arXiv subjects

Angela Bragaglia

Publications and source records attributed to Angela Bragaglia.

103 records · Page 6Linked to original sources

arXiv2004

Abundances of C, N, O in slightly evolved stars in the globular clusters NGC 6397, NGC 6752 and 47 Tuc

Abundances of C and N are derived from features due to the CH G-band and to the UV CN band measured on high resolution (R ~ 40,000) UVES spectra of more than 40 dwarfs and subgiants in NGC 6397, NGC 6752 and 47 Tuc. Oxygen abundances (or upper limits) are available for all stars in the sample. Isotopic ratios 12C/13C were derived from the CH molecular band. This is the first determination of this ratio in unevolved dwarf stars in globular clusters. By enlarging the sample of subgiants in NGC 6397 studied in Gratton et al. (2001), we uncovered, for the first time, large variations in both Na and O also in this cluster. The origin of the chemical inhomogeneities must be searched for outside the stars under scrutiny. Our data indicate that in unevolved or slightly evolved stars in these clusters C abundances are low but not zero, also in stars with large N-enhancements and O-depletions, and that the isotopic ratios 12C/13C are low, but never reach the equilibrium value of the CN-cycle. When coupled with the run of O and Na abundances, these findings possibly require that, in addition to CNO burning and p-captures, some triple-alpha process is also involved: previously evolved intermediate-mass AGB stars are then the most likely polluters.

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arXiv2004

Star-to-star Na and O abundance variations along the red giant branch in NGC 2808

We report for the first time Na and O abundances from high-resolution, high S/N echelle spectra of 20 red giants in NGC 2808, taken as part of the Science Verification program of the FLAMES multi-object spectrograph at the ESO VLT. In these stars, spanning about 3 mag from the red giant branch (RGB) tip, large variations are detected in the abundances of oxygen and sodium, anticorrelated with each other; this is a well known evidence of proton-capture reactions at high temperatures in the ON and NeNa cycles. One star appears super O-poor; if the extension of the Na-O anticorrelation is confirmed, NGC 2808 might reach O depletion levels as large as those of M 13. This result confirms our previous findings based on lower resolution spectra (Carretta et al. 2003) of a large star-to-star scatter in proton capture elements at all positions along the RGB in NGC 2808, with no significant evolutionary contribution. Finally, the average metallicity for NGC 2808 is [Fe/H]= -1.14 +/- 0.01 dex (rms=0.06) from 19 stars.

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arXiv2004

The type IIn supernova 1994W: evidence for the explosive ejection of a circumstellar envelope

We present and analyse spectra of the Type IIn supernova 1994W obtained between 18 and 203 days after explosion. During the luminous phase (first 100 d) the line profiles are composed of three major components: (i) narrow P-Cygni lines with the absorption minima at -700 km/s; (ii) broad emission lines with BVZI ~4000 km/s; and (iii) broad, smooth wings, most apparent in H-alpha. These components are identified with an expanding circumstellar (CS) envelope, shocked cool gas in the forward post-shock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P-Cygni lines from the supernova is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. We model the supernova deceleration and Thomson scattering wings to recover the density, radial extent and Thomson optical depth of the CS envelope during the first month. We reproduce the light curve with a hydrodynamical model and find it to be powered by a combination of internal energy leakage after the explosion of an extended pre-supernova (~10^15 cm) and luminosity from circumstellar interaction. We recover the pre-explosion kinematics of the CS envelope: it is close to homologous expansion with outer velocity ~1100 km/s and a kinematic age of ~1.5 yr. The CS envelope's high mass and kinetic energy, combined with its small age, strongly suggest that the CS envelope was explosively ejected about 1.5 yr before the supernova explosion.

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arXiv2003

Abundance Analysis of Turn-Off and Early Subgiant Stars in the Globular Cluster 47 Tuc (NGC 104)

We used the UVES spectrograph on Kueyen (VLT UT2) to perform the abundance analysis of stars at the base of the giant branch (RGB) and at the main sequence turn-off in the globular cluster 47 Tuc (NGC 104). High dispersion spectra (R ~ 40,000) for 3 dwarfs and 9 subgiants were analyzed. We found an overall [Fe/H] value of -0.67 +/-0.01 +/-0.04, rms=0.05 dex. The mean value obtained from stars at the base of the RGB is virtually the same as that obtained from the dwarfs. Na and O abundances show a star-to-star scatter and are anti-correlated, not unlike previous finding in more metal-poor clusters. The extension of the anticorrelation is not as extreme as found in other clusters, but it is clearly present in unevolved stars. Al abundances do not show a significant spread; this could be indicative that p-captures in high temperature regions are less likely to occur in metal-rich clusters. The [alpha/Fe] ratios suggest a slight excess of Ti with respect to field stars of similar metallicities. Finally, we found an enhanced odd-even effect for Fe-group elements. In particular, the [Mn/Fe] ratio is almost 0.2 dex deficient with respect to the mean value of the dissipative component of field stars in the solar neighborhood. While the theoretical and observational framework for Mn is still poorly understood, the chemistry of 47 Tuc, coupled with its kinematics and its somewhat younger age, might give hints favouring an origin in a formely independent, possibly larger, subsystem of our Galaxy.

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arXiv2003

UBVI photometry of the intermediate age open cluster NGC 6939

We present CCD UBVI photometry of the nearby, intermediate age open cluster NGC 6939. Using the synthetic Colour - Magnitude Diagrams technique we estimate the following parameters: age between 1.3 and 1.0 Gyr (depending on whether or not overshooting is considered), reddening 0.34<E(B-V)<0.38 and distance modulus 11.3<mod_0<11.4 .

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arXiv2003

SN 1994W: evidence of explosive mass ejection a few years before explosion

We present and analyse spectra of the Type IIn supernova 1994W obtained between 18 and 202 days after explosion. During the first 100 days the line profiles are composed of three major components: (i) narrow P Cygni lines with absorption minima at -700 km/s; (ii) broad emission lines with BVZI ~4000 km/s; (iii) broad wings most apparent in H-alpha. These components are identified with the expanding circumstellar (CS) envelope (Sollerman, Cumming & Lundqvist 1998), shocked cool gas in the forward postshock region, and multiple Thomson scattering in the CS envelope, respectively. The absence of broad P Cygni lines from the supernova (SN) is the result of the formation of an optically thick, cool, dense shell at the interface of the ejecta and the CS envelope. Models of the SN deceleration and Thomson scattering wings are used to recover the Thomson optical depth of the CS envelope and its radial extent, 4E15 cm. The light curve, which we reproduce by a hydrodynamical model, is powered by a combination of internal energy leakage after the explosion of an extended presupernova (~1E15 cm) and luminosity from circumstellar interaction. We recover the pre-explosion kinematics of the CS envelope and find it to be close to homologous expansion with outmost velocity ~1100 km/s and a kinematic age of \~1.5 yr. The high mass and kinetic energy of the CS envelope strongly suggest that the CS envelope was explosively ejected only a few years before explosion.

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arXiv2003

Proton capture elements in the globular cluster NGC 2808 I. First detection of large variations in Sodium abundances along the Red Giant Branch

We have used spectra obtained as part of the Science Verification program of the FLAMES multi-object spectrograph mounted on Kueyen (VLT-UT2) to perform an abundance analysis of stars along the giant branch (RGB) in the globular cluster NGC 2808. Sodium abundances are derived from Na D lines for a sample of 81 cluster stars spanning a range of about 2 magnitudes from the tip of the RGB. Our results show that a large star-to-star scatter does exist at all positions along the RGB, suggesting large variations in the abundance of proton capture elements down to luminosities comparable to the red horizontal branch (HB). The distribution of Na abundances along the RGB seems to point out that in this cluster most of the observed spread has a primordial origin. Overimposed evolutionary effects, if any, must be only at ``noise'' level, at odds with results from a similar analysis in M 13 (Pilachowski et al. 1996). This study is a first step towards ascertaining if a link exists between the distribution of chemical anomalies in light elements along the RGB and the global properties of globular clusters, in particular the HB morphology.

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arXiv2003

Distances and ages of NGC 6397, NGC 6752 and 47 Tuc

Distances and absolute ages for the globular clusters NGC6397, NGC6752, and 47 Tuc are obtained using Main Sequence Fitting. Accurate reddenings and metal abundances (errors of 0.005 mag and 0.04 dex) are obtained by comparing B-V, b-y colours and UVES spectra for clusters TO-stars and subgiants with similar data for Hipparcos subdwarfs, using a reddening free temperature indicator (Halpha). Errors in distances and ages are 0.07 mag and 1 Gyr. NGC6397 and NGC6752 have ages of 13.5\pm 1.1 and 13.4\pm 1.1 Gyr, while 47 Tuc is probably about 2.6 Gyr younger, in agreement with results obtained by techniques sensitive to relative ages. Considering possible systematic errors, the age of the oldest globular clusters is 13.4\pm 0.8\pm 0.6 Gyr (random and systematic errors). This age is fully compatible with WMAP results, and indicates that the oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big Bang, corresponding to a redshift of z\geq 2.5, in a standard LambdaCDM model. The epoch of formation of the (inner halo) globular clusters lasted about 2.6 Gyr, ending at a time corresponding to a redshift of z\geq 1.3. Once combined with values of Ho given by WMAP and by the HST Key Project, our age estimate provides a robust upper limit at 95% level of confidence of Omega_M<0.57, indicating the need for a vacuum energy independent of results from type Ia SNe and clusters of galaxies. Using cluster distances, we derive a value of M_V(HB)=(0.22\pm 0.05)([Fe/H]+1.5)+(0.56\pm 0.07) for the zero point of the relation between the horizontal branch absolute magnitude and metallicity. When combined with the apparent average luminosity of the RR Lyrae stars in the LMC by Clementini et al. (2003), this zero point provides a new estimate of the distance modulus to the LMC: (m-M)o=18.50\pm 0.09.

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arXiv2003

Intermediate age open clusters: Collinder 110

We present CCD BV photometry of the intermediate age open cluster Collinder 110, a nearby, scarcely populated, and poorly studied system. There is no literature information on the metallicity, so we tested several possibilities, and found a slight evidence of sub-solar abundances. Using the synthetic Colour - Magnitude Diagrams technique we estimate the following parameters: age between 1.1 and 1.5 Gyr, reddening 0.38 <~ E(B-V) <~ 0.45, distance modulus (m-M)o between 11.45 and 11.7 if the cluster metallicity is sub-solar.

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arXiv1999

Mixing along the Red Giant Branch in Metal-poor Field Stars

We have determined Li, C, N, O, Na, and Fe abundances, and 12C/13C isotopic ratios for a sample of 62 field metal-poor stars (plus 43 taken from the literature). This large sample was used to show that small mass lower-RGB stars (i.e., fainter than the RGB bump) have abundances of light elements in agreement with theoretical predictions from classical evolutionary models. A second, distinct mixing episode occurs just after the RGB bump, reaching regions of incomplete CNO burning. No O-Na anticorrelation, as observed in globular cluster stars, is found in field stars. This means that the mixing episode is not deep enough to reach regions where ON-burning occurs.

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arXiv1998

First Dredge-Up and further Mixing Mechanisms along the Red Giant Branch in Field Stars

In this research we use a large sample of field metal-poor stars ([Fe/H]<-1) in different evolutionary phases to prove observationally that (small mass) subgiants (stars brighter than the first dredge-up and fainter than the RGB bump) have abundances of light elements in agreement with predictions from classical evolutionary models. A further mixing episode occurs just after the RGB bump, in agreement with the predictions by Charbonnel (1994, A&A, 282, 811; 1995, ApJ, 453, L41) The O-Na anti-correlation, typical signature observed amongst globular cluster giants, is not present amongst field stars.

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arXiv1997

On the calibration of Red Giant Branch metallicity indicators in Globular Clusters: new relations based on an improved abundance scale

We present an improved calibration of photometric metallicity indicators, derived from the new metallicity scale for Globular Clusters presented by Carretta & Gratton (1997) and based on direct high resolution spectroscopy of 160 stars in 24 globular clusters. We have carefully recalibrated the traditional abundance indices based upon the red giant branch (RGB) morphology, both in the V,B-V and V,V-I planes, namely the dereddened colour at the luminosity level of the horizontal branch (HB), and the magnitude difference between the HB and the RBG at a given dereddened colour. Finally, we give new accurate relations to employ in the Simultaneous Metallicity Reddening method by Sarajedini (1994), also tied to the Carretta & Gratton (1997) abundance scale.

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arXiv1996

The White Dwarf Distance to the Globular Cluster NGC 6752 (and its Age) with the Hubble Space Telescope (Based on observations made with the NASA/ESA Hubble Space Telescope (HST))

Deep Hubble Space Telescope (HST) observations with WFPC2 of the nearby globular cluster NGC 6752 have allowed us to obtain accurate photometry for the cluster white dwarfs (WD). A sample of local WDs of known trigonometric parallax and mass close to that of the cluster WDs have also been observed with WFPC2. Matching the cluster and the local WD sequences provides a direct measure of the distance to the cluster: $(m-M)_\circ=13.05$, with an uncertainty less than $\pm0.1$ mag which allows a substantial reduction in the uncertainty in the age of the cluster. Indeed, coupling this value of the cluster distance to the cluster metallicity, helium abundance and $α$-element enhancement [$α$/Fe]=0.5 yields an age of 15.5 Gyr and 14.5 Gyr using evolutionary models that do not include or do include helium diffusion, respectively. The uncertainty affecting these age determinations is $\sim 10\%$. The majority of the cluster WDs appear to be of the DA variety, while the color-magnitude location of two WDs is consistent with the DB type. This suggests a cluster DB/DA ratio similar to that of WDs in the solar neighborhood.

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