arXiv subjects

L. Di Fabrizio

Publications and source records attributed to L. Di Fabrizio.

At least 37 records · Page 2Linked to original sources

arXiv2010

A New Census of the variable star population in the Globular Cluster NGC 2419

We present B, V and I CCD light curves for 101 variable stars belonging to the globular cluster NGC2419, 60 of which are new discoveries, based on datasets obtained at the TNG, SUBARU and HST telescopes. The sample includes 75 RR Lyrae stars (of which 38 RRab, 36 RRc and one RRd), one Population II Cepheid, 12 SX Phoenicis variables, 2 DeltaScuti stars, 3 binary systems, 5 long-period variables, and 3 variables of uncertain classification. The pulsation properties of the RR Lyrae variables are close to those of Oosterhoff type II clusters, consistent with the low metal abundance and the cluster horizontal branch morphology,disfavoring (but not totally ruling out) an extragalactic hypotesis for the origin of NGC2419. The observed properties of RR Lyrae and SX Phoenicis stars are used to estimate the cluster reddening and distance, using a number of different methods. Our final value is mo(NGC2419)=19.71+/-0.08mag (D= 87.5+/-3.3kpc), with E(B-V)=0.08+/-0.01mag, [Fe/H]=-2.1dex in the Zinn & West metallicity scale, and a value of Mv that sets mo(LMC)=18.52mag. This value is in good agreement with most recent literature estimates of the distance to NGC 2419.

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arXiv2009

The Leo IV dwarf spheroidal galaxy: color-magnitude diagram and pulsating stars

We present the first V, B-V color-magnitude diagram of the Leo IV dwarf spheroidal galaxy, a faint Milky Way satellite recently discovered by the Sloan Digital Sky Survey. We have obtained B,V time-series photometry reaching about half a magnitude below the Leo IV turnoff, which we detect at V= 24.7 mag, and have performed the first study of the variable star population. We have identified three RR Lyrae stars (all fundamental-mode pulsators, RRab) and one SX Phoenicis variable in the galaxy. In the period-amplitude diagram the Leo IV RR Lyrae stars are located close to the loci of Oosterhoff type I systems and the evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. However, their mean pulsation period, $ $=0.655 days, would suggest an Oosterhoff type II classification for this galaxy. The RR Lyrae stars trace very well the galaxy's horizontal branch, setting its average magnitude at $ = 21.48 \pm 0.03$ mag (standard deviation of the mean). This leads to a distance modulus of $μ_{0}=20.94 \pm 0.07$ mag, corresponding to a distance of $154 \pm 5$ kpc, by adopting for the Leo IV dSph a reddening $E(B-V) = 0.04 \pm 0.01$ mag and a metallicity of [Fe/H] = -2.31 $\pm$ 0.10.

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arXiv2009

Pulsating Variable Stars in the Coma Berenices dwarf spheroidal galaxy

We present B, V, I time-series photometry of the Coma Berenices dwarf spheroidal galaxy, a faint Milky Way satellite, recently discovered by the Sloan Digital Sky Survey. We have obtained V, B-V and V, V-I color-magnitude diagrams that reach V~23.0-23.2 mag showing the galaxy turnoff at V~21.7 mag, and have performed the first study of the variable star population of this new Milky Way companion. Two RR Lyrae stars (a fundamental-mode -RRab- and a first overtone -RRc- pulsator) and a short period variable with period P=0.12468 days were identified in the galaxy. The RRab star has a rather long period of P_ab=0.66971 days and is about 0.2 mag brighter than the RRc variable and other non-variable stars on the galaxy horizontal branch. In the period-amplitude diagram the RRab variable falls closer to the loci of Oosterhoff type-II systems and evolved fundamental-mode RR Lyrae stars in the Galactic globular cluster M3. The average apparent magnitude of the galaxy horizontal branch, =18.64+-0.04 mag, leads to a distance modulus for the Coma dSph mu_0=18.13+-0.08 mag, corresponding to a distance d=42^{+2}_{-1} kpc, by adopting a reddening E(B-V) = 0.045 +- 0.015 mag and a metallicity [Fe/H]=-2.53 +- 0.05 dex.

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arXiv2007

On the newly discovered Canes Venatici II dSph galaxy

We report on the detection of variable stars in the Canes Venatici II (CVn II) dwarf spheroidal galaxy, a new satellite of the Milky Way recently discovered by the Sloan Digital Sky Survey. We also present a V, B-V color-magnitude diagram that reaches V = 25.5 mag, showing the galaxy's main sequence turn off at V = 24.5 mag and revealing several candidate blue straggler stars. Two RR Lyrae stars have been identified within the half-light radius of CVn II,a fundamental-mode variable (RRab) with period P_ab = 0.743 days, and a first-overtone (RRc) RR Lyrae star with P_c = 0.358 days. The rather long periods of these variables along with their position on the period-amplitude diagram support an Oosterhoff type II classification for CVn II. The average apparent magnitude of the RR Lyrae stars, = 21.48 +/- 0.02 mag, is used to obtain a precision distance modulus of mu_0 = 21.02 +/- 0.06 mag and a corresponding distance of 160(+4,-5} kpc, for an adopted reddening E(B-V) = 0.015 mag.

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arXiv2007

NGC 2419: an "intergalactic wanderer" or a simple Galactic Globular Cluster?

We have carried out a new photometric study of the remote Galactic globular cluster NGC 2419, using proprietary and archive B,V,I time-series CCD photometry of the cluster, that allowed us to discover a large number of new variable stars and to obtain a new colour magnitude diagram that reaches V ~26 mag over a field of 50X43 square arcmin centered on NGC 2419. The new variables include 39 RR Lyrae and 11 SX Phoenicis stars. The pulsation properties of the new RR Lyrae stars confirm and strengthen the classification of NGC 2419 as an Oosterhoff type II cluster.

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arXiv2007

On the remote Galactic globular cluster NGC 2419

We present a new, deep (V ~ 26) study of the Galactic globular cluster NGC 2419 based on B,V,I time-series CCD photometry over about 10 years and extending beyond the cluster published tidal radius. We have identified 101 variable stars of which 60 are new discoveries, doubling the known RR Lyrae stars and including 12 SX Phoenicis stars. The average period of the RR Lyrae stars ( =0.662 d, and =0.366 d, for fundamental-mode -RRab- and first-overtone pulsators, respectively), and the position in the period-amplitude diagram both confirm that NGC 2419 is an Oosterhoff II cluster. The average apparent magnitude of the RR Lyrae stars is =20.31 +/- 0.01 (sigma=0.06, 67 stars) and leads to the distance modulus (m-M)o=19.60 +/- 0.05. The Color-Magnitude Diagram, reaching about 2.6 mag below the cluster turn-off, does not show clear evidence of multiple stellar populations. Cluster stars are found until r~ 10.5', and possibly as far as r~15', suggesting that the literature tidal radius might be underestimated. No extra-tidal structures are clearly detected in the data. NGC 2419 has many blue stragglers and a well populated horizontal branch extending from the RR Lyrae stars down to an extremely blue tail ending with the "blue-hook", for the first time recognized in this cluster. The red giant branch is narrow ruling out significant metallicity spreads. Our results seem to disfavor the interpretation of NGC 2419 as either having an extragalactic origin or being the relict of a dwarf galaxy tidally disrupted by the Milky Way.

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arXiv2006

Variable stars in the newly discovered Milky Way satellite in Bootes

We present $V,I$ light curves for 12 variable stars identified in the newly discovered satellite of the Milky Way in the Bootes constellation (Belokurov et al. 2006).Our sample includes 11 RR Lyrae stars (5 first overtone, 5 fundamental mode and 1 double mode pulsator),and one long period variable close to the galaxy red giant branch tip. The RR Lyrae stars trace very well the average $V$ luminosity of the galaxy horizontal branch, leading to a true distance modulus for the galaxy of $μ_0$=19.11 $\pm$ 0.08 mag for an assumed metal abundance of [Fe/H]=-2.5 (Monoz et al. 2006), and for $E(B-V)$=0.02 mag. Average periods are =0.69 d and =0.37 d for {\it ab-} and {\it c-} type RR Lyrae stars, respectively, making of Bootes the second pure Oosterhoff type II (OoII) dSph after Ursa Minor. The location of the double mode RR Lyrae (RRd) in the Petersen diagram is consistent with RRd stars in OoII clusters, and corresponds to an intrinsic luminosity of $log L/logL\odot$=1.72 (for Z=10$^{-4}$ and M=0.80 M$\odot$) according to Bono et al. (1996) pulsation models.

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arXiv2006

The old anticentre open cluster Berkeley 32: membership and fundamental parameters

We have obtained medium-low resolution spectroscopy and BVI CCD imaging of Berkeley 32, an old open cluster which lies in the anticentre direction. From the radial velocities of 48 stars in the cluster direction we found that 31 of them, in crucial evolutionary phases, are probable cluster members, with an average radial velocity of +106.7 (sigma = 8.5) km/s. From isochrone fitting to the colour magnitude diagrams of Berkeley 32 we have obtained an age of 6.3 Gyr, (m-M)0 = 12.48 and E(B-V) = 0.10. The best fit is obtained with Z=0.008. A consistent distance, (m-M)0 ~= 12.6 +/- 0.1, has been derived from the mean magnitude of red clump stars with confirmed membership; we may assume (m-M)0 ~= 12.55 +/- 0.1. The colour magnitude diagram of the nearby field observed to check for field stars contamination looks intriguingly similar to that of the Canis Major overdensity.

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arXiv2005

Variable stars in the globular cluster NGC 2419

We have used DOLORES at the TNG to obtain B,V time series photometry of NGC 2419, one of the most distant and bright clusters in the Galactic halo. These data will be used to study its variable star population in order to check whether the cluster could be the relic of an extragalactic system accreted by the Milky Way. Using the Image Subtraction technique (Alard 2000) we have identified about 300 candidate variables, many of which are in the cluster central regions. Several of the variables appear to be RR Lyrae stars, but we detected variability also around the tip of the red giant branch, and in other regions of the colour-magnitude diagram. To improve the light curve sampling and to resolve variables in the cluster inner regions, the TNG data were combined with HST archive data. Preliminary results are presented on the light curves from the combined data set.

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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

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

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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arXiv2003

Variable stars as tracers of stellar populations in Local Group galaxies: Leo I and NGC 6822

Results are presented of a study of the variable star populations in the dwarf spheroidal galaxy Leo I and in the dwarf irregular galaxy NGC 6822, based on time series photometry obtained with the Wide Field Imager of the 2.2 m ESO/MPI telescope (Leo I) and the Very Large Telescope (NGC 6822). We found about 250 (lower limit) variables in Leo I most of which are RR Lyrae stars. In NGC 6822 we identified 450 candidate variables among which about 20 are RR Lyrae stars, and many are low-luminosity, small-amplitude Cepheids.

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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

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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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

Anomalous RR Lyrae stars(?). III. CM Leonis

Time series of B,V,I CCD photometry and radial velocity measurements from high resolution spectroscopy (R=30,000) covering the full pulsation cycle are presented for the field RR Lyrae star CM Leonis. The photometric data span a 6 year interval from 1994 to 1999, and allow us to firmly establish the pulsation mode and periodicity of the variable. The derived period P=0.361699 days (+/- 0.000001) is very close to the value published in the Fourth Edition of the General Catalogue of Variable Stars (P=0.361732 days). However, contrary to what was previously found, the amplitude and shape of the light curve qualify CM Leo as a very regular first overtone pulsator with a prominent hump on the rising branch of its multicolour light curves. According to an abundace analysis performed on three spectra taken near minimum light (0.42 < phase < 0.61), CM Leo is a metal-poor star with metal abundance [Fe/H]=-1.93 +/- 0.20. The photometric and radial velocity curves of CM Leo have been compared with the predictions of suitable pulsational models to infer tight constraints on the stellar mass, effective temperature, and distance modulus of the star. We derive a true distance modulus of CM Leo of (m-M)0=13.11 +/- 0.02 mag and a corresponding absolute magnitude of Mv=0.47 +/- 0.04. This absolute magnitude, once corrected for evolutionary and metallicity effects, leads to a true distance modulus of the Large Magellanic Cloud of (m-M)0=18.43 +/- 0.06 mag, in better agreement with the long astronomical distance scale.

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