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Search Results: 1 - 10 of 22415 matches for " Roberto Silvotti "
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On a conjecture of Varchenko
Roberto Silvotti
Mathematics , 1995, DOI: 10.1007/s002220050096
Abstract: In this note we generalize and prove a recent conjecture of Varchenko concerning the number of critical points of a (multivalued) meromorphic function $\phi$ on an algebraic manifold. Under certain conditions, this number turns out to coincide with the Euler characteristic (up to a sign) of the complement of the divisor of $\phi$. A few variants of this basic situation are also discussed. Two independent proofs are given, respectively using Chern classes and Morse theory In its original form, this counting problem arises (in special cases) in evaluating the asymptotics of correlation functions of Wess--Zumino--Witten conformal field theory.
The EXOTIME targets HS0702+6043 and HS0444+0458
Ronny Lutz,Sonja Schuh,Roberto Silvotti
Physics , 2010, DOI: 10.1063/1.3570968
Abstract: Pulsations in subdwarf B (sdB) stars are an important tool to constrain the evolutionary status of these evolved objects. Interestingly, the same data used for this asteroseismological approach can also be used to search for substellar companions around these objects by analyzing the timing of the pulsations by means of a so-called O-C diagram. Substellar objects around sdB stars are important for two different reasons: they are suspected to be able to influence the evolution of their host-star and they are an ideal test case to examine the properties of exoplanets which have survived the red giant expansion of their host stars.
White Dwarf Planets from GAIA
Roberto Silvotti,Alessandro Sozzetti,Mario Lattanzi
Physics , 2010, DOI: 10.1063/1.3556222
Abstract: We investigate the potential of high-precision astrometry with GAIA for detection of giant planetary companions to nearby white dwarfs. If one considers that, to date, no confirmed planets around single white dwarfs are known, the results from GAIA will be crucial to study the late-stage evolution of planetary systems and to verify the possibility that 2nd-generation planets are formed.
EXOTIME: searching for planets and measuring Pdot in sdB pulsators
Ronny Lutz,Sonja Schuh,Roberto Silvotti
Physics , 2013, DOI: 10.1002/asna.201211793
Abstract: We review the status of the EXOTIME project (EXOplanet search with the TIming MEthod). The two main goals of EXOTIME are to search for sub-stellar companions to sdB stars in wide orbits, and to measure the secular variation of the pulsation periods, which are related to the evolutionary change of the stellar structure. Now, after four years of dense monitoring, we start to see some results and present the brown dwarf and exoplanet candidates V1636 Ori b and DW Lyn b.
Detectability of substellar companions around white dwarfs with Gaia
Roberto Silvotti,Alessandro Sozzetti,Mario Lattanzi,Roberto Morbidelli
Physics , 2014,
Abstract: To date not a single-bona fide planet has been identified orbiting a single white dwarf. In fact we are ignorant about the final configuration of >95% of planetary systems. Theoretical models predict a gap in the final distribution of orbital periods, due to the opposite effects of stellar mass loss (planets pushed outwards) and tidal interactions (planets pushed inwards) during the RGB and the AGB stellar expansions. Over its five year primary mission, Gaia is expected to astrometrically detect the first (few tens of) WD massive planets/BDs giving first evidence that WD planets exist, at least those in wide orbits. In this article we present preliminary results of our simulations of what Gaia should be able to find in this field.
A Recent Class of Chemosensory Neurons Developed in Mouse and Rat
Lucia Silvotti, Eleonora Cavalca, Rita Gatti, Riccardo Percudani, Roberto Tirindelli
PLOS ONE , 2011, DOI: 10.1371/journal.pone.0024462
Abstract: In most animal species, the vomeronasal organ ensures the individual recognition of conspecifics, a prerequisite for a successful reproduction. The vomeronasal organ expresses several receptors for pheromone detection. Mouse vomeronasal type-2 receptors (V2Rs) are restricted to the basal neurons of this organ and organized in four families. Family-A, B and D (family ABD) V2Rs are expressed monogenically (one receptor per neuron) and coexpress with either Vmn2r1 or Vmn2r2, two members of family-C V2Rs. Thus, basal neurons are characterized by specific combinations of two V2Rs. To investigate this issue, we raised antibodies against all family-C V2Rs and analyzed their expression pattern. We found that six out of seven family-C V2Rs (Vmn2r2-7) largely coexpressed and that none of the anti-Vmn2r2-7 antibodies significantly stained Vmn2r1 positive neurons. Thus, basal neurons are divided into two complementary subsets. The first subset (Vmn2r1-positive) preferentially coexpresses a distinct group of family-ABD V2Rs, whereas the second subset (Vmn2r2-7-positive) coexpresses the remaining group of V2Rs. Phylogenetic reconstruction and the analysis of genetic loci in various species reveal that receptors expressed by this second neuronal subset are recent branches of the V2R tree exclusively present in mouse and rat. Conversely, V2Rs expressed in Vmn2r1 positive neurons, are phylogenetically ancient and found in most vertebrates including rodents. Noticeably, the more recent neuronal subset expresses a type of Major Histocompatibility Complex genes only found in murine species. These results indicate that the expansion of the V2R repertoire in a murine ancestor occurred with the establishment of a new population of vomeronasal neurons in which coexists the polygenic expression of a recent group of family-C V2Rs (Vmn2r2-7) and the monogenic expression of a recent group of family-ABD V2Rs. This evolutionary innovation could provide a molecular rationale for the exquisite ability in individual recognition and mate choice of murine species.
Planetary companions to sdB stars
Roberto Silvotti,Roy H. Ostensen,John H. Telting,Christophe Lovis
Physics , 2014,
Abstract: The formation of single sdB stars is an unresolved issue and the presence of close sub-stellar companions could explain the stellar envelope ejection near the tip of the RGB, that is needed to form an sdB star. In the last 6 years the first planets orbiting sdB stars were detected using different methods. In this article we discuss these recent discoveries, review the different detection methods used, and present some preliminary results obtained with Harps-N, which allows to reach an unprecedented accuracy in radial velocity measurements.
The Potential of the Timing Method to Detect Evolved Planetary Systems
Roberto Silvotti,Robert Szabo,Pieter Degroote,Roy H. Ostensen,Sonja Schuh
Physics , 2010, DOI: 10.1063/1.3570966
Abstract: The timing method, using either stellar pulsations or eclipse timing of close binaries as a clock, is proving to be an efficient way to detect planets around stars that have evolved beyond the red giant branch. In this article we present a short review of the recent discoveries and we investigate the potential of the timing method using data both from ground-based facilities as well as from the Kepler and CoRoT space missions.
HS2324+3944: discovery of non-radial pulsations in a hydrogen-rich PG1159 star
R. Silvotti
Physics , 1996,
Abstract: HS\,2324+3944 is a peculiar PG\,1159 star, with a high amount of H in its atmosphere (Dreizler et al. 1995). Its location in the $\log T_{\rm eff}$ -- $\log g$ plane is well inside the GW Vir instability strip. In this paper I report the results of two photoelectric observations of HS\,2324+39, which clearly show that the luminosity of this star presents periodical variations with a period of (2140 $\pm$ 11) s. This photometric behaviour is most easily explained by the presence of non-radial oscillations. Therefore HS\,2324+39 is a new member of the GW Vir group, and is characterized by the longest pulsation period found among these stars. Moreover HS\,2324+39 appears to be the first pulsating PG\,1159 star with a high H abundance in its atmosphere (H/He = 2 by number, Dreizler et al. 1995). Were the pulsation mechanism based on the C/O cyclic ionization (Starrfield et al. 1984) at work, the H abundance should drop to zero sharply in the driving regions. Such a strong decrease of hydrogen looks quite unlikely; for this reason the presence of pulsations in HS\,2324+39 seems to be a very interesting phenomenon.
V391 Peg: identification of the two main pulsation modes from ULTRACAM u'g'r' amplitudes
Roberto Silvotti,Suzanna Randall,Vik S. Dhillon,Tom R. Marsh,Chris D. Savoury,Sonja Schuh,Gilles Fontaine,Pierre Brassard
Physics , 2010, DOI: 10.1002/asna.201011451
Abstract: V391 Peg (HS2201+2610) is an extreme horizontal branch subdwarf B (sdB) star, it is an hybrid pulsator showing p- and g-mode oscillations, and hosts a 3.2/sini M_Jup planet at an orbital distance of about 1.7 AU. In order to improve the characterization of the star, we measured the pulsation amplitudes in the u'g'r' SLOAN photometric bands using ULTRACAM at the William Herschel 4.2 m telescope and we compared them with theoretical values. The preliminary results presented in this article conclusively show that the two main pulsation periods at 349.5 and 354.1 s are a radial and a dipole mode respectively. This is the first time that the degree index of multiple modes has been uniquely identified for an sdB star as faint as V391 Peg (B=14.4), proving that multicolor photometry is definitely an efficient technique to constrain mode identification, provided that the data have a high enough quality.
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