The Blue Straggler Population in Dwarf Galaxies

In this chapter I review the recent developments regarding the study of Blue Stragglers (BSS) in dwarf galaxies. The loose density environment of dwarf galaxies resembles that of the Galactic Halo, hence it is natural to compare their common BSS properties. At the same time, it is unescapable to compare with the BSS properties in Galactic globular clusters, which constitute the reference point for BSS studies. Admittedly, the literature on BSS in dwarf galaxies is not plentiful. The limitation is mostly due to the large distance to even the closest dwarf galaxies. Nevertheless, recent studies have allowed a deeper insight on the BSS photometric properties that are worth examining

Science Verification for the VISIR Upgrade

The Very Large Telescope spectrometer and imager for the mid-infrared (VISIR) was upgraded in 2015 with new detectors and several new modes were added. Science Verification (SV) is carried out for new ESO instruments as well as for substantial upgrades to existing instruments. Sparse aperture masking and coronagraphy in the mid infrared have now been added to VISIR’s capabilities and during SV these new observational modes, together with the recommissioned burst mode, were used to demonstrate the observational capabilities of the instrument. The SV process for VISIR is briefly described and some results from the successful observations are presented. All SV data are publicly available

A Plague of Magnetic Spots Among the Hot Stars of Globular Clusters

Six decades and counting, the formation of hot ~20,000-30,000 K Extreme Horizontal Branch (EHB) stars in Galactic Globular Clusters remains one of the most elusive quests in stellar evolutionary theory. Here we report on two discoveries shattering their currently alleged stable luminosity. The first EHB variability is periodic and cannot be ascribed to binary evolution nor pulsation. Instead, we here attribute it to the presence of magnetic spots: superficial chemical inhomogeneities whose projected rotation induces the variability. The second EHB variability is aperiodic and manifests itself on time-scales of years. In two cases, the six-year light curves display superflare events a mammoth several million times more energetic than solar analogs. We advocate a scenario where the two spectacular EHB variability phenomena are different manifestations of diffuse, dynamo-generated, weak magnetic fields. Ubiquitous magnetic fields, therefore, force an admittance into the intricate matrix governing the formation of all EHBs, and traverse to their Galactic field counterparts. The bigger picture is one where our conclusions bridge similar variability/magnetism phenomena in all radiative-enveloped stars: young main-sequence stars, old EHBs and defunct white dwarfs.Comment: Author's version of the main article (23 pages) and Supplementary Information (22 pages) combined into a single pdf (45 pages). Readers invited to read the Nature Astronomy Published version available at this url: https://www.nature.com/articles/s41550-020-1113-

The Return of the Mid-infrared to the VLT: News from the VISIR Upgrade

The VLT mid-infrared imager and spectrometer VISIR returns to science operations following an extended upgrade period. Among the most important modifications are: the imaging and spectroscopic detectors have been replaced with larger AQUARIUS (1024 by 1024 pixel) detector arrays; the N-band low-resolution grating has been exchanged; and support is now provided for precipitable water vapour monitoring, in order to select the best observing conditions. The AQUARIUS detectors stem from a development for very low background applications which result in excess noise under ground-based conditions. A series of interventions was needed to find a scheme that effectively exploits these detectors for ground-based use, involving the implementation of faster chopping. VISIR has been returned to service at the VLT with enhanced capabilities

A lack of close binaries among hot horizontal branch stars in globular clusters M 80 and NGC 5986

Context: Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC 6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. Aims: The present paper extends our study of NGC 6752 to M 80 and NGC 5986, to establish whether the unexpected properties of EHB stars in NGC 6752 are also present in other clusters. Methods: Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC 5986 and 31 in M 80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations and evaluated statistical and systematic errors. Temperatures, gravities, and helium abundances were also measured. Results: By means of a statistical analysis of the observed radial velocity variations, we detected one EHB close binary candidate per cluster. In M 80, the best estimate of the close binary EHB fraction is f=12%, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC 5986, although our best estimate is rather low (f=25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC 6752, our analysis of M 80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature, although the trend for the hottest stars is still unclear. Conclusions: Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC 6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations. Based on observations with the ESO Very Large Telescope at Paranal Observatory, Chile (proposal ID 69.D-0682)

BVRIJHK photometry and proper motion analysis of NGC 6253 and the surrounding field

Context: We present a photometric and astrometric catalog of 187 963 stars located in the field around the old super-metal-rich Galactic open cluster NGC 6253. The total field-of-view covered by the catalog is 34”×33”. In this field, we provide CCD BVRI photometry. For a smaller region close to the cluster's center, we also provide near-infrared JHK photometry. Aims: We analyze the properties of NGC 6253 by using our new photometric data and astrometric membership. Methods: In June 2004, we targeted the cluster during a 10 day multi-site campaign, which involved the MPG/ESO 2.2 m telescope with its wide-field imager and the Anglo-Australian 3.9 m telescope, equipped with the IRIS2 near-infrared imager. Archival CCD images of NGC 6253 were used to derive relative proper motions and to calculate the cluster membership probabilities. Results: We have refined the cluster's fundamental parameters, deriving (V_0-M_v)=11.15, E(B - V) = 0.15, E(V - I) = 0.25, E(V - J) = 0.50, and E(V - H) = 0.55. The color excess ratios obtained using both the optical and near infrared colors indicate a normal reddening law in the direction of NGC 6253. The age of NGC 6253 at 3.5 Gyr, determined from our best-fitting isochrone appears to be slightly older than the previous estimates. Finally, we estimated the binary fraction among the cluster members to be ~20%-30% and identified 11 blue straggler candidates. Based on observation made at the European Southern Observatory, La Silla, Chile and at the Anglo-Australian Observatory, Siding Spring, Australia. The catalog presented in this paper is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/505/112

A MAD view of Trumpler 14

We present adaptive optics (AO) near-infrared observations of the core of the Tr 14 cluster in the Carina region obtained with the ESO multi-conjugate AO demonstrator, MAD. Our campaign yields AO-corrected observations with an image quality of about 0.2 arcsec across the 2 arcmin field of view, which is the widest AO mosaic ever obtained. We detected almost 2000 sources spanning a dynamic range of 10 mag. The pre-main sequence (PMS) locus in the colour-magnitude diagram is well reproduced by Palla & Stahler isochrones with an age of 3 to 5 1E+05 yr, confirming the very young age of the cluster. We derive a very high (deprojected) central density n0~4.5(+/-0.5) \times 10^4 pc^-3 and estimate the total mass of the cluster to be about ~4.3^{+3.3}_{-1.5} \times 10^3 Msun, although contamination of the field of view might have a significant impact on the derived mass. We show that the pairing process is largely dominated by chance alignment so that physical pairs are difficult to disentangle from spurious ones based on our single epoch observation. Yet, we identify 150 likely bound pairs, 30% of these with a separation smaller than 0.5 arcsec (~1300AU). We further show that at the 2-sigma level massive stars have more companions than lower-mass stars and that those companions are respectively brighter on average, thus more massive. Finally, we find some hints of mass segregation for stars heavier than about 10 Msun. If confirmed, the observed degree of mass segregation could be explained by dynamical evolution, despite the young age of the cluster.Comment: 15 pages, 24 figures, accepted for publication in A&

VizieR Online Data Catalog: Abundances of red giant stars in NGC6093 (M80) (Carretta+, 2015)

For a sample of 143 stars in M80 observed with FLAMES@VLT we give here: identifications, coordinates, BV magnitudes, heliocentric RV (table2). For members we also give: atmospheric parameters, [Fe/H] (table 3); [O/Fe], [Na/Fe], [Mg/Fe], [Al/Fe] (table 5); the alpha-elements Si, Ca, and Ti (table 6); the Fe-peak elements Sc, V, Cr, Mn, Co, Ni, Cu, and Zn (table 7); the n-capture elements Y, Zr, La, Ce, Pr, Nd, Sm, (table 8, only UVES stars); Ba (table 9). (7 data files)

VizieR Online Data Catalog: Horizontal branch stars in NGC 6723 (Gratton+, 2015)

We observed a total of 58 candidate HB stars of NGC 6723 with FLAMES + GIRAFFE at the VLT. The instrument was used in MEDUSA mode, with fibres pointing to each star and several (~20) fibres used for determining the local sky background. Observations were made between 2011-07-11 and 2011-08-27. (5 data files)