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Register a new userThe Na I D resonance lines in main sequence late-type stars
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Rodrigo D\\'iaz Mr.
Publication date
2007
fields
Physics
and research's language is
English
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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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The young $sigma$-Orionis cluster is an important location for understanding the formation and evolution of stars, brown dwarfs, and planetary-mass objects. Its metallicity, although being a fundamental parameter, has not been well determined yet. We present the first determination of the metallicity of nine young late-type stars in $sigma$-Orionis. Using the optical and near-infrared broadband photometry available in the literature we derive the effective temperatures for these nine cluster stars, which lie in the interval 4300--6500 K (1--3 Msuno). These parameters are employed to compute a grid of synthetic spectra based on the code MOOG and Kurucz model atmospheres. We employ a $chi^2$-minimization procedure to derive the stellar surface gravity and atmospheric abundances of Al, Ca, Si, Fe, Ni and Li, using multi-object optical spectroscopy taken with WYFFOS+AF2 at at the William Herschel Telescope ($lambda/deltalambdasim7500$). The average metallicity of the $sigma$-Orionis cluster is [Fe/H] $ = -0.02pm0.09pm0.13$ (random and systematic errors). The abundances of the other elements, except lithium, seem to be consistent with solar values. Lithium abundances are in agreement with the cosmic $^7$Li abundance, except for two stars which show a $log epsilon(mathrm{Li})$ in the range 3.6--3.7 (although almost consistent within the error bars). There are also other two stars with $log epsilon(mathrm{Li})sim 2.75$. We derived an average radial velocity of the $sigma$-Orionis cluster of $28pm4$km/s. The $sigma$-Orionis metallicity is roughly solar.
We construct a sample of nearly 30,000 main-sequence stars with 4500K $<Trm_{eff}<$ 5000K and stellar ages estimated by the chromospheric activity$-$age relation. This sample is used to determine the age distribution in the $R-Z$ plane of the Galaxy, where $R$ is the projected Galactocentric distance in the disk midplane and $Z$ is the height above the disk midplane. As $|Z|$ increases, the percentage of old stars becomes larger. It is known that scale-height of Galactic disk increases as $R$ increases, which is called flare. A mild flare from $R$ $sim$ 8.0 to 9.0 kpc in stellar age distribution is found. We also find that the velocity dispersion increases with age as confirmed by previous studies. Finally we present spiral-shaped structures in $Z-upsilon_{Z}$ phase space in three stellar age bins. The spiral is clearly seen in the age bin of [0, 1] Gyr, which suggests that a vertical perturbation to the disk probably took place within the last $sim$ 1.0 Gyr.
This work aims to detect and classify stellar flares and potential stellar coronal mass ejection (CME) signatures in optical spectra provided by the Sloan Digital Sky Survey (SDSS) data release 14. The sample is constrained to all F, G, K, and M main-sequence type stars, resulting in more than 630,000 stars. This work makes use of the individual spectral exposures provided by the SDSS. An automatic flare search was performed by detecting significant amplitude changes in the $Halpha$ and $Hbeta$ spectral lines after a Gaussian profile was fit to the line core. CMEs were searched for by identifying asymmetries in the Balmer lines caused by the Doppler effect of plasma motions in the line of sight. We identified 281 flares on late-type stars (spectral types K3 to M9). We identified six possible CME candidates showing excess flux in Balmer line wings. Flare energies in $Halpha$ were calculated and masses of the CME candidates were estimated. The derived $Halpha$ flare energies range from $3 times 10^{28} - 2 times 10^{33}$ erg. The $Halpha$ flare energy increases with earlier types, while the fraction of flaring times increases with later types. Mass estimates for the CME candidates are in the range of $6 times 10^{16} - 6 times 10^{18}$ g, and the highest projected velocities are $sim300 - 700:$ km/s. The low detection rate of CMEs we obtained agrees with previous studies, suggesting that for late-type main-sequence stars the CME occurrence rate that can be detected with optical spectroscopy is low.
(Original) Recent high-resolution spectra of the Type Ia SN 2006X have revealed the presence of time-variable and blueshifted Na I D features, interpreted by Patat et al. as originating in circumstellar material within the progenitor system. The variation 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.
In studies on intermediate- and high-resolution spectra of Type Ia supernovae (SNe Ia), some objects exhibit narrow Na-I D absorptions often blueshifted with respect to the rest wavelength within the host galaxy. The absence of these in other SNe Ia may reflect that the explosions have different progenitors: blueshifted Na-I D features might be explained by the outflows of single-degenerate systems (binaries of a white dwarf with a non-degenerate companion). In this work, we search for systematic differences among SNe Ia for which the Na-I D characteristics have been clearly established in previous studies. We perform an analysis of the chemical abundances in the outer ejecta of 13 spectroscopically normal SNe Ia (five of which show blueshifted Na lines), modelling time series of photospheric spectra with a radiative-transfer code. We find only moderate differences between blueshifted-Na, redshifted-Na and no-Na SNe Ia, so that we can neither conclusively confirm a one-scenario nor a two-scenario theory for normal SNe Ia. Yet, some of the trends we see should be further studied using larger observed samples: Models for blueshifted-Na SNe tend to show higher photospheric velocities than no-Na SNe, corresponding to a higher opacity of the envelope. Consistently, blueshifted-Na SNe show hints of a somewhat larger iron-group content in the outer layers with respect to the no-Na subsample (and also to the redshifted-Na subsample). This agrees with earlier work where it was found that the light curves of no-Na SNe - often appearing in elliptical galaxies - are narrower, i.e. decline more rapidly.
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