Star AB Aur Research Articles

UV-optical Emission of AB Aur b Is Consistent with Scattered Stellar Light

The proposed protoplanet AB Aur b is a spatially concentrated emission source imaged in the millimeter-wavelength disk gap of the Herbig Ae/Be star AB Aur. Its near-infrared spectrum and absence of strong polarized light have been interpreted as evidence supporting the protoplanet interpretation. However, the complex scattered-light structures in the AB Aur disk pose challenges in resolving the emission source and interpreting the true nature of AB Aur b. We present new images of the AB Aur system obtained using the Hubble Space Telescope Wide Field Camera 3 in the ultraviolet (UV) and optical bands. AB Aur b and the known disk spirals are recovered in the F336W, F410M, and F645N bands. The spectral energy distribution of AB Aur b shows absorption in the Balmer jump, mimicking that of early-type stars. By comparing the colors of AB Aur b to those of the host star, the disk spirals, and predictions from scattered light and self-luminous models, we find that the emission from AB Aur b is inconsistent with planetary photospheric or accretion shock models. Instead, it is consistent with those measured in the circumstellar disks that trace scattered light. We conclude that the UV and visible emission from AB Aur b does not necessitate the presence of a protoplanet. We synthesize observational constraints on AB Aur b and discuss inconsistent interpretations among different data sets. Considering the significance of the AB Aur b discovery, we advocate for further observational evidence to verify its planetary nature.

Relation between metallicities and spectral energy distributions of Herbig Ae/Be stars

Context. Most studies devoted to Herbig Ae/Be stars (HAeBes) assume solar metallicity. However, the stellar metallicity, [M/H], is a fundamental parameter that can strongly differ depending on the source and may have important implications for planet formation. It has been proposed that the deficit of refractory elements observed in the surfaces of some HAeBes may be linked to the presence of cavities in their disks and is likely caused by Jovian planets that trap the metal-rich content. Aims. This work aims to provide a robust test on the previous proposal by analyzing the largest sample of HAeBes characterized by homogeneously derived [M/H] values and stellar and circumstellar properties. Methods. The spectra of 67 HAeBes, along with their well-known properties drawn from our previous work, have been collected from the ESO Science Archive Facility. Their [M/H] values were derived based on the comparison with Kurucz synthetic models. Statistical analyses were carried out with the aim to test the potential relation between [M/H] and the Meeus group I sources, with spectral energy distributions (SEDs) associated with the presence of cavities potentially carved by giant planets. We critically analyzed the eventual link between [M/H], the SED groups, and the presence of such planets. Results. Our statistical study robustly confirms that group I sources tend to have a lower [M/H] (typically ~ −0.10) than that of group II HAeBes (~ +0.14). A similar analysis involving SED-based transitional disks, with infrared excess only at wavelengths of ≥2.2 µm, does not reveal such a relation with [M/H]. This result indicates that not all processes capable of creating holes in the inner dust disks end up having an effect on the stellar abundances. The spatial distributions of group I and II sources are similar, at least within the available range of distances to the galactic centre and the galactic plane, for which the observed [M/H] differences are not driven by environmental effects. In addition, group I sources tend to have stronger (sub-) mm continuum emission presumably related to the presence of giant planets. Indeed, literature results indicate that disk substructures probably associated with the presence of giant planets are up to ten times more frequent in group I HAeBes than in group II. Finally, along with the metallicities derived for the whole sample, surface gravities and projected rotational velocities are additional outcomes reported in this work. Conclusions. We provide indirect evidence to suggest that giant planets are more frequent around group I/low [M/H] stars than around the rest of the HAeBes. However, a direct test of the previous hypothesis requires multiple detections of forming planets in their disks. Such detections have so far been limited to the candidate around the metal depleted ([M/H] = −0.35 ± −0.25) group I HAeBe star AB Aur, which is consistent with our findings.

AB Aur, a Rosetta stone for studies of planet formation

Context. The sulfur abundance is poorly known in most environments. Yet, deriving the sulfur abundance is key to understanding the evolution of the chemistry from molecular clouds to planetary atmospheres. We present observations of H2S 110–101 at 168.763 GHz toward the Herbig Ae star AB Aur. Aims. We aim to study the abundance of sulfuretted species toward AB Aur and to constrain how different species and phases contribute to the sulfur budget. Methods. We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and H2S 110–101 line at 168.763 GHz toward AB Aur. We derived radial and azimuthal profiles and used them to compare the geometrical distribution of different species in the disk. Assuming local thermodynamical equilibrium (LTE), we derived column density and abundance maps for H2S, and we further used Nautilus to produce a more detailed model of the chemical abundances at different heights over the mid-plane at a distance of r = 200 au. Results. We have resolved H2S emission in the AB Aur protoplanetary disk. The emission comes from a ring extending from 0.67″ (~109 au) to 1.69″ (~275 au). Assuming T = 30 K, nH = 109 cm−3, and an ortho-to-para ratio of three, we derived a column density of (2.3 ± 0.5) × 1013 cm−2. Under simple assumptions, we derived an abundance of (3.1 ± 0.8) × 10−10 with respect to H nuclei, which we compare with Nautilus models to deepen our understanding of the sulfur chemistry in protoplanetary disks. Chemical models indicate that H2S is an important sulfur carrier in the solid and gas phase. We also find an important transition at a height of ~12 au, where the sulfur budget moves from being dominated by ice species to being dominated by gas species. Conclusions. We confirm that present-day models still struggle to simultaneously reproduce the observed column densities of the different sulfuretted species, and the observed abundances are still orders of magnitude away from the cosmic sulfur abundance. Studying sulfuretted species in detail in the different phases of the interstellar medium is key to solving the issue.

AB Aur, a Rosetta stone for studies of planet formation

Context.AB Aur is a Herbig Ae star that hosts a prototypical transition disk. The disk shows a plethora of features connected with planet formation mechanisms, such as spiral arms, dust cavities, and dust traps. Understanding the physical and chemical characteristics of these features is crucial to advancing our knowledge of the planet formation processes.Aims.We aim to characterize the gaseous disk around the Herbig Ae star AB Aur. A complete spectroscopic study was performed using NOEMA to determine the physical and chemical conditions with high spatial resolution.Methods.We present new NOrthern Extended Millimeter Array (NOEMA) interferometric observations of the continuum and12CO,13CO, C18O, H2CO, and SO lines obtained at high resolution. We used the integrated intensity maps and stacked spectra to derive reliable estimates of the disk temperature. By combining our13CO and C18O observations, we computed the gas-to-dust ratio along the disk. We also derived column density maps for the different species and used them to compute abundance maps. The results of our observations were compared with a set of Nautilus astrochemical models to obtain insight into the disk properties.Results.We detected continuum emission in a ring that extends from 0.6′′ to ~2.0′′, peaking at 0.97′′ and with a strong azimuthal asymmetry. The molecules observed show different spatial distributions, and the peaks of the distributions are not correlated with the binding energy. Using H2CO and SO lines, we derived a mean disk temperature of 39 K. We derived a gas-to-dust ratio that ranges from 10 to 40 along the disk. Abundance with respect to13CO for SO (~2 × 10−4) is almost one order of magnitude greater than the value derived for H2CO (1.6 × 10−5). The comparison with Nautilus models favors a disk with a low gas-to-dust ratio (40) and prominent sulfur depletion.Conclusions.From a very complete spectroscopic study of the prototypical disk around AB Aur, we derived, for the first time, the gas temperature and the gas-to-dust ratio along the disk, providing information that is essential to constraining hydrodynamical simulations. Moreover, we explored the gas chemistry and, in particular, the sulfur depletion. The derived sulfur depletion is dependent on the assumed C/O ratio. Our data are better explained with C/O ~ 0.7 and S/H = 8 × 10−8.

Results of Many-year Photometry of the Herbig Ae Star AB Aur in the Near Infrared

The results of many-year infrared observations of the Herbig AeBe star AB Aur at 1-5 μm (JHKLM bands) are presented. The duration of the photometric series, together with observations published earlier, is about 18 years. The observations demonstrate that, in contrast to the optical, the star is quite active in the studied infrared range. The variability amplitude increases with wavelength, reaching ΔM = 0.72m. Modeling shows that both the amount of IR radiation from the inner part of the disk and its temperature increase when the star becomes brighter. A periodogram analysis demonstrates that variations of the IR flux from AB Aur display significant evidence for periodic changes, whose stability is uncertain. This behavior is most probably associated with transient quasi-oscillations related to instability processes in the inner disk. The estimated correlation timescale, τ = 130 ± 40d, can be considered as an approximate characteristic of the duration of such processes. The analysis results provide no certain grounds to believe that the inner disk contains any massive objects in permanent orbits, such as protoplanets. The existing trend in the variations of the IR flux, most clearly expressed in the KLM bands, provides evidence for the presence of large-scale perturbations in the disk. Conditions that could bring about a cyclic character in the accretion of gas from the circumstellar disk onto the young star are briefly discussed.

A disk wind in AB Aurigae traced with Hαinterferometry

Context. A crucial issue in star formation is understanding the physical mechanism by which mass is accreted onto and ejected by a young star, then collimated into jets. Hydrogen lines are often used to trace mass accretion in young stars, but recent observations suggest that they could instead trace mass outflow in a disk wind.Aims. Obtaining direct constraints on the HI line formation regions is crucial in order to disentangle the different models. We present high angular and spectral resolution observations of the Hα line of the Herbig Ae star AB Aur to probe the origin of this line at sub-AU scales, and to place constraints on the geometry of the emitting region.Methods. We use the visible spectrograph VEGA at the CHARA long-baseline optical array to resolve the AB Aur circumstellar environment from spectrally resolved interferometric measurements across the Hα emission line. We developed a 2D radiative transfer model to fit the emission line profile and the spectro-interferometric observables. The model includes the combination of a Blandford & Payne magneto-centrifugal disk wind and a magnetospheric accretion flow.Results. We measure a visibility decrease within the Hα line, indicating that we clearly resolve the Hα formation region. We derive a Gaussian half width at half maximum between 0.05 and 0.15 AU in the core of the line, which indicates that the bulk of the Hα emission has a size scale intermediate between the disk inner truncation radius and the dusty disk inner rim. A clear asymmetric differential phase signal is found with a minimum of −30° ± 15° towards the core of the line. We show that these observations are in general agreement with predictions from a magneto-centrifugal disk wind arising from the innermost regions of the disk. Better agreement, in particular with the differential phases, is found when a compact magnetospheric accretion flow is included.Conclusions. We resolve the Hα formation region in a young accreting intermediate mass star and show that both the spectroscopic and interferometric measurements can be reproduced well by a model where the bulk of Hα forms in a MHD disk wind arising from the innermost regions of the accretion disk. These findings support similar results recently obtained in the Brγ line and confirm the importance of outflows in the HI line formation processes in young intermediate mass stars.

AN ORDERED MAGNETIC FIELD IN THE PROTOPLANETARY DISK OF AB Aur REVEALED BY MID-INFRARED POLARIMETRY

ABSTRACT Magnetic fields (B-fields) play a key role in the formation and evolution of protoplanetary disks, but their properties are poorly understood due to the lack of observational constraints. Using CanariCam at the 10.4 m Gran Telescopio Canarias, we have mapped out the mid-infrared polarization of the protoplanetary disk around the Herbig Ae star AB Aur. We detect ∼0.44% polarization at 10.3 μm from AB Aur's inner disk (r < 80 au), rising to ∼1.4% at larger radii. Our simulations imply that the mid-infrared polarization of the inner disk arises from dichroic emission of elongated particles aligned in a disk B-field. The field is well ordered on a spatial scale, commensurate with our resolution (∼50 au), and we infer a poloidal shape tilted from the rotational axis of the disk. The disk of AB Aur is optically thick at 10.3 μm, so polarimetry at this wavelength is probing the B-field near the disk surface. Our observations therefore confirm that this layer, favored by some theoretical studies for developing magneto-rotational instability and its resultant viscosity, is indeed very likely to be magnetized. At radii beyond ∼80 au, the mid-infrared polarization results primarily from scattering by dust grains with sizes up to ∼1 μm, a size indicating both grain growth and, probably, turbulent lofting of the particles from the disk mid-plane.

Cyclic phenomena in the circumstellar envelope of the A0e/B9e Herbig star AB Aur

An analysis of spectra of the well known Herbig Ae/Be star AB Aur obtained with a high resolution Coude spectrograph (R~20000-25000) installed on the 2.6-m Shejn telescope at the Crimean Astrophysical Observatory (Ukraine) and a medium resolution echelle spectrometer (R~14000) on the 2-m Zeiss telescope at the Shemakha Astrophysical Observatory of the National Academy of Sciences of Azerbaijan yielded an unexpected result. It turns out the the variations in the equivalent widths EW of the HeI 5876 and D NaI emission lines have an inverse correlation (r=-0.67±0.14 and -0.73±0.10 according to the data from the CrAO and the ShAO, respectively), while the correlation is direct for the Hα and HeI 5876 lines (r=+0.68±0.14 and +0.59±0.14 according to the data from the CrAO and the ShAO, respectively). The Lafler-Kinman method gives a period P=123.7 days for the variation in the EW of these lines. On phase diagrams plotted for this period, the variations in the EW are sinusoidal, with the variations in EW(HeI) and EW(DNaI) in counterphase, while EW(HeI) and EW(Hα) are in phase. A possible interpretation of this phenomenon is examined. The need for new spectral observations of AB Aur in order to confirm the observed effects on a larger statistical sample is emphasized.

DIRECT IMAGING OF FINE STRUCTURES IN GIANT PLANET-FORMING REGIONS OF THE PROTOPLANETARY DISK AROUND AB AURIGAE

We report high-resolution 1.6 $\micron$ polarized intensity ($PI$) images of the circumstellar disk around the Herbig Ae star AB Aur at a radial distance of 22 AU ($0."15$) up to 554 AU (3.$"$85), which have been obtained by the high-contrast instrument HiCIAO with the dual-beam polarimetry. We revealed complicated and asymmetrical structures in the inner part ($\lesssim$140 AU) of the disk, while confirming the previously reported outer ($r$ $\gtrsim$200 AU) spiral structure. We have imaged a double ring structure at $\sim$40 and $\sim$100 AU and a ring-like gap between the two. We found a significant discrepancy of inclination angles between two rings, which may indicate that the disk of AB Aur is warped. Furthermore, we found seven dips (the typical size is $\sim$45 AU or less) within two rings as well as three prominent $PI$ peaks at $\sim$40 AU. The observed structures, including a bumpy double ring, a ring-like gap, and a warped disk in the innermost regions, provide essential information for understanding the formation mechanism of recently detected wide-orbit ($r$ $>$20 AU) planets.

The Hαline forming region of AB Aurigae spatially resolved at sub-AU with the VEGA/CHARA spectro-interferometer

A crucial issue in star formation is to understand the physical mechanism by which mass is accreted onto and ejected by a young star. The visible spectrometer VEGA on the CHARA array can be an efficient means of probing the structure and the kinematics of the hot circumstellar gas at sub-AU. For the first time, we observed the Herbig Ae star AB Aur in the H$\alpha$ emission line, using the VEGA low spectral resolution on two baselines of the array. We computed and calibrated the spectral visibilities between 610 nm and 700 nm. To simultaneously reproduce the line profile and the visibility, we used a 1-D radiative transfer code that calculates level populations for hydrogen atoms in a spherical geometry and synthetic spectro-interferometric observables. We clearly resolved AB Aur in the H$\alpha$ line and in a part of the continuum, even at the smallest baseline of 34 m. The small P-Cygni absorption feature is indicative of an outflow but could not be explained by a spherical stellar wind model. Instead, it favors a magneto-centrifugal X-disk or disk-wind geometry. The fit of the spectral visibilities could not be accounted for by a wind alone, so we considered a brightness asymmetry possibly caused by large-scale nebulosity or by the known spiral structures, inducing a visibility modulation around H$\alpha$. Thanks to the unique capabilities of VEGA, we managed to simultaneously record for the first time a spectrum at a resolution of 1700 and spectral visibilities in the visible range on a target as faint as $m_{V}$ = 7.1. It was possible to rule out a spherical geometry for the wind of AB Aur and provide realistic solutions to account for the H$\alpha$ emission compatible with magneto-centrifugal acceleration. The study illustrates the advantages of optical interferometry and motivates observations of other bright young stars to shed light on the accretion/ejection processes.

Chemistry in disks

We study the molecular content and chemistry of a circumstellar disk surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim is to reconstruct the chemical history and composition of the AB Aur disk and to compare it with disks around low-mass, cooler T Tauri stars. We observe the AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D- configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO isotopes. Using an iterative minimization technique, observed columns densities and abundances are derived. These values are further compared with results of an advanced chemical model that is based on a steady-state flared disk structure with a vertical temperature gradient, and gas-grain chemical network with surface reactions. We firmly detect HCO+ in the 1--0 transition, tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+ and 13CO column densities as well as the upper limits to the column densities of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and those from previous studies. The AB Aur disk possesses more CO, but is less abundant in other molecular species compared to the DM Tau disk. This is primarily caused by intense UV irradiation from the central Herbig A0 star, which results in a hotter disk where CO freeze out does not occur and thus surface formation of complex CO-bearing molecules might be inhibited.

The use of genetic algorithms to model protoplanetary discs

The protoplanetary discs of T Tauri and Herbig Ae/Be stars have previously been studied using geometric disc models to fit their spectral energy distribution (SED). The simulations provide a means to reproduce the signatures of various circumstellar structures, which are related to different levels of infrared excess. With the aim of improving our previous model, which assumed a simple flat-disc configuration, we adopt here a reprocessing flared-disc model that assumes hydrostatic, radiative equilibrium. We have developed a method to optimize the parameter estimation based on genetic algorithms (GAs). This paper describes the implementation of the new code, which has been applied to Herbig stars from the Pico dos Dias Survey catalogue, in order to illustrate the quality of the fitting for a variety of SED shapes. The star AB Aur was used as a test of the GA parameter estimation, and demonstrates that the new code reproduces successfully a canonical example of the flared-disc model. The GA method gives a good quality of fit, but the range of input parameters must be chosen with caution, as unrealistic disc parameters can be derived. It is confirmed that the flared-disc model fits the flattened SEDs typical of Herbig stars; however, embedded objects (increasing SED slope) and debris discs (steeply decreasing SED slope) are not well fitted with this configuration. Even considering the limitation of the derived parameters, the automatic process of SED fitting provides an interesting tool for the statistical analysis of the circumstellar luminosity of large samples of young stars.

High-resolution X-ray spectroscopy of T Tauri stars in the Taurus-Auriga complex

Context. Di erences have been reported between the X-ray emission of accreting and non-accreting stars. Some observations have suggested that accretion shocks could be responsible for part of the X-ray emission in classical T Tauri stars (CTTS). Aims. We present high-resolution X-ray spectroscopy for nine pre-main sequence stars in order to test the proposed spectroscopic di erences between accreting and non-accreting pre-main sequence stars. Methods. We used X-ray spectroscopy from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. We interpret the spectra using optically thin thermal models with variable abundances, together with an absorption column density. For BP Tau and AB Aur we derive electron densities from the Ovii triplets. Results. Using the Ovii/Oviii count ratios as a diagnostic for cool plasma, we find that CTTS display a soft excess (with equivalent electron temperatures of 2:5 3 MK) when compared with WTTS or zero-age main-sequence stars. Although the Ovii triplet in BP Tau is consistent with a high electron density (3:4 10 11 cm 3 ), we find low density for the accreting Herbig star AB Aur (ne < 10 10 cm 3 ). The element abundances of accreting and non-accreting stars are similar. The Ne abundance is found to be high (4-6 times the Fe abundance) in all K and M-type stars. In contrast, for the three G-type stars (SU Aur, HD 283572, and HP Tau/G2), we find an enhanced Fe abundance (0.4-0.8 times solar photospheric values) compared to later-type stars. Conclusions. Adding the results from our sample to former high-resolution studies of T Tauri stars, we find a soft excess in all accreting stars, but in none of the non-accretors. On the other hand, high electron density and high Ne/Fe abundance ratios do not seem to be present in all accreting pre-main sequence stars.

Possible Molecular Spiral Arms in the Protoplanetary Disk of AB Aurigae

The circumstellar dust disk of the Herbig Ae star AB Aur has been found to exhibit complex spiral-like structures in the near-IR image obtained with the Subaru Telescope. We present maps of the disk in both 12CO (3-2) and dust continuum at 345 GHz with the Submillimeter Array at an angular resolution of 10 × 07 (144 × 100 AU). The continuum emission traces a dust disk with a central depression and a maximum overall dimension of 450 AU (FWHM). This dust disk exhibits several distinct peaks that appear to coincide with bright features in the near-IR image, in particular the brightest inner spiral arm. The CO emission traces a rotating gas disk of size 530 × 330 AU with a deprojected maximum velocity of 2.8 km s-1 at 450 AU. In contrast with the dust disk, the gas disk exhibits an intensity peak at the stellar position. Furthermore, the CO emission in several velocity channels traces the innermost spiral arm seen in the near-IR. We compare the observed spatial-kinematic structure of the CO emission to a simple model of a disk in Keplerian rotation and find that only the emission tracing the main spiral arm clearly lies outside the confines of our model. This emission has a net outward radial motion compared with the radial velocity predicted by the model at the location of the main spiral arms. The disk of AB Aur is therefore quite different from the Keplerian disks seen around many Herbig Ae stars. The spiral-like structures of the disk with non-Keplerian motions we revealed in 12CO (3-2), together with the central depression of the dust disk, could be explained to be driven by the possible existence of a giant planet forming in the disk.

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.

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.

Azimuthal structures in the wind and chromosphere of the Herbig Ae star AB Aurigae. Results from the MUSICOS 1992 campaign

The observations of the MUSICOS 1992 campaign concerned three scientific programs, among which was the study of azimuthal structures in the wind and chromosphere of the pre-main sequence Herbig Ae star AB Aur. The He I 5876 Aline of AB Aur, which is formed in the expanding chromosphere of this star, most probably in the innermost parts of its wind, was continuously monitored at a spectral resolution of 30000 for about 4 days. The line was discovered to be variable in a spectacular way, the profile changing from pure emission to a composite profile including a deep absorption component in the course of a few hours. In this paper we present strong clues that suggest a rotational modulation of the line, with a period corresponding to the stars rotation period of 32 hrs. We confirmed the covariation of several non-photospheric spectral lines, formed at very different radial distances of the star, which strengthens the idea that the variability is the signature of azimuthal structures in the wind of AB Aur, most probably due to magnetically confined streams or loops emanating from the stellar surface. We present the data collected during the MUSICOS 1992 campaign, but also a data set of the He I D3 line obtained with FOE at KPNO during the years 1991–1994 and a series of IUE spectra containing the Mg II h & k UV lines obtained one month prior to the campaign. A thorough discussion of the possible interpretations of the spectacular variations of the He I 5876 A line is presented.

Azimuthal structures in the wind and chromosphere of the herbig Ae star ab aur. preliminary results from the MUSICOS 1992 campaign

MUSICOS (for MUlti-SIte COntinuous Spectroscopy) is an international project to facilitate and organize world-wide multi-site campaigns in high resolution spectroscopy, in view of obtaining a complete time coverage of various types of variable stellar phenomena. In the framework of this project a vast spectroscopic campaign was organized in December 1992, involving 8 sites well distributed in longitude around the Earth. The observations concerned three scientific programs, among which was the study of azimuthal structures in the wind and chromosphere of the pre-Main-Sequence Herbig Ae star AB Aur. The Hei 5876 A line of AB Aur, which is formed in the expanding chromosphere of this star, in the innermost parts of its wind, was monitored at a resolution of 30000, nearly continuously for about 4 days. A spectacular variability of this line was discovered, the profile changing from pure emission to a composite profile including a deep absorption component in the course of a few hours. This variability can be the signature of azimuthal structures in the wind of AB Aur. We present the data collected during the campaign, and discuss possible interpretations of the spectacular variations of the Hei 5876 A line.

Constraints on astronomical silicate dust

view Abstract Citations (19) References (16) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Constraints on Astronomical Silicate Dust Sorrell, Wilfred H. Abstract Numerical radiative-transfer models are used to discuss the properties of circumstellar dust grains around the premain-sequence star AB Aur (HD 31293). It is assumed that the dust consists of a silicate-graphite mixture with Draine and Lee (1984) optical properties. The modeling technique is to match the observed FUV through FIR energy distribution with the spectral energy distribution predicted for a spherical dust shell around a luminous hot star. Special attention is given to matching the observed 10-micron silicate emission feature and the observed circumstellar absorption curve at UV wavelengths, making it possible to strengthen constraints on dust-grain opacity and chemical composition. It is concluded that, although silicate grains can explain the observed 10-micron emission feature, the Draine and Lee silicate-graphite mixture cannot explain the observed FUV circumstellar absorption at the same time. The dust shell around AB Aur contains an additional population of small particles, the most likely candidate being amorphous carbon grains in a nonhydrogenated form. Publication: The Astrophysical Journal Pub Date: September 1990 DOI: 10.1086/169177 Bibcode: 1990ApJ...361..150S Keywords: Astronomical Models; Interstellar Matter; Pre-Main Sequence Stars; Silicates; Stellar Envelopes; Graphite; Infrared Spectra; Optical Properties; Radiative Transfer; Ultraviolet Spectra; Visible Spectrum; Astrophysics; INFRARED: SPECTRA; INTERSTELLAR: GRAINS; RADIATIVE TRANSFER; STARS: CIRCUMSTELLAR SHELLS; STARS: INDIVIDUAL CONSTELLATION NAME: AB AURIGAE full text sources ADS | data products SIMBAD (2)