arXiv subjects

R. G. Gratton

Publications and source records attributed to R. G. Gratton.

At least 55 records · Page 3Linked to original sources

arXiv2006

Detecting Extrasolar Planets with Integral Field Spectroscopy

Observations of extrasolar planets using Integral Field Spectroscopy (IFS), if coupled with an extreme Adaptive Optics system and analyzed with a Simultaneous Differential Imaging technique (SDI), are a powerful tool to detect and characterize extrasolar planets directly; they enhance the signal of the planet and, at the same time, reduces the impact of stellar light and consequently important noise sources like speckles. In order to verify the efficiency of such a technique, we developed a simulation code able to test the capabilities of this IFS-SDI technique for different kinds of planets and telescopes, modelling the atmospheric and instrumental noise sources. The first results obtained by the simulations show that many significant extrasolar planet detections are indeed possible using the present 8m-class telescopes within a few hours of exposure time. The procedure adopted to simulate IFS observations is presented here in detail, explaining in particular how we obtain estimates of the speckle noise, Adaptive Optics corrections, specific instrumental features, and how we test the efficiency of the SDI technique to increase the signal-to-noise ratio of the planet detection. The most important results achieved by simulations of various objects, from 1 M_J to brown dwarfs of 30 M_J, for observations with an 8 meter telescope, are then presented and discussed.

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arXiv2006

Na-O Anticorrelation And HB. III. The abundances of NGC 6441 from FLAMES-UVES spectra

The aim of the present work is to determine accurate metallicities for a group of red giant branch stars in the field of the bulge Globular Cluster NGC 6441. This is the third paper of a series resulting from a large project aimed at determining the extent of the Na-O anticorrelation among Globular Cluster stars and exploring its relationship with HB morphology. We present an LTE abundance analysis of these objects, based on data gathered with the FLAMES fiber facility and the UVES spectrograph at VLT2. Five of the thirteen stars observed are members of the cluster. The average Fe abundance for these five stars is [Fe/H]=$-0.39\pm 0.04\pm 0.05$~dex, where the first error bar includes the uncertainties related to star-to-star random errors, and the second one the systematic effects related to the various assumptions made in the analysis.The overall abundance pattern is quite typical of Globular Clusters, with an excess of the $α-$elements and of Eu. There is evidence that the stars of NGC 6441 are enriched in Na and Al, while they have been depleted of O and Mg, due to H-burning at high temperatures, in analogy with extensive observations for other Globular Clusters: in particular, one star is clearly Na and Al-rich and O and Mg-poor. We obtained also quite high V abundances, but it is possible that this is an artifact of the analysis, since similar large V abundances are derived also for the field stars. These last are all more metal-rich than NGC 6441 and probably belong to the bulge population.

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arXiv2005

Photometric and spectroscopic study of the intermediate age open cluster NGC 3960

We present CCD UBVI photometry and high-resolution spectroscopy of the intermediate age open cluster NGC 3960. The colour - magnitude diagrams (CMDs) derived from the photometric data and interpreted with the synthetic CMD method allow us to estimate the cluster parameters. We derive: age = 0.9 or 0.6 Gyr (depending on whether or not overshooting from convective regions is included in the adopted stellar models), distance (m-M)0 = 11.6 +/- 0.1, reddening E(B-V) = 0.29 +/- 0.02, differential reddening Delta E(B-V) = 0.05 and approximate metallicity between solar and half of solar. We obtained high resolution spectra of three clump stars, and derived an average [Fe/H] = -0.12 (rms 0.04 dex), in very good agreement with the photometric determination. We also obtained abundances of alpha-elements, Fe-peak elements, and of Ba. The reddenings toward individual stars derived from the spectroscopic temperatures and the Alonso et al. calibrations give further support to the existence of significative variations across the cluster.

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arXiv2005

Line bisectors and radial velocity jitter from SARG spectra

We present an analysis of spectral line bisector variations for a few stars observed in the SARG high precision radial velocity planet survey, and discuss their relationship with differential radial velocities. The spectra we consider are the same used for determining radial velocities. The iodine cell lines employed in the measurement of radial velocities were removed before bisector analysis. The line bisectors were then computed from average absorption profiles obtained by cross correlation of the stellar spectra with a mask made from suitable lines of a solar catalog. Bisector velocity spans were then determined: errors in these quantities compare well with theoretical expectations based on resolution, S/N and line shape. The plot of bisector velocity span against radial velocity was studied to search for correlations between line asymmetries and radial velocity variations. A correlation was seen for HD 166435 due to stellar activity, and for HD 8071B due to spectral contamination by the companion. No correlation was seen for 51 Peg and rho CrB, stars hosting planets. We conclude that this technique may be useful to separate radial velocity variations due to barycenter motion from spurious signals in spectra acquired with the iodine cell.

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arXiv2005

The metal abundance distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy

(abridged): Low resolution spectroscopy obtained with FORS2 at the Very Large Telescope (VLT) has been used to measure individual metal abundances ([Fe/H]) for 107 RR Lyrae stars, and trace the metal distribution of the oldest stellar component in the Sculptor dwarf spheroidal galaxy. Their metallicities have an average value of [Fe/H]=-1.83 +/- 0.03 (r.m.s. 0.26 dex) and cover the metallicity range -2.40<[Fe/H]<-0.85 (only 1 star with [Fe/H]>-1). The star-to-star scatter is larger than typical errors on individual metallicities (+/- 0.15-0.16 dex), indicating a real spread in metal abundances. The radial velocities have a dispersion of 12.9 km/s, consistent with the dispersion derived in Sculptor by Tolstoy et al. (2004). This along with the metallicity distribution, suggests that most of the RR Lyr's arise from the same burst of stellar formation that produced the metal-poor component giving origin to the galaxy blue horizontal branch, and only a few (if any) come from the centrally concentrated metal-rich red horizontal branch population. The spectroscopic metallicities and the apparent luminosities were used to study the luminosity-metallicity relation, for which we derive a shallow slope of 0.09 mag/dex. This result can be due to a high level of evolution off the zero age horizontal branch of the RR Lyrae stars in this galaxy, again in agreement with their origin from the blue horizontal branch population.

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arXiv2005

Metal abundances of RR Lyrae stars in the metal rich globular cluster NGC 6441

Low resolution spectra have been used to measure individual metal abundances of RR Lyrae stars in NGC 6441, a Galactic globular cluster known to have very unusual horizontal branch morphology and periods of the RR Lyrae stars for its high metallicity. We find an average metal abundance of [Fe/H]=-0.69 +/- 0.06 (r.m.s.=0.33 dex) and [Fe/H]=-0.41 +/- 0.06 (r.m.s.=0.36 dex) on Zinn & West and Carretta & Gratton metallicity scales, respectively, consistent with the cluster metal abundance derived by Armandroff & Zinn. Most of the metallicities were extrapolated from calibration relations defined for [Fe/H] < -1; however, they are clearly high and contrast with the rather long periods of the NGC 6441 variables, thus confirming that the cluster does not fit in the general Oosterhoff classification scheme. The r.m.s. scatter of the average is larger than observational errors (0.15-0.16 dex) possibly indicating some spread in metallicity. However, even the metal poor variables, if confirmed to be cluster members, are still more metal rich than those commonly found in the Oosterhoff type II globular clusters.

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arXiv2005

Sulphur abundance in Galactic stars

We investigate sulphur abundance in 74 Galactic stars by using high resolution spectra obtained at ESO VLT and NTT telescopes. For the first time the abundances are derived, where possible, from three optical multiplets: Mult. 1, 6, and 8. By combining our own measurements with data in the literature we assemble a sample of 253 stars in the metallicity range -3.2 < [Fe/H] < +0.5. Two important features, which could hardly be detected in smaller samples, are obvious from this large sample: 1) a sizeable scatter in [S/Fe] ratios around [Fe/H] ~ -1 ; 2) at low metallicities we observe stars with [S/Fe] ~ 0.4, as well as stars with higher [S/Fe] ratios. The latter do not seem to be kinematically different from the former ones. Whether the latter finding stems from a distinct population of metal-poor stars or simply from an increased scatter in sulphur abundances remains an open question.

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arXiv2005

Li in NGC 6752 and the formation of globular clusters

Li abundances for 9 Turnoff (TO) stars of the intermediate metallicity cluster ([Fe/H]=-1.4) NGC6752 are presented. The cluster is known to show abundance anomalies and anticorrelations observed in both evolved and main sequence stars. We find that Li abundance anticorrelates with Na (and N) and correlates with O in these Turn-Off stars. For the first time we observe Pop II hot dwarfs systematically departing from the Spite plateau. The observed anticorrelations are in qualitative agreement with what is expected if the original gas were contaminated by Intermediate Mass AGB - processed material. However, a quantitative comparison shows that none of the existing models can reproduce all the observations at once. The very large amount of processed gas present in the cluster does not imply a 'pollution', but rather that the whole protocluster cloud was enriched by a previous generation of stars. We finally note that the different abundance patterns in NGC 6397 and NGC 6752 imply different ejecta of the preenrichment composition for the two clusters.

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arXiv2005

Early star formation in the Galaxy from beryllium and oxygen abundances

We investigate the evolution of the star formation rate in the early Galaxy using beryllium and oxygen abundances in metal poor stars. Specifically, we show that stars belonging to two previously identified kinematical classes (the so-called ``accretion'' and ``dissipative'' populations) are neatly separated in the [O/Fe] vs. Log(Be/H) diagram. The dissipative population follows the predictions of our model of Galactic evolution for the thick disk component, suggesting that the formation of this stellar population occurred on a timescale significantly longer (by a factor ~5-10) than the accretion component. The latter shows a large scatter in the [O/Fe] vs. Log(Be/H) diagram, probably resulting from the inhomogeneous enrichment in oxygen and iron of the protogalactic gas. Despite the limitation of the sample, the data suggest that the combined use of products of spallation reactions (like beryllium) and elemental ratios of stellar nucleosynthesis products (like [O/Fe]) can constrain theoretical models for the formation and early evolution of our Galaxy.

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arXiv2005

Precise reddening and metallicity of NGC6752 from FLAMES spectra

(abridged) Accurate reddenings for Globular Clusters could be obtained by comparing the colour-temperature obtained using temperatures from reddening-free indicator (Halpha), with that given by standard colour-temperature calibrations. From a single 1300 seconds exposure with FLAMES at VLT2 we obtained spectra centred on Halpha (R=6000, 5<S/N<50) for 120 stars near the turn-off of NGC6752. This set of spectra was used to derive effective temperatures from fittings of Halpha profiles with typical errors of about +/- 200 K and reddening estimates with individual errors of 0.05 mag. Averaging all individual reddenings, a high precision reddening estimate has been obtained for the cluster: E(B-V)=0.046 +/- 0.005. The same exposure provided UVES spectra of seven stars near the red giant branch bump (R=40000, 20<S/N<40) that provided Fe abundances with internal errors of 0.026 dex, and with average metallicity [Fe/H]= -1.48 +/- 0.01 +/- 0.06 dex (random + systematic). Abundances were obtained for several other elements, allowing e.g. an accurate estimate of the ratio between the alpha-elements and Fe ([alpha/Fe]=+0.27 +/- 0.01). The O-Na anticorrelation is evident from our UVES data, in agreement with past results. This analysis shows the power of FLAMES for analysis of globular clusters: the accurate reddenings and metal abundances obtained by a procedure like that described here, combined with distance determinations from cluster dynamics or main sequence fitting, and high quality colour-magnitude diagrams, could allow derivation of ages with errors below 1 Gyr for individual globular clusters.

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arXiv2005

Abundance anomalies in hot horizontal branch stars of the galactic globular cluster NGC 1904

We present abundance measurements, based on high-resolution optical spectroscopic data obtained with the Ultraviolet and Visual Echelle Spectrograph mounted on Kueyen (Very Large Telescope UT2), for ten stars in the extended blue horizontal branch of the Galactic globular cluster NGC 1904 (M79). In agreement with previous findings for other clusters, we obtain normal abundances for stars cooler than Teff ~ 11,000 K and largely anomalous abundances for hotter stars: large He depletions, and overabundances of Fe, Ti, Cr, P and Mn. The abundances of Mg, Si and Ca are roughly normal, in the hot stars as well as in the cooler ones. This abundance pattern can be attributed to the onset of diffusion and to radiation pressure in the stable atmospheres of hot horizontal branch stars.

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arXiv2004

Heavy elements and chemical enrichment in globular clusters

High resolution (R > 40 000) and high S/N spectra have been acquired with UVES on the VLT-Kueyen (Paranal Observatory, ESO Chile) for several main sequence turnoff stars (V ~ 17 mag) and subgiants at the base of the Red Giant Branch (V ~ 16 mag) in three globular clusters (NGC 6397, NGC 6752 and 47 Tuc/NGC 104) at different metallicities (respectively [Fe/H] = -2.0; -1.5; -0.7$). Spectra for a sample of 25 field halo subdwarves have also been taken with equal resolution, but higher S/N. These data have been used to determine the abundances of several neutron-capture elements in these three clusters: strontium, yttrium, barium and europium. This is the first abundance determination of these heavy elements for such unevolved stars in these three globular clusters. These values, together with the [Ba/Eu] and [Sr/Ba] abundance ratios, have been used to test the self-enrichment scenario. A comparison is done with field halo stars and other well known Galactic globular clusters in which heavy elements have already been measured in the past, at least in bright giants (V > 11-12 mag). Our results show clearly that globular clusters have been uniformly enriched by r- and s-process syntheses, and that most of them seem to follow exactly the same abundance patterns as field halo stars, which discards the ``classical'' self-enrichment scenario for the origin of metallicities and heavy elements in globular clusters.

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arXiv2004

Beryllium in turnoff stars of NGC6397: early Galaxy spallation, cosmochronology and cluster formation

We present the first detection of beryllium in two turnoff stars of the old, metal-poor globular cluster NGC 6397. The beryllium lines are clearly detected and we determine a mean beryllium abundance of log(Be/H)=-12.35 +/- 0.2. The beryllium abundance is very similar to that of field stars of similar Fe content. We interpret the beryllium abundance observed as the result of primary spallation of cosmic rays acting on a Galactic scale, showing that beryllium can be used as a powerful cosmochronometer for the first stellar generations. With this method, we estimate that the cluster formed 0.2-0.3 Gyr after the onset of star formation in the Galaxy, in excellent agreement with the age derived from main sequence fitting. From the same spectra we also find low O (noticeably different for the two stars) and high N abundances, suggesting that the original gas was enriched in CNO processed material. Our beryllium results, together with the N, O, and Li abundances, provide insights on the formation of this globular cluster, showing that any CNO processing of the gas must have occurred in the protocluster cloud before the formation of the stars we observe now. We encounter, however, difficulties in giving a fully consistent picture of the cluster formation, able to explain the complex overall abundance pattern.

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arXiv2004

Metal abundances of RR Lyrae stars in the bar of the Large Magellanic Cloud

Metallicities ([Fe/H]) from low resolution spectroscopy obtained with the Very Large Telescope (VLT) are presented for 98 RR Lyrae and 3 short period Cepheids in the bar of the Large Magellanic Cloud. Our metal abundances have typical errors of +/-0.17 dex. The average metallicity of the RR Lyrae stars is [Fe/H]=-1.48 +/- 0.03 +/- 0.06 on the scale of Harris (1996). The star-to-star scatter (0.29 dex) is larger than the observational errors, indicating a real spread in metal abundances. The derived metallicities cover the range -2.12 < [Fe/H] <-0.27, but there are only a few stars having [Fe/H] > -1. For the ab-type variables we compared our spectroscopic abundances with those obtained from the Fourier decomposition of the light curves. We find good agreement between the two techniques, once the systematic offset of 0.2 dex between the metallicity scales used in the two methods is taken into account. The spectroscopic metallicities were combined with the dereddened apparent magnitudes of the variables to derive the slope of the luminosity-metallicity relation for the LMC RR Lyrae stars: the resulting value is 0.214 +/- 0.047 mag/dex. Finally, the 3 short period Cepheids have [Fe/H] values in the range -2.0 < [Fe/H] <-1.5 . They are more metal-poor than typical LMC RR Lyrae stars, thus they are more likely to be Anomalous Cepheids rather than the short period Classical Cepheids that are being found in a number of dwarf Irregular galaxies.

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arXiv2004

No Planet Around HD 219542 B

The star HD 219542 B has been reported by us (Desidera et al. 2003) to show low-amplitude radial velocity variations that could be due to the presence of a Saturn-mass planetary companion or to stellar activity phenomena. In this letter we present the results of the continuation of the radial velocity monitoring as well as a discussion of literature determinations of the chromospheric activity of the star (Wright et al. 2004). These new data indicate that the observed radial velocity variations are likely related to stellar activity. In particular, there are indications that HD 219542 B underwent a phase of enhanced stellar activity in 2002 while the activity level has been lower in both 2001 and 2003. Our 2003 radial velocity measurements now deviate from our preliminary orbital solution and the peak in the power spectrum at the proposed planet period is severely reduced by the inclusion of the new data. We therefore dismiss the planet hypothesis as the cause of the radial velocity variations.

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arXiv2004

Iron abundances from high-resolution spectroscopy of the open clusters NGC 2506, NGC 6134, and IC 4651

This is the first of a series of papers devoted to derive the metallicity of old open clusters in order to study the time evolution of the chemical abundance gradient in the Galactic disk. We present detailed iron abundances from high resolution (R~40000) spectra of several red clump and bright giant stars in the open clusters IC 4651, NGC 2506 and NGC 6134. We observed 4 stars of NGC 2506, 3 stars of NGC 6134, and 5 stars of IC 4651 with the FEROS spectrograph at the ESO 1.5 m telescope; moreover, 3 other stars of NGC 6134 were observed with the UVES spectrograph on Kueyen (VLT UT2). After excluding the cool giants near the red giant branch tip (one in IC 4651 and one in NGC 2506), we found overall [Fe/H] values of -0.20 +/- 0.01, rms = 0.02 dex (2 stars) for NGC 2506, +0.15 +/- 0.03, rms = 0.07 dex (6 stars) for NGC 6134, and +0.11 +/- 0.01, rms = 0.01 dex (4 stars) for IC 4651. The metal abundances derived from line analysis for each star were extensively checked using spectrum synthesis of about 30 to 40 Fe I lines and 6 Fe II lines. Our spectroscopic temperatures provide reddening values in good agreement with literature data for these clusters, strengthening the reliability of the adopted temperature and metallicity scale. Also, gravities from the Fe equilibrium of ionization agree quite well with expectations based on cluster distance moduli and evolutionary masses.

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arXiv2003

Heavy elements abundances in turn-off stars and early subgiants in NGC 6752

High resolution spectra (R > 40 000) for 9 main sequence turn-off stars and 9 subgiants in the globular cluster NGC 6752 were acquired with UVES on the VLT-Kueyen (UT2). These data have been used to determine the abundances of some heavy elements (strontium, yttrium, barium and europium). This paper presents for the first time accurate results for heavy elements in this globular cluster. We did not find any systematic effect between the abundances found in turn-off stars, subgiants, and giants. We obtain the following mean abundances for these elements in our sample (turn-off stars and subgiants): [Sr/Fe] = 0.06 +/- 0.16, [Y/Fe] = -0.01 +/- 0.12, [Ba/Fe] = 0.18 +/- 0.11, and [Eu/Fe] = 0.41 +/- 0.09. The dispersion in the abundance ratios of the different elements is low and can be totally explained by uncertainties in their derivation. These ratios are in agreement with the values found in field halo stars with the same metallicity. We did not observe any correlation between the [n-capture/Fe] ratios and the star-to-star variations of the O and Na abundances. The [Ba/Eu] and [Sr/Ba] ratios show clearly that this globular cluster has also been uniformly enriched by r- and s-process synthesis.

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arXiv2003

Mass motions and chromospheres of RGB stars in the globular cluster NGC 2808

We present the results of the first observations, taken with FLAMES during Science Verification, of red giant branch (RGB) stars in the globular cluster NGC 2808. A total of 137 stars was observed, of which 20 at high resolution (R=47,000) with UVES and the others at lower resolution (R=19,000-29,000) with GIRAFFE in MEDUSA mode, monitoring ~ 3 mag down from the RGB tip. Spectra were taken of the Halpha, Na I D and Ca II H and K lines. This is by far the largest and most complete collection of such data in globular cluster giants, both for the number of stars observed within one cluster, and for monitoring all the most important optical diagnostics of chromospheric activity/mass motions.Evidence of mass motions in the atmospheres was searched from asymmetry in the profiles and coreshifts of the Halpha, Na I D and Ca II K lines, as well as from Halpha emission wings. We have set the detection thresholds for the onset of Halpha emission, negative Na D_2 coreshifts and negative K_3 coreshifts at log L/Lsun ~ 2.5, 2.9 and 2.8, respectively. These limits are significantly fainter than the results found by nearly all previous studies. Also the fraction of stars where these features have been detected has been increased significantly with respect to the previous studies. Our observations confirm the widespread presence of chromospheres among globular cluster giants, as it was found among Population I red giants. Some of the above diagnostics suggest clearly the presence of outward mass motions in the atmosphere of several stars.

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