Star AB Research Articles

AB Aurigae Resolved: Evidence for Spiral Structure

We obtained high angular resolution (~2) images of the 13CO(J=1-0) line and 2.7 millimeter continuum emission, and slightly lower resolution images of 12CO(J=1-0) and C18O(J=1-0) line emission toward the Herbig Ae star AB Aurigae. We resolve a circumstellar disk of diameter 780 AU (FWHM) with a velocity pattern consistent with a purely rotational disk at inclination 21.5 degrees and position angle 58.6 degrees. Using Keplerian disk models, we find a central source dynamical mass of 2.8+-0.1 Msun and a cutoff radius of 615 AU for the 13CO emission. Inclination, mass, and radius determined from 12CO and C18O observations agree with those values, given optical depth and abundance effects. As a result of the high angular resolution of our observations, we confirm the existence of spiral structure suggested by near-IR scattered light images and show that the spiral arms represent density contrasts in the disk.

Inferring Coronal Structure from X‐Ray Light Curves and Doppler Shifts: AChandraStudy of AB Doradus

The Chandra X-Ray Observatory continuously monitored the single cool star AB Dor for a period lasting 88 ks (1.98Prot) in 2002 December with the Low-Energy Transmission Grating HRC-S. The X-ray light curve shows rotational modulation with three peaks that repeat in two consecutive rotation cycles. These peaks may indicate the presence of compact emitting regions in the quiescent corona. Centroid shifts as a function of phase in the strongest line profile, O VIII λ18.97, indicate Doppler rotational velocities with a semiamplitude of 30 ± 10 km s-1. By taking these diagnostics into account along with constraints on the rotational broadening of line profiles (provided by archival Chandra High-Energy Transmission Grating Fe XVII and Far Ultraviolet Spectroscopic Explorer Fe XVIII profiles), we can construct a simple model of the X-ray corona that requires two components. One of these components is responsible for 80% of the X-ray emission and arises from the pole and/or a homogeneously distributed corona. The second component consists of two or three compact active regions that cause modulation in the light curve and contribute to the O VIII centroid shifts. These compact regions account for 16% of the emission and are located near the stellar surface with heights of less than 0.3R*. At least one of the compact active regions is located in the partially obscured hemisphere of the inclined star, while another of the active regions may be located at 40°. High-quality X-ray data such as these can test the models of the coronal magnetic field configuration as inferred from magnetic Zeeman Doppler imaging.

The Coronae of AB Doradus and V471 Tauri: Primordial Angular Momentum versus Tidal Spin‐up

The zero-age main-sequence star AB Dor and the K dwarf component of the V471 Tau close binary have essentially identical rotation rates and spectral types. An analysis of their high-resolution Chandra X-ray spectra reveals remarkably similar coronal characteristics in terms of both temperature structure and element abundances. Both stars show depletions of low first ionization potential (FIP) elements by factors of ~3, with higher FIP elements showing more mild depletions. No evidence for enhancements of very low FIP (<7 eV) elements, such as Na, Al, and Ca, as compared to other low-FIP elements, was found. The abundance anomaly pattern for AB Dor and V471 Tau is similar to, although less extreme than, the abundance anomalies exhibited by active RS CVn-type binaries. While we find statistically significant structure in the underlying differential emission measure distributions of these stars over narrow temperature intervals, this structure is strongly dependent on the lines used in the analysis and is probably spurious. On the basis of their X-ray similarities, we conclude that the exact evolutionary state of a star has little effect on coronal characteristics and that the parameters that dominate coronal structure and composition are simply the rotation rate and spectral type.

Probing the circumstellar structures of T Tauri stars and their relationship to those of Herbig stars

We present Hα spectropolarimetry observations of a sample of 10 bright T Tauri stars, supplemented with new Herbig Ae/Be star data. A change in the linear polarization across Hα is detected in most of the T Tauri (9/10) and Herbig Ae (9/11) objects, which we interpret in terms of a compact source of line photons that is scattered off a rotating accretion disc. We find consistency between the position angle (PA) of the polarization and those of imaged disc PAs from infrared and millimetre imaging and interferometry studies, probing much larger scales. For the Herbig Ae stars AB Aur, MWC 480 and CQ Tau, we find the polarization PA to be perpendicular to the imaged disc, which is expected for single scattering. On the other hand, the polarization PA aligns with the outer disc PA for the T Tauri stars DR Tau and SU Aur and FU Ori, conforming to the case of multiple scattering. This difference can be explained if the inner discs of Herbig Ae stars are optically thin, whilst those around our T Tauri stars and FU Ori are optically thick. Furthermore, we develop a novel technique that combines known inclination angles and our recent Monte Carlo models to constrain the inner rim sizes of SU Aur, GW Ori, AB Aur and CQ Tau. Finally, we consider the connection of the inner disc structure with the orientation of the magnetic field in the foreground interstellar medium: for FU Ori and DR Tau, we infer an alignment of the stellar axis and the larger magnetic field direction.

Flare X-Ray Observations of AB Doradus: Evidence of Stellar Coronal Opacity

X-ray spectra of the late-type star AB Dor obtained with the XMM-Newton satellite are analyzed. AB Dor was particularly active during the observations. An emission measure reconstruction technique is employed to analyze flare and quiescent spectra, with emphasis on the Fe XVII 15-17 Å wavelength region. The Fe XVII 16.78 Å/15.01 Å line ratio increases significantly in the hotter flare plasma. This change in the ratio is opposite to the theoretical predictions and is attributed to the scattering of 15.01 Å line photons from the line of sight. The escape probability technique indicates an optical depth of ≈0.4 for the 15.01 Å line. During the flare, the electron density is 4.4 × 1010 cm-3, and the fractional Fe abundance is 0.5 ± 0.1 of the solar photospheric value. Using these parameters, a path length of ≈8000 km is derived. There is no evidence of opacity in the quiescent X-ray spectrum of the star.

Theory of chromatographic separation of linear and star-shaped binary block-copolymers

Equations for the distribution coefficient of heteroarm stars are derived by using a model of an ideal chain in a slit-like pore; these equations together with those previously reported for linear block-copolymers are applied to describe chromatography of such copolymers. According to the theory, the retention generally depends on molar mass, composition, and architecture (microstructure and topology) of copolymers, on pore size and on adsorption interaction of chain units A and B. Three special modes of chromatography are studied in detail. (i) If interactions for A and B are close to the critical point of adsorption (CPA), the retention practically does not depend on architecture, and high molar mass copolymers can be separated by composition. (ii) At SEC condition for B and strong adsorption for A, copolymers in principle can be separated by architecture; better separation is expected in wide pores. Retention of linear block-copolymers decreases with increasing of the number of blocks; for heteroarm stars the theory predicts retention decreasing as: AB > StarAAB > StarABB; StarAAAB > StarABBB > StarAABB; StarAAAAB > StarABBBB > StarAAABB > StarAABBB. (iii) At the CPA for B copolymers AB, BAB and heteroarm stars regardless molar mass of B, M(B), can be separated by M(A). The same is true for ABA and ABAB...A in narrow pores. While the retention of AB, BAB, Star AB...B and StarAAB...B is the same, copolymers AB, ABA and linear multiblock-copolymers can be separated, as well as symmetric and very asymmetric triblock-copolymers ABA.

A dynamical calibration of the mass–luminosity relation at very low stellar masses and young ages

Mass is the most fundamental parameter of a star, yet it is also one of the most difficult to measure directly. In general, astronomers estimate stellar masses by determining the luminosity and using the 'mass-luminosity' relationship, but this relationship has never been accurately calibrated for young, low-mass stars and brown dwarfs. Masses for these low-mass objects are therefore constrained only by theoretical models. A new high-contrast adaptive optics camera enabled the discovery of a young (50 million years) companion only 0.156 arcseconds (2.3 au) from the more luminous (> 120 times brighter) star AB Doradus A. Here we report a dynamical determination of the mass of the newly resolved low-mass companion AB Dor C, whose mass is 0.090 +/- 0.005 solar masses. Given its measured 1-2-micrometre luminosity, we have found that the standard mass-luminosity relations overestimate the near-infrared luminosity of such objects by about a factor of approximately 2.5 at young ages. The young, cool objects hitherto thought to be substellar in mass are therefore about twice as massive, which means that the frequency of brown dwarfs and planetary mass objects in young stellar clusters has been overestimated.

New insights in the FUV into the activity of the Herbig Ae star HD 163296

We present an analysis of the FUSE spectra of HD 163296, a young isolated Herbig Ae star. The spectra in the far UV spectral range are dominated by strong emission lines of C  and O , which exhibit broad and complex asymmetric profiles with wings extending over more than 1000 km s −1 .T he H Ly α and the C  emission profiles show evidence for an outflow with a terminal velocity of 270 km s −1 . We also identified several narrower emission lines due to fluorescent decays in Fe  .U sing an HST/STIS archive spectrum, we checked that the H  Ly α line, which has a clear type III P Cygni emission profile, is broad enough to provide the photons required to photo-excite the Fe  upper levels. As previously explored for the well-known Herbig Ae star AB Aur, most of the spectral features including the fluorescent lines are consistent with the presence of a chromosphere above the photosphere. However, the over-ionized species cannot be produced in this region alone and we discuss two alternative mechanisms to explain such spectral signatures: magnetospheric accretion and a magnetically confined wind. Both interpretations involve a large-scale magnetic field whose detection is now a challenge for the new generation of spectropolarimeters such as ESPaDOnS.

Polar caps on active stars: magnetic flux emergence and transport

In recent years, Zeeman Doppler imaging of rapidly rotating solar-like stars has shown high-latitude magnetic flux patterns of intermingled magnetic polarities. Such high-latitude intermingling of positive and negative flux is inconsistent with our present understanding of how magnetic flux emerges on the Sun and is transported poleward. To determine how these patterns may arise, magnetic flux transport simulations are carried out. These simulations follow the evolution of the radial magnetic field at the surface of the star as new magnetic bipoles emerge and are advected poleward by the surface effects of differential rotation, meridional flow and supergranular diffusion. To produce intermingling of flux at high latitudes, key parameters such as the emergence profiles and transport coefficients are varied from presently used solar values. In doing so, it is found that, in order to explain the high-latitude intermingling, at least two of these parameters must be changed. First, the emergence profile must be extended to higher latitudes (λ = 50°-70° ), and secondly the value of the meridional flow must be increased by around a factor of 10 (∼100 m s -1 ). The results show that the observed intermingling of high-latitude flux can only occur through a flux emergence and transport process that is significantly different from that which occurs on the Sun. Observable features produced by both of these changes are considered, and the significance of the simulations to future observing programmes discussed. Finally the emergence profile and transport coefficients that best fit the observations of the young active star AB Dor (period = 0.514 d) are put forward.

Spiral Structure in the Circumstellar Disk around AB Aurigae

We present a near-infrared image of the Herbig Ae star AB Aur obtained with the Coronagraphic Imager with Adaptive Optics mounted on the Subaru Telescope. The image shows a circumstellar emission extending out to a radius of r = 580 AU, with a double spiral structure detected at r = 200-450 AU. The surface brightness decreases as r-3.0±0.1, steeper than the radial profile of the optical emission possibly affected by the scattered light from the envelope surrounding AB Aur. This result, together with the size of the infrared emission similar to that of the 13CO (J = 1-0) disk, suggests that the spiral structure is indeed associated with the circumstellar disk but is not part of the extended envelope. We identified four major spiral arms, which are trailing if the brighter southeastern part of the disk is the near side. The weak gravitational instability, maintained for millions of years by continuous mass supply from the envelope, might explain the presence of the spiral structure at the relatively late phase of the pre-main-sequence period.

CO Emission from Disks around AB Aurigae and HD 141569: Implications for Disk Structure and Planet Formation Timescales

We present a comparison of CO fundamental rovibrational lines (observed in the M band near 4.7 μm) from the inner circumstellar disks around the Herbig AeBe stars AB Aur and HD 141569. The CO spatial profiles and temperatures constrain the location of the gas for both stars to a distance of less than 50 AU. The CO emission from the disk of the ~4 Myr star AB Aur shows at least two temperature components, the inner disk at a rotational temperature of 1540 ± 80 K and the outer disk at 70 ± 10 K. The hot gas is located near the hot bright inner rim of the disk and the cold gas is located in the outer disk from 8-50 AU. The relative intensities of low-J lines suggest that the cold gas is optically thick. The excitation of CO in both temperature regimes is dominated by infrared fluorescence (resonant scattering). In the more evolved disk around HD 141569, the CO is excited by UV fluorescence. The relative intensity of the CO emission lines implies a rotational temperature of 190 ± 30 K. The resulting column density is ~ 1011 cm-2, indicating approximately 1019 g of CO. The observed line profiles indicate that the inner disk has been cleared of CO gas by stellar radiation out to a minimum of 17 AU. The residual mass of CO suggests that the inner disk of HD 141569 is not in an active phase of planet building but it does not rule out the possibility that giant planet building has previously occurred.

Quiescent and Flaring Structure in RS Canum Venaticorum Stars

Four of the most active RS CVn stars (V711 Tau, II Peg, σ Gem, and UX Ari) have been observed for a total of 3 Ms with the Extreme Ultraviolet Explorer satellite (EUVE) between 1992 and 2000 January. Flaring and quiescent states of extreme ultraviolet spectra (λλ70-740) and light curves (λλ75-175) have been analyzed to provide emission measure distributions (EMD) for these systems in the range log Te(K) ~ 5.6-7.4, based principally on iron lines. Flux measurements obtained with IUE and the Orbiting and Retrievable Far and Extreme Ultraviolet Spectrometer (ORFEUS) complete the EMD in the lower temperature range [log Te(K) ~ 4.0-5.6]. Frequent flaring activity has been found in the systems, including an increase during the rise phase by a factor of ~9 in the flux of σ Gem, the largest flare enhancement observed with EUVE. Analyses of the EUVE emission in the active single star AB Dor and the low-rotation giant star β Cet are also included. The EMDs are remarkably similar among all the stars, showing a narrow enhancement or bump around log Te(K) ~ 6.9. These narrow bumps are apparently unrelated to rotation rate, spectral type, binarity, or evolutionary stage. Significant material is found at log Te(K) 7.0 for the most active stars. Modulation of the EUV flux outside of flaring occurs in four of the stars (σ Gem, V711 Tau, UX Ari, AB Dor). The electron density ranges between Ne ~ 1012 and ~1013 cm-3, measured at log Te(K) ~ 7.0, and may reach higher values during flares. These densities and EMD values imply small scale sizes for emitting regions.

The coronal FeXXI λ1354.094 line in AB Doradus

The active late-type star AB Doradus was observed in February 1996 with the Goddard High Resolution Spectrograph of the Hubble Space Telescope using the low resolution G140L grating. The observations covered one half of the star's rotation cycle (P =0 :514 d) with 11.5 min time resolution. The strong coronal FeXXI 1354.094 line formed at 10 7 K was analysed and its emission measure (EM) derived. This EM is much higher than that derived from recent XMM-Newton observations (Gudel et al. 2001), and earlier EXOSAT (Collier Cameron et al. 1988) and ASCA/EUVE (Mewe et al. 1996) data, as well, requiring a variability by a factor of 5. The physical reason for the variability remains unknown, since (outside flares) the observed broad band variability of AB Dor is much smaller.

Halpha spectropolarimetry of the Herbig Ae star AB Aurigae

We present spectropolarimetric observations, obtained at Hα, of the Herbig Ae star AB Aurigae. Changes in linear polarization across the Hα line probe structure in the immediate circumstellar environment of the central star, down to scales of the order of one to a few stellar radii. In the case of AB Aurigae the observed polarimetric signature is complex. After applying a correction for foreground continuum polarization, we find that there is a linear-polarized Hα emission component intrinsic to the source. Rotation of the angle of polarization through the emission-line profile suggests scattering in a rotating circumstellar disc. The magnetic accretor model commonly applied to T Tauri stars shows promise of explaining these data.

ITAL]Far Ultraviolet Spectroscopic Explorer[/ITAL] Observations of the Active Cool Star AB Doradus

Far-ultraviolet spectra were obtained of the active cool star AB Doradus (HD 36705) during the calibration and checkout period of the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite. Observations in this early phase of the mission were taken at a resolving power of 12,000-15,000 (~20-25 km s-1) and covered the spectral range 905-1187 A. The integrated spectrum exhibits strong, rotationally broadened stellar emission from C III (λλ977, 1175) and O VI (λλ1032, 1037) and many weaker lines. Strong emission lines of C III and O VI exhibit broad wings. The C III λ977 profile shows blueshifted absorption at ~30 km s-1, and C II λ1036 absorption appears superposed on emission in the wing of O VI λ1037. Rotational modulation of C III and O VI is present, in harmony with its photometric variability. Flares were detected in the brightest lines, and subexposures were analyzed to examine flux and profile variations. Downflows that extend to 600 km s-1 during a flare are found in the O VI profiles. These early observations demonstrate that FUSE will be an exceptional instrument for studying chromospheres in cool stars.

X-Ray spectroscopy of stellar coronae with BeppoSAX

We report on preliminary results of stellar coronae observations by BeppoSAX with emphasis on our own observations of the RS CVn binary UX Arietis and of the young rapidly rotating star AB Doradus. We discuss the temperature, emission measure distribution and elemental abundances derived from these observations and compare the results with those obtained previously by ASCA. We also report on the detection by BeppoSAX of hard (>20 keV) X-ray emission from large stellar flares, and discuss briefly possible interpretations of this detection.

ITAL]Hubble Space Telescope[/ITAL] Space Telescope Imaging Spectrograph Coronagraphic Imaging of the Herbig Ae Star AB Aurigae

We present the first broadband, coronagraphic Hubble Space Telescope images of the bright, optically visible, isolated Herbig Ae star AB Aurigae. The Space Telescope Imaging Spectrograph (STIS) images reveal extended circumstellar nebulosity (r ≈ 1300 AU) covering the region of the millimeter continuum and CO disk. The structure is observed in the disk on spatial scales down to 01 (14 AU) and exhibits a north-south asymmetry. A comparison of the STIS data with scattering models for flared disks or disks + envelopes suggests that the disk inclination is i ≤ 45° from the plane of the sky.

Multisite observations of surface structures on AB Doradus in 1994 November

We present time-resolved optical spectroscopy and broad-band photometry of the rapidly rotating southern K0 dwarf star AB Doradus, obtained during 1994 November. The data were obtained as part of a collaboration dedicated to MUlti-SIte COntinuous Spectroscopy (MUSICOS), and entailed coordinated observations on three continents to obtain the fullest phase coverage possible subject to limitations of local weather conditions. The Doppler images from the three consecutive nights of the run show excellent mutual agreement, with a dark polar cap and numerous intermediate- and low-latitude features. Simultaneous optical photometry showed numerous short-duration U-band flares, and two longer duration optical flares with durations of the order of hours. The latter produced broad-band continuum enhancements throughout the optical spectrum. Where simultaneous spectroscopy was available, both types of flare were seen to have counterparts in Ha and the Caii H line. Simultaneous time-resolved ultraviolet spectroscopy from the Goddard High Resolution Spectrograph (GHRS) aboard the Hubble Space Telescope, reported elsewhere, shows that at least one of the short-duration U-band flares was also observed in Civ with the GHRS. Time-series Ha spectra showed significant evolution of the circumstellar prominence system over five consecutive stellar rotations. One prominence underwent a dramatic increase in distance from the stellar rotation axis. We speculate that this event may have been associated with one of the long-duration flares.

Phase-resolved Simultaneous [ITAL]ORFEUS[/ITAL] Far-Ultraviolet and [ITAL]ROSAT[/ITAL] X-Ray Observations of the Active Star AB Doradus

We report phase-resolved simultaneous ORFEUS far-ultraviolet (FUV) and ROSAT soft X-ray observations of the rapidly rotating young star AB Doradus (HD 36705), obtained with the FUV spectrometer flown on board ORFEUS II and the High Resolution Imager (HRI) on board ROSAT. The lines of C III at 977 and 1176 A , O V at 1218 A, and O VI at 1032 and 1038 A, as well as N II at 1085 A and Si III at 1206 A, are clearly detected in the ORFEUS spectra. The X-ray flux intrinsically varied during the observation and FUV and X-ray fluxes are found to correlate. No changes in the line profiles are detectable, but the line profiles of C III λ977 and O VI λ1032 are definitely broadened with respect to the instrumental line profile. The observed broadening does not exceed the photospheric v sin i value, suggesting that the observed C III and O VI emission, formed at a characteristic temperature of ≈ 80,000-300,000 K, is produced close to the star's surface. The C III λ1176/λ977 line ratio is found to be larger than that in the Sun, indicating electron densities (at ≈ 80,000 K) of ne ~ 1011 cm−3 or higher.

ITAL]Extreme[/ITAL] [ITAL]Ultraviolet[/ITAL] [ITAL]Explorer[/ITAL] Investigation of Three Short-Period Binary Stars

Extreme Ultraviolet Explorer spectroscopic observations (SW, MW, and LW bands, covering 80–160, 170–350, and 300–450 Å with resolutions 0.5, 1, and 2 Å, respectively) and photometric observations (Deep Sky Survey [DSS], broadband 70–140 Å) have been obtained for two contact binaries, 44i Boo B and VW Cep, and one close but detached binary, ER Vul. All three systems have orbital periods shorter than 1 day and thus are expected to show "saturated" levels of chromospheric, transition region, and lower corona emissions. Only moderate flaring activity was seen in the DSS light curves of all three stars. The phased DSS light curve of 44i Boo B shows one minimum at the eclipse of the hotter, smaller component, while the phased light curve of ER Vul has a slightly higher level at the first maximum, following the deeper eclipse. An unsuccessful attempt has been made to determine the temperature dependence of the differential emission measure (DEM) of the coronal plasma for 44i Boo B on the basis of iron ion emission lines. It utilized the singular value decomposition formalism developed by Schmitt et al. to handle the inverse problem of the spectrum-to-DEM mapping. Implementation of this formalism suffered from the low number of available spectral features and possibly also inconsistencies in the line strengths, which become amplified in the mathematical inversion process. However, experiments with the direct reconstruction of the spectra, which were generated with the DEM functions available from the literature, favor the distribution of Dupree et al. in preference to the two-temperature model of McGale et al. A comparison of the relative line fluxes fline/fbol for selected strongest chromospheric, transition region, and low-corona emission lines in 44i Boo B, VW Cep, ER Vul, and the single, rapidly rotating star AB Dor shows that the currently available data are consistent with the period-independent ("saturated") levels of activity for features forming at log T ≤ 5 in all four stars, while features formed at log T ≃ 6.8–7.1 are definitely the weakest in AB Dor.