Surface inhomogeneities and line variability on DF Tau

We have mapped surface inhomogeneities on the classical T Tauri star DF Tau, using the Li  i doublet at 670.8 nm, the Ca  i lines at 612.2 nm and 643.9 nm and a calcium and iron blend at 646.3 nm. We find compelling evidence that there are hotspots with temperatures of more than 5000 K. Two of the hotspots produce line-profile deformations that can be traced as they move through the cross-correlated profiles. When one of the hotspots crosses the stellar disc, redshifted absorption components appear in the Na D lines. As these redshifted absorption features are usually tracers for mass-infall we interpret this hotspot as an accretion shock close to the stellar surface.   Parts of the surface of DF Tau are covered with a hot chromosphere that is visible in the Ca  ii infrared triplet lines and the narrow component of He  i . We find no correlation between the veiling and the lines that originate from the hot chromosphere, suggesting that the veiling and the chromospheric emission are produced in physically distinct regions.     

Chromospheric Ca II H and K emission among subdwarfs

Echelle spectra have been obtained of the Ca  II H and K lines for a sample of metal-poor subdwarf stars as well as for a number of nearby Population I dwarfs selected from among those included in the Mount Wilson HK survey. The main conclusion of this paper is that Ca  II H- and K-line emission does occur among subdwarfs. It is particularly notable among those subdwarfs with colours of B − V ≥0.75; all such stars observed exhibit chromospheric emission, although emission is observed among some subdwarfs bluer than this colour. The Ca  II K emission profile in most subdwarfs exhibits an asymmetry of V / R >1, similar to that seen in the integrated light of the solar disc. Two quantitative indicators of the contrast between the peaks in the emission profile and the neighbouring photospheric line profile are introduced. Measurements of these indicators show that the level of Ca  II emission among the subdwarfs is similar to that among low-activity Population I dwarfs.     

The role of non-thermal electrons in the hydrogen and calcium lines of stellar flares

There is observational evidence showing that stellar and solar flares occur with a similar circumstance, although the former are usually much more energetic. It is expected that the bombardment by high-energy electrons is one of the chief heating processes of the flaring atmosphere. In this paper we study how a precipitating electron beam can influence the line profiles of Ly α , H α , Ca  ii K and λ 8542. We use a model atmosphere of a dMe star and make non-LTE computations taking into account the non-thermal collisional rates owing to the electron beam. The results show that the four lines can be enhanced to different extents. The relative enhancement increases with increasing formation height of the lines. Varying the energy flux of the electron beam has different effects on the four lines. The wings of Ly α and H α become increasingly broad with the beam flux; change of the Ca  ii K and λ 8542 lines, however, is most significant in the line centre. Varying the electron energy (i.e. the low-energy cut-off for a power-law beam) has a great influence on the Ly α line, but little on the H α and Ca  ii lines. An electron beam of higher energy precipitates deeper, thus producing less enhancement of the Ly α line. The Ly α /H α flux ratio is thus sensitive to the electron energy.     

Echelle spectroscopy of Ca ii HK activity in Southern Hemisphere planet search targets

We present the results of ultraviolet echelle spectroscopy of a sample of 59 F, G, K and M stars from the Anglo-Australian Planet Search target list. Ca  ii activity indices, which are essential in the interpretation of planet detection claims, have been determined for these stars and placed on the Mount Wilson R '_HK system.     

IPHAS discoveries of young stars towards Cyg OB2 and its southern periphery

We report on the discovery of over 50 strong Hα emitting objects towards the large OB association Cyg OB2 and the H  ii region DR 15 on its southern periphery. This was achieved using the INT Photometric Hα Survey of the Northern Galactic Plane (IPHAS), combined with follow-up spectroscopy using the MMT multi-object spectrometer HectoSpec. We present optical spectra, supplemented with optical r ',  i ' and H α photometry from IPHAS, and near-infrared J ,  H and K photometry from Two Micron All Sky Survey. The position of the objects in the ( J − H ) versus ( H − K ) diagram strongly suggests most of them are young. Many show Ca  ii infrared triplet emission indicating that they are in a pre-main-sequence phase of evolution of T Tauri and Herbig Ae nature. Among these, we have uncovered pronounced clustering of T Tauri stars roughly a degree south of the centre of Cyg OB2, in an arc close to the H  ii region DR 15, and the radio ring nebula G79.29+0.46, for which we discuss its candidacy as a luminous blue variable. The emission-line objects towards Cyg OB2 itself could be the brightest most prominent component of a population of lower mass pre-main-sequence stars that has yet to be uncovered. Finally, we discuss the nature of the ongoing star formation in Cyg OB2 and the possibility that the central OB stars have triggered star formation in the periphery.     

Empirical calibration of the near-infrared Ca ii triplet – III. Fitting functions

Using a near-infrared stellar library of 706 stars with a wide coverage of atmospheric parameters, we study the behaviour of the Ca  ii triplet strength in terms of effective temperature, surface gravity and metallicity. Empirical fitting functions for recently defined line-strength indices, namely CaT*, CaT and PaT, are provided. These functions can be easily implemented into stellar population models to provide accurate predictions for integrated Ca  ii strengths. We also present a thorough study of the various error sources and their relation to the residuals of the derived fitting functions. Finally, the derived functional forms and the behaviour of the predicted Ca  ii are compared with those of previous works in the field.     

Optical spectroscopy and photometry of SAX J1808.4−3658 in outburst

We present phase resolved optical spectroscopy and photometry of V4580 Sagittarii, the optical counterpart to the accretion powered millisecond pulsar SAX J1808.4−3658, obtained during the 2008 September/October outburst. Doppler tomography of the N  iii λ4640.64 Bowen blend emission line reveals a focused spot of emission at a location consistent with the secondary star. The velocity of this emission occurs at  324 ± 15 km s⁻¹  ; applying a ' K -correction', we find the velocity of the secondary star projected on to the line of sight to be  370 ± 40 km s⁻¹  . Based on existing pulse timing measurements, this constrains the mass ratio of the system to be  0.044^+0.005_−0.004  , and the mass function for the pulsar to be  0.44^+0.16_−0.13 M_⊙  . Combining this mass function with various inclination estimates from other authors, we find no evidence to suggest that the neutron star in SAX J1808.4−3658 is more massive than the canonical value of  1.4 M_⊙  . Our optical light curves exhibit a possible superhump modulation, expected for a system with such a low mass ratio. The equivalent width of the Ca  ii H and K interstellar absorption lines suggest that the distance to the source is ∼2.5 kpc. This is consistent with previous distance estimates based on type-I X-ray bursts which assume cosmic abundances of hydrogen, but lower than more recent estimates which assume helium-rich bursts.     

Chromospheric changes in K stars with activity

We study the differences in chromospheric structure induced in K stars by stellar activity, to expand our previous work for G stars, including the Sun as a star. We selected six stars of spectral type K with  0.82 < B − V < 0.90  , including the widely studied Epsilon Eridani and a variety of magnetic activity levels. We computed chromospheric models for the stars in the sample, in most cases in two different moments of activity. The models were constructed to obtain the best possible match with the Ca  ii K and the Hβ observed profiles. We also computed in detail the net radiative losses for each model to constrain the heating mechanism that can maintain the structure in the atmosphere. We find a strong correlation between these losses and   S _Ca II  , the index generally used as a proxy for activity, as we found for G stars.     

Excitation of Sr ii lines in Eta Carinae

We study the nature of the peculiar [Sr  ii ] and Sr  ii emission filament found in the ejecta of Eta Carinae. To this purpose we carry out ab initio calculations of radiative transition probabilities and electron impact excitation rate coefficients for Sr  ii . Then we build a multilevel model for the system which is used to investigate the physical condition of the filament and the nature of the observed allowed and forbidden Sr  ii optical emission. It is found that the observed spectrum is consistent with the lines being pumped by the continuum radiation field in a mostly neutral region with electron density near 10⁷ cm⁻³. Under these conditions, the observed emission can be explained without the need for a large Sr overabundance.     

K-band imaging of strong Ca ii-absorber host galaxies at z∼1

We present K -band imaging of fields around 30 strong Ca  ii absorption-line systems, at  0.7 < z < 1.2  , three of which are confirmed damped Lyman α systems. A significant excess of galaxies is found within 6.0 arcsec (≃50 kpc) from the absorber line of sight. The excess galaxies are preferentially luminous compared to the population of field galaxies. A model in which field galaxies possess a luminosity-dependent cross-section for Ca  ii absorption of the form  ( L / L *)^0.7  reproduces the observations well. The luminosity-dependent cross-section for the Ca  ii absorbers appears to be significantly stronger than the established  ( L / L *)^0.4  dependence for Mg  ii absorbers. The associated galaxies lie at large physical distances from the Ca  ii -absorbing gas; we find a mean impact parameter of 24 kpc  ( H ₀= 70 km s⁻¹ Mpc⁻¹)  . Combined with the observed number density of Ca  ii absorbers the large physical separations result in an inferred filling factor of only ∼10 per cent. The physical origin of the strong Ca  ii absorption remains unclear, possible explanations vary from very extended discs of the luminous galaxies to associated dwarf galaxy neighbours, remnants of outflows from the luminous galaxies, or tidal debris from cannibalism of smaller galaxies.     

Further images of α Persei G dwarfs

We present two images of intermediate and low axial inclination G dwarfs (AP 149 and AP 193) in the young open cluster α Persei, and compare these with previous images of intermediate and high axial inclination objects in this cluster. All stars show starspots at high latitudes, with one star exhibiting a strong polar spot. Although low-latitude features are found on all stars to some degree, the detection of spots on AP 193 is marginal. The apparent difference in starspot morphology from one object to the next is probably the result of a stellar magnetic cycle, although the exact effect on the starspot distribution throughout a cycle is unknown.
Polar spots are found in many Doppler images of rapidly rotating cool stars. In the past, their existence has been called into question, and it has been suggested that they could be the manifestations of NLTE (e.g. chromospheric filling in of line profiles) effects rather than real photospheric features. We assume the polar spots to be real photospheric features, and conclude that the flat-bottomed nature of the profile shape can be attributed to photospheric polar spots. The degree of truncation of the profile depends not only on spot size and strength, but also on the effective foreshortening of the polar region, a function of axial inclination.
H α is in emission on AP 149 which shows a double peak at most phases. The time-series of the profile shows an s-wave pattern as the position of these peaks changes throughout the rotation cycle. We attribute this to coronal clouds located above the stellar surface in synchronous orbit. A maximum-entropy tomogram is derived revealing four distinct emission regions located near and above the corotation radius.     

Search for rapid spectral feature variations in chemically peculiar star HR 4330

The chemically peculiar star HR 4330 (=HD 96707, EP-UMa) was observed using the 2-m Himalayan Chandra Telescope at the Indian Astronomical Observatory, Hanle, India, for studying the variations in its spectral features. It was found that the spectral features of Ca  ii 3933 Å and Sr  ii 4077 Å were showing rapid variations in their equivalent widths with a periodicity of  0.28 ± 0.02 d  . Such short-term variations suggest that there could be a dynamically active surface with rapid variation in the conditions of the stellar atmosphere.     

δ Orionis: further temporal variability and evidence for small-scale structure in the interstellar medium

We report here the detection of both spatial and temporal variations in interstellar absorption in the line of sight to δ Orionis. First, we present new high-resolution ( R ≈110 000) observations of the interstellar D lines of Na  i towards both δ Ori A and C. Comparison of these spectra highlights variations in absorption between the two stars, indicative of small-scale spatial structure in the interstellar medium in this direction over distances of less than ≈15 000 au (the projected separation of the two stars). Components with the largest Na  i column densities and lowest velocity dispersions are, in general, found to be subject to the greatest differences; in fact the narrowest component detected is only observed in one of the sightlines. This effect has also been reported by Meyer & Blades. Secondly, we present new ultra-high-resolution ( R ≈900 000) Na  i D₁ observations and high-resolution ( R ≈110 000) Ca  ii H & K observations of δ Ori A which, through ultra-high-resolution work conducted between 1994 and 2000, has been shown to exhibit a time-variable interstellar Na  i absorption component. These new observations, while revealing the further reduction in intensity of the time-variable Na  i absorption, indicate constant Ca  ii absorption over the same period. This results in a dramatic reduction in the Na°/Ca⁺ abundance ratio, perhaps indicating the line of sight to be gradually probing a less-dense outer region of an absorbing filament.     

Scandium and chromium in the strontium filament in the Homunculus of η Carinae

We continue a systematic study of chemical abundances of the strontium filament found in the ejecta of η Carinae. To this end we interpret the emission spectrum of Sc  ii and Cr  ii using multilevel non-local thermodynamic equilibrium models. Since the atomic data for these ions were previously unavailable, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. The observed spectrum is consistent with an electron density of the order of 10⁷ cm⁻³ and a temperature between 6000 and 7000 K, conditions previously determined from [Ni  ii ], [Ti  ii ] and [Sr  ii ] diagnostics. The observed spectrum indicates an abundance of Sc relative to Ni more than 40 times the solar value, while the Cr/Ni abundance ratio is roughly solar. Various scenarios of depletion and dust destruction are suggested to explain such abnormal abundances.     

Infrared spectroscopy of cataclysmic variables – III. Dwarf novae below the period gap and nova-like variables

We present K -band spectra of the short-period dwarf novae YZ Cnc, LY Hya, BK Lyn, T Leo, SW UMa and WZ Sge, the nova-like variables DW UMa, V1315 Aql, RW Tri, VY Scl, UU Aqr and GP Com, and a series of field dwarf stars with spectral types ranging from K2 to M6.
The spectra of the dwarf novae are dominated by emission lines of H  i and He  i . The large velocity and equivalent widths of these lines, in conjunction with the fact that the lines are double-peaked in the highest inclination systems, indicate an accretion disc origin. In the case of YZ Cnc and T Leo, for which we obtained time-resolved data covering a complete orbital cycle, the emission lines show modulations in their equivalent widths that are most probably associated with the bright spot (the region where the gas stream collides with the accretion disc). There are no clear detections of the secondary star in any of the dwarf novae below the period gap, yielding upper limits of 10–30 per cent for the contribution of the secondary star to the observed K -band flux. In conjunction with the K -band magnitudes of the dwarf novae, we use the derived secondary star contributions to calculate lower limits to the distances to these systems.
The spectra of the nova-like variables are dominated by broad, single-peaked emission lines of H  i and He  i – even the eclipsing systems we observed do not show the double-peaked profiles predicted by standard accretion disc theory. With the exception of RW Tri, which exhibits Na  i , Ca  i and ¹²CO absorption features consistent with a M0V secondary contributing 65 per cent of the observed K -band flux, we find no evidence for the secondary star in any of the nova-like variables. The implications of this result are discussed.     

Empirical calibration of the near-infrared Ca ii triplet – I. The stellar library and index definition

A new stellar library at the near-IR spectral region developed for the empirical calibration of the Ca  ii triplet and stellar population synthesis modelling is presented. The library covers the range λλ 8348–9020 at 1.5-Å (FWHM) spectral resolution, and consists of 706 stars spanning a wide range in atmospheric parameters. We have defined a new set of near-IR indices, CaT*, CaT and PaT, which mostly overcome the limitations of previous definitions, the former being specially suited for the measurement of the Ca  ii triplet strength corrected for the contamination from Paschen lines. We also present a comparative study of the new and the previous Ca indices, as well as the corresponding transformations between the different systems. A thorough analysis of the sources of index errors and the procedure to calculate them is given. Finally, index and error measurements for the whole stellar library are provided together with the final spectra.     

[Ti ii] and [Ni ii] emission from the strontium filament of η Carinae

We study the nature of the [Ti  ii ] and [Ni  ii ] emission from the so-called strontium filament found in the ejecta of η Carinae. To this purpose, we employ multilevel models of the Ti  ii and Ni  ii systems, which are used to investigate the physical condition of the filament and the excitation mechanisms of the observed lines. For the Ti  ii ion, for which no atomic data were previously available, we carry out ab initio calculations of radiative transition rates and electron impact excitation rate coefficients. It is found that the observed spectrum is consistent with the lines being excited in a mostly neutral region with an electron density of the order of 10⁷ cm⁻³ and a temperature around 6000 K. In analysing three observations with different slit orientations recorded between 2000 March and 2001 November, we find line ratios that change among various observations, in a way consistent with changes of up to an order of magnitude in the strength of the continuum radiation field. These changes result from different samplings of the extended filament due to the different slit orientations used for each observation, and yield clues on the spatial extent and optical depth of the filament. The observed emission indicates a large Ti/Ni abundance ratio relative to solar abundances. It is suggested that the observed high Ti/Ni ratio in gas is caused by dust–gas fractionation processes and does not reflect the absolute Ti/Ni ratio in the ejecta of η Carinae. We study the condensation chemistry of Ti, Ni and Fe within the filament and suggest that the observed gas phase overabundance of Ti is likely the result of selective photoevaporation of Ti-bearing grains. Some mechanisms for such a scenario are proposed.     

Doppler and modulation tomography of XTE J1118+480 in quiescence

We present Doppler and modulation tomography of the X-ray nova XTE J1118+480 with data obtained during quiescence using the 10-m Keck II telescope. The hotspot where the gas stream hits the accretion disc is seen in Hα, Hβ, He  i λ5876 and Ca  ii λ8662, thus verifying the presence of continued mass transfer within the system. The disc is clearly seen in Hα and Ca  ii λ8662. We image the mass-donor star in narrow absorption lines of Na  i  λλ5890, 5896, 8183, 8195  and Ca  ii λ8662, implying an origin from the secondary itself rather than the interstellar medium. We also detect deviations in the centroid of the double peak of Hα akin to those found by Zurita et al. suggesting disc eccentricity.     

Composite spectra Paper 15: HD 216572, a triple system containing a short-period, double-lined secondary

We analyse 81 optical spectra of the composite-spectrum binary HD 216572, and show that the primary is a cool giant of type G8 III while the secondary is a double-lined binary consisting of two nearly identical B9 dwarfs in a 1.18-d orbit. The inner system undergoes partial eclipses, whose photometry we model to derive the physical parameters of both secondary stars. The outer system does not eclipse. We isolate the combined spectrum of the secondary by spectral subtraction, and from 48 separate radial-velocity measurements of both secondary components we obtain a triple-lined orbit solution from which we determine the individual masses of all three stars and the inclinations of both the inner and the outer orbits. The period of the outer system is 55 d, which is surprisingly short for a giant star, and our detection of small but non-negligible amounts of variable chromospheric emission in the Ca  ii K line is not unlike that detected in other systems with comparably short periods. The secondary components are in a circular orbit and are rotating at about  95 ± 10 km s⁻¹  ; although their surface-to-surface separation is only  4 R_⊙  the stars are not noticeably distorted geometrically by such close proximity. All three stars appear to be in synchronous rotation in their respective orbits. We derive fairly accurate Hertzsprung–Russell diagram positions for all three stars and compare them to evolutionary tracks calculated for the respective stellar masses, but cannot reconcile the age of the cool giant with that of the B stars.     

Detection of a variable interstellar absorption component towards δ Orionis A

Observations of δ Ori A made with the UHRF in its highest resolution mode ( R ≈900 000) have revealed the presence of a cool ( T _k350 K) variable absorption component at a heliocentric velocity of +21.3 km s⁻¹. The component is detected in Na  i D₁, where clear hyperfine splitting is seen, and Ca  ii K. Comparison of our data with existing spectra suggests that the component has consistently increased in strength from 1966 to 1994, and subsequently reduced in intensity by 1999. Following a discussion of the possible origins of this component it is concluded that an interstellar, rather than circumstellar, origin is most likely. This is one of very few detections of variable interstellar absorption reported in the literature, and we suggest an origin within filamentary material associated with the expanding H  i shell surrounding the Orion–Eridanus superbubble.