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Register a new userHigh resolution spectroscopy for Cepheids distance determination. III. A relation between gamma-velocities and gamma-asymmetries
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Galactic Cepheids in the vicinity of the Sun have a residual line-of-sight velocity, or gamma-velocity, which shows a systematic blueshift of about 2 km/s compared to an axisymmetric rotation model of the Milky Way. This term is either related to the space motion of the star and, consequently, to the kinematic structure of our Galaxy, or it is the result of the dynamical structure of the Cepheids atmosphere. We aim to show that these residual gamma-velocities are an intrinsic property of Cepheids. We observed nine galactic Cepheids with the HARPS spectroscope, focusing specifically on 17 spectral lines. For each spectral line of each star, we computed the gamma-velocity (resp. gamma-asymmetry) as an average value of the interpolated radial velocity (resp. line asymmetry) curve. For each Cepheid in our sample, a linear relation is found between the gamma-velocities of the various spectral lines and their corresponding gamma-asymmetries, showing that residual gamma-velocities stem from the intrinsic properties of Cepheids. We also provide a physical reference to the stellar gamma-velocity: it should be zero when the gamma-asymmetry is zero. Following this definition, we provide very precise and physically calibrated estimates of the gamma-velocities for all stars of our sample. To understand this very subtle gamma-asymmetry effect, further numerical studies are needed. Cepheids atmosphere are strongly affected by pulsational dynamics, convective flows, nonlinear physics, and complex radiative transport. Hence, all of these effects have to be incorporated simultaneously and consistently into the numerical models to reproduce the observed line profiles in detail.
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The ratio of pulsation to radial velocity (the projection factor) is currently limiting the accuracy of the interferometric Baade-Wesselink method. This work aims at establishing a link between the line asymmetry evolution over the Cepheids pulsation cycles and their projection factor, with the final objective to improve the accuracy of the Baade-Wesselink method for distance determinations. We present HARPS high spectral resolution observations of nine galactic Cepheids having a good period sampling. We fit spectral line profiles by an asymmetric bi-Gaussian to derive radial velocity, Full-Width at Half-Maximum in the line (FWHM) and line asymmetry for all stars. We then extract correlations curves between radial velocity and asymmetry. A geometric model providing synthetic spectral lines, including limb-darkening, a constant FWHM (hereafter sigma_c) and the rotation velocity is used to interpret these correlations curves. For all stars, comparison between observations and modelling is satisfactory, and we were able to determine the projected rotation velocities and sigma_c for all stars. We also find a correlation between the rotation velocity (Vrot sin i) and the period of the star: Vrot sin i = (11.5 +- 0.9) log(P) + (19.8 +- 1.0) [km/s]. Moreover, we observe a systematic shift in observational asymmetry curves (noted gamma_O), related to the period of the star, which is not explained by our static model: gamma_O = (10.7+-0.1) log(P) + (9.7+-0.2) [in %] . For long-period Cepheids, in which velocity gradients, compression or shock waves seem to be large compared to short- or medium period Cepheids we observe indeed a greater systematic shift in asymmetry curves. (abridged)
The GALILEO $gamma$-ray spectrometer has been constructed at the Legnaro National Laboratory of INFN (LNL-INFN). It can be coupled to advanced ancillary devices which allows nuclear structure studies employing the variety of in-beam $gamma$-ray spectroscopy methods. Such studies benefit from reactions induced by the intense stable beams delivered by the Tandem-ALPI-PIAVE accelerator complex and by the radioactive beams which will be provided by the SPES facility. In this paper we outline two experiments performed within the experimental campaign at GALILEO coupled to the EUCLIDES Si-ball and the Neutron Wall array. The first one was aimed at spectroscopic studies in A=31 mirror nuclei and the second one at measurements of lifetimes of excited states in nuclei in the vicinity of $^{100}$Sn.
The separation between RR Lyrae (RRLs) and Type II Cepheid (T2Cs) variables based on their period is debated. Both types of variable stars are distance indicators and we aim to promote the use of T2Cs as distance indicators in synergy with RRLs. We adopted new and existing optical and Near-Infrared (NIR) photometry of wcen~to investigate several diagnostics (colour-magnitude diagram, Bailey diagram, Fourier decomposition of the light curve, amplitude ratios) for their empirical separation. We found that the classical period threshold at 1 day is not universal and does not dictate the evolutionary stage: V92 has a period of 1.3 days but is likely to be still in its core Helium-burning phase, typical of RRLs. We also derived NIR Period-Luminosity relations and found a distance modulus of 13.65$pm$0.07 (err.)$pm$0.01 ($sigma$) mag, in agreement with the recent literature. We also found that RRLs and T2Cs obey the same PL relations in the NIR. This equivalence gives the opportunity to adopt RRLs+T2Cs as an alternative to classical Cepheids to calibrate the extragalactic distance scale.
A new generation of Ge-based high-resolution gamma-ray spectrometers has allowed accurate measurements to be made of the profiles, widths and energies of the gamma-ray lines emitted in the impulsive phases of solar flares. Here we present measurements in two flares of the energies of the de-excitation lines of 12C and 16O at 4.4 and 6.1 MeV respectively by the Ge spectrometer SPI on board INTEGRAL, from which Doppler shifts are derived and compared with those expected from the recoil of 12C and 16O nuclei which were excited by the impacts of flare-accelerated ions. An anomalous Doppler measurement (in terms of recoil theory) has been reported by the Ge spectrometer RHESSI in a flare near the east limb, and explained by a tilt of the magnetic field lines at the footpoint of a magnetic loop away from the vertical, and towards the observer. This might be interpreted to imply a significant difference between the Doppler shifts on the east and west limbs, if it is a general phenomenon. SPI observed both east and west limb flares and found no significant difference in Doppler shifts. We also measured the shapes and fluences of these lines, and their fluence ratio to the 2.2 MeV line from the capture of flare-generated neutrons. Analyses of both quantities using thick-target models parametrized by solar physical and geometric quantities suggest that the two flares studied here also have magnetic fields tilted towards the observer, though the significance of the measurements is not high.
We have compared the parsec-scale jet linear polarization properties of the Fermi LAT-detected and non-detected sources in the complete flux-density-limited (MOJAVE-1) sample of highly beamed AGN. Of the 123 MOJAVE sources, 30 were detected by the LAT during its first three months of operation. We find that during the era since the launch of Fermi, the unresolved core components of the LAT-detected jets have significantly higher median fractional polarization at 15 GHz. This complements our previous findings that these LAT sources have higher apparent jet speeds, brightness temperatures and Doppler factors, and are preferentially found in higher activity states.
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