F. Cusano

Pre-main sequence spectroscopic binaries suitable for VLTI observations

Context. A severe problem for research in star-formation is that the masses of young stars are almost always estimated from evolutionary tracks alone. Since the tracks published by different groups differ, it is often only possible to give a rough estimate of the masses of young stars. It is thus crucial to test and calibrate the tracks. Up to now, only a few tests of the tracks could be carried out. However, it is now possible with the VLTI to set constrains on the tracks by determining the masses of many young binary stars precisely. Aims. In order to use the VLTI efficiently, a first step is to find suitable targets, which is the purpose of this work. Given the distance of nearby star-forming regions, suitable VLTI targets are binaries with orbital periods between at least 50 days and a few years. Although a number of surveys for detecting spectroscopic binaries have been carried out, most of the binaries found so far have periods that are too short. Methods. We thus surveyed the Chamaeleon, Corona Australis, Lupus, Sco-Cen, and ρ Ophiuci star-forming regions in order to search for spectroscopic binaries with periods longer than 50 days, which are suitable for the VLTI observations. Results. As a result of the 8 year campaign, we discovered 8 binaries with orbital periods longer than 50 days. Amongst the newly discovered long-period binaries is CS Cha, which is one of the few classical T Tauri stars with a circumbinary disk. The survey is

ANGULAR DIAMETERS AND EFFECTIVE TEMPERATURES OF 25 K GIANT STARS FROM THE CHARA ARRAY

Using Georgia State Universitys Center for High Angular Resolution Astronomy Array interferometer, we measured angular diameters for 25 giant stars, six of which host exoplanets. The combination of these measurements and Hipparcos parallaxes produces physical linear radii for the sample. Except for two outliers, our values match angular diameters and physical radii estimated using photometric methods to within the associated errors with the advantage that our uncertainties are significantly lower. We also calculated the effective temperatures for the stars using the newly measured diameters. Our values do not match those derived from spectroscopic observations as well, perhaps due to the inherent properties of the methods used or because of a missing source of extinction in the stellar models that would affect the spectroscopic temperatures.

CHARACTERIZING FAINT GALAXIES IN THE REIONIZATION EPOCH: LBT CONFIRMS TWO L < 0.2 L* SOURCES AT z = 6.4 BEHIND THE CLASH/FRONTIER FIELDS CLUSTER MACS0717.5+3745

We report the LBT/MODS1 spectroscopic confirmation of two images of faint Lyα emitters at z = 6.4 behind the Frontier Fields galaxy cluster MACSJ0717.5+3745. A wide range of lens models suggests that the two images are highly magnified, with a strong lower limit of μ > 5. These are the faintest z > 6 candidates spectroscopically confirmed to date. These may also be multiple images of the same z = 6.4 source as supported by their similar intrinsic properties, but the lens models are inconclusive regarding this interpretation. To be cautious, we derive the physical properties of each image individually. Thanks to the high magnification, the observed near-infrared (restframe ultraviolet) part of the spectral energy distributions and Lyα lines are well detected with S/N(m _(1500)) ≳ 10 and S/N(Lyα) ≃ 10-15. Adopting μ > 5, the absolute magnitudes, M _(1500), and Lyα fluxes are fainter than –18.7 and 2.8 × 10^(–18) erg s^(–1) cm^(–2), respectively. We find a very steep ultraviolet spectral slope β = –3.0 ± 0.5 (F _λ = λ^β), implying that these are very young, dust-free, and low metallicity objects, made of standard stellar populations or even extremely metal poor stars (age ≾ 30 Myr, E(B – V) = 0 and metallicity 0.0-0.2 Z/Z_☉). The objects are compact (<1 kpc^2) and with a stellar mass M_* < 10^8 M_☉. The very steep β, the presence of the Lyα line, and the intrinsic FWHM (<300 km s^(–1)) of these newborn objects do not exclude a possible leakage of ionizing radiation. We discuss the possibility that such faint galaxies may resemble those responsible for cosmic reionization.

VARIABLE STARS IN THE ULTRA-FAINT DWARF SPHEROIDAL GALAXY URSA MAJOR I

We have performed the first study of the variable star population of Ursa Major I (UMa I), an ultra-faint dwarf satellite recently discovered around the Milky Way (MW) by the Sloan Digital Sky Survey. Combining time series observations in the B and V bands from four different telescopes, we have identified seven RR Lyrae stars in UMa I, of which five are fundamental-mode (RRab) and two are first-overtone pulsators (RRc). Our V, B ? V color-magnitude diagram of UMa I reaches V ~ 23?mag (at a signal-to-noise ratio of ~6) and shows features typical of a single old stellar population. The mean pulsation period of the RRab stars P ab = 0.628, ? = 0.071?days (or P ab = 0.599, ? = 0.032?days, if V4, the longest period and brightest variable, is discarded) and the position on the period-amplitude diagram suggest an Oosterhoff-intermediate classification for the galaxy. The RR Lyrae stars trace the galaxy horizontal branch (HB) at an average apparent magnitude of V(RR) = 20.43 ? 0.02?mag (average on six stars and discarding V4), giving in turn a distance modulus for UMa I of (m ? M)0 = 19.94 ? 0.13?mag, distance d = ?kpc, in the scale where the distance modulus of the Large Magellanic Cloud is 18.5 ? 0.1?mag. Isodensity contours of UMa I red giants and HB stars (including the RR Lyrae stars identified in this study) show that the galaxy has an S-shaped structure, which is likely caused by the tidal interaction with the MW. Photometric metallicities were derived for six of the UMa I RR Lyrae stars from the parameters of the Fourier decomposition of the V-band light curves, leading to an average metal abundance of [Fe/H] = ?2.29?dex (? = 0.06?dex, average on six stars) on the Carretta et al. metallicity scale.

Spitzer/IRAC view of Sh 2-284. Searching for evidence of triggered star formation in an isolated region in the outer Milky Way

Aims. Using Spitzer/IRAC observations of a region to be observed by the CoRoT satellite, we have unraveled a new complex star-forming region at low metallicity in the outer Galaxy. We perform a study of S284 in order to outline the chain of events in this star-forming region. Methods. We used four-band Spitzer/IRAC photometry as well as H alpha imaging obtained with INT/WFC. Combining these data with the optical photometry obtained in the frame of CoRoTs preparation and the 2MASS catalog we analysed the properties and distribution of young stellar objects (YSOs) associated with point-like sources. We also studied the SEDs of regions of extended emission, complementing our dataset with IRAS and MSX data. Results. We find that S284 is unique in several ways: it is very isolated at the end of a spiral arm and both the diffuse dust and ionized emission are remarkably symmetric. We have partially resolved the central clusters of the three bubbles present in this region. Despite the different scales observed in its multiple-bubble morphology, our study points to a very narrow spread of ages among the powering high-mass clusters. In contrast, the particular sawtooth structure of the extended emission at the rim of each ionized bubble harbours either small lower-mass clusters with a younger stellar population or individual young reddened protostars. In particular, triggered star formation is considered to be at work in these regions.

Multi-object spectroscopy of stars in the CoRoT fields - II. The stellar population of the CoRoT fields IRa01, LRa01, LRa02, and LRa06

Context. With now more than 20 exoplanets discovered by CoRoT, it has often been considered strange that so many of them are orbiting F-stars, and so few of them K-or M-stars. Up to now, studies of the relation between the frequency of extrasolar planets and the spectral types, or masses of their host stars has been the realm of radial velocity surveys. Although transit search programs are mostly sensitive to short-period planets, they are ideal for verifying these results. This is because transit search programs have different selection biases than radial velocity surveys. To determine the frequency of planets as a function of stellar mass, we also have to characterize the sample of stars that was observed. Aims. We study the stellar content of the CoRoT-fields IRa01, LRa01 (=LRa06), and LRa02 by determining the spectral types of 11 466 stars. Nine planet-host stars have already been identified in these fields. Determing the spectral types of thousands of stars of which CoRoT obtained high-precision light-curves also makes a wide variety of other research projects possible. Methods. We used spectra obtained with the multi-object spectrograph AAOmega and derived the spectral types by using template spectra with well-known parameters. Results. We find that 34.8 +/- 0.7% of the stars observed by CoRoT in these fields are F-dwarfs, 15.1 +/- 0.5% G-dwarfs, and 5.0 +/- 0.3% K-dwarfs. We conclude that the apparent lack of exoplanets of K- and M-stars is explained by the relatively small number of these stars in the observed sample. We also show that the apparently large number of planets orbiting F-stars is similarly explained by the large number of such stars in these fields. Given the number of F-stars, we would have expected to find even more F-stars with planets. Our study also shows that the difference between the sample of stars that CoRoT observes and a sample of randomly selected stars is relatively small, and that the yield of CoRoT specifically is the detection one hot Jupiter amongst 2100 +/- 700 stars. Conclusions. We conclude that transit search programs can be used to study the relation between the frequency of planets and the mass of the host stars, and that the results obtained so far generally agree with those of radial velocity programs.

VARIABLE STARS AND STELLAR POPULATIONS IN ANDROMEDA XXV. III. A CENTRAL CLUSTER OR THE GALAXY NUCLEUS

We present B and V time-series photometry of Andromeda XXV, the third galaxy in our program on the Andromedas satellites, that we have imaged with the Large Binocular Cameras of the Large Binocular Telescope. The field of Andromeda XXV is found to contain 63 variable stars, for which we present light curves and characteristics of the light variation (period, amplitudes, variability type, mean magnitudes, etc.). The sample includes 58 RR Lyrae variables (46 fundamental-mode

CoRoT 102699796, the first metal‐poor Herbig Ae pulsator: a hybrid δ Sct–γ Dor variable?★†

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Lyman continuum escape fraction of faint galaxies at z ~ 3.3 in the CANDELS/GOODS-North, EGS, and COSMOS fields with LBC

RRab, and 12 first-overtone

VARIABLE STARS AND STELLAR POPULATIONS IN ANDROMEDA XXI. II. ANOTHER MERGED GALAXY SATELLITE OF M31

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