Elena Sabbi

Discovery of Another Peculiar Radial Distribution of Blue Stragglers in Globular Clusters: The Case of 47 Tucanae*

We have used the high-resolution Wide Field Planetary Camera (WFPC2) on the Hubble Space Telescope (HST) and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) in the globular cluster 47 Tuc spanning the entire radial extent of the cluster. The BSS distribution is highly peaked in the cluster center, rapidly decreases at intermediate radii, and finally rises again at larger radii. The observed distribution closely resembles that discovered in M3 by Ferraro and coworkers. To date, complete BSS surveys covering the full radial extent (from HST for the center and wide-field CCD, ground-based observations for the exterior) have only been performed for these two clusters. Both show a bimodal radial distribution despite their different dynamical properties. BSS surveys covering the full spatial extent of more globular clusters are clearly required to determine how common bimodality is and what its consequences are for theories of BSS formation and cluster dynamics.

The Resolved Stellar Populations in NGC 1705

We present HST photometry of the resolved stellar population in the dwarf irregular galaxy NGC 1705. The galaxy has been observed with both WFPC2 and NICMOS, and successful images have been obtained in the F555W, F814W, F110W, and F160W bands. The optical fields cover most of the galaxy, while the infrared field (NIC2) maps only its central regions. The optical photometry provides ~20,000 objects down to mF555W 29 in the PC field of view and ~20,000 in the three WFCs. In the infrared we have been able to resolve ~2400 stars down to mF110W, mF160W ≈ 26. A subsample of 1834 stars has been unambiguously measured in all four bands. The corresponding color-magnitude diagrams confirm the existence of an age gradient, showing that NGC 1705 hosts both young (a few megayears) and very old (up to 15 Gyr) stars, with the former strongly concentrated toward the galactic center and the latter present everywhere, but much more easily visible in the external regions. The tip of the red giant branch (TRGB) is clearly visible both in the optical and in the infrared CMDs and allows us to derive the galaxy distance. Taking into account the uncertainties related to both the photometry and the TRGB magnitude-distance relation, we find that the distance modulus of NGC 1705 is (m - M)0 = 28.54 ± 0.26, corresponding to a distance D = 5.1 ± 0.6 Mpc.

The Puzzling Dynamical Status of the Core of the Globular Cluster NGC 6752

We have used high-resolution Hubble Space Telescope Wide Field Planetary Camera 2 and ground-based wide-field images to determine the center of gravity and construct an extended radial density and brightness profile of the cluster NGC 6752 including, for the first time, detailed star counts in the very inner region. The barycenter of the nine innermost X-ray sources detected by Chandra is located only 19 off the new center of gravity. Both the density and the brightness profile of the central region are best fitted by a double King model, suggesting that NGC 6752 is experiencing a post-core-collapse bounce. Taking advantage of our new optical data, we discuss the puzzling nature of the accelerations displayed by the innermost millisecond pulsars detected in this cluster. We discuss two possible origins to the accelerations: (1) the overall cluster gravitational potential, which would require a central projected mass-to-light ratio of the order of 6-7 and the existence of a few thousand solar masses of low-luminosity matter within the inner 0.08 pc of NGC 6752, and (2) the existence of a local perturber(s) of the pulsar dynamics, such as a recently proposed binary black hole of intermediate (100-200 M☉) mass.

The Small Blue Straggler Star Population in the Dense Galactic Globular Cluster NGC 6752

We used high-resolution Hubble Space Telescope (HST)-WFPC2 and wide-field ground-based observations to construct a catalog of blue straggler stars (BSSs) that spans the entire radial extent of the globular cluster NGC 6752. The BSS sample is the most extensive ever obtained for this cluster. Although NGC 6752 is a high-density cluster with a large binary population, we found that its BSS content is surprisingly low: the specific number of BSSs is among the lowest ever measured in a cluster. The BSS distribution is highly peaked in the cluster center, shows a rapid decrease at intermediate radii, and rises again at larger distances. This distribution closely resembles those observed in M3 and 47 Tuc by Ferraro and coworkers. To date, BSS surveys covering the central regions with HST and the outer regions with wide-field CCD ground-based observations have been performed for only these three clusters. Despite the different dynamical properties, a bimodal radial distribution has been found in each. A detailed comparison of observed BSS luminosity and temperature distributions with theoretical models reveals a population of luminous, hot BSSs that is not easily interpreted.

The Complex Star Formation History of NGC 1569

We present new results on the star formation history of the dwarf irregular galaxy NGC 1569. The data were obtained with Hubble Space Telescope’s NICMOS/NIC2 in the F110W (J) and F160W (H) near-infrared (NIR) filters and interpreted with the synthetic color-magnitude diagram method. The galaxy experienced a complex star formation (SF) activity. The best fit to the data is found assuming three episodes of activity in the last 1 − 2 Gyr. The most recent and strong episode constrained by these NIR data started � 3.7 � 10 7 yr ago and ended � 1.3 � 10 7 yr ago, although we cannot exclude that up to three SF episodes occurred in this time interval. The average star-formation rate (SFR) of

The Complex H? Line Profile of the Bright Companion to PSR J1740?5340 in NGC 6397

We present a detailed study of the Hα 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 Hα 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.

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 this enables us to derive the most accurate mass ratio (MPSR/MCOM = 5.85 ± 0.13) for any nonrelativistic binary system containing a neutron star. Assuming a pulsar mass in the range 1.3-1.9 M☉, the mass of COM J1740-5340 spans the interval 0.22-0.32 M☉, the inclination of the system is constrained within 56° i 47°, and the Roche lobe radius is rRL ~ 1.5-1.7 R☉. 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, which are 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.

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 burn- ing 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 7 Li has been produced on the stellar surface.

Gaia DR2 reveals a very massive runaway star ejected from R136

A previous spectroscopic study identified the very massive O2 III star VFTS 16 in the Tarantula Nebula as a runaway star based on its peculiar line-of-sight velocity. We use the Gaia DR2 catalog to measure the relative proper motion of VFTS 16 and nearby bright stars to test if this star might have been ejected from the central cluster, R136, via dynamical ejection. We find that the position angle and magnitude of the relative proper motion (0.338 +/- 0.046 mas/yr, or approximately 80 +\- 11 km/s) of VFTS 16 are consistent with ejection from R136 approximately 1.5 +/- 0.2 Myr ago, very soon after the cluster was formed. There is some tension with the presumed age of VFTS 16 that, from published stellar parameters, cannot be greater than 0.9 +0.3/-0.2 Myr. Older ages for this star would appear to be prohibited due to the absence of He I lines in its optical spectrum, since this sets a firm lower limit on its effective temperature. The dynamical constraints may imply an unusual evolutionary history for this object, perhaps indicating it is a merger product. Gaia DR2 also confirms that another very massive star in the Tarantula Nebula, VFTS 72 (alias BI253; O2 III-V(n)((f*)), is also a runaway on the basis of its proper motion as measured by Gaia. While its tangential proper motion (0.392 +/-0.062 mas/yr or 93 +/-15 km/s) would be consistent with dynamical ejection from R136 approximately 1 Myr ago, its position angle is discrepant with this direction at the 2

Is the massive star cluster Westerlund 2 double? - A high resolution multi-band survey with the Hubble Space Telescope

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