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Register a new userChromospheric features of LQ Hydrae from H-alpha line profiles
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A. Frasca
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We analyze the H-alpha spectral variability of the rapidly-rotating K1-dwarf LQ Hya using high-resolution H-alpha spectra recorded during April-May 2000. Chromospheric parameters were computed from the H-alpha profile as a function of rotational phase. We find that all these parameters vary in phase, with a higher chromospheric electron density coinciding with the maximum H-alpha emission. We find a clear rotational modulation of the H-alpha emission that is better emphasized by subtracting a reference photospheric template built up with a spectrum of a non-active star of the same spectral type. A geometrical plage model applied to the H-alpha variation curve allows us to derive the location of the active regions that come out to be close in longitude to the most pronounced photospheric spots found with Doppler imaging applied to the photospheric lines in the same spectra. Our analysis suggests that the H-alpha features observed in LQ Hya in 2000 are a scaled-up version of the solar plages as regards dimensions and/or flux contrast. No clear indication of chromospheric mass motions emerges.
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We investigate retrieval of the stellar rotation signal for Proxima Centauri. We make use of high-resolution spectra taken with uves and harps of Proxima Centauri over a 13-year period as well as photometric observations of Proxima Centauri from asas and hst. We measure the H{alpha} equivalent width and H{alpha} index, skewness and kurtosis and introduce a method that investigates the symmetry of the line, the Peak Ratio, which appears to return better results than the other measurements. Our investigations return a most significant period of 82.6 $pm$ 0.1 days, confirming earlier photometric results and ruling out a more recent result of 116.6 days which we conclude to be an alias induced by the specific harps observation times. We conclude that whilst spectroscopic H{alpha} measurements can be used for period recovery, in the case of Proxima Centauri the available photometric measurements are more reliable. We make 2D models of Proxima Centauri to generate simulated H{alpha}, finding that reasonable distributions of plage and chromospheric features are able to reproduce the equivalent width variations in observed data and recover the rotation period, including after the addition of simulated noise and flares. However the 2D models used fail to generate the observed variety of line shapes measured by the peak ratio. We conclude that only 3D models which incorporate vertical motions in the chromosphere can achieve this.
In this study we test the possibility that the asymmetry in the profiles of the H-alpha and Ca II K lines in red giant stars is due to the presence of an active chromosphere rather than to mass loss. To this end, we compare line profiles computed using relevant model chromospheres to profiles of the H-alpha and Ca II K lines observed in five red giant stars of the globular cluster NGC 2808. The spectra were taken with FLAMES during Science Verification, using the UVES mode at high resolution (R=43,000) for the H-alpha line, and GIRAFFE in MEDUSA mode (R=20,000) for the Ca II K line. We find that the observed profiles are better described if a negative (outward) velocity field is included in the model chromospheres. This leads to mass loss rates of a few 10**(-9) solar masses per year, very close to the requirements of the stellar evolution theory.
We present temporal variations of the Si IV line profiles at the flare ribbons in three solar flares observed by the Interface Region Imaging Spectrograph (IRIS). In the M1.1 flare on 2014 September 6 and the X1.6 flare on 2014 September 10, the Si IV line profiles evolve from wholly redshifted to red-wing enhanced with the flare development. However, in the B1.8 flare on 2016 December 2, the Si IV line profiles are wholly redshifted throughout the flare evolution. We fit the wholly redshifted line profiles with a single Gaussian function but the red-asymmetric ones with a double Gaussian function to deduce the corresponding Doppler velocities. In addition, we find that hard X-ray emission above 25 keV shows up in the two large flares, implying a nonthermal electron beam heating. In the microflare, there only appears weak hard X-ray emission up to 12 keV, indicative of a thermal heating mostly. We interpret the redshifts or red asymmetries of the Si IV line at the ribbons in the three flares as spectral manifestations of chromospheric condensation. We propose that whether the line appears to be wholly redshifted or red-asymmetric depends on the heating mechanisms and also on the propagation of the condensation.
[Abridged] We present the first comprehensive study of short-timescale chromospheric H-alpha variability in M dwarfs using the individual 15 min spectroscopic exposures for 52,392 objects from the Sloan Digital Sky Survey. Our sample contains about 10^3-10^4 objects per spectral type bin in the range M0-M9, with a total of about 206,000 spectra and a typical number of 3 exposures per object (ranging up to a maximum of 30 exposures). Using this extensive data set we find that about 16% of the sources exhibit H-alpha emission in at least one exposure, and of those about 45% exhibit H-alpha emission in all of the available exposures. Within the sample of objects with H-alpha emission, only 26% are consistent with non-variable emission, independent of spectral type. The H-alpha variability, quantified in terms of the ratio of maximum to minimum H-alpha equivalent width (R_EW), and the ratio of the standard deviation to the mean (sigma_EW/<EW>), exhibits a rapid rise from M0 to M5, followed by a plateau and a possible decline in M9 objects. In particular, R_EW increases from a median value of about 1.8 for M0-M3 to about 2.5 for M7-M9, and variability with R_EW>10 is only observed in objects later than M5. For the combined sample we find that the R_EW values follow an exponential distribution with N(R_EW) exp[-(R_EW-1)/2]; for M5-M9 objects the characteristic scale is R_EW-1approx 2.7, indicative of stronger variability. In addition, we find that objects with persistent H-alpha emission exhibit smaller values of R_EW than those with intermittent H-alpha emission. Based on these results we conclude that H-alpha variability in M dwarfs on timescales of 15 min to 1 hr increases with later spectral type, and that the variability is larger for intermittent sources.
We present zELDA(redshift Estimator for Line profiles of Distant Lyman-Alpha emitters), an open source code to fit Lyman-Alpha (Lya) line profiles. The main motivation is to provide the community with an easy to use and fast tool to analyze Lya line profiles uniformly to improve the understating of Lya emitting galaxies. zELDA is based on line profiles of the commonly used shell-model pre-computed with the full Monte Carlo radiative transfer code LyaRT. Via interpolation between these spectra and the addition of noise, we assemble a suite of realistic Lya spectra which we use to train a deep neural network. We show that the neural network can predict the model parameters to high accuracy (e.g.,.0.34 dex HI column density for R=12000) and thus allows for a significant speedup over existing fitting methods. As a proof of concept, we demonstrate the potential of zELDA by fitting 97 observed Lya line profiles from the LASD data base. Comparing the fitted value with the measured systemic redshift of these sources, we find that Lya determines their rest frame Lya wavelength with a remarkable good accuracy of 0.3A (75 km/s). Comparing the predicted outflow properties and the observed Lya luminosity and equivalent width, we find several possible trends. For example, we find an anticorrelation between the Lya luminosity and the outflow neutral hydrogen column density, which might be explained by the radiative transfer process within galaxies
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