Chromospheric activity properties and search for subdwarfs and extreme subdwarfs based on LAMOST stellar spectral survey

Big data obtained from a stellar spectroscopic survey carried out using the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST, also known as Guo Shou Jing telescope) provide important information for studying chromospheric activity, variability of chromospheric activity, and chromospheric statistical properties, and for searching subdwarfs and extreme subdwarfs. Using as chromospheric activity indicator the H_α line, we detected 6391 active M stars among the 99741 stars in the M-star catalogue of the LAMOST survey. We also obtained the relationship between the fraction of active stars and the spectral types, which is consistent to previous results. We also studied the effects of activity on broadband photometric colors, and we did not see significant differences between active and inactive M stars. Using as spectroscopic molecular indicators the CaH123 and TiO5 lines, we found 1288 subdwarfs (including 120 active subdwarfs). We also found 15 extreme subdwarf (2 active extreme subdwarf) candidates. Our subdwarf candidates are slightly redder by about 0.05 mag in g-r compared with dwarfs using the g-r and r-i, and g-r and i-z color diagrams. Using our active M-star catalogue, we found that 898 stars exhibited H_α emission in at least two exposures (170 of them in at least three exposures). Among these 170 stars, 163 of them show variability in H_α emission on long timescales (more than 2.5 h). Furthermore, 34 stars show variability over short timescales (less than 2.5 h), and 29 actives show variability over both short and long time scales.     

Recent observations of AB Dor and interpretation

We use minimal empirical modelling techniques to interpret recent (2006–2007) photometry and spectroscopy of AB Dor. We compare, in particular, broadband (B and V) maculation effects with emission features in high‐resolution Ca II K‐line spectroscopy. We also compare emission effects in the Ca II Kand Hα lines observed at different rotational phases. We refer to a broader multiwavelength campaign, of which these optical data were a part, involving X‐ray and microwave observations to be published later. The broadband light curves are characterized by one outstanding macula, whereas the emission lines suggest 4 possible main chromospheric activity sites (‘faculae’). These appear at a similar latitude and with comparable size to the main umbra, but there are significant displacements in longitude. However, one strong facular concentration near phase zero may have a physical relationship to the main macula. The derived longitudes of these features would have been affected by differential rotation operating over the several months between the spectroscopic and photometric observations, but the difference of at least ∼30° between facula and umbra appears too great to allow their coincidence. The possibility of a large bipolar surface structure is considered, keeping in mind the bipolar character of solar activity centres: the activity of rapidly rotating cool stars being generally compared with that of the Sun, scaled up by a few orders of magnitude. Observed microwave activity may link to this same main photospheric and chromospheric centre picked up by the optical analysis. Characterization of macular and facular contributions in stellar activity sites would be improved with a closer timing of observations and higher signal to noise ratios in emission line data (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

A photometric and spectroscopic study of the binary V1430 Aql

We present the first infrared light curves of the binary V1430 Aql, in the bands J and K, plus V, R and I light curves and spectra covering the ranges of H_β, H_α and Ca II-IRT lines. Our VRIJK data, together with published radial velocity curves, are analyzed to determine the orbital and stellar parameters of the system. Both stellar components present spectroscopic evidence of chromospheric activity, with emission excesses in the H_β, H_α and Ca II-IRT lines. The measured ratio of the lines H_β/H_α emission excesses can be interpreted as originated in plages. Our light curves also show photometric evidence of cool spots at least on one of the stars.     

Physical properties of solar chromospheric plages

We propose chromospheric models of plages to explain profiles of the Ca ii H, K, λl8498, λ8542, and λ8662 lines described in Paper I. These models are consistent with boundary conditions imposed by the photosphere and the Lyman continuum. We find that increasing emission in these lines is consistent with a picture of increasing temperature gradient in the low chromosphere and the resulting increase in pressure and electron density at similar line optical depths. With this picture we suggest how to empirically determine the distribution of chromospheric parameters across the solar disk directly from Ca ii filtergrams. We also propose that the high density aspects of solar activity are produced by steep temperature gradients in the low chromosphere and thus by the enhanced heating mechanisms that steepen these gradients.     

Winds and mass-loss from evolved,low-gravity cool stars

We summarize results from several programs utilizing the Goddard High Resolution Spectrograph (GHRS) on the Hubble Space Telescope (HST) to study winds and mass-loss from evolved, low-gravity cool stars. We have found that: (i) the photons for thermally and fluorescently excited UV emission lines are created below the region of wind acceleration, (ii) the self-reversals in optically thick emission lines indicate an outflowing wind with mean velocities of 9–25 km/s, (iii) the profiles of optically thin emission lines indicate a mean chromospheric macroturbulence of 24–35 km/s, anisotropically distributed along the radial-tangential directions, (iv) significant emission from hot material (≈10⁵ K) is seen in both non-coronal and hybrid stars to the right of the Linsky-Haisch dividing line, (v) the weakness of Fe II emission lines in the carbon stars, combined with the presence of the Fe I 2807 Å feature only in carbon stars, suggests that the ionization fraction of iron is significantly lower in the outer atmospheres of carbon stars than in O-rich stars, and (vi) Fe II line profile variations indicate changes in mass-loss rate and wind opacity on a timescale of several years in two typical late-type, low-gravity stars.     

Slitless spectrograph for small telescopes: First results

A slitless UBVR spectrograph has been built for use on small telescopes. Test observations on the Zeiss-600 telescope at the Terskol peak demonstrated that the spectrograph is an efficient instrument for studying high-speed processes in variable stars with a high temporal resolution. The spectrograph resolving power R ≈ 100 in the vicinity of λ = 480 nm and the error in the determination of the wavelength is approximately 3 nm. The spectrograph provides a moderate signal-to-noise ratio for stars up to 16 ^m . It permits one to measure equivalent widths of unblended lines down to 0.1 nm. The developed special software based on the theory of quantum statistics makes it possible to detect relative variations in the spectrum of approximately 10^?5–10^?6 of the bolometric flux of the star. Observations with the spectrograph made it possible to detect variations of emissions in Balmer lines and Ca II H, K lines in the EV Lac flare star in the subsecond range. The spectroscopic monitoring permits one to study stellar flares with small amplitude, to carry out a comprehensive colorimetric analysis of flare plasma, and to determine temperatures and sizes of flares in the light intensity maximum. Observations of the transit of the HAT-P-1 B exoplanet demonstrate that the chromospheric activity power of the parent star does not vary during the transit. The slitless spectrograph with a low resolving capacity opens new prospects in studying active processes occurring on stars’ surfaces.     

Flare stars: Postflare behaviour of emission lines

The problem of the postflare behaviour of chromospheric emission lines in UV Cet-type flare stars is considered. It is shown that the postflare behaviours of different emission lines differ essentially from one another. In particular, the intensity of hydrogen and helium lines must be greatest at flare peak in continuum, while the intensities of 2800 Mg_II and H and K Ca_II lines reach their maximum magnitudes in the intermediate period between two flares. Theoretical postflare light curves are derived for the most important chromospheric lines: helium, hydrogen ionized magnesium and calcium (Figures 8, 9, 11, and 16). The definite regularities in sequence of these light curves are established (Figure 17). Methods of obtaining the mass concentration,n _*, in the chromospheres of flare stars based on the analysis of hydrogen and calcium light curves are elaborated. Values ofn _* obtained for a group of UV Cet-type flare stars are listed in Table VII. The conditions of the intensification of emission lines during the flare are examined, as is the possibility of the existence of an empirical relationship between chromospheric concentration,n _* and flare frequency,f _U .     

Stars with discrepant v sin i as derived from Ca II λ 3933 Å and Mg II λ 4481 Å lines. III. Stars with v sin i (λ 3933 Å) < v sin i (λ 4481 Å)

Axial rotation of a star plays an important role in its evolution, physical conditions in its atmosphere and the appearance of its spectrum. We analyzed CCD spectra of nine stars for which the projected rotational velocity derived from the Ca II line at λ 3933 Å was remarkably lower than the one derived from the MgII line at λ 4481 Å. We derived effective temperatures and surface gravities using published uvbyβ photometries, and computed synthetic spectra. Comparing the observed line profiles of the two lines with the computed ones, we estimated the values of v sin i. One of the stars, HD44783, is a Be-star which, besides the narrow absorptions in the spectrum originating in its circumstellar envelope, also has lines of interstellar origin. We also found indications of circumstellar matter in the spectrum of HD25152. In the spectra of the remaining seven stars the narrow components in the Ca II λ 3933 Å line as well as narrow absorptions in the Na I λ 5889.951 Å (D1) and λ 5895.924 Å (D2) lines are of interstellar origin. In HD114376 there are two systems of interstellar components, thus disclosing two different interstellar clouds in the direction of the star. In the spectrum of HD138527 signs of a possible companion were detected, the emission of which contributes 15% to the total light of the system.     

Chromospheric Activity of Weak-Lined T Tauri Stars

A summary of our ongoing high and medium resolution optical observational program, devoted to the study of chromospheric activity of pre-main sequence stars, is reported here. The study is centered in bona-fid weak-lined T Tauri stars (WTTS) in Taurus with known rotational period, and in WTTS recently discovered by the ROSAT All-Sky Survey. It is our purpose to quantify the phenomenology of the chromospheric activity of each star determining stellar surface fluxes in the more important chromospheric activity indicators (Ca II H & K, Hβ, Hα, Ca II IRT) as well as to obtain the Li I abundance, a better determination of the stellar parameters, spectral type, and possible binarity. By using these chromospheric excess emissions we will study the flux-flux and rotation-activity relationships in order to get insights into the mechanisms which drive solar-like stellar activity in this kind of low mass pre-main sequence stars. This revised version was published online in July 2006 with corrections to the Cover Date.     

Stellar chromospheric temperatures

The emission doublet 2800 Mgii of stellar chromospheric origin is always stronger than the emission in the Lα line of hydrogen. At the same time, the ratio of their fluxes,Q=F(Mgii)/F(Lα), varies over a wide range— from 2 up to 20 in stars of one and the same type. Assuming that both emission structures, 2800 Mgii and Lα, are excited in one and the same region of the chromosphere, an attempt is made to represent the ratioQ as a function from one physical parameter only—chromospheric electron temperature. Such an approach explains easily the observed behaviour of the ratioQ in different stars.     

The chromospheric emission of solar-type stars in the young open clusters IC 2391 and IC 2602

In this paper we present chromospheric emission levels of the solar-type stars in the young open clusters IC 2391 and IC 2602. High-resolution spectroscopic data were obtained for over 50 F, G and K stars from these clusters over several observing campaigns using the University College London Echelle Spectrograph on the 3.9-m Anglo-Australian Telescope. Unlike older clusters, the majority (28/52) of the solar-type stars in the two clusters are rapid rotators  ( v  sin  i > 20 km s⁻¹)  with five of the stars being classified as ultra-rapid rotators  ( v  sin  i > 100 km s⁻¹)  . The emission levels in the calcium infrared triplet lines were then used as a measure of the chromospheric activity of the stars. When plotted against the Rossby number ( N _R), the star's chromospheric emission levels show a plateau in the emission for  log( N _R) ≲−1.1  indicating chromospheric saturation similar to the coronal saturation seen in previously observed X-ray emission from the same stars. However, unlike the coronal emission, the chromospheric emission of the stars shows little evidence of a reduction in emission (i.e. supersaturation) for the ultra-rapid rotators in the clusters. Thus we believe that coronal supersaturation is not the result of an overall decrease in magnetic dynamo efficiency for ultra-rapid rotators.     

On the mass motions and the atmospheric states of moustaches

Analyses of broad moustache profiles of Balmer lines and Ca ii H and K lines are performed based upon our spectroscopic observation under good seeing conditions. Hα emission profiles are found to consist of three components, i.e., a central absorption, a Gaussian core and a power-law wing. Each of them has a different Doppler shift from others. From the data of Doppler shifts, mass motions with velocity of about 6 km s^?1 are found to be present in chromospheric levels of moustache atmospheres. Computations of Hα emission profiles radiated from a variety of model atmospheres are made. Comparison of computed profiles with the observed ones leads us to the conclusion that a broad Hα profile is due to a formation of heated (ΔT = 1500 K) and condensed (?/? ₀ = 5) chromospheric layers relative to the normal.     

Rotations of the sun and stars and their activity

An equation for determining the magnetic activity cycle for the lower main-sequence stars is derived on the basis of the results of an analysis of magnetic structure drift with the solar activity cycle. The equation correlates the star activity cycle with the period of its rotation T _rot, the B?V color index, and the average chromospheric emission level 〈R′_HK>. The activity cycles for 30 stars (14 young and 16 old) entering the Wilson sample are calculated. The calculated magnetic activity cycles are found to be in good agreement with the observed ones.     

X-ray stars observed in LAMOST spectral survey

X-ray stars have been studied since the beginning of X-ray astronomy. Investigating and studying the chromospheric activity from X-ray stellar optical spectra is highly significant in providing insights into stellar magnetic activity. The big data of LAMOST survey provides an opportunity for researching stellar optical spectroscopic properties of X-ray stars. We inferred the physical properties of X-ray stellar sources from the analysis of LAMOST spectra. First, we cross-matched the X-ray stellar catalogue (12254 X-ray stars) from ARXA with LAMOST data release 3 (DR3), and obtained 984 good spectra from 713 X-ray sources. We then visually inspected and assigned spectral type to each spectrum and calculated the equivalent width (EW) of H\(\alpha\) line using the Hammer spectral typing facility. Based on the EW of H\(\alpha\) line, we found 203 spectra of 145 X-ray sources with H\(\alpha\) emission above the continuum. For these spectra we also measured the EWs of H\(\beta\), H\(\gamma\), H\(\delta\) and Ca ii IRT lines of these spectra. After removing novae, planetary nebulae and OB-type stars, we found there are 127 X-ray late-type stars with H\(\alpha\) line emission. By using our spectra and results from the literature, we found 53 X-ray stars showing H\(\alpha\) variability; these objects are Classical T Tauri stars (CTTs), cataclysmic variables (CVs) or chromospheric activity stars. We also found 18 X-ray stars showing obvious emissions in the Ca ii IRT lines. Of the 18 X-ray stars, 16 are CTTs and 2 are CVs. Finally, we discussed the relationships between the EW of H\(\alpha\) line and X-ray flux.     

Non-LTE modeling of narrow emission components of He and Ca lines in optical spectra of classical T Tauri stars

Using LTE calculations of the structure of T Tauri stellar atmospheres heated by radiation from an accretion shock (Dodin and Lamzin 2012), we have calculated the spectrum of the hot spot emerging on the stellar surface by taking into account non-LTE effects for He I, He II, Ca I, and Ca II. Assuming the pre-shock gas density N ₀ and velocity V ₀ to be the same at all points of the accretion stream cross section, we have calculated the spectrum of the star+circular spot system at various N ₀, V ₀, and parameters characterizing the star and the spot. Using nine stars as an example, we show that the theoretical optical spectra reproduce well the observed veiling of photospheric absorption lines as well as the profiles and intensities of the so-called narrow components of He II and Ca I emission lines with an appropriate choice of parameters. The accreted gas density in all of the investigated stars except DK Tau has been found to be N ₀ > 10¹² cm^?3. We have managed to choose the parameters for eight stars at a calcium abundance in the accreted gas ξ _Ca equal to the solar one, but we have been able to achieve agreement between the calculations and observations for TW Hya only by assuming ξ _Ca to be approximately a factor of 3 lower than the solar one. The estimated parameters do not depend on interstellar extinction, because they have been determined from the spectra normalized to the continuum level. The calculated intensity of Ca II lines has turned out to be lower than the observed one, but this contradiction can be eliminated by assuming that, in addition to the accreted gas with a high density N ₀, a more rarefied gas also falls onto the star. The theoretical equivalent widths and relative intensities of the subordinate He I lines disagree significantly with the observations. This is apparently because non-LTE effects should be taken into account when calculating the structure of the upper layers of the hot spot, the accuracy of the cross sections for collisional processes from upper levels is insufficient, and the spot inhomogeneity should probably be taken into account.     

Toward Understanding the Formation of Molecular Clouds

Based on a comparison of similar colour indices we investigate the effects of luminosity and metallicity that influence the Strömgren and Vilnius photometric quantities for different spectral types. We notice significant differences for similar indices, apparently due to small deviations of central wavelengths and band-widths of the filters. For the G-K type stars of moderate luminosity we interpret the difference introduced by v and X pass-bands as an log g effect, reflecting the presence of H&;K Ca II stellar lines in these spectral types. We discuss a possible influence of the H&;K Ca II lines on the luminosities derived from the Vilnius classification diagrams.     

Flare model chromospheres and photospheres

Homogeneous plane-parallel model atmospheres for solar flares have been constructed to approximately simulate observations of flares. The wings of the Ca II lines have been used to derive flare upper photosphere models, which indicate temperature increases of ～100 K over the temperature distribution in the pre-existing facula at a height of 300 km above τ₅₀₀₀ = 1. In the case of flares covering sunspots the temperature rise seems to occur much higher in the atmosphere. We solve the transfer and statistical equilibrium equations for a three-level hydrogen atom and a five-level calcium atom in order to obtain the chromospheric flare models. The general properties of flares, including n _e, N ₂, linear thickness, and Lyman continuum intensity are approximately reproduced. We find that with increasing flare importance the height of the upper chromosphere and transition region occur lower in the solar atmosphere, accounting for the factor of 60–600 increase in pressure in these regions relative to the quiet Sun. The Ca II line profiles agree with observations only by assuming a macro-velocity distribution that increases with height. Also the chromospheric parts of flares appear to be highly inhomogeneous. We show that shock and particle heated flare models do not agree with the observations and propose a thermal response model for flares. In particular, it appears that heating in the photosphere is an essential aspect of flares.     

Ca II K Emission Line Asymmetries Among Red Giants

In the spectra of red giants the chromospheric emission feature found in the core of the Ca II K line often exhibits an asymmetric profile. This asymmetry can be documented by a parameter V/R which is classified as > 1, 1, or < 1 if the violet wing of the emission profile is of greater, equal, or lower intensity than the redward wing. A literature search has been conducted to compile a V/R dataset which builds on the large survey of bright field giants made by Wilson (1976). Among stars of luminosity classes II–III–IV the majority of those with V/R > 1 are found to be bluer than B-V =1.3, while those with V/R < 1 are mostly redder than this colour. Stars with nearly symmetric profiles, V/R≈ 1, are found throughout the colour range 0.8 < B-V < 1.5. There is no sharp transition line separating stars of V/R > 1 and < 1 in the colour-magnitude diagram, but rather a ‘transition zone’ centered at B-V ≈ 1.3. The center of this zone coincides closely with a ‘coronal dividing line’ identified by Haish, Schmitt and Rosso (1991) as the red envelope in the H–R diagram of giants detected in soft x-ray emission by ROSAT. It is suggested that both the transition to a Ca II K emission asymmetry of V/R < 1 and the drop in soft x-ray activity across the coronal dividing line are related to changes in the dynamical state of the chromospheres of red giants. By contrast, the onset of photometric variability due to pulsation occurs among stars of early-M spectral type, that are redward of the mid-point of the Ca II V/R ‘transition zone’, suggesting that the chromospheric motions which produce an asymmetry of V/R < 1 are established prior to the onset of pulsation. This revised version was published online in July 2006 with corrections to the Cover Date.     

Probing the Quiet Solar Atmosphere from the Photosphere to the Corona

We investigate the morphology and temporal variability of a quiet-Sun network region in different solar layers. The emission in several extreme ultraviolet (EUV) spectral lines through both raster and slot time-series, recorded by the EUV Imaging Spectrometer (EIS) on board the Hinode spacecraft is studied along with \(\mbox{H}\upalpha\) observations and high-resolution spectropolarimetric observations of the photospheric magnetic field. The photospheric magnetic field is extrapolated up to the corona, showing a multitude of large- and small-scale structures. We show for the first time that the smallest magnetic structures at both the network and internetwork contribute significantly to the emission in EUV lines, with temperatures ranging from \(8\times 10^{4}~\mbox{K}\) to \(6\times 10^{5}~\mbox{K}\). Two components of transition region emission are present, one associated with small-scale loops that do not reach coronal temperatures, and another component that acts as an interface between coronal and chromospheric plasma. Both components are associated with persistent chromospheric structures. The temporal variability of the EUV intensity at the network region is also associated with chromospheric motions, pointing to a connection between transition region and chromospheric features. Intensity enhancements in the EUV transition region lines are preferentially produced by \(\mbox{H}\upalpha\) upflows. Examination of two individual chromospheric jets shows that their evolution is associated with intensity variations in transition region and coronal temperatures.     

Influence of departures from LTE on calcium,titanium, and iron abundance determinations in cool giants of different metallicities

We have performed statistical equilibrium calculations for Ca I–Ca II, Ti I–Ti II, and Fe I–Fe II by taking into account the nonequilibrium line formation conditions (the non-LTE approach) in model atmospheres of giant stars with effective temperatures 4000 K ≤ T _eff ≤ 5000 K and metal abundances ?4 ≤ [Fe/H] ≤ 0. The dependence of departures from LTE on atmospheric parameters has been analyzed. We present the non-LTE abundance corrections for 28 Ca I lines, 42 Ti I lines, 54 Ti II lines, and 262 Fe I lines and a three-dimensional interpolation code to obtain the non-LTE correction online for an individual line and specified atmospheric parameters.