arXiv subjects

L. Pasquini

Publications and source records attributed to L. Pasquini.

At least 73 records · Page 4Linked to original sources

arXiv2010

The subgiant branch of omega Cen seen through high-resolution spectroscopy. I. The first stellar generation in omega Centauri?

We analysed high-resolution UVES spectra of six stars belonging to the subgiant branch of omega Centauri, and derived abundance ratios of 19 chemical elements (namely Al, Ba, C, Ca, Co, Cr, Cu, Fe, La, Mg, Mn, N, Na, Ni, Sc, Si, Sr, Ti, and Y). A comparison with previous abundance determinations for red giants provided remarkable agreement and allowed us to identify the sub-populations to which our targets belong. We found that three targets belong to a low-metallicity population at [Fe/H]~-2.0 dex, [alpha/Fe]~+0.4 dex and [s/Fe]~0 dex. Stars with similar characteristics were found in small amounts by past surveys of red giants. We discuss the possibility that they belong to a separate sub-population that we name VMP (very metal-poor, at most 5% of the total cluster population), which - in the self-enrichment hypothesis - is the best-candidate first stellar generation in omega Cen. Two of the remaining targets belong to the dominant metal-poor population (MP) at [Fe/H]~-1.7 dex, and the last one to the metal-intermediate (MInt) one at [Fe/H]~-1.2 dex. The existence of the newly defined VMP population could help to understand some puzzling results based on low-resolution spectroscopy (Sollima et al., Villanova et al.) in their age differences determinations, because the metallicity resolution of these studies was probably not enough to detect the VMP population. The VMP could also correspond to some of the additional substructures of the subgiant-branch region found in the latest HST photometry (Bellini et al.). After trying to correlate chemical abundances with substructures in the subgiant branch of omega Cen, we found that the age difference between the VMP and MP populations should be small (0+/-2 Gyr), while the difference between the MP and MInt populations could be slightly larger (2+/-2~Gyr).

astro-ph.SR↗

arXiv2010

NGC 1866: a milestone for understanding the chemical evolution of stellar populations in the LMC

We present new FLAMES@VLT spectroscopic observations of 30 stars in the field of the LMC stellar cluster NGC 1866. NGC 1866 is one of the few young and massive globular cluster that is close enough so that its stars can be individually studied in detail. Radial velocities have been used to separate stars belonging to the cluster and to the LMC field and the same spectra have been used to derive chemical abundances for a variety of elements, from [Fe/H] to the light (i.e. Na, O, Mg...) to the heavy ones. The average iron abundance of NGC 1866 turns out to be [Fe/H]= -0.43+-0.01 dex (with a dispersion of 0.04 dex), from the analysis of 14 cluster-member stars. Within our uncertainties, the cluster stars are homogeneous, as far as chemical composition is concerned, independent of the evolutionary status. The observed cluster stars do not show any sign of the light elements 'anti-correlation' present in all the Galactic globular clusters so far studied, and also found in the old LMC stellar clusters. A similar lack of anti-correlations has been detected in the massive intermediate-age LMC clusters, indicating a different formation/evolution scenario for the LMC massive clusters younger than ~3 Gyr with respect to the old ones. Also opposite to the Galactic globulars, the chemical composition of the older RGB field stars and of the young post-MS cluster stars show robust homogeneity suggesting a quite similar process of chemical evolution. The field and cluster abundances are in agreement with recent chemical analysis of LMC stars, which show a distinctive chemical pattern for this galaxy with respect to the Milky Way. We discuss these findings in light of the theoretical scenario of chemical evolution of the LMC.

astro-ph.SR↗

arXiv2010

Gravitational redshifts in main-sequence and giant stars

Gravitational redshifts in solar-type main-sequence stars are expected to be some 500 ms$^{-1}$ greater than those in giants. Such a signature is searched for between groups of open-cluster stars which share the same average space motion and thus have the same average Doppler shift. 144 main-sequence stars and cool giants were observed in the M67 open cluster using the ESO FEROS spectrograph, obtaining radial velocities by cross correlation with a spectral template. M67 dwarf and giant radial-velocity distributions are well represented by Gaussian functions, sharing the same apparent average radial velocity within $\simeq$ 100 ms$^{-1}$. In addition, dwarfs in M67 appear to be dynamically hotter ($σ$ = 0.90 kms$^{-1}$) than giants ($σ$ = 0.68 kms$^{-1}$). Explanations for the lack of an expected signal are sought: a likely cause is the differential wavelength shifts produced by different hydrodynamics in dwarf and giant atmospheres. Radial-velocity differences measured between unblended lines in low-noise averaged spectra vary with line-strength: stronger lines are more blushifted in dwarfs than in giants, apparently 'compensating' for the gravitational redshift. Synthetic high-resolution spectra are computed from 3-dimensional hydrodynamic model atmospheres for both giants and dwarfs, and synthetic wavelength shifts obtained. In agreement with observations, 3D models predict substantially smaller wavelength-shift differences than expected from gravitational redshift only. The procedures developed could be used to test 3D models for different classes of stars, but will ultimately require high-fidelity spectra for measurements of wavelength shifts in individual spectral lines.

astro-ph.SR↗

arXiv2010

Li - O anti-correlation in NGC 6752: evidence for Li-enriched polluting gas

Elemental correlations and anti-correlations are known to be present in globular clusters (GCs) owing to pollution by CNO cycled gas. Because of its fragility Li is destroyed at the temperature at which the CNO cycling occurs, and this makes Li a crucial study for the nature of the contaminating stars. We observed 112 un-evolved stars at the Turnoff of the NGC6752 cluster with FLAMES at the VLT to investigate the presence and the extent of a Li-O correlation. This correlation is expected if there is a simple pollution scenario. Li (670.8 nm) and O triplet (771 nm) abundances are derived in NLTE. All stars belong to a very narrow region of the color-magnitude diagram, so they have similar stellar parameters (Teff, log g). We find that O and Li correlate, with a high statistical significance that confirms the early results for this cluster. At first glance this is what is expected if a simple pollution of pristine gas with CNO cycled gas (O-poor, Li-poor) occurred. The slope of the relationship, however, is about 0.4, and differs from unity by over 7 Sigma. A slope of one is the value predicted for a pure contamination model. We confirm an extended Li-O correlation in non evolved stars of NGC 6752. At the same time the characteristic of the correlation shows that a simple pollution scenario is not sufficient to explain the observations. Within this scenario the contaminant gas must have been enriched in Li. This would rule out massive stars as main polluters, and favor the hypothesis that the polluting gas was enriched by intermediate or high-mass AGB stars, unless the former can be shown to be able to produce Li. According to our observations, the fraction of polluting gas contained in the stars observed is a considerable fraction of the stellar mass of the cluster.

astro-ph.GA↗

arXiv2010

An investigation of chromospheric activity spanning the Vaughan--Preston gap: impact on stellar ages

Chromospheric activity is widely used as an age indicator for solar-type stars based on the early evidence that there is a smooth evolution from young and active to old and inactive stars. We analysed chromospheric activity in five solar-type stars in two open clusters, in order to study how chromospheric activity evolves with time. We took UVES high-resolution, high S/N ratio spectra of 3 stars in IC 4756 and 2 in NGC 5822, which were combined with a previously studied data-set and reanalysed here. The emission core of the deep, photospheric Ca II K line was used as a probe of the chromospheric activity. All of the 5 stars in the new sample, including those in the 1.2 Gyr-old NGC 5822, have activity levels comparable to those of Hyades and Praesepe. A likely interpretation of our data is that solar-type-star chromospheric activity, from the age of the Hyades until that of the Sun, does not evolve smoothly. Stars change from active to inactive on a short timescale. Evolution before and after such a transition is much less significant than cyclical and long-term variations. We show that data presented in the literature to support a correlation between age and activity could be also interpreted differently in the light of our results.

astro-ph.SR↗

arXiv2010

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651

The simultaneous investigation of Li and Be in stars is a powerful tool in the study of the evolutionary mixing processes. Here, we present beryllium abundances in stars along the whole evolutionary sequence of the open cluster IC 4651. This cluster has a metallicity of [Fe/H] = +0.11 and an age of 1.2 or 1.7 Gyr. Abundances have been determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. Lithium abundances for the same stars were determined in a previous work. Confirming previous results, we find that the Li dip is also a Be dip. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions, as does the Li dilution. Theoretical hydrodynamical models are able to reproduce well all the observed features.

astro-ph.SR↗

arXiv2010

Beryllium abundances and the formation of the halo and the thick disk

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.

astro-ph.SR↗

arXiv2009

Evidence for early disk-locking among low-mass members of the Orion Nebula Cluster

We present high-resolution spectroscopic observations for 91 PMS stars in ONC with masses in the range 0.10-0.25Msun carried out with the multi-fiber spectrograph FLAMES@ESO. Our aim is to better understand the disk-locking scenario in very low-mass stars. We have derived radial velocities, vsini, and full width at 10% of the Halpha emission peak. Using published measurements of infrared excess as disk tracer, and equivalent width of the NIR CaII line lambda8542, mid-IR difference [3.6]-[8.0]micron derived by Spitzer data, and 10% Halpha width as diagnostic of the level of accretion, we have looked for any correlation between vsini divided by the radius and presence of disk and accretion. Four low-mass stars are SB2 systems. The distribution of rotation periods derived from our vsini measurements is unimodal with a peak of few days. Our is lower than the one expected for a random distribution. We find no evidence for a population of fast rotators close to the break-up velocity. A clear correlation between vsini/R and Delta(Ic-K) has been found. While for stars with no circumstellar disk a spread in the rotation rates is seen, stars with a circumstellar disk show an abrupt drop in their rotation rates by a factor of ~5. On the other hand, only a partial correlation between vsini and accretion is observed when other indicators are used. The X-ray coronal activity level shows no dependence on vsini/R suggesting that all stars are in a saturated regime limit. The critical velocity is probably below our vsini detection limit of 9 km/s. The ONC low-mass stars in our sample at present seem to be not locked, but the clear correlation we find between rotation and IR color excess suggests that they were locked once. In addition, the percentage of accretors seems to scale inversely to the stellar mass.

astro-ph.SR↗

arXiv2009

A view of the Galactic halo using beryllium as a time scale

Beryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined.

astro-ph.GA↗

arXiv2009

Planetary companions around the K giant stars 11 UMi and HD 32518

11 UMi and HD 32518 belong to a sample of 62 K giant stars that has been observed since February 2004 using the 2m Alfred Jensch telescope of the Thüringer Landessternwarte (TLS) to measure precise radial velocities (RVs). The aim of this survey is to investigate the dependence of planet formation on the mass of the host star by searching for planetary companions around intermediate-mass giants. An iodine absorption cell was used to obtain accurate RVs for this study. Our measurements reveal that the RVs of 11 UMi show a periodic variation of 516.22 days. The RV curve of HD 32518 shows sinusoidal variations with a period of 157.54 days. The HIPPARCOS photometry as well as our Hαcore flux measurements reveal no variability with the RV period. Thus, Keplerian motion is the most likely explanation for the observed RV variations for both giant stars. An exoplanet with a minimum mass of 10.5 Jupiter masses orbits the K giant 11 UMi. The K1 III giant HD 32518 hosts a planetary companion with a minimum mass of 3.0 Jupiter masses in a nearly circular orbit. These are the 4th and 5th planets published from this TLS survey.

astro-ph.EP↗

arXiv2009

Beryllium abundances and star formation in the halo and in the thick disk

[abridged] Beryllium is a pure product of cosmic ray spallation. This implies a relatively simple evolution in time of the beryllium abundance and suggests its use as a time-like observable. We study the evolution of Be in the early Galaxy and its dependence on kinematic and orbital parameters. We investigate the formation of the halo and the thick disk of the Galaxy and the use of Be as a cosmochronometer. Beryllium abundances are determined from high resolution, high signal to noise UVES spectra with spectrum synthesis in the largest sample of halo and thick disk stars analyzed to date. We present our observational results in various diagrams. 1) In a log(Be/H) vs [Fe/H] diagram we find a marginal statistical detection of a real scatter, above what expected from measurement errors, with a larger scatter among halo stars. The detection of the scatter is further supported by the existence of pairs of stars with identical atmospheric parameters and different Be abundances. 2) In an log(Be/H) vs [alpha/Fe] diagram, the halo stars separate into two components; one is consistent with predictions of evolutionary models, while the other has too high alpha and Be abundances and is chemically indistinguishable from thick disk stars. This suggests that the halo is not a single uniform population where a clear age-metallicity relation can be defined. 3) In diagrams of Rmin vs [alpha/Fe] and log(Be/H) the thick disk stars show a possible decrease of [alpha/Fe] with Rmin, whereas no dependence of Be with Rmin is seen. This anticorrelation suggests that the star formation rate was lower in the outer regions of the thick disc, pointing towards an inside-out formation. The lack of correlation for Be indicates that it is insensitive to the local conditions of star formation.

astro-ph.GA↗

arXiv2009

Planetary companion candidates around the K giant stars 42 Dra and HD 139357

For the past 3 years we have been monitoring 62 K giant stars using precise stellar radial velocity (RV) measurements with the 2m Alfred Jensch telescope of the Thüringer Landessternwarte Tautenburg (TLS). To probe the dependence of planet formation on stellar mass by finding planets around intermediate-mass giant stars. We present high accuracy RV measurements of the K1.5 III star 42 Dra and the K4 III star HD 139357. The wavelength reference for the RV measurements was provided by an iodine absorption cell placed in the optical path of the spectrograph. Our measurements reveal that the time series of the radial velocity of 42 Dra shows a periodic variation of 479.1 days. HD 139357 shows periodic RV variations of 1125.7 days. Keplerian motion is the most likely explanation for the observed radial velocity variations for these stars. Thus the K giant stars 42 Dra and HD 139357 host extrasolar planets.

astro-ph.EP↗

arXiv2009

Solar analogues in open clusters: The case of M67

Solar analogues are fundamental targets for a better understanding of our Sun and our Solar System. Usually, this research is limited to field stars, which offer several advantages and limitations. In this work, we present the results of a research of solar twins performed for the first time in a open cluster, namely M67. Our analysis allowed us to find five solar twins and also to derive a solar colour of (B-V)0=0.649+/-0.016 and a cluster distance modulus of 9.63+/-0.08. This study encourages us to apply the same method to other open clusters, and to do further investigations for planet search in the solar twins we find.

astro-ph.SR↗

arXiv2008

Abundances of four open clusters from solar stars

We present the abundance measurements of several elements (Fe, Ca, Na, Ni, Ti, Al, Cr, Si) for 20 solar--type stars belonging to four Galactic open clusters: NGC 3680, IC 4651, Praesepe, and M 67. Oxygen abundances were in addition measured for most of the stars in each cluster apart from IC 4651. For NGC 3680 accurate abundance determinations using high--resolution spectra covering a large spectral domain are computed for the first time. We used UVES high--resolution, high signal to noise ratio spectra and performed a differential analysis with respect to the sun, by measuring equivalent widths and assuming LTE. The most surprising result is a significant supersolar metallicity for Praesepe ([Fe/H]=0.27+-0.10). As for the other clusters, we confirm a supersolar metallicity for IC 4651 ([Fe/H]=0.12+-0.05), a solar metallicity for M 67 ([Fe/H]=0.03+-0.04) and a slight subsolar metallicity for NGC 3680 ([Fe/H]=-0.04+-0.03). We find that the abundance ratios of almost all elements are solar, with the notable exception of oxygen in NGC 3680 and Praesepe, supersolar in the former cluster ([O/Fe]=0.2+-0.05) and as low as [O/Fe]=-0.4+-0.1 in the latter. Observations of several objects per cluster is required to obtain robust results, especially for those elements with a limited number of suitable lines.

astro-ph↗

arXiv2008

True solar analogues in the open cluster M67

The solar analogues are fundamental targets for a better understanding of our Sun and Solar System. Notwithstanding the efforts, this research is usually limited to field stars. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very similar to those of our star. In this work, we analyze FLAMES@VLT spectra of about one hundred of M67 main sequence stars with the aim to identify solar analogues. We first determine cluster members which are likely not binaries, by combining both proper motions and radial velocity measurements. Then, we concentrate our analysis on the determination of stellar effective temperature, using the analyzes of line-depth ratios and Halpha wings. Finally, we also compute lithium abundance for all the stars. Thanks to the our analysis, we find ten solar analogues, which allow us to derive a solar color (B-V)=0.649+/-0.016 and a cluster distance modulus of 9.63+/-0.08, very close to values found by previous authors. Among them, five are the best solar twins with temperature determinations within 60 K from the solar values. Our results lead us to do further spectroscopic investigations because the solar analogues candidates are suitable for planet search.

astro-ph↗

arXiv2008

Solar twins in M67

The discovery of true solar analogues is fundamental for a better understanding of the Sun and of the solar system. The open cluster M67 offers a unique opportunity to search for solar analogues because its chemical composition and age are very similar to those of the Sun. We analyze FLAMES spectra of a large number of M67 main sequence stars to identify solar analogues in this cluster.We first determine cluster members which are likely not binaries, by combining proper motions and radial velocity measurements. We concentrate our analysis on the determination of stellar effective temperature, using analyses of line-depth ratios and H$α$ wings, making a direct comparison with the solar spectrum obtained with the same instrument. We also compute the lithium abundance for all the stars.Ten stars have both the temperature derived by line-depth ratios and H$α$ wings within 100 K from the Sun. From these stars we derive, assuming a cluster reddening $E(B-V)=0.041$, the solar colour $(B-V)_\odot=0.649\pm0.016$ and a cluster distance modulus of 9.63. Five stars are most similar (within 60 K) to the Sun and candidates to be true solar twins. These stars have also a low Li content, comparable to the photospheric abundance of the Sun, likely indicating a similar mixing evolution. We find several candidates for the best solar analogues ever. These stars are amenable to further spectroscopic investigations and planet search. The solar colours are determined with rather high accuracy with an independent method, as well as the cluster distance modulus.

astro-ph↗

arXiv2008

N in turn-off stars of NGC 6397 and NGC 6752

We measure the nitrogen abundance in 5 Turn Off(TO) stars of the Globular Clusters NGC 6397 and NGC 6752, and compare the cluster abundances with those of field stars of comparable metallicity. We determine the nitrogen abundance from the band head system at 3360 Å, using spectra of resolution R=45000 obtained with the UVES spectrograph on the VLT. We apply the same method previously used on field stars, to allow a direct comparison of the results. Nitrogen is found to have the same abundance in two of the NGC 6397 stars, in spite of a difference of one order of magnitude in oxygen abundance between them. In a third star of NGC 6397 the value is slightly lower, but compatible with the other two, within the uncertainties. All the stars in NGC 6397 are N-rich with respect to field objects of similar metallicity. The two stars in NGC 6752 show a difference in nitrogen abundance by over one order of magnitude. The same stars differ in the abundances of other elements such as Na, O and Li, only by a factor 3-4. The behaviour of N is different in the two clusters: no variation is observed NGC 6397, while a large variation is observed in NGC 6752. This is consistent with a picture in which the stars in NGC 6752 have been formed by a mixture of ``pristine'' material and material which has been processed by an early generation of stars, referred to as ``polluters''. The N abundances here reported will help to constrain the properties of the polluters. In the case of NGC 6397 a simple pollution history is probably not viable, since the observed variations in O abundances are not accompanied by corresponding variations in N or Li.

astro-ph↗

arXiv2008

Ground-based CCD astrometry with wide field imagers. II. A star catalogue for M67: WFI@2.2m MPG/ESO astrometry, FLAMES@VLT radial velocities

The solar-age open cluster M67 (C0847+120, NGC2682) is a touchstone in studies of the old Galactic disk. Despite its outstanding role, the census of cluster membership for M67 at fainter magnitudes and their properties are not well-established. Using the proprietary and archival ESO data, we have obtained astrometric, photometric, and radial velocities of stars in a 34'x 33' field centered on the old open cluster M67. The two-epoch archival observations separated by 4 years and acquired with the Wide Field Imager at the 2.2m MPG/ESO telescope have been reduced with our new astrometric techniques, as described in the first paper of this series. The same observations served to derive calibrated BVI photometry in M67. Radial velocities were measured using the archival and new spectroscopic data obtained at VLT. We have determined relative proper motions and membership probabilities for ~2,400 stars. The precision of proper motions for optimally exposed stars is ~2 mas/yr, gradually degrading down to ~5 mas/yr at V= 20. Our relatively precise proper motions at V>16 are first obtained in this magnitude range for M67. Radial velocities are measured for 211 stars in the same field. We also present a detailed comparison with recent theoretical isochrones from several independent groups. For M67 area we provide positions, calibrated BVI photometry, relative proper motions, membership probabilities, and radial velocities. We demonstrate that the ground-based CCD mosaic observations just a few years apart are producing proper motions, allowing a reliable membership determination. We produced a catalogue that is made electronically available to the astronomical community.

astro-ph↗