arXiv subjects

A. Bragaglia

Publications and source records attributed to A. Bragaglia.

At least 199 records · Page 11Linked to original sources

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

Berkeley 22, an old and distant open cluster towards the Galactic anticentre

We present deep CCD BVI photometry of the distant, old open cluster Berkeley 22, covering from the red giants branch (RGB) to about 6 magnitudes below the main sequence (MS) turn-off. Using the synthetic Colour - Magnitude Diagram method with three different types of stellar evolutionary tracks, we estimate values and theoretical uncertainty of distance modulus mod0, reddening E(B-V), age tau and approximate metallicity. The best fit to the data is obtained for 13.8 <= mod0 <= 14.1, 0.64 <= E(B-V) <= 0.65, 2 <= tau <= 2.5 Gyr (depending on the amount of overshooting from convective regions adopted in the stellar models) and solar metallicity.

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arXiv2004

Structure of the Galactic Halo towards the North Galactic Pole

We have used RR Lyrae and Blue HB stars as tracers of the old Galactic halo, in order to study the halo structure and the galactic rotation as a function of height above the plane. Our sample includes 40 RR Lyrae and 80 BHB stars that are about 2 to 15 kpc above the plane, in a roughly 250 sq. deg. area around the North Galactic Pole (NGP). We use proper motions (derived from the GSC-II database) and radial velocities to determine the rotation of the halo. From the whole sample the motion appears to be significantly more retrograde than the samples in the solar neighborhood, confirming Majewski (1992) results and our own preliminary results based on 1/3 the present sample (Kinman et al. 2003; Spagna et al. 2003). However, the better statistics has now revealed the likely existence of two components, whose characteristics need an accurate analysis of systematic errors on the proper motions in order to be assessed in detail.

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arXiv2004

Variable stars in the bar of the Large Magellanic Cloud: the photometric catalogue

The catalogue of the Johnson-Cousins B,V and I light curves obtained for 162 variable stars (135 RR Lyrae, 4 candidate Anomalous Cepheids, 11 Classical Cepheids, 11 eclipsing binaries and 1 delta Scuti star) in two areas close to the bar of the Large Magellanic Cloud is presented along with coordinates, finding charts, periods, epochs, amplitudes, and mean quantities (intensity- and magnitude-averaged luminosities) of the variables with full coverage of the light variations. A star by star comparison is made with MACHO and OGLE II photometries based on both variable and constant stars in common, and the transformation relationships to our photometry are provided. The pulsation properties of the RR Lyrae stars in the sample are discussed in detail. Parameters of the Fourier decomposition of the light curves are derived for the fundamental mode RR Lyrae stars with complete and regular curves (29 stars). They are used to estimate metallicities, absolute magnitudes, intrinsic (B-V)o colours, and temperatures of the variable stars, according to Jurcsik and Kovacs (1996), and Kovacs and Walker (2001) method. Quantities derived from the Fourier parameters are compared with the corresponding observed quantities. In particular, the "photometric" metallicities are compared with the spectroscopic metal abundances derived by Gratton et al. (2004) from low resolution spectra obtained with FORS at the Very Large Telescope.

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arXiv2004

Radial velocities and membership of stars in the old, distant open cluster Berkeley 29

Multi slit spectroscopy at the Telescopio Nazionale Galileo was employed to measure radial velocities for 20 stars in the direction of the old open cluster Berkeley 29, the farthest known in our Galaxy. Membership information was derived for stars along all the red giant branch, in particular near its tip, and on the red clump. The sample of bona-fide cluster members was used to revise the cluster distance to about 15 kpc, on the basis of an empirical comparison with the red clump in open clusters with known distances. A metallicity [Fe/H] = -0.74 +/- 0.18 was also estimated using the colours of spectroscopically confirmed red giant stars.

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arXiv2004

Berkeley 29, the most distant old open cluster

We present CCD BVI photometry of the old open cluster Berkeley 29, located in the anticentre direction. Using the synthetic Colour - Magnitude Diagrams technique we estimate at the same time its age, reddening, distance, and approximate metallicity using three types of stellar evolutionary tracks. The best solutions give: age=3.4 or 3.7 Gyr, (m-M)_0 = 15.6 or 15.8 with E(B-V) = 0.13 or 0.10, and metallicity lower than solar (Z=0.006 or 0.004), depending on the adopted stellar models. Using these derived values, Be 29 turns out to be the most distant open cluster known, with Galactocentric distance R_GC= 21.4 to 22.6 kpc. Hence, Be 29 qualitatively follows both the age-metallicity relation and the metal abundance gradient typical of Galactic disc objects. The cluster position and radial velocity, however, appear to link Be 29 to the family of the Canis Major debris.

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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

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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arXiv2003

The chemical composition of the peculiar companion to the millisecond pulsar in NGC 6397

We present the chemical composition of the bright companion to the millisecond pulsar J1740-5340 in NGC 6397, based on high resolution spectra. Though the large rotation velocity of the star broadens the lines and complicates the analysis, the derived abundances are found fully compatible with those of normal unperturbed stars in NGC 6397, with the exception of a few elements (Li, Ca, and C). The lack of C suggests that the star has been peeled down to regions where incomplete CNO burning occurs, favouring a scenario where the companion is a turn-off star which has lost most of its mass. In addition we found an unexpected large Li abundance, which suggests that fresh 7Li has been produced on the stellar surface.

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arXiv2003

TNG photometry of the open cluster NGC 6939

We present CCD UBVI photometry of the intermediate age open cluster NGC 6939 for three TNG-DOLORES fields. The fields A and B cover the center of the cluster; the third one is located about 30 arcmin away, and is used for field stars decontamination. The V-I, B-V and U-B color-magnitude diagrams (CMDs), obtained joining very different exposures show: i) a Main Sequence (MS) extending down to V = 24, much deeper (~ 5 magnitudes) than any previous study; ii) a clearly defined Turn Off (TO) and iii) a well populated Red Giant Clump (RC) at about V = 13.

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arXiv2003

Distance scale and variable stars in Local Group Galaxies: LMC and Fornax

We briefly review our photometric and spectroscopic study of RR Lyrae variable stars in the bar of the Large Magellanic Cloud (LMC), that allowed us to reconcile the so-called "short" and "long" distance moduli of the LMC on the value mod_LMC=18.51 +/- 0.085 mag. Then we present preliminary results from the photometric study of a 33' x 34' area in the Fornax dwarf spheroidal galaxy containing the stellar clusters Fornax 3 (NGC 1049) and 6. We identified about 1000 candidate variables in this field of Fornax, and report the first detection and measure of about 60 RR Lyrae variable stars in the globular cluster Fornax 3.

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arXiv2003

The complex Halpha line profile of the bright companion to PSR J1740-5340 in NGC 6397

We present a detailed study of the Halpha and He I spectral features of COM J1740-5340 (the companion to PSR J1740-5340 in the Galactic Globular Cluster NGC 6397), exploiting a series of high resolution spectra obtained at different orbital phases. The Halpha absorption line shows a complex two-component structure, revealing that optically thin hydrogen gas resides outside the Roche lobe of COM J1740-5340. The line morphology precludes the existence of any residual disk around the millisecond pulsar, and suggests the presence of a stream of material going from the companion toward the neutron star. This material never reaches the neutron star surface, being driven back by the pulsar radiation far beyond COM J1740-5340. By analyzing the He I absorption lines as a function of orbital phase, we infer the presence of an overheated longitudinal strip (about 150 times narrower than it is long) on the COM J1740-5340 surface facing the radio pulsar.

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arXiv2003

Variable Stars in the LMC: the Photometric Catalogue

New B, V, I photometry was obtained for a sample of 152 variables (125 RR Lyrae's, 4 anomalous Cepheids, 11 classical Cepheids, 11 eclipsing binaries and a delta Scuti star) in two regions near the bar of the Large Magellanic Cloud (LMC). We derived complete and well sampled B and V light curves, very accurate periods, epochs of maximum light, amplitudes, intensity- and magnitude-averaged apparent luminosities for about 95% of the RR Lyrae stars. All these informations are contained in the photometric catalogue. Metallicities from the Fourier decomposition of the V light curves were derived for 29 of the ab type RR Lyrae's and are compared with the DeltaS metal abundances from low resolution spectra obtained with the Very Large Telescope (VLT).

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arXiv2003

Accurate mass ratio and heating effects in the dual-line millisecond binary pulsar in NGC 6397

By means of high-resolution spectra we have measured radial velocities of the companion (hereafter COM J1740-5340) to the eclipsing millisecond pulsar PSR J1740-5340 in the Galactic globular cluster NGC 6397. The radial-velocity curve fully confirms that COM J1740-5340 is orbiting the pulsar and enables us to derive the most accurate mass ratio (M_ PSR/M_COM=5.85+/-0.13) for any non-relativistic binary system containing a neutron star. Assuming a pulsar mass in the range 1.3-1.9 Msun, the mass of COM J1740-5340 spans the interval 0.22-0.32 Msun, the inclination of the system is constrained within 56 deg <= i <= 47 deg and the Roche lobe radius is r_RL ~ 1.5-1.7 Rsun. A preliminary chemical abundance analysis confirms that COM J1740-5340 has a metallicity compatible with that measured for other stars in this metal-poor globular, but the unexpected detection of strong He I absorption lines implies the existence of regions at T>10,000 K, significantly warmer than the rest of the star. The intensity of this line correlates with the orbital phase, suggesting the presence of a region on the companion surface, heated by the millisecond pulsar flux.

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arXiv2002

Distance to the Large Magellanic Cloud: the RR Lyrae stars

(abridged) New photometry and spectroscopy for more than a hundred RR Lyrae stars in two fields located close to the bar of the Large Magellanic Cloud (LMC) are used to derive new accurate estimates of the average magnitude, the local reddening, the luminosity-metallicity relation, and of the distance to the LMC. Metallicities from low resolution spectra obtained with the VLT have been derived for 101 RR Lyr's; the average value is [Fe/H]=-1.48+/-0.03 (sigma=0.29). An estimate of the reddening was obtained with 2 different methods, and we found: E(B-V)=0.116+/-0.017, and 0.086+/-0.017 mag in field A and B, respectively. The dereddened apparent average luminosity of the RR Lyrae's defined by the present photometry is _0=19.064+/-0.064 at [Fe/H]=-1.5. We determine the slope of the luminosity-metallicity relation for the RR Lyrae stars: Delta[M_V(RR)]/ Delta[Fe/H]=0.214+/-0.047, with no clear evidence for a change in slope at [Fe/H]=-1.5. When coupled with the absolute magnitude derived from the Baade-Wesselink and the statistical parallaxes methods, both methods known to favour the short distance scale, this value leads mod(LMC)=18.38+/- 0.16 and 18.30+/-0.14, respectively. If we use instead the absolute magnitude from the new Main Sequence Fitting of Galactic globular clusters [M_V(RR)=0.61+/-0.07 mag at [Fe/H]=-1.5] we derive mod(LMC)=18.45+/-0.09. The average I luminosity of the clump stars is I_0=18.12+/-0.06, and moves the clump distance modulus to the LMC to mod(LMC)=18.42+/-0.07 and 18.45+/-0.07 when two metallicity-I relations are used. All these values are only 1 sigma shorter than provided by the Pop I distance indicators, and allow to reconcile the short and long distance scale on a common value for the distance modulus of the LMC of 18.515+/-0.085 mag.

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arXiv2002

The Vertical Structure of the Halo Rotation

New GSC-II proper motions of RR Lyrae and Blue Horizontal Branch (BHB) stars near the North Galactic Pole are used to show that the Galactic Halo 5 kpc above the Plane has a significantly retrograde galactic rotation.

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