The helium spread in the Globular cluster 47 Tuc

Spectroscopy has shown the presence of the CN band dicothomy and the Na-O anticorrelations for 50--70% of the investigated samples in the cluster 47 Tuc, otherwise considered a "normal" prototype of high metallicity clusters from the photometric analysis. Very recently, the re-analysis of a large number of archival HST data of the cluster core has been able to put into evidence the presence of structures in the Sub Giant Branch: it has a brighter component with a spread in magnitude by $\sim$0.06 mag and a second one, made of about 10% of stars, a little fainter (by $\sim$0.05 mag). These data also show that the Main Sequence of the cluster has an intrinsic spread in color which, if interpreted as due to a small spread in helium abundance, suggests $\Delta$Y$\sim$0.027. In this work we examine in detail whether the Horizontal Branch morphology and the Sub Giant structure provide further independent indications that a real --although very small-helium spread is present in the cluster. We re--analyze the HST archival data for the Horizontal Branch of 47 Tuc, obtaining a sample of $\sim$500 stars with very small photometric errors, and build population synthesis based on new models to show that its particular morphology can be better explained by taking into account a spread in helium abundance of 2% in mass. The same variation in helium is able to explain the spread in luminosity of the Sub Giant Branch, while a small part of the second generation is characterized by a small C+N+O increase and provides an explanation for the fainter Sub Giant Branch. We conclude that three photometric features concur to form the paradigm that a small but real helium spread is present in a cluster that has no spectacular evidence for multiple populations like those shown by other massive clusters.Comment: Accepted for publication in the MNRAS on 2010 June 8. Received 2010 May 19; in original form 2010 February 9. 7 pages and 3 figures. No table

HST Astrometry of M4 and the Galactic Constant V_0/R_0

From multi-epoch WFPC2/HST observations we present astrometric measurements of stars in the Galactic globular cluster M4 (NGC 6121) and in the foreground/background. The presence of an extragalactic point source allows us to determine the absolute proper motion of the cluster, and, through use of the field stars in this region only 18 degree from the Galactic center, to measure the difference between the Oort constants, A-B. We find: (mu_alpha cos dec, mu_dec)_J2000 = (-13.21 +/- 0.35, -19.28 +/- 0.35) mas/yr, and A-B = V_0/R_0 = 27.6 +/- 1.7 km / s / kpc.Comment: 20 pages, 6 figures, A.J.__ACCEPTED__, 1 April, 2003, (...!

Wind Speed and Direction Detection by Means of Solid-state Anemometers Embedded on Small Quadcopters

This work describes the application of a compact, MEMS-based, 2D anemometer to the estimation of a quadrotor's airspeed. Correcting for the vehicle's ground speed provided by internal GPS and inertial units allows this low cost, mobile platform to provide local wind speed estimates. A series of initial, bench-top tests were performed to characterize and calibrate the sensor, which is an improved version of a recently proposed and novel device. Additional full-scale wind tunnel experiments were performed with the sensor mounted on a fixed quadrotor to test the effect of the propellers on the sensor's performance

HST color-magnitude diagrams of 74 galactic globular clusters in the HST F439W and F555W bands

We present the complete photometric database and the color-magnitude diagrams for 74 Galactic globular clusters observed with the HST/WFPC2 camera in the F439W and F555W bands. A detailed discussion of the various reduction steps is also presented, and of the procedures to transform instrumental magnitudes into both the HST F439W and F555W flight system and the standard Johnson B and V systems. We also describe the artificial star experiments which have been performed to derive the star count completeness in all the relevant branches of the color magnitude diagram. The entire photometric database and the completeness function will be made available on the Web immediately after the publication of the present paper.Comment: 21 pages, 77 figures. High resolution version of this paper can be retrived at http://dipastro.pd.astro.it/globular

Wind Speed and Direction Detection by Means of Solid-state Anemometers Embedded on Small Quadcopters

This work describes the application of a compact, MEMS-based, 2D anemometer to the estimation of a quadrotor's airspeed. Correcting for the vehicle's ground speed provided by internal GPS and inertial units allows this low cost, mobile platform to provide local wind speed estimates. A series of initial, bench-top tests were performed to characterize and calibrate the sensor, which is an improved version of a recently proposed and novel device. Additional full-scale wind tunnel experiments were performed with the sensor mounted on a fixed quadrotor to test the effect of the propellers on the sensor's performance. Keywords: wind sensor; quadcopter; MEMS; UA

Photometry and spectroscopy of multiple populations along the AGB of NGC2808 and NGC6121 (M4)

We present a photometric and spectroscopic study of multiple populations along the asymptotic-giant branch (AGB) of the intermediate-metallicity globular clusters (GCs) NGC2808 and NGC6121 (M4). Chemical abundances of O, Na, Mg, Al, Si, Ca, Sc, Ti, V, Cr, Fe, Co, Ni, Zn, Y, and Ce in AGB stars from high-resolution FLAMES+UVES@VLT spectra are reported for both clusters. Our spectroscopic results have been combined with multi-wavelength photometry from the HST UV survey of Galactic GCs and ground-based photometry, plus proper motions derived by combining stellar positions from ground-based images and Gaia DR1. Our analysis reveals that the AGBs of both clusters host multiple populations with different chemical composition. In M4 we have identified two main populations of stars with different Na/O content, lying on distinct AGBs in the mF438W vs. C_F275W,F336W,F438W and the V vs.C_U,B,I pseudo-CMDs. In the more massive and complex GC NGC2808 three groups of stars with different chemical abundances occupy different locations on the so-called "chromosome map" photometric diagram. The spectroscopic+photometric comparison of stellar populations along the AGB and the red giants of this GC suggests that the AGB hosts stellar populations with a range in helium abundances spanning from primordial up to high contents of Y~0.32. On the other hand, from our dataset, there is no evidence for stars with extreme helium abundance (Y~0.38) on the AGB, suggesting that the most He-rich stars of NGC2808 do not reach this phase.Comment: 33 pages, 8 figures, accepted for publication in Ap

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 \~20000 objects down to m_F555W ~ 29 in the PC field of view and ~ 20000 in the three WFCs. In the infrared we have been able to resolve ~ 2400 stars down to m_{F110W}, m_{F160W} ~ 26. A subsample of 1834 stars have been unambiguously measured in all the four bands. The corresponding color-magnitude diagrams (CMDs) confirm the existence of an age gradient, showing that NGC 1705 hosts both young (a few Myr old) and very old (up to 15 Gyr old) 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.Comment: aastex, 37 pages, 13 (reduced resolution) .ps figures. Accepted by AJ. Full resolution figures are available upon request to the first autho

The SUMO project I. A survey of multiple populations in globular clusters

We present a general overview and the first results of the SUMO project (a SUrvey of Multiple pOpulations in Globular Clusters). The objective of this survey is the study of multiple stellar populations in the largest sample of globular clusters homogeneously analysed to date. To this aim we obtained high signal-to-noise (S/N>50) photometry for main sequence stars with mass down to ~0.5 M_SUN in a large sample of clusters using both archival and proprietary U, B, V, and I data from ground-based telescopes. In this paper, we focus on the occurrence of multiple stellar populations in twenty three clusters. We have defined a new photometric index cubi= (U-B)-(B-I), that turns out to be very effective for identifying multiple sequences along the red giant branch (RGB). We found that in the V-cubi diagram all clusters presented in this paper show broadened or multimodal RGBs, with the presence of two or more components. We found a direct connection with the chemical properties of different sequences, that display different abundances of light elements (O, Na, C, N, and Al). The cubi index is also a powerful tool to identify distinct sequences of stars along the horizontal branch and, for the first time in the case of NGC104 (47 Tuc), along the asymptotic giant branch. Our results demonstrate that i) the presence of more than two stellar populations is a common feature among globular clusters, as already highlighted in previous work; ii) multiple sequences with different chemical contents can be easily identified by using standard Johnson photometry obtained with ground-based facilities; iii) in the study of GC multiple stellar populations the cubi index is alternative to spectroscopy, and has the advantage of larger statistics.Comment: 23 pages, 20 figures, accepted for publication in MNRA

Photometry of the Globular Cluster NGC 5466: Red Giants and Blue Stragglers

We present wide-field BVI photometry for about 11,500 stars in the low-metallicity cluster NGC 5466. We have detected the red giant branch bump for the first time, although it is at least 0.2 mag fainter than expected relative to the turnoff. The number of red giants (relative to main sequence turnoff stars) is in excellent agreement with stellar models from the Yonsei-Yale and Teramo groups, and slightly high compared to Victoria-Regina models. This adds to evidence that an abnormally large ratio of red giant to main-sequence stars is not correlated with cluster metallicity. We discuss theoretical predictions from different research groups and find that the inclusion or exclusion of helium diffusion and strong limit Coulomb interactions may be partly responsible. We also examine indicators of dynamical history: the mass function exponent and the blue straggler frequency. NGC 5466 has a very shallow mass function, consistent with large mass loss and recently-discovered tidal tails. The blue straggler sample is significantly more centrally concentrated than the HB or RGB stars. We see no evidence of an upturn in the blue straggler frequency at large distances from the center. Dynamical friction timescales indicate that the stragglers should be more concentrated if the cluster's present density structure has existed for most of its history. NGC 5466 also has an unusually low central density compared to clusters of similar luminosity. In spite of this, the specific frequency of blue stragglers that puts it right on the frequency -- cluster M_V relation observed for other clusters.Comment: 51 pages, 21 figures, 1 electronic table, accepted to Ap

The Galactic globular cluster NGC 1851: its dynamical and evolutionary properties

We have completely mapped the globular cluster NGC 1851 with VI CCD photometry, obtaining a CMD for ca. 20500 stars. From the apparent luminosity of the HB we derive a distance modulus (m-M)o=15.44+/-0.20. The presence of 7 "supra-HB" stars is confirmed. Six of them are identified as evolved descendants from HB progenitors. Synthetic HR diagrams demonstrate that the problem of the HB morphology could be solved by assuming a bimodal efficiency of the mass loss along the RGB. We find evidence that the radial distribution of the blue HB stars is different from that of the red HB and SGB stars. The first measurement of the mean absolute I magnitude for 22 known RR Lyr variables (=0.12+/-0.20 at [Fe/H]=-1.28), is given. We found 7 new RR Lyr candidates (6 ab type and 1 c type), lowering the ratio N(c)/N(ab) to 0.38. From a sample of 25 globular clusters (including NGC 1851) a new calibration for DV(bump,HB) as a function of metallicity is derived, and we find some evidence for an age-metallicity relation among GCs. We identify 13 blue straggler stars, which do not show any sign of variability. The blue stragglers are less concentrated than the SGB stars with similar magnitudes for r>80". Finally, a radial dependence of the LF, a sign of mass segregation, is found. Transforming the LF into a mass function and correcting for mass segregation by means of multi-mass King-Michie models, we find a global MF exponent x(0)=0.2+/-0.3.Comment: 19 pages, 5 figures, to be published in Astronomy & Astrophysics. Appendices A and B are only available in electronic form at the CDS via anonymous ftp to ftp://cdsarc.u-strasbg.fr or via http://cdsweb.u-strasbg.fr/Abstract.htm