Line Core Emission Research Articles

Searching for signatures of H α spicule-like features in the solar transition region

ABSTRACT New instruments and telescopes covering the optical and ultraviolet spectral regions have revealed a range of small-scale dynamic features, many which may be related. For example, the range of spicule-like features hints towards a spectrum of features and not just two types; however, direct observational evidence in terms of tracking spicules across multiple wavelengths is needed in order to provide further insight into the dynamics of the Sun’s outer atmosphere. This paper uses H α data obtained with the CRisp Imaging SpectroPolarimeter instrument on the Swedish 1-m Solar Telescope, and in the transition region using the Interface Region Imaging Spectrograph with the SJI 1400 Å channel plus spectral data via the Si iv 1394 Å line to track spicules termed rapid blueshifted excursions (RBEs). The RBEs as seen in the H α blue wing images presented here can be subdivided into two categories: a single or multithreaded feature. Based on the H α spectra, the features can be divided into events showing broadening and line core absorption, events showing broadening and line core emission, events with a pure blueshifted H α profile without any absorption in the red wing, and broadened line profile with the absorption in the blue stronger compared to the red wing. From the RBE-like events that have a Si iv 1394 Å line profile, 78 per cent of them show a Si iv line flux increase. Most of these features show a second broadened Si iv component that is slightly blueshifted.

Rotation of solar‐like stars in the immediate solar neighborhood

AbstractAlthough photometric space‐based missions such as CoRoT or Kepler have yielded rotation measurements of many thousands of late‐type stars during the last decade, the rotational properties of the bulk of the G star population remain undetected by these missions. From the Sun (when viewed as a star), we know that rotation measurements in the ultraviolet are the most promising, or more general, measurements in wavelength regions very sensitive to plage areas on the stars. Therefore, the “classical” S‐index, that is, the strength of the Ca II H&K line core emission, is still the most viable activity and rotation indicator, and with robotic spectroscopy telescopes, such monitoring measurements can be carried out efficiently and economically. We define a complete volume‐limited sample of solar stars in the immediate solar environment and present period measurements in Ca II H&K, both from archival Mount Wilson data and new data obtained with our robotic TIGRE facility.

Quiet-Sun Explosive Events Observed in He ii λ304 with MOSES-06

Abstract In this paper the unique data from the Multi-Order Solar Extreme-Ultraviolet Spectrograph (MOSES) are used to investigate transition region explosive events in the He ii λ304 spectral line. Particular attention is paid to two example events: one blueshifted jet and one bidirectional jet. Observations suggest that these events consist exclusively of high-velocity (∼100 km s−1) plasma. These two and other examples presented here exhibit a striking lack of emission in the line core. No acceleration phase is observed at the onset of either event. In total, 41 examples of explosive events are identified, including 5 blueshifted jets, 2 redshifted jets, and 10 bidirectional jets. The remaining 24 events resist simple classification, but observations indicate compact, highly Doppler-shifted emission. Event spatial scales and lifetimes are consistent with published explosive event characteristics. Data from the Michelson Doppler Imager provide magnetic context to the MOSES observations. Bidirectional jets lacking line core emission are interesting because they are predicted in models of Petschek reconnection in the transition region.

Generation of a circumstellar gas disc by hot Jupiter WASP-12b

Observations of transiting extra-solar planets provide rich sources of data for probing the in-system environment. In the WASP-12 system, a broad depression in the usually-bright MgII h&k lines has been observed, in addition to atmospheric escape from the extremely hot Jupiter WASP-12b. It has been hypothesized that a translucent circumstellar cloud is formed by the outflow from the planet, causing the observed signatures. We perform 3D hydrodynamic simulations of the full system environment of WASP-12, injecting a planetary wind and stellar wind from their respective surfaces. We find that a torus of density high enough to account for the lack of MgII h&k line core emission in WASP-12 can be formed in approximately 13 years. We also perform synthetic observations of the Lyman-alpha spectrum at different points in the planet's orbit, which demonstrate that significant absorption occurs at all points in the orbit, not just during transits, as suggested by the observations.

THE MOUNT WILSON OBSERVATORY S-INDEX OF THE SUN

ABSTRACT The most commonly used index of stellar magnetic activity is the instrumental flux scale of singly ionized calcium H & K line core emission, S, developed by the Mount Wilson Observatory (MWO) HK Project, or the derivative index . Accurately placing the Sun on the S scale is important for comparing solar activity to that of the Sun-like stars. We present previously unpublished measurements of the reflected sunlight from the Moon using the second-generation MWO HK photometer during solar cycle 23 and determine cycle minimum , amplitude , and mean . By establishing a proxy relationship with the closely related National Solar Observatory Sacramento Peak calcium K emission index, itself well correlated with the Kodaikanal Observatory plage index, we extend the MWO S time series to cover cycles 15–24 and find on average , , . Our measurements represent an improvement over previous estimates that relied on stellar measurements or solar proxies with non-overlapping time series. We find good agreement from these results with measurements by the Solar-Stellar Spectrograph at Lowell Observatory, an independently calibrated instrument, which gives us additional confidence that we have accurately placed the Sun on the S-index flux scale.

SELF-CONSISTENT MAGNETIC STELLAR EVOLUTION MODELS OF THE DETACHED, SOLAR-TYPE ECLIPSING BINARY EF AQUARII

We introduce a new one-dimensional stellar evolution code, based on the existing Dartmouth code, that self-consistently accounts for the presence of a globally pervasive magnetic field. The methods involved in perturbing the equations of stellar structure, the equation of state, and the mixing-length theory of convection are presented and discussed. As a first test of the code's viability, stellar evolution models are computed for the components of a solar-type, detached eclipsing binary (DEB) system, EF Aquarii, shown to exhibit large disagreements with stellar models. The addition of the magnetic perturbation corrects the radius and effective temperature discrepancies observed in EF Aquarii. Furthermore, the required magnetic field strength at the model photosphere is within a factor of two of the magnetic field strengths estimated from the stellar X-ray luminosities measured by ROSAT and those predicted from Ca II K line core emission. These models provide firm evidence that the suppression of thermal convection arising from the presence of a magnetic field is sufficient to significantly alter the structure of solar-type stars, producing noticeably inflated radii and cooler effective temperatures. The inclusion of magnetic effects within a stellar evolution model has a wide range of applications, from DEBs and exoplanet host stars to the donor stars of cataclysmic variables.

Observation and modelling of main-sequence star chromospheres - XIX. FIES and FEROS observations of dM1 stars

We present 187 high-resolution spectra for 62 different M1 dwarfs from observations obtained with the FIbre-fed Echelle Spectrograph (FIES) on the Nordic Optical Telescope (NOT) and from observations with the Fibre-fed Extended Range Echelle Spectrograph (FEROS) from the European Southern Observatory (ESO) data base. We also compiled other measurements available in the literature. We observed two stars, Gl 745A and Gl 745B, with no Ca ii line core emission and Hα line equivalent widths (EWs) of only 0.171 and 0.188 Å, respectively. We also observed another very low activity M1 dwarf, Gl 63, with an Hα line EW of only 0.199 Å. These are the lowest activity M dwarfs ever observed and are of particular interest for the non-local thermodynamic equilibrium radiative transfer modelling of M1 dwarfs. Thanks to the high signal-to-noise ratio of most of our spectra, we were able to measure the Ca ii H&K full width at half-maximum (FWHM) for most of our stars. We find good correlations between the FWHM values and the mean Ca ii line EW for dM1 stars. Then the FWHM seems to saturate for dM1e stars. Our previous models of M1 dwarfs can reproduce the FWHM for dM1e stars and the most active dM1 stars, but fail to reproduce the observations of lower activity M1 dwarfs. We believe this is due to an effect of metallicity. We also investigate the dependence of the Hα line FWHM as a function of its EW. We find that the models globally agree with the observations including subwarfs, but tend to produce too narrow profiles for dM1e stars. We re-investigate the correlation between the Ca ii line mean EW and the absolute magnitude. With our new data that notably include several M1 subdwarfs, we find a slightly different and better correlation with a slope of −0.779 instead of −0.909. We also re-investigate the variations of the Hα line EW as a function of radius and find that the EW increases continuously with increasing radius. This confirms our previous finding that the level of magnetic activity in M1 dwarfs increases with the radius. For the first time, we investigate the Wilson–Bappu correlation for a given spectral type. We find a rather linear correlation for stars of absolute magnitude greater than 9.6, but below this value the FWHM seems to saturate. In fact, we show that these Wilson–Bappu type correlations are activity–FWHM correlations and are due to the diminishing column mass of the transition region with decreasing activity level.

Chromospheric polarity reversal on sunspots: New insight from spectro-polarimetric measurements

We present here spectro-polarimetric observations of chromospheric and photospheric lines on an active region. We show that the presence of polarity reversal between photosphere and chromosphere cannot be detected relying on magnetograms and broad band and even narrow band filters only. We demonstrate that opposite signs in CaII magnetograms in sunspots compare to photospheric magnetograms are not due to a reversal of the magnetic field, but rather due to the presence of line core emission as suggested from theoretical arguments by Sánchez Almeida ([CITE]).

Wavelet analysis of stellar differential rotation

Among the several methods to study differential rotation on stellar surfaces the observation of a stellar butterfly diagram, i.e. monitoring the rotation period over a complete activity cycle, is the only method which canbe applied to stars rotating as slow as the Sun. However, the method requires active belts on the stellar surface which are moving continously in stellar latitude during a cycle. In addition, an appropriate activity tracer showing rotational modulation is needed. Tracers which even work for slowly rotating, i.e. weakly active, stars are the emission cores of certain spectral lines. However, the question is whether surface differential rotation can be determined from this kind of observations and furthermore, which method of analysis is best suited to yield a correct result. We investigate observations of the Sun as a star, i.e. disk-integrated measurements, to answer these questions. In a Paper I it was demonstrated that time-frequency analysis using the wavelet transform is possibly a suitable method for monitoring the stellar butterfly diagram. It was also shown that - in comparison with Fourier analysis - wavelet analysis of disk-integrated solar Ca II K line core emission measures yield a much more realistic pattern of the solar differential rotation. In this Paper II disk-integrated solar measures in Mg II h+k and Lyman a taken from public UARLS Solstice data are analysed using the same methods as in Paper I. In these data solar rotational modulation is much more pronounced than in earlier Ca II K time-series. Wavelet analysis yields the following results: from the beginning of the time-series in 1991 until the end of solar cycle 22 the period of rotational modulation remains stable at 26.9-27.0 days. It is followed by a jump to 27.6 days when the new cycle 23 starts. Then the period rapidly decreases to 26.9 days again until the end of the time-series in 1999. However, the analysis is hampered by frequent period splitting into two modes. It can be shown that this kind of splitting results from phase jumps in the time-series coming from active region growth and decay at 180° shifted solar longitudes.

Small‐Scale Density and Velocity Structure of a Molecular Cloud Edge

This paper presents a detailed analysis of a random piece of molecular gas, chosen in a weakly CO– emitting part of a cloud edge in the Perseus-Auriga complex. The data set consists of high angular resolution observations in the 12CO J = 1 − 0, J = 2 − 1, J = 3 − 2, and J = 4 − 3 and CS J = 2 − 1 and J = 3 − 2 lines, combined with CO (J = 3 − 2) and 13CO (J = 2 − 1) observations obtained previously. The observational results can be summarized as follows : (i) At many locations the CO line profiles exhibit weak, broad line wings, superimposed on a narrow intense line core. (ii) The CO line emission is highly structured down to the resolution of the observations (~0.014 pc), but the spatial distribution of the line core emission is different from that of the wing emission. The former is concentrated mostly in two distinct structures of size ~0.06 pc, while the latter is concentrated in a long filamentary structure (l ~ 0.2 pc) that crosses the mapped field and is barely resolved in its transverse dimension. (iii) The J = 2–1 to J = 1–0 CO line ratio is approximately constant across the entire field and has the same value, R = 0.62±0.08, in the CO-bright areas as in the almost 10 times weaker areas. (iv) A weak CS (2–1) line has been detected at the core velocity, and only an upper limit has been obtained for the CS (3–2) line, and (v) the low column density gas that emits in the line wings has been detected clearly in CO (3–2) and detected tentatively in the CO (4–3) line. The present work strengthens the results of our previous study, that the edges of molecular clouds are weak CO emitters when observed at the parsec scale, say, because the CO emission there is beam-diluted emission of dense (nH2 > 104 cm−3), cold (Tk < 15 K) structures. The moderate CO beam-averaged optical depth and the smoothness of the CO profiles set an upper limit of ~35 AU for the size of the cells within which each CO photon interacts with the gas. This result, though, does not necessarily imply that the CO-free regions are structured similarly and, indeed, a moderate-opacity region of CO-free gas and dust is required to surround the CO-emitting structures in order to provide shielding from the interstellar radiation field. Our work also reveals, for the first time, that the line wing emission also originates in small-scale structure. The CO (4–3) wing measurement is critically important in that it constrains the wing gas temperature to be in the range from Tk ~ 25 K, for dense gas at nH2 ~ 103 cm−3, up to Tk ~ 250 K for nH2 ~ 200 cm−3. Its filamentary morphology is consistent with the idea that it is dilute and warm gas confined to the specific structures in which the energy of turbulence is being dissipated, in an intermittent way. The large body of observational results presently available on the CO emission properties of non-starforming molecular clouds, from the smallest to the largest scales, is not consistent with a picture of randomly moving dense clumps immersed in a lower density medium. The AU scale dense CO cells have to be distributed on a fractal set with correlated velocities. They are likely to be dynamically connected to the turbulent velocity field of the gas that fills the volume (atomic and molecular hydrogen) and probably trace the active regions of large vorticity either because they would tend to be trapped in these structures or because those may be regions of enhanced CO formation rate.

Rotation of solar-like main sequence stars

The results of the photometric survey of 16 solar-type, active, field stars are presented. During our observations 9 stars showed appreciable light variability with amplitudes of a few hundredths of a magnitude but for three of them periods could not be determined. Most of the observed variable stars have periods shorter than about 10 days. It is suggested that, similarly as is observed in the Hyades cluster, small amplitude light variations are quite common among active field solar-type dwarfs with rotation periods around one week or less. A strong modulation of amplitude of some variable stars over the time scale of years is demonstrated. The amplitude may sometimes decrease even below the detectability threshold. A special case is HD 17576 – a visual binary consisting of a G0 dwarf and a much fainter hot subdwarf. It has the largest amplitude of all the stars observed, which suggests an intense spot activity, a very strong H and K line core emission and a very high X-ray emission flux, close to the saturation limit. Yet its variability period is equal to 18.74 days – almost an order of magnitude longer than expected for such an active dwarf.

Relations between the photospheric magnetic field and the emission from the outer atmospheres of cool stars. I - The solar CA II K line core emission

Observations of a solar active region complex and its surroundings are used to establish a quantitative relation between the Ca II K line core intensity and magnetic flux density. The Ca II K line core intensity is transformed to a Ca II H + K line core flux density to facilitate a comparison of solar and stellar data. A new absolute calibration for the Mount Wilson Ca II H + K fluxes for G-type dwarfs is derived. The minimum Ca II K flux, found in the centers of supergranulation cells in quiet regions on the sun, is identical to the minimum flux that is observed for solar-type stars. An expression is presented for the nonlinear trend between the Ca II H + K line core excess flux density and the absolute value of the magnetic flux density. Models that explain the nonlinearity of the mean trend and the large intrinsic scatter about it are discussed. The solar data define a relation that is similar to the relation between stellar hemisphere-average magnetic flux densities and Ca II H + K excess flux densities.

Relations between broad-band linear polarization and CA II H and K emission in late-type dwarf stars

Broadband UBV linear polarization data acquired for a sample of late-type dwarfs are compared with contemporaneous measurements of Ca II H and K line core emission. A weighted average of the largest values of the polarization degree is shown to be the best parameter for chromospheric activity diagnosis. The average maximum polarization in the UV is found to increase from late-F to late-G stars. It is noted that polarization in the U band is considerably more sensitive to activity variations than that in the B or V bands. The results indicate that stellar magnetic fields and the resulting saturation in the Zeeman-sensitive absorption lines are the most probably source of linear polarization in late-type main-sequence stars.