arXiv subjects

S. Ortolani

Publications and source records attributed to S. Ortolani.

At least 73 records · Page 4Linked to original sources

arXiv2011

Near-infrared thermal emissivity from ground based atmospheric dust measurements at ORM

We present an analysis of the atmospheric content of aerosols measured at Observatorio del Roque de los Muchachos (ORM; Canary Islands). Using a laser diode particle counter located at the Telescopio Nazionale Galileo (TNG) we have detected particles of 0.3, 0.5, 1.0, 3.0, 5.0 and 10.0 um size. The seasonal behavior of the dust content in the atmosphere is calculated. The Spring has been found to be dustier than the Summer, but dusty conditions may also occur in Winter. A method to estimate the contribution of the aerosols emissivity to the sky brightness in the near-infrared (NIR) is presented. The contribution of dust emission to the sky background in the NIR has been found to be negligible comparable to the airglow, with a maximum contribution of about 8-10% in the Ks band in the dusty days.

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arXiv2011

Alpha element abundances and gradients in the Milky Way bulge from FLAMES-GIRAFFE spectra of 650 K giants

We obtained FLAMES-GIRAFFE spectra (R=22,500) at the ESO Very Large Telescope for 650 bulge red giant branch (RGB) stars and performed spectral synthesis to measure Mg, Ca, Ti, and Si abundances. This sample is composed of 474 giant stars observed in 3 fields along the minor axis of the Galactic bulge and at latitudes b=-4, b=-6, b=-12. Another 176 stars belong to a field containing the globular cluster NGC 6553, located at b=-3 and 5 degrees away from the other three fields along the major axis. Our results confirm, with large number statistics, the chemical similarity between the Galactic bulge and thick disk, which are both enhanced in alpha elements when compared to the thin disk. In the same context, we analyze [alpha/Fe] vs. [Fe/H] trends across different bulge regions. The most metal rich stars, showing low [alpha/Fe] ratios at b=-4 disappear at higher Galactic latitudes in agreement with the observed metallicity gradient in the bulge. Metal-poor stars ([Fe/H]<-0.2) show a remarkable homogeneity at different bulge locations. We have obtained further constrains for the formation scenario of the Galactic bulge. A metal-poor component chemically indistinguishable from the thick disk hints for a fast and early formation for both the bulge and the thick disk. Such a component shows no variation, neither in abundances nor kinematics, among different bulge regions. A metal-rich component showing low [alpha/Fe] similar to those of the thin disk disappears at larger latitudes. This allows us to trace a component formed through fast early mergers (classical bulge) and a disk/bar component formed on a more extended timescale.

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arXiv2010

Fraction of clear skies above astronomical sites: a new analysis from the GOES12 satellite

Comparing the number of clear nights (cloud free) available for astronomical observations is a critical task because it should be based on homogeneous methodologies. Current data are mainly based on different judgements based on observer logbooks or on different instruments. In this paper we present a new homogeneous methodology on very different astronomical sites for modern optical astronomy, in order to quantify the available night time fraction. The data are extracted from night time GOES12 satellite infrared images and compared with ground based conditions when available. In this analysis we introduce a wider average matrix and 3-Bands correlation in order to reduce the noise and to distinguish between clear and stable nights. Temporal data are used for the classification. In the time interval 2007-2008 we found that the percentage of the satellite clear nights is 88% at Paranal, 76% at La Silla, 72.5% at La Palma, 59% at Mt. Graham and 86.5% at Tolonchar. The correlation analysis of the three GOES12 infrared bands B3, B4 and B6 indicates that the fraction of the stable nights is lower by 2% to 20% depending on the site.

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arXiv2010

Abundances for a large sample of red giants in NGC 1851: hints for a merger of two clusters?

We present the abundance analysis of a sample of more than 120 red giants in the globular cluster (GC) NGC 1851, based on FLAMES spectra. We find a small but detectable metallicity spread. This spread is compatible with the presence of two different groups of stars with a metallicity difference of 0.06-0.08 dex, in agreement with earlier photometric studies. If stars are divided into these two groups according to their metallicity, both components show a Na-O anticorrelation (signature of a genuine GC nature) of moderate extension. The metal-poor stars are more concentrated than the metal-rich ones. We tentatively propose the hypothesis that NGC 1851 formed from a merger of two individual GCs with a slightly different Fe and alpha-element content, and possibly an age difference up to 1 Gyr. This is supported also by number ratios of stars on the split subgiant and on the bimodal horizontal branches. The distribution of n-capture process elements in the two components also supports the idea that the enrichment must have occurred in each of the structures separately, and not as a continuum of events in a single GC. The most probable explanation is that the proto-clusters formed into a (now dissolved) dwarf galaxy and later merged to produce the present GC.

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arXiv2010

Insights on the Milky Way bulge formation from the correlations between kinematics and metallicity

Two main scenarios for the formation of the Galactic bulge are invoked, the first one through gravitational collapse or hierarchical merging of subclumps, the second through secular evolution of the Galactic disc. We aim to constrain the formation of the Galactic bulge through studies of the correlation between kinematics and metallicities in Baade's Window (l=1, b=-4) and two other fields along the bulge minor axis (l=0, b=-6 and b=-12). We combine the radial velocity and the [Fe/H] measurements obtained with FLAMES/GIRAFFE at the VLT with a spectral resolution of R=20000, plus for the Baade's Window field the OGLE-II proper motions, and compare these with published N-body simulations of the Galactic bulge. We confirm the presence of two distinct populations in Baade's Window found in Hill et al. 2010: the metal-rich population presents bar-like kinematics while the metal-poor population shows kinematics corresponding to an old spheroid or a thick disc one. In this context the metallicity gradient along the bulge minor axis observed by Zoccali et al. (2008), visible also in the kinematics, can be related to a varying mix of these two populations as one moves away from the Galactic plane, alleviating the apparent contradiction between the kinematic evidence of a bar and the existence of a metallicity gradient. We show evidences that the two main scenarios for the bulge formation co-exist within the Milky Way bulge.

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arXiv2010

The end of the white dwarf cooling sequence in M 67

In this paper, we present for the first time a proper-motion-selected white dwarf (WD) sample of the old Galactic open cluster M 67, down to the bottom of the WD cooling sequence (CS). The color-magnitude diagram is based on data collected with the LBC-Blue camera at the prime focus of LBT. As first epoch data, we used CFHT-archive images collected 10 years before LBC data. We measured proper motions of all the identified sources. Proper motions are then used to separate foreground and background objects from the cluster stars, including WDs. Finally, the field-object cleaned WD CS in the V vs. B-I color-magnitude diagram is compared with the models. We confirm that the age derived from the location of the bottom of the WD CS is consistent with the turn off age.

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arXiv2009

On a new Near-Infrared method to estimate the absolute ages of star clusters: NGC3201 as a first test case

We present a new method to estimate the absolute ages of stellar systems. This method is based on the difference in magnitude between the main sequence turn-off (MSTO) and a well defined knee located along the lower main sequence (MSK). This feature is caused by the collisionally induced absorption of molecular hydrogen and it can be easily identified in near-infrared (NIR) and in optical-NIR color-magnitude diagrams of stellar systems. We took advantage of deep and accurate NIR images collected with the Multi-Conjugate Adaptive Optics Demonstrator temporarily available on the Very Large Telescope and of optical images collected with the Advanced Camera for Surveys Wide Field Camera on the Hubble Space Telescope and with ground-based telescopes to estimate the absolute age of the globular NGC3201 using both the MSTO and the Delta(MSTO-MSK). We have adopted a new set of cluster isochrones and we found that the absolute ages based on the two methods agree to within one sigma. However, the errors of the ages based on the Delta(MSTO-MSK) method are potentially more than a factor of two smaller, since they are not affected by uncertainties in cluster distance or reddening.Current isochrones appear to predict slightly bluer (~0.05mag) NIR and optical-NIR colors than observed for magnitudes fainter than the MSK.

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arXiv2009

Analysis of the fraction of clear sky at the La Palma and Mt Graham sites

The fraction of available telescope time is one of the most important requirements for selecting astronomical sites affecting the performance of ground based telescopes. A quantitative survey of clouds coverage at La Palma and Mt.Graham is presented using both ground and satellite based data. The aim of this work is deriving clear nights for the satellite infrared channels and verifying the results using ground based observations. At La Palma we found a mean percentage of clear nights of 62.6% from ground and 71.9% from satellite. Taking into account the fraction of common nights we found a concordance of 80.7% clear nights from ground and satellite. At Mt.Graham we found a 97% of agreement between Columbine heliograph and night time observing log. From Columbine heliograph and TOMS-OMI satellite we found about 45% of clear nights, while satellite data (GOES, TOMS) are much more dispersed than those ones of La Palma. Setting a statistical threshold we retried a comparable seasonal trend between heliograph and satellite.

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arXiv2009

Chemical abundances for 11 bulge stars from high-resolution, near-IR spectra

It is debated whether the Milky Way bulge has the characteristics of a classical bulge sooner than those of a pseudobulge. Detailed abundance studies of bulge stars is a key to investigate the origin, history, and classification of the bulge. The aim is to add to the discussion on the origin of the bulge and to study detailed abundances determined from near-IR spectra for bulge giants already investigated with optical spectra, the latter also providing the stellar parameters which are very significant for the results of the present study. Especially, the important CNO elements are better determined in the near-IR. High-resolution, near-infrared spectra in the H band are recorded using the CRIRES spectrometer on the Very Large Telescope. The CNO abundances can all be determined from the numerous molecular lines in the wavelength range observed. Abundances of the alpha elements are also determined from the near-IR spectra. [O/Fe], [Si/Fe] and [S/Fe] are enhanced up to metallicities of at least [Fe/H]=-0.3, after which they decline. This suggests that the Milky Way bulge experienced a rapid and early star-formation history like that of a classical bulge. However, a similarity between the bulge trend and the trend of the local thick disk seems present. Such a similarity could suggest that the bulge has a pseudobulge origin. Our [C/Fe] trend does not show any increase with [Fe/H] which could have been expected if W-R stars have contributed substantially to the C abundances. No "cosmic scatter" can be traced around our observed abundance trends; the scatter found is expected, given the observational uncertainties.

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arXiv2009

Na-O Anticorrelation and HB. VII. The chemical composition of first and second-generation stars in 15 globular clusters from GIRAFFE spectra

We present abundances of Fe, Na, and O for 1409 red giant stars in 15 galactic globular clusters, derived from the homogeneous analysis of high resolution FLAMES/GIRAFFE spectra. Combining the present data with previous results, we obtained a total sample of 1958 stars in 19 clusters, the largest and most homogeneous database of this kind to date. Our GCs have [Fe/H] from -2.4 to -0.4, with a wide variety of global parameters (morphology of the horizontal branch, mass, concentration, etc). For all clusters we find the Na-O anticorrelation, the classical signature of proton-capture reactions in H-burning at high temperature in a previous generation of more massive stars, now extinct. Using quantitative criteria (from the morphology and extension of the Na-O anticorrelation), we can define 3 components of the stellar population in GCs: a primordial component (P) of first-generation stars, and 2 components of second-generation stars (intermediate I and extreme E populations from their different chemical composition). The P component is present in all GCs, and its fraction is almost constant at about one third. The I component represents the bulk of the cluster population. The E component is not present in all GCs, and it is more conspicuous in some (but not in all) of the most massive ones. We discuss the fractions and spatial distributions of these components in our sample and in two additional clusters (M3 and M13) from the literature. We also find that the slope of the anti-correlation (defined by the minimum O and maximum Na abundances) changes from cluster-to-cluster, a change that is represented well by a bilinear relation on cluster metallicity and luminosity. This second dependence suggests a correlation between average mass of polluters and cluster mass.

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arXiv2009

VLT-FLAMES Analysis of 8 giants in the Bulge Metal-poor Globular Cluster NGC 6522: Oldest Cluster in the Galaxy?

NGC 6522 has been the first metal-poor globular cluster identified in the bulge by W. Baade. Despite its importance, very few high resolution abundance analyses of stars in this cluster are available in the literature. The bulge metal-poor clusters may be important tracers of the early chemical enrichment of the Galaxy. The main purpose of this study is the determination of metallicity and elemental ratios in individual stars of NGC 6522. High resolution spectra of 8 giants of the bulge globular cluster NGC 6522 were obtained at the 8m VLT UT2-Kueyen telescope with the FLAMES+GIRAFFE spectrograph. Multiband V,I,J,Ks} photometry was used to derive effective temperatures as reference values. Spectroscopic parameters are derived from FeI and FeII lines, and adopted for the derivation of abundance ratios. The present analysis provides a metallicity [Fe/H] = -1.0+-0.2. The alpha-elements Oxygen, Magnesium and Silicon show [O/Fe]=+0.4, [Mg/Fe]=[Si/Fe]= +0.25, whereas Calcium and Titanium show shallower ratios of [Ca/Fe]=[Ti/Fe]=+0.15. The neutron-capture r-process element Europium appears to be overabundant by [Eu/Fe]=+0.4. The neutron-capture s-elements La and Ba are enhanced by [La/Fe]=+0.35 and [Ba/Fe]=+0.5. The large internal errors, indicating the large star-to-star variationin the Ba and Eu abundances, are also discussed. The moderate metallicity combined to a blue Horizontal Branch (BHB), are characteristics similar to those of HP~1 and NGC 6558, pointing to a population of very old globular clusters in the Galactic bulge. Also, the abundance ratios in NGC 6522 resemble those in HP 1 and NGC 6558. The ultimate conclusion is that the bulge is old, and went through an early prompt chemical enrichment.

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arXiv2009

Li-rich RGB stars in the Galactic Bulge

We present Lithium abundance determination for a sample of K giant stars in the galactic bulge. The stars presented here are the only 13 stars with detectable Lithium line (6767.18 A) among ~400 stars for which we have spectra in this wavelength range, half of them in Baade's Window (b=-4) and half in a field at b=-6. The stars were observed with the GIRAFFE spectrograph of FLAMES@VLT, with a spectral resolution of R~20,000. Abundances were derived via spectral synthesis and the results are compared with those for stars with similar parameters, but no detectable Li line. We find 13 stars with a detectable Li line, among which 2 have abundances A(Li)>2.7. No clear correlations were found between the Li abundance and those of other elements. With the exception of the two most Li rich stars, the others follow a fairly tight A(Li)-T_eff correlation. It would seems that there must be a Li production phase during the red giant branch (RGB), acting either on a very short timescale, or selectively only in some stars. The proposed Li production phase associated with the RGB bump cannot be excluded, although our targets are significantly brighter than the predicted RGB bump magnitude for a population at 8 kpc

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arXiv2009

On the various origins of close-in extrasolar planets

The extrasolar planets (EPs) so far detected are very different to the planets in our own Solar System. Many of them have Jupiter-like masses and close-in orbits (the so-called hot planets, HPs), with orbital periods of only a few days. In this paper, we present a new statistical analysis of the observed EPs, focusing on the origin of the HPs. Among the several HP formation mechanisms proposed so far, the two main formation mechanisms are type II migration and scattering. In both cases, planets form beyond the so-called snow-line of the protoplanetary disk and then migrate inward due to angular momentum and energy exchange with either the protoplanetary disk or with companion planets. Although theoretical studies produce a range of observed features, no firm correspondence between the observed EPs and models has yet been established. In our analysis, by means of principal component analysis and hierarchical cluster analysis, we find convincing indications for the existence of two types of HPs, whose parameters reflect physical mechanisms of type II migration and scattering.

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arXiv2008

Ground-based CCD astrometry with wide field imagers. III. WFI@2.2m proper-motion catalog of the globular cluster omega Centauri

Omega Centauri is the most well studied Globular Cluster because of its numerous puzzling features. Intensive spectroscopic follow-up observing campaigns targeting stars at different positions on the color-magnitude diagram promises to clarify some of these peculiarities. To able to target cluster members reliably during spectroscopic surveys and both spatial and radial distributions in the cluster outskirts without including field stars, a high quality proper-motion catalog of omega Cen and membership probability determination are required. The only available wide field proper-motion catalog of omega Cen is derived from photographic plates, and only for stars brighter than B~16. Using ESO archive data, we create a new, CCD-based, proper-motion catalog for this cluster, extending to B~20. We used the high precision astrometric software developed specifically for data acquired by WFI@2.2m telescope and presented in the first paper of this series. We achieved a good cluster-field separation with a temporal base-line of only four years. We corrected our photometry for sky-concentration effects. We provide calibrated photometry for UBVRI wide-band data plus narrow-band filter data centered on H_alpha for almost 360 000 stars. We can confirm that the omega Cen metal-poor and the metal-rich components have the same proper motion, and demonstrate that the metal-intermediate component in addition exhibits the same mean motion as the other RGB stars. We provided membership probability determination for published omega Cen variable star catalogs. Our catalog extends the proper-motion measurements to fainter than the cluster turn-off luminosity, and covers a wide area (~33'x33') around the center of omega Cen. Our catalog is now electronically available to the astronomical community.

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arXiv2008

Astronomical site selection: On the use of satellite data for aerosol content monitoring

The main goal of this work is the analysis of new approaches to the study of the properties of astronomical sites. In particular, satellite data measuring aerosols have recently been proposed as a useful technique for site characterization and searching for new sites to host future very large telescopes. Nevertheless, these data need to be critically considered and interpreted in accordance with the spatial resolution and spectroscopic channels used. In this paper we have explored and retrieved measurements from satellites with high spatial and temporal resolutions and concentrated on channels of astronomical interest. The selected datasets are OMI on board the NASA Aura satellite and MODIS on board the NASA Terra and Aqua satellites. A comparison of remote sensing and in situ techniques is discussed. As a result, we find that aerosol data provided by satellites up to now are not reliable enough for aerosol site characterization, and in situ data are required.

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arXiv2008

Multi-Conjugate Adaptive Optics VLT imaging of the distant old open cluster FSR1415

We employ the recently installed near infrared Multi-Conjugate Adaptive optics demonstrator (MAD) to determine basic properties of a newly identified, old and distant, Galactic open cluster (FSR1415). The MAD facility remarkably approaches the diffraction limit, reaching a resolution of 0.07 arcsec (in K), that is also uniform in a field of ~1.8 arcmin in diameter. The MAD facility provides photometry that is 50% complete at K~19. This corresponds to about ~2.5 mag below the cluster main sequence turn-off. This high quality data set allows us to derive an accurate heliocentric distance of ~8.6 kpc, a metallicity close to solar, and an age of ~2.5 Gyr. On the other hand, the deepness of the data allow us to reconstruct (completeness-corrected) mass functions indicating a relatively massive cluster, with a flat core MF. The VLT/MAD capabilities will therefore provide fundamental data in identifying/analyzing other faint and distant open clusters in the Galaxy III and IV quadrants.

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arXiv2008

Obscured clusters. II. GLIMPSE-C02 - A new metal rich globular cluster in the Milky Way

The estimated total number of Milky Way globulars is 160+-20. The question of whether there are any more undiscovered globular clusters in the Milky Way is particularly relevant with advances in near and mid-IR instrumentation. This investigation is a part of a long-term project to search the inner Milky Way for hidden star clusters and to study them in detail. GLIMPSE-C02 (G02) is one of these objects, situated near the Galactic plane (l=14.129deg, b=-0.644deg). Our analysis is based on SOFI/NTT JHKs imaging and low resolution (R~1400) spectroscopy of three bright cluster red giants in the K atmospheric window. We derived the metal abundance by analysis of these spectra and from the slope of the RGB. The cluster is deeply embedded in dust and undergoes a mean reddening of Av~24.8+-3 mag. The distance to the object is D=4.6+-0.7kpc. The metal abundance of G02 is [Fe/H](H96)=-0.33+-0.14 and [Fe/H](CG)=-0.16+-0.12 using different scales. The best fit to the radial surface brightness profile with a single-mass King's model yields a core radius rc=0.70 arcmin (0.9pc), tidal radius rt=15 arcmin (20pc), and central oncentration c=1.33. We demonstrate that G02 is new Milky Way globular cluster, among the most metal rich globular clusters in the Galaxy. The object is physically located at the inner edge of the thin disk and the transition region with the bulge, and also falls in the zone of the "missing" globulars toward the central region of the Milky Way.

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arXiv2008

The Metal Content of Bulge Field Stars from FLAMES-GIRAFFE Spectra. I. Stellar parameters and Iron Abundances

We determine the iron distribution function (IDF) for bulge field stars, in three different fields along the Galactic minor axis and at latitudes b=-4 deg, b=-6 deg, and b=-12 deg. A fourth field including NGC6553 is also included in the discussion. About 800 bulge field K giants were observed with the GIRAFFE spectrograph of FLAMES@VLT at spectral resolution R~20,000. Several of them were observed again with UVES at R~45,000 to insure the accuracy of the measurements. The LTE abundance analysis yielded stellar parameters and iron abundances that allowed us to construct an IDF for the bulge that, for the first time, is based on high-resolution spectroscopy for each individual star. The IDF derived here is centered on solar metallicity, and extends from [Fe/H]~ -1.5 to [Fe/H]~ +0.5. The distribution is asymmetric, with a sharper cutoff on the high-metallicity side, and it is narrower than previously measured. A variation in the mean metallicity along the bulge minor axis is clearly between b=-4 deg and b=-6 deg ([Fe/H] decreasing by ~ 0.6 dex per kpc). The field at b=-12 deg is consistent with the presence of a gradient, but its quantification is complicated by the higher disk/bulge fraction in this field. Our findings support a scenario in which both infall and outflow were important during the bulge formation, and then suggest the presence of a radial gradient, which poses some challenges to the scenario in which the bulge would result solely from the vertical heating of the bar.

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