arXiv subjects

G. Bono

Publications and source records attributed to G. Bono.

At least 199 records · Page 11Linked to original sources

arXiv2004

The influence of chemical composition on the properties of Cepheid stars I - Period-Luminosity relation vs iron abundance

We have assessed the influence of the stellar iron content on the Cepheid Period-Luminosity (PL) relation by relating the V band residuals from the Freedman et al (2001) PL relation to [Fe/H] for 37 Galactic and Magellanic Clouds Cepheids. The iron abundances were measured from FEROS and UVES high-resolution and high-signal to noise optical spectra. Our data indicate that the stars become fainter as metallicity increases, until a plateau or turnover point is reached at about solar metallicity. Our data are incompatible with both no dependence of the PL relation on iron abundance, and with the linearly decreasing behavior often found in the literature (e.g. Kennicutt et al 1998, Sakai et al 2004). On the other hand, non-linear theoretical models of Fiorentino et al (2002) provide a fairly good description of the data.

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arXiv2004

Spectroscopic survey of the Galaxy with Gaia I. Design and performance of the Radial Velocity Spectrometer

The definition and optimisation studies for the Gaia satellite spectrograph, the Radial Velocity Spectrometer (RVS), converged in late 2002 with the adoption of the instrument baseline. This paper reviews the characteristics of the selected configuration and presents its expected performance. The RVS is a 2.0 by 1.6 degree integral field spectrograph, dispersing the light of all sources entering its field of view with a resolving power R=11 500 over the wavelength range [848, 874] nm. The RVS will continuously and repeatedly scan the sky during the 5 years of the Gaia mission. On average, each source will be observed 102 times over this period. The RVS will collect the spectra of about 100-150 million stars up to magnitude V~17-18. At the end of the mission, the RVS will provide radial velocities with precisions of ~2 km/s at V=15 and \~15-20 km/s at V=17, for a solar metallicity G5 dwarf. The RVS will also provide rotational velocities, with precisions (at the end of the mission) for late type stars of sigma_vsini ~5 km/s at V~15 as well as atmospheric parameters up to V~14-15. The individual abundances of elements such as Silicon and Magnesium, vital for the understanding of Galactic evolution, will be obtained up to V~12-13. Finally, the presence of the 862.0 nm Diffuse Interstellar Band (DIB) in the RVS wavelength range will make it possible to derive the three dimensional structure of the interstellar reddening.

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arXiv2004

The Araucaria Project. An improved distance to the Sculptor spiral galaxy NGC 300 from its Cepheid variables

In a previous paper, we reported on the discovery of more than a hundred new Cepheid variables in the Sculptor Group spiral NGC 300 from wide-field images taken in the B and V photometric bands at ESO/La Silla. In this paper, we present additional VI data, derive improved periods and mean magnitudes for the variables, and construct period-luminosity relations in the V, I and the reddening-independent (V-I) Wesenheit bands using 58 Cepheid variables with periods between 11 and 90 days. We obtain tightly defined relations, and by fitting the slopes determined for the LMC Cepheids by the OGLE II Project we obtain reddening-corrected distances to the galaxy in all bands. We adopt as our best value the distance derived from the reddening-free Wesenheit magnitudes, which is 26.43 $\pm$ 0.04 (random) $\pm$ 0.05 (systematic) mag. We argue that our current distance result for NGC 300 is the most accurate which has so far been obtained using Cepheid variables, and that it is largely free from systematic effects due to metallicity, blending, and sample selection. It agrees very well with the recent distance determination from the tip of the red giant branch method obtained from HST data by Butler et al. (2004), and it is consistent with the Cepheid distance to NGC 300 which was derived by Freedman et al. (2001) from CCD photometry of a smaller sample of stars.

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arXiv2004

The distance to the LMC cluster Reticulum from the K-band Period-Luminosity-Metallicity relation of RR Lyrae stars

We present new and accurate Near-Infrared J and Ks-band data of the Large Magellanic Cloud cluster Reticulum. Data were collected with SOFI available at NTT and covering an area of approximately (5 x 5) arcmin^2 around the center of the cluster. Current data allowed us to derive accurate mean K-band magnitudes for 21 fundamental and 9 first overtone RR Lyrae stars. On the basis of the semi-empirical K-band Period-Luminosity-Metallicity relation we have recently derived, we find that the absolute distance to this cluster is 18.52 +- 0.005 (random) +- 0.117 (systematic). Note that the current error budget is dominated by systematic uncertainty affecting the absolute zero-point calibration and the metallicity scale.

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arXiv2003

The Red Giant Branch Luminosity Function Bump

We present observational estimates of the magnitude difference between the luminosity function RGB bump and the HB (Delta(bump-zahb)), and of star counts in the bump region (R_bump), for a sample of 54 Galactic globular clusters (GCs) observed by the HST. The large sample of stars resolved in each cluster, and the high photometric accuracy of the data allowed us to detect the bump also in a number of metal poor clusters. To reduce the photometric uncertainties, empirical values are compared with theoretical predictions obtained from a set of updated canonical stellar evolution models which have been transformed directly into the HST flight system. We found an overall qualitative agreement between theory and observations. Quantitative estimates of the confidence level are hampered by current uncertainties on the GCs metallicity scale, and by the strong dependence of Delta(bump-zahb) on the cluster metallicity. In case of the R_bump parameter, which is only weakly affected by the metallicity, we find a very good quantitative agreement between theoretical canonical models and observations. For our full cluster sample the average difference between predicted and observed R_bump values is practically negligible, and ranges from -0.002 to -0.028, depending on the employed metallicity scale. The observed dispersion around these values is entirely consistent with the observational errors on R_bump. As a comparison, the value of R_bump predicted by theory in case of spurious bump detections would be \~0.10 smaller than the observed ones. We have also tested the influence on the predicted Delta(bump-zahb) and R_bump values of an He-enriched component in the cluster stellar population. We find that the predicted Delta(bump-zahb) and R_bump values are only marginally affected.

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arXiv2003

A pulsational approach to near infrared and visual magnitudes of RR Lyrae stars

In this paper we present an improved theoretical scenario concerning near infrared and visual magnitudes of RR Lyrae variables, as based on up-to-date pulsating models. On this basis, we revisit the case of the prototype variable RR Lyr, showing that the parallax inferred by this new pulsational approach appears in close agreement with HST absolute parallax. Moreover, available K and V measurements for field and cluster RR Lyrae variables with known reddening and metal content are used to derive a relation connecting the K absolute magnitude to period and metallicity, as well as a new calibration of the M_V-[Fe/H] relation. The comparison between theoretical prescriptions and observations suggests that RR Lyrae stars in the field and in Galactic Globular Clusters should have quite similar evolutionary histories. The comparison between theory and observations also discloses a general agreement that supports the reliability of current pulsational scenario. On the contrary, current empirical absolute magnitudes based on the Baade-Wesselink (BW) method suggest relations with a zero-point that is fainter than predicted by pulsation models, together with a milder metallicity dependence. However, preliminary results based on a new calibration of the BW method provided by Cacciari et al. (2000) for RR Cet and SW And appear in a much better agreement with the pulsational predictions.

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arXiv2003

Near-Infrared photometry of LMC cluster Reticulum

We present near-infrared (JKs) time series data of the Large Magellanic Cloud (LMC) cluster Reticulum. The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of the order of 0.02 mag at limiting magnitudes typical of RR Lyrae stars. We are interested in Reticulum, since it hosts a sizable sample of RR Lyrae (32), and therefore the use of the K-band Period-Luminosity-Metallicity (PLZ_K) relation will allow us to supply an accurate LMC distance evaluation. The main advantages in using this method is that it is marginally affected by off-ZAHB evolutionary effects and by reddening corrections. As a preliminary but robust result we find a true distance in good agreement with the LMC Cepheid distance scale, i.e. DM = 18.47 +- 0.07 mag.

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arXiv2003

The IR counterpart to the Anomalouos X-ray Pulsar 1RXS J1708-4009

We report the discovery of the likely IR counterpart to the Anomalous X-ray Pulsar (AXP) 1RXS J1708-4009, based on the combination of Chandra HRC-I X-ray position, and deep optical/IR observations carried out from ESO telescopes and the Canada France Hawaii Telescope (CFHT) during 1999-2002. Within the narrow uncertainty region we found two relatively faint (K'=20.0 and K'=17.53) IR objects. Based on their color and position in the J-K' versus J-H diagram only the brighter object is consistent with the known IR properties of the counterparts to other AXPs. No variability was detected for this source, similarly to what is observed in the case of 4U 0142+614. Like in other AXPs, we found that the IR flux of 1RXSJ1708-4009 is higher then expected for a simple blackbody component extrapolated from the X-ray data. If confirmed, this object would be the fourth IR counterpart to a source of the AXP class, and would make the IR excess a likely new characteristic of AXPs.

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arXiv2003

The Carina Project: II. Stellar Populations

We present a new (V,B-V) Color-Magnitude Diagram (CMD) of the Carina dwarf spheroidal (dSph) that extends from the tip of the Red Giant Branch (RGB) down to about V=25 mag. Data were collected with the Wide Field Imager (WFI) available at 2.2m ESO/MPI telescope and cover an area of about 0.3 square degrees around the center of the galaxy. We confirm the occurrence of a substantial amount of old stars with ages around 11 Gyr, together with an intermediate-age population around 5 Gyr. Moreover, we also detected a new well-defined blue plume of young Main-Sequence (MS) stars with an age at most of the order of 1 Gyr. At odds with previous results available in the literature we found thatstars along the RGB of the old and of the intermediate-age stellar populationindicate a mean metallicity roughly equal to Z=0.0004 and a small dispersion around this value. We find evidence for a smooth spatial distribution of the "intermediate-age" stellar population, which appears more centrally concentrated than the oldest one.

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arXiv2003

The Carina Project: I. Bright Variable Stars

We present new BV time series data of the Carina dwarf spheroidal galaxy. Current data cover an area of about 0.3 square degrees around the center of the galaxy and allowed us to identify 92 variables. Among them 75 are RR Lyrae stars, 15 are "bona fide" Anomalous Cepheids, one might be a Galactic field RR Lyrae, and one is located along the Carina Red Giant Branch. Expanding upon the seminal photographic investigation by Saha, Monet & Seitzer (1986) we supply for the first time accurate estimates of their pulsation parameters (periods, amplitudes, mean magnitude and colors) on the basis of CCD photometry. Approximately 50% of both RR Lyrae and Anomalous Cepheids are new identifications. Among the RR Lyrae sample 6 objects are new candidate double-mode variables. On the basis of their pulsation properties we estimated that two variables (V152, V182) are about 50% more massive than typical RR Lyrae stars, while the bulk of the Anomalous Cepheids are roughly a factor of two more massive than fundamental mode RR Lyrae stars. This finding supports the evidence that these objects are intermediate-mass stars during central He burning phases. The data for Carina, together with data available in the literature, strongly support the conclusion that dSph galaxies can barely be classified into the classical Oosterhoff dichotomy. The mean period of fundamental mode RR Lyrae in Carina resembles that found for Oosterhoff type II clusters, whereas the ratio between first overtones and total number of RR Lyrae is quite similar to that found in Oosterhoff type I clusters.

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arXiv2003

"The Carina Project": color magnitude diagram and radial distribution

We present B, V photometric data of the Carina Dwarf Spheroidal Galaxy collected with the Wide Field Imager (WFI) available at the 2.2m MPI/ESO telescope. We briefly discuss the main features of the color magnitude diagram and in particular the mix of stellar populations present in this galaxy. A preliminary analysis of the spatial distribution of these populations over a substantial fraction of the body of the galaxy is also presented.

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arXiv2002

Multiwavelength Time Series Data of the LMC Cluster Reticulum

We present accurate multiwavelength UBVI time series data of the LMC cluster Reticulum. Data cover a time interval of ~ 6 yr and have been collected with SUSI1/2 and SOFI at NTT/ESO. For each band we collected approximately 30 short/long exposures and the total exposure times range from roughly 3500 (U, B), 6000s (V) to 8300s (I). The observing strategy and data reduction (DAOPHOTII/ALLFRAME) allowed us to reach a photometry accuracy of 0.02 magnitude from the tip of the Red Giant Branch well below the Turn-Off region. Even though this cluster presents a very low central density, we found a sizable sample of Blue Stragglers (BSs). We also selected stars with a variability index larger than 2, and interestingly enough we detected together with the RR Lyrae stars a large sample of variable stars around and below the TO region. Preliminary analysis on the luminosity variation indicate that these objects might be binary stars.

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arXiv2002

Cosmic Distances: Current Odds and Future Perspectives

We discuss recent theoretical and empirical results concerning cosmic distances and discuss the problems affecting the Cepheid and the RR Lyrae distance scales. In particular, we outline the key role that first overtone Cepheids can play to improve the accuracy of distance determinations to nearby galaxies. We also address the positive features of the K-band Period-Luminosity-Metallicity (PLZ_K) relation of RR Lyrae stars when compared to the M_V vs [Fe/H] relation. Moreover, we discuss the impact that accurate multiband data in external galaxies can have on the evolutionary properties of intermediate and low-mass stars. Finally, we introduce possible avenues of future research in stellar astrophysics.

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arXiv2002

A New Feature Along the Extended Blue Horizontal Branch of NGC 6752

In this letter we report on the detection of a new feature in the complex structure of the horizontal branch (HB) of the Galactic globular cluster NGC 6752. In the U vs. (U-V) plane, the HB shows a discontinuity (``jump'') at U-V ~= -1.0 (corresponding T_e ~ 23,000 K). This ``second U-jump'' adds to the u-jump identified by Grundahl et al. (1999) in a dozen of clusters at T_e ~ 11,500 K. We show that this new discontinuity might be due to the combination of post zero age HB evolution and diffusion effects. We identify 11 AGB-manque stars. The comparison between post-HB star counts and evolutionary lifetimes, as predicted by canonical stellar models, shows good agreement, at variance with similar estimates for NGC 6752 available in the literature.

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arXiv2002

On the distance of Magellanic Clouds: first overtone Cepheids

We present a detailed comparison between predicted and empirical PL_{I,K} relations and Wesenheit function for Galactic and Magellanic Clouds (MCs) First Overtone (FO) Cepheids. We find that zero-points predicted by Galactic Cepheid models based on a noncanonical (mild overshooting) Mass-Luminosity (ML) relation are in very good agreement with empirical zero-points based on HIPPARCOS parallaxes, while those based on canonical (no overshooting) ML relation are about 0.2-0.3 mag brighter. We also find that predicted and empirical PL_K relation and Wesenheit function give, according to optical (V,I OGLE) and near-infrared (NIR, K, 2mass) data, mean distances to the MCs that agree at the 2% level. Individual distances to the Large and the Small Cloud are: 18.53+-0.08-19.04+-0.11 (theory) and 18.48+-0.13-19.01+-0.13 (empirical). Moreover, predicted and empirical FO relations do not present, within the errors, a metallicity dependence. Finaly, we find that the upper limit in the FO period distribution is a robust observable to constrain the accuracy of pulsation models. Current models agree within 0.1 in log P with the observed FO upper limits.

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arXiv2002

BVI time series data of the Galactic Globular Cluster NGC3201. I. RR Lyrae stars

We present Johnson BV, and Kron-Cousins I-band time series data collected over three consecutive nights in a region of 13 arcmin^2 centered on the Galactic Globular Cluster (GGC) NGC3201. The time sampling of current CCD data allowed us to derive accurate light curves, and in turn mean magnitudes and colors for a sample of 53 RR Lyrae. To overcome the thorny problem of differential reddening affecting this cluster, we derived new empirical relations connecting the intrinsic (B-V) and (V-I) colors of fundamental ($RR_{ab}$) RR Lyrae to the luminosity amplitude, the metallicity, and the pulsation period. The key features of these relations are the following: i) they rely on stellar parameters which are not affected by reddening; ii) they supply accurate estimates of intrinsic colors across the fundamental instability strip and cover a wide metallicity range; iii) they were derived by neglecting the RR Lyrae that are affected by amplitude modulation. Moreover, the zero-point of the E(B-V) reddening scale was empirically checked using the large sample of RR Lyrae in M3 (Corwin & Carney 2001), a GGC affected by a vanishing reddening. According to these relations we estimated individual reddenings for RR Lyrae in our sample and the main results we found are the following: i) the mean cluster reddening based on E(B-V) color excesses is =0.30\pm0.03. iii) According to current individual E(B-V) and E(V-I) reddenings and theoretical predictions for Horizontal-Branch stars, we found that the true distance modulus for this cluster is 13.32\pm0.06 mag. iv) The comparison between present luminosity amplitudes and estimates available in the literature discloses that approximately 30% of fundamental RR Lyrae are affected by amplitude modulation (Blazhko effect).

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arXiv2002

On the pulsation parallax of the variable star RR Lyr

We show that a straightforward application of the predicted relation connecting the absolute K magnitude of fundamental RR Lyrae variables to their period and metal content, together with current evolutionary predictions on the mass and luminosity of Horizontal Branch stars, supply a distance estimate to the prototype star RR Lyr in close agreement with the recent parallax determination by HST (Benedict et al. 2002), largely unaffected by the interstellar extinction correction. Adopting =0.12+-0.10 mag as a safe estimate of the extinction correction to RR Lyr, we obtain a "pulsation" parallax of 3.858+-0.131 mas which agrees quite well, and with a smaller formal error, with the HST measurement (3.82+-0.20 mas) and with the weighted average of HST, Hipparcos, and ground-based determinations (3.87+-0.19 mas). This result suggests that near-infrared magnitudes and pulsation models could constrain the distance to well-studied RR Lyrae stars, both in the field and in globular clusters, with an accuracy better than current direct trigonometric measurements.

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arXiv2002

Theoretical models for Bump Cepheids

We present the results of a theoretical investigation aimed at testing whether full amplitude, nonlinear, convective models account for the I-band light curves of Bump Cepheids in the Large Magellanic Cloud (LMC). We selected two objects from the OGLE sample that show a well-defined bump along the decreasing (short-period) and the rising (long-period) branch respectively. We find that current models do reproduce the luminosity variation over the entire pulsation cycle if the adopted stellar mass is roughly 15 % smaller than predicted by evolutionary models that neglect both mass loss and convective core overshooting. Moreover, we find that the fit to the light curve of the long-period Cepheid located close to the cool edge of the instability strip requires an increase in the mixing length from 1.5 to 1.8 Hp. This suggests an increase in the efficiency of the convective transport when moving toward cooler effective temperatures. Current pulsation calculations supply a LMC distance modulus ranging from 18.48 to 18.58 mag.

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