Search

Since 1995, the extra-solar planet search has driven the high resolution spectroscopy community to build more and more stable spectrographs in order to reach the photon statistics limit in radial velocity measurements. This situation opened the possibility of asteroseismic observations of stellar p mode pulsations in solar-like stars. In this contribution we summarize the high precision radial velocity measurements of two solar type stars (α CMi and μ Her) using the SARG spectrograph at TNG equipped with an iodine cell. The analyzed spectra show individual measurement errors of about 1.0 m/s (very close to the theoretical photon noise limit). Further we discuss the synergy between high precision radial velocity asteroseismology campaigns and the search for Super Earths. Individual Objects: Procyon, μ Her Radial velocity uncertainties with SARG SARG (Spettrografo Alta Risoluzione Galileo) is the high resolution optical spectrograph of the Italian Telescopio Nazionale Galileo (TNG). Instrument characteristics include a high spectral resolution (maximum about 160 000), high efficiency (peak at about 13%), rather large spectral coverage in a single shot. SARG was designed as a multipurpose instrument. The instrument, which mounts an iodine absorbing cell, is particularly suited for precise radial velocity programs, such as planet search and asteroseismology (Gratton et al. 2001). Radial velocities are obtained by means of the AUSTRAL code (Endl et al. 2000). We evaluate radial velocity uncertainties for SARG using the appropriate quality factor (see Bouchy et al. 2001) for spectrograph resolution, spectral type and rotational broadening. For stars brighter than 3.5 magnitude the resulting RV uncertainties range between 0.9 to 3 m/s depending on magnitude and spectral type of the star. This result affects also the efficiency of the instrument in detecting oscillations maintaining the number of spectra confined inside a reasonable number necessary to detect oscillation of amplitude νosc over a 4 σ threshold. We are aware that the limits of RV measurements are also set by spectrograph instabilities, but analyzing the results obtained with the subgiant star Procyon and μ Her we can limit the instrumental contribution to RV uncertainty lower than 1 m/s. 22 Asteroseismology of solar–type stars with SARG@TNG

The G5 subgiant star μ Herculis was observed in June 2006 by means of the high-resolution spectrograph SARG operating at the 3.6m Italian telescope TNG (Telescopio Nazionale Galileo) at the Canary Islands, exploiting the iodine cell technique. A time series of about 1200 spectra was acquired during 7 observing nights. Data analysis of the radial velocity time series (Bonanno et al. 2008) has shown a significant power excess centred at 1.2 mHz, with ∼ 0.9 ms−1 peak amplitude. Here we present an analysis of the Equivalent Width time series of Hα and other lines. Individual Objects: μ Her Method We considered the red part of echelle spectrum of μ Her (between 600 and 900 nm), since it is insensitive to the iodine cell. This allows us to use the Equivalent Width (EW) variations to evaluate the presence of stellar pulsations and granulation. The analyzed lines were: Fe I (λ=6393.2, 6545.8, 6677.6 A), Ni I (6643.25 A) and Hα (6563 A). The Hα line is on the rim of two adjacent spectral orders of the echelle spectrum. In order to have the red and blue wings of the line both on the same spectrum, we merged the two parts. To do this in a safe way, we correct the two orders for the continuum before merging them. For all the lines we evaluate the EW in the usual way, using the following expression: EW = Z λ2 λ1 (1− Rλ)dλ, where Rλ = f (λ) f (c) Time series and power spectra of EWs Figure 1 shows the EW time series of one of the analyzed lines (Hα). The observations were conducted on 7 nights in June 2006 using SARG, the high resolution spectrograph of the TNG. As expected the computed power spectrum of the EW time series shows a very low amplitude. Figure 2 shows as example the Hα Power Spectrum. The power spectrum does not show evidence of a power excess due to pulsation. Nevertheless a concentration of power is present at low frequencies. This feature is similar to the background power found by Kjeldsen et al. (1999) for α Cen A and by Leccia et al. (2007) for Procyon A. These authors hypothesized this noise to be due to stellar granulation. A more detailed analysis of the time series of EWs and the estimate of the stellar granulation using by power density spectrum is in progress. 20 μ Herculis: Analysis of EW time series of a solar-type pulsator Figure 1: Time series of EWs of Hα. Figure 2: Power spectra of EWs of Hα. References Bonanno, A., Benatti, S., Claudi, R., et al. 2008, ApJ, 676, 1248 Kjeldsen, H., Bedding, T. R., Frandsen, S., & Dall, T. H. 1999, MNRAS, 303, 579 Leccia, S., Kjeldsen, H., Bonanno, A., et al. 2007, A&A, 464, 1059

The F5 IV-V subgiant Procyon was observed over 6 nights with SARG at TNG. We took sequences of 10 s exposures with an average dead time in between of 127 s. In total, 956 spectra were collected with a typical signal-to-noise ratio ranging between 250-330. Data reduction was performed using IRAF tasks devoted to echelle spectra. Successively the modelling of the star plus iodine absorption spectra was performed by using AUSTRAL code (Endl et al. 2000, A&A, 362, 585) in order to obtain the radial velocities time series. The Scargle and Lomb modified periodogram (Lomb 1976, Ap&SS, 39, 447, Scargle 1982, ApJ, 263, 835) shows two prominent frequencies: ν1 = 0.2346 mHz and ν2 = 0.8387 mHz both with a significance level greater than 99% . Frequency ν2 corresponds to the 20 min period found by Brown et al. (1991 ApJ, 368, 599).