AB Aur Research Articles

Spatially Resolved Circumstellar Structure of Herbig Ae/Be Stars in the Near‐Infrared

We have conducted the first systematic study of Herbig Ae/Be stars using the technique of long baseline stellar interferometry in the near-infrared, with the objective of characterizing the distribution and properties of the circumstellar dust responsible for the excess near-infrared fluxes from these systems. The observations for this work have been conducted at the Infrared Optical Telescope Array (IOTA). The principal result of this paper is that the interferometer resolves the source of infrared excess in 11 of the 15 systems surveyed. A new binary, MWC 361-A, has been detected interferometrically for the first time. The visibility data for all the sources has been interpreted within the context of four simple models which represent a range of plausible representations for the brightness distribution of the source of excess emission: a Gaussian, a narrow uniform ring, a flat blackbody disk with a single temperature power law, and an infrared companion. We find that the characteristic sizes of the near-infrared emitting regions are larger than previously thought (0.5-5.9 AU, as given by the FWHM of the Gaussian intensity). A further major result of this paper is that the sizes measured, when combined with the observed spectral energy distributions, essentially rule out accretion disk models represented by blackbody disks with the canonical T(r) ∝ r-3/4 law. We also find that, within the range observed in this study, none of the sources (except the new binary) shows varying visibilities as the orientation of the interferometer baseline changes. This is the expected behavior for sources which appear circularly symmetric on the sky, and for the sources with the largest baseline position angle coverage (AB Aur, MWC 1080-A) asymmetric brightness distributions (such as inclined disks or binaries) become highly unlikely. Taken as an ensemble, with no clear evidence in favor of axisymmetric structure, the observations favor the interpretation that the circumstellar dust is distributed in spherical envelopes (the Gaussian model) or thin shells (the ring model). This interpretation is also supported by the result that the measured sizes, combined with the excess near-infrared fluxes, imply emission of finite optical depth, as required by the fact that the central stars are optically visible. The measured sizes and brightnesses do not correlate strongly with the luminosity of the central star. Moreover, in two cases, the same excess is observed from circumstellar structures that differ in size by more than a factor of 2 and surround essentially identical stars. Therefore, different physical mechanisms for the near-infrared emission may be at work in different cases, or alternatively, a single underlying mechanism with the property that the same infrared excess is produced on very different physical scales.

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.

Infrared Interferometric Observations of Young Stellar Objects

We present infrared observations of four young stellar objects using the Palomar Testbed Interferometer (PTI). For three of the sources, T Tau, MWC 147 and SU Aur, the 2.2 micron emission is resolved at PTI's nominal fringe spacing of 4 milliarcsec (mas), while the emission region of AB Aur is over-resolved on this scale. We fit the observations with simple circumstellar material distributions and compare our data to the predictions of accretion disk models inferred from spectral energy distributions. We find that the infrared emission region is tenths of AU in size for T Tau and SU Aur and ~1 AU for MWC 147.

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.

Sub–Astronomical Unit Structure of the Near-Infrared Emission from AB Aurigae

We present near-infrared, long-baseline, interferometric observations of AB Aurigae, a well-known Herbig Ae/Be star, obtained with the Infrared Optical Telescope Array. The near-infrared emission from this source has been spatially resolved for the first time, and the observations indicate a characteristic diameter of 5 mas for the emission, corresponding to 0.7 AU at the distance of AB Aur. The data appear to be most consistent with models in which the circumstellar material around AB Aurigae lies in a flattened structure with a central large hole or optically thin region of radius about 0.3 AU.

Transient Infall Events in the Disk of AB Aurigae: The β Pictoris Phenomenon at 2–4 Megayears

Surveys of Herbig Ae/Be stars have indicated that infalling circumstellar gas similar to that routinely detected toward beta Pic is preferentially observed close to the disk midplane. Recent millimeter interferometry of the historically prototypical Herbig Ae star, AB Aur, indicates that this star is viewed slightly above the disk midplane (i = 76 deg). The wealth of archival spectroscopic data for this star makes it an ideal test of the geometry for infalling or accreting gas in these stars. We present a review of the extant IUE and Hubble Space Telescope GHRS spectral data for AB Aur and find, in contrast to previous studies of this star, which focused on Mg II, that infalling gas is detected at all epochs with covering factors similar to those reported for HD 100546 and that an episode of enhanced infall was serendipitously detected during the 1990 monitoring run.

High‐Resolution Far‐Infrared Studies of Intermediate‐Mass Pre–Main‐Sequence Objects

We have obtained high-resolution far-infrared maps of nine regions with 10 Herbig Ae/Be stars (intermediate-mass pre-main-sequence stars). Similar maps were obtained for 10 embedded IRAS sources with Sν(100 μm) > Sν(60 μm) and L ~ 200 L☉, which are possible evolutionary precursors of Herbig Ae/Be stars. Single far-infrared sources were found in most maps. The embedded sources have positions in agreement with those of the IRAS PSC, but some of the Herbig Ae/Be stars are offset significantly from the position of peak far-infrared emission. For all objects where it was possible to obtain 100 μm flux densities, they are consistent with those observed by IRAS, but derived 50 μm flux densities are larger than expected. The far-infrared maps reveal that objects in at least 17 of 19 emission regions are significantly extended at the 30''-40'' resolution of the Kuiper Airborne Observatory at 100 μm. Only sources associated with AB Aur and possibly IRAS 05338-0624 have unresolved far-infrared emission. Detailed analyses of the flux densities and positions from our maps suggest the far-infrared emission in regions with Herbig Ae/Be stars may not immediately surround these stars in all cases. Instead, far-infrared emission from these objects may originate from dust heated externally by the Herbig stars, or from dust heated internally by other sources. For other objects arguably surrounded by far-infrared emission, the Herbig stars or embedded IRAS objects have similar mean deconvolved sizes (i.e., 0.10-0.15 pc), but possibly have different mean deconvolved shapes (i.e., aspect ratios). Thus, far-infrared emission here may originate from flattened dust envelopes; the appearance of a far-infrared object as either a Herbig Ae/Be star or an embedded IRAS source may be merely a matter of viewing orientation.

Stellar Winds in Herbig Ae/Be Stars

The winds observed in the pre-main-sequence phase of intermediate-mass young stars are discussed in light of a model originally developed by Mestel for describing stellar winds ejected by rotating stars. In addition to the acceleration mechanisms of thermal expansion and stellar radiation pressure, which typically play an important role in driving the winds of low- and high-mass stars, respectively, centrifugal acceleration by a corotating magnetic field and the presence of hydromagnetic waves are taken into account. The relevance of these mechanisms for the acceleration of stellar winds from the surface of young stars is discussed in the context of growing evidence for the presence of magnetic fields and surface convection in pre-main-sequence evolutionary phases. In the particular case of Herbig Ae/Be stars, the deposition of a fraction of the convection energy in Alfven waves seems to be required in order to explain the winds observed in these pre-main-sequence objects. Under some simplifying assumptions, the velocity and density fields around the central stellar object are derived and the emerging line profiles are computed. A test case is discussed in which the observed line profiles of the prototypical object AB Aur are used to constrain the model parameters, showing that the magnetic rotator model offers a physically consistent description of this system.

A High‐Resolution Study of Gas and Dust around Young Intermediate‐Mass Stars: Evidence for Circumstellar Disks in Herbig Ae Systems

As part of a long-term program of observations to search for and characterize disks of gas and dust around intermediate-mass counterparts to solar-mass T Tauri stars, we have probed the environments of seven pre-main-sequence stars of spectral type Ae using millimeter-wave continuum and molecular line aperture synthesis imaging. In each case we identify a compact region of thermal continuum emission centered on the star. Upper limits to radii are in the range 200-300 AU for five members of our sample, and 680 AU for the distant source HD 245185. We identify an elongated continuum source around HD 163296, with a semimajor axis of 110 AU. Adopting relatively high values for dust grain opacities, we obtain minimum masses of circumstellar dust and gas in the range 0.005-0.034 M☉ for the seven sources, assuming that the observed continuum emission is optically thin. We detect molecular line emission from gas regions centered on four of the stars. Two of these regions are spatially resolved and are found to be elongated, with semimajor axes of 310 and 450 AU for HD 163296 and AB Aur, respectively. Ordered velocity gradients along the major axes of both of these structures point strongly to the presence of orbiting material in disklike configurations, and we argue that the nebular environments of our entire sample include substantial disk components.

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.

Spatially Resolved Mid-Infrared Observations of Circumstellar Dust around AB Aurigae

Spatially Resolved Mid-Infrared Observations of Circumstellar Dust around AB Aurigae

Coronagraphic imaging of pre-main-sequence stars: Remnant evvelopes of star formation seen in reflection

We have obtained R- and I-band coronagraphic images of the vicinities of 11 pre-main sequence (PMS) stars to search for faint, small-scale reflection nebulae. The inner radius of the search and the field of view are 1.9 arcsec and 1x1 arcmin, respectively. Reflection nebulae were imaged around RY Tau, T Tau,DG Tau, SU Aur, AB Aur, FU Ori, and Z CMa. No nebulae were detected around HBC 347, GG Tau, V773 Tau, and V830 Tau. Categorically speaking, most of the classical T Tauri program stars and all the FU Orionis-type program stars are associated with the reflection nebulae, while none of the weak-line T Tauri program stars are associated with nebulae. The detected nebulae range in size from 250 to 37 000 AU. From the brightness ratios of the stars and nebulae, we obtain a lower limit to the visual extinction of PMS star light through the nebulae of (A<SUB>V</SUB>)<SUB>neb</SUB> = 0.1. The lower limits of masses and volume densities of the nebulae associated with the classical T Tauri stars are 10(exp-6) Solar mass and N<SUB>H</SUB> = 10<SUP>5</SUP>/cu cm, respectively. Lower limits for the nebulae around FU Orionis stars are 10<SUP>-5</SUP> Solar mass and n<SUB>H</SUB> = 10 <SUP>5</SUP>/cu cm, respectively. Some reflection nebulae may trace the illuminated surfaces of the optically thick dust nebulae, so these mass estimates are not stringent. All the PMS stars with associated nebulae are strong far-infrared emitters. Both the far-infrared emission and the reflection nebulae appear to originate from the remnant envelopes of star formation. The 100 micrometers emitting regions of SU Aur and FU Ori are likely to be cospatial with the reflection nebulae. A spatial discontinuity between FU Ori and its reflection nebula may explain the dip in the far-infrared spectral energy distribution at 60 micrometers. The warped, disk-like nebulae around T Tau and Z CMa are aligned with and embrace the inner star/circumstellar disk systems. The arc-shaped nebula around DG Tau may be in contact with the coaligned inner star/disk system. These three-reflection nebulae may trace the surfaces of pseudo-disks from which matter accretes onto the stars or the inner circumstellar disks. 19 stellar objects brighter than I = 19 were detected around 9 program stars. Using a color-magnitude diagram, we have identified three new PMS candidates aroun Z CMa and one previously known PMS candidate, GG Tau/c.

Inhomogeneities in the circumstellar envelope of the A0E herbig star AB aur

Simultaneous high-resolution spectroscopy in Hα andUBVRI polarimetric observations are proposed as an effective method for the search for circumstellar inhomogeneities in A0-type Herbig stars. The new results for AB Aur are presented as a successful example of the use of this method. The analysis of about 100 CCD Hα profiles (R = 30 000) and more than 150 polarimetric measurements obtained in January, 1994 allowed to discover a long-lived stream-like inhomogeneity in the circumstellar gaseous envelope.

Observations of Short-term Dips in the Orion Variables DD Serpentis and AB Aurigae

AbstractShort-term fadings (dips) in the range of a few seconds with amplitudes from 0m.3 to 1m.2 were discovered in the two T Orions type variables DD Ser and AB Aur. Similar features were observed in T Tau and R CrB by other observers.

Submillimetre observations of Herbig Ae/Be systems

Pre-main-sequence, intermediate-mass stars have been observed in continuum emission between 0.35 and 1.3 mm in an effort to study the geometrical distribution of circumstellar dust. Flux measurements were made for a total of 13 Herbig Ae/Be systems, and the use of eight filter bandpasses has now essentially filled-in much of the submm portion of the spectral energy distributions of these objects for the first time. Eight of the sources are classified as Group I (possibly disc-dominated) systems by Hillenbrand et al. The remaining five sources belong to Group II and are characterized by substantial far-IR and mm-wave emission from extended dusty envelopes. The new measurements of 1 .3-mm flux provide rough estimates of the total mass of dust and gas in each source. It is shown that flat, disc-like distributions of material would be consistent with these masses for the Group I sources. However, it is also found that, with the exception of the Herbig Ae/Be system HD 163296, the same masses distributed within spherical envelopes matched to the 20-arcsec-FWHM beam would lead to V-band extinctions that are consistent with observed values towards these stars. To decide between the two geometries, the observed Group I submm spectral energy distributions (SEDs) have been compared with SEDs computed using a standard disc model |$[T(r)\propto r^{-3/4}]$|⁠. Two of the Group I sources - AB Aur and MWC 147 - can be modelled by assuming that a disc either dominates, or is entirely responsible for, all excess emission. Other geometrical distributions are not ruled out for these sources, but it is claimed that a pure disc model is not inconsistent with available measurements. Four of the Group I sources - MWC 137, Lk Hα 215, HD 163296 and MWC 1080 - have measured 0.35- or 0.45-mm flux densities which are significantly in excess of those which can be generated even by standard discs which are entirely optically thick at all wavelengths: long-wavelength emission from these sources is possibly dominated by flared discs and/or extended dust components. Finally, it is speculated that possible optically thin mm/submm disc emission in HD 163296 may provide indirect evidence of the presence of large grains.

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.

Rapid line profile variability of Hα in two A0e Herbig stars of the P Cyg-subgroup and a model for their circumstellar envelope

The present report gives the results on the Hα line profile investigation of two the most well-known A0e Herbig stars of the so-called P Cyg -subgroup: AB Aur and HD 163296. 35 high-resolution CCD spectra (R ~ 50 000) of HD 163296 (ESO, CAT+CES, July 1991 and 1992) and 43 CCD spectra (R ~ 30 000) of AB Aur (Crimean Observatory, 2.6-meter telescope, January 1993) were obtained. A striking profile variability is discovered in both objects on the timescale from one our to a few days. The most dominant part of rapid variations (τ ~ hours) is the monotonous flux drift of different profile components. Positional shift of sharp spectral bumps, found in HD 163296 is probably connected with local inhomogeneities moving in the envelope. Shape variability of the absorption P Cyg -component of the Hα -line in AB Aur is suspected to be periodic with P = 35 or 70 hours. A model for a circumstellar envelope is proposed to explain the observed variability in this type of objects. It supposes the existence of an active non-stable region near the star, formed by equatorially concentrated stellar wind and an outer cool shell.

Short-Term Spectral Variability in the Herbig Ae Star Ab Aur: Preliminary Analysis of Chromospheric Lines

The Herbig Ae stars are PMS objects of intermediate mass. Their location in the H-R diagram indicates that they are in the radiative phase of their quasi-static contraction toward the main sequence, i.e. that they do not possess outer convective zones, according to the standard stellar evolution theory (Iben, 1965; Gilliland, 1986). In spite of the expected absence of subphotospheric convective envelopes, these stars show remarkable signs of activity: emission in the Mg II h and k lines, presence of the CIV resonance lines at 1550 A and He I 5875.7 A line, Ca II IR triplet in emission, etc... Considering that stellar activity, witnessed by the same type of indicators in other parts of the H-R diagram, is generally attributed to dynamo magnetic fields and/or acoustic waves generated in the convection zone, these active phenomena are quite paradoxical in the Herbig Ae stars.The main question concerns the origin of their activity: is this activity linked to phenomena occurring within the stars, like e.g. dynamo-generated magnetic fields, or to an external agent, like e.g. a boundary layer between an accretion disk and the stellar surface? We already have some indirect clues that the activity of the Herbig Ae stars might be of magnetic origin (Praderie et al., 1986; Catala et al. 1986) observed a rotational modulation of lines formed in the wind of AB Aur, prototype of the Herbig Ae stars. By analogy with the solar wind, they proposed that the modulation might be due to the corotation of azimuthal structures in the wind, controlled by a surface magnetic field.

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)

Fiber Fed Spectrograph for Line Variability Studies

In our studies of activity in pre-main sequence Herbig Ae/Be stars we are mainly interested in searching rotational modulation of line profile. If the period of star rotation is of the order of one or two days, the data collected from a single site is insufficient. This led us to start correlated observations from two or three sites spread as much as possible in longitude. Our first bi-site observations started in 1982 on AB Aur from two observatories located 11 hours apart.: CFHT in Hawaï and OHP in France. To achieve a high flexibility and to gain access to telescopes without attached spectrographs. We built an instrument that is mobile and specially designed for line profile studies.