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تسجيل مستخدم جديدThe Na I D resonance lines in main sequence late-type stars
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We study the sodium D lines (D1: 5895.92 AA; D2: 5889.95 AA) in late-type dwarf stars. The stars have spectral types between F6 and M5.5 (B-V between 0.457 and 1.807) and metallicity between [Fe/H] = -0.82 and 0.6. We obtained medium resolution echelle spectra using the 2.15-m telescope at the argentinian observatory CASLEO. The observations have been performed periodically since 1999. The spectra were calibrated in wavelength and in flux. A definition of the pseudo-continuum level is found for all our observations. We also define a continuum level for calibration purposes. The equivalent width of the D lines is computed in detail for all our spectra and related to the colour index (B-V) of the stars. When possible, we perform a careful comparison with previous studies. Finally, we construct a spectral index (R_D) as the ratio between the flux in the D lines, and the bolometric flux. We find that, once corrected for the photospheric contribution, this index can be used as a chromospheric activity indicator in stars with a high level of activity. Additionally, we find that combining some of our results, we obtain a method to calibrate in flux stars of unknown colour.
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ation seen in SN 2006X induces relatively large changes in the total Na I D equivalent width ($Deltarm{EW}approx 0.5 unicode{x212B}$ in just over two weeks), that would be detectable at lower resolutions. We have used a large data set comprising 2400 low-resolution spectra of 450 Type Ia supernovae (SNe Ia) obtained by the CfA Supernova Program to search for variable Na I D features. Out of the 31 SNe Ia (including SN 2006X) in which we could have detected similar EW variations, only one other (SN 1999cl) shows variable Na I D features, with an even larger change over a similar ~10-day timescale ($Deltarm{EW} = 1.66 pm 0.21 unicode{x212B}$). Interestingly, both SN 1999cl and SN 2006X are the two most highly-reddened objects in our sample, raising the possibility that the variability is connected to dusty environments. (Erratum) The large variation in the Na I D equivalent width observed in SN 1999cl results in fact from a measurement error. Our new measurements show that the EW variation is significantly lower, at $0.43 pm 0.14 unicode{x212B}$. While the EW variation remains statistically significant (3.1$sigma$ different from zero), it is now below the detection threshold of 0.5 $unicode{x212B}$ derived from the Monte Carlo simulations published in the original paper. As a result, SN 1999cl should no longer be considered as an object displaying variable Na I D lines in our study. The fraction of SNe Ia in our sample displaying Na I D lines thus goes from $sim$6% (2/31) in the original study to $sim$3% (1/31) in the revised analysis, SN 2006X being the only SN Ia in our sample with variable Na I D lines.
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