L. Pulone

Spectroscopic identification of r-process nucleosynthesis in a double neutron star merger

The merger of two neutron stars is predicted to give rise to three major detectable phenomena: a short burst of γ-rays, a gravitational-wave signal, and a transient optical–near-infrared source powered by the synthesis of large amounts of very heavy elements via rapid neutron capture (the r-process). Such transients, named ‘macronovae’ or ‘kilonovae’, are believed to be centres of production of rare elements such as gold and platinum. The most compelling evidence so far for a kilonova was a very faint near-infrared rebrightening in the afterglow of a short γ-ray burst at redshift z = 0.356, although findings indicating bluer events have been reported. Here we report the spectral identification and describe the physical properties of a bright kilonova associated with the gravitational-wave source GW170817 and γ-ray burst GRB 170817A associated with a galaxy at a distance of 40 megaparsecs from Earth. Using a series of spectra from ground-based observatories covering the wavelength range from the ultraviolet to the near-infrared, we find that the kilonova is characterized by rapidly expanding ejecta with spectral features similar to those predicted by current models. The ejecta is optically thick early on, with a velocity of about 0.2 times light speed, and reaches a radius of about 50 astronomical units in only 1.5 days. As the ejecta expands, broad absorption-like lines appear on the spectral continuum, indicating atomic species produced by nucleosynthesis that occurs in the post-merger fast-moving dynamical ejecta and in two slower (0.05 times light speed) wind regions. Comparison with spectral models suggests that the merger ejected 0.03 to 0.05 solar masses of material, including high-opacity lanthanides.

Chandra X-Ray Observatory observations of the globular cluster M28 and its millisecond pulsar PSR B1821-24

We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). 46 X-ray sources are detected, of which 12 lie within one core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources for which we determine the X-ray luminosity function down to a limit of about 6xE30 erg/s. For the first time the unconfused phase-averaged X-ray spectrum of the 3.05-ms pulsar B1821--24 is measured and found to be best described by a power law with photon index ~ 1.2. Marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum is found, which could be interpreted as cyclotron emission from a corona above the pulsars polar cap if the the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5--8.0 keV band is ~3.5xE-13 ergs/s/cm^2. Spectral analysis of the 5 brightest unidentified sources is presented. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T_eff^\infty = 90^{+30}_{-10} eV and the radius R_NS^\infty = 14.5^{+6.9}_{-3.8} km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core of M28. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts.We report here the results of the first Chandra X-Ray Observatory observations of the globular cluster M28 (NGC 6626). We detect 46 X-ray sources, of which 12 lie within 1 core radius of the center. We show that the apparently extended X-ray core emission seen with the ROSAT HRI is due to the superposition of multiple discrete sources, for which we determine the X-ray luminosity function down to a limit of about 6 � 10 30 ergs s � 1 . We measure the radial distribution of the X-ray sources and fit it to a King profile finding a core radius of rc;X � 11 00 . We measure for the first time the unconfused phase-averaged X-ray spectrum of the 3.05 ms pulsar B1821� 24 and find that it is best described by a power law with photon index � ’ 1:2. We find marginal evidence of an emission line centered at 3.3 keV in the pulsar spectrum, which could be interpreted as cyclotron emission from a corona above the pulsar’s polar cap if the magnetic field is strongly different from a centered dipole. The unabsorbed pulsar flux in the 0.5–8.0 keV band is � 3:5 � 10 � 13 ergs s � 1 cm � 2 . We present spectral analyses of the five brightest unidentified sources. Based on the spectral parameters of the brightest of these sources, we suggest that it is a transiently accreting neutron star in a low-mass X-ray binary, in quiescence. Fitting its spectrum with a hydrogen neutron star atmosphere model yields the effective temperature T 1 eff ¼ 90 þ30 � 10 eV and the radius R 1 ¼ 14:5 þ6:9 � 3:8 km. In addition to the resolved sources, we detect fainter, unresolved X-ray emission from the central core. Using the Chandra-derived positions, we also report on the result of searching archival Hubble Space Telescope data for possible optical counterparts. Subject headings: globular clusters: individual (M28) — pulsars: general — stars: neutron — X-rays: stars

A Pulsational Distance to ω Centauri Based on Near-Infrared Period-Luminosity Relations of RR Lyrae Stars*

We present new near-infrared (J and K) magnitudes for 114 RR Lyrae stars in the globular cluster ω Centauri (NGC 5139), which we combine with data from the literature to construct a sample of 180 RR Lyrae stars with J and K mean magnitudes on a common photometric system. This is currently the largest such sample in any stellar system. We also present updated predictions for J- and K-band period-luminosity relations for both fundamental and first-overtone RR Lyrae stars, based on synthetic horizontal branch models with metal abundance ranging from Z = 0.0001 to 0.004. By adopting for the ω Cen variables with measured metal abundances an α-element enhancement of a factor of 3 (≈0.5 dex) with respect to iron, we find a true distance modulus μ0 = 13.70 ± 0.06 ± 0.06 (random and systematic errors, respectively), corresponding to a distance d = 5.5 ± 0.03 ± 0.03 kpc. Our estimate is in excellent agreement with the distance inferred for the eclipsing binary OGLEGC 17, but differ significantly from the recent distance estimates based on cluster dynamics and on high-amplitude δ Scuti stars.

The evolution of He-burning stars - Horizontal and asymptotic branches in Galactic globulars

A grid of theoretical evolutionary models covering the horizontal-branch (HB) and the asymptotic-giant-branch (AGB) phases of globular cluster stars is presented. The computations were performed for a fixed amount of the original He and for metallicity values. For each value of the assumed metallicity, the evolutionary structure at the He flash was used to construct an initial set of He-burning models with the same mass of the central He core but with decreasing masses of the H-rich envelopes. Results are presented of the evolution of these models through the exhaustion of central He up to the minimum in luminosity marking the reignition of the H shell. The luminosity of the AGB clump and the number ratio of AGB to HB stars was found to remain fairly constant with the cluster metallicity. The parameter R is recalibrated, and it is suggested that there is a small increase in the adopted value of the original He in Galactic globulars and a correlation of helium and metals in 47 Tuc. 26 refs.

The Distance to the Large Magellanic Cloud 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.

Strömgren Photometry of Galactic Globular Clusters. I. New Calibrations of the Metallicity Index

We present a new calibration of the Stromgren metallicity index m(1) using red giant (RG) stars in four globular clusters (GCs: M92, M13, NGC 1851, 47 Tuc) with metallicity ranging from - 2.2 to - 0.7, marginally affected by reddening [E( B - V) <= 0: 04] and with accurate (u, v, b, y) photometry. The main difference between the new metallicity-index-color (MIC) relations and similar relations available in the literature is that we have adopted the u - y and v - y colors instead of b - y. These colors present a stronger sensitivity to effective temperature, and the MIC relations show a linear slope. The difference between photometric estimates and spectroscopic measurements for RGs in M71, NGC 288, NGC 362, NGC 6397, and NGC 6752 is 0: 04 +/- 0: 03 dex (sigma = 0: 11 dex). We also apply the new MIC relations to 85 field RGs with metallicity ranging from - 2.4 to -0.5 and accurate reddening estimates. We find that the difference between photometric estimates and spectroscopic measurements is -0.14 +/- 0.01 dex (sigma = 0.17 dex). We also provide two sets of MIC relations based on evolutionary models that have been transformed into the observational plane by adopting either semiempirical or theoretical color-temperature relations. We apply the semiempirical relations to the nine GCs and find that the difference between photometric and spectroscopic metallicities is 0.04 +/- 0.03 dex (sigma = 0.10 dex). A similar agreement is found for the sample of field RGs, with a difference of -0.09 +/- 0.03 dex (with sigma = 0.19 dex). The difference between metallicity estimates based on theoretical relations and spectroscopic measurements is -0.11 +/- 0.03 dex (sigma = 0.14 dex) for the nine GCs and -0.24 +/- 0.03 dex (sigma = 0.15 dex) for the field RGs. Current evidence indicates that new MIC relations provide metallicities with an intrinsic accuracy better than 0.2 dex.

ON A NEW NEAR-INFRARED METHOD TO ESTIMATE THE ABSOLUTE AGES OF STAR CLUSTERS: NGC 3201 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 turnoff (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 easily be 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 NGC 3201 using both the MSTO and the ?(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 1?. However, the errors of the ages based on the ?(MSTO-MSK) method are potentially more than a factor of 2 smaller, since they are not affected by uncertainties in cluster distance or reddening. Current isochrones appear to predict slightly bluer (0.05 mag) NIR and optical-NIR colors than observed for magnitudes fainter than the MSK.

STROMGREN PHOTOMETRY OF GALACTIC GLOBULAR CLUSTERS. II. METALLICITY DISTRIBUTION OF RED GIANTS IN ω CENTAURI

We present new intermediate-band Str?mgren photometry based on more than 300 u, v, b, y images of the Galactic globular cluster ? Cen. Optical data were supplemented with new multiband near-infrared (NIR) photometry (350 J, H, Ks images). The final optical-NIR catalog covers a region of more than 20 ? 20 arcmin squared across the cluster center. We use different optical-NIR color-color planes together with proper-motion data available in the literature to identify candidate cluster red-giant (RG) stars. By adopting different Str?mgren metallicity indices, we estimate the photometric metallicity for 4000 RGs, the largest sample ever collected. The metallicity distributions show multiple peaks ([Fe/H]phot = ?1.73 ? 0.08, ?1.29 ? 0.03, ?1.05 ? 0.02, ?0.80 ? 0.04, ?0.42 ? 0.12, and ?0.07 ? 0.08 dex) and a sharp cutoff in the metal-poor (MP) tail ([Fe/H]phot ?2 dex) that agree quite well with spectroscopic measurements. We identify four distinct subpopulations, namely, MP ([Fe/H] ? ?1.49), metal-intermediate (MI; ?1.49 < [Fe/H] ? ?0.93), metal-rich (MR; ?0.95 < [Fe/H] ? ?0.15), and solar metallicity ([Fe/H] 0). The last group includes only a small fraction of stars (~8% ? 5%) and should be confirmed spectroscopically. Moreover, using the difference in metallicity based on different photometric indices, we find that the 19% ? 1% of RGs are candidate CN-strong stars. This fraction agrees quite well with recent spectroscopic estimates and could imply a large fraction of binary stars. The Str?mgren metallicity indices display a robust correlation with ?-elements ([Ca+Si/H]) when moving from the MI to the MR regime ([Fe/H] ?1.7 dex).

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.

The 'Red Giant Clock' as an indicator for the efficiency of central mixing in horizontal-branch stars

New evolutionary evaluations have been compared with observational data for the globular cluster M5 in order to assess the kind of mixing (semiconvection with and without the inclusion of the breathing pulses or overshooting) which takes place in real stars during the central He-burning phase. It is suggested that the evolutionary time scale on the red giant branch should be used to assess the absolute time scale of evolution on the asymptotic giant branch. On this basis, it is found that ordinary semiconvection without breathing pulses leads to the best match to the observed data for M5. 18 refs.