution of the Galactic bulge is treated here in the context of an inside-out model for the Galaxy formation. We assume that this central region evolved even faster than the Galactic halo and test the effect of changing the slope of the IMF on the predicted stellar metallicity distribution for bulge stars. An IMF favoring the formation of massive stars is found to improve the agreement with the most recently observed metallicity distribution of bulge K giants. Then, we make specific predictions about temporal evolution of several light and heavy species in the Galactic bulge. We predict that alpha-elements should be enhanced relative to Fe for most of the [Fe/H] range, with different degrees of enhancement due to the different nucleosynthetic history of each element, and show that Lithium abundance should follow a trend with metallicity similar to that found for the solar neighbourhood. Several possible stellar Li sources are discussed.

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arXiv1998

Nitrogen Abundances in Damped Ly alpha Galaxies

Nitrogen abundances have been derived in Damped Ly alpha (DLA) galaxies at Zabs = 2.309, 2.827 and 3.025 toward the QSOs 0100+1300, 1425+6039 and 0347-3819 respectively. The behaviour of nitrogen relative to iron-peak and alpha-elements has been investigated by considering all the extant NI determinations for a total of 9 DLA galaxies. We have estimated the fraction of iron locked into dust grains to convert the observed [N/Fe] ratios into overall (dust plus gas) relative abundances, [N/Fe]corr. The ratios [N/alpha] have been mostly determined by using sulphur as a tracer of alpha-elements which is unaffected by dust. The [N/Fe] and [N/alpha] ratios show high dispersions, of one order of magnitude or more, which have no equivalent in other element-to-element ratios in DLAs. The lowest values of the [N/Fe]corr and [N/alpha] ratios are at variance with the values measured in Galactic halo stars of similar metallicity suggesting that part of the DLA galaxies do not follow the chemical evolution of the Milky Way. The DLA nitrogen abundances and their dispersion show some similarities with those observed in dwarf galaxies. The behaviour of nitrogen abundance ratios can be ascribed, in general to the delayed release of nitrogen in the course of evolution. However it is difficult to conciliate this interpretation with the lowest [N/alpha] values measured, since an expected enhancement of alpha-elements respect to the iron-peak elements is not observed simultaneously in these DLA galaxies. In two cases, relatively high [N/alpha] values are observed which require also a more complex chemical evolution to be explained.

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arXiv1997

CS 22957-027: A carbon-rich extremely-metal-poor star

We present a high-resolution spectroscopic analysis of the extremely-metal-poor star CS 22957-027, which Beers et al (1992) noted to exhibit a rather strong G-band. For Teff = 4839 K, derived from broadband photometry, our analysis obtains log g = 2.25, and a very low metallicity of [Fe/H]=-3.43 +/- 0.12. The carbon-to-iron ratio is found to be enhanced by about 2 dex relative to the solar value, similar to the handful of other carbon-rich metal-deficient stars discussed by Norris et al (1997a) and Barbuy et al (1997). From the 12C13C (1,0) Swan bandhead and 13CH lines the isotope ratio 12C/13C is about 10, indicating a 13C enrichment. Nitrogen is also found to be enhanced by about 1 dex. The s-process elements Sr and Ba are, surprisingly, found to be under-abundant relative to solar, with [Sr/Fe]=-0.91 and [Ba/Fe]=-0.93. The star's low luminosity requires that the chemical enrichment arises from mass transfer from an evolved companion, rather than self-polluting dredge-up processes. However, the s-process elements Sr and Ba are unusually low, with a solar ratio to one another, at variance with what found in the ``classical'' CH stars. Finally we note that a feature due to 13CH is present in coincidence with the Th II 401.9129 nm resonance line. The blend is of relevance for the nucleo-chronology use of the Th II line in these extremely metal poor stars, when significant 13C is present.

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arXiv1997

The $z=0.558$ absorption system towards PKS 0118-272: A candidate Damped Ly $α$ system at low redshift

We present a spectroscopic and imaging study of the z=0.558 MgII absorption system towards the BL Lac PKS 0118-272. At the absorber redshift we detect lines of the low ionization species MgI, MgII, CaII, TiII, MnII, and FeII. The column densities of these species are typical of the interstellar gas in the disk of the Galaxy. By assuming dust-free gas with solar abundances we infer N(HI) > 2.5 x 10^19 cm^-2. The high value of this conservative lower limit indicates that the absorber is a Damped Ly alpha system. We find [Ti/Fe] = +0.3, in agreement with [Ti/Fe] measurements in DLA absorbers, but [Mn/Fe] = +0.4, at variance with the values [Mn/Fe]<0 common to DLA systems. The measured [Ti/Fe] and [Mn/Fe] ratios match remarkably well the differential depletion pattern of low-density interstellar clouds in the Galaxy. Unlike high-redshift DLA systems (z>2), the z=0.558 absorber seems to originate in a galaxy that has already attained the abundances and dust content of present-day disk galaxies. The analysis of our imaging data lends support to the presence of an intervening galaxy. After careful subtraction of the BL Lac image, an object at 1.6 arcsec from PKS 0118-272 is detected. At the absorber redshift the projected distance (14 h_50^-1 kpc) and the absolute magnitude (M_R ~ -22.3) of this companion are consistent with those found for galaxies associated with low-redshift DLA systems.

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arXiv1997

Lithium in very metal poor thick disk 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.

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arXiv1997

The stellar content of the Sagittarius Dwarf Galaxy

We present V,I deep CCD photometry for three fields of the dwarf galaxy in Sagittarius (Sgr). One of the fields is centered on the globular cluster NGC 6715 (M54), which lies in one of the dense clumps of the Sgr galaxy. Comparing the CMD of Sgr with those of globular clusters which are believed to be kinematically associated with the dwarf galaxy, we conclude that the stellar population of Sgr presents a spread in metallicity, and that the dominant population (about 10 Gyr old) is extremely similar to the star content of the associated globular cluster Terzan 7. The estimated distance to Sgr is about d=24.55 Kpc.

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arXiv1996

New beryllium observations in low-metallicity stars

We present observations of the Be II 313.0 nm resonance doublet in 14 halo and old disk stars with metallicities ranging from [Fe/H]=-0.4 to about -3.0 obtained with the CASPEC spectrograph of the ESO 3.6m telescope at a FWHM about 8.6 km/s resolution. Abundances are derived by means of the synthetic spectra technique employing Kurucz (1993) atmospheric models, with enhanced alpha-elements and no overshooting. The derived abundances together with those available in literature show that for -2.7 < [Fe/H] < -0.8 Be correlates linearly with iron [Be] proportional to 1.07(+/- 0.08)[Fe/H], giving strength to previous results. However, a steeper correlation is still possible at metallicities lower than [Fe/H]<-1.4 with [Be] proportional to 1.6(+/- 0.44)[Fe/H]. When iron is replaced with oxygen, Be is found tracking closely oxygen up to solar values, without signs of breaking in correspondence of the onset of the Galactic disk. No evidence of intrinsic dispersion is found, ought to the large errors involved in the Be abundance determinations, but for three stars (HD 106516, HD 3795, HD 211998) a significant upper limit in the Be abundance can be placed at about 1 dex below the mean trend of the Be-Fe relation. For such stars non conventional mixing is required to explain Be depletion. Be observations can be used to discriminate strongly Li-depleted stars. These are the stars which show less Li than that expected by high energy cosmic rays production as deduced from Be observations. The available Be observations imply that some of the stars which contribute to the scatter in the Li-Fe diagramme are Li-depleted stars.

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arXiv1996

Chemical evolution of DLA systems

High redshift DLA systems suggest that the relative abundances of elements might be roughly solar, although with absolute abundances of more than two orders of magnitude below solar. The result comes from observations of the [SII/ZnII] ratio, which is a reliable diagnostic of the true abundance, and from DLA absorbers with small dust depletion and negligible HII contamination. In particular, in two DLA systems nitrogen is detected and at remarkably high levels (Vladilo et al. 1995, Molaro et al. 1995, Green et al. 1995, Kulkarni et al. 1996). Here we compare the predictions from chemical evolution models of galaxies of different morphological type with the abundances and abundance ratios derived for such systems. We conclude that solar ratios and relatively high nitrogen abundances can be obtained in the framework of a chemical evolution model assuming short but intense bursts of star formation, which in turn trigger enriched galactic winds, and a primary origin for nitrogen in massive stars. Such a model is the most successful in describing the chemical abundances of dwarf irregular galaxies and in particular of the peculiar galaxy IZw18. Thus, solar ratios at very low absolute abundances, if confirmed, seem to favour dwarf galaxies rather than spirals as the progenitors of at least some of the DLA systems.

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