Herbig Ae Star AB Aur Research Articles

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.

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.

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.

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.

The well-known Herbig AE star AB Aur. I. Investigation of variability of the emission lines in the region 0.37?0.88 ?m

The variability of the profiles, equivalent widths (EW/sub /lambda//), and relative intensities of of the emission lines in the spectrum of AB Aur has been investigated with a photoelectric scanner. During a 20-day period of observations, EW/sub /lambda//H/sub /alpha// varied from 23.20 to 35.35 /angstrom/. The mean value was EW/sub /lambda//H/sub /alpha// = 27.25 /angstrom/, the rms fluctuation 3.60 /plus minus/ 0.07 /angstrom/. The shortest variability time was 1 h. The chromospheric lines of the infrared Ca II triplet, K Ca II, the emission lines H/beta/-H13, P12-P20, OI 8446.5 /angstrom/ and others were investigated using spectra obtained photographically or with an image tube. The intensities of these lines vary by factors 2-4 during a time from one hour to several years. The nature of the variability of the emission lines of AB Aur suggests that the deep chromosphere is not a centrally symmetric or axisymmetric formation but rather a conglomerate of gas condensations of different densities and velocities.

Short-term spectral variability in AB Aurigae: Clues for activity in Herbig AE stars. II - The CA II K line

view Abstract Citations (63) References (29) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Short-Term Spectral Variability in AB Aurigae: Clues for Activity in Herbig AE Stars. II. The CA II K Line Catala, C. ; Felenbok, P. ; Czarny, J. ; Talavera, A. ; Boesgaard, A. M. Abstract The Herbig Ae star AB Aur (A0ep) was monitored with the CFH 3.6 m and the Observatoire de Haute-Provence 1.52 m telescopes in the Ca II K line, for six nights in 1982 October. Some of the 39 recorded spectra were simultaneous with ultraviolet spectra obtained with IUE. The Ca II K line is variable and modulated with a period of 32±4 hours, corresponding to the estimated rotation period of the star. The difference between the 45 hour period found in the Mg II data (Paper I) and the 32 hour period in the Ca II data is assumed as due to a differential rotation in the envelope of the star. This differential rotation allows an estimate of upper limits for the magnetic field and the Alfvén radius in the envelope of AB Aur, as well as for the rate of angular-momentum loss. Publication: The Astrophysical Journal Pub Date: September 1986 DOI: 10.1086/164551 Bibcode: 1986ApJ...308..791C Keywords: A Stars; Abundance; B Stars; Line Spectra; Stellar Spectra; Stellar Spectrophotometry; Calcium; Emission Spectra; K Lines; Red Shift; Stellar Rotation; Astrophysics; STARS: EMISSION-LINE; STARS: INDIVIDUAL CONSTELLATION NAME: AB AURIGAE; STARS: ROTATION; STARS: SPECTRUM VARIABLES full text sources ADS | data products SIMBAD (1) Related Materials (1) Part 1: 1986ApJ...303..311P

Short-term spectral variability in AB Aurigae - Clues for activity in Herbig AE stars. I - The ultraviolet lines of MG II and Fe II

IUE satellite monitoring of the Herbig Ae star AB Aur in 1982 and 1984, with attention to the variability of both the Mg II 2795 A wind profile and a number of Fe II UV lines, has indicated temporal variations in the Mg II blue wing velocity by 30-50 percent at both epochs. By contrast, the Fe II spectrum was constant in 1982, but variable in 1984. There is no evidence of photometric variability in AB Aur during either period. An analysis of the Mg II blue wing velocity vs time shows that the variability can be fitted with a sine curve of period 45 + or - 6 yr, which is interpreted to be the stellar rotation period. The observed rotational modulation of the blue wing velocity of Mg II 2795 A in AB Aur's wind is the first direct evidence of a nonaxisymmetric wind in an A-type premain sequence object.