Long-term radial-velocity variations of the Sun as a star: The HARPS view

Stellar radial velocities play a fundamental role in the discovery of extrasolar planets and the measurement of their physical parameters as well as in the study of stellar physical properties. We investigate the impact of the solar activity on the radial velocity of the Sun using the HARPS spectrograph to obtain measurements that can be directly compared with those acquired in the extrasolar planet search programs. We use the Moon, the Galilean satellites, and several asteroids as reflectors to measure the radial velocity of the Sun as a star and correlate it with disc-integrated chromospheric and magnetic indexes of solar activity that are similar to stellar activity indexes. We discuss in detail the systematic effects that affect our measurements and the methods to account for them. We find that the radial velocity of the Sun as a star is positively correlated with the level of its chromospheric activity at about 95 percent significance level. The amplitude of the long-term variation measured in the 2006-2014 period is 4.98 \pm 1.44 m/s, in good agreement with model predictions. The standard deviation of the residuals obtained by subtracting a linear best fit is 2.82 m/s and is due to the rotation of the reflecting bodies and the intrinsic variability of the Sun on timescales shorter than the activity cycle. A correlation with a lower significance is detected between the radial velocity and the mean absolute value of the line-of-sight photospheric magnetic field flux density. Our results confirm similar correlations found in other late-type main-sequence stars and provide support to the predictions of radial velocity variations induced by stellar activity based on current models.Comment: 11 pages, 7 figures, 2 tables, 1 Appendix; accepted by Astronomy and Astrophysic

The Spectra of Main Sequence Stars in Galactic Globular Clusters II. CH and CN Bands in M71

Spectra with a high signal-to-noise ratio of 79 stars which are just below the main sequence turnoff of M71 are presented. They yield indices for the strength of the G band of CH and the ultraviolet CN band at 3885 \AA. These indices are each to first order bimodal and they are anti-correlated. There are approximately equal numbers of CN weak/CH strong and CN strong/CH weak main sequence stars in M71. It is not yet clear whether these star-to-star variations arise from primordial variations or from mixing within a fraction of individual stars as they evolve.Comment: Accepted for publication in the AJ to appear back to back with paper I. 14 pages with 5 figure

Nucleosynthesis in Type II supernovae and the abundances in metal-poor stars

We explore the effects on nucleosynthesis in Type II supernovae of various parameters (mass cut, neutron excess, explosion energy, progenitor mass) in order to explain the observed trends of the iron-peak element abundance ratios ([Cr/Fe], [Mn/Fe], [Co/Fe] and [Ni/Fe]) in halo stars as a function of metallicity for the range $-4 \le$ [Fe/H] $\le -2.5$. [Cr/Fe] and [Mn/Fe] decrease with decreasing [Fe/H], while [Co/Fe] behaves the opposite way and increases. We show that such a behavior can be explained by a variation of mass cuts in Type II supernovae as a function of progenitor mass, which provides a changing mix of nucleosynthesis from an alpha-rich freeze-out of Si-burning and incomplete Si-burning. This explanation is consistent with the amount of ejected $^{56}$Ni determined from modeling the early light curves of individual supernovae. We also suggest that the ratio [H/Fe] of halo stars is mainly determined by the mass of interstellar hydrogen mixed with the ejecta of a single supernova which is larger for larger explosion energy and the larger Str\"omgren radius of the progenitor.Comment: 17 pages, LaTeX, Accepted for publication in the Astrophysical Journal, more discussion on the Galactic chemical evolutio

Beryllium abundances and the formation of the halo and the thick disk

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 - Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C. Chiappini, eds

A Frequency Comb calibrated Solar Atlas

The solar spectrum is a primary reference for the study of physical processes in stars and their variation during activity cycles. In Nov 2010 an experiment with a prototype of a Laser Frequency Comb (LFC) calibration system was performed with the HARPS spectrograph of the 3.6m ESO telescope at La Silla during which high signal-to-noise spectra of the Moon were obtained. We exploit those Echelle spectra to study the optical integrated solar spectrum . The DAOSPEC program is used to measure solar line positions through gaussian fitting in an automatic way. We first apply the LFC solar spectrum to characterize the CCDs of the HARPS spectrograph. The comparison of the LFC and Th-Ar calibrated spectra reveals S-type distortions on each order along the whole spectral range with an amplitude of +/-40 m/s. This confirms the pattern found by Wilken et al. (2010) on a single order and extends the detection of the distortions to the whole analyzed region revealing that the precise shape varies with wavelength. A new data reduction is implemented to deal with CCD pixel inequalities to obtain a wavelength corrected solar spectrum. By using this spectrum we provide a new LFC calibrated solar atlas with 400 line positions in the range of 476-530, and 175 lines in the 534-585 nm range. The new LFC atlas improves the accuracy of individual lines by a significant factor reaching a mean value of about 10 m/s. The LFC--based solar line wavelengths are essentially free of major instrumental effects and provide a reference for absolute solar line positions. We suggest that future LFC observations could be used to trace small radial velocity changes of the whole solar photospheric spectrum in connection with the solar cycle and for direct comparison with the predicted line positions of 3D radiative hydrodynamical models of the solar photosphere.Comment: Accept on the 15th of October 2013. 9 pages, 10 figures. ON-lINE data A&A 201

Fine-structure diagnostics of neutral carbon toward HE 0515-4414

New high-resolution high signal-to-noise spectra of the $z=1.15$ damped Lyman $\alpha$ (DLA) system toward the quasi-stellar object HE 0515-4414 reveal absorption lines of the multiplets 2 and 3 in \ion{C}{i}. The resonance lines are seen in two components with total column densities of $\log N=13.79\pm0.01$ and $\log N=13.36\pm0.01$, respectively. The comparision of theoretical calculations of the relative fine-structure population with the ratios of the observed column densities suggests that the \ion{C}{i} absorbing medium is either very dense or exposed to very intense UV radiation. The upper limit on the local UV energy density is 100 times the galactic UV energy density, while the upper limit on the \ion{H}{i} number density is 110 cm$^{-3}$. The excitation temperatures of the ground state fine-structure levels of $T=15.7$ and $T=11.1$ K, respectively, are consistent with the temperature-redshift relation predicted by the standard Friedmann cosmology. The cosmic microwave background radiation (CMBR) is only a minor source of the observed fine-structure excitation.Comment: 5 pages, 5 figures, uses A&A macro package, gzipped tar archive, accepted by A&

Boron in Very Metal-Poor Stars

We have observed the B I 2497 A line to derive the boron abundances of two very metal-poor stars selected to help in tracing the origin and evolution of this element in the early Galaxy: BD +23 3130 and HD 84937. The observations were conducted using the Goddard High Resolution Spectrograph on board the Hubble Space Telescope. A very detailed abundance analysis via spectral synthesis has been carried out for these two stars, as well as for two other metal-poor objects with published spectra, using both Kurucz and OSMARCS model photospheres, and taking into account consistently the NLTE effects on the line formation. We have also re-assessed all published boron abundances of old disk and halo unevolved stars. Our analysis shows that the combination of high effective temperature (Teff > 6000 K, for which boron is mainly ionized) and low metallicity ([Fe/H]<-1) makes it difficult to obtain accurate estimates of boron abundances from the B I 2497 A line. This is the case of HD 84937 and three other published objects (including two stars with [Fe/H] ~ -3), for which only upper limits can be established. BD +23 3130, with [Fe/H] ~ -2.9 and logN(B)_NLTE=0.05+/-0.30, appears then as the most metal-poor star for which a firm measurement of the boron abundance presently exists. The evolution of the boron abundance with metallicity that emerges from the seven remaining stars with Teff < 6000 K and [Fe/H]<-1, for which beryllium abundances were derived using the same stellar parameters, shows a linear increase with a slope ~ 1. Furthermore, the B/Be ratio found is constant at a value ~ 20 for stars in the range -3<[Fe/H]<-1. These results point to spallation reactions of ambient protons and alpha particles with energetic particles enriched in CNO as the origin of boron and beryllium in halo stars.Comment: 38 pages, 11 Encapsulated Postscript figures (included), uses aaspp4.sty. Accepted for publication in The Astrophysical Journal. The preprint is also available at: http://www.iac.es/publicaciones/preprints.htm

Detection of the Li I 6104 Angstroem transition in the Population II star HD 140283

Lithium is one of the few primordially produced elements. The value of the primordial Li is taken to be that observed in metal--poor dwarfs, where it is not contaminated by stellar Li sources which act on longer time scales. The atmospheric abundance is currently derived from the LiI 6707 Angstroem resonance transition and the validity of the models employed has been questioned (Kurucz 1995). In this letter we report the first detection of the Li I 6104 Angstroem 2^2P - 3^2D subordinate transition in the prototype population II star HD 140283. The same Li abundance of (Li/H) =1.4x10^{-10} is found consistent with both the resonance and subordinate lines. The two lines form at different depths in the atmosphere implying that the 1-D homogeneous atmospheric models used in the abundance determination are essentially correct. When coupled with the standard big bang yields, the Li in the halo dwarfs provides two solutions for the baryon-to-photon ratio \eta_{10}= n_{b}/n_{\gamma} x 10^{10}and for the present baryon density \Omega_b h_{70}^2=0.0748\eta_{10}: a) a first solution at \eta_{10} about 1.8, consistent with the \eta_{10} implied by the high deuterium values D/H\approx 2 x 10^{-4}observed in some quasar absorption systems (Webb et al 1997) and b) a second solution at \eta_{10} about 4 which is consistent, within the errors, with the low deuterium D/H =3.4x10^{-5} measured in other quasar absorption systems (Burles & Tytler 1998).Comment: To appear in ApJ Letters, 11 pages 3 figures, uses aasms4.st